Crystal structure of compound

- фактор розбіжності R=7,311%.

Ключові слова: рентгеноструктурний аналіз, геометрія зйомки Брег-Бертран, метод Ритвельда, кристалічна структура, склад K3TiOF.

Introduction

Ferroelectric materials are used in variable capacitors. The dielectric constant of ferroelectric materials is not only tunable but also very high. Therefore, ferroelectric capacitors are much smaller in size and have a higher electrical capacity than dielectric capacitors. In comparison, a number of materials with ferroelectric properties have recently been synthesized, including K₃TiOF₅ [1]. Starting materials KF (Ventron, purity > 99.9%), oxides TiO₂, Nb₂O₅, Ta₂O₅, WO₃ (Cerac, 99,95%) pre-dried in vacuum at 473K for 20 hours. Oxyfluorides TiOF₂, NbO₂F, TaO₂F are obtained by the action of a 40% solution of hydrofluoric acid on the corresponding oxides in a Teflon bath, after complete evaporation of the solution in a sand bath at 373K. Residual solids are degassed in vacuum at a temperature of 473K. About 18 g of the mixture is weighed in stoichiometric proportions, ground in an agate mortar in an oven, then placed in a Biconical platinum 10% rhodium crucible. The compound is obtained in the following reaction 3KF+TiOF₂^ K₃TiOF₅. After degassing in vacuum for 20 hours at 473K and then sealing in a dry oxygen atmosphere, the crucible is fired at a reaction temperature for 24 hours and then at a melting temperature of + 50K [1].

The object of research is the crystal structure of the K₃TiOF₅compound.

PDF-2 database for 2009 contains indexed diffraction spectrum obtained for the K₃TiOF₅ compound. The crystal structure of this spectrum is unknown.

The aim and objectives of research

The aim of research is to propose a structural model for the diffraction spectrum of the K₃TiOF₅ compound under the number 00-023-0506 in the PDF-2 database for 2009.

To achieve this aim, it is necessary to solve the following objectives:

1. Determine the periods of the lattice and the crystal system in which the studied compound crystallizes.

2. Select the space group of symmetry and propose a structural model for the given spectrum of the compound.

3. Carry out the refinement of microstructural parameters for the selected model by the Rietveld method.

Solvothermal synthesis of K₃TiOF₅ was carried out in a stainless steel autoclave with a Teflon lining at reduced pressure. All reagents (K0H, KF-2H₂0, Ti02, Ti, NH₄HF₂, H₂0₂) and methanol were analytically pure and used without further purification (purchased by Shanghai Chemical Reagent Company). The mixture is placed in a stoichiometric ratio in a teflon-lined stainless steel autoclave of milliliters, which is then filled with methanol to 80 times the total volume. The autoclave was quickly closed and heated at 200°C for 24 or 36 hours and cooled naturally to room temperature. The precipitates were collected and washed with ethanol, distilled water, and dried in a vacuum at 60°C for 2 hours [3].

Diffraction patterns were taken from the obtained sample by the powder method with the Bragg-Bertrand geometry. The diffraction spectrum corresponds to K₃TiOF₅ numbered 00-023-0506 in the PDF-2 database [2; 3], indexed in the tetragonal system, with lattice periods a=6,102°A, c=8,655°A.

The results of the analysis of the literature indicate that the crystal structure of the test compound is unknown. With its electrical properties it can be used as a ferroelectric.

Methods of research

The diffraction spectra of the compounds for the study were generated using High- ScorePlus 3.0 and the attached PDF-2 database for 2009 in UDF format.

Analysis of the proposed structural model of this spectrum was performed using the program HighScorePlus 3.0 by the Rietveld method.

Research results

The diffraction spectrum of the compound K₃TiOF₅ is indexed in tetragonal syngony with lattice periods a=6.086 A°; b=6.086 A°; c=8.675 A. Possible symmetry group I41 (80).

The correct system of points and their specified coordinates for this spectrum are presented in Table 1.

Table 1 Microstructural parameters of K3TiOF5 for spectrum 00-023-0506 in PDF-2 database up to 2009

Note: Wyck. - correct point system; s.o.f. - position filling factor with atoms; x, y, z - coordinates of atoms in the fate of lattice periods (x=X/a; y=Y/b; z=Z/c); Uisoa - temperature factor

The values of observed and calculated interplanar distances and integral intensities of the diffraction spectra are given in Table 2.

Table 2 The values of interplanar distances and intensities are given in [2], and calculated according to this model

Disagreement factor R = 7.311%.

Table 3 shows the interatomic distances of the proposed structural model for the K₃TiOF₅ compound for spectrum 00-023-0506 in the PDF-2 database for 2009.

Table 3 Interatomic distances of the 3TiOF5 compound

Fig. 1 shows diffraction patterns generated and calculated from the structural simulator for the K₃TiOF compound.

Fig. 2 shows an image of the proposed model of the crystal structure of the investigated compound.

The space group of symmetry І4_г (80) has a rotary axis of symmetry of the 2nd order parallel to 001, a helical axis of the 4th order with translation 1/3 с parallel to 001. crystal ferroelectric physical

Also, partial filling of the correct systems of points in the structure under study may indicate that the stoichiometric composition of the compound may be slightly changed. So, the structure of the connection requires further investigation.