Lattice thermal expansion of zircon-type LuPO4 and LuVO4: A comparative study

We report the lattice thermal expansion of zircon-(xenotime-)type LuPO4 and LuVO4 in the temperature range of 25-1000 degrees C from high-temperature powder XRD studies. The details of the high-temperature crystal chemistry of both phases have been determined from Rietveld analysis of the powder XRD data. Both the compounds show appreciably higher thermal expansion than analogous zircon-type silicates. Despite isomorphism, the axial thermal expansion of LuVO4 shows significant anisotropy compared to LuPO4. In the studied temperature range, the average axial thermal expansion coefficients of LuPO4 are alpha(a) = 6.0 x 10(-6) and alpha(c) = 7.2 x 10(-6) (degrees C-1) and those of LuVO4 are alpha(a) = 3.6 x 10(-6) and alpha(c) = 11.8 x 10(-6) (degrees C-1). However, the average volume thermal expansion coefficients are almost identical. The differences in the thermal expansion behavior of the two structures originate from differences in the expansion and distortion of the LuO8 polyhedra. The LuO8 polyhedron in LuVO4 shows about 30% higher thermal expansion than that in LuPO4. The overall thermal expansion behaviors of these two Structures are predominantly related to the distortion in the LuO8 unit, inter-cation distances and spatial arrangement of the Lu-O bonds in the structure.