High-pressure Form Research Articles

Rare-earth compounds with mixed valencies

Abstract This paper reviews the properties of certain rare-earth compounds in which the 4f band has mixed valency, notably SmB6 and the high-pressure forms of SmS, SmSe and SmTe. The metal-insulator transitions of the last three materials under pressure are discussed. It is suggested that the low-pressure form of SmS is an excitonic insulator. In SmB6 and high-pressure SmS a very small gap separates occupied from unoccupied states, this in our view being due to hybridization of 4f and 5d bands. The electrical properties are discussed; if kT is greater than the gap energy, then the gap does not affect the metallic behaviour. Finally metallic compounds such as CeAl3 are described, in which there is no magnetic ordering at low temperatures, and it is suggested that this must always occur if the Kondo temperature is higher than the RKKY interaction. In this case, as in compounds with mixed valency, the Fermi energy will pass through the 4f band, and there is a very large enhancement of the effective mass. The relationship to the side-band model is discussed.

Structure of Cerium Metal at High Pressure

The high-pressure form of cerium metal stable above 56 kbar is found to be orthorhombic with the $\ensuremath{\alpha}$-uranium type of structure. The effective metallic radius and the observed compressibility correspond to full tetravalency for cerium. A second, metastable form of cerium metal with a monoclinic structure has also been observed above 60 kbar.

A comparative study of the motions of methane molecules and argon atoms in liquid and high-pressure gas states by neutron scattering

The microscopic motions in methane in the liquid state, T = 102 K, and in the high-pressure gas state at 1640 bar, T = 295 K, as well as in high-pressure argon at 1815 bar, T = 295 K, mixed with methane of partial pressure of 82 bar were investigated by scattering of slow neutrons of wavelengths 4.18 and 4.73 Å. The data were carefully corrected for instrumental effects and multiple scattering in sample and container walls. The corrected data were compared to a few simple models for the micro-dynamics: Langevin diffusion and argon-type motion for the centre-of-mass motion, Langevin diffusion and free rotation for the rotational motion. Translational and rotational motions were supposed to be uncoupled. The centre-of-mass motion described as the argon-type of motion was derived from existing experimental data on liquid argon by use of the law of corresponding states. It is found that the molecular motions in liquid methane are well described by the argon type of centre-of-mass motion combined with the Langevin type of rotational diffusion. In contrast, the Langevin-diffusion model for both translational and rotational motion fits the data reasonably well in high-pressure methane as well as in high-pressure argon. It is, however, fair to state that a model for rotational motion inbetween free rotation and Langevin rotational diffusion would give the best fit to the gas data. It is thus demonstrated that a clear distinction exists between the dynamics of the liquid and the high-pressure gas forms of argon and methane. The experiments on high-pressure argon and methane are the first ones to reveal a case in which Langevin diffusion describes the translational motions in a dense fluid to a very good approximation. A comparison of the values of the diffusion constants obtained from these neutron studies to values calculated by use of Enskog's theory as well as other data is carried through. The various values of the diffusion constants agree reasonably well. The gap between microscopic and macroscopic measurements of the diffusion constant is thereby overbridged.

Caracterisation et Etude par effet Mössbauer d'une nouvelle variete haute pression de FeOOH

A new high pressure form of FeOOH, isostructural with InOOH, has been prepared by hydrothermal synthesis under very high pressure conditions. Characterization of the new phase was performed by D.T.A., T.G.A., infrared and Mössbauer spectroscopy. The compound behaves as an antiferromagnet with a Néel temperature of approximately 570° K. The hyperfine magnetic field at 0° K is about 525 kOe. Thermal decomposition occurs below the Néel temperature. Comparison with other FeOOH phases is made.

Phase relations of TlClO 4 and TlBF 4 to high pressures

Orthorhombic TlBF 4 III transforms at 201.7°C to cubic TlBF 4 II, and at 26.6°C, 6.73 ± 0.60 kbar to a high-pressure phase TlBF 4 IV similar to Bridgman's high-pressure form of TlClO 4. The high-pressure phase diagrams of TlClO 4 and TlBF 4 were studied by means of differential thermal analysis and volumetric techniques, and found to be closely similar to those of RbClO 4 and RbBF 4.

