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Two bismuth niobates containing Bi2O2 segments
Journal Pre-proof Two bismuth niobates containing Bi2O2 segments Jingfang Zhou, Rukang Li PII:
S0022-4596(19)30606-1
DOI:
https://doi.org/10.1016/j.jssc.2019.121101
Reference:
YJSSC 121101
To appear in:
Journal of Solid State Chemistry
Received Date: 15 October 2019 Revised Date:
28 November 2019
Accepted Date: 29 November 2019
Please cite this article as: J. Zhou, R. Li, Two bismuth niobates containing Bi2O2 segments, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.121101. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Title Page Title: Two Bismuth Niobates containing Bi2O2 Segments
Authors: Jingfang Zhou and Rukang Li* Affiliation: Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China and Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P.R. China *Corresponding author: Rukang
Li
Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China Email address: [email protected]
Two Bismuth Niobates containing Bi2O2 Segments Jingfang Zhou and Rukang Li* Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China and Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P.R. China *corresponding author. Email address: [email protected]
ABSTRACT: (I) and Two new compounds Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)-Nb2O31 (II) are obtained which are constituted by octahedral NbO6 chains and Bi2O2 segments. I and II are iso-structural and crystallize in the tetragonal space group P42/nmc with cell parameters of a = 11.3333(16) Å, c = 10.854(2) Å and a = 11.3003(1) Å, c = 10.8186(1) Å, respectively. The Bi2O2 segment in this structure resembles those in the Aurivillius or Sillen structures whereas the connection of NbO6 chains is closely related to the pyrochlore structures. Na+ and Ca2+ cations were found to replace about half of the Bi(II) sites in the Bi2O2 segments. The two compounds have similar band gaps around 2.8 eV, which may find potential applications as visible-light-driven photocatalysts. Keywords: Aurivillius structure; Sillen structure; Bi2O2 segments; NbO6 chains; photo-catalyst; 1. Introduction Bi2O2 layer is formed by edge-sharing connection of BiOn polyhedrons and exists in various compounds with excellent physical and chemical properties. Bi2O2 layer is present in Bi-based high-TC oxide superconductors with general formula Bi2O2 . Can-1 . (SrO)2 . (CuO2)n (n = 1-3), which are built by alternating stacks of the rock-salt Bi2O2 layers and the CuO2 layers [1-5]. Aurivillius type compounds (general formula: (Bi2O2)2+(An-1BnO3n+1)2-) could be depicted structurally as alternately stacking of Bi2O2 slabs and perovskite blocks [6-8]. Due to their excellent physical properties including fatigue-free, piezoelectric with high values of TC, Aurivillius oxides were widely applied in ferroelectric random-access memory (FRAM) and high-temperature piezoelectric devices [9-14]. Bi2O2 sheet is also found in the Sillén series, which can be expressed by [Bi2O2][Xm] (m= 1-3, X = halogen) [15-16]. Sillén series exhibit outstanding photocatalytic activity for water splitting and organic pollutant decomposition under exposure of ultraviolet or visible light [17-18]. Recently, it has been found that compounds with Bi2O2 sheets interleaved with OH- and NO3groups or IO3- groups show large nonlinear optical effects [19-20]. In addition to NO3- and IO3- as NLO active units, Bi2O2 with stereochemically active lone pair Bi3+ also contributes to a large part for the second-order susceptibility. Furthermore, LiNbO3 and KNbO3 are typical perovskite ferroelectric with distorted NbO6 octahedra through corner-sharing connection. Both of them are also known as nonlinear optical and electro-optic crystals with large second order susceptibilities [21-23]. It was proposed that the cations with second-order Jahn-Teller effect like Nb and Ta, usually forming distorted NbO6 or TaO6 octahedron, as an asymmetric unit, could increase the chance to find noncentro-symmetric structures [24].
Thus, we speculated that new compounds with the cooperation of Bi2O2 layers and distorted NbO6 octahedra may exhibit strong NLO effects and useful properties. Based on the aforementioned idea, we paid attention to searching for new compounds in the Bi-Nb-O system. In the process of exploring Bi-M-Nb-O system (M = Na, Ca), two new compounds with compositions of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) contained one-dimensional NbO6 chains and Bi2O2 segments were discovered. 2. Experimental section 2.1. Materials and Instruments: All reagents including H3BO3, LiF, NaF, Bi2O3, Nb2O5, Na2CO3, CaCO3 with analytical grade purities were purchased from Sinopharm Chemical ReagentCo., Ltd. Powder X-ray diffraction (PXRD) was performed with a Bruker D8 focus X-ray diffractometer equipped with Kα radiation from Cu target at room temperature. Typical PXRD patterns were collected with scanning step width of 0.02° and scanning rate of 0.05 s/step. The optical diffuse-reflection spectra of I and II were tested on Agilent Cary 7000 UV-vis-NIR spectrometer with the wavelength covering from 200 nm to 2500 nm. Thermal stability of I were investigated with a NETZSCH STA 409 CD differential scanning calorimetry (DSC) and thermogravimetric analyzer.
