Effect of bismuth substitution on crystal chemistry, photocatalysis and conductivity in Sr₃V₂O₈: a new structural type in palmierite class

Abstract

The effect of Bi substitution has been investigated in the palmierite type system Sr₃V₂O₈, wherein, a new solid solution Sr_3−xBi_2x/3V₂O₈ (0 ≤ x ≤ 0.4) has been established. The structure of the composition x = 0.20 (Sr_2.8Bi_0.13V₂O₈) was solved using single-crystal X-ray diffraction (XRD) technique. The compound crystallizes in the trigonal crystal system R3̄m with a palmierite structural type with a = 5.6067(4) Å, c = 20.0099(13) Å, V = 580.64(5) ų and Z = 3. The substituent Bi occupies a new unique 18h site resulting in a new series in palmierite class of compounds. The new solid solution has been characterized by synchrotron X-ray powder diffraction, solid-state UV-Vis diffuse-reflectance spectra, scanning electron microscopy and ionic conductivities and photocatalytic activities has been investigated. The solid solution compounds exhibited photocatalytic activity towards various classes of anionic dyes under UV radiation.

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Introduction

Alkaline-earth metal phosphate, vanadates, arsenates and chromates with the general formula A₃M₂O₈ (A = Sr, Ba; M = P, V, As, Cr) have been extensively investigated for their interesting optical, transport and electrical properties and for potential application in materials science, for example, as luminophors, as host materials for solid-state lasers, and as luminescent lamp coatings in television tubes.^1–8 Strontium orthovanadate (Sr₃V₂O₈) belongs to the palmierite (space group R3̄m) type of compounds⁹ and consists of [VO₄]³⁻ tetrahedra and two different kinds of Sr sites (Scheme 1). While one Sr atom occupies the 3̄m Wyckoff site, the other one sits at the 3m position, with coordination numbers of 6 and 10, respectively. It is observed that compounds of the type A₂Ln_2/3(VO₄)₂ belong to the palmierite class of compounds consisting of double vanadate units in the binary system A₃V₂O₈–LnVO₄, where A = Ca, Sr, Ba; Ln = La, Nd.^10,11 In these compounds, Ln³⁺ and the A atom are statistically distributed in 6c (3̄m) and 3a (3m) positions. The fact that the A atom sites are differently occupied, suggests that substitution at the A site would lead to solid solutions of varying compositions, leading to the possibility of forming hitherto unknown functional materials. For example, doping of Mn⁵⁺ and Eu³⁺ in Sr₃V₂O₈ has been observed to enhance the near IR-laser action and rare-earth activated luminescence, respectively.^3,12

Scheme 1 General structure of palmierite showing polyhedral linkages.

Sr₃V₂O₈ undergoes a pressure-induced first-order phase transition at around 150 kbar.⁶In situ high-pressure Raman studies reveal that the compound is transformed to the spinelloid type of structure by the mechanism in which the alternate V⁵⁺ ions pass through the triangular faces of their original distorted tetrahedral sites.⁶ Recently, a series of compounds with the formula Sr_3−3xLa_2x(V_1−yP_yO₄)₂ have been synthesized, and their cationic and electrical conductivities have been measured.^13,14 The incorporation of Bi³⁺ in Sr sites in Sr₃V₂O₈ has been described by Zhuravlev et al.¹⁵ The powder diffraction analysis has assigned 3a Wyckoff site to Bi atom (occupancy 0.667) and 6c site to Sr atom with full occupancy. However, it has been shown by our recent work using an accurate single-crystal data set that substitution of Bi³⁺ at the 3a Wyckoff site is not allowed in Pb₃V₂O₈.¹⁶

Indeed, Bi³⁺ occupies a different Wyckoff site 18h in this solid solution and thus stabilizing the high-temperature γ form of Pb₃V₂O₈. It is obvious that the position and occupancy of Bi³⁺ in relation to the A atom significantly influences the properties exhibited by these systems. Thus, a careful single-crystal study becomes a prerequisite to unequivocally establish the Bi site location and occupancy in substituted Sr₃V₂O₈, to evaluate the structure–property correlation. In this study, Sr_3−xBi_2x/3V₂O₈ solid solutions have been synthesized and the limits of the solid solution and precise Bi atomic position have been determined through accurate single-crystal diffraction study, followed by the evaluation of photocatalytic and conductivity properties.

Dye pollutants from the textile industry are an important source of environmental contamination in the form of wastewater. Photocatalysis is widely used for the cleaning of wastewater and extensive research is being carried out to overcome the problem of remediation of these wastes by developing new photocatalysts.^17–19 Several factors such as band gap energies, and features associated with cation vacancy, oxygen deficiency and surface area influence the performance of materials used in photocatalytic degradation. Sr_3−xBi_2x/3V₂O₈, with its variability in site occupancies and structural features, may serve as a good candidate for use as a photodegradation material.

Results and discussion

X-Ray powder diffraction

Phase purity of the solid solution (Sr_3−xBi_2x/3V₂O₈, x ≤ 0.4) members were confirmed by PXRD. Fig. 1 shows the powder X-ray diffractograms of various members of the solid solution. To establish the solid solution limit, Sr_3−xBi_2x/3V₂O₈, x = 0.45 has been synthesized and a comparison of the powder pattern with the solid-solution members x = 0.45 and x = 0.4 (see Fig. S1, ESI†) clearly shows the presence of impurity phases Bi₂O₃ and BiVO₄ at x = 0.45, suggesting that the solid solution limit is x ≅ 0.4. Fig. 2 describes the changes in cell parameters obtained after profile refinement. While the length of the a axis increases, the length of the c axis decreases with the composition, indicating an opposite trend in the a and c axes with an increase in the Bi content in the series Sr_3−xBi_2x/3V₂O₈, 0 ≤ x ≤ 0.4. These observations are similar to those observed for the palmirite type structure Pb₃V₂O₈.¹⁶

Fig. 1 Powder X-ray diffractogram for the various members of solid solution Sr_3−xBi_2x/3V₂O₈, 0 ≤ x ≤ 0.4.

Fig. 2 Variation in the cell parameters with the composition.

Single-crystal structure

Single-crystal XRD data on the composition x = 0.20 of the solid solution were indexed in a trigonal crystal system, space group R3̄m, with a = 5.6067(4) Å, c = 20.0099(13) Å, V = 580.64(5) ų and Z = 3 (Table 1). Initially, the positions of heavy atoms, Sr (Sr1 and Sr2) and V were obtained by Patterson methods using SHELXL97,²⁰ and were found to be in similar positions as those found in the known structure of Sr₃V₂O₈.²¹ The positions were refined using all observed reflections during the refinement cycles. All oxygen atoms were located from the subsequent difference Fourier synthesis. In order to account for the presence of Bi in the crystal structure, occupancies of Sr1 and Sr2 were allowed to refine keeping their respective displacement parameters constant. While no change in the occupancy value was observed for Sr1 (Wyckoff site 3a), the occupancy of Sr2 (Wyckoff site 6c) decreased considerably. The Bi atom was hence assigned to the 6c site along with the Sr2 atom and the occupancies were refined keeping the thermal parameters fixed. The resulting R factor was 0.0478 and the corresponding residual electron density map shows fairly significant residual density Δρ , ranging from +2.34 to −1.62 e Å⁻³. Fig. 3a shows the residual density map where the maximum is located at the 18h site. At this stage, it was thought worthwhile to consider the structure of the Bi-substituted Pb₃V₂O₈ as a starting model since the parent structures Pb₃V₂O₈ and Sr₃V₂O₈ are isostructural. It is observed that upon substitution of Bi, the symmetry of Pb2 in Pb₃V₂O₈ is lowered from the 6c to 18h site.¹⁶ Bi occupies the identical Wyckoff site (18h) as that of the atom Pb2. However, refinements on the Bi-substituted Sr₃V₂O₈ based on this model become unsteady, suggesting that both Bi and Sr atoms cannot be present simultaneously at the 18h site unlike in the case of Bi-substituted Pb₃V₂O₈. It may be argued that since Bi and Pb are neighbors in the periodic table, the assignment of the same site for Bi and Pb2 is appropriate in Bi substituted Pb₃V₂O₈. Thus, it was decided that in further refinements to assign the Sr2 atom to the 6c site with lower occupancy and the Bi atom (lower occupancy) to the 18h site. However, there are no significant differences between the coordinates of Bi2 before and after the refinements of anisotropic displacement parameters. Refinements carried out with these features resulted in a R factor 0.0286 with Δρ_min, max = 0.86, −0.96 e Å⁻³. This featureless residual map (Fig. 3b) thus provides proof for the formation of a new variant in the Palmierite class of compounds. The atomic coordinates and the thermal parameters are listed in Table 2. Fig. 4 shows the packing diagram depicting the nature of coordination in each unit cell and the extended network as viewed down the c-axis. Fig. 5 shows the extended features of the coordination environment for the metal atoms. High-resolution powder diffraction data collected at the Elettra synchrotron source for the composition x = 0.2 were modeled based on these new variant coordinates and subsequent Rietveld refinements gave a reasonable fit substantiating the observation (see Fig. S2, ESI†).

