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Aug 1, 2016 - and Kui Wu. †. †. Key Laboratory of Functional Materials and Devices for Special Environments of CAS; Xinjiang Key Laboratory of Ele...
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KBa (BO)F: A New Potassium Barium Borate Fluoride with a Perovskite-Like Structures Lili Liu, Yun Yang, Qun Jing, Xiaoyu Dong, Zhihua Yang, Shilie Pan, and Kui Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05489 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 4, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

K5Ba10(BO3)8F: A New Potassium Barium Borate Fluoride with a Perovskite-Like Structures Lili Liu,a,b Yun Yang,a* Qun Jing,c Xiaoyu Dong,a Zhihua Yang,a Shilie Pan,a* Kui Wua a

Key Laboratory of Functional Materials and Devices for Special Environments of

CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry of CAS, 40-1 South Beijing Road, Urumqi 830011, China b

University of Chinese Academy of Sciences, Beijing 100049, China

c

Department of Physics, College of Sciences, Shihezi University, Shihezi, 832000,

China

ABSTRACT: A new borate fluoride, K5Ba10(BO3)8F, has been discovered through spontaneous crystallization, which is the first reported compound in the potassium barium borate fluorides system. The single-crystal X-ray structural analysis shows that K5Ba10(BO3)8F crystallizes in the trigonal space group R 3 c. The parameters of the trigonal unit cell are a=15.293(2) Å, c=22.699(3) Å, and Z=6. The structure of K5Ba10(BO3)8F features a perovskite-like structure, and it can be written as [K3(K/Ba)6(BO3)8]FBa6. Meanwhile, the structure of K5Ba10(BO3)8F exhibits an intricate three-dimensional (3D) network composed of interpenetrated K-B-O net and Ba-O-F net. K5Ba10(BO3)8F possesses a large experimental band gap of 6.05 eV and an ultraviolet (UV) cut-off edge of about 205 nm proved by diffuse reflection spectrum. The birefringence of K5Ba10(BO3)8F was also evaluated by the first-principles

calculations.

Thermal

behavior

and

Raman

spectrum

of

K5Ba10(BO3)8F were also reported in this work.

INTRODUCTION Borate crystals have drawn enormous attention attributable to their wide range of

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applications as laser hosts, phosphors, electrode materials, birefringent optics and nonlinear optical (NLO) materials.1-3 The structural possibilities of borates are immense, which stems from the ability of boron to coordinate to three or four oxygen atoms forming trigonal planar or tetrahedral building blocks.4 These two basic building blocks can be connected by means of corner- or edge-sharing oxygen atoms5-6 to form infinite varieties of chains, sheets, or 3D frameworks, which endow borates rich structural chemistry and diverse applications. We focused on synthesizing new compounds with isolated B-O groups, as these materials may possess interesting functional properties.7-9A statistical analysis of borate fundamental building blocks, carried out by P. Becker,10 indicated that compounds with isolated borates can be obtained by increasing the ratio of cations/boron (M/B). Furthermore, attributable to relatively large electron affinity differences between boron and oxygen, borate crystals often show a wide transparency range and a high resistance to laser or other radiation damage, especially when the A-site cations are alkaline or alkaline earth metals.11-15 Moreover, the strong electronegativity of fluorine atoms would cause a large energy band gap, thus resulting in short UV cut-off edge in the compound, as well as expand the structural diversity.16-18 Considering the merits of alkaline and/or alkaline earth borates containing fluoride, a great deal of effort has been expended by our group to explore these systems. After a series of systematic experiments, a new potassium barium borate fluoride, K5Ba10(BO3)8F, was discovered. K5Ba10(BO3)8F is the first material reported in K-Ba-B-O-F system. Its M/B ratio is around 1.88. The large M/B ratio and smaller electronegativity of K+ and Ba2+ ions together lead to the exclusive existence of isolated BO3 triangles.5 The crystal structure of K5Ba10(BO3)8F consists of two (4,6)-connected nets, a Ba-O-F net and a K-B-O net, and it displays a perovskite-like structure.19 Herein, we report the synthesis, crystal structure, thermal stabilities, and Raman and UV-Vis-NIR diffuse reflectance spectra for K5Ba10(BO3)8F. Theoretical calculations were also performed to analyze the relationships between crystal structure and optical properties. Moreover, the crystal structures of other existing complex alkaline and alkaline earth borate fluorides are also discussed in this

