RU2442143C1 - Method for creating reference gages for non-destructive spectral methods of measuring impurity concentration in crystals - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: according to the method, the roentgenofluorescence intensity of the analytic line is measured in the tested sample (crystal) and the roentgenofluorescence intensity of the same line is measured in working mixture containing a known amount of the measured element. A calibration chart is built and the absolute concentration of the impurities in the sample. At the same time spectral measurements of the sample are carried out, their results are compared with the results of roentgenofluorescence measurement and a reference gage is acquired for future use in impurity spectral analysis in industrial-scale production of the crystals.

EFFECT: creation of reference gages (reference crystal samples) while retaining measurement accuracy for controlled crystal properties.

4 dwg

Description

The invention relates to the field of chemistry, physics, crystal growth and can be used to create activated crystalline materials with predictable properties for the needs of photonics, quantum electronics and optics. The need to control the concentration of impurities in active elements follows from the dependence of the output characteristics of the generation of laser materials on the concentration of active ions and the uniformity of their distribution along the length of the crystal.

Currently known methods for determining the concentration of impurities in crystals using exemplary measures. Thus, in [M.Fornoni, A. Bulou, J.M. Breteau, J.Y. Gesland, and M. Rousseau, "Neodymium concentration measurements in Nd: YLF laser rods: a nondestructive method". -Applied Optics, 1990. V.29, N12, pp.1758-1764] a method for determining the concentration of neodymium ions in LiYF4 (YLF) crystals is proposed, based on the linear dependence of the line intensities of the luminescence spectra of the impurity being determined on its concentration. In this case, to normalize the luminescence signal, we used the lines of Raman spectra of the crystal under study.

The practical application of this technique is associated with significant difficulties. Raman signals are of low intensity and require the use of powerful pump sources, and the luminescence signal under these conditions ceases to be proportional to the concentration, since processes such as self-quenching of luminescence, nonlinear effects associated with excited absorption and nonlinear interaction will influence its magnitude. as well as reabsorption of luminescence. The exclusion of these effects imposes restrictions on the range of detectable impurity concentrations in crystals — the concentration should not exceed 0.5 at.%. As a rule, such low concentrations are not used in practice, for example, in the manufacture of laser active elements, activator ion concentrations in crystals of 1% or more atomic percent are required.

Thus, the known method is not acceptable for practical use, especially for applied purposes for determining the concentration of impurities in crystals.

The closest analogue that we selected as a prototype for the combination of characteristic features and the expected technical result are studies conducted on LYF4 double fluoride crystals (YLF) activated by neodymium ions, published in [Tkachuk AM, AV Poletimova, Petrova M.A., Egorov V.Yu., Korolev N.E. "The entry of Nd3 + ions into the LiYF4 matrix and the determination of the Nd3 + concentration by a non-destructive spectrophotometric method." - Optics and spectroscopy, 1991. V.70, issue 6. S.1230-1235]. In this work, the occurrence of neodymium ions in the YLF lattice was studied and a spectrophotometric method was proposed for determining both the average integral concentration of activator ions in crystals and the form of its distribution along the length of the active element. The spectrophotometric technique for determining the neodymium concentration in LYF4 crystals is based on measuring the peak optical density D on the reference line with a wavelength λ from the known, or determined by other methods, value of the peak absorption cross section σ. In this case, the average concentration of neodymium C in a sample 1 cm ^-3 long is calculated by the formula:

As can be seen from formula (1), the accuracy of determining the concentration mainly depends on the degree of accuracy of determining σ, since the optical density D is measured with a much higher accuracy (<0.5-1%) than the absorption cross section σ (~ 10-20 %). Thus, there is a significant drawback in determining the accuracy of the concentration of impurities in crystals, which prevents obtaining reliable information on the crystals under study, which in turn affects the quality of the devices (lasers) themselves.

