CA2370381A1

CA2370381A1 - Calcium fluoride

Info

Publication number: CA2370381A1
Application number: CA002370381A
Authority: CA
Inventors: Daniel Russell Tovey; Neil John Curwen Spooner
Original assignee: Individual
Current assignee: University of Sheffield
Priority date: 1999-04-19
Filing date: 2000-04-17
Publication date: 2000-10-26
Also published as: AU4577800A; GB2349147B; EP1187884A1; GB2349147A; GB9908841D0; WO2000063301A1

Abstract

The present invention relates to a doped calcium fluoride and in particular to a calcium fluoride precipitate doped with Eu and to a method for producing such a phosphor. Conventionally techniques such as disclosed in, "Ion-beam induced white luminescence of calcium fluoride implanted with both Eu and Tb ions", Aono et al, Jpn. J. Appl. Phys. Vol.32 (1993) pp 3851-3853, Part 1, N o. 9A, September 1993 (Aono) produce Eu and Tb implanted calcium fluoride (CaF2 ). However, there are significant disadvantages associated with using the Aono technique such as the duration of manufacture and the quality of the resulti ng product. Therefore, an embodiment of the present invention provides a calciu m fluoride precipitate doped with ions. Preferably, the calcium fluoride precipitate is doped with at least one of a plurality of lanthanide ions, preferably Eu+2, Eu+3, ions of Tb or Dy, or at least one group IIIb ion, preferably, ions of T1. Accordingly, an embodiment of the present invention provides a method for manufacturing a calcium fluoride precipitate, the meth od comprising the steps of producing a dopant solution using a salt of at least a first dopant and a solvent for that salt; producing a solution of CaCl2; mixing the dopant solution and CaCl2 solution with hydrogen fluoride to produce a calcium fluoride precipitate doped with ions of the first dopant.< /SDOAB>