Crystal and molecular structure of CCl4 III: A high pressure polymorph at 10 kbar

The crystal and molecular structure of a high pressure form of carbon tetrachloride (CCl4 III) was determined at approximately 10 kbar using a diamond-anvil beryllium pressure cell and a modified Buerger-type precession camera. CCl4 III crystallizes in the monoclinic system with a unit cell of the following dimensions: a=9.079± 0.012 Å, b=5.764± 0.003 Å, c=9.201± 0.004 Å, and β=104.29±0.05°. The space group is P21/c with four molecules per unit cell. Observed infrared spectra indicate that the CCl4 molecule exhibits regular tetrahedral symmetry at 10 kbar. By using an approximation to the repulsion energy in a least-squares refinement procedure considering only nonbonded Cl–Cl interactions, an approximate structure was obtained. Subsequent structure factor calculations using a grid-point sampling procedure yielded a final structure with a reliability factor of 9.56%. CCl4 III is isostructural with SnBr4, and contains closest nonbonded Cl–Cl distances of 3.49 Å, significantly less than the normal Van der Waals separation of 3.6 Å. Taking four molecules per unit cell, the calculated density is 2.190 g cm−3. A more compact phase than CCl4 III was predicted in the CCl4 system on the basis of packing efficiency and the predicted phase (CCl4 IV) was subsequently verified by visual observation in microscopic studies at pressures in the 35–40 kbar range and temperatures up to 500°C.

The electronic structure of ZnSe in two phases

A non-adjusted pseudopotential is used to calculate the band structure of ZnSe in two phases. For the zinc-blende phase the calculated band structure agrees extremely well with experiment, while the high pressure (rock salt) form is shown to have an energy gap quite close to zero. The effect of ionicity on the electronic spectrum is also studied.

A shock-induced phase change in iron-silicate garnet

Hugoniot measurements on iron-silicate garnet (Fe 0.79, Mg 0.14, Ca 0.04, Mn 0.03) 3 Al 2Si 3O 12 initial density 4.18 g/cm 3, demonstrate that above a shock pressure of205 ± 20kbar transition to a high-pressure phase(s), having an apparent zero-pressure density and bulk modulus of4.44 ± 0.06g/cm 3and3.3 ± 0.3Mbar, occurs. Crystal chemical systematics and Debye-Scherrer X-ray patterns of a recovered phase(s), which may be the shock-induced high-pressure form, suggest possible formation of a phase with a density of∼ 4.48/cm 3. Occurrence of such a polymorph of garnet in the mantle would give rise to an increase in density and seismic velocity below ∼ 600km in the earth.

The lunar interior

The compressional velocities are estimated for materials thought to be important in the lunar interior and compared with lunar seismic results. The lower lunar crust has velocities appropriate for basalts or anorthosites. Anorthosite is preferred if lunar basalts result from a small degree of partial melting. The high velocities associated with the uppermost mantle imply high densities and a change to a lighter assemblage at depths of the order of 120 km. Ca- and Al-rich minerals are important components of both the lower crust and the upper mantle. Most of the Moon may have accreted from refractory material rich in Ca, Al, U, and the rare-earth elements. The important mineral of the upper mantle is garnet; possible accessory minerals are kyanite, spinel and rutile. If the seismic results stand up, the high-velocity layer in the Moon is more likely to be a high-pressure form of anorthosite ( garnet ± kyanite ± quartz) than eclogite, pyroxenite or dunite. The thickness of the layer is of the order of 50 km. The deep interior of the Moon may also be rich in Ca and Al especially if temperatures are near solidus. Achondrites such as eucrites and howardites and some inclusions in Type III carbonaceous chondrites have more of the required characteristics of the lunar interior than carbonaceous chondrites. A density inversion in the Moon is a strong possibility.

The tetragonal high-pressure form of TaOPO 4

A high-pressure form of TaOPO 4 has been synthesized at 25 to 60 kbar and 500 to 1500° C. It was found to be isostructural with the low-temperature form of NbOPO 4. A structure refinement of this more dense, tetragonal modification of TaOPO 4 [ a = 6.425(3) A ̊ and c = 4.001(3) A ̊ ] has been carried out.

Structure and properties of the high and low pressure forms of SrIrO 3

The structure of the atmospheric pressure form of SrIrO 3 is shown to be a monoclinic distortion of the hexagonal BaTiO 3 structure ( a = 5.604 A ̊ , b = 9.618 A ̊ , c = 14.17 A ̊ , β = 93.26°). The cell dimensions have been studied to 1000°C and the coefficients of thermal expansion given. The structure transforms at 40 kbar and 1000°C to an orthorhombic perovskite ( a = 5.60 A ̊ , b = 5.58 A ̊ , c = 7.89 A ̊ ) with a 3% decrease in volume. This high pressure phase only retransforms slowly at atmospheric pressure and 1200°C and exhibits metallic conductivity and Pauli paramagnetism.

Generation of Elastic Shocks Accompanied with a Phase Change on NH4F

In order to study whether or not the elastic shocks can be radiated from the material undergoing phase chnage, an experiment was carried out by the use of NH4F for the working material. NH4F changes the crystal structure at about 3.8kb and at room temperature. An amount of volume change at this phase change is about 28 percent. When the polycrystalline NH4F specimen was supercompressed or superdecompressed, a sudden shock occurred followed by many rectangular pulses with a short pulse width of about 50 milliseconds. After these shocks propagated in the pressure medium, a shock with a large pulse width of about 2 seconds appeared. This last shock seems to have been due to the absorption of a high frequency component. The direction of initial motion for the shock at the phase change from a low pressure form to a high pressure form is opposite to that for the shock at the phase change from a high pressure form to a low pressure form. This fact shows that the shocks were caused directly by the volume change at the phase change. These elastic shocks seem to occur because of the properties of the large hysteresis in NH4F on the transition pressure of phase change.The number of shocks at the phase change was counted. By the use of the relation between the cumulative number of shocks and time, the reaction rate at the phase change of NH4F was calculated based on the theory of reaction kinetics. If the reaction rate is represented by the equation; dX/dt=K(1-X)p, where X is mole fraction of product, t is time, K is the reaction constant and p is the order of reaction, p_??_1 and K_??_10-1/sec. The equation where p=1 means a first order reaction in terms of reaction kinetics.

High pressure synthesis of ilmenite and perovskite type MnVO 3 and their magnetic properties

Ilmenite and perovskite type Mn 2+V 4+O 3 are newly synthesized at high pressures and high temperatures. The low pressure form of MnVO 3 I is supposed to have a trichlinic symmetry and to be isomorphous with the vanadate ilmenites CoVO 3 I and CuVO 3 I. Unit cell dimensions are a = 5.072 (5) Å, b = 5.550 (5) Å, c = 5.023 (4) Å, α = 90°0′ (6′), β = 118°38′ (4′), γ = 63°4′ (4′) and V = 106.3 (1) Å 3. The high pressure form of MnVO 3 II, stabilized above about 45 kbar, has the orthorhombic perovskite structure with the possible space group of D 16 2h-Pbnm or C 9 2v-Pbn2 1. Unit cell dimensions are a = 5.106 (1) Å, b = 5.265 (1) Å, c = 7.387 (1) and V = 198.6 (1) Å 3. MnVO 3 II is the first perovskite containing Mn 2+ ions in the larger cationic sites in the perovskite structure. Magnetic and electric properties of both MnVO 3 I and II are studied. Magnetic susceptibilities of both substances obey the Curie-Weiss law. Weak ferromagnetic moments are found in both substances at low temperatures. MnVO 3 I shows semiconducting behaviour, whereas MnVO 3 II relatively low electric resistivity.

Mise en evidence d'une nouvelle variete “haute pression” des oxyfluorures de terres rares TOF

A high pressure, high temperature form of the lanthanide oxide fluorides has been observed. This modification is orthorhombic with a PbCl 2-like structure, space group Pmnb. The stability of these high pressure form is discussed as a function of the temperature.

The migration of protons in solids at high pressures in the presence of an electric field

The protonic electrical conductivity at 1592 Hz has been measured for KHF 2, KH 2PO 4 and H 3BO 3 at pressures up to 60 kbar and respective temperatures up to 350°C, 280°C and 220°C. For KHF 2 and KH 2PO 4 the overall activation volumes are respectively 2·5 and −1·1 cc mole −1. Orthoboric acid is converted to two new forms under pressure, metastable on reducing the pressure to atmospheric. The structures of these have been studied by powder X-ray diffraction. Form II, produced at 41·8 kbar at 45°C is orthorhombic (a = 10·18 A ̊ , b = 8·00 A ̊ , c = 7·45 A ̊ ) ; form III, prepared at 57 kbar at 45°C is tetragonal (a = b = 9·04 A ̊ , c = 8·06 A ̊ ) . The activation volume for the high pressure form (III) is virtually zero. Orthoboric acid(I) was too little conducting to give significant results for the activation volume. It appears that protonic conductors are differentiated from typical ionic conductors by the effect of pressure, the activation volume being abnormally low for the former.

Synthesis and structure of a new high-pressure form of Rh 2O 3

A new high-pressure form of Rh 2O 3 has been prepared at 65 k bar and 1200°C. Four-probe resistivity measurements made on a single crystal show Rh 2O 3 II to be a semiconductor with a room-temperature resistivity of 130 Ω cm and an activation energy of 0.16 eV. Rh 2O 3 II is orthorhombic with a = 5.1686 ± 3, b = 5.3814 ± 4, c = 7.2426 ± 4 Å, Z = 4, D obsd = 8.28 gm/cc, D c = 8.36 gm/cc, and space group, Pbna. Although the unit cell is similar to that of an orthorhombic perovskite, the structure, refined by least squares, is similar to that of corundum in that it contains pairs of RhO 6 octahedra which share faces. However, only two edges of each octahedra are shared with other octahedra. The structure may also be described as containing layers of the corundum structure cut parallel to (101̄1), and stacked together to form the Rh 2O 3 II structure.

Wadsley defects and crystallographic shear in hexagonally close-packed structures

Different shear structures are derived from the PdF 3, rutile, and α-PbO 2 structure types, which all have the anions in an approximately hexagonally close packed arrangement. Using the principles derived the corundum structure is described as a shear compound. A new simple series M n X 3 n−1 , with BNb 2O 5 as a known member ( n = 2) is described as the high-pressure form of the series with known members Nb 3O 7F ( n = 3) and R-Nb 2O 5 ( n = 2). Nonstoichiometry can be explained by Wadsley defects in these structures.

A new high pressure form of DyF 3

A new high-pressure phase of DyF 3 has been formed at 11.6 kb at 265°C in a hexahedral pressure apparatus. The new form shows a characteristic principal x-ray reflection corresponding to d-spacing 3.35Å, and is metastable at ambient conditions for short periods of time.

Synthesis, structure, and superconducting properties of new high-pressure forms of tin phosphide

Synthesis, structure, and superconducting properties of new high-pressure forms of tin phosphide

Unit cell dimensions and infra-red absorption spectra of MgSiO3 polymorphs

Three MgSiO3 polymorphs were synthesized and their unit cell dimensions and infra-red absorption spectra were determined. Protoenstatite with 2 mol % Fe2O3 which does not transform to clinoenstatite by quenching or grinding was sucessfully synthesized. Unit cell dimensions of three MgSiO3 polymorphs synthesized at normal pressure are close to those synthesized under high pressure. The clinoenstatite synthesized in this study is confirmed to be the high pressure and low temperature form. The densities are found to increase in the order of proto, ortho and clinoenstatite. It is also shown that distinction between three polymorphs can be easily made by X-ray powder diffraction patterns. Absorption spectra of these polymorphs are reasonably explained by Lazarev's empirical rule. In protoenstatite, two peaks due to the stretching of bridging Si-O are observed in 550-750 cm-1 region, whereas in ortho and clinoenstatite, four peaks due to the stretching of bridging Si-O in non crystallographic equivalent (SiO3) chain are observed.