Abstract We investigated the structure, equation of state, thermodynamics, and elastic properties of tremolite amphibole [Ca2Mg5Si8O22(OH)2] up to 10 GPa and 2000 K, using first principles simulations based on density functional perturbation theory. We found that at 300 K, the pressurevolume results can be adequately described by a thirdorder BirchMurnaghan equation of state with bulk moduli K0 of 78.5 and 66.3 GPa based on local density approximation (LDA) and generalized gradient approximation (GGA), respectively. We also derived its coefficients of the elastic tensor based on LDA and GGA and found that the LDA result is in good agreement with the experimental results. At 300 K, the shear modulus G0 is 58.0 GPa based on LDA. The pressure derivative of the bulk modulus K′ is 5.9, while that of the shear modulus G′ is 1.3. The second Grüneisen parameter, or δT = [–1/(αKT)](∂KT/∂T)P, is 3.3 based on LDA. We found that at ambient conditions, tremolite is elastically anisotropic with the compressional wave velocity anisotropy AVP being 34.6% and the shear wave velocity anisotropy AVS being 27.5%. At higher pressure corresponding to the thermodynamic stability of tremolite, i.e., ~3 GPa, the AVP reduces to 29.5%, whereas AVS increases to 30.8%. To evaluatemore »
Experimental and theoretical PVT equation of state for Os _{2} B _{3}
Thermoelastic behavior of transition metal boride Os2B3 was studied under quasihydrostatic and isothermal conditions in a ParisEdinburgh cell to 5.4 GPa and 1273 K. Insitu Energy Dispersive Xray diffraction was used to determine interplanar spacings of the hexagonal crystal structure and thus the volume and axial compression. PVT data were fitted to a 3rd Order BirchMurnaghan equation of state with a temperature modification to determine thermal elastic constants. The bulk modulus was shown to be K0 = 402 ± 21 GPa when the first pressure derivative was held to K0’ = 4.0 from the room temperature PV curve. Under a quadratic fit α=α_0+α_1 Tα_2 T^(2), the thermal expansion coefficients were determined to be α_0=1.862×10^(5) K1, α_1=0.841×10^(9) K2, and α_2=0.525 K. Density functional theory (DFT) with the quasiharmonic approximation (QHA) were further employed to study Os2B3, including its PVT curves, phonon spectra, bulk modulus, specific heat, thermal expansion, and the Grüneisen parameter. A good agreement between the firstprinciple theory and experimental observations was achieved, highlighting the success of the ArmientoMattsson 2005 generalized gradient approximation functional employed in this study and QHA for describing thermodynamic properties of Os2B3.
 Award ID(s):
 1904164
 Publication Date:
 NSFPAR ID:
 10207502
 Journal Name:
 High Pressure Research
 Page Range or eLocationID:
 1 to 12
 ISSN:
 08957959
 Sponsoring Org:
 National Science Foundation
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High pressure study on ultrahard transitionmetal boride Os2B3 was carried out in a diamond anvil cell under isothermal and nonhydrostatic compression with platinum as an Xray pressure standard. The ambientpressure hexagonal phase of Os2B3 is found to be stable with a volume compression V/V0 = 0.670 ± 0.009 at the maximum pressure of 358 ± 7 GPa. Anisotropic compression behavior is observed in Os2B3 to the highest pressure, with the caxis being the least compressible. The measured equation of state using the 3rdorder BirchMurnaghan fit reveals a bulk modulus K0= 397 GPa and its first pressure derivative K0'= 4.0. The experimental lattice parameters and bulk modulus at ambient conditions also agree well with our densityfunctionaltheory (DFT) calculations within an error margin of ~1%. DFT results indicate that Os2B3 becomes more ductile under compression, with a strong anisotropy in the axial bulk modulus persisting to the highest pressure. DFT further enables the studies of charge distribution and electronic structure at high pressure. The pressureenhanced electron density and repulsion along the Os and B bonds result in a high incompressibility along the crystal caxis. Our work helps to elucidate the fundamental properties of Os2B3 under ultrahigh pressure for potential applications in extrememore »

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