First-order phase transitions in $\mathrm{EuCo_2P_2}$ and $\mathrm{SrNi_2P_2}$

Abstract

First-order phase transitions with strong and extremely anisotropic changes of the lattice parameters were observed in the ThCr2Si2 structure-type compounds EuCo2P2 and SrNi2P2. At room temperature, with increasing pressure the phase transition occurs in SrNi2P2 at 4 kbar and in EuCo2P2 at 30 kbar which is in the latter probably accompanied by a valence change of Eu. On the basis of single-crystal data of ACo2P2 (A = Ca, Sr, La, Ce, Pr, Nd, Eu) at ambient pressure and temperature we discuss the pressure dependence of the bond lengths in these compounds