Fig. 3 An xy section of the difference Fourier map around Sr2 atom. Contour lines are drawn in the interval of 0.5 e Å⁻³.

Fig. 4 Packing diagram of the compound Sr_3−xBi_2x/3V₂O₈, x = 0.2.

Fig. 5 Coordination environment around different metal atoms.

Table 1 Crystallographic refined parameters obtained from single-crystal data

Empirical formula	Sr2.8Bi0.13V2O8
M r	503.08
Crystal habit, colour	Block, pale yellow
Crystal size/mm	0.15 × 0.12 × 0.09
T/K	120(2)
Radiation	Mo-Kα
Monochromator	Graphite
λ/Å	0.71069
Crystal system	Trigonal
Space group	R3̄m
a/Å	5.6067(4)
c/Å	20.0099(13)
V/Å^3	580.64(5)
Z	3
D c/g cm^−3	4.601
F(000)	682
Scan mode	ω scan
θ max/°	29.10
hkl min, max	−7 to 7, −7 to 7, −27 to 26
No of reflns measured	2772
No of unique reflns	213
No of parameters	25
Absorption correction	Numerical from crystal shape
μ/mm^−1	26.137
Refinement	F ^2
R (all), R (obs)	0.0345, 0.0285
wR2 (all), wR2 (obs)	0.0558, 0.0551
GoF	1.208
Δρmax, min/e Å^−3	0.890, −0.960

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Table 2 Atomic coordinates and isotropic displacement parameters (Ų) for Sr_2.8Bi_0.13V₂O₈ obtained from single-crystal X-ray diffraction

Atom (Wyckoff position)	x	y	z	U eq/Å^2	Occupancy (site multiplicity)
Sr1 (3a)	0	0	0	0.0154(4)	1 (12)
Sr2 (6c)	0	0	0.20132(6)	0.0072(3)	0.9 (6)
Bi2 (18h)	0.513(5)	0.257(8)	0.1335(11)	0.060(7)	0.022 (2)
V1 (6c)	1/3	2/3	0.07265(8)	0.0079(4)	1 (6)
O1 (6c)	1/3	2/3	−0.0113(4)	0.0229(17)	1 (6)
O2 (18h)	0.1682(5)	0.3364(10)	0.1009(2)	0.0231(14)	1 (2)

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In VO₄ tetrahedra there are two kinds of V–O distances making the tetrahedra distorted. The apical V–O1 [O atom shared by both Sr2, Bi2 and V] bond length (1.680(8) Å) is shorter that the other three bonds (V–O2 = 1.700(5) Å). These bond lengths are comparable with those reported for the analogous compound Pb_3−xBi_2x/3V₂O₈.¹⁶ The atom Sr1 forms an octahedron with six equal bond distances of 2.597(5) Å while Sr2 forms a SrO₁₀ polyhedron with axial Sr2–O1 distance of 2.415(8) Å and hexagonal set Sr2–O2 at distances of 2.872(1) Å respectively. The other three Sr2–O2 bond distances are 2.589(5) Å. Similarly Bi2 also forms BiO₁₀ polyhedra and the axial bond length of Bi2–O1 (2.557(10) Å) is longer than the corresponding Bi2–O1 (2.420(14) Å) reported for Pb_3−xBi_2x/3V₂O₈. The three similar Bi2–O2 bond lengths (2.434(18) Å) are shorter than Pb2/Bi2–O2 bond (2.668(8) Å) whereas the six similar bond lengths are 2.972(47) Å, longer than the bonds in Pb_3−xBi_2x/3V₂O₈.¹⁶

UV-Visible spectra and morphology

Room-temperature UV-visible spectra were collected in the diffuse-reflectance mode for various compositions in the series Sr_3−xBi_2x/3V₂O₈. The optical absorption was represented by the Kubelka–Munk function (F(R) = (1 − R)²/2R) calculated from the diffuse reflectance spectra at constant scattering (Fig. 6). The steep shape of the spectra signified that absorption was due to the band-gap transition. The experimental band gap for Sr₃V₂O₈ was calculated to be 3.45 eV. It is of interest to note that upon substitution, the band gap of the material decreases and reaches a value 3.15 eV for the composition x = 0.4. This can be attributed to the fact that the Bi 6p orbital makes a significant contribution to the conduction band.²² Fig. S3 (ESI†) is an SEM micrograph showing particle sizes in the μm range and single-crystal images for the composition Sr_3−xBi_2x/3V₂O₈ (x = 0.2).

