1. Introduction

Lanthanide ion doped glasses have great interest for their use in telecommunication systems as glass fibers lasers, and optical amplifiers. In particular, the search for developing new solid-state lasers based on rare-earth (RE)-doped glass matrices is nowadays the subject of intense research [1,2]. The investigation of new laser wavelengths or new RE-doped glass matrices thus becomes a prior requirement in order to satisfy the demand of further outcomings in the development of this kind of solid-state lasers.

Since the first report on laser action in glass [3], many different glass compositions have been used (silicates, phosphates, fluorides) as matrix materials for trivalent rare-earth ions to produce active optical devices. Among them, rare-earth aluminosilicate and calcium aluminosilicate glasses have been studied for many years due to their high elastic modulus and hardness, high refraction index, high thermal conductivity, high glass transition temperature, excellent optical properties and good corrosion resistance [4–11]. Moreover, they are known for their ability to accommodate high concentration of rare-earth active ions such as Nd³⁺ or Er³⁺ as well as their low phonon energy due to the aluminate network. These properties have led to various applications in solid state lasers, optical waveguides, luminescence probes, and even in medicine as in vivo radiation delivery vehicles for cancer treatment of internal organs [4].

Among the possible rare-earth ions, neodymium has been studied in a large variety of glasses due to its potential applications in the field of infrared optical amplification related to the radiative efficiency of the ⁴F_3/2→⁴I_11/2 emission at around 1.06 μm [12]. In addition, the properties observed for Nd³⁺, related to the modification in composition and microstructure, are also applicable to other rare-earths. It is well known that spectroscopic properties of trivalent rare-earth ion depend on the chemical composition of the glass matrix, which determines the structure and nature of the bonds [12,13].

The aim of this work is to assess thermo-mechanical and spectroscopic properties of neodymium doped (Mg_xCa_1-x)₃Al₂Si₃O₁₂ glasses, for x = 0, 0.5 and 1, produced by the laser floating zone technique (LFZ). These compounds always adopt an amorphous state when they are prepared at room pressure but at high pressure and temperature conditions these materials may form crystals. In fact, the end members of the solution (x = 0 and 1) can be found in nature as the well-known crystalline minerals of Grossular, Ca₃Al₂Si₃O₁₂ and Pyrope, Mg₃Al₂Si₃O₁₂, both adopting garnet structure. The temperatures and pressures required to acquire this crystalline structure range 1000 - 1400°C and 2.5 - 4 GPa, respectively [14,15]. These P-T conditions have been reproduced at lab-scale by means of hydrothermal and dry techniques using piston-cylinder and multi-anvil press devices, and Grossular and Pyrope garnets have been successfully prepared departing from glasses [16–19]. On the other hand, it was recently demonstrated that it is possible to create extreme pressure and temperature conditions in table-top laboratory experiments with ultrashort laser pulses focussed inside transparent materials, with intensity in the focal spot significantly above the threshold for optical breakdown. These laser processing conditions transform the matter into a high-entropy state of dense plasma, a so called Warm Dense Matter (WDM), the state of matter in the core of planets and stars [20–22]. Tightly focalization of few femtosecond laser pulses can deposit energy densities of several MJcm⁻³ into a submicron volume, several times higher than the strength of any material. This causes the solid to superheat yielding to the formation of plasma. This confined plasma explodes and generates a powerful shock wave that expands out of the focal volume and compresses the surrounding material. The shock wave of the expanding plasma can reach pressures of over 10 megabar (1 TPa), triggering dramatic changes in the material properties. The modified material remains confined in a strongly localized region inside the unmodified surrounding bulk material so that it can be investigated later [20–22].

It is our aim to compare thermo-mechanical and spectroscopic results obtained in this work with laser synthesised optical glass-ceramics developed in a future work. These laser synthesised glass-ceramics are intended to be used as laser matrices since garnet structure is one of the most suitable materials to be used as active medium in optical amplification and solid-state lasers. In addition, the nature of these alkaline-earth aluminosilicate glasses have allow studying in the same glass network former, for the first time to the best of our knowledge, the local structure surrounding the Nd³⁺ ions and the covalence of the Nd-O bond related to the nephelauxetic effect in terms of absorption and emission spectral shifts, Judd-Ofelt intensity parameters and ionic packing ratio. All obtained results agree and confirm the higher covalency of the Nd-O bond in the Grossular glass.

