# Solid-state synthesis of stoichiometric and dense CoV2O4 targets

Lamiae El Khabchi, Antonio Peña Corredor, François Roulland, Marc Lenertz, Cédric Leuvrey, Jérôme Robert, Gilles Versini, Laurent Schlur, Christophe Lefèvre, Nathalie Viart

► To cite this version:

Lamiae El Khabchi, Antonio Peña Corredor, François Roulland, Marc Lenertz, Cédric Leuvrey, et al. Solid-state synthesis of stoichiometric and dense CoV2O4 targets. Journal of the European Ceramic Society, 2025, 45 (8), pp.117286. 10.1016/j.jeurceramsoc.2025.117286. hal-05093029

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# Solid-state synthesis of stoichiometric and dense CoV₂O₄ targets

Lamiae El Khabchi ID, Antonio Peña Corredor, François Roulland, Marc Lenertz ID,
Cédric Leuvrey, Jérôme Robert ID, Gilles Versini, Laurent Schlur, Christophe Lefèvre,
Nathalie Viart ID

Université de Strasbourg, CNRS, IPCMS UMR 7504, Strasbourg F-67034, France

## ARTICLE INFO

**Keywords:**

Spinel vanadium oxides
Magnetically frustrated materials
Solid state synthesis
Target synthesis

## ABSTRACT

The CoV₂O₄ (CVO) spinel is a magnetically frustrated compound for which thin films, with their associated substrate-induced strains, can bring challenging new magnetic structures. Research on CVO thin films remains scarce to date, probably hindered by the complex synthesis of CVO stoichiometric targets necessary for their deposition. The difficulty to have V in its +III valence is a serious challenge. We present here a solid-state method for the production of polycrystalline stoichiometric single-phased and dense pellets of CVO. The procedure involves controlling the heat treatment atmospheres to ensure single-phased CoV₂O₄ powder and homogenising grains size by attrition to promote dense pellet sintering. The significant density of the prepared pellets allows their use as targets for the growth of thin films by techniques such as pulsed laser deposition (PLD) or sputtering and opens the way to alternative thin films depositions.

## 1. Introduction

Spinel vanadium oxides of general formula AV₂O₄ (A being a transition metal), in which itinerancy and magnetic frustration can be tailored by the V-V distance ($d_{V,V}$) [1,2], constitute a compelling playground for the condensed matter community. In their Fd$\overline{3}$m normal spinel structure, the A²⁺ ions occupy the tetrahedral sites and form a diamond-like sublattice, while the V³⁺ ions occupy the octahedral sites and form a geometrically frustrated pyrochlore sublattice [3]. When A²⁺ is a non-magnetic ion (Zn²⁺, Mg²⁺, Cd²⁺), AV₂O₄ undergoes two phase transitions with decreasing temperatures. The first one, structural, from cubic to tetragonal symmetry, originates from the relief of the orbital degeneracy of the V³⁺ t₂g orbitals which leads to an orbital ordering. The second one is an antiferromagnetic ordering of the V³⁺ spins [4–6]. When A²⁺ is a magnetic ion (Fe²⁺, Mn²⁺), additional competing magnetic exchange interactions, $J_{A2^+ - A2^+}$ and $J_{A2^+ - V3^+}$, come into play alongside with the already existing $J_{V3^+ - V3^+}$. Their presence on such a geometrically frustrated lattice leads to complex magnetic ordering, with the possibility for non-collinear spin states where the V³⁺ moments cant away from their collinear initial state [7].

Among the spinel vanadates, CoV₂O₄ (CVO), with magnetic Co²⁺ ions (3d⁷, (e)⁴(t₂g)³, S = 3/2) occupying the tetrahedral sites (A sites) and magnetic and orbitally active V³⁺ ions (3d², (t₂g)²(eg)⁰, S = 1) occupying

the octahedral sites (B sites) (Fig. 1), grabs extra attention because it presents the shortest V-V distances. With a $d_{V,V} = 2.9724$ Å, it lies at the crossover from insulating to metallic behavior [8] and thus provides a unique platform where localized spins and itinerant electrons compete in a geometrically frustrated spinel, leading to complex structural and magnetic behaviours.

