Igor Yakimov

NaF-KF-AlF3 System: Phase Transition in K2NaAl3F12 Ternary Fluoride

Phase formation in the NaF-KF-AlF3 system, in the vicinity of the K2NaAl3F12 composition, has been studied. The samples have been prepared by melting the starting components at 650 °C. A new phase has been revealed, which appeared to be a low-temperature form of the well-known K2NaAl3F12 ternary fluoride obtained by the hydrothermal synthesis method. The high-temperature form melts at 598 °C and is stable in a narrow temperature region of about 15 deg below the melting point. Thermal analysis, high temperature X-ray diffraction, IR-spectroscopy, X-ray fluorescence, and X-ray powder diffraction crystal structure analysis have been applied to study the composition, crystal structure, and thermal properties of the low-temperature phase. The crystal structure consists of trigonal-hexagonal two-dimensional (2D) grids built from the [AlF6] octahedra connected via vertices. The 2D grids have a specific wave-like conformation with a wavelength of 11.88 Å and an amplitude of 0.46 Å. There is a shift of the adjacent grids relative to each other. Because of this shift, the space between the grids changes. The shift leads to the formation of pores adapted to potassium and sodium ions. The reasons for the wave-like structure of layers are discussed. It is shown that the two polymorphic forms differ in the order of cation occupations.

LiNa2AlF6: a powder structure solution.

Lithium sodium aluminium fluoride was obtained as a white powder by melting a stoichiometric mixture of AlF(3), NaF and LiF at 1223 K, and then cooling to 923 K and sintering at this temperature for 4 h. The ab initio crystal structure determination was carried out using X-ray powder diffraction techniques. The monoclinic structure of LiNa(2)AlF(6) can be related to cubic elpasolite. The Li and Al atoms lie on inversion centres. The main octahedral AlF(6) structural elements are not deformed, but are rotated slightly with respect to the unit-cell axes. The Li atoms have octahedral coordinations, whereas the Na atoms have cubo-octahedral coordinations. The Na coordination polyhedron is distorted in comparison with that of elpasolite.

Genetic Algorithm for Automated X-Ray Diffraction Full-Profile Analysis of Electrolyte Composition on Aluminium Smelters

Aluminium is produced by means of the electrolysis of alumina in molten fluoride salts . A certain proportion of the fluoride compounds continuously evaporates, and this negatively impacts on the optimal composition of the electrolyte in the electrolytic baths. It means that a regular adjustment of the electrolyte composition is required by the addition of fluorides based on the results of the automatic express analysis of the electrolyte. The XRD phase analysis of crystallized electrolyte samples automatically performs the control of the main composition characteristics. This method, most frequently implemented in conjunction with aluminium smelters, necessitates periodic calibration with reference samples, whose phase composition is known exactly. The preparation of such samples is relatively complex since samples include 5–6 different phases with variable microcrystalline structure. One further diffraction method is the Rietveld method, which can be implemented without the use of reference samples. The method is based on the modelling of the experimental powder patterns of crystalline samples as the sum of the powder patterns of comprised phases, calculated from their atomic crystal structure. Included in the simulation is a refinement of the profile parameters and crystal structure of phases using the nonlinear least squares method. The difficulty associated with the automation of this approach is that a set of initial values for the parameters must be inputted that must be automatically refined by LSM to exact values. In order to resolve this problem, an optimization method was put forward by the article based on an evolutionary choice of initial values of profile and structural parameters using a genetic algorithm. The criterion of the evolution is the minimization of the profile R-factor, which represents the weighted discrepancy between the experimental and model powder patterns of the electrolyte sample. It is established that this approach achieves the required level of accuracy and complete automation of the electrolyte composition control.

Method of evolutionary structure-sensitive quantitative X-ray phase analysis of multiphase polycrystalline materials

A method of quantitative X-ray phase analysis (QXPA) based on multiobjective evolutionary optimization for automatization and increase in accuracy of QXPA that is based on full-profile analysis by the Rietveld method is proposed. The two-level genetic algorithm performs the probabilistic evolution of profile and structural parameters by the criterion of minimization of the profile R factor and controls their refinement by the method of full-profile difference-differential analysis (a version of the Rietveld method). The method of evolutional QXPA is discussed by an example of a test mixture of minerals that was used in Round Robin on QPA CPD IUCr.

