CaCl2 Structure Research Articles

Structural, Magnetic, and Electronic Properties of the Cr1-xTixO2 Solid.

Cr1-xTixO2 (0 ≤ x ≤ 1) solid solution was synthesized by a high-pressure and high-temperature method, whereafter systematic experimental and computational studies were conducted on the Cr1-xTixO2 system. The crystal structure of the samples where 0 ≤ x ≤ 0.4 and x = 1 was of a rutile structure (P42/mnm), while samples where 0.5 ≤ x ≤ 0.9 crystallized in a CaCl2 structure (Pnnm). The structural transformation from rutile-type to CaCl2-type structure should be due to the combined action of positive chemical pressure and physical pressure. The saturation magnetization of the Cr1-xTixO2 samples decreased linearly with the increase of x because Ti4+ is nonmagnetic. In addition, the Curie temperature of the Cr1-xTixO2 samples also decreased noticeably with the increase of x. When nonmagnetic Ti4+ randomly replaced Cr4+ and occupied its position, the net exchange coupling in Cr1-xTixO2 would decrease. When Ti4+ occupied the majority in the system, Cr4+ ions would be separated by nonmagnetic Ti4+ ions far enough for the long-range ferromagnetic order to weaken or even disappear with the increase of x, causing the Cr1-xTixO2 system to finally approach a paramagnetic state. Density functional theory calculations were performed for the Cr1-xTixO2 system, and the predicted trends of the magnetic properties agreed well with the experimental results. These calculations also showed that Cr1-xTixO2 was still half-metallic until x reached 0.6.

Phase transition of GeO2 crystal at high pressure: An ab initio molecular dynamics study

The phase transition of GeO2 structure was studied under constant pressure by using ab initio calculation and the generalized gradient approximation (GGA). A structural phase transition was observed at 30 GPa for the first time under the constant hydrostatic pressure, and P42/mnm structure was transformated into a CaCl2 (Pnnm) structure. This phase transition was also investigated using total energy, enthalpy, and the phase shift calculations. It was determined that the obtained transformation parameters and bulk modulus properties were compatible with the previous experimental and theoretical first principle calculations.

Pressure-induced phase transition in CrO2.

The ab initio constant pressure molecular dynamics technique and density functional theory with generalized gradient approximation (GGA) was used to study the pressure-induced phase transition of CrO2. The phase transition of the rutile (P42/mnm) to the orthorhombic CaCl2 (Pnnm) structure at 30 GPa was determined successfully in a constant pressure simulation. This phase transition was analyzed from total energy calculations and, from the enthalpy calculation, occurred at around 17 GPa. Structural properties such as bulk modules, lattice parameters and phase transition were compared with experimental results. The phase transition at 12 ± 3 GPa was in good agreement with experimental results, as was the phase transition from the orthorhombic CaCl2 (Pnnm) to the monoclinic (P21/c) structure also found at 35 GPa.

Atomistic modeling of the low-temperature atom-beam deposition of magnesium fluoride.

We model the deposition and growth of MgF(2) on a sapphire substrate as it occurs in a low-temperature atom-beam-deposition experiment. In the experiment, an (X-ray) amorphous film of MgF(2) is obtained at low temperatures of 170-180 K, and upon heating, this transforms to the expected rutile phase via the CaCl(2)-type structure. We confirm this from our simulations and propose a mechanism for this transformation. The growth process is analyzed as a function of the synthesis parameters, which include the substrate temperature, deposition rate of clusters, and types of clusters deposited. Upon annealing an initially amorphous deposit, we observe the formation of two competing nanocrystalline modifications during this process, which exhibit the CaCl(2) and CdI(2) structure types, respectively. We argue that this joint growth of the two nanocrystalline polymorphs stabilizes the kinetically unstable CaCl(2)-type structure on the macroscopic level long enough to be observed in the experiment.

