The crystal structure and thermal expansion of novel substitutionally disordered Ca10TM0.5(VO4)7 (TM = Co, Cu) orthovanadates

The whitlockite-related materials have attracted researchers' attention because of their potential application in various fields, especially in optoelectronics. In the present work, the structure of novel whitlockite-related oxides Ca₁₀TM_0.5(VO₄)₇ (TM = Co, Cu) is studied at room and high temperatures, using X-ray powder diffraction. These compounds form by fractional substitution of divalent transition metal atoms into the Ca₃(VO₄)₂ lattice. Rietveld refinements provided the structural details. The lattice parameters are a = 10.78074(6) Å, c = 37.8196(2) Å, and V = 3806.67(4) ų for Ca₁₀Co_0.5(VO₄)₇ and a = 10.78710(7) Å, c = 37.8997(3) Å, and V = 3819.23(4) ų for Ca₁₀Cu_0.5(VO₄)₇. Structure refinement results show that among the five available sites (M1-M5), the M²⁺ ions select the M5 site. This finding is confirmed by analysis of interatomic distances: due to the difference in size between TM and Ca ions sharing the M5 site, the M5-O distance shortens by about 5.0% for Ca₁₀Co_0.5(VO₄)₇ and 2.7% for Ca₁₀Cu_0.5(VO₄)₇ with respect to the unsubstituted parent compound, Ca₃(VO₄)₂. The observed trends in the crystallographic properties of the studied crystals are in line with those of previously reported structurally related phosphates, Ca_10.5-xM_x(PO₄)₇ (M = Mg or divalent transition metal). Moreover, the observed tendency for occupation of M5 by small divalent ions follows the earlier theoretical results. For cobalt and copper substituted orthovanadate and orthophosphate whitlockite related materials, a linear variation in the unit cell size is demonstrated. The common equation for evaluation of volume is applicable to the substitution of the two transition metals in orthovanadate and orthophosphate whitlockite related materials. Thermal expansion is investigated for both compounds. The variations of the lattice parameters and the thermal expansion coefficient with temperature are determined in the 300-810 K range. The lattice parameter, a, expands by 0.80% for Ca₁₀Co_0.5(VO₄)₇ and 0.74% for Ca₁₀Cu_0.5(VO₄)₇ in this range. The lattice parameter, c, enlarges by about 0.70% for both samples. In the studied temperature range, the volume thermal expansion coefficient of Ca₁₀Co_0.5(VO₄)₇ increases from 37.2 to 44.8 MK^-1 and for Ca₁₀Cu_0.5(VO₄)₇, it increases from 35.1 to 45.2 MK^-1; the observed expansion anisotropy is smaller than those of other related compounds.