CRYSTAL-CHEMISTRY AND MAGNETIC-PROPERTIES OF THE K2NIF4 TYPE DILUTED SOLID-SOLUTION YCAAL1-XCRXO4 ESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.10) - EVIDENCE FOR CR3+ CLUSTERING

The K2NiF4 type diluted solution YCa(Al1-xCrx)O4 (0 less-than-or-equal -to X less-than-or-equal-to 0.10) is studied with respect to both crys tal chemistry and magnetic properties, mainly in order to obtain some evidence of Cr3+ segregation phenomena in the octahedral layers. The A l3+ --> Cr3+ replacement results in a strong anisotropic variation of the cell constants in terms of an increase and a decrease in the a and c parameters, respectively, of the tetragonal cell. Such behaviour is largely dependent on the M-0(apical) distances in the (Al,Cr)-O6 octa hedra, as well as in the (Y,Ca)-O9 polyhedra, which show a coupling of their variation. On the basis of the temperature (80, 400 K) and Cr3 concentration dependence of the magnetic susceptibility, which is use d in the Heisenberg-Dirac-Van Vleck approach for modeling the magnetic exchange interactions, a significant deviation of the Cr3+ distributi on is found with respect to a statistical distribution. The amount of Cr3+ clusters is lower than in the perovskite solid solution YAl1-xCrx O3, owing to the lower dimensionality of the K2NiF4 type structure of the YCaAl1-xCrxO4 series. The 2D character of the exchange interaction s is found to be stronger than in pure chromite, YCaCrO4.