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Register a new userThermal expansion and compressibility of monogermanides of 3d-metals
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G. A. Valkovskiy
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Synchrotron diffraction as a function of temperature and pressure, specific heat, magnetic susceptibility and small-angle neutron scattering experiments have revealed an anomalous response of MnGe. Similar but less pronounced behavior has also been observed in Mn$_{1-x}$Co$_x$Ge and Mn$_{1-x}$Fe$_x$Ge solid solutions. Spin density fluctuations and Mn spin state instability are discussed as possible candidates for the observed effects.
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We describe a web-based tool (PASCal; Principal Axis Strain Calculator) aimed at simplifying the determination of principal coefficients of thermal expansion and compressibilities from variable-temperature and variable-pressure lattice parameter data. In a series of three case studies, we use PASCal to re-analyse previously-published lattice parameter data and show that additional scientific insight is obtainable in each case. First, the two-dimensional metal-organic framework Cu-SIP-3 is found to exhibit the strongest area-negative thermal expansion (NTE) effect yet observed; second, the widely-used explosive HMX exhibits much stronger mechanical anisotropy than had previously been anticipated, including uniaxial NTE driven by thermal changes in molecular conformation; and, third, the high-pressure form of the mineral malayaite is shown to exhibit a strong negative linear compressibility (NLC) effect that arises from correlated tilting of SnO6 and SiO4 coordination polyhedra.
We present temperature dependent inelastic neutron scattering measurments, accompanied byab-initio calculations of phonon spectra and elastic properties as a function of pressure to understand anharmonicity of phonons and to study the mechanism of negative thermal expansion and negative linear compressibility behaviour of ZnAu2(CN)4. The mechanism is identified in terms of specific anharmonic modes that involve bending of the Zn(CN)4-Au- Zn(CN)4 linkage. The high-pressure phase transition at about 2 GPa is also investigated and found to be related to softening of a phonon mode at the L-point at the Brillouin zone boundary and its coupling with a zone-centre phonon and an M-point phonon in the ambient pressure phase. Although the phase transition is primarily driven by a L-point soft phonon mode, which usually leads to a second order transition with a 2 x 2 x 2 supercell, in the present case the structure is close to an elastic instability that leads to a weakly first order transition.
Recent reported very high thermal conductivities in the cubic boron arsenide (BAs) and boron phosphide (BP) crystals could potentially provide a revolutionary solution in the thermal management of high power density devices. To fully facilitate such application, compatible coefficient of thermal expansion (CTE) between the heat spreader and device substrate, in order to minimize the thermal stress, need to be considered. Here we report our experimental CTE studies of BAs and BP in the temperature range from 100K to 1150K, through a combination of X-ray single crystal diffraction and neutron powder diffraction. We demonstrated the room temperature CTE, 3.6 $pm$ 0.15 $times$ 10E-6 /K for BAs and 3.2 $pm$ 0.2 $times$ 10E-6 /K for BP, are more compatible with most of the semiconductors including Si and GaAs, in comparison with diamond, and thus could be better candidates for the future heat spreader materials in power electronic devices.
We have investigated the anisotropic thermal expansion of graphite using ab-initio calculation of lattice dynamics and anharmonicity of the phonons, which reveal that the negative thermal expansion (NTE) in the a-b plane below 600 K and very large positive thermal expansion along the c-axis up to high temperatures arise due to various phonons polarized along the c-axis. While the NTE arises from the anharmonicity of transverse phonons over a broad energy range up to 60 meV, the large positive expansion along the c-axis occurs largely due to the longitudinal optic phonon modes around 16 meV and a large linear compressibility along the c-axis. The hugely anisotropic bonding in graphite is found to be responsible for wide difference in the energy range of the transverse and longitudinal phonon modes polarized along the c-axis, which are responsible for the anomalous thermal expansion behavior. This behaviour is in contrast to other nearly isotropic hexagonal structures like water-ice, which show anomalous thermal expansion in a small temperature range arising from a narrow energy range of phonons.
The simplest anharmonic characteristics of Ir and Rh are discussed in the framework of a previously developed simple pseudopotential model which describes the elastic moduli, phonon spectra and the lattice heat capacity in the harmonic approximation of these metals succesfully. The microscopic Gruneisen parameters, the dependences of the elastic moduli on pressure, the coefficient of thermal expansion and the equations of state at the finite temperatures have been calculated. The ab initio calculations of the energy-band structure and the equation of state for Ir at T=0 have been done to test the model for adequacy at high pressures. The values of different contributions (zero-point oscillations, quasiharmonic, etc.) in the considered thermodynamic characteristics of Ir and Rh are discussed.
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