EP2370353A2

EP2370353A2 - Cerium and/or terbium phosphate, optionally with lanthanum, phosphor resulting from said phosphate, and methods for making same

Info

Publication number: EP2370353A2
Application number: EP09752387A
Authority: EP
Inventors: Valérie BUISSETTE; Thierry Le-Mercier
Original assignee: Rhodia Operations SAS
Current assignee: Rhodia Operations SAS
Priority date: 2008-11-20
Filing date: 2009-11-18
Publication date: 2011-10-05
Also published as: US20110272635A1; FR2938526A1; KR20110079729A; JP2012509379A; WO2010057924A3; CN102216210A; CA2741980A1; CA2741980C; FR2938526B1; KR20130081307A; WO2010057924A2; JP5480909B2; KR101407783B1; KR101342358B1

Abstract

The invention relates to a rare earth element phosphate (Ln), where Ln is either at least one rare earth element selected from cerium and terbium, or lanthanum in combination with at least one of the above two rare earth elements, having a crystalline structure of the rhabdophane type or of the monazite type with a lithium content of 300 ppm at most. The phosphate is obtained by the precipitation of a rare earth element chloride at a constant pH lower than 2, and then calcining and redispersing the same in hot water. The invention also relates to a phosphor obtained by calcining the phosphate at at least 1000°C.

Description

CERIUM PHOSPHATE AND / OR TERBIUM, POSSIBLY

WITH LANTHANUM, LUMINOPHORE FROM THIS PHOSPHATE AND

METHODS OF PREPARATION THEREOF

The present invention relates to a cerium and / or terbium phosphate, optionally with lanthanum, a phosphor derived from this phosphate and processes for their preparation.

Mixed phosphates of lanthanum, terbium and cerium and mixed phosphates of lanthanum and terbium, hereinafter generally referred to as LAP, are well known for their luminescence properties. For example, when they contain cerium and terbium, they emit a bright green light when they are irradiated by certain energetic radiations of wavelengths lower than those of the visible range (UV or VUV radiation for lighting systems or visualization). Phosphors exploiting this property are commonly used on an industrial scale, for example in fluorescent tri-chromium lamps, in backlight systems for liquid crystal displays or in plasma systems. Several processes for the preparation of LAPs are known. These methods are of two types. There are first of all so-called "dry" processes in which a mixture of oxides or a mixed oxide is phosphated in the presence of diammonium phosphate. These processes, which may be relatively long and complicated, pose a particular problem for the control of the size and chemical homogeneity of the products obtained. The other type of process groups together those referred to as "wet processes" where a synthesis is carried out. in a liquid medium, a mixed rare earth phosphate or a mixture of rare earth phosphates.

These different syntheses lead to mixed phosphates requiring, for their application in luminescence, a heat treatment at high temperature, about 1100 ⁰ C, under a reducing atmosphere, generally in the presence of a fluxing agent or flux. Indeed, for the mixed phosphate is a phosphor as effective as possible, it is necessary that terbium and, where appropriate, cerium are in the 3+ oxidation state. The aforementioned dry and wet methods have the disadvantage of leading to phosphors of uncontrolled granulometry, in particular insufficiently tightened, which is further accentuated by the need for the high temperature activation heat treatment, under flux and under a reducing atmosphere, which, in general, still induces disturbances in the particle size, thus leading to non-homogeneous phosphor particles in sizes, which may furthermore contain greater or lesser amounts of impurities bound in particular to the use of the flux, and finally having insufficient luminescence performance.

It has been proposed in the patent application EP 0581621 a process for improving the particle size of the LAPs, with a narrow particle size distribution, which leads to particularly efficient phosphors. The method described uses nitrates more particularly as rare earth salts and recommends the use of ammonia as a base which has the disadvantage of a rejection of nitrogen products. Consequently, if the process leads to efficient products, its implementation can be made more complicated to comply with increasingly stringent environmental legislation that outlaws or limits such discharges.

It is certainly possible to use, in particular, strong bases other than ammonia, such as alkali hydroxides, but these induce the presence of alkalis in the LAPs and this presence is considered likely to degrade the luminescence properties of the phosphors during their use, especially in mercury vapor lamps.

There is therefore currently a need for preparation processes using little or no nitrates or ammonia, or not requiring the use of flux during the preparation of phosphors and this without negative consequences on the properties of luminescence of the products obtained.

An object of the invention is the development of a process for the preparation of LAP limiting the rejection of nitrogen products, or even without rejecting these products.

Another object of the invention is to provide phosphors which nevertheless have the same properties as those of currently known phosphors, or even superior properties.

