Ba₇(BO₃)₃GeO₄X (X = Cl, Br): borogermanate halides with rigid GeO₄ tetrahedra and flexible XBa₆ octahedra

Abstract

The first borogermanate halides, Ba₇(BO₃)₃GeO₄X (X = Cl, Br), have been synthesized using a high temperature solution method. Ba₇(BO₃)₃GeO₄X (X = Cl, Br) are isostructural and crystallize in the orthorhombic space group Pbam (no. 55). They feature a Ba-based 3D framework, while isolated GeO₄ tetrahedra and BO₃ triangles fill in the space created by the framework. Significantly, Ba₇(BO₃)₃GeO₄X (X = Cl, Br) possess a similar formula to the borosilicate halides, Ba₇(BO₃)₃SiO₄X (X = Cl, Br), but they are not isostructural. An interpretation for the structural transformation between them is demonstrated here, considering rigid GeO₄ tetrahedra and flexible XBa₆ (X = Cl, Br) octahedra. IR spectroscopy, UV-Vis-NIR diffuse reflectance spectroscopy and first-principles calculations have been carried out on the title compounds.

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Introduction

Borates have been widely investigated owing to their rich chemical structures.^1–9 In particular, borates with planar BO₃ units have attracted considerable attention, because the π-delocalization of the BO₃ group is beneficial for a strong second-harmonic generation (SHG) response and large birefringence based on anionic group theory. A statistical analysis of the fundamental building blocks of borate, carried out by P. Becker,¹⁰ indicates that borates with isolated BO₃ groups will be found in systems with appropriate cation/boron ratios (>1). Thus, in the alkaline-earth borates, compounds with the formula M₇(BO₃)₃F₅ (M = Ba or Sr), with a large cation/boron ratio, have attracted the interest of scientists.

In the M₇(BO₃)₃F₅ (M = Ba or Sr) system, two compounds, Ba₇(BO₃)₃F₅ (ref. 11) and Ba₃Sr₄(BO₃)₃F₅ (ref. 11), were first identified by Keszler et al. in 1994. In 2008, Zhang’s group reinvestigated the crystal structure of Ba₃Sr₄(BO₃)₃F₅ and grew large crystals using the top-seeded solution growth method, which showed an SHG intensity about 0.5 times as large as that of KH₂PO₄ (KDP).¹² Then, in 2012, Bekker et al. systematically studied the M₇(BO₃)₃F₅ (M = Ba or Sr) system. They found that a disordered state between (4F)⁴⁻ and (BO₃F)⁴⁻ exists in the system and a series compounds of Ba₇(BO₃)_4−xF_2+3x and Ba_4−xSr_3+x(BO₃)_4−yF_2+3y (ref. 13 and 14) were identified. However, the existence of disorder in the crystal structure is not favorable for excellent physical or chemical properties.

The SiO₄ tetrahedra possess the same charge as (4F)⁴⁻ or (BO₃F)⁴⁻ and can be introduced into borate to replace them, which may solve the disorder in the crystal structure and improve the properties. Inspired by this, the compounds Ba₇(BO₃)₃SiO₄X (X = Cl, Br)¹⁵ have been synthesized by our group, in which the structural disorder disappears. Ba₇(BO₃)₃SiO₄X (X = Cl, Br) have a SHG response as strong as that of KDP. However, as is well-known, the high reaction temperature and high viscosity of the silicate are not conducive to the growth of crystals. Compared to silicates, germanates have a lower reaction temperature, and in previous research, many alkaline and alkaline-earth metal borogermanates have been reported.^16–20 In addition, the Ge and Si elements are in the same main group, and possess similar coordination environments. Guided by these ideas, the first borogermanate halides, Ba₇(BO₃)₃GeO₄X (X = Cl, Br), have been successfully synthesized. Owing to the large cation/boron ratio (7 : 3), the BO₃ triangles and GeO₄ tetrahedra are all isolated, which is quite rare in inorganic borogermanates.²¹ It is worth noting that although the Ge and Si atoms possess similarity, Ba₇(BO₃)₃GeO₄X (X = Cl, Br) and Ba₇(BO₃)₃SiO₄X (X = Cl, Br) crystallize in different space groups, centrosymmetric Pbam (no. 55) and polar P6₃mc (no. 186), respectively.

