Metallic liquid hydrogen and likely Al<Subscript>2</Subscript>O<Subscript>3</Subscript> metallic glass

Dynamic compression has been used to synthesize liquid metallic hydrogen at 140 GPa (1.4 million bar) and experimental data and theory predict Al₂O₃ might be a metallic glass at ∼ 300 GPa. The mechanism of metallization in both cases is probably a Mott-like transition. The strength of sapphire causes shock dissipation to be split differently in the strong solid and soft fluid. Once the 4.5-eV H-H and Al-O bonds are broken at sufficiently high pressures in liquid H₂ and in sapphire (single-crystal Al₂O₃), electrons are delocalized, which leads to formation of energy bands in fluid H and probably in amorphous Al₂O₃. The high strength of sapphire causes shock dissipation to be absorbed primarily in entropy up to ∼400 GPa, which also causes the 300-K isotherm and Hugoniot to be virtually coincident in this pressure range. Above ∼400 GPa shock dissipation must go primarily into temperature, which is observed experimentally as a rapid increase in shock pressure above ∼400 GPa. The metallization of glassy Al₂O₃, if verified, is expected to be general in strong oxide insulators. Implications for Super Earths are discussed.     

Compressibility of Fe1.087Te: a high pressure X-ray diffraction study

Fe_1.087Te exhibits three phases in the pressure range from ambient to 16.6?GPa and becomes amorphous at higher pressures. All three phases have tetragonal symmetry. The low pressure T-phase is stable in the pressure range 0≤P<4.1?GPa and is found to be relatively soft having zero pressure bulk modulus B ₀=36(1)?GPa. The intermediate cT-phase is less compressible with B ₀=88(5)?GPa and stable in the pressure range 4.1≤P<10?GPa while a more compressible phase was observed between 10 and 16.6?GPa.     

High-pressure radial X-ray diffraction study of osmium to 58 GPa

Nonhydrostatic compression behavior of osmium (Os) was investigated up to 58.2 GPa using radial X-ray diffraction (RXRD) together with lattice strain theory in a diamond-anvil cell. The apparent bulk modulus of Os derived from RXRD data varies from 262 GPa to 413 GPa, depending on Ψ, the orientation of the diffraction planes with respect to the loading axis. Fitting to the third-order Birch-Murnaghan equation of state, the RXRD data obtained at Ψ = 54.7° yields a bulk modulus K₀ = 390 ± 6 GPa with pressure derivative K^′ ₀ fixed at 4. The ratio of differential stress to shear modulus t/G ranges from 0.024 to 0.029 at the pressures of 15.7–58.2 GPa. The yield strength was observed to increase with compression and reach the value of 11.7 GPa at the highest pressure. This confirms that Os is the strongest known pure metallic material compared with the reported stiff elemental metals such as W, Mo and Re. It was found that the apparent c/a ratio changed with the nonhydrostatic compression, as well as the orientation Ψ in our experiments. Moreover, the aggregate moduli of Os at high pressure were determined from the RXRD measurements.     

Anomalous pressure-induced phase transformation in nano-crystalline erbium sesquioxide (Er2O3): partial amorphization under compression

Nanophase materials have novel physical and chemical properties, differing from bulk materials. It is of exceptional interest to investigate the size effect on structural stability in nanocrystals. Here, we investigated pressure-induced phase transitions in nanosized Er₂O₃ using angle-dispersive synchrotron X-ray diffraction up to 40.6 GPa. Nano-Er₂O₃ has enhanced transition pressure and higher bulk modulus (K₀) than its bulk counterparts. Amorphous Er₂O₃ nanoclusters with traces of monoclinic phase are obtained upon compression. This is the first time that partial amorphous structure under compression was observed in nano-Er₂O₃, indicating a kinetic trapping of partial amorphous Er₂O₃ on pressurizing.     

Elastic properties of high-temperature superconductors Pb2Sr2ACu3O8 (A=Ho,Y,Y0.5Ca0.5) derived from high-pressure X-ray diffraction

The crystal structure of the superconducting compounds Pb₂Sr₂ACu₃O₈, where A=Ho, Y and Y_0.5Ca_0.5, has been studied at room temperature in the pressure range up to 50 GPa. A transition from orthorhombic to tetragonal structure is observed at about 10 GPa for all these components. The transition is reversible and without any discontinuous volume change. The value of the bulk modulus for the two compounds with A=Ho and Y is in the range 160–164 GPa, whereas for A=Y_0.5 Ca_0.5 a larger value of 195 GPa has been measured.     

