Temperature-induced Metal-insulator Transition Research Articles

Boosting Polysulfide Redox Kinetics by Temperature-Induced Metal-Insulator Transition Effect of Tungsten-Doped Vanadium Dioxide for High-Temperature Lithium-Sulfur Batteries.

The practical application of Li-S batteries is still severely restricted by poor cyclic performance caused by the intrinsic polysulfides shuttle effect, which is even more severe under the high-temperature condition owing to the inevitable increase of polysulfides' solubility and diffusion rate. Herein, tungsten-doped vanadium dioxide (W-VO2) micro-flowers are employed with first-order metal-insulator phase transition (MIT) property as a robust and multifunctional modification layer to hamper the shuttle effect and simultaneously improve the thermotolerance of the common separator. Tungsten doping significantly reduces the transition temperature from 68 to 35 °C of vanadium dioxide, which renders the W-VO2 easier to turn from the insulating monoclinic phase into the metallic rutile phase. The systematic experiments and theoretical analysis demonstrate that the temperature-induced in-suit MIT property endows the W-VO2 catalyst with strong chemisorption against polysulfides, low energy barrier for liquid-to-solid conversion, and outstanding diffusion kinetics of Li-ion under high temperatures. Benefiting from these advantages, the Li-S batteries with W-VO2 modified separator exhibit significantly improved rate and long-term cyclic performance under 50°C. Remarkably, even at an elevated temperature (80°C), they still exhibit superior electrochemical performance. This work opens a rewarding avenue to use phase-changing materials for high-temperature Li-S batteries.

A near-field study of VO2/(100)TiO2 film and its crack-induced strain relief

Temperature-induced metal–insulator transition (MIT) in vanadium dioxide (VO2) has been under intense research interest for decades both theoretically and experimentally. Due to the complex nature of electron correlations, the underlying physics behind the MIT in VO2 has yet to be fully grasped. In this work, we utilize the fine resolution of the scattering-type scanning near-field optical microscope to investigate the MIT in an epitaxial VO2 thin film on the (100)R TiO2 substrate with mid-infrared light. Bidirectional tweed-like metal–insulator phase coexistence patterns are observed and understood under the Landau free energy paradigm. More interestingly, delayed metallic nucleation is observed near the surface cracks due to local strain relief. This research proposes ideas in investigating the temperature–pressure phase diagram and tuning the interplay between local strain and MIT in oxide thin films.

Magnetically driven band shift and metal-insulator transition in spin-orbit-coupled Sr3(Ir1−xRux)2O7

We report a combined infrared and angle-resolved photoemission study of the electronic response of $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{I}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{R}{\mathrm{u}}_{x})}_{2}{\mathrm{O}}_{7}$ ($x=0$, 0.22, 0.34). The low-temperature optical conductivities of the three compounds exhibit the characteristic feature of the effective total angular momentum ${J}_{\mathrm{eff}}=1/2$ antiferromagnetic Mott state. As the temperature increases across the antiferromagnetic ordering temperature ${T}_{\mathrm{N}}$, the indirect gap gradually closes whereas the direct gap remains open. In the optical conductivity of $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{I}{\mathrm{r}}_{0.66}\mathrm{R}{\mathrm{u}}_{0.34})}_{2}{\mathrm{O}}_{7}$ which shows a thermally driven insulator-metal transition at ${T}_{\mathrm{N}}$, a Drude-like response from itinerant carriers is registered in the paramagnetic phase. We observe in angle-resolved photoemission data of $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{I}{\mathrm{r}}_{0.66}\mathrm{R}{\mathrm{u}}_{0.34})}_{2}{\mathrm{O}}_{7}$ that the valence band shifts continuously toward the Fermi energy with the weakening of the antiferromagnetic order and crosses the Fermi level in the paramagnetic phase. Our findings demonstrate that the temperature-induced metal-insulator transition of the $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{I}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{R}{\mathrm{u}}_{x})}_{2}{\mathrm{O}}_{7}$ system should be attributed to a magnetically driven band shift.

Bipolar resistive switching in room temperature grown disordered vanadium oxide thin-film devices

We demonstrate bipolar switching with high OFF/ON resistance ratios (>104) in Pt/vanadium oxide/Cu structures deposited entirely at room temperature. The SET (RESET) process occurs when negative (positive) bias is applied to the top Cu electrode. The vanadium oxide (VOx) films are amorphous and close to the vanadium pentoxide stoichiometry. We also investigated Cu/VOx/W structures, reversing the position of the Cu electrode, and found the same polarity dependence with respect to the top and bottom electrodes, which suggests that the bipolar nature is linked to the VOx layer itself. Bipolar switching can be observed at 100°C, indicating that it not due to a temperature-induced metal–insulator transition of a vanadium dioxide second phase. We discuss how ionic drift can lead to the bipolar electrical behavior of our junctions, similar to those observed in devices based on several other defective oxides. Such low-temperature processed oxide switches could be of relevance to back-end or package integration processing schemes.