For this purpose, according to a first aspect, the invention provides a rare earth phosphate (Ln) Ln representing either at least one rare earth chosen from cerium and terbium, or lanthanum in combination with at least one of the two rare earths mentioned above, characterized in that it has a crystalline structure of either rhabdophane or mixed rhabdophane / monazite type and in that it contains lithium, the lithium content being at most 300 ppm.

The invention also relates to a rare earth phosphate (Ln), Ln having the same meaning as above, and which is characterized in that has a crystalline structure of monazite type and in that it contains lithium, the lithium content being at most 300 ppm.

According to another aspect, the invention also relates to a phosphor based on a rare earth phosphate (Ln), Ln having the same meaning as above, and which is characterized in that it has a crystalline structure of monazite type and in that it contains lithium, the lithium content being at most 75 ppm.

The phosphors of the invention, despite the presence of an alkali, lithium, have good luminescence properties and good durability. They may even have a better luminescence yield than the known products.

Other features, details and advantages of the invention will appear even more fully on reading the description which follows, as well as various concrete but non-limiting examples intended to illustrate it. It is also specified for the remainder of the description that, unless otherwise indicated, in all ranges or limits of values that are given, the values at the terminals are included, the ranges or limits of values thus defined thus covering any value at least equal to and greater than the lower bound and / or at most equal to or less than the upper bound. With regard to the lithium contents mentioned in the following description for phosphates and phosphors, it will be noted that minimum values and maximum values are given. It should be understood that the invention covers any range of lithium content defined by any of these minimum values with any one of these maximum values.

For rare earth is meant for the remainder of the description the elements of the group consisting of yttrium and the elements of the periodic classification of atomic number inclusive between 57 and 71.

It is also specified here and for the whole of the description that the measurement of the lithium content is made according to two techniques. The first is the X-ray fluorescence technique and it can measure lithium levels that are at least about 100 ppm. This technique will be used more particularly for phosphates or precursors or phosphors for which the lithium contents are the highest. The second technique is the Inductively Coupled Plasma (ICP) technique - AES (Atomic Emission Spectroscopy) or ICP - OES (Optical Emission Spectroscopy). This technique will be used more particularly here for the precursors or the phosphors for which the lithium contents are the lowest, especially for contents of less than about 100 ppm.

As has been seen above, the invention relates to two types of products: phosphates, also called precursors, and phosphors obtained from these precursors. The luminophores themselves have sufficient luminescence properties to make them directly usable in the desired applications. The precursors have no luminescence properties or possibly luminescence properties that are too low for use in these same applications. These two types of products will now be described more precisely.

Phosphates or precursors

The phosphates of the invention are in two embodiments which differ from each other in the crystallographic structure of the products. The characteristics common to these two embodiments will be described first.

The phosphates of the invention are essentially, the presence of other residual phosphate species being indeed possible, and preferably completely orthophosphate type of formula LnPO ₄ , Ln being as defined above.

The phosphates of the invention are cerium or terbium phosphates or a combination of these two rare earths. It can also be lanthanum phosphates in combination with at least one of these two rare earths mentioned above and it can also be very particularly lanthanum, cerium and terbium phosphates.

The respective proportions of these different rare earths may vary within wide limits and, more particularly, in the range of values that will be given below. Thus, the phosphates of the invention essentially comprise a product which can satisfy the following general formula (1):

La _x Ce _y Tb _z PO ₄ (1) wherein the sum x + y + z is equal to 1 and at least one of y and z is different from 0.

In formula (1) above, x may be more particularly between 0.2 and 0.98 and even more particularly between 0.4 and 0.95.

The presence of the other phosphate species mentioned above may result in the molar ratio Ln (all the rare earths) / PO ₄ being less than 1 for all the phosphate. If at least one of x and y is different from O in formula (1), preferably z is at most 0.5, and z may be between 0.05 and 0.2 and more preferably between 0. , 1 and 0.2.

If y and z are both different from 0, x may range from 0.2 to 0.7 and more particularly from 0.3 to 0.6.

If z is 0, y may be more particularly between 0.02 and 0.5 and even more particularly between 0.05 and 0.25.

If y is 0, z may be more particularly between 0.05 and 0.6 and even more particularly between 0.08 and 0.3. If x is equal to 0, z can be more particularly between 0.1 and

0.4.

