ИСТИНА

Due to the fine tuning of the physical and chemical properties of single crystals through the correct choice of composition and structure, they are successfully used in many fields of science and technology, but their potential has not yet been exhausted. The creation or improvement of materials for known or new devices is impossible without understanding the fundamental relationship "composition-structure-method (conditions) of synthesis-properties", the establishment of which is the main goal of modern materials science. Knowledge of all the links of this chain makes it possible to purposefully obtain compounds and materials based on them with desired characteristics and parameters, to control their functional properties, to simulate new crystalline systems taking into account the conditions of their growth and behavior in different conditions, including extreme ones. Determination of the actual composition and real structure of large-sized single crystals obtained by melt or solution methods and having a commercial focus is a non-trivial task that differs significantly from the study of the same single-crystal objects, but with small sizes. It is necessary to know a priori phase diagrams to determine the melting parameters and the distribution coefficients of the components along the length and cross-section of the crystal, to take into account the directions of growth and possible growth defects (facet effect, growth banding), the potential influence of the growth conditions (composition of the initial charge, method of introduction of impurity, growth atmosphere, post-growth processing), etc. However, even the provision of all the above factors does not guarantee the growth of single crystals of uniform composition within a specific structure: in the overwhelming majority of cases, the composition of the synthesized crystals (the actual composition, taking into account all types of point defects and main associates), which should be associated with the parameters of functional properties, differs significantly from composition of the initial charge. This does not allow establishing the correct composition-property relationship and directionally growing crystals with the required set of properties. On the other hand, the synthesis conditions can change the statistical structure, which is determined by the methods (techniques) of structural analysis, and the local environment of the components, revealed by X-ray absorption spectroscopy. It is the local structure that is the forerunner of the crystal structure and its stability. Further, the examples show the efficiency of either one or both methods in the study of single crystals of different compositions and structures. The obtained details of the crystal and local structures made it possible to explain the functional properties of the objects under investigation and to find important correlations. 1. For a long time, it was believed that compounds of the huntite family with the general composition RESc3(BO3)4 with RE = La – Gd, obtained by melt methods, crystallize in space group R32, which was due to widespread survey on laboratory diffractometers or rejection of weak reflections in the analysis of diffraction patterns. A detailed structural study of single crystals and the same powdered crystals using synchrotron radiation revealed a decrease in the symmetry caused by the order-disorder structural transition accompanied by the distribution of Sc (RE) ions over two (space group P321) rather than one (space group R32) crystallographic sites of the huntite structure. 2. Different coordination environments of Na1+ and Gd3+ ions was established in the structures of single crystals of the scheelite family with the initial compositions (Na1/2Gd1/2)MoO4 and (Na2/7Gd4/71/7)MoO4 and, respectively, refined (actual) compositions (Na0.498(2)Gd0.502)(Mo0,999(4)0.001)O4 and (Na0.348(8)Gd0.5280.124)(Mo0.996(3)0.004)O4 (-vacancies). This seemed obvious from the crystal-chemical point of view due to the difference in the formal charges of the Na1+ and Gd3+ ions, but it was proved for the first time, which made it possible to explain the observed kinetic processes in this family of crystals. 3. The method of introducing activators affects their content, formal charge, and structural location in the host crystal. Thus, Mn activator ions introduced over stoichiometry (up to 1.0 wt%) into a crystal melt with a whitlockite-like structure Ca3(VO4)2 (CVO:Mn) or by high-temperature diffusion annealing of Ca3(VO4)2 in the presence of a Mn2O3 solid phase (CVO:Mn2O3) showed different structural behavior. A higher content of manganese was found in CVO:Mn2O3. In addition, different formal charges (Mn3+ and Mn(2+)+, respectively, in CVO:Mn and CVO:Mn2O3) and different coordination environments (elongated Jahn-Teller octahedron and tetrahedron, respectively, in CVO:Mn and CVO:Mn2O3) were revealed. It should be noted that, according to the structural analysis, manganese ions occupy coordination polyhedra unusual for them (mono- and two-capped trigonal prisms). Only the use of X-ray absorption spectroscopy made it possible to remove this contradiction. 4. The most interesting effect is "homeopathic", which we first observed: a significant difference in the local structure of single crystals doped with ions of rare earth or transition metals in small (<~ 5 wt%) or relatively large (~> 5 wt%) amounts over stoichiometry. Thus, in the structure of Dy3+-doped sillenite crystals Вi4Ge3O12:Dy (BGO:Dy) with actual compositions (Bi3+3.972(5)Dy0.028)Ge3O12 (BGO:1.0%Dy) and (Bi3+3.993(5)Dy0.007)Ge3O12 (BGO:0.1%Dy), different polyhedra are formed: DyO5 (derivative of a trigonal prism) and BiO6 (distorted octahedron), DyO7 (derivative of an octahedron) and BiO5 (defective octahedron), respectively. Numerous studies on single crystals of the garnet, perovskite, scheelite, huntite, whitlockite and many other families have shown that structural analysis and X-ray absorption spectroscopy are a powerful tandem for applied and fundamental materials science.