1. Introduction

In the course of recent decades, there has been much interest in vanadium oxides because of their rich crystal chemistry ¹, they show some basic solid state phenomena such as; metal-insulator transitions ², Spin-Peierls transitions ³, doping promoted ferromagnetism in vanadium oxide nanotubes (VONT’s) ⁴; more recently, novel technical applications based on these compounds have been reported ^5,6. On the other hand, GeO2 exhibits many interesting physicochemical properties, such as photoluminescence, high values of dielectric constant and refractive index, good thermal stability and mechanical strength which make it suitable for applications in optical, electronic and optoelectronic devices ^7,8,9,10,11, in particular, as transparent glasses their structural and specific optical properties, make it a valuable candidate for the construction of planar waveguides for optoelectronic communications ^12,13,14.

As mixed oxides, the V₂O₅ and the GeO2 have been synthesized as vitreous materials for several years and their physical and chemical properties have been investigated systematically ^15,16,17. Later, amorphous coatings of mixtures of these oxides were prepared by the route of sol-gel synthesis and characterized using different spectroscopic techniques ^18,19. Although there is published work related to Ge-V-O based crystalline compounds (see ^20,21 and references therein), the information on crystalline solid solutions of type Ge(1-x) V _x O₂ is scarce, so, we start a systematic study of the physical and chemical properties of these solid solutions synthesized from high temperature reaction of mixtures of vanadium and germanium oxide powders.

In this paper we present the characterization of the nonstoichiometric Ge_0.74V_0.21 □_0.05O₂ compound synthesized by our group some years ago ²². The characterization was performed through the following analysis techniques: X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance ultraviolet-visible spectroscopy (DRUV-VIS) and electron spin resonance (ESR) spectroscopy.

2. Experimental Details

The Ge_0.74V_0.21 □_0.05O₂ polycrystalline compound was prepared by conventional solid-state reactions, starting from quartz-type GeO₂ (Cerac 99.999%) and V₂O₅ (J.T. Baker 99.7%). To set up the phase transition of q-GeO₂ to r-GeO₂, we employ the K₂CO₃ (J.T. Baker 99.9%) reactive as a catalyst. The educts were mixed and ground in an agate mortar in order to obtain a fine powder, which was placed on a platinum crucible and then heated in a furnace in air at 1123 K for two days. At the end of the heat treatment, a solidified mixture of crystalline and amorphous phases was obtained. The amorphous component was easily removed by dissolution, stirring and washing the mixture wth hot distilled water.

The X-ray powder diffraction measurements were performed on a Bruker D8 Advance Diffractometer: Cu K _al radiation, and a goniometer with a lynx-eye detector. The data were collected from 2θ=7∘ to 110° and with a step size of 0.019°, the operating conditions of the X-ray generator being 35 kV and 25 mA. The Scanning electron microscopy (SEM) observations were performed on a JEOL JSM 5600 LV microscope.

The XPS measurements were performed at ultra-high vacuum conditions in a Thermo-VG SCALAB 250 system equipped with a monochromatic AlKα source (hv = 86.6 eV) and a hemispherical analyzer. The DRUV-VIS spectra of synthesized powders were taken with a Cary 5000 UV-VI-NIR spectrophotometer, equipped with a DRA-900 integration sphere accessory and politetrafluoroetilen as reflective reference material. The ESR spectra of the synthesized powders were recorded at 310 K using an ESR JEOL (JES-RE3X) spectrometer equipped with a cylindrical TE 011 mode cavity in X band (9.6 or 9.21192 GHz) with a 100 kHz field modulation. The V⁴⁺ concentration was determined from the double integration of the first derivative of the ESR spectrum of powders and by a comparison with a KCl-pitch standard.

3. Results

After the washing process described above, the “beige” recovered powders (Fig. 1) were subjected to an X-ray diffraction analysis. The Rietveld refinement of the X-ray pattern (see Fig. 2), clearly matches with that of the crystalline model previously reported by Rosales et al. ²² for the rutile-type compound Ge_0.74V_0.21 □_0.05O₂. In that compound, some percentage of Ge⁴⁺ is replaced by V⁵⁺ ions and the charge neutrality is accomplished by the presence of cation vacancies. On the other hand, SEM images show that the synthesized powder consists of long faceted rectangular crystallites (Fig. 3), a shape frequently observed on rutile type nano-crystals grown by sol-gel methods in alkaline environments ²³.

