Effect of LaF₃ on Thermal Stability of Na-Aluminosilicate Glass and Formation of Low-Phonon Glass-Ceramics

Highlights

• Na-aluminosilicate glasses containing La₂O₃ and LaF₃ were obtained.
• Na-aluminosilicate glasses provide a thermally stable matrix for the incorporation of lanthanides, either in the form of oxides or fluorides.
• The addition of La₂O₃ or LaF₃ instead of Na₂O increases the glass transition temperature.
• For glass containing 7.5 mol% LaF₃, a low-phonon phase can be obtained through a controlled crystallization process.

Abstract

This study examines the impact of varying the content of lanthanum oxide and lanthanum fluoride on the formation of glass-ceramics and their effect on the thermal stability of Na-aluminosilicate glasses, depending on the type and concentration of the raw material used. The aim of this study is to obtain a fluoride crystalline phase in the glassy matrix. Such a phase, due to its low phonon energy, increases the probability of radiative transitions (decay) of optically active lanthanide dopants, thereby enhancing luminescence. The scope of the work included the preparation of two glass series with varying amounts of La₂O₃ and LaF₃ to determine the glass-forming range and to identify the characteristic temperatures of the glasses using Differential Thermal Analysis. It was found that increasing the La₂O₃ content above 10 mol% in this glass leads to exceeding the target melting temperature (1400 °C) of the glass batch. In contrast, the introduction of 10 mol% LaF₃ prevents the formation of homogeneous glass. Based on these results, a controlled crystallization process was designed, and the resulting crystalline phases were identified using X-ray diffraction (XRD). In the base glass, two crystalline phases were identified: Na₂O·Al₂O₃·SiO₂ and Na₂SiO₃. For the La-oxide series, the crystallization of NaAlSiO₄ and La₂SiO₅ was confirmed. In the case of the La-fluoride series, the formation of LaF₃ was observed. It was found that by introducing an appropriate amount of LaF₃ (7.5 mol%) into the aluminosilicate network, it is possible to obtain a glass suitable for controlled crystallization, leading to the formation of a low-phonon LaF₃ phase.

1. Introduction

Aluminosilicate glasses are characterized by good thermal stability [1], high thermal [2] and chemical resistance [3], as well as high mechanical strength [4]. Due to the significant polymerization of the glass network and the strong bonding within [SiO₄] and [AlO₄] tetrahedra, these glasses exhibit high phonon energy (~1100 cm⁻¹) [5]. As a result, when doped with optically active ions, they show low luminescence efficiency because the probability of non-radiative relaxation processes is high [6,7].

On the other hand, fluoride glasses [8] (such as ZBLAN [9]), which have low phonon energy [10] (<600 cm⁻¹), serve as excellent host matrices for the development of optically active glasses [11]. However, their poor thermal stability [12] and the complexity of their melting process, which requires highly pure raw materials and an oxygen-free atmosphere, pose significant challenges in their fabrication [13].

The development of oxyfluoride glasses [14] could represent a step forward in creating materials that combine the advantages of both glass types, provided that controlled crystallization of the fluoride phase can be achieved. This crystalline phase, in the form of nanocrystals, can serve as a suitable host for optically active lanthanide dopants [15]. Although lanthanum itself does not exhibit luminescent properties, in the form of LaF₃ it can act as an excellent host for doping with other optically active lanthanides [16,17,18].

Thus, the aim of this study was to investigate the influence of lanthanum addition on the thermal stability of aluminosilicate glass and to determine the differences resulting from the use of La₂O₃ and LaF₃ as lanthanum-containing raw materials in the context of forming low-phonon phases based on fluorides. The formation of such phases would enable the incorporation of optically active lanthanide ions into their structure.

The study focused on examining the impact of varying the content of lanthanum oxide and lanthanum fluoride on the formation of glass-ceramics and their effect on the thermal stability of aluminosilicate glasses, depending on the type and concentration of the raw material used.

The scope of the work included the preparation of two glass series with varying amounts of La₂O₃ and LaF₃ to determine the glass-forming range and to identify the characteristic temperatures of the glasses using Differential Thermal Analysis (DTA). Based on these results, a controlled crystallization process was designed, and the resulting crystalline phases were identified using X-ray diffraction (XRD). This study also included structural analysis based on Fourier Transform Infrared Spectroscopy (FT-IR).