Thus, we have investigated the substitution of Ge for Si in other reported borates, determined from single crystal X-ray diffraction structures.^{17,18,22–28} We found that most of the compounds are isostructural (Table S1 in the ESI†). In these borates,^{17,18,22–28} we define a parameter of R (O/M, M = Si or Ge), which represents the mole ratio of the O element and the substitution element. In general, a higher value of R means a lower mole ratio of the substituted element in the compounds, which results in a smaller change due to the substitution and more likely results in isostructural substitution. Therefore, when R is higher than 3, all the compounds are isostructural.^17,23–28 It is unusual that Ba₇(BO₃)₃GeO₄X (X = Cl, Br) and Ba₇(BO₃)₃SiO₄X (X = Cl, Br) are not isostructural, with a quite high R value of 13. The structural transformation between Ba₇(BO₃)₃GeO₄X (X = Cl, Br) and Ba₇(BO₃)₃SiO₄X (X = Cl, Br) has been discussed in detail in this paper. We have also reported the spectra and the first-principles calculations of the title compounds.

Experimental

Solid-state synthesis

Polycrystalline samples of Ba₇(BO₃)₃GeO₄X (X = Cl, Br) were synthesized via conventional solid-state reactions. Stoichiometric mixtures of BaCO₃, BaCl₂, GeO₂, and B₂O₃ at a molar ratio of 6.5 : 0.5 : 1 : 1.5 for Ba₇(BO₃)₃GeO₄Cl, and BaCO₃, BaBr₂, GeO₂, and B₂O₃ at a molar ratio of 6.5 : 0.5 : 1 : 1.5 for Ba₇(BO₃)₃GeO₄Br, were ground and packed into alumina crucibles. The raw materials were preheated at 600 °C for 14 h. The mixtures were heated to 900 °C for Ba₇(BO₃)₃GeO₄Cl and 950 °C for Ba₇(BO₃)₃GeO₄Br, then held for 4 days, and finally cooled to room temperature. During the sintering processes the samples were ground thoroughly. The purity of the samples was confirmed by powder X-ray diffraction (XRD), which agrees well with the theoretical patterns of the compounds (Fig. S1 in the ESI†).

Crystal growth

Single crystals of Ba₇(BO₃)₃GeO₄X (X = Cl, Br) were grown from the high temperature solutions using NaCl–LiCl, NaBr–LiBr and B₂O₃ as flux systems. The solutions were obtained in platinum crucibles by melting mixtures of BaCO₃, GeO₂, B₂O₃, NaCl and LiCl at a molar ratio of 7 : 1 : 2.5 : 2 : 5 for Ba₇GeB₃O₁₃Cl, and BaCO₃, GeO₂, B₂O₃, NaBr and LiBr at a molar ratio of 7 : 1 : 2.5 : 1 : 4 for Ba₇GeB₃O₁₃Br. The solutions were heated to 900 °C, maintained at this temperature for 10 h, then slowly cooled to 700 °C at a rate of 3 °C h⁻¹ and finally cooled down to room temperature at a rate of 10 °C h⁻¹. Crystals of Ba₇(BO₃)₃GeO₄X (X = Cl, Br) were obtained, which were used for the single-crystal X-ray diffraction analysis.

Structure determination

The crystal structures of Ba₇(BO₃)₃GeO₄X (X = Cl, Br) were determined using a Bruker SMART APEX II CCD diffractometer with monochromatic Mo Kα radiation at 293(2) K, and were further integrated with the SAINT program.²⁹ The calculations were completed using programs from the SHELXTL crystallographic software package, while the structures were determined using direct methods with SHELXS-97. Final least-squares refinements are on F_o² with data having F_o² ≥ 2σ(F_o²). PLATON has been used to check for missing symmetry.³⁰ Crystal data and structure refinement information are shown in Table 1, and the final refined atomic positions and isotropic thermal parameters are displayed in Table S2 in the ESI.†

Table 1 Crystal data and structure refinements for Ba₇(BO₃)₃GeO₄Cl and Ba₇(BO₃)₃GeO₄Br

a R1 = ∑||Fo| − |Fc||/∑|Fo| and wR2 = [∑w(Fo^2 − Fc^2)^2/∑wFo^4]^1/2 for Fo^2 > 2σ(Fo^2).
Empirical formula	Ba7(BO3)3GeO4Cl	Ba7(BO3)3GeO4Br
Temperature	296(2) K
Wavelength	0.71073 Å
Crystal system	Orthorhombic
Space group	Pbam
Formula weight	1309.85	1354.31
a (Å)	20.353(18)	20.381(8)
b (Å)	7.416(6)	7.477(3)
c (Å)	11.123(10)	11.175(4)
Z, volume (Å^3)	4, 1679(3)	4, 1702.9(11)
ρcalcd (mg m^−3)	5.183	5.283
μ (mm)	18.121	20.054
R(int)	0.0776	0.0651
Goodness-of-fit on F^2	1.037	1.043
Final R indices [Fo^2 > 2σ(Fo^2)]^a	R1 = 0.0351	R1 = 0.0315
	wR2 = 0.0612	wR2 = 0.0585
R indices (all data)^a	R1 = 0.0545	R1 = 0.0426
	wR2 = 0.0675	wR2 = 0.0630
Extinction coefficient	0.00061(4)	0.00207(7)
Largest diff. peak and hole (e Å^−3)	2.403 and −2.209	2.158 and −2.070

