⇦Chinese Physics Letters, 2019, Vol. 36, No. 8, Article code 084203 Preparation and 1.06 µm Fluorescence Decay of Nd$^{3+}$-Doped Glass Ceramics Containing NaYF$_{4}$ Nanocrystallites * Xing-Yong Huang (黄兴勇)1,2, Da-Qin Chen (陈大钦)3, Bi-Zhou Shen (沈必舟)4, Hai-Zhi Song (宋海智)1,5** Affiliations 1Southwest Institute of Technical Physics, Chengdu 610041 2School of Physics and Electronic Engineering, Yibin University, Yibin 644007 3College of Physics and Energy, and Fujian Provincial Key Laboratory of Quantum Manipulation and New Energy Materials, Fujian Normal University, Fuzhou 350117 4ENREACH Education (ChengDu) of Dipont Education Management Group, Chengdu 617000 5Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054 Received 24 May 2019, online 22 July 2019 *Supported by the National Key Research and Development Program of China under Grant No 2017YFB0405302, the Scientific Research Fund of Sichuan Provincial Education Department under Grant No 15ZB0294, the Key Technical Project of Yibin City in 2015, the Scientific Research Project of Yibin University under Grant No 2015PY02, the 1000 Talents Plan of Sichuan Province, and the Rongpiao Plan of Chengdu City.
**Corresponding author. Email: hzsong1296@163.com Citation Text: Huang X Y, Chen D Q, Chen B Z and Song H Z 2019 Chin. Phys. Lett. 36 084203    Abstract Considered to be a candidate for large-size bulk materials used in lasers and other fields, Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites are prepared. Using x-ray diffraction and transmission electron microscopy, we show that pure cubic NaYF$_{4}$ is well precipitated in the glass matrix. To obtain the optical property of this material at 1.06 μm, the fluorescence decay of $^{4}\!F_{3/2}$ energy levels is measured and analyzed. It is found that the fluorescence lifetime decreases first and then increases with the increasing dopant concentration due to the existing but finally weakening energy dissipation. As a result, a long radiation lifetime of about 191–444 μs is obtained at 1.06 μm in the prepared material. It is thus revealed that Nd$^{3+}$-doped glass ceramic containing NaYF$_{4}$ nanocrystallites is a potential candidate as a near-infrared laser material. DOI:10.1088/0256-307X/36/8/084203 PACS:42.70.-a, 78.55.-m, 78.45.+h, 78.30.-j, 61.82.Rx © 2019 Chinese Physics Society Article Text Nd$^{3+}$ is well known as an active ion in optical sensing, optical amplifiers and solid-state lasers.[1–4] Its possible applications cover a wavelength range from ultraviolet to near-infrared emission,[5] most significantly at wavelengths of $\sim $0.9, $\sim $1.06 and $\sim $1.35 µm,[6] which correspond to the transitions $^{4}\!F_{3/2}\to^{4}\!I_{9/2}$, $^{4}\!F_{3/2}\to$ $^{4}\!I_{11/2,}$ and $^{4}\!F_{3/2}\to^{4}\!I_{13/2}$, respectively.[7] The 0.9 µm band is expected to be used in the monolithic integration of laser emission,[7] and the 1.35 µm band has caused much interest in respect of its application to O-band optical signal amplification.[7,8] The 1.06 µm band, in particular, has been well studied in laser applications,[7,9,10] but still has much more potential to be explored. So far, the doped matrix can be divided into two main categories: oxides and non-oxides. Oxides such as the traditional silicate glass have the advantages of excellent mechanical strength, mechanical stability and chemical stability,[8] while non-oxides such as fluoride[11] have a low phonon energy and wide transmission window.[7] Recently, transparent glass ceramic has attracted much attention as a new optical material.