Electrical and Magnetic Properties of NbxTi1-xO2

Sakata, Kimiko; Nishida, Isao; Matsushima, Masaaki; Sakata, T.

doi:10.1143/jpsj.27.506

Cited by 22 publications

(27 citation statements)

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“…Both phases are correctly described within density functional theory (DFT) and the LDA. The results strongly support the band theoretical point of view of the MIT as arising predominantly from a Peierls-type instability of the rutile phase.Evidence for the MIT being driven by strong metalmetal bonding was first inferred from the small and almost temperature independent paramagnetic susceptibility below the transition temperature as well as from the enhanced thermal expansion of the c lattice parameter just above T c [18,19,20,21]. Additional support came from strong decrease of T c , nonlinear variation of c, and crossover of the magnetic susceptibility to Curie-Weiss behaviour on substitution of Ti for Nb, which destroys the metal-metal bonding [19,20,21].…”

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confidence: 99%

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The metal-insulator transition of NbO ₂ : An embedded Peierls instability

Eyert

2002

Europhys. Lett.

101

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Results of first principles augmented spherical wave electronic structure calculations for niobium dioxide are presented. Both metallic rutile and insulating low-temperature NbO2, which crystallizes in a distorted rutile structure, are correctly described within density functional theory and the local density approximation. Metallic conductivity is carried to equal amounts by metal t2g orbitals, which fall into the one-dimensional d band and the isotropically dispersing e π g bands. Hybridization of both types of bands is almost negligible outside narrow rods along the line X-R. In the lowtemperature phase splitting of the d band due to metal-metal dimerization as well as upshift of the e π g bands due to increased p-d overlap remove the Fermi surface and open an optical band gap of about 0.1 eV. The metal-insulator transition arises as a Peierls instability of the d band in an embedding background of e π g electrons. This basic mechanism should also apply to VO2, where, however, electronic correlations are expected to play a greater role due to stronger localization of the 3d electrons.PACS numbers: 71.30.+h, 72.15.Nj Despite intense work over decades the metal-insulator transition (MIT) of VO 2 has remained a matter of controversy [1,2,3,4,5,6,7]. This is related to the simultaneous occurrence of a structural transformation from the high-temperature rutile phase to a distorted monoclinic structure, which is characterized by (i) pairing of the metal atoms within chains parallel to the rutile c axis and (ii) their lateral zigzag-like displacement [8]. Electronic states near the Fermi energy are of mainly V 3d t 2g character. They separate into the d band, which mediates V-V overlap along the metal chains, and the remaining e π g bands [3]. At the transition, splitting of the d band and upshift of the e π g bands due to increased metal-oxygen overlap produce a finite band gap. Dispute centers about the question, whether the d splitting is caused by metal dimerization or by increased electronic correlations resulting from the reduced screening by the e π g electrons [3,4]. State of the art band calculations gave strong hints of a structural instability but were not able to reproduce the insulating gap due to the shortcomings of the local density approximation (LDA) [5]. Interestingly, only very few studies have dealt with the neighbouring dioxides, which display related phenomena. NbO 2 likewise undergoes a MIT (at 1081 K) and a simultaneous structural transition from rutile to a distorted variant having a body-centered tetragonal (bct) lattice [9,10]. Despite the differences in long range order, local deviations from rutile are the same as in VO 2 , i.e. niobium atoms dimerize and experience lateral displacements at the transition [11,12,13]. In contrast, the metallic oxides MoO 2 , WO 2 , TcO 2 , and α-ReO 2 all crystallize in the same monoclinic structure as VO 2 [14]. No doubt the phase transitions of VO 2 and NbO 2 as well as the destabilization of the rutile structure in all these dioxides call for a uni...

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“…Additional support came from strong decrease of T c , nonlinear variation of c, and crossover of the magnetic susceptibility to Curie-Weiss behaviour on substitution of Ti for Nb, which destroys the metal-metal bonding [19,20,21].…”

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The metal-insulator transition of NbO ₂ : An embedded Peierls instability

Eyert

2002

Europhys. Lett.