Highly depressed temperature-induced metal-insulator transition in synthetic monodisperse 10-nm V2O3 pseudocubes enclosed by {012} facets

Monodisperse 10-nm V(2)O(3) pseudocubes enclosed by {012} facets were successfully synthesized for the first time via a novel and facile solvothermal method, offering the first opportunity to elucidate the effect of finite-size and facet on the temperature-induced reversible metal-insulator transition (MIT) behavior of V(2)O(3). Very excitingly, the transition temperature of these V(2)O(3) pseudocubes drastically depressed from 133 K to 36 K and their corresponding hysteresis width highly narrowed from 17 K to 5 K, compared to the MIT behaviors of other irregular V(2)O(3) particles with average sizes of 10 nm, 20 nm, 40 nm, 170 nm and 2 μm. Notably, the size-related surface energy, grain boundary connectivity and volume expansion could be used to account for their strong size-dependent transition temperature and hysteresis width. Moreover, the improved grain boundary connectivity associated with well-defined {012} facets enabled these 10-nm V(2)O(3) pseudocubes to display a 10 times higher resistivity jump (at the order of 10(5)) and by nearly one-half smaller hysteresis width of 5 K than the irregular 10-nm V(2)O(3) particles with randomly exposed facets, directly evidencing the pronounced influence of facets on the MIT behavior. Briefly, the present work not only develops an effective strategy for synthesizing high-quality nanocrystals but also provides an excellent platform to investigate the size- and facet-dependent temperature-induced MIT behavior, enabling to design smart electrical switching nano-devices in the rapidly developing ultra-low temperature field.

X-ray absorption spectroscopic analysis of CuIr 2S 4

The thiospinel CuIr 2S 4 undergoes a temperature-induced metal–insulator transition around 230 K with structure transformation. We report on the results of X-ray absorption fine structure (XAFS) studies at the Cu K-edge and Ir L III-edge of CuIr 2S 4. X-ray absorption spectra were measured by a laboratory XAFS apparatus over the temperature range from 50 to 300 K. The Cu K-edge X-ray absorption near edge structure (XANES) spectra verified that Cu in CuIr 2S 4 is monovalent, whereas Cu in spinel-type oxide CuRh 2O 4 is divalent. Chemical shift of Ir L III-edge XANES spectrum of CuIr 2S 4 was larger than IrCl 3, and smaller than IrO 2. This finding supports the presence of both Ir 3+ and Ir 4+ in CuIr 2S 4.

Observation of spin-singlet state in [formula omitted

Bi x V 8 O 16 shows a temperature-induced metal–insulator transition (MIT) in the Bi composition range of 1.72 ⩽ x ⩽ 1.80 . The magnetic susceptibility χ of the compound with 1.72 ⩽ x ⩽ 1.80 shows an anomaly at the MIT temperature T MIT . Below T MIT , χ shows a rapid increase with decreasing temperature, following a Curie–Weiss law. The effective moment estimated from susceptibility below T MIT is about 0.4 μ B /(V atom). In order to reveal the origin of the magnetic moment below T MIT , we performed a V 51 -NMR study. Observation of the V 51 -NMR signal below T MIT indicates the existence of non-magnetic vanadium sites. The marked decrease of the absolute value of the Knight shift at low temperatures suggests the spin-singlet nature of the ground state. The origin of the small magnetic moment is attributed to the formation of the spin-singlet state.

Thickness-dependent metal–insulator transition in V2O3 ultrathin films

In this study, V2O3 ultrathin films about 5–20 nm thick were prepared on Al2O3 (0001) substrates through a reactive evaporation process. Auger electron spectroscopy and x-ray photoelectron spectroscopy have been used in situ to characterize their compositions and chemical states. Electric resistance measurements show that V2O3 films transform from metallic to semiconducting with the decrease of film thickness, which results from the a1g level rising because the lattice mismatch between the substrate and the film expands the c/a parameter ratio. No temperature-induced metal–insulator transition (like that in bulk V2O3) was observed in V2O3 thin films at low temperature. We conclude that stress plays a major role in suppressing the temperature-induced metal–insulator transition.

Metal–Insulator Transition in BixV8O16:51V NMR Study

The 51 V nuclear magnetic resonance and relaxation (NMR) are measured for Bi x V 8 O 16 in the temperature range from 4.2 K to 280 K. The Bi x V 8 O 16 system with 1.60 ≤ x ≤1.80 is known to crystallize in the Hollandite structure with a [V 8 O 16 ] framework which is composed of double chains formed by edge-shared VO 6 octahedra. In this system, we observed temperature-induced Metal–Insulator transition (MIT) in the composition range of 1.72 ≤ x ≤1.80. In order to reveal the mechanism of MIT in this system, 51 V NMR measurements have been performed on Bi 1.77 V 8 O 16 as well as on the simple Pauli paramagnetic compound Bi 1.60 V 8 O 16 for a comparative study: the Knight shift K and the spin–lattice relaxation rate 1/ T 1 of 51 V NMR have been measured to investigate the magnetic and electric properties. By applying the random phase approximation (RPA) method the absolute values of 1/ T 1 T are analyzed in the Pauli paramagnetic regime of Bi 1.77 V 8 O 16 above T MIT and Bi 1.60 V 8 O 16 for whole tempe...