[12–15] The material of crystalline fluoride embedded in glass matrix is a glass ceramic that bears the advantages of both oxide- and non-oxide-doped matrices.[16] With Nd$^{3+}$ doping to modulate the optical property, glass ceramic is very worthy of investigation as a novel optical material.[17,18] In this work, Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites are successfully prepared, and the fluorescence decay study suggests its great potential as a near-infrared laser material. The sample preparation started from the following mixture (the composition ratio in mol%): 40SiO$_{2}+$25Al$_{2}$O$_{3}$+18Na$_{2}$CO$_{3}$+10YF$_{3}$+7NaF+$x$NdF$_{3}$ ($x =0.1$ 0.5, 1.0, 2.0 and 3.0). About 15 g of the original material was fully mixed per batch and was kept at 1450$^{\circ}\!$C for 1 h to cause melting in a closed platinum crucible, then placed into a brass mold for quenching. The samples were then annealed at 400$^{\circ}\!$C to remove the internal stress. Finally, the crystallization process occurred under the condition of heat treatment at 620$^{\circ}\!$C for 2 h to obtain transparent nanocrystallite-containing glass ceramic samples. The crystalline phase was characterized by x-ray diffraction (XRD) using a powder diffractometer (DMAX2500 Rigaku) with Cu $K_{\alpha}$ ($\lambda =1.54$ Å) radiation rays. A transmission electron microscope (TEM) was measured to analyze the microstructures of the samples using JEM-2010, and it was equipped with an energy dispersive x-ray spectroscopy (EDS) system. Fluorescence decays were measured using a near-infrared photomultiplier tube (PMT) detector (R5509) and an 801 nm exciting laser, which was obtained with a 450 W xenon pumping lamp. All measurements were performed at room temperature. The XRD results of the prepared materials doped with 1 mol% Nd$^{3+}$ are illustrated in Fig. 1. Through the comparison with card no. 77–2042 of Powder Diffraction Standards (JCPDS) of the Joint Committee, all the diffraction peaks clearly demonstrate that the sample measured is of cubic NaYF$_{4}$ crystalline phase ($\alpha$-NaYF$_{4}$). No significant complex peaks are observed in Fig. 1, indicating that the $\alpha$-NaYF$_{4}$ crystallites have been well precipitated in the glass matrix.
cpl-36-8-084203-fig1.png
Fig. 1. XRD spectrum of the 1 mol% Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites.
cpl-36-8-084203-fig2.png
Fig. 2. TEM bright field image of the glass ceramic doped with 1 mol% Nd$^{3+}$ (a), particle size distribution histogram (b), and the corresponding SAED pattern (c).
For further observation of the internal structure, a TEM bright field image of the sample doped with 1 mol% Nd$^{3+}$ is shown in Fig. 2(a). It can be clearly observed that the spherical nanoscale particles are uniformly distributed in the glass matrix. The size distribution of the sample is shown in Fig. 2(b). The distribution rate of particle sizes from 22 to 30 nm is more than 50%. Normal distribution analysis shows that the average size of NaYF$_{4}$ crystallites is 28.7$\pm$5.9 nm. Here, the error was deduced by statistical standard deviation. In the corresponding selected area electron diffraction (SAED) pattern shown in Fig. 2(c), the diffraction characteristics of the polycrystalline rings indicate that the crystalline phase structure is cubic, confirming well-formed $\alpha$-NaYF$_{4}$ nanocrystallites. The samples with 0.1, 0.5, 2 and 3 mol% of Nd$^{3+}$ doping are observed to be similar, thus it is clear that Nd$^{3+}$-doped glass ceramic containing NaYF$_{4}$ nanocrystallites has been effectively prepared.
cpl-36-8-084203-fig3.png
Fig. 3. EDS spectra from glass ceramic doped with 1 mol% Nd$^{3+}$.