Fig. 6 UV-visible diffuse-reflectance spectra for various members of the solid solution.

Photocatalysis

The photocatalytic behavior of Sr₃V₂O₈ was studied under UV radiation by degrading anionic and cationic dyes. Dye solutions were stirred in the presence of the catalyst for 1 h prior to the UV radiation exposure to monitor the changes in dye concentration due to adsorption. No significant adsorption (<5%) was observed. Experiments were also carried out in the absence of catalyst (photolysis) and less than 5% degradation was observed.

The inset to Fig. 7 shows the degradation profile for cationic dyes MB and RB and anionic dye OG in the presence of Sr₃V₂O₈. After 1 h of UV radiation, less than 10% degradation was observed for cationic dyes MB and RB while 30% degradation was observed for the anionic dye OG. It was therefore concluded that the catalyst may be active for only anionic dyes. Further to study the effect of Bi substitution, OG was degraded under UV light using all compositions of Bi-substituted Sr₃V₂O₈ (Fig. 7). It is observed that upon increased substitution the photocatalytic activity decreases. Further, photocatalytic dye degradation experiments were carried out using unsubstituted Sr₃V₂O₈. Fig. 8 shows the degradation profile for different anionic dyes under UV light in the presence of Sr₃V₂O₈. Complete degradation was observed in case of IC within 30 min of UV exposure while EY was completely degraded in 60 min. For RBBR and AB, degradation was 60 and 35%, respectively, while degradation was around 30% for both OG and AG. Therefore, from the degradation profile we can conclude that the degradation rate follows the trend IC > EY > RBBR > AB > AG ≅ OG. This observation is consistent with the order observed with TiO₂.²³ The powder pattern (Fig. S4, ESI†) of the compound remains identical even after several degradation experiments, suggesting that the material is stable under the degradation conditions. The degradation of EY was compared with commercial catalysts Degussa P-25 and the activity was found to be similar (Fig. 8).

Fig. 7 Degradation profile of OG in presence of different compositions of Sr₃V₂O₈ (inset degradation profile for cationic dyes MB and RB and anionic dye OG in the presence of Sr₃V₂O₈).

Fig. 8 Degradation profiles of different anionic dyes in the presence of Sr₃V₂O₈. The open diamonds represent the degradation profile EY in presence of Degussa P 25.

The structure of the dye plays an important role in its degradation rate. The rate of degradation of IC is higher than that of the other dyes and is attributed to the very reactive central –C=C– bond.²³ It is found that intermolecular hydrogen bonds in water disrupt the planarity and the central bond is readily cleaved leading to rapid degradation. The degradation of AG is higher than OG because of the role of the sulfate ions. The sulfonyl group attached to the benzene ring in AG makes it more reactive than when it is attached to naphthalene, as in the case of OG.²³ Similarly, the rate of OG and AB can be directly compared because the former is a monoazo dye while the later is a diazo dye. AB has two azo groups functions, namely an amino and a nitro group, which can be decolorized faster with the evolution of ammonium ion, nitrates and nitrogen compared to OG, which has only one azo group. The higher rate of degradation of the diazo relative to the monoazo dye is consistent with previous studies.^23,24 The mechanism of degradation of the anthraquinonic dye, RBBR, suggests that it degrades faster than azo dyes.²⁵ An important reaction during photocatalysis is the photo-Kolbe decarboxylation reaction. This involves the reaction of the carboxyl radical with a valance band hole. As the carboxyl ion is present in EY the reaction is dominant and leads to a higher degradation of EY.

