XPS and Mössbauer studies on BaSn 1x Nb x O 3 (x 0.100) Prabhakar Singh a , Benjamin J. Brandenburg b , C. Peter Sebastian c , Devendra Kumar d , Om Parkash d, * a School of Sciences, Indira Gandhi National Open University, Maidan Garhi, New Delhi 110068, India b Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, 48149-Münster, Germany c Institut für Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, 48149-Münster, Germany d Department of Ceramic Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India Received 27 April 2007; received in revised form 31 July 2007; accepted 19 September 2007 Available online 25 September 2007 Abstract A few compositions of perovskite oxide BaSn 1x Nb x O 3 (with x 0.10) system, prepared by solid state ceramic method, have been studied employing XPS and Mössbauer spectroscopy techniques. Mössbauer spectra of these compositions in the temperature range of 78–300 K reveal that the oxidation state of Sn is +4. In XPS measurements, compositions with x 0.010 show no evidence of Nb 5+ signal whereas the compositions with x 0.050 show clear evidence of Nb 5+ signals indicating some unreacted Nb 2 O 5 component in the system. This confirms the earlier report where presence of small amount of unreacted Nb 2 O 5 was predicted. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; C. Mössbauer spectroscopy; D. Electronic structure 1. Introduction Barium stannate is an ABO 3 -type perovskite oxide having cubic unit cell. This material is important for both fundamental as well as from the point of view of materials technology due to its useful dielectric and semiconducting behaviour [1,2]. BaSnO 3 forms a component of thermally stable capacitors based on BaTiO 3 [3]. It has been reported that thin film of barium stannate can be used for sensing humidity [4], carbon monoxide [5] and nitrogen oxide gases [6]. Recently BaSnO 3 is reported as a promising material for sensing of liquefied petroleum gas [7,8]. Though barium stannate is technologically important material even in undoped form, doping modifies its dielectric, semiconducting and sensing properties. Synthesis, electrical and dielectric properties of doped and undoped BaSnO 3 have been extensively studied in our group for more than last one decade [9,10]. These studies on the doped and undoped BaSnO 3 reveal many useful and interesting properties. In this report, we present our XPS and Mossbauer studies on niobium-doped barium stannate system, namely BaSn 1x Nb x O 3 (with x 0.100). In our previous investigations on this system we have reported its synthesis [11] electrical [12] and dielectric [13] behaviour. A strong correlation of microstructure and electrical conduction behaviour with defect structure has been reported for this system [14]. All the compositions were synthesized based on www.elsevier.com/locate/matresbu Materials Research Bulletin 43 (2008) 2078–2084 * Corresponding author. Tel.: +91 542 2307043; fax: +91 542 2368428. E-mail address: [email protected](O. Parkash). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.09.018
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XPS and Mössbauer studies on BaSn1�xNbxO3 (x � 0.100)
Prabhakar Singh a, Benjamin J. Brandenburg b, C. Peter Sebastian c,Devendra Kumar d, Om Parkash d,*
a School of Sciences, Indira Gandhi National Open University, Maidan Garhi, New Delhi 110068, Indiab Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, 48149-Münster, Germany
c Institut für Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Westfälische Wilhelms-Universität Münster,Corrensstrasse 30, 48149-Münster, Germany
d Department of Ceramic Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India
Received 27 April 2007; received in revised form 31 July 2007; accepted 19 September 2007
Available online 25 September 2007
Abstract
A few compositions of perovskite oxide BaSn1�xNbxO3 (with x � 0.10) system, prepared by solid state ceramic method, havebeen studied employing XPS and Mössbauer spectroscopy techniques. Mössbauer spectra of these compositions in the temperaturerange of 78–300 K reveal that the oxidation state of Sn is +4. In XPS measurements, compositions with x � 0.010 show no evidenceof Nb5+ signal whereas the compositions with x � 0.050 show clear evidence of Nb5+ signals indicating some unreacted Nb2O5
