Structure and Magnetism inAB10O15+x(A= Sr, Ba;B= V, Cr;x= 0, ∼1): Evidence for Geometric Frustration

Some members of the seriesAB10O15+x,A= Sr, Ba,B= V, Cr,x= 0 and ∼1, have been studied and are found to possess unusual magnetic properties which can be traced to geometric frustration of theB-site magnetic sublattice. These compounds crystallize inCmcaand the structure can be described as a hexagonal close packing of alternatingAO7and O8layers with theBions occupying 5/8 of the octahedral sites. The presence ofA2+ions in the close packedAO7layers directs the pattern ofB-site occupation and octahedral sites near theA2+ions are forbidden. The resultingBsublattice consists ofB10clusters of edge-sharing, distorted tetrahedra connected by corner-sharing to adjacent clusters to form a three-dimensional network. SrV10O15shows a sharp susceptibility maximum at 50K and powder neutron diffraction provides evidence for long range magnetic order. In contrast, BaCr10O15shows only a broad susceptibility maximum at 25K but a field-cooled–zero-field cooled divergence at 30K, i.e., spin glass-like behavior. Neutron diffraction detects only short range (SRO) antiferromagnetic (AF) order in the form of diffuse scattering consistent with a correlation length corresponding to the distance between nearest neighbors. The AFSRO sets in below 180K and surprisingly, ferromagnetic SRO appears below 8K. The properties of BaCr10O15and BaCr10O15+xare compared to Cr2O3which is also based on an hcp lattice with nearly the same octahedral site occupation rate, 2/3 versus 5/8, and which shows no evidence of frustration.