CONTROL OF MAGNETISM BY CRYSTAL-CHEMISTRY IN T'-PHASE R2CU1-XPDXO4 (R= ND, SM, EU, GD LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.2)

Polycrystalline samples of the T'-phases R2Cu1-xPdxO4 (R=Nd, Sm, Eu, G d; 0 less than or equal to x less than or equal to 0.2) have been char acterized by x-ray powder diffraction (XRPD) and magnetometry. Doping with diamagnetic Pd induces orthorhombic symmetry and hence weak ferro magnetism in the Sm (x greater than or equal to 0.1) and Eu (x greater than or equal to 0.05) phases, which are antiferromagnetic for x = 0. 0. The structural distortion is too subtle to be observed directly by XRPD, but it is shown that the derivative partial derivative(c/a)/part ial derivative x (positive for orthorhombic, ferromagnetic phases and negative for the tetragonal phases) can be used as a diagnostic. The d opant concentration necessary to induce the transition is determined b y the ratio of ionic radii r(R)/r(M), where r(M) is the mean radius of the four coordinate cation (Pd/Cu). Samples having r(R)/r(M)<1.87 are weak ferromagnets; Nd3+ is too large for this condition to be satisfi ed in the composition range studied, whereas Gd3+ is too small to stab ilize tetragonal T'-phases. There is evidence for re-entrant spin-glas s behavior in Pd-free Eu2CuO4. [S0163-1829(98)06929-X].