Thermal etching of CaWO4 crystals

Abstract

(011) cleavages of single crystals of calcium tungstate, grown by the flux-evaporation method in this laboratory, have been subjected to thermal etching between temperatures of 1110 and 1430 °C in a vacuum chamber at a pressure of 10−5 Torr. The etch figures thus obtained successfully reveal the emergence sites of dislocations in the crystals. An excellent correlation between thermal and chemical etch pittings has also been obtained on mirror cleavages. Quantitative measurements made on the etch rate and weight loss with temperature show that the process of the removal of ledges from the cleavage surface has an activation character. The activation energy for evaporation at dislocation sites is less than that responsible for overall evaporation from the exposed surface. The implications are discussed.

Transferability of Empirical Force Fields in Silicates: Lattice-Dynamical Evaluation of Atomic Displacement Parameters and Thermodynamic Properties for the Al2OSiO4 Polymorphs

“…This is another proof of the reliability of lattice-dynamical calculations and of our procedure in particular. The excellent results found for a group of silicates by other authors, such as Winkler, Dove & Leslie (1991) and Patel, Price & Mendelssohn (1991), by lattice dynamics, concerning transferability of empirical potentials within a wide group of substances, are also confirmed.…”

Atomic displacement parameters for garnets: a lattice-dynamical evaluation

“…This unusual behaviour of the Mg atom has also been confirmed by X-ray diffraction studies, especially by AGL and Sawada (1993): the latter author had to apply the Gram-Charlier series expansion up to the fourthrank tensor to account for the atomic displacement satisfactorily. In their lattice-dynamical work, Patel et al (1991) also find a notable disagreement in the lowtemperature simulations for pyrope, in contrast to generally very good agreement for other silicates. This disagreement is ascribed to the effect of a particularly limited sampling of the Brillouin zone, where only eight points were considered for saving computing time in view of the large unit cell.…”

“…Furthermore, the possibility of deriving the values of thermodynamic functions by applying statistical mechanics, using the phonon density of states obtained from a calculation extended to the whole Brillouin zone, is particularly interesting: this procedure is essentially similar (and parallel) to the calculation of atomic thermal parameters and can be carried out at the same time, as a by-product of the same routine (see, for instance, PDG93). These estimates of thermodynamic functions can be compared with those obtained by other authors using similar methods, such as the lattice-dynamical calculations in connection with energy-minimization carried out by Winkler, Dove & Leslie (1991), Patel, Price & Mendelssohn (1991) or, again, with the results of much simpler statistical mechanical procedures.…”

Etch pits on basal cleavage faces of apophyllite crystals

“…The logarithmic value of Vs<a00> is given as a function of inverse temperature in Fig. 4b, from which the activation energy for dissolution was calculated (Patel and Arora, 1974). Values of activation energies for different concentration of NH4F in the base etchant are tabulated in Table 2.…”