Vanadium oxides - properties and applications

I. Introductory remarks

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Abstract: This paper considers several issues related to the quantitative analysis of the properties of vanadium oxides and their applications. These issues will be addressed in more detail in follow-up papers in terms of the structural and defect chemistry properties as well as the related electronic, electrical and transport properties. This set of papers includes both a major collection of literature references and own unpublished data concerning the properties and potential practical applications of vanadium oxides as well as their critical analysis.

Keywords: vanadium oxide electronics, metal-insulator transition, thin films, electronic structure, point defect structure, chemical diffusion

1. INTRODUCTION

Solid metal oxides are the most abundant materials in Earth’s crust. Their structure varies, and so do their chemical, mechanical, electrical, optical and magnetic properties. Metal oxide semiconductors are strikingly different from conventional inorganic semiconductors such as silicon and III-V compounds with respect to material design concepts, defect structure, electronic structure and charge transport mechanisms. This allows them to serve both conventional and completely new functions. Recently, remarkable advances in oxide electronics have been achieved. The term ‘oxide electronics’ had emerged not too long ago, but its place in the literature of the subject has already been firmly established [1].

The unprecedented diversity of physical properties exhibited by transition metal oxides offers immense prospects for various electronic applications. This is all the more significant given the fact that the modern IT revolution has been based on technological progress that has allowed the performance of electronic devices to grow at an exponential rate. Over the history of the development of electronic components ranging from a vacuum diode to modern highly integrated circuits with nanosized individual elements, the question of the impact of physical limitations on further progress in this area has become increasing relevant. The objective of the presented series of papers [2-6] was to analyse the available experimental and theoretical material on vanadium oxides. This introductory paper gives a short overview of the critical issues on this topic, which will be addressed more comprehensively in papers [2-6].

2. VANADIUM OXIDE ELECTRONICS

The outstanding physical and chemical properties of vanadium oxides give them an exceptional position among oxide materials [1, 7]. About 14 vanadium oxides are known. They can be classified as:

single-valence vanadium oxides: VO, V2O3, VO2 and V2O5

double valence vanadium oxides: VnO2n-1 (2 < n < 10) and VnO2n+1 (n = 3, 4, 6) known as Magnéli and Wadsley series, respectively.

Reference [2] includes a vanadium-oxygen phase diagram, and it describes the thermodynamics of the formation of vanadium oxides as well as the thermodynamics of the partial reduction of vanadium pentoxide, which leads to the synthesis of respective lower valence vanadium oxides. Moreover, the crystallographic structure of vanadium oxides is discussed.

3. METAL-INSULATOR TRANSITIONS in VANADIUM OXIDES

One of the most spectacular phenomena observed for vanadium oxides is an abrupt change in electrical conductivity from one typical of an insulator or semiconductor to that typical of a metal phase. This phenomenon, called the metal-insulator transition (MIT), is observed for all vanadium oxide phases with the exception of VO and V7O13. Reference [3] describes a Mott-Hubbard metal-insulator transition. The temperature dependence of the metal-insulator transition, TMIT, versus the chemical composition of vanadium oxide was analysed. A detailed description of the MIT in VO2 and V2O5 and its practical applications is included.

4. THIN FILMS – PREPARATION, PROPERTIES, APPLICATIONS

Vanadium dioxide, VO2, has been intensively studied due to its unique properties and the fact that it undergoes the metal-insulator transition (MIT) at near room temperature. It is currently considered one of the most promising materials for oxide electronics. Both planar and sandwich thin-film Metal-Oxide – Metal (MOM) devices based on VO2 exhibit electrical switching with an S-shaped I-V characteristics, and this switching effect is associated with the MIT. In an electrical circuit containing such a switching device, relaxation oscillations are observed if the load line intersects the I–V curve at a unique point in the negative differential resistance (NDR) region [8]. Each of these effects is an advantage when designing various devices based on oxide electronics, especially the Mott-FET (field effect transistor based on the Mott MIT material) [9] or elements of dynamic neuron networks based on coupled oscillators [10]. Reference [4] describes the preparation, properties, and applications of vanadium oxides thin films.

5. ELECTRONIC STRUCTURE

Reference [5] describe the electronic properties of vanadium oxides. The electronic structure of the three main vanadium oxides – V2O3, VO2 and V2O5 – is reviewed. The electronic properties were studied via optical measurements. Optical transmittance and reflectance spectra were measured over a wide wavelength range with a Lambda 19 Perkin-Elmer double beam spectrophotometer equipped with a 150 mm integrating sphere. The optical properties such as energy band gap, of vanadium pentoxide thin films were determined. It was found that a direct allowed (DA) transition is the most probable one in the studied films.

6. DEFECT STRUCTURE and ELECTRICAL PROPERTIES of VANADIUM OXIDES

The electrical properties of vanadium oxides were investigated by analyzing the complex impedance spectra (IS) as a function of temperature (T), oxygen partial pressure (p(O2)) and equilibration time (t) of electrical conductivity, σ. Based on the dependence σ = f[p(O2)], the predominant type of point defects was determined. The dependence σ = f(T) provided values of the activation energy of conductivity and the corresponding enthalpy of point defect formation. On the other hand, σ = f(t) made it possible to determine the chemical diffusivity of point defects.

7. CONCLUSIONS

The physicochemical properties of vanadium oxides have been the subject of many conflicting reports. The presented series of papers [2-7] provides an extensive analysis of the available experimental data on the their structure and the related properties, in particular, the electrical properties that determine the feasibility of the practical application of vanadium oxides as materials used for the production of oxide electronics.

