Crystal structure of the high-pressure phase of the oxonitridosilicate chloride Ce4[Si4O3 + xN7 − x]Cl1 − xOx, x≃0.2

The structural compression mechanism of Ce4[Si4O3 + xN7 − x]Cl1 − xOx, x≃ 0.2, was investigated by in situ single-crystal synchrotron X-ray diffraction at pressures of 3.0, 8.5 and 8.6 GPa using the diamond–anvil cell technique. On increasing pressure the low-pressure cubic structure first undergoes only minor structural changes. Between 8.5 and 8.6 GPa a first-order phase transition occurs, accompanied by a change of the single-crystal colour from light orange to dark red. The main structural mechanisms, leading to a volume reduction of about 5% at the phase transition, are an increase in and a rearrangement of the Ce coordination, the loss of the Ce2, Ce3 split position, and a bending of some of the inter-polyhedral Si—N—Si angles in the arrangement of the corner-sharing Si tetrahedra. The latter is responsible for the short c axis of the orthorhombic high-pressure structure compared with the cell parameter of the cubic low-pressure structure

Elastic properties of single crystal Bi12SiO20 as a function of pressure and temperature and acoustic attenuation effects in Bi12 MO20 (M = Si, Ge and Ti)

A comprehensive study of sillenite Bi12SiO20 single-crystal properties, including elastic stiffness and piezoelectric coefficients, dielectric permittivity, thermal expansion and molar heat capacity, is presented. Brillouin-interferometry measurements (up to 27 GPa), which were performed at high pressures for the first time, and ab initio calculations based on density functional theory (up to 50 GPa) show the stability of the sillenite structure in the investigated pressure range, in agreement with previous studies. Elastic stiffness coefficients c 11 and c 12 are found to increase continuously with pressure while c 44 increases slightly for lower pressures and remains nearly constant above 15 GPa. Heat-capacity measurements were performed with a quasi-adiabatic calorimeter employing the relaxation method between 2 K and 395 K. No phase transition could be observed in this temperature interval. Standard molar entropy, enthalpy change and Debye temperature are extracted from the data. The results are found to be roughly half of the previous values reported in the literature. The discrepancy is attributed to the overestimation of the Debye temperature which was extracted from high-temperature data. Additionally, Debye temperatures obtained from mean sound velocities derived by Voigt-Reuss averaging are in agreement with our heat-capacity results. Finally, a complete set of electromechanical coefficients was deduced from the application of resonant ultrasound spectroscopy between 103 K and 733 K. No discontinuities in the temperature dependence of the coefficients are observed. High-temperature (up to 1100 K) resonant ultrasound spectra recorded for Bi12 MO20 crystals revealed strong and reversible acoustic dissipation effects at 870 K, 960 K and 550 K for M = Si, Ge and Ti, respectively. Resonances with small contributions from the elastic shear stiffness c 44 and the piezoelectric stress coefficient e 123 are almost unaffected by this dissipation. © 2020 The Author(s). Published by IOP Publishing Ltd

Single-crystal elastic and thermodynamic properties of gamma-LiAlO2

ABSTRACT: The elastic properties of γ-LiAlO2 were reinvestigated with the aid of resonant ultrasound spectroscopy (RUS) at ambient conditions. A strong discrepancy of the elastic coefficients derived by RUS can be found from the experimental results from literature, where c12 and c13 deviate from our results by about 15 % (24 GPa) and 60 % (42 GPa), respectively. In contrast to the experimental cij from literature we can recognize a good agreement between the elastic coefficients derived from RUS and the values using density functional theory (DFT). The dielectric permittivity was measured on large plane-parellel plates and the piezoelectric stress coefficient e123 = 0.14 C m−2 was derived from RUS measurements at ambient conditions. The heat capacity between 4 K – 398 K has been obtained by microcalorimetry using a relaxation calorimeter. The Debye temperature was derived from heat capacity measurements (⊝ Cp = 676 K) and from RUS measurements (⊝ elastic = 688 K).The authors are grateful to J. Schreuer (Ruhr-Universit¨at Bochum, Germany) for providing the computer program RUSREF used in the refinement of resonant ultrasound spectra. The authors gratefully acknowledge the Leibniz-institute for crystal growth (IKZ, Berlin, Germany) for the provision of a single crystal of γ-LiAlO2 and the DFG for financial support of this investigation (HA 5137/3 and HA 5137/5). J.R.-F. acknowledges the MCINN (PGC2018-097520-A-100)

