Influence of Mixed Na2O/K2O on Chemical Durability and Spectral Properties of P2O5-Al2O3-BaO-K2O-Na2O-Nd2O3 Phosphate Glasses

A series of 56P2O5-7.5Al2O3-5.9BaO-(28.56-x)K2O-xNa2O-1.51Nd2O3 phosphate glasses with different Na/(Na+K) ratios, which were specially designed for high-power laser application, were prepared by a high-temperature melting method. Except for the density, refractive index, glass transition temperature, and DC conductivity, the chemical durability and spectral properties, as emphasized by high-power and high-energy laser material, were further measured and analyzed. Regarding the chemical durability, the dissolution rates of these glasses do not show an evident mixed alkali effect with increasing the Na/(Na+K) ratio, although the effect is obvious for the glass transition temperature and DC conductivity. To better understand the nature of the dissolution mechanism, the ionic release concentrations of every element are determined. Both Na and K undergo ion exchange, but the ion exchange rate of K is much larger than that of Na. In terms of the spectral properties, the J–O parameters, emission cross-section, radiation lifetime, fluorescence lifetime, effective bandwidth, fluorescence branching ratio, and quantum efficiency are determined from absorption and emission spectra. The trend of Ω2 deviating from linearity indicates that the coordination environment symmetry of Nd3+ ions and the covalence of Nd-O also present an evident mixed alkali effect. The most important finding is that the emission cross-section and fluorescence lifetime of Nd3+ ions at 1053 nm were not affected by the change in the Na/K ratio. According to the above experimental results, the optimized value of the Na/K ratio was determined, based on which the 56P2O5-7.5Al2O3-5.9BaO-(28.56-x)K2O-xNa2O-1.51Nd2O3 glass maintains a high emission cross-section with good chemical durability.


Introduction
Nd-doped phosphate glass has become the preferred laser gain medium for largescale high-power laser systems due to its advantages such as moderate phonon energy, high solubility of rare earth ions, good spectral performance, small nonlinear coefficient, large stimulated emission cross-section, and facile large-scale preparation [1][2][3]. Typical representatives of commercial neodymium glass are the LHG-8 of Hoya company [4], LG-770 of Schott company [5], and N31 neodymium glass of Shanghai Institute of Optics and precision machinery, Chinese Academy of Sciences [6,7]. Generally, neodymium glass consists mainly of (58-62) P 2 O 5 -(8-12) Al 2 O 3 -(12-16) M 2 O-(8-12) MO-(1-2) Nd 2 O 3 . The continuous development of large-scale laser systems has led to higher requirements for laser gain, and the laser gain of neodymium glass is positively correlated with the emission cross-section (σ ems ) and negatively correlated with the nonlinear refractive index (n 2 ). Thus, laser neodymium glass with high σ ems and low n 2 has become the prime focus in this field of research [7]. The largest difference between high-gain N51 glass and N31 is the decrease in the Al 2 O 3 molar content, which causes an increase in the emission cross-section of Nd 3+ ions [8], simultaneously, a decrease in the chemical durability of glass [9][10][11]. Therefore, maintaining the emission cross-section and improving the chemical durability of N51 glass are of significant concern.
The addition of transition metal ions or alkaline earth metals and intermediate oxides [12][13][14] to the glass matrix can improve its chemical durability. However, controlling the chemical durability of glasses without significantly affecting other properties remains a problem. Studies have shown that glasses containing two different types of network modifiers have unusual ionic mobilities [15,16]; this phenomenon is commonly known as the mixed alkali effect (MAE) [17][18][19]. This effect is more significant in conductivity, ion diffusion, chemical durability, glass transition temperature, viscosity, and other characteristics related to mobility, this effect does not cause large structural differences in glasses [20]. The MAE has been studied for improving the chemical durability of simple silicate systems [21][22][23], but research on the chemical durability of phosphate glass is rare [24][25][26]. Yang [24], Fang [25], and others found that the chemical durability of Fe-P glass did not show a distinct mixed alkali effect, whereas Guo et al. [26] showed that the chemical durability of Zn-P glass present an apparent mixed alkali effect. In general, the influence of mixed alkalis on the chemical durability of multicomponent phosphate glass has not been systematically studied.
