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# Structure and properties of gallate and germanate langasite infrared optical glass materials

Florian Calzavara<sup>a</sup>, Pierre Florian<sup>b</sup>, Franck Fayon<sup>b</sup>, Sonia Buffière<sup>a</sup>, Marc Dussuze<sup>c</sup>, Véronique Jubera<sup>a</sup>, Thierry Cardinal<sup>a</sup>, Evelyne Fargin<sup>a,∗</sup>

## ARTICLE INFO

### Keywords:

- Oxide glasses

- Glass structure

- Gallate

- Germanate

## ABSTRACT

Glasses in the system BaO – La₂O₃ – Ga₂O₃ – GeO₂ and Langasite crystalline phases with the same stoichiometry have been studied considering the possible use of these glasses for laser-induced space-selective growth of crystalline architectures in the glass compositions. For better understanding of mechanisms involved in such controlled crystallization process, crystalline and glass compounds with same stoichiometry and different La/Ba ratios have been synthesized to compare their local structure. Solid-state Nuclear Magnetic Resonance evidence 4- and 5-coordinated gallium units in the glass matrix, denying unambiguously the formation of 6-coordinated gallium sites observed in crystalline phases. Moreover vibrational spectroscopies suggest a link between a 3D network formed by 4- and 5-coordinated gallium, connected to each other and to 4- coordinated Q4 and Q3 germanium units in the glass and their congruent crystallization. The transition from 4- and 5-coordinated gallium to 6-coordinated gallium sites observed in Langasite crystalline phases remains not still elucidated.

## 1. Introduction

Materials covering a transparency window from the ultra – violet (UV) up to the mid – infrared (MIR) domain are of interest. Gallate-(Ga₂O₃-) and germanate (GeO₂-) based optical glasses are suitable candidates since they exhibit a transparency domain from the UV (~280 nm) up to the MIR (~5–6 µm) with mechanical assets comparable to silica glasses [1] while being capable to be shaped as sub-millimetric-size as optical glass fibers [2]. In addition, several compositions have demonstrated a promising potential for hosting a relatively large amount of rare-earth emitting centers (i.e. ~25 Er₂O₃ – 25 La₂O₃ – 50 Ga₂O₃ mol% glass for example [3]) enabling key lumencence properties ranging from the visible up to the MIR [3,4]. Nonetheless, the random atomic arrangement of glasses with an inversion symmetry prevents the generation of active functional properties such as second – order optical nonlinearity or piezoelectricity for example. The crystallization control providing surface or bulk precipitation of crystalline phases in glasses is a well-known method for the development of novel functional materials [5]. In the two recent decades, numerous studies of laser-induced space-selective crystallization

of glass and growth of crystalline architectures in glass by direct laser writing have been efficiently realized as reviewed by Komatsu and Honma [6]. These methods open the way to the fabrication of 2D and 3D components of integrated optical circuits in glass matrix which obtain functional properties of crystals such as high second-order susceptibility, linear electrooptic effect, ferroelectricity, etc. For this purpose, the study of controlled crystallization of langasite-type (LGS) crystalline phases has been promoted in this study since their local structure provide a high temperature use while combining optical, electro – optical and multiferroic properties [7–11]. Previously, the langasite-type phases were singly crystallized in the corresponding stoichiometric multi-component glass systems (BaO – La₂O₃ – Ga₂O₃ – GeO₂) after thermal treatment [12], opening the way for future laser-induced controlled space-selective crystallization [13]. Furthermore, we selected this langasite – type solid solution since the main constituent from those crystal compositions can emerge from germanate to gallate glass network formers. In this glass system the precipitation of single LGS crystalline phase in the glass of composition 16.43 BaO-17.14 LaO₃/₂–40.00 GaO₃/₂–26.43 GeO₂ (at.%) was recently demonstrated [14] and the local structure of the glass matrix was studied by combined high – field solid –

<footnote><sup>a</sup> Institut de Chimie de la Matière Condensée de Bordeaux, UMR 5026, 87 Avenue du Dr Schweitzer, Pessac F-33608, France</footnote>

<footnote><sup>b</sup> Conditions Extrèmes et Matériaux: Haute Température et Irradiation, UPR 3079, Université d'Orléans, 1 Avenue de la Recherche Scientifique, 45100 Orléans, France</footnote>

<footnote><sup>c</sup> Institut des Sciences Moléculaires, UMR 5255, Université de Bordeaux, 351 cours de la Libération, Talence Cedex 33405, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: evelyne.fargin@u-bordeaux.fr (E. Fargin).</footnote>
<caption>**Table 1**</caption>

Nominal theoretical and experimental compositions of the corresponding LGS – type BaₓGa₅₋ₓGe₁₋ₓLa₃₋ₓO₁₄ – glass and – crystal materials with x = 0, 1, 2, 3. Glass compositions have been measured using induced coupled plasma optical emission spectroscopy (ICP – OES) with a standard deviation of ± 2 at.%. Crystal compositions have been verified by Energy Dispersive X – Ray spectroscopy (EDX) mapping with a standard deviation of ± 2 at.%. “Th” and “Exp” correspond to the abbreviation of “Theoretical” and “Experimental” respectively.

