CRYSTAL STRUCTURES OF NEW TRIPLE Ca9CoМ(PO4)7 (M = Li, Na, K) PHOSPHATES

Abstract

The progress of inorganic chemistry is syntheses of new triple phosphates Ca₉CoM(PO₄)₇ (M = Li, Na, K) were synthesized by solid state method. Their crystal structures were determined by Rietveld analysis. They are related to tricalcium phosphate and crystallize into trigonal system (space group R3c) with unit-cell parameters a = 10.3276(1) Е, с = 37.100(1) Е, M = Li; a = 10.3515(1) Е, с = 37.073(1) Е, M = Na; a = 10.4017(1) Е, с = 37.009(1) Е, M = K. All five independent cation sites are filled in Ca₉CoM(PO₄)₇ (M = Li, Na, K). Cobalt occupies the octahedral site M(5). Alkali metal cations occupy the M(4) Site. The features of the crystal structures of β-Ca₃(PO₄)₂  give the possibility for iso- and heterovalent substitutions of Ca²⁺ by M⁺, Me²⁺, R³⁺, and R⁴⁺ cations. These substitutions lead to the formation of the solid solutions on the base of the β-Ca₃(PO₄)₂ structure. The solid solutions of Ca₃-_xMe_x(PO₄)₂ (Me = Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd, Sr, Pb, Ba) were studied by Nord. The schemes of heterovalent substitutions are described in details. Among the triple phosphates with the structure of β-Ca₃(PO₄)₂ crystal structures were studied only for Ca₉MgM(PO₄)₇ (M = Li, Na, K) and Ca₁₈Na₃Fe(PO₄)₁₄.

The main idea of this work is syntheses phosphates. The phosphates Ca₉CoM(PO₄)₇ (M = Li, Na, K) were synthesized by solid state reactions from stoichiometric mixtures of Ca₂P₂O₇, CaCO₃, Co₃O₄, and corresponding carbonates M₂CO₃ (M = Li, Na, K) at 1273 К for 50-90 h in air. The compounds obtained were light violet in color and single phase. The indexing of the powder diffraction patterns of Ca₉CoM(PO₄)₇ (M = Li, Na, K) has been submitted to the Powder Diffraction Files. The corresponding unit-cell parameters and figures of merit are a = 10.3275(1) Е c= 37.103(1) Е, F₃₀ = 161.1 (0.0058, 32), M = Li; a = 10.3514(1) Е, с = 37.073(1) Е, F₃₀ = 142.6 (0.0062, 34), M = Na; a = 10.4015(1) Е, с = 37.012(1) Е, F₃₀ = 210.8 (0.0038, 37), M = K.

The atomic coordinates of Ca_9.5Co(PO₄)₇ were used as starting parameters for structure refinements. The cobalt cations were placed in the M(5) site as in Ca_9,5Me(PO₄)₇ (Me = Co, Cu). Alkali metal cations were placed in the M(4) site as it was obtained for Ca₉MgM(PO₄)₇, Ca₁₀M(PO₄)₇ (M= Li, Na, K), and Ca₁₀K(VO₄)₇. The structure refinements in these models gave a good agreement between the observed and calculated patterns and reasonable values of isotropic thermal atomic parameters for all cations. Final plots of observed electron density maps did not show residual peaks. The electron density on different electron density maps did not exceed ±0.8 e/Е³. The details of data collection and refinement are given. The final atomic parameters are listed, presents selected interatomic distances and angles.