Three magnetic and two structural transitions are observed in CVO. The exact temperatures at which the transitions occur slightly vary from one study to another, the largest differences being probably due to some stoichiometric variations, as pointed out by Ishibashi et al. [9]. Table 1 gathers the most significant reported values observed in the literature for both polycrystalline powders and single crystals. With decreasing temperature, CVO first undergoes a paramagnetic (PM) to collinear ferrimagnetic (CL FIM) transition at T₁ ~ 142 K, still retaining the Fd$\overline{3}$m cubic structure, then a CL FIM to non-collinear ferrimagnetic (NC FIM) transition accompanied with a cubic Fd$\overline{3}$m to tetragonal I4₁/amd structural transition at T* ~ 95 K, and, finally, a transition towards an antiferro-orbital ordering (AF-OO) of the V³⁺ ions, accompanied with an I4₁/amd to I4₁/a structural transition at T₂ ~ 59 K [9]. It is believed that the strong itinerant character of CVO makes it very particular in that it has a lower need for orbital degeneracy relief [7]. It consequently shows a not very demonstrative cubic to tetragonal transition, with an extremely small crystal distortion of (1 - c/a) < 0.06 %, only recently

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: nathalie.viart@ipcms.unistra.fr (N. Viart).</footnote>
experimentally brought to light [9,10], after many years of thinking that, contrary to other AV₂O₄, CVO remained cubic down to the lowest temperatures [8].

CVO is thus a very unique system in which both conductivity and magnetism can be driven by the application of external perturbations such as pressure [7,8]. One way of applying a pressure is the substrate-induced stress when the material is deposited in the form of a thin film. This explains the recent interest observed in the literature for the deposition of CVO in thin films [11–14].

The targets used for these thin films deposition are either metal Co and V targets, in the case of radio frequency magnetron sputtering [11] or a CoV₂O₆ target, in the case of pulsed laser deposition (PLD) [12–14], but never a stoichiometric CoV₂O₄ target. This is probably due to the difficulty to obtain stoichiometric and dense CVO pellets which could be used as targets. The pitfalls mostly originate from the multivalence of both Co and V ions, with an additional difficulty lying in the fact that V₂O₅ melts at 690 °C [15–17]. The synthesis of polycrystalline CVO powder itself is only scarcely detailed in the literature. The source of Co²⁺ ions can be Co₃O₄ [9,22], a mixture of Co and Co₃O₄ [18], a mixture of Co and Co₂O₃ [19], or CoO [20,21,28]. That of V³⁺ can be V₂O₃ [18,20,28], a mixture of V₂O₃ and V [9,22], or V₂O₅ [21]. The heat treatment is usually performed at rather low temperatures such as 900°C [9,19,22] or 950°C [18] for 24 h in an evacuated sealed capsule to avoid the re-oxidation of V into V⁵⁺. Such low temperatures are not compatible with the sintering of dense pellets.

In this study we have set up a solid-state chemistry synthesis route for dense pellets of CVO which can be used as targets for PLD or sputtering depositions, starting from some of the most common and stable sources of Co and V, which are CoCO₃·xH₂O and V₂O₅. We detail all the intermediate steps, and check the composition, structure, heat capacity, magnetic behaviour, and density of the obtained CVO pellet.

## 2. Materials and methods

The precursors used for the synthesis of CVO polycrystalline powder are commercial CoCO₃·xH₂O (Sigma-Aldrich, Co 43–47 %) and V₂O₅ (Sigma-Aldrich, > 99.6 %) compounds, which are first transformed into CoO and V₂O₃, respectively. A CVO pellet was then produced from the sintering of the CoV₂O₄ powder, after homogenization of its grain size by attrition. The particle size distribution of the powder was measured using laser granulometry with a Mastersizer 2000 coupled with a small volume sample dispersion unit and a dispersion unit controller (Malvern Instruments). The crystalline structure of the synthesized compounds was determined at each step of the synthesis by room temperature θ–2θ X-ray diffraction measurements, using a D8 Discover Brucker

<caption>**Table 1**</caption>

Literature reported values for the structural and magnetic transitions of CVO in both single crystal and polycrystalline samples.

<table><thead><tr><th>CVO sample</th><th>T<sub>1</sub><br>(K)</th><th>T*<br>(K)</th><th>T<sub>2</sub><br>(K)</th><th>Reference</th></tr></thead><tbody><tr><td>CoV<sub>2</sub>O<sub>4</sub> (single crystal)</td><td>150</td><td>75</td><td>-</td><td>[8]</td></tr><tr><td>CoV<sub>2</sub>O<sub>4</sub> (polycrystalline)</td><td>142</td><td>100</td><td>59</td><td>[19]</td></tr><tr><td>CoV<sub>2</sub>O<sub>4</sub> (polycrystalline with minimal<br>cation site disorder)</td><td>156</td><td>90</td><td>-</td><td>[10]</td></tr><tr><td>CoV<sub>2</sub>O<sub>4</sub> (polycrystalline)</td><td>142</td><td>95</td><td>59</td><td>[9]</td></tr><tr><td>Co<sub>1.1</sub>V<sub>1.9</sub>O<sub>4</sub> (single crystal)</td><td>152</td><td>75</td><td>47</td><td></td></tr></tbody></table>

diffractometer in the Bragg-Brentano configuration, equipped with a copper anode ($K_{\alpha1}$ = 0.154056 nm), a quartz front monochromator and an energy resolved Lynxeye XE-T detector. Le Bail and Rietveld refinements of the diffraction patterns were performed using the FullProf suite [23] with Thompson-Cox-Hastings pseudo-Voigt profile functions. The pellet was imaged with both a Zeiss GeminiSEM 500 and a JEOL 6700 F scanning electron microscope (SEM). Its stoichiometry, i.e. the Co/V ratio, was determined by energy dispersive spectroscopy (EDS) coupled with these microscopes. Temperature dependent specific heat capacity measurements, $C_p(T)$, of the obtained CoV₂O₄ pellet were performed using a physical property measurement system (PPMS Dynacool, Quantum Design). The magnetic properties of the elaborated pellet were evaluated with a superconducting quantum interference device magnetometer (SQUID VSM, Quantum Design). Measurements of magnetization as a function of temperature in both zero-field cooled (ZFC) and field cooled (FC) modes were performed in a temperature range between 2 and 300 K and under several different magnetic fields between 0.01 and 5 T. Measurements of the magnetization (M) as a function of the applied field (H) were also carried out at several fixed temperatures (40, 70, 90, 110, 130 and 170 K). For all the magnetic measurements, the CoV₂O₄ powder was mixed with a small amount of eicosane to block any movement of the grains.

## 3. Results and discussion

### 3.1. Chemical syntheses

Even if Co is already in the desired +II valence in CoCO₃·xH₂O compound, it is recommended to avoid any possible impact of the important release of CO₂ and H₂O related to the use of such a precursor during the heat treatment. CoO was thus first obtained from the heat treatment of hydrated cobalt carbonate CoCO₃·xH₂O in an alumina crucible for 7 h at 900°C under an argon atmosphere and with a gas flow

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** (a) Crystal structure of the normal cubic spinel CoV₂O₄ at room temperature and (b) Electronic configuration of Co²⁺ and V³⁺ at the tetrahedral and octahedral sites, respectively.</figcaption></figure>
of 100 sccm (Eq. (1)).

$$CoCO_3 \cdot x H_2O \xrightarrow{900^{\circ}C. Ar. 7h} CoO \quad (1)$$

The purity of the obtained precursor was checked by X-ray diffraction (Fig. 2). No spurious phase was observed and a lattice parameter of $a_{CoO} = (4.262 \pm 0.001)$ Å was determined from the Le Bail fitting, in agreement with the reported value in literature [PDF #00–048–1719], confirming the good quality of the precursor.

The second precursor, $V_2O_3$, was synthesized through the reduction of high purity $V_2O_5$ commercial powder as summarized in Eq. (2). It is important to mention that $V_2O_3$ can be commercially purchased but presents some severe stability issues, being prompt to absorb oxygen in air and convert to $V_2O_4$, with the additional risk of igniting upon heating in air. This explains the need to synthesize it right before its use.

$$ \begin{array}{ccc} & \text{Ar(97.5%)/H}_2(2.5\%) & \\ \text{V}_2\text{O}_5 \xrightarrow{\begin{array}{c} \text{a) } 600^{\circ}\text{C (10 h)} \\ \text{b) } 1100^{\circ}\text{C (10 h)} \end{array}} & & \text{V}_2\text{O}_3 \end{array} \quad (2) $$

$V_2O_5$ was calcinated in a platinum crucible under a mildly reducing atmosphere composed of Ar (97.5 %)/H$_2$ (2.5 %). A first calcination step was performed by heating the powder to 600°C, reached with a speed of 100°C/h, and kept for 10 h. This first step at 600°C is crucial as it allows the reduction of the valence of vanadium from $V^{5+}$ to $V^{3+}$ before heating the powder to higher temperatures, because $V_2O_5$ melts at 690°C [15–17]. A second step was performed by then heating the powder to 1100°C reached with a 100°C/h speed and maintained for another 10 h. This second step of maintaining the powder for 10 h in the reducing atmosphere helps the stabilization of the vanadium’s valence in the +III state. Finally, the powder was cooled down to room temperature at a 200°C/h speed, still in the Ar (97.5 %)/H$_2$ (2.5 %) mildly reducing atmosphere.

Fig. 3 shows the ($\theta - 2\theta$) X-ray diffraction pattern of the obtained $V_2O_3$ precursor, together with its Le Bail’s refinement with lattice parameters of $a_{V_2O_3} = b_{V_2O_3} = (4.954 \pm 0.001)$ Å, $c_{V_2O_3} = (14.005 \pm 0.002)$ Å, in good agreement with the values reported in literature [PDF #00–034–0187], confirming the good quality of the obtained precursor.

Stoichiometric amounts of the thus synthesized CoO and $V_2O_3$ precursors were reacted to yield $CoV_2O_4$ according to Eq. (3).

$$CoO + V_2O_3 \rightarrow CoV_2O_4 \quad (3)$$

The precursors were first ground in ethanol in an agate stone mortar for 15 min. The obtained mixed powder was then placed into a platinum crucible and introduced into a tubular furnace to undergo a first

calcination thermal cycle under inert Ar atmosphere with a gas flow of 100 sccm maintained during the whole thermal cycle. The fact that both precursors are in their final expected valences, $Co^{2+}$ in CoO and $V^{3+}$ for $V_2O_3$, explains the choice of the inert Ar atmosphere. The presence of oxygen, even in traces, is to be avoided during the heating since it could re-oxidize the vanadium to its +V state.

The powder is heated to 600°C at a 100°C/h speed and kept at this temperature for 10 h prior to be taken to 1100°C, still at a 100°C/h speed, for 10 h. It is then allowed to cool down to room temperature with a 200°C/h speed. The whole heating process is performed under an Ar atmosphere with a purity above 99.995 % vol. and an $O_2$ content below 5 ppm$_v$ (Ar 4.5, Linde gas). A 2 h Ar flush is performed prior to any heating and a Ti crucible was placed upstream of the reagent crucible, to serve as a getter in case Ar contained traces of $O_2$.

The thus obtained powder is checked to be $CoV_2O_4$ by X-ray diffraction. Before performing a sintering calcination step at higher temperature in order to obtain a dense pellet, we sought to obtain a uniform small grain size of the powder (ca. 1 µm diameter) through an attrition step. The attrition was performed with 1 mm diameter zirconia (ZrO$_2$) balls in ethanol [24]. Ethanol is used owing to its good dispersive properties for oxide powders, adding to this the fact that it easily vaporizes. Since the grinding engenders a heating of the mixture, the attrition takes place at a stabilized ambient temperature thanks to a cooling system which compensates the heating caused by the friction of the zirconia balls. For 1 g of powder, 10 mL of zirconia balls and 40 mL of ethanol were used. (Figure S1 in Supplementary Material shows a representative scheme of the attrition system).

A study of the size of the grains with respect to the attrition duration was performed, in order to determine the time required to obtain homogeneous grain sizes of around 1 µm. Fig. 4 shows a clear decrease of the particle size distribution with the attrition duration.

