Synthesis, dopant solubility, and thermostructural properties of Ca_10.5-TM(VO₄)₇ (TM = Co, Cu) as function of...

Crystals of Ca_10.5-TM(VO₄)₇ (TM = Co, Cu), belonging to the whitlockite family, were synthesized by solid-state reaction and studied as a function of the TM content (x) for the first time. The structure was refined at ambient conditions and at high temperatures up to 1200 K using the Rietveld method. The unit cell size significantly decreases with increasing TM content up to the solubility limit, x_lim, which is 0.78(3) for TM = Co and 0.75(4) for TM = Cu. Occupancy factors show a preference for the M5 site by Co/Cu. The unit cell size varies smoothly with temperature, while the axial ratio exhibits nonlinear behaviour above approximately 800 K. The thermal expansion coefficient was determined from 300-1100 K. Atomic arrangement modifications at higher temperatures are indicated by changes in the axial ratio, the thermal expansion coefficient, and the reduction of fractional TM occupancy at the M5 site at specific temperatures.