component in the system. This confirms the earlier report where presence of small amount of unreacted Nb2O5 was predicted.# 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics; C. Mössbauer spectroscopy; D. Electronic structure
1. Introduction
Barium stannate is an ABO3-type perovskite oxide having cubic unit cell. This material is important for bothfundamental as well as from the point of view of materials technology due to its useful dielectric and semiconductingbehaviour [1,2]. BaSnO3 forms a component of thermally stable capacitors based on BaTiO3 [3]. It has been reportedthat thin film of barium stannate can be used for sensing humidity [4], carbon monoxide [5] and nitrogen oxide gases[6]. Recently BaSnO3 is reported as a promising material for sensing of liquefied petroleum gas [7,8]. Though bariumstannate is technologically important material even in undoped form, doping modifies its dielectric, semiconductingand sensing properties. Synthesis, electrical and dielectric properties of doped and undoped BaSnO3 have beenextensively studied in our group for more than last one decade [9,10]. These studies on the doped and undoped BaSnO3
reveal many useful and interesting properties.In this report, we present our XPS and Mossbauer studies on niobium-doped barium stannate system, namely
BaSn1�xNbxO3 (with x � 0.100). In our previous investigations on this system we have reported its synthesis [11]electrical [12] and dielectric [13] behaviour. A strong correlation of microstructure and electrical conductionbehaviour with defect structure has been reported for this system [14]. All the compositions were synthesized based on
above formula, viz. BaSn1�xNbxO3, which assumes electronic charge compensation (Nb is a pentavalent ion ontetravalent Sn site) as under
2BaOþ Nb2O5! 2Ba�Ba þ 2Nb�Sn þ 2e0 þ 12O2 " (1)
where, all the species are written in accordance with Kroger Vink notation of defects. The study of electrical
conductivity indicated that resistivity decreases with x upto x � 0.01 and thereafter it increases with x [12]. This was
explained on the basis that charge compensation occurs electronically for compositions with x � 0.01. For
compositions with x � 0.05, the charge compensation occurs ionically as below by vacancies in Sn sublattice in
accordance with the formula BaSn1�5x/4NbxO3
5BaOþ 2Nb2O5! 5Ba�Ba þ 4Nb�Sn þ V000Sn þ 15O0 (2)
Since diffusion of Sn vacancies is very slow, therefore they do not contribute to the conductivity and resistivityincreases due to scattering of charge carriers (electrons) by Nb5+ on Sn4+ sites. Similarly it was observed that averagegrain size of the compositions with x � 0.01 was in the range 2–4 mm while it is in submicron range for x = 0.05 and0.10. This has been again explained due to segregation of Nb5+ and V000Sn at grainboundaries, which reduces theirmobility and hence inhibit grain growth. This is in conformity with our results of microstructure as mentioned above.This means that we should have prepared the compositions with x � 0.05 as per the formula BaSn1�5x/4NbxO3 ratherthan BaSn1�xNbxO3. It was therefore predicted that compositions with x = 0.05 and 0.10 should contain someunreacted Nb2O5. Any unreacted Nb2O5 could not be detected in the XRD patterns of these compositions. In thepresent investigation we have studied XPS and Mössbauer spectra of these materials. XPS studies indeed show thepresence of some unreacted Nb2O5, which could not be detected by powder XRD. In this paper, the Mössbaur and XPSspectroscopic investigations on the system BaSn1�xNbxO3 (with x � 0.100) have been reported which seems to be thefirst report on these materials to the best of our knowledge.
2. Experimental details
Compositions of BaSn1�xNbxO3, with x = 0.001, 0.005, 0.010, 0.050, 0.100 were prepared using conventional solidstate ceramic route. The details of synthesis and sample preparation are described in our earlier publications [11,12].
All XPS experiments were performed using an Axis-Ultra XPS Spectrophotometer (Kratos Analytical) fitted withan unmonochromated Al-anode X-ray source (Al Ka hn = 1486.6 eV) operated at 150 W. The photoelectrons werecollected with a concentric hemispherical analyser and detected with eight channeltrons. The total energy resolutionwas estimated to be about 0.1 eV. During all the measurements a charge neutraliser was running. For calibration of theobtained spectra, the Sn 4d peak, set at 26.8 eV, was used as an internal reference.
