The $3d^1$ system ${\mathrm{B}\mathrm{a}\mathrm{V}\mathrm{S}}_3$ undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at $T_{\mathrm{M}\mathrm{I}}=70\mathrm{K}$ is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a $2k_F=c^{*}/2$ charge-density wave (CDW) instability of the $d_{z^2}$ electron gas. We discuss the formation below $T_{\mathrm{M}\mathrm{I}}$ of an unconventional CDW state involving the condensation of the other ${\mathrm{V}}^{4+}$ $3d^1$ electrons of the quasidegenerate $e(t_{2g})$ orbitals. This study stresses the role of the orbital degrees of freedom in the physics of ${\mathrm{B}\mathrm{a}\mathrm{V}\mathrm{S}}_3$ and reveals the inadequacy of current first principle band calculations to describe its electronic ground state.

• Received 8 October 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.196401