# Determination of the Na\_3AlF\_6 - Y\_2O\_3 Phase Diagram and Its Implications for Low-Temperature YAG/Nd:YAG Synthesis

Michal Korenko, František Šimko, Mathieu Allix, Aydar Rakhmatullin,

Michael J Pitcher, Graham King

► To cite this version:

Michal Korenko, František Šimko, Mathieu Allix, Aydar Rakhmatullin, Michael J Pitcher, et al. Determination of the Na\_3AlF\_6 - Y\_2O\_3 Phase Diagram and Its Implications for Low-Temperature YAG/Nd:YAG Synthesis. Crystal Growth & Design, 2024, 10.1021/acs.cgd.4c00684. [https://doi.org/10.1021/acs.cgd.4c00684](https://doi.org/10.1021/acs.cgd.4c00684)

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# Determination of the Na₃AlF₆–Y₂O₃ Phase Diagram and its Implications for Low - Temperature YAG / Nd:YAG Synthesis

Michal Korenko*,†,‡,†, František Šimko*,†,§, Mathieu Allix‡, Aydar Rakhmatullin‡, Michael J. Pitcher‡, and Graham King#

KEYWORDS: YAG, Y₃Al₅O₁₂, fluoride melts, cryolite, Na₃AlF₆–Y₂O₃, solid state NMR

**ABSTRACT:** A new way of preparing YAG, Y₃Al₅O₁₂, materials at low - temperatures has been discovered using a molten Na₃AlF₆–Y₂O₃ reaction mixture. For the successful synthesis of YAG, a precise examination of the cryolite part of the phase diagram of the Na₃AlF₆–Y₂O₃ system was carried out up to 45 mol% of Y₂O₃ using thermal analysis with a larger amount of sample (12 g). The phase diagram of the Na₃AlF₆–Y₂O₃ system was discovered to likely be a simple eutectic system with one inflection point on the liquidus curve (coordinates: 22.0 mol% Y₂O₃, 920 °C) and one eutectic point (coordinates: 43.0 mole% Y₂O₃, 620 °C). The spontaneously solidified samples of Na₃AlF₆–Y₂O₃ after thermal analysis have been investigated using solid - state NMR (¹⁹F, ²³Na, and ²⁷Al) spectroscopy and X - ray powder diffraction over a broad range of compositions. The minimal synthesis temperature used in this work for the preparations of YAG was 630 °C and the Y₂O₃ concentration was 43 mol%. Besides the synthesis of YAG, the molten Na₃AlF₆–Y₂O₃ system with the addition of Nd₂O₃ has been successfully used also for the preparation of the neodymium - doped YAG powders (Nd:YAG). Rietveld refinement has been used to quantitatively assess the incorporation of neodymium into YAG and β-Na(Y₁.₅Na₀.₅)F₆ materials.

## INTRODUCTION

Yttrium aluminum garnet Y₃Al₅O₁₂ (YAG) is a widely used material that has significant commercial and technological applications in white LED lighting<fnref n="1" />, solid - state lasers², and scintillation detectors.³ YAG structure with the unit the cell parameter 12.0089 Å consists of three Y³⁺ cations per formula unit exclusively in eightfold dodecahedral coordination by oxide at the A site (YO₈), with five Al³⁺ cations distributed between the octahedral B site (AlO₆) and tetrahedral C site (AlO₄). This offers chemical flexibility as the A (Y³⁺) site is able to accommodate most of the rare earth element (REE³⁺) dopants, whereas the B and C (Al³⁺) sites can be replaced with other transition- and post-transition metals, resulting in a number of characteristic emission bands of this garnet material that potentiate its practical use. Besides the abovementioned characteristics of YAG, chemical and thermal stability, mechanical strength, and other optical characteristics are the main practical advantages of this material.⁴

The conventional method for synthesizing YAG is high - temperature solid state reaction using oxide raw materials. The initial raw materials must be mixed homogeneously before synthesis and sintered at high temperature (>1600 °C) to produce a single YAG phase without the two coexisting yet not desirable intermediate phases, YAlO₃ (yttrium aluminum perovskite, YAP) and Y₄Al₂O₉ (yttrium aluminum monoclinic, YAM).⁵ As a result, time- and energy- intensive crushing and grinding processes are required to get the desired particle size of the powders prior to further operation.⁶, ⁷ Unfortunately, these extra procedures can introduce impurities and defects, resulting in a possibly final significant decrease in YAG emission efficiency.

Inorganic molten salts as high - temperature ionic liquids have various beneficial physicochemical qualities as solvents and reaction media, including a higher oxidizing potential, high mass transfer, high heat and electrical conductivity, and significantly lower viscosities, densities, and vapor pressure. Thus, when compared to conventional

<footnote>† Institute of Inorganic Chemistry, Slovak Academy of Sciences, 845 36 Bratislava, Slovakia.</footnote>

<footnote>‡ CNRS, UPR3079 CEMHTI, 1D Av. de la Recherche Scientifique, 45071 Orléans, France</footnote>

<footnote n="1">¹Loire Valley Institute for Advanced Studies, 1 Rue Dupanloup, 45000 Orléans, France</footnote>

<footnote>§ Centre of Excellence for Advanced Materials Application - CEMEA, Slovak Academy of Sciences, Dúbravská cesta 5807/9, 845 11 Bratislava, Slovakia</footnote>

<footnote># Material and Chemical Sciences, Canadian Light Source, Saskatoon, Saskatchewan S7N 2V3, Canada</footnote>
solid-state reactions (or high - temperature reactions from molten oxides), molten salt synthesis is one of the simplest methods for fast, cost - effective, low - temperature ways for obtaining crystalline, chemically relatively pure, oxide/non-oxide powders.