In this study, we prepared a series of 56P 2 O 5 -7.5Al 2 O 3 -5.9BaO-(28.56-x)K 2 O-xNa 2 O-1.51Nd 2 O 3 phosphate glasses with different Na/(Na+K) ratios, measured their oxide composition, density, refractive index, glass transition temperature (T g ), conductivity (σ dc ), glass dissolution rate (D r ), ionic release concentrations in solution, infrared transmission spectrum, absorption spectrum, fluorescence spectrum, as well as decay curves, and then systematically studied the influence of MAE on the chemical durability of phosphate glasses and spectral properties of Nd 3+ ions.

Sample Preparation
A series of 56P 2 O 5 -7.5Al 2 O 3 -5.9BaO-(28.56-x)K 2 O-xNa 2 O-1.51Nd 2 O 3 phosphate glasses were prepared by a high-temperature melting and quenching method. The samples were numbered PKNi (i = 0, 4, 8, 12, 16, 20, 24, 28.56) according to the molar content of Na 2 O present, the starting materials of which were anhydrous reagent powders of Al(PO 3 ) 3 , Ba(H 2 PO 4 ) 2 , KPO 3 , NaPO 3 , P 2 O 5 , and Nd 2 O 3 . The 500 g of raw materials were fully mixed and poured into a preheated corundum crucible. Furthermore, the corundum crucible was transferred to a silicon carbide rod electric furnace at the melting temperature of 1050 • C for 60 min. After dehydration with CCl 4 (1050 • C, 1 h) and clarification at a high temperature (1150 • C, 1 h), the glass liquid was poured into a preheated steel mold. Finally, the formed glass was placed in an annealing furnace for 24 h, and the annealing temperature was varied according to the Na/K ratio.

Experimental Method
The annealed glass was processed into 20 mm × 20 mm × 2 mm flakes (polished on both sides) for testing spectral properties and into 14.7 mm × 14.7 mm × 12.1 mm bulk samples (polished on six sides) for hydrolysis. The ion concentrations of P 5+ , Al 3+ , Ba 2+ , Na + , K + , and Nd 3+ in the glass samples were determined using an inductively coupled plasma emission spectrometer (ICP-OES), and the values are listed in Table 1. The density (ρ) was measured with the ELECTRONIC DENSIMETER SD-200L (ALFA MIRAGE, Fukuoka, Japan) instrument. The PRECISON REFRACTOMETER KPR-2000 high-precision refractometer (SHIMADZU, Shimane, Japan) was used to test the refractive index (n) according to the V prism method. The glass transition temperature (T g ) was determined from DSC curve using a differential scanning calorimeter (DSC, sta449/C, Netzsch, Selb, Germany) with a heating rate of 10 • C/min. The conductivity was measured by HIOKI 3522-50lCR testing instrument (HIOKI, Kagoshima, Japan) with experimental test frequency range and the test temperature of 0-100 kHz and 237 • C, respectively. The chemical durability [27,28] was tested by placing bulk samples polished on six sides in a test tube containing 100 mL of high-purity deionized water. After heating in a 90 • C water bath for 48 h, the glass dissolution rate (D r ) and the ionic release concentrations of each ion in the test tube were measured. The glass dissolution rate (Dr = ∆w/At [26]) was defined as the mass loss per unit surface area and unit time (µg·cm −2 ·h −1 ). The absorption spectrum was measured using the Lambda 950 UV/VIS/NIR spectrophotometer (Perkin−Elmer, Waltham, MA, USA) with a test range of 200-1000 nm and a scan step of 1 nm. The infrared transmission spectrum was measured using a Nicolet FTIR infrared spectrometer (Thermo Scientific, Waltham, MA, USA) between 2000-4000 cm −1 . An Edinburgh instrument FLSP920 steady-state/transient fluorescence spectrometer was used to test the fluorescence spectrum and lifetime, whereby a Xe-lamp was used as the pump source. The fluorescence spectrum testing range was 850-1500 nm, and the fluorescence lifetime excitation and testing wavelengths were 808 and 1053 nm, respectively. Figure 1a,b exhibits the density (ρ) and the refractive index (n d ) at 656 nm of the PKNi glass with different Na/(Na+K) ratios, respectively. With an increase in Na/(Na+K) ratio, the ρ and n d gradually increase. However, when Na completely replaces K, the ρ and n d decrease slightly. The Nd 3+ ion concentration can be calculated by Equation (1):

Density and Refractive Index
where ρ is the density of the glass sample, M is the molar mass of the rare earth oxide, w t is the weight percentage of the rare earth oxide measured by ICP, and N A is the Avogadro constant. The Nd 3+ ion concentration (N 0 ) and refractive index (n d ) were used for the subsequent calculation of J-O parameters (Ω t ) and emission cross-sections (σ ems ).  