# Synthesis of garnet tellurate powders by coprecipitation: the case of yafsoanite Ca3Zn3Te2O12

G. Guy, A. Baillard, D. Jouglard, A. Ballestero, Gaelle Delaizir, L. Boyer, Yann Launay, O. Durand, V. Couderc, Philippe Thomas, et al.

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G. Guy, A. Baillard, D. Jouglard, A. Ballestero, Gaelle Delaizir, et al. Synthesis of garnet tellurate powders by coprecipitation: the case of yafsoanite Ca3Zn3Te2O12. Ceramics International, 2025, 51 (21), pp.33290-33300. 10.1016/j.ceramint.2025.05.062. hal-05404996

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# Synthesis of garnet tellurate powders by coprecipitation: the case of yafsoanite Ca₃Zn₃Te₂O₁₂

G. Guy<sup>a</sup>, A. Baillard<sup>a,d</sup>, D. Jouglard<sup>b</sup>, A. Ballesterob<sup>b</sup>, G. Delaizir<sup>a</sup>, L. Boyer<sup>b</sup>, Y. Launay<sup>a</sup>, O. Durand<sup>b</sup>, V. Couderc<sup>c</sup>, P. Thomas<sup>a</sup>, J. Jouin<sup>a</sup>, F. Rémondière<sup>a,**</sup>, J.-R. Duclère<sup>a,*</sup>

## ARTICLE INFO

Handling Editor: Dr P. Vincenzini

**Keywords:**

Calcium zinc tellurate garnet

Coprecipitation

Monodisperse

Nanopowders

## ABSTRACT

The calcium-zinc garnet tellurate phase Ca₃Zn₃Te₂O₁₂ (CZT) was successfully synthesized via the reverse coprecipitation method conducted at room temperature, followed by a series of *ex situ* heat treatments under atmospheric conditions. Orthotelluric acid, with calcium and zinc nitrates served as cationic precursors, and the precipitating agents corresponded to a mixture of ammonium hydrogen carbonate (NH₄HCO₃) with ammonium hydroxide (NH₄OH). Maintaining the pH around a value of ~9 during the synthesis, through regular manual additions of a few drops of NH₄OH, was actually crucial in order to obtain chemically adequate and low agglomerated nanopowders. Both X-ray diffraction and Scanning Electron Microscopy data collected on the dried precipitated powder indicate that the latter consists mainly in nanocrystalline CZT grains, which are mixed with CaCO₃ submicronic grains. Heat-treating the powder incrementally up to a maximum temperature of 900 °C will then allow to stabilize almost purely the CZT phase, with only remaining traces of the calcium tellurate phase Ca₃TeO₆. Both raw and calcined powders were further characterized employing diverse techniques, such as thermal and elemental analysis, laser granulometry, BET surface area, as well as Raman, infrared and optical spectroscopies.

## 1. Introduction

Garnets, occurring both synthetically and naturally (gemstones), form a very wide range of inorganic compounds and display a multitude of applications. To illustrate that statement, Ce³⁺-doped garnets are for instance employed as phosphors in fluorescent tubes, afterglow materials, colour converters in White Light-Emitting Diodes (WLEDs) and scintillators, as reviewed by Xia et al. [1]. Among such family of materials, Yttrium aluminum garnet (YAG – Y₃Al₂O₅) can be probably regarded as the archetype of the garnet structure, possessing a cubic structure with Ia-3d symmetry [2]. YAG is indeed noteworthy due to its importance as a host material, and the doping by rare-earth ions opens numerous opportunities; some already mentioned above (see Ref. [3] as a pioneer reference) and others, such as the possibility to fabricate competitive laser materials, in relation with the full transparency of polycrystalline YAG ceramics [4,5].

In terms of process, the solid-state reaction technique is a well-recognized route for elaborating YAG nanopowders that will be further sintered in the aim to fabricate polycrystalline YAG ceramics [4,5]. As a supplementary illustration, Chandra et al. successfully used different routes (among them, the solid-state route) to elaborate as well YAG powders [6]. The authors also evaluated the characteristics of the elaborated ceramics, including their transparency.

Without being exhaustive, besides solid-state reaction, various wet chemical routes were also employed in order to fabricate YAG powders. For instance, Xu et al. elaborated by the solvothermal method some monodispersed YAG crystallites with well developed and controllable crystal faces [7]. The tested solvents were either pure water, pure ethanol or a water-ethanol mixture. The range of temperatures fell in between 250 °C and 300 °C, for an experimental duration varying from 5h to 24h. In addition, a part of the same authors successfully synthesized Europium doped YAG crystallites with a good dispersity via some

<footnote><sup>a</sup> Institut de Recherche sur Les Céramiques, UMR 7315, CNRS / Université de Limoges, Centre Européen de La Céramique, Limoges, France</footnote>

<footnote><sup>b</sup> Centre de Transfert de Technologies Céramiques, Parc D'Ester, 7 Rue Soyouz, 87000, Limoges, France</footnote>

<footnote><sup>c</sup> Institut XLIM, UMR, CNRS 7252 – Université de Limoges, 87060, Limoges, France</footnote>

<footnote><sup>d</sup> Current Address: Centre de Recherche sur Les Ions, Les Matériaux et La Photonique, UMR CNRS 6252 – Université de Caen Normandie, 14076, Caen, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: fabien.remondiere@unilim.fr (F. Rémondière), jean-rene.duclere@unilim.fr (J.-R. Duclère).</footnote>
hydrothermal method conducted at 300 °C during 24h [8].

