Influence of Defects on Ferroelastic Phase Transition in InI Single Crystals

Abstract

InI single crystals (space group D172h) are typical layered A 3Β7 compounds with a pronounced anisotropy of the crystalline stucture and the corresponding optical properties [1-5]. At the same time, the recent revelation of the low temperature phase transitions in these crystals [6, 7] can present them as convenient model objects for the creation of phase transitions theory in high anisotropy media including those with the incommensurate phase. In the circumstances, it is very substantial that similar twodimensional systems are most susceptible to the appearance of the translational symmetry perturbations of different kind, above all at the expense of appearance of defects, dislocations and vacancies. The last ones are in particular fashion exhibited in modulated superstructures [8]. The goal of this work is to describe the influence of the defect subsystem on phase transitions in InI single crystals, because the layering of structure allows one to control smoothly the corresponding parameters. The initial samples were synthesized using reaction between the pure metallic indium and iodine in a hermetical quartz ampoule. After the band cleaning and distillation in the vacuum, single crystals have been grown by the Bridgman method [9]. Independently of crystal perfection all the cleaved monocrystalline specimens possessed mirror surfaces of high quality in αc-plane, which did not require additional polishing. The stuctural parameters were checked using the X-ray DROΝ-05 diffractometer. The temperature measurements were performed in the range 4.2-500 K and were stabilized within 0.1-0.2 K•min -1 . The Senarmon method (λ = 0.6328 μm) was used for measuring of the piezooptical constants. Testing of samples quality was performed by the X-ray method. The broadening of X-ray satellite reflexes permits to select crystals of different quality. To obtain smooth change of the specimens quality, they were annealed in non-inertial