Crystal structures and photocatalysis of the triclinic polymorphs of BiNbO4 and BiTaO4

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Abstract

The high-temperature polymorphs of two photocatalytic materials, BiNbO4 and BiTaO4 were synthesized by the ceramic method. The crystal structures of these materials were determined by single-crystal X-ray diffraction. BiNbO4 and BiTaO4 crystallize into the triclinic system P1¯ (No. 2), with a=5.5376(4) Å, b=7.6184(3) Å, c=7.9324(36) Å, α=102.565(3)°, β=90.143(2)°, γ=92.788 (4)°, V=326.21 (5) Å3, Z=4 and a=5.931 (1) Å, b=7.672 (2) Å, c=7.786 (2) Å, α=102.94 (3)°, β=90.04 (3)° γ=93.53 (3)°, V=344.59 (1) Å3 and Z=4, respectively. The structures along the c-axis, consist of layers of [Bi2O2] units separated by puckered sheets of (Nb/Ta)O6 octahedra. Photocatalytic studies on the degradation of dyes indicate selectivity of BiNbO4 towards aromatics containing quinonic and azo functional groups.

Graphical abstract

Crystal structures of Bi(Nb/Ta)O4 along b-axis: triclinic form.

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Introduction

BiNbO4 and BiTaO4 belong to the family of the A3+B5+O4 compounds. The triclinic forms of BiNbO4 and the iso-structural BiTaO4 were synthesized at 1149 °C [1]. Both BiNbO4 and BiTaO4 undergo an irreversible orthorhombic–triclinic transition. The low-temperature orthorhombic form synthesized at 900 °C transforms to the triclinic high-temperature form at 1020 °C for BiNbO4 and at 1150 °C for BiTaO4 [2], [3]. Single crystals of the orthorhombic form of BiNbO4 were grown from a flux containing BiOF below 900 °C [4].

The first attempt towards structure determination of the high temperature triclinic form of BiNbO4, was made by Keve and Skapski who reported the single-crystal X-ray structure in the P1¯ space group with a=7.61 (1) Å, b=5.53 (6) Å, c=7.91 (9) Å, α=89.88°, β=77.43°, γ=87.15° and Z=4 [5]. Since the data was collected on a twinned crystal, an absorption correction was not applied. This resulted in negative thermal parameters for Bi(1) (B=−0.01 (8) Å2) and the oxygen atoms, O(1), O(2), O(4), O(6) and O(7) [5]. Subsequently, the crystal structure of the iso-structural BiTaO4 was obtained by Rietveld refinement using the structural parameters of BiNbO4 [5] in the space group P1 [6]. In another report of the Rietveld refinement of BiTaO4, Lee et al. [7] used the space group P1¯, but obtained high standard deviations for the positional parameters of oxygen atoms and unacceptable bond distances from powder X-ray data. Subsequently, photocatalytic activity of the solid solution of the type, BiTa1−xNbxO4 in the range (0⩽x⩽1) [8], [9] was reported.

As explained above, the structural reports on the high temperature forms of both BiNbO4 and BiTaO4 are ambiguous. Since BiNbO4 and BiTaO4 are potential photo-catalytic materials, an accurate structure determination of these materials becomes extremely important in order to co-relate the structure property relationships. In this context, our objective was to re-determine the crystal structures of BiNbO4 and BiTaO4 by single-crystal X-ray diffraction. Further, photocatalytic degradation of dyes like Orange G (OG), Methyl violet (MV) and Alizarin green (AG) were studied in the presence of BiNbO4 and BiTaO4.

Section snippets

Materials

Bi2O3, Nb2O5 and Ta2O5 (all of them obtained from Fluka, 99.9%) were dried at 600 °C before use. Dyes, Methyl Violet MS, Orange G (all from S.D. fine-Chem Ltd., India) and Alizarin cyanine Green (Rolex, India) were used as such. Water was double distilled and filtered before use.

Synthesis and crystallization

Polycrystalline samples of BiNbO4 and BiTaO4 were synthesized by the ceramic route from Bi2O3, Nb2O5 and Ta2O5 in stoichiometric quantities. The reaction mixtures were ground well in an agate mortar and heated in a

Crystal structures

BiNbO4 and BiTaO4 crystallize in the triclinic system P1¯ (No. 2) with a=5.5376 (4) Å, b=7.6184 (3) Å, c=7.9324 (36) Å, α=102.565 (3)°, β=90.143 (2)°, γ=92.788 (4)°, V=326.21 (5) Å3, Z=4 and a=5.931 (1) Å, b=7.672 (2) Å, c=7.786 (2) Å, α=102.94 (3)°, β=90.04 (3)°, γ=93.53 (3)°, V=344.59 (1) Å3 and Z=4, respectively. The crystal structure of Bi(Nb/Ta)O4 along the c-axis is shown in Fig. 1a. It consists of pseudo layers of [Bi2O2] units connected to each other and surrounded by sheets of puckered NbO6

Conclusion

The crystal structures of the high temperature polymorphs of two important photocatalytic materials, BiNbO4 and BiTaO4 were re-determined by single-crystal X-ray diffraction. Both the compounds are iso-structural and consist of layers of [Bi2O2] units separated by pseudo-layers of puckered sheets of Nb/TaO6 octahedra. The crystal structures depict subtle variations with respect to co-ordination of the (Nb/Ta)O6 octahedra around [Bi2O2] units. The low-temperature orthorhombic and high

Acknowledgments

Single-crystal X-ray data collection on the CCD facility under the IRPHA-DST program at the Indian Institute of Science is gratefully acknowledged. B. Muktha thanks CSIR for senior research fellowship.

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