Calibration of the methodology for determining the concentration in the work [Tkachuk A.M., Poletimova A.V., Petrova M.A., Egorov V.Yu., Korolev N.E. "The entry of Nd3 + ions into the LiYF4 matrix and the determination of the Nd3 + concentration by a non-destructive spectrophotometric method." - Optics and spectroscopy, 1991. V.70, issue 6. S.1230-1235] was carried out according to the control series of samples of YLF-Nd crystals grown by the Stockbarger method. The manufactured samples of the control series are polished and oriented relative to the optical axis crystalline disks with a thickness of 1 mm and a diameter of 6-10 mm, cut from various places of the crystal boule. For all samples, polarized absorption spectra were recorded, and then the concentration of neodymium ions was determined in them by the same method of x-ray spectral analysis. The measurements were carried out on a Camebax electron probe microanalyzer. By measuring the intensity of the absorption spectra of oriented YLF-Nd plates and knowing the concentration of neodymium from the data of X-ray spectral analysis, we calculated the values of peak absorption cross sections σ for reference lines with the lowest degree of polarization in different spectral regions. The average concentration of neodymium in atomic percent was calculated from the expression:

Недостатком известной методики, изложенной в прототипе, является то, что:

The disadvantage of the known methods outlined in the prototype is that:

- no calibration data is given for the microanalyzer;

- there is no reliable information on the assessment of accuracy in determining the concentration of neodymium ions in samples of the control series;

- there is no information on the subsequent calculation of the values of the absorption cross sections σ.

As a result of the foregoing, it is significantly difficult to assess the quality of the obtained crystals of laser substances and the actual devices (laser systems) in which they are used for their intended purpose. Since the intensity of the x-ray fluorescence of the ions determined depends on the depth of penetration of the x-ray radiation used to excite the fluorescence deep into the sample, as well as on the absorption of the X-ray fluorescence quanta by other chemical elements present in the samples, it is necessary to calibrate the XRD device for each sample of similar chemical composition and with a known concentration determined impurities. Based on the above arguments, we can conclude that this factor is a necessary and determining condition for increasing the accuracy in measuring the concentration of impurities in crystals.

Thus, the average error given in the work in measuring the concentration of neodymium ions, positioned within 10%, does not seem reasonable. At the same time, the given error value is a rather large value, which narrows the field of practical application of the proposed technique and minimizes the possibility of creating high-quality products (laser crystals) and, accordingly, significantly reduces the quality of the devices themselves (lasers).

In addition, the authors of the work [Tkachuk A.M., Poletimova A.V., Petrova M.A., Egorov V.Yu., Korolev N.E. "The entry of Nd3 + ions into the LiYF4 matrix and the determination of the Nd3 + concentration by a non-destructive spectrophotometric method." - Optics and spectroscopy, 1991. V.70, issue 6. S.1230-1235] it was not proposed to create an exemplary measure for its subsequent use in determining the concentration of neodymium ions by various non-destructive spectroscopic methods. The authors performed only a single calibration of the samples by X-ray spectral analysis in order to determine the value of the peak absorption cross-section σ on the test sample.

Therefore, we can state the fact that the authors of the work did not set the task of creating exemplary measures (standard samples of crystals) with known (measured) properties. And this does not contribute to the possibility of a significant reduction in the time interval for identifying quantitative indicators of crystals, such as the concentration of impurities in the crystal, and, consequently, the qualitative properties of the obtained crystals.

In addition, the authors of the aforementioned work did not seek to improve the accuracy in determining the absorption cross section, and therefore there was no task of increasing the accuracy in determining the concentration of neodymium ions through the use of a statistical method, measuring σ values from different parts of the crystal under study that differ in the concentration of impurity ions, and therefore, the absorption coefficient. This indicates that the known method is not intended to solve the special issue of creating exemplary measures (standard crystal samples) with known (measured) properties.

Although there is currently an urgent problem (need) in the market for the production of laser crystals in the creation of a new express method for determining the properties of grown crystals, namely, an express method for determining the concentration of impurities in crystals.

The proposed method allows you to create a series (line) of exemplary measures of crystals (standard samples of crystals) with known properties that could be used to automate the selection process from a number of grown crystals, crystals with the desired properties. By comparing the optical spectra of standard samples (crystals) with the optical spectra of newly grown crystals, it becomes possible to quickly select and rank them according to the required qualitative (quantitative) parameters of crystals (for example, the coefficient of entry, transparency, uniformity of crystals, etc.) without a long time of the research process of their properties. In order to ensure the possibility of using the new claimed method (method) to reduce unproductive costs both in the time of research of crystals, and with the possibility of maintaining the guaranteed quality of measurement of controlled parameters of crystals, and thereby making it possible to create a low-cost, but effective method of replicating crystals in conditions of mass (mass) production.