It has been demonstrated that molten salt synthesis can reduce the synthesis temperature more efficiently than the typical solid - state reaction approach. Another important and preferential factor is that low - temperature, solution - based synthesis below 1000 °C can help stabilize newly discovered, highly nonstoichiometric forms of YAGs⁸, as well as new, unusually metastable doped YAGs.⁹

To synthesize YAG powders at lower temperatures, several wet chemical synthesis methods such as sol – gel⁶, co-precipitation⁷,¹⁰, spray pyrolysis¹¹, and solvothermal¹² have been developed in recent years. However, these approaches still have drawbacks such as expensive equipment, difficult synthesis procedures, and environmentally hazardous precursors and solvents. Molten salt synthesis is a relatively recent synthesis approach that has been successfully used in synthesizing various ceramic powders and phosphors, including Eu³⁺ doped Y₂O₃ and Lu₂O₃.¹³⁻¹⁷

Yang et al.¹⁸ and Wu et al.¹⁹ reported molten salt synthesis of Ce:YAG phosphor employing Y, Ce, and Al nitrate hydrates or chloride hydrates as starting materials, calcined at 1100°C. Lin et al.⁴ reported using molten chlorides and nanosized oxide particles to synthesize YAG nanopowders. Nevertheless, hazardous gasses such as nitrogen oxides or hydrogen chloride may be formed during the process of heating especially when using molten nitrate hydrates or molten chloride hydrates. Therefore, it is still required to investigate new molten salt approaches for the synthesis of YAG at low temperatures.

To the best of our knowledge, there are no publications on the preparation of YAG or other functional materials using the molten fluoride synthesis method. The utilization of molten fluorides for oxides synthesis has one advantage over widely used molten chlorides and other molten salts¹⁷: molten fluorides, notably sodium cryolite, are very good solvents of oxides (e.g. Al₂O₃, SiO₂, MgO, La₂O₃, Ce₂O₃, etc.).²⁰⁻²¹ Sodium cryolite, sodium hexafluoroaluminate (Na₃AlF₆), is the complex species present in the binary system of NaF and AlF₃ at a molar ratio of 1:3. Sodium cryolite is the most famous fluoroaluminate highly reactive agent used in industrial molten salt chemistry. Its best known application is the use as an electrolyte for the industrial production of primary aluminum from Al₂O₃ in the so-called Hall-Héroult process.²⁰

The usefulness of a simple molten fluoride synthesis process based on molten cryolite for the preparation of YAG (Y₃Al₅O₁₂) and Nd doped YAG (Nd:YAG) was investigated in this study. It is for the first time demonstrated that crystallized YAG and Nd doped YAG may be produced without the use of any alumina at temperatures as low as 630 °C (compared to 1600 °C of the solid - state conversion) utilizing only the molten cryolite (Na₃AlF₆) - yttria (Y₂O₃) reaction mixture where cryolite works as a solvent and source of Al and yttria as a solute and source of Y and O. The

possible mechanism of the YAG synthesis is also addressed. Another valuable piece of information presented and published for the first time in this work is the experimental determination of the cryolite - reach part of the phase diagram of the Na₃AlF₆-Y₂O₃ system. Although molten cryolitic systems are one of the most studied molten salt systems with a lot of phase equilibrium data collected in literature²¹, there is no information available specifically about the solubility of yttria in molten cryolite. With no literature data about the phase equilibria in the Na₃AlF₆-Y₂O₃ system, the first objective prior to the synthesis of YAG itself was to find out the solubility of the Y₂O₃ in molten cryolite as the possible reactant for the preparation of YAG. It will be shown that the molten fluoride synthesis procedure using molten Na₃AlF₆ could be a feasible way for the synthesis of YAG and potentially other oxide materials.

# EXPERIMENTAL SECTION

## Materials and chemicals, used in experiments

For the preparation of the samples, the following chemicals were used: sodium fluoride (NaF, CAS number: 7681-49-4, Sigma-Aldrich, 99.9%, Germany), aluminum fluoride (AlF₃, CAS number: 7784-18-1, resublimated and dried at 300 °C for min. 4 hours, Slovalco, 99.0 %, Slovakia), yttrium oxide (Y₂O₃, CAS number: 1314-36-9, Sigma-Aldrich, 99.99 %, Germany), neodymium oxide (Nd₂O₃, CAS number: 1313-97-9, Sigma-Aldrich, 99.9 %, Germany). All chemicals were stored in a glove box under an argon atmosphere (Ar, CAS number: 7440-37-1, Siad, 99.999 %, Slovakia) and maintained below 0.3 ppm of moisture and 0.1 - 0.5 ppm of oxygen.

## Thermal analysis measurements and the chemicals preparation

The phase equilibria of the examined system were determined using a thermal analysis method. All samples were prepared in a glove box with an inert argon environment (Ar, 99.999%, SIAD, Slovakia). The 12 g powdered samples were homogenized in an inert environment before being transported in a platinum crucible to a preheated (about 90 °C) electric resistance furnace with a dry argon atmosphere (99.999%, SIAD, Slovakia). One platinum crucible holds the sample, while the other contains the reference material (high purity Al₂O₃ powders). The temperature of the furnace was controlled by a PtioRh/Pt thermocouple put into the reference material. The furnace was at the beginning heated at a rate of 7 °C/min to a temperature of 50 °C higher than the melting point of the investigated mixture. The samples were then maintained at this temperature for around 50 minutes. The temperature of primary crystallization and other heat effects were measured and recorded at a cooling rate of 1.4 °C/min. Such a slow cooling rate is critical for molten systems that tend to undercool. A larger amount of samples (12 grams) and a higher bulk-to-surface ratio improve the sensitivity of enthalpic change detection and minimize the influence of evaporation of the measured systems. A computerized measuring instrument (Multi-component model for thermal analysis
data collections - National Instruments™ where the data collections run online under Labview™ software environment) built at the Institute of Inorganic Chemistry SAS, Slovakia, was used to regulate the furnace's temperature and collect data. The sample's temperature was controlled using a Pt10Rh/Pt thermocouple validated based on the known melting points of the following pure chemicals: NaCl, NaF, and Li₂CO₃. The accuracy of the temperature measurement of the measured thermocouples was found to be ±2 °C. More details about the method can be found elsewhere.²²