<table><thead><tr><th rowspan="2">Glass compositions</th><th rowspan="2"></th><th colspan="4">Nominal cationic molar composition (cat mol %)</th></tr><tr><th>GaO<sub>3/2</sub> ± 2%</th><th>GeO<sub>2</sub> ± 2%</th><th>BaO ± 2%</th><th>LaO<sub>3/2</sub> ± 2%</th></tr></thead><tbody><tr><td rowspan="2">La<sub>2</sub>BaGa<sub>4</sub>Ge<sub>2</sub>O<sub>14</sub> – glass</td><td>Th</td><td>44.44</td><td>22.22</td><td>11.11</td><td>22.22</td></tr><tr><td>Exp</td><td>45.(5)</td><td>19.(4)</td><td>11.(6)</td><td>23.(6)</td></tr><tr><td rowspan="2">LaBa<sub>2</sub>Ga<sub>3</sub>Ge<sub>3</sub>O<sub>14</sub> – glass</td><td>Th</td><td>33.33</td><td>33.33</td><td>22.22</td><td>11.11</td></tr><tr><td>Exp</td><td>33.(3)</td><td>31.(9)</td><td>23.(4)</td><td>11.(4)</td></tr><tr><td rowspan="2">Ba<sub>3</sub>Ga<sub>2</sub>Ge<sub>4</sub>O<sub>14</sub> – glass</td><td>Th</td><td>22.22</td><td>44.44</td><td>33.33</td><td>0</td></tr><tr><td>Exp</td><td>22.(4)</td><td>43.(7)</td><td>34.(0)</td><td>–</td></tr><tr><td>Crystal compositions</td><td>GaO<sub>3/2</sub> ± 2%</td><td>GeO<sub>2</sub> ± 2%</td><td>BaO ± 2%</td><td>LaO<sub>3/2</sub> ± 2%</td><td></td></tr><tr><td rowspan="2">La<sub>3</sub>Ga<sub>5</sub>GeO<sub>14</sub> – crystal</td><td>Th</td><td>55.56</td><td>11.11</td><td>0</td><td>33.33</td></tr><tr><td>Exp</td><td>57.(9)</td><td>9.(6)</td><td>0</td><td>32.(5)</td></tr><tr><td rowspan="2">La<sub>2</sub>BaGa<sub>4</sub>Ge<sub>2</sub>O<sub>14</sub> – crystal</td><td>Th</td><td>44.44</td><td>22.22</td><td>11.11</td><td>22.22</td></tr><tr><td>Exp</td><td>46.(8)</td><td>22.(2)</td><td>10.(4)</td><td>20.(6)</td></tr><tr><td rowspan="2">LaBa<sub>2</sub>Ga<sub>3</sub>Ge<sub>3</sub>O<sub>14</sub> – crystal</td><td>Th</td><td>33.33</td><td>33.33</td><td>22.22</td><td>11.11</td></tr><tr><td>Exp</td><td>37.(9)</td><td>28.(8)</td><td>19.(6)</td><td>13.(8)</td></tr><tr><td rowspan="2">Ba<sub>3</sub>Ga<sub>2</sub>Ge<sub>4</sub>O<sub>14</sub> – crystal</td><td>Th</td><td>22.22</td><td>44.44</td><td>33.33</td><td>0</td></tr><tr><td>Exp</td><td>25.(9)</td><td>41.(9)</td><td>32.(1)</td><td>–</td></tr></tbody></table>

state ⁷¹Ga NMR and vibrational spectroscopies. In this paper, the main spectral features in the glasses and the crystalline LGS compounds are investigated.

## 2. Experimental parts

### 2.1. Glass synthesis

Glasses were elaborated using high purity powders Ga₂O₃ (Fox Chemicals, ≤ 99.999%), GeO₂ (Fox Chemicals, ≤ 99.999%), La₂O₃ (Stream Chemicals, ≤ 99.99%) and BaCO₃ (Fox Chemicals, ≤ 99.99%). Before being weighed out in stoichiometric proportions corresponding to the glass compositions BaₓGa₅₋ₓGe₁₋ₓLa₃₋ₓO₁₄ with x = 0, 1, 2, 3. Lanthanum oxide has been pre-heated at a temperature equal to 900 °C for a duration of about 2 h. One notes that the heat treatment of lanthanum oxide have been made since La₂O₃ is very hygroscopic and may also absorb carbonates to form La₂O₂CO₃. Thus La₂O₃ suffers significantly from weight changes when used in manufacturing processes as compared to other reagents. The mixtures were then grinded in an agate mortar. Then the powders were placed in the same platinum crucible before being heated up to the melting temperature for a duration of ~ 1–2 h in an electrical furnace with a heating rate of ~ 10 °C. min⁻¹. Melting temperatures were increased from ~ 1450 °C to ~ 1600 °C according to the decrease in the x fraction in order to homogenize the liquid melt. The use of a lot of thermal energy may be justified since gallium-rich germanate glasses possess a combination of unique superior properties which are the mid-infrared transparency up to 5.5 µm, relatively high nonlinear third order optical properties, a glass ability to be shaped as optical fibres and superior mechanical properties and a high rare-earth solubility up to 20–25 mol%. The platinum crucible has been further quenched in water at room temperature. The glasses were taken out of the Pt container by slightly moving the walls of the Pt crucible. Using this method, the glasses gently detached from the crucible walls. Transparent glasses exhibiting a yellowish tint were obtained for LGS-type glasses with x = 1, 2, 3. We propose that some platinum from the crucible may be dissolved in the glass as Pt⁴⁺ during the manufacturing process at high temperatures. This may explain the yellowish tint since platinum as Pt⁴⁺ presents absorption bands in the visible range at 357 nm and 425 nm [15]. The glass corresponding to the Ga₅GeLa₃O₁₄ (at.%) composition could not be obtained even with a melting temperature increased up to ~ 1600 °C. The low amount of glass former GeO₂ can prevent the glass formation of composition Ga₅GeLa₃O₁₄ (at.%). The optical glasses were subsequently cut and polished on both parallel faces for further characterizations. Glass compositions

have been verified using induced coupled plasma optical emission spectroscopy (ICP – OES) measurement with a standard deviation of ± 2 atomic percent as shown in Table 1. Glass Samples for ICP – OES measurements were prepared by microwave mineralization.