Thus, there is an unrealized need on the market for the creation (development) of a new express method that allows, at the minimum cost of resources, the opportunity to evaluate a set of qualitative indicators (properties) of the obtained crystals and their ranking according to qualitative and other controlled indicators.

The purpose of the claimed technical solution is:

1. a method of creating exemplary measures (standard samples of crystals); (for measuring with a high degree of accuracy both the absolute concentration of cerium subgroup lanthanide ions in activated crystals of the composition LiY _1-x Lu _x F _4, where x is continuously changing from 0 to 1 by spectroscopic methods, and other controlled parameters of crystals (for example, optical quality, namely, the number of bubbles, transparency, the presence of stubs) the presence of mechanical damage);

2. maintaining the guaranteed quality of measurement of controlled indicators of crystals;

3. reducing unproductive costs and improving economic performance in the serial production of industrial crystals (cost reduction while maintaining high product quality);

4. ensuring the possibility of industrial production of crystals with controlled and (or) specified parameters.

The claimed method provides the ability to:

- eliminating the need for expensive equipment, such as x-ray fluorescence, mass spectroscopic equipment;

- maintaining the guaranteed quality of measurement of controlled indicators of crystals;

use of industrial production of crystals with controlled and (or) specified parameters.

The goal set by the applicants is achieved due to the fact that a technological process (method) for creating exemplary measures (verified crystals) with known concentrations of the above ions in a wide concentration range (a line of standard crystal samples) is proposed to enable the express methodology in determining controlled parameters of crystals, such as the concentration of the impurity of the cerium subgroup (Re) lanthanides in LiY _1-x Lu _x F ₄ : Re crystals by both optical and / or EPR spectroscopy, excluding I am at the same time the stage of x-ray fluorescence analysis (XRF).

The claimed technical solution is as follows. An X-ray spectrofluorimeter is pre-calibrated on a charge for crystal growth with a given x with a known (by weight) content of the detected impurity. According to the data obtained, a calibration graph is built, then according to this graph, using the XRD method, the absolute concentration of the element being determined in the samples of crystals grown from the same mixture is determined. Thus, the obtained crystalline samples with a known concentration of the detected impurity can then serve as an exemplary measure for the subsequent determination of the same impurity in other crystals of the same chemical composition by comparison.

The claimed technical solution is illustrated by the following materials:

- in figure 1. a sketch is presented illustrating the cutting pattern of a sample for measuring the absolute concentration of neodymium impurity ions in LiY1-xLuxF4: Nd3 + crystals (x = 0 ... 1) by spectroscopy and X-ray fluorescence analysis;

- figure 2 presents the normalized intensities of lines belonging to Nd3 + ions in the X-ray fluorescence spectra of the samples depending on the concentration of Nd3 + ions in the LiF-YF3-NdF3 charge;

- figure 3 shows the absorption spectrum of Nd3 + ions in a LiYF4 crystal in the region of transitions between 4I9 / 22-G7 / 2 multiplets, σ is the polarized radiation, T = 300 K;

- figure 4 presents the dependence of the integrated absorption coefficient of Nd ions from the concentration of Nd ions in the crystal.

The essence of the claimed technical solution is that to create exemplary measures for determining the impurity concentration in LiY _1-x Lu _x F ₄ crystals by non _- destructive spectroscopic methods, the intensities of the analytical X-ray fluorescence line in the test sample (crystal) and the X-ray fluorescence intensities of the same line in the charge are measured, with the known content of the element being determined, a calibration graph is built and the absolute concentration of the impurity in the sample is determined, characterized in that at the same time In the same sample, spectroscopic studies are carried out, they are compared with X-ray fluorescence studies and an exemplary measure is obtained for its subsequent use to determine the impurity concentration by non-destructive spectroscopic methods in the industrial production of crystals.

Example.

