Alka B. Garg

High-pressure structural behaviour of HoVO4: combined XRD experiments and ab initio calculations

We report a high-pressure experimental and theoretical investigation of the structural properties of zircon-type HoVO4. Angle-dispersive x-ray diffraction measurements were carried out under quasi-hydrostatic and partial non-hydrostatic conditions up to 28 and 23.7 GPa, respectively. In the first case, an irreversible phase transition is found at 8.2 GPa. In the second case, the onset of the transition is detected at 4.5 GPa, a second (reversible) transition is found at 20.4 GPa, and a partial decomposition of HoVO4 was observed. The structures of the different phases have been assigned and their equations of state (EOS) determined. Experimental results have also been compared to theoretical calculations which fully agree with quasi-hydrostatic experiments. Theory also suggests the possibility of another phase transition at 32 GPa; i.e. beyond the pressure limit covered by present experiments. Furthermore, calculations show that deviatoric stresses could trigger the transition found at 20.4 GPa under non-hydrostatic conditions. The reliability of the present experimental and theoretical results is supported by the consistency between the values yielded for transition pressures and EOS parameters by the two methods.

Investigation of the phase stability of LuVO4 at high pressure using powder x-ray diffraction measurements and lattice dynamical calculations

High pressure angle dispersive x-ray diffraction measurements are carried out on LuVO4 in a diamond anvil cell up to 33 GPa at the Elettra synchrotron radiation source. The measurements show that LuVO4 undergoes a zircon to scheelite structure phase transition with a volume change of about 11% at about 8 GPa. A second transition to a monoclinic fergusonite structure occurs above 16 GPa. The data are also recorded while releasing the pressure, and indicate that the scheelite phase is metastable under ambient conditions. The equations of state and changes in internal structural parameters are reported for various phases of LuVO4. Lattice dynamical calculations based on a transferable interatomic potential were also performed and the results support the stability of the scheelite structure at high pressures. The calculated structure, equation of state and bulk modulus for all the phases are in fair agreement with the experimental observations.

Phase stability of YbVO4 under pressure: In situ x-ray and Raman spectroscopic investigations

The phase stability of YbVO4 under pressure has been investigated using synchrotron based angle dispersive x-ray diffraction and Raman spectroscopic techniques up to 34.5 and 26.5 GPa, respectively. The results indicate that the compound transforms from the ambient pressure zircon structure to the scheelite structure above 5.9 GPa with 11.8% volume discontinuity. The coexistence of the two phases is observed over a large pressure range. At 15.8 GPa, the (011) peak of the scheelite phase develops asymmetry, and the pattern at further high pressures could be fitted to a fergusonite-type monoclinic structure. On reducing the pressure, the fergusonite phase reverses back to the scheelite phase; the latter phase could be recovered as a metastable phase at ambient pressure. The refined structural parameters along with the equation of state are given for various phases of YbVO4. Changes in the vibrational properties across these transitions, particularly across the scheelite↔fergusonite transition, have been inv...

Electrical resistance measurements in a diamond anvil cell to 40 GPa on ytterbium

An easily assembled setup employing diamond anvil cell, stainless steel gasket and leads, and mylar embedded Al2O3 (alumina) pressure medium for the measurement of electrical resistance of materials under pressure is described. The use of a mylar sheet prevents the alumina layer from sticking to the anvil in the precompacting stage of Al2O3 and also reduces the pressure gradients in the final assembly. The technique is used to reproduce the known transition in Si, Ge, and Fe. The results of measurements of electrical resistance of ytterbium up to 40 GPa are reported. In the hcp phase of ytterbium the electrical resistance increases with pressure. Efforts are made to explain the variation of resistance with pressure from known band structure calculations.

Electronic topological transition in AuX2 (X = In, Ga and Al) compounds at high pressures

We present accurate x-ray diffraction data at high pressures for AuIn2,AuGa2 and AuAl2, obtained using a diamond anvil cell with the ELETTRA synchrotron source. The resulting P–V data obtained from the d-values were used to get the universal equation of state (UEOS), which is compared with theoretical estimates. Deviation from linearity is evident in the UEOS curves of AuIn2 and AuGa2, thus verifying that some of the observed anomalies in these systems below 5 GPa are due to electronic topological transitions.