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The Journal of Physical Chemistry

paper.17,20-33

EXPERIMENTAL AND CALCULATION METHODOLOGY Synthesis. Single crystals of K5Ba10(BO3)8F were grown from a high temperature solution by using KF-B2O3 as flux. This solution was prepared in a platinum crucible by melting a mixture of K2CO3, BaCO3, BaF2 and B2O3 at a molar ratio of 2.5:0.5:1.5:1. The mixture was heated in a programmable tube furnace to 800 ºC to ensure transparency of the melt, and held at this temperature for 10 h to ensure homogeneity. The homogenized solution was cooled slowly (2 ºC/h) to 500 ºC, and then cooled to room temperature at a rate of 10 ºC/h. Using this cooling profile, small crystals were obtained (Figure 1).

Figure 1. Small crystals of K5Ba10(BO3)8F (the minimum scale of the ruler is one millimeter).

Single-Crystal X-ray Diffraction. A transparent block-shaped crystal with dimensions of 0.106×0.085×0.105 mm3 was mounted on a glass fiber with epoxy for the structure determination. The crystal structure of K5Ba10(BO3)8F was determined at room temperature by single-crystal XRD on an APEX II CCD diffractometer using monochromatic Mo-Kα radiation (λ=0.71073 Å). Moreover, absorption corrections were carried out using the SCALE program for area detector and integrated with the SAINT program.34 All calculations were performed with programs from the SHELXTL crystallographic software package.35 All of atoms were refined using full matrix least-squares techniques with anisotropic thermal parameters, and final least-squares refinement is on Fo2 with data having Fo2≥2σ(Fo2). The crystal structure of K5Ba10(BO3)8F was checked for missing symmetry elements by the program

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PLATON,36 and no higher symmetries were found. During the refinement we determined that the K(2) and Ba(2) atoms were set to share the same sites with the same anisotropic displacement parameters, and this situation has also appeared in some other crystal structures, such as KBa2(CO3)2F,[18] KBaY(BO3)2,[37] and KBaRE(B3O6)2 (RE=Y, Eu, and Tb).[38] The refined site occupation factors converge to 0.333/0.667 for K(2)/Ba(2). The EDS elemental analyses on single crystals of K5Ba10(BO3)8F confirmed that K(2)/Ba(2) molar ratio of 5.7:11.2, that is in good agreement with the stoichiometric proportions from single-crystal X-ray structural analyses. Relevant crystallographic data, atomic coordinates, and equivalent isotropic displacement parameters are listed in Tables 1 and 2. Interatomic bond lengths and angles are given in Table S1 in the Supporting Information.

Table 1. Crystal data and structure refinement for K5Ba10(BO3)8F. Temperature (K)

296(2)

Crystal system, space group a (Å) b (Å) c (Å) Volume (Å3) Z, Calculated density (Mg/m3) Absorption coefficient (mm-1) F(000) Crystal size (mm3) θ range for data collection (˚)

Trigonal, R 3 c 15.2929(17) 15.2929(17) 22.699(3) 4597.4(9) 6, 4.461 13.395 5376 0.106×0.085×0.105 2.36 to 27.45

Limiting indices R(int) Goodness-of-fit on F2 R/wR [I>2σ(I)][a] R/wR (all data)[a] Extinction coefficient [a]

-18≤h≤19 -19≤k≤13 -29≤l≤21 0.0493 1.099 0.0339/0.0936 0.0381/0.0965 0.00036(3)

R1=Σ||Fo|-|Fc||/Σ|Fo| and wR2=[Σw(Fo2-Fc2)2/ΣwFo4]1/2 for Fo2>2σ(Fo2)

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Powder X-ray Diffraction. X-ray diffraction patterns were obtained on an automated Bruker D2 X-ray diffractometer equipped with a diffracted beam monochromator set for Cu-Kα radiation (λ=1.5418 Å) at room temperature in the angular range of 2θ=10~70° with a scan step of 0.01° and a fixed counting time of 0.5 s/step. The collected XRD intensity data were analyzed by the GSAS package.39 The powder XRD pattern for the pure powder samples and Rietveld refinement of K5Ba10(BO3)8F are displayed in Figure S1 in the Supporting Information. As seen in Figure S1 in the Supporting Information, the experimental data and calculated data from the single crystal modes are in excellent agreement indicating the structural model is correct. Raman Spectroscopy. The Raman spectra of the ground crystals were measured on a LABRAM HR Evolution spectrometer using 532 nm radiations at an integration time of 15 s. The laser power of 50 mW with beam diameter about 35 µm was used. UV-Vis-NIR Diffuse Reflectance Spectrum. UV-Vis-NIR diffuse reflectance data of

K5Ba10(BO3)8F

were

collected

on

a

Shimadzu

SolidSpec-3700DUV

spectrophotometer over the 190~2600 nm spectral range at room temperature. Tetrafluoroethylene was used as a diffuse reflectance standard. The reflectance spectrum was converted to absorption using the Kubelka-Munk function: F(R)=(1-R)2/2R=K/S, where R is the reflectance, K is the absorption, and S is the scattering.40

Table 2. Atomic coordinates and equivalent isotropic displacement parameters (Å) for K5Ba10(BO3)8F. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. Atom

Wyck. Occupancy x/a

y/b

K(1)

18e

1

0.51706(13) 0.18373(13) 0.0833

2(1)

Ba(1)

36f

1

0.52585(4)

0.16547(4)

0.25898(2)

12(1)

K(2)/Ba(2) 36f

0.33/0.67

0.66054(5)

0.07319(5)

-0.00223(3) 13(1)

B(1)

36f

1

0.6971(7)

-0.1083(7)

-0.0397(4)

12(2)

B(2)

12c

1

2/3

1/3

-0.0031(7)

11(3)

O(1)

36f

1

0.6522(5)

0.2367(5)

-0.0054(3)

20(1)

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z/c

U(eq)

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O(2)

36f

1

0.4581(5)

0.0301(5)

0.1762(3)

20(1)

O(3)

36f

1

0.5158(5)

-0.0015(5)

0.0842(3)

18(1)

O(4)

36f

1

0.7851(5)

-0.0182(5)

-0.0449(3)

21(1)

F(1)

6b

1

2/3

1/3

1/3

21(3)

Thermal Analyses. The thermal behavior of K5Ba10(BO3)8F was investigated on a NETZSCH STA 449C simultaneous thermal analyzer at a temperature range of 40~1400 °C with a heating rate of 5 °C min-1 under a constant flow of N2. Theoretical Calculations. The electronic structure and the optical property of K5Ba10(BO3)8F were calculated using the CASTEP package.41-42 As shown in Table 2, there is one K/Ba ion sitting in the same position, hence the virtual crystal approximation (VCA)43 method was used to calculate the band structures and density of states (DOS). During the calculation, the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional was adopted.44 Under the norm-conserving pseudopotential (NCP),45-46 the following orbital electrons were treated as valence electrons: B:2s22p1, O:2s22p4, F:2s22p5, K:3s23p64s1, and Ba:5s25p66s2. The kinetic energy cutoffs of 940 eV was chosen, and the numerical integration of the Brillouin zone was performed using a 2×2×4 Monkhorst-Pack k-point sampling. The other calculation parameters and convergent criteria were the default values of the CASTEP code. In order to evaluate the refractive indices and birefringence, two modified models were built. It is well known that for an alkaline/alkali-earth metal borates, the contribution to the birefringence from the alkaline/alkaline earth metal atoms are very small.47-48 The modified models can be built from: (1) the atoms on the K/Ba position were set as Ba atoms with excess charges, named KBaBOF-Basurplus; (2) all the atoms at the K/Ba position were set as K with excess charges, named KBaBOF-Ksurplus. The refractive indices and birefringence of these models were then calculated using the CASTEP package. After the electronic structures obtained, the imaginary and real part of the dielectric constant and then the refractive indices can be obtained.49-50

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The Journal of Physical Chemistry

RESULTS AND DISCUSSION Crystal Structure of K5Ba10(BO3)8F. Title compound K5Ba10(BO3)8F crystallizes in a trigonal crystal system with a centric space group of R 3c (no. 167). Its unit cell and asymmetric unit are shown in Figure S2 in the Supporting Information. In the asymmetric unit, K5Ba10(BO3)8F contains two K atoms, two Ba atoms, two B atoms and four O atoms (Figure S2b in the Supporting Information). The K(1) cation is bonded to eight O atoms to form a [K(1)O8] polyhedron, and the K-O bonds range in 2.704(7)~2.972(7) Å. The Ba(1) atom coordinates with eight O atoms and one F(1) atom to from a [Ba(1)O8F] polyhedron. The Ba-O bond distances range from 2.598(6) to 3.092(7) Å and the Ba-F bond length is 2.923(1) Å. For K(2) and Ba(2), they share the same sites with a mixed occupancy model of Ba 67%/K 33%, and it is connected to eight O atoms to form the [K(2)/Ba(2)O8] polyhedron. The length of K(2)/Ba(2)-O bonds ranges from 2.568(6) to 3.031(7) Å. The two unique B atoms are all bonded to three O atoms to form BO3 triangles, with the B-O distances ranging from 1.367(11) to 1.391(11) Å (Figure S3 and Table S1 in the Supporting Information). Because the BO3 triangles are isolated, then we can describe the structure of K5Ba10(BO3)8F as a cationic framework. The whole crystal structure of K5Ba10(BO3)8F can be regarded as a perovskite-like structure. In the structure, six Ba(1) and one F(1) atoms form a FBa(1)6 octahedra centered F atom, which is associated with TiO6 octahedra of perovskite (Figure 2). It is well known that the molecular formula of perovskite is CaTiO3. By analogy with CaTiO6, the positions of Ca cations are occupied by the large [K3(K/Ba)6(BO3)8] clusters, the positions of Ti atoms are similar to those of F atoms, and the positions of O atoms are similar to the positions of Ba atoms, and K5Ba10(BO3)8F can be written as [K3(K/Ba)6(BO3)8]FBa6 (Figure 2).

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Figure 2. (a) The perovskite-like structure of K5Ba10(BO3)8F; (b) the perovskite structure of CaTiO3.

From another aspect, the structure of K5Ba10(BO3)8F exhibits an intricate 3D network composed of [Ba(1)6O30F] clusters, [K(1)3(BO3)8] clusters and [K(2)/Ba(2)O8] groups, and can also be described as two networks (Ba(1)-O-F net and K(1)-B-O net) that are interweaved (Figure 3 and Figures S4, S5 and S6 in the Supporting Information). A topological approach has been applied to better understand the nature of the anionic structure, so the 3D Ba(1)-O-F net and K(1)-B-O net can be simplified as (4,6)-connected nets with topological type of pcu and Schläfli symbol51-54 of {412·63}, by considering the [Ba(1)6O30F] and [K(1)3O18] clusters as six-connected (6c) nodes (Figure 3). K(2)/Ba(2) atoms locate in the space of Ba(1)-O-F net (Figure S7 in the Supporting Information). Figure S8 in the Supporting Information describes the geometry pattern and aihedral angle of BO3 groups. B(1)O3 groups form a coaxial sturcture along c-axis, and the angles between two B(1)O3 groups is 7.0(3)°, 76.6(3)°, 81.7(3)° and 87.2(3)°. B(2)O3 groups are all parallel with a-b plane and vertical to c-axis, and the relative rotation angles between neighboring B(2)O3 group are about 60.1(4)°and 16.5(5)°. The angle between B(1)O3 and B(2)O3 is about 60.8(3)°.

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Figure 3. (a) View of the pcu Ba(1)-O-F net; (b) view of the pcu K(1)-B-O net; (c) the interpenetrated pcu Ba(1)-O-F net and pcu K(1)-B-O net. Color code: green, [Ba(1)6O30F] nodes; fuchsia, [K(1)3(BO3)8] nodes.

Crytal Structure Comparition. Based on a survey of the Inorganic Crystal Structure Database

(ICSD,

http://www2.fiz-karlsruhe.de/icsd_web.html)

and

recent

publications, alkaline and/or alkaline earth borates containing fluorine are summarized in Table S2 in the Supporting Information. According to whether fluorine atom is connected with boron atom or not, these alkaline and/or alkaline earth borates containing fluorine can be classed into two genres, “fluoroborate” and “borate fluoride”.16 Based on their different cationic elements, the fluoborate and borate fluorides listed in Table S2 in the Supporting Information can be divided into three classes: 1) compounds containing both alkaline and alkaline earth metals: there are 20 members in this group which have also been listed in Table 3, such as KBe2BO3F2,17 Ca3Na4LiBe4B10O24F,30 KMg2Ba7B14O28F5,33 Li3Ca9(BO3)7·2(LiF),27 and they are all borate fluorides; 2) compounds containing only alkali metals: there is one borate fluoride (Li6RbB2O6F), and five fluoborates (KBOF2,56 LiB6O9F,57 Li2B6O9F2,58 Li2B3O4F359 and Na3B3O3F660) in this group; 3) compounds containing only alkaline earth metals: in this family, there is only one fluoborate (BaBOF361), and 20 borate fluorides, such as Ba4B10O21F,62 Sr3B6O11F2,63 Ba3B6O11F217 and Sr5(BO3)3F.64

Table 3. The existing complex alkaline and alkaline earth borate fluorides, space group, cation:boron ratio, and borate groups. Cation : Boron Ratio

Compounds

Space group

1

NaBe2(BO3)F220

C2

3

isolated BO3

2

KBe2(BO3)F221

R32

3

isolated BO3

3

RbBe2(BO3)F217

R32

3

isolated BO3

4

CsBe2(BO3)F217

R32

3

isolated BO3

B-O groups

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5

KBe2(BO3)F222

R 3c

3

isolated BO3

6

RbBe2(BO3)F223

R 3c

3

isolated BO3

7

Sr0.23Ca0.77Na2Be2B2O6F224

R3

2.5

isolated BO3

8

NaBe3Sr3(BO3)3F425

R3m

2.33

isolated BO3

9

KCaBe2(BO3)2F26

P 3 1c

2

isolated BO3

10 KSrBe2(BO3)2F26

P63/m

2

isolated BO3

11 NaCaBe2(BO3)2F26

Cc

2

isolated BO3

12 Li3Ca9(BO3)7(LiF)227

P1

2

isolated BO3

13 LiBa12(BO3)7F428

I4/mcm

1.86

isolated BO3

14 NaBa12(BO3)7F428

I4/mcm

1.86

isolated BO3

15 Li0.8Mg2.1B2O5F29

P21/n

1.45

isolated B2O5

16 LiNa4Be4Ca3B10O24F30

R3

1.2

isolated BO3 and [B12O24]

17 Na3Ba2(B3O6)2F31

P63/m

0.833

isolated [B3O6]

18 Rb18Mg6(B5O10)3(B7O14)2F32 C2/c

0.828

isolated [B5O10] and [B7O14]

19 Cs18Mg6(B5O10)3(B7O14)2F32

C2/c

0.828

isolated [B5O10] and [B7O14]

20 KMg2Ba7B14O28F533

C2/c

0.71

isolated [B7O14]

For the existing complex alkaline and alkaline earth borate fluorides, the existence form of B-O anionic groups in the crystal structures of borate fluorides were summarized as shown in Table 3. It is well known that increasing the number of cations can decompose the dimension of B-O frame and restrain the polymerization of B-O anions effectively.65 After comparison, it is found that B-O frameworks of the compounds listed in Table 3 are divided into isolated pieces by the quantitatively dominant cation, and their M/B ratio range from 0.71 to 3. The first 14 compounds in Table 3 content isolated BO3 units exclusively and corresponding M/B is from 1.86 to 3. With the ratio M/B decreasing slight, B-O groups in Li0.8Mg2.1B2O5F change into [B2O5] dimer. When M/B is 1.2, a large cluster [B12O24] emerges and coexists with BO3 group for LiNa4Be4Ca3B10O24F. In the structures of Rb18Mg6(B5O10)3(B7O14)2F

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and Cs18Mg6(B5O10)3(B7O14)2F, there are two kinds of relatively high-polymerized polyborate B-O group, [B5O10] and [B7O14] with M/B value of 0.83. For KMg2Ba7B14O28F5,

M/B

value

is

down

to

0.71,

which

results

that

higher-polymeric-level [B7O14] units remain exclusively with lower-polymeric-level [B5O10] groups disappearing (Figure S9 in the Supporting Information). Besides cation content, it should be noticed that, the configuration of B-O groups might be associated with the electronegativity of cations. The liability of cations transferring their electronic charge to the neighbor O atoms would be beneficial for the formation of the BO3 planar groups.5, 66 For these isolated B-O anions-containing compounds, their resemblance is that, the isolated B-O groups raise the number of nonbonding O atoms, which would be harmful the cut-off edge of materials.67 So for the Be-containing compounds in Table 3, their isolated B-O groups banded with Be atoms to construct Be-B-O layer or framework to eliminate all the original dangling bonds, which endow KBe2BO3F family broad band gap reaching deep-UV region. MgO4 groups can also act as linkers to joint isolated B-O groups together to remove the dangling bonds due to the relatively strong covalence of Mg-O bond, like Li0.8Mg2.1B2O5F, KMg2Ba7B14O28F5, Rb18Mg6(B5O10)3(B7O14)2F Li3Ca9(BO3)7(LiF)2,