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Infrared and UV-Vis-NIR diffuse reflectance spectroscopy

Infrared spectroscopy in the range of 400–4000 cm⁻¹ has been performed on a Shimadzu IR Affinity-1 Fourier transform infrared spectrometer. UV-Vis-NIR diffuse reflectance data have been obtained on a Solid Spec-3700DUV spectrophotometer with a range from 190 to 2600 nm. Polytetrafluoroethylene was used as a standard. Finally, we converted the reflectance spectra to absorbance using the function, F(R) = (1 − R)²/2R. R represents the reflectance while F(R) represents the Kubelka–Munk remission function.³¹

Numerical calculation details

The electronic structure calculations were performed using a plane-wave basis set and pseudopotentials within density functional theory (DFT), implemented in the total-energy module CASTEP.³² The exchange and correlation effects were treated with Perdew–Burke–Ernzerhof (PBE) in the generalized gradient approximation (GGA).^33–35 The interactions between the ionic cores and the electrons were described by ultrasoft pseudopotentials. The following orbital electrons were treated as valence electrons: Ba 5s²5p⁶6s²5d⁰, Ge 4s²4p², B 2s²2p¹, O 2s²2p⁴, Cl 3s²3p⁵ and Br 4s²4p⁵. The number of plane waves included in the basis were determined by a cutoff energy of 340 eV, and the numerical integration of the Brillouin zone was performed using a 4 × 4 × 3 Monkhorst–Pack scheme³⁶ k-point grid sampling for Ba₇(BO₃)₃GeO₄Cl and Ba₇(BO₃)₃GeO₄Br. Our test showed that these computational parameters ensure a good convergence in the present studies.

The linear optical response properties of Ba₇(BO₃)₃GeO₄X (X = Cl, Br) were examined by calculating the complex dielectric function ε(ω) = ε₁(ω) + iε₂(ω). The imaginary part of the dielectric function ε₂ is given in the following equation:³⁷

The real part ε₁(ω) can be obtained from the imaginary part ε₂(ω) by the Kramers–Kronig transformation. All the other optical constants, such as the absorption spectrum, refractive index, and reflectivity are derived from ε₁(ω) and ε₂(ω).

Results and discussion

Structure

Both of the title compounds crystallize in an orthorhombic space group, Pbam (no. 55). As Ba₇(BO₃)₃GeO₄X (X = Cl, Br) are isostructural, only the structure of Ba₇(BO₃)₃GeO₄Br is described in detail here. There are six unique Ba sites, two unique B sites, one unique Ge site, one unique Br site and eight unique O sites in the asymmetric unit (Table S2 in the ESI†). In the structure of Ba₇(BO₃)₃GeO₄Br, BaO_n (n = 8, 9) and BaO₇Br₂ polyhedra are interconnected by shared oxygen atoms to form the 3D framework (Fig. 1a), while isolated GeO₄ tetrahedra and BO₃ triangles fill in the space created by the framework (Fig. 1b).

Fig. 1 The structure of Ba₇(BO₃)₃GeO₄Br. (a) Ba-based 3D framework. (b) 3D network of Ba₇(BO₃)₃GeO₄Br. All Ba–O and Ba–Br bonds have been omitted for clarity.

Selected bond distances and angles for Ba₇(BO₃)₃GeO₄X (X = Cl, Br) are presented in Fig. S3 in the ESI.† The Ba–O, B–O, Ge–O and Ba–Br distances vary over 2.567(7)–3.217(5) Å, 1.362(12)–1.391(9) Å, 1.737(7)–1.766(5) Å and 3.1424(15)–3.7463(13) Å, respectively. These values are observed in other reported compounds.^16a,38

Comparing the structure of Ba₇(BO₃)₃GeO₄X (X = Cl, Br) with Ba₇(BO₃)₃SiO₄X (X = Cl, Br)

Ba₇(BO₃)₃SiO₄X (X = Cl, Br) are isostructural and crystallize in the hexagonal P6₃mc space group, while Ba₇(BO₃)₃GeO₄X (X = Cl, Br) crystallize in orthorhombic Pbam. Three unique Ba atoms, one unique B atom, one unique Si atom, four unique O atoms and one unique Br atom are in the asymmetric unit of Ba₇(BO₃)₃SiO₄Br. The structure of Ba₇(BO₃)₃SiO₄Br is composed of a 3D Ba(3)-based framework with tunnels viewed along the c axis, in which the BrBa(2)₃ chains reside (Fig. 2a). In addition, _∞¹[Ba(1)O₆(SiO₄)] single chains and isolated BO₃ triangles fill in the framework to form its 3D network structure (Fig. 2b and c). In the structure, the _∞¹[Ba(1)O₆(SiO₄)] single chains consist of isolated SiO₄ tetrahedra and Ba(1)O₁₀ polyhedra.