### 2.2. Crystal synthesis

The La₃Ga₅GeO₁₄ LGS phase (x = 0) was prepared through classic solid state route. Powder precursors were weighed out in stoichiometric proportions before being carefully grinded in an agate mortar. The powder was subsequently shaped as a pellet before being sintered at 1350 °C during 12 h. The remaining LGS crystals with x = 1, 2, 3 belonging to the solid solution BaₓGa₅₋ₓGe₁₋ₓLa₃₋ₓO₁₄ were obtained by congruent crystallization of the previous obtained glasses previously crushed in powder. BaGa₄Ge₂La₂O₁₄, Ba₂Ga₃Ge₃LaO₁₄, Ba₃Ga₂Ge₄O₁₄ were obtained after a heat treatment of 14, 8 and 10 h respectively at a temperature of 1000 °C since it ensures the full glass crystallization. No additional longer thermal treatment has been done in order to verify the full crystallization of the glasses. Crystal compositions have been verified by Energy Dispersive X-Ray spectroscopy (EDX) mapping using a MET JEOL 2200FS with a standard deviation of ± 2 atomic percent.

## 2.3. Thermal and physical characterizations

Thermal properties including the onset of the glass transition temperature (T_g) and the onset of the crystallization temperature (T_x) were measured on a Netzsch DSC Pegasus 404 F3 apparatus in a platinum pan using glass powder. A heating rate of 10 °C.min⁻¹ has been used. The onset of the T_g and T_x were determined from the DSC trace from the tangents crossing. The error has be estimated to be equal to ± 2 °C. The density ρ was obtained from the average of five measurements per sample using Archimedes method by immerging a glass chunk in diethyl phthalate at room temperature on a Precisa XT 220A weighing scale. The estimated errors were estimated around ± 0.01 g.cm⁻³. Optical properties have been investigated by transmission measurements. The infrared transmission spectra were recorded on a spectrometer Brüker Equinox 55 from 4000 cm⁻¹ to 900 cm⁻¹ with a spectral resolution of 4 cm⁻¹. The spectra have been recorded from an average of 32 scans. The linear absorption coefficient α has been obtained from the transmission spectra according to the relation α = − ln(I/I₀) / x, where x is the sample’s thickness and I/I₀ the transmission percentage. Fresnel reflections have been subtracted. The IR absorption edges were determined for an absorption coefficient equal to α = 10 cm⁻¹. The ultraviolet– visible transmission spectra were recorded on a Cary5000 (Varian)
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Evolution of the a –, b – and c – lattice parameters in the solid solution BaₓGa₅ₓGe₁₁ₓLa₃ₓO₁₄.</figcaption></figure>

from 200 nm to 2500 nm with a spectral resolution of 1 nm. The sample thickness of Ba₃Ga₂Ge₄O₁₄ (at.%), Ba₂Ga₃Ge₃LaO₁₄ (at.%) and BaGa₂Ge₄La₂O₁₄ (at.%) glasses were respectively 0.050 cm, 0.234 cm and 0.047 cm.

## 2.4. Structural characterizations

The amorphous state of the glass samples was verified by powder X-Ray Diffraction (XRD) which were collected on a PANalitycal X'pert PRO MPD diffractometer used in a Bragg-Brentano θ – θ geometry and equipped with a secondary monochromator and X'Celerator multi-strip detector. Measurements were performed within an angular range of 2θ= 8°–80°. Lebail refinements were performed on crystalline phases. The powders have been sieved to a size of 40 µm. Measurements were performed within an angular range of 2θ= 10.0042°–129.9995° with a step interval equals to 0.008356°. ⁷¹Ga solid-state NMR spectra were obtained on a wide bore 20.0 T Bruker Neo spectrometer using a rotorsynchronized solid echo excitation with a full echo acquisition (i.e. T₉₀ – τ – T₉₀ – acq.) to avoid any ambiguity related to phasing and baseline correction. All powdered samples have been packed in 1.3 mm zirconia rotors, spun at 64.0 kHz and the delay set to 12 rotor periods (i.e. 187 µs). A short, selective, pulse T₉₀ of 0.9 µs has been used at a radio-frequency field of 100 kHz ensuring a full excitation of the spectra. A recycle delays were 0.5 s (spin-lattice relaxation times estimated ≤ 100 ms) and between 8192 and 16,384 transients were accumulated for all compositions. ⁷¹Ga chemical shift is referenced to a 1 M solution of Ga(NO₃). All spectra were simulated using DMFit based on either the so-called “Czjzek” (or Gaussian Isotropic) model for glassy compounds or the “Czjzek extended” model for the crystalline one, both models rendering the quadrupolar-broadened central (1/2, -1/2) transition in

the finite spinning-speed regime (i.e. taking into account its spinning sidebands [16]). Unpolarized Raman spectra were recorded with a confocal micro–Raman spectrometer LabRAM HR Evolution (Horiba Jobin Yvon) equipped with a Synapse CCD detector cooled down to –70 °C using a 532 nm radiation from a diode pumped solid state laser (output power = 20 mW). The incident laser beam was focused onto the sample through a microscope with a 50x objective (NA = 0.50, Olympus). Scattered light was dispersed by 1200 grooves. mm⁻¹ grating system. Raman spectra have been corrected by the Bose–Einstein factor. The reflectance infrared spectra of the glasses were recorded with an incident angle of 11° from 200 to 1000 cm⁻¹ with a spectral resolution of 4 cm⁻¹ using a Fourier Transform Spectrometer Vertex 70 V. A MIR source has been used (globar type). All measurements were performed in vacuum. The spectra have been averaged from 200 measurements. Kramers–Kronig transforms have been performed in order to extract the optical constant *k* from the imaginary part of the refractive index *n* according to the relation *n* = *n*₀ + *i* *k* with *n*₀ the linear refractive index.

# 3. Results

## 3.1. Synthesis

Glass and crystal synthesis are reported in **Section 2**. Nominal theoretical and experimental compositions are presented in **Table 1**. One observes a relatively good agreement between the targeted and the experimental glass and crystal compositions.

