First-order phase transitions in EuCo P-2(2) and SrNi2P2

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 ACo(2)P(2) (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.