All the samples were fixed with conductive glue to a special copper carrier and before measuring, etched by Ar-sputtering in UHV until no change in composition was observed. For etching, a Minibeam I was used with anacceleration current of 3.11 kV. The base pressure while etching was allowed to be about 5 � 10�8 mbar whereas thebase pressure while analysing was better than 5 � 10�10 mbar. For measurements at 140 K, liquid nitrogen waspumped through the sample holder continually with an external-pumping unit.
A Ca119mSnO3 source was available for the 119Sn Mössbauer spectroscopic investigations. The samples wereplaced within thin-walled PVC containers at a thickness of about 10 mg Sn/cm2. A palladium foil of 0.05 mmthickness was used to reduce the tin K X-rays concurrently emitted by this source. The measurements were conductedin the usual transmission geometry at 78 K. Transmission integral fits were obtained using the Normos-90 softwarepackage, resulting in the parameters of isomer shift and line width.
3. Results and discussion
3.1. XPS studies
Figs. 1, 2 and 3 show the measured XPS spectra of Ba 3d, Sn 3d and Nb 3d core levels, respectively measured forBaSn0.90Nb0.10O3 at 298 K. All the three spectra show a simple spin–orbit doublet structure typical for 3d core levels.After fitting and deconvolution of the measured emission lines, the signals could be identified as Ba2+, Sn4+ and Nb5+,by using reference measurements of pure BaSO4, SnO2, NbO2 and Nb2O5. Table 1 shows the binding energies of Ba2+,
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Fig. 1. Ba 3d spectra of BaSn0.90Nb0.10O3 at 298 K.
Fig. 2. Sn 3d spectra of BaSn0.90Nb0.10O3 at 298 K.
Table 1Measured binding energies of Ba2+, Sn4+ and Nb5+ at 298 K and after Ar-sputtering 140 K
Note all binding energies are referenced to Sn 4d emission line set at 26.8 eV
Sn4+ and Nb5+ at 298 and 140 K for the system BaSn1�xNbxO3. This table reveals that the binding energies of Ba2+ andSn4+ at both the temperatures slightly increase upto x = 0.005 and thereafter it remains constant. A similar dependencewas observed for BaPb1�xBixO3 system reported by Namatame et al. [15]. X-ray diffraction study of the systemBaSn1�xNbxO3 [13] depicts that the cell parameters for the compositions with x > 0.010 remain almost constant. Thismight explain the similarity of the XPS spectra of BaSn1�xNbxO3 for x � 0.005 as a possible initial state effect [16].Moreover, from Table 1, we do not find any Nb5+ signals for the composition x � 0.010 but for x � 0.050, Nb5+ signalscan be clearly seen, which could not be observed in the XRD study of the system [14]. This probably indicates someunreacted Nb2O5 components in the system. The results show the significance of XPS study in detecting small amountof unreacted components in complex oxide systems, which cannot be detected by powder X-ray diffraction. Nb 3dspectra (Fig. 3) show one set of signals typical for Nb5+. Neither disproportion of the dopant to Nb4+, nor any electronicinfluence of the dopant on the binding energy of Ba2+ and Sn4+ could be observed (Table 1). Claessen et al. [17] havereported similar results in the system BaSn1�xSbxO3, they found no evidence for a charge disproportionation into Sb3+
and Sb5+ and only Sb5+ was identified in the system.Indeed etching the compositions BaSn0.95Nb0.05O3 and BaSn0.90Nb0.10O3 by Ar-sputtering at 140 K gave rise to a
second niobium species (Fig. 4, Table 1), which was identified to be Nb4+. Comparing with the values from Table 1, the
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Fig. 4. XPS spectra of Nb 3d core levels at 298 K and 140 K after Ar-sputtering for BaSn0.90Nb0.10O3. For better survey, spectra are normalised andshifted towards each other.
