Introduction

V₂O₅ is a binary transition metal oxide with the highest oxidation state (+V) among vanadium oxides. At room temperature, this oxide is an odorless solid crystalline powder whose color varies from pale yellow to dark orange depending on its state of division (Figure 1)^[1][2]. V₂O₅ is hazardous for health and environment, but it has properties that are used in many industry applications, such as a cathode in lithium-ion batteries^[2][3], a component in special glasses^[4], and a catalyst in the production of various acids such as sulfuric^[1][5]. And although this is only a part of all possible applications, vanadium pentoxide has found the greatest distribution (about 85% of all world vanadium) in the production of springs and cutting-tool steels, where it is mixed with Fe₂O₃ and Al to increase their toughening^[1][6].

Synthesis

Vanadium pentoxide can be obtained in several ways, for example, by heating metallic vanadium in oxygen under pressure as shown in following equation:

4 V + 5 O₂ → 2 V₂O₅

However, this method does not produce the purest vanadium pentoxide, since in addition to V₂O₅, other vanadium oxides are formed during this heating, which are difficult to separate from each other^[7].

Another method is to calcinate other vanadium oxides such as VO, V₂O₃ and VO₂ in oxygen as shown in the example below^[8]

4 VO₂ + O₂ → 2 V₂O₅

The purest V₂O₅ is obtained by a multi-step reaction. First, vanadium slag is mixed with NaCl and roasted, which leads to the oxidation of vanadium to form V(V) and the formation of sodium vanadate. Further, with the help of sulfuric acid, the pH is lowered, which leads to the formation of sodium polyvanadate Na₄H₂V₁₀O₂₈. An increase in temperature triggers the hydrolysis of sodium polyvanadate, which leads to the precipitation of a phase called "red cake" (Figure 2) due to the characteristic reddish color:

6 Na₄H₂V₁₀O₂₈ + 7 H₂SO₄ + (n+13) H₂O → 5 Na₂V₁₂O₃₁ · n H₂O + 7 Na₂SO₄+13 H₂O

Next, the "red cake" is dissolved in sodium hydroxide, which leads to an increase in pH, and NH₄Cl is added, which leads to the precipitation of ammonium metavanadate:

VO₃^- + NH₄⁺ → NH₄VO₃

Finally, the ammonium metavanadate is calcined, resulting in V₂O₅ with a purity of about 98.5%^[6][8]:

2 NH₄VO₃ → V₂O₅ + NH₃ + H₂O

Structure

At ambient pressure, vanadium pentoxide crystallizes in the orthorhombic crystal system^[2][9][10], however, there are also reports of the existence of monoclinic V₂O₅ ^[11][12]. The difference between the methods for obtaining an orthorhombic and monoclinic structure is that a much higher pressure (about 2-6 GPa) is required to obtain a monoclinic structure, which allows the orthorhombic structure to be reformed into a denser form. Alternatively, there is another method to obtain monoclinic V₂O₅, namely by adding sufficient oxygen deficiencies to the crystal structure^[13]. Below are images of V₂O₅ crystal structures in the orthorhombic system (Figs. 3 and 4) and in the monoclinic system (Figs. 5 and 6). As can be seen from these figures, the difference between these systems, in addition to the unit cell parameters and crystal symmetry, lies in different polyhedra.

It should be noted that in the ICSD data for the orthorhombic structure from which the figures below are made, in addition to five V-O bonds, there is also a sixth bond with a length significantly longer than the rest (2.81 Å versus 2.02 Å), which makes the polyhedra octahedral. Earlier studies pointed to the presence of this bond in the structure^[9], however, later studies have shown that this bond is actually a weak interaction, and polyhedra are in fact square pyramids with common corners and edges^[10]. Thus, if the orthorhombic structure is viewed from a greater distance (Fig. 4), it is possible to see that it forms two-dimensional nets.

Figure 3. Close-up structure of orthorhombic V₂O₅ (Data from ICSD, visualized with VESTA. Figures: Nikita Jamkin).

Figure 4. Distant orthorhombic V₂O₅ structure (Data from ICSD, visualized with VESTA. Figures: Nikita Jamkin).

In turn, the structure of monoclinic V₂O₅ (here δ-V₂O₅^[11]) is a three-dimensional structure, in which the polyhedra are in the form of distorted octahedrons with the central atom displaced towards the edge, and form paired zigzag chains connected by common corners and, in some places, by common edges.

Figure 5. Close-up structure of monoclinic V₂O₅ (Data from ICSD, visualized with VESTA. Figures: Nikita Jamkin).

Figure 6. Distant structure of monoclinic V₂O₅ (Data from ICSD, visualized with VESTA. Figures: Nikita Jamkin).

Properties

The magnetic nature of V₂O₅ requires more detailed studies. At the moment, it is believed that the bulk phase of V₂O₅ is diamagnetic, since V5+ ions do not have unpaired electrons and, therefore, have a balanced spin equal to zero, which, in turn, means that V₂O₅ does not have its own magnetic moment^[14]. On the other hand, some practical studies show that V₂O₅ is a paramagnet mainly due to oxygen vacancies present in the structure, and an increase in the number of oxygen vacancies can even lead to antiferromagnetism under certain conditions^[13].

It is mainly due to oxygen vacancies that V₂O₅ can be used as an n-type semiconductor, since a decrease in the amount of oxygen in the lattice will lead to an increase in the number of charge carriers^[15]. Below is Figure 7, according to which V₂O₅ is a wide-gap semiconductor with an indirect band gap of 2.3 eV.

Figure 7. V₂O₅  band structure (License: CC BY)^[16]

Moreover, due to its high oxidation state, V₂O₅ is an amphoteric compound and, depending on the environment, behaves like a base or an acid and therefore is also highly soluble in acids and bases^[17], but in addition to this, and unlike most other metal oxides, it is slightly soluble in water^[7][18].

Finally, powder X-ray diffraction patterns of both V₂O₅ crystal systems using CuKα radiation are presented below (Fig. 8).

Figure 8. Powder XRD patterns for orthorhombic (left, license: CC BY)^[19] and monoclinic (right; calculated data from ICSD, Figure: Nikita Jamkin) V₂O₅ systems.

References