1) Several charge compositions are prepared (set of compositions) for growing crystals with the chemical formula LiY _1-x Lu _x F ₄ : Re, where the value x = 0 ... 1 is determined by the desired chemical composition of the exemplary measure and does not change in the set, and the concentration of rare-earth impurities (Re) varies from 0 to 10 at.%. Each charge is a mixture of powder reagents LiF, YF ₃ , LuF ₃ and ReF ₃ (Re = Ce, Pr, Nd ... Gd). Moreover, the concentration of each of the cations included in its composition is determined by weighed amounts of these components with an accuracy better than 0.03 at.%.

2) A small portion of each charge composition from the kit is used to calibrate the XRD spectrometer. For this, the dependences of the integrated line intensities in the X-ray fluorescence spectra belonging to the determined impurity lanthanide ions (I _Re ) and the crystal lattice cations Y and Lu (I _Y and I _Lu ) on the concentrations of all these elements in the charge are studied. Since the intensities in the X-ray fluorescence spectra linearly depend on the number of analyzed elements, and hence on the amount of substance in the sample, recorded on an X-ray powder diffraction spectrometer, the integral intensity I _{Re was} normalized to the integral X-ray fluorescence intensities of Y and Lu ions, the concentration of which in the charge is known and is a measure of the amount substances. Based on the measurement results, calibration graphs I _Re / I _Lu and / or I _Re / I _Y are plotted against the known concentration of lanthanides in the charge.

3) Most of the charge is used for growing LiY _1-x Lu _x F ₄ : Re crystals by the Bridgman-Stockbarger method in graphite crucibles in a growth installation of a filament type with a growth rate of 1 mm / h and a crucible rotation speed of 10 rpm. As a result of the study, it was found that with the used methods of growing crystals, there is no heterogeneity of the impurity over the diameter of the crystal, i.e. the dependence of the impurity concentration in the grown crystalline boules of the composition LiY _1-x Lu _x F ₄ : Re remains constant over the diameter of the crystal.

4) Samples are made from the crystals grown by boules, for investigation using the XRD method and spectroscopic methods. On boules along the growth direction (Z axis) marks were applied with a step of 4 mm, which served as guidelines for its subsequent cutting. Samples are made by two longitudinal (along the crystallization direction) sections of the side surfaces of the crystal boules, as shown in FIG. The direction of the slices is chosen so that the optical axis of the crystal is located along the direction of the cut. Then, the cut off side parts are sawn into 4 mm long plates, which, in turn, are divided into two parts: one plate (sample I) for analysis by the XRD method, and the other plate (sample II) for studying the EPR spectra. The resulting flat side surfaces of the remaining central part of the crystal boules (sample III) were ground and polished in order to perform spectrophotometric studies. At the same time, the marking of the remaining part of the boule was preserved.

5) The result is several series of two samples I and II. Each series differs from another series by the place in the crystalline boulevard from which it is cut out (by the Z coordinate), since the nonisomorphic impurity is distributed unevenly along the crystallization direction [Kirgintsev AN, Isaenko LI, Isaenko VA Impurity distribution in directional crystallization. - Novosibirsk: “Science”, 1977, 256 pp.]. Moreover, in accordance with the data of Section 3, samples I and II in each series corresponding to the same Z coordinate have the same content of the analyzed impurity ions - c _I (z) = c _II (z). Similarly, depending on the Z coordinate, different regions of sample III contain different concentrations of the analyzed impurity c _III (z). Moreover, the concentration of the detected impurity in all samples I, II, and III at the same Z coordinate, in accordance with the data of Section 3, are equal to each other (2):

6) Sample I of each of the series was ground in an agate mortar to obtain a powder for which an X-ray fluorescence spectrum was recorded. Then, the ratios of the integrated intensities of the X-ray fluorescence bands of the determined ions (elements) I _Re to the integrated intensities of the X-ray fluorescence bands of the Lu and Y ions - I _Re / I _Lu and / or I _Re / I _{Y were} calculated and the desired curve was determined (see Section 2) absolute concentration of Re in the sample.