2. Experimental

2.1 Samples fabrication

Glass rods were obtained by the laser floating zone technique (LFZ). This technique has been described in detail elsewhere [23]. The laser floating zone system includes a 600 W CO₂ semisealed laser (Electronic Engineering Blade 600) emitting at 10.6 µm and an in-house built growth chamber with gold coated metal mirrors for beam focussing, and two vertical axes for cylinder displacement. Both axes have independent rotation and translation movements. The mirror system inside the chamber consists of a reflaxicon that transforms the solid beam into a ring which is deflected by a flat mirror at 45° and focused on the ceramic rod by a parabolic mirror producing a homogenous heating. The correct optical alignment is obtained with the aid of a red diode laser coaxial with the infrared beam. Once the precursor is placed in the upper axis, the growth process starts by heating its lower end. When a drop is formed, a small seed placed in the lower axis is approached until a liquid bridge between the precursor and the seed is established. Then the seed is moved away while the precursor is moved towards the molten zone, and the volume of the liquid zone is kept constant. Identical feed and growth rates can be used when equal precursor and eutectic rod diameters are required. To increase or decrease the eutectic rod diameter, the growth rate must be lower or higher respectively than the precursor speed. The precursor and the grown rod are counter rotated to improve the heat distribution in the molten zone. This technique permits to control the solidification rate, providing high axial and radial thermal gradients in the liquid-solid interface, of paramount importance in the microstructure domain, and opens up the possibility of fabricating glasses and glass-ceramics samples [24,25].

Grossular, Ca₃Al₂Si₃O₁₂ (Ca-Glass), Pyrope, Mg₃Al₂Si₃O₁₂ (Mg-Glass) and the intermediate composition Ca_1.5Mg_1.5Al₂Si₃O₁₂ (CaMg-Glass) precursor rods were obtained by mixing stoichiometric amounts of Al₂O₃ (Sigma-Aldrich, 99.99%), SiO₂ (Aldrich, 99.6%), CaO (Aldrich, 99.9%) and MgO (Alfa, 99.5%) powders. Furthermore, 1 wt% of Nd₂O₃ (Aldrich, 99.99%) was added to the composites to obtain the doped samples. The resulting powders were isostatically pressed at 200 MPa for 3 minutes to obtain ceramic rods which were sintered at 1200°C for 12 hours.

2.2 Characterization techniques

Microstructure and composition were determined by means of field emission scanning electron microscopy (FESEM) using a Carl Zeiss Merlin equipped with an EDX detector. The amorphous character of the glass samples was analysed by X-ray powder diffraction (XRD) using a X´Pert Pro MPD diffractometer (Cu-kα radiation). Mechanical characterization was determined by a microhardness tester Matsuzawa MXT-70, and a nano-indenter Agilent Technologies G200. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) tests were carried out using the SDT 600 Thermobalance from TA Instruments. Absorption spectra were determined by using a 1050 Perkin Elmer spectrophotometer, and emission and excitation spectra were obtained by exciting the sample with a 3900S Spectra Physics Ti-sapphire. The fluorescence was analysed with a SpectraPro-500i monochromator. Lifetime measurements were obtained by exciting the samples with a pulsed Ti-sapphire laser and recording the decay times with an oscilloscope. Density was measured by Archimedes method and refractive index by dark mode technique.

3. Results and discussion

3.1 Compositional and microstructural characterization

Semi-quantitative chemical composition analysis of the glass samples was realised by EDX microanalysis to verify that the composition of the processed samples was around the theoretical composition. Table 1 shows the amount of oxides present in each sample. Composition of the samples was close to the nominal value. The amorphous nature of the samples was confirmed by XRD.

Table 1. Theoretical (T) and experimental composition (E) of the ceramic glasses in at%.