2.2. Solid-state synthesis The polycrystalline powder of I and II were prepared by traditional solid-state synthesis from sintering the mixture of starting reactants in ideal stoichiometric ratios. The mixture was preheated at 500 °C for 10 hours in order to remove H2O and CO2 from raw materials. Then the temperature was elevated to 850 °C for I and 950 °C for II, respectively, at a heating rate of 2 °C/ min and held for 12 hours at the set temperatures. Several reheating and re-grindings were needed. The experimental PXRD patterns of I and II were in good agreement with the simulated PXRD patterns deduced from the single-crystal and Rietveld structure analyses (seen in Fig. 1.).
Fig. 1. Calculated and experimental PXRD patterns of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) (b). 2.3. Growth of single crystals The single crystals of I were successfully grown from B2O3-LiF-NaF flux by spontaneous crystallization. The samples were prepared in corundum crucible by the mixture of H3BO3, LiF, NaF, Bi2O3 and Nb2O5 at a molar ratio of 1:9:9:1:0.6. Large amounts of the fluorides and the boric acid were found necessary to lower the melting temperature of the mixtures to facilitate the crystal growth. The mixture was melt at 800 °C and kept for 12 hours to create a homogeneous solution. Subsequently, the temperature of the melt was cooled slowly to 650°C at a rate of 2 °C/h and further to room temperature in 24 hours. Light yellow, transparent crystals of I were observed on the melt surface and were mechanically separated from the surface. 2.4. Structure determination A transparent yellow crystal of I was chosen for single-crystal X-ray diffraction on a Rigaku AFC10 CCD diffractometer with an incident wavelength of 0.71075 Å at 153 K. Collection of the intensity data, refinement of cell parameters and reduction of data were carried out with the CrystalClear software. The basic structure with heavy atoms was solved and refined in the SHELX-2018 package [25]. During the refinement, the temperature factors of the Nb atoms and most of the oxide ions went negative and one of the Bi site went very large, strongly indicative of deficiency and/or substitution of light atoms occurred. Finally it is found about 40% of Bi(II) site is replaced by Na and about 10% of Nb replaced by Bi, leading to a nominal formula of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 , in consistent with elemental analysis result with a molar ratio of Bi, Na, Nb, O of 10.5:1.9:5.8:31 (shown in Fig. S2). By knowing the chemical formula, we tried to prepare a stoichiometric compound with an
ideal composition of Bi8Ca4Nb6O31. By repeated heating and checking the purity with PXRD, it was found slightly reducing the Nb content, as Bi8Ca4Nb5.7O31, was necessary to get a phase pure sample. A prolonged scanning of powder X-ray diffraction (PXRD) was carried out with PANalytical X’Pert PRO powder X-ray diffractometer at room temperature. Structure determination from this data with importing the structure model of I from the single XRD data was performed with the GSAS software [26]. It is found for this sample, Ca goes to both of the Bi sites while only Nb(I) site is replaced by 5% of Bi, corresponding to a nominal formula of (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (here charge balance was not deliberately constrained, for Bi could go to Bi5+ and O could have deficiencies). Final refinement with meaningful structure moiety and temperature factors was achieved with reasonable agreement indices of Rwp = 0.0632 and χ2=2.332. The crystallographic data of I and II are summarized in Table 1. Atomic coordinates and equivalent isotropic temperature factors are given in Table S1-2. Selected bond lengths are listed in Table S3-4. Table 1 Crystallographic data for Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) formula
Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31
(Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31
Space group
P 42/nmc (No.137)
P 42/nmc (No.137)
a (Å)
11.3333(16)
11.3003(1)
c (Å)
10.854(2)
10.8186(1)
V (Å ）
1394.2(5)
1381.50(2)
Z
2
2
λ (Å)
0.71073
1.54178
T (K)
153(2)
295(2)
3
2
Rwp, χ (GSAS) R1,wR2 (SHELX-2018)
0.0632, 2.332 0.0571, 0.1337
3. Results and discussion 3.1. Structure description I and II are isostructural and crystallized in the tetragonal space group of P42/nmc with cell parameters of a = 11.3333(16) Å, c = 10.854(2) Å; a = 11.3003(1) Å, c = 10.8186(1) Å, respectively. The structure of I is shown in Fig. 2c. In this structure, Nb5+ cations are bonded to six O atoms with bond lengths ranging from 1.83(2) Å to 2.34(2) Å. Distorted NbO6 octahedra are further connected with O1 and O5 atoms to form one-dimensional chains, which are perpendicular to each other in the x and y axes. The intersection of NbO6 chains along x and y axes form [Nb4O18]16- blocks, which are formed by four NbO6 octahedra (Fig. 2a). This structure of approximate tetrahedral block [Nb4O18]16- is similar with that in the pyrochlore structure. Bi3+ atoms are coordinated in two ways (Fig. 2d). One is to adopt a regular hepta-coordinated
connection to form BiO7 polyhedron with three shorter Bi-O bonds (Bi-O3 : 2.185 (8) Å; Bi-O4 : 2.179 (11) Å; Bi-O7 :2.164 (8) Å) and four longer Bi-O bonds (Bi-O2 : 2.667 (12) Å; Bi-O4 : 2.621 (12) Å; Bi-O6 : 2.647 (15) Å; Bi-O5 :2.699 (14) Å). The other is co-occupied with Na and five-coordinated to O atoms in the forms of (Bi0.6Na0.4)O5 tetragonal pyramids with three shorter Bi-O bonds (Bi-O3 : 2.399 (11) Å; Bi-O3 :2.399 (11) Å; Bi-O8 : 2.230 (2) Å) and two longer Bi-O bonds (Bi-O6 : 2.59 (2) Å; Bi-O7 : 2.70 (2) Å). If only the shorter Bi-O bonds in BiO7 polyhedra and (Bi0.6Na0.4)O5 tetragonal pyramids are considered, Bi2O2 segments (Fig. 2e,f) are formed similar to the Bi2O2 layers in the Aurivillius family. It’s worth noting that Bi and Na co-occupy the Bi (II) site, which was rarely seen in Bi2O2 layer in the Aurivillius phases or high TC Bi-cuprates, except for Pb/Sn for Bi [27-30]. II with more Ca substitution shrinks the unit cell volume by 0.9% but still maintains the similar three-dimension structure made up of NbO6 octahedra, (BiCa)O6 polyhedra and (CaBi)O5 tetragonal pyramids via corner-sharing or edge-sharing connection and the phenomenon of co-occupancy in the Bi2O2 segments is also vividly manifested in II. As aforementioned, in II Ca goes to both of the Bi sites while only Nb(I) site is slightly replaced forming (Nb0.975Bi0.025)O6 octahedra. Bi(I) site with Bi0.75Ca0.25 is linked with six O atoms by three shorter bonds (Bi-O3 : 2.153 (19) Å; Bi-O4 : 2.140 (25) Å; Bi-O6 : 2.279 (13) Å) and three longer bonds(Bi-O2 : 2.695 (26) Å; Bi-O5 : 2.691 (20) Å; Bi-O7 : 2.519 (3) Å), whereas Bi(II) site with Ca0.525Bi0.475 is bonded to five O atoms via three shorter bonds (Bi-O3 : 2.371 (24) Å; Bi-O3 : 2.371 (24) Å; Bi-O8 : 2.291 (7) Å) and two longer bonds (Bi-O6 : 2.520 (34) Å; Bi-O7 : 2.576 (7) Å). Similarly, we can observe Bi2O2 segments in the structure of compound II, if the longer bonds are ignored.
Fig. 2. the structure of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 with the [Nb4O18]16- blocks (a); the layer formed by NbO6 chains (b); highlighting the Nb-O bonding (c); the basic structural units in I (d); the Bi2O2 segments (e) and highlighting the Bi-O bonding (f) Highlighting the Nb-O bonding, the structure of I and II can also be described as a layer structure formed by the interconnected NbO6 chains (Fig. 2c). As aforementioned, the intersection of NbO6 chains along x and y axes form [Nb4O18]16- blocks, which is related to the Ca2Nb2O7 with
a pyrochlore structure [31-32]. The structure of Ca2Nb2O7 can be described as NbO6 chains with [Nb4O18]16- joints interconnected in 3 dimensions to form a NbO6 network (Fig. 3a). Starting from the NbO6 net, using the Bi2O2 segments as “scissors” to break the [Nb4O18]16- joints in one direction, leaving the NbO6 layers with NbO6 chains cross linked in the remaining 2 dimensions, then the structure of I or II is obtained (Fig. 3b).
Fig. 3. the structure of pyrochlore Ca2Nb2O7 (a) and its relationship with the structure of the title compounds (b) 3.2. Thermal Analysis For I, the DSC curve (Fig. 4) reveals some features around 900°C and 980°C and a broad endothermic peak located at 1030°C in the heating process and an exothermic peak located at 810°C in the cooling process, suggesting that I is an incongruent compound. Powder XRD diffraction pattern on the residues of I demonstrates that it decomposes to Bi2.5Na0.5Nb2O9 above 1030°C, further verifying that I melts incongruently. Test by PXRD, II is also an incongruent melting compound, which decomposes to Bi2CaNb2O9 after melting (seen in Fig. S3).
Fig. 4. The DSC curve for Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and PXRD patterns for the residues and the simulated one (b).
3.3. Diffuse-Reflectance Spectroscopy As depicted in Fig. 5, I and II present high optical reflectance in the range of 500 - 2500 nm (R% more than 80%) and their cut-off edges are located at about 415 nm and 409 nm, respectively. The band gap (Eg) of a material is a significant criterion of photocatalyst to determine its applied wavelength regions. For powdered oxide samples, assuming a direct transition, their band gaps can be determined by the following equation [33]: [F ℎ ] = ℎ − where F is Kubelka-Munk equation, ℎ is the photon energy, B is the proportionality is the energy of a direct band gap. constant and The band gap energies of I (2.82 eV) and II (2.85 eV) are obtained by plotting [F ℎ ] against photon energies ℎ (Fig. 5 inset). The two compounds have appropriate band gaps (1.23 eV < Eg < 3.0 eV) to harvest visible light, which accounts for more than 50% of solar energy. Meanwhile, layered structures, such as Aurivillius type, Sillén type and mixed Sillén-Aurivillius type, are considered to be favorable of generating and separating the charge carries, contributing to high photocatalytic activities. In addition, Nb-based oxides, with d10-metal cations, are also widely applied as photocatalysts [34]. Thus, the two compounds reported here with Bi2O2 segments and NbO6 chains may be employed as potential visible-light-driven photocatalysts.
Fig. 5. The optical diffuse-reflection spectra for Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) (b), the insets show plots of [ F ( R∞ ) hv ]2 - hv .
Band engineering is generally recognized as an effective approach to design and obtain potential photocatalysts. For Bi-based oxide, EVB is located at more negative potential compared to other semiconductor oxides, which is ascribed to the interaction of O 2p orbitals and Bi 6s orbitals [35]. It is found that Pb2+ and Sn2+ with lone pairs also can interact with O 2p orbitals to achieve positive shift of valence band energy EVB, while conductive band energy ECB can undergo negative shift with the size of substituted cations in Bi2O2 layers increasing. For example, ECB of Sr2Bi3M2O11Cl (M=Nb, Ta) is higher than that of Ba2Bi3M2O11Cl (M=Nb, Ta), while EVB is maintained [35]. For I and II, Na-for-Bi and Ca-for-Bi substitution are observed in Bi2O2
segments, which is a signature of the possibility to other cation substitutions (Rb+, Cs+, Tl+ for I; Sr2+, Ba2+, Pb2+, Sn2+ for II ). Thus, we anticipate that a series of I and II with narrower bang gap could be obtained via band engineering to achieve perfectly fitting band gaps to the sunlight spectrum. 4. Conclusions In summary, two new compounds of I and II were found during exploration of Bi-Nb-O system. These two compounds feature complicated three dimensional structure containing NbO6 chains and Bi2O2 segments, similar to Bi2O2 layers in Aurivillius. Na+ and Ca2+ cations were found to replace Bi sites in the Bi2O2 segments. The two compounds have appropriate (Eg)s around 2.8 eV, which may find potential applications as visible light-active photocatalysts due to the cooperative effects of d10-metal cations Nb5+ and Bi3+. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51772304) and Fujian Institute of Innovation, Chinese Academy of Sciences (FJCXY18030101).
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Figure Captions Fig. 1. Calculated and experimental PXRD patterns of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) (b). Fig. 2. the structure of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 with the [Nb4O18]16- blocks (a); the layer formed by NbO6 chains (b); highlighting the Nb-O bonding (c); the basic structural units in I (d); the Bi2O2 segments (e) and highlighting the Bi-O bonding (f). Fig. 3. the structure of pyrochlore Ca2Nb2O7 (a) and its relationship with the structure of the title compounds (b) Fig. 4. The DSC curve for Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and PXRD patterns for the residues and the simulated one (b). Fig. 5. The optical diffuse-reflection spectra for Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) (b), the insets show plots of [ F ( R∞ ) hv ]2 - hv .
Author contributions ZJF: carried out the synthesis, crystal growth, data collection and some data analysis, written up the draft of the manuscript LRK: Supervised the work and solved the structures from single crystal and powder XRD data, and some data analysis, editing and final version approval of the manuscript.
Declaration of interests ☐ √The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
None
S0022-4596(19)30606-1
DOI:
https://doi.org/10.1016/j.jssc.2019.121101
Reference:
YJSSC 121101
To appear in:
Journal of Solid State Chemistry
Received Date: 15 October 2019 Revised Date:
28 November 2019
Accepted Date: 29 November 2019
Please cite this article as: J. Zhou, R. Li, Two bismuth niobates containing Bi2O2 segments, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/j.jssc.2019.121101. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Title Page Title: Two Bismuth Niobates containing Bi2O2 Segments
Authors: Jingfang Zhou and Rukang Li* Affiliation: Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China and Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P.R. China *Corresponding author: Rukang
Li
Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China Email address: [email protected]
Two Bismuth Niobates containing Bi2O2 Segments Jingfang Zhou and Rukang Li* Beijing Center for Crystal Research and Development, Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China and Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P.R. China *corresponding author. Email address: [email protected]
ABSTRACT: (I) and Two new compounds Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)-Nb2O31 (II) are obtained which are constituted by octahedral NbO6 chains and Bi2O2 segments. I and II are iso-structural and crystallize in the tetragonal space group P42/nmc with cell parameters of a = 11.3333(16) Å, c = 10.854(2) Å and a = 11.3003(1) Å, c = 10.8186(1) Å, respectively. The Bi2O2 segment in this structure resembles those in the Aurivillius or Sillen structures whereas the connection of NbO6 chains is closely related to the pyrochlore structures. Na+ and Ca2+ cations were found to replace about half of the Bi(II) sites in the Bi2O2 segments. The two compounds have similar band gaps around 2.8 eV, which may find potential applications as visible-light-driven photocatalysts. Keywords: Aurivillius structure; Sillen structure; Bi2O2 segments; NbO6 chains; photo-catalyst; 1. Introduction Bi2O2 layer is formed by edge-sharing connection of BiOn polyhedrons and exists in various compounds with excellent physical and chemical properties. Bi2O2 layer is present in Bi-based high-TC oxide superconductors with general formula Bi2O2 . Can-1 . (SrO)2 . (CuO2)n (n = 1-3), which are built by alternating stacks of the rock-salt Bi2O2 layers and the CuO2 layers [1-5]. Aurivillius type compounds (general formula: (Bi2O2)2+(An-1BnO3n+1)2-) could be depicted structurally as alternately stacking of Bi2O2 slabs and perovskite blocks [6-8]. Due to their excellent physical properties including fatigue-free, piezoelectric with high values of TC, Aurivillius oxides were widely applied in ferroelectric random-access memory (FRAM) and high-temperature piezoelectric devices [9-14]. Bi2O2 sheet is also found in the Sillén series, which can be expressed by [Bi2O2][Xm] (m= 1-3, X = halogen) [15-16]. Sillén series exhibit outstanding photocatalytic activity for water splitting and organic pollutant decomposition under exposure of ultraviolet or visible light [17-18]. Recently, it has been found that compounds with Bi2O2 sheets interleaved with OH- and NO3groups or IO3- groups show large nonlinear optical effects [19-20]. In addition to NO3- and IO3- as NLO active units, Bi2O2 with stereochemically active lone pair Bi3+ also contributes to a large part for the second-order susceptibility. Furthermore, LiNbO3 and KNbO3 are typical perovskite ferroelectric with distorted NbO6 octahedra through corner-sharing connection. Both of them are also known as nonlinear optical and electro-optic crystals with large second order susceptibilities [21-23]. It was proposed that the cations with second-order Jahn-Teller effect like Nb and Ta, usually forming distorted NbO6 or TaO6 octahedron, as an asymmetric unit, could increase the chance to find noncentro-symmetric structures [24].
Thus, we speculated that new compounds with the cooperation of Bi2O2 layers and distorted NbO6 octahedra may exhibit strong NLO effects and useful properties. Based on the aforementioned idea, we paid attention to searching for new compounds in the Bi-Nb-O system. In the process of exploring Bi-M-Nb-O system (M = Na, Ca), two new compounds with compositions of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) contained one-dimensional NbO6 chains and Bi2O2 segments were discovered. 2. Experimental section 2.1. Materials and Instruments: All reagents including H3BO3, LiF, NaF, Bi2O3, Nb2O5, Na2CO3, CaCO3 with analytical grade purities were purchased from Sinopharm Chemical ReagentCo., Ltd. Powder X-ray diffraction (PXRD) was performed with a Bruker D8 focus X-ray diffractometer equipped with Kα radiation from Cu target at room temperature. Typical PXRD patterns were collected with scanning step width of 0.02° and scanning rate of 0.05 s/step. The optical diffuse-reflection spectra of I and II were tested on Agilent Cary 7000 UV-vis-NIR spectrometer with the wavelength covering from 200 nm to 2500 nm. Thermal stability of I were investigated with a NETZSCH STA 409 CD differential scanning calorimetry (DSC) and thermogravimetric analyzer.