An attrition time of about 6 h is required in order to have a homogeneous monomodal particle size distribution with a grain size around 1 µm. The powder was dried in an oven at 100°C for one night and pressed uniaxially under a 60 bars load to produce 15 mm diameter and 4 mm thick pellets. Polyvinyl alcohol was used as a binder, to improve ease of unmolding and mechanical resistance of the green pellets. The pellet was finally sintered for 20 h at 1300°C (reached at 100 °C/h) and then cooled down at room temperature with a 200 °C/h speed, under Ar.

The X-ray diffraction pattern of the obtained $CoV_2O_4$ pellet, measured at room temperature, exhibits only the presence of the $CoV_2O_4$ spinel single phase and no impurities or residues from the precursors, as shown in Fig. 5. In order to probe the cationic distribution within the structure, Rietveld refinement has been performed on the obtained CVO X-ray diffraction pattern. The Rietveld refinement allows concluding

<figure><img src="image_4.png" /><figcaption>Fig. 2. X-ray diffraction pattern (red circle) of the synthesized CoO precursor with its Le Bail refinement (black full line) and the difference (blue full line), with the resulting parameters of the fit. Positions of the Bragg reflections are represented by green vertical bars.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. X-ray diffraction pattern of the synthesized V₂O₃ precursor (red circle), together with its Le Bail refinement (black full line) and the difference (blue full line), with the resulting parameters of the fit. Positions of the Bragg reflections are represented by green vertical bars.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. Particle size distribution of the CoV₂O₄ powder for several attrition times.</figcaption></figure>

that all V atoms are in the octahedral sites, while the Co ones are in the tetrahedral ones. Bearing in mind that the two elements are very close electron-wise, refinements of the cation distribution did not reveal any vanadium in the tetrahedral site. Moreover, the Rietveld refinement allowed the obtention of a lattice parameter of a = (8.411 ± 0.001) Å, consistent with the reported value in the literature [PDF #04-002-4958].

### 3.2. *Physical properties characterization*

The pellet reaches 71.2 % of the theoretical density (See *Supplementary Material* for details on the calculation), fully compatible with a possible further use as a target in thin films deposition methods. The pellet stoichiometry was checked by SEM-EDS measurements and the expected Co:V ratio of 1:2 was measured (See details in *Supplementary Material*). The morphology and microstructure of the pellet in cross-section, imaged by SEM (Fig. 6), indicate a significant density, consistent with the experimentally determined one.

Specific heat capacity measurements were performed to search for phase transitions. Two transitions, at T₂ ≈ 55 K and T₁ ≈ 145 K, are clearly observed in Fig. 7. These values are in very good agreement with those reported in the literature for polycrystalline or single-crystalline stoichiometric CoV₂O₄ [9,19,22]. In order to get information concerning the possible magnetic nature of these transitions, ZFC and FC

<figure><img src="image_7.png" /><figcaption>Fig. 5. (θ – 2θ) X-ray diffraction pattern (red circle) of the CoV₂O₄ pellet together with its Rietveld refinement (black full line), the difference (blue full line) and the resulting parameters of the fit. Positions of the Bragg reflections are represented by green vertical bars.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 6. SEM image of a cross-section of the CoV₂O₄ pellet.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. Specific heat capacity measurements of the CoV₂O₄ pellet.</figcaption></figure>

magnetic measurements were also performed.

Fig. 8 presents the ZFC and FC measurements performed on the CoV₂O₄ pellet for different applied magnetic fields. Magnetic transitions

<figure><img src="image_10.png" /><figcaption>Fig. 8. ZFC and FC measurements performed on the CoV₂O₄ pellet with several applied magnetic fields.</figcaption></figure>

are clearly observed for all applied magnetic fields for T₂ ≈ 55 K and T₁ ≈ 145 K, consistent with previous studies [9,19,22]. A third transition, not present for the specific heat measurements, is detected at T* ≈ 87 K and seems to be field-dependent as it vanishes with higher applied fields (μ₀H ≥ 0.5 T). This again is in perfect agreement with what has been observed for perfectly stoichiometric samples [9].