Pressure‐induced structural transformations in In2‐xYx(MoO4)3 systems

In this work, we report results of high‐pressure Raman experiments (P < 8 GPa) on In_2‐xY_xMo₃O₁₂ for x = 0.0 and 0.5. A crystalline to crystalline structural phase transition and pressure‐induced amorphization (PIA) have been identified. The structural phase transition takes place at 1.5 and 1.0 GPa for In₂(MoO₄)₃ and In_1.5Y_0.5(MoO₄)₃, respectively, resulting in the change of structure from monoclinic P2₁/a to a more denser structure. The PIA started at 5 and 3.4 GPa for In₂Mo₃O₁₂ and In_1.5Y_0.5Mo₃O₁₂, respectively. The amorphization process takes place in two stages in the case of In_1.5Y_0.5Mo₃O₁₂ phase, while for In₂Mo₃O₁₂, it is not complete until the pressure is as high as 7 GPa. Our results also suggest that with increase of ionic size of the A³⁺ ions, the octahedral distortion increases and consequently larger local structural disorder is introduced in the A₂(MoO₄)₃ system, where A is a trivalent ion (In, Y³⁺, Sc³⁺, Fe³⁺, etc.). Copyright © 2015 John Wiley & Sons, Ltd.     

Magnetic structure and properties of a bulk Fe72Al5P10Ga2C6B4Si1 alloy in the amorphous and nanocrystalline states

The structure forming under controlled crystallization of a bulk Fe₇₂Al₅P₁₀Ga₂C₆B₄Si₁ amorphous alloy has been studied using differential scanning calorimetry, transmission electron microscopy, and x-ray diffraction. Crystallization of the alloy was established to result in the formation of a nanocrystalline structure consisting of three phases. The domain structure and magnetic properties of amorphous and nanocrystalline samples were investigated using the magnetooptic indicating film technique (MOIF) and a vibrating-sample magnetometer. The coercive force and the saturation magnetization of the amorphous samples were found to be 1 Oe and 130 emu/g, respectively. It was shown that the formation of the nanocrystalline structure entails a dramatic decrease in domain size (down to 1–4 μm) as compared to an amorphous sample (∼1 mm). Simultaneously, a decrease in the saturation magnetization and a strong increase in the coercive force of the samples were observed. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 5, 2004, pp. 858–863. Original Russian Text Copyright ? 2004 by Abrosimova, Aronin, Kabanov, Matveev, Molokanov.     

Hyperfine characterization of the crystallization of nanocrystalline NiFe2O4 from the amorphous silica gel

Nanocrystalline NiFe₂O₄ was in‐situ prepared in amorphous silica using tetramethylor‐thosilicate and nickel (iron) nitrate hydrate as the starting materials in a sol‐gel reaction. The magnetic nanocrystals in the amorphous silica glasses grew slowly with increasing temperature. Above 600^○C, nickel ferrite nanoparticles began to precipitate from the amorphous silica matrix. Mössbauer spectroscopy of the nanocomposites suggested that in the silica glasses, Fe ions were present exclusively as Fe³⁺ in octahedral coordination, and the chemical environment of the Fe³⁺ ions appeared to remain unchanged until the crystallization of nickel ferrite nanocrystals. The formation of NiFe₂O₄ nanocrystals was the result of partial transformation of the FeO₆ octahedra to FeO₄ tetrahedra. The nanocrystalline NiFe₂O₄ are characterized by super‐paramagnetic behaviour at room temperature.     

Compression of volume and lattice parameters of 2H-CsCdBr3 under high pressure

Abstract

Powder x-ray diffraction experiments have been performed on 2H-CsCdBr₃. at room temperature up to 25 GPa. At normal pressure this compound shows unidimensional electronic properties. Such unidimensional behaviour is not evident in terms of elastic bulk properties under pressure. No phase transformation occurs in this pressure range. The a and c lattice parameters steadily decrease with pressure; their ratio lowers by only 2% up to 25 GPa. The bulk modulus is low, 21.2 GPa, and is in very good agreement with the bulk modulus-volume systematics for ionic compounds. The value of the first pressure derivative is also typical of ionic compounds.     

Far-infrared spectral studies of magnesium and aluminum co-substituted lithium ferrites

Polycrystalline Mg_xAl_2xLi_0.5(1−x)Fe_2.5(1−x) O₄ (x = 0.0, 0.2, 0.5, 0.6 and 0.7) ferrites were prepared by standard ceramic method, and characterized by X-ray diffraction and infrared absorption spectroscopy. The spectra show two significant absorption bands in the wave number range of 400–1000 cm⁻¹ arising from interatomic vibrations in the tetrahedral and octahedral coordination compounds. The decrease in intensity and increase in broadness of bands with concentration (x) are explained on the basis of cation distribution. The force constants and bulk modulus are found to decrease with Mg-Al content (x) which suggested weakening of interatomic bonding. An alternate method for the determination of bulk modulus, longitudinal and transverse velocities is suggested. The magnetic and electrical properties of these compounds are explained in the light of structural and optical properties.     

