Combined X-ray and Neutron Powder Diffraction Study on B-Site Cation Ordering in Complex Perovskite La₂(Al_1/2MgTa_1/2)O₆

Complex perovskite La-2(Al1/2MgTa1/2)O-6 (LAMT) crystallizes in a monoclinic unit cell with space group P2(1)/n at room temperature. Its B-site cations are ordered in a rock-salt-type arrangement. Previously, the full occupancy of Mg on the 2c-Wyckoff position was deduced from powder X-ray diffraction (PXRD). However, conventional X-rays could not properly resolve the mixed occupation on the B-site, since there is little scattering contrast between the neighbouring elements Mg and Al of the periodic table. Hence, complementary neutron diffraction studies were carried out to verify the exact B-site cation ordering in the unit cell. In this specific configuration of the B-cations, with its occupancy ratio and the presence of a heavy element Ta as well as neighbouring elements Mg and Al, only the strategy of a combined Rietveld analysis using both the X-ray and neutron diffraction data simultaneously succeeded in elucidating an accurate B-site cation ordering in this complex perovskite system. A full occupancy of Mg on the 2c-Wyckoff position and each a half occupancy of Al and Ta on the 2d-Wyckoff position could be resolved for the rock-salt-type ordering of the B-site cations in the monoclinic unit cell of LAMT.