2. Methods

The crystallization behavior of aluminosilicate glasses modified with La₂O₃ or LaF₃ was investigated using a PerkinElmer DTA-7 system (PerkinElmer, Connecticut, USA) operating in DSC heat flux mode. The DTA curves enabled the identification of distinct thermal regions associated with crystallization, annealing, and phase transitions. For each measurement, approximately 50 mg of powdered glass was placed in a platinum crucible and heated at a constant rate of 10 K·min⁻¹ under a nitrogen atmosphere.

The crystalline phases formed after thermal treatment were identified by powder X-ray diffraction (XRD), performed on a Philips X’Pert diffractometer (Philips Analytical X-Ray B.V., Amsterdam, Netherlands) using CuKα radiation. Prior to analysis, glass-ceramic samples were finely ground to particle sizes below 1 μm to ensure adequate resolution.

Mid-infrared (MIR) spectra were acquired using a Bruker Vertex 70v FTIR spectrometer (Bruker, Massachusetts, USA). Measurements were conducted at room temperature over the 4000–400 cm⁻¹ range, with a resolution of 4 cm⁻¹. Each spectrum was averaged over 128 scans to enhance the signal-to-noise ratio. Samples were prepared via the standard potassium bromide (KBr) pellet technique to ensure reproducibility. All spectra were normalized to standardize the integrated intensities of absorption bands, allowing for consistent comparison across different samples.

3. Materials

All glass samples were synthesized using a conventional melt-quenching method. The batches were prepared by thoroughly mixing high-purity reagents (SiO₂, Al₂O₃, Na₂CO₃, La₂O₃, and LaF₃) with a purity of at least 99%, procured from Sigma-Aldrich, for the synthesis of the glasses. Prior to melting, the raw materials were homogenized using a ceramic mortar to ensure compositional uniformity. Each 30 g batch was melted in a covered platinum crucible, heated in an electric furnace up to 1400 °C over a period of 2 h, and held at that temperature for 90 min. The melting process was carried out in an air atmosphere. The resulting melts were cast onto a preheated brass plate, producing bulk glass samples with a thickness of 2–3 mm. The glasses were subsequently annealed at 500 °C to relieve internal stresses.

The chemical compositions of the synthesized glasses are provided in

Table 1. Nominal composition of aluminosilicate glasses with the addition of La₂O₃ or LaF₃.

Glass No.	Reference Chemical Composition of Glass, % mol
	SiO2	Al2O3	Na2O	LaO1.5	LaF3
Base glass	45	15	40	0	0
2.5 La	45	15	37.5	2.5	0
5 La	45	15	35	5	0
7.5 La	45	15	32.5	7.5	0
10 La	45	15	30	10	0
2.5 LaF	45	15	37.5	0	2.5
5 LaF	45	15	35	0	5
7.5 LaF	45	15	32.5	0	7.5
10 LaF	45	15	30	0	10

. The La₂O₃ content was recalculated to LaO_1.5 in order to maintain the same lanthanum concentration as in the glass series containing LaF₃.

4. Results and Discussion

4.1. Composition

For this study, a glass system composed of SiO₂, Al₂O₃, Na₂O, La₂O₃, and LaF₃ was selected. The key variable was the increasing content of lanthanum-containing compounds, introduced at the expense of sodium carbonate, which served as the source of Na₂O in the glass composition. Sodium oxide is essential for obtaining aluminosilicate glass, as it balances the charge of the aluminum ion (Al³⁺) within the tetrahedral unit [19]. Additionally, it lowers the melting temperature of the batch. Two series of glass samples were prepared: one series in which lanthanum was added in the form of La₂O₃, and another in which LaF₃ served as the dopant. The purpose of the La₂O₃-based series was to provide a reference system, while the LaF₃-containing series enabled the evaluation of the effect of fluoride incorporation on glass properties. This approach allowed for a comparative assessment of whether substituting La₂O₃ with LaF₃ yields similar structural and thermal behavior and to what extent the presence of fluoride influences the overall characteristics of the resulting glasses.

It was found that increasing the La₂O₃ content above 10 mol% in this glass leads to exceeding the target melting temperature (1400 °C) of the glass batch. In contrast, the introduction of 10 mol% LaF₃ prevents the formation of homogeneous glass.