Temperature-induced metal–insulator transition on a triangular binary system: Sn/Si(111)

WereportthatametallicSn/Si(111)- × R30° surface undergoes a metal–semiconductor transition upon annealing the surface above the transition temperature of 760 °C. The unique loss peak initially observed weakly from the metallic surfaces in our high-resolution electron-energy-loss spectra shifts gradually towards higher loss energy with increasing temperature. The peak eventually reaches 2.0 eV, revealing a band gap of about 1.2 eV at 810 °C. We find that a two-dimensional (2D) disordered alloy model, where the semiconducting surface is a 2D mixture of Sn and Si substitutions, is quite consistent with our experimental data.

Scalings and phase diagram in the vicinity of the first-order metal–insulator transition in Y 1− xCa xTiO 3

Magnetic susceptibility and electrical resistivity in the metal–insulator (MI) transition of a mixed-valent Ti-oxide system, Y 1− x Ca x TiO 3, are investigated at high pressure. This system exhibits obviously the first-order temperature-induced MI transition at x=0.35–0.4. In the low temperature metallic phase, susceptibility, χ( T), is strongly suppressed with pressure. On the other hand, in the high temperature semiconductiong phase χ( T) obeying the Curie–Weiss law is affected weakly by applying pressure. Below the MI transition temperature resistivity, ρ( T), shows the characteristic Fermi liquid behavior, ρ= ρ 0+ AT 2. The coefficient of the T 2-term in resistivity, A, and the residual resistivity, ρ 0, decrease rapidly with pressure, while following the simple relation of A(x, P)∝ρ 0(x, P) . A scaled temperature-composition phase diagram shows that the first-order transition terminates at both the lower and upper critical points and A, ρ 0 and χ are enhanced enormously at the lower critical point. We present scalings between susceptibility and resistivity in the metallic state taken into account the distribution of the Kondo temperature.

Optical response of CuIr 2X 4 (X=S,Se) due to metal–insulator transition

CuIr 2X 4 (X=S,Se) are ones of ternary metal chalcogen compounds with a thiospinel structure. CuIr 2S 4 undergoes a temperature-induced metal–insulator transition around 226 K. We have measured the temperature dependence of the optical reflection spectra of both samples in the energy regions of 0.005–6 eV in order to study the electronic structure very close to the Fermi level ( E F) and its change across the metal–insulator transition (X=S). Obtained optical conductivity spectrum, σ( ω), of CuIr 2S 4 below 221 K showed a growth of a new band around 0.4 eV instead of a band at 0.7 eV in the metallic phase. The growth of a new band which compensates the decrease in the spectral intensity of the Drude component suggests the appearance of the localized band below the E F level.

Phase transition in perovskites

Transport and magnetic properties of have been studied to construct the magnetic phase diagram. is found to be a weak ferromagnet with K. Anomalous magnetic behaviour of x = 0 and x = 0.1 compositions is attributed to the Nd-sublattice magnetic contribution. Materials with and are suggested to be mixtures of antiferromagnetic and ferromagnetic states. The inhomogeneous ferromagnets exhibit the giant magnetoresistance effect without the temperature-induced metal-insulator transition. The results are discussed in terms of the superexchange interaction model proposed by Goodenough.

Pressure Effect on Metal/Insulator Transition in Y1-xCaxTiO3.

The magnetic susceptibility and electrical resistivity in the metal-insulator (MI) transition of a mixed-valent Ti-oxide system, Y1-x CaxRTiO3, is investigated as a function of pressure. This system exhibits obviously the temperatureinduced MI transition at x=0. 3-0. 4. At the low temperature metallic phase, susceptibility, X (T), is strongly suppressed with pressure. On the other hand, in the high temperature semiconductiong phase X (T) obeying the Curie-Weiss law is not affected strongly with pressure. The coefficient of the T2-term in resistivity, A, and the residual resistivity, ρ (0), are decreases rapidly with pressure. At x=0. 375 and 0. 39, we recognize the relation of A∝X (10 K) 2 at the low temperature metallic state.

Heat diffusivity of La1−xCaxMnO3 epitaxial layers

The Seebeck effect and the thermal diffusivity of La0.67Ca0.33MnO3−δ were investigated in the temperature range 35<T<300 K and in particular in the regime of the high magnetoresistance and maximum resistivity ρmax. We find a sign change of the thermoelectric power S(T) and a step in heat conductivity near ρmax which we attribute to a temperature-induced metal–insulator transition at about 80 K, suggesting that the large giant magnetoresistance found in these substances is due to a field-induced metal–insulator transition.