Figure 1. Powders of the synthesized Ge_0.74V_0.21 □_0.05O₂ compound 

Figure 2. Experimental X-ray pattern obtained in the recovered microcrystalline powders compared with that reported by Rosales et al. ²². 

Figure 3. SEM image of synthesized microcrystalline powders of the Ge_0.74V_0.21 □_0.05O₂ compound. 

In many compounds based on vanadium oxides it is very common to find the vanadium ion in different oxidation states. This leads to the well known chromophoric effect observed in both, glassy and crystalline materials ^24,25, then, the “beige” color observed in the recovered powders, is probably caused by combined oxidation states of vanadium ions hosted in the crystal lattice. So, to determine the oxidation state of vanadium we perform an analysis of powders by XPS.

The XPS spectra we obtained for this compound in the binding energy interval 510-540 eV, shows the characteristics Vp3/2, Vp1/2, Os1 bands assigned to vanadium and oxygen (Fig. 4). A detailed analysis through the deconvolution of the band corresponding to Vp3/2 zone (Fig. 5) clearly shows that this consists of two peaks located at 516 eV and 517 eV which have been assigned to V⁴⁺ and V⁵⁺ ions respectively ²⁶, so we can assert that this two types of vanadium ions are present, at least, in the first atomic layers of synthesized microcrystals if we consider that the XPS is a surface analysis technique. Additionally, DRUV-VIS spectra taken on the recovered powders show broad absorption bands in the interval of 200 to 800 nm, which invariably can be matched by six Gaussian-shape bands centered at 230, 310, 400, 505, 620 and 751 nm (see Fig. 6). In this spectral region charge-transfer (C-T) and d-d transition bands of the V⁵⁺ and V⁴⁺ ions hosted in based vanadium oxide compounds have been reported ²⁷.

Figure 4. V2p and O1s bands of vanadium and oxygen detected on the Ge_0.74V_0.21 □_0.05O₂ compound. 

Figure 5. Fitting of V2p3/2 band with two peaks assigned to V⁴⁺ and V⁵⁺ oxidation states of vanadium. 

Figure 6. DRUV-VIS spectrum (displayed in absorbance) of recovered powders of the Ge_0.74V_0.21 □_0.05O₂ compound. 

Figure 7. ESR spectrum taken on the synthesized powders of the compound Ge_0.74V_0.21 □_0.05O₂ compound. 

On the other hand, it is well known that some physical properties of compounds based on vanadium oxide such as electronic conductivity and magnetic susceptibility are linked with the presence of vanadium ions in mixed oxidation states (as an example see Ref. ²⁸), so to understand the physical processes that give rise to these properties, it is first necessary to determine the crystalline sites occupied by the different vanadium ions.

As we saw, the XPS measurements performed on the nonstoichiometric Ge_0.74V_0.21 □_0.05O₂ compound detected the presence of V⁴⁺ ions hosted, at least, in the first layers of crystallites. Taking into account that V⁴⁺ is a paramagnetic ion, from ESR measurements it is possible to obtain information about the environment of the crystalline sites occupied by these ions trough the analysis of the spin Hamiltonian data.

The paramagnetic V⁴⁺ ion has the 3d¹ electronic configuration and an electronic spin S = 1/2, and a nuclear spin for its V 51 isotope (natural abundance 99.5%) of I = 7/2, then an eight component hyperfine structure is expected in the ESR spectra of synthesized compound.

In this case, we used a full spin Hamiltonian to describe the spectra of V⁴⁺:

where β is the Bohr magneton; Hx, Hy and Hz are the components of the static magnetic field; Sx, Sy, and Sz are the spin operators of the electron; Ix, Iy, and Iz are the nuclear spin operators of the nucleus; gx, gy, and gz are the values of the anisotropic g-tensor; Ax, Ay and Az are the parallel and perpendicular hyperfine values of the hyperfine A-tensor. The experimental ESR spectrum taken on a sample of synthesized powders shows three sets of eight hyperfine transitions (Fig. 5), one of them due to a parallel site and the other two due to perpendicular sites for the applied external magnetic field.

