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

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 (Tg ≥ 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 4G5/2 translated to the lower states of 6H5/2, 6H7/2, 6H9/2, and 6H11/2, 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.


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 Al 2 O 3 -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 Al 2 O 3 -SrO glassy systems would promote the understanding of its multifunctional properties, such as the thermal, mechanical and optical properties.
Sm 3+ is well known as an important rare earth activator that exhibits unique properties, such as strong reddish-orange emission due to its 4 G 5/2 → 6 H J (J = 5/2, 7/2, 9/2, 11/2) transition, high quantum and luminescence efficiencies. Sm 3+ ions doped glasses, such as single alkali and mixed alkali fluoro tungsten tellurite glasses [12], sodiumfluoro-phosphate glasses [13], Li 2 O-MO-B 2 O 3 (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 Sm 3+ 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 Sm 3+ ions into the Al 2 O 3 -SrO glassy system is of great significance for developing new optical devices with specific utility and enhanced performance.
In this work, we focused on Sm 3+ doped Al 2 O 3 -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., ∆T) 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 Sm 3+ -ligand bond in the glass can be identified. Moreover, based on the emission spectra, the 4 G 5/2 → 6 H 7/2 (607 nm) transition exhibits the maximum intensity, indicating that the Sm 3+ doped Al 2 O 3 -SrO glasses (namely Al 2 O 3 -(3-x)SrO: xSm 3+ glasses) show a bright orange-reddish emission under an UV source. In addition, a moderate amount of Sm 3+ 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.

Experimental Procedure
Sm 3+ doped Al 2 O 3 -(3-x)SrO: xSm 3+ (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 Al 2 O 3 , SrO and Sm 2 O 3 (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 CO 2 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).

Structural Properties
Considering the similar structural characteristics of Al 2 O 3 -(3-x)SrO: xSm 3+ 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 Al 2 O 3 -(3-x)SrO: xSm 3+ 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 Al 2 O 3 -(3-x)SrO: xSm 3+ samples. These results suggest that uniform transparent Al 2 O 3 -(3-x)SrO: xSm 3+ glasses could be successfully prepared by a containerless process.   Figure 3 depicts the Vicker's hardness and density (ρ) of Al 2 O 3 -(3-x)SrO: xSm 3+ glasses. Both Vicker's hardness and density of the as-prepared glasses increased with rising Sm 3+ content, with values from 5.01 to 7.03 GPa and 3.99 to 4.12 g/cm 3 for x = 0 to 0.2, respectively. The density of Sm 2 O 3 (8.35 g/cm 3 ) is larger than the SrO (4.70 g/cm 3 ), content, and hence, the substitution of the lighter SrO by the heavier Sm 2 O 3 can lead to an increase in density. In addition, the ionic radius of Sm 3+ (96 pm) is slightly smaller than Sr 2+ (118 pm). The replacement of Sr 2+ ions by Sm 3+ 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 Sm 3+ doped oxide glasses [21] and exhibits good potential for mechanical applications.

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 Al 2 O 3 -(3-x)SrO: xSm 3+ (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 ∆T (defined as ∆T = T x − T g ), are summarized in Table 1. ∆T has been used to evaluate glassforming ability, and higher values of ∆T 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 Sm 2 O 3 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 Sm 3+ 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 Sm 2 O 3 content, which leads to the widening of the supercooled liquid region. For instance, ∆T 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 Sm 2 O 3 is followed, such as x = 0.06, 0.1 and 0.2, the values of ∆T increased significantly and reached 99 • C, 121 • C and 127 • C, respectively. The booming supercooled liquid region ∆T well confirmed the prosperous glass-forming ability when SrO is partially replaced by Sm 2 O 3 . Therefore, a suitable amount of Sm 2 O 3 modified Al 2 O 3 -SrO glasses exhibit high thermal stability and glass-forming ability, which are of benefit to their applications in coating fields [4]. x  Figure 5 presents the absorption spectra of Al 2 O 3 -(3-x)SrO: xSm 3+ (x = 0, 0.01, 0.03, 0.06, 0.1, 0.2) glasses in the UV-Vis-NIR region. In the current investigation, all Sm 3+ ions doped glasses exhibit nearly the same absorption spectra, with several inhomogeneous bands corresponding to the characteristic f-f transitions from the ground state 6 H 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 4 D 3/2 , 6 P 7/2 , 6 P 3/2 , 6 F 9/2 , 6 F 7/2 , 6 F 5/2 , 6 F 3/2 and 6 H 15/2 transitions, respectively, similar to those reported in other Sm 3+ 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 Sm 3+ 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 6 H 5/2 → 6 F 7/2 seems to be stronger in the NIR region, whereas, in the UV-Vis region, the transition 6 H 5/2 → 6 P 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 Sm 3+ ions in the as-prepared samples and investigate the nature of Sm 3+ -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 RE 3+ 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 β using the following expression [31]:

Absorption Spectra and Nephelauxetic Effect
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 RE 3+ -ligand bond. The calculated values of β and δ for the Al 2 O 3 -(3-x)SrO: xSm 3+ glasses are shown in Table 2. The negative δ values reflect the ionic nature of the prepared Sm 3+ -doped glasses and the ionicity gradually decreases with the increase of Sm 3+ ions concentration, which indicates that a higher amount of Sm 3+ results in altering the dominant form of bonding. A similar ionic bonding nature has also been observed in Sm 3+ -doped zinc fluorophosphate glasses [32], sodium potassiumborate glasses [27] and zinc alumino bismuth borate glasses [33].

Photoluminescence Properties
The excitation spectra are often used to extract the efficient luminescence properties and recognize the higher energy levels of Sm 3+ ions. Figure 6 exhibits the photoluminescence excitation spectra of Al 2 O 3 -(3-x)SrO: xSm 3+ 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 Sm 3+ ions are observed at 316, 344, 360, 375, 404, 420, 470 and 488 nm, corresponding to the transitions from the ground state 6 H 5/2 to 4 P 3/2 , 4 D 7/2 , 4 D 3/2 , 6 P 7/2 , 6 P 3/2 , 6 P 5/2 , ( 4 I 13/2 + 4 I 11/2 ) and 4 I 9/2 , respectively [19,34]. Clearly, the highest intensity of the excitation spectra is the 6 H 5/2 → 6 P 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 Al 2 O 3 -(3-x)SrO: xSm 3+ glasses excited at 404 nm ( 6 H 5/2 → 6 P 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 4 G 5/2 → 6 H 5/2, 6 H 7/2 , 6 H 9/2 and 6 H 11/2 , respectively, which are similar to the emission characteristics of the other Sm 3+ doped glasses [13,35]. Among these four transitions, the 4 G 5/2 → 6 H 7/2 (607 nm) transition exhibits the maximum intensity, indicating that the Sm 3+ doped Al 2 O 3 -(3-x)SrO: xSm 3+ 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 4 G 5/2 → 6 H 7/2 transition is found to increase with the rising content of Sm 3+ ion up to x = 0.03, and then a luminescence quenching behavior is observed beyond the critical Sm 3+ 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 Sm 3+ -Sm 3+ 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 Sm 3+ emission decay curves of Al 2 O 3 -(3-x)SrO: xSm 3+ glasses for 4 G 5/2 → 6 H 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 Sm 3+ 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: where I is the luminescence intensity at time t, I 0 is the initial emission intensity, A 1 , A 2 are scalar constants obtained from the curve fitting, and τ 1 , τ 2 are the lifetimes related to the fast and slow decays. The average values of decay time (τ av ) of 4 G 5/2 excited level of Sm 3+ ions can be calculated as [37,38]:  [39]. The τ av value decreases with the rising concentration of Sm 3+ ions. Quenching of the 4 G 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 Sm 3+ ions [40]. 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 Sm 3+ doped glasses [15,27,37].

Conclusions
Sm 3+ doped Al 2 O 3 -(3-x)SrO: xSm 3+ (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 Sm 3+ ions augmented the emissions first and then reduced them when x reached 0.03, verifying the concentration quenching effect. A bright orangereddish luminescence was observed in Al 2 O 3 -(3-x)SrO: xSm 3+ glasses, indicating that these glasses are promising materials for developing visible orange-reddish lasers and smart photoluminescent coating films.  Data Availability Statement: Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.