Growth and characterization of Ba3InB9O18 single crystals

Growth and characterization of Ba3InB9O18 single crystals

Chemical Physics Letters 660 (2016) 136–142 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/lo...

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Chemical Physics Letters 660 (2016) 136–142

Contents lists available at ScienceDirect

Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Research paper

Growth and characterization of Ba3InB9O18 single crystals Xian-Shun Lv a,b, Qing-Gang Li a,c, Bing Liu a,b, Yuan-Yuan Zhang a,b, Lei Wei a,d, Yu-Guo Yang a,b, Xu-Ping Wang a,b,⇑, Jian-Hua Xu a,b, Ling Ma b, Ji-Yang Wang a,b a

Advanced Materials Institute, Shandong Academy of Sciences, Jinan 250014, China Key Laboratory for Light Conversion Materials and Technology of Shandong Academy of Sciences, Jinan 250014, China c Shandong Key Laboratory for High Strength Lightweight Metallic Materials, Jinan 250014, China d State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China b

a r t i c l e

i n f o

Article history: Received 6 July 2016 In final form 4 August 2016 Available online 4 August 2016 Keywords: Crystal growth Electronic structure Raman spectra DFT calculations Ba3InB9O18

a b s t r a c t Ba3InB9O18 is expected to be potential ultraviolet optical materials. In this work, a crack free and colorless transparent Ba3InB9O18 single crystal has been grown by the Czochralski technique. The as-grown Ba3InB9O18 crystals show good UV transparency with the cutoff wavelength locating at 195 nm. An indirect band gap of 5.25 eV was obtained from the calculated electronic structure results. The factor group analysis results show that the total lattice modes are 15Au + 14Bg + 36E1u + 28E2u + 34E2g + 26E1g + 17Bu + 16Ag, of which 17E2g + 13E1g + 16Ag are Raman-active. The strongest recorded Raman peak at 642 cm1 was assigned with the assistance of the first-principle calculation and factor group analysis results. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction With the increasing requirements of laser application, the demand of ultraviolet nonlinear optical crystal especially below 200 nm is growing. Scientists devote continuously to the prediction of new material properties in the recent years to make up for the disadvantages of time-consuming and blindness in exploring for new optical materials. It is well known that borate crystals have been widely used in linear and nonlinear optics due to their outstanding properties, such as wide transmittance range in the ultraviolet (UV) region, larger birefringence, low absorption edge, high optical damage threshold, good thermal stability and chemical stability, as well as many other interesting optical properties [1–3]. One theory named anionic group theory proves that the excellent optical properties of borates are mainly determined by the existence and arrangement of boron oxygen groups [3,4]. In borates, B atoms link with O atoms and construct BO3 triangles and/or BO4 tetrahedrons. BO3 triangles and BO4 tetrahedrons interconnect as building blocks forming more complicate oxoanions 5 6 such as B3O3 6 rings, triborates B3O7 , tetraborates B4O9 , or endless (B3O5)n chains. Based on the theoretical understanding of UV transmittance of the borates, we now know that the borate crystals containing B3O6 groups within the O dangling bonds of the borate ⇑ Corresponding author at: Advanced Materials Institute, Shandong Academy of Sciences, Jinan 250014, China. E-mail address: [email protected] (X.-P. Wang). http://dx.doi.org/10.1016/j.cplett.2016.08.008 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.