The spin Hamiltonian parameters deduced from computer simulation for the three sets of eight hyperfine transitions are: The parallel site to the z axis, has a hyperfine splitting A _zz = 146.54 gauss with a gzz=1.9623. The two perpendicular sites have splittings of; A _xx = 40.69 gauss whit gxx=1.9514 and A _yy = 19.84 gauss with gyy=1.9198. Therefore it is evident that recovered powder contains V⁴⁺ ions hosted at crystalline sites with orthorhombic ligand field symmetry, i.e.., the V⁴⁺ ions are possibly located at sites with a bi-axially distorted octahedral symmetry.

The ESR spectra of V⁴⁺ ions at crystalline sites with this kind of ligand symmetry are characterized by A_iso and g_iso values in (60-90) and (1.920-1.95) intervals respectively ²⁹. In our case, the calculated A_iso (69.02) and g_iso (1.945) values fall within these ranges, and are very similar to those reported in a pioneering research related to a paramagnetic resonance in polycrystalline and single crystals of rutile-type GeO₂ slightly doped with V⁴⁺ ions ^30,31.

Finally, following the quantification procedure described in the experimental section, the V⁴⁺/V_total proportion was estimated as 0.002.

4. Discussion

To prepare the polycrystalline Ge_0.74V_0.21 □_0.05O₂, we employed K₂CO₃ as flux and catalyst. This carbonate decomposes in K₂O and CO₂ at temperatures reached in our synthesis procedure, then, the microcrystals were grown from the melted mixtures of quartz-type GeO2 and V₂O₅ in an alkaline ambient and in a reducing atmosphere. This would account for the shape of crystallites obtained, as in the case of sol-gel methods of synthesis of rutile nanoparticles in alkaline environments ²³ and for the reduction of V₅₊ to V₄₊, which diffuses in small quantities into crystallite lattice, as we detected by ESR.

The deconvolution of the XPS spectra taken on recovered powders of synthesized compound, invariably shown the presence of vanadium as V⁵⁺ and V⁴⁺ ions, whose electronic configuration are [Ar] 3d⁰ and [Ar] 3d¹ respectively. It is known that the V⁴⁺ reacts readily to form some oxy-cations such as the VO²⁺, this molecular oxy-cation can be thought as consisting of a V⁴⁺ (3d¹) ion and a closed shell O^2- ion. The VO²⁺ ion has a single d electron, which gives raise the free ion term ²D. In a crystal field of octahedral geometry, this electron occupies the t2g orbital. When the electron absorbs energy it is excited to eg orbital, so, in this geometry only one optical absorption band is expected. Nevertheless, due to the non-symmetrical alignment of V=O bond in the molecular oxy-cation VO²⁺, this ion could be hosted in lower symmetrical sites such as tetragonal (C _4v ) or rhombic (C _2v ). In these cases, three absorption bands are expected for tetragonal and four for the rhombic symmetry ²⁷ (see Fig. 8).

Figure 8. Diagram of energy levels with the assignment of the optical transitions of VO²⁺ in the symmetry C_2V. 

As we see, the absorption spectra taken in the Ge_0.74V_0.21 □_0.05O₂ compound we synthesized, invariably can be adjusted with four bands in the region of d-d transitions of the V⁴⁺ ion. This result could indicate that this ion is hosted as VO²⁺ oxy-cation with a (C _2v ) rhombic symmetry in the crystal lattice of the rutile-type Ge_0.74V_0.21 □_0.05O₂ compound. Moreover, a ESR spectrum with three sets of hyperfine lines are expected for VO⁴⁺ in a (C _2v ) symmetry ²⁷, just as we from our ESR measurements on the Ge_0.74V_0.21 □_0.05O₂ compound. Similar absorption bands and ESR signals have been reported for the VO²⁺ cation hosted in sites of (C _2v ) symmetry on CsCl single crystals ³².

In a subsequent paper we will make a detailed theoretical analysis of our ESR and DRUV-VIS results.

5. Conclusions

The characterization of microcrystalline powders of the Ge_0.74V_0.21 □_0.05O₂ by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance ultraviolet-visible spectroscopy (DRUV-VIS) and electron spin resonance spectroscopy (ESR) let us conclude: