Synthesis and Characterization of a New Pentaborate, KNa2CdB5O10①

CHEN Xue-An ZHANG Y-Hu CHANG Xin-An XIAO Wei-Qing

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Synthesis and Characterization of a New Pentaborate, KNa2CdB5O10①

CHEN Xue-Ana②ZHANG Ya-HuaaCHANG Xin-AnaXIAO Wei-Qiangb

a(College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China)b(Institute of Microstructure and Property of Advanced Materials,Beijing University of Technology, Beijing 100124, China)

A new pentaborate, KNa2CdB5O10,has been synthesized and its crystal structure was determined by single-crystalXRD method with the following crystal data:KNa2CdB5O10,,= 18.4555(13),= 8.3040(10),= 12.8772(9) ?,= 1973.5(3) ?3,= 8,M= 411.53,= 2.770 g/cm3,= 2.765 mm-1,(000) =1552,= 0.0648 and= 0.1756 for 1684 observed reflections and 175 variables. It contains the double ring [B5O10]5-groups composed of one BO4tetrahedron and four BO3triangles that arebridged by tetrahedralCd2+centerscommon O atoms to form 2D [CdB5O10]n3n-layers, with A+(A = K/Na, Na) cations situated between and within the layers. IR spectrum confirms the existence of BO3and BO4groups.UV-VIS diffuse reflectance spectrum shows a band gap of about 3.44 eV and solid-state fluorescence spectrum exhibits a broad emission band at around 386 nm. Band structure calculations indicate that KNa2CdB5O10is an indirect band-gap insulator, with the calculated energy gap (3.68 eV) close tothe experimental one.

KNa2CdB5O10, borate, synthesis, X-ray diffraction, crystal structure;

1 INTRODUCTION

Borate materialshave attracted widespread atten- tion during the last several decades because of their promising applications in nonlinear optical (NLO) materials, birefringent materials, ferroelectric and piezoelectric materials, and host materials for luminescence[1-8]. In addition to that, structurally, a boron atom may adopt a triangular or tetrahedral coordinationenvironment. The BO3and BO4groups may be further linkedcommon oxygen atoms to form isolated rings and cages or polymerize into infinite chains, sheets and networks, leading to the rich structural chemistry, which has raised the interest of many solid-state chemists.The pentabo- rates with general composition A3MB5O10(A = Na, K; M = Mg, Zn, Cd, Co, and Fe) have been widely studied because they exhibit intriguing structural diversity and some of them show interesting electrochemical properties[9-14]. All of these com- pounds contain topologically identical [MB5O10]n3n-layers constructed by [B5O10]5-groups and MO4tetrahedracommon O atoms.The anionic layers are stacked to build the unit cell, with alkali metalcationsA+sitting inside and between these layers to provide charge compensation. Depending on ionic radii of A+and M2+cations, three kinds of space groups have been previously reported for this family of compounds, i.e., K2NaZnB5O10, Na3MB5O10(M = Zn, Co) and K3MB5O10(M = Zn, Cd) crystallize in the monoclinic system with different space groups (2/for the first phase and21/for the rest, res- pectively)[9-13], whereas Na3MB5O10(M = Mg, Fe) has orthorhombic structures with space group[11, 14].

In our effort to search for new borate materials to study their structure-property relationships, we have obtained a new member of this family of compounds, KNa2CdB5O10. Our single-crystal X-ray structural analyses indicated that KNa2CdB5O10is the first example of pentaborates crystallizing in the orthor- hombic space group. Insofar as we know, no XRD, IR, UV-VIS diffuse reflectance, and fluore- scence spectra for this compound are available in the literature. Herein we report the results of experi- mental measurements mentioned above for the first time. Moreover, the band electronic structure calcu- lations have also been carried out so as to help in understanding the chemical bonding properties and electronic origins of the optical transitions.;

