Abstract
We used temperature-dependent x-ray absorption fine structure (XAFS) measurements to examine the local structural properties around vanadium atoms at the V K edge from VO2 films. A direct comparison of the simultaneously-measured resistance and XAFS regarding the VO2 films showed that the thermally-driven structural transition occurred prior to the resistance transition during a heating, while this change simultaneously occured during a cooling. Extended-XAFS (EXAFS) analysis revealed significant increases of the Debye-Waller factors of the V-O and V-V pairs in the {111} direction of the R-phase VO2 that are due to the phonons of the V-V arrays along the same direction in a metallic phase. The existance of a substantial amount of structural disorder on the V-V pairs along the c-axis in both M1 and R phases indicates the structural instability of V-V arrays in the axis. The anomalous structural disorder that was observed on all atomic sites at the structural phase transition prevents the migration of the V 3d1 electrons, resulting in a Mott insulator in the M2-phase VO2.
Highlights
Vanadium dioxide (VO2) is a typical metal-to-insulator transition (MIT) material, and it is accompanied by a first-order structural phase transition (SPT) from a monoclinic (M1) phase to a rutile (R) phase via a distorted-monoclinic(M2) phase
This study strongly suggests that the phonons in the {111} direction of the R-phase VO2 play a key role in the delocalization of the V 3d1 electrons, and that the structural disorder, at V sites, prevents the propagation of electrons as well as phonons near the SPT temperature
Using the temperature-dependent x-ray absorption fine structure (XAFS) measurements with simultaneously-measured resistance, it has been demonstrated that the SPT, the local density of states (LDOS) change of the V 3d orbitals, and the MIT do not occur at the same temperature during a heating, while the MIT nearly coincides with the SPT and LDOS change during a cooling
Summary
Vanadium dioxide (VO2) is a typical metal-to-insulator transition (MIT) material, and it is accompanied by a first-order structural phase transition (SPT) from a monoclinic (M1) phase to a rutile (R) phase via a distorted-monoclinic(M2) phase. Tao et al showed metallic properties that could be induced in the M1 phase via structural strain[29], and other researchers have reported an observation of insulating properties in the R phase near the MIT temperature (Tc)[17,30,31,32,33]. Simultaneous measurements of the XAFS at the V K edge and resistance from VO2 films were conducted to directly compare the structural and electrical properties. From the direct comparison of the simultaneously measured XAFS and resistance, the following findings were observed: 1) The SPT is congruent with neither the MIT nor the pre-edge peak shift during a heating, while the three transitions occur nearly at the same temperature during a cooling. From the direct comparison of the simultaneously measured XAFS and resistance, the following findings were observed: 1) The SPT is congruent with neither the MIT nor the pre-edge peak shift during a heating, while the three transitions occur nearly at the same temperature during a cooling. 2) Insulating properties are evident in the R phase near the SPT. 3) The bond-length changes of the six V-O pairs on a V-O octahedron are non-rigid. 4) Two of the bonds of the V-O pairs are slightly longer than the other four bonds of a V-O octahedron in the R phase. 5) Anomalous structural disorder exists on all atomic sites at the SPT. 6) The Debye-Waller factor (σ2) of the V(0)-V(2) pairs along the {111} direction is larger by approximately 1.7 times in the R phase compared with that in the M1 phase, while on the V(0)-V(1) pairs along the c-axis, it remains at a constant value in the M1 and the R phases. 7) A substantial amount of structural disorder exists on the V(0)-V(1) pairs, compared to that on the V(0)-V(2) pairs
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