MoP\(_3\)SiO\(_{11}\): a \(4d^3\) honeycomb antiferromagnet with disconnected octahedra

We report the crystal structure and magnetic behavior of the \(4d^3\) spin-\(\frac32\) silicophosphate MoP\(_3\)SiO\(_{11}\) 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 (\(R\bar 3c\)) along with the geometrically perfect honeycomb lattice of the Mo\(^{3+}\) ions residing in disconnected MoO\(_6\) octahedra. Long-range antiferromagnetic order with the propagation vector \(\mathbf k=0\) observed below \(T_N=6.8\) K is a combined effect of the nearest-neighbor in-plane exchange coupling \(J\simeq 2.6\) K, easy-plane single-ion anisotropy \(D\simeq 2.2 \) K, and a weak interlayer coupling \(J_c\simeq 0.8\) K. The 12% reduction in the ordered magnetic moment of the Mo\(^{3+}\) ions and the magnon gap of \(\Delta\simeq 7\) 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.