The O-Ta (Oxygen-Tantalum) system

Garg, S.P.; Krishnamurthy, N.; Awasthi, Alok K.; Venkatraman, M.

doi:10.1007/bf02648373

Cited by 179 publications

(110 citation statements)

References 60 publications

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“…According to XSMA data the Ta : O ratio averaged over the surface is different for local XSMA for regions of the carrier adjacent to the surface clusters of metals. The Ta : O ratios obtained by local XSMA data ( Table 2) indicate the presence in the adjacent layer of oxygen-deficient oxides based on Ta 2 O 5-x (x = 0.1-0.03 and 0.12-0.15), fragment of structures of which, according to [9], are identical to those observed in the structure of Ta These values are close to those determined earlier for the defect phase TaO 2-x , belonging to the rutile structure type (space group P4 2 /mnm with parameters a = 4.609 Å and c = 3.088 Å) (see [11,12]). …”

supporting

confidence: 74%

Properties of the surface layer and catalytic activity of Ta2O5 with Pt or Pd additives in the oxidation of hydrogen

Lesnyak¹,

Yatsimirskii²,

Boldyreva³

et al. 2008

Theor Exp Chem

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The catalytic activity in the oxidation of hydrogen (in the gaseous state in the presence of excess oxygen) has been studied for samples of Pt(Pd)/Ta 2 O 5-x , formed by reduction with hydrogen. The samples obtained had greater activity than the traditional catalysts Pt(Pd)/Al 2 O 3 . According to X-ray diffraction analysis and electron microscopic studies, Ta 2 O 5-x becomes amorphous with the formation of more reduced non-stoichiometric oxygen-deficient tantalum oxides with a surface layer of catalyst.It is known [1-3] that higher oxides of transition metals of groups V and VI, modified by small amounts of precious metals, particularly platinum or palladium, on partial reduction with hydrogen or light hydrocarbons, are capable of forming surface compounds which are highly active in various heterogeneous catalytic processes.We have shown previously [4, 5] that formation of tungsten (molybdenum) oxide bronzes and/or oxygen deficient phases by the reduction of tungsten or molybdenum trioxides with hydrogen in the presence of metallic Pt or Pd leads to a considerable increase in both the specific surface of the composite catalysts and in its catalytic activity in the oxidation of small molecules (H 2 , CO, and CH 4 ).At the present time there is no information on the state of the surface layer of oxides of transition metals of group Vvanadium, niobium and tantalum obtained on reduction with hydrogen in the presence of platinum metals, nor data on their catalytic activity in the oxidation of small molecules in gaseous mixtures with an excess of oxygen.The objective of the present work was to investigate the properties of the surface layer and the catalytic activity of Ta 2 O 5 with a layer of Pt or Pd, reduced with hydrogen, on the oxidation of H 2 .Samples were obtained by impregnating tantalum pentaoxide (Alfa-Aesar 99.995 mass %) with solutions of H 2 PtCl 6 or PdCl 2 to 0.5 mass % of platinum metal in the composition of the sample. The samples calcined at 393 K (1 h) and reduced with hydrogen (10 vol.% H 2 in Ar) for 1 h at 673 K. The specific surface of the samples was determine by low temperature adsorption of argon. To compare the activity of the catalysts prepared with traditional catalysts, samples with 0.5 mass % Pt(Pd)/Al 2 O 3 (S sp Al 2 O 3 = 3.6 m 2 /g) were prepared by the same method. Treatment of these catalysts with hydrogen was carried out by the same regime as for the Pt(Pd)/Ta 2 O 5 samples.Catalytic activity of the samples was investigated with a flow-type apparatus at atmospheric pressure with a reaction mixture with the composition 1 vol.% H 2 + 20 vol.% O 2 and 79 vol.% Ar. The rate of flow of the reaction mixture was 0.1 L/min and the mass of catalysts was 0.5 g. Analysis of the reaction components (H 2 and O 2 ) was by chromatography (LKhM-8MD chromatograph with a thermal conductivity detector). The temperature required to achieve a particular extent of conversion of H 2 into H 2 O served as the characteristic of catalytic activity.

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supporting

confidence: 74%

Properties of the surface layer and catalytic activity of Ta2O5 with Pt or Pd additives in the oxidation of hydrogen

Lesnyak¹,

Yatsimirskii²,

Boldyreva³

et al. 2008

Theor Exp Chem

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“…Ta 2 O 5 has an orthorhombic phase up to ∼ 1350 • C, above which a transition to a tetragonal phase occurs [7,8]. Although the low-temperature orthorhombic phase is of technological relevance, it has proven difficult to characterize structurally.…”

mentioning

confidence: 99%

Hidden Structural Order in OrthorhombicTa2O5

Lee

Kim

et al. 2013

Phys. Rev. Lett.