Fig. 2 The structure of Ba₇(BO₃)₃SiO₄Br. (a) _∞¹[BrBa₃]. (b) Ba(3)-based 3D framework. (c) 3D network of Ba₇(BO₃)₃SiO₄Br. All Ba–O bonds have been omitted for clarity.

Similarly, the structure of Ba₇(BO₃)₃GeO₄Br is composed of a 3D Ba(1, 4, 5)-based framework with tunnels viewed along b axis, in which the BrBa(2, 6)₃ chains are located (Fig. 3a). In addition, _∞¹{[Ba(3)O₄(GeO₄)]₂} double chains and isolated BO₃ triangles fill in the framework to form its 3D network structure (Fig. 3b and c). The _∞¹{[Ba(3)O₄(GeO₄)]₂} double chains are built of isolated GeO₄ tetrahedra and Ba(3)O₁₀ polyhedra.

Fig. 3 The structure of Ba₇(BO₃)₃GeO₄Br. (a) _∞¹[BrBa₃]. (b) Ba-based 3D framework. (c) 3D network of Ba₇(BO₃)₃GeO₄Br. All Ba–O bonds have been omitted for clarity.

Accordingly, the structural transformation between Ba₇(BO₃)₃SiO₄Br and Ba₇(BO₃)₃GeO₄Br will be discussed from the following two perspectives: (A) the arrangements of chains and (B) the Ba-based 3D frameworks.

(A) The transformation from _∞¹[Ba(1)O₆(SiO₄)] single chains to _∞¹{[Ba(3)O₄(GeO₄)]₂} double chains. In the structure of Ba₇(BO₃)₃SiO₄Br, the parallel single chains of _∞¹[Ba(1)O₆(SiO₄)] are linked by Ba(3)O₉ polyhedra (Fig. 4a). When the Si atoms are replaced by the Ge atoms, it causes a slight squeeze between the GeO₄ tetrahedra and Ba(3)O₉ polyhedra owing to the dimensional effect (the average bond length of Si–O is 0.155 Å smaller than that of Ge–O). In addition, as we know, the rigid units are hard to compress and squeeze, while the flexible ones are easy. In the structure, the SiO₄ and GeO₄ tetrahedra can be regarded as rigid due to their small bond distance variations compared to those of Ba–O polyhedra. However, compared with Ba–O polyhedra, the BrBa₆ octahedra can be regarded as flexible units due to their largest bond distance variations in the structures. Thus, the Ba(3)O₉ polyhedra are squeezed by the rigid GeO₄ tetrahedra to the flexible BrBa₆ octahedra (Fig. 4b). As a result, the Ba(3)O₉ polyhedra have been squeezed out of the chains, which leads the single chains to connect with each other to form the double chains (Fig. 4c and d).

Fig. 4 The transformation from _∞¹[Ba(1)O₆(SiO₄)] single chains to _∞¹{[Ba(3)O₄(GeO₄)]₂} double chains. (a) _∞¹[Ba(1)O₆(SiO₄)] single chains. (b) The arrangement of single chains and BrBa₆ octahedra in Ba₇(BO₃)₃SiO₄Br. (c) _∞¹{[Ba(3)O₄(GeO₄)]₂} double chains. (d) The arrangement of single chains and BrBa₆ octahedra in Ba₇(BO₃)₃GeO₄Br.

(B) The transformation of the Ba-based 3D frameworks. Both Ba₇(BO₃)₃SiO₄Br and Ba₇(BO₃)₃GeO₄Br have a Ba : Si or Ba : Ge molar ratio of 7 : 1, but the Ba atoms are arranged in different configurations. In the structure of Ba₇(BO₃)₃SiO₄Br, the BrBa₆ octahedra are in the center of six neighboring SiO₄ tetrahedra and the SiO₄ tetrahedra connect with the Ba(3) atoms to form a single chain viewed along the c axis (Fig. 5a). From Fig. 5b, we can also observe that every six Ba atoms are arranged around one SiO₄ tetrahedron. The rigid SiO₄ tetrahedra have pushed the Ba(2) atoms to the Br atoms, which makes the six Ba atoms have a pattern like an equilateral triangle (Fig. 5b). Six equilateral triangles constitute a hexagonal configuration template. This suggests that Ba₇(BO₃)₃SiO₄Br crystallizes in the hexagonal crystal system. Similarly, in the structure of Ba₇(BO₃)₃GeO₄Br, the BrBa₆ octahedra are located at the center of the SiO₄ tetrahedra and the Ba atoms lie around the SiO₄ tetrahedra (Fig. 5c and d). Owing to the squeeze from the SiO₄ tetrahedra to the Ba(2) and Ba(6) atoms, the Ba atoms form a pattern analogous to a rectangle. Four rectangles compose a bigger one, which can become a rectangular template (Fig. 5d). This implies that Ba₇(BO₃)₃GeO₄Br crystallizes in the orthogonal crystal system.

