Gregory Kh. Rozenberg

Mott transition in CaFe 2 O 4 at around 50 GPa

ossbauer spectroscopy (MS), Raman spectroscopy, and electrical resistance measurements. These studies have shown the onset of the Mott transition (MT) at a pressure of around 50 GPa, leading to the collapse of Fe 3+ magnetic moments and to the insulator-metal (IM) transition. The observed onset of the MT corroborates with the recently reported isostructural transition accompanied by a 12% decrease in the Fe polyhedral volume. An analysis of the alterations of the electrical transport, magnetic, and structural properties with pressure increase and at the transition range suggests that the coinciding IM transition, magnetic moment, and volume collapse at around 50 GPa are caused by the closure of the Hubbard gap driven by the high-spin to low-spin (HS-LS) transition. At that, since MS did not reveal any evidence of a preceding LS state, it could be inferred that the HS-LS transition immediately leads to an IM transition and complete collapse of magnetism.

Pressure-induced structural phase transition of the iron end-member of ringwoodite (γ-Fe2SiO4) investigated by X-ray diffraction and Mössbauer spectroscopy

Abstract We have carried out X-ray diffraction and Mössbauer spectroscopy measurements on the spinel phase g-Fe2SiO4 (ringwoodite) at ambient temperature and pressures up to 66 GPa using diamond anvil cells. At pressures above 30 GPa, a previously unknown structural phase transition to a rhombohedrally distorted spinel phase has been observed (space group R3̄mR). Mössbauer spectroscopy measurements reveal two different Fe2+ sites at high pressure with an abundance ratio of 3:1, in agreement with the two crystallographic sites occupied by the iron in this distorted spinel structure. The unit-cell volume of the low-pressure spinel phase as a function of pressure results in a bulk modulus of K0 = 197(3) GPa using the second-order Birch-Murnaghan equation of state, and K0 = 201(8) GPa and K′ = 3.7(7) when using a third-order equation of state. The pressure evolution of the unit-cell volume and the Mössbauer hyperfine parameters are in good agreement with previous studies, which were limited to a lower pressure range.

The Mott insulators at extreme conditions; structural consequences of pressure-induced electronic transitions

Abstract Electronic/magnetic transitions and their structural consequences in Fe-based Mott insulators in a regime of very high static density are the main issue of this short review paper. The paper focuses on the above-mentioned topics based primarily on our previous and ongoing experimental HP studies employing: (i) diamond anvil cells, (ii) synchrotron X-ray diffraction, (iii) 57Fe Mössbauer spectroscopy, (iv) electrical resistance and (v) X-ray absorption spectroscopy. It is shown that applying pressure to such strongly correlated systems leads to a number of changes; including quenching of the orbital moment, quenching of Jahn-Teller distortion, spin crossover, inter-valence charge transfer, insulator–metal transition, moment collapse and volume collapse. These changes may occur simultaneously or sequentially over a range of pressures. Any of these may be accompanied by or be a consequence of a structural phase transition; namely, a change in crystal symmetry. Analyzing this rich variety of phenomena we show the main scenarios which such strongly correlated systems may undergo on the way to a correlation breakdown (Mott transition). To illustrate these scenarios we present recent results for MFeO3 (M = Fe, Ga, Lu, Eu, Pr) and CaFe2O4 ferric oxides; FeCl2 and FeI2 ferrous halides, and FeCr2S4 sulfide. Fe3O4 is given as an example case for the impact of Mössbauer Spectroscopy on High Pressure Crystallography studies.

Pressure-driven collapse of the relativistic electronic ground state in a honeycomb iridate