Figure 2a,b displays the glass transition temperature (T g ) and the DC conductivity (σ dc ) of PKNi glass with different Na/(Na+K) ratios, respectively. Glass transition temperature is the temperature corresponding to the transition from glass state to rubbery state. With the increase in Na/(Na+K) ratio, the T g decreases first and then increases, reaching a minimum (383 • C) at Na/(Na+K) = 0.42, as shown in Figure 2a. The T g minimum indicates that there is an MAE in the glass system [19,29]. The MAE is particularly significant in the DC conductivity (σ dc ), and the DC conductivity can be obtained by fitting the AC conductivity. The DC conductivity (σ dc ) of glasses with different Na/(Na+K) ratios are shown in Figure 2b. With an increase in the Na/(Na+K) ratio, σ dc decreases first and then increases, reaching a minimum at Na/(Na+K) = 0.42, and PKN28.56 glass is higher than that of PKN0, which is consistent with the observations from Figure 2a. The dynamic structural mismatch model [30,31] can comprehensively explain the change in conductivity of mixed alkali glasses: (1) With the gradual replacement of Na, some K + ions did not migrate successfully, and therefore produced a sharp decline in conductivity. (2) When the Na/(Na+K) ratio was close to 0.5, the number of effective sites were minimal, which resulted in minimal conductivity. (3) When Na/(Na+K) was greater than 0.5, the effective sites of Na + ions began to increase, which was reflected in a sharp increase in the conductivity. Owing to the small radius, the conductivity of PKN28.56 glass was higher than PKN0.

Glass Dissolution Rate and Ion Release Concentration in Solution
The interaction between glass and water will dissolve the glass. It is generally believed that there are two reactions in the dissolution process [32]. (1) Dealkylation reaction: At the glass interface, H + ions in water exchange with alkali metal R + ions on the glass surface, thus triggering the dissolution reaction of glass. A hydration layer will be formed on the glass surface due to the selective dissolution of R 2 O; this process is called the hydration process. (2) Grid dissolution: In the hydration layer on the glass surface, the bridge oxygen bond in the glass structure is constantly attacked by H + , and eventually the glass network is destroyed, leading to continuous glass dissolution. The diffusion rate of water molecules in the glass determines the rate of the ion exchange reaction, thereby limiting the formation and development rate of the hydration layer on the glass surface, which is the control step of glass dissolution.
The most studies on the chemical durability of phosphate glass have focused on the glass dissolution rate or ionic release concentration in the solution. Figure 3 shows the glass dissolution rate of PKNi glass with different Na/(Na+K) ratios in a 90 • C water bath for 48 h. The glass dissolution rate (logD r ) curve is divided into three steps: (1) When 0 ≤ Na/(Na+K) ≤ 0.42, the dissolution rate decreases rapidly, and the logD r decreases from 2.74 to 1.28. (2) When 0.42 ≤ Na/(Na+K) ≤0.7, the logD r starts to decrease slowly from 1.28 to 1.13, reaching the minimum point when Na/(Na+K) = 0.7. (3) 0.7 ≤ Na/(Na+K) ≤ 1, the dissolution rate increases slowly from 1.13 to 1.36. The specific values are listed in Table 2.  To better understand the release value of each element after the glass was heating in water, we conducted ICP tests on the solution. The ionic release concentration in the solution of the PKNi glass with different Na/(Na+K) ratios in a 90 • C water bath for 48 h is exhibited in Figure 4. When Na/(Na+K) = 0, the ionic release concentrations of P, Al, Ba, and K were the highest, with concentrations of 687, 4.95, 76.14, and 578 µg/mL, respectively. When 0 ≤ Na/(Na+K) ≤ 0.42, the release concentration decreases rapidly, and the values of P, Al, Ba, and K decreased to 41.29, 0.78, 8.13, and 20.16 µg/mL, respectively. When 0.42 ≤ Na/(Na+K) ≤ 1, the ionic release concentration varied slowly. The release concentration of P, which is the glass network former, reached a minimum when Na/(Na+K) = 0.7, while that of Al, which is the glass network intermediate, reached a minimum when Na/(Na+K) = 0.84. The change in Na + ion release concentration is completely different relative to that of other ions, presenting a trend with an initial increase followed a decrease, that then stabilized before finally increasing. The specific values of each ion precipitation concentration are listed in Table 2. The nature of the dissolution mechanism can be further examined by determining the degree of congruence (DOC) of the glass formers with other elements [33,34]. Based on the original ion concentration in the glass, and the released concentration in the solution after boiling, the DOC is defined as: number cation in solution number P 5+ in solution − number cation in glass number P 5+ in glass where N a is the empirical index of ion exchange overflow, N 1 is the ionic release concentration ratio of cations to P 5+ in the solution, and N 2 is the concentration ratio of cations to P 5+ in the original glass composition. If the ionic release concentration ratio of M + /P 5+ in the solution is the same as that in the original glass composition, namely N a = 0, the type of hydrolysis attack is grid breakdown; if the release concentration ratio of M + /P 5+ in the solution is greater than that in the original glass composition, namely N a > 0, it is inferred that an additional part is released through ion exchange [34]. Therefore, the cations released in the solution can be categorized into two parts: release by grid breakdown or by ion exchange. Furthermore, N 1 can be expressed as: N b is expressed as follows: N b is the ratio of cations released through ion exchange to those through grid breakdown (sum is the cations release concentration in the solution).