By way of examples only, the following particular compositions may be mentioned:

Lao _{, 44} Ceo _, 4Tbo _, i3PO ₄ The _o , 57Ce _o , 29Tbo, -I ₄ PO ₄

The phosphate of the invention may comprise other elements that typically play a role, in particular a promoter of the luminescence properties or of stabilizing the oxidation levels of the cerium and terbium elements. By way of example of these elements, mention may be made more particularly of boron and other rare earths such as scandium, yttrium, lutetium and gadolinium. When lanthanum is present, the aforementioned rare earths may be more particularly present in substitution for this element. These promoter or stabilizer elements are present in an amount generally of at most 1% by weight of element relative to the total weight of the phosphate of the invention in the case of boron and generally at most 30% for the others. elements mentioned above.

The phosphates of the invention are also characterized by their granulometry.

They consist in fact of particles generally having an average size of between 1 and 15 μm, more particularly between 2 μm and 6 μm.

The average diameter referred to is the volume average of the diameters of a particle population.

The granulometry values given here and for the remainder of the description are measured by means of a Malvern type laser granulometer. on a sample of particles dispersed in ultrasonic water (130 W) for 1 minute 30 seconds.

Moreover, the particles preferably have a low dispersion index, typically at most 0.5, preferably at most 0.4. By "dispersion index" of a population of particles is meant, for the purposes of this description, the ratio I as defined below: where: 0s ₄ is the particle diameter for which 84% of the particles have a diameter less than 0s ₄ ; 016 is the particle diameter for which 16% of the particles have a diameter less than 0-ι ₆ ; and

050 is the average particle diameter, diameter for which 50% of particles have a diameter less than 0 ₅ o

This definition of the dispersion index given here for the precursor particles also applies for the remainder of the phosphor description.

An important characteristic of the phosphates of the invention and common to both embodiments is the presence of lithium. The amount of lithium depends on the embodiment. It may be thought that lithium is not present in phosphate simply in mixture with the other constituents of it but that it is chemically bound with one or more constituent chemical elements of phosphate.

According to a particular embodiment, the phosphates contain only lithium as an alkaline element. The two embodiments of the phosphates of the invention also have more specific characteristics which will be described below.

For the first embodiment of the invention, the phosphate has a crystalline structure of rhabdophane type or mixed type rhabdophane / monazite. The crystalline structures referred to herein and in the remainder of the description can be demonstrated by the X-ray diffraction technique (XRD).

The phosphates can thus have a rhabdophane type structure and they can be in this case phasically pure, that is to say that the XRD diagrams show only a single phase rhabdophane. Nevertheless, the phosphates of the invention may also not be phasically pure and in this case, the X-ray product diagrams show the presence of very minor residual phases. Phosphates may also have a mixed-type rhabdophane / monazite structure.

The rhabdophane or mixed rhabdophane / monazite structure corresponds to phosphates that have not undergone heat treatment at the end of their preparation or have undergone heat treatment at a temperature generally below 600 ^° C., in particular between 400 ^° C. and 500 ° C. The lithium content of the phosphate according to this first embodiment is at most 300 ppm, more particularly at most 250 ppm and even more particularly at most 100 ppm. This content is expressed, here and for the entire description, in mass of lithium element relative to the total mass of phosphate.

The minimum lithium content is not critical. It may correspond to the minimum value detectable by the analysis technique used to measure the lithium content. However, generally this minimum lithium content is generally at least 10 ppm and more particularly at least

90 ppm.

The phosphate of rhabdophane-type crystalline structure consists of particles that are themselves composed of an aggregation of crystallites whose size, measured in the (012) plane, is at least 25 nm, more particularly at least 30 ppm. . This size may vary depending on the temperature of the heat treatment or the calcination suffered by the precursor during its preparation.

It is specified here and for the whole of the description that the value measured in XRD corresponds to the size of the coherent domain calculated from the width of the main diffraction line corresponding to the crystallographic plane (012). The Scherrer model is used for this measurement, as described in the book "Theory and technique of radio-stallography", A. Guinier, Dunod, Paris, 1956.

It should be noted that the description that has just been given on the crystallite size applies essentially to the case of phosphates of rhabdophane structure because the determination of this size by the XRD technique becomes much more difficult in the case of a structure of mixed type rhabdophane / monazite.

This size of crystallite, which is greater than that of phosphates of the prior art obtained after a heat treatment at the same temperature and which may also have the same particle size, results in a better crystallization of the products. The phosphates of the first embodiment which have not undergone heat treatment are generally hydrated; however, simple drying, operated for example between 60 and 100 ⁰ C, sufficient to remove most of this residual water and lead to substantially anhydrous rare earth phosphates, the minor amounts of remaining water being removed by calcinations conducted at higher temperatures and above about 400 ^° C.

For the second embodiment, the phosphates have a monazite-type crystalline structure which corresponds to products which are obtained after a higher heat treatment than in the case of the phosphates of the first mode and operated at a temperature of at least 600 ° C. C, advantageously between 700 and 1000 ^° C.

