TEMPERATURE-INDUCED VALENCE INSTABILITIES IN TERNARY EU-PNICTIDES - ACOMPREHENSIVE VIEW

We report on measurements of the lattice constants, magnetic susceptib ility, LIII X-ray absorption and Massbauer-effect on EuNiP and EuPdP, which crystallize in the hexagonal layered Ni,In structure. In both co mpounds the Eu valence above 510 K is 2.33. With decreasing temperatur e they show one (EuPdP) or two (EuNiP) first order phase transitions w ith a valence increase of about 0.16. At the same time the c-axis shri nks while the a-axis even increases. From Mossbauer measurements we sh ow that the nature of the valence mixing is static. In contrast, the v alence mixing in isostructural EuPtP is static at low temperatures, to o, but it becomes homogeneous valent above a first order phase transit ion at 235 K. The behaviour of these compounds (as well as that of EuP dAs) is explained in a new model of electrostatically charged layers. In this model we can explain the temperature dependence of the lattice constants, the static valence mixing and the occurrence of preferred valences of the order 2n/6. Together with the compression shift model of Hirst we can also understand the mechanism of the phase transitions . A comparison with EuT2Si2 compounds in ThCr2Si2 structure shows that in EuTX compounds only the electronic structure of the transition ele ments is relevant for the occurrence of mixed valency.