Comparative studies of mechanical properties of stishovite and sapphire single crystals by nanoindentation

Comparative nanoindentation tests of stishovite and sapphire single crystals have been performed. It has been found that nanohardness of the (0001) plane of sapphire at a depth of 200 nm is 29.1 ± 0.1 GPa, while the nanohardness of the (110) plane of stishovite is 38.1 ± 0.6 GPa, the tests conditions being the same. The specifics of elastoplastic transition in sapphire and stishovite single crystals and alumina-based polycrystalline ceramic when plastic deformation is localized in a submicron region have been studied. Anomalous elastoplastic transition in stishovite has been revealed. A possibility that anomalous mechanical behavior of stishovite at the nanoscale may be due to a strong softening of the lattice as a precursor of the formation of a post-stishovite phase of silica with the CaCl2 structure in an indent has been discussed.

Mid‐mantle layering from SKS receiver functions

Many seismic data on mid‐mantle discontinuities are related only to subduction zones. Here we use a method that is applicable outside subduction zones. The idea is to use SKS‐to‐P converted phases that are generated in the receiver regions and can be detected in S receiver functions. In two regions (southern Africa and Western Europe) we have detected apparently the same ‘1200‐km’ discontinuity, but at a depth that varies from 1170 km (Africa) to 1240–1270 km (Europe). The S velocity contrast at this discontinuity is ∼0.1 km/s, i.e., half of the value in subduction zones. The ‘1200‐km’ discontinuity may correspond to the phase transition of SiO2 from stishovite to CaCl2 structure. The small S velocity contrast is compatible with scenarios in which small remnants of old subducted oceanic lithosphere, thermally equilibrated and partly reacted, are finely dispersed in a bulk mantle matrix.

Elastic and magnetic effects on the infrared phonon spectra ofMnF2

We measured the temperature dependent infrared reflectivity spectra of MnF2 between 4 K and room temperature. We show that the phonon spectrum undergoes a strong renormalization at TN. The ab-initio calculation we performed on this compound accurately predict the magnitude and the direction of the phonon parameters changes across the antiferromagnetic transition, showing that they are mainly induced by the magnetic order. In this material, we found that the dielectric constant is mostly from phonon origin. The large change in the lattice parameters with temperature seen by X-ray diffraction as well as the A2u phonon softening below TN indicate that magnetic order induced distortions in MnF2 are compatible with the ferroelectric instabilities observed in TiO2, FeF2 and other rutile-type fluorides. This study also shows the anomalous temperature evolution of the lower energy Eu mode in the paramagnetic phase, which can be compared to that of the B1g one seen by Raman spectroscopy in many isostructural materials. This was interpreted as being a precursor of a phase transition from rutile to CaCl2 structure which was observed under pressure in ZnF2.

The thermodynamic stability of three near-degenerate phases of platinum dioxide

ABSTRACTThe thermodynamic stability of the three nearly energy degenerate crystal structures of PtO2 is studied here with first-principles-based calculations of their free energies. For P = 0 the α-(CdI2) structure is the thermodynamically stable phase at low temperature, while the β-(CaCl2) structure is stable at high pressure. The β'-(rutile) structure represents an unstable fixed point on the potential energy surface, or is possibly just barely bound. These results reconcile seemingly contradictory findings and answer longstanding questions about PtO2.

First-principles studies of pressure-induced phase transitions in SiO2

Structural stability is investigated for the CaCl2 structure of SiO2 by using a first-principleslinear response theory. Softening of modes is found in the direction of the Γ–X (100) axisin k-spaceat 810 GPa. The results of the present work strongly support the assertion that theP21/nstructure is very reasonable suggestion for the post-CaCl2 structure.

Dynamic compression of SiO2: A new interpretation

In light of recent discoveries of post‐stishovite phases of SiO2, with the CaCl2 and α‐PbO2 structures, we have reassigned the regimes along the Hugoniots for initial quartz, coesite, cristobalite, porous coesite, and fused silica. Calculated Hugoniots for fused silica, cristobalite and porous coesite indicate transition to stishovite, and then melt. Hugoniots for crystal quartz and coesite indicate that transition occurs to stishovite, then the CaCl2 structure and finally to melt.