Method of regularized multipeak reference intensity ratio for quantitative X-ray phase analysis of polycrystalline materials

The development of the regularized multipeak modification of the reference intensity ratio method (RRIR) is reported. The RRIR method consists in the simulation of the experimental X-ray pattern by a linear combination of the reference standard spectra of the identified phases taken from the PDF ICDD database, application of the regularized Tikhonov LSM and the data on quantitative elemental composition upon the optimization of the model spectrum, and harnessing of the scaling coefficients of reference spectra to calculate the phase concentrations. The peculiarities of application and accuracy of RRIR achieved in the analysis of texturized multiphase mixture of minerals used in Round Robin on QPA CPD IUCr are discussed.

On the Application of Co-Operative Swarm Optimization in the Solution of Crystal Structures from X-Ray Diffraction Data

A co-operative method based on five biology-related optimization algorithms is used in solving crystal structures from X-ray diffraction data. This method does not need essential effort for its adjustment to the problem in hand but demonstrates high performance. This algorithm is compared with a sequential two-level genetic algorithm, a multi-population parallel genetic algorithm and a self-configuring genetic algorithm as well as with two problem specific approaches. It is demonstrated on a special crystal structure with 7 atoms and 21 degrees of freedom on which the co-operative swarm optimization algorithm exhibits comparative reliability but works faster than other used algorithms. Perspective directions for improving the approach are discussed.

Multipopulation Genetic Algorithm for Determining Crystal Structures Using Powder Diffraction Data

A method for the automatic determination of crystal structures using powder diffraction data by the multipopulation genetic algorithm is proposed. The advantage of using coevolution with exchange by better individuals over using evolution within a single genetic algorithm without interpopulation exchange is demonstrated. The process of searching for a structural solution using the multipopulation genetic algorithm is illustrated and analyzed by the example of the known Ca2Al3O6F crystal structure (sp. gr. R3̄, a = 17.3237(1) Å, c = 7.0002(0) Å, Z = 6, and V = 1819.38(1) Å3). The fitness functions for the best structural models and atomic position maps at different algorithm operation stages are plotted.

System NaF-KF-AlF3: Solid Solutions Based on the Сhiolite Structure

The formation of solid solutions in the NaF-KF-AlF3 system has been studied by X-ray diffraction on samples obtained experimentally. For the first time the formation of solid solutions based on structure of chiolite with the composition (Na(5-x)Kx)Al3F14, 0 < x < 0,4) has been established. The change of a crystal lattice of chiolite occurs in the range: (a) – from 7,010 (3) to 7,050 (3) Å, (c) from 10,365 (10) Å to 10,400 (10) Å. The sodium potassium substitution occurs at only a 2-fold sodium position by an amount ~ 40 % established by refinement of the crystal structure. The limit dissolution of chiolite in less than 5 % (wt.) KF. The solid solution is stable in the range from the melting temperature to the room.

Genetic Algorithm based X-Ray Diffraction Analysis for Chemical Control of Aluminium Smelters Baths

Aluminium production is based on the high-temperature electrolysis of alumina in molten fluoride salts. Part of the fluoride compounds continuously evaporates, which violates the optimal composition of the electrolyte in the electrolytic baths. It causes a technological necessity for regular adjustment of the electrolyte composition by the addition of fluorides according to results of automatic express analysis of the electrolyte. Control of the main composition characteristics is performed automatically by XRD phase analysis of crystallized electrolyte samples. The XRD method, usually used on aluminium smelters, requires periodic calibration with reference samples, whose phase composition is exactly known. The preparation of such samples is a rather complicated problem because samples include 5–6 different phases with variable microcrystalline structure. An alternative diffraction method is the Rietveld method, which does not require reference samples to be used. The method is based on the modelling of the experimental powder patterns of electrolyte samples as the sum of the phase of component powder patterns, calculated from their atomic crystal structure. The simulation includes a refinement of the profile parameters and crystal structure of phases by the nonlinear least squares method (LSM). The problem with the automation of this approach is the need to install a set of initial values of the parameters that can and should be automatically refined by LSM to exact values. To solve this problem, the article proposed an optimization method based on an evolutionary choice of initial values of profile and structural parameters using a genetic algorithm. The criterion of the evolution is the minimization of the profile R-factor, which represents the weighted discrepancy between the experimental and model powder patterns of the electrolyte sample. It is shown that this approach provides the necessary accuracy and complete automation of the electrolyte composition control.