Fm3m Phases Research Articles

Low-temperature polymorphic transition in highly compressed uranium dioxide

The Mayer group expansion method is extended on the low-temperature solids where quantum effects cannot be ignored and applied to the prediction of polymorphic transition line between cubic Fm3m and orthorhombic Pnma phases in the highly compressed ionic crystal of uranium dioxide. The temperature dependence of the transition pressure and volume changes are evaluated and the results are compared with existing experimental data and ab initio predictions.

Design optimization of spinel-rocksalt intergrown high entropy oxide structure for enhanced electrochemical properties

The advantages of each phase can be utilized to prepare a dual-phase intergrown anode consisting of spinel (Fd-3m) and rocksalt (Fm-3m) phases through the solid-state method. This approach leverages the high redox activity and lithium storage capacity of the rocksalt phase, as well as the good kinetics and stability of the spinel structure. By adjusting the sintering temperature, the ratio of the spinel phase (S-HEO) to the rocksalt phase (R-HEO) in the novel intergrown high entropy oxide structure (d-HEO) can be modulated, allowing for control over its electrochemical properties. Through this, an optimized S-HEO/R-HEO ratio can be achieved, resulting in high capacity coupled with desired cyclic stability and rate performance. This work demonstrates an effective strategy for constructing spinel-rocksalt intergrown structures. Furthermore, optimizing the structural properties of the dual-phase may shed light on material design for electrochemical energy storage and new applications of advanced HEOs.

Synthesis and study of single crystals and optical ceramics based on the AgCl0.25Br0.75 – AgI system

To obtain non-toxic, non-hygroscopic materials used in IR fiber optics and optoelectronics, a comprehensive study of the AgCl0.25Br0.75 – AgI system was carried out. The study of the AgCl0.25Br0.75 – AgI system's phase diagram showed the presence of a homogeneity region with a content 0–16 mol.% AgI in AgCl0.25Br0.75 and heterogeneity region 16–98 mol.% AgI in AgCl0.25Br0.75. The phases in the homogeneous region have a cubic structure of the Fm3m type. The phases in the heterogeneous region are a mixture of cubic Fm3m and hexagonal P63mc phases. Within each of the regions, single-crystals samples with compositions of 4, 8, 16, and 20 mol.% AgI in AgCl0.25Br0.75 and optical ceramics ones of composition 20 and 33 mol.% AgI in AgCl0.25Br0.75 were synthesized. For a sample of 20 mol.% AgI in AgCl0.25Br0.75, different synthesis modes were used, as a result it was possible to obtain both a single-crystal and ceramics. The materials' band gap and the optical transmission range are determined. The band gap decreases from 2.5195 to 2.3724 with a rise in the fraction of AgI in AgCl0.25Br0.75. The materials are transparent in the range of 0.492 - 54 μm. Single-crystals are characterized by a stepwise increase in transmission up to 50–57% at the short-wavelength absorption edge. For optical ceramics, the transmission initially reaches 1% stepwise and then increases until reaching a maximum value in the IR of 15–20 μm. It is associated with scattering by grains of different phases. A uniform shift of the transmission spectrum to longer wavelengths from 492 to 525 nm in the visible and from 47 to 54 μm in the far IR with a rise in the proportion of AgI in AgCl0.25Br0.75 is shown. The single-crystals' and optical ceramics' highest transmission level reaches 75–78% in the wavelength range from 10 to 40 µm.

Phase transition and dielectric properties of (1-x)Pb(Mg1/2W1/2)O3-xPb(Yb2/3W1/3)O3 solid solutions

ABSTRACT (1-x)Pb(Mg1/2W1/2)O3-xPb(Yb2/3W1/3)O3 ceramics have been prepared by a conventional ceramic process. Their crystal structures, phase transitions and dielectric properties have been investigated in this paper. Limited solid solutions (x ≤ 6 mol%) could be formed. The x = 0.02 composition demonstrates mixture of orthorhombic (Pmcn) and cubic (Fm-3m) phases, while apparent single-phase solid solution with B-site 1:1 ordering (space group: Fm-3m) could be observed for the 0.02 < x ≤ 0.06 compositions at room temperature. The solid solution demonstrates a first-order phase transition. Local orthorhombic distortion in the cubic matrix above transition temperature, manifested in the large temperature factors of Pb2+ and O2− observed in the structure refinements, could be further confirmed by Raman analysis. The doping of (Yb2/3W1/3)4+ leads to the decrease in orthorhombic-cubic phase transition temperature. The microscopic nature affecting the driving force of the phase transition was discussed from the point of stereochemistry.

