Mixed Occupancy Research Articles

A site with mixed occupation: Neanderthals and carnivores at Érd (Hungary)

The Middle Palaeolithic open-air site of Érd, near Budapest (Transdanubia, Hungary), with its specific lithic industry on quartzite pebbles, combustion structures and abundant animal bone material dominated by cave bears, was long time considered as one of the best example of a long-term camp of Neanderthals specializing in cave bear hunting. This interpretation of the site was based on the knowledge of the late 1960s. However, the association of carnivore remains and lithic artefacts found in cave or sheltered sites of the Palaeolithic period raises questions about the formation processes of the bone accumulations and about the relationships between these different agents. The development in methodology, both in archaeozoology and lithic technology, allows to apply new approaches in the study of this complex problematic. Independent new analyses of the lithic assemblage and the osteological material of Érd resulted completely new conclusions concerning past human activities at the site as well as the presence and the role of cave bear and cave hyena. An interpretation of the site as a refuge or den for these predators together with the undeniable presence of humans implies a mixed functioning through recurrent occupations.

Low-Dimensional Magnetic Semimetal Cr0.65Al1.35Se3.

While exploring novel magnetic semiconductors, the new phase Cr0.65Al1.35Se3 was discovered and characterized by both structural and physical properties. Cr0.65Al1.35Se3 was found to crystallize into orthorhombic CrGeTe3-type structure with space group Pnma (no. 62). Vacancies and mixed occupancies were tested, and the results show that one of the 4c sites accommodates a mixture of Cr and Al atoms, while the other 4c site is fully occupied by Al atoms. Unique structural features include a T-shaped channel network created from the edge-sharing Cr/Al@Se6 and Al@Se4 polyhedra and a zipper effect of the puckered Se atoms inside the columnar channels. The round peak observed in the temperature-dependent magnetic susceptibility (χg) plot at ∼8(1) K corresponds to the antiferromagnetic-type transition in Cr0.65Al1.35Se3. However, the positive θCW indicates an additional ferromagnetic interaction, which is highly likely due to the complex magnetic structure arising from the mixed Cr/Al occupancies on the 4c site. Electrical resistivity measurements confirm that Cr0.65Al1.35Se3 is a semimetal with a positive magnetoresistance. Here we present the characterization and determination of the crystal structure and physical properties for this new material.

NMR, magnetization, and heat capacity studies of the uniform spin- 12 chain compound Bi6V3O16

We report the local (NMR) and bulk (magnetization and heat capacity) properties of the vanadium-based S=1/2 uniform spin chain compound Bi6V3O16(Bi4V2O10.66). In the low-temperature {\alpha} phase, the magnetic ions (V4+) are arranged in one-dimensional chains. The magnetic susceptibility shows a broad maximum around 50 K signifying a short-range magnetic order. Heat capacity measurements also reveal low-dimensional magnetism. The 51V magic angle spinning nuclear magnetic resonance measurements clearly show that the magnetic V4+ and nonmagnetic V5+ species are located on different crystallographic sites with no mixed occupation. The spin susceptibility calculated from the shift of the 51V NMR spectra reproduces the behavior observed in magnetic susceptibility and agrees well with the S=1/2 uniform spin chain model with J=113(5) K.

