Sr 4 Os 3 O 12 — A Layered Osmate(V,VI) that is Magnetic close to Room Temperature

Black crystals of the mixed‐valence osmate(V,VI) Sr 4 Os 3 O 12 were grown via a gas phase reaction in an evacuated silica tube. The material is attracted to a permanent magnet at room temperature (295±5 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 μ B per formula unit. Resistivity measurements show that Sr 4 Os 3 O 12 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 m and lattice parameters a r =554.64(2) pm and c r =2700.1(1) pm at 300(1) K. The compound crystallizes in the La 4 Ti 3 O 12 structure type and is isostructural to Sr 4 CoRe 2 O 12 . 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° favors superexchange that leads to antiferromagnetic coupling between the osmium atoms with 5 d 3 and 5 d 2 configurations. Sr 4 Os 3 O 12 exhibits strong lattice dynamics. One aspect is the antiferro‐rotative lattice modes of the corner‐sharing [OsO 6 ] 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 [OsO 6 ] 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.