Structural, mechanical and electronic properties study on group 5 transition metals ternary mononitrides from first-principles calculations

Various possible structures of ternary mononitrides of group 5 transition metals (V0.5Ta0.5N, V0.5Nb0.5N and Nb0.5Ta0.5N) are systematically explored in 0–300 GPa pressure range through CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method. Three possible phases with Pmm2, Cmc21, and Fm-3m space group have been uncovered. Their dynamical, mechanical and thermodynamic stabilities are checked through calculations of phonon spectra, elastic constants and formation enthalpy. The elastic anisotropies are fully investigated by the dependence of linear bulk, Young's and shear moduli on the crystal orientations. To study the strength of these three compounds, the compressive, tensile and indentation shear strengths are systematically studied from the first-principles calculations. The mononitrides with Pmm2 and Cmc21 structures exhibit much better mechanical property relative to VN, NbN and TaN with traditional B1 (NaCl-like) structure. Our results reveal that the Pmm2 and Cmc21 phases of these three compounds possess the strength values up to 30 GPa under the Vickers indentation simulation calculations, which are comparable to well-known WB3, WB4 and ReB2 reported as potential superhard transition metal borides. Additionally, the electronic structures of these mononitrides are also studied to explore the intrinsic chemical bonding nature, and a novel transition of semiconductor-to-conductor from V0.5Ta0.5N, V0.5Nb0.5N to Nb0.5Ta0.5N is exposed for the Fm-3m phases of these compounds.

Neutron and X-ray Diffraction Texture Analysis of Novel Al-Si-Mg Alloy

Crystallographic texture and microstructure of as-casted specimen of a novel aluminum-based alloy is analyzed. Pole figures recorded by means of neutron and X-ray diffraction methods are used as the primary data in texture analysis. The data collection is performed on the SmartLab diffractometer (Rigaku) diffractometer with Cu rotating anode X-ray tube, and the KSN-2 neutron diffractometer located at the research reactor LVR-15 in the Nuclear Research Institute, plc. Rez, Czech Republic. Software tools GSAS, X’Pert Texture, and MTEX are applied in the subsequent data processing. Additional characterization methods applied include elemental composition analysis by the instrumental neutron activation analysis, phase analysis by X-ray fluorescence and X-ray diffraction, and microstructure analysis by scanning electron microscopy and metallographic optical microscopy. Elemental composition of the alloy is dominated by Al (79.91 wt.%), Si (9.90 wt. %) and Mg (4.19 wt. Aluminum (cubic, Fm3m), silicon (cubic, Fd3m) and magnesium silicide (Mg2Si, cubic, Fm3m) phases are then identified as the main crystallographic phases. Crystallographic preferential orientation of the main Al phase is only analyzed and discussed; the other two phases do not provide well-resolved diffraction maxima necessary for a reliable texture analysis.

Transport properties of mixing conduction in CaF2 nanocrystals under high pressure**Project supported by the National Natural Science Foundation of China (Grant Nos. 11374131, 11674404, 11404137, and 61378085), Program for the Development of Science and Technology of Jilin Province, China (Grant Nos. 201201079 and 20150204085GX), Twentieth Five-Year Program for Science and Technology of Education Department of Jilin Province, China (Grant No.

We report on the intriguing electrical transport properties of compressed CaF2 nanocrystals. The diffusion coefficient, grain and grain boundary resistances vary abnormally at about 14.37 GPa and 20.91 GPa, corresponding to the beginning and completion of the Fm3m–Pnma structural transition. Electron conduction and ion conduction coexist in the transport process and the electron conduction is dominant. The electron transference number of the Fm3m and Pnma phases increases with pressure increasing. As the pressure rises, the F− ion diffusion and electronic transport processes in the Fm3m and Pnma phases become more difficult. Defects at grains play a dominant role in the electronic transport process.

Antiferromagnetic behaviour and dielectric relaxation of xBa2FeNbO6–(1-x)LaFeO3 [x = 0.1, 0.3, 0.5

In this work we have investigated the magnetic and dielectric properties of xBa2FeNbO6–(1-x)LaFeO3 [x = 0.1, 0.3, 0.5] prepared by the solid state reaction technique. The Rietveld refinement of the X-ray diffraction pattern of the samples is performed to confirm the phase formation as well as the crystal structure of the materials. The x = 0.1 and 0.5 materials have orthorhombic Pbnm and cubic Fm3m crystal symmetry respectively. Whereas x = 0.3 material has both Pbnm (83.64%) and Fm3m (16.36%) phases. The effect of lattice distortion on the magnetic and dielectric properties has been discussed. The temperature dependent magnetization study reveals the anti-ferromagnetic behaviour of the materials. The magnetic field dependence magnetization curve indicates the canting of Fe3+ spins in these materials like LaFeO3. The value of magnetization has increased with respect to pure LaFeO3. The dielectric relaxation of the materials is analyzed within impedance and electric modulus mechanisms. The value of conductivity and dielectric constant increases with the increase of x value. Two semi-circular arcs in the complex impedance plane plot originate due to the grain and grain-boundary contributions which give rise to two dispersion regions in the frequency dependence conductivity spectra of the materials. The complex impedance plane plot is described by an electrical equivalent circuit consisting of two parallel RC elements connected in series.

