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Phase transitions, dielectric properties, and vibrational study of stannates perovskites Sr1−xErxSnO3−δ
Materials Research Bulletin 51 (2014) 136–140
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Phase transitions, dielectric properties, and vibrational study of stannates perovskites Sr1xErxSnO3d S. Ouni a, S. Nouri a, H. Khemakhem b, R. Ben Hassen a,* a Unite´ de recherche´ de chimie des mate´riaux et de l’environnement UR11ES25, ISSBAT, Universite´ de Tunis ElManar 9, Avenue Dr. Zoheir Safi, 1006 Tunis, Tunisia b Laboratoire des mate´riaux ferroe´lectriques (LFM), De´partement de Physique, Faculte´ des Sciences de Sfax, Sfax, Tunisia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 June 2013 Received in revised form 13 November 2013 Accepted 5 December 2013 Available online 12 December 2013
A polycrystalline stannates perovskites Sr1xErxSnO3d (x = 0.00, 0.01 and 0.03) were synthesized by sol– gel method and their vibrational properties were investigated using Raman scattering. The substitution of Er in the Sr site, results in a slight change of the position of the Raman spectrum bands. The differential scanning calorimetry (DSC) shows two phase transitions at 532 K and 634 K for Sr0.99Er0.01SnO3d. The dielectric behavior of each solid solution (x = 0.01 and x = 0.03) has been studied as a function of temperature and frequency and has confirmed the observed phase transitions. The diffuseness of the phase transitions of these materials enhances with the increasing of erbium content. The dielectric constant showed a strong increase near the phase transitions temperature which value depends with frequency. This phenomenon is usually observed in relaxor materials. The conduction and the dielectric relaxation are attributed to hopping of electrons among Sn2+ and Sn4+ ions. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Sol–gel chemistry C. Raman spectroscopy D. Dielectric properties D. Phase transition
1. Introduction SrSnO3 is the most common ferroelectric oxide in the perovskite ABO3 structure, which is used as various electronic devices such as capacitors, thermistors, transducers and non-volatile memories in semiconductor industries because of its dielectric and ferroelectric properties [1]. Indeed, the alkaline earth stannates, ASnO3 (A = Ca, Sr and Ba), have recently been investigated as potential capacitor components with a small temperature coefficient of capacitance [2]. The interesting properties of the perovskites structure are known to be strongly linked to subtle structural variations. For many perovskites, the distortions from the ideally cubic structure are found to be due to the tilting of the octahedra. A good example of this is that varying the degree of octahedral tilting in a SrSnO3 perovskite changes the extent of orbital overlap through the BO6 octahedral network there by affecting electronic properties such as conductivity, magnetism and dielectric properties [1]. The alkaline earth stannates therefore give good insights into the evolution of the perovskite structure by octahedral tilting and deformation on substitution at the A site [3]. At room temperature, SrSnO3 adopts the Pbnm orthorhombic structure but with only a very small distortion from the cubic form,
* Corresponding author. Tel.: +216 98692745; fax: +216 71573526. E-mail address: [email protected] (R. Ben Hassen). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.12.012
the typical laboratory of X-ray diffraction does not resolve the lower symmetry distortion [4,5]. Mountstevens et al. [3] have shown that SrSnO3 undergoes two structural phase transitions, as follows: Pnma–Imma transition at 909 K, and interpreted as a continuous order–disorder transition, and a first-order transition to a tetragonal I4/mcm phase at 1173 K. And they have emphasized that this would be unique for an octahedron-tilting transition in oxide perovskite, as they have always been found to be displacive (‘Glazer’s rule’). Further, Singh et al. [6] have reported a continuous or nearly continuous order–disorder transition from orthorhombic (pseudo-tetragonal) to orthorhombic structure on heating in SrSnO3 at 650 K, and a discontinuous transition near 533 K. The transition at 650 K is manifested in an order of magnitude increase in Raman linewidths and in a l-shaped anomaly in the specific heat. Their work on Raman scattering and differential thermal analysis showed that the disorder sets in at much lower temperature 650 K, so that the 909 K transition is a disorder– disorder or a displacive transition within an already highly disordered structure. Hence it may not be as unusual as had been claimed, and Glazer’s rule remains inviolate. The discontinuous transition observed at 533 K is manifest primarily in the Raman spectroscopy and verified by dielectric measurements. The dielectric behavior of stannate perovskites has been studied as a function of temperature and frequency. The frequency dependence of dielectric constant and dielectric loss in these materials indicates that space charge polarization contributes
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significantly to their observed dielectric parameters [6–8]. Moreover, the electric and dielectric properties of SrSnO3 can be modified by doping with various isovalent cations on both A and B sites [9–12]. The isovalent A site dopants such as Sr2+ are effective in displacing or shifting the Curie temperature but do not have a dramatic effect on dielectric maximum. In this work, vibrational property by Raman scattering is studied and dielectric properties of Sr1xErxSnO3d system with different erbium contents are investigated in the frequency range 1 kHz to 1 MHz for temperatures ranging from 300 to 800 K. 2. Experimental The solid solutions Sr1xErxSnO3d (x = 0.00, 0.01 and 0.03) were prepared by the sol–gel method. To examine the structure of the Sr1xErxSnO3d powder, an X-ray diffraction (XRD) study was carried out; the details of the preparation method and crystal structure have been described in detail elsewhere [13]. Raman scattering data were obtained using a Lab Ram HR 800 spectrometer (Horiba-Jobin-Yvon) with an excitation wavelength of 633 nm. A differential scanning calorimetry (DSC) analysis was carried out by Setaram DSC 131 evo equipment in flowing protective nitrogen gas at a heating rate of 10 K min1 up to 673 K. Series of experiments were performed using 77.7 mg sample size. To measure the dielectric properties, silver electrodes were formed on both surfaces of sintered disk. The relative dielectric permittivity (e) and dielectric loss (tan d) were measured as parametric functions of frequency vs. temperature in the range 298–800 K and frequency from 1 kHz to 1 MHz using an Agilent 4284A component analyzer. The electrical direct current resistivity measurements were carried out on sintered pellet using a Lucas Labs 302 fourpoint probe with a Keithley 2400 digital Source Meter (Keithley Instruments Inc., Cleveland, OH) in the temperature range 350– 713 K. 3. Results and discussion 3.1. Crystal structure Erbium-substituted strontium stannate having a general formula Sr1xErxSnO3d (0 = 0, 0.01 and 0.03) has been prepared by sol–gel method. In order to identify the phase composition and to refine the unit cell parameters of Sr1xErxSnO3d, the samples were examined by X-ray diffraction as discussed in our earlier publication [13]. The distribution of Er3+ in the crystal structure has been studied. The XRD patterns of these solid solution revealed extra lines of reflections of cassiterite phase SnO2. At room temperature, Rietveld refinement of the data revealed that the crystal structure of the representative compound (Sr0.97Er0.03SnO3d) is orthorhombic perovskite with point group mmm (D2h) and space group Pbnm. The cell dimensions are: a = 5.7152(1) A˚, b = 5.7092(1) A˚ and c = 8.0710(2) A˚. The Er doped SrSnO3 oxide contains some SnO2 minor impurity phase (0.13%). Hence the solubility limit of erbium in strontium stannate is up to 3 mol%. Tilting of the SnO6 octahedra generally occurs depending on the amount of erbium added [13]. 3.2. Raman spectroscopy In order to study the structural instability due to octahedral tilting and its effect on the optical phonons of the Sr1xErxSnO3d (x = 0.00, 0.01 and 0.03), we performed Raman scattering measurements as discussed below. Fig. 1 shows the Raman spectra obtained for x = 0.00, 0.01 and x = 0.03. X-ray diffraction structural determination indicates that the Er-doping did not change the orthorhombic structure of the SrSnO3 material. At room temperature
Fig. 1. Raman spectra of Sr1xErxSnO3 (x = 0.00, 0.01 and 0.03) at the room temperature.
Sr1xErxSnO3d (x = 0.01 and 0.03) were characterized [13] by orthorhombic symmetry with the point group mmm (D2h) and the space group Pbnm (D162h). The distribution of ions in crystallographic positions can be summarized using the Wyckoff notation: Sr2+/Er3+ ions in 4(c), Sn4+ ions in 4(b), and O2 ions in 4(c) and 8(d). The B-site Sn4+ ion is surrounded with the six neighboring O2 ions in a SnO6 octahedron. The orientation of the corner-sharing SnO6 octahedra in Sr1xErxSnO3d (x = 0.01 and 0.03) can be described by using the tilting Glazer system [14] a+ac+ with in-phase tilting about the direction of the c-axis. The Raman active modes for this Pbnm structure are GRaman = 7Ag + 5B1g + 7B2g + 5B3g. These can be classified as two symmetric and four antisymmetric octahedral stretching modes, four bending modes, and six rotation or tilt modes of the octahedra. The other eight modes are associated with the strontium/erbium cations. Our study is based by comparing the spectra, mainly in the range between 50 and 300 cm1. The peaks of SrSnO3 compound which appear at around 91, 117, 155, 173, 223 and 260 cm1 were well detailed by Moreira et al. [15]. In Sr1xErxSnO3d (x = 0.01 and 0.03), all these modes are conserved except the one at 155 cm1 which disappears completely. The intense peak at 224 cm1 (for x = 0.01 and x = 0.03) can be assigned to an Ag mode corresponding to the scissors movement of Sn–O–Sn groups along the c-axis. The peak at 92 cm1 is due to an Ag mode which may correspond to strong librational character of the SnO6 octahedra, more or less coupled to Sr/Er translations, as observed in perovskite structure calculations [15,16]. Because we considered, in our description of X-ray diffraction and in Infrared studies [13] that the SnO6 octahedra, the units with the more covalent bonds, to be the vibrational unit, no internal modes could exist for the distorted alkali-earth/rare earth cations-oxygen cage. The modes observed at 117 and 173 cm1 (for x = 0.00 spectrum) are directly related to the thermally induced disorder [17]. These two peaks begin to disappear in the spectrum with x = 0.01 and vanished totally in spectrum with x = 0.03. So, we can conclude that the substitution Sr/Er in the alkaline earth cage modifies this thermally induced disorder. The peaks at 261 cm1 (for x = 0.01), and 264 cm1 (for x = 0.03) are related to O–Sn–O bending within the ab plane and Sn–O–Sn scissoring perpendicular to the c-axis [15]. On the other hand, the mode observed at 224 cm1 should be very sensitive to any distortion and disorder in the oxygen atom sublattice. The full width at half height (FWHH 10–16 cm1) of this peak increases with the increase of Er content indicates to a strong short-range disorder, in agreement with the presence of vacancies and/or related to the Sr/Er substitutional disorder, as usually observed in Raman spectra of perovskites [16,18].
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Fig. 2. Differential thermal analysis (DSC) of Sr0.99Er0.01SnO3d at elevated temperatures.
Fig. 4. Thermal variation of the real permittivity (e0 ) for Sr0.97Er0.03SnO3d compound.
In order to improve the dielectric and sensing behavior of strontium stannate, it is worthwhile to study the effect of donor doping on the dielectric behavior of the SrSnO3 in basing on the literatures [9,11,19]. Figs. 3–6 show the temperature dependences
of dielectric constant and losses of the Sr1xErxSnO3d materials with different erbium contents