Analysis of the solidified samples after thermal analysis and the preparation of the synthesis experiments

The solidified samples after thermal analysis were moved to a glovebox, homogenized, and analyzed by XRD analysis and solid - state NMR. Also, the samples for the synthesis experiments (reaction mixtures) were prepared in the glove box in the platinum crucible covered with a lid before being transferred to the resistance furnace and heated to the appropriate synthesis temperature. Some synthesis experiments were also performed in the air. Different reaction durations and temperatures were used for synthesis experiments. All mixtures were then spontaneously cooled to 100 °C, quickly moved to a glovebox, homogenized, and analyzed by XRD analysis and/or by solid - state NMR.

### X-ray diffraction analysis

X-ray powder diffraction (XRD) patterns of spontaneously cooled samples were measured using an Empyrean PANalytical diffractometer with Cu Kα₁,₂ radiation in Bragg-Brennero geometry and a β filter (Ni). A solid-state PXcel detector was utilized to record XRD patterns. The measurements were taken at room temperature, with 2θ ranging from 5 to 80° with a step of 0.016°. Phase analysis was carried out using Oxford Cryosystem Crystallographica Search-Match 2.1 software and the PDF2 2011 database.

### Solid - state NMR analysis

All NMR measurements were performed using a Bruker Avance NEO 850 (20 T) NMR spectrometer. ²⁷Al, ²³Na, and ¹⁹F spectra were acquired using 1.3 mm diameter rotors at the rotor frequencies of 60 kHz. The measured chemical shifts are referenced to CFCI₃, 0.1 M NaCl, and Al(NO₃)₃ for ¹⁹F, ²³Na, and ²⁷Al, respectively. The NMR parameters (chemical shifts, line widths, and quadrupolar coupling parameters) were fitted to the observed spectra employing the DMfit2023 program.²³

## RESULTS

The initial limitation to using molten cryolite for YAG synthesis was a lack of knowledge regarding the solubility of yttria in such a molten environment. We initially planned to use cryolite's capacity to dissolve alumina and other oxides. With no literature data, the first goal was to determine the phase diagram of the Na₃AlF₆-Y₂O₃ system. The thermal analysis was used to achieve this goal.

### Phase equilibria of the Na₃AlF₆-Y₂O₃ system

The results of the thermal analysis experiment on the investigated system Na₃AlF₆-Y₂O₃ up to 45 mol% of Y₂O₃ are

shown in Fig. 1. The phase diagram depicted in this picture was constructed based on the information of the temperatures of the related heat effects obtained on the particular cooling curves acquired during the thermal analysis measurement. The Na₃AlF₆-Y₂O₃ system was discovered to be a simple eutectic system with one eutectic point, E₁, and one inflection point on the liquidus curve, I₁. The system also presents a solid - solid transformation between α and β forms of Na₃AlF₆, T₁. The coordinates of these phase equilibria characteristics are as follows:

I₁: 22.0 mol% Y₂O₃, 78.0 mol% CR, t(I₁) = 920 °C,

E₁: 43.0 mole% Y₂O₃, 57.0 mol% CR, t(E₁) = 620 °C,

T₁: solid-solid phase transitions of α and β form of Na₃AlF₆, t(T₁) = 560°C

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Fig. 1. The cryolite - rich part of the phase diagram of the system Na₃AlF₆-Y₂O₃. Full circles: temperatures of primary crystallizations; Triangles: temperatures of the second heat effect, which divide two different phase fields of the coexistence of the solid and liquid phases; Open circles: eutectic line; Open squares: temperature of the α and β cryolite solid-solid transformation; Black lines: formal lines, Dashed line: expected Y₂O₃ solidus line.</figcaption></figure>

Fig. 2 shows the results of the room temperature X - ray diffraction analysis of the investigated spontaneously solidified samples performed after thermal analysis measurement. The results of the XRD analysis can be divided into two groups. The results of the samples with the concentration up to the inflection point (22.0 mol% of Y₂O₃) and the results of the samples with the higher concentrations. The presence of the following species has been detected in the samples in the first group (concentration up to the inflection point).

Na₃AlF₆ (decreasing intensity of reflections with the increasing concentration of Y₂O₃),

NaF (increasing intensity of reflections with the increasing content of Y₂O₃),

β-Na(Y₁.₅Na₀.₅)F₆ (increasing intensity of reflections with the increasing content of Y₂O₃),

β-YOF (relatively constant intensity of reflections vs. concentration of Y₂O₃),
$NaAl_{11}O_{17}$ (relatively constant intensity of reflections vs. concentration of $Y_2O_3$),

YAG, $Y_3Al_5O_{12}$ (first detection of YAG in the sample with 20 mol% concentration of the $Y_2O_3$).

With the increasing concentration of $Y_2O_3$ in the samples (beyond the inflection point, i.e. >22.0 mol% of $Y_2O_3$), the following patterns have been identified (Fig. 2).

<figure><img src="image_3.png" /><figcaption>**Figure 2.** The XRD patterns of the spontaneously solidified samples of the $Na_3AlF_6-Y_2O_3$ system taken after the thermal analysis as a function of $Y_2O_3$ concentration. 43A: the softer part of the sample, 43B: the harder part of the sample, 43: XRD pattern of the related sample when both parts were analyzed together.</figcaption></figure>

$Na_3AlF_6$ (decreasing intensity of reflections with the increasing concentration of $Y_2O_3$, no reflection of $Na_3AlF_6$ in the sample for the eutectic composition),

NaF (relatively constant intensity of reflections vs. concentration of $Y_2O_3$),

$\beta$-Na($Y_{1.5}Na_{0.5})F_6$ (relatively constant intensity of reflections vs. concentration of $Y_2O_3$),

$\beta$-YOF (decreasing intensity of reflections with the increasing concentration of $Y_2O_3$, no reflection of $\beta$-YOF in the sample for the eutectic composition),

YAG, $Y_3Al_5O_{12}$ (increase of reflections with the increasing concentration of $Y_2O_3$).