In our work, we are focusing on the coprecipitation route, as it is a rather simple experimental method to handle. To start with, the PhD thesis manuscript of C. Marlot offers a nice concatenated work focused on the synthesis by coprecipitation of Er-doped YAG powders and the subsequent densification by Spark Plasma Sintering in the aim to fabricate transparent ceramics [9]. Many other interesting contributions can be then quoted. For instance, YAG powders were synthesized using diverse precipitating agents, such as NaOH [10], NH₄HCO₃ [11], NH₄HCO₃ or NH₄OH [12], and as well NH₄HCO₃ or urea [13]. In particular, J.-G. Li et al. claimed that YAG powders synthesized using NH₄HCO₃ are less aggregated and might therefore further lead to transparent ceramics after sintering [12]. Moreover, as discussed by Apte et al., the reverse coprecipitation route offers some great interest due to the favoured simultaneous precipitation of Y³⁺ and Al³⁺ cations [14]. Furthermore, several research groups have underlined the importance of carefully maintaining constant the pH value during the coprecipitation experiment [14–16].

In this work, we are particularly motivated by tellurate materials (tellurium oxides, with Te having the highest + VI oxidation degree) [17], as they can present interesting vibrational, conductivity and/or dielectric properties [18]. And among such tellurates, we specifically concentrate on cubic garnet tellurates [19], especially the Ca₃Zn₃Te₂O₁₂ (CZT) composition, which corresponds to the yafoanite structure, initially described by Kim et al. [20] and later by Jarosch and Zemann [21], for the lead-contained system.

Up to date, the elaboration of either undoped or doped CZT was systematically conducted via the traditional solid-state reaction which necessitates high temperatures, typically beyond 1000 °C [17,19–27]. More precisely, Kasper elaborated CZT at 1050 °C for 1h, and simply mentioned the lattice constant value (a = 12.58 Å) and the fact that the sample displayed a white colour [19]. Zhang et al. synthesized Eu³⁺-CZT red emitting phosphors after following this experimental protocol: the initial mixture of precursors was heated at 650 °C for 5 h; the obtained powder was further mixed and then heated up to 850 °C for 5h in air; after that, the sample was thoroughly mixed and heated once more up to 1100 °C for 15h [22]. The authors further studied the photoluminescence (PL) properties of the phosphor material, which exhibited a strong red emission. Deng et al. elaborated (Dy³⁺, Na⁺) co-doped yafoanite CZT phosphor after reacting and sintering the precursors at 1050 °C for 12 h under air [23]. In particular, through thermal quenching experiments, for one specific composition, they showed the excellent thermal stability of the PL intensity up to 423K. Vilesh et al. calcined their powder at 1000 °C for 10h, before sintering it over 4h at different temperatures, ranging from 1025 °C up to 1125 °C [24]. They systematically observed the microstructure by Scanning Electron Microscopy and finally performed vibrational studies and measured microwave dielectric properties for the CZT ceramic sintered at 1100 °C during 4h [24]. Zhang et al. synthesized Ca₃Zn₃(TeO₆)₂:Sm³⁺ phosphors for WLEDs from the powdery precursors sintered at 1050 °C for 10h, under air [25]. The authors demonstrated that such material displays a high colour purity and exhibits some excellent thermal stability. Cao et al. fabricated new co-doped Ca₃Zn₃Te₂O₁₂:Eu³⁺/Bi³⁺ phosphors with adjustable luminescence properties [26]. They first calcined the mixed chemical precursors at 650 °C, for 4 h in air, before cooling down to room temperature (RT), regrinding the mixture, and finally sintering at 1100 °C for 4h again. The resulting samples were investigating in order to access structural information and evaluate the optical properties. To complete this brief literature overview, recently, Xie et al. published on the synthesis, using a conventional high-temperature solid-phase method, of a novel Eu³⁺ activated Ca₃Zn₃Te₂O₁₂ red-emitting phosphor [27]. The authors heated up the precursors in a muffle furnace at 5 K min⁻¹ up to 1323 K (hence 1050 °C), for 10 h, in atmospheric conditions. They also reported the high red colour purity and the excellent thermal stability, as the CIE colour coordinates hardly changed with temperature.