(Invited) Mn4+ r-Line Emission Intensity: The Ways of Its Enhancement

Mn4+-based red phosphors are excellent materials to be used for making white LEDs with warm white light. They can be excited by the blue or ultraviolet light to produce red photoluminescence (PL), which is due to the parity and spin-forbidden 2Eg→4A2g transition. However, in many host materials, the intensity of the 2Eg→4A2g zero-phonon line (R-line) is very weak due to the presence of the inversion center of the crystallographic site occupied by the manganese ions. As a result, the red-shifted Stokes components dominate in the emission spectra of such hosts, but this leads to the decreased overlap of the PL spectra with human eye sensitivity curve and decreased intensity of the phosphor (lower lumen/Watt value). Therefore, finding the recipes of enhancement of the R-line intensity and production of brighter phosphors is an important scientific and technological task. In the present work we establish the key parameters responsible for the energy and the intensity of the Mn4+ R-line. Thorough consideration and analysis of the optical spectra for a large number of the Mn4+-based phosphors allowed to draw the following conclusions: The energetic position of the R-line is determined by the “Mn4+-ligand” bonding covalence. Most phosphors for the solid state lighting can be conditionally divided into fluorides and oxides (where the fluorine or oxygen ions are the Mn4+ nearest neighbors, respectively). In ionic fluorides, the Mn-F bonding covalence is weak, which shifts the R-line to higher energies, with the emission wavelength at about 620 nm. In covalent oxides, the stronger Mn-O bonding covalence lowers the energy of the 2Eg state, moving the emission to about 650 nm.In the host crystals, where the Mn4+ ions are located at the sites without inversion center, the R-line intensity is higher compared with the crystals possessing inversion symmetry at the Mn4+ Absence of the inversion center relaxes the parity section rule, which enhances the R-line intensity. As a result, the phosphor luminosity is increased considerably. The inversion center can be removed by cation co-doping, when different cations in the second coordination sphere around the Mn4+ ion remove the inversion center. As an example, a figure below shows the PL spectra of K2SiF6:Mn4+ and NaKSiF6:Mn4+. In the former case the ideal MnF6 octahedron with the inversion center does not allow for high R-line intensity. It the latter case structural disorder and mixed occupation at the K/Na site results in the presence of different cations in the second coordination sphere. As a result, the intensity of the R-line is increased by several times. Therefore, to get the phosphors with increased luminosity one has to focus on the host materials without inversion center at the dopant sites. If, however, the inversion symmetry exists, it should be broken by additional co-doping that would produce structural disorder around the dopant. Figure 1

Ultralow Thermal Conductivity and High-Temperature Thermoelectric Performance in n-Type K2.5Bi8.5Se14

We studied the narrow band-gap (0.55 eV) semiconductor K2.5Bi8.5Se14 as a potential thermoelectric material for power generation. Samples of polycrystalline K2.5Bi8.5Se14 prepared by spark plasma sintering exhibit exceptionally low lattice thermal conductivities (κlat) of 0.57–0.33 W m–1 K–1 in the temperature range of 300–873 K. The physical origin of such low κlat in K2.5Bi8.5Se14 is related to the strong anharmonicity and low phonon velocity caused by its complex low symmetry, large unit cell crystal structure, and mixed occupancy of Bi and K atoms in the lattice. High-resolution scanning transmission electron microscopy studies and microanalysis indicate that the K2.5Bi8.5Se14 sample is a single phase without intergrowth of the structurally related K2Bi8Se13 phase. The undoped material exhibits an n-type character and a figure-of-merit (ZT) value of 0.67 at 873 K. Electronic band structure calculations indicate that K2.5Bi8.5Se14 is an indirect band-gap semiconductor with multiple conduction bands clo...

Single-Crystal Automated Refinement (SCAR): A Data-Driven Method for Determining Inorganic Structures.

Single-crystal diffraction is one of the most common experimental techniques in chemistry for determining a crystal structure. However, the process of crystal structure determination and refinement is not always straightforward. Methods for simplifying and rationalizing the path to the most optimal crystal structure model have been incorporated into various data processing and crystal structure solution software, with the focus generally on aiding macromolecular or protein structure determination. In this work, we propose a new method that uses single-crystal data to determine the crystal structures of inorganic, extended solids called "single-crystal automated refinement" (SCAR). The approach was developed using data mining and machine learning methods and considers several structural features common in inorganic solids, like atom assignment based on physically reasonable distances, atomic statistical mixing, and crystallographic site deficiency. The output is a tree of possible solutions for the data set with a corresponding fit score indicating the most reasonable crystal structure. Here, the foundation for SCAR is presented followed by the implementation of SCAR to determine two newly synthesized and previously unreported phases, ZrAu0.5Os0.5 and Nd4Mn2AuGe4. The structure solutions are found to be comparable with those produced by manually solving the data set, including the same refined mixed occupancies and atomic deficiency, supporting the validity of this automatic structure solution method. The proposed SCAR program is thus verified as being a fast and reliable assistant in determining even complex single-crystal diffraction data for extended inorganic solids.

