First-order phase transitions in the ThCr2Si2-type phosphides ARh(2)P(2) (A = Sr, Eu)

We observed for the first time a first-order phase transition with strong and extremely anisotropic changes of the lattice parameters in compounds crystallizing in the ThCr2Si2 structure type. The phase transition occurs in SrRh2P2 with increasing pressure at 6 GPa (300 K) and in EuRh2P2 with increasing temperature at 810 K (ambient pressure). On the basis of single-crystal data of ARh(2)P(2) (A = Ca, Sr, Ba, Eu) at ambient pressure and temperature we discuss the P-P distance in the framework of band-structure calculations at the first-order phase transition the P-P state changes from a 'no-bond' to a 'single-bond' state.