Sr4Os3O12- A Layered Osmate (V,VI) that is Magnetic close to Room Temperature

TL;DR

This study reports on Sr4Os3O12, a layered osmate that exhibits near-room-temperature magnetism, semiconducting behavior, and complex structural and electronic properties influenced by strong spin-orbit coupling and lattice dynamics.

Contribution

The paper presents the synthesis, detailed structural analysis, and magnetic and electronic characterization of Sr4Os3O12, revealing its unique layered structure and magnetic properties close to room temperature.

Findings

Sr4Os3O12 is attracted to magnets at 295 K but loses magnetism when heated or cooled.

It is a small band gap semiconductor with about 0.26 eV at room temperature.

The compound exhibits a rhombohedral structure with layered blocks and complex lattice dynamics.

Abstract

Black crystals of the mixed-valence osmate(V,VI) Sr4Os3O12 were grown via a gas phase reaction in an evacuated silica tube. The material is attracted to a permanent magnet at room temperature (295 K) but loses this property when heated or cooled. In this temperature interval, soft magnetic behavior with vanishing coercivity is observed, but the saturation magnetization is only 0.05 MuB per formula unit. Resistivity measurements show that Sr4Os3O12 is a semiconductor with a small band gap of about 0.26 eV at room temperature. X-ray diffraction on a single-crystal revealed a rhombohedral structure with the space group R-3m and lattice parameters ar = 554.64(2) pm and cr = 2700.1(1) pm at 300(1) K. The compound crystallizes in the La4Ti3O12 structure type and is isostructural to Sr4CoRe2O12. The structure is a rhombohedral stack of layered blocks, each of which can be considered as a section of three layers of the cubic perovskite structure cut perpendicular to one of its threefold axes. The Os-O-Os bond angle of about 175{/deg} favors superexchange that leads to antiferromagnetic coupling between the osmium atoms with 5d3 and 5d2 configurations. Sr4Os3O12 exhibits strong lattice dynamics. One aspect is the antiferro-rotative lattice modes of the corner-sharing [OsO6] octahedra, which freeze upon cooling. At 100 K, an ordered, yet twinned triclinic superstructure is formed. Strong spin-orbit coupling and other effects cause deformations of the [OsO6] octahedra that differ significantly at 300 K and 100 K, suggesting a change in the electronic configuration. Competing ground states could also explain the temperature dependent band gap and the magnetic fluctuations manifested in the magnetization peak at room temperature.