High pressure X-ray diffraction study of potassium azide

Crystal structure and compressibility of potassium azide was investigated by in-situ synchrotron powder X-ray diffraction in a diamond anvil cell at room temperature up to 37.7 GPa. In the body-centered tetragonal (bct) phase, an anisotropic compressibility was observed with greater compressibility in the direction perpendicular to the plane containing N₃⁻ ions than directions within that plane. The bulk modulus of the bct phase was determined to be 18.6(7) GPa. A pressure-induced phase transition may occur at 15.5 GPa.     

Evidence of polymorphic transformations of Sn under high pressure

The high-pressure polymorphs and structural transformation of Sn were experimentally investigated using angledispersive synchrotron x-ray diffraction up to 108.9 GPa. The results show that at least at 12.8 GPa β-Sn→bct structure transformation was completed and no two-phase coexistence was found. By using a long-wavelength x-ray, we resolved the diffraction peaks splitting and discovered the formation of a new distorted orthorhombic structure bco from the bct structure at 31.8 GPa. The variation of the lattice parameters and their ratios with pressure further validate the observation of the bco polymorph. The bcc structure appears at 40.9 GPa and coexists with the bco phase throughout a wide pressure range of40.9 GPa–73.1 GPa. Above 73.1 GPa, only the bcc polymorph is observed. The systematically experimental investigation confirms the phase transition sequence of Sn as β-Sn→bct→bco→ bco + bcc→bcc upon compression to 108.9 GPa at room temperature.     

Magnetic,structural and electrical properties of ordered and disordered Co50Fe50 films

Co₅₀Fe₅₀ films with thickness varying from 100 to 500 Å were deposited on a glass substrate by sputtering process, respectively. Two kinds of CoFe films were studied: one was the as-deposited film, and the other the annealed film. The annealing procedure was to keep the films at 400 °C for 5 h in a vacuum of 5×10⁻⁶ mbar. From the X-ray study, we find that the as-deposited film prefers the CoFe(1 1 0) orientation. Moreover, the body-centered cubic (bcc) CoFe(1 1 0) line is split into two peaks: one corresponding to the ordered body-centered tetragonal (bct) phase, and the other, the disordered bcc phase. After annealing, the peak intensity of the ordered bct phase becomes much stronger, while that of the disordered bcc phase disappears. The annealing has also caused the ordered CoFe(2 0 0) line to appear. When the amount of the ordered bct phase in Co₅₀Fe₅₀ is increased, the saturation magnetization (M_s) and coercivity (H_c) become larger, but the electrical resistivity (ρ) decreases. From the temperature coefficient of resistance (TCR) measurement, we learn that the bct grains in the CoFe film start to grow at temperature 82 °C.     

High-pressure structural phase stability in Hg TO 1 TPa (10 Mbar)

From first-principles total-energy calculations on Hg over a wide volume range, we predict: (i) a pressure-induced bct (β-Hg) to hcp phase transition near 100 GPa (1 Mbar); (ii) a stable, low-temperature hcp structure from 100 GPa to at least 1 TPa (10 Mbar), with a high but increasing c/a ratio approaching a limiting value of 1.83; and (iii) three additional distorted, metastable phases (including α-Hg), all possibly energetically accessible at high temperature.     

Pressure-induced phase transition of zinc-blende AlN: An ab initio molecular dynamics study

An ab initio constant pressure technique is carried out to study the pressure-induced phase transition of the zinc blende AlN (aluminum nitride). A first order phase transformation into a rock salt structure is observed in the constant pressure simulations. The transformation is accompanied by an initial tetragonal distortion and a subsequent shearing, similar to that found in the other zinc blende structured materials. This phase transition should occur around 6.2 GPa based upon the enthalpy calculations.     

Magnetic configurations of hexagonal iron rhenium multilayers

We present ab initio calculation of the electronic structure and magnetic properties of Fe/Re multilayers assuming hcp crystallographic phase. The system is modeled according to recent structural analysis which shows that Fe/Re has bct(0 0 1) ordering for 0⩽t_Re⩽8 Å after which a structural phase transition occurs to an hcp phase. Antiferromagnetic and ferromagnetic orders appear more stable than the nonmagnetic (NM) state, only when the Fe atoms begin to adopt an expanded volume of about 27% accompanied by an axial distortion (c/a=1.42). The sizeable magnetic moments calculated on Fe atoms contrast with the paramagnetic bulk hcp. These results are discussed and compared with recent experimental data on hcp Fe/Re superlattices and also with magnetic properties of Fe/Ru multilayers.     

