Structural fluctuations in NbO2at high temperatures

Abstract

Inelastic neutron scattering has been used to study the fluctuations associated with the structural phase transformation at $T_c=808.5\mathrm{}$ °C in Nb${\mathrm{O}}_2$. Above $T_c$ there is strong quasielastic scattering centered upon the reciprocal-lattice position ${\overrightarrow{\mathrm{q}}}_p=(\frac{1}{4},\frac{1}{4},\frac{1}{2})$, but lying along rods oriented parallel to (110) directions. The temperature dependence of the anisotropic correlation lengths and energy widths have been determined. We show that the structural fluctuations consist of strong-coupled antiferrodistortive distortions within (110) planes in the parent rutile structure with much weaker correlations of the fluctuations from plane to plane. A simple mean-field pseudospin treatment can be used to discuss the data semiquantitatively and aids in understanding the relationship between the structural instabilities in Nb${\mathrm{O}}_2$ and its isoelectronic compound V${\mathrm{O}}_2$. No evidence was found for a soft-phonon branch.