Antiferromagnetic phase transition in garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12

Ferrimagnetism has been extensively studied in garnets, whereas it is rare to find the antiferromagnet. Present work will demonstrate antiferromagnetism in the two Mn–V-garnets. Antiferromagnetic phase transition in AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ has been found, where the magnetic Mn²⁺ ions locate only on octahedral A site. The heat capacity shows sharp peak due to antiferromagnetic order with the Néel temperature T_N=23.8 K for AgCa₂Mn₂V₃O₁₂ and T_N=14.2 K for NaPb₂Mn₂V₃O₁₂. The magnetic entropy change over a temperature range 0–50 K is 13.9 J K^?1 mol-Mn²⁺-ions^?1 for AgCa₂Mn₂V₃O₁₂ and 13.6 J K^?1 mol-Mn²⁺-ions^?1 for NaPb₂Mn₂V₃O₁₂, which are in good agreement with calculated value of Mn²⁺ ion with spin S=5/2. The magnetic susceptibility shows the Curie–Weiss behavior over the range 29–350 K. The effective magnetic moment μ_eff and the Weiss constant θ are μ_eff=6.20 μ_B Mn²⁺-ion^?1 and θ=?34.1 K (antiferromagnetic sign) for AgCa₂Mn₂V₃O₁₂ and μ_eff=6.02 μ_B Mn²⁺-ion^?1 and θ=?20.8 K for NaPb₂Mn₂V₃O₁₂.     

A model for the metal-to-insulator transition in V2O3

The metal-to-insulator transition in V₂O₃ is described by a model that is based on the electronic band structure of this material. The vanadium 3d-t _2g band decomposes in the trigonal symmetry into two bands, a _1g and e _π. The a _1g band consists of orbitals connecting pairs of c-axis neighbouring atoms, while the e _π band consists of orbitals in the plane perpendicular to the c-axis. The change in distance between c-axis neighbours changes the nature of the a _1g band from molecular (delocalized) to atomic (localized). The localization of the a _1g electrons causes through the atomic exchange interaction also the localization of the e _π electrons, and this localization creates a gap in the e _π band which causes the material to become insulating. This model is treated in the Hartree-Fock approximation (the ‘Excitonic’ model) at zero and finite temperatures, and various aspects of the transition, such as changes in the c/a ratio, the creation of magnetic moments, changes in covalency, the effect of pressure and the order of the transition, are investigated.     

Orbital switching and the first-order insulator-metal transition in paramagnetic V2O3

The first-order metal-insulator transition (MIT) in paramagnetic V2O3 is studied within the ab initio scheme LDA+DMFT, which merges the local density approximation (LDA) with dynamical mean field theory (DMFT). With a fixed value of the Coulomb U=6.0 eV, we show how the abrupt pressure driven MIT is understood in a new picture: a pressure-induced decrease of the trigonal distortion within the strong correlation scenario (which is not obtained within LDA). We find good quantitative agreement with (i) switch of the orbital occupation of (a(1g),e(pi)(g1),e(pi)(g2)) and the spin state S=1 across the MIT, (ii) thermodynamics and dc resistivity, and (iii) the one-electron spectral function, within this new scenario.     

An X-ray investigation of the high temperature anomaly in pure V2O3

An X-ray investigation of pure V₂O₃ in the temperature region 300 to 700 K has revealed the presence of more than one phase coexisting from ≈450 to at least 700 K. This observation can be correlated with the high temperature resistivity anomaly and with recent nmr studies on this material.     

Hyperfine spectroscopic study of the metal-insulator transition in V2O3

The metal-insulator (M-I) transition in vanadium sesquioxide V₂O₃ has been investigated by time differential perturbed angular correlation measurements of the electric fieldgradient (EFG) and the magnetic hyperfine field at dilute¹¹¹Cd impurities. The EFG undergoes a first-order change at the M-I transition at T_t=160 K, but does not reflect the high temperature resistivity anomaly. The increase of the EFG with temperature in the metallic phase can be attributed to thermal variations of the oxygen sublattice. The temperature dependence of the magnetic hyperfine field in the insulating phase follows a Brioullin function with a saturation value of H_hf(O)=15 KOe and an extrapolated Neel temperature, which, depending on the impurity concentration, varies between 188 and 230 K.     

Local structure changes in V2O3 below and above the metal-insulator transition

According to diffraction measurements of the average structure, V₂O₃ changes on heating from monoclinic to trigonal, accompanied by a 1.4% volume decrease, where some V - V distances decrease by about 0.11Å, favoring the Mott - Hubbard mechanism of the phase transition from insulator to metal. Our x-ray absorption fine structure measurements of the local structure of the single crystal V₂O₃ show the same decrease in volume but no change in local symmetry in the transition, indicating that the phase transition contains a significant order-disorder component, contrary to the purely displacive model based on diffraction results.     

Effect of the metal-insulator transition on vanadium K-photoabsortion spectrum in V2O3

The effect of the metal-insulator phase transition on the X-ray K-absorption spectrum of V in V₂O₃ was measured by synchrotron radiation. A different behavior of the transition to 3d vs 4p states in the two phases was observed. These results have been correlated with the change in the screening of the 1s hole due to the variation of the dielectric constant ?(q, ω) in the two phases.     