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3.2 Thermo-mechanical characterization

Table 2 presents the thermo-mechanical properties for the three glass samples. Mechanical properties were assessed in terms of Vickers hardness, HV, and toughness, K_IC. Furthermore, Young’s elastic modulus was determined from nano-indentation tests. Vickers hardness tests performed in the three glasses showed hardness values ranging between 5.68 and 7.37 GPa. Toughness was evaluated indirectly by indentation method making direct measurements of crack lengths created by the diamond indenter during hardness test [26]. Recorded values were between 0.77 and 1.24 MPa m^1/2. Nano-indentation tests carried out on the three samples reported Young’s modulus between 82.10 and 99.90 GPa. With regards to the thermal characterization, DTA analyses performed on the samples reported glass transition temperatures, Tg, and peak crystallization temperatures, Tc, between 795 and 823 °C, and 1095 and 1101 °C respectively. It is worth mentioning that Ca-Glass did not show peak crystallization temperature. The three samples showed similar thermo-mechanical properties, and in the same range of magnitude if compared to other aluminosilicate glasses [7,9,11].

Table 2. Mechanical and thermal properties of Ca₃Al₂Si₃O₁₂, Ca_1.5Mg_1.5Al₂Si₃O₁₂ and Mg₃Al₂Si₃O₁₂ glasses.

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3.3 Optical characterization

3.3.1 Absorption properties

Room temperature absorption spectra were obtained for the three samples in the 200-1000 nm range. Figure 1 shows the absorption cross-section spectra as a function of wavelength for the glass samples doped with 1.049x10²⁰, 1.014x10²⁰ and 1.018x10²⁰ at./cm³ of Nd³⁺ ions for Ca-Glass, CaMg-Glass and Mg-Glass respectively. The absorption bands correspond to the 4f-4f optical excitations between the ⁴I_9/2 ground level and their excited states of the Nd³⁺ ions. The absorption spectra show an absorption edge shift to the UV range for the Ca-Glass with respect to the Mg-Glass which is related to the higher polarizability of the Ca²⁺ ions [27]. Worth noting is that the absorption edge of the intermediate glass is placed between the extreme compositions.

 

Fig. 1 Room temperature absorption cross-section spectra as a function of wavelength for Ca₃Al₂Si₃O₁₂, Ca_1.5Mg_1.5Al₂Si₃O₁₂ and Mg₃Al₂Si₃O₁₂ glasses doped with Nd³⁺ ions.

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Generally speaking absorption and emission spectra of RE-doped glasses show spectral shifts which depend on the choice of the glass composition and the degree of covalent bonding of the RE ion so that the spectroscopic properties of RE ions change with glass composition. Spectral shifts arise from the nephelauxetic or cloud expanding effect, where the red shift means an increase of the covalency related to the expansion of the 4f orbital cloud [28–30]. They are generally affected most by the glass network former. However, within a given glass composition, further variations are possible by the selection of the network-modifier cations [29,30].

The estimation of the covalency between the rare-earth ion and its ligand can be carried out by means of the shift of the absorption peak wavenumber of the 4f-4f transitions [30,31]. For this purpose and for the case of Nd³⁺-doped glasses, the shift of the hypersensitive transition ⁴I_9/2→⁴F_7/2,⁴S_3/2 is selected. Conversely to other rare-earth hypersensitive transitions, ⁴I_9/2→⁴F_7/2,⁴S_3/2 transition of Nd³⁺ shows two peaks by the Stark splitting so that the peak wavenumber cannot be determined correctly. In this case, the relative intensity ratio between the peaks is used as covalency parameter of Nd³⁺-ion sites since this ratio varies with glass composition [29,30]. Figure 2 shows the normalized ⁴I_9/2→⁴F_7/2,⁴S_3/2 absorption transition, where a spectral shift and a difference in the relative peak intensity ratio is observed. Peak intensities, designated as I_S and I_L, correspond to the peak intensities at the shortest and longest wavelengths. Increase of the ratio I_L/I_S indicates a shift of the centre of gravity of the absorption spectra to lower energies, i.e., longer wavelengths. This red shift means an increase in the covalency of the Nd-O bond [30]. In particular, the I_L/I_S ratio is 1.06 and 1.04 for the Ca-Glass and Mg-Glass respectively so that the covalency of the Nd-O bond is higher for the Ca-Glass than for the Mg-Glass.