Fig. 5 The comparison of the different Ba-based frameworks in Ba₇(BO₃)₃SiO₄Br and Ba₇(BO₃)₃GeO₄Br. (a) The arrangement of Si and Br atoms in Ba₇(BO₃)₃SiO₄Br. (b) The arrangement of Si, Ba and Br atoms in Ba₇(BO₃)₃SiO₄Br. (c) The arrangement of Ge and Br atoms in Ba₇(BO₃)₃GeO₄Br. (d) The arrangement of Ge, Ba and Br atoms in Ba₇(BO₃)₃GeO₄Br.

Based on the above analyses, rigid SiO₄ tetrahedra and flexible BrBa₆ octahedra play key roles in the structural transformation from hexagonal Ba₇(BO₃)₃SiO₄Br to orthogonal Ba₇(BO₃)₃GeO₄Br.

Infrared and UV-Vis-NIR diffuse reflectance spectroscopy

The infrared spectra of Ba₇(BO₃)₃GeO₄X (X = Cl, Br) are quite same. For both of them, the bands that appeared in the ranges of 1350–1450 cm⁻¹ and 1150–1250 cm⁻¹ can be assigned to the asymmetric stretching vibration of the BO₃ groups (Fig. S2 in the ESI†). The peaks observed in the region of 733–641 cm⁻¹ are attributed to the out-of-plane bending of BO₃. The peaks in the region of 860–900 cm⁻¹ can be assigned to an asymmetrical stretch of Ge–O of the tetrahedral germanium. The absorption bands in the range of 450–600 cm⁻¹ correspond to the bending vibrations of the Ge–O bonds. The infrared spectra further confirm the existence of BO₃ triangles and GeO₄ tetrahedra, which is consistent with the results obtained from single crystal X-ray structural analyses.

The UV-Vis-NIR diffuse reflectance spectra of Ba₇(BO₃)₃GeO₄X (X = Cl, Br) are shown in Fig. S3 in the ESI.† They indicate that the experimental optical band gaps of Ba₇(BO₃)₃GeO₄Cl and Ba₇(BO₃)₃GeO₄Br are 5.28 and 5.31 eV, respectively. The large band gaps of the title compounds suggest that they may be used as optical window materials.

Band structures, density of states and optical properties

The band structures of Ba₇(BO₃)₃GeO₄X (X = Cl, Br) are presented in Fig. S4 in the ESI.† The valence band maximum (VBM) and the conduction band minimum (CBM) are located at the Γ point of the Brillouin zone indicating that both of the two compounds belong to direct band-gap semiconductors. The extrapolated experimental optical gaps of 5.28 eV for Ba₇(BO₃)₃GeO₄Cl and 5.31 eV for Ba₇(BO₃)₃GeO₄Br are slightly larger than the calculated values of 4.93 eV and 4.91 eV, respectively. The underestimation of the band gap is generally due to insufficient accuracy of the exchange correlation energy under DFT methods.³⁹ We can see that the results of our calculations are good and give reasonable explanations for the optical absorption spectra. The highest occupied and lowest unoccupied orbitals determine the route of electronic transitions and the absorption edge. It is shown in Fig. S5 in the ESI† that the absorption edges of Ba₇(BO₃)₃GeO₄Cl and Ba₇(BO₃)₃GeO₄Br are mainly decided by the O 2p states in the BO₃ units.

The total and partial densities of states (TDOS and PDOS) are shown in Fig. 6. The top region of the VBs extends in a wide range from −6.7 eV to the VBM. These bands mostly originate from the Ge 4s4p, B 2s2p and O 2s2p states with mixing of the Ba 5s5p and Cl 3p/Br 4p states, especially near the Fermi level. It is worthy of note that strong hybridizations occur among the Ge 4s4p, B 2s2p and O 2s2p states in the range of −6.7 to −0.7 eV. For Ba₇(BO₃)₃GeO₄Cl and Ba₇(BO₃)₃GeO₄Br, the conduction bands from the CBM to 8.5 eV are derived from the Ba 5d, Ge 4s4p and B 2p states with mixing of the Ba 5s5p, B 2s and O 2s2p states, implying strong interactions in the Ge–O and B–O bonds of the compounds. The valence bands and conduction bands near the gap are dominated by O 2p and Ba 5d, respectively. Accordingly, the absorption spectrum near the UV-visible cut-off wavelength can be assigned as the charge transfers from the O 2p states to the Ba 5d states.