X – Ray diffraction patterns of the langasite crystals obtained either via glass crystallization or classic solid state route in the solid solution BaₓGa₅ₓGe₁₁ₓLa₃ₓO₁₄ with x = 0, 1, 2, 3 are shown in ref [14] and Supplementary Data. The LGS crystal structure belongs to the structure type of calcium germanate Ca₃Ga₂Ge₄O₁₄ consistent with the space group P321. The evolution of the a –, b – and c – lattice parameters reported in Fig. 1. Those parameters have been obtained using the Lebail least square refinements.

In accordance with ref [12], we obtain a linear increase of the lattice parameters while displacing the solid solution from La₃Ga₅GeO₁₄ towards Ba₃Ga₂Ge₄O₁₄ phases in accordance with the common Vegard's rule. The similar measured composition (Table 1) between the glass and the crystal and the linear evolution of the lattice parameters suggests a congruent crystallization. The increase in the a-, b-, c- lattice parameters with x can be mostly ascribed to the larger ionic radius of Ba²⁺ (r²⁺Ba = 1.38 Å [17]) than La³⁺ (r³⁺La = 1.05 Å [17]) involving more drastic evolution on a-, b- lattice parameters. However a contradiction appears considering the larger ionic radius of gallium in tetrahedral or octahedral environment compared to germanium ionic radius in the same site. Indeed LGS phases are reported in the chemical formula A₃BC₃D₂O₁₄ where A and B cations are respectively located on decahedral site (3e) and octahedral site (1a) while C and D are located in tetrahedral site respectively 3f and 2d Wyckoff's positions [18]. In La₃Ga₅GeO₁₄, Ge is mostly located in 2d smaller tetrahedral site whereas in compositions with larger ratio Ge/Ga like Ca₃Ga₂Ge₄O₁₄ or Sr₃Ga₂Ge₄O₁₄, Ge is also

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** La₃Ga₅GeO₁₄ crystalline phases obtained from the refined XRD pattern according to the (a) a – b and (b) b – c planes.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** $^{71}$Ga Solid – State Nuclear Magnetic Resonance (SSNMR) spectra of the crystalline compositions (a) La$_{3}$Ga$_{5}$GeO$_{14}$ (b) Ba$_{1}$Ga$_{4}$Ge$_{2}$La$_{2}$O$_{14}$ (c) Ba$_{3}$Ga$_{2}$Ge$_{4}$O$_{14}$. The La$_{3}$Ga$_{5}$GeO$_{14}$ crystalline phase spectra is identical to the one already published under similar experimental conditions assigned to only GaO$_{4}$ and GaO$_{6}$ coordinated gallium units [22].</figcaption></figure>

expected in larger site [19]. LGS crystal structure can then be described by layers formed with six membered rings made of three CO$_{4}$ and two DO$_{4}$ tetrahedrons, bonded to each other by the edges to one BO$_{6}$ octahedron. The CO$_{4}$ tetrahedrons have only bridging oxygen anions while

DO$_{4}$ tetrahedrons have one nonbridging oxygen anion. Eight coordinated La$^{3+}$ or Ba$^{2+}$ cations occupy AO$_{8}$ sites in between the tetrahedral layers. As an illustration, the La$_{3}$Ga$_{5}$GeO$_{14}$ LGS structure is presented in Fig. 2a–b. The nature of the atoms (Ga, Ge) position in their sites evolves according to the increasing Ge/Ga ratio with x in the solid solution Ba$_{x}$Ga$_{5-x}$Ge$_{1+x}$La$_{3-x}$O$_{14}$. According to ref [19], we suggest that the decrease of AO$_{8}$vol with x decrease could be ensured thanks to exchanges of Ge between 2d and 1a sites and a great volume flexibility of 3f sites when occupied by Ga, with different result on (a-, b-) and c-cell parameters.

The $^{71}$Ga NMR spectrum of La$_{3}$Ga$_{5}$GeO$_{14}$ shows a very complex line shape pattern (Fig. 3a) arising from strong electric field gradients felt by this nuclei. A very reasonable decomposition can nevertheless be obtained, using the so-called "extended-Czjzek" model, i.e. taking into account a random distribution of quadrupolar interaction around a well-defined set of ($C_Q, \eta_Q$) values. In other words this an attempt to simulate small local structural variations at well-defined crystallographic sites, a procedure which has been successfully applied in the case of $^{139}$La NMR in La$_{2}$Si$_{2}$O$_{7}$-Y$_{2}$Si$_{2}$O$_{7}$ solid solution [20]. Under this assumption, the simulation reveals three components with respective isotropic chemical shifts of 10 ppm, 279 ppm and 233 ppm and based on those numbers the first component is assigned to the GaO$_{6}$ site in 1a (site B). The two other components are then expected to be GaO$_{4}$ units, and this is confirmed by their high isotropic chemical shift values. They are markedly different not only in their $C_Q$ values (resp. 17.2 Mhz and 25.5 MHz), but also in their integrated intensity (resp. 18 % and 64 %) leading to an overall population very close to 20 %:20 %:60 % ratios for the three sites, in excellent agreement with the 1:1:3 expected ratio for the 1a:2d:3f sites when taking into account the partial occupancy of the 2d one. We therefore assign the (233 ppm, 25.5 MHz) site to GaO$_{4}$ in 3f position (site C) and the (279 ppm, 17.2 MHz) one to GaO$_{4}$ in 2d position (site D). It is striking to observe a $C_Q$ value as high as 25.5 MHz and it suggests a highly distorted GaO$_{4}$ tetrahedra [21], again in agreement with the crystallographic data and the distribution of O–Ga–O bond angle found for this 3f site (site C) as opposed to the more symmetric 2d site (site D). It is worth noting that only the 3f (site C) site sees the (Ga,Ge)O$_{4}$ partial occupancy of the 2d site (site D) as a second neighbor and is connected to GaO$_{6}$ units, which leads to even more structural distortion than the average picture given by the XRD analysis. On the other hand, the presence of one non-bridging oxygen on the 2d (site D) site does not lead to a significant increase in the distortion of this GaO$_{4}$ tetrahedra.