Theory of the crystal structures of cerium and the light actinides

First-principles theory, based on the density-functional approach, is used to study the crystal structures of Ce and the light actinides (Th-Pu) at low temperatures as a function of hydrostatic pressure. Calculated ground-state properties, such as crystal structure, atomic volume and bulk modulus, are shown to be very well described within this theory. We present the following pressureinduced phase transitions: Ce, fcc -> bct -> hcp; Th, fcc -> bct -> hcp; Pa, bct -> alphaU bct -> hcp; U, alpha-U -> bct -> bcc; Np, alpha-Np -> beta-Np -> bcc; Pu, alpha-Pu -> alphaNp -> beta-Np -> bcc. We explain the occurrence of low-symmetry (complex) structures in these metals as a consequence of a symmetry-breaking mechanism that shows similarities to a Peierls distortion. The ultimate high-pressure phases are well accounted for in a canonical model for the f bands for these metals.     

高温高压下过渡金属Ru的结构相变

采用基于密度泛函理论的第一性原理和准简谐晶格动力学方法对Ru的六角密排 (hcp)、面心立方 (fcc)、体心四方 (bct) 和体心立方 (bcc) 结构的磁性、晶格结构稳定性和高温高压下的相变进行了系统的研究. 计算获得了各相结构的磁性基态及其稳定性范围, 结果表明: 零温下在计算的压力范围内, NM-hcp 结构是Ru最稳定的结构, 压力的单独作用下并没有相变的发生; NM-fcc结构是Ru的亚稳定结构, 而NM-bcc和FM-bct结构在动力学上并不稳定. 高温高压下Ru将发生从NM-hcp到NM-fcc结构的相变, 并给出了Ru的温度压力相图. 关键词： 相变 晶格稳定性 磁性 第一性原理     

Effect of pressure on the magneto-optical properties of bcc and bct iron

We have studied the effect of pressure on the electronic structure of bcc and bct iron within the local spin density approximation (LSDA) for the electronic exchange and correlation. We report calculations of the magneto-optical properties of ferromagnetic iron in the bcc and bct phases using the full potential linearized augmented plane wave (FPLAPW) method as implemented in the WIEN2K code. In both phases, we find two prominent minima in Kerr rotation. In bcc phase, both the calculated Kerr minima shift towards higher energies with increase in pressure while in bct phase, the low energy minimum in Kerr rotation spectra 2 eV becomes less pronounced and the minimum at 7 eV moves towards higher energies on increasing pressure.     

Fluctuating lattice constants of indium under high pressure

Recent high-pressure investigations of elemental In have yielded controversial results. We show that the observed high-pressure face-centered orthorhombic (fco) structure can be explained as an intermediate state between two body-centered tetragonal (bct) structures with different c/a ratios (c/a < square root [2] and c/a > square root [2], respectively). In a pressure range from about 50 to 200 GPa these two bct structures correspond to local minima of the total energy with respect to orthorhombic distortion of the ground-state bct In structure. The fco saddle point represents a tiny barrier and even at low temperatures rapid structural fluctuations should occur. Such a situation has not been identified in any other elemental metal.     

A study on the formation of crystalline phases during solidification and crystallisation in the bulk metallic glass of Zr53Cu21Al10Ni8Ti8 composition

The present paper considers the formation of crystalline phases during solidification and crystallisation of the Zr₅₃Cu₂₁Al₁₀Ni₈Ti₈ alloy. Solidification was carried out by a copper mould casting technique, which yielded a partially crystalline microstructure comprising a ‘big cube phase’ in a dendritic morphology and a bct Zr₂Ni phase. Detailed high-resolution microscopy was carried out to determine possible mechanisms for the formation of the crystalline phases. Based on microstructural examinations, it was established that the dendrites grew by the attachment of atomistic ledges. The bct Zr₂Ni phase, formed during solidification and crystallisation, showed various types of faults depending on the crystallite size, and its crystallography was examined in detail. It has been shown that the presence of these faults could be explained by anti-site occupancy in the bct lattice of the Zr₂Ni phase.     