High‐pressure elastic properties of major materials of Earth's mantle from first principles

The elasticity of materials is important for our understanding of processes ranging from brittle failure, to flexure, to the propagation of elastic waves. Seismologically revealed structure of the Earth's mantle, including the radial (one‐dimensional) profile, lateral heterogeneity, and anisotropy are determined largely by the elasticity of the materials that make up this region. Despite its importance to geophysics, our knowledge of the elasticity of potentially relevant mineral phases at conditions typical of the Earth's mantle is still limited: Measuring the elastic constants at elevated pressure‐temperature conditions in the laboratory remains a major challenge. Over the past several years, another approach has been developed based on first‐principles quantum mechanical theory. First‐principles calculations provide the ideal complement to the laboratory approach because they require no input from experiment; that is, there are no free parameters in the theory. Such calculations have true predictive power and can supply critical information including that which is difficult to measure experimentally. A review of high‐pressure theoretical studies of major mantle phases shows a wide diversity of elastic behavior among important tetrahedrally and octahedrally coordinated Mg and Ca silicates and Mg, Ca, Al, and Si oxides. This is particularly apparent in the acoustic anisotropy, which is essential for understanding the relationship between seismically observed anisotropy and mantle flow. The acoustic anisotropy of the phases studied varies from zero to more than 50% and is found to depend on pressure strongly, and in some cases nonmonotonically. For example, the anisotropy in MgO decreases with pressure up to 15 GPa before increasing upon further compression, reaching 50% at a pressure of 130 GPa. Compression also has a strong effect on the elasticity through pressure‐induced phase transitions in several systems. For example, the transition from stishovite to CaCl2 structure in silica is accompanied by a discontinuous change in the shear (S) wave velocity that is so large (60%) that it may be observable seismologically. Unifying patterns emerge as well: Eulerian finite strain theory is found to provide a good description of the pressure dependence of the elastic constants for most phases. This is in contrast to an evaluation of Birch’s law, which shows that this systematic accounts only roughly for the effect of pressure, composition, and structure on the longitudinal (P) wave velocity. The growing body of theoretical work now allows a detailed comparison with seismological observations. The athermal elastic wave velocities of most important mantle phases are found to be higher than the seismic wave velocities of the mantle by amounts that are consistent with the anticipated effects of temperature and iron content on the P and S wave velocities of the phases studied. An examination of future directions focuses on strategies for extending first‐principles studies to more challenging but geophysically relevant situations such as solid solutions, high‐temperature conditions, and mineral composites.

Ab initio elasticity of three high‐pressure polymorphs of silica

Full elastic constant tensors of three high‐pressure polymorphs of silica: stishovite, CaCl2‐type and columbite‐type (α‐PbO2 structure); are determined at lower mantle pressures from first‐principles using the plane wave pseudopotential method within the local density approximation. The calculated zero pressure athermal elastic moduli are within a few percent of the experiments. We find that the elastic properties of silica are strongly pressure dependent. The shear wave velocity decreases rapidly (by 60%) and the anisotropy increases rapidly (by a factor of five) between 40 and 47 GPa prior to the transition from stishovite to the CaCl2 structure at 47 GPa. At this phase transition, the isotropically averaged shear wave velocity changes discontinuously by 60%, while the S‐wave polarization anisotropy decreases by a factor of two. The transformation of the CaCl2 phase to the columbite phase at 98 GPa is accompanied by a discontinuous change of 1–2% in elastic wave velocity and decrease by a factor of two in anisotropy. We suggest that even a small amount of silica in the lower mantle may contribute significantly to observed seismic anisotropy, and may provide an explanation of observed seismic reflectivity near 1000 km.

SiO2stishovite under high pressure: Dielectric and dynamical properties and the ferroelastic phase transition

The pressure dependence of the dielectric and dynamical properties of SiO2 stishovite and its pressure induced ferroelastic phase transition to the CaCl2 structure are investigated using density-functional theory. The pressure dependence of the dielectric permittivity tensors and the Born effective charges shows that the local atomic environments in stishovite, more compressible along the a axis than along the c axis, become less anisotropic as pressure increases. The phonon frequencies at the Gamma point increase with increasing pressure, except the B-1g mode. The phonon band structure at 73 GPa, the study of elastic constants as a function of pressure, and the structural relaxation at various pressure provide evidence for spontaneous elastic deformation, i.e., a ferroelastic phase transition at 64 GPa of stishovite in the tetragonal rutile structure to the orthorhombic CaCl2 structure. The on-site and interatomic force constants are found to be consistent with the predicted structural phase transition. The dielectric properties and the phonon frequencies at the Gamma point are also computed in the CaCl2 structure.