As for the previous embodiment, the phosphates can in this case be phasically pure, that is to say that the XRD diagrams show only one single monazite phase. Nevertheless, the phosphates of the invention may also not be phasically pure and in this case, the X-ray product diagrams show the presence of very minor residual phases.

The lithium content of the phosphate according to this second embodiment is at most 300 ppm, more particularly at most 250 ppm.

As for the first embodiment, the minimum lithium content is not critical and may correspond to the minimum value detectable by the analysis technique used to measure this content.

However, generally this minimum lithium content is generally at least 10 ppm, more particularly at least 90 ppm.

Phosphates of monazite crystalline structure consist of particles themselves consisting of an aggregation of crystallites whose size, measured in the (012) plane, is at least 70 nm, more particularly at least 80 nm, this size also varies according to the calcination temperature suffered by the precursor during its preparation. Here again, the same remark can be made here as before concerning the best crystallization of the phosphates of the invention compared with phosphates of the same structure of the prior art.

Although the phosphates or precursors according to the invention have, for those having undergone a calcination or a heat treatment at a temperature generally greater than 600 ° C., and advantageously between 800 and 900 ° C., luminescence properties at lengths of variable wave according to the composition of the product and after exposure to a radius of given wavelength (for example emission at a wavelength of about 550 nm, ie in green after exposure to a wavelength of 254 nm for lanthanum phosphate, of cerium and terbium), it is possible and even necessary to further improve these luminescence properties by proceeding with the products to post-treatments, and this in order to obtain a real luminophore directly usable as such in the desired application.

It is understood that the boundary between a single rare earth phosphate and a real phosphor remains arbitrary, and depends on the only luminescence threshold from which it is considered that a product can be directly implemented in a manner acceptable to a user.

In the present case, and in a rather general manner, it is possible to consider and identify as phosphor precursors rare earth phosphates according to the invention which have not been subjected to heat treatments greater than about 900 ^° C., since such The products generally have luminescence properties which can be judged as not satisfying the minimum brightness criterion of commercial phosphors which can be used directly and as such without any subsequent processing. On the other hand, rare earth phosphates which, possibly after being subjected to appropriate treatments, develop suitable glosses which are sufficient to be used directly by an applicator, for example in lamps or lamps, can be qualified as phosphors. television screens.

The description of the phosphors according to the invention will be made below.

The luminophores

The phosphors of the invention have characteristics in common with the phosphates or precursors which have just been described.

Thus, they have the same granulomethane characteristics as these, ie an average particle size of between 1 and 15 μm with a dispersion index of at most 0.5. All that has been described above about granulometry for precursors applies likewise here.

They also have, in a form of orthophosphate of the same formula as that given above, a composition substantially identical to that of the precursors. The relative proportions of lanthanum, cerium and terbium which have been given above for the precursors also apply here. Similarly, they may include the promoters or stabilizers that have been mentioned above for phosphates and in the proportions indicated.

The phosphors have a crystalline structure of the monazite type. According to a preferred embodiment, the phosphors of the invention are phasically pure, that is to say that the XRD diagrams show only the single monazite phase. Nevertheless, the phosphors of the invention may also not be phasically pure and in this case, the X-ray product diagrams show the presence of very minor residual phases. They contain lithium in a content of not more than 75 ppm, more particularly not more than 50 nm. This content is also expressed in mass of lithium element relative to the total mass of the phosphor.

The minimum lithium content is not critical. Here again, as for phosphates, it can correspond to the minimum value detectable by the analysis technique used to measure the content of this element. However, generally this minimum lithium content may be at least 10 ppm.

According to a particular embodiment, the phosphors do not contain any other alkaline element than lithium.

The phosphors of the invention consist of particles whose coherence length, measured in the (012) plane, is at least 300 nm.

This length, which is measured by the same technique as for the precursors, may vary depending on the temperature of the heat treatment or calcination undergone by the phosphor during its preparation.

This coherence length may be more particularly at least 350 nm.

As for the precursors, it is also observed here that this length of coherence is greater than those of the phosphors of the prior art obtained after heat treatment at the same temperature and may also have the same particle size. This again reflects a better crystallization of the products which is beneficial for their luminescence property, especially for the luminescence yield.

The particles constituting the phosphors of the invention may have a substantially spherical shape. These particles are dense.

The processes for preparing precursors and phosphors of the invention will now be described. The processes for preparing the phosphates or precursors The first method which will be described concerns the preparation of the precursors according to the first embodiment, that is to say those with a crystalline structure, either of the rhabdophane type or of the mixed rhabdophane / monazite type. This method is characterized in that it comprises the following steps:

a first solution containing rare earth chlorides (Ln) is continuously introduced into a second solution containing phosphate ions and having an initial pH of less than 2;

during the introduction of the first solution in the second, the pH of the medium thus obtained is checked at a constant value and less than 2, whereby a precipitate is obtained, the pH at second solution for the first step or the pH control for the second step or both being carried out at least in part with lithium hydroxide;

the precipitate thus obtained is recovered and, if appropriate, calcined at a temperature of less than 600 ^° C.

the product obtained is redispersed in hot water and then separated from the liquid medium.

The different steps of the process will now be detailed. According to the invention, a direct precipitation is carried out at controlled pH of a rare earth phosphate (Ln), and this by reacting a first solution containing chlorides of one or more rare earths (Ln), these elements being then present in the proportions required to obtain the desired composition product, with a second solution containing phosphate ions. According to a first important characteristic of the process, a certain order of introduction of the reagents must be respected, and more precisely still, the chlorides solution of the rare earth element (s) must be introduced, progressively and continuously, into the solution containing the phosphate ions. According to a second important characteristic of the process according to the invention, the initial pH of the solution containing the phosphate ions must be less than 2, and preferably between 1 and 2.

According to a third characteristic, the pH of the precipitation medium must then be controlled at a pH value of less than 2, and preferably of between 1 and 2.

By "controlled pH" is meant a maintenance of the pH of the precipitation medium to a certain value, constant or substantially constant, by addition of a basic compound in the solution containing the phosphate ions, and this simultaneously with the introduction into the latter of the solution containing the rare earth chlorides. The pH of the medium will thus vary by at most 0.5 pH units around the fixed setpoint, and more preferably by at most 0.1 pH units around this value. The fixed set value will advantageously correspond to the initial pH (less than 2) of the solution containing the phosphate ions.

Precipitation is preferably carried out in an aqueous medium at a temperature which is not critical and which is advantageously between room temperature (15 ° C - 25 ° C) and 100 ^° C. This precipitation takes place with stirring. reaction medium.

The concentrations of rare earth chlorides in the first solution can vary within wide limits. Thus, the total concentration of rare earths can be between 0.01 mol / liter and 3 mol / liter.

Note finally that the solution of rare earth chlorides may further comprise other metal salts, including chlorides, such as salts of the promoter or stabilizer elements described above, that is to say boron and other rare earths.

Phosphate ions intended to react with the solution of the rare earth chlorides can be provided by pure or in solution compounds, for example phosphoric acid, alkali phosphates or other metallic elements giving with the anions associated with the rare earths a soluble compound.

The phosphate ions are present in such a quantity that there is, between the two solutions, a molar ratio PO ₄ / Ln greater than 1, and advantageously between 1, 1 and 3.

As pointed out above in the description, the solution containing the phosphate ions must initially have (ie before the beginning of the introduction of the solution of rare earth chlorides) a pH of less than 2, and preferably included between 1 and 2. Also, if the solution used does not naturally have such a pH, it is brought to the desired suitable value either by adding a basic compound or by adding an acid (for example, hydrochloric acid, in the case of an initial solution of too high pH).

Subsequently, during the introduction of the solution containing the rare earth chloride (s), the pH of the precipitation medium gradually decreases; also, according to one of the essential features of the process according to the invention, in order to maintain the pH of the precipitation medium at the desired constant work value, which must be less than 2 and preferably between 1 and 2, is introduced simultaneously into this medium a basic compound.

According to another characteristic of the process of the invention, the basic compound which is used either to bring the initial pH of the second solution containing the phosphate ions to a value of less than 2 or to control the pH during the precipitation is, at less in part, lithium hydroxide. By "at least in part" is meant that it is possible to use a mixture of basic compounds of which at least one is lithium hydroxide. The other basic compound may be, for example, ammonia. According to a preferred embodiment, a basic compound which is solely lithium hydroxide is used and according to another even more preferred embodiment, lithium hydroxide alone is used and for the two abovementioned operations, that is to say both to bring the pH of the second solution to the proper value and to control the pH of precipitation. In these two preferred embodiments, the rejection of nitrogen products which may be provided by a basic compound such as ammonia is reduced or eliminated.

At the end of the precipitation step, a phosphate of rare earth (Ln) is obtained directly, possibly additive by other elements. The overall concentration of rare earths in the final precipitation medium is then advantageously greater than 0.25 mol / liter.

At the end of the precipitation, it is possible to carry out a maturing process by maintaining the reaction medium obtained previously at a temperature situated in the same temperature range as that at which the precipitation took place and for a period which can be understood between a quarter of an hour and an hour for example.

The phosphate precipitate can be recovered by any means known per se, in particular by simple filtration. Indeed, under the conditions of the process according to the invention, a non-gelatinous rare earth phosphate is precipitated and easily filtered.

The recovered product is then washed, for example with water, and then dried.

The product can then be subjected to heat treatment or calcination. The temperature and duration of this calcination is a function of the desired crystalline structure for the phosphate that will be derived therefrom. Generally a calcination temperature up to about 400 ⁰ C provides a product rhabdophane structure, which is also that presented by the non-calcined product from precipitation. For the structure mixed rhabdophane / monazite, the calcination temperature is generally at least 400 ⁰ C and can go up to a temperature below 600 ⁰ C; it can thus be between 400 ° C. and 500 ^° C.

By way of example only, this duration can be between 1 and 3 hours.

Heat treatment is usually done under air. The crystallite size of the phosphate will be greater the higher the calcination temperature.

According to another important characteristic of the invention, the product resulting from the calcination or from the precipitation in the absence of heat treatment is then redispersed in hot water.

This redispersion is done by introducing the solid product into the water and stirring. The suspension thus obtained is kept stirring for a period which may be between 1 and 6 hours, more particularly between 1 and 3 hours.

The temperature of the water may be at least 30 ° C, more preferably at least 60 ° C and may be between about 30 ^° C and 90 ° C, preferably between 60 ° C and 90 ^° C under atmospheric pressure. It is possible to carry out this operation under pressure, for example in an autoclave, at a temperature which can then be between 100 ° C. and 200 ° C., more particularly between 100 ^° C. and 150 ° C.

In a final step is separated by any known means, for example by simple filtration of the solid liquid medium. It is possible to repeat the redispersion step one or more times under the conditions described above, possibly at a temperature different from that at which the first redispersion was conducted.

The separated product can be washed, especially with water, and can be dried. This gives the rare earth phosphate (Ln) structure rhabdophane or rhabdophane / monazite of the invention and having the required lithium levels.

The process for the preparation of rare earth phosphates (Ln) according to the second embodiment of the invention, that is to say with a crystalline structure of monazite type, is very similar to that which has just been described. It differs only in that the product resulting from the precipitation is calcined at a temperature of at least 600 ° C. What has been described above for the preceding steps in the process for the preparation of the phosphates of the first mode is therefore likewise applicable here for the process for the preparation of phosphates of the second mode. The heat treatment or calcination can be carried out more particularly at a temperature of between 800 and 900 ^° C.

Here again, the crystallite size of the phosphate will be greater the higher the calcination temperature.

The rest of the process and in particular the redispersion step in water is identical to that described above for the phosphates according to the first embodiment.

The process for preparing phosphors The phosphors of the invention are obtained by calcination at a temperature of at least 1000 ^° C. of a phosphate or precursor according to the two embodiments as described above or of a phosphate or precursor obtained by the methods which have also been described previously. This temperature can be between 1000 ° C and 1300 ⁰ C. By this treatment, the phosphates or precursors are converted into effective phosphors.

The calcination can be carried out under air, under an inert gas but also and preferably under a reducing atmosphere (H ₂ , N ₂ / H ₂ or Ar / H ₂ for example), in the latter case, to convert all the species This and Tb at their oxidation state (+ III).

In a known manner, the calcination can be carried out in the presence of a flux or fluxing agent such as, for example, lithium fluoride, lithium tetraborate, lithium chloride, lithium carbonate, lithium phosphate, chloride of potassium, ammonium chloride, boric oxide and boric acid and ammonium phosphates, and mixtures thereof.

In the case of the use of a flux, a luminophore is obtained which exhibits luminescence properties which, generally, are at least equivalent to those of known phosphors. The advantage of the most important invention here is that the phosphors come from precursors which are themselves derived from a process that rejects less nitrogen products than known methods or not at all.

It is also possible to carry out the calcination in the absence of any flux, thus without prior mixing of the fluxing agent with the phosphate, which contributes to reducing the level of impurities present in the phosphor. In addition, it avoids the use of products that may contain nitrogen or whose implementation must be done in strict safety standards given their possible toxicity, which is the case for a large number of agents. fondants mentioned above. Still in the case of calcination without flux it is found that the precursors of the invention make it possible to obtain phosphors whose luminescence properties are at least equivalent to those of phosphors obtained from precursors of the prior art. After treatment, the particles are advantageously washed, so as to obtain the purest phosphor possible and in a deagglomerated or weakly agglomerated state. In the latter case, it is possible to deagglomerate the phosphor by subjecting it to deagglomeration treatment under mild conditions. The luminophores of the invention exhibit intense luminescence properties for electromagnetic excitations corresponding to the various absorption domains of the product.