2.2. Solid-state synthesis The polycrystalline powder of I and II were prepared by traditional solid-state synthesis from sintering the mixture of starting reactants in ideal stoichiometric ratios. The mixture was preheated at 500 °C for 10 hours in order to remove H2O and CO2 from raw materials. Then the temperature was elevated to 850 °C for I and 950 °C for II, respectively, at a heating rate of 2 °C/ min and held for 12 hours at the set temperatures. Several reheating and re-grindings were needed. The experimental PXRD patterns of I and II were in good agreement with the simulated PXRD patterns deduced from the single-crystal and Rietveld structure analyses (seen in Fig. 1.).
Fig. 1. Calculated and experimental PXRD patterns of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) (b). 2.3. Growth of single crystals The single crystals of I were successfully grown from B2O3-LiF-NaF flux by spontaneous crystallization. The samples were prepared in corundum crucible by the mixture of H3BO3, LiF, NaF, Bi2O3 and Nb2O5 at a molar ratio of 1:9:9:1:0.6. Large amounts of the fluorides and the boric acid were found necessary to lower the melting temperature of the mixtures to facilitate the crystal growth. The mixture was melt at 800 °C and kept for 12 hours to create a homogeneous solution. Subsequently, the temperature of the melt was cooled slowly to 650°C at a rate of 2 °C/h and further to room temperature in 24 hours. Light yellow, transparent crystals of I were observed on the melt surface and were mechanically separated from the surface. 2.4. Structure determination A transparent yellow crystal of I was chosen for single-crystal X-ray diffraction on a Rigaku AFC10 CCD diffractometer with an incident wavelength of 0.71075 Å at 153 K. Collection of the intensity data, refinement of cell parameters and reduction of data were carried out with the CrystalClear software. The basic structure with heavy atoms was solved and refined in the SHELX-2018 package [25]. During the refinement, the temperature factors of the Nb atoms and most of the oxide ions went negative and one of the Bi site went very large, strongly indicative of deficiency and/or substitution of light atoms occurred. Finally it is found about 40% of Bi(II) site is replaced by Na and about 10% of Nb replaced by Bi, leading to a nominal formula of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 , in consistent with elemental analysis result with a molar ratio of Bi, Na, Nb, O of 10.5:1.9:5.8:31 (shown in Fig. S2). By knowing the chemical formula, we tried to prepare a stoichiometric compound with an
ideal composition of Bi8Ca4Nb6O31. By repeated heating and checking the purity with PXRD, it was found slightly reducing the Nb content, as Bi8Ca4Nb5.7O31, was necessary to get a phase pure sample. A prolonged scanning of powder X-ray diffraction (PXRD) was carried out with PANalytical X’Pert PRO powder X-ray diffractometer at room temperature. Structure determination from this data with importing the structure model of I from the single XRD data was performed with the GSAS software [26]. It is found for this sample, Ca goes to both of the Bi sites while only Nb(I) site is replaced by 5% of Bi, corresponding to a nominal formula of (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (here charge balance was not deliberately constrained, for Bi could go to Bi5+ and O could have deficiencies). Final refinement with meaningful structure moiety and temperature factors was achieved with reasonable agreement indices of Rwp = 0.0632 and χ2=2.332. The crystallographic data of I and II are summarized in Table 1. Atomic coordinates and equivalent isotropic temperature factors are given in Table S1-2. Selected bond lengths are listed in Table S3-4. Table 1 Crystallographic data for Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) formula
Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31
(Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31
Space group
P 42/nmc (No.137)
P 42/nmc (No.137)
a (Å)
11.3333(16)
11.3003(1)
c (Å)
10.854(2)
10.8186(1)
V (Å ）
1394.2(5)
1381.50(2)
Z
2
2
λ (Å)
0.71073
1.54178
T (K)
153(2)
295(2)
3
2
Rwp, χ (GSAS) R1,wR2 (SHELX-2018)
0.0632, 2.332 0.0571, 0.1337
