G. Kh. Rozenberg

Applications of perforated diamond anvils for very high-pressure research

To reduce the large absorption effect in diamond anvil pressure cells for soft x rays, perforated anvils employed as diamond-backing plates (DBPs) used in conjunction with miniature anvils (MAs) made of 1/100 carat diamonds were tested for high-pressure efficacy. Static pressures beyond 100 GPa were generated using a piston/cylinder cell having 0.2 mm culets. Tests were carried out in 0.1 mm cavities drilled in a Re gasket, using Ar samples and ruby chips for manometry. Except for a single failure of a 0.3 mm culet MA, no damage was detected in the DBPs drilled with truncated conical holes tapering from 0.3 mm to 1, 1.5, and 2 mm diameter. Another arrangement in which one anvil was partially drilled leaving a 0.5 mm thick wall behind the culet achieved 100 GPa. Detailed discussions are given concerning the benefits of the DBP/MA cells for high-pressure studies with soft x rays and for background reduction in Raman, IR, UV, and 57Fe Mossbauer spectroscopies.

The structure of the metallic high-pressure Fe3O4 polymorph: experimental and theoretical study

The structure of the metallic high-pressure Fe3O4 polymorph : experimental and theoretical study

A multipurpose miniature piston-cylinder diamond-anvil cell for pressures beyond 100 GPa

A miniature piston-cylinder diamond-anvil cell (DAC) was constructed and tested for pressure operation at and beyond 100 GPa. Its advantages compared to other piston-cylinder DACs are its compactness (22-mm diam by 21-mm high), self-contained force generator, and simple way of operation. Tungsten carbide backing plates are used for supporting the anvils; one with a hemispherical shape allowing for parallelism alignment, and one with a flat circular shape allowing for lateral alignment of the anvils’ culets. The force is generated by six M3 Allen screws and is conveyed to the piston via force rings. Pressures to 130 GPa were achieved with beveled culets having 300-μm flats and Re gaskets. Design features, mode of operation, and performance are described. The latter has been demonstrated for the particular case of Mossbauer spectroscopy in La57FeO3.

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.

High-pressure phase transition detection in diamond anvil cell using the method of ellipsometry

It is suggested to use the ellipsometric technique to measure the electronic properties of static highly compressed matter. The information derived from this technique is important for the theoretical and phenomenological research for the equation of state. This diagnostic was found to be useful and sensitive for measuring pressure induced phase transitions. In particular, high-pressure ellipsometry with a diamond anvil cell was used to detect the α⇄e phase transition in iron. The polarization of the light reflected from an iron foil was analyzed in the pressure domain of 0–24 GPa and the α⇄e transition in compression and decompression was detected. These results are in good agreement with Mossbauer spectroscopy, x-ray diffraction, and resistivity measurements.

Pressure-Induced Metallization of the Perovskite Sr3Fe2O7

Electrical, magnetic and structural properties of the antiferromagnetic semiconductor Sr3Fe2O7 (Fe4+, d4) were probed by resistance, Mossbauer spectroscopy (MS) and X-ray diffraction (XRD) measurements up to P ≈ 40 GPa using diamond-anvil cells. A sluggish pressure-induced insulator–metal (IM) transition is observed with a clear incipient metallic state at P ≥ 20 GPa. The Fe(IV) 3d magnetic moments remain unaltered across the transition as deduced from MS, and XRD studies show no structural symmetry change up to 40 GPa. The results are consistent with carrier delocalization due to p–p gap closure, e.g., ligand-to-ligand charge transfer that does not involve the d-states and structural symmetry changes. A mechanism to account for the IM transition in the strontium ferrates is discussed.

HIGH-PRESSURE METALLIZATION AND AMORPHIZATION OF THE MOLECULAR CRYSTAL SN(IBR)2

An insulator-to-metal transition concurring with amorphization is found in the cubic (Pa{bar 3}) molecular crystal Sn(IBr){sub 2} at P {approx} 20 GPa. Measurements were carried out with diamond-anvil cells at pressures up to {approximately}30 GPa using resistance measurements, X-ray diffraction (XRD), and {sup 119}Sn Moessbauer spectroscopy (MS). With increasing pressure a new crystalline phase is observed in the 10--23 GPa range; at P {approx} 16 GPa a gradual onset of structural disorder is first observed, and full amorphization takes place at P {ge} 21 GPa. Both electronic properties as measured by R(P,T) and MS data are consistent with a gradual growth of disordered (SnI{sub 2}Br{sub 2}){sub n} polymeric chains, formed by intermolecular I{single_bond}I bonding allowing for electronic delocalization to occur. Upon decompression both XRD and {sup 119}Sn MS show a significant pressure hysteresis.

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.

Crystallographic transitions related to magnetic and electronic phenomena in TM compounds under high pressure

Structural aspects of magnetic/electronic transitions in strongly correlated systems in a regime of very high static density are the main issues of this article. To achieve this objective, we have carried out series of X-ray diffraction (XRD) measurements up to 120 GPa using diamond anvil cells (DACs) to probe structural features specifically related to pressure-induced (PI) magnetic/electronic phenomena in transition metal (TM) compounds. The types of phenomena are the Mott transition (MT), high-spin (HS) to low-spin (LS) transition, valence transformations, the quenching of the orbital term, and Verwey transition. In all these cases, the electronic transition may induce or be a consequence of structural alterations. These studies provide essential information concerning: (i) structural alterations attributed to the valence transformation in some TM compounds; (ii) mechanisms and precursors responsible for the PI MT; features of the structural transformation specifically attributed to the MT for different types of the electronic transitions (Mott–Hubbard and Charge-Transfer); (iii) the mechanism of the PI degradation of the magnetic state due to HS–LS transition and their influence on the structural properties of material; (iv) mechanism of Verwey transition in magnetite; (v) structure of new phases at high pressure. As example cases we present recent results in some Fe-oxides, halides, and sulfides.

Mott transition and magnetic collapse in iron-bearing compounds under high pressure

ABSTRACT We discuss the electronic, magnetic, and related structural transitions in the iron-based Mott insulators under high pressures relevant to the Earths lower mantle conditions. The paper focuses on the above-mentioned topics based primarily on our theoretical analysis and various experimental studies employing synchrotron X-ray diffraction,57Fe Mössbauer spectroscopy, and electrical transport measurements. We review the main theoretical tools employed for the analysis of the properties of materials with strongly interacting electrons and discuss the problems of theoretical description of such systems. In particular, we discuss a state-of-the-art method for calculating the electronic structure of strongly correlated materials, the method, which merges standard band-structure techniques (DFT) with dynamical mean-field theory of correlated electrons (DMFT). We employ this method to study the pressure-induced magnetic collapse in Mott insulators, such as wüstite (FeO), magnesiowüstite (FeMg)O (x=0.25 and 0.75) and goethite (FeOOH), and explore the consequences of the magnetic collapse for the electronic structure and phase stability of these materials. We show that the paramagnetic cubic -structured FeO and (Fe,Mg)O and distorted orthorhombic (Pnma) FeOOH exhibit upon compression a high- to low-spin (HS-LS) transition, which is accompanied by a simultaneous collapse of local moments. However, the HS-LS transition is found to have different consequences for the electronic properties of these compounds. For FeO and (FeMg)O, the transition is found to be accompanied by a Mott insulator-to-metal phase transition. In contrast to that, both (FeMg)O and FeOOH remain insulating up to the highest studied pressures, indicating that a Mott insulator to band insulator phase transition takes place. Our combined theoretical and experimental studies indicate a crossover between localized to itinerant moment behavior to accompany magnetic collapse of Fe ions.