Description

Calcium Fluoride The present' invention relates to calcium fluoride, and in particular to ion doped calcium fluoride and methods of manufacture thereof.
"Ion-beam induced white luminescence of calcium fluoride implanted with both Eu and Tb ions", Aono et al, Jpn. J. Appl.
Phys. Vo1.32 (1993) pp 3851-3853, Part 1, No. 9A, September 1993 (Aono) discloses a technique for manufacturing Eu and Tb ion implanted calcium fluoride (CaF2). A single crystal substrate of CaF2 is implanted using fluences of 1x1019-1x101s ions/cm2 at 100 keV at room temperature with a relatively low beam current density of 0 . C2 r.tl1/ cm2. P;hen the implanted single crystal substrate was subjected to Ar bombardment light was output at between 400-460 nm and 600-700 nm due to the presence of Eu+z and Eu+3 ions . The ion implantation technique used in Aono does not represent a very practical method for the manufacture of commercially viable quantities of ion doped CaF2 or an ion doped CaFz phosphor. Furthermore, the Aono ion implantation technique allows neither the manufacture of CaFz nor a CaF2 powder within a relatively short period of time or which has a relatively uniform grain size. It will be appreciated that any such ion implantation technique is both expensive and slow. Furthermore, the ion bombardment will also produce lattice damage in the substrate or crystal being implanted.
If the single crystal Eu ion implanted substrate of Aono is crushed to produce a CaFz powder, the light output efficiency and intensity of the resulting powder are relatively poor.
Producing a CaFz powder by such crushing also has the disadvantage that the powder is not uniformly doped with Eu ions.
It will be appreciated that there is a physical lower limit to the grain size that can be produced by crushing bulk grown CaF2. Attempting to produce grain sizes below this lower limit leads '~o contamination of the resulting powder with material constituting the crushing or grinding surfaces.
It is an object of the present invention to at least mitigate some of the problems of the prior art.
Accordingly, a first aspect of the present invention provides a calcium fluoride precipitate doped with ions.
Preferably, an embodiment provides a calcium fluoride precipitate which is doped with at least one of the lanthanide ions, preferably Eu+Z, Eu+3, ions of Tb or Dy, or at least one group IIIb ion, preferably, ions of T1.
AonU does not allow the manufacture of a CaF2 powder having a controllable or relatively uniform grain size.
Advantageously, an embodiment of the present provides a calcium fluoride precipitate and phosphor wherein the grain size of the calcium fluoride is between 5 nm and 10,000 nm and is relatively uniform. It will be appreciated that the actual grain size intended to be manufactured will depend upon the application of the CaF2 powder or phosphor.
Given the macroscopic size of the substrates used for implantation, it is very likely that the substrates contain stresses which, in turn, lead to relatively inferior luminescence properties. Furthermore, the crushing process used to produce crushed bulk grown CaFz also impairs or deforms crystal structure, which, again, results in poor luminescence properties. In contrast, the present invention produces good quality crystals having little or no defects. Still further, it has been found that the smaller grain size of the present invention results in better performance of a CASPAR detector by affecting the nuclear/electron recoil discrimination.
Preferably, the grain size is substantially 800 nm for CAS PAR
applications.
An embodiment provides a calcium fluoride precipitate capable of being used to generate and a phosphor capable of generating visible light, preferably having a wavelength of between 360 nm and 780 nm.
It has been found that the CaFz(Eu) or calcium fluoride phosphor derived therefrom has a photoluminescence light output efficiency of greater than or of the order of five times that of crushed bulk grown CaFz(Eu) crystal.
Accordingly, a further aspect of the present invention provides a calcium fluoride phosphor powder comprising a calcium fluoride phosphor derived from a CaFz(Eu) precipitate.
The stresses within crystals of the embodiments of the present inver~~ion are substantially reduced or eliminated.
Typically, the Aono process must use single crystal substrates that are contaminated with at least 10 ppb U and Th. This, in low level background scintillator applications, leads to an undesirable level of background radiation. Therefore, an embodiment provides a calcium fluoride phosphor containing impurities of less than 10 ppb of at least one of either U or Th.
As discussed above, the ion implantation process is expensive, slow and produces inefficient CaF2 having a relatively poor light output.
Accordingly, a second aspect of the present invention provides a method for manufacturing a calcium fluoride precipitate doped with ions and phosphor derived therefrom, the method comprising the steps of producing a dopant solution using a salt of at least a first dopant and a solvent for that salt:
producing a solution of CaCl2;
mixing the dopant solution and CaCl2 solution with Hydrogen Fluoride to produce a precipitate.
The precipitate comprises grains of CaFZ doped with ions of the first dopant. It will be appreciated that the doped calcium fluoride is made and doped in solution in a single step.
An embodiment further comprises the step of treating the precipitate to activate the dopant ions by incorporating them into the CaFz crystal lattice thereby producing a phosphor doped with ions of the first dopant.
An embodiment provides a method wherein the salt of the first dopant is in the form of a powder.
A further embodiment provides a method wherein the salt of the dopant is selected such that a required ionisation state is not readily oxidisable in the so?vent at above a predeterminable rate of oxidation. A dopant ion is considered to be not readily oxidisable if not more than 10~ oxidisation of the dopant ion occurs, preferably, during the immersion in the solvent used from the time of first dissolving the dopant salt in the solvent to the time the HF is added to it (and the CaCl2 solution).
Still, further, there is provided an embodiment wherein the salt of the dopant is a salt of a lanthanide or of a group IIIb element. Preferably, the lanthanide is Dy, Tb, Eu'2 or Eu'3 and/or the group IIIb element is T1.
A still further embodiment provides a method wherein is used a number of moles of the first dopant equal to between 0.05 and 10~ of the number of moles of CaCl2 used. A preferred embodiment provides a method wherein the number of moles of the first dopant is equal to between 0.5$ and 5$ of the number of moles of CaCl2. A still more preferred embodiment provides a method wherein the number of moles of the first dopant represents about 1$ of the number of moles of CaCl2.
Yet another embodiment provides a method wherein the step of producing the dopant solution comprises the step of adding an acetic acid solution, preferably a glacial acetic acid solution. Advantageously, the acetic acid is a solvent for EuCl2 that does not oxidise the Eu+2. Preferably, an embodiment is provided wherein 1 cm3 of acetic acid solution is provided per gram of salt of the first dopant.