The N a and N b trends of Na + and K + ions for PKNi glasses with different Na/(Na+K) ratios are shown in Figure 5a,b. When Na/(Na+K) = 0, the N a value of K + ion is 0.30. As Na + started to replace K + , the N a values of K + and Na + were 0.29 and 0.002, respectively. With the increase in the Na/(Na+K) ratio, the N a of K + ions decreased continuously, while the Na + ions increased slowly. This indicates that the Na + and K + ions in the glass continuously underwent ion exchange as ion exchange rate of K + is much larger than that of Na + , which is related to the high ion field strength of Na + . It can be observed from Figure 5b that with an increase in the Na/(Na+K) ratio, the N b values of K + and Na + ions fluctuate at approximately 0.6 and 0.02, respectively, indicating that the unit [PO 4 ] grid dissolution will be accompanied by a 0.6-unit K + -H + and 0.02-unit Na + -H + ion exchange, which has a weak relationship with the concentration of Na + and K + ions. The value α is defined as the extent of MAE: where a 1 and a 2 represent the dissolution coefficients of Na and K (calculated when the alkali metals in the glass were Na and K, a 1 = 0.7 and a 2 = 19.67), respectively. M 1 and M 2 represent the molar content of Na and K (ICP data), respectively. A and B represent the theoretical and actual ionic release concentration values of the total alkali metal in the solution, respectively. Using the above formula, the α of MAE plotted in Figure 5c. When Na/(Na+K) = 0.42, the MAE is most evident, which is consistent with the trends of conductivity and T g . Therefore, we divided the phosphate glass system with different Na/(Na+K) ratios into three sections for discussion: (1) When 0 ≤ Na/(Na+K) ≤ 0.42, some K + ions in the grid are replaced by Na + ions. Since the ion exchange rate and grid dissolution rate of Na are much smaller than those of K, and the MAE gradually increases, the glass dissolution rate decreases rapidly and chemical durability increases. (2) When 0.42 ≤ Na/(Na+K) ≤ 0.7, Na + ions gradually begin to dominate. However, the MAE is the strongest in the ratio range, resulting in a slow decrease in the dissolution rate of the glass. (3) When 0.7 ≤ Na/(Na+K) ≤ 1, Na+ ions dominate in the grid and the MAE gradually weakens, resulting in a slight increase in the glass dissolution rate.

