Investigations of Thermal Stability and Spectroscopic Features of Sm³⁺ Doped Strontium Aluminate Glasses

Abstract

In the present work, a series of Sm³⁺ doped transparent strontium aluminate glasses with the composition Al₂O₃-(3-x)SrO: xSm³⁺ (x = 0, 0.01, 0.03, 0.06, 0.1, 0.2) were fabricated by a containerless process using an aerodynamic levitation furnace. The structural characteristics, density, Vicker’s hardness, and thermal and spectroscopic behaviors of these glasses were investigated. All the glasses exhibit excellent thermal stabilities (T_g ≥ 792 °C) and the glass-forming ability is enhanced with the increasing content of Sm³⁺. The emission spectra recorded under an excitation of 404 nm show four emission transitions as a result of ⁴G_5/2 translated to the lower states of ⁶H_5/2, ⁶H_7/2, ⁶H_9/2, and ⁶H_11/2, and a bright orange-reddish luminescence can be observed in Al₂O₃-(3-x)SrO: xSm³⁺ glasses. The high thermal stability, good glass-forming ability and excellent hardness provide new options for the development of visible orange-reddish lasers and smart photoluminescent glass coating materials.

1. Introduction

Oxide glasses containing luminescent rare-earth (RE) ions have been widely investigated due to their excellent photoluminescence properties, outstanding optical properties and high thermal and chemical stabilities. Numerous applications as functional photonic devices [1,2,3] and smart glass coating materials [4,5] have been carried out in these glasses. However, the spectroscopic properties of RE ions in these glasses are susceptible to the surrounding environment and the distribution of ions doped in the glassy matrix [6]. Aluminate-based materials have been widely concerned for their high stability, quantum efficiency, high transparency in the UV-Vis range and decent mechanical property [4,5,7,8,9]. Studies have shown that the presence of alkaline earth is of great benefit to the chemical resistance of the glass substrate [10], and alkaline earth aluminate-based luminescent materials have been extensively applied as host materials. It was noticed that the Al₂O₃-SrO system can form solid solutions with various metal oxides, which results in certain modifications in crystal structure and optical properties [11]. Therefore, in-depth study of RE-doped Al₂O₃-SrO glassy systems would promote the understanding of its multifunctional properties, such as the thermal, mechanical and optical properties.

Sm³⁺ is well known as an important rare earth activator that exhibits unique properties, such as strong reddish-orange emission due to its ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) transition, high quantum and luminescence efficiencies. Sm³⁺ ions doped glasses, such as single alkali and mixed alkali fluoro tungsten tellurite glasses [12], sodiumfluoro-phosphate glasses [13], Li₂O-MO-B₂O₃ (M = Mg/Ca/Sr/Ba) glasses [14], lead fluoro-borophosphate glasses [15], borate glasses [16], etc., have been investigated for decades to explore their structural, spectroscopic and luminescent properties. Numerous applications of Sm³⁺ doped glasses in the field of visible solid state lasers, optical memory devices, submarine communication and display devices have been developed [17,18,19]. Predictably, the introduction of Sm³⁺ ions into the Al₂O₃-SrO glassy system is of great significance for developing new optical devices with specific utility and enhanced performance.

In this work, we focused on Sm³⁺ doped Al₂O₃-SrO glasses to explore the effect of samarium concentration on the structural, mechanical and thermal stability, as well as the absorption and emission spectra of these glasses. The difference between the crystallization temperature T_x and glass transition temperature T_g (i.e., ΔT) was measured to evaluate the glass-forming ability. Additionally, the nephelauxetic effect β and bonding parameters δ were analyzed from the absorption spectral features, from which the nature of Sm³⁺-ligand bond in the glass can be identified. Moreover, based on the emission spectra, the ⁴G_5/2 → ⁶H_7/2 (607 nm) transition exhibits the maximum intensity, indicating that the Sm³⁺ doped Al₂O₃-SrO glasses (namely Al₂O₃-(3-x)SrO: xSm³⁺ glasses) show a bright orange-reddish emission under an UV source. In addition, a moderate amount of Sm³⁺ doping can enhance the Vicker’s hardness (with a value of 7.03 GPa) and thermal stability (with high T_g values of over 792 °C) of these glasses, which provide more options for the manufacture of novel visible orange-reddish lasers and smart photoluminescent glass coatings.

2. Experimental Procedure

Sm³⁺ doped Al₂O₃-(3-x)SrO: xSm³⁺(x = 0, 0.01, 0.03, 0.06, 0.1, 0.2) luminescent glasses were successfully prepared by a containerless process using an aerodynamic levitation furnace (ALF, Shanghai Institute of Ceramics, Shanghai, China). Stoichiometric amounts of high purity (>99.9%) raw materials of Al₂O₃, SrO and Sm₂O₃ (China New Metal Materials Technology Co., Ltd., Beijing, China) were milled for two hours with ZrO balls using alcohol as a medium. The speed of the ball mill is 200 rpm, and the quality of the ball mill is about 5 times that of the raw material. After being blended well, the mixture of about 1 g was compacted into disks of diameter 10 mm, melted by a CO₂ laser device (ALF, Shanghai Institute of Ceramics, Shanghai, China) and levitated by oxygen gas flow. The samples were kept in a molten state for ~20 s to ensure homogenization. Then, the melts were rapidly cooled down to room temperature by shutting off the laser power, and transparent glassy-spheres with a diameter of approximately 2–4 mm were obtained. Part of the glassy spheres were carefully polished into disks for structural and performance analyses.

The structures of the samples were examined by X-ray diffraction (XRD, Smartlab, Rigaku D/MAX/2500/PC, Tokyo, Japan) analysis. A high-resolution TEM (HRTEM) image and the electron diffraction pattern were observed by transmission electron microscopy (TEM, Tecnai G2 F20, FEI, Phoenix, AZ, USA). Bulk densities of the glasses were measured by the Archimedes method. Vicker’s hardness was tested for each sample via a digital hardness tester (MHV-50Z/V2.0, Sctmc, Beijing, China) equipped with a Vickers indenter with an applied load of 500 g for 10 s. Differential thermal analysis (DTA) was obtained by using an Evolution system (Setsys, Setaram, Lyon, France) at a heating rate of 10 °C/min in the temperature range of 600 °C to 1000 °C in an argon atmosphere. Optical absorption spectra in the wavelength range of 300–1800 nm were detected by an UV-Vis spectrometer (Cary5000, Varian, Palo Alto, CA, USA). The luminescence properties and fluorescence decay curves were measured by fluorescence spectrofluorometers (F-7000, Hitachi, Tokyo, Japan, and F-7100, Hitachi, Tokyo, Japan, respectively).

3. Results and Discussion

3.1. Structural Properties

Considering the similar structural characteristics of Al₂O₃-(3-x)SrO: xSm³⁺ glasses, typical XRD patterns of selective x = 0, 0.06 and 0.2 are shown in Figure 1. It can be seen that each sample exhibits a broad diffusive diffraction peak centered at 2θ ≈ 31°, indicating the amorphous nature of Al₂O₃-(3-x)SrO: xSm³⁺ glasses. In order to obtain a high-resolution image and further confirm the structural characteristics, the HRTEM micrograph and the corresponding electron diffraction pattern of a randomly selected composition of x = 0.03 are captured and shown in Figure 2. It is observed that no significant grains, second phases or residual pores are detected in Figure 2a and only a broad diffuse halo is presented in Figure 2b, which further confirms the complete glassy state of Al₂O₃-(3-x)SrO: xSm³⁺ samples. These results suggest that uniform transparent Al₂O₃-(3-x)SrO: xSm³⁺ glasses could be successfully prepared by a containerless process.

3.2. Density and Vicker’s Hardness

Figure 3 depicts the Vicker’s hardness and density (ρ) of Al₂O₃-(3-x)SrO: xSm³⁺ glasses. Both Vicker’s hardness and density of the as-prepared glasses increased with rising Sm³⁺ content, with values from 5.01 to 7.03 GPa and 3.99 to 4.12 g/cm³ for x = 0 to 0.2, respectively. The density of Sm₂O₃ (8.35 g/cm³) is larger than the SrO (4.70 g/cm³), content, and hence, the substitution of the lighter SrO by the heavier Sm₂O₃ can lead to an increase in density. In addition, the ionic radius of Sm³⁺ (96 pm) is slightly smaller than Sr²⁺ (118 pm). The replacement of Sr²⁺ ions by Sm³⁺ ions decreases the spatial distance between ions, which is inversely proportional to hardness [20] and results in the rise of Vicker’s hardness. The highest value of Vicker’s hardness is 7.03 GPa for x = 0.2, which is larger than the reported Sm³⁺ doped oxide glasses [21] and exhibits good potential for mechanical applications.