groups are saturated by ions with large electro-negativities, which is a better candidate for deep UV applications [5]. Recently, Cai et al. synthesized a new compound Ba3InB9O18 and studied its crystal structure [6]. In Ba3InB9O18, whose structure is made up by planar B3O6 rings parallel to each other along the (0 0 1) direction, regular InO6 octahedral units, and irregular BaO6 and BaO9 polyhedra, the BAO dangling bonds of B3O6 are tightly bonded to the large electronegativities In3+ ions. This means that the Ba3InB9O18 crystal ought to possess UV absorption edges with shorter wavelength. Ba3InB9O18 has been synthesized as not only potential phosphors but also new kinds of scintillation materials under vacuum ultraviolet (VUV) or X-ray excitation in 2008 [6]. The X-ray excited luminescence spectra indicate that Ba3InB9O18 is a promising scintillation material. Its light yield is about 75% as large as that of BGO powders under the same measurement conditions, and it exhibits a broad and intense emission band in 360–500 nm range peaking at 400 nm. In 2010, Eu3+/Tb3+ doped Ba3InB9O18 has been investigated under ultraviolet excitation as a phosphor material for the first time [7]. The dominant emission of Eu3+ and Tb3+ ions doped Ba3InB9O18 is at 590 nm and 550 nm under the excitation at 227 nm. Ba3InB3O9:Tb3+ is a potential green phosphor used in UV-LED chips for its chemical stabilization, simple preparation and 370 nm emission under 543 nm excitation [8]. DTA and TGA curves for Ba3InB9O18 show that it is a chemically stable and congruent melting compound, which means this crystal can be grown easily by the Czochralski method. However, the above research is focused

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on the luminescence properties. To the best of our knowledge no previous work on the crystal growth of Ba3InB9O18 has appeared in the literature. Theoretical exploration has shown that the electronic structure of indium borate is very sensitive to the cationic system [9] and under some conditions it may show promising nonlinear optical properties. In this paper, we present the crystal growth, X-ray powder diffraction (XRD) analysis, transmission spectrum, band structure, density of states and charge distribution of the single crystal of Ba3InB9O18. Further, the Raman spectra of Ba3InB9O18 are studied.

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The crystal Raman spectrum was recorded by using a LABRAM HR800 Raman spectrometer (Jobin Yvon, France). A He-Ne laser source (632.8 nm) with a power of 800 mW was introduced into the crystal with the incident beam perpendicular to the (0 0 1) crystal face. The Raman spectrum was collected in the range of 100–1800 cm1 in a backscattering configuration with a spectral resolution better than 2 cm1. The diameter of the laser beam was less than 2 lm. The spectral acquisition, under accumulated mode, was 10 s each time with 10 repetition times. 3. Computational details

2. Experimental details Ba3InB9O18 crystals were grown by the Czochralski technique. The starting materials were prepared by mixing BaCO3, H3BO3 and In2O3 powders with A.R purity in stoichiometric ratios according to the reaction:

6BaCO3 þ 18H3 BO3 þ In2 O3 ¼ 2Ba3 InB9 O18 þ 6CO2 " þ27H2 O "

Band structure, density of states, charge distribution as well as the Raman spectra were performed with the CASTEP code, based on density functional theory (DFT) [10]. The generalized gradient approximation with the scheme of Perdew–Bruke–Ernzerhof (GGA-PBE) [11] was chosen to represent the exchange–correlation functional in the DFT formalism. The structure reported by Cai was adopted as the initial structure and was optimized using the Broy den-Fletcher-Goldfarb-Shannon (BFGS) method [6]. Norm conserving pseudopotential is employed to describe the electron–ion interactions and plane-wave basis sets are used to represent electronic wave functions. The orbitals of B (2s2 2p1), O (2s2 2p4), In (5s2 5p1) and Ba (5s2 5p6 6s2) were treated as valence electrons. The k-point set was chosen as 3  3  3 Monkhorst-Pack grid; the energy cutoff of plane wave basis set was selected as 990 eV and the energy convergence criterion of electronic self-consistency was chosen as 5.0  107 eV/atom.

