Disorder-Induced Breaking of the Local Inversion Symmetry in Rhombohedral Pyrochlores MLaSbO (M = Mg or Ca): A Structural and Spectroscopic Investigation.

A detailed investigation of the overall crystal structure, and in particular of the local structure around the cations in MLaSbO (M = Mg, Ca) was accomplished using X-ray diffraction, steady state luminescence spectroscopy and decay kinetics, and state of the art density functional calculations. The computational tool was also used to investigate the structure of MnLaSbO. The Eu dopant ion was employed as an optical probe of the local symmetry at the cationic sites. The use of these complementary techniques shows that the antimonates under investigation belong to the rhombohedral pyrochlore family with space group R3̅ m (No. 166), but while MgLaSbO and MnLaSbO show an ordered cationic configuration, the Ca and La of CaLaSbO are disordered because of their similar ionic radii. In both the Mg- and the Ca-based compounds, the Eu ions formally occupy centrosymmetric sites, but in the case of CaLaSbO the presence of disorder in the outer coordination spheres removes the local inversion symmetry in these sites. This has a strong influence on the Eu luminescence spectrum and on the radiative decay rate of the D emitting level.