3.3. Thermal Properties

Thermal stability determines the service conditions of glasses. The differential thermal analyses (DTA) curves were measured to study the thermal stability, glass-forming ability and crystallization behavior of Al₂O₃-(3-x)SrO: xSm³⁺ (x = 0, 0.01, 0.03, 0.06, 0.1, 0.2) glasses, as shown in Figure 4. The intersection of tangents overlaid across the endothermic peaks is specified as the glass transition temperature T_g, and the exothermic peak is assigned to the crystallization temperature T_p. The detailed T_g, crystallization onset temperature T_x, crystallization temperatures T_p1 and T_p2, as well as the supercooled liquid region ΔT (defined as ΔT = T_x − T_g), are summarized in

Table 1. Thermal properties, T_g, T_x, T_p1, T_p2 and ΔT of Al₂O₃-(3-x)SrO: xSm³⁺ (x = 0, 0.01, 0.03, 0.06, 0.1, 0.2) glasses.

x	Tg (°C)	Tx (°C)	Tp1 (°C)	Tp2 (°C)	ΔT (°C)
0	804	858	885	947	54
0.01	800	854	884	950	54
0.03	798	854	882	963	56
0.06	798	897	925	962	99
0.1	797	918	938	964	121
0.2	792	919	940	971	127

. ΔT has been used to evaluate glass-forming ability, and higher values of ΔT correspond to superior glass-forming ability [22,23]. It can be seen that T_g slightly declines with increasing x, as a sign of thermal stability reduction, which suggests that the substitution of Sm₂O₃ for SrO may weaken the network connectivity of matrix glass. Nevertheless, it can be observed that each T_g of the glasses is still above 792 °C, significantly superior to those Sm³⁺ doped phosphate-based glasses [24], zinc magnesium sulfophosphate glasses [17] and calcium sulfoborophosphate glasses [25], confirming a higher thermal stability. Meanwhile, T_x tends to increase with rising Sm₂O₃ content, which leads to the widening of the supercooled liquid region. For instance, ΔT for x = 0, 0.01 and 0.03 are lower than 60 °C, which means the inferior glass-forming ability of these glasses. When the higher content of Sm₂O₃ is followed, such as x = 0.06, 0.1 and 0.2, the values of ΔT increased significantly and reached 99 °C, 121 °C and 127 °C, respectively. The booming supercooled liquid region ΔT well confirmed the prosperous glass-forming ability when SrO is partially replaced by Sm₂O₃. Therefore, a suitable amount of Sm₂O₃ modified Al₂O₃-SrO glasses exhibit high thermal stability and glass-forming ability, which are of benefit to their applications in coating fields [4].

3.4. Absorption Spectra and Nephelauxetic Effect

Figure 5 presents the absorption spectra of Al₂O₃-(3-x)SrO: xSm³⁺ (x = 0, 0.01, 0.03, 0.06, 0.1, 0.2) glasses in the UV-Vis-NIR region. In the current investigation, all Sm³⁺ ions doped glasses exhibit nearly the same absorption spectra, with several inhomogeneous bands corresponding to the characteristic f-f transitions from the ground state ⁶H_5/2 to various excited states. The absorption peaks centered at about 360, 375, 404, 1066, 1214, 1356, 1460 and 1523 nm can be assigned to the ⁴D_3/2, ⁶P_7/2, ⁶P_3/2, ⁶F_9/2, ⁶F_7/2, ⁶F_5/2, ⁶F_3/2 and ⁶H_15/2 transitions, respectively, similar to those reported in other Sm³⁺ ions doped glasses [26,27]. For the present system, most of the absorption peaks are induced electric dipole contributions with the selection rule ∆J ≤ 6 and a few magnetic dipole transitions followed the selection rule ∆J = 0, ±1. According to Boehm et al. [28], the absorption bands of Sm³⁺ ion can be divided into two groups: a low-energy group in the NIR region and a high-energy group in the UV-Vis region. The transition ⁶H_5/2 → ⁶F_7/2 seems to be stronger in the NIR region, whereas, in the UV-Vis region, the transition ⁶H_5/2 → ⁶P_3/2 appears to be the most intense compared with other transitions in the UV-Vis and NIR regions.

