Single crystals of ternary mixed alkali halides (KCI)x(KBr)0.9-x(NaI)0.1 of different compositions were grown by slow evaporation technique. All the crystals were grown under identical conditions. The crystals obtained were checked by XRD pattern. EDAX data has been taken for the samples to know the elemental analysis. Microhardness measurements have been carried out using Zwick hardness tester fitted with a Vicker diamond pyramidal indenter. The dielectric constant and dielectric loss measurement were made using LCR meter which operates in the range IkHz to 10 MHz . It is observed that the formation of a ternary mixed crystal is accompanied by an increase in hardness and it varies non-linearly with composition. From the hardness measurement, the Elastic stiffness constant (C11) of the crystals for various loads have been estimated. In order to investigate the nature of defects, dielectric loss and dielectric constant measurements have been done which showed that the mixed crystals are more defective, containing a high concentration of dislocations, low angle grain boundaries and vacancies as compared to the end products