High pressure impedance spectroscopy of SrF2 nanocrystals

ABSTRACTThe charge transport behavior of strontium fluoride nanocrystals has been investigated by in situ impedance measurement up to 41 GPa. It was found that the parameters changed discontinuously at each phase transition. The charge carriers in SrF2 nanocrystals include both F− ions and electrons. Pressure makes the electronic transport more difficult. The defects at grains dominate the electronic transport process. Pressure could make the charge–discharge processes in the Fm3m and Pnma phases more difficult.

Strong correlation effect on the thermodynamic and mechanical properties of ytterbium dihydride

The strong correlation effect of the thermodynamic and mechanical properties of the Pnma and Fm-3m phases of ytterbium dihydride (YbH2) has been investigated using density functional theory plus Hubbard U method. The equilibrium geometries, elastic constants, various moduli, Poisson's ratio, and elastic wave velocities of the two phases of YbH2 are systematically studied. The Pnma and Fm-3m phases of YbH2 exhibit brittle characteristics. Phonon calculations verify that the two phases are thermodynamically stable. The low-frequency region of the two phases share similar features, whereas the high-frequency region shows a remarkable difference between the two phases. Given the phonon contribution to the Helmholtz free energy, the temperature dependences of equilibrium volume, bulk modulus, entropy, and heat capacity at constant pressure are obtained using quasiharmonic approximation. The calculated trends of equilibrium volume and head capacity at constant pressure are consistent with experimental data.

Building Less Tortuous Ion-Conduction Pathways Using Block Copolymer Electrolytes with a Well-Defined Cubic Symmetry

We investigated the effects of morphology on the ion transport properties of polymer electrolytes. A single sulfonated block copolymer displayed a series of ordered morphologies with three-dimensional symmetry, i.e., cubic, Fddd (O70), and Fm3m (fcc) phases, upon the addition of ionic liquids. The unique phase behavior was understood on the basis of the selective swelling of the sulfonated blocks by ionic liquids and the changes in segregation strength with the modulation of the ionic interactions in the ionic phases. The type of three-dimensional lattice was revealed to play an important role in determining the ion transport properties of ionic liquid-containing sulfonated block copolymers. For example, the sample with disordered spherical lattices exhibited the highest tortuosity (∼2) for ion conduction, indicative of the considerable diffusion barriers in the conducting phases. On the contrary, the samples with well-defined orthorhombic and face-centered cubic symmetries, i.e., O70 and fcc phases, rev...

High-pressure synthesis and electrochemical properties of lithium transition metal oxides with layered rock-salt structure

Li-rich layered transition metal oxides with the nominal composition of Li1+x[Li0.2MnαCoβNiγ]O2 with 0.0 ≤ x ≤ 0.7 are synthesized using a high-pressure technique. The chemical compositions are determined using inductively coupled plasma (ICP) spectroscopy and the structure is identified from X-ray diffraction measurements as a mixture of rhombohedral R-3m and cubic Fm-3m phases. Rietveld refinement analysis reveals that the layered to cubic phase ratio increases as x tends towards 0.7, with an almost complete cubic phase present for x ≥ 0.6. Electrochemical charge–discharge experiments performed using coin cells and show capacities of around 200 mAh g−1 for 0.0 ≤ x ≤ 0.4 samples, although the capacity quickly decreases for x ≥ 0.5, which is consistent with the trend towards a predominantly cubic phase. The plateau for oxygen generation, which appears at the 4.4 V threshold, is reduced as x increases.

STRUCTURAL TRANSITIONS OF BiI3 UNDER PRESSURE

High pressure studies of BiI 3 at 0 K are performed using first-principles pseudopotential calculations within the framework of density functional theory. The calculations indicate that BiI 3 undergoes a structural transition from rhombohedral R-3 phase to monoclinic P2 1/c phase at 7 GPa which is accompanied by a 5.8% volume collapse. In addition, we find that P2 1/c phase prevails about 60 GPa range and transforms to cubic Fm-3m phase at 68 GPa, and finally takes the orthorhombic Pnma phase at high pressures up to 133 GPa. The structural and electronic properties of four competitive structures are also calculated. The analysis of density of states reveals that BiI 3 has semiconductor-metal transition at about 61 GPa, which also demonstrates the metallic nature of both Fm-3m and Pnma phases.