Raman, EPR and structural studies of novel CuZrNb2O8 ceramic for LTCC applications

Abstract To determine the mechanism of microwave response in AZrNb2O8 (A: Mn, Zn, Mg, Co, et al.) wolframites, CuZrNb2O8 ceramic with low sintering temperature (920 °C) is successfully fabricated by solid-state reaction method. Meanwhile, its structural stability is synthetically evaluated, the vibration modes are also calibrated. Except for the extrinsic factor like sintering temperature, the condition of Jahn-Teller ion Cu2+ as intrinsic part is characterized for the first time in relation to the microwave dielectric properties of the system. In virtue of Raman and EPR spectra, it is predicted that the electronic structure of transition metal ions, as the less concerned factor, may be influential on the dielectric properties of ceramics at microwave range, especially in the structure with mixed ion occupancy. In addition, the CuZrNb2O8 ceramic is well matched with the silver electrode, which makes it a prospective candidate for temperature compensation material of LTCC (low-temperature co-fired ceramic) passive integration technology.

Detuning the Honeycomb of the α-RuCl3 Kitaev Lattice: A Case of Cr3+ Dopant.

Fine-tuning chemistry by doping with transition metals enables new perspectives for exploring Kitaev physics on a two-dimensional (2D) honeycomb lattice of α-RuCl3, which is promising in the field of quantum information protection and quantum computation. The key parameters to vary by doping are both Heisenberg and Kitaev components of the nearest-neighbor exchange interaction between the Jeff = 1/2 Ru3+ spins, depending strongly on the peculiarities of the crystal structure. Here, we present crystal growth by chemical vapor transport and structure elucidation of a solid solution series Ru1- xCr xCl3 (0 ≤ x ≤ 1), with Cr3+ ions coupled to the Ru3+ Kitaev host. The Cr3+ substitution preserves the honeycomb type lattice of α-RuCl3 and creates mixed occupancy of Ru/Cr sites without cationic order within the layers as confirmed by single-crystal X-ray diffraction and transmission electron microscopy investigations. In contrast to high-quality single crystals of α-RuCl3 with ABAB-stacked layers, the ternary compounds demonstrate a significant stacking disorder along the c-axis direction as evidenced by X-ray diffraction and high resolution scanning transmission electron microscopy (HR-STEM). Raman spectra of substituted samples are in line with the symmetry conservation of the parent lattice upon chromium doping. At the same time, our magnetic susceptibility data indicate that the Kitaev physics of α-RuCl3 is increasingly suppressed by the dominant spin-only driven magnetism of Cr3+ ( S = 3/2) in Ru1- xCr xCl3.

ASc[SeO3]2 (A = Na – Cs): Pure Alkali‐Metal Scandium Oxoselenates(IV) and Their Representatives With Mixed Alkali‐Metal Occupation

Alkali‐metal scandium oxoselenates(IV) ASc[SeO3]2 (A = Na – Cs) are known since a few years and a hydrothermal synthesis was used to obtain them. In our new studies we applied a flux‐supported solid‐state reaction and produced colorless single crystals as well. All representatives ASc[SeO3]2 with A = Na – Cs crystallize in the orthorhombic space group Pnma, in contrast to earlier reports for hexagonal RbSc[SeO3]2. Furthermore we have extended this field with some crystals showing a mixed occupation on the alkali‐metal site, namely (K,Na)Sc[SeO3]2, (Rb,K)Sc[SeO3]2, and (Cs,Rb)Sc[SeO3]2. Since all of them contain [ScO6]9– octahedra and [SeO3]2– ψ1‐tetrahedra the diverse connectivity of the distinct alkali‐metal centered oxygen polyhedra differentiates the compounds with the smaller alkali metals (A′ = Na and K) from those with the bigger ones (A′′ = Rb and Cs). For the mixed crystals the amount of smaller or bigger alkali metal is responsible, which design is chosen by the system. This forces the mixed crystal (Rb,K)Sc[SeO3]2 with a higher amount of potassium instead of rubidium to crystallize isotypically with KSc[SeO3]2 and NaSc[SeO3]2, whereas the pure rubidium compound RbSc[SeO3]2 adopts the CsSc[SeO3]2‐type structure. These findings are supported by single‐crystal Raman spectroscopy.

Contributions to the stereochemistry of zirconium oxysalts—part II: syntheses and crystal structures of Zr(SeO3)(SeO4), Zr4(SeO3)(SeO4)7, and Zr3(SeO3)(SeO4)5·2H2O

Three new compounds Zr(SeO3)(SeO4), Zr4(SeO3)(SeO4)7, and Zr3(SeO3)(SeO4)5·2H2O were synthesized at low-hydrothermal conditions (Teflon-lined steel vessels, 220 °C) from mixtures of Zr2O2(CO3)(OH)2, H2SeO4, and minor contents of water. Colorless single crystals up to several tenth of a mm in size, obtained within 1 week, were studied by single crystal X-ray techniques. Zr(SeO3)(SeO4) crystallizes in the orthorhombic space group Pbca (No. 61), with a = 8.291 (2) A, b = 9.458 (2) A, c = 15.357 (3) A, V = 1204.2 (5) A3, Z = 8, R1 = 0.0322. Zr4(SeO3)(SeO4)7 is monoclinic, space group P21/n (No. 14), with a = 5.313 (1) A, b = 10.704 (2) A, c = 10.484 (2) A, β = 104.13 (1)°, V = 578.2 (1) A3, Z = 1, R1 = 0.0172. Two independent selenium atoms are present in this structure: one forming a SeO4 tetrahedron, and the other one exhibiting mixed occupation by ¾ Se6+ and ¼ Se4+; its coordination is, therefore, partially disordered. Zr3(SeO3)(SeO4)5·2H2O crystallizes in the triclinic space group P1 (No. 1), with a = 5.273 (1) A, b = 8.079 (2) A, c = 11.959 (2) A, α = 82.60 (1)°, β = 88.27 (1)°, γ = 89.87 (1)°, V = 505.1 (1) A3, Z = 1, R1 = 0.0235, but exhibits strong centrosymmetric pseudosymmetry; the inversion center is violated only by replacement of one selenate(VI) tetrahedron by a trigonal pyramidal selenite(IV) group as pseudo-centric counterpart. Hydrogen bonds in this compound show donor–acceptor distances within the range of 2.67–2.81 A. All three framework structures are unique and built up from corner-sharing polyhedra. In all three compounds, mean cation-oxygen bond lengths (Zr[6]: 2.062 and 2.067 A; Zr[7]: 2.132, 2.137 and 2.139 A; Se4+[3]: 1.675 and 1.680 A, excluding the disordered group; Se6+[4]: 1.621–1.641 A) are comparatively short, resulting in rather high bond valence sums.

Limits of Cation Solubility in AMg₂Sb₂ (A = Mg, Ca, Sr, Ba) Alloys.

compounds that crystallize in the CaAlSi structure type have emerged as a promising class of n- and p-type thermoelectric materials. Alloying on the cation (A) site is a frequently used approach to optimize the thermoelectric transport properties of compounds, and complete solid solubility has been reported for many combinations of cations. In the present study, we investigate the phase stability of the AMgSb system with mixed occupancy of Mg, Ca, Sr, or Ba on the cation (A) site. We show that the small ionic radius of Mg leads to limited solubility when alloyed with larger cations such as Sr or Ba. Phase separation observed in such cases indicates a eutectic-like phase diagram. By combining these results with prior alloying studies, we establish an upper limit for cation radius mismatch in alloys to provide general guidance for future alloying and doping studies.

The transition between antiferromagnetic order and spin-glass state in layered chalcogenides KFeAgCh2 (Ch = Se, S)

Investigation on the magnetic interaction in Fe-based ThCr2Si2-type compounds is very important not only for understanding the structure-property relationship in this family but also for exploring the superconducting mechanism in related materials. In this work, we report the synthesis and physical properties of two new Fe-based chalcogenides KFeAgSe2 and KFeAgS2. These two compounds adopt ThCr2Si2-type structure with I4/mmm space group and contain the Fe/Ag mixed occupation in the centre of edge-sharing Fe/AgCh4 tetrahedron (Ch = Se, Te). The lattice parameters are refined to be a = 4.1866(2) Å, c = 13.7731(4) Å and a = 3.9845(3) Å, c = 13.3610(4) Å for KFeAgSe2 and KFeAgS2 respectively. Both two compounds show semiconducting behavior with electrical resistivity increased as the temperature decreased, while the resistivity of KFeAgS2 is much larger (~61 Ω cm at room temperature) than that of KFeAgSe2 (~2.5 Ω cm at room temperature). Most interestingly, despite the similar electronic structure of these two compounds, KFeAgSe2 features a long-range antiferromagnetic order below 26 K while KFeAgS2 shows a spin-glass state below 19 K. It is indicated that the long-range magnetic ordering can be suppressed in these ThCr2Si2-type transition metal chalcogenides via compressing the unit cell, that is to say, by chemical pressure. Our work not only gives a new sight to understand the structure-magnetism relationship in ThCr2Si2-type transition metal chalcogenides, but also provides clue to explore the mechanism of superconductivity in Fe-based family.

Ternary transition metal gallides with TiNiSi, ZrBeSi and MgZn2-type structure

AbstractA series of ternary transition metal gallides around the equiatomic composition have been synthesized from the elements by arc-melting and subsequent annealing. The compounds crystallize with site occupancy variants of the hexagonal Laves phase MgZn2, with the hexagonal ZrBeSi or the orthorhombic TiNiSi type. All samples have been characterized on the basis of their lattice parameters, determined by X-ray powder diffraction (Guinier technique). The structures of NbCr1.58Ga0.42and NbFe1.51Ga0.49(MgZn2type,P63/mmc), NbRhGa (ZrBeSi type,P63/mmc), and ScNiGa, ScPtGa and ScAuGa (TiNiSi type,Pnma) were refined from single crystal X-ray diffractometer data. The ScPtGa and ScAuGa crystals showed trilling formation. Mixed site occupancies were only observed in the Laves phases while all other crystals were well ordered. A striking structural motif of NbRhGa is the formation of niobium chains (264 pm Nb–Nb) along thecaxis. Several gallides were magnetically characterized. They are Pauli paramagnets. The two crystallographically independent iron sites in the Laves phase TaFeGa could be distinguished in the57Fe Mössbauer spectrum. The isomer shifts of 0.06(3) (Fe1) and –0.02(3) (Fe2) mm s−1indicate metallic iron.

A new descendant of the γ-brass family in the zinc rich Ni-Zn-In system

The first ternary intermetallic phase - Ni11.4+xZn19.8In14.9-y (0 ≤ x ≤ 0.2; 0 ≤ y ≤ 0.1) in the Ni-Zn-In system is presented. The structure of the phase has been analysed by single crystal X-ray diffraction. Due to the similar X-ray scattering factors for Ni and Zn, a combination of X-ray and neutron diffraction has been employed to identify accurate sites of Ni and Zn in the structure of the Ni-Zn-In phase. The phase is structurally complex and adopts a narrow homogeneity range. The structure crystallizes in the orthorhombic space group Cmcm (63) and contains 46 atoms per unit cell which bears all types of disorder phenomena (vacancies, mixed site occupancies, and positional disorders) that is typical for complex metallic alloys. The structure is mostly tetrahedrally close-packed and is generated mainly by two types of endohedral clusters: a 12-atom cluster that is closely related to an icosahedron and an 11-atom polyhedron that is related to twinned cuboctahedron. The phase is stabilized at valance electron concentration between 1.82 and 1.83 e/a.

Housing in “Intramural Favelas”: Considerations on New Forms of Urban Expansion in Contemporary Times

This paper develops a deeper look into new residential appropriations of space in marginalized areas of a large Brazilian city, while highlighting the subjective importance of housing and its meaning beyond the idea of shelter. Firstly, it presents a brief history of Rio de Janeiro’s favelas – the local version of slums – and its relationship with vacant land over the past 100 years. Then, it explains the value of self-built housing and its contribution to the consolidation of multiple and hybrid territories, highlighting their subjective character. Lastly, it presents a case study called Portelinha, located in a set of favelas known as the Mare Complex, stressing how this mixed occupation has transformed the local urban fabric, leading to the emergence of what is referred to as an “intramural favela”. This phenomenon consists of the self-construction of a smaller-scale set of houses within the walls of a former factory turned into an industrial void in the 1990s. The analysis shows how this housing appropriation is articulated with other activities, especially cultural ones, leading to a diversity of social actors, alliances and conflicts, turning it into a real disputed territory. Cases like this reflect the challenges with which architects and planners need to deal with when working in the unequal urban contexts that are so common in the Global South.

Band Engineering of SrTiO3: Effect of Synthetic Technique and Site Occupancy of Doped Rhodium

It is well known that doping of Rh into the SrTiO3 lattice introduces 4d donor levels within the band gap, which causes reduction in the gap and extends the photocatalytic activity to the visible region of the solar spectrum. The mid-gap states formed also act as recombination centers and diminish the efficiency of the material. Herein, we present a combined theoretical and experimental approach to avoid the formation of the so-called acceptor mid-gap states. For the first time, we study the effect of occupancy of Rh in the Sr site. First-principles calculations reveal that mixed occupancies of Rh into Sr and Ti sites lead to the introduction of acceptor levels within the band gap, leading to decrease in photocatalytic efficiency. A facile one-pot solvothermal approach by avoiding high-temperature calcinations is reported to obtain Rh-doped SrTiO3 nanoparticles in Rh3+ states, suppressing the formation of Rh4+ states by directing Rh toward Sr sites. The photocatalytic activity of Rh-doped SrTiO3 nanoparti...

Ba6(MN4)N2–x (M = MoVI/TaV), a Subvalent Nitridometalate with Perovskite‐like Crystal Structure

The subvalent nitridometalate Ba6[(Mo1–xTax)N4]N0.86 was prepared from mixtures of Mo powder with Ba, Na, and Ba2N at 600 °C in Ta ampoules. It crystallizes in space group Cmcm with a = 11.672(3), b = 10.177(2) and c = 10.8729(19) Å. Its crystal structure exhibits an orthorhombically distorted Perovskite topology with [Ba6N] building units forming the ReO3‐type lattice via common vertices, and the nitridometalate anions occupying half of the available distorted cuboctahedral interstices. [MN4] anions show statistically mixed occupancy of M by MoVI and TaV. They show no notable deviation from nitridometalate anions in known ionic nitridomolybdates and ‐tantalates, and the metrics of the [Ba6N] octahedra correspond to those found in similar subvalent compounds. The nitrogen atom position centering the [Ba6N] octahedra is underoccupied. Band structure calculations corroborate the subvalent character of the compound and the two individual anionic structural building units.

Structure and properties of (Na0.5Bi0.5)ZrO3 (NBZ) lead-free perovskite compound

The structure of NBZ was studied by neutron diffraction. NBZ is monoclinic, (SG #11 P21/m) am = 5.696(8) Å, bm = 8.166(9) Å, cm = 5.790(4) Å and βm = 89.97(4)°. Being centrosymmetric, NBZ is neither piezoelectric nor ferroelectric. No phase transition was evidenced between 100 K and 923 K. NBZ presents a relaxor behavior below room temperature attributed to the mixed occupancy of the A-site by both Na+ and Bi3+ ions. The relative permittivity of NBZ appears noticeably stable (for 1 kHz −2.9% at −55 °C/218 K and +5.3% at 85 °C/358 K compared to the 25 °C/298 K value). NBZ appears thus as promising for the elaboration of capacitors requiring high thermal stability.

Short-range versus long-range structure in Cu(In,Ga)Se2, Cu(In,Ga)3Se5, and Cu(In,Ga)5Se8

The Cu-poor phases Cu(In,Ga)3Se5 and Cu(In,Ga)5Se8 play an important role both for understanding the Cu-(In,Ga)-Se material system and for growing high-efficiency Cu(In,Ga)Se2 thin film solar cells. Using extended X-ray absorption fine structure spectroscopy, we have studied the element-specific short-range structure of Cu(In,Ga)Se2, Cu(In,Ga)3Se5, and Cu(In,Ga)5Se8 alloys spanning the entire compositional range. The materials feature different local atomic arrangements and the element-specific average bond lengths remain nearly constant despite significant changes of the lattice constants with increasing In to Ga ratio and decreasing Cu content. In particular, the average bond lengths of Cu-Se and Ga-Se are almost identical while the average In-Se bond length is significantly longer in all three phases. The distance between lattice sites with mixed site occupation therefore corresponds to the weighted average of different element-specific bond lengths rather than to the individual bond lengths themselves. Furthermore, the increasing number of vacancies with decreasing Cu content lead to both a significant unit cell contraction and a slight bond length expansion. The crystallographic long-range structure and the element-specific short-range structure thus describe different structural aspects that are certainly interrelated but obviously not identical.

Synthesis and structure determination of β-K0.399V2.623W0.377O7.5 with mixed occupation of vanadium and tungsten elements

Synthesis and structure determination of β-K_0.399V_2.623W_0.377O_7.5 with mixed occupation of vanadium and tungsten elements