The $^{71}$Ga NMR spectra of BaLa$_{2}$Ga$_{4}$Ge$_{2}$O$_{14}$ (Fig. 3b) bears strong resemblance with that of La$_{3}$Ga$_{5}$GeO$_{14}$ with an increased importance of the disordered contribution to the electric field gradient (as measured with the $\epsilon$ parameter), in agreement with the increased configurational degree of freedom induced by the presence of Ba atoms in the structure. While the relative intensities are staying the same suggesting a similar structure, one observes a significant increased shielding (downward shift of –30 ppm) of the isotropic chemical shift for the 3f sites (site C). This is also the case for the Ba$_{3}$Ga$_{2}$Ge$_{4}$O$_{14}$ (Fig. 3c) compound, with an additional deviation from the 1:1:3 initial ratio between the crystallographic sites. Fitting parameters are given in Table 2.

## 3.2. Glass physico – chemical properties

Glass thermal properties and their density are presented in Table 3. The glass transition temperatures $T_g$ increase from 665 °C to 738 °C upon the x decrease in the glass system Ba$_x$Ga$_{5-x}$Ge$_{1+x}$La$_{3-x}$O$_{14}$. The density increases as well from 4.87 g.cm$^{-3}$ up to 5.34 g.cm$^{-3}$. Such variations could be characteristic of important changes in the network glass structure.

A transparent region from the UV (~ 280 nm) up to the mid infrared (MIR) domain is observed for the glass Ba$_{3}$Ga$_{2}$Ge$_{4}$O$_{14}$ in Fig. 4. The linear absorption coefficient $\alpha$ in the MIR window is reported in inset. All glass present an IR transparency up to 5.5–6 µm due to the low phonon energy compounds Ga$_{2}$O$_{3}$ (~ 650 cm$^{-1}$) and GeO$_{2}$ (~
<table><caption>**Table 2**

SSNMR parameters from line shape fitting of the investigated langasite – type glasses.</caption><thead><tr><th>Crystalline sample</th><th>Wyckoff position (Site)</th><th>Proportion (%)</th><th>δ<sub>iso</sub> (ppm)</th><th>Δδ<sub>iso</sub> (ppm)</th><th>C<sub>Q</sub> (MHz)</th><th>ΔC<sub>Q</sub> (MHz)</th><th>η<sub>Q</sub></th><th>ε</th></tr></thead><tbody><tr><td rowspan="3">La<sub>3</sub>Ga<sub>5</sub>GeO<sub>14</sub></td><td>2d</td><td>18</td><td>279.0</td><td>5.8</td><td>17.18</td><td>4.66</td><td>0.15</td><td>0.15</td></tr><tr><td>1a</td><td>18</td><td>10.3</td><td>17.3</td><td>9.44</td><td>6.20</td><td>0.65</td><td>0.15</td></tr><tr><td>3f</td><td>64</td><td>232.7</td><td>173.5</td><td>25.48</td><td>10.96</td><td>0.0</td><td>0.90</td></tr><tr><td rowspan="3">BaGa<sub>4</sub>Ge<sub>2</sub>La<sub>2</sub>O<sub>14</sub></td><td>2d</td><td>24</td><td>268.8</td><td>12.1</td><td>19.08</td><td>4.69</td><td>0.15</td><td>0.90</td></tr><tr><td>1a</td><td>14</td><td>15.7</td><td>19.3</td><td>10.54</td><td>6.99</td><td>0.65</td><td>0.15</td></tr><tr><td>3f</td><td>62</td><td>203.6</td><td>173.5</td><td>27.78</td><td>11.93</td><td>0.2</td><td>0.90</td></tr><tr><td rowspan="3">Ba<sub>3</sub>Ga<sub>2</sub>Ge<sub>4</sub>O<sub>14</sub></td><td>2d</td><td>18</td><td>221.4</td><td>11.6</td><td>19.89</td><td>8.05</td><td>0.20</td><td>0.90</td></tr><tr><td>1a</td><td>7</td><td>14.2</td><td>11.6</td><td>13.32</td><td>4.17</td><td>0.50</td><td>0.20</td></tr><tr><td>3f</td><td>75</td><td>164.2</td><td>11.6</td><td>30.67</td><td>5.74</td><td>0.30</td><td>0.20</td></tr></tbody></table>

<table><caption>**Table 3**

Thermal properties and density of the corresponding langasite – type Ba<sub>x</sub>Ga<sub>5-x</sub>Ge<sub>1+x</sub>La<sub>3-x</sub>O<sub>14</sub> glasses.</caption><thead><tr><th></th><th>T<sub>g</sub> (± 2 °C)</th><th>T<sub>x</sub> (± 2 °C)</th><th>ΔT = T<sub>x</sub> - T<sub>g</sub> (± 4 °C)</th><th>P (± 0.01 g. cm<sup>-3</sup>)</th></tr></thead><tbody><tr><td>Ba<sub>1</sub>Ga<sub>4</sub>Ge<sub>2</sub>La<sub>2</sub>O<sub>14</sub></td><td>738</td><td>834</td><td>96</td><td>5.34</td></tr><tr><td>Ba<sub>2</sub>Ga<sub>3</sub>Ge<sub>3</sub>La<sub>1</sub>O<sub>14</sub></td><td>718</td><td>840</td><td>122</td><td>4.98</td></tr><tr><td>Ba<sub>3</sub>Ga<sub>2</sub>Ge<sub>4</sub>O<sub>14</sub></td><td>665</td><td>777</td><td>112</td><td>4.87</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Transmission spectrum of the germanate stoichiometric Ba<sub>3</sub>Ga<sub>2</sub>Ge<sub>4</sub>O<sub>14</sub> glass. The linear absorption coefficient α in the mid – infrared range from 2500 nm up to 7000 nm of the langasite – type glass samples are shown in the inset.</figcaption></figure>