2 EXPERIMENTAL

2.1 Synthesis

Single crystals of the title compound were grown by the high-temperature solution reaction. All reagents used in the syntheses were commercially available with analytic grade and used without further purification. In a typical synthesis, a powder mixture of the starting materials K2B4O7?4H2O, Na2CO3, CdO, H3BO3, Bi2O3, and PbOat the molar ratio K:Na:Cd:B:Bi:Pb = 1:2:1:5:4:2 was finely ground in an agate mortar, then transferred to a platinum crucible of 40 mm in diameter and 40 mm in height, which was subsequently placed in the center of a vertical, programmable temperature furnace. The sample was slowly heated to 750 ℃ and held at that temperature for one week to ensure that the charge melts completely and mixes homogeneously.Subsequently, it was slowly cooled to 500℃at a rate of 0.5℃/h, to 300℃ at 1.0℃/h, followed by cooling to room temperature rapidly at 20℃/h. Several colorless prismatic crystals were physically removed from the crucible for structure determinations. The crystals were also checked by energy-dispersive X-ray analyses in a scanning electron microscope, which gave an approximate atomic composition K/Na/Cd = 1.1:1.8:0.9, con-sistent with the results obtained from single-crystal structural analysis.

A bulk phase was prepared by direct reaction of a stoichiometric mixture of the starting materials (K2CO3, Na2CO3, CdO, and H3BO3) at 600°C for four weeks with several intermediate re-mixings, and its phase purity was examined by powder XRD analyses in a Bruker D8 ADVANCE diffractometer.

2. 2 Powder and single-crystal X-ray diffraction

At room temperature, powder XRD data were collected in a continuous mode using the monochro- matized Curadiation (40 KV, 40 mA) of a Bruker D8 ADVANCE (Bruker AXS) diffractometer.Data were collected in the 2range of 5～50°with a step size of 0.02°. Fig. 1 shows the observed powder XRD pattern of the title compound together with that calculatedfrom the single-crystal data for com- parison. It is clear that the observed XRD pattern is in good agreement with the theoretical one, confirming the existence of this phase. However, some diffraction peaks such as those at about 2= 33.44o and 33.77o in the calculated pattern were merged into one broad peak with 2= 33.60o [(33.44o + 33.77o)/2] in the observed pattern, which may be associated with the poor crystallinity of the powder sample due to its low preparation tempera- ture of 600℃.

For single-crystal XRD measurements,a colorless crystal with approximate dimensions of 0.20mm × 0.10mm × 0.10mm was put on an automated Rigaku AFC7R four-circle diffractometer equipped with a graphite-monochromatic Moradiation (= 0.71073 ?). The data were collected at room tempe- rature by using an2scan mode in the range of 2.21≤≤28.00°. A total of 3346 reflections were collected, of which 2380 were independent (int= 0.0712) and 1684 with> 2() were considered as observed. The absorption correction based on empirical PSI-scan technique was applied, and the crystal structure was solved by direct methods and refined in SHELX-97 system[15]by full-matrix least-squares techniques onF2. All atoms were refined anisotropically. The final cycle of refinement converged to= 0.0648,= 0.1756 (= 1/[2(F2) + (0.0789)2+ 29.4449], where= (F2+ 2F2)/3), (?/)max= 0.000,= 1.087, (?)max= 3.342 and (?)min= –3.412 e/?3. The selected bond distances and bond angles are listed in Table 1.

Fig. 1. XRD patterns of KNa2CdB5O10observed from powder polycrystalline sample and calculated from the single-crystal data

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Note: K(1)/Na(1) and K(2)/Na(2) have the compositions K0.685(11)Na0.315(11)and K0.63(2)Na0.37(2), respectively. #1:–1/2, –+1,+1/2; #2:+1/2,+3/2,; #3:+1/2,,–1/2;#4:–+1/2, –+1/2,; #5:–+1/2,,+1/2;#6:, –+1/2,+1/2; #7:–,+1/2, –+1/2;#8:–1/2,+1/2, –+1;#9:, –+3/2,+1/2

Refinements of atomic occupancy parameters indicated that K and Na atoms were statistically distributed over two atomic sites. For these two atomic sites, in an alternative LS refinement, K and Na positions were allowed to be refined freely. The refined K and Na positions are very close (K(1)×××Na(1)= 0.300 ?; K(2)×××Na(2)= 0.367 ?). Therefore, in the final model, the occupancies of the disordered K/Na sites were constrained to 1.0, with the same coordinates and displacement parameters for the two types of metals. The refined ratios were illustrated in Table 1. The K+/Na+disorder is not surprising because both potassium and sodium belong to the same group elements in the Periodic Table and have similar coordination geometries. The fact that K+and Na+cations are located on the same atomic siteshas also been previously observed in a lot of compounds, e.g., Na(Na0.17K0.83)2(B3O5)3, (Na0.25K0.75)4B8O14and (K0.48Na0.52)2Al2B2O7[16-18]. Our attempts to refine the crystal structure in lower-symmetry space groups with an ordered distribution of the cations have been unsuccessful. In addition, during our single-crystal XRD data collection, a larger unit cell with symmetry lower than orthorhombic that would allow all of atomic positions to be ordered has not been found. Therefore, the disorder model was finally assumed. Positional parametershave also been checked using the program ADDSYM of PLATON[19]. No potential additional symmetry was found, supporting our space group assignment.

2. 3 Spectral measurements

Infrared spectrum was recorded on a Perkin Elmer 1730 FT-IR spectrometer using the KBr pellet technique in the spectral range of 450～4000 cm-1.UV-VIS diffuse reflectance spectrum wasrecorded on the finely ground sample, with a Shimadzu UV-3101PC double-beam, double-monochromator spectrophotometer using BaSO4powder as a standard material for the baseline correctionin the spectral range of200～1000 nm. Reflectance spectrum was converted to absorbance using the Kubelka-Munk equation[20]. The minima in the second-derivative curves of the Kubelka-Munk function are taken as the position of the absorption bands. The emission spectrum was measured on an F-7000 time-resolved fluorescence spectrometer using a Xe lamp at roomtemperature.

2. 4 Electronic structure calculations

The electronic band structure and density of states (DOS) were calculated on the basis of our XRD data. The calculations were made using the first-principle plane-wave pseudopotential technique based on density functional theory (DFT) with CASTEP code distributed inside a computational commercial pack[21]. Local density approximation (LDA) of CA-PZscheme was employed to evaluate exchange- correlation energy[22, 23]. Norm-conserving pseudopo- tentials were employed for all of the atoms in the reciprocal representation[24]. The Monkhorst-Pack k-point grid sampling with 1′2′2 was applied in the DFT calculations. A kinetic energy cut-off of 600.0 eV was used to determine the plane wave basis set. Pseudoatomic calculations were performed for K 323641, Na 222631, Cd 41052, B 2221, and O 2224. The rest parameters used in the calculations were set by the default values of the CASTEP code[21].

3 RESULTS AND DISCUSSION

3.1 Description of the structure

KNa2CdB5O10has the pentaborate group [B5O10]5-as the basic structural unit, which is a double ring formed by one BO4tetrahedron (T) and four BO3triangles (D) with the shorthand notation 4D1T:<2DT>-<2DT>[25], as illustrated in Fig. 2. Two hexagonal rings are approximately perpendicular to each other, the angle between the planes defined by B(1), O(2), B(3), O(4), B(2), O(3) and by B(3), O(6), B(4), O(8), B(5), O(7) being 82.7(3)o. The four terminal O atoms are nearly coplanar with the corresponding hexagonal rings, with the maximum displacement from the ring planes being 0.280(14) ? at O(5) and 0.467 (15) ? at O(9) atoms, respectively.

Fig. 2. Asymmetric unit of KNa2CdB5O10supplemented by additional oxygen atoms to show the full coordination around Cd1 site. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: #2:+1/2+3/2; #4:–+1/2–+1/2; #5:–+1/2+1/2

Each [B5O10]5-group is linked to four different CdO4tetrahedra and likewise each CdO4tetrahedron is connected to four different [B5O10]5-groups by sharing O atoms to form a 2D infinite [CdB5O10]n3n-layer, as depicted in Fig. 3. The cadmium borate anionic layer is not flat because there exists aglide plane perpendicular to the-axis which lies on the middle of this layer and CdO4tetrahedra are located at both sides of the glide plane pointing in the opposite direction. These cadmium boratelayers extend within the crystallographicalplane and stack along the-axis. K(1)/Na(1)+and Na(3)+cationsare located between the layers to balance charge and also to hold the layers togetherelectrostatic interactions. The rest of alkali metalcations [K(2)/Na(2)+and Na(4)+] occupy the intersecting open channels that run along the [010] and [001] directions of the layers (Fig. 4).