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We investigate using first-principles calculations the atomic structure of the orthorhombic phase of Ta2O5. Although this structure has been studied for decades, the correct structural model is controversial owing to the complication of structural disorder. We identify a new low-energy highsymmetry structural model where all Ta and O atoms have correct formal oxidation states of +5 and −2, respectively, and the experimentally reported triangular lattice symmetry of the Ta sublattice appears dynamically at finite temperatures. To understand the complex atomic structure of the Ta2O3 plane, a triangular graph-paper representation is devised and used alongside oxidation state analysis to reveal infinite variations of the low-energy structural model. The structural disorder of Ta2O5 observed in experiments is attributed to the intrinsic structural variations, and oxygen vacancies that drive collective relaxation of the O sublattice.Tantalum pentoxide (Ta 2 O 5 ) is one of the most extensively studied transition metal oxides due to its potential for technological applications such as anti-reflection coatings, photocatalysis, and high-k dielectrics for highdensity transistors [1][2][3]. It has recently attracted interest as the material of choice for resistance-change memory and the related memristor [4][5][6], however, its crystalline structure, which is critical to its unique structural and electronic properties, is still unclear.Ta 2 O 5 has an orthorhombic phase up to ∼ 1350 • C, above which a transition to a tetragonal phase occurs [7,8]. Although the low-temperature orthorhombic phase is of technological relevance, it has proven difficult to characterize structurally. Early X-ray diffraction studies [9,10] found that the orthorhombic Ta 2 O 5 phase consists of two-dimensional (2D) layers of Ta 2 O 3 in the ab plane, and two-fold coordinated O atoms that connect the Ta atoms in adjacent layers (forming linear chains of -Ta-O-Ta-O-Ta-) along the c direction. In the Ta 2 O 3 plane, it was understood that the Ta atoms form a triangular arrangement, but the arrangement of O atoms was unresolved, with many weak superstructure lines [10]. Later, Roth and coworkers reported that the Ta 2 O 3 plane consists of a disordered array of Ta atoms in octahedral and pentagonal bipyramidal arrangements, with an 11-fold increase of the lattice parameters in the b direction [11]. The superstructure periodicity was shown to change when aliovalent cations, such as W or Li, were incorporated in small amounts to stabilize the phase [12,13]. The superstructures consist of a set of small-unit basic structures and invoked the concept of infinitely adaptive structures [14] for nonstoichiometric Ta 2 O 5−x thatwithin certain composition limits-every composition orders into a different superstructure.Extensive theoretical studies have been made with regards to the bulk properties, dielectric response, and oxygen vacancies (and their diffusion) of Ta 2 O 5 [15][16][17][18][19][20][21][22][23]. In these studies, the low-temperature Ta 2 O 5 pha...

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“…Their findings also suggest that a-Ta 2 O x provides the best binary TMO system to exhibit a continuously variable degree of electron localization and therefore the best opportunity to tune conduction as a function of x. Similarly, the binary simplicity of the ambient Ta-O phase diagram [23] is widely-cited as the explanation for the good repeatability and endurance characteristics of Ta 2 O x memristors for basic ON/OFF operation.…”

Section: Tablesmentioning

confidence: 88%

“…Despite the simplicity of the Ta-O phase diagram, the crystal structure of Ta 2 O 5 remains a subject of current debate [23][24][25] [27,28].…”

Section: Tablesmentioning

confidence: 99%

Electrical conductivity in oxygen-deficient phases of transition metal oxides from first-principles calculations.

Bondi¹,

Desjarlais²,

Thompson³

et al. 2013

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Density-functional theory calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula are applied to predict electrical conductivity in Ta 2 O x (0 ≤ x ≤ 5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (V O n ; n=0,1+,2+). Our calculations of DC conductivity at 300K agree well with experimental measurements taken on Ta 2 O x thin films and bulk Ta 2 O 5 powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta 2 O 5 electronic structure provide further theoretical basis to substantiate V O 0 as a donor dopant in Ta 2 O 5 and other metal oxides. Furthermore, this dopant-like behavior appears specific to neutral V O cases in both Ta 2 O 5 and TiO 2 and was not observed in other oxidation states. This suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for V O 0 in transition metal oxides.

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The O-Ta (Oxygen-Tantalum) system

Cited by 179 publications

References 60 publications

Properties of the surface layer and catalytic activity of Ta2O5 with Pt or Pd additives in the oxidation of hydrogen

Properties of the surface layer and catalytic activity of Ta2O5 with Pt or Pd additives in the oxidation of hydrogen

Hidden Structural Order in OrthorhombicTa2O5

Electrical conductivity in oxygen-deficient phases of transition metal oxides from first-principles calculations.

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