Low temperature structural and thermal expansion studies on α-cristobalite type Al0.5Ga0.5PO4 by powder neutron diffraction

The orthorhombic (α) low-cristobalite type Al0.5Ga0.5PO4, prepared by co-precipitation followed by high-temperature annealing of the amorphous precipitate has been investigated by powder X-ray diffraction (XRD) at room temperature and variable temperature neutron diffraction technique down to 20 K. In the temperature range of 20-300 K, the crystal structure of this compound has been analyzed by Rietveld refinements of powder neutron diffraction data. This composition crystallizes in the orthorhombic lattice with unit cell parameters: a = 7.030(1), b = 7.015(1), c = 6.923(1) Å and V = 341.4(1) Å3 (space group C2221) (at ambient temperature). The unit cell parameters show a gradual decrease with decreasing temperature and the Al3+ and Ga3+ ions are statistically distributed over a single crystallographic site forming an ideal solid solution of AlPO4 and GaPO4 down to 20 K. The structural details and low-temperature thermal expansion behavior of this compound are reported in this communication. The results are also compared with our earlier thermal expansion studies of cristobalite type phosphates above ambient temperature.