Nb${\mathrm{O}}_2$, heated in air, shows sharply decreasing electrical resistance until oxidation occurs at about 486°C and forms $L$-${\mathrm{Nb}}_2$${\mathrm{O}}_5$. This oxidation is accompanied by a first-order increase in electrical resistance. Further heating causes only a slight decrease in resistance until the occurrence of a minimum resistance anomaly, centered at about 800°C, and showing for the most part a second-order character of the singularity type. The magnitude of this effect is considerably larger in the cooling direction, and the recovered product gives the x-ray pattern of $L$-${\mathrm{Nb}}_2$${\mathrm{O}}_5$ rather than the high-temperature $H$ phase of ${\mathrm{Nb}}_2$${\mathrm{O}}_5$. Heating the sample under flowing argon postponed the reaction to ${\mathrm{Nb}}_2$${\mathrm{O}}_5$ to about 750°C; however, in the cooling direction, the reversal of the anomaly near 800°C is clearly observed. The minimum resistance anomaly is interpreted to arise from an intermediate stage of a phase transformation centered about reduction and reoxidation phenomena. Heating $\beta$-${\mathrm{Nb}}_2$${\mathrm{O}}_5$ in flowing argon causes a reversible first-order decrease in resistance beginning at about 450°C associated with the $\beta \to \delta$ phase transformation followed by a minimum in resistance at 700-1000°C interpreted as above. In the cooling direction the minimum in resistance occurs between 900 and 700°C, and the recovered product gives the x-ray pattern of ${\mathrm{Nb}}_2$${\mathrm{O}}_5$ ($105M$) which is a structure very similar to the $H$ phase.

• Received 20 July 1981

DOI:https://doi.org/10.1103/PhysRevB.26.3954