3. Results and discussion 3.1. Structure description I and II are isostructural and crystallized in the tetragonal space group of P42/nmc with cell parameters of a = 11.3333(16) Å, c = 10.854(2) Å; a = 11.3003(1) Å, c = 10.8186(1) Å, respectively. The structure of I is shown in Fig. 2c. In this structure, Nb5+ cations are bonded to six O atoms with bond lengths ranging from 1.83(2) Å to 2.34(2) Å. Distorted NbO6 octahedra are further connected with O1 and O5 atoms to form one-dimensional chains, which are perpendicular to each other in the x and y axes. The intersection of NbO6 chains along x and y axes form [Nb4O18]16- blocks, which are formed by four NbO6 octahedra (Fig. 2a). This structure of approximate tetrahedral block [Nb4O18]16- is similar with that in the pyrochlore structure. Bi3+ atoms are coordinated in two ways (Fig. 2d). One is to adopt a regular hepta-coordinated
connection to form BiO7 polyhedron with three shorter Bi-O bonds (Bi-O3 : 2.185 (8) Å; Bi-O4 : 2.179 (11) Å; Bi-O7 :2.164 (8) Å) and four longer Bi-O bonds (Bi-O2 : 2.667 (12) Å; Bi-O4 : 2.621 (12) Å; Bi-O6 : 2.647 (15) Å; Bi-O5 :2.699 (14) Å). The other is co-occupied with Na and five-coordinated to O atoms in the forms of (Bi0.6Na0.4)O5 tetragonal pyramids with three shorter Bi-O bonds (Bi-O3 : 2.399 (11) Å; Bi-O3 :2.399 (11) Å; Bi-O8 : 2.230 (2) Å) and two longer Bi-O bonds (Bi-O6 : 2.59 (2) Å; Bi-O7 : 2.70 (2) Å). If only the shorter Bi-O bonds in BiO7 polyhedra and (Bi0.6Na0.4)O5 tetragonal pyramids are considered, Bi2O2 segments (Fig. 2e,f) are formed similar to the Bi2O2 layers in the Aurivillius family. It’s worth noting that Bi and Na co-occupy the Bi (II) site, which was rarely seen in Bi2O2 layer in the Aurivillius phases or high TC Bi-cuprates, except for Pb/Sn for Bi [27-30]. II with more Ca substitution shrinks the unit cell volume by 0.9% but still maintains the similar three-dimension structure made up of NbO6 octahedra, (BiCa)O6 polyhedra and (CaBi)O5 tetragonal pyramids via corner-sharing or edge-sharing connection and the phenomenon of co-occupancy in the Bi2O2 segments is also vividly manifested in II. As aforementioned, in II Ca goes to both of the Bi sites while only Nb(I) site is slightly replaced forming (Nb0.975Bi0.025)O6 octahedra. Bi(I) site with Bi0.75Ca0.25 is linked with six O atoms by three shorter bonds (Bi-O3 : 2.153 (19) Å; Bi-O4 : 2.140 (25) Å; Bi-O6 : 2.279 (13) Å) and three longer bonds(Bi-O2 : 2.695 (26) Å; Bi-O5 : 2.691 (20) Å; Bi-O7 : 2.519 (3) Å), whereas Bi(II) site with Ca0.525Bi0.475 is bonded to five O atoms via three shorter bonds (Bi-O3 : 2.371 (24) Å; Bi-O3 : 2.371 (24) Å; Bi-O8 : 2.291 (7) Å) and two longer bonds (Bi-O6 : 2.520 (34) Å; Bi-O7 : 2.576 (7) Å). Similarly, we can observe Bi2O2 segments in the structure of compound II, if the longer bonds are ignored.
Fig. 2. the structure of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 with the [Nb4O18]16- blocks (a); the layer formed by NbO6 chains (b); highlighting the Nb-O bonding (c); the basic structural units in I (d); the Bi2O2 segments (e) and highlighting the Bi-O bonding (f) Highlighting the Nb-O bonding, the structure of I and II can also be described as a layer structure formed by the interconnected NbO6 chains (Fig. 2c). As aforementioned, the intersection of NbO6 chains along x and y axes form [Nb4O18]16- blocks, which is related to the Ca2Nb2O7 with
a pyrochlore structure [31-32]. The structure of Ca2Nb2O7 can be described as NbO6 chains with [Nb4O18]16- joints interconnected in 3 dimensions to form a NbO6 network (Fig. 3a). Starting from the NbO6 net, using the Bi2O2 segments as “scissors” to break the [Nb4O18]16- joints in one direction, leaving the NbO6 layers with NbO6 chains cross linked in the remaining 2 dimensions, then the structure of I or II is obtained (Fig. 3b).
Fig. 3. the structure of pyrochlore Ca2Nb2O7 (a) and its relationship with the structure of the title compounds (b) 3.2. Thermal Analysis For I, the DSC curve (Fig. 4) reveals some features around 900°C and 980°C and a broad endothermic peak located at 1030°C in the heating process and an exothermic peak located at 810°C in the cooling process, suggesting that I is an incongruent compound. Powder XRD diffraction pattern on the residues of I demonstrates that it decomposes to Bi2.5Na0.5Nb2O9 above 1030°C, further verifying that I melts incongruently. Test by PXRD, II is also an incongruent melting compound, which decomposes to Bi2CaNb2O9 after melting (seen in Fig. S3).