Ionic conductivity

The measured conductivity vs. temperature of Sr₃V₂O₈ is shown in Fig. 9 in the form of an Arrhenius plot (log σ vs. 1000/T). The value of the bulk ionic conductivity was calculated from the intercept of the single semicircular arcs obtained in the complex impedance plots of Re(Z′)–Im (Z′′) plots (Z = impedance). The conductivity values showed a linear relationship with the temperature. The bulk ionic conductivity of Sr₃V₂O₈ reaches a value of 10⁻³ Ω⁻¹ cm⁻¹ at 750 °C while the substituted compounds showed a decrease in the ionic conductivity value in the range 10⁻⁴–10⁻⁵ Ω⁻¹ cm⁻¹.The activation energy calculated from the Arrhenius plot was found to be 0.61 eV whereas the activation energy for all the substituted compounds were found to be around 1 eV (x = 0.2, E_a = 1.10 eV; x = 0.3, E_a = 1.05 eV; x = 0.4, E_a = 1.00 eV). Sr ion conductivity is also observed in similar materials Sr_3−3xLa_2x□_x(VO₄)₂²⁶ La_1−xSr_xCo_1−yFe_yO_3−δ,²⁷ and La_1−xSr_xFeO_3−δ.²⁸ It has been earlier reported that the conduction mechanism is primarily the result of the formation of vacancies and interstitial Sr ions with an increase in the temperature in a solid solution, Sr_2.4La_0.4(V_1−yP_yO₄)₂, with the Palmierite structural type.¹⁴ A similar mechanism can be invoked in case of the Bi-substituted Sr₃V₂O₈. It is interesting to note that the substituent Bi³⁺ (1.17 Å) is smaller in size than Sr²⁺ (1.36 Å, 10 coordination) but heavier than strontium. However, the presence of the substituted heavy Bi³⁺ ions causes a reduction in the number of Sr²⁺ ions and subsequently the vacancies, resulting in the observed decrease in conductivity. There is no clear trend observed in case of differently substituted Bi compounds as their conductivity values lie within the error limits.

Fig. 9 Ionic conductivity from ac-impedance spectroscopy for Sr_3−xBi_2x/3V₂O₈ (x = 0–0.40).

Experimental

Materials

Bi₂O₃ (Aldrich, 99.9%) was dried at 350 °C for 6 h before use while V₂O₅ and SrCO₃ (from S. D. Fine-Chem Ltd., India, 99%) were used as received. Methylene blue (MB), Rhodamine B (RB), Remazol brilliant blue R (RBBR), Orange G (OG), Eosin Y (EY), Amido black 10B (AB) (all from S. D. Fine Chem Ltd, India), Alizarin Green (AG), Indigo carmine (IC) (all from Rolex Labs, India) were used as supplied without further purification. Water was distilled twice and filtered through a Millipore membrane before use.

Preparation and crystal growth

Polycrystalline samples of Sr_3−xBi_2x/3V₂O₈ (x = 0, 0.1, 0.2, 0.3 and 0.4) were synthesized by a solid-state route using the starting materials SrCO₃, Bi₂O₃ and V₂O₅ in stoichiometric ratios. The starting reactants were initially ground well in an agate mortar and pestle and were subjected to heat treatment at 650 °C for 6 h to decompose the carbonates. The resultant mixture was ground well and subjected to heat treatment for 6 h at 750 °C, at 900 °C for 24 h and at 950 °C for 24 h with two intermediate regrindings. All the syntheses were carried out under an oxygen atmosphere. The unsubstituted Sr₃V₂O₈ was colorless, while Bi-substituted products were light yellow. The progress of the reaction was monitored by powder X-ray diffraction that confirmed the formation of a single phase in case of Sr_3−xBi_2x/3V₂O₈. Single-crystals of the composition x = 0.2 were obtained by melting a polycrystalline sample at 1100 °C for 1 h in a platinum crucible followed by slow cooling at 5 °C h⁻¹ to 900 °C and then furnace cooling to room temperature.

Powder XRD

Powder XRD data were collected using a Philips X-pert Pro diffractometer with Cu-Kα radiation over the 2θ range 5–100°, with a step width of 0.0167°. Le Bail profile fitting followed by Rietveld analysis were carried out using the XRD data by the JANA2000²⁹ suite of programs. The background was estimated by a Legendre polynomial function consisting of 15 coefficients, and the peak shapes were described by a pseudo-Voigt function, varying five profile coefficients. A scale factor, a zero error factor, and a shape function were refined.

High-resolution Powder XRD data were collected at the Powder Diffraction Beam line (MCX) equipped with fast scintillator detector (Bede) at synchrotron Elettra, Trieste, Italy. The wavelength used for the experiment was 1.0 Å, calibrated against silicon (NIST-SRM660a) standards. The data was collected with a step size of 0.02° in the range 5–60°. The samples were loaded in a capillary and sonicated to pack the capillary uniformly. The capillary was rotated during data accumulation.