An embodiment provides a method wherein the step of producing a solution of CaCl2 comprises the steps of dissolving CaClz.6H20 crystals in a solvent; and adding hydrogen fluoride.
Preferably, the step of adding HF comprises adding a stoichiometric quantity of HF at a predeterminable concentration. Preferably, the concentration is 48~.
A further embodiment provides a method wherein the step of treating comprises the steps of annealing the prccipii:ate at a predeterminable temperature. Preferably, an embodiment provides a method wherein the predetermined temperature is between 700°C and 1200°C, preferably 800°C to 1000°C. Still more preferably, the predetermined temperature is 900°C. The temperature or temperature range are selected to balance the activation of the dopant in the shortest period of time while reducing sintering of grains that occurs at higher temperatures.
An embodiment provides a method wherein the step of heating spans a predeterminable period of time. Preferably, the duration of the predeterminable period of time is set according to the required ionisation states of at least the first dopant and/or the required light output characteristics.
A further embodiment provides a method wherein the step of annealing is performed in a predetermined atmosphere or under a vacuum. An embodiment is provided wherein the atmosphere is an inert gas, preferably He.
A still further embodiment provides a method further comprising the step of subjecting the mixture of dopant solution, CaCl2 solution and Hydrogen Fluoride to an ultrasonic field to produce a precipitate having a predeterminable range of grain sizes. The characteristics of the ultrasonic field are such that grains of a required size are produced.
A still further embodiment provides a method further comprising the step of processing the CaCl2 solution to remove impurities, for example, actinide impurities, preferably, by passing the solution through a DIPHONIX column.
The calcium fluoride precipitate or phosphor derived therefrom can be used for many purposes. Therefore, an embodiment of the present invention provides a method for making a fluorescent transparent polycrystalline solid comprising the steps of pressing or sintering it, with or without a binding agent (LOr example, potassium bromide) using a technique such as Hot lsostatic Pressing (HIP).
Further, an embodiment of the present invention provides a method for making a fluorescent paint comprising the steps of mixing a calcium fluoride phosphor with a binding agent.
Still further, an embodiment provides a method for making a fluorescent polymer comprising the steps of mixing a calcium fluoride phosphor with a polymer, for example, polytri-fluorochloroethylene (PTFCE).
A further embodiment provides a method for making a fluorescent transparent liquid or a fluorescent gel comprising a liquid or gel having a refractive index matched to the calcium fluoride phosphor. Preferably, the tolerance of the matching is such that the difference in the refractive indices is less than 0.05 and still more preferably less than 0.01.
Dioxan and naphthalene could be used for the above purposes.
Once an efficient, high-light output calcium fluoride phosphor powder has been produced, it can be applied in numerous applications. Accordingly, embodiments of the present invention provide, for example, a VDU or television comprising a tube with a coating of a calcium fluoride phosphor, a flat-panel or field-emission display containing a coating of a calcium fluoride phosphor, a paint comprising a calcium fluoride phosphor, a light source comprising a calcium fluoride phosphor, preferably a white light source. It is not always desirable to have a white light source. Accordingly, an embodiment provides a visible light phosphor comprising a calcium fluoride phosphor.
Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:
figure 1 shows the photoluminescence spectra of raw CaF2 (Eu+z) powder and annealed CaFz (0.5gM Eu'2) according to an embodiment of the present invention;
figure 2 shows the photoluminescence spectra for annealed CaF2(1$M Eu+Z) according to embodiments of the present invention as compared to crushed bulk CaF2(Eu) and undoped CaF2:
figure 3 shows the photoluminescence spectra for annealed CaF2(Eu+3) according to an embodiment of the present invention showing a strong 610 nm emission; and figure 4 shows the photoluminescence spectra for sintered CaF2(Eu+3) according to an embodiment of the present invention having reduced peaks at 610 nm and increased peaks at 435 nm.
Sufficient EuCl2 powder for doping the CaFZ is obtained. In the specific embodiment, 1~ of the total number of moles of initial CaC12.6HZ0 was used. Thus to prepare lg (1.28x10-2 moles) of CaFz product, 0.0285g (1.28x10-" moles) of EuClz was used along with a stoichiometric quantity of CaC12.6Hz0 (2.8g or 1.28x10-Z moles). Other embodiments can use equivalent molar fractions between 0.05$ and 10$. However molar fractions between 0.5~ and 5~ are preferred. The percentage doping represents a balance between too little doping, which will result in poor light output, and too much doping, which will cause self absorption of the phosphor's own fluorescence, that is, quenching of the light output.
It will be appreciated that the molar quantity of CaClz.6H20 used will be identical to that of the final CaF2 product assuming stoichiometric quantities of CaClz.6H20 and HF and assuming that all Ca+2 ions form CaF2. Thus a 1$ molar fraction of EuClz relative to the number of moles of CaC12.6Hz0 is, under these assumptions, equivalent to a 1$ molar fraction relative to the number of moles of CaF2 product. Then, if it is assumed that all Eu dopant ions are taken up into the CaF2, the molar concentration of dopant ions in the CaF2 is also about l~s.
A concentrated solution of EuCl2 is produced by dissolving the EuCl2 in a small amount of distilled water. Typically, approximately 3 cm3 of water is used per gram of EuCl2 powder.
Preferably, a quantity, preferak~ly i cm', of glacial acetic acid solution is also added to the solution. The EuCl2 concentration selected represents a balance between the need to avoid oxidation (which occurs at too low a concentration) and the ability to dissolve the powder (which will not occur at too high a concentration). The solution is vigorously shaken until the powder has been completely dissolved. It is thought, without wishing to be bound by any particular theory, that maintaining the Eu ions in solution until the HF is added significantly reduces the resulting grain size.
A solution of CaCl2 is added to the shaken solution.
Preferably, a small amount of acetic acid is also added. The CaCl2 solution is prepared using CaClz.6H20 crystal. The CaCl2 solution is highly concentrated. Preferably, the solution is a 5 mole/litre solution. Preferably, concentrated hydrogen fluoride is added to the solution whilst stirring vigorously, using a mechanical stirring rod set at 5 revolutions sec-1, until precipitation of the CaFz results. The concentration of the hydrogen fluoride governs the grain size; the higher the concentration, the quicker the reaction and the smaller the grain size.
The CaF2 precipitate is then washed and dried very thoroughly.
Preferably, the washing process utilises water. Preferably, the drying process uses a centrifuge/vacuum oven for drying.
In an embodiment, a MSE Centaur 2 centrifuge and a Gallenkamp vacuum oven were used for washing and drying. Care should be taken to ensure that the drying process does not cause caking of the doped calcium fluoride.
Finally, the powder is annealed at a high temperature in an inert atmosphere. Alternatively, the powder may be annealed in a vacuum. The preferred annealing temperature is between 800°C and 1000 °C, preferably, 900°C. The annealing period is for between 10 and 15 minutes. The annealing period is arranged to activate the dopant in the shortest period of time to at least reduce and preferably avoid the sintering of grains that occurs at higher temperatures. After annealing the dopant ions are incorporated into the crystal lattice thereby activating the characteristic light output of the doped CaFz. A preferred annealing atmosphere is Helium.
Although the embodiments above have been described with reference to specific values of concentration or temperature etc, the present invention is not limited thereto.
Embodiments can equally well be realised using the variants described below.
Other dopants can be selected provided they satisfy certain ionisation requirements. The ionisation requirements are such that the dopant must not be readily oxidisable in or by water.
A salt of the required dopant should preferably be made in water to produce the required starting ion. For example, use of a salt of Eu+3 may be used to produce Eu+3 in a red phosphor, using heat if necessary, see, for example, figure 3. It will be appreciated that the use of heat or otherwise represents a balance between the speed of dissolving of the dopant salt and the oxidation of ions prone to oxidation. If oxidised ions are required for doping or if the required ions are stable against oxidation, then heat may be applied until the dopant salt has been dissolved in the solvent. Other possibilities include using Tb ions for a green phosphor and other Lanthanides such as Dy. A Dy doped powder may be useful in Thermoluminescent Dosimetry applications, for example, radiation detection badges.
5 Other phosphors can be made by adding several different desired dopants to the starting CaClz solution; each dopant being selected according to a desired emission wavelength.
Alternatively, once phosphor powders having respective dopants have been produced, they can be mixed to produce a phosphor 10 having various light emission wavelengths.
A prolonged heating step at a high temperature of, for example, 1000 °C, for several days can reduce any given ions to ano~her desired state. The initial dopants, annealing came and temperature may be used to vary the different dopant and ionisation states that may be present simultaneously after annealing thereby enabling a mixed light output to be realised. For example, a phosphor having blue 435 nm + red 600 nm output characteristics can result from Eu+3 initial dopant ions partially reduced to Eu+2. Eu" can be converted to Eu+2 by annealing for approximately 3 days at a temperature of 1000 °C, as illustrated in figure 4.
The grain size of Eu doped CaFz can be selected by performing the reaction with the hydrofluoric acid in an ultrasonic field and by varying the concentrations of the reactants. This permits the formation of very small, 5 nm to 10,000 nm, crystals of excellent cubic crystal structure. The preferred grain size varies according to the application of the phosphor. For example, for neutron/gamma discrimination in CASPAR the grain size should be preferably approximately 800 nm.
An embodiment of the present invention used 48~ hydrogen fluoride, 5M CaCl2 solution and an ultrasonic field of 25 kHz of sufficient power to produce cavitation. A preferred embodiment used 500 watts to produce a grain size of 100 nm.