Thus, the cerium and terbium phosphors of the invention can be used in lighting or visualization systems having an excitation source in the UV range (200-280 nm), for example around 254 nm . Of particular note are mercury vapor trichromatic lamps, backlighting lamps for liquid crystal systems, in tubular or planar form (LCD Back Lighting). They exhibit a high gloss under UV excitation, and a lack of luminescence loss as a result of thermal post-treatment. Their luminescence is particularly stable under UV at relatively high temperatures (100 - 300 ⁰ C).

The phosphors based on terbium and lanthanum or lanthanum, cerium and terbium of the invention are also good candidates as green phosphors for VUV (or "plasma") excitation systems, such as for example plasma screens. and mercury-free trichromatic lamps, especially Xenon excitation lamps (tubular or planar). The luminophores of the invention have a high green emission under VUV excitation (for example, around 147 nm and 172 nm). The phosphors are stable under VUV excitation.

The phosphors of the invention can also be used as green phosphors in LED devices. They can be used especially in systems excitable in the near UV. They can also be used in UV excitation labeling systems.

The luminophores of the invention can be implemented in lamp and screen systems by well known techniques, for example by for example by screen printing, spraying, electrophoresis or sedimentation.

They can also be dispersed in organic matrices (for example, plastic matrices or transparent polymers under UV ...), mineral (for example, silica) or organo-mineral hybrids.

According to another aspect, the invention also relates to luminescent devices of the above-mentioned type, comprising, as a source of green luminescence, the phosphors as described above or the phosphors obtained from processes described above. Examples will now be given.

In these examples, the lithium content is determined, as indicated above, by two measurement techniques. For the X-ray fluorescence technique, it is a semi-quantitative analysis performed on the powder of the product as such. The apparatus used is PANalytical's MagiX PRO PW 2540 Fluorescence Spectrometer. The ICP-AES (or OES) technique is carried out by performing a quantitative assay by additions dosed with a ULTIMA device of JOBIN YVON. The samples are first subjected to mineralization (or digestion) in nitric-perchloric medium assisted by microwaves in closed reactors. (MARS system - CEM).

The luminescence efficiency is measured on powdered products by comparing the areas under the emission spectrum curve between 380 nm and 750 nm recorded with a spectrofluorometer under excitation of 254 nm and assigning a value of 100% to the obtained for the comparative product.

EXAMPLE 1 COMPARATIVE

This example relates to the preparation of a lanthanum phosphate, cerium and terbium according to the prior art. In 1 L of a solution of 1.73 mol / L of phosphoric acid H ₃ PO ₄ analytical grade previously brought to pH 1, 6 by addition of ammonia and brought to 60 ⁰ C, are added in one hour 1 L of a solution of rare earth nitrates of purity 4N, of overall concentration 1, 5 mol / L and decomposing as follows: 0.66 mol / L of lanthanum nitrate, 0.65 mol / L of cerium nitrate and 0.20 mol / L of terbium nitrate. The pH during the precipitation is regulated to 1, 6 by addition of ammonia.

At the end of the precipitation step, the mixture is further maintained for 1 h at 60 ^° C. The resulting precipitate is then recovered by filtration, washed with water and then dried at 60 ^° C. under air and then subjected to a heat treatment for 2 h at 840 ^° C. under air. Is obtained at the end of this step a precursor composition (Lao, ₄₄ Ceo, ₄ 3Tbo, i3) PO _4.

EXAMPLE 2

This example relates to the preparation of a lanthanum phosphate, cerium and terbium according to the invention.

In 1 L of a solution of 1.5 mol / L of phosphoric acid H ₃ PO ₄ analytical grade previously brought to pH 1, 6 by addition of LiOH and heated to 60 ° C, are added in one hour 1 L of a solution of rare earth chlorides of 4N purity, of overall concentration 1.3 mol / L and decomposing as follows: 0.57 mol / L of lanthanum chloride, 0.56 mol / L of cerium chloride and 0.17 mol / L of terbium chloride. The pH during the precipitation is regulated to 1, 6 by adding LiOH. After the precipitation step, the mixture is further maintained for 15 minutes at 60 ^° C. The resulting precipitate is then recovered by filtration, washed with water and then dried at 60 ° C in air and then subjected to a heat treatment of 2h to 840 ° C under air. At the end of the calcination, the product obtained is redispersed in water at 80 ^° C. for 3 hours, then washed and filtered, and finally dried. Is obtained at the end of this step a precursor composition (Lao, ₄₄ Ceo, ₄ 3 ^~ rbo, i3) PO _4.