Generally, the nephelauxetic effect results from the expansion of partially filled f-shells and is used to identify the covalency of the RE-O bond in the host matrix [29,30]. To further confirm the absorption transitions of Sm³⁺ ions in the as-prepared samples and investigate the nature of Sm³⁺-ligand bond in the glass, the nephelauxetic ratio β and bonding parameter δ were evaluated. The nephelauxetic ratio is given by the ratio of the observed wave number for a particular absorption transition of the RE³⁺ ion in the host under investigation (ν_c in cm⁻¹) to the same transition of the aquoion (ν_a in cm⁻¹). Then, the bonding parameter δ can be determined from the average nephelauxetic ratio $\overline{\beta }$ using the following expression [31]:

$$\delta =\left(\frac{1-\overline{\beta }}{\overline{\beta }}\right)\times 100$$

(1)

The δ can be presented as a positive or a negative value and determined by the ligand field environment, thus implying the corresponding covalent or ionic nature of RE³⁺-ligand bond. The calculated values of $\overline{\beta }$ and δ for the Al₂O₃-(3-x)SrO: xSm³⁺ glasses are shown in

Table 2. Band positions of observed ν_c and aqueous solution ν_a (cm⁻¹), Nephelauxetic effect (β) and bonding parameters (δ) of Al₂O₃-(3-x)SrO: xSm³⁺ (x = 0.01, 0.03, 0.06, 0.1, 0.2) glasses.

Transition 6H5/2 →	νc (x = 0.01)	νc (x = 0.03)	νc (x = 0.06)	νc (x = 0.1)	νc (x = 0.2)	νa [29]
4D3/2	27,778	27,778	27,701	27,701	27,701	27,700
6P7/2	26,667	26,667	26,667	26,667	26,525	26,750
6P3/2	24,814	24,752	24,691	24,691	24,691	24,950
6F9/2	9390	9363	9372	9372	9381	9200
6F7/2	8244	8230	8251	8237	8237	8000
6F5/2	7380	7375	7375	7375	7375	7100
6F3/2	6849	6854	6849	6854	6854	6630
6H15/2	6566	6570	6562	6566	6566	6508
$\overline{\beta }$	1.01585	1.01504	1.01459	1.01454	1.01400	-
$\delta$	−1.56021	−1.48135	−1.43759	−1.43296	−1.38035	-

. The negative δ values reflect the ionic nature of the prepared Sm³⁺-doped glasses and the ionicity gradually decreases with the increase of Sm³⁺ ions concentration, which indicates that a higher amount of Sm³⁺ results in altering the dominant form of bonding. A similar ionic bonding nature has also been observed in Sm³⁺-doped zinc fluorophosphate glasses [32], sodium potassiumborate glasses [27] and zinc alumino bismuth borate glasses [33].

3.5. Photoluminescence Properties

The excitation spectra are often used to extract the efficient luminescence properties and recognize the higher energy levels of Sm³⁺ ions. Figure 6 exhibits the photoluminescence excitation spectra of Al₂O₃-(3-x)SrO: xSm³⁺ glasses in the region of 300–550 nm under the emission wavelength of 607 nm. The excitation bands arising caused by the f-f transitions of Sm³⁺ ions are observed at 316, 344, 360, 375, 404, 420, 470 and 488 nm, corresponding to the transitions from the ground state ⁶H_5/2 to⁴P_3/2, ⁴D_7/2, ⁴D_3/2, ⁶P_7/2, ⁶P_3/2, ⁶P_5/2, (⁴I_13/2 + ⁴I_11/2) and ⁴I_9/2, respectively [19,34]. Clearly, the highest intensity of the excitation spectra is the ⁶H_5/2 → ⁶P_3/2 transition centered at 404 nm, which is selected as an excitation source for the measurement of emission spectra.

Figure 7 shows the emission spectra of Al₂O₃-(3-x)SrO: xSm³⁺ glasses excited at 404 nm (⁶H_5/2 → ⁶P_3/2) within the spectral range 550–750 nm. The emission spectra consist of potential green, orange-reddish and red emission bands centered at 568, 607, 655 and 712 nm. These bands can be assigned to the emission transition ⁴G_5/2 → ⁶H_5/2, ⁶H_7/2, ⁶H_9/2 and ⁶H_11/2, respectively, which are similar to the emission characteristics of the other Sm³⁺ doped glasses [13,35]. Among these four transitions, the ⁴G_5/2 → ⁶H_7/2 (607 nm) transition exhibits the maximum intensity, indicating that the Sm³⁺ doped Al₂O₃-(3-x)SrO: xSm³⁺ glasses show a bright orange-reddish emission under an UV source. These results are significant for color displays, medical diagnostics and high-density optical data storage [36]. In addition, the photoluminescence intensity relative to ⁴G_5/2 → ⁶H_7/2 transition is found to increase with the rising content of Sm³⁺ ion up to x = 0.03, and then a luminescence quenching behavior is observed beyond the critical Sm³⁺ ion concentration of x = 0.03. Generally, the active ions can aggregate with others along with the increasing concentration and result in the cross-relaxation process between Sm³⁺-Sm³⁺ ions, leading to emission quenching [15].

