Flux Exploration, Growth, and Optical Spectroscopic Properties of

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Flux exploration, growth and optical spectroscopic properties of large size LaBSiO5 and Eu3+ doped LaBSiO5 crystals Ling-Yun Li, Yanping Lin, Lizhen Zhang, Qiuchan Cai, and Yan Yu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01204 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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Crystal Growth & Design

Flux exploration, growth and optical spectroscopic properties of large size LaBSiO5 and Eu3+ doped LaBSiO5 crystals Lingyun Lia,b,c,*, Yanping Lina,b, Lizhen Zhangc, Qiuchan Caia,b and Yan Yua,b, * a

Key Laboratory of Eco-materials Advanced Technology, Fuzhou University, Fuzhou,

Fujian 350108, China. b

College of Materials Science and Engineering, Fuzhou University, No.2 Xueyaun

Road, Minhou, Fujian 350108, China; c

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute

of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujain, 350002, China; In this work, large size Eu3+ doped LaBSiO5 crystal has been successfully grown by using the flux method for the first time. Meanwhile, a kind of experiment design technology that based on mixture design has been developed to optimize the composition of the flux system. A type of complex flux system comprising LaBO3, SiO2 and Li2MoO4 was investigated in this article. By the direction of mixture design method, the composition of complex flux is optimized by two rounds of testing experiment. Based on the exploration result of flux compositions, a large size Eu3+ doped LaBSiO5 crystal with dimension of 12*10*8mm3 has been grown by TSSG method. As far as we know, it is the first report that active ion doped LaBSiO5 bulk crystal is obtained from an environmentally friendly flux without fluorides. The optical spectral and electronic transition properties of Eu3+ doped LaBSiO5 crystals are studied by analyzing the excitation, emission and luminescence decay curves systematically. The radiative lifetime, transition probabilities and J-O intensity parameters of Eu3+ optical transition have been investigated by using the J-O theory.

The composition region of flux used to grow LaBSiO5 crystals and photo of Eu3+ doped LaBSiO5 crystal * Corresponding author: Fax: +86 591 22866534; E-mail: [email protected]; [email protected];

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Flux exploration, growth and optical spectroscopic properties of large size LaBSiO5 and Eu3+ doped LaBSiO5 crystals Lingyun Lia,b,c,*, Yanping Lina,b, Lizhen Zhangc, Qiuchan Caia,b and Yan Yua,b, * a

Key Laboratory of Eco-materials Advanced Technology, Fuzhou University, Fuzhou,

Fujian 350108, China. b

College of Materials Science and Engineering, Fuzhou University, No.2 Xueyaun

Road, Minhou, Fujian 350108, China; c

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute

of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujain, 350002, China; *Corresponding author. Fax: +86 591 22866534; E-mail: [email protected]; [email protected]; Abstract: This paper reports the flux composition optimizing, crystal growth and optical spectroscopic properties of LaBSiO5 and Eu3+ doped LaBSiO5 crystals. The mixture design technology was successfully applied in the crystal growth area for the first time. By the direction of mixture design method, the composition of a complex flux comprising LaBO3, SiO2 and Li2MoO4 was optimized and used for growth of LaBSiO5 crystals. A large size of Eu3+ doped LaBSiO5 crystal with dimension of 12*10*8mm3 has been prepared by TSSG method. As far as we know, it is the first report that active ion doped LaBSiO5 bulk crystal was obtained from an environmentally friendly flux without fluorides. The optical spectroscopy and electronic transition properties of Eu3+ doped LaBSiO5 crystals are studied thoroughly by analyzing the excitation, emission spectra and luminescence decay curves. The luminescence of Eu3+ ions are characteristic of intense red light emission corresponding to transitions of 5D0→7F1 and 5D0→7F2. Meanwhile, the splitting of emission bands mentioned above is apparent because of the lower symmetry of lattice sites where active ion locates. Furthermore, the symmetry properties of the lattice sites where Eu3+ ions locate are investigated by using Judd-Ofelt theory. The value of intensity parameters of Eu3+ doped LaBSiO5 ranges from 4.19 to 4.89 when the doping concentration varies.


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1, Introduction LaBSiO5 (Abbreviated as LBSO) is a member of serial rare earth compounds ReBO(SiO4) (Re=rare-earth element) with stillwellite structure, which were first found in Australia, 1955[1]. The LBSO crystal has attracted many research interest because it could be used in many functional material areas such as ferroelectric, non-linear optical, piezoelectric and laser applications. As a candidate of multifunctional materials, the synthesis, structure properties and the crystal growth have been investigated thoroughly. The compound LBSO undergoes a ferroelectric phase transition at 412K with space group changing from P3121 at high temperature to P31 at low temperature. The structure of LBSO crystal with space group of P31 is characteristic of infinite helical chains that composed by BO4 and SiO4 groups connected with shared O atoms along the c axis. The neighboring helical chains connect with each other by nine-coordinated La atoms filling among them. The crystal structure of low temperature phase LBSO is illustrated in Fig. 1[2,3]. The vibrational properties of LBSO glass and LBSO glass-crystal composite was investigated by I. Kratochvílová-Htubá et al. in the temperature range 25-260 oC. The results reveal that the BO4 chain arrangement and the bending vibrations of SiO4 are influenced by the loss of long-range order in glass phase LBSO. Meanwhile, the rotation of BO4 tetrahedra makes the LBSO crystal structure disorder to some degree[4]. Further more, B. A. Strukov et al. studied the ferroelectric phase transition property by measuring and analyzing the thermal and dielectric characterizations. It was found that the phase transition is of a displacement character and it was a order-disorder transition from the point of view of structure[5]. Many kinds of phosphors based on LBSO powders were studied for their potential application in the fields of luminescence display. Optically active ions, such as Eu3+, Bi3+, Sm3+, Dy3+, Ce3+ and Tb3+, were doped into LBSO powders that prepared by sol-gel method or high temperature solid state reaction[6-10]. The obtained phosphors exhibit optical spectroscopy diversity because of the inclusive of the LBSO crystal structure. It was demonstrated that the LBSO polycrystal powders could be used as matrix of red or white LEDs. On the other hand, LBSO crystal with large size also attracted research interest because they have more important practical applications. N. I. Leonyuk et al. tried to prepare LBSO single crystals by using high temperature flux method[11]. The authors used K2Mo3O10 and KF as solvent in order to reduce the melting temperature of the flux that containing LBSO. Single crystals of



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LBSO with size up to 3mm were obtained and the structure properties were refined. Unfortunately, large LBSO crystals with size up to centimeter have never been grown up to now, which has severely restricted the investigation of their macroscopic performance and their practical application. The difficulty on the growth of large size LBSO crystals is mainly arose from the nature its structure, since the chain or network arrangement of borates and silicates in the raw materials usually make them to be glass or ceramic phase when the melting chemicals cooled down. One possible solution is to reduce the viscosity of the flux at high temperature. The solvent that used by N. I. Leonyuk et al. is composed by K2Mo3O10 and KF, which are both easily volatile and could reduce the viscosity of the melting flux. However, the volatile KF is harmful to the environment and human health. The use of KF should be avoid in the flux for the growth of a crystal which has the potential of practical engineering application. The other difficulty of the growth of large size LBSO crystals is the optimization of the complex flux composition. A complex flux, which is always composed of three or more kinds of compounds, is usually applied to grow crystals of complicated compounds that melt incongruently or could not be grown directly from the melting of themselves. The Na3VO2B6O11 crystal, which is a candidate of nonlinear optical materials, was grown by a Na2CO3-V2O5 flux by X. Y. Fan et al[12]; An important ferroelectric material, KTaO3 single crystal, was successfully grown from a complex flux that containing K2CO3 and B2O3 by S. Zlotnik[13]; Recently, a new kind of nonlinear optical crystal named Rb3Ba3Li2Al4B6O20F, belonging to the KBBF-family, was obtained by Y. G. Shen et al. using a Li2O-BaF2-B2O3 self-flux system[14]; Another important nonlinear optical crystal, La2CaB10O19, was also reported to be grown from a complex flux composed by CaO, Li2O and B2O3[15-17]. Meanwhile, a famous nonlinear crystal Cs2B4SiO9 was also grown through flux method by using Cs2CO3 as the solvent[18]. It could be seen that the complex flux is crucial to grow large size crystals. Nevertheless, the composition of the fluxes mentioned above is usually determined empirically, which means that the exploration of flux formulation is usually a time-consuming and laborious procedure. So, a methodology that could guide the exploration and optimization of flux composition for crystal growth is necessary since it could make the experiments more efficiently and reduce the workload considerably. For example, the application of orthogonal test and uniform design in the material field makes the optimizing of material preparation and processing procedure more precise and


