LOCAL-STRUCTURE AND ELECTRONIC-CHARGE DISTRIBUTION AT THE RARE-EARTH SITE OF COMPOUNDS (GD1-XRX)2CUO4, R = PR, ND, SM, AND EU

Mixed compounds (Gd1-xRx)2CuO4, R = Pr, Nd, Sm, and Eu, with T' structure, were studied by x-ray diffraction and by 155 Gd Mossbauer spectroscopy. Structural parameters (unit cell volume, c/3a) differ considerably for compounds with different rare earths. In contrast, the variation of the isomer shift delta(IS) and of the electric field gradient V(zz) at Gd-155 nuclei with Gd concentration is practically identical in the systems with Pr, Nd, and Sm. These results demonstrate that delta(IS) and V(zz) in the compounds R2CuO4 are primarily determined by contributions of the local electronic-charge distribution at the rare-earth site, which depends weakly upon the positions of the ions as determined by the structural parameters. Furthermore, our results indicate the possibility of an influence of the degree of covalency upon the transition temperature T(c) achieved in Ce-doped compounds.