Fig. 3. View of the[CdB5O10]n3n-layer approximately along the [100] direction. Cd atoms: green balls; BO3groups: navy triangles; BO4groups: magenta tetrahedra

Fig. 4. Crystal structure of KNa2CdB5O10projected along the [010] (a) and [00-1] (b) directions, respectively. K/Na atoms: big purple balls; Na atoms: middle black balls; Cd atoms: small green balls; BO3groups: navy triangles; BO4groups: magenta tetrahedra

As seen from Fig. 2, the asymmetric unit of KNa2CdB5O10contains 20 independent sites, i.e., 2K/Na, 2Na, 1Cd, 5B, and 10O. Among them, K and Na atoms are statisticallydistributed over two crystallographic sites that are K-rich with the compositions K(1)/Na(1) [K0.685(11)Na0.315(11)] and K(2)/Na(2) [K0.63(2)Na0.37(2)], respectively. The K(1)/Na(1) atom occupies a crystallographic general position, thereby giving eight distinctK/Na–O distances (2.633(7)～3.154(9) ?), while K(2)/Na(2) is located on a 2-fold axis, resulting in four sets of K/Na–O bond lengths (2′2.708(6), 2′2.807(6), 2′3.071(10), and 2′3.107(8) ?, Table 1). These geometric parameters are comparable to those observed in the structures of Na(Na0.17K0.83)2(B3O5)3(K/Na–O = 2.710(8)～3.130(9) ?) and (Na0.25K0.75)4B8O14(K/Na–O = 2.587(10)～3.161(9) ?), where (K/Na)O8groups have also been observed[16, 17]. The 8-fold coordination around K(1)/Na(1) can be described as an irregular polyhedron, while the coordination geometry around K(2)/Na(2) is a deformed dodecahedron. Of the two independent Na atoms, Na(3) is coordinated to five O forming a strongly distorted square pyramid and Na(4) bonded to six O atoms in a highly distorted octahedralconfiguration. The fact that the Na atom has a lower coordination number relative to the K/Na atom is associated with the difference in their atomic size.The average Na–O distance of 2.434 ? (CN = 5) for Na(3) is slightly shorter than that for Na(4) (2.443 ?, CN = 6) as expected and both are very reasonable when compared with the values 2.38 and 2.40 ? computed from crystal radii for a 5- and 6-coordinated Na+ion, respectively[26]. Bond valence sum (BVS) calculations using Brown’s formula[27]produced BVS values of 0.96 and 1.11 for Na(3) and Na(4) atoms, respectively,supporting the choice of five- and six-fold coordination to describe the Na environment.

There is one crystallographically unique Cd atom, which has four O nearest-neighbors arranged into a stronglydistorted tetrahedron, with the 109.5o tetrahedral angles being 93.7(3)～122.8(3)o (average 108.8o). Note that tetrahedral, trigonal bipyramidal, and octahedral coordinationenvironmentshave been previously reported for Cd atoms in the cadmium oxides. Among them, CdO5trigonal bipyramids are rare, but have been encountered in the structure of LiCdBO3[28], while CdO4tetrahedra and CdO6octahedra are more popular: the former have been observed in CdTeMoO6as well as CdB4O7[29, 30]and the latter inCd2B2O5as well as Cd3(BO3)2[31, 32]. Cd–O distances fall in the rangeof 2.157(7)～2.161(7) ? (average 2.158 ?), which is in the similar range as reported for the 4-coordinated Cd2+in CdTeMoO6(4′2.160(10) ?) and CdB4O7(2.182(13)～2.217(15) ?, average 2.200 ?)[29, 30]. Five distinct boron atoms are divided into two sets: B(3) in tetrahedral coordination and the rest of B atoms in triangular configuration. The average B–O bond lengths of 1.471 ? in the tetrahedron and 1.364～1.372 ? in the triangles are in good agreement with those reported in the case of borate minerals (1.476 and 1.370 ? for the tetrahedrally and trigonally coordinated boron atoms, respectively)[33]. The average O–B–O angles are approximately equal to the ideal values of 109.5o or 120o, respectively, indicating that both BO4and BO3groups are almost regular.The calculated BVS concerning Cd and B atoms are also reasonable, at 1.71 and 2.99～3.08, respectively.