and

Cs18Mg6(B5O10)3(B7O14)2F.68

LiBa12(BO3)7F4,

NaBa12(BO3)7F4

and

In title

terms

of

compound

K5Ba10(BO3)8F, the higher M/B value and lower-electronegativity cations generate the compositions of isolated BO3 groups and indelible dangling bonds. Raman Spectrum. As shown in Figure S10 in the Supporting Information, Raman spectrum displays strong absorption peaks at 583, 897 and 932 cm−1 for the title compound. The peaks around 583 cm-1 are characteristics of the bending vibration of triangular BO3. The peaks of 897 and 932 cm−1 can be assigned to the characteristic absorptions of the symmetric stretching vibration of BO3.69 UV-Vis-NIR Diffuse Reflectance Spectrum. The UV-Vis-NIR diffuse reflectance spectrum of K5Ba10(BO3)8F and the plots of [F(R)hʋ]1/2 versus hʋ were created,70-71 which illustrate that the title compound has a UV cut-off edge of 205 nm with a moderate band gap of 6.05 eV (Figure S11 in the Supporting Information). From the

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description of crystal structure, it was found that all O atoms in K5Ba10(BO3)8F have dangling bonds, which maybe the reason why the cut-off edge of K5Ba10(BO3)8F just reaches the UV region rather than deep-UV region. Similar to K5Ba10(BO3)8F, all the O atoms in Li3Ca9(BO3)7(LiF)2 and LiBa12(BO3)7F4 (NaBa12(BO3)7F4) also have dangling bonds, and their cut-off edges are about 230 and 220nm, respectively.27-28 However, for KBe2(BO3)F2, its dangling bonds have been removed through introducing Be, and its transmittance windows go deep into 147 nm.8 So the existence of dangling bonds has a negative influence on their transmitting to deep-UV region. Thermal Behavior Analysis. The TG and DSC curves of polycrystalline samples of K5Ba10(BO3)8F are shown in Figure S12a in the Supporting Information. K5Ba10(BO3)8F exhibits no weight loss and stable up to 1055 °C. The DSC curve for title compound exhibits one endothermic peak upon heating to 1200 °C which is at about 1055 °C. To verify the endothermic peak corresponds to which kind of thermodynamic

phenomenon

(for

melt

or

decomposition),

polycrystalline

K5Ba10(BO3)8F (5 g) were placed in a platinum crucible and heated to 1050 °C and held at this temperature for 10 h, and then slowly cooled to room temperature. During above process, the samples were not melt, and powder XRD data of the residuals reveal that the residuals at 1050 °C are mainly the mixture of K6Ba4B8O19 and Ba3B2O6 (Figure S12b in the Supporting Information). The result illuminates that the peak at 1055 °C is a decomposition peak, and K5Ba10(BO3)8F is an incongruent compound. Band Structures and Density of States. Using the method described above, the projected density of states (PDOS) and band structures of the solid solution K5Ba10(BO3)8F have been obtained (shown in Figure 4 and S13 in the Supporting Information). It is clearly shown that the K5Ba10(BO3)8F is an indirect band gap semiconductor whose GGA-PBE band gap obtained by VCA method is 3.55 eV. The obtained GGA-PBE band gap is smaller than the experimental values. This is not surprising, because the unoccupied eigenvalues of the electronic states was not accurately described, resulting in quantitative underestimation of band gaps.72-74 Since the electronic transition among the states near Fermi level mainly determines

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the optical properties, the electronic states at valence-band maximum (VBM) and conduction band minimum (CBM) are particularly analyzed. As shown in Figure 4a, the PDOS graph of K5Ba10(BO3)8F is divided into VB-1~2 (from -6 eV to the Fermi level) and CB-1~2 (at about 4~10 eV) energy regions. In VB-1, O-s, p, B-s, p and F-p states dominate this region, and cations also have certain contribution. In VB-2, it mainly includes O-p nonbonding states. Moreover, very little states from cations (K-s, p and Ba-s, p, d states) exist in VB-2. In CB-1, namely in the CBM, it is obvious that K(2)/Ba(2)-s states paly the main role, and K(1)-s, p and Ba(1)-s states have little contribution. In CB-2, K-s, p and Ba-s, p, d states occupy this region, and the contribution coming from anionic groups is less than that of cations. Based on above analysis, the BO3 anionic groups determine the energy band gap and optical properties of K5Ba10(BO3)8F. Meanwhile, the effect of alkaline and alkaline earth metal cations cannot be ignored.