Single-crystal XRD

Good quality crystals were chosen under a polarizing microscope for single-crystal X-ray diffraction studies. Single-crystal X-ray diffraction data were collected on Xcalibur™ Mova E diffractometer with a four-circle kappa goniometer and Eos CCD detector in a CrysAlis CCD, Oxford Diffraction apparatus employing graphite-monochromated Mo-Kα (λ = 0.7107 Å) radiation at 120(2) K with X-ray generator (50 kV and 0.8 mA). The cell refinement and data reduction were accomplished using CrysAlis RED.³⁰ The diffraction intensities were corrected for Lorentz and polarization effects. The data were reduced using CrysAlisRED (special programs available with the diffractometer), the shape was determined with the video microscope attached to the diffractometer, and an analytical absorption correction (after Clark and Reid) was applied. The structures were solved by the direct method using SHELXL97²⁰ using the program suite WINGX (version 1.70.01).³¹ The packing diagrams were generated using package Diamond, Version 2.1c.³²

Characterization

Morphology and composition analysis of all samples of Sr_3−xBi_2x/3V₂O₈ were carried out with the help of FEI Quanta scanning electron microscope. UV-visible diffuse-reflectance spectra were recorded on a Perkin-Elmer Lambda 35 UV-visible spectrophotometer.

Photocatalytic experiments

The photochemical reactor used in this study was made of a jacketed quartz tube of 3.4 cm internal diameter, 4 cm outer diameter and 21 cm length and an outer Pyrex glass reactor of 5.7 cm internal diameter and 16 cm length. A high-pressure 125 W mercury vapor lamp (Philips, India) was placed inside the jacketed quartz tube after the removal of the outer shell. To avoid fluctuations in the input, the supply ballast and capacitor were connected in series with the lamp. Ice water was circulated through the annulus of the quartz tube to avoid heating of the solution. The lamp radiated predominantly at 365 nm corresponding to an energy of 3.4 eV and photon flux of 6.1 × 10⁻⁶ mol photons s⁻¹.

Aqueous solutions of all dyes and organic compounds were taken in the reactor with a catalyst loading of 1 g L⁻¹. 100 ml of the solution was taken in the outer reactor and continuously stirred to ensure that the suspension of the catalyst was uniform. The mixture was stirred for 12 h in the dark and the resulting concentration was taken as the initial concentration after adsorption. The reactions were carried out at ∼30 °C, which was maintained by circulating water in the annulus of the jacketed quartz reactor. No adjustment of pH was done during the experiment. The degradation experiment was carried out for 1 h during which samples were collected at regular time intervals. The collected samples were then filtered and centrifuged for about an hour to remove any catalyst particles before the analysis. All dyes were analyzed with a UV-visible spectrometer (Lambda 35 Perkin-Elmer). The calibration for MB, RB, RBBR, OG, EY, AB, AG and IC were based on the Beer–Lambert law at their maximum absorption wavelength, λ_max of 664, 554, 592, 480, 517, 618, 640 and 661 nm, respectively.

Ionic conductivity from ac-impedance spectroscopy

The ac-impedance measurements were carried out using pelletized powder samples. Prior to measurement, the pellets were annealed at 900 °C for 6 h. Silver paste was applied on both sides of the pellets for better ohmic contact. The coated pellets were heated at 200 °C for 1 h to remove the organic component of the Ag-paste. The diameter and thickness of the pellet were approximately 10 mm and 1 mm, respectively. The annealed pellets were then placed between two steel electrodes of a homemade conductivity cell. The ac-impedance measurements were carried out (Novocontrol Alpha-A) in the frequency range 1 to 3 × 10⁶ Hz (signal amplitude = 0.05 V) from 30 to 700 °C at intervals of 50 °C up to 750 °C. The temperature of samples was equilibrated for 30 min at each temperature point (±2 °C, programmable Thermolyne furnace) prior to the impedance measurements.

Conclusions

The effect of substitution of Bi atom (varying size, oxidation state and atomic number) in Sr₃V₂O₈ is found to result in a new variant in palmeirite type compounds with varying properties. Accurate location of the substituent can only be achieved from single-crystal data using difference Fourier methods. Structural basis for the selective photodegradation in dyes reveals the importance of the location of the substituent in the solid solution.

Acknowledgements

We thank DST-FIST (level II) for funding the Oxford diffraction X-ray facility. D. S. thanks DST-India and Sincrotrone Trieste for financial support during the synchrotron data collection at Trieste, Italy. D. S. also thanks Dr Jasper Plaisier for scientific assistance during the synchrotron data collection. T. N. G. thanks DST-India for a J. C. Bose fellowship for funding.

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Footnote

† Electronic supplementary information (ESI) available: Supporting figures S1–S4. CCDC reference number 882405. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra20937c

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