Information about the decay behaviors and lifetimes of the excited state of rare-earth ions can be provided by analyzing the emission decay curves. The Sm³⁺ emission decay curves of Al₂O₃-(3-x)SrO: xSm³⁺ glasses for ⁴G_5/2 → ⁶H_7/2 (607 nm) transition under 404 nm excitation are displayed in Figure 8. It is noticed that the decay curves deviate from the single exponential law and exhibit non-exponential behavior for all glasses. This non-exponential behavior may be attributed to the energy transfer through cross-relaxation between the Sm³⁺ ions in different sites [37]. The non-radiative decay rate can be evaluated experimentally by combining the lifetime measurement. Given that the best fit is obtained by using bi-exponential expression for all the decay curves, as follows:

$$I=I_0+A_1{e}^{\frac{-t}{{\tau }_1}}+A_2{e}^{\frac{-t}{{\tau }_2}}$$

(2)

where I is the luminescence intensity at time t, I₀ is the initial emission intensity, A₁, A₂ are scalar constants obtained from the curve fitting, and τ₁, τ₂ are the lifetimes related to the fast and slow decays. The average values of decay time (τ_av) of ⁴G_5/2 excited level of Sm³⁺ ions can be calculated as [37,38]:

$$\tau =\frac{A_1{\tau }_1{}^2+A_2{\tau }_2{}^2}{A_1{\tau }_1+A_2{\tau }_2}$$

(3)

The calculated τ_av values of Sm³⁺doped Al₂O₃-(3-x)SrO: xSm³⁺ glasses are given in the inset of Figure 8. As shown, the τ_av values are found to be 0.98 (x = 0.01), 0.69 (x = 0.03), 0.35 (x = 0.06), 0.18 (x = 0.1) and 0.02 ms (x = 0.2), which are in the order of milliseconds, corresponding to the characteristics of the f → f transition of samarium ions [39]. The τ_av value decreases with the rising concentration of Sm³⁺ ions. Quenching of the ⁴G_5/2 lifetime and the non-exponential nature of the decay curves are the characteristic features for the existence of concentration quenching caused by the energy transfer among Sm³⁺ ions [40].

Finally, the CIE (Commission Internationale Eclairage 1931) chromaticity diagram of Al₂O₃-(3-x)SrO: xSm³⁺ (x = 0.01, 0.03, 0.06, 0.1, 0.2) glasses is calculated and shown in Figure 9 to investigate the dominant emission color. The CIE chromaticity coordinates (x, y) are presented as (0.59, 0.41), (0.60, 0.40), (0.60, 0.40), (0.59, 0.41) and (0.58, 0.42) for x = 0.01, 0.03, 0.06, 0.1 and 0.2 in Al₂O₃-(3-x)SrO: xSm³⁺(x = 0.01, 0.03, 0.06, 0.1, 0.2) glasses, respectively, and are located in the orange-reddish region of the visible spectrum. The chromatic color coordinates of the obtained glasses are found to be consistent with other Sm³⁺ doped glasses [15,27,37].

4. Conclusions

Sm³⁺ doped Al₂O₃-(3-x)SrO: xSm³⁺ (x = 0, 0.01, 0.03, 0.06, 0.1, 0.2) transparent glasses with high mechanical hardness were successfully prepared by a containerless process. The excellent glass-forming ability and thermal stability can be detected in these glasses. Elevating the content of Sm³⁺ ions augmented the emissions first and then reduced them when x reached 0.03, verifying the concentration quenching effect. A bright orange-reddish luminescence was observed in Al₂O₃-(3-x)SrO: xSm³⁺ glasses, indicating that these glasses are promising materials for developing visible orange-reddish lasers and smart photoluminescent coating films.