Effect of aluminum substitution on the structural and magnetic properties of cobalt ferrite synthesized by sol-gel auto combustion process

Aluminum substituted cobalt ferrite powders (CoFe_2−xAl_xO₄) with varying composition from 0.0 to 1.0 in the step of 0.2 have been obtained by sol-gel auto combustion technique using citric acid as a fuel. The metal nitrate to fuel ratio was maintained 1:4 throughout the synthesis of CoFe_2−xAl_xO₄. The thermal analysis of as prepared samples is done by TGA technique. The compositional stoichiometry of the prepared samples is confirmed by Energy dispersive X-ray analysis technique. Single phase cubic spinel structure and nano phase structure of the synthesized powders were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallite size of 16-26 nm was obtained using Scherrer formula. SEM analysis shows the formation of uniform grain growth. The grain size obtained from SEM results is of the order of 30 nm. Maximum specific surface area was observed to be of the order of 52 m²/gm. The highest value of saturation magnetization and coercivity was observed for pure cobalt ferrite sample and it decreases as the aluminum content x increases. A strong co-relation between the saturation magnetization and aluminum content was observed. The decrease in magnetic properties is due to the substitution of aluminum ions in place of Fe³⁺.     

Synthesis and structure of aluminum substituted MoSi2

Mo(Si_1−xAl_x)₂ compositions have been prepared by a modified SHS route under pressure. The compositional phase analysis indicates a body centered tetragonal C11_b phase below x<0.05 substitution, above which a biphasic region of C11_b and C40 structure coexists. Occurrence of pure hexagonal C40 phase is observed for x>0.1. The aluminum substitution leads to a continuous increase in the lattice parameters in both these structures. In the biphasic region, redistribution of aluminum concentration takes place with the evolution of aluminum rich C40 phase as indicated by energy dispersive X-ray analysis and supported by micro-hardness measurements.     

Determination of temperature-induced changes in the structure of the nearest environment of aluminum in mordenite zeolites from x-ray absorption near-edge structure spectra

The structure of the nearest environment of aluminum in mordenite zeolites was investigated at temperatures up to 1000 K using the Al K x-ray absorption near-edge structure (XANES) spectra. The interatomic Al-O distances and the number of coordinating oxygen atoms were determined by Fourier transformation of Al K-XANES spectra and fitting of the contribution of the nearest oxygen atoms. The exactness and stability of the obtained values of structural parameters were verified using the Al K-XANES spectrum of the model Na-mordenite zeolite. To determine the coordination of aluminum in H-mordenites in the temperature range under study, fitting of the contribution of coordinating oxygen atoms was performed using both the single-sphere model of the nearest environment of aluminum and two-sphere fitting for the most probable structural models of the nearest environment of aluminum. Such models were chosen based on the analysis of the specific preedge features and fine structure of the XANES spectra, as well as NMR data. With this approach, the simultaneous presence of tetra-and hexacoordinated aluminum in H-mordenite at T = 300 K was established and the concentrations of such mixed states were determined. The presence of strong distortions of the oxygen tetrahedron coordinating the aluminum atom in dehydrated H-mordenite at 575 K was shown and the Al-O distances for such distortions were determined. The simultaneous presence of tri-and tetracoordinated aluminum in H-mordenite at 985 K was established and the fraction of tricoordinated aluminum was estimated quantitatively.     

Structure and mechanical properties of crystals of partially stabilized zirconia after thermal treatment

The phase composition and morphology of the twin structure of the Y₂O₃-stabilized zirconia crystals (from 2.8 to 4.0 mol %) after the thermal treatment at 1600°C have been investigated by X-ray diffractometry and transmission electron microscopy. It is shown that as the concentration of the stabilizing Y₂O₃ impurity increases, the character of the twin structure changes, and the amount of the untransformed phase t′ increases. The dependence of the hardness and crack resistance of the crystals of partially stabilized zirconia on the Y₂O₃ concentration and the indenter orientation is investigated using the microindentation method. The sample with the lowest concentration of the stabilizing Y₂O₃ impurity turned out the most crack resistant. This can be explained by a high content of tetragonal phase t in it, which provides the transformation strengthening mechanism of the material, and by a more multilevel character of twinning.     