In situ Resonant Ultrasound Spectroscopy during irradiation of solids with relativistic heavy ions

Samples of steel, fused silica, CaF2, NaCl, and a (La,Eu)PO4 monazite ceramic were irradiated with Bi-209 (130 GeV) and U-238 (60 GeV) ions up to total fluences of 6 x 10(11) cm(-2). During beam exposure, resonant ultrasound spectra were recorded. No radiation-induced changes in the density and elastic stiffness coefficients were observed when comparing samples before and after irradiation. The irradiation caused fully reversible shifts of the resonance frequencies in all samples except NaCl, silica and monazite irradiated with U ions. These reversible shifts are due to a temperature increase during irradiation. The heating process was modelled quantitatively by an energy balance model. The average thickness traversed by the ions was estimated from geometrical considerations and the energy deposition was calculated with the SRIM software. The results from the model and these calculations are in good agreement. For NaCl, silica and monazite, a degradation of the samples was observed. Hence, in situ Resonant Ultrasound Spectroscopy can be used to monitor sample integrity and temperatures in harsh radiation environments. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Compression Behavior of Sm2Ti2O7Sm_{2}Ti_{2}O_{7}-Pyrochlore up to 50 GPa: Single-Crystal X-ray Diffraction and Density Functional Theory Calculations

Single-crystal X-ray diffraction at pressures up to 50 GPa has been employed to study the compression behavior of Sm2Ti2O7-pyrochlore. In contrast to earlier reports, we observed no pressure-induced amorphization or pressure-induced anion disorder up to 50 GPa. The experimental study has been complemented by density functional theory-based calculations. A combination of the theoretical and experimental data yields a bulk modulus of about 185 GPa, significantly higher than a value which had been reported earlier. In comparison to earlier work, the current study provides more reliable data due to the use of neon as a pressure medium, which provides a morehydrostatic pressure than the aluminum, which had been employed as a pressure medium in the earlier studies. An analysis of the compressibility of Al2B2O7 pyrochlores shows an approximately linear dependence of the bulk modulus on the unit cell volume

Experimental and theoretical study of the vibrational properties of diaspore (α-AlOOH)

Vibrational properties of diaspore, α-AlOOH, have been re-investigated using room-temperature single-crystal Raman spectroscopy and low-temperature powder infrared (IR) transmission spectroscopy. First-principles harmonic calculations based on density functional theory provide a convincing assignment of the major Raman peaks and infrared absorption bands. The large width of the Raman band related to OH stretching modes is ascribed to mode–mode anharmonic coupling due to medium-strength H-bonding. Additional broadening in the powder IR spectrum arises from depolarization effects in powder particles. The temperature dependence of the IR spectrum provides a further insight into the anharmonic properties of diaspore. Based on their frequency and temperature behavior, narrow absorption features at ~2,000 cm−1 and anti-resonance at ~2,966 cm−1 in the IR spectrum are interpreted as overtones of fundamental bending bands

Elastic stiffness coefficients of thiourea from thermal diffuse scattering

The complete elastic stiffness tensor of thiourea has been determined from thermal diffuse scattering (TDS) using high‐energy photons (100 keV). Comparison with earlier data confirms a very good agreement of the tensor coefficients. In contrast with established methods to obtain elastic stiffness coefficients (e.g. Brillouin spectroscopy, inelastic X‐ray or neutron scattering, ultrasound spectroscopy), their determination from TDS is faster, does not require large samples or intricate sample preparation, and is applicable to opaque crystals. Using high‐energy photons extends the applicability of the TDS‐based approach to organic compounds which would suffer from radiation damage at lower photon energies.The elastic stiffness coefficients of thiourea are determined from thermal diffuse scattering. imag