4.2. Thermal Analysis

Analysis of DTA curves indicates that the obtained glasses do not exhibit sharp crystallization effects (Figure 1 and Figure 2). Instead, the structural ordering process proceeds slowly, as evidenced by broad exothermic effects. This behavior results from a strong, polymerized aluminosilicate network characterized by robust covalent–ionic bonding. The strength of this network decreases with the introduction of LaF₃, as indicated by the increase in crystallization enthalpy (ΔH).

The base glass exhibits good thermal stability. It is generally accepted that if ΔT exceeds 100 °C, the glass is considered stable and suitable for forming processes. In the case of the glass series with La₂O₃, a non-monotonic change in this parameter is observed. The addition of 2.5 mol% La₂O₃ lowers the ΔT value below 100 °C, but with 7.5 mol% La₂O₃, the threshold is exceeded again (

Table 2. Thermal parameters for aluminosilicate glasses with the addition of La₂O₃.

Glass No.	Tg (±3 °C) 1	Tx (±3 °C) 2	Tc (±3 °C) 3	ΔT (±3 °C) 4	ΔH (J/g) 5	ΔCp (J/(g·K)) 6
Base glass	512	617	682	105	125	0.146
La-oxide series
2.5 La	559	640	702	81	115	0.691
5 La	576	683	780	107	108	0.677
7.5 La	602	-	-	-	-	0.686
10 La	589	687	687	98	104	0.953
La-fluoride series
2.5 LaF	549	696	774	147	154	0.530
5 LaF	573	690	760	117	414	0.235
7.5 LaF	581	616	665	35	21	0.431

¹ The glass transformation temperature (T_g) is defined as the temperature corresponding to the deflection point in the thermal curve; ² the onset of crystallization temperature (T_x) marks the initiation of the crystallization process; ³ the peak crystallization temperature (T_c) corresponds to the maximum crystallization rate; ⁴ the thermal stability parameter ΔT, defined as T_x—T_g; ⁵ the enthalpy of crystallization (ΔH); and ⁶ the change of heat capacity (ΔC_p) in the region of transformation.

).

In contrast, the glass series containing LaF₃ shows the opposite trend. A small addition of lanthanum fluoride increases thermal stability, which is then gradually reduced, reaching a minimum value of 35 °C for the sample with 7.5 mol% LaF₃ (Table 2).

Additionally, the introduction of lanthanum significantly increases Δc_p. This increase is more pronounced in the series with lanthanum oxide than in the one with lanthanum fluoride. This parameter serves as an indicator of stress relaxation within the glass network during the transformation process.

4.3. XRD Characterization

Based on the obtained DTA curves (Figure 1 and Figure 2), the heat-treatment temperature was determined for each glass composition in order to induce glass-ceramic formation. Figure 3 presents an example of the 7.5 La sample before and after ceramization.

Figure 4 and Figure 5 show the diffraction patterns of the heat-treated glass samples. All glasses were crystallized at the maximum crystallization temperature determined from the DTA. In the base glass, two crystalline phases were identified: Na₂O·Al₂O₃·SiO₂ and Na₂SiO₃. Due to the high Na₂O content in the glass, the first one crystallizes as the main phase. This results from the strong affinity of sodium ions for aluminum and their role in compensating the charge deficiency of Al³⁺ in the [AlO₄] tetrahedron. The remaining sodium acts as a network modifier, breaking the oxygen bridges between [SiO₄] tetrahedra. During the crystallization process, this leads to the formation of sodium silicate. In the 10 La sample, two different crystalline phases were identified: NaAlSiO₄ and La₂SiO₅. NaAlSiO₄ (nepheline) exhibits a zeolite-like structure based on a three-dimensional tetrahedral network of [SiO₄] and [AlO₄] units [20], with Na⁺ ions occupying interstitial sites within the framework. La₂SiO₅ is of particular interest due to its well-known application as a host matrix for rare-earth dopant ions such as Eu³⁺, Ce³⁺, Tb³⁺, and Dy³⁺ [21,22], which impart photoluminescent properties. For example, La₂SiO₅:Eu³⁺ is a widely studied red phosphor used in LED technologies and display screens [18]. For the glass-ceramic 2.5 LaF and 5 LaF samples were observed to have crystallization of the NaAlSiO₄ and NaLa₄(SiO₄)₃F, respectively. The latter belongs to the apatite group. Apatite structures are known for their flexible crystal frameworks, allowing for extensive ionic substitution, making them attractive for applications in optics [23], ceramics [24], biomaterials [25], and as phosphor hosts [26]. The silicate units [SiO₄]⁴⁻ form a stable tetrahedral backbone, while La³⁺ and Na⁺ occupy highly symmetric crystallographic sites. This structure facilitates the incorporation of various ions, especially rare-earth elements (e.g., Eu³⁺, Ce³⁺, Tb³⁺, and Sm³⁺), making this apatite a promising host for optically active ions. Finally, in the sample labeled 7.5 LaF, lanthanum fluoride (LaF₃) was identified as the only phase present. Thus, for this glass, a low-phonon phase can be obtained through controlled crystallization. Increasing the amount of LaF₃ in the glass batch composition leads to spontaneous crystallization of LaF₃ during melt cooling.