The characteristics of the products of Examples 1 and 2 are presented in Table 1 below.

Table 1

The precursor phosphate of the invention is better crystallized than that of the prior art while maintaining similar granulometric characteristics. COMPARATIVE EXAMPLE 3

This example relates to the preparation of a phosphor according to the prior art obtained from the phosphate of Example 1.

The precursor phosphate obtained in Example 1 is reprocessed under a reducing atmosphere (Ar / H 2) for 2 h at 1000 ^° C. The calcination product obtained is then washed in hot water at 80 ^° C. for 3 h, then filtered and dried. .

EXAMPLE 4 This example relates to the preparation of a luminophore according to the invention obtained from the phosphate of Example 2.

The precursor phosphate obtained in Example 2 is reprocessed under the same conditions as those of Example 3.

The characteristics of the products of Examples 3 and 4 are presented in Table 2 below.

Table 2

The luminescence yield of the product of the invention 4 is given with respect to the comparative product 3.

Claims

1-rare earth phosphate (Ln) Ln representing either at least one rare earth selected from cerium and terbium, or lanthanum in combination with at least one of the two rare earths mentioned above, characterized in that it has a crystalline structure either of the rhabdophane type or of the mixed rhabdophane / monazite type and in that it contains lithium, the lithium content being at most 300 ppm.

2- phosphate according to claim 1 characterized in that it consists of crystallites whose size, measured in the plane (012), is at least 25 nm.

3- rare earth phosphate (Ln) Ln representing either at least one rare earth chosen from cerium and terbium, or lanthanum in combination with at least one of the two rare earths mentioned above, characterized in that it has a crystalline structure of the monazite type and in that it contains lithium, the lithium content being at most 300 ppm.

4- phosphate according to one of claims 1 to 3, characterized in that its lithium content is at least 10 ppm.

5. Phosphate according to one of claims 1 to 3, characterized in that its lithium content is at least 90 ppm.

6. Phosphate according to one of claims 3 to 5 characterized in that it consists of crystallites whose size, measured in the plane (012), is at least 70 nm.

7. The phosphate as claimed in one of the preceding claims, characterized in that it consists of particles having a mean size of between 1 and 15 μm, preferably with a dispersion index of at most 0.5.

8. The phosphate according to one of the preceding claims, characterized in that it comprises a product of general formula (1) below:

La _x Ce _y Tb _z PO ₄ (1) wherein the sum x + y + z is equal to 1 and at least one of y and z is different from 0, x can be understood more particularly between 0 and 0.4 95. 9- phosphor based on a rare earth phosphate (Ln) Ln representing either at least one rare earth chosen from cerium and terbium, or lanthanum in combination with at least one of the two rare earths mentioned above, characterized in it has a crystalline structure of monazite type and in that it contains lithium, the lithium content being at most 75 ppm.

10- phosphor according to claim 9, characterized in that it consists of particles whose coherence length, measured in the plane (012), is at least 300 nm.

11 - phosphor according to claim 9 or 10, characterized in that its lithium content is at least 10 ppm.

12- Process for preparing a phosphate according to one of claims 1, 2 and 4, 5, 7 or 8 in combination with one of claims 1 or 2, characterized in that it comprises the following steps:

during the introduction of the first solution in the second, the pH of the medium thus obtained is controlled at a constant value and less than 2, whereby a precipitate is obtained, the setting at a pH below 2 of the second solution for the first step or pH control for the second step or both being carried out at least in part with lithium hydroxide;

13- Process for preparing a phosphate according to claim 3 or one of claims 4 to 8 in combination with claim 3, characterized in that it comprises the following steps:

during the introduction of the first solution in the second, the pH of the medium thus obtained is checked at a constant value and less than 2, whereby a precipitate is obtained, bringing the second solution for the first step or the pH control for the second step or both to at least a portion with lithium hydroxide at a pH of less than 2;

the precipitate thus obtained is recovered and calcined at a temperature of at least 600 ^° C .;

14- Process for the preparation of a luminophore according to one of claims 9 to 11, characterized in that calcined at a temperature of at least 1000 ⁰ C a phosphate according to one of claims 1 to 8 or a phosphate obtained by the process according to claim 12 or 13.

15. Process according to claim 14, characterized in that the calcination is carried out under a reducing atmosphere.

16- Plasma system device, mercury vapor lamp, backlighting lamp for liquid crystal systems, mercury-free trichromatic lamp, light-emitting diode-driven device, UV-activated marking system, characterized in that it comprises or is manufactured using a luminophore according to one of claims 9 to 11 or a luminophore obtained by the process according to claim 14 or 15.