The H3BO3 was overweighed by 2 wt% in order to compensate for its evaporation loss during growth. The reactants were ground, mixed separately in an agate motor and preheated at 500 °C for 12 h. After triturating to uniform powder, the compounds were heated again at 950 °C for 24 h and cooled to room temperature. Then, an intermediate product was obtained. The above process was repeated to make sure that the chemicals react thoroughly. Finally, these compounds were pressed into pieces and placed into a Pt crucible with diameter of 40 mm and height of 60 mm. The raw material was slowly heated to 1100 °C and kept at this temperature for 12 h. Subsequently, the temperature was decreased to the crystallizing point and a Ba3InB9O18 seed crystal was introduced into the melt. The seed crystal can be obtained by contacting a platinum wire and then slowing down the temperature. During the crystal growth, the crystal rotational speed was kept at 8– 15 rpm and the pulling rate was 0.1–1 mm/h. After the growth was completed, the crystal was drawn out of the melt and cooled down to room temperature at a rate of 15–30 °C/h. The as-grown Ba3InB9O18 crystals were cut into slices and polished for various characterizations (Fig. 1). Fig. 1 shows the grown crystal was crack free and optically transparent. A small crystal sample was cut from the obtained Ba3InB9O18 crystal, and ground thoroughly for XRD (X-ray diffraction) characterization. The data were collected using Rigaku D/max-rB X-ray diffractometer with Cu Ka radiation (k = 1.54056 Å) in the 2h range 10–80° with a 0.02° 2h-step. The transmission spectrum of the Ba3InB9O18 crystal was measured by the UV–VIS–NIR Spectrometer (VARIAN CARY-5E) at room temperature from 175 to 2500 nm.

The XRD pattern (shown in Fig. 2(a)) is in good accordance with the reported data (shown in Fig. 2(b)) of Ba3InB9O18 [1], indicating the obtained powder is pure hexagonal Ba3InB9O18 phase. As illustrated in Fig. 3(a), Ba3InB9O18 crystallizes in a centric space group P63/m (no.176) with two molecular formula per unit cell. Three boron atoms are coordinated to six neighbor oxygen atoms to form a planar B3O6 ring (Fig. 3(b)). The planar B3O6 rings are perpendicular to the c-axis and parallel to each other in the opposite direction forming the B3O6 layers. In atoms are coordinated to six oxygen atoms belonging to a different B3O6 group to form a regular octahedron InO6, at equal bond distances of 2.1817 Å. The Ba atoms are bound to six or nine O atoms to form BaO6 plane hexagons or nine-vertex irregular BaO9 polyhedron. Thus, the basic units of Ba3InB9O18 are regular InO6 octahedra, irregular BaO6 hexagon,

Fig. 1. (a) Optical sample cut from Ba3InB9O18 crystal grown by the Czochralski method.

Fig. 2. Powder XRD pattern of pure Ba3InB9O18 crystal.

4. Results and discussion 4.1. X-ray diffraction analysis (XRD) and geometry optimization

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Fig. 3. (a) Primitive cell of Ba3InB9O18 and (b) the B3O6 planar six-membered rings.

BaO9 polyhedra and planar hexagonal [B3O6]3 ring. The building units are stack along the c-axis in the sequence of regular InO6 octahedra, planar [B3O6]3 rings, irregular BaO6 hexagon, planar [B3O6]3 rings, BaO9 polyhedra and planar [B3O6]3 rings. 4.2. Transmission spectrum of Ba3InB9O18 As mentioned before, the terminal O ions of BAO dangling bonds of B3O6 are bonded to In3+ ions, which means the Ba3InB9O18 crystal ought to possess high transmittance in the UV region. A polished Ba3InB9O18 crystal sheet with a thickness of 1.5 mm is used for the transmission spectrum measurement. Fig. 4 shows the transmission curve of Ba3InB9O18, which exhibits a high and constant transmittance in the measured wavelength region. The transparency is over 80% in the whole range of 300–3000 nm and decreases to 40% at 200 nm. The cutoff wavelength is about 195 nm for Ba3InB9O18 crystals, which can be used in deep ultraviolet optical device. 4.3. Band structure, density of states The calculated energy band structure and an enlarged band structure near the Fermi level are shown in Fig. 5(a) and (c). The maximum of valence band (VB) is located at the M point of the

Fig. 4. Transmittance spectrum of Ba3InB9O18.

