PRESSURE-DRIVEN VALENCE CHANGE IN TERNARY EU PNICTIDES

We observed a structural phase transition with extremely anisotropic c hanges of the lattice parameters as a function of pressure at 2.6 GPa in EuPdP, which crystallizes in the hexagonal layered Ni2In structure type. On the basis of the results of pressure-dependent x-ray diffract ion experiments on the isostructural series APdP and APdAs (A = Sr or a trivalent rare-earth element) we show that the phase transition in E uPdP is accompanied by a valence change of the Eu. Strong but continuo us changes of the lattice parameters with increasing pressure, which a re due to increase of the Eu valence, were observed in EuNiP, EuPtP an d EuPdAs, too. An estimation of the average Eu valence in these compou nds leads to preferred values of the order of 2n/6.