According to our findings, fluoride can modify the glass structure by influencing the anionic subnetwork. Introducing fluorine into the glass matrix reduces its degree of polymerization, which leads to the weakening of network bonds and increased ion mobility. Fluorine forms weaker bonds with network-forming cations (e.g., Si⁴⁺ and P⁵⁺), resulting in a more open and less ordered glass structure. This allows for the formation of lanthanide fluoride phases [27].

4.4. FT-IR Study

In all the glass spectra there appear three bands (Figure 6a). They derive from the bridging oxygen atom vibrations: stretching (at about 1000 cm⁻¹), bending (700–800 cm⁻¹), and rocking (about 500–420 cm⁻¹). The first of these bands is the most intensive, and it usually represents a superposition of some bands situated close to each other, which is visible for glass-ceramics (Figure 6b).

For silicate glasses, the position of the Si–O–Si band, typically observed around 1100 cm⁻¹, varies depending on the type and concentration of network-modifying cations (Na⁺, Ca²⁺, and La³⁺) [28,29]. In our samples, this band appears as a right-hand shoulder of the main absorption band. Silicate glasses generally exhibit higher wavenumber values compared to aluminosilicate glasses [30]. The introduction of modifiers in the form of oxides (e.g., Na₂O and CaO) also leads to the appearance of bands at lower wavenumbers, attributed to symmetric stretching vibrations of terminal Si–O bonds [31,32]. The presence of a band near 980 cm⁻¹ is characteristic of orthosilicates [33], while bending vibrations typically appear around 660 cm⁻¹ [34], as observed in the spectra of the glass-ceramics. With increasing La₂O₃ content—but not in the case of LaF₃—these bands become more pronounced.

By comparing the spectra of glasses from the La₂O₃ and LaF₃ series, it can be observed that the shift of the main band maximum toward lower wavenumbers is less pronounced for the LaF₃ series. This is attributed to the weaker interaction between lanthanum ions and oxygen ions in terminal Si–O ··· and Al–O ··· bonds. Spectral analysis confirms that lanthanum is surrounded by fluoride ions, and the crystallization of lanthanum fluoride can occur in this glass.

5. Conclusions

Aluminosilicate glasses provide a thermally stable matrix for the incorporation of lanthanides, either in the form of oxides or fluorides. Increasing the content of either lanthanum oxide or lanthanum fluoride at the expense of Na₂O leads to an increase in the melting temperature of the glass batch. However, these dopants exhibit distinct effects on the glass transition temperature (Tg), thermal stability (ΔT), and crystallization tendency, as indicated by the enthalpy of crystallization (ΔH).

A small amount of lanthanum fluoride improves the thermal stability of the glass; however, higher concentrations also result in spontaneous crystallization of the LaF₃ phase during melt quenching.

It was found that by introducing an appropriate amount of LaF₃ (7.5 mol%) into the aluminosilicate network, it is possible to obtain a glass suitable for controlled crystallization, leading to the formation of a low-phonon LaF₃ phase. Thanks to the chemical affinity of lanthanide ions for fluorine, it becomes possible for rare-earth dopants to incorporate into the LaF₃ nanocrystals, leading to the formation of transparent materials with enhanced luminescence.