Recently, Zhaohave reported a new potas- sium europium borate K3EuB6O12, whichcrystallizes in a rhombohedral space group32with= 13.2610(17),= 15.3822(19) ?,= 2342.6(7) ?3and= 9, and has the potential to be a red phosphor pumped by near-UV LED chips[34]. This compound can be formulated as K5/2Eu5/6(B5O10). The asymmetric unit comprises 13 independent sites, i.e., one disordered (K1/2Eu1/2), one Eu, three K, three B, and five O sites. In this structure, each [B5O10]5-group is linked to four different EuO6octahedra and each EuO6octahedron connected to six neighboring [B5O10]5-groups by sharing O atoms to form a 3D [Eu2/3B5O10]n3n-framework. The 3D nextwork also affords 1D open channels running parallel to the [211] direction where K+and (K1/2Eu1/2)2+cationsreside. It is the difference in the coordination geometry around the metal ions that enables the crystal structure of K3EuB6O12to be significantly different from that of KNa2CdB5O10in this work.

As mentioned in Introduction, Na3ZnB5O10crys- tallizes in the monoclinic21/group (= 6.6725(7),= 18.1730(10),= 7.8656(9) ?,= 114.604(6)°,= 867.18(14) ?3), while KNa2CdB5O10has the orthorhombic structure with space group(= 18.4555(13),= 8.3040(10),= 12.8772(9) ?,= 1973.5(3) ?3).Their crystal structures can be illustrated by group-subgroup relationship as follows: KNa2CdB5O10structure in this work (, a, b, c,= 8) → Na3ZnB5O10structure (21/, which is an I-type maximal non-isomorphic subgroup (index 2) of, 0.5b + 0.5c, a, 0.5b – 0.5c,= 4); To the best of our knowledge, no pentaborates with space grouphave been found, thus KNa2CdB5O10represents a new structure type (Pearson symbol152).

3. 2 Spectrum properties

Infrared spectrum of the title compound is shown in Fig. 5, where the characteristic absorption bands of both BO3and BO4groups are indeed observed. They are BO3asymmetric stretching ((BO3)) vibrations in the frequency range of 1435.8～1197.2 cm-1, BO4asymmetric stretching ((BO4)) modes from 1063.9 to 1019.0 cm-1, BO3symmetric stretching ((BO3)) modes lying at 941.6 and 926.9 cm-1, BO4symmetric stretching ((BO4)) mixed with BO3out-of plane bending ((BO3)) modes locating at about 772.6 cm-1, and the overlappedBO3and BO4bending vibrations occurring below 725.6 cm-1, respectively. These assignments are consistent with the literature[35].Because BO3groups are distorted from the ideal3hand BO4distorted from ideal Tdsymmetry, the degeneracy of the infrared active vibrations is removed, thus a large number of absorption bands are observed. This also enables the non-active vibrations(BO3) and(BO4) to absorb energy in the infrared region.

Fig. 5. Infrared spectrum ofKNa2CdB5O10

In order to investigate the optical properties, UV-VIS diffuse reflectance spectrum was measured and reflectance data were converted to absorbance using the Kubelka-Munk function: F() = (1 –)2/2= K/S, where R(), K(), and S()are the reflectance,absorption, and scattering coefficient, respectively. Fig. 6 shows an F() –(nm) plot, where no absorption peaks have been observed above 400 nm,consistent with the fact that the crystals are transparent under the visible light. A steep absorption edge was observed, confirming the insulator character as predicted by the electron precise nature of the chemical formula.Extrapolating the linear part of the spectral absorption edge to zero results in the optical band gap of about 3.44 eV (360 nm).

Fig. 6. Optical absorption spectrum of KNa2CdB5O10

In addition, we have also examined the solid-state luminescent properties of the titlecompound at room temperatureunder the excitation of a wavelength of 300 nm. It can be seen from Fig. 7 that KNa2CdB5O10shows a broad emission band from 340 to 450 nm with the emitted main peak lying at about 386 nm (3.21 eV). Since the emission energy is less than the energy corresponding to the optical absorption edge, we can deduce that the emitted fluorescence probably originates from the defects or excitons.

Fig. 7. Luminescence emission spectrum of KNa2CdB5O10

3. 3 Band structures and density of states

The calculated band structure of KNa2CdB5O10along high-symmetry points within the first Brillouin zone (BZ) is plotted in Fig. 8. It is observed that the valence bands (VBs) are relatively flat, while the conduction bands (CBs) have some oscillating. The valence band maximum (VBM) is located at the X (0, 1/2, 0) pointand the conduction bandminimum (CBM) at the G (0, 0, 0) point of BZ, indicating KNa2CdB5O10to be an indirect band material.The calculated energy gap is3.68 eV, in good agreement with the experimental one measured from the UV- VIS diffuse reflectance spectrum (3.44 eV).

