Crystal structures and high-temperature phase-transitions in SrNdMRuO_6 (M=Zn,Co,Mg,Ni) new double perovskites studied by symmetry-mode analysis

摘要

Crystal structures of SrNdZnRuO_6, SrNdCoRuO_6, SrNdMgRuO_6 and SrNdNiRuO_6 double perovskites have been studied by X-ray, synchrotron radiation and neutron powder diffraction method, at different temperatures, and using the symmetry-mode analysis. All compounds adopt the monoclinic space group P21 at room-temperature, and contain a completely ordered array of the tilted MO_6 and RuO_6 octahedra, whereas Sr/Nd cations are completely disordered. The analysis of the structures in terms of symmetry-adapted modes of the parent phase allows the identification of the modes responsible for the phase-transition. The high-temperature study (300-1250 K) has shown that the compounds present a temperature induced structural phase-transition: P2_1→P4_2→Fm3?m.