Fig. 2 also shows the results of the room temperature X-ray diffraction analysis of the spontaneously solidified sample after thermal analysis with eutectic composition (43, 43A, 43B). The solidified sample had at first sight two separable parts: a harder, lower part (43B) and a softer, upper part (43A). The softer part contains only reflections of $\beta$-Na($Y_{1.5}Na_{0.5})F_6$ and NaF. The harder part contains on the other hand the dominant reflections of YAG and $\beta$-Na($Y_{1.5}Na_{0.5})F_6$ and smaller reflections of NaF.

The relatively unusual shape (with the inflection point) of the liquidus curve of the cryolite in the phase diagram of $Na_3AlF_6-Y_2O_3$ (Fig. 1) can be in some way interrelated with the formation of YAG. The intensity of the reflections of $Na_3AlF_6$ continually decreases with the increasing concentration of $Y_2O_3$ in the samples till the eutectic point where $Na_3AlF_6$ completely disappears. The intensity of reflections of YOF is on the other hand relatively constant in the whole spectrum of concentration of $Y_2O_3$ between 5 mol% and 35 mol% with complete disappearing at the concentration of the eutectic point (43 mol%). The above-mentioned observation suggests that the formation of $\beta$-YOF may in this system act as an intermediate step in the production of YAG. When the content of $Y_2O_3$ reaches the critical concentration in the system (inflection point), $\beta$-YOF may start to work as a precursor for the formation of YAG.

Fig. 3 and Fig. 4 show solid-state $^{27}$Al, $^{23}$Na, and $^{19}$F NMR spectra of the spontaneously solidified samples after thermal analysis measurements (in the concentration range of 10 - 43 mol% of $Y_2O_3$). The assignment of resonances was made based on the following references and reflections: YAG ($^{27}$Al), $^{24}\beta$-$Al_2O_3$ ($^{27}$Al), $^{25}$YOF, $^{26-27}$, $\beta$-Na($Y_{1.5}Na_{0.5})F_6$.$^{28}$ A significant overlap between the $^{23}$Na signals of eight coordination environment $Na_3AlF_6$ and both nine coordinated sodium sites of $\beta$-Na($Y_{1.5}Na_{0.5})F_6$ can be seen in Fig 3 (chemical shift between -5 ppm and -10 ppm). However, at 43 mol% of $Y_2O_3$, the presence of cryolite disappeared so we can assign the signals between -5 ppm and -10 ppm also to $\beta$-Na($Y_{1.5}Na_{0.5})F_6$.

NMR analysis confirms the XRD results. The first signals of YAG appeared in the sample with 20 mol% of $Y_2O_3$ (inflection point in the phase diagram, Fig 1) and became more pronounced in the samples with higher concentrations of $Y_2O_3$. $\alpha$-$Al_2O_3$ appeared in the samples up to 35 mol% of $Y_2O_3$ and $\beta$-$Al_2O_3$ was found only in the sample with the lowest concentration of $Y_2O_3$ (10 mol%). $\beta$-Na($Y_{1.5}Na_{0.5})F_6$, NaF, $\beta$-YOF and $Na_3AlF_6$ were identified in all investigated samples. The intensity of the $\alpha$-$Al_2O_3$ peaks increases until the concentration related to the inflection point (20 mol%) and then decreases and completely disappears in the sample with the highest concentration of $Y_2O_3$. In the sample with the highest concentration of $Y_2O_3$, besides dominant signals of YAG, $\beta$-Na($Y_{1.5}Na_{0.5})F_6$, and NaF, only small signals of $Na_3AlF_6$ and $\beta$-YOF appeared.
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Solid state <sup>27</sup>Al (lower part) and <sup>23</sup>Na (upper part) NMR spectra of the Na<sub>3</sub>AlF<sub>6</sub>-Y<sub>2</sub>O<sub>3</sub> solidified samples after thermal analysis. The assignment of resonances was made based on the following references and reflections; YAG (<sup>27</sup>Al),<sup>24</sup> β'-Al<sub>2</sub>O<sub>3</sub> (<sup>27</sup>Al).<sup>25</sup></figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Solid state <sup>19</sup>F NMR spectra of the Na<sub>3</sub>AlF<sub>6</sub>-Y<sub>2</sub>O<sub>3</sub> solidified samples after thermal analysis. The assignment of resonances was made based on the following references and reflections; YOF,<sup>26-27</sup> β-Na(Y<sub>1.5</sub>Na<sub>0.5</sub>)F<sub>6</sub>;<sup>28</sup> * - spinning sidebands.</figcaption></figure>

## YAG Synthesis experiments

Na<sub>3</sub>AlF<sub>6</sub>-Y<sub>2</sub>O<sub>3</sub>

The previous XRD and NMR analysis of the solidified samples after thermal analysis clearly showed that Na<sub>3</sub>AlF<sub>6</sub>-Y<sub>2</sub>O<sub>3</sub> may be considered a very promising system for the low temperature synthesis of YAG materials. To find out the most effective route for the YAG synthesis, several experiments with different reactant compositions, temperatures, and holding times were performed in a platinum crucible in an open-air furnace. The most effective route in this regard means the lowest synthesis temperature possible with the highest yield of YAG in the reaction mixture.