Modification of hyperfine field on57Fe nuclei in substituted ferrimagnetic garnets

We review the results of the NMR on⁵⁷Fe nuclei in a number of ferrimagnetic garnets Y_3−x R_xFe₅O₁₂ (R =rare earth or Bi) and Y₃Fe_5−xM_xO₁₂ (M=Al, Ga, In). For suitable concentrations of substituted ions (x∼0.1–0.4) satellite lines in the NMR spectra of both tetrahedral and octahedral Fe ions are observed. These satellites correspond to Fe ions in the vicinity of which single substitution is situated. The splitting between the satellite and the parent line has its origin in the change of the dipolar field and in the change of crystal field effects (including the transfer of electrons). While the dipolar field is easily calculated, once the magnetic moments and the geometry is known, the origin of the change of the crystal field effects is much less clear. Experimental results show that it is anisotropic and that in some cases the anisotropy may be substantial. This anisotropy is discussed within the framework of the semiempirical “independent bond” method.     

High-Pressure Mössbauer Measurements on Nanocrystalline Perovskite (La,Sr)(Mn,Fe)O3

We report here the Mössbauer measurements on nanocrystalline perovskite structured manganite La_0.8Sr_0.2Mn_0.8Fe_0.19 ⁵⁷Fe_0.01O₃ as a function of pressure up to 10 GPa at room temperature. The nanocrystalline sample, prepared by sol–gel technique found to have crystallite sizes of ∼138 ± 10 Å. Zero-field electrical resistivity measurements with temperature support the nanocrystalline nature. At ambient pressure, Fe³⁺ as well as Fe⁴⁺ ions are distributed in two different environments – Fe³⁺ in low symmetric site surrounded by Mn³⁺ ions only while Fe⁴⁺ in high symmetric site with at least one Mn³⁺ ion. Pressure seems to affect the higher symmetric site. A sudden increase in isomer shift at 0.52 GPa indicates the first order phase transition representing the transformation of Fe⁴⁺ to Fe³⁺. Another transition at 3.7 GPa, represents the presence of Fe³⁺ in single kind of environment. Pressure dependence of electrical resistivity measurements verifies the transitions attributing the first order transition to the cross over of localized-electron to band magnetism.     

Compression behavior of nanocrystalline anatase TiO2

We present a synchrotron X-ray diffraction study of pressure-induced changes in nanocrystalline anatase (with a crystallite size of 30-40 nm) to 35 GPa. The nanoanatase was observed to a pressure above 20 GPa. Direct transformation to the baddeleyite-TiO₂ polymorph was seen at 18 GPa. A fit of the pressure versus volume data to a Birch-Murnaghan equation yielded the following parameters: zero-pressure volume, V₀=136.15 Å³, bulk modulus, K_T=243(3) GPa, and the pressure derivative of bulk modulus, K′=4 (fixed). The bulk modulus value obtained for the nanocrystalline anatase is about 35% larger than that of the macrocrystalline counterpart.     

High-density strontium hydride: An experimental and theoretical study

Powder x-ray diffraction experiments and first-principles calculations have been carried out to investigate the possibility of a structural phase transition, characterized by a change from ionic to covalent bonding, in strontium hydride at pressures greater than 50 GPa. The powder x-ray diffraction results confirm a previously reported transition from the cotunnite structure to the Ni₂In structure at approximately 8 GPa. The Ni₂In phase remained stable up to the maximum experimental pressure of 113 GPa. The first-principles calculations, however, predict that under hydrostatic conditions a transition from the Ni₂In structure to the AlB₂ structure will occur at 115 GPa. A comparison of the pressure-dependent volume yielded by the respective experimental and theoretical studies suggests that in many cases the bulk modulus obtained from experiments carried out under non-hydrostatic conditions may be overestimated. Raman spectroscopy experiments corroborated the previously proposed Ni₂In structure, as the spectra obtained at pressures greater than 8 GPa exhibited two Raman-active modes, consistent with those expected from the Ni₂In structure.     