Absorption Spectrum
The absorption spectra of Nd 3+ ion in PKNi glass with different Na/(Na+K) ratios are shown in Figure 6. The absorption at 523, 582, 684, 746, 801, and 870 nm corresponds to the transitions from 4 I 9/2 to ( 4 G 9/2 + 4 G 7/2 + 2 K 13/2 ), ( 4 G 5/2 + 2 G 7/2 ), ( 4 F 9/2 ), ( 4 F 7/2 + 4 S 3/2 ), ( 2 H 9/2 + 4 F 5/2 ), and 4 F 3/2 , respectively [35][36][37]. According to the Judd-Ofelt (J-O) theoretical model [38,39], the J-O parameters were calculated by standard least square fitting of the experimental oscillator strength (f exp ) and calculated oscillator strength (f cal ). The experimental oscillator strength (f exp ) can be calculated by Equation (7) [40]: where m e and e are the mass and charge of an electron, respectively, c is the speed of light, is the central wavelength, N 0 is the ion concentration of Nd 3+ , l is the thickness of the glass sample, and OD (λ) is the optical density. The calculated oscillator strength f cal from the initial J state to the final J' state is calculated using Equation (8) where h is the Planck constant, n is the glass refractive index, 2J+1 is the degeneracy of the originating level of the transition, S ed and S md are the electric and magnetic dipole line strengths, respectively,|(S,L)JU t (S ,L )J | 2 is the doubly reduced matrix elements of the tensor transition operator, which is determined by rare earth elements and unrelated to the host material [41], and Ω t (t = 2,4,6) represents the J-O parameters. The values of Ω t in PKNi glass with different Na/(Na+K) ratios are given in Table 3.   The J-O parameter (Ω 2 ) reflects the symmetry of the rare earth ion coordination environment and covalency of the Nd-O bonds, while Ω 4 and Ω 6 reflects the glass rigidity. The larger the Ω 2 , the lower the symmetry of the Nd 3+ ion coordination environment and the greater the covalency of the Nd-O bonds. When Na/(Na+K) = 0, Ω 2 has the maximum value of 3.79 × 10 −20 cm 2 , which indicates that the Nd 3+ ion coordination environment symmetry is the lowest and the covalency of the Nd-O bonds is the strongest in PKN0 glass. With an increase in the Na/(Na+K) ratio, Ω 2 shows an initially increasing and then decreasing trend, reaching a minimum point when Na/(Na+K) ratio is 0.56. When Na/(Na+K) = 1, Ω 2 is 3.14 × 10 −20 cm 2 . Moreover, Ω 2 deviates from the linear trend, indicating that the symmetry of the Nd 3+ ion coordination environment and the covalence of the Nd-O bonds have an obvious MAE. However, the values of Ω 4 and Ω 6 do not change significantly with different Na/(Na+K) ratios, indicating that the glass rigidity is basically not affected by the mixed alkali conditions. Figure 7a shows the infrared (IR) transmission spectrum of the PKNi glass with different Na/(Na+K) ratios. A higher hydroxyl absorption coefficient enhances the nonradiative transitions of the upper energy level and affects the lifetime of rare earth ions, which influences the luminescence performance of laser glass. The absorption coefficient of hydroxyl OHis determined by the following Equation (11):

Infrared Transmittance and Fluorescence Lifetime
where T 0 refers to the maximum infrared transmittance of the glass, T is the transmittance at 3000 cm −1 , and L represents the thickness of the sample. The values of α(OH − ) of these glass samples are less than 0.2 cm −1 , indicating that the whole group of glass exhibits beneficial water removal properties.  Figure 7b shows the fluorescence decay curve of Nd 3+ ion at 1053 nm in PKNi glasses with different Na/(Na+K) ratios. With an increase in the Na/(Na+K) ratio, the fluorescence decay curves of the Nd 3+ ions almost coincide, and do not change significantly, indicating that the MAE has little effect on the fluorescence lifetime. The fluorescence intensity of the Nd 3+ ions generally conform to single exponential decay, and the fluorescence lifetime of PKNi glasses with different Na/(Na+K) ratios obtained by fitting are given in Table 3.

Fluorescence Properties
The fluorescence spectra of the Nd 3+ ions in PKNi glasses with different Na/(Na+K) ratios are shown in Figure 8a. Excited by the Xe-lamp at 808 nm, the Nd 3+ ions in the ground state 4 I 9/2 are first pumped to the excited state energy levels 4 F 5/2 and 2 H 9/2 , then undergo non-radiative transition to the 4 F 3/2 level, and finally undergo radiative transition to the 4 I 9/2 , 4 I 11/2 , and 4 I 13/2 levels, corresponding to the fluorescence peaks at 892, 1053, and 1330 nm, respectively. With an increase in Na/(Na+K) ratio, the intensities of the fluorescence peaks at 892, 1053, and 1330 nm did not change significantly. Figure 8b presents the normalized fluorescence spectrum of Nd 3+ ions at 1053 nm. The full width at half maxima (FWHM) of the Nd 3+ ions at 1053 nm changes slightly with an increase in Na/(Na+K) ratio and the effective bandwidth (∆λ eff ) can be obtained from the normalized fluorescence spectrum. According to the J-O parameter (Ω t ) obtained from the previous fitting, the radiation transition probabilities (A) from 4 F 3/2 to 4 I 9/2 , 4 I 11/2 , and 4 I 13/2 can be obtained by Equation (12) [42]: where J = 3/2 and J' = 9/2, 11/2, and 13/2, respectively, and λ is the central wavelength. The fluorescence branching ratios (β) are the ratios of the radiative transition probabilities to the sum of all radiative transition probabilities, which can be calculated using Equation (13): The stimulated emission cross-section (σ ems ) from 4 F 3/2 to 4 I 11/2 can be determined according to Equation (14): where c is speed of light, n is the glass refractive index, λ is the peak wavelength of emission, and ∆λ eff is the effective bandwidth, which can be obtained using Equation (15): where I(λ) represents the luminescence intensity at wavelength λ, and I max is the maximum intensity.