To examine whether NaYF$_{4}$ nanocrystallites contain Nd$^{3+}$ ions, EDS spectra from the glass ceramic are shown in Fig. 3. It can be seen that Nd, Na, F and Y were detected in individual NaYF$_{4}$ nanocrystallites. Naturally, since the glass matrix surrounds NaYF$_{4}$ nanocrystallites, the signals of O, Al, and Si elements are also detected. In contrast, no significant signals of the element Nd were observed in the glass matrix, mainly due to the low level of Nd below the detection limit. The result implies that the Nd concentration in NaYF$_{4}$ crystallites is much higher than that in the glass matrix. To evaluate the optical property of the prepared glass ceramic, the fluorescence decay was studied at $\sim $1.06 µm. Figure 4 shows the fluorescence decay data of $^{4}\!F_{3/2} \to^{4}\!I_{11/2}$ (corresponding to the wavelength of 1056 nm) for the glass ceramics with different Nd$^{3+}$ doping concentrations. The decay procedure changes greatly with the Nd$^{3+}$ concentration, probably due to the complex environment (glass phase, crystal phase and others) where rare earth ions lie. In these glass ceramic samples, the fluorescence radiation may contain multiple forms of phonon relaxation and energy transfer to the impurities or crystal defects. To understand the detailed decay mechanisms, we use four models to analyze the fluorescence decay data. Model 1: The experimental fluorescence lifetime ($\tau_{\rm df}$) can be simply defined as the first $e$-time of the decay curve.[19] In other words, the time corresponding to $1/e$ of the initial intensity is the fluorescence lifetime. Model 2: It is generally considered that most of the rare earth ions would go into the expected crystal phase (a single environment), thus the fluorescence decay approximately takes a single exponential behavior. The fluorescence lifetime ($\tau_{\rm se}$) can be expressed as[20] $$\begin{align} I(t)=I(0)\exp \Big(-\frac{t}{\tau_{\rm se}}\Big),~~ \tag {1} \end{align} $$ where $I(t$) is the fluorescence intensity at a time $t$, and $I$(0) is the fluorescence intensity at $t=0$. Model 3: Given that rare earth ions may exist in both the glass phase and the crystalline phase, the double-exponential decaying behavior used to evaluate the fluorescence lifetime is expressed as[21] $$\begin{align} I(t)=a_{1}\exp (-t/\tau_{1})+a_{2}\exp (-t/\tau_{2}),~~ \tag {2} \end{align} $$ where $I(t)$ means the fluorescence intensity, $a_{1}$ and $a_{2}$ are the weighting parameters, and $\tau_{1}$ and $\tau_{2}$ are the short- and long-decay lifetimes. Here we may further define two weight proportion factors $A_{1}$ and $A_{2}$ as $a_{1}/(a_{1}+a_{2})$ and $a_{2}/(a_{1}+a_{2})$, respectively. The reduced fluorescence lifetime $(\tau_{\rm de})$ can be defined as[21–23] $$\begin{align} \tau_{\rm de}=\frac{a_{1}\tau_{1}^{2}+a_{2}\tau_{2}^{2}} {a_{1}\tau_{1}+a_{2}\tau_{2}}.~~ \tag {3} \end{align} $$ Model 4: Since the rare earth ions Nd$^{3+}$ may not be completely incorporated into the NaYF$_{4}$ crystal phase, an average lifetime ($\tau_{\rm av}$) is thus needed to evaluate the fluorescence decay, which reads[7,10] $$\begin{align} \tau_{\rm av}=\int_0^\infty \frac{I(t)}{I_{0}} dt,~~ \tag {4} \end{align} $$ where $I(t)$ means the experimental fluorescence intensity at a time $t$, and $I_{0}$ is the maximum of $I(t)$. As can be seen from Fig. 4, the experimental data can be more or less fitted by the single exponential function (model 2). The fluorescence lifetime $\tau_{\rm se}$ ranges from 164.94 to 380.50 µs. It was reported that the luminescence decay lifetimes of $^{4}\!F_{3/2}$ level in Nd$^{3+}$-doped NaYF$_{4}$ matrix and glass can be $\sim $300 and $\sim $150 µs, respectively.[23,24] The above results thus suggest that a large number of Nd$^{3+}$ ions has already been in the NaYF$_{4}$ crystal phase, and this is the same as the EDS result. At the same time, it can be observed that the double exponential fitting curves are in great agreement with the experimental data, as shown by the green curves in Fig. 4. The fitted short lifetime $\tau_{1}$ appears to be 10.45–198.49 µs, while the fitted long lifetime $\tau_{2}$ varies from 282.64 to 539.56 µs. This implies that the fluorescence decay process of Nd$^{3+}$ contains two different carrier recombination paths. Figure 5(a) shows the fluorescence lifetime values calculated by all the models. One can see that the fitted lifetimes by model 1 $\tau_{\rm df}$, model 2 $\tau_{\rm se}$, model 3 $\tau_{\rm de}$ and model 4 $\tau_{\rm av}$ are longer than $\tau_{1}$ and shorter than $\tau_{2}$. Moreover, $\tau _{\rm df}$, $\tau _{\rm se}$ and $\tau _{\rm av}$ are very close to each other. According to the reported works, when Nd$^{3+}$ is doped in fluoride, the fluorescence lifetime of the $^{4}\!F_{3/2}$ level will decrease with the increasing Nd$^{3+}$ concentration due to the energy transfer between Nd$^{3+}$ ions,[2,23,25,26] and the fluorescence lifetime of low-Nd$^{3+}$-doped NaYF$_{4}$ may exceed 300 µs.[24] The good lifetime estimation for a doping concentration lower than 1 mol% implies that models 1, 2 and 4 are highly effective in reflecting the fluorescence decay process of glass ceramics. They show that the fluorescence lifetime of Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites is roughly 191.14–444.12 µs (model 4). Certainly, for the same calculation method of fluorescence lifetime (model 4) and the same doping concentration, the energy level lifetime of Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites is longer than that of the as-prepared glass.[17]
cpl-36-8-084203-fig4.png
Fig. 4. Normalized fluorescence decay data of the transition $^{4}\!F_{3/2} \to^{4}\!I_{11/2}$ for the glass ceramics doped with different Nd$^{3+}$ concentrations, and the fitting curves to different models.