Therefore, in this work, we report for the first time on the elaboration of the CZT garnet phase by the reverse coprecipitation route, by using a mixture of ammonium hydrogenocarbonate (NH₄HCO₃) and ammonium hydroxide (NH₄OH) as the precipitating agents. Like [14–16], we also illustrate the necessity of carefully controlling and maintaining the pH value constant during the entire duration of the synthesis (i.e. just before starting the maturation step) in order to obtain the most adequate powders. We finally show in this article that the obtained coprecipitated powder is perfectly adapted to produce almost pure CZT powders after a series of calcinations step limited to a maximum temperature of 900 °C.

## 2. Experimental section

### 2.1. Materials synthesis

In this work, calcium and zinc nitrates were used (Ca(NO₃)₂·4H₂O from Sigma Aldrich, 99.0 %; Zn(NO₃)₂·6H₂O from Sigma Aldrich, 99.0 %), as well as orthotelluric acid (H₆TeO₆ from Sigma Aldrich, 99.0 %). All the cationic precursors were dissolved in deionized water in stoichiometric proportions, to respectively reach the following concentrations: 0.75 mol L⁻¹, 0.75 mol L⁻¹ and 0.5 mol L⁻¹. 20 mL of a pH = 9.2 buffer mixture made of NH₄HCO₃ from Alfa Aesar, 98 % and NH₄OH from ThermoFischer, 35 % (density = 0.88), used as precipitating agents, was prepared at a concentration of 1 mol L⁻¹.

Fig. 1 displays the schematic representation of the experimental setup we used for the reverse strike coprecipitation route. The cationic solution is added dropwise into the buffer solution. At the same time to compensate the acidification of the mixture, the pH is manually adjusted to a value of 9.0 by a few drops of NH₄OH. The precipitation occurs immediately, and the total reaction time lies in between 15 and 20 min. The maturation step will last for 24h, under constant magnetic stirring at RT.

The suspension is then filtered using a Buchner device and dried overnight at 90 °C, under air.

An approximative mass of ~3.75 g is collected, and since only traces of metals were detected into the filtrate of the filtrated suspension, we can confirm that the global process yield is relatively good (typically beyond 85 %).

Finally, after the synthesis, the raw dried powder was heated incrementally up to a maximum of 900 °C, under air, in alumina crucibles, using the following protocol: heating and cooling ramps (10 °C/min), successive and cumulative calcinations: 1h at 500 °C, then 1h at 700 °C, followed by 1h at 800 °C and finally 1h at 900 °C. At each step, the powder was characterized.

### 2.2. Characterization methods

X-Ray Powder Diffraction (XRPD) data were recorded on a Bragg Brentano D8 Advance Bruker diffractometer (CuKα radiation) equipped with a LynxEye XE detector over a 2θ angular range of 10° < 2θ < 70° for phase identification, with a 0.02° step and 1.05s step equivalent time.

ThermoGravimetric and Differential Thermal Analyses (TGA-TDA) were performed using a NETZSCH STA449 F3 JUPITER apparatus on the raw ceramic powder recovered after the drying step, using a heating rate of 10 °C/min, up to 900 °C in air.

Both Horiba EMGA-830 and EMIA-321V analyzers were respectively employed for (N, H) and C elemental analysis. In order to perform correct analyses, approximately 0.015 g of raw powder was needed for the dual nitrogen and hydrogen analysis, whereas roughly 0.250 g of raw powder was used for the quantification of carbon. Each time, three different measurements were conducted and the resulting data were averaged.

Particle size analysis was performed by laser scattering granulometry (Horiba Partica La-950), using Fraunhofer approximation. The powder was dispersed in deionized water by sonication for 1 min.

BET specific surface is measured with Micromeritics ASAP 2020
<figure><img src="image_3.png" /><figcaption>Fig. 1. Schematic representation depicting the reverse strike coprecipitation method employed in this work: focus on the different steps (synthesis, maturation, filtration, drying).</figcaption></figure>

apparatus. Before measurement, the powder is degassed at 80 °C for 15h.

Employing an InVia Reflex Renishaw Raman spectrophotometer, micro-Raman spectroscopy experiments were performed on calcined powders. The spectra were collected from ~50 to 1300 cm⁻¹, using a x20 objective and a 532 nm excitation wavelength, under a continuous wave laser power of ~4 mW at the sample. Spectral resolution is 2.5 cm⁻¹. Each displayed spectrum finally results from the average of 5 different sets of collected data points.

NIR Spectroscopy measurements were performed on a NICOLET iS10 with a Smart OMNI-Transmission accessory. Measurements are made on KBr pellets consisting of small quantity (~5–10 mg) of CZT sample powder dispersed within the KBr powder.

A Quanta 450 FEG Scanning Electron Microscope was used to observe the microstructure of the CZT powders (raw and calcined), mainly to derive the actual size of the individual grains, but also to confront the acquired SEM images with XRPD data. Prior to any SEM observation, a brief flash-evaporation of carbon was done in order to reduce surface charging effects and therefore improve the overall quality of the images. In addition, Energy Dispersive X-ray (EDX) analyses were conducted under an acceleration voltage of 10 kV, in order to control the actual cationic composition of the elaborated powders.

