Phonon anomalies and the magnetic transition in pure and Cr-dopedV2O3

We have studied the lattice dynamics, the magnetic ordering and excitations, and the crystallographic distortions associated with the metal-insulator transitions in pure and Cr-doped vanadium sesquioxide using neutron-scattering techniques. In the paramagnetic metallic phase of pure ${\mathrm{V}}_2$ ${\mathrm{O}}_3$ and in the higher-temperature paramagnetic metallic phase of 1.5-at. %-Cr-doped ${\mathrm{V}}_2$ ${\mathrm{O}}_3$ we see an anomalous splitting of the transversely polarized acoustic phonon mode, propagating along the hexagonal [hh0] direction near the center of the Brillouin zone. Upon lowering the temperature, a substantial softening of a large portion of the transverse acoustic branch in the [00l] direction is observed in Cr-doped samples in the paramagnetic insulating phase as the transition to the antiferromagnetic insulating phase is approached. The loss of antiferromagnetic order with increasing temperature in 3-at. %-Cr-doped and 6-at. %-Cr-doped samples appears to be nearly continuous. The temperature dependence of the spin-wave energy gap in the 3-at. %-Cr-doped sample appears to approach zero continuously up to the Néel temperature. The monoclinic distortion of the trigonal phase at this transition is much reduced in the 6-at. %-Cr-doped sample compared to pure ${\mathrm{V}}_2$ ${\mathrm{O}}_3$. These and other results are described in detail, and discussed in connection with the recent theoretical work on metal-insulator transitions.