The two components of the crystallographic transition in VO2

Two distinguishable mechanisms of the monoclinic-to-tetragonal transition in VO₂ are identified: an antiferroelectric-to-paraelectric transition at a temperature T_t as well as a change from homopolar to metallic VV bonding at a temperature T_t′. In pure VO₂ at atmospheric pressure, the two transitions occur at the same temperature, T_t′ = T_t = 340°K. However, a T_t′ < T_t may be induced by atomic substitutions; and in the intermediate temperature range the structure is orthorhombic, the antiferroelectric distortions being somewhat different in the absence of homopolar VV bonding. From energy-band considerations, the semiconductor-to-metal transition is to be associated with the transition at T_t; but below T_t′ the semiconducting energy gap should be larger and the charge-carrier mobilities should be smaller. The existence of two transition temperatures in doped VO₂ is distinguished from the claim of two transition temperatures in the Magnéli phases V_nO_2n−1. In this latter case, the appearance of two transitions reflects a two-phase region consisting of two adjoining Magnéli phases.