References

Oxide electronics and functional properties of transition metal oxides, A. Pergament, ed., New York: NOVA Sci. Publishers, 2014.

K. Schneider, Vanadium oxides – properties and applications. II Vanadium oxide electronics, this issue.

K. Schneider, Vanadium oxides – properties and applications. III. Metal-Insulator Transition (MIT), vanadium oxides, this issue.

K. Schneider, Vanadium oxides – properties and applications. IV. Thin films: preparation, properties, applications, this issue.

K. Schneider, Vanadium oxides – properties and applications. V. Electronic structure, this issue.

K. Schneider, Vanadium oxides – properties and applications. VI Defect structure and electrical properties, this issue

V.E. Henrich, P.A. Cox, The surface science of metal oxides, University Press, Cambridge, 1994.

A. Pergament, G. Stefanovich, and A. Velichko, Oxide electronics and vanadium dioxide perspective: A review, J. Sel. Top. Nano Electron. Comput. 1, 24 (2013).

Z. Yang, C. Ko, and S. Ramantham, Oxide electronics utilizing ultrafast metal-insulator transitions, Ann. Rev. Mater. Res. 41(2011) 337-367.

A. Beaumont, J. Leroy, J.-C. Orlianges, and A. Crunteanu, Current-induced electrical self-oscillations across out-of-plane threshold switches based on VO2 layers integrated in crossbars geometry, J. Appl. Phys. 115, 154502 (2014).

II. Vanadium oxide electronics

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Abstract

Vanadium oxides can exist as single- and mixed-valence compounds with a large variety of structures. They exhibit diverse physicochemical properties which make them important with regard to oxide electronic material technology. This paper presents a broad overview of single-valence vanadium oxides and the mixed-valence variants, which form the Magnéli and Wadsley homologous series. Under certain ambient conditions (including temperature), phase transformations between these oxides can occur. Based on the available thermodynamic data, the specific conditions required to obtain particular oxides were determined.

Keywords: vanadium oxides, single-valence oxides, Magnéli phases, Wadsley phases thermodynamics, crystal structure

1. INTRODUCTION

Vanadium belongs to the transition metal group with the electronic configuration [Ar]3d³4s², which means that in compounds this element can assume the valence of +2 (V²+ with the [Ar]3d³ electron configuration), +3 (V³+ [Ar]3d²), +4 (V⁴+ [Ar]3d¹) and +5 (V⁵+ [Ar]). There is a large number of vanadium oxide phases that take the form of either single-valence binary oxides (VO, V2O3, VO2, V2O5) or oxides with two different oxidation states of vanadium, known as Magnéli or Wadsley phases.

2. VANADIUM-OXYGEN SYSTEM – PHASE DIAGRAM

The phase diagram of vanadium-oxide was the subject of several papers [1-4]. Based on both these data and the newest results on the melting temperatures of vanadium oxides, the phase diagram for an oxygen-to-vanadium ratio corresponding to 40-72 at.% of O was constructed (Fig. 1).

Little is known about the phase diagram for low oxygen content (below 40 at.%). Metal vanadium may dissolve relatively large amounts of oxygen (up to 3 at.% at 1770 K) [5].

Fig.1 Vanadium-oxygen phase diagram. Data compiled from papers [1-4].

3. Thermodynamics of V+O2 reactions

3.1 Definition of terms

Thermodynamics is a branch of physics and chemistry concerned with the interdependences between heat, work, temperature and energy in physical processes and chemical reactions. Knowledge of the energy transfer that takes place during physical and chemical transitions makes it possible to predict the type of changes that are likely to occur. The general rule is based on the second law of thermodynamics: the physical-chemical processes will (or can) proceed spontaneously if the change in the total entropy (S) of the universe resulting from this process is non-negative. In the case of chemical reactions that occur at a constant temperature (T) and pressure (p), this rule assumes the following form: at constant T and p, the Gibbs free energy (G) attains a minimum. From the rule which defines the change in the Gibbs free energy for a chemical reaction as:

(1)

the following criteria can be formulated:

if 𝛥G < 0, then the reaction is possible

if 𝛥G = 0, the system is in equilibrium

if ΔG > 0, the reaction is impossible.

In particular, Eq. (1) can be applied to the reaction of the formation of the VmOn vanadium oxide:

(2)

(3)

where 𝜇o = Go(A) [J/mole] is the chemical potential of pure component A, R is the gas constant R = 8.3144 J/(mole⋅K), T represents temperature [K], and pO2 [atm.] is oxygen activity, which can be expressed as oxygen partial pressure.

Based on the first and third laws of thermodynamics as well as on the experimentally determined temperature dependence of specific heat, the values of chemical potential vs. temperature were determined for various phases, including gases, liquids and solids of elements and chemical compounds.

3.2 Thermodynamics of vanadium oxide formation

When the ambient gas atmosphere exhibits certain characteristics, i.e. oxygen activity, pO2, and temperature (T), phase transformations between vanadium oxides can occur. These conditions can be estimated via the thermodynamic calculation of the changes in Gibbs energy (ΔG) that accompany chemical reactions. The most comprehensive thermodynamic data on chemical potentials (𝜇o) for inorganic substances are collected in [1]. In the case of the vanadium-oxygen system, the available thermodynamic data include those for V and O2, those for single-valence vanadium oxides (VO, V2O3, VO2 and V2O5), and those for oxides that belong to the Magnéli series – V3O5 and V4O7 [1].

Fig. 2 illustrates the standard Gibbs energy (Go corresponding to pO2 = 1 atm.) of the formation of principal (single-valence) vanadium oxides as a function of temperature.