The open-shell p-electron compound Cs4O6 features anionic charge ordering with a concomitant jump in electronic conductivity.

The square-planar coordination of transition metals has been assumed to require the d 8 with Ir(IV) in d 5 configuration, and characterize the chemical bonding by experiment and ab initio calculations. We find that Na4IrO4 in its ground state evolves a square-planar coordination for Ir(IV) because of the weak Coulomb repulsion of Ir-5d electrons. In contrast, in its 3d counterpart, Na4CoO4, Co(IV) is in tetrahedral coordination, due to strong electron correlation. Na4IrO4 thus may serve as a simple paradigmatic platform for studying the ramifications of Hubbard type Coulomb interactions on local geometries.Tetrahedral geometry is by far the prevailing coordination geometry encountered with isolated entities [MO4] n-and is electrostatically favored. Consequently, other coordination geometries such as square planar require special local electronic configurations and covalent bonding contributions to be stabilized. It has been quite easy to rationalize the occurrence of square-planar coordination, even in purely qualitative terms, from electron counting. The d 8 and d 9 electron configurations are frequently associated to quadrate surroundings of transition metals, [1] while for main group elements, the combination of four covalently bonded ligands and the presence of two lone pairs stringently directs toward such a topology (VSEPR model).[2] Recent work on oxygen-depleted perovskites has somewhat blurred this clear picture. Using soft chemistry routes, a square-planar local geometry has been realized for Fe(II), for example, in SrFeO2. [3] However, this compound is reported to be metastable, and its structure does not represent the ground-state configuration. Moreover, the coordination polyhedra are not solitary, but linked via vertices to form 2D sheets. It is thus difficult to judge whether the local geometry is intrinsically stable or rather, is supported by extended lattice effects.The family of A4IrO4 iridates(IV), synthesized previously with A = Na, K and Cs [4] by Rudolf Hoppe, and featuring the first examples of square-planar mono-oxoanions for a transition metal with an electron configuration different from d 8 or d 9 , is raising deeper concern in this context. As a particularly puzzling fact, Co 4+ in the lighter homologue, Na4CoO4, is as expected tetrahedrally coordinated in the high-spin d 5 state.[5] Therefore, it is compelling to investigate why Ir 4+ can be stabilized in the square-planar structure, rather than tetrahedral. Furthermore it would be interesting to determine the factors that induce different local structures for IrO4 and CoO4 entities in the same oxidation state, and whether they can be attributed as an effect of spin-orbit coupling (SOC), Coulomb interactions or others. Moreover, how these different structures affect the characteristic electronic and magnetic properties.To unravel the apparent conundrum, we revisit this exotic class of square-planar iridates (IV) by validating earlier structural work, followed by an in depth theoretical analysis of the chemical bonding. Our current...

No abstract

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.

Local environments and valence electron counts primarily determine the electronic states and physical properties of transition-metal complexes.F or example,square-planar coordination geometries found in transition-metal oxometalates such as cuprates are usually associated with the d 8 or d 9 electron configuration. In this work, we address an unusual square-planar single oxoanionic [IrO 4 ] 4À species,a s observed in Na 4 IrO 4 in which Ir IV has ad 5 configuration, and characterize the chemical bonding through experiments and by ab initio calculations.W ef ind that the Ir IV center in groundstate Na 4 IrO 4 has square-planar coordination geometry because of the weak Coulomb repulsion of the Ir-5d electrons. In contrast, in its 3d counterpart Na 4 CoO 4 ,t he Co IV center is tetrahedrally coordinated because of strong electron correlation. Na 4 IrO 4 may thus serve as asimple yet important example to study the ramifications of Hubbard-type Coulomb interactions on local geometries.The tetrahedral geometry is by far the most common coordination geometry encountered in isolated [MO 4 ] nÀ moieties and is electrostatically favored. Consequently, other coordination geometries,such as square planar,require special local electronic configurations and covalent bonding contributions to be stabilized. It has been quite easy to rationalize the occurrence of square-planar coordination, even in purely qualitative terms,from electron counting. The d 8 and d 9 electronic configurations for transition-metal centers are frequently found to form square-planar complexes, [1] whereas for main group elements,t he combination of four covalently bonded ligands and the presence of two lone pairs stringently directs toward the same topology (valence-shell electron-pair repulsion model). [2] Recent work on oxygen-depleted perovskites has somewhat blurred this clear picture. Using soft chemistry routes,asquare-planar local geometry has been realized for Fe II ,for example,inSrFeO 2 . [3] However, this compound is reported to be metastable,and its structure does not represent the ground-state configuration. Moreover, the coordination polyhedra are not solitary,b ut linked via vertices to form 2D sheets.Itisthus difficult to judge whether the local geometry is intrinsically stable or rather is supported by extended lattice effects.Thef amily of A 4 Ir IV O 4 iridates,s ynthesized previously with A = Na, K, and Cs [4] by Hoppe and co-workers, [4] featured the first examples of square-planar single oxoanions for at ransition-metal center with an alternative electron configuration to d 8 or d 9 .Asaparticularly puzzling fact, Co IV in the lighter homologue Na 4 CoO 4 is,a se xpected, tetrahedrally coordinated in the high-spin d 5 state. [5] Therefore,i t is compelling to investigate why Ir IV can be stabilized in the square-planar rather than tetrahedral coordination geometry. Furthermore it would be interesting to determine the factors that induce different local structures for IrO 4 and CoO 4 species in the same oxidation state and to deter...