Stress-Induced Martensitic Transformation in Na₃YCl₆

Martensitic transformation with volume expansion plays a crucial role in enhancing the mechanical properties of steel and partially stabilized zirconia. We believe that a similar concept could be applied to unexplored nonoxide materials. Herein, we report the stress-induced martensitic transformation of monoclinic Na₃YCl₆ with an ∼3.4% expansion. In situ synchrotron X-ray diffraction and atomistic simulations showed that anisotropic crystallographic transformation from monoclinic to rhombohedral Na₃YCl₆ occurs exclusively under uniaxial pressure; no effect is observed under hydrostatic pressure conditions. The uniaxially pressed powder compact of monoclinic Na₃YCl₆ showed a large indentation impression and low Young's modulus, in contrast to its high bulk modulus, suggesting that these unique mechanical properties are induced by the martensitic transformation.