Perovskite and pyrochlore modifications of Pb₂MnReO₆: synthesis, structure, and electronic properties

Abstract

We describe the synthesis and investigations of Pb₂MnReO₆ in perovskite and pyrochlore modifications. Both the modifications are formed at atmospheric pressure, the perovskite (I) at 550 °C and the pyrochlore (II) at 600 °C. Rietveld refinements of the structures using the powder XRD data show that oxide I adopts the monoclinic (P2₁/n) double perovskite structure with partial ordering (~72%) of Mn^II/Re^VI at the octahedral sites, while oxide II crystallizes in a defect-pyrochlore structure where Mn^II/Re^VI are disordered at the octahedral sites. The frustrated cation sublattice precludes a long-range ordering of Mn/Re in the pyrochlore structure. I is a semiconductor showing evidence for a ferrimagnetic ordering at T_c ~100 K, but the small saturation magnetic moment (<S> = 0.81) and the divergence between FC and ZFC magnetization data are consistent with spin-glass-like behavior and a partially ordered Mn/Re in the double perovskite structure. On the other hand, both the Curie-Weiss paramagnetism down to ~5 K and non-Arrhenius electrical resistivity behavior of II suggest a spin-liquid/spin-glass ground state that is consistent with the frustrated cation sublattice of the pyrochlore structure.