cpl-36-8-084203-fig5.png
Fig. 5. Fluorescence lifetime (a) and weight proportion (b) varying with doping concentration.
Furthermore, Fig. 5(a) also shows that the lifetime values calculated by model 1 $\tau_{\rm df}$, model 2 $\tau_{\rm se}$ and model 4 $\tau_{\rm av}$ have the same variation trend, i.e. decreasing first and then increasing with the increase of doping concentration. This phenomenon is different from that seen in previous reports,[23,25,26] and can be explained as follows. Generally speaking, carrier recombination can be divided into radiation recombination and non-radiation recombination. The former produces light emission, while the latter causes photon energy to be dissipated. Here, all the factors (vibration atoms, unsaturated bonds, crystal defects, concentration quenching and so on[22]) causing non-radiative recombination are defined as energy dissipation centers. When the doping concentration increases from 0.1 mol% to 1 mol%, the increased activation centers also result in an increasing chance of energy being absorbed by the energy dissipation centers. Meanwhile, the results of the weight $A_{1}$ increase and $A_{2}$ decrease presented in Fig. 5(b) mean that the component of short (long)-lifetime radiation is relatively increasing (decreasing). As a result, non-radiative recombination dominates the whole recombination process and the fluorescence lifetime decreases. However, when the doping concentration further increases from 1 mol% to 3 mol%, the trend of weight proportion is decreasing for $A_{1}$ while for $A_{2}$ it is increasing (Fig. 5(b)). Referring to the analysis of ceramic glass formation,[16] this process may be explained such that more Nd$^{3+}$ ions can induce more nucleation centers and increase the volume percentage fraction of NaYF$_{4}$ crystallites.[27] In other words, increasing the doping concentration may increase the percentage of Nd$^{3+}$ ions in the NaYF$_{4}$ lattice.[27] This will not only increase the value of $A_{2}$ but also re-establish the dominance of radiation recombination. Accordingly, the fluorescence lifetime increases from 1 to 3 mol% of Nd$^{3+}$ doping.
Table 1. Comparison of the maximum fluorescence lifetime ($\tau_{\max}$) of Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites with $\tau_{\max}$ of many other Nd$^{3+}$-doped matrices.
Doped matrices $\tau_{\max}$ Model
NaYF$_{4}$ glass ceramics (This work) 444.12 Model 4
NaLu(MoO$_{4}$)$_{2}$ laser crystal[28] 134 Model 2
Oxyfluorosilicate glasses[23] 181 Model 3
Fluorogallate glass[19] 191 Model 1
Lead boro-tellurite glass[29] 100 Model 4
LaMgAl$_{11}$O$_{19}$ crystal[30] 321 Model 2
SiO$_{2}$-B$_{2}$O$_{3}$-Na$_{2}$CO$_{3}$-NaF-CaF$_{2}$ glasses[31] 257 Model 2
TeO$_{2}$–PbF$_{2}$–AlF$_{3}$ glasses[32] 174 Model 4
TeO$_{2}$–TiO$_{2}$–Nb$_{2}$O$_{5}$ glass[33] 146 Model 4
In Table 1, the maximum fluorescence lifetime ($\tau_{\max}$) of the fabricated Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites is compared with the $\tau_{\max}$ of many other Nd$^{3+}$-doped matrices. It is observed that models 2 and 4 are the main methods of calculating the 1.06 µm fluorescence lifetime of Nd$^{3+}$, by which a reasonable result can be easily obtained. Meanwhile, Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites have longer fluorescence lifetime, which reveals that these materials are suitable for the ion population on the energy level and easily form a population inversion for lasing. Our results thus suggest