The radio purity of the resulting Eu doped CaF2 phosphor can be improved by extracting actinide impurities using chelating agents . The levels of U, TH and K are reduced to improve the radio purity of the resulting phosphor.
Further modifications to the present invention to improve the purity, grain size and light output of the resulting Eu doped CaF2 may result by varying the following parameters:
i) the proportion of doping materials used;
ii) the proportion of water/acetic acid solution or other solvents needed to dissolve the starting materials;
iii) the concentration of CaCl2 and its pH;
iv) the relative quantity of concentrated hydrogen fluoride used;
v) the speed and type of mechanical stirrer used in the reaction;
vi) the use of ultrasound;
vii) the washing and drying techniques employed including the centrifuge techniques, the use of a vacuum oven, the use of distilled water as the wash and viii) the annealing technique, including the temperature, time and atmosphere used.
In summary, the material concentrations, speeds of reaction/stirring and the use of ultrasound may be used to control the CaF2 grain size. The proportion of doping materials, water and acetic acid solution and annealing can be varied to influence the colour and magnitude of light output through efficiency and valency of dopant ions. It will be appreciated that the annealing affects the quality of the powder as it determines the amount of sintering of grains.
The washing and drying techniques affect the quality of the powder and also its optical properties through the introduction of impurities into the powder.
Advantageously, the light-output power of calcium fluoride phosphor doped with Eu is substantially five times the light-output power of crushed bulk CaFz. The increase in luminescence arises from the rapid production of small grain sizes in the solution.
A still further advantage of the present invention resides in the production of uniform mono-crystals produced during precipitation on the addition of hydrogen fluoride. Still further, since the reactants are in solution, they can be purified in advance.
Without wishing to be bound by any theory it is thought that doping with other ions should be possible using the present invention providing they are soluble and stable in water or some other sol~Tent for short periods and their ionic radii and charge are similar to Ca+z.
Referring to figure 1 there is shown the photoluminescence spectra 100 of raw CaFz(0.5~M Eu*z) precipitate and annealed CaFz(0.5gM Eu+z) precipitate according to embodiments of the present invention. It can be seen from the plot 110 for the raw CaFz(0.5$M Eu+z) as compared to the plot 120 for annealed CaFz(0.5~aM Eu+z) that the latter has a significantly greater photoluminescence.
With reference to figure 2, there is shown photoluminescence spectra 200 for annealed CaFz(1$M Eu+z) according to an embodiment of the present invention as compared to bulk crushed CaFz (Eu) and undoped CaFz. It can be seen from the photoluminescence plot 210 for the CaFz (1~SM Eu+z) that the light output intensity is significantly greater that the light output intensities 220 and 230 of bulk crushed CaFz(Eu) and undoped CaFz respectively.
Figure 3 illustrates photoluminescence spectra 300 for annealed CaFz(Eu+3) according to an embodiment of the present invention. It can be from the photoluminescence plot 310 that there is a strong 610 nm emission.