Honeycomb-lattice quantum magnets with strong spin-orbit coupling are promising candidates for realizing a Kitaev quantum spin liquid. Although iridate materials such as Li2IrO3 and Na2IrO3 have been extensively investigated in this context, there is still considerable debate as to whether a localized relativistic wavefunction (Jeff = 1/2) provides a suitable description for the electronic ground state of these materials. To address this question, we have studied the evolution of the structural and electronic properties of α-Li2IrO3 as a function of applied hydrostatic pressure using a combination of x-ray diffraction and x-ray spectroscopy techniques. We observe striking changes even under the application of only small hydrostatic pressure (P ≤ 0.1 GPa): a distortion of the Ir honeycomb lattice (via X-ray diffraction), a dramatic decrease in the strength of spin-orbit coupling effects (via X-ray absorption spectroscopy), and a significant increase in non-cubic crystal electric field splitting (via resonant inelastic X-ray scattering). Our data indicate that α-Li2IrO3 is best described by a Jeff = 1/2 state at ambient pressure, but demonstrate that this state is extremely fragile and collapses under the influence of applied pressure.Kitaev spin liquids: Understanding the ground stateA Kitaev quantum spin liquid is an exotic state of matter in which spins do not order even at very low temperature — it can be realized in materials with a honeycomb lattice and strong spin-orbit coupling, such as Li2IrO3. Two different descriptions have been put forward to describe the electronic ground state of this material, one involving localized electrons, the other itinerant electrons. Only the localized picture is compatible with the realization of a Kitaev spin liquid. To discriminate between these scenarios, Young-June Kim at the University of Toronto, Canada and colleagues studied the effect of applying hydrostatic pressure combining different X-ray techniques. They found that a localized electronic state is observed at room pressure, but it is very fragile and extremely small pressures are sufficient to disrupt it.

Suppression of Jahn-Teller distortion and insulator-to-metal transition in LaMnO3 at high pressures

Structural, vibrational and electronic properties of LaMnO 3 under pressures up to 38 GPa have been studied by synchrotron X-ray powder diffraction, Raman spectroscopy, optical reflectivity, and transport measurements. The cooperative Jahn-Teller distortion of the MnO 6 octahedra of the perovskite-type structure is continuously suppressed with increasing pressure, a process which appears completed at ∼20 GPa. The system remains insulating to 32 GPa, where an insulator-metal transition is observed. This transition is attributed to strengthened Mn--O--Mn interactions due to the increasing overlap of atomic orbitals.

High-pressure structural and electronic transitions in lithium ferrites

Electronic, magnetic and structural transitions in strongly correlated transition-metal compounds have been among the main topics of condensed-matter research over recent decades, being especially relevant to understanding high-temperature superconductivity as well as heavy-fermion behavior. The definitive electronic phenomenon in such compounds is the breakdown of delectron localization, causing a Mott (Mott-Hubbard) insulator-to-metal transition typically accompanied by a collapse of magnetic moments [1]. Such a transition does not necessarily imply a rearrangement of atoms, but in fact often exhibits an appreciable collapse in volume or even symmetry change [2]. The classic Mott transition observed in many systems involves a simultaneous insulator–metal transition, magnetic moment collapse and volume collapse. Here, we have report structural, magnetic and electronic properties of the disordered α-LiFeO2 and ordered LiF5O8 compounds, which crystallize in the cubic (Fd3m and P4332 space group, respectively) structure, and ordered TLiFeO2 (space group I41/amd), at pressures up to about 1 Mbar. The work is based on our experimental high-pressure studies employing: (i) diamond anvil cells, (ii) synchrotron powder and single crystal x-ray diffraction, (iii) 57Fe Mössbauer spectroscopy, (iv) electrical resistance, and (v) Raman spectroscopy. For the disordered LiFeO2 system, the crystal structure is stable at least up to 82 GPa, though a significant change in compressibility has been observed above 50 GPa. The changes in the structural properties are found to be on a par with a sluggish Fe3+ highto low-spin (HS-LS) transition (S=5/2 → S=1/2) starting at 50 GPa and not completed even at ~100 GPa. The HS-LS transition is accompanied by an appreciable resistance decrease; however, the material remains a semiconductor up to 115 GPa and is not expected to be metallic even at about 200 GPa [3]. These features of the structural and electronic transition in α-LiFeO2 strongly contradict with the case of ordered TLiFeO2, which undergoes a first-order isostructural transition above 50 GPa. For the ordered spinel LiF5O8, an irreversible structural phase transition from the cubic phase to the orthorhombic (space group Cmcm) post-spinel structure has been observed above 40 GPa accompanied by about 4% volume reduction. Another noticeable change in the V(P) data, namely: a steeper decrease of unit-cell volume with pressure increase occurs above 60 GPa corroborating with a significant change of the electronic and magnetic properties resulting in the gradual formation of the nonmagnetic metallic high pressure state on the Fe3+ octahedral sites [4]. With this, 40% of Fe3+ occupying bicapped trigonal prism sites remain in the HS state. Thus, our studies demonstrate that in a material with a complex crystal structure, containing transition metal cation(s) in different environments, delocalization/metallization of the 3d electrons does not necessarily occur simultaneously and may propagate through different crystallographic sites at different degrees of compression. The effect of Fe3+ nearest and next nearest neighbors on the features of the electronic transition is discussed.