Optical spectroscopy of europium molybdate in the crystalline and amorphous States

Comparative spectroscopic studies of crystalline and amorphous samples of Eu₂(MoO₄)₃ were carried out. Amorphous samples were obtained through exposure of the β' crystal phase to a high pressure of ∼9 GPa. It was established that the transition to the amorphous state is accompanied by substantial changes both in the luminescence spectrum and the luminescence excitation spectrum. The long-wavelength absorption edge is estimated to shift by ∼0.8 eV, which is much more significant than in the case of amorphization of classical semiconductors.     

Structural and magnetic phases of Bi<Subscript>1−x</Subscript>A<Subscript>x</Subscript>FeO<Subscript>3−</Subscript><Subscript>δ</Subscript> (A = Sr,Pb) perovskites

The Bi_1−xA_xFeO_{3− δ} (A = Sr, Pb) systems have been studied using the X-ray, neutron powder diffraction and magnetization measurements in a magnetic field up to 14 T. It was found that around x ∼ 0.06 the crystal symmetry changes from a rhombohedral (space group R3c) to pseudo-tetragonal. In the composition range 0.07 ≤ x ≤ 0.14 the phases with different symmetry of the unit cell coexist independent of synthesis conditions. The neutron powder diffraction shows that the iron ions have average oxidation state close to 3+. The magnetic structure for Bi_0.5Sr_0.5FeO_{3− δ} is found to be G-type antiferromagnetic with magnetic moment of about 3.8 μ_B/Fe³⁺. The weak ferromagnetic state due to magnetoelectric interactions was revealed in the lightly doped rhombohedrally distorted compositions. No evidence for a spontaneous magnetization was observed for the pseudo-tetragonal phases. These compositions show irreversible nonlinear magnetization vs. field behavior apparently due to small local deviations from the collinearity of the magnetic moments.     

Pressure induced amorphization of AlPO4

AlPO₄ has been compressed to pressures of 16 GPa in a diamond anvil cell and its X-ray diffraction pattern studied by the energy-dispersive technique. The compound is observed to become amorphous at ∼ 12 GPa. This explains the loss of Raman spectrum of AlPO₄ reported by Jayaraman and coworkers (1987).     

Internal friction and change in Young’s modulus in the alloy Mg-Ni-Y due to a transition from an amorphous to a nanocrystalline state

The effect on the internal friction and Young’s modulus of the evolution of the structure of the amorphous alloy Mg₈₄Ni_12.5Y_3.5 (relaxation of internal stresses, devitrification, nucleation and decay of nanocrystalline phases) was investigated as it was heated. The measurements were performed on ribbon samples by flexural oscillations. Irreversible peaks of the internal friction and anomalies in the behavior of Young’s modulus as a function of temperature were observed. The position of the anomalies correlates with the characteristic temperatures of restructurings observed by differential thermal and x-ray diffraction. Possible internal-friction mechanisms associated with various types of structural relaxation and nanocrystallization processes in the alloy are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 561–566 (April 1999)     

Compression curve of BaBiO3

The compression curve of BaBiO₃ was measured by X-ray diffraction method using a diamond anvil cell apparatus up to about 8 GPa. The diffraction pattern did not change in this pressure range, but the bulk modulus suddenly decreased near 4 GPa. The initial value of the bulk modulus, 157 GPa was estimated from the data up to 4 GPa.     

Volume and structural study of Fe64Mn36 anti-ferromagnetic Invar alloy under high pressure

We have investigated the pressure variation of the volume and structure of an FCC Fe₆₄Mn₃₆ anti-ferromagnetic Invar alloy. The inclination of the pressure-volume (P-V) curve of the FCC structure becomes discontinuous at a pressure of 4 GPa. According to the bulk modulus at zero pressure estimated by the Birch-Murnaghan equation of state, the pressure between 4 and 10 GPa is 33 GPa larger than that at a pressure below 4 GPa. Considering previous experiments on magnetism at high pressure the Neel temperature at 4 GPa almost decreases to room temperature. These results suggest that the increase in the bulk modulus by 33 GPa can be attributed to the pressure-induced magnetic phase transition from anti-ferromagnetism to paramagnetism. Volume at zero pressure was estimated using the Birch-Murnaghan equation of state. The volume of FCC structure in the anti-ferromagnetic state was 1.17% larger than the volume in the paramagnetic state, namely, the spontaneous magnetostriction was 1.17%. Pressure-induced structural transition from FCC to HCP occurs with an increase in the pressure, especially at up to 5 GPa. The value of c/a is 1.62; this value almost corresponds to that of an ideal HCP structure. The bulk modulus of the HCP structure estimated by the Birch-Murnaghan equation of state is larger than that of the FCC structure, and the volume/atom ratio is smaller than that of the FCC structure.