Pressure-induced phase transition and structural properties of CrO2

The structural properties and pressure-induced phase transitions of CrO2 have been investigated using the pseudopotential plane-wave method based on the density functional theory (DFT). The rutile-type (P42/mnm), CaCl2-type (Pnnm), pyrite-type (Pā3), and CaF2-type (Fm-3m) phases of CrO2 have been considered. The structural properties such as lattice parameters, bulk moduli and its pressure derivative are consistent with the available experimental data. The second-order phase-transition pressure of CrO2 from the rutile phase to CaCl2 phase is 10.9 GPa, which is in good agreement with the experimental result. The sequence of these phases is rutile-type → CaCl2-type → pyrite-type → CaF2-type with the phase-transition pressures 10.9, 23.9, and 144.5 GPa, respectively. The equation of state of different phases has also been presented. It is more difficult to compress with the increase of pressure for different phases of CrO2.

Effects of structural change on half-metallicity of CoMnZ (Z=P, As, Sb): A first-principle study

The C1b and Fm-3m phases of CoMnZ (Z=P, As, Sb) were studied via first-principle calculations. The C1b CoMnZ exhibits half-metallicity and is more energetically stable than Fm-3m CoMnZ. Under volume distortions, CoMnP, CoMnAs, and CoMnSb can maintain half-metallicity for volume alterations of −16% to 16%, −20% to 11%, and −32% to 0%, respectively. Under tetragonal distortions, CoMnP, CoMnAs, and CoMnSb can maintain half-metallicity from a change of in-plane lattice constants of 0.500–0.580, 0.515–0.585, and 0.545–585nm, respectively.

Fulleride superconductors and orientational order: T c vs lattice constant in Na 2Rb xCs 1− xC 60

The effect of orientational order on fulleride superconductivity was studied by performing DC SQUID magnetometry and high-resolution x-ray diffraction on the title quaternary phases. These are all simple cubic, Pa3space group, with the same molecular orientational order as occurs in pure C 60. The T c vs. lattice constant correlation is much steeper than is found for the merohedrally-disordered Fm3m phases, implying the possibility of large T c enhancements with only minor additional dilation of the sc lattice. The effect of orientational order can be rationalized as primarily due to a density of states effect.

Infrared spectroscopic study of the rotation of the NH+4 ion in ammonium halides

Infrared correlation functions, have been obtained from the analysis of band shapes of the 1400 cm−1 bending mode of NH4Cl, NH4Br and NH4I in both the Pm3m and Fm3m phases. The NH 4 + ion seems to undergo relatively free rotation in the high temperature Fm3m phases of these halides.

Melting and Polymorphism of KHF2, RbHF2, and CsHF2 to High Pressures

The high-pressure phase diagrams of KHF2, RbHF2, and CsHF2 were studied by means of differential thermal analysis and volume displacement. The tetragonal / Fm3m transition lines of KHF2 and RbHF2 rise steeply with pressure to meet the melting curves of the Fm3m phases near 2 kbar. The resulting melting curves of the tetragonal phases are almost immediately terminated by the appearance of new phases which appear to be similar to the high-temperature forms with space group Pm3m of CsHF2 and the corresponding azides. In the case of KHF2 the tetragonal / Pm3m transition line meets the melting curve of the Pm3m phase at a triple point at 29 kbar, 451°C. A new high-pressure phase CsHF2 IV appears above 2.7 kbar at 20°C. This phase may be similar to CsN3 III. The phase diagram of CsHF2 is closely similar to that of CsN3.

Defect energies in CsCl and its solid solutions with KCl, RbCl, and CsBr

The formation energies of Schottky defects in the CsCl (Pm3m) and the NaCl (Fm3m) structures of CsCl are found to be 1·4 and 2·0 eV respectively. The ground-state interaction energies between Sr2+Cs+ and the V–Cs+ in the Pm3m and Fm3m phases are ca. 0·45 and 0·35 eV respectively. The cation migration energies in CsCl are affected by the incorporation of K+, Rb+ or Br–. The formation energy of Schottky defects in the solid solutions of CsCl with KCl or RbCl is low (1·4 eV) in the Pm3m phase, but attains a much higher value (2 eV) at compositions where the Fm3m structure gets stabilized.