Fig. 4. The DSC curve for Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and PXRD patterns for the residues and the simulated one (b).
3.3. Diffuse-Reflectance Spectroscopy As depicted in Fig. 5, I and II present high optical reflectance in the range of 500 - 2500 nm (R% more than 80%) and their cut-off edges are located at about 415 nm and 409 nm, respectively. The band gap (Eg) of a material is a significant criterion of photocatalyst to determine its applied wavelength regions. For powdered oxide samples, assuming a direct transition, their band gaps can be determined by the following equation [33]: [F ℎ ] = ℎ − where F is Kubelka-Munk equation, ℎ is the photon energy, B is the proportionality is the energy of a direct band gap. constant and The band gap energies of I (2.82 eV) and II (2.85 eV) are obtained by plotting [F ℎ ] against photon energies ℎ (Fig. 5 inset). The two compounds have appropriate band gaps (1.23 eV < Eg < 3.0 eV) to harvest visible light, which accounts for more than 50% of solar energy. Meanwhile, layered structures, such as Aurivillius type, Sillén type and mixed Sillén-Aurivillius type, are considered to be favorable of generating and separating the charge carries, contributing to high photocatalytic activities. In addition, Nb-based oxides, with d10-metal cations, are also widely applied as photocatalysts [34]. Thus, the two compounds reported here with Bi2O2 segments and NbO6 chains may be employed as potential visible-light-driven photocatalysts.
Fig. 5. The optical diffuse-reflection spectra for Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) (b), the insets show plots of [ F ( R∞ ) hv ]2 - hv .
Band engineering is generally recognized as an effective approach to design and obtain potential photocatalysts. For Bi-based oxide, EVB is located at more negative potential compared to other semiconductor oxides, which is ascribed to the interaction of O 2p orbitals and Bi 6s orbitals [35]. It is found that Pb2+ and Sn2+ with lone pairs also can interact with O 2p orbitals to achieve positive shift of valence band energy EVB, while conductive band energy ECB can undergo negative shift with the size of substituted cations in Bi2O2 layers increasing. For example, ECB of Sr2Bi3M2O11Cl (M=Nb, Ta) is higher than that of Ba2Bi3M2O11Cl (M=Nb, Ta), while EVB is maintained [35]. For I and II, Na-for-Bi and Ca-for-Bi substitution are observed in Bi2O2
segments, which is a signature of the possibility to other cation substitutions (Rb+, Cs+, Tl+ for I; Sr2+, Ba2+, Pb2+, Sn2+ for II ). Thus, we anticipate that a series of I and II with narrower bang gap could be obtained via band engineering to achieve perfectly fitting band gaps to the sunlight spectrum. 4. Conclusions In summary, two new compounds of I and II were found during exploration of Bi-Nb-O system. These two compounds feature complicated three dimensional structure containing NbO6 chains and Bi2O2 segments, similar to Bi2O2 layers in Aurivillius. Na+ and Ca2+ cations were found to replace Bi sites in the Bi2O2 segments. The two compounds have appropriate (Eg)s around 2.8 eV, which may find potential applications as visible light-active photocatalysts due to the cooperative effects of d10-metal cations Nb5+ and Bi3+. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51772304) and Fujian Institute of Innovation, Chinese Academy of Sciences (FJCXY18030101).
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Figure Captions Fig. 1. Calculated and experimental PXRD patterns of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) (b). Fig. 2. the structure of Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 with the [Nb4O18]16- blocks (a); the layer formed by NbO6 chains (b); highlighting the Nb-O bonding (c); the basic structural units in I (d); the Bi2O2 segments (e) and highlighting the Bi-O bonding (f). Fig. 3. the structure of pyrochlore Ca2Nb2O7 (a) and its relationship with the structure of the title compounds (b) Fig. 4. The DSC curve for Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and PXRD patterns for the residues and the simulated one (b). Fig. 5. The optical diffuse-reflection spectra for Bi8(Bi2.4Na1.6)(Nb5.4Bi0.6)O31 (I) (a) and (Bi6Ca2)(Bi1.9Ca2.1)(Nb3.9Bi0.1)Nb2O31 (II) (b), the insets show plots of [ F ( R∞ ) hv ]2 - hv .
Author contributions ZJF: carried out the synthesis, crystal growth, data collection and some data analysis, written up the draft of the manuscript LRK: Supervised the work and solved the structures from single crystal and powder XRD data, and some data analysis, editing and final version approval of the manuscript.
Declaration of interests ☐ √The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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