Fig. S1 (in *Supporting information*) shows the coordinates of these synthesis experiments within the Na<sub>3</sub>AlF<sub>6</sub>-Y<sub>2</sub>O<sub>3</sub> phase diagram. Based on these compositions the synthesis experiments can be divided into three groups (4 mol%, 43 mol%, and 80 mol% of Y<sub>2</sub>O<sub>3</sub>). Based on the synthesis route, the samples can be divided into two groups: the liquid synthesis route and the solid/partly - melted state synthesis. The visual observation of the reaction mixture after the synthesis experiments was able to clearly show whether the particular reaction mixture was or was not fully melted during the holding time in the furnace.

Figs. 5 - 7 show the results of the XRD analysis of the samples after synthesis experiments.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** The XRD patterns of the samples Na<sub>3</sub>AlF<sub>6</sub>-Y<sub>2</sub>O<sub>3</sub> (4.8 mol% of Y<sub>2</sub>O<sub>3</sub>) melted at 1020 °C for 4 hours. A: the XRD results of the sample itself; B: the XRD of the condensate found on the lid.</figcaption></figure>

The samples from the synthesis used fully liquid route show results that correlate very well to the results of the XRD analysis taken after the thermal analysis (Fig. 2). The results of the sample after the synthesis with the lowest concentration of Y<sub>2</sub>O<sub>3</sub> (4.8 mol%) contain the reflections of Na<sub>3</sub>AlF<sub>6</sub>, NaF, β-Na(Y<sub>1.5</sub>Na<sub>0.5</sub>)F<sub>6</sub>, and β-YOF (Fig. 5). Moreover, a crystalline condensate was also found on the lid of the Pt crucible after this synthesis. The XRD analysis of the condensate subsequently showed the presence of NaAl<sub>5</sub>O<sub>8</sub> (Na-β'-alumina) and Al<sub>2</sub>O<sub>3</sub>.
The results of the synthesis of the sample with coordinates 680 °C and 43 mol% of Y₂O₃ (eutectic composition) are depicted in Fig. 6 (B₁ and B₂). The sample was like in the case of the thermal analysis also separable into two different parts. The results from this liquid synthesis experiment also correlate with the XRD results obtained after the thermal analysis, although for the synthesis experiments, an open-air atmosphere was used. The patterns of the harder part (B₁) contain the reflections of YAG, β-Na(Y₁.₅Na₀.₅)F₆, and NaF. The softer part (B₂) contains only reflections of β-Na(Y₁.₅Na₀.₅)F₆ and NaF and practically no reflections of YAG.

<figure><img src="image_8.png" /><figcaption>**Figure 6.** The XRD patterns of the samples with 43 mol% Y₂O₃ synthesized at 600 °C (A), and at 680 °C (B₁ - the harder part of the sample, B₂ - the softer part of the sample). Time of the synthesis in both cases: 4 hours in open-air atmosphere.</figcaption></figure>

XRD analysis of the sample prepared in the solid state (below the eutectic temperature, Fig. 6A) showed a different picture: a relatively strong reflection of β-YOF, and then smaller reflections of YAlO₃, NaF, and Na₃AlF₆. YAlO₃ is yttrium aluminum oxide with a perovskite structure (YAP), known as an intermediary in the formation of YAG. Considering the results of both synthesis experiments with the eutectic composition (43 mol%), it appears that the temperature of 600 °C (solid state route) is insufficient for the successful formation of YAG, so raising the temperature of the reaction mixture above the melting point appears to be critical for the formation of YAG.

In the sample with 80 mol% of Y₂O₃ in cryolite and synthesized at 750 °C, the XRD patterns contain NaF reflections (small intensity reflections), β-YOF (strong intensity reflections), and unreacted Y₂O₃ (strong intensity reflections) (Fig. 7). Additionally, the same sample was heated even to 1020 °C (for 4 hours), and analyzed. Its XRD pattern contains reflections of the same phases, NaF, β-YOF, and Y₂O₃. Besides that, the pattern contains a reflection of a new phase, YAlO₃. With a further increase in temperature to 1200 °C, the reflections of NaF in this sample disappeared and the reflection intensities of YAlO₃ increased (Fig. 7, C).

Based on the latest results, it is possible to conclude that decreasing the Na₃AlF₆/Y₂O₃ ratio in the reaction mixture prevents the YAG synthesis in favor of the synthesis of YAP.

<figure><img src="image_9.png" /><figcaption>**Figure 7.** The XRD patterns of the samples Na₃AlF₆-Y₂O₃ (80 mol% of Y₂O₃) melted at 750 °C (A), 1020 °C (B), and 1250 °C (C) during 4 hours in open-air atmosphere.</figcaption></figure>

### YAG synthesis experiments with different cryolite ratios (NaF/AlF₃)

To characterize the real potential of a whole molten NaF-AlF₃ system as a reaction medium for the preparation of YAG, in which molten cryolite represents only tiny part, a series of synthesis experiments with different NaF/AlF₃ ratios has been performed (NaF-Y₂O₃, AlF₃-Y₂O₃, NaF-Na₃AlF₆-Y₂O₃, Na₅Al₃F₁₄-Y₂O₃). The phase diagram of the NaF-AlF₃ system (Fig. S2) contains two eutectic points, E₁ (75 mol% NaF, t_eut = 888 °C) and E₂ (58 mol% NaF, t_eut = 695 °C), and two complex species: congruently melting cryolite (Na₃AlF₆, 75 mol% NaF, t MP = 1011 °C) and incongruently melting chiolite (Na₅Al₃F₁₄, 62 mol%, t MP = 738 °C).²¹ From the parameters of both complex species and parameters of the eutectic points in the NaF-AlF₃ system, it is clear that there might still be some room, within the frame of the NaF-AlF₃ system, for decreasing the reaction
temperature (e.g. using the eutectic composition and the eutectic temperature of the points E1 or E2 for the preparation of YAG, Fig. S2). The analysis of the “boundary” reaction mixtures; NaF-Y₂O₃ and AlF₃-Y₂O₃, may at the same time help to elucidate and better understand the particular roles of NaF and AlF₃ during the preparation of YAG.

