Mop3sio11: A 4d(3) Honeycomb Antiferromagnet With Disconnected Octahedra

We report the crystal structure and magnetic behavior of the 4d(3) spin-3/2 silicophosphate MoP3SiO11 studied by high-resolution synchrotron x-ray diffraction, neutron diffraction, thermodynamic measurements, and ab initio band-structure calculations. Our data revise the crystallographic symmetry of this compound and establish its rhombohedral space group (R (3) over barc) along with the geometrically perfect honeycomb lattice of the Mo3+ ions residing in disconnected MoO6 octahedra. Long-range antiferromagnetic order with the propagation vector k = 0 observed below T-N = 6.8 K is a combined effect of the nearest-neighbor in-plane exchange coupling J similar or equal to 2.6 K, easy-plane single-ion anisotropy D similar or equal to 2.2 K, and a weak interlayer coupling J(c) similar or equal to 0.8 K. The 12% reduction in the ordered magnetic moment of the Mo3+ ions and the magnon gap of Delta similar or equal to 7 K induced by the single-ion anisotropy further illustrate the impact of spin-orbit coupling on the magnetism. Our analysis puts forward single-ion anisotropy as an important ingredient of 4d(3) honeycomb antiferromagnets despite their nominally quenched orbital moment.