PHASE-TRANSITIONS AND MAGNETIC-BEHAVIOR OF R(1-X)CA(1-Y OR SM) -OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.5)(X)CRO(4) OXIDES (R)

The RCaCrO(4) (R=Sm or Y) stoichiometric oxides crystallize with ortho rhombic symmetry and space group Bmab. In the case of the SmCaCrO4 at 1073 K a structural phase transition of the type Bmab (orthorhombic)-- >I4/mmm (tetragonal) has been observed. Below room temperature at abou t 190 K a second Bmab-->P4(2)/ncm (tetragonal) phase transition has be en detected from X-ray diffraction data. The magnetic properties of bo th SmCaCrO4 and YCaCrO4 reveal the existence of antiferromagnetic orde ring above room temperature. The appearance of the ferromagnetic compo nent at the temperature at which the low temperature phase transition takes place has been explained as the result of the canting of the ant iferromagnetically ordered chromium sublattice. A field-induced magnet ic transition has been observed at 50 K and 14 kOe for SmCaCrO4. The d oping of the samarium sublattice, which increases the Ca/Sm ratio from 1 to 1.5 to give mixed-valence compounds of the type Sm1-xCa1+xCrO4, yields very important changes in the structural properties.