Multiferroic CuCrO2 under high pressure: In situ X-ray diffraction and Raman spectroscopic studies

The compression behavior of delafossite compound CuCrO2 has been investigated by in situ x-ray diffraction (XRD) and Raman spectroscopic measurements up to 23.2 and 34 GPa, respectively. X-ray diffraction data show the stability of ambient rhombohedral structure up to ∼23 GPa. Material shows large anisotropy in axial compression with c-axis compressibility, κc = 1.26 × 10−3(1) GPa−1 and a-axis compressibility, κa = 8.90 × 10−3(6) GPa−1. Our XRD data show an irreversible broadening of diffraction peaks. Pressure volume data when fitted to 3rd order Birch-Murnaghan equation of state give the value of bulk modulus, B0 = 156.7(2.8) GPa with its pressure derivative, B0′ as 5.3(0.5). All the observed vibrational modes in Raman measurements show hardening with pressure. Appearance of a new mode at ∼24 GPa indicates the structural phase transition in the compound. Our XRD and Raman results indicate that CuCrO2 may be transforming to an ordered rocksalt type structure under compression.

High-pressure resistance and equation-of-state anomalies in Zn: a possible Lifshitz transition

Experimental results on electrical resistance variation with pressure up to 25 GPa are presented for zinc; they reveal the signature of an electronic topological transition. Theoretical estimates based on the results of first-principles band structure calculations corroborate the observed variation of the resistance. The Lifshitz transition, which is hardly observable in experimental P-V data, becomes discernible via the equation of state, in the universal as well as in Holzapfels form. Another anomaly in the measured resistance is also observed around 20 GPa pressure. The resistance variation with pressure exhibits considerable hysteresis.

Pressure-induced phase transition and band-gap collapse in the wide-band-gap semiconductor InTaO4

This paper was partially supported by the Spanish Ministerio de Economia y Competitividad (MINECO) under Grants No. MAT2013-46649-C04-01/02/03 and No. MAT2015-71070-REDC (MALTA Consolider). The XRD experiments were performed at the MSPD-BL04 beamline at ALBA Synchrotron with the collaboration of ALBA staff. We thank S. Agouram from SC-SIE at Universitat de Valencia for technical support with the transmission electron microscope measurements.

Pressure-induced phase transformation in zircon-type orthovanadate SmVO4 from experiment and theory.

The compression behavior of zircon-type samarium orthovanadate, SmVO4, has been investigated using synchrotron-based powder x-ray diffraction and ab initio calculations of up to 21 GPa. The results indicate the instability of ambient zircon phase at around 6 GPa, which transforms to a high-density scheelite-type phase. The high-pressure phase remains stable up to 21 GPa, the highest pressure reached in the present investigations. On pressure release, the scheelite phase is recovered. The crystal structure of the high-pressure phase and the equations of state for the zircon- and scheelite-type phases have been determined. Various compressibilities, such as the bulk, axial and bond compressibilities, estimated from the experimental data are found to be in good agreement with the results obtained from theoretical calculations. The calculated elastic constants show that the zircon structure becomes mechanically unstable beyond the transition pressure. Overall there is good agreement between the experimental and theoretical findings.

Phase stability of Heusler compound Co2FeSi under pressure: An in-situ x-ray diffraction investigation

Results of ambient temperature synchrotron based in-situ x-ray powder diffraction measurements up to 24.8 GPa on the structural stability of the ferromagnetic shape memory Heusler alloy Co2FeSi is reported. The compound is structurally stable up to the highest pressure of the present investigations; however, the pressure (P) vs. volume (V) data shows an anomalous linearity beyond 4.7 GPa. The P-V data up to 4.7 GPa, when fitted to Birch-Murnaghan equation of states, gives the value of bulk modulus (B) as 240 GPa (B′ = 4). The P-V data beyond 4.7 GPa can be fitted to a straight line implying a constant bulk modulus (B = 279.5 GPa, B′ = 0.0) as seen in several metallic samples with highly correlated electrons.