Crystallochemical modeling, synthesis, and study of new tantalum and niobium phosphates with a framework structure

A crystallochemical approach is used to model the compositions of phosphates of pentavalent elements with the expected structure. New phosphates with a framework structure of the T₂^IT₃^2/IIIT₁^2/V (PO₄)₃ type (T^I = Na or K; T^III = Al, Cr, or Fe; and T^V = Nb or Ta) are synthesized and characterized by X-ray diffraction analysis (including high-temperature diffraction) and IR spectroscopy. It is established that, depending on the nature of the alkali cation (Na or K), these compounds are crystallized in two structural modifications: rhombohedral and cubic (sp. gr. R $$\bar 3$$ c and P2₁3, respectively). The unit-cell parameters and the thermal expansion coefficients of the phosphates under study are determined and the dependences of these characteristics on the nature of cations are established.