The magnetic hysteresis loops performed at different temperatures and for applied fields in the range between + 7 T and -7 T are shown in Fig. 9. A total saturated magnetic moment of ca. 1.5 µB/f.u. is observed in the temperature range 40–90 K, consistent with literature [19]. This value is in good agreement with a scenario in which the spins of Co²⁺ in the A sites are anti-ferromagnetically coupled to the spins of V³⁺ in the B sites. If the Co²⁺ and V³⁺ had their expected moments of 3 and 2 µB, respectively, the total saturated magnetic moment would be of 2 × 2 – 3 = 1 µB/f.u. Lower values than 2 µB have however been reported for the moment of the V atoms, and one can find values such as 0.85, 1.3, and 0.63 µB for FeV₂O₄ [25], MnV₂O₄ [26], and ZnV₂O₄ [27], respectively, in the literature. These discrepancies are mostly attributed to geometrical frustration [9]. The value of ca. 1.5 µB/f.u. observed here corresponds to an effective value of ca. 0.75 µB/V ion, in perfect agreement with what is usually observed for spinel vanadates, and in particular for the cobalt vanadate [9].

## 4. Conclusion

We report here a rationalized solid-state elaboration of a stoichiometric single-phased polycrystalline spinel CoV₂O₄ pellet which reaches 71.2 % of the theoretical density. Specific heat capacity measurements showed two clear transitions at T₂ ≈ 55 K and T₁ ≈ 145 K, detected also in ZFC and FC magnetic measurements under weak applied magnetic fields. A third magnetic transition is detected at T* ≈ 87 K and disappears when applying higher magnetic fields (μ₀H ≥ 0.5 T). The physical properties of the prepared CoV₂O₄ pellet were thus shown to be in perfect agreement with those reported in the literature for single- and poly-crystalline CoV₂O₄. The significant density of the elaborated pellet allows its use as a target for thin film growth methods such as PLD or sputtering. This opens new paths for the production of CVO thin films out of a stoichiometric spinel CoV₂O₄ target.

## CRediT authorship contribution statement

Lamiae El Khabchi: Writing – review & editing, Writing – original draft, Visualization, Validation, Investigation, Formal analysis, Data curation. Antonio Peña Corredor: Methodology, Investigation, Data

<figure><img src="image_11.png" /><figcaption>Fig. 9. Hysteresis loops measured for the CoV₂O₄ pellet at different temperatures. The curves plot shows the magnetic moment per formula unit, calculated from the measured magnetization versus applied magnetic field data.</figcaption></figure>
curation, Conceptualization. **François Roulland:** Writing – review & editing, Validation, Supervision, Investigation, Data curation. **Marc Lenertz:** Writing – review & editing, Validation, Investigation, Data curation. **Cédric Leuvrey:** Writing – review & editing, Validation, Investigation, Data curation. **Jérôme Robert:** Validation, Investigation, Data curation. **Gilles Versini:** Validation, Investigation, Data curation. **Laurent Schlur:** Writing – review & editing, Validation, Investigation, Data curation. **Christophe Lefèvre:** Writing – review & editing, Validation, Supervision, Methodology, Data curation, Conceptualization. **Nathalie Viart:** Writing – review & editing, Writing – original draft, Validation, Supervision, Project administration, Investigation, Funding acquisition, Data curation, Conceptualization.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This work was supported by the Interdisciplinary Thematic Institute QMat, which as part of the ITI 2021 2028 program of the University of Strasbourg, CNRS and Inserm, was supported by IdEx Unistra (ANR 10 IDEX 0002), and by SFRI STRAT'US project (ANR 20 SFRI 0012) and ANR-11-LABX-0058-NIE and ANR-17-EURE-0024 under the framework of the French Investments for the Future Program. It is also supported by France 2030 government investment plan managed by the French National Research Agency under grant reference PEPR SPIN – SPINMAT ANR-22-EXSP-0007. The authors acknowledge experimental support from the XRD, MEB-CRO, MagTransCS, and PLD platforms of the IPCMS, as well as technical support from Didier Burger.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jeurceramsoc.2025.117286.

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