The pressure-induced ferroelastic phase transition of SiO2 stishovite

Using a newly developed accurate ab initio technique, we study the stability and the lattice dynamics of SiO2 polymorph stishovite as a function of pressure. We observe a ferroelastic phase transition to CaCl2 structure at 64 GPa driven by a mechanical instability associated with a vanishing shear modulus. Other lattice instabilities due to phonon anomalies are theoretically excluded along with a proposed softening of the B1g phonon mode.

Transformation of stishovite to a denser phase at lower-mantle pressures

WHETHER stishovite, the highest-pressure polymorph of SiO2 known from natural samples, transforms to a denser structure at higher pressures has long been of interest. A suggested transition from rutile to the CaCl2 structure driven by a vibrational-mode instability1 was supported by the observation of a frequency decrease (softening) of a Raman mode with increasing pressure2. Subsequent X-ray diffraction measurements provided evidence3 for stability of the CaCl2 phase near 100 GPa. Electronic-structure calculations predict, however, that the transition occurs at much lower pressure, where a shear modulus vanishes and before the Raman mode softens completely4. Here we use in situ Raman spectroscopy and a new theoretical model to investigate the high-pressure behaviour of stishovite. At 50 GPa, the pressure dependence of the soft B1g mode abruptly changes and the Eg mode splits, as predicted for transformation to the CaCl2 structure. Our results demonstrate that any free silica in the, deep mantle (below 1,200-1,500 km) will exist in the CaCl2 structure at considerably lower pressures than previously thought3.

Calculations of pressure‐induced phase transitions in silica

The crystal structures and energies of the quartz, stishovite and CaCl2 phases of silica were calculated as a function of pressure with the polarization‐included electron gas (PEG) model. The calculated structures are in good agreement with experiment throughout the pressure range in which experimental results exist. The calculated pressure of the quartz to stishovite phase transition is about 3.5 GPa, as compared to an experimental value of 5.5 GPa. The calculated pressure of the stishovite to CaCl2 structure phase transition is slightly less than 200 GPa, higher than the value suggested by experiments of about 100 GPa. The compressibility of quartz, the quartz to stishovite transition pressure, and the stishovite to CaCl2 structure transition pressure are all calculated more accurately with the PEG model, which allows the ions to distort from spherical symmetry, than with the modified electron gas model, which constrains the ions to be spherical.

A new, post-stishovite highpressure polymorph of silica

THE possibility of ‘post-stishovite’ phases of silica (SiO2) has been the subject of extensive study during the past two decades, not only because of the intrinsic crystallographic interest but also because of its relevance to the geochemistry of the lower mantle. Here in situ X-ray diffraction observations of SiO2 compressed to pressures of up to 124 GPa using a laser-heated diamond-anvil cell. Stishovite was formed by heating α-quartz or amorphous silica at pressures of up to 92 GPa; on heating at 108 and 124 GPa, however, the materials crystallized into a CaCl2-type structure, which results from a distortion of the Stishovite structure and is slightly denser. The quenched and recovered sample was always Stishovite; thus the transformation from Stishovite to the CaCl2 structure is reversible on release of pressure.

New pressure-induced structural transformations in silica obtained by computer simulation

TO reproduce and predict crystal structures from first principles has been a longstanding problem in solid-state physics. We have recently shown1 that a first-principles many-body calculation for clusters can be used to extract effective pairwise interatomic potentials, which were then used in a molecular dynamics study of the stability of crystalline silica (SiO2). Here we use this approach in a molecular dynamics study of pressure-induced structural transitions at room temperature for various polymorphs of silica. We predict new structural transitions for low-quartz, low-cristobalite and coesite, in which some of the new phases, appearing without the occurrence of diffusion, comprise mixed arrays of fourfold and sixfold Si–O coordinations. Stishovite, the densest known polymorph of silica, persists up to 250 GPa, with deformation to the CaCl2 structure. Although a recent theoretical calculation predicts a possible polymorph denser than stishovite, which may be of importance in the Earth's interior2, this phase is not obtained by (simulated) compression of stishovite at room temperature, presumably because of the potential barrier between the two structures.