 

Fig. 2 Normalized ⁴I_9/2→⁴F_7/2,⁴S_3/2 absorption transition for Ca₃Al₂Si₃O₁₂, and Mg₃Al₂Si₃O₁₂ glasses doped with Nd³⁺ ions.

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3.3.2 Judd-Ofelt parameters and emission properties

The forced electric-dipole transition probability between 4f levels of rare-earth ion is estimated from absorption spectra on the basis of Judd-Ofelt theory [32,33]. Transition oscillator strengths can be experimentally determined from the optical absorption spectra using the equation

where m and e are the electron mass and charge respectively, c is the light velocity, α (λ) the optical absorption coefficient as a function of wavelength λ and N is the concentration of the absorbing ions. Table 3 shows such oscillator strengths for the three compositions at 300 K together with the assignment of the bands to particular transitions.

Table 3. Experimental (f_exp) and calculated (f_cal) oscillator strength of Nd³⁺ in Ca₃Al₂(SiO₄)₃, Mg₃Al₂(SiO₄)₃ and Ca_1.5Mg_1.5Al₂(SiO₄)₃ glasses.

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According to the Judd-Ofelt theory, the oscillator strength f (J, J’) of the J→J’ transition occurring at the mean frequency ν is given by

where n is the refractive index, X_ed = n(n² + 2)²/9 is the local field correction for electric-dipole transitions and X_md = n³ for magnetic-dipole ones. Magnetic-dipole transitions give only a negligible contribution to the transition bands of Nd³⁺ so that they are not taken into account [34,35].

The electric-dipole strength is given by

where Ω₂, Ω₄ and Ω₆ are the Judd-Ofelt, JO, intensity parameters. The reduced matrix elements <\\U^(t)\\> are almost insensitive to the environment and the values provided by Carnall et al. have been used [35]. Intensity parameters Ω₂, Ω₄ and Ω₆, obtained by the least-squares method, are shown in Table 4 and compared to those obtained for other silicate glasses in which the alkali and alkaline-earth oxide modifiers have been exchanged between Li⁺, Na⁺, K⁺ and Mg²⁺ and Ca²⁺ respectively. The calculated oscillator strength values are also compared to the experimental ones in Table 3.

Table 4. Judd-Ofelt intensity parameters (in 10⁻²⁰cm²) for Nd³⁺ in Ca₃Al₂(SiO₄)₃, Mg₃Al₂(SiO₄)₃ and Ca_1.5Mg_1.5Al₂(SiO₄)₃ glasses.

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The JO intensity parameters contain implicitly odd-symmetry crystal field terms, radial integrals, and perturbation denominators. These coefficients, regarded as phenomenological parameters, are usually used to evaluate spontaneous emission probability, induced-emission cross-section, quantum efficiency, and branching ratios. The close relationship between JO parameters and the nature of the glass host allows tailoring, within limits, these optical properties based on the suitable choice of the glass composition. JO parameters are related to local structures in the vicinity of rare-earth ions and/or the covalency of the rare-earth-ion site [12,13,28–30,36–42]. For instance, Ω₂ is sensitive to the local structures in the vicinity of rare-earth ions relating to the anion structures of host glasses and reflects the asymmetry around the rare-earth ions. However, the spontaneous emission probabilities are independent of Ω₂. On the other hand, Ω₆ is of particular interest because this parameter affects the values of the calculated spontaneous emission probabilities for the ⁴F_3/2→⁴I_11/2 transition of Nd³⁺. The Ω₄ parameter is affected by the factors causing changes in both Ω₂ and Ω₆. Especially, if the changes of Ω₂ and Ω₆ are opposite, the resolution of the respective effects on Ω₄ is difficult. Therefore, Ω₂ and Ω₆ are mostly studied for local structure investigations [42]. With regards to the Ω₆ parameter, this intensity parameter is insensitive to local structures but related to the covalency of the rare-earth-ion site [13,28–30,36–41]. Tanabe et al. stated that Ω₆ of Nd³⁺ and of Er³⁺ decreased with increasing the covalency of the rare-earth-ion sites in oxide glasses [39,40]. On the contrary, Takebe et al. reported that in silicate, borate and phosphate glasses Ω₆ of Nd³⁺ increased and Ω₆ of Er³⁺ decreased when the covalency between the rare-earth ion and the oxygen ion increased [29,30,41].

Despite the numerous works on compositional dependence of intensity parameters, the correlation between intensity parameters and glass structure has not yet been completely clarified. As an example, Table 4 shows that Ω₂ increases in the order Ca < Mg independently of the alkali or Al₂O₃ network-modifier used. However, there is not any trend in the increasing order for Ω₄ and Ω₆. In the Ca-Glass and Mg-Glass studied in this work, Ω₄ and Ω₆ increase in the order Mg < Ca as in the silicate glasses reported by Izumitani et al. [36]. On the contrary, the values reported by Nageno et al. [29] show that there is not any pattern in the variation of Ω₄ and Ω₆ with the type of the alkali oxide. In the glasses with Li⁺ and K⁺ the increasing order for Ω₄ is Ca < Mg, but in the glass with Na⁺ the increasing order is Mg < Ca. With regards to Ω₆, in Li⁺ and Na⁺ glasses the increasing order is Ca < Mg but the in glass with K⁺ the increasing order is the contrary. Moreover, the increasing order for Ω₆ reported by Nageno et al. for Na⁺ glasses mismatches the values reported by Izumitani et al. for the same glass system.

According to Nageno et al. Ω₂ is independent of the I_L/I_S intensity ratio, i.e., the covalency of the Nd-O bond. Furthermore, it is well known that Ω₂ is sensitive to local structure in the vicinity of Nd³⁺ ions. In this way, in ternary alkali alkaline-earth glasses, Ω₂ increases monotonically with increasing difference in the field strength between alkali and alkaline-earth ions. So the distortion in the vicinity of Nd³⁺ ions is increased due to the combination of alkali and alkaline-earth oxides, and as a result Ω₂ is increased. This behaviour is also observed in the Mg-Glass and Ca-Glass, as shown in Table 4, but in this case is due to the higher field strength between aluminate and Mg²⁺ ion.

On the other hand, Ω₆ behaviour is related to the covalency of the rare-earth-ion site in glasses. The absorption peak wavenumber of the transitions whose intensities are determined mainly by Ω₆, such as the ⁴I_9/2→⁴F_7/2,⁴S_3/2 for the Nd³⁺, is shifted systematically with glass composition, due to the nephelauxetic effect. As seen in the previous section, the covalency of the Nd-O bond increased with increasing the intensity ratio I_L/I_S. Furthermore, according to Takebe et al., Ω₆ increased with the covalency of the Nd-O bond. These results are in agreement for the JO parameters Ω₆ obtained in this work, since Ω₆ is higher for the Ca-Glass than for the Mg-Glass, as shown in Table 4 and hence the covalency of the Nd-O bond is higher for the Ca-Glass.

In addition, the covalency of the rare-earth-ion site can also be correlated to the ionic packing ratio, V_p, which can be expressed as [29]

where r_i is the ionic radius taken as the effective ionic radius r(O^2-) = 0.138 nm [43], n_i is the molar fraction, N_A is the Avogadro number, V_m = M/ρ is the molar volume, M is the molar weight, and ρ is the density of the glass host. This calculation gives an ionic packing ratio V_P of 55.9% and 58.7% for the Mg-Glass and Ca-Glass respectively. The ionic packing ratio, which is a macroscopic property of a glass host, greatly affects Ω₆ and the spontaneous emission probabilities of Nd³⁺ and Er³⁺ in silicate and borate glasses [29,31]. In particular, Ω₆ parameter and the total spontaneous emission probability increase with increasing ionic packing ratio which corresponds to an increase in the covalency of the Nd-O bond. This result is also in agreement with the obtained in this work since V_P is higher for the Ca-Glass, confirming the higher covalency of the Nd-O in the Ca-Glass.

Once Ω_i are obtained, the evaluation of the system for optical applications in terms of radiative transition probabilities, emission branching ratios, radiative lifetimes, quantum efficiency and peak induced-emission cross-section can be carried out.

The radiative transition probability from the initial manifold |(S,L)J〉 to the terminal manifold |(S’,L’)J’〉 is given by

Fluorescence branching ratios for transitions originating on a specific initial manifold |(S,L)J〉 are defined by

where the summation is over all terminal manifolds |(S’,L’)J’〉. The sum represents the total transition probability for radiative decay from the initial manifold. Obtained calculated values are listed in Table 5. Fluorescence branching ratio to ⁴I_11/2 level should be as favourable as possible in order to optimise laser action. Since intensity of ⁴F_3/2→⁴I_11/2 laser transition is dependent only on the Ω₄ and Ω₆ parameters because of the triangle rule [44], the branching ratio can be expressed in terms of the ratio Ω₄/Ω₆ so that the optimal intensity to ⁴I_11/2 level is obtained for the lower Ω₄/Ω₆ ratio [13]. For the Mg-Glass and Ca-Glass this ratio is 0.92 and 1.26 respectively. As consequence of the lower Ω₄/Ω₆ ratio for the Mg-Glass, the branching ratio to the ⁴I_11/2 level is higher than for the Ca-Glass, specifically a 47.7 and 44.5% respectively, Table 5.

Table 5. Radiative transitions probabilities and branching ratios of Nd³⁺ in Ca₃Al₂(SiO₄)₃, Mg₃Al₂(SiO₄)₃ and Ca_1.5Mg_1.5Al₂(SiO₄)₃ glasses.

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Radiative lifetime is related to the total radiative transition probabilities A_rad by its reciprocal

obtaining values of 294, 410 and 445 μs for the Ca-Glass, CaMg-Glass and Mg-Glass respectively. The radiative quantum efficiency of the |(S,L)J〉 manifold defined as

where τ_f is the fluorescence lifetime, which is due to all relaxation processes, and was found to be 272, 261 and 273 μs for the Ca-Glass, CaMg-Glass and Mg-Glass respectively giving rise to quantum efficiencies of 0.93, 0.63 and 0.61 for the three compositions respectively. In this way, according to the previous results the higher the covalency of the rare-earth-ion site the shorter the radiative lifetime and the higher the quantum efficiency.

The ⁴F_3/2→⁴I_11/2 steady-state fluorescence spectra at room temperature were measured by exciting the samples at 808 nm in resonance with the ⁴I_9/2→⁴F_5/2,²H_9/2 absorption band. Figure 3 shows the differences in the normalized emission spectra for the three samples in terms of fluorescence peak wavelength, λ_p, and effective bandwidth, Δλ_eff, the latter obtained by integrating the fluorescence band and dividing by the peak wavelength. The obtained fluorescence bands were inhomogeneously broadened due to site-to-site variations in the local field seen by rare-earth ions in the glass, and asymmetric, with a shift to high energy values in the emission maximum for the Mg-Glass compared to Ca-Glass. These features yield to a shorter effective bandwidth, which is a measure of the overall extent of the Stark splitting of the J manifolds. In particular, Mg-Glass, CaMg-Glass and Ca-Glass peak wavelengths were placed at 1060, 1061 and 1062 nm, with effective bandwidths of 42, 43 and 44 nm respectively, Table 6.

 

Fig. 3 Normalized fluorescence spectra of the ⁴F_3/2→⁴I_11/2 laser transition at room temperature measured by exciting the samples at 808 nm.

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Table 6. Room Temperature emission properties of Nd³⁺ in Ca₃Al₂(SiO₄)₃, Mg₃Al₂(SiO₄)₃ and Ca_1.5Mg_1.5Al₂(SiO₄)₃ glasses.