We performed a series of synthesis experiments with various NaF/AlF₃ ratios to assess the molten NaF-AlF₃ system as a reaction medium for YAG preparation. These experiments explored how different NaF/AlF₃ compositions influence YAG synthesis. The main findings of those experiments are listed below. More details of those experiments including the XRD analysis can be found in Supporting Information (Fig. S3-S6).

*   **NaF-Y₂O₃:** In experiments conducted at 1020°C with Y₂O₃ concentrations of 1 and 2 mol%, the low solubility of Y₂O₃ in molten NaF was evident, with β-YOF forming as the primary product, alongside NaF and unreacted Y₂O₃.

*   **NaF-Na₃AlF₆-Y₂O₃:** Two experiments at 600°C and 630°C were performed to explore the possibility of reducing the YAG synthesis temperature. At 600°C, β-YOF was the primary product, while at 630°C, YAG and β-Na(Y₁.₅Na₀.₅)F₆ were formed. This suggests that while NaF addition does not significantly lower the synthesis temperature, it plays a role in facilitating the formation of YAG.

*   **Na₅Al₃F₁₄-Y₂O₃:** Chiolite was tested as a reaction medium at 860°C. The results showed β-YOF, Na₃AlF₆, and Al₂O₃ as the main products, indicating that chiolite behaves differently than cryolite, likely due to the low solubility of Y₂O₃ in molten chiolite.

*   **AlF₃-Y₂O₃:** An experiment at 1000°C without NaF aimed to synthesize YAG using a solid-state reaction between AlF₃ and Y₂O₃. The products included YF₃, Al₂O₃, and Y₅O₄F₇, with no YAG formation. This highlights the importance of NaF in successful YAG synthesis, even at elevated temperatures.

**Nd: YAG synthesis experiment**

Na₃AlF₆-Y₂O₃-Nd₂O₃

To check the suitability of molten fluorides for the preparation of YAG - doped materials, a Nd:YAG synthesis experiment was performed with Na₃AlF₆, Y₂O₃, and Nd₂O₃. Yttrium aluminum garnet doped with neodymium (Nd:YAG) is the most well-known oxide crystal used as the active medium in solid-state lasers.²⁹⁻³⁰ The Nd³⁺ dopant typically replaces a small percentage of the yttrium ions in the host crystal structure of YAG and it is precisely the Nd ions that provide the leasing activity in the crystal. Molten cryolite can be used based on the literature as a good solvent for Nd₂O₃ (the eutectic concentration of the Na₃AlF₆-Nd₂O₃ system is around 12 mol%).³¹ The other characteristics of the experiments were as follows; composition of the reaction mixture: Na₃AlF₆, Y₂O₃ (42 mol%), and Nd₂O₃ (7 mol%), temperature: 900 °C, duration: 4 hours, atmosphere: open air. The powdered starting materials were all with purity greater than 99%.

In the YAG crystal structure, Y³⁺ ions inhabit dodecahedral positions, while Al³⁺ ions inhabit octahedral and tetrahedral positions. This arrangement results from variances in ionic radii: O²-(1.40 Å), Y³+(1.02Å), and Al³+(0.51 Å).³²⁻³⁵ Because the ionic radii of Y³⁺ ions and rare-earth ions are comparatively similar, trivalent ions of rare earth can substitute Y³⁺ ions to a minor extent. The most common rare earth ion used in this regard is Nd³⁺ (1.12 Å) with a doping molar concentration in YAG between 0.2 and 1.4%. Since the radius of the Nd³⁺ ion is bigger than the radius of Y³⁺, it may be sometimes complicated to incorporate neodymium in the required amount into the yttrium sites of the crystal lattice.³³, ³⁶

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Rietveld refinement fit to the undoped sample (Na₃AlF₆ and 42 mol% Y₂O₃) synthesized at 900 °C for 4 hours (open air atmosphere). The black circles are data points, the red line is the fit, and the difference curve is beneath. The tick marks from top to bottom are the hkl positions for YAG, β-Na₁.₅Y₁.₅F₆, NaF, Na₃AlF₆, and YOF. R<sub>wp</sub> = 9.06%.</figcaption></figure>

The Rietveld refinement of the XRD patterns of the solidified eutectic mixture of Na₃AlF₆ and Y₂O₃ with and without the addition of Nd₂O₃ are shown in Fig. 8 and Fig. 9. The undoped sample (Na₃AlF₆ and 42 mol% Y₂O₃) had the same parameters of the synthesis (900 °C, 4 hours in an open-air atmosphere) as the Nd doped sample. The final parameters of the refinement are listed in Table 1 and Table 2. The Rietveld refinement of the undoped sample had an R<sub>wp</sub> of 9.06%. The R<sub>wp</sub> of the fit of the doped sample was 11.91%. Both samples contain the reflections of YAG as well as the reflections of β-Na(Y₁.₅Na₀.₅)F₆, NaF, and β-YOF. Small peaks of Na₃AlF₆ were also recorded in the undoped sample. No extra diffraction peaks were recorded in the sample with doped neodymium.