Anomalies in the X-ray diffuse scattering from Cr-doped V2O3 at the high temperature metal—insulator transition

Several anomalies in the X-ray diffuse scattering of Cr-doped V₂O₃ have been observed in proximity of the high temperature metal—insulator transition. It is speculated that charge density waves might be involved. The original interpretation in terms of a Mott transition is brought up again for discussion.     

Metal-insulator transition investigation in V2O3 by nuclear magnetic resonance and relaxation

The high temperature metal-insulator transition in pure V₂O₃ has been investigated by nuclear magnetic resonance and relaxation. The relaxation rate in the range 160–320K was found to satisfy the Korringa relaxation and at temperature above 550K to be constant as expected for a paramagnetic insulator. In the intermediate temperature range the high field resonance line shape showed broadening which we interpret as due to the coexistence of a metallic and an insulating phase.     

Magnetically mediated transparent conductors: In2O3 doped with Mo

First-principles band structure investigations of the electronic, optical, and magnetic properties of Mo-doped In2O3 reveal the vital role of magnetic interactions in determining both the electrical conductivity and the Burstein-Moss shift which governs optical absorption. We demonstrate the advantages of the transition metal doping which results in smaller effective mass, larger fundamental band gap, and better overall optical transmission in the visible as compared to commercial Sn-doped In2O3. Similar behavior is expected upon doping with other transition metals opening up an avenue for the family of efficient transparent conductors mediated by magnetic interactions.     

Quantum phase transition in the itinerant antiferromagnet (V0.9Ti0.1)2O3

Quantum-critical behavior of the itinerant electron antiferromagnet (V0.9Ti0.1)2O3 has been studied by single-crystal neutron scattering. By directly observing antiferromagnetic spin fluctuations in the paramagnetic phase, we have shown that the characteristic energy depends on temperature as c1 +c2T3/2, where c1 and c2 are constants. This T3/2 dependence demonstrates that the present strongly correlated d-electron antiferromagnet clearly shows the criticality of the spin-density-wave quantum phase transition in three space dimensions.     

Search for new transparent conductors: Effect of Ge doping on the conductivity of Ga2O3, In2O3 and Ga1.4In0.6O3

Only a small amount (≤3.5 mol%) of Ge can be doped in Ga₂O₃, Ga_1.4In_0.6O₃ and In₂O₃ by means of solid state reactions at 1400 °C. All these samples are optically transparent in the visible range, but Ge-doped Ga₂O₃ and Ga_1.4In_0.6O₃ are insulating. Only Ge-doped In₂O₃ exhibits a significant decrease in resistivity, the resistivity decreasing further on thermal quenching and H₂ reduction. The resistivity of 2.7% Ge-doped In₂O₃ after H₂ reduction shows a metallic behavior, and a resistivity of ～1 mΩ cm at room temperature, comparable to that of Sn-doped In₂O₃.     

Mott-hubbard metal-insulator transition in paramagnetic V2O3: an LDA+DMFT(QMC) study

The electronic properties of paramagnetic V2O3 are investigated by the computational scheme LDA+DMFT(QMC). This approach merges the local density approximation (LDA) with dynamical mean-field theory (DMFT) and uses quantum Monte Carlo simulations (QMC) to solve the effective Anderson impurity model of DMFT. Starting with the crystal structure of metallic V2O3 and insulating (V0.962Cr0.038)2O3 we find a Mott-Hubbard transition at a Coulomb interaction U approximately 5 eV. The calculated spectrum is in very good agreement with experiment. Furthermore, the orbital occupation and the spin state S = 1 determined by us agree with recent polarization dependent x-ray-absorption experiments.     

Low-temperature resistance anomaly at SrTiO3 grain boundaries: evidence for an interface-induced phase transition

Variable temperature transport between 1.4 and 300 K, structural imaging, and theoretical calculations were used to characterize the properties of electrically active 24 degrees and 36.8 degrees [001] tilt SrTiO3 grain boundaries with 0.1 at. % niobium doping. An anomaly in boundary resistance and capacitance characteristics typical of a positive temperature coefficient effect is observed. This behavior is indicative of interface-induced dipole ordering. The detailed atomic structures of these grain boundaries were determined from a comparison of ab initio calculations and Z-contrast TEM images. The number of excess electrons at the boundaries determined experimentally and theoretically agrees and is associated with the boundary structural units.     

Formation and Evolution of Carbonate Species in Na-Doped Al2O3 and V2O5-Al2O3

A study is carried out by FT-IR spectroscopy of the carbonate species formed upon interaction of CO₂ with alumina and vanadia-alumina catalysts doped with sodium. It is found that the presence of sodium enhances the ability of the catalyst surface to adsorb CO₂, yielding to carbonate formation. The species formed changes in the presence of vanadium, shifting the ν_COO stretching bands towards higher wavenumbers than those recorded in Na-Al₂O₃ systems.