Abstract
The pentachloride MoCl5 adopts several molecular crystal structures, all comprising isolated Mo2Cl10 units with well-separated Mo–Mo magnetic dimers. Using magnetization measurements, single-crystal X-ray diffraction, and first-principles calculations, we confirm ferromagnetism with strong anisotropy below a Curie temperature of 22 K in α-MoCl5, and report a fifth polymorph, ϵ-MoCl5 , that we find to be ferromagnetic below 14 K. Magnetization measurements indicate unquenched orbital moments antialigned with the spins. This is confirmed by first-principles calculations, which also predict an unusually strong magnetocrystalline anisotropy in α-MoCl5 arising from spin–orbit coupling. An anisotropy field near 80 T is calculated, while a smaller but still substantial anisotropy field exceeding 12 T is realized experimentally. Further, increased anisotropy and Curie temperature are predicted when W is substituted for Mo. Similarly, strong magnetism and anisotropy are predicted for isolated Mo2Cl10 molecules, indicating the potential for true molecular magnetism. Together, these results identify Mo1–xWxCl5 as novel molecular crystals that combine spin 1/2 with strong magnetic anisotropy and exhibit surprisingly high Curie temperatures, considering their molecular nature.
