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Functional oxides whose physicochemical properties may be reversibly changed at standard conditions are potential candidates for the use in next‐generation nanoelectronic devices. To date, vanadium dioxide (VO2) is the only known simple transition‐metal oxide that demonstrates a near‐room‐temperature metal–insulator transition that may be used in such appliances. In this work, we synthesized and investigated the crystals of a novel mixed‐valent iron oxide with an unconventional Fe5O6 stoichiometry. Near 275 K, Fe5O6 undergoes a Verwey‐type charge‐ordering transition that is concurrent with a dimerization in the iron chains and a following formation of new Fe−Fe chemical bonds. This unique feature highlights Fe5O6 as a promising candidate for the use in innovative applications. We established that the minimal Fe−Fe distance in the octahedral chains is a key parameter that determines the type and temperature of charge ordering. This model provides new insights into charge‐ordering phenomena in transition‐metal oxides in general. [ABSTRACT FROM AUTHOR]

Functional oxides whose physicochemical properties may be reversibly changed at standard conditions are potential candidates for the use in next‐generation nanoelectronic devices. To date, vanadium dioxide (VO2) is the only known simple transition‐metal oxide that demonstrates a near‐room‐temperature metal–insulator transition that may be used in such appliances. In this work, we synthesized and investigated the crystals of a novel mixed‐valent iron oxide with an unconventional Fe5O6 stoichiometry. Near 275 K, Fe5O6 undergoes a Verwey‐type charge‐ordering transition that is concurrent with a dimerization in the iron chains and a following formation of new Fe−Fe chemical bonds. This unique feature highlights Fe5O6 as a promising candidate for the use in innovative applications. We established that the minimal Fe−Fe distance in the octahedral chains is a key parameter that determines the type and temperature of charge ordering. This model provides new insights into charge‐ordering phenomena in transition‐metal oxides in general. [ABSTRACT FROM AUTHOR]

The discovery of superconductivity in hafnium pentatelluride HfTe5 under high pressure is reported. Two structural phase transitions and metallization with superconductivity developing at around 5 GPa are observed. A maximal critical temperature of 4.8 K is attained at a pressure of 20 GPa, and superconductivity persists up to the maximum pressure of the study (42 GPa). The combination of electrical transport and crystal structure measurements as well as theoretical electronic structure calculations enables the construction of a phase diagram of HfTe5 under high pressure. [ABSTRACT FROM AUTHOR]

• Structural phase transitions in the FeSe0.89S0.11 superconductor at high pressures up to 18.5 GPa. • Most sample part's recrystallization into the hexagonal phase (sp. gr. P63/mmc) at decompression. • Strong hysteresis of the structural properties during a phase transition under pressure. We report on the structural phase transitions in the FeSe 0.89 S 0.11 superconductor with T C = 11 K observed by powder synchrotron X-ray diffraction at high pressures up to 18.5 GPa under compression and decompression modes. It was found that at ambient pressure and room temperature, FeSe 0.89 S 0.11 has a tetragonal structure (space group P4/n). Under compression, in the region of 10 GPa, a phase transition from the tetragonal into the orthorhombic structure (sp. gr. Pnma) is observed, which persists up to 18.5 GPa. Our results strongly suggest that, at decompression, as the applied pressure decreases to 6 GPa and then is completely removed, most of the sample recrystallizes into the hexagonal phase of the structural type NiAs (sp. gr. P6 3 /mmc). However, the other part of the sample remains in the high pressure orthorhombic phase (sp. gr. Pnma), while the tetragonal phase (sp. gr. P4/n) is not restored. These observations illustrate a strong hysteresis of the structural properties of FeSe 0.89 S 0.11 during a phase transition under pressure. [ABSTRACT FROM AUTHOR]

SiO 2 exhibits a high-pressure-high-temperature polymorphism, leading to an increase in silicon coordination number and density. However, for the related compound SiS 2 such pressure-induced behavior has not been observed with tetrahedral coordination yet. All four crystal structures of SiS 2 known so far contain silicon with tetrahedral coordination. In the orthorhombic, ambient-pressure phase these tetrahedra share edges and achieve only low space filling and density. Up to 4 GPa and 1473 K, three phases can be quenched as metastable phases from high-pressure high-temperature to ambient conditions. Space occupancy and density are increased first by edge and corner sharing and then by corner sharing alone. The structural situation of SiS 2 up to the current study resembles that of SiO 2 in 1960: Then, in its polymorphs only Si-O 4 tetrahedra were known. But in 1961, a polymorph with rutile structure was discovered: octahedral Si-O 6 coordination was established. Now, 50 years later, we report here on the transition from 4-fold to 6-fold coordination in SiS 2 , the sulfur analogue of silica.