The peak position shifts to lower diffraction angles in the X-ray diffraction patterns are apparent as a result of the substitution of smaller Y³⁺ ions by larger Nd³⁺ ions in the host lattice (Nd:YAG has larger cell parameters than YAG). It needs to be stressed at this point that the 10% ionic radius difference between Y³⁺ and Nd³⁺ makes it relatively difficult to form highly doped Nd:YAG since solid solutions
from the molten liquid usually only form if the difference of radii is smaller than 5 %.³⁵

<table><caption>**Table 1.** Final parameters of Rietveld refinement of undoped sample (Na₃AlF₆ and 42 mol% Y₂O₃) prepared at 900 °C for 4 hours in an open air atmosphere.</caption><thead><tr><th></th><th>Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub></th><th>Na<sub>1.5</sub>Y<sub>1.5</sub>F<sub>6</sub></th><th>NaF</th><th>Na<sub>3</sub>AlF<sub>6</sub></th><th>YOF</th></tr></thead><tbody><tr><td>Phase fraction / wt%</td><td>36.5(1)</td><td>37.1(1)</td><td>19.1(1)</td><td>7.0(1)</td><td>0.37(1)</td></tr><tr><td>a (Å)</td><td>12.00819(9)</td><td>5.9707(3)</td><td>4.63367(9)</td><td>5.4068(6)</td><td>3.8205(9)</td></tr><tr><td>b (Å)</td><td>-</td><td>-</td><td>-</td><td>5.5938(7)</td><td>-</td></tr><tr><td>c (Å)</td><td>-</td><td>3.52901(5)</td><td>-</td><td>7.7685(8)</td><td>18.762(4)</td></tr><tr><td>β (°)</td><td>-</td><td>-</td><td>-</td><td>90.16(1)</td><td>-</td></tr><tr><td>V (Å<sup>3</sup>)</td><td>1731.54(3)</td><td>108.952(6)</td><td>99.489(6)</td><td>234.96(3)</td><td>237.16(6)</td></tr></tbody></table>

The Nd³⁺ ions replace Y³⁺ in YAG without the need for charge adjustment. The increased size of the Nd³⁺ ions produces polyhedra with larger sides than Al³⁺ polyhedra. This affects the lattice, limiting the maximum feasible neodymium doping concentration to a few atomic weight percent.

The concentration of doped neodymium in our sample can be estimated to be around 0.32 wt% of Nd. This assessment is based on the comparison of the change of the cell parameter of YAG/Nd:YAG in our work (0.046 %) and the work of Kostić et al. (0.116 %).³³ The rest of Nd³⁺ was probably intercalated into the other compound present in our samples, β-Na(Y₁.₅Na₀.₅)F₆. The reflection shifts of the XRD peaks of this compound toward lower angles (increase of the cell parameters) were also observed.

β-Na(Y₁.₅Na₀.₅)F₆ compound is one of the two polymorphic forms of NaYF₄ known as hexagonal β-NaYF₄. The structure of that compound has three cation sites, one for Y³⁺ ions, one for both Y³⁺ and Na⁺ ions, and the third for only Na⁺ ions.³⁷ Previous research indicates that NaYF₄ is an excellent host lattice for various optically active lanthanide ions for upconversion luminescence applications (higher energy light emission upon lower energy excitation, e.g. emit visible light upon infrared excitation). The hexagonal β-form outperforms the cubic β-NaYF₄ for the luminescence of optically active lanthanide ions.³⁸⁻³⁹ A wide variety of inorganic structures has been already proposed in the literature as matrices for the lanthanides based luminescent materials. Among the various potential inorganic host matrices, different fluoride substrates are being widely used due to their low phonon energy. According to the literature, β-Na(Y₁.₅Na₀.₅)F₆ is a good multicolor luminescent matrix that can be doped with trivalent lanthanides to form the hexagonal Na((YₓLnᵧ)₁.₅Na₀.₅)F₆, where x+y=1.⁴⁰⁻⁴²

<figure><img src="image_11.png" /><figcaption>**Figure 9.** Rietveld refinement fit to the Nd-doped sample (Na₃AlF₆, Y₂O₃ (42 mol%), and Nd₂O₃ (7 mol%)), temperature: 900 °C, duration: 4 hours, atmosphere: open air. The black circles are data points, the red line is the fit, and the difference curve is beneath. The tick marks from top to bottom are the hkl positions for YAG, Na₁.₅Y₁.₅F₆, NaF, and YOF. R<sub>wp</sub> = 11.91%.</figcaption></figure>

<table><caption>**Table 2.** Final parameters of Rietveld refinement of Nd-doped sample (Na₃AlF₆, 42 mol% Y₂O₃, and 7 mol% Nd₂O₃) prepared at 900 °C for 4 hours in an open air atmosphere.</caption><thead><tr><th></th><th>Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub></th><th>Na<sub>1.5</sub>Y<sub>1.5</sub>F<sub>6</sub></th><th>NaF</th><th>YOF</th></tr></thead><tbody><tr><td>Phase fraction / wt%</td><td>36.7(1)</td><td>28.8(1)</td><td>30.5(2)</td><td>4.04(4)</td></tr><tr><td>a (Å)</td><td>12.01373(13)</td><td>5.9881(8)</td><td>4.63293(10)</td><td>3.8371(4)</td></tr><tr><td>c (Å)</td><td>-</td><td>3.5552(1)</td><td>-</td><td>18.943(2)</td></tr><tr><td>V (Å<sup>3</sup>)</td><td>1733.94(5)</td><td>110.40(2)</td><td>99.377(3)</td><td>241.53(4)</td></tr></tbody></table>

## CONCLUSIONS

In this work, we tried to use molten sodium cryolite (Na₃AlF₆) flux for low-temperature molten salt synthesis of YAG and Nd-doped YAG. The main idea behind that approach is based on the fact that molten fluorides, particularly sodium cryolite, are excellent solvents of oxides (Al₂O₃, SiO₂, La₂O₃, etc.). To the best of our knowledge, no papers have yet been published on the preparation of YAG materials (or any other functional oxides) utilizing the molten fluoride synthesis process.

