LaMnO(3) was studied by synchrotron x-ray diffraction, optical spectroscopies, and transport measurements under pressures up to 40 GPa. The cooperative Jahn-Teller (JT) distortion is continuously reduced with increasing pressure. There is strong indication that the JT effect and the concomitant orbital order are completely suppressed above 18 GPa. The system, however, retains its insulating state to approximately 32 GPa, where it undergoes a bandwidth-driven insulator-metal transition. Delocalization of electron states, which suppresses the JT effect but is insufficient to make the system metallic, appears to be a key feature of LaMnO(3) at 20-30 GPa.

Electronic and structural properties of the high-pressure phase of Fe 2 O 3 were determined by combining the methods of Mössbauer spectroscopy, x-ray diffraction, and electrical resistance, R͑P, T͒, to 80 GPa. Because of a first-order phase transition taking place in the 50 -75 GPa range and accompanied by a volume decrease of ϳ10%, a breakdown of the electronic d-d correlation occurred, leading to a Mott transition, a metallic and a nonmagnetic single Fe 31 electronic state. The highpressure structure is of the distorted Rh 2 O 3 -II type. The accommodation of the denser phase within this six-coordinated structure is attributable to the metallic state. [S0031-9007(99)09286-8] PACS numbers: 62.50. + p, 71.30. + h, 76.80. + y More than half of the binary transition metal (TM) compounds and the majority of the undoped TM oxides are magnetic insulators that order antiferromagnetically below a certain Néel temperature (T N ). The localization of carriers in those so-called Mott insulators results from strong on-site Coulomb repulsion within the 3d band. Its insulating nature [1] is commensurate to an effective energy-gap U, far exceeding the d-band width W, splitting the d band into upper (empty) and lower (filled) subbands. In view of the "Coulomb" nature of the gap, it is expected that in ionic insulators, such as the binary TM oxides, very high pressures will be needed to reach the W͞U ¿ 1 regime. In this regime the d-electron correlation collapses, giving rise to an insulating-metal transition concurring with a magnetic moment breakdown, a phenomenon often called the Mott transition. Indeed, as shown by Cohen et al. [2] for the case of binary TM oxides, the Mott transition could be attained only at pressures close to 200 GPa. Pasternak et al. experimentally observed a pressure-induced Mott transition in the highly covalent NiI 2 , [3] using the combined methods of 129 I Mössbauer spectroscopy (MS), x-ray diffraction (XRD), and electrical conductivity in diamond anvil cells (DAC's). The isostructural transition was second order and quasi-isochoric. The question then arises whether in the case of a first-order phase transition resulting in a reduction in volume, such a TM-oxide Mott insulator might undergo a Mott transition.In this Letter we report the observation of such a transition attributable to a huge volume reduction, the first-order phase transition in a-Fe 2 O 3 (hematite) at 50 GPa. As will be shown, this work is important not only to the elucidation of such a basic phenomenon as the Mott transition, but also to ultimately untangling the baffling nature of the high-pressure phase of hematite.Fe 2 O 3 , a wide-gap antiferromagnetic insulator (T N 956 K [4]) can be regarded as archetypal of a Mott insulator. Motivated primarily by its importance in the earth sciences, high-pressure studies of this corundumtype mineral have been extensively performed since the early days of high-pressure physics. As early as in the mid-1960s shock-wave experiments were performed by McQueen and Marsh [5] who reached energy...

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Using electrically- and laser-heated diamond anvil cells at pressures above 40 GPa we synthesized the pure high-pressure Fe3O4 phase (h-Fe3O4)and performed an in situ structural refinement. In good agreement with our ab initio calculations we found that h-Fe3O4 adopts the CaTi2O4-type structure (space group Bbmm). Electrical resistivity measurements show that h-Fe3O4 is metallic up to at least 70 GPa.

Comprehensive x-ray powder diffraction studies were carried out in magnetite in the 80-150 K and 0-12 GPa ranges with a membrane-driven diamond anvil cell and helium as a pressure medium. Careful data analyses have shown that a reversible, cubic to a distorted-cubic, structural transition takes place with increasing pressure, within the (P,T) regime below the Verwey temperature TV(P). The experimental documentation that TV(P)=Tdist(P) implies that the pressure-temperature-driven metal-insulator Verwey transition is caused by a gap opening in the electronic band structure due to the crystal-structural transformation to a lower-symmetry phase. The distorted-cubic insulating phase comprises a relatively small pressure-temperature range of the stability field of the cubic metallic phase that extends to 25 GPa.

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The electronic ground state in many iridate materials is described by a complex wave-function in which spin and orbital angular momenta are entangled due to relativistic spin-orbit coupling (SOC) [1,2]. Such a localized electronic state carries an effective total angular momentum of J ef f = 1/2 [3,4]. In materials with an edge-sharing octahedral crystal structure, such as the honeycomb iridates Li2IrO3 and Na2IrO3, these J ef f = 1/2 moments are expected to be coupled through a special bonddependent magnetic interaction [5][6][7], which is a necessary condition for the realization of a Kitaev quantum spin liquid [8]. However, this relativistic electron picture is challenged by an alternate description, in which itinerant electrons are confined to a benzene-like hexagon, keeping the system insulating despite the delocalized nature of the electrons [9,10]. In this quasi-molecular orbital (QMO) picture, the honeycomb iridates are an unlikely choice for a Kitaev spin liquid. Here we show that the honeycomb iridate Li2IrO3 is best described by a J ef f = 1/2 state at ambient pressure, but crosses over into a QMO state under the application of small (∼0.1 GPa) hydrostatic pressure. This result illustrates that the physics of iridates is extremely rich due to a delicate balance between electronic bandwidth, spin-orbit coupling, crystal field, and electron correlation. arXiv:1803.04056v1 [cond-mat.str-el]