900 cm<sup>-1</sup>). Several absorption bands located around ~ 3300 nm, ~ 4400 nm, ~ 6400 nm and ~ 7600 nm can be noticed. The absorption band located around ~ 3300 nm and the absorption band located at ~ 4400 nm could be characteristic of Ge–OH or Ga–OH combined stretching and bending vibration modes with the hydrogen being either strong or very strong bonded [23]. The assignation of the absorption bands located at higher wavelengths (~ 6400 nm and ~ 7600 nm) could be characteristic of overtones corresponding to fundamental IR vibrational modes [24].

One observes two main effects upon the x decrease in the Ba<sub>x</sub>Ga<sub>5-x</sub>Ge<sub>1+x</sub>La<sub>3-x</sub>O<sub>14</sub> solid – solution: (i) a change in the line shape of the absorption band located at ~ 3300 nm combined with a decrease of the absorption band located around ~ 4400 nm and (ii) a red-shift of the IR absorption edge from ~ 5800 nm up to ~ 6100 nm for a linear absorption coefficient α equals to 10 cm<sup>-1</sup> upon the compositional change from langasite barium germanate (x = 3) to the lanthanum gallate (x = 1). Spectral differences can be mostly assigned to an important modification in the glass structure from the germanate – rich to the gallate – rich glass skeleton.

### 3.3. Glass structural properties

#### 3.3.1. Solid – state nuclear magnetic resonance spectroscopy

⁷¹Ga solid-state Nuclear Magnetic Resonance (SSNMR) spectra collected at high magnetic field (20.0 T) and very fast spinning (64 kHz) are presented in Fig. 5a–c, along with some of their crystalline counterpart when available. The differences in the spectra obtained for the glasses and the crystals are striking: GaO₆ environments are clearly absent from the BaLa₂Ga₄Ge₂O₁₄ glass, and the GaO₄ site with large Cₚ is, within a few percent, not seen in Ba₃Ga₂Ge₄O₁₄. In order to better understand those spectral variations, deconvolution of the characteristic line shapes are made using the so-called “Czjzek” model applicable to half-integer nuclei in fully randomly disordered environments. As can be seen in the Fig. 5a–c, those latter ones are dominated one main peak with a maximum around 180 ppm and a discontinuity around 70 ppm [25–28]. Assuming the presence of only two sites leads to an excellent simulation of the experimental spectrum, including the spinning side-bands, which points to GaO₄ (δ<sub>iso</sub> ~ 210 ppm) and GaO₅ (δ<sub>iso</sub> ~ 110 ppm) types of environments and the absence of GaO₆ units (which isotropic chemical shifts are around 50–60 ppm) [25–28]. The absence of the later can be related to the absence of GaO₆ units in the glass and to which this site was connected in the crystalline compounds. As seen in Table 4, those simulations show that upon increasing the La/Ba ratio the GaO₄ site is decreasing with respect to GaO₅, and both sites average (most probable) isotropic chemical shifts move to higher values (decreasing shielding). This chemical shift change can be assigned to a change in the nature of the second coordination sphere neighbor of Ga, and hence to the changes in the nature of the cation performing the charge compensation of the (Ga,Ge)–O–(Ga,Ge) glass skeleton. The obtained fitting parameters indicate that the x decrease from 3 to 1 in the glass compositions Ba<sub>x</sub>Ga<sub>5-x</sub>Ge<sub>1+x</sub>La<sub>3-x</sub>O<sub>14</sub> leads to increase significantly the fraction of GaO₅ units of about ~ 5% up to ~ 24%. This important increase in the GaO₅ fraction can be partly explained due to the higher cationic field strength Z defined as the ratio of the cationic charge out of the ionic radius of Lanthanum (Z = 2.72 Å<sup>-2</sup> [17]) as compared to Barium cations (Z = 1.05 Å<sup>-2</sup> [17]). This increased the structural disorder with higher field strength cation is confirmed with the increase of σ<sub>c</sub> with increasing La content since σ<sub>c</sub>² is the variance of the distribution of the quadrupolar interaction and hence measures the extent of the atomic-level disorder present in the sample.

#### 3.3.2. Vibrational spectroscopy

Unpolarized Raman and Fourier Transformed InfraRed (FTIR) spectra of the investigated langasite – type glasses are presented in Fig. 6a–b. Those spectra were respectively normalized with respect to their total area between 200 cm<sup>-1</sup> and 1000 cm<sup>-1</sup>. A perfect agreement on Fig. 6a- is observed with Raman spectra obtained by [12] for x = 1 and x = 3 while the spectrum for x = 2 is of better quality unlike it was polluted by a weak Eu³⁺ photoluminescence. Then a continuous evolution of FTIR and Raman spectra can be insured for x increasing from 1 to 3. These spectra can be usually decomposed in three main regions which correspond respectively to the frequency domains between 200–430 cm<sup>-1</sup>, 430–600 cm<sup>-1</sup> and 600–1000 cm<sup>-1</sup>. The lowermost
<figure><img src="image_7.png" /><figcaption>(c) Ba₃Ga₂Ge₄O₁₄- glass</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>(b) Ba₂Ga₃Ge₃LaO₁₄- glass</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 5. ⁷¹Ga Solid – State Nuclear Magnetic Resonance (SSNMR) spectra of the glass compositions (a) BaGa₄Ge₂La₂O₁₄ (b) Ba₂Ga₃Ge₃LaO₁₄ (c) Ba₃Ga₂Ge₄O₁₄.</figcaption></figure>

