Dimerization of a linear Heisenberg chain in the insulating phases ofV1−xCrxO2

The nuclear magnetic resonance (NMR) of ${\mathrm{V}}^{51}$ has been studied in the series of compounds ${\mathrm{V}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_2$ between 100 and 350 K. Three insulating phases are clearly distinguished. In the low-temperature $M_1$ phase only one V site is seen with a positive Knight shift and electric-field gradient identical to the insulating phase of pure V${\mathrm{O}}_2$. At temperatures just below the metal-insulator transition a second phase $M_2$ is stable in which two sites are resolved. One site has a small positive Knight-shift characteristic of a paired ${\mathrm{V}}^{4+}$ site while the other has a negative Knight shift showing a localized ${\mathrm{V}}^{4+}$ site. These two sites are identified as the V atoms on the paired chains and the equispaced chains in $\frac{C2\mathrm{}}{m}$ structure of Marezio et al. At intermediate temperatures a transitional phase $T$ is stable in which two sites can be resolved by their electric-field gradients. The two sites are progressively differentiated by increasing temperature and Cr concentration and are interpreted as arising from two sets of inequivalent paired chains, one of which is depairing with increasing temperature and Cr concentration. These results are inconsistent with the monoclinic symmetry and a disordered bond model proposed previously for the transitional phase. Triclinic splittings were observed recently by Villeneuve et al. and additional crystallographic evidence supporting this result is presented. The magnetic properties are interpreted as a set of noninteracting linear Heisenberg chains and the $M_2-T$ transition as a dimerization of the linear Heisenberg chain. The results demonstrate a breakdown of the band model in the insulating phases.