Structural distortions in the low-temperature phase ofNbO2

At about 810°C niobium dioxide undergoes an apparently second-order transition between a high-temperature (HT) rutile structure and a low-temperature (LT) phase whose structure is slightly distorted from rutile. The primary order parameters of the transition are the amplitudes of sinusoidal displacement waves with wave vectors ${\overrightarrow{\mathrm{q}}}_p\equiv (\frac{1}{4},\pm \frac{1}{4},\frac{1}{2})$. These waves give rise to a series of superlattice reflections in diffraction patterns obtained with the LT phase. Additional superlattice reflections occur at wave vectors which are related by the rutile point group to ${\overrightarrow{\mathrm{q}}}_m\equiv (\frac{1}{2},\frac{1}{2},0)$ and ${\overrightarrow{\mathrm{q}}}_x\equiv (\frac{1}{2},0,0)$. The latter reflections may be diffraction harmonics or they may represent the presence of secondary distortions in the LT phase. A detailed neutron diffraction study at room temperature has allowed the LT structure to be determined and has demonstrated that both primary and secondary distortions are required for an adequate description of this phase. The general symmetry properties of the primary distortion are discussed and this distortion is discribed in terms of a simple model involving dimerization of $c$-axis chains of Nb atoms.