The structure of an induced electrorheological solid

We have extended a method devised by Rayleigh and calculate the effective dielectric constants (ϵ_eff) of face-centred-cubic (fcc), body-centred tetragonal (bct) and hexagonal close-packed (hcp) structures of spheres of a dielectric material (ϵ_p) in a dielectric matrix (ϵ_f) specially. The comparison of the (ϵ_eff − ϵ_f)/f_c of these three types of lattice shows that the body-centred tetragonal structure is favoured in energy, where f_c is the volume fraction of particles that are touching for a given lattice.     

Formation of a Cmcm phase in SnS at high pressure; an ab initio constant pressure study

The stability of SnS at high pressure is studied using a constant pressure ab initio technique. For the first time, a pressure-induced phase transformation from the Pnma structure to a Cmcm structure with the application of pressure is predicted through the simulations in this material. The Cmcm phase is still a layered structure, consisting of rocksalt-like bilayers, similar to that formed at high temperatures. The Cmcm structure is fivefold coordinated. This phase transformation gradually proceeds and is due to the significant decrease of the second neighbor distances. This phase change is also studied by total energy calculations.     

Local variation of the dielectric properties of poly(vinylidene fluoride) during the α- to β-phase transformation

The phase transformation from the non-polar α-phase to the polar electroactive β-phase of polyvinylidene fluoride (PVDF) has been investigated using the fluorescence from Nile red. Films of α-PVDF doped with Nile red were stretched at controlled rates at a temperature of 80 °C to produce the α- to β-phase transition. The thermo/mechanical dependent changes in the crystalline structure are related to the physical rotation of the polar (CH₂-CF₂) group, which can be monitored by steady state fluorescence techniques. The degree of phase transformation is related to variation in the fluorescence, which in turn is linked to local dielectric constant of the polymer. The variation of the refractive index is more associated to the alignment of the polymeric chains than to the phase transformation. Thus, fluorescence is a suitable technique to monitor phase transitions coupled to a variation in the polarity of the dielectric medium.     

In situ X-ray observation of phase transformation in Fe2O3 at high pressures and high temperatures

A laser-heated sample in a diamond anvil cell and synchrotron X-ray radiation was used to carry out structural characterization of the phase transformation of Fe₂O₃ at high pressures (30-96 GPa) and high temperature. The Rh₂O₃(II) (or orthorhombic perovskite) structure transforms to a new phase, which exhibits X-ray diffraction data that are indicative of a CaIrO₃-type structure. The CaIrO₃-type structure exhibited an orthorhombic symmetry (space group: Cmcm) that was stable at temperatures of 1200-2800 K and pressure of 96 GPa (the highest pressure used). Unambiguous assignment of such a structure requires experimental evidence for the presence of two Fe species. Based on the equation of state of gold, the phase boundary of the CaIrO₃-type phase transformation was P (GPa)=59+0.0022×(T−1200) (K).     

First-principles study of the elastic and thermodynamic properties of thorium hydrides at high pressure

The high pressure behaviors of Th_4H_(15) and ThH_2 are investigated by using the first-principles calculations based on the density functional theory(DFT). From the energy–volume relations, the bct phase of ThH_2 is more stable than the fcc phase at ambient conditions. At high pressure, the bct ThH_2 and bcc Th_4H(15) phases are more brittle than they are at ambient pressure from the calculated elastic constants and the Poisson ratio. The thermodynamic stability of the bct phase ThH_2 is determined from the calculated phonon dispersion. In the pressure domain of interest, the phonon dispersions of bcc Th_4H(15) and bct ThH_2 are positive, indicating the dynamical stability of these two phases, while the fcc ThH_2 is unstable. The thermodynamic properties including the lattice vibration energy, entropy, and specific heat are predicted for these stable phases. The vibrational free energy decreases with the increase of the temperature, and the entropy and the heat capacity are proportional to the temperature and inversely proportional to the pressure. As the pressure increases, the resistance to the external pressure is strengthened for Th_4H_(15) and ThH_2.