The bands can be assigned according to the total and partial densities of states (TDOS, PDOS) as plotted in Fig. 9, where there exist 456 bands below the Fermi level, which can be divided into six regions. The lowest VB region with energies located at around –47.9 eV corresponds with Na 2electronic states and the second region lying near –27.0 eV has K 3character. They are not shown in Fig. 9 for brevity. The third region lying around –20.8 eV is derived from Na 2orbitals. The fourth region ranging from –20.0 to –15.8 eV mainly results from O 2, with some mixtures of B 2/2states. The fifth region which is located at about –11.0 eV is basically of K 3character. The highest VB regionextendingfrom –8.8 eV up to Fermi energy (EF) is primarily composed of O 2and Cd 4states with small mixtures of B 2/2states. In this region, the bands near EFmostly originate from the O 2states and those with energies at about –6.6 eV have significant contributions from the Cd 4states, while the B 2/2states are dispersed in the energy range between –8.8 and –2.0 eV.The CBs just above the Fermi level ranging from 3.6 to 7.8 eV strongly involve unoccupied B 2, Cd 5/5, K 3/3, Na 2/2states and less unoccupied O 2states, with the CBM controlled by Cd 5states. Thus, the electronic transitions from the occupied O 2to the complicated mixed states of unoccupied B 2, Cd 5/5, K 3/3, Na 2/2orbitals are believed to make main contributions to the optical absorption near the UV-VIS edge for the compound KNa2CdB5O10.

Fig. 8. Calculated band structure of KNa2CdB5O10, where the Fermi level (EF) is set at 0 eVand the labeled k-points are present as: G (0, 0, 0), Z (0, 0, 1/2), T (-1/2, 0, 1/2), Y (-1/2, 0, 0), S (-1/2, 1/2, 0), X (0, 1/2, 0), U (0, 1/2, 1/2), R (-1/2, 1/2, 1/2)

Fig. 9. Total and partial density of states of KNa2CdB5O10

From the angular momentum decomposition of the atom-projected density of states, one can see a strong hybridization between B 2/2and O 2states in the energy range between –20.0 and –15.8 eV. The B 2/2states also hybridize strongly with O 2states from –8.8 to –2.0 eV and Cd 4hybridizes with O 2states at energies around –6.6 eV in VBs. In contrast, Na 2and K 3electronic states are localized within very narrow energy ranges in VBs where there exists almost no mixing of O 2/2states. This means that B–O and Cd–O bonds have strong covalent character, while Na–O and K–O interactions are basically ionic, consistent with the general bonding properties we usually consider.

4 CONCLUSION

Single crystals of a new pentaborate, KNa2CdB5O10, have been grown by the high-temperature solution method and its crystal structure, IR, UV-VIS diffuse reflectance, and emission spectra as well as the band electronic structure calculations have been investigated. It is a layered compound containing the 2D [CdB5O10]n3n-sheet and crystallizes in a new structure type. IR spectrum contains the characteristic absorption bands of both BO3and BO4groups. The fact that the crystals of the title compound are transparent under the visible light has been confirmed by the UV-VIS diffuse reflectance spectrum, where the optical band gap of 3.44 eV is obtained. This band gap for the materials suggests insulating behavior. The observed absorption peaks near the absorption edge of UV-VIS diffused reflectance spectrum of KNa2CdB5O10are assigned as the electronic transitions from the occupied O 2to the complicated mixed states of unoccupied B 2, Cd 5/5, K 3/3, Na 2/2orbitals in view of our calculations of band structures and density of states. Moreover, solid-state lumine- scent properties of the title compound have also been examined and the emission is ascribed to the defects or excitons

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12 September 2017;

30 November 2017 (ICSD 433798)

the National Natural Science Foundation of China (No. 20871012)

.Chen Xue-An, born in 1965, associate professor, majoring in inorganic chemistry. E-mail: xueanchen@bjut.edu.cn

10.14102/j.cnki.0254-5861.2011-1826