Stress-Induced Martensitic\nTransformation in Na<sub>3</sub>YCl<sub>6</sub>

Martensitic transformation with volume expansion plays\na crucial\nrole in enhancing the mechanical properties of steel and partially\nstabilized zirconia. We believe that a similar concept could be applied\nto unexplored nonoxide materials. Herein, we report the stress-induced\nmartensitic transformation of monoclinic Na<sub>3</sub>YCl<sub>6</sub> with an ∼3.4% expansion. In situ synchrotron X-ray diffraction\nand atomistic simulations showed that anisotropic crystallographic\ntransformation from monoclinic to rhombohedral Na<sub>3</sub>YCl<sub>6</sub> occurs exclusively under uniaxial pressure; no effect is\nobserved under hydrostatic pressure conditions. The uniaxially pressed\npowder compact of monoclinic Na<sub>3</sub>YCl<sub>6</sub> showed\na large indentation impression and low Young’s modulus, in\ncontrast to its high bulk modulus, suggesting that these unique mechanical\nproperties are induced by the martensitic transformation.