Systematics in the metal-insulator transition temperatures in vanadium oxides

• The mono-valence VO2 and V2O3 and the mixed-valence VnO2n−1(n=3−6,8,9)VnO2n−1(n=3−6,8,9) exhibit first-order metal-insulator transitions (MIT).
• The relative roles of the Mott-like or Peierls-like interactions in these MIT were under debate for several decades.
• An unexpectedly simple systematics of TMITTMIT is presented here: For TMIT=|TM−TP|TMIT=|TM−TP| all data points of TM−TPTM−TP versus 1/n lie on, or close to, two simple straight lines.

Nine of the known vanadium oxides, VO2−1/nVO2−1/n (n   – a positive or negative integer) with n=2−6,8,9,∞n=2−6,8,9,∞ and −6, undergo metal-insulator transitions accompanied by structural transitions, at various temperatures TMIT (V7O13 is metallic above T=0). Among the persistent efforts to determine the driving force(s) of these transitions, electron–electron (Mott-like) and electron–phonon (Peierls-like) interactions, there were several attempts to find systematics in TMIT as function of n. Here we present an unexpectedly simple and illuminating systematics that holds for positive n: if TMIT is the absolute value of the difference between TM(n) and TP(n), which represent the contributions of electron–electron and electron–phonon interactions, respectively, all data points of TM-TP versus 1/n   lie on, or close to, two simple straight lines; one is TM−TP=T∞(7/n−1)TM−TP=T∞(7/n−1) for V3O5, V4O7, V5O9, V7O13, V8O15, V9O17 and VO2 and the other is TM−TP=T∞(3/n−1)TM−TP=T∞(3/n−1) for V2O3, V6O11 and VO2.