Referring to figure 4, there is shown the photoluminescence spectra 400 for sintered CaF2(Eu+3) according to an embodiment of the present invention. It can be seen from the plot 410 that the intensities of the emissions 420 at 610 nm have been reduced and the intensities of the emissions 430 at 435 nm have been increased as compared to the photoluminescence spectra shown in figure 2.

Claims

1. A coprecipitant comprising calcium fluoride and Europium ions.

2. A coprecipitant as claimed in claim 1, doped with at least one of Eu+2, Eu+3 ions .

3. A coprecipitant as claimed in any preceding claim, wherein the grain size of the calcium fluoride is between 5 nm and 10000 nm.

4. A coprecipitant as claimed in any preceding claim capable of generating light, preferably having a wavelength of between 360 nm and 780 nm.

5. A coprecipitant as claimed in any preceding claim having a photoluminescence light output efficiency exceeding that of crushed bulk grown Europium doped calcium fluoride.

6. A coprecipitant as claimed in claim 5, wherein the light output efficiency exceeds that of crushed bulk grown Europium doped calcium fluoride by at least five time.

7. A coprecipitant as claimed in any preceding claim having an impurity of less than 10 ppb of at least one of either U or Th.

8. A coprecipitant phosphor substantially as described herein.

9. A calcium fluoride phosphor derived from a coprecipitant as claimed in any preceding claim.

10. A calcium fluoride phosphor powder comprising a calcium fluoride phosphor as claimed in claim 9.

11. A calcium fluoride phosphor or powder substantially as described herein.

12. A VDU, television or field-emission/flat-panel display with a coating of a calcium fluoride phosphor as claimed in any of claims 9 to 11.

