Thermal Expansion of Alkali Halides at Low Temperatures

Anharmonic thermal oscillations of the electron momentum distribution in lithium fluoride

Anharmonic thermal effects on the electron momentum distribution of a lithium fluoride single crystal are experimentally measured through high-resolution Compton scattering and theoretically modeled with ab initio simulations, beyond the harmonic approximation to the lattice potential, explicitly accounting for thermal expansion. Directional Compton profiles are measured at two different temperatures, 10 and 300 K, with a high momentum space resolution (0.10 a.u. in full width at half maximum), using synchrotron radiation. The effect of temperature on measured directional Compton profiles is clearly revealed by oscillations extending almost up to |p|=4  a.u., which perfectly match those predicted from quantum-mechanical simulations. The wave-function-based Hartree-Fock method and three classes of the Kohn-Sham density functional theory (local-density, generalized-gradient, and hybrid approximations) are adopted. The lattice thermal expansion, as described with the quasiharmonic approach, is found to entirely account for the effect of temperature on the electron momentum density within the experimental accuracy.

Extended temperature dependence of elastic constants in cubic crystals

To extend the theory of the temperature dependence of the elastic constants in cubic crystals beyond the second- and third-order elastic constants, the fourth-order elastic constants, as well as the non-linearity in the thermal expansion temperature dependence, have been taken into account. Theoretical results were represented as temperature functions of the effective elastic constants and compared with experimental data for a number of cubic crystals, such as alkali metal halides, and elements gold and silver. The relations obtained give a more accurate description of the experimental temperature dependences of second-order elastic constants for a number of cubic crystals, including deviations from linear behavior. A good agreement between theoretical estimates and experimental data has been observed.

Dielectric constants of ionic crystals and their variations with temperature and pressure

Measurements of the audio- and radio-frequency dielectric constants of the alkali and thallium halides have been made over the range of temperature 1.5 to 350 K and at pressure up to 4 kbar (400 MN m -2 ), on crystals of high purity. The results are used to separate, for each compound, that part of the variation in dielectric constant which is explicitly dependent on temperature from that part which is explicitly dependent on volume. The observed variations at constant temperature are used to provide measures of the Gruneisen parameters for the long-wave transverse-optic phonons, and the variations at constant volume are discussed in terms of the anharmonic self-energies of these phonons. The materials studied here are all simple highly ionic compounds but, at room temperature, the self-energies of some prove to be positive while for others they are negative. It is shown how this can be interpreted in terms of competing three-phonon and four-phonon decay and scattering processes.

Thermal expansion data for eight optical materials from 60 K to 300 K

Coefficients of linear thermal expansion are reported, in the range 60 K to room temperature, for eight optical materials: Polytran potassium chloride and Polytran calcium fluoride-Harshaw; chemical-vapor-deposited (CVD) zinc sulfide and zinc selenide-Raytheon; germanium (single-crystal and polycrystal); crystalline magnesium fluoride, potassium dihydrogen phosphate (KDP), and lithium niobate-Harshaw. The last three are anisotropic crystals; thermal expansion was measured both parallel and perpendicular to the c axis.