Fig. 3. Nb 3d spectra of BaSn0.90Nb0.10O3 at 298 K.
new Nb-state did not affect the binding energies of Ba2+ and Sn4+, as the binding energies seem to be unchanged. Thenew Nb4+ state was caused by a so-called sputtering effect [18], a chemical reduction of Nb5+ to Nb4+ during ionbombardment, which is independent from the chemical system. When the samples are allowed to warm up to 298 K,the formed Nb4+ state disappeared. At 298 K this new Nb4+ state is obviously thermodynamically unstable. Fig. 5shows the XPS spectra of O 1s core levels of BaSn1�xNbxO3 for x = 0.010, 0.050 and 0.100 after Ar-sputtering at140 K. This figure rules out any detectable effect of this new Nb-state on oxygen, because a change in composition ofoxygen cannot be seen at all.
3.2. Mössbauer studies
119Sn Mössbauer spectra of niobium-doped BaSnO3 at different concentrations are presented at liquid nitrogentemperature in Fig. 6 together with transmission integral fits. The hyperfine parameters extracted from these data byleast squares fitting are summarised in Table 2.
All the spectra could be fitted with a single signal with a small amount of quadrupole splitting. The isomer shiftsgradually increases from 0.019 to 0.031 mm/s as niobium content increases, indicating increase of the ‘s’ electrondensity at the tin nuclei in the case of doped niobium–barium stannate at higher concentration. This means that the selectron charge distribution of Sn ions is slightly influenced by niobium doping. The experimental line width is moreor less close to the natural line width of the Sn in 119Sn Mössbauer spectroscopy. BaSn0.95Nb0.05O3 andBaSn0.90Nb0.10O3 show the higher line width as compared with the other compositions.
Even though all the measured compounds are cubic, a negligible amount of the quadrupole splitting values is shownwhich is increasing as the amount of the niobium increases. BaSn0.999Nb0.001O3 is having the quadrupole splitting
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Fig. 5. XPS spectra of O 1s core levels of BaSn1�xNbxO3 for x = 0.010, 0.050 and 0.100 at 140 K all after Ar-sputtering under same conditions. Forbetter survey, spectra are normalised and shifted towards each other.
Table 2Fitting parameters of 119Sn Mössbauer measurements for the Nb-doped BaSnO3
BaSn1�xNbxO3 (x) Chemical shift, d (mm/s) Quadrupole splitting, DEQ Experimental line width, G (mm/s) Goodness of fit, x2
Numbers in parentheses represent the statistical errors in the last digit; d, isomeric shift; G, experimental line width.
value 0.18 mm/s, which is 0.48 mm/s in higher doped sample (BaSn0.90Nb0.10O3). This clearly points out thatprominent non-cubical site symmetry is created in the higher doped samples, BaSn0.95Nb0.05O3 and BaSn0.90Nb0.10O3.The isomer shift value near zero suggests that the oxidation state of the Sn in these compounds is 4+. This confirms theresult of XPS measurements. Within the temperature interval 78 K < T < 300 K no significant changes in the line-shape parameters are observed.
4. Conclusions
In correspondence to Mössbauer spectra, XPS confirms the existence of Sn4+ within all the samples. Although inthe compositions BaSn0.95Nb0.05O3 and BaSn0.90Nb0.10O3 dopant (Nb) can be found by XPS only, the bindingenergies of all the elements in this series of samples seem to be nearly equal and are not influenced by the amount ofdopant at all. This agrees well with XRD results. Structural similarities might explain these identical positionsof binding energies in XPS. Disproportion of the dopant could not be observed at all. High energy Ar-sputteringat 140 K indeed forms some stable Nb4+, but has been shown to be a typical sputtering effect which disappearsat 298 K.
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Fig. 6. 119Sn Mössbauer spectra of the system BaSn1�xNbxO3 obtained at 78 K. Points represent the experimental data and the solid lines representtransmission integral fits.
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