Ultra-Violet and Visible (UV-Vis) diffuse reflectance data were acquired using an integration sphere placed inside a Cary 5000 spectrophotometer commercialized by Agilent. In order to extract an estimation of the optical band gap, the Kubelka-Munk formula was applied and plotted as a function of the photon energy (see Section 3).

# 3. Results and discussion

## 3.1. Titration curves

The pH meter was calibrated using standard buffer solutions at pH values of 4.0, 7.0 and 10.0. Fig. 2a shows titration curves of 0.2 M solution of NaOH or NH₄HCO₃ neutralized by an aqueous cationic solution. The cationic solution is prepared from Ca(NO₃)₂·4H₂O (0.5 M), Zn(NO₃)₂·6H₂O (0.5 M) and Te(OH)₆ (0.33 M) with a cationic ratio equal to 3:3:2. Precipitation occurs immediately after the first drop. For both titration curves, the pH decreases when the cationic solution is added. In the case of NaOH, the pH curve shows two equivalences. The equivalence points occur for an equivalent volume Veq,1 = 3.8 mL (pH ~ 9.0) and for an equivalent volume Veq,2 = 5.7 mL (pH ~ 5.4). These equivalent volumes delimit three zones, denoted I, II and III in Fig. 2a. In terms of added cationic solution, it is important to note that the extension of zone I is twice as wide as zone II, meaning that reaching the second equivalent point requires only half of the cations that were delivered during the first equivalence. In the case of NH₄HCO₃, the pH curve shows only one equivalence, difficult to accurately determine, and occurring at an equivalent volume Veq ~7.4 mL (pH ~ 7.1).

XRPD of resulting precipitates were measured for specific points of

the experiment. The resulting diagrams are shown in Fig. 2b for the sample collected at the end of the first equivalence for NaOH (point A) and Fig. 2c for the samples collected at the end of the titrations for both systems (points B and C, respectively). In NaOH, after the first equivalence, yafsoanite was identified (Fig. 2b), while at the end of the titration, dagenaisite was formed whatever the solution (Fig. 2c).

Based on these results, in the case of NaOH, the first pH region (I) seems to correspond to the formation of yafsoanite Ca₃Zn₃Te₂O₁₂ according to Equation (1).

$$12 \text{HO}^{-} + \{3 \text{Ca}^{2+} + 3 \text{Zn}^{2+} + 2 \text{Te(OH)}_{6}\} \rightarrow \text{Ca}_{3}\text{Zn}_{3}\text{Te}_{2}\text{O}_{12} + 12\text{H}_{2}\text{O}(1)$$

The second pH region (II) spreads below pH = 9.0 down to 5.4 under the effect of the acidic cationic solution. It corresponds in fact to the progressive transformation of Ca₃Zn₃Te₂O₁₂ into the dagenaisite Zn₃TeO₆ phase in the suspension (Cf. Equation (2)).

$$\mathrm{Ca}_{3}\mathrm{Zn}_{3}\mathrm{Te}_{2}\mathrm{O}_{12} + (1/23\mathrm{Ca}^{2+} + 3\mathrm{Zn}^{2+} + 2\mathrm{Te(OH)}_{6}) \rightarrow (3/2) \mathrm{Zn}_{3}\mathrm{TeO}_{6} + (9/2) \mathrm{Ca}^{2+} + (3/2) \mathrm{Te}^{6+} + 3\mathrm{H}_{2}\mathrm{O} \quad (2)$$

In the case of NH₄HCO₃, yafsoanite also precipitates at pH = 9.0 (see Fig. 2a), but the pH value does not remain at 9.0 long enough to ensure a good reaction yield. At the end of the titration, like in the case of NaOH, the dagenaisite is the only formed compound.

However, the nature of the precipitating agent is essential. Previous studies on co-precipitation synthesis of garnets [9] have indeed shown that the use of NH₄HCO₃ gives the desired phase. Besides, the precipitate is loosely agglomerated with better dispersity and excellent sinterability. Unlike NaOH, which can pollute the system with Na⁺ ions (even inside the crystal structure), NH₄HCO₃ can be more easily removed by some washing and heat treatment steps [28,29]. As it was previously shown, below pH = 9.0, we tend to obtain the dagenaisite phase. Therefore, ammonia NH₄OH was manually added during co-precipitation to keep the pH constant and stabilize mainly the yafsoanite phase.

