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.