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Register a new userPressure-Induced Phase Transformation in $beta$-Eucryptite: an X-Ray Diffraction and Density Functional Theory Study
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Certain alumino-silicates display exotic properties enabled by their framework structure made of corner-sharing tetrahedral rigid units. Using textit{in situ} diamond-anvil cell x-ray diffraction (XRD), we study the pressure-induced transformation of $beta$ eucryptite, a prototypical alumino-silicate. $beta$ eucryptite undergoes a phase transformation at moderate pressures, but the atomic structure of the new phase has not yet been reported. Based on density functional theory stability studies and Rietveld analysis of XRD patterns, we find that the pressure-stabilized phase belongs to the Pna2$_1$ space group. Furthermore, we discover two other possible pressure-stabilized polymorphs, P1c1 and Pca2$_1$.
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SrMoO4 was studied under compression up to 25 GPa by angle-dispersive x-ray diffraction. A phase transition was observed from the scheelite-structured ambient phase to a monoclinic fergusonite phase at 12.2(9) GPa with cell parameters a = 5.265(9) A, b = 11.191(9) A, c = 5.195 (5) A, and beta = 90.9, Z = 4 at 13.1 GPa. There is no significant volume collapse at the phase transition. No additional phase transitions were observed and on release of pressure the initial phase is recovered, implying that the observed structural modifications are reversible. The reported transition appeared to be a ferroelastic second-order transformation producing a structure that is a monoclinic distortion of the low-pressure phase and was previously observed in compounds isostructural to SrMoO4. A possible mechanism for the transition is proposed and its character is discussed in terms of the present data and the Landau theory. Finally, the EOS is reported and the anisotropic compressibility of the studied crystal is discussed in terms of the compression of the Sr-O and Mo-O bonds.
There is considerable controversy about swift heavy ion (SHI) irradiation induced displacive phase transitions in thermally insulating oxides. We present here unambiguous evidence for tetragonal to monoclinic and rhombohedral to monoclinic phase transitions in BaTiO3 under swift heavy ion irradiation (120MeV 108Ag+9 ions) using in-situ x-ray powder diffraction (XRPD) studies. The anomalous splitting/broadening of 111/222pc, 200pc and 220pc pseudocubic peaks for fluences greater than 3*1012 ions/cm2 reveal structural changes before amorphization at higher fluences. Lebail analysis of XRPD profiles confirm that the monoclinic phase is of MA type in the Cm space group. Shear stress for the structural phase transition is estimated to be ~ 430MPa, which we believe is generated as a result of stopping of the SHI.
We report the evolution of charge density wave states under pressure for two NbS3 phases triclinic (phase I) and monoclinic (phase II) at room temperature. Raman and X-ray diffraction (XRD) techniques are applied. The x-ray studies on the monoclinic phase under pressure show a compression of the lattice at different rates below and above 7 GPa but without a change in space group symmetry. The Raman spectra of the two phases evolve similarly with pressure; all peaks almost disappear in the 6-8 GPa range, indicating a transition from an insulating to a metallic state, and peaks at new positions appear above 8 GPa. The results suggest suppression of the ambient charge-density waves and their subsequent recovery with new orderings above 8 GPa.
The effect of lithium vacancies in the hexagonal structure of $alpha-$Li$_3$N, is studied within the framework of density functional theory. Vacancies ($square$) substituting for lithium in $alpha-$Li$_2$(Li$_{1-x}square_x$)N are treated within the coherent potential approximation as alloy components. According to our results long range N($p$)-ferromagnetism ($sim 1$ $mu_B$) sets in for vacancy substitution within the [Li$_2$N] layers ($x ge 0.7$) with no significant change in unit cell dimensions. By total energies differences we established that in-plane exchange couplings are dominant. Vacancies substituting inter-plane Li, leads to a considerable structural collapse ($c/a approx 0.7$) and no magnetic moment formation.
Cubic boron phosphide BP has been studied in situ by X-ray diffraction and Raman scattering up to 55 GPa at 300 K in a diamond anvil cell. The bulk modulus of B0 = 174(2) GPa has been established, which is in excellent agreement with our ab initio calculations. The data on Raman shift as a function of pressure, combined with equation-of-state data, allowed us to estimate the Gruneisen parameters of the TO and LO modes of zinc-blende structure, {gamma}GTO = 1.16 and {gamma}GLO = 1.04, just like in the case of other AIIIBV diamond-like phases, for which {gamma}GTO > {gamma}GLO = 1. We also established that the pressure dependence of the effective electro-optical constant {alpha} is responsible for a strong change in relative intensities of the TO and LO modes from ITO/ILO ~0.25 at 0.1 MPa to ITO/ILO ~2.5 at 45 GPa, for which we also find excellent agreement between experiment and theory.
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