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efficient[19-24]. This indicates that we could introduce the experiment design technology into the exploration of flux formulation for crystal growth. The mixture design, an important type of experiment design technology, has been widely used in the field of food processing, pharmacy and composite materials design. M. Yolmeh et al. studied the multiple-strain mixture of Lactobacillus with the highest antimicrobial activity against common food-borne pathogenic bacteria. In their study, cell-free supernatant of L. plantarum, L. brevis (1) and L. brevis (2) were selected as the constituents and the experiments were performed under the direction of mixture design. The relationship between the components and the responses was studied using least square multiple regression method[25]. S. Saoudi et al. investigated the combined effect of carnosol, rosmarinic acid and thymol on the oxidative stability of soybean oil using a simplex centroid mixture design method, they found that the bioactive compounds are effective in maintaining oxidative stability of soybean oil[26]. The mechanical and cooking properties of cooked rice noodle as function of pasta composition was researched by M. A. Loubes and co-authors. In their study, the composition of pasta comprising gelatinized corn starch, guar gum and xanthan gum was formulated under the guidance of mixture design technology[27]. A simplex centroid mixture design was also used by O. J. Coker et al. to optimize ingredient combination comprising cassava starch, high quantity cassava flour and fish flour in the production of cassava-fish crackers[28]. Meanwhile, the mixture design method has also been successfully applied into the formulation of drugs such as matrix tablets containing diclofenac sodium, ispaghula husk powder, lactose, microcrystalline cellulose[29] and improved thymol containing soybean lecithin, stearic acid and sodium taurodeoxycholate[30]. In the materials science and engineering area, the mixture design was usually used in optimizing the preparation of composited materials that exhibit excellent performance. A composite material that formed by graphene, TiO2 and ZSM-5 were synthesized by X. Y. Hu et al. through mixture design experiments. The relation between the formulation of the composites and the photocatalytic performance was studied[31]. T. Skopak et al. used mixture design technology as a multivariate data analysis tool to investigate properties of gallate glasses containing Ga3O2, GeO2 and Na2O[32]. Meanwhile, the mixture design method was also applied to improving the performance of steel material, blend films and waterborne binders[33-35]. The research methods mentioned above all include changing mixture composition and exploring how such changes will affect the properties of the



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mixture. The number of components in the mixture are three or more and the portion of every factor in the mixture varies from 0 to 1. Most of all, the components could not be manipulated completely independent of one another because the sum of all of the factors should be 100%. In a typical flux system for crystal growth, the portion of solvents and solute are also 100%. Meanwhile, the solubility of the flux and quantity of the grown crystal are always determined by the relative content of the composition of flux. That’s to say, the mixture design technology could also be used in the crystal growth area to guide the exploration of flux formulating. In this paper, the growth of LBSO and Eu3+ doped LBSO crystals are studied. In order to grow LBSO and active ion doped LBSO crystals, compounds of LaBO3, SiO2 and Li2MoO4 are selected as constituents of the complex flux. By using the mixture design technology, the flux composition are screened and optimized. Meanwhile, the optical spectroscopy and electronic transition properties of LBSO:Eu3+ crystals are also investigated.

2, Experimental 2.1 Materials and methods LaBO3, SiO2 and Li2MoO4 were selected as the constituents of the flux to grow LBSO crystal. The starting materials, La2O3, H3BO3, SiO2, MoO3, Li2CO3, Eu2O3 with purity of 99.99% were purchased from Sinopharm Chemical Reagent Co. China and used without any further treatment. For each testing experiment of flux exploration, the sum amounts of the raw materials was constrained to be 30 gram when all of the volatile matters disappear. In a typical procedure of flux composition testing experiment, the raw materials were weighted and grinded thoroughly in an agate mortar. After a period of 30min grinding, the raw materials were packed into a platinum crucible. Then, the loaded crucible was transferred into a vertical furnace which has two observation windows on the top. The temperature of the furnace was raised to 1050 oC from room temperature in 6 hours. Once the furnace was heated up to 1050oC, it was kept at this temperature for about 10 hours to make the materials react thoroughly. Then, the furnace was cooled down to 850oC by 5-10 oC/h and then to room temperature rapidly. The precipitation of solutes in the flux occurs spontaneously during the cooling procedure. When the flux composition testing experiment done, the transparent precipitates of the mixture were collected and then treated by ultrasonic in an aqueous solution to make the attached flux remove thoroughly.


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The Eu2O3 was used as the starting material to prepare Eu3+ doped LBSO crystals (LBSO:Eu3+). Since the Eu3+ ions replace La3+ ions in the lattice of LBSO, the amount of Eu2O3 was calculated by stoichiometric formula of La1-xEuxBSiO5. Once the composition of the flux was optimized, the top-seeded solution growth (TSSG) method described in Ref. 36 and 37 was used to grow large size crystals. 2.2 The experimental design of flux composition exploration for LBSO crystal growth The experiments of flux composition exploration for LBSO crystal growth was performed by mixture design method. In a typical region of experimental design map, the portion of each component varies from 0 to 1, and the sum of all the portions equals to 100%[38, 39]. The composition of the complex flux containing LaBO3, SiO2 and Li2MoO4 was optimized by two rounds of testing experiments. In the first round of flux screening experiments, a regular region was used and all of the component varies from 0 to 1. The test points are determined by simplex lattice mixture method. The distribution of the test points and the experimental formulations are illustrated in Fig. 1 and Tab. 1, respectively. Meanwhile, the solubility and transparency of the flux system at 1050 oC was considered as the response and was quantitatively defined as follow: When the flux melts thoroughly and homogeneously at 1050 oC, the response value of the testing flux experiment is defined as 10; while the response value is defined as 0 when the flux never melt at 1050 oC. Once the first round of testing experiments done, the suitable content of LaBO3, SiO2 and Li2MoO4 in the flux for LBSO growth could be estimated roughly, which means that one or more relational constrains should be applied to get an irregular region in which the second round of mixture design experiments could be conducted. During the second round of testing experiments, the response of the testing experiment is defined by two factors: the solubility and the quantity of the LBSO crystals grown spontaneously during the cooling procedure. If there is no LBSO crystals grown during the cooling procedure, the response value of the flux is 0; when the quantity of the grown LBSO crystals enhances, the response value of the testing experiment increases but never exceeds 10.



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Fig. 1 The framework of low temperature phase LBSO crystal (The red balls are O atoms, green B atom, cyan Si atom and yellow La atom).

Fig. 2 The distribution of the test points in the regular region of mixture design, where A represents LaBO3, B Li2MoO4 and C SiO2. Tab. 1 The compositions of the test point in the first round of mixture design (molar ratio) Batch No.

A（LaBO3）

B（Li2MoO4）

C（SiO2）

1

1/3

1/3

1/3

2

0

1/2

1/2

3

1/4

1/4

1/2

4

1/2

0

1/2

5

0

1/4

3/4

6

1/4

1/2

1/4

7

1/4

3/4

0

8

0

0

1

9

1/2

1/4

1/4

10

3/4

0

1/4

11

0

3/4

1/4

12

3/4

1/4

0

13

2/3

1/6

1/6

14

1

0

0


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15

1/4

0

3/4

16

1/2

1/2

0

17

1/6

2/3

1/6

18

1/6

1/6

2/3

19

0

1

0

After two rounds of mixture design experiments, the region of the flux compositions will be optimized and the verification experiments could be performed. 2.3 Characterization The phase of the precipitates collected from the flux composition testing experiments were identified by powder XRD technology. The diffraction data were collected by using a Miniflex 600 type X-ray diffractometer operating with Cu Kα1 radiation and a graphite diffracted beam monochromator. The concentration of Eu3+ ion in the Eu3+ doped LBSO crystals were measured by the inductively coupled plasma atomic emission spectrometry (ICP-AES) method. The optical excitation and emission spectra of the LBSO:Eu3+ crystals were measured using the Edinburgh Analysis Instruments FLS920 spectrophotometer with Xenon lamp as light source. The luminescence decay curves of the LBSO:Eu3+ crystals were recorded by using a customized ultraviolet (UV) to mid-infrared steady-state laser and a phosphorescence lifetime spectrometer (FSP920–C, Edinburgh) equipped with a digital oscilloscope (TDS3052B, Tektronix) and a tunable midband Optical Parametric Oscillator (OPO) pulse laser as the excitation source (410–2400 nm, 10 Hz, pulse width of 5 ns, Vibrant 355II, OPOTEK). 3, Results and discussion 3.1 The testing results of the flux exploration Since the mixture design is a special technology of response surface experiment, the response of the experiments could reveal the relationships among variables and the influence of variables on the observed performance. In the first round of flux composition testing experiment, the values of the response y, namely the solubility of the flux, was assigned based on the observed transparency of the flux. As mentioned above, the values of x1, x2 and x3, representing the content of LaBO3, Li2MoO4 and SiO2, all ranges from 0 to 1, and the relation x1+x2+x3=1 is the only constrain.



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Fig. 3 Contour (a) and 3D (b) plots of the solubility in a regular region, the first round of flux testing based on mixture design, where A is the content of LaBO3 (x1), B is the content of Li2MoO4 (x2) and C is the content of SiO2 (x3). The response between x and y is: y=0.101x1+9.943x2+0.237x3+3.356x1x2+2.931x1x3+2.782x2x3-7.984x1x2x3-3.096x1x2(x1 -x2)+0.484x1x3(x1-x3)+3.580x2x3(x2-x3), R2 =95.13%. In order to reveal the influence of the variations of compositions of LaBO3, Li2MoO4 and SiO2 on the solubility of the complex flux, the response of the testing experiment of first round is graphically shown by a contour and 3-dimensional plots in Fig. 3. As exhibited in this figure, the content of Li2MoO4 seems to be the most important factor to improve the solubility of the flux, since the response of the testing experiment increases monotonically when the content of Li2MoO4 enlarges. Meanwhile, the solubility of the complex flux reduces when the relative content of LaBO3 or SiO2 becomes larger. The relative simple response surface curves with little singular points demonstrates that the interactions among the components LaBO3, Li2MoO4 and SiO2 is numerically weak. The high melting temperature of SiO2, LaBO3 and LBSO might the main reason for the decrease of solubility of the complex flux. It is found that the flux would melt mostly when the content of Li2MoO4 is larger than 0.7. The reliability of the fitting result was checked by the statistical method i.e. residual analysis. The residual plots of the flux solubility, including normal probability plot of residuals, plot of residuals versus the predicted values, histogram of frequency versus residual and plot of residual versus experiment order are displayed in Fig. 4. As shown in this figure, most of the plotted points are close to the fitted distribution line and also close together. Meanwhile, the residuals distribute evenly and randomly near the centerline. There is no obvious anomalous distribution


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in the residual plots, which means that the fitness of the flux solubility could be accepted.

Fig. 4 The residual error of the testing experiments in the first round: (a) Normal probility plot; (b) plot of residuals versus the predicted values; (c) histogram of frequency versus residual and (d) plot of residual versus experiment order Aiming to find the most suitable region of flux composition to get LBSO crystals, we performed the second round of testing experiments. The distributions of the testing point were also planed by the mixture design using the extreme vertices method. Based on the screening result of the first round, an additional constrain was introduced for optimizing: 0

Flux Exploration, Growth, and Optical Spectroscopic Properties of

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