Electronic entropy contribution to the metal insulator transition in VO2

Abstract

VO 2 experiences a metal-insulator transition at 340 K. Discontinuities in electronic transport properties, such as the Seebeck coefficient and the electronic conductivity, suggest that there is a significant change in the electronic structure upon metallization. However, the thermodynamic nature of this transformation remains difficult to describe using conventional computational and experimental methods. This has led to disagreement over the relative importance of the change in electronic entropy with respect to the overall transition entropy. A method is presented that links measurable electronic transport properties to the change in electronic state entropy of conduction electrons. The change in electronic entropy is calculated to be 9.2 ± 0.7 J/mol K which accounts for 62%-67% of the total transition entropy. This compares favorably with some estimates of the vibrational entropy.

Basic aspects of the metal–insulator transition in vanadium dioxide VO2: a critical review

“…However, the most recent experimental determinations remain controversial. In the work of [147], the variation of phonon entropy, ∆S ph , at the R-M 1 transition is estimated at 68% of transition entropy while the work of [179], using the variation of Seebeck coefficients between R and M 1 , the variation of electronic entropy, ∆S elec , is estimated at 65% of transition entropy. The addition of these two determinations is 30% larger the experimental transition entropy.…”

Voltage hysteresis loop as a fingerprint of slow kinetics Co2+-to-Co3+ transition in layered NaxCox/2Ti1−x/2O2 cathodes for sodium batteries

“…47 Such an insulator-to-metal transition is usually accompanied by an entropy rise of 10–15 J mol −1 K −1 due to a high contribution of the electronic degree of freedom (62–67% of the total entropy of transition) because of an increased amount of free charge carriers, and an increased vibrational degree of freedom (around 33%) as it was elaborately discussed for VO 2 . 48 Therefore, further entropy gain may be expected in the Ti 4+ /Ti 3+ redox activity region due to a possible composition-induced narrowing the band gap of O3-Na x Co 0.33 Ti 0.67 O 2 at the Fermi level because of the Ti 4+ to Ti 3+ reduction, and corresponding elongation of Ti–O bond distances, visible from the rising lattice parameter a in this range of compositions (Fig. 3).…”

Thermal Evolution of the Eubaco2o5.5 Crystal Structure

“…If both octahedra were merging into one HS octahedron, their spin-state conversion entropy ΔS spin ¼ ½ ΔS LS→HS þ ½ ΔS IS→HS ¼ 1.61R per EuBa-Co 2 O 5.5 would be complemented to the above-idealized DSC-based estimate of 3R by the entropy of delocalizing the electron or its fraction from topmost Co 3þ orbital and corrected for that electron's magnetic moment turning into the relatively weaker contribution from enhanced Pauli paramagnetism. A recent paper on the insulator-metal transition entropy in d 1 VO 2 [38] reports the electronic entropy change of about 1R (that represents ~2/3 of the total entropy of that transition). Given two Co 3þ per mole of the oxide, none of the alternatives is impossible.…”