W. M. Xu

On the compressibility of ferrite spinels: a high-pressure X-ray diffraction study of MFe2O4 (M=Mg, Co, Zn)

High-pressure synchrotron X-ray diffraction studies were carried out on a series of ferrite spinels MFe2O4 (M=Mg, Co, Zn) up to ∼ 70 GPa using diamond anvil cells. He and Ne, used as pressure media, provided quasi-hydrostatic conditions, resulting in a high-quality fit to both second- and third-order equations of state (EOSs). A quality fit to the second-order EOS allows a comparison between the compressibility of these spinels and that of other spinels found in the literature. Fitting to the second-order Birch–Murnaghan EOS results in the values of K 0=170.5(8), 176.1(6) and 174(2) GPa for M=Mg, Co and Zn, respectively. A linear dependence of K 0 is obtained as a function of the normalized average cationic-sphere volume S N of each spinel with a slope of 490(15) GPa/Å3. A number of previous studies of the same and similar spinels exhibit a strong deviation from K 0(S N), which can be attributed to a lack of hydrostatic conditions.

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.

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.

Crystallographic transitions related to high-pressure magnetic and electronic phenomena in Fe-based compounds

The main purpose of this paper is to address the issue of crystallographic transitions related to magnetic/electronic phenomena in strongly correlated transition metal (TM) compounds in a regime of very high static density. The experimental tools used were: synchrotron X-ray diffraction, Mössbauer spectroscopy and electrical resistivity. We focus on the following cases: (i) high-spin to low-spin transition which could lead to a significant reduction of the TM ionic radii and therefore even a structural transition; (ii) sluggish structural phase transitions in antiferromagnetic insulators FeI2 and FeCl2 attributed to the onset of a Mott transition; (iii) volume dependence of the orbital term of the moment in FeI2 and FeCl2 resulting in its eventual collapse, which is accompanied by a significant lattice distortion; and (iv) pressure-induced metal–metal intervalence charge transfer in the antiferromagnetic Cu1+Fe3+O2 as a result of the increase in overlap of atomic orbitals.

Magnetism in FeCl2 at High Pressures

Anhydrous FeCl2 has been studied by 57Fe Mossbauer effect up to 61 GPa and at 5 42 GPa the onset of a diamagnetic phase is observed, signifying a total collapse of magnetism at P > 61 GPa.

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.

Structural features related to pressure-induced electronic/magnetic phenomena in TM compounds

Minerals, metals, inorganic materials, ices, gas hydrates and clathrates are ’’traditional’’ systems for high-pressure studies. On the other hand, high-pressure studies of complex biological systems, in particular, of proteins, are becomingmore andmore common. The effects of high pressure on organic molecular crystals were described in the early classical works by P. Bridgman and L. Vereshchagin, but these systems remained very little studied until the systematic work initiated by A. Katrusiak in the late 1970s, who has combined precise X-ray single-crystal diffraction studies, spectroscopy and calorimetry. In the last years, however, more and more research groups join the highpressure studies of organicmolecular crystals, and in this presentation I shall try to illustrate on several examples, mainly from my own experience, why these studies are so challenging and promising. In particular, I shall consider: the studies of the anisotropy of structural strain within the range of stability of the same polymorph as a tool to study intermolecular interactions, thermodynamic and kinetic aspects of pressure-induced phase transitions, comparison of the low-temperature and highpressure behaviour, high-pressure studies of the polymorphism of drugs in relation to tabletting procedure in industry, organic molecular crystals as biomimetics high-pressure behaviour in relation to folding. Acknowledgments: Financial support has been obtained from Alexander von Humboldt Foundation, RFBR (05-03-32468), BRHE Program, and a grant from SB RAS.

Electronic, Structural, and Magnetic Properties of Transition-Metal Insulators at Very High Pressures

By simultaneously combining the methods of X-ray diffraction for structural phase transitions and EOS measurements, 57Fe Mossbauer spectroscopy (MS) as a sitesensitive probe, and resistivity measurements for studying insulating-metal transitions we are able to study the effect of extreme pressures and at varying temperature on magnetic and electronic properties of Transition Metal Compounds (TMC). Studies are carried out with specially tailored diamond anvils and diamond anvil cells, reaching pressures beyond 100 GPa. Our studies allow investigating the most basic phenomenon of quantum effect of magnetism in insulating antiferromagnets, the Mott insulators: the high to low spin crossovers, quenching of magnetic moments’ orbital term, and the collapse of the MottHubbard state. Examples of these phenomena will be given in cases of ferrous and ferric oxides, ferrous-halides and the rare-earth iron perovskites.

The effect of pressure-induced collapse of correlation and Hund’s rules on structure and electronic properties of transition-metal compounds

Mossbauer spectroscopy in 57Fe, resistance, and synchrotron-XRD methods were combined with high-pressure diamond anvil cells for detailed studies of the pressure-induced breakdown of Hunds rules and the strong electrons correlation in transition-metal compounds. It is shown that Hunds rules are not applicable in a regime of high density induced by external pressure. Following a short introduction on Mott insulators the effect of correlation breakdown, at very high-pressures, results in magnetism collapse concurring with insulatormetal transition. The outcome of these two pressure-induced phenomena in matter and particularly in geological species will be discussed. Examples are given for the some cases of ferric oxides and ferrous halides and oxides. Details concerning the methods used are presented.

Pressure-Induced Spin-Crossover in EuFeO3

EuFeO3 perovskite has been studied by using 57Fe Mössbauer effect and X-ray diffraction under pressures up to 90 GPa and in the temperature range of 4.2–300 K. A high-pressure-induced first-order phase transition is observed in the pressure range of 45–52 GPa as manifested by a discontinuous volume and IS reduction and dramatic changes of the hyperfine interaction parameters. The high-pressure phase of Fe3+ is established as a low-spin state (S=1/2, 2T2g) characterized by spin-spin magnetic relaxation spectra to the lowest temperature. The nature and mechanism of the magnetic relaxation are discussed.