<table><caption>**Table 4**

SSNMR parameters from line shape fitting of the investigated langasite – type glasses.</caption><thead><tr><th>Glass sample</th><th>Coordination number</th><th>Proportion (%)</th><th>δ<sub>iso</sub> (ppm)</th><th>Δ δ<sub>iso</sub> (ppm)</th><th>σ<sub>c</sub> (MHz)</th></tr></thead><tbody><tr><td>Ba<sub>3</sub>Ga<sub>2</sub>Ge<sub>4</sub>O<sub>14</sub> - glass</td><td>4</td><td>95</td><td>207.2</td><td>42.9</td><td>7.58</td></tr><tr><td>Ba<sub>3</sub>Ga<sub>3</sub>Ge<sub>3</sub>LaO<sub>14</sub> - glass</td><td>5</td><td>5</td><td>97.2</td><td>42.4</td><td>7.50</td></tr><tr><td>Ba<sub>3</sub>Ga<sub>4</sub>Ge<sub>2</sub>La<sub>2</sub>O<sub>14</sub> - glass</td><td>4</td><td>93</td><td>213.4</td><td>49.0</td><td>8.75</td></tr><tr><td>Ba<sub>3</sub>Ga<sub>4</sub>Ge<sub>2</sub>La<sub>2</sub>O<sub>14</sub> - glass</td><td>5</td><td>7</td><td>113.9</td><td>68.4</td><td>8.24</td></tr><tr><td>Ba<sub>3</sub>Ga<sub>4</sub>Ge<sub>2</sub>La<sub>2</sub>O<sub>14</sub> - glass</td><td>4</td><td>76</td><td>217.0</td><td>52.8</td><td>9.52</td></tr><tr><td>Ba<sub>3</sub>Ga<sub>4</sub>Ge<sub>2</sub>La<sub>2</sub>O<sub>14</sub> - glass</td><td>5</td><td>24</td><td>126.9</td><td>77.8</td><td>10.0</td></tr></tbody></table>

200–430 cm⁻¹ region is often assigned to vibrational signature of heavy cations such as barium and lanthanum oxide in an oxygenated site [29–31]. The intermediate frequency window between 400–600 cm⁻¹ is mainly characteristic to a combination of stretching ν (T–O–T) and bending δ (T–O–T) modes of an oxygen surrounded by two units being either gallate or germanate ones. The highest frequency region between 600–1000 cm⁻¹ contains mainly localized stretching features of germanium sites involved respectively in Qn=1,2,3,4 germanate units [32] and gallate -based polyhedra units [14,29,33]. This regions contains also stretching frequencies of an oxygen surrounded by gallate and/or germanate units ν (T[4]–O–T[4]) bridges [34,35] and Ga–O vibrations in GaO₄,₅ polyhedra including probably a non-bridging oxygen Onb [14, 29]. According to the spectral variations, one observes that the decrease of x from 3 to 1 in the langasite-type BaₓGa₅ₓGe₁ₓLa₃ₓO₁₄ compositions leads to: (i) broaden the vibrational signatures located in the low frequency region, (ii) shift the intermediate vibrational band from ~512 cm⁻¹ to ~526 cm⁻¹ Raman and from ~537 cm⁻¹ to ~508 cm⁻¹ IR signatures in the intermediate frequency region and (iii) decrease the intensity and to shift the high frequency peak from ~817 cm⁻¹ to ~793 cm⁻¹ and from ~769 cm⁻¹ to ~725 cm⁻¹ for Raman and IR signatures respectively. This vibrational changes appears for the benefit of a new signature in the 600 cm⁻¹–700 cm⁻¹ range.

### 3.3.3. Raman comparative analysis with langasite crystal phases

Unpolarized Raman scattering spectra measured on the langasite-type glasses and crystallized phases are shown in Fig. 7. As observed by [12] the vibrational signature of the langasite crystal phases can be superposed with the corresponding glass ones, suggesting similar short range order in the glass structure.

In the low frequency region related to the Ba₃Ga₂Ge₄O₁₄ langasite crystal, three main vibrational modes located around ~267 cm⁻¹, ~300 cm⁻¹, ~337 cm⁻¹ that could be assigned to Ba–O vibrational modes. Upon the x decrease from 3 to 0 in the glass system BaₓGa₅ₓGe₁ₓLa₃ₓO₁₄, one observes a broadening and a shift of those Raman vibrational modes upon the introduction of lanthanum cations. The appearance of those vibrational modes around ~240 cm⁻¹ and ~324 cm⁻¹ can be assigned mostly to stretching La–O vibrational modes [29–31].

Regarding the intermediate frequency region, one observes two main vibrational modes around ~487 cm⁻¹ and ~559 cm⁻¹ in the Ba₃Ga₂Ge₄O₁₄ Raman spectrum that could be respectively assigned to a combination of stretching and bending vibrational modes of T[4]–O–T[4] bridges with T = Ga, Ge corner- shared tetrahedra and T[4]–O–T[6] edge- share tetrahedra and octahedra units [36]. Those vibrational peaks shifts from ~487 cm⁻¹ to ~499 cm⁻¹ and from ~559 cm⁻¹ to ~590 cm⁻¹ upon the x decrease. This may traduce a change in the network connectivity from Ge[4]–O–Ge[4] and Ge[4]–O–Ge[4,6] in the germanate Ba₃Ga₂Ge₄O₁₄ composition to T[4]–O–T[4] and T[4]–O–T[4,6] bridges in the gallate La₃Ga₅GeO₁₄.