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The stimulated emission cross-section of the laser transition can be determined from spectral parameters and from the calculated radiative transition probability by making use of [13]

where λ_p is the fluorescence peak wavelength, n is the refractive index, Δλ_eff is the effective bandwidth of the ⁴F_3/2→⁴I_11/2 transition, and A[(⁴F_3/2);(⁴I_11/2)] is the radiative transition probability for this transition. The stimulated emission cross-sections for the ⁴F_3/2→⁴I_11/2 transition for the three samples are presented in Table 6. The higher cross-section of the Ca-Glass is because large emission cross-section requires Ω₄ and Ω₆ intensity parameters as large as possible and, as shown in Table 4, this glass has the largest intensity parameters. According to these results the higher the covalency of the rare-earth-ion site the higher effective bandwidth and emission cross-section. Furthermore, the estimation of the covalency between the rare-earth ion and its ligand can also be carried out by means of the peak wavelength shift of the fluorescence spectrum of Nd³⁺. Jacobs and Weber studied the differences in the ⁴F_3/2→⁴I_11/2 fluorescence spectra for several glass network formers and network-modifier ions. They reported that the shift of the fluorescence peak wavelength to longer wavelengths was caused by the increase of the covalency between the Nd³⁺ ion and the oxygen ion in oxide glasses, i.e., the so called nephelauxetic effect. However, They did not find any variation in the fluorescence peak wavelength for alkaline-earth alkali ternary silicate glasses when the alkaline-earth additive was exchanged between Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ ions [13]. Furthermore, they found that the effective bandwidth decreased in the order Ca < Mg, conversely to the results found in this work, see Table 6. Brecher et al. also studied the correlation between the nephelauxetic effect with the fluorescence peak wavelength shift by comparing the fluorescence line narrowing spectra between oxide and fluoride glasses [45]. However, there have not been found previous works, to our best knowledge, in which the exchange of the alkaline-earth cations in the same glass network former had been used to explain the correlation between fluorescence peak shift and nephelauxetic effect. This work shows for the first time that in aluminosilicate network former the modification of the alkaline-earth network-modifier gives rise to a fluorescence peak wavelength shift so that it can be used to assess the covalency of the Nd-O bond. In particular, the red shift observed in the fluorescence peak wavelength for the Ca-Glass with regards to the Mg-Glass indicates the higher covalency of the Nd-O bond in the Ca-Glass, in good agreement with the results obtained in previous sections.

4. Conclusions

In this work Ca₃Al₂Si₃O₁₂, Mg₃Al₂Si₃O₁₂ and Ca_1.5Mg_1.5Al₂Si₃O₁₂ glasses doped with Nd³⁺ ions were grown by the laser floating zone technique. Thermo-mechanical and spectroscopic characterization were carried out in order to establish their properties as laser matrix and to used them as starting point before realizing laser synthesis processes in their inside.

The three glass samples showed good thermo-mechanical properties, with similar hardness, toughness and glass transition temperature, and in the same range when compared to other aluminosilicate glasses. Optical characterization revealed absorption and fluorescence spectral shifts arisen from the nephelauxetic effect. These spectral shifts together with Judd-Ofelt intensity parameters and ionic packing ratio were used to study the covalency of the Nd-O bond in the glass samples. The close relationship between intensity parameters and the nature of the glass host allowed evaluating the local structure surrounding the Nd³⁺ ions and the covalency of the Nd-O bond. The higher distortion in the vicinity of Nd³⁺ ions caused by the higher field strength between aluminate and Mg²⁺ ion was explained in terms of the increasing of Ω₂ intensity parameter. The higher values obtained for Ω₆ intensity parameter and ionic packing ratio in the Ca₃Al₂Si₃O₁₂ glass agree with the higher covalency of the Nd-O site in this glass. Emission properties were also calculated for the three glass samples. The higher the covalency of the rare-earth-ion site the shorter the radiative lifetime and the higher the quantum efficiency obtained. Furthermore, the broader effective bandwidth and the higher emission cross-section were also found in the Ca₃Al₂Si₃O₁₂ glass.