New Sr 5 Os 3 O 13 , as synthesized from binary constituents, exhibits several uncommon features. Its crystal structure is dominated by quasi-2D poly-oxoanions that correspond to unprecedented cutouts of the perovskite type of structure, where corner sharing (OsO 6 ) octahedra aggregate to form terraced slabs.The Os 5+ /Os 6+ mixed valence oxide displays a particular charge ordering scheme. One osmium atom (Os1) per formula unit is in the valence state of 5+ in the whole temperature range studied, while the two remaining sites (Os2A and Os2B) show full charge disorder at high temperatures, resulting in an average charge of 5.5+. The latter, however, apparently undergo a process of continuous charge ordering at cooling. Full charge order appears to be established concomitantly with a phase transition to an antiferromagnetically ordered state at T (Néel) = 170 K. This kind of temperature dependent continuous charge ordering is reflected by structural changes with temperature as well as by changes in paramagnetic response above T (Néel) . Disentangling the intimate interplay between magnetic and charge ordering degrees of freedom will require applying sophisticated spectroscopy and (neutron) diffraction techniques.

The new osmates(VI), Sr2OsO5 and Sr7Os4O19, feature quasi-1-D polyoxo anions, consisting of corner sharing [OsO6] octahedra. In both compounds, the magnetic moment at T = 300 K is significantly lower (1.2-1.3 μB/Os-atom) than the value expected for S = 1. For neither of the new osmates(VI) is any evidence for long-range magnetic order found. For Sr7Os4O19, magnetic susceptibility suggests an antiferromagnetic ordering at TN = 43(3) K; however, no corresponding anomaly is visible in specific heat. Both compounds are semiconductors.

Synthesis of elusive K4O6 has disclosed implications of crucial relevance for new solid materials discovery. K4O6 forms in equilibrium from K2O2 and KO2, in an all‐solid state, endothermic reaction at elevated temperature, undergoing back reaction upon cooling to ambient conditions. This tells that the compound is stabilized by entropy alone. Analyzing possible entropic contributions reveals that the configurational entropy of “localized” electrons, i.e., of polaronic quasi‐particles, provides the essential contribution to the stabilization. We corroborate this assumption by measuring the relevant heats of transformation and tracking the origin of entropy of formation computationally. These findings challenge current experimental and computational approaches towards exploring chemical systems for new materials by searching the potential energy landscape: one would fail in detecting candidates that are crucially stabilized by the configurational entropy of localized polarons.

Single crystals of a new twinned hexagonal perovskite compound Sr8Os6.3O24 have been synthesized, and structural and magnetic properties have been determined. The compound crystallizes in a hexagonal cell with lattice parameters a = 9.6988(3) Å and c = 18.1657(5) Å. The structure is an eight-layered hexagonal B-site deficient perovskite with the layer sequence (ccch)2 and represents the first example of a hexagonal structure among 5d oxides adopting a twin option.The sample shows spontaneous ferromagnetic magnetization below 430 K with a small saturation moment of 0.11 μB/Os ion. This is the highest Curie temperature (TC) reported for any bulk perovskite containing only 5d ions at the B site.

No abstract