Fig. 6 TDOS and PDOS plots of Ba₇(BO₃)₃GeO₄Cl (a) and Ba₇(BO₃)₃GeO₄Br (b).

On the basis of the electronic structures, the linear optical properties can be estimated. The imaginary part of the dielectric function ε₂ is calculated, and its real part ε₁ is determined using the Kramers–Kronig transform, from which the refractive indices and the birefringence Δn were obtained.⁴⁰ The dispersion curves of the refractive indices are calculated using the formula n²(ω) = ε(ω), displaying a similar optical anisotropy behavior, n^x ≈ n^y > n^z, for Ba₇(BO₃)₃GeO₄Cl and Ba₇(BO₃)₃GeO₄Br. The birefringence (Δn) versus wavelength is shown in Fig. S6 in the ESI.† It can be seen that the birefringence values of Ba₇(BO₃)₃GeO₄Cl and Ba₇(BO₃)₃GeO₄Br are about 0.03 with a wavelength of 1064 nm.

Conclusion

The first borogermanate halides, Ba₇(BO₃)₃GeO₄X (X = Cl, Br), have been obtained using a high temperature solution method. We have discussed the structural transformation from Ba₇(BO₃)₃SiO₄X (X = Cl, Br) to Ba₇(BO₃)₃GeO₄X (X = Cl, Br). The unusual structural transformation mainly originates from rigid GeO₄ tetrahedra and flexible XBa₆ (X = Cl, Br) octahedra. We believe that this interpretation can give new insights into structural transformation. The first-principles calculations show that the calculated band gaps (4.93 eV for Ba₇(BO₃)₃GeO₄Cl and 4.91 eV for Ba₇(BO₃)₃GeO₄Br) agree well with the experimental ones (5.28 and 5.31 eV). The top of the valence band is dominated by a mixture of B 2p and O 2p states, while the Ba 5d, Ge 4s4p, B 2s and O 2s2p states dominate the bottom of the conduction band.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant nos 21201176, U1303392, 51425206, 51172277, U1129301), 973 Program of China (Grant no. 2014CB648400), the Xinjiang Program of Cultivation of Young Innovative Technical Talents (Grant no. 2014711001), the Western Light of CAS (Grant no. XBBS201214), the Outstanding Young Scientists Project of Chinese Academy of Science, the Xinjiang International Science & Technology Cooperation Program (Grant no. 20146001), the Funds for Creative Cross & Cooperation Teams of CAS, and the Xinjiang Key Laboratory Foundation (Grant no. 2014KL009).