## 3.2. X-ray powder diffraction (XRPD)

XRPD patterns of the raw powder and of powders calcined at various cumulated temperatures (500, 700, 800 and 900 °C) for 1h are shown in Fig. 3a. Both the raw and the calcined powders mostly consist of the CZT phase. Besides, the raw powder (after drying) displays extra Bragg peaks corresponding to the calcite CaCO₃ secondary phase. As the temperature increases, the intensity of these peaks decreases until they latter completely disappear beyond 700 °C (see the highlighted 17–31° range in Fig. 3b). Another phase corresponding to the calcium tellurate Ca₃TeO₆ appears at 700 °C, and remains present only in small amounts up to 900 °C. Based on the literature, Ca₃TeO₆ can be obtained by reacting CaCO₃ with TeO₂ around 700 °C in air [30,31], therefore totally in agreement with our observations.

The integral breaths of the reflections were measured, after refining
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (a) Titration curves of 0.2 M solution of NaOH and NH₄HCO₃ titrated with an aqueous cationic solution. (b) XRPD pattern of the dried precipitate collected in point A. (c) XRPD patterns of the dried precipitates collected in points B and C.</figcaption></figure>

their profile using a Voigt function and taking into account the $K_{α1}-K_{α2}$ contributions from the X-ray source. They confirm the sharpening of the peaks, associated with a better crystallinity. However, the phenomenon is not linear with the temperature increase. Fig. 3c shows the evolution of the integral breadth of the (220) reflection of the diagram, which corresponds to the first intense peak of the CZT pattern, situated around 20° in 2θ. At RT, the peak is broad. Using the well-known Scherrer equation [32] and neglecting the instrument resolution function, the spatial extension of the crystallites was estimated to be about 19 nm. The broadening does not evolve at all up to 500 °C. Beyond that temperature, the peaks sharpen rapidly, thus reflecting a higher degree of crystallinity achieved with increasing calcination temperatures. It is to be noted that for the RT and the 500 °C samples, the Gaussian contribution to the

Voigt function was negligible, which can also be related with a higher degree of structural disorder in the particles.

Moreover, the CZT unit cell parameters for each pattern were refined, based on the structural model published by Jarosh et al. (ICSD 67045) [21], using 1a-3d space group. The resulting values are reported in Table 1. The refined unit cell parameters differ from the Jarosh’s model, which was defined from a natural yafsoanite that could contain lead, but are in fact very similar to the Kasper’s values from synthetic yafsoanite of higher chemical purity (PDF card 04-007-0234 ¹⁹). The values remain close to the values reported in the PDF card with a variation inferior to 0.5 %, and they are very stable at each temperature, showing that the CZT phase does not drastically evolve during the calcination steps.
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** (a) X-ray powder diffraction patterns of the raw and calcined powders at successive increasing temperatures for 1h. (b) Zoomed X-ray powder diffraction patterns of the raw and calcined powders between 17 and 31°. (c) Measured integral breadth of the (2θ) reflection (2θ ~ 20°) plotted as a function of the calcination temperature. Please note that the dashed line is there only as a guide for the eyes.</figcaption></figure>

Finally, based on the XRPD data, it was possible to extract the vol-
ume fraction of the calcite which is present in the raw powder. For this,
structural models of yafsoanite [21] (ICSD 67045) and calcite [33]
(ICSD 40107) were taken from Inorganic Crystal Structure Database
[34]. The background of the diagram, unit cells, and the Lorentzian part
of the peak profiles were then refined on the full diagram, using
Jana2006 software [35]. Then, the amount of crystalline calcite was
estimated from the intensity of its main peaks, leading to a CZT:Calcite
volume ratio of 94:6 (±1).

3.3. *Thermal and elemental analyses*

The thermal analysis (TGA-DTA) thermograms up to 900 °C in air,
starting from the raw powder, are shown in Fig. 4. Three zones are
clearly observed on the TGA curve, with the respective weight loss
indicated each time in terms of percentage of the initial mass.

The first zone (labeled A) corresponds to the evaporation of solvents and physisorbed molecules (mainly water). It ranges up to ~350 °C and is equivalent to a weight loss of ~3.8 %. It is connected with a series of endothermic peaks (represented by black arrows) on the DTA curve; the
<table><caption>**Table 1**
Refined cell parameters, unit cell volume, variation from the PDF card reference [19]</caption><thead><tr><th>Temperature (°C)</th><th>a (Å)</th><th>V (Å<sup>3</sup>)</th><th>Variation from (%)</th></tr></thead><tbody><tr><td>RT</td><td>12.574(4)</td><td>1987.9</td><td>-0.15</td></tr><tr><td>500</td><td>12.596(3)</td><td>1998.4</td><td>0.38</td></tr><tr><td>700</td><td>12.598(4)</td><td>1999.7</td><td>0.44</td></tr><tr><td>800</td><td>12.593(1)</td><td>1997.0</td><td>0.31</td></tr><tr><td>900</td><td>12.592(1)</td><td>1996.4</td><td>0.28</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** TGA-DTA curves recorded starting from the raw powder up to 900 °C in air, with a heating rate of 10 °C/min.</figcaption></figure>

main endothermic peak being detected below 100 °C. Based on the elemental analysis (Table 2), an overall H content of 0.53 wt % contained in the raw powder would represent a potential weight loss of ~ 4.77 % (once again, when assuming mostly the removal of water molecules). Thus, only 80 % of the total H content will serve to explain this ~3.8 % weight loss within zone A.