Figure 4. The PDOS of K5Ba10(BO3)8F.

Because there are two kinds of B-O groups in the structure of K5Ba10(BO3)8F, the PDOS of B(1)O3 and B(2)O3 have also displayed in Figure 4b to investigate the contribution from the B(1)O3 and B(2)O3 groups to the energy band gap and optical properties. Obviously, the electronic states of B(1)O3 and B(2)O3 have similar distribution in the region of -5~0 eV. While the PDOS of B(1)O3 is stronger than that of B(2)O3 (shown in Figure 4b), implying the B(1)O3 groups may play an important

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role in determining the optical properties. The stronger PDOS of B(1)O3 groups may result from the larger density of B(1)O3 groups, compared with that of B(2)O3 groups (Figure S5c in the Supporting Information).

Figure 5. The refractive indices and birefringence of KBaBOF-Ba surplus (left) and KBaBOF-Ksurplus (right).

Using the method described above, the refractive indices and birefringence of KBaBOF-Basurplus and KBaBOF-Ksurplus were obtained (shown in Figure 5) to evaluate the birefringence of the solid solution K5Ba10(BO3)8F. The obtained birefringence of these two models are all very small (approaching zero) from 200 nm to 1100 nm. The result is not surprising, because the values of birefringence are closely related to the configuration of anionic groups. It is well known that, the nearly coplanar or coaxial arrangement of BO3 or B3O6 groups are benefical to get relative large birefringence, while the inclined arragement of BO3 or B3O6 groups are easy to get smaller birefrigence.75-76 For example, BO3 groups in KBe2(BO3)F217 and B3O6 groups in Na3Ba2(B3O6)2F31 are all coplanar, which endows the two materials with considerable birefringence. However, for the title compound K5Ba10(BO3)8F, although coaxial B(1)O3 groups and coplannar B(2)O3 groups were found in its crystal structure, while the dihedral angle between B(1)O3 groups and B(2)O3 groups are as large as 60.8(3)°, which make K5Ba10(BO3)8F own small optical anisotropic birefringence (Figure S8 in the Supporting Information).

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CONCLUSION A new complex alkaline and alkaline earth borate fluoride K5Ba10(BO3)8F has been obtained by spontaneous crystallization. K5Ba10(BO3)8F exhibits a perovskite-like structure featuring an intricate 3D network with a self-coupled (4,6)-connected net with the Schläfli symbol of {412·63}. Diffuse reflection spectrum proves that K5Ba10(BO3)8F possesses an experimental band gap of 6.05 eV, and the theoretical calculations also reveal that it has an indirect band gap. Raman spectrum detects the existence of BO3 unit. Thermal analysis and melt experiment verify that it melts incongruently and remains stable before 1050 °C. Meanwhile, combining experiments, theoretical calculations were also performed to analyze the relationships between crystal structure and optical property, which illustrates that inclined arragement of BO3 groups gives a smaller birefrigence to the title compounds.

ASSOCIATED CONTENT Supporting Information CCDC-number 1436122 for K5Ba10(BO3)8F; CIF file; atomic coordinates and equivalent isotropic displacement parameters; alkaline and/or alkaline-earth borate fluorides and fluoborates; selected bond lengths and angles; XRD patterns; crystal structures of K5Ba10(BO3)8F; Raman spectrum and UV−Vis-NIR diffuse reflectance spectrum; the TG/DSC curves; calculated band structure and birefrigence of K5Ba10(BO3)8F.

AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]

(Shilie

Pan).

Tel:

(86)-991-3810816.

(86)-991-3838957. *E-mail: [email protected] (Yun Yang).

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grant No. U1303193 , 51562036 , 51402352), 973 Program of China (Grant No. 2014CB648400),

University

scientific

research

project

of

Xinjiang

(XJEDU2014S073), Foundation for High-level Talents in Shihezi University (RCZX201511), and Applied Basic Research Foundation of Science and Technology in Shihezi University (2015ZRKXYQ07). REFERENCES (1) Huang, H. W.; Liu, L. J.; Jin, S. F.; Yao, W. J.; Zhang, Y. H.; Chen, C. T. Deep-Ultraviolet

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