Microscopic magnetic properties of metallic and insulating ${\mathrm{V}}_4$${\mathrm{O}}_7$ and ${\mathrm{V}}_7$${\mathrm{O}}_{13}$

The local magnetic properties of the various atomic sites in metallic and insulating ${\mathrm{V}}_4$ ${\mathrm{O}}_7$ and ${\mathrm{V}}_7$ ${\mathrm{O}}_{13}$ have been determined by means of ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$ nuclear magnetic resonance. Remarkably free-spin-like paramagnetic behavior is found in metallic ${\mathrm{V}}_4$ ${\mathrm{O}}_7$ and ${\mathrm{V}}_7$ ${\mathrm{O}}_{13}$, although there is not full differentiation into 3+ and 4+ charge states on the different sites. Below the ${\mathrm{V}}_4$ ${\mathrm{O}}_7$ metal-insulator transition, one group of ${\mathrm{V}}^{4+}$ ions forms nonmagnetic singlet spin pairs in the insulating phase while the remainder of the sites are paramagnetic. In the antiferromagnetic insulating state two magnetic vanadium sites with hyperfine fields 70.1 and 78.4 kOe and one nonmagnetic vanadium site are found. These results are analyzed in terms of various theoretical models, i.e., a modified Hubbard model and small-polaron model. None of the theoretical models is satisfactory.