Chemical and thermal properties of VO2 mechanochemically derived from V2O5 by co-milling with paraffin wax

A novel mechanochemical reduction process of V₂O₅ to VO₂ was established by milling with paraffin wax (PW, average molecular weight 254–646), serving as a reductant. The reduction progressed with increasing milling time and mass ratio V₂O₅ : PW (MRVP). The mechanochemically derived VO₂ became phase pure after milling for 3 h with an MRVP of 30 : 1 and exhibited a reversible polymorphic transformation between tetragonal and monoclinic phases at around 53–60 °C and 67–79 °C during heating and cooling, respectively. The latent heat was above 20 J g⁻¹ in both processes, being superior to those of commercial VO₂. Doping of starting V₂O₅ with Cr, Mo or W at 1 at% in the form of oxide did not increase the latent heat. This is another difference from the conventionally prepared doped VO₂. These anomalous heat storage properties of mechanochemically derived VO₂ were discussed mainly on the basis of X-ray photoelectron spectroscopy V_2p3/2 peaks combined with ion etching. The observed relatively high heat storage capacity of undoped VO₂ is primarily ascribed to the abundance of V⁴⁺ ionic states introduced during milling with PW, which were stabilized with simultaneously introduced structural degradation throughout the entire particles. The possible role of a remaining small amount of PW was also discussed.

Reduction of V₂O₅via a mechano-chemical route brings about unique electronic states of vanadium. The resulting VO₂ exhibits high latent heat storage during heating (a) and cooling (b).

Discovering correlated fermions using quantum Monte Carlo

It has become increasingly feasible to use quantum Monte Carlo (QMC) methods to study correlated fermion systems for realistic Hamiltonians. We give a summary of these techniques targeted at researchers in the field of correlated electrons, focusing on the fundamentals, capabilities, and current status of this technique. The QMC methods often offer the highest accuracy solutions available for systems in the continuum, and, since they address the many-body problem directly, the simulations can be analyzed to obtain insight into the nature of correlated quantum behavior.

On the electronic structure of the low lying electronic states of vanadium trioxide

The electronic structure of transition metal oxides is frequently studied using density functional theory. Nonetheless, the electronic structure of VO3 has been found to be sensitive to the choice of functional. As a consequence, the basic question of whether or not the ground electronic state exhibits a Jahn-Teller distortion has yet to be resolved. Using basis sets of triple zeta quality and multireference configuration interaction wave functions as large as 700 million configuration state functions, we determine that the ground electronic state of VO3 is a (2)A2 state in C3v symmetry. The first two excited electronic states are also characterized and found to be the components of a degenerate (2)E state, in C3v symmetry, which exhibits a small Jahn-Teller distortion. The Jahn-Teller stabilization energy is only 40 cm(-1) and the barrier to pseudo-rotation is 9 cm(-1). This (2)E state exhibits some unexpected properties. In the vicinity of the minimum energy conical intersection, the local topography appears almost quadratic, rather than linear, in the Jahn-Teller active coordinates. This gives rise to three symmetry-related seams of conical intersections in addition to the symmetry-required seam and results in the suppression of the geometric phase effect. These features, attributable to small linear Jahn-Teller parameters, are usually found in states characterized by e(2) (or e(3)e(')) electron configurations rather than the e(3) configuration found here. In addition to its Jahn-Teller minimum, the first excited state exhibits a second minimum with a structure significantly distorted from C3v. A conical intersection with Cs symmetry connects the two minima and puts an upper limit of 190 cm(-1) on the barrier connecting these minima.

Electron Structure Quantum Monte Carlo

The diffusion quantum Monte Carlo method (DMC) is capable of calculating accurately the electronic energy for molecules and molecular aggregates. An overview is given on recent developments for the optimization of the guide function that determines the accuracy of the method. Furthermore, the versatility of DMC is shown with applications to Rydberg states, transition metal compounds, and weakly interacting systems.