13. A paint or coating comprising a calcium fluoride phosphor as claimed in any of claims 9 to 11.

14. A light source comprising a calcium fluoride phosphor as claimed in any of claims 9 to 11.

15. A light source as claimed in claim 14, wherein the light source is a fluorescent tube.

15. A visible light phosphor comprising a calcium fluoride phosphor as claimed in any of claims 9 to 11.

17. A visible light phosphor as claimed in claim 16, wherein the visible light is white light.

18. A method for manufacturing a coprecipitant of calcium fluoride and Europium; the method comprising the steps of producing a dopant solution using a salt of at least Europium and a solvent for that salt;
producing a solution of CaCl2; and mixing the dopant solution and CaCl2 solution with hydrogen fluoride to produce a coprecipitant calcium fluoride doped with ions of Europium.

19. A method as claimed in claim 18, wherein the salt of the Europiurn dopant is in the form of a powder.

20. A method as claimed in claim 18 wherein the number of moles of the Europium dopant represents between 0,05% and 10%
of the number of moles of CaCl2, preferably between 0.5% and 5%
of the number of moles of CaCl2.

21. A method as claimed in claim 20, wherein the number of moles of the Europium dopant is about 1% of the number of moles of CaCl2.

22. A method as claimed in any of claims 18 to 21, wherein the step of producing the dopant solution comprises the step of adding an acetic acid solution, preferably a glacial acetic acid solution.

23. A method as claimed in claim 22, wherein 1 cm3 of acetic acid solution is provided per gram of salt of the Europium dopant.

24. A method as claimed in any of claims 18 to 23, wherein the step of producing a solution of CaCl2 comprises the steps of dissolving CaCl2,6H2O crystals in a solvent; and adding HF.

25. A method as claimed in claim 24, wherein the step of adding HF comprises adding a stoichiometric amount of HF at a predeterminable concentration, preferably at a concentration of 48%.

26. A method as claimed in any of claims 18 to 25, further comprising the step of treating the coprecipitant to activate the dopant ions and hence produce a calcium fluoride phosphor from the coprecipitant.

27. A method as claimed in claim 26, wherein the step of treating comprises the step of annealing or sintering the coprecipitant at a predeterminable temperature.

28. A method as claimed in claim 27, wherein the predetermined temperature is between 700°C and 1200°, preferably between 800°C and 1000°C.

29. A method as claimed in claim 28, wherein the predetermined temperature is 900°C.

30. A method as claimed in any of claims 27 to 29, wherein the step of heating spans a predeterminable period of time.

31. A method as claimed in claim 30, wherein the duration of the predeterminable period of time is set according to the required ionisation states of said Europium dopant and/or the required light output characteristics of the resulting powder or phosphor.

32. A method as claimed in any of claims 27 to 31, wherein the step of annealing is performed in a predetermined atmosphere or under a vacuum.

33. A method as claimed in claim 32, wherein the atmosphere is an inert gas.

34. A method as claimed in claim 32, wherein the inert gas is He.

35. A method as claimed in any of claims 18 to 34, further comprising the step of subjecting the mixture of dopant solution, CaCl2 solution and hydrogen fluoride to an ultrasonic field to produce a coprecipitant having a predeterminable range of grain sizes.

36. A method as claimed in any of claims 18 to 35, further comprising the step of processing the CaCl2 solution to remove impurities, preferably actinide impurities.

37. A method for producing a coprecipitant of calcium fluoride and Europium, a calcium fluoride phosphor or a calcium fluoride phosphor powder doped with Europium substantially as described herein.

38. A method for making a fluorescent paint comprising the steps of mixing a calcium fluoride as claimed in any of claims 1 to 17 or made according to a method as claimed in any of claims 18 to 37 with a binding agent.

39. A method for making a fluorescent polymer comprising the steps of mixing a calcium fluoride as claimed in any of claims 1 to 17 or as made according to a method as claimed in any of claims 18 to 37 with a polymer.

40. A method for making a fluorescent transparent liquid or a fluorescent gel using a polymer as claimed in claim 39, wherein the liquid or gel is refractive index matched to the calcium fluoride phosphor.

41. A method for making a fluorescent transparent polycrystalline solid comprising the steps of pressing or sintering doped calcium fluoride as claimed in any of claims 1 to 17 or as made according to any of claims 18 to 37, with or without a binding agent (for example, potassium bromide), using a Hot Isostatic Pressing (HIP) technique.