Pseudoelasticity of SrNi$_2$P$_2$ micropillar via Double Lattice Collapse and Expansion

The maximum recoverable strain of most crystalline solids is less than 1% because plastic deformation or fracture usually occurs at a small strain. In this work, we show that a SrNi$_2$P$_2$ micropillar exhibits pseudoelasticity with a large maximum recoverable strain of ~14% under uniaxial compression via unique reversible structural transformation, double lattice collapse-expansion that is repeatable under cyclic loading. Its high yield strength (~3.8$\pm$0.5 GPa) and large maximum recoverable strain bring out the ultrahigh modulus of resilience (~146$\pm$19MJ/m$^3$) a few orders of magnitude higher than that of most engineering materials. The double lattice collapse-expansion mechanism shows stress-strain behaviors similar with that of conventional shape memory alloys, such as hysteresis and thermo-mechanical actuation, even though the structural changes involved are completely different. Our work suggests that the discovery of a new class of high performance ThCr$_2$Si$_2$-structured materials will open new research opportunities in the field of pseudoelasticity.