The second zone (labeled B) is comprised between ~350 °C and ~550 °C and is linked with a weak endothermic signature. A weight loss of ~2.2 % is noticed. We believe that such segment is related with the desorption of the chemisorbed species, such as ammonium, nitrate and mostly carbonate groups. Effectively, 20 % of the H content still needs to be removed, thus likely under the form of ammonium groups. As well, although the synthesized powder has been thoroughly rinsed with H₂O and ethanol, nitrate groups could still be adsorbed at the surface. Furthermore, carbonate groups must be also invoked when considering

<table><caption>**Table 2**
Elemental analysis for H, N and C elements. Results are expressed in wt. % and the value in between brackets represent the standard deviation issued from three measurements.</caption><thead><tr><th>Element</th><th>H</th><th>N</th><th>C</th></tr></thead><tbody><tr><td>Analysis in wt. %</td><td>0.53 (0.02)</td><td>0.50 (0.03)</td><td>0.77 (0.01)</td></tr></tbody></table>

the overall C content detected by the elemental analysis (Table 2). Indeed, in the last zone (labeled C), only half of the C content will serve to explain the weight loss with the CO₂ departure (see below). Hence, the other half of the carbon is supposed to be adsorbed at the raw powder surface, under the form of carbonate species. Such assumption, supported by IR spectroscopy data (see further in the 3.6 section, with Fig. 7), seems therefore reasonable.

The last weight loss (zone labeled C) observed on the TGA curve corresponds to the thermal decomposition of calcite CaCO₃ occurring between 600 °C and 700 °C. It is associated with the CO₂ departure, according to the following chemical reaction: CaCO₃(s) → CaO(s) + CO₂(g). This process is related with the neat endothermic peak (black arrow in the third part) around 650 °C. The shape of the peak indicates that decomposition begins slowly between 500 °C and 600 °C, before accelerating beyond [36,37]. These results agree with the XRPD data, which show that CaCO₃ Bragg peaks have almost completely vanished after the calcination at 700 °C. Moreover, based on the ~-1.4 % weight loss associated with the CO₂ departure, and considering the molecular weights of CO₂ and CaCO₃, it is possible to derive the weight proportion of calcite contained in the raw powder: the latter is thus equal to ~3.18 %. In addition, taking into account the relative densities of 2.71 and 5.09 for CaCO₃ and CZT, respectively, the volume fraction of calcite present in the raw powder can be estimated to ~5.8 %. Such volume fraction falls then in very good agreement with the previous estimation of 6.0 ± 1.0 % done using XRPD data.

Finally, it is worth mentioning that after heating up to 900 °C, the TGA curve indicates that global weight losses represent ~7-7.5 % of the initial weight.

### 3.4. Particle size distribution and BET specific surface area

The particle size distribution was measured using laser scattering granulometry analysis, for the initial raw powder and for the powder calcined up to 900 °C. The corresponding results are presented in Fig. 5.

Amazingly, comparing raw (d(v0.5) = 1.2 µm) and calcined (d(v0.5) = 1.3 µm) powders indicates that the average particle size d₅₀ is in fact rather close. On another hand, using the BET method, the measurement of the specific surface area conducted on the same aforementioned powders reveals real differences. Indeed, the corresponding values obtained were 43.50 ± 0.26 m²/g and 4.94 ± 0.02 m²/g, respectively. This important decrease of the surface area with the heat treatment is partially explained by the stronger agglomeration of the particles, as evidenced in Fig. 5b with the increasing population of larger agglomerates (50–60 µm), but also by a grain growth mechanism that was already suggested in Fig. 3c. Hereafter, the Scanning Electron
<figure><img src="image_7.png" /><figcaption>Fig. 5. Particle size distribution (volume and number) for the raw powder (a) and the calcined powder up to 900 °C (b).</figcaption></figure>

Microscopy (SEM) observations depicted in Fig. 6 will provide even more evidence. Such value for the calcined powder has then to be compared to the $16.14 \pm 0.18 \text{ m}^2/\text{g}$ measured for the solid-state CZT powder after multiple grinding steps using high energy ball milling [38].

### 3.5. Scanning electron microscopy

Fig. 6 shows the SEM images of raw and powders incrementally calcined at 500, 700, 800 and finally 900 °C. Up to a calcination temperature of 700 °C, the CZT powder can be described as micrometer-sized agglomerates composed of clusters of nanometric primary particles measuring barely 20–30 nm (though at 700 °C, such nanograins already start to grow). Beyond 700 °C, granular coarsening is

unambiguously observed, and at 800 °C and even more at 900 °C, the CZT powder consists now in dense individual grains, which correspond to the initial clusters and remain typically below a size of 200 nm. Moreover, at 900 °C (Fig. 6e), the faceting of some grains can be clearly evidenced. The granular coarsening is finally fully consistent with the XRPD data, in particular with the reduction of the width of the (220) Bragg reflection (Fig. 3c) noticed after heating up to 700 °C and beyond.

