Thermal expansion, Gruneisen functions and static lattice properties of quartz

Abstract

Reports measurements of the thermal expansion of quartz both parallel and perpendicular to the trigonal axis, at temperatures between 1.7 and 30K, between 57.5 and 85K, and a 283K. These measurements confirm the qualitative inadequacy of the SiO4 tetrahedral tilt model. Together with other thermodynamic data, the results are used to derive Gruneisen functions gamma perpendicular to (T) and gamma /sub ///(T) for temperatures up to 923K. gamma perpendicular to (T), gamma /sub ///(T) and the heat capacity are analysed to give the moments ( omega n) of the vibrational frequency distribution, the equivalent Debye temperature theta D(n) and the Gruneisen functions gamma perpendicular to (n) and gamma /sub ///(n). These can be used to estimate various crystal properties. For the equilibrium static lattice, the energy and lattice constants are respectively: EL=-1876+or-5 kJ mol-1; aL=4.875+or-0.005 AA; cL=5.380+or-0.005 AA. The temperature variation of the Gruneisen functions is discussed in the light of available spectroscopic data, and the positive expansion of alpha -quartz is attributed to high values of normal mode Gruneisen parameters in the upper parts of the acoustic branches and in some of the lower optical branches.