The stimulated emission cross-section, fluorescence lifetime are important parameters for evaluating laser performance. The stimulated emission cross-section (σ ems ), the effective bandwidth (∆λ eff ), and the fluorescence lifetime (τ f ) of the Nd 3+ ions at 1053 nm in PKNi glass are shown in Figure 9. With an increase in the Na/(Na+K) ratio, the σ ems , ∆λ eff , and τ f show an initially increasing and then decreasing trend, an initially decreasing and then increasing trend, and an initially rising and then decreasing trend, respectively; however, the overall difference in the range is negligible. The laser neodymium glass for high-power laser systems requires a large σ ems and long τ f , while in the range of 0.28 ≤ Na/(Na+K) ≤ 0.84, Nd 3+ ions have a large emission cross-section and fluorescence lifetime. The dissolution rate D r of PKNi glass with Na/(Na+K) = 0.7 was 13.542 µg·cm −2 ·h −1 (listed in Table 2), which was 1/40 that of PKN0 glass (549.167 µg·cm −2 ·h −1 ), and depended on the low ion exchange rate of Na + -H + , the low [PO 4 ] grid dissolution rate, and the strong MAE. The glass size of this experiment and LHG-8 used for chemical durability test [43] were 14.7 mm × 14.7 mm × 12.1 mm and 26 mm × 26 mm × 12 mm, respectively, and deionized water temperature were 90 • C and 50 • C, respectively. Although the test temperature of PKNi glasses was higher than that of LHG-8 [43], the dissolution rate, D r , of PKNi glass with Na/(Na+K) = 0.7 was close to that of the LHG-8 neodymium glass, which was 20.833 µg·cm −2 ·h −1 [43]. As is well known, LHG-8 is a very good commercialized Nd laser glass material, which is widely used in high-energy and high-power laser systems. By the mixed alkali effect, the dispersion rate of PKNi glass was successfully adjusted to the same level as that of LHG8. Therefore, the chemical durability of PKNi glass with Na/(Na+K) = 0.7 should be guaranteed in practical laser applications.
A series of studies on the Na/K ratio in 56P 2 O 5 -7.5Al 2 O 3 -5.9BaO-(28.56-x)K 2 O-xNa 2 O-1.51Nd 2 O 3 glass greatly improve the chemical durability and spectral characteristics of N51 glass. Although N51 Nd-doped phosphate laser glass is still under further improvement before scale production, N51 has been successfully trialed in liquid-cooled KJ-class laser applications [44].

Conclusions
The influences of the mixed alkali effect (MAE) on the chemical durability and spectral properties of Nd 3+ ions in 56P 2 O 5 -7.5Al 2 O 3 -5.9BaO-(28.56-x)K 2 O-xNa 2 O-1.51Nd 2 O 3 phosphate glasses were studied systematically. With an increase in Na/(Na+K) ratio, the glass dissolution rate and ionic release concentration in the solution show an initial rapid decreasing trend, then decreased slowly, and lastly, increased slightly, depending on the combined action of the ion exchange rate, [PO 4 ] grid dissolution rate, and mixed alkali effect. With an increase in Na/(Na+K) ratio, the Ω 2 related to the symmetry of Nd 3+ ions and covalent nature of the Nd-O bonds initially decreased and then increased, while the Ω 4 and Ω 6 , related to glass rigidity, did not change significantly. The emission cross-section (σ ems ) fluctuates between 3.89 × 10 −20 cm 2 and 4.11 × 10 −20 cm 2 , and the lowest value is still higher than that of LHG-8 commercial glass. The radiation lifetime (τ rad ), fluorescence lifetime (τ f ), effective bandwidth (∆λ eff ), fluorescence branching ratios (β), and quantum efficiency (η) do not change significantly with Na/Na+K ratio.