Notes and references

1. (a) P. Becker, Adv. Mater., 1998, 10, 979; (b) C. T. Chen, B. C. Wu, A. Jiang and G. M. You, Sci. Sin., Ser. B, 1985, 28, 235; (c) C. T. Chen, Y. C. Wu, A. Jiang, B. C. Wu, G. M. You, R. K. Li and S. J. Lin, J. Opt. Soc. Am. B, 1989, 6, 616; (d) Y. C. Wu, T. Sasaki, S. Nakai, A. Yokotani, H. Tang and C. T. Chen, Appl. Phys. Lett., 1993, 62, 2614.
2. (a) Y. Mori, I. Kuroda, S. Nakajima, T. Sasaki and S. Nakai, Appl. Phys. Lett., 1995, 67, 1818; (b) C. T. Chen, Y. B. Wang, B. C. Wu, K. C. Wu, W. L. Zeng and L. H. Yu, Nature, 1995, 373, 322; (c) N. Ye, W. R. Zeng, J. Jiang, B. C. Wu, C. T. Chen, B. H. Feng and X. L. Zhang, J. Opt. Soc. Am. A, 2000, 17, 764; (d) L. Y. Li, G. B. Li, Y. X. Wang, F. H. Liao and J. H. Lin, Chem. Mater., 2005, 17, 4174.
3. (a) H. W. Huang, L. J. Liu, S. F. Jin, W. J. Yao, Y. H. Zhang and C. T. Chen, J. Am. Chem. Soc., 2013, 135, 18319; (b) H. W. Huang, Y. He, Z. S. Lin, L. Kang and Y. H. Zhang, J. Phys. Chem. C, 2013, 117, 22986; (c) R. H. Cong, J. L. Zhu, Y. X. Wang, T. Yang, F. H. Liao, C. Q. Jin and J. H. Lin, CrystEngComm, 2009, 11, 1971.
4. (a) S. G. Zhao, P. F. Gong, S. Y. Luo, S. J. Liu, L. N. Li, M. A. Asghar, T. Khan, M. C. Hong, Z. S. Lin and J. H. Luo, J. Am. Chem. Soc., 2015, 137, 2207; (b) S. L. Pan, J. P. Smit, M. R. Marvel, E. S. Stampler, J. M. Haag, J. Baek, P. S. Halasyamanin and K. R. Poeppelmeier, J. Solid State Chem., 2008, 181, 2087; (c) T. Pilz and M. Jansen, Z. Anorg. Allg. Chem., 2011, 637, 1.
5. (a) H. P. Wu, H. W. Yu, Z. H. Yang, X. L. Hou, X. Su, S. L. Pan, K. R. Poeppelmeier and J. M. Rondinelli, J. Am. Chem. Soc., 2013, 135, 4215; (b) X. W. Zhang, H. W. Yu, H. P. Wu, S. L. Pan, A. Q. Jiao, B. B. Zhang and Z. H. Yang, RSC Adv., 2014, 4, 13195; (c) H. Y. Li, H. P. Wu, X. Su, H. W. Yu, S. L. Pan, Z. H. Yang, Y. Lu, J. Han and K. R. Poeppelmeier, J. Mater. Chem. C, 2014, 2, 21704; (d) K. Wu, S. L. Pan and Z. H. Yang, RSC Adv., 2015, 5, 33646.
6. (a) C. Y. Bai, S. J. Han, S. L. Pan, B. B. Zhang, Y. Yang, L. Li and Z. H. Yang, RSC Adv., 2015, 5, 12416; (b) Y. Yang, S. L. Pan, X. Su, Y. Wang, Z. H. Yang, J. Han, M. Zhang and Z. H. Chen, CrystEngComm, 2014, 16, 1978; (c) Z. Wang, Q. Jing, M. Zhang, X. Y. Dong, S. L. Pan and Z. H. Yang, RSC Adv., 2014, 4, 54194; (d) H. P. Wu, H. W. Yu, S. L. Pan, A. Q. Jiao, H. Y. Li, J. Han, K. Wu and S. J. Han, Dalton Trans., 2014, 43, 4886.
7. (a) A. H. Reshak, X. A. Chen, S. Auluck, H. Kamarudin, J. Chyský, A. Wojciechowski and I. V. Kityk, J. Phys. Chem. B, 2013, 117, 14141; (b) X. A. Chen, H. Yin, X. N. Chang, H. G. Zang and W. Q. Xiao, J. Solid State Chem., 2010, 183, 2910.
8. (a) C. D. McMillen, J. T. Stritzinger and J. W. Kolis, Inorg. Chem., 2012, 51, 3953; (b) C. Heyward, C. McMillen and J. W. Kolis, Inorg. Chem., 2012, 51, 3956.
9. (a) H. Emme, M. Valldor, R. Pöttgen and H. Huppertz, Chem. Mater., 2005, 17, 2707; (b) J. S. Knyrim, H. Emme, M. Döblinger, O. Oeckler, M. Weil and H. Huppertz, Chem.–Eur. J., 2008, 14, 6149; (c) M. J. Xia, B. Xu and R. K. Li, J. Cryst. Growth, 2014, 404, 65.
10. P. Becker, Z. Kristallogr., 2001, 216, 523.
11. D. A. Keszler, A. Akella, K. I. Schaffers and T. Alekel, Mater. Res. Soc. Symp. Proc., 1994, 329, 15.
12. G. C. Zhang, Z. L. Liu, J. X. Zhang, F. D. Fan, Y. C. Liu and P. Z. Fu, Cryst. Growth Des., 2009, 9, 3139.
13. T. B. Bekker, S. V. Rashchenko, V. V. Bakakin, Y. V. Seryotkin, P. P. Fedorov, A. E. Kokha and S. Y. Stonogaa, CrystEngComm, 2012, 14, 6910.
14. S. V. Rashchenko, T. B. Bekker, V. V. Bakakin, Y. V. Seryotkin, V. S. Shevchenko, A. E. Kokh and S. Y. Stonoga, Cryst. Growth Des., 2012, 12, 2955.
