Elastic constants and thermal expansion were measured using large single crystals of monoclinic Na2H4(C6H8O4)3 · 2 H2O. These properties show a considerable anisotropy, which is strongly correlated to features of the crystal structure. Essential details of the structure are discussed by comparing results from a structure determination by neutron diffraction (Gao, Clancy, Weber, Craven, McMullan, 1991) with our results using X-ray diffraction. Special emphasis is given to the different models of disorder proposed in the neutron and the X-ray work, which have to be discussed in the light of different scattering properties of atoms for the two radiations used. The temperature dependence of the elasticity tensor is anomalous, indicating a ferroelastic phase transition at about 270K. The critical shear stiffness belongs to a transverse mode propagating along the twofold axis.
The geometries of the most stable isomers of gold telluride systems AuTe, Au2Te, and AuTe2 are determined using the MP2 method. The aspect of gold—telluride interaction, the electron correlation, and relativistic effects on geometry and stability are investigated at the MP2 and CCSD(T) theoretical levels. The results show that the electron correlation and relativistic effects are responsible not only for gold—gold attraction but also for additional gold—telluride interaction. The gold—telluride interaction is strong enough to modify the known pattern of bare gold clusters. Both effects are essential for determining the geometry and relative stability of this type of systems.