Multiple lattice collapses in the family of SrNi₂P₂

Recently, it was discovered that SrNi₂P₂ has a high recoverable strain rate of ~14%, which is very stable against repetition[1]. Common values for such strain rates are less than one percent and limited by plastic deformation or fracture. For SrNi₂P₂, a double lattice collapse was proposed as the underlying mechanism. We perform ab initio density functional theory calculations for AT₂X₂ (A = (earth) alkaline, T = transition metal, X = atom of the C or N group) under various strain conditions and find that the strain-free ground state exists in an orthorhombic unit cell in which both collapsed and uncollapsed P-bonds are present. Upon strain, a structural transition to a tetragonal unit cell of the ThCr₂Si₂ family occurs. This transition occurs not only by compressive but also by tensile strain, where a change in the hybridization of the P atoms forms either collapsed or uncollapsed bonds respectively. We will discuss comparisons with experimental observations.