101

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“…Some of the binary oxides of these metals have been studied in electrochemical environments. 95,[99][100][101][102][103][104][105][106][107][108] In most cases, preparation of bulk powders is easily carried out under conditions that fix the oxygen stoichiometry to 2 per cation. For example, sealing the appropriate ratio of precursors in a silica tube is sufficient to fix the oxygen stoichiometry during synthesis: eg.…”

Section: Oxides As Supports and Catalystsmentioning

confidence: 99%

Cornell Fuel Cell Institute: Materials Discovery to Enable Fuel Cell Technologies

Abruña¹,

DiSalvo²

2012

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IntroductionThe discovery and understanding of new, improved materials to advance fuel cell technology are the objectives of the Cornell Fuel Cell Institute (CFCI) research program. CFCI was initially formed in 2003. This report highlights the accomplishments from 2006-2009. Many of the grand challenges in energy science and technology are based on the need for materials with greatly improved or even revolutionary properties and performance. [1][2][3][4][5][6] This is certainly true for fuel cells, which have the promise of being highly efficient in the conversion of chemical energy to electrical energy. Fuel cells offer the possibility of efficiencies perhaps up to 90 % based on the free energy of reaction.Thus, there is considerable interest in fuel cells which, however, have yet to live up to their promise, not only for efficiency, but also for cost, durability, performance, etc. [7][8][9][10][11][12] Here, the challenges are clearly in the materials used to construct the heart of the fuel cell: the membrane electrode assembly (MEA). The MEA consists of two electrodes separated by an ionically conducting membrane. Each electrode is a nanocomposite of electronically conducting catalyst support, ionic conductor and open porosity, that together form three percolation networks that must connect to each catalyst nanoparticle; otherwise the catalyst is inactive.While there are a number of different fuel cell technologies 12, 13 , the CFCI is focused on polymer electrolyte membrane fuel cells (PEMFCs). The broadest applications of these fuel cells in portable power or individual transportation vehicles The highest interest is in fuels that are carbon neutral, such as hydrogen, which could be carbon neutral if it is generated from sunlight, rather than from natural gas, as is done now. Fuels derived from algal or plant matter are also of interest, if we could discover catalysts that enable complete oxidation of those fuels, or if they could be efficiently converted to hydrogen.Our research program is divided into four main areas: Electrocatalysts and Supports, Self-Assembly and Meso-Structured Electrodes, Membranes and Theory. The first section is further sub-divided into combinatorial synthesis and screening, oxide supports, mechanistic studies and surface characterization. This report highlights selected advances rather than give an exhaustive account of previous results. A full appreciation of the scope and advances of our program can be garnered from our publications

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“…NbO 2 undergoes a phase transition at around 1073 K, which is, in all respects, akin to that for VO 2 at 341 K (Sakata, Sakata & Nishida, 1967;Sakata, 1969a). The high-temperature (HT) phase has a simple rutile-type structure (space group P42/mnm) and the low-temperature (LT) phase a slightly deformed one, characterized by a series of superlattice reflections on an Xray diffraction pattern.…”

Section: Introductionmentioning

confidence: 96%

Structural features of Nb1−xTixO2

Sakata¹

1979

Acta Crystallogr Sect B

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X-ray structural analysis with Mo Ka radiation was carried out for Nb~_xTixO 2 (0 < x < 0.2) in its lowtemperature deformed rutile phase 14~/a. Crystals were prepared by chemical transport, with TeC1 a as the transport agent. Atomic parameters determined for NbO2 are in good agreement with those obtained by neutron diffraction. For superlattice reflections with h, k = 2n + 1 and l = 2n in the system Nbt_xTixO2, the discrepancy between F o and F c is appreciable. Attempts to clarify this situation are made and discussed. Analysis of interatomic distances for NbO 2 showed that Nb-Nb pair bonds exist along z and strong Nb-O-O-Nb bonds exist along x and y in the form of non-intersecting linear chains. With such a molecular-chain model the structural features and phase transition in the Nbt_xTixO 2 system are discussed.

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Electrical and Magnetic Properties of Nb_xTi_1-xO₂

Cited by 22 publications

References 4 publications

The metal-insulator transition of NbO ₂ : An embedded Peierls instability

The metal-insulator transition of NbO ₂ : An embedded Peierls instability

Cornell Fuel Cell Institute: Materials Discovery to Enable Fuel Cell Technologies

Structural features of Nb1−xTixO2

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Electrical and Magnetic Properties of NbxTi1-xO2

Cited by 22 publications

References 4 publications

The metal-insulator transition of NbO 2 : An embedded Peierls instability

The metal-insulator transition of NbO 2 : An embedded Peierls instability

Cornell Fuel Cell Institute: Materials Discovery to Enable Fuel Cell Technologies

Structural features of Nb1−xTixO2

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Electrical and Magnetic Properties of Nb_xTi_1-xO₂

The metal-insulator transition of NbO ₂ : An embedded Peierls instability

The metal-insulator transition of NbO ₂ : An embedded Peierls instability