Cs4O6 is a mixed-valence molecular oxide with a cubic structure, comprising valency-delocalized O2 4/3– units and with properties highly sensitive to cooling protocols. Here we use neutron powder diffraction to authenticate that, while upon deep quenching the cubic phase is kinetically arrested down to cryogenic temperatures, ultraslow cooling results in an incomplete structural transition to a contracted tetragonal phase. Two dioxygen anions in a 1:2 ratio are identified, providing evidence that the transition is accompanied by charge and orbital order and stabilizes a Robin–Day Class II mixed-valence state, comprising O2 2– and O2 – anions. The phenomenology of the phase change is consistent with that of a martensitic transition. The response of the low-temperature phase assemblage to heating is complex, involving a series of successive interconversions between the coexisting phases. Notably, a broad interconversion plateau is present near 260 K, signifying reentrant kinetic arrest of the tetragonal phase upon heating because of the combined effects of increased steric hindrance for molecular rotation and melting of charge and orbital order. The geometrically frustrated pyrochlore lattice adopted by the paramagnetic S = 1/2 O2 – units provides an intimate link between the crystal and magnetic properties of charge-ordered Cs4O6, naturally accounting for the absence of magnetic order.

Micro-structural changes in zinc oxide (ZnO) nanoparticles induced by the substitution of Zn 2+ in ZnO by a rare earth (RE) metal ion, Sm 3+ , are investigated. Both pristine and Sm-doped ZnO with a nominal doping concentration of 1, 2 and 4% of Sm using a simple wet-chemical synthetic route followed by calcination at a high temperature of 900 • C, are synthesized. Structural investigations are primarily conducted using X-ray powder diffraction (XRPD) and scanning electron microscopy techniques. Evolution of structural parameters (unit cell parameters, average crystallite size, crystallinity percentage, lattice strain, stress, energy density and atomic packing factor) upon Sm doping is investigated together with Rietveld refinement and Le Bail analysis techniques. XRPD data confirmed that the synthesized nanostructures crystallize in a wurtzite hexagonal structure, the dopant Sm is incorporated into the Zn lattice and the annealing treatment plays a crucial role in determining the structural and optical properties of RE-metal-doped nanoparticles. Values of the optical band gap energy estimated from optical absorbance measurements reveal a widening of the band gap.

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The ternary fulleride, Rb[Formula: see text]Cs[Formula: see text]C[Formula: see text], is the most expanded member of the family of face-centered cubic (fcc) structured superconducting fullerides ever accessed with superconductivity surviving at ambient pressure closest to the Mott insulator boundary. Here, we study the evolution of its structural and electronic properties with temperature. At ambient temperature, Rb[Formula: see text]Cs[Formula: see text]C[Formula: see text] lies in the Mott–Jahn–Teller (MJT) insulating part of the phase diagram. High-resolution synchrotron X-ray diffraction shows that its structure remains strictly cubic at all temperatures, but the transition to the metallic state at [Formula: see text] 50 K — evident in the evolution of the magnetic susceptibility with temperature — is accompanied by a lattice collapse, [Formula: see text]V/V0 of [Formula: see text]. Bulk superconductivity then emerges on further cooling with a T[Formula: see text] of 25.9 K. The results permit the extension of the electronic phase diagram of A3C[Formula: see text] fullerides as close as possible to the metal–insulator (M–I) crossover.