Breakdown of the Mott-Hubbard State inFe2O3: A First-Order Insulator-Metal Transition with Collapse of Magnetism at 50 GPa

Abstract

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...

Single-crystal X-ray diffraction at megabar pressures and temperatures of thousands of degrees

“…At ambient pressure trigonal hematite α-Fe 2 O 3 (space group R3c) is a wide-gap antiferromagnetic insulator and can be considered as an archetype of a Mott-insulator [24]. Due to its significance in the solid state and mineral physics, the high pressure behavior of Fe 2 O 3 has been intensively investigated (see [25][26][27] and references therein).…”

“…Due to its significance in the solid state and mineral physics, the high pressure behavior of Fe 2 O 3 has been intensively investigated (see [25][26][27] and references therein). On compression at ambient temperature above 50 GPa, it undergoes a sluggish structural phase transition [24][25][26] into a phase with an orthorhombic symmetry. The same phase has been reported [25] to appear upon heating above about 30 GPa.…”

“…Two structural models have been proposed [24][25][26] -that of the high pressure Rh 2 O 3 -II-type structure (space group Pbna, #60) and the orthorhombic perovskite-type structure (space group Pbnm, #62). While the Mössbauer spectroscopic data seem to support the Rh 2 O 3 -II-type structure, the powder diffraction data collected by different groups at different synchrotron facilities over the last several decades were not able to unambiguously assign the structural type (see references in [24][25][26]). We compressed a single crystal of hematite Fe 2 O 3 in a DAC loaded with Ne as the pressure-transmitting medium to 25.6(5) GPa and collected the X-ray diffraction data in 40 • ω-scans with a 0.5 • step.…”

Pressure-induced Jahn-Teller suppression and simultaneous high-spin to low-spin transition in the layered perovskiteCsMnF4

“…[2][3][4] Similar to Earth's interior conditions, high-pressure techniques have emerged as efficient tools to attain structural conditions for spin change, even dealing with systems where ⌬Ӷ⌬ SCO . Although we know the relevance of the spin state of Fe 2+ in ͑Mg,Fe͒O, the second most abundant phase in lower mantle, on the radiative conductivity 5,6 or that highspin Fe 3+ ͑S =5/2͒ transits to the low-spin state ͑S =1/2͒ in Fe 2 O 3 at pressures around 50 GPa, 7,8 the problem of predicting spin-crossover pressures in materials science is still a challenge. A comprehensive characterization of high-spin to low-spin ͑or intermediate spin͒ 1 phenomena requires knowledge of the electronic structure of materials, a major problem because optical absorption measurements in extreme conditions are tricky.…”

“…Spin-transition phenomena have been intensively investigated in materials providing ligand fields close to the spin crossover. [1][2][3][4][5][6][7][8][9] Current research of this kind is focused on materials involving transition-metal complexes with C, O, or N ligands. Their strong neuphelaxetic effect makes those complexes well suited to exhibiting bistability around ambient conditions.…”