15. X. X. Lin, F. F. Zhang, S. L. Pan, H. W. Yu, F. Y. Zhang, X. Y. Dong, S. J. Han, L. Y. Dong, C. Y. Bai and Z. Wang, J. Mater. Chem. C, 2014, 2, 4257.
16. (a) X. Xu, C. L. Hu, F. Kong, J. H. Zhang and J. G. Mao, Inorg. Chem., 2011, 50, 8861; (b) D. B. Xiong, J. T. Zhao, H. H. Chen and X. X. Yang, Chem.–Eur. J., 2007, 13, 9862.
17. (a) J. H. Zhang, F. Kong and J. G. Mao, Inorg. Chem., 2011, 50, 3037; (b) Y. C. Hao, C. L. Hu, X. Xu, F. Kong and J. G. Mao, Inorg. Chem., 2013, 52, 13644; (c) J. B. Parise and T. E. Gier, Chem. Mater., 1992, 4, 1065.
18. Z. E. Lin, J. Zhang and G. Y. Yang, Inorg. Chem., 2003, 42, 1797.
19. (a) H. X. Zhang, J. Zhang, S. T. Zheng, G. M. Wang and G. Y. Yang, Inorg. Chem., 2004, 43, 6148; (b) D. B. Xiong, H. H. Chen, M. R. Li, X. X. Yang and J. T. Zhao, Inorg. Chem., 2006, 45, 9301.
20. (a) P. Becker, Cryst. Res. Technol., 2001, 2, 119; (b) B. Petermüller, L. L. Petschnig, K. Wurst, G. Heymann and H. Huppertz, Inorg. Chem., 2014, 53, 9722.
21. B. F. Dzhurinskii, A. B. Pobedina, K. K. Palkina and M. G. Komova, Russ. J. Inorg. Chem., 1998, 43, 1488.
22. M. Ihara and F. Kamei, J. Ceram. Soc. Jpn., 1980, 88, 32.
23. H. P. Wu, H. W. Yu, S. L. Pan, Z. J. Huang, Z. H. Yang, X. Su and K. R. Poeppelmeier, Angew. Chem., Int. Ed., 2013, 52, 3406.
24. X. Xu, C. L. Hu, F. Kong, J. H. Zhang, J. G. Mao and J. L. Sun, Inorg. Chem., 2013, 52, 5831.
25. T. Berger and K. J. Range, Z. Naturforsch., B: J. Chem. Sci., 1996, 51, 172.
26. C. Heyward, C. D. McMillen and J. J. Kolis, Solid State Chem., 2013, 203, 166.
27. V. R. Samygina, E. A. Genkina, B. A. Maksimov and N. I. Leonyuk, Kristallografiya, 1993, 38, 61.
28. A. A. Kaminskii, B. V. Mill, E. L. Belokoneva and A. V. Butashin, Izv. Akad. Nauk SSSR, Energ. Transp., 1990, 26, 1105.
29. SAINT, Version 7.60A, Bruker Analytical X-ray Instruments, Inc., Madison, WI, 2008.
30. A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
31. (a) P. Kubelka and F. Z. Munk, Tech. Phys., 1931, 12, 593; (b) J. Tauc, Mater. Res. Bull., 1970, 5, 721.
32. S. J. Clark, M. D. Segall, C. J. Pickard, P. J. Hasnip, M. J. Probert, K. Rrfson and M. C. Payne, Z. Kristallogr., 2005, 220, 567.
33. J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77, 3865.
34. J. S. Lin, A. Qteish, M. C. Payne and V. Heine, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 47, 4174.
35. A. M. Rappe, K. M. Rabe, E. Kaxiras and J. D. Joannopoulos, Phys. Rev. B: Condens. Matter Mater. Phys., 1990, 41, 1227.
36. H. J. Monkhorst and J. D. Pack, Phys. Rev. B: Condens. Matter Mater. Phys., 1976, 13, 5188.
37. E. D. Palik, Handbook of Optical Constants of Solids, Academic Press, Orlando, New York, 1985.
38. X. Y. Dong, H. P. Wu, Y. J. Shi, H. W. Yu, Z. H. Yang, B. B. Zhang, Z. H. Chen, Y. Yang, Z. J. Huang, S. L. Pan and Z. X. Zhou, Chem.–Eur. J., 2013, 19, 7338.
39. R. W. Godby, M. Schluter and L. J. Sham, Phys. Rev. B: Condens. Matter Mater. Phys., 1987, 36, 6497.
40. F. Wooten, Optical Properties of Solid, Academic, New York, 1972.

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Footnote

† Electronic supplementary information (ESI) available: Calculated and observed XRD patterns, IR and UV-Vis-NIR diffuse transmittance spectra, substitution of Ge for Si in borates, final refined atomic positions and isotropic thermal parameters, selected bond distances and angles and first-principles calculations for Ba₇(BO₃)₃GeO₄X (X = Cl, Br). CCDC 1058752 and 1058753. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra07450a

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