EDS analyses were also carried out, only for the powder calcined up to 900 °C, and shaped as a pellet. Table 3 regroups the experimental results obtained for all the cationic elements (Ca, Zn and Te). These data are in fact issued from the average of a series of 9 measurements conducted on 9 different and separate grains. Hence, it appears clear that experimental values are close to the theoretical ones, thus in accordance
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** SEM images of the raw powder (a) and of powders calcined at 500 °C (b), 700 °C (c), 800 °C (d) and 900 °C (e) for 1h. a'), b') and c') are respectively zoomed-in areas of raw and calcined powders at 500 °C and 700 °C in order to better highlight that smaller nanograins form larger agglomerates.</figcaption></figure>

once more with the XRPD data and the extracted lattice parameter, which was found close to the theoretical one. As well, it further confirms that the successful synthesis of the CZT phase can be achieved via such reverse coprecipitation route.

### 3.6. IR and Raman spectroscopies

IR spectra of raw and calcined powders at 900 °C of the CZT garnet tellurate are presented in Fig. 7. For both powders, all the bands between 750 and 400 $cm^{-1}$ are typical of vibrations issued from the yafsoanite structure [18,24]. In the 750-600 $cm^{-1}$ region, the peaks observed correspond to the stretching vibration of TeO$_6$ octahedra while the 500-400 $cm^{-1}$ region is composed of a mixture of bending vibrations of TeO$_6$ octahedra and ZnO$_4$ tetrahedra [18,22]. In addition, in the 750-600 $cm^{-1}$ region, it is rather clear that the different specific absorptions are better resolved for the calcined powder in respect with the raw powder, reflecting mainly the higher degree of crystallinity, but also the granular coarsening. This falls once more in complete agreement

with the XRPD data and the suggested evolution of the crystallite size with the increasing calcination temperature (Cf. Fig. 3).

For the raw powder, other bands can be observed which do not correspond to the CZT garnet tellurate. First, the very broad band at 3420 $cm^{-1}$ is typical of water (-OH groups) [39]. Then, a series of three sharp bands, reflecting a better defined crystalline environment, are positioned at 1795 $cm^{-1}$, 1384 $cm^{-1}$ and 879 $cm^{-1}$: they are all attributed to vibrations related to the secondary phase CaCO$_3$ [40,41]. More precisely, based on the work of Kim et al. [41], the bands at 1384 $cm^{-1}$ and 879 $cm^{-1}$ respectively correspond to the asymmetric stretching and bending vibrations of $CO_3^{2-}$ entities. The fact that these latter absorption bands are all absent in the calcined powder testifies once again that the calcite phase has completely disappeared, in agreement with both the XRPD and the thermal analysis data. In addition, based on the existing literature [42], two larger bands positioned at 1637 $cm^{-1}$ and 1430 $cm^{-1}$ testify to the presence of a variety of carbonate species. The high IR absorption at 1430 $cm^{-1}$ likely suggests that carbonate groups are strongly adsorbed at the surface of the raw
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Transmittance IR spectra collected for the raw powder and the calcined powder up to 900 °C for 1h. Please note that the actual curves almost overlap and that the transmission curve related to the raw powder has been intentionally down-shifted for clarity reasons.</figcaption></figure>

<caption>**Table 3**</caption>

EDS analyses for all the cationic elements (Ca, Zn and Te) for the powder calcined up to 900 °C. σ represents the measured standard deviation for each cationic element.

<table><thead><tr><th>Element</th><th></th><th>Ca</th><th>Zn</th><th>Te</th></tr></thead><tbody><tr><td>Theoretical</td><td></td><td>37.5%</td><td>37.5%</td><td>25.0%</td></tr><tr><td rowspan="2">Calcined powder</td><td>A%</td><td>36.9%</td><td>38.1%</td><td>25.0%</td></tr><tr><td>σ</td><td>0.70</td><td>0.84</td><td>0.23</td></tr></tbody></table>

powder, thus supporting the assumption formulated in the 3.3 section. Furthermore, such observation appears somehow “logical” because of the use of NH₄HCO₃ as one of the precipitating agents. Finally, from the IR spectroscopy data, the presence of nitrate (NO₃⁻) ions does not seem very conclusive [39,43].

Raman spectra collected for the raw powder and the powder calcined

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Baseline corrected Raman spectra collected for the raw powder and the calcined powder up to 900 °C for 1h.</figcaption></figure>

up to 900 °C for 1h are shown in Fig. 8. For both powders, bands at 773, 731, 661, 648, 504, 430, 408, 329, 314, 327, 138 and 122 cm⁻¹ corresponds to specific vibrations related to the CZT tellurate garnet. The Raman spectrum collected on the powder calcined up to 900 °C is indeed in excellent agreement with the data published by Rulmont et al. [18], and as well by Vilesh et al. [24], thus confirming by this way the macroscopic XRPD data which have shown that the powder is almost single phased. For the raw powder, bands located at 1086, 712, 282 and 155 cm⁻¹ are characteristic of the calcite CaCO₃ and the band at 1049 cm⁻¹ could be associated with NO₃⁻ ions [39], though their presence was not very convincing based on IR data.

