When do structural anomalies of ionic crystals begin?

Abstract

The present work contributes to the search of anomalous effects in ionic crystals, and possible explications of those effects observed in pure NaCl and CuCl:NaCl system. Single nanocrystals can only be used if their positions arefixed in space which needs forces between a nanocrystals and its enviroment. Therfore, the research of nano-crystals structures requres also the study of these forces. Structural and optical anomalies are determinated by measurements in single crystals of NaCl which contain a low concentration of CuCl. Embedded nanocrystals serve as sources for volume forces and their optical signal is used for the study of local forces. Samples of NaCl powder have been prepared be pulverizing pieces of single crystals by pestle in a mortar. They have been studied by X-ray diffraction. The observed line-widths of Bragg reflexes are about ten time larger than expacted from the well-known data of single crystals which have been subjected of plastic flow. The widths of Bragg reflexes decreases with increasing CuCl concentration, this effect is a true anomaly for each crystal growth process. Pure NaCl presents a storage strain effect due to the existence of the dislocations and the missing effect in CuCl:NaCl system. The optical birefringence effect and a strong correlation data between two opposite faces in CuCl:NaCl system and the missing of this correlation in pure NaCl have been observed. Obviously, a minimal concentration of identical sources of local stresses is needed to develop the collective phenomena which manifest a spatial dependence of birefringence. CuCl nanocrystals are trated as quasi-stable elements which are embedded in the NaCl matrix. The signature of the crystalline state of CuCl becomes detectable by optical absorption measurements and a break of linearity at 100K have been observed. Two different types of forces drive structural changes during cooling process: anion-forces and thermal forces.