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 str ong and extremely anisotropic changes of the lattice parameters in com pounds crystallizing in the ThCr2Si2 structure type. The phase transit ion occurs in SrRh2P2 with increasing pressure at 6 GPa (300 K) and in EuRh2P2 with increasing temperature at 810 K (ambient pressure). On t he 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 fr amework of band-structure calculations at the first-order phase transi tion the P-P state changes from a 'no-bond' to a 'single-bond' state.