7) Since the impurity concentration in the samples of the series is the same c _I (z) = c _II (z), this means that the absolute concentration of impurity ions in sample II is also known, and the sample itself can serve as an exemplary measure for determining the desired absolute concentration of cerium lanthanide ions subgroups in other crystals of the studied composition by comparison. In particular, sample II can be used as an exemplary measure for measuring the absolute concentration of cerium subgroup lanthanide ions by EPR.

8) Similarly, sample III can serve as an exemplary measure for determining the absolute concentration of cerium subgroup lanthanide ions in LiY _1-x Lu _x F ₄ : Re crystals (x = 0 ... 1) by absorption optical spectroscopy.

9) Given that for the same coordinates Z c ₁ (z) = c _II (z) = c _III (z), and knowing the integral absorption coefficient of the analyzed impurity ions from their absorption spectra, we can calculate the integral absorption cross section for each of the determined ions cerium subgroup lanthanides in these crystals according to the formula:

where c is the known concentration of impurity ions, k (λ) is the integral absorption coefficient in the selected spectral range λ, σ is the integral absorption cross section of the ion being determined in this crystal, the value of which was determined to be (1.9 ± 0.1) × 10- 19 cm2 and in the future can be used to determine the concentration of the same ion in other crystals of this composition, knowing its absorption coefficient in the selected spectral range.

10) To increase the accuracy in determining the absolute concentration of impurity ions in crystals, using relative spectroscopic methods, it is necessary to conduct many measurements on various samples with subsequent statistical processing of the results. In this case, it is advisable to use a sufficiently large number of samples differing in the content of the studied impurity and varying as much as possible. To ensure the necessary statistical sampling, we used the experimentally established fact of the uneven distribution of the impurity along the length of the crystal boule. In this case, the concentration of impurity ions in the initial part of the boule turns out to be lower than in the final, which does not contradict the theory of incorporation of an isomorphic impurity described in [Kirgintsev AN, Isaenko LI, Isaenko VA Impurity distribution in directional crystallization. - Novosibirsk: “Science”, 1977, 256 pp.].

Thus, a set of samples with different impurity concentrations is equivalent to different sections of the same crystalline boule, which can significantly reduce the complexity of manufacturing model measures and limit ourselves to growing not a dozen crystals, but only one crystal. The data obtained in this case is quite sufficient to increase the accuracy to hundredths of a percent, in contrast to the prototype indicators when determining the absolute concentration of an impurity in crystals, and therefore, in determining the integral absorption cross section of a determined ion in a given crystal.

Thus, the applicant was able to create exemplary measures to determine the absolute concentration of cerium subgroup lanthanide ions in LiY _1-x Lu _x F ₄ : Re crystals (x = 0 ... 1) with high accuracy. The claimed technical solution meets the criterion of "novelty," since for LiY _1-x Lu _x F ₄ : Re crystals (x = 0 ... 1) activated by cerium subgroup lanthanide ions, no such technical solutions have been found that have the indicated set of features leading to implementation of the stated goals.

The claimed technical solution meets the criterion of "inventive step", as for specialists in the claimed field of technology, this technical solution is not obvious due to the fact that obtaining and analyzing each grown crystal requires very significant costs, both time and the use of a number of expensive devices, apparatus and lengthy settlements. In addition, the claimed technical solution allows one-time production of standard samples of crystals (a line of crystals) and use it in the future as standard samples, eliminating the unproductive costs in the serial production of industrial lasers.

The claimed technical solution meets the criterion of "industrial applicability", although the claimed method is implemented in laboratory conditions on the territory of the Kazan Federal (Volga) University, but it has been tested on standard samples of crystals with known properties.

The inventive method is universal in nature and can be used to create exemplary measures for other impurity ions and in other crystals.

Claims (1)

A method of creating exemplary measures for non-destructive spectroscopic methods for determining the concentration of an impurity in crystals, which consists in measuring the intensity of the analytical X-ray fluorescence line in the test sample (crystal) and the X-ray fluorescence intensity of the same line in the charge, with a known content of the element being determined, constructing a calibration curve and determining the absolute concentration of the impurity in the sample, characterized in that at the same time spectroscopic studies are carried out on the same sample , compare them with X-ray fluorescence studies and obtain an exemplary measure for its use, subsequently, for determining the concentration of an impurity by non-destructive spectroscopic methods in the industrial production of crystals.

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