Brillouin zone with a secondary maximum at the L point, while the minimum of the conduction band (CB) is located at the G point. Thus, Ba3InB9O18 is found to be an indirect band gap material with a large band gap of 5.25 eV. However, the experimental band gap is about 6.36 eV (corresponding to the experimental cutoff wavelength of 195 nm), which is larger than the calculated value. This dispersion is caused by the discontinuity of the exchangecorrelation in generalized gradient approximation. The total density of states (DOS) and orbital projected partial DOS (PDOS) for indium, oxygen, barium and boron are shown in Fig. 5(b) and (d). By comparing the total density of states and the partial density of states, we can conclude that the CB minimum is directly determined by the In 5s orbitals while the VB maximum is exclusively occupied by the O 2p orbitals. The PDOS can be divided into five regions: (1) two strong localized regions locating at 24.7 eV (Ba 5s) and 10.0 eV (Ba 5p), the electrons in these regions would be difficult to stimulate by external perturbations; (2) the energy states ranging from 20.25 to 16.58 eV mainly consist of B 2s2p and O 2s orbitals, and the large hybridization between B 2s2p and O 2s confirms the formation of strong covalent B O bonds; (3) the electronic structure between -7.82 eV and Fermi energy (EF) is mainly B 2p and O 2p states with small contributions of In 5s and In 5p states; and (4) a conduction band between 5.25 and 6.83 eV composing of In 5s, Ba 5d, B 2p and O 2p levels. 4.4. Charge density distribution We perform charge density analysis to investigate the bond behavior in Ba3InB9O18 crystal. For Ba3InB9O18 crystal, we chose two planes perpendicular to c-axis: (1) a plane containing the B3O6 rings and Ba atoms; (2) a plane containing the In atoms. As shown in Fig. 6, three B atoms bond with six O atoms and form B3O6 ring. The B3O6 ring is bonded with Ba and In atoms in the adjacent layers. For all BAO bonds, the charge distribution of oxygen is deformed toward the boron atoms, which shows the BAO bonds in B3O6 rings have typical covalent character. The charge distribution among the O atoms is average resulting from their structure equivalence. The charge distribution of the Ba and In sites is almost spherical as shown in Fig. 6(a) and (b). This reveals that the bonding between Ba or In atoms and oxygen is of significant ionic character. 4.5. Raman spectra analysis In order to further confirm the coordination surroundings of BAO in the Ba3InB9O18 structure, a Raman spectrum for Ba3InB9O18 is measured at room temperature (as shown in Fig. 7(a)) and assigned with the assistance of calculated Raman spectra (as shown in Fig. 7(b)) and atom displacement. Ba3InB9O18 crystallizes in a centric space group P63/m (no.176) with two molecules (62 atoms) per unit cell resulting in 186 vibrational normal modes. The site symmetry of In atoms is S6 whereas Ba atoms possess two difference site symmetry (C3 and C6h) and [B3O6]3 rings possess C3 and C3h, respectively. Analysis of the vibrations of an isolated B3O6 anion with a point symmetry group D3h yields three A01 modes (m14-symmetric BAO0 stretching mode, m13-BAOAB bending mode and m12-symmetric BAO0 stretching mode), two A02 modes (m11-in plane bending mode and m10-in plane bending mode), two A02 modes (m9-out of plane bending mode and m8-out of plane bending mode), five E0 modes (m7-out of plane bending mode, m6-Out of plane bending mode, m5-antisymmetric BAO0 stretching mode, m4-antisymmetric BAO stretching mode and m3-antisymmetric BAO stretching mode) and two E0 modes (m2-OABAO0 bending mode and m1-BAOAB bending mode). The BAO refers to ring oxygen atoms and BAO0 to extra ring oxygen atoms. From the 21

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Fig. 5. (a) The calculated energy band structure of Ba3InB9O18, (b) PDOS of constituent atoms of Ba3InB9O18, (c) the enlarged energy band in the vicinity of the Fermi level and (d) the calculated PDOS results of Ba3InB9O18.