Finally, the optical band gap of CZT was derived from diffuse reflectance measurements. The acquisition was done only for the powder calcined up to 900 °C and the corresponding data are displayed in Fig. 9a. Such data can be then treated by applying the Kubelka-Munk formula, which can be expressed as follows (Equation (3)):

$$F(R) = \frac{K}{S} = \frac{(1 - R)^2}{2R} \quad (3)$$

and where R is the reflection coefficient (<1) produced by the powder sample (supposed semi-infinite) at a given wavelength, K is the absorption coefficient and S the scattering coefficient, respectively. F(R) corresponds to the Kubelka-Munk function.

The Kubelka-Munk function is then plotted in Fig. 9b, as a function of the photon energy. Though the Kubelka-Munk approach strictly requires that grains size is larger than 5 µm, still, it provides a good estimation of the optical band gap. The latter was then extracted from the intersection of the tangent line with the abscissa axis, as can be seen in Fig. 9b. We were able to derive a value of about 4.74 ± 0.05 eV for the optical band gap, which can be compared favourably to the 4.51 eV value measured by Xie et al., using the same technique, on undoped CZT powder [27].

## 4. Conclusion

In this paper, the synthesis of the calcium-zinc tellurate garnet Ca₃Zn₃Te₂O₁₂ (CZT) phase by the reverse coprecipitation method has been successfully achieved. The successive calcination steps, limited to a maximum temperature of 900 °C, were sufficient to elaborate almost pure CZT powders, with an excellent crystallinity. However, it must be stressed that the as-prepared powder was already crystallized at room temperature, at the end of the maturation step. The choice of maintaining the pH of the reaction mixture at the value of 9.0 during the synthesis was fixed as a compromise between, on one side, the risk to promote the *in situ* transformation of the yafsoanite phase into the dagaaisite one when the pH drops in the acidic range, and on the other side, the extensive formation of calcite when the pH exceeds the pKₐ₂ value of the (HCO₃⁻/CO₃²⁻) acid/base couple. Although the choice of the precipitating agent led to the presence of small quantities of calcite, mainly, our process made it possible to produce agglomerates that were homogeneous in size by a few micrometers and composed of crystalline primary nanoparticles. This disadvantageous production of moderate amounts of calcite necessitated next an additional calcination step, and from 800 °C onwards, the study showed the possibility to envisage its decomposition without the average size of the agglomerates varying too much. Opportunely, this preserves the sinterability of the final powder. To summarize, our process allows producing CZT garnet at a lower temperature range, in comparison with the literature data, where calcination temperatures beyond 1000 °C are systematically required to reach the same crystalline quality in the case of solid-state elaborated CZT powders.

We believe this work will pave the way towards a more general usage of soft-chemistry routes, leading then to the successful synthesis of garnet tellurate powders, even at lower temperatures.
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** UV-Vis diffuse reflectance data of the CZT powder calcined under air up to 900 °C. (a) Raw reflectivity measurements. (b) Kubelka-Munk function F(R) displayed versus the photon energy (hν). The tangent line intersects the abscissa axis at about 4.74 eV.</figcaption></figure>

CRediT authorship contribution statement

**G. Guy:** Writing – original draft, Visualization, Methodology, Investigation. **A. Baillard:** Methodology, Investigation. **D. Jouglard:** Methodology, Investigation. **A. Ballestero:** Writing – review & editing, Methodology, Investigation. **G. Delaizir:** Writing – review & editing, Validation. **L. Boyer:** Writing – review & editing, Validation. **Y. Launay:** Investigation. **O. Durand:** Writing – review & editing, Validation. **V. Couderc:** Writing – review & editing, Supervision. **P. Thomas:** Writing – review & editing, Project administration. **J. Jouin:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Investigation. **F. Rémondière:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Investigation, Data curation, Conceptualization. **J.-R. Duclère:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Investigation, Funding acquisition, Data curation, Conceptualization.

Funding sources and Acknowledgments

This work was supported by institutional grants from the ANR (LabEX SigmaLim (ANR-10-LABX-0074-01). As well, this work has been financially supported by the Nouvelle-Aquitaine Region via the MON ECOLE (AAPR2021-2020-11934110) project. Graziella Guy also benefited from an institutional grant from the National Research Agency under the investments for the future program with reference ANR-18-EURE-0017 TACTIC.

Some of the authors would like to acknowledge both Marina Soustre and Julie Cornette (IRCER) for their experimental support on the elemental analyses and the IR/Raman spectral acquisitions, respectively.

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.

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