For the low-temperature synthesis of YAG with the best yield, a molten Na₃AlF₆-Y₂O₃ reaction mixture has been used with the Y₂O₃ concentration of 43 mol% which is approximately the eutectic composition of the system Na₃AlF₆-Y₂O₃. The other products were β-Na(Y₁.₅Na₀.₅)F₆ and NaF. While β-Na(Y₁.₅Na₀.₅)F₆ is also a valuable product (as laser material), NaF can be easily washed away after synthesis from the reaction mixture with water. Rapid cooling of the reaction mixture ex-post synthesis may be however the way to separate/prepare single-phase YAG material. Nonetheless, more experimental work is still
needed in this regard to identify effective and selective solvents/methods.

The phase equilibria and phase diagram of the Na₃AlF₆–Y₂O₃ system have been measured prior to the synthesis experiments since there is so far no information in the literature about the phase equilibria in the Na₃AlF₆–Y₂O₃ system and solubility of Y₂O₃ in molten Na₃AlF₆. The thermal analysis within the investigated concentration range - 45 mol% Y₂O₃ revealed that the Na₃AlF₆–Y₂O₃ system is probably a simple eutectic system with one inflection (22.0 mol% Y₂O₃, 920°C) point and one eutectic point with the following coordinates: 43 mol% Y₂O₃, 620 °C.

The lowest temperature used in this work for the synthesis of YAG was 630 °C and the holding time was 4 hours in an open air atmosphere. The synthesis in the solid state was on the other hand not successful in terms of the preparation of YAG. The solid - state synthesis in this regard means either to have the reaction temperature below the eutectic temperature of the Na₃AlF₆–Y₂O₃ system or the synthesis with a higher concentration of Y₂O₃ in the reaction mixture (beyond the eutectic concentration of Y₂O₃). The products in the solid - state synthesis were β-YOF, YAlO₃ (YAP, yttrium aluminum oxide with a perovskite structure), and unreacted Y₂O₃. The proper concentration ratio between Na₃AlF₆ and Y₂O₃, as well as a preparation temperature above the melting point (wet route), are both critical for the successful synthesis of YAG, as demonstrated by the results of all synthesis experiments.

It can be also concluded, that the change in the cryolite ratio (NaF/AlF₃) in the reaction mixture from pure cryolite (1:3) towards higher concentrations of NaF did not substantially decrease the temperature of the synthesis of YAG. When we moved with cryolite ratio to the other side, towards AlF₃, using chiolite (Na₅Al₃F₁₄) and yttria for the synthesis in the liquid state (860 °C), only β-YOF, Na₃AlF₆, and Al₂O₃ were identified in the reaction mixture. When we used pure AlF₃ and yttria as reactants for the synthesis at 1000 °C (only solid - state reaction is possible in this system due to AlF₃ sublimation), the reaction mixture after synthesis contained YF₃, Al₂O₃, and Y₅O₄F₇.

When we reflect on doing these tests, the cryolite yttria systems with the composition around the eutectic point look finally like the best synthesis option (in terms of temperature and yield) for the synthesis of YAG materials.

For the low - temperature Nd:YAG synthesis, the molten Na₃AlF₆–Y₂O₃ reaction mixture with 7 mol% Nd₂O₃ has been used. The other products besides neodymium doped YAG were in this experiment β-YOF, β-Na(Y₁.₅Na₀.₅)F₆, and NaF. The incorporation of Nd into the YAG matric re - increased the cell parameters (shifts to lower diffraction angles in the XRD patterns). Some Nd³⁺ ions were however intercalated also into the β-Na(Y₁.₅Na₀.₅)F₆ matrix making Nd:Na(Y₁.₅Na₀.₅)F₆ which is however also a valuable functional material.

We believe that the low - temperature synthesis from the molten fluorides could be an interesting alternative to the already developed methods of the synthesis of different

functional oxide materials. The main advantage of that approach is clear: an excellent solubility of different oxides and other compounds in fluoride melts, a disadvantage, especially compared to chlorides, is the relatively low solubility of some fluorides in water to clean a final product after synthesis easily. Identifying the best reaction mixture(s) to prepare a single - phase YAG material (or other functional oxides/materials) and YAG materials doped with different dopants (e.g. Yb, Er, Ce, etc.) will be a matter of continuing future work.

## ACKNOWLEDGMENT

The authors acknowledge support from the LE STUDIUM Loire Valley Institute for Advanced Studies in Orléans, France. The work was financially supported by the Slovak grant agencies APVV-19-0270, APVV-19-0461, and VEGA 2/0046/22. Financial support from the IR INFRANALYTICS FR2054 for conducting the research is also gratefully acknowledged. NMR measurements realized through the project: Pan-European solid-state NMR Infrastructure for Chemistry-Enabling Access - PANACEA.

## ASSOCIATED CONTENT

### Supporting Information

Phase diagrams with coordinates of the synthesis experiments. The details of the synthesis experiments with different cryolite ratios and XRD patterns from systems NaF–Y₂O₃, NaF–Na₃AlF₆–Y₂O₃, Na₅Al₃F₁₄–Y₂O₃, and AlF₃–Y₂O₃. The Supporting Information is available free of charge on the ACS Publications website.

## AUTHOR INFORMATION

### Corresponding Authors

* Michal Korenko, ORCID: 0000-0001-9800-8913, Tel: +421 2 59465 462, E-mail: uachmiko@savba.sk, František Šimko, ORCID: 0000-0003-2390-1349, Tel: +421 2 59465 495, E-mail: uachsim@savba.sk

### Author Contributions

M.K. with M.A. and M.P. conceptualized and planned the project. M.K. and F.Š. prepared the samples. F.Š. did TA measurements. M.K. and F.Š did the synthesis experiments A.R. carried out the NMR experiments and performed analysis of the NMR data. M.K., F.Š., M.A., and G.K. performed an analysis of the structural data. The manuscript was conceptualized and written by M.K. and by F.Š., with the contribution of all co-authors.

### Notes

The authors declare no competing financial interest.

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