that Nd$^{+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites have a high potential for application in near-infrared lasers. In summary, Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites have been successfully prepared and optically characterized as a possible optical material. The pure $\alpha$-NaYF$_{4}$ nanocrystallite particles are precipitated in a glass matrix. Four different methods are used to analyze the fluorescence decay curves of $^{4}\!F_{3/2}$ energy levels, and the expectations of three fluorescence lifetime analysis methods are in agreement with the experimental results, and show that Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites have a long fluorescence lifetime of about 191–444 µs (model 4). The analysis implies that the energy dissipation exists but finally weakens, suggesting the good quality of the prepared material. Therefore, Nd$^{3+}$-doped glass ceramics containing NaYF$_{4}$ nanocrystallites have good application prospects in near-infrared lasers. References Emission cross section and optical gain of 1.06mm laser Nd 3+ doped borate glassesMechanism for broadening and enhancing Nd 3+ emission in zinc aluminophosphate laser glass by addition of Bi 2 O 3Optical Transition Probability of Nd 3+ Ions Doped in Ferroelectric Plzt for Active Electro-Optical ApplicationsUltraviolet to near-infrared energy transfer in NaYF4:Nd3+,Yb3+ crystalsFluorescence and Judd–Ofelt analysis of Nd3+ ions in oxyfluoride glass ceramics containing CaF2 nanocrystals915 nm all-fiber laser based on novel Nd-doped high alumina and yttria glass @ silica glass hybrid fiber for the pure blue fiber laserThe enhanced near-infrared fluorescence of Nd 3+ -doped tellurite glassOptimizing Nd/Er ratio for enhancement of broadband near-infrared emission and energy transfer in the Er3+–Nd3+ co-doped transparent silicate glass-ceramicsSilver nanoparticles enhanced photoluminescence of Nd 3+ doped germanate glasses at 1064 nmSpectroscopic investigations of Nd3+ doped gadolinium calcium silica borate glasses for the NIR emission at 1059 nmSpectroscopy and energy transfer dynamics of Nd3+ strengthened Er3+: 2.7μm emission in modified fluoride glassEffect of microstructure on up-conversion luminescent of Tb3+/Yb3+ co-doped phosphate glass and glass-ceramicUp-conversion Luminescence Properties of Er3+/Yb3+ Co-Doped Oxyfluoride Glass CeramicA review on X‐ray detection using nanomaterialsUltrapure NIR-to-NIR single band emission of β-PbF2: Yb3+/Tm3+ in glass ceramicsCrystallization of a highly viscous multicomponent silicate glass: Rigidity percolation and evidence of structural heterogeneitySpectroscopic calculation of NaYF4 contained transparent glass ceramics doped with different content of Nd3+Dynamics analysis of laser radiation in Nd3+:NaYF4 nanostructure glass ceramicThermal and spectroscopic properties of Nd3+-doped novel fluorogallate glassCrystal structure, morphology and luminescent properties of rare earthion-doped SrHPO4 nanomaterialsVisible upconversion luminescence of Tb3+ ions in Y2O3 nanoparticles induced by a near-infrared femtosecond laserLuminescence enhancement by energy transfer in melamine-Y 2 O 3 :Tb 3+ nanohybridsOptical absorption and emission properties of Nd 3+ -doped oxyfluorosilicate glasses for solid state lasersSynthesis and spectral properties of colloidal Nd3+ doped NaYF4 nanocrystalsSynthesis and the luminescent properties of the Nd3+ ions doped three kinds of fluoride nanocrystals in organic solventsNd3+ doped new oxyfluoro tellurite glasses and glass ceramics containing NaYF4 nano crystals − 1.06 µm emission analysisCrystallization behavior and microstructure investigations on LaF3 containing oxyfluoride glass ceramicsAssessment of the spectroscopic properties and laser performance of a prominent Nd 3+ :NaLu(MoO 4 ) 2 laser crystalNd3+-doped lanthanum lead boro-tellurite glass for lasing and amplification applicationsGrowth, spectral properties, and diode-pumped laser operation of a Nd 3+ -doped LaMgAl 11 O 19 crystalA study of low threshold and high gain Nd 3+ ions doped SiO 2 -B 2 O 3 -Na 2 CO 3 -NaF-CaF 2 glasses for NIR laser applicationsNd3+-doped TeO2–PbF2–AlF3 glasses for laser applicationsSpectroscopy and frequency upconversion in Nd 3+ -doped TeO 2 –TiO 2 –Nb 2 O 5 glass
[1] Rajagukguk J, Sinaga B, Sihombing E, Djamal M and Kaewkhao J 2018 Mater. Today: Proc. 5 14998
[2] Wang Y, Cao J, Li X, Li J, Tan L, Xu S and Peng M 2019 J. Am. Ceram. Soc. 102 1694
[3] Li B Z et al 2005 Chin. Phys. Lett. 22 80
[4] Dong X, You F, Peng H and Huang S 2016 J. Rare Earths 34 863
[5] Chen D, Wang Y, Yu Y, Ma E and Liu F 2007 J. Phys. Chem. Solids 68 193
[6] Wang Y, Zhang Y, Cao J, Wang L, Peng X, Zhong J, Yang C, Xu S, Yang Z and Peng M 2019 Opt. Lett. 44 2153
[7] Zhou Z, Zhou Y, Zhou M, Su X and Cheng P 2017 J. Non-Cryst. Solids 470 122
[8] Dan H K, Zhou D, Yang Z, Song Z, Yu X and Qiu J 2015 J. Non-Cryst. Solids 414 21
[9] Kassab L R P, Silva D M, Garcia J A M, da Silva D S and de Araújo C B 2016 Opt. Mater. 60 25
[10] Kesavulu C R, Kim H J, Lee S W, Kaewkhao J, Wantana N, Kaewnuam E, Kothan S and Kaewjaeng S 2017 J. Alloys Compd. 695 590
[11] Zhang L, Yang B and Hu L 2014 J. Quant. Spectrosc. Radiat. Transfer 147 47
[12] Zhao M, Zhang H, Zou X, Jia W and Su C 2019 Mater. Lett. 243 73
[13] Deng Y and Niu C 2019 J. Lumin. 209 39
[14] Zhao J, Huang L, Zhao S and Xu S 2019 J. Am. Ceram. Soc. 102 1720
[15] Fu Y, Zhao L, Guo Y, Wu B, Dong H and Yu H 2019 J. Lumin. 208 33
[16] Liu S, Kong Y, Tao H and Sang Y 2017 J. Eur. Ceram. Soc. 37 715
[17] Liu F, Chen D, Wang Y, Ma E and Yu Y 2007 J. Alloys Compd. 443 143
[18] Huang X, Xiong Y, Hu Y and Xiao Y 2016 Optik 127 8847
[19] Xue T, Zhang L, Hu J, Liao M and Hu L 2015 Opt. Mater. 47 24
[20] Wang M, Tian B, Yue D, Lu W, Yu M, Li C, Li Q and Wang Z 2015 J. Rare Earths 33 355
[21] Lü Q, Wu Y, Ding L, Zu G, Li A, Zhao Y and Cui H 2010 J. Alloys Compd. 496 488
[22] Stagi L, Chiriu D, Ardu A, Cannas C, Carbonaro C M and Ricci P C 2015 J. Appl. Phys. 118 125502
[23] Ramachari D, Rama M L and Jayasankar C K 2014 Infrared Phys. Technol. 67 555
[24] Bednarkiewicz A, Wawrzynczyk D, Nyk M and Strek W 2011 Opt. Mater. 33 1481
[25] Chen Z, Tian C, Bo S, Liu X and Zhen Z 2015 Opt. Mater. 48 86
[26] Rajesh D and de Camargo A S S 2019 J. Lumin. 207 469
[27] Hu Z, Wang Y, Bao F and Luo W 2005 J. Non-Cryst. Solids 351 722
[28] Yi Y, Fan X, Zhang X, Zhu X, Yuan J, Yu H, Wang G and Shan X 2016 J. Alloys Compd. 683 506
[29] Madhu A, Eraiah B, Manasa P and Srinatha N 2018 Opt. Mater. 75 357
[30] Pan Y, Zhou S, Wang J, Xu B, Liu J, Song Q, Xu J, Li D, Liu P, Xu X and Xu J 2018 Appl. Opt. 57 9657
[31] Megala R, Gowthami T, John Sushma N, Kamala S and Deva Prasad Raju B 2018 Infrared Phys. Technol. 90 221
[32] Lalla E A, Rodríguez-Mendoza U R, Lozano-Gorrín A D, Sanz-Arranz A, Rull F and Lavín V 2016 Opt. Mater. 51 35
[33] Balda R, Fernández J, Arriandiaga M A and Fernández-Navarro J M 2007 J. Phys.: Condens. Matter 19 086223