Regarding the high frequency region, on the Ba₃Ga₂Ge₄O₁₄ Raman spectrum two main strong active intense bands around ~813 cm⁻¹ and ~800 cm⁻¹. According to a previous study [35], one mostly assigns those vibration modes to Q₃ germanate units. Upon the x decrease, several spectral variations which can be described as follows (i) a shift from ~863 cm⁻¹ to ~824 cm⁻¹ of the most intense vibrational band and (ii) an increase of the vibrational modes located in the 600–700 cm⁻¹ regions. Based on those spectral variations, both previous items can be mostly ascribed to a change in the crystal skeleton from
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Vibrational (a) Raman and (b) FTIR spectra of the investigated LGS glass compositions.</figcaption></figure>

germanate (Ge–O–Ge) to gallate-based (Ga–O–Ga) structure with the disappearance of the vibrational modes in the 800–900 cm⁻¹ domain. The spectral variations in the 600–700 cm⁻¹ region were mostly assigned to the diverse coordination environments of the oxygen atoms in a gallium-rich glass according to Yoshimoto et al. [29,33]. The authors [12] suggested the presence of the octahedral GaO₆ units in the La₃Ga₅GeO₁₄ – Ba₃Ga₂Ge₄O₁₄ system glass, as would be expected from the crystal structure of the langasite-type phase and concluded to the coexistence of octahedral GaO₆ and GeO₄ units in the gallo-germanate glasses. The present NMR analysis of glass compositions infirm this conclusion since no GaO₆ units have been evidenced in the glasses, thus proving the increase in the pentahedral to tetrahedral units ratio of gallium GaO₅/GaO₄ when Ba is substituted to La. Then the increase of the 600–700 cm⁻¹ band intensity can be definitely correlated to the evolution of this ratio.

## 4. Summary

Heavy Metal Oxide germanate and gallate oxide langasite-type compositions in the system BaₓGa₅ₓGe₁+xLa₃ₓO₁₄ with x = 0, 1, 2, 3

were successfully synthesized either by glass recrystallization or the traditional solid-state route with relatively good agreement factors. In accordance with the refined crystalline structures with x = 0 and x = 1, those ones can be described as a layered structure including mainly corner-shared GaO₄ tetrahedra perpendicular to the crystallographic c-axis connected through their vertices. These layers are connected by octahedrally coordinated GaO₆ in which eighthy-coordinated Lanthanum or Barium lie, separating then the tetrahedra layers. Similar vibrational signatures can be observed between both crystalline and glass compositions, suggesting the local glass structure contains similar local ordering than the corresponding crystal. In particular, one has observed characteristic vibrational signature of corner-shared oxygenated gallate and/or germanate tetrahedra units. This similar short range order is also consistent with the SSNMR data in which most gallium are found to be four-coordinated. Nonetheless, the large number of GaO₅ around 24 percent units in the glass matrix Ba₁Ga₄Ge₂La₂O₁₄ as compared to the BaₓGa₅ₓGe₁+xLa₃ₓO₁₄ compositions in which with x = 2 and 3 can be mostly ascribed to the increase of lanthanum oxide in the glass matrix which presents a higher field strength (~ 2.72 Å²) as compared to barium (~ 1.05 Å²) cations. This increase in highly coordinated gallate GaO₅ units leads to increase significantly the glass transition temperature values as well as a higher glass packing density. Those structural changes from GaO₄ to GaO₅ units when adding La³⁺ ions are thus in good agreement with the increase of the glass transition temperatures and the densities, favoring the glass network polymerization. Furthermore, such increase seems to favor the glass crystallization as can be seen with the reduction of the ΔT values upon the introduction of lanthanum. In opposition to the conclusions of the authors in ref [12], NMR characterization of the glass matrix evidenced 4- and 5-coordinated gallium units and denied unambiguously the formation of 6-coordinated gallium sites [12].

## 5. Conclusion

Following our previous work [2,14,35], we investigated the properties and the local glass structure of BaₓGa₅ₓGe₁+xLa₃ₓO₁₄ compositions with x = 0, 1, 2, 3 in the glass system BaO – La₂O₃ – Ga₂O₃ – GeO₂ considering the optical properties that can be induced by the controlled crystallization of the single Langasite phase having the same stoichiometry. NMR characterization of the glass matrix has evidenced 4- and 5-coordinated gallium units in glasses and denied unambiguously the formation of 6-coordinated gallium sites existing in the crystalline phases. According to vibrational spectroscopies, the glass network can be considered as a 3D network formed by 4- and 5- coordinated gallate units connected to 4- coordinated germanate units. Interconnection between gallate units and the diverse environments of gallium can be at the origin of congruent crystallization towards Langasite phases, though the transition from 4- and 5-coordinated gallium to 6-coordinated gallium sites observed in crystalline phases is not still elucidated.

### CRediT authorship contribution statement

**Florian Calzavara:** Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Conceptualization.
**Pierre Florian:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Methodology, Investigation, Formal analysis.
**Franck Fayon:** Writing – review & editing, Visualization, Supervision, Resources, Investigation, Formal analysis.
**Sonia Buffière:** Writing – review & editing, Visualization, Validation, Investigation, Formal analysis.
**Marc Dussauze:** Writing – review & editing, Visualization, Validation, Resources, Methodology, Funding acquisition, Formal analysis.
**Véronique Jubera:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Formal analysis, Conceptualization.
**Thierry Cardinal:** Writing – review & editing, Visualization, Validation, Supervision,
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Raman scattering spectra of the langasite – type crystallized and glass compositions.</figcaption></figure>

Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Evelyne Fargin:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization.

**Declaration of competing interest**

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

**Data availability**

No data was used for the research described in the article.

**Acknowledgement**

Financial support from the IR INFRANALYTICS FR2054 for conducting the research is gratefully acknowledged.

**Supplementary materials**

Supplementary material associated with this article can be found, in

the online version, at doi:10.1016/j.jnoncrysol.2024.123204.

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