Fig. 6. Charge density distribution of (a) (B3O6)3 anion, (b) Mg2+ anion and (c) In3+ anion in Ba3InB9O18 crystal perpendicular to c-axis.

normal modes, the A01 and E0 species are only Raman active, the A02 are only IR active, and the E0 are both Raman and IR active, whereas the A02 species are inactive. Considering each primitive cell contains six B3O6 units, we conclude that the internal modes of the B3O6 units give rise to 162 lattice vibrations (9Au + 9Bg + 12E1u + 9E2u + 12E2g + 9E1g + 12Bu + 12Ag). Similarly, we can deduce that

18 lattice vibrations (2Au + 2Bg + 2E1u + 1E2u + 2E2g + 1E1g + 1Bu + 1Ag) originate from the translational modes of the B3O6 units, and 18 lattice vibrations (2Au + 2Bg + 1E1u + 2E2u + 1E2g + 2E1g + 1Bu + 1Ag) originate from the rotational modes. Ba atoms are in C3v symmetry positions, which results in 1Au + 2Bg + E1u + 2E2u + 2E2g + 1E1g + 2Bu + Ag modes for each of them; In atoms

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X.-S. Lv et al. / Chemical Physics Letters 660 (2016) 136–142 Table 2 Comparison of calculated and observed vibrational frequencies and assignments of Raman-active vibrational modes of Ba3InB9O18. Symmetry

Mode

mcal

mexp

Approximate descriptiona

Ag(C3)

A01

1529

m14 symmetric BAO0

1499 766 768 681 629

1513,1526, 1543,1552 n 783 n n 642

1110 1074 579 600

n n 597 n

m11 in plane bending

Ag(C3h) Ag(C3) Ag(C3h) Ag(C3) Ag(C3h)

Fig. 7. (a) Experimental and (b) calculated Raman spectra of the Ba3InB9O18 crystal.

occupying highly symmetric C6h positions generate 2Au + 2E1u modes. Summing all these modes and subtracting the 1Au + 1E1u acoustic modes, one gets the following optical vibrational modes of the crystal:

Copt ¼ 14Au þ 14Bg þ 17E1u þ 14E2u þ 17E2g þ 13E1g þ 17Bu þ 16Ag in which 14Au + 17E1u are IR active ones, 17E2g + 13E1g + 16Ag are Raman active modes, and 14Bg + 17Bu + 14E2u are spectroscopically inactive (silent modes). E1u, E2u, E2g and E1g modes are twofold degenerated and often considered as one mode. The correspondence between modes in the free ring and those in the lattice is established in Table 1. The calculated Raman spectra are shown in Fig. 7(b). Using the Lorentz fitting procedure, the calculated frequencies of Raman peaks are extracted and listed in Table 2. A comparison of the calculated and observed Raman spectra shows good agreement. Thus, the observed Raman-active fundamentals can be assigned more reliably by contrasting the experimental and calculated Raman intensities. Based on previous reported results [12–15], we identify and assign the Raman spectra of Ba3InB9O18 by comparing the vibrational frequencies and atom displacements of free [B3O6]3 metaborate ring and in crystal. The recorded Bands above 400 cm1 should be due to internal vibration arising from the B3O6 planar ring structures. As mentioned above, [B3O6]3 rings possess two different site symmetry C3 and C3h. Thus, due to crystalline field effect, the same modes arising from the B3O6 ring will split into two or three modes, which lead to two or three different shifts. However, only one line can be distinguished in both

a

stretching

Ag(C3) Ag(C3)

A02

728 162

n n

m9 out of plane bending m8 out of plane bending

E1g(C3) E2g(C3) E2g(C3h) E1g(C3) E2g(C3) E2g(C3h) E1g(C3) E2g(C3) E2g(C3h) E1g(C3) E2g(C3) E2g(C3h) E1g(C3) E2g(C3) E2g(C3h)

E0

1379 1378 1400 1213 1201 1242 950 946 959 499.4 499.9 478.4 414.3 414 402

1366 n n n n n n n n 485 499 n n 419 n

m7 out of plane bending

E1g(C3) E2g(C3) E2g(C3h) E1g(C3) E2g(C3) E2g(C3h)

E0

685 676 651 247 243 218

n 662 n n n n

m2 OABAO0 bending

m10 in plane bending

m6 out of plane bending m5 antisymmetric BAO0 stretching

m4 antisymmetric BAO stretching

m3 antisymmetric BAO stretching

m1 BAOAB bending

BAO refers to ring oxygen atoms and BAO0 to extra ring oxygen atoms.

calculated spectra and experimental spectra because of the slight discrepancy in frequencies. As an example, we analysis the three Ag lines arising from the A01 modes of the B3O6 ring. The symmetries of the Raman active modes, as well as their calculated and experimental Raman shifts are listed in Table 2. Fig. 8 schematically displays the atomic displacements of the internal modes. The atomic displacements of other internal modes are shown in Figs. S1–S3. The strongest experimental Raman peak locates at 642 cm1, which is assigned

Site symmetry C 3

A1 '

A

A2'

E

Factor group symmetry C 6h Ag Au Bg Bu

Site symmetry C 3h A'

E"

m12 symmetric BAO0

A02

Free ion symmetry D 3h

E'

m13 BAOAB bending

Ag(C3) Ag(C3h) Ag(C3) Ag(C3h)

Table 1 Correlation diagram for the B3O6 internal vibrations in Ba3InB9O18.

A2"

stretching

A" E' E"

E 1g E 1u E 2g E 2u

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Fig. 8. Calculated atomic displacements of the internal vibrational modes of the Ba3InB9O18 crystal arising from the A01 mode modes of the B3O6 rings.

to the breathing vibration of the B3O6 ring. The Raman peak is commonly regarded as the characteristic Raman shift of the B3O6 ring because this Raman shift is insensitive to the environment around the B3O6 ring (m12). For example, the values are 630 cm1, 637 cm1, 636 cm1 and 645 cm1 in a-BBO [16], b-BBO [14], YBa3B9O18 [13] and Ba2Mg(B3O6)2 [12] crystals (all of them contain B3O6 rings), respectively. Four Raman peaks located at 1508 cm1,

1520 cm1, 1539 cm1 and 1559 cm1 arise from one vibration, the stretching vibration of the extra-ring BAO bonds (m12). The vibration splits into four peaks in the experimental spectrum because of an isotopic effect. According to the reported results [12,14], the Raman shifts of 1508 cm1, 1520 cm1, 1539 cm1 and 1559 cm1 originate from the metaborate rings 11B3O6, 10B11B2O6, 10 B211BO6 and 10B3O6 rings, respectively.

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5. Conclusions

References

Colorless and transparent single crystals of Ba3InB9O18 were grown by the Czochralski technique. X-ray transmission and electronic structure calculations based on first principles method show that Ba3InB9O18 is an indirect band gap material with band gap of 5.25 eV. The nature of chemical bonding analysis indicates that the interaction between boron and oxygen is mainly covalent, while BaAO and InAO bonds have significant ionic character. According to the electronic structure, the top of valence band is mainly from the contribution of the O 2p orbitals, and the In 5 s orbitals determine the CB bottom. Factor group analysis show that 46 lattice vibrations (17E2g + 13E1 g + 16Ag) are Raman active. First-principles calculations are carried out to interpret the Raman spectrum by using a hybrid DFPT/finite displacement method. According to the calculated results, the vibrations with the Raman shift above 400 cm1 are linked to the internal modes of the B3O6 rings. All of the recorded Raman peaks have been assigned with the assistance of the calculated atom displacements. The strongest experimental peak at 642 cm1 is attributed to the breathing vibration of the B3O6 rings.

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Acknowledgments This work is supported financially by the National Science Foundation of China (Grant No. 51302158), Natural Science Foundation of Shandong Province (Grant No. ZR2014EMQ004), Youth Foundation of Shandong Academy of Sciences (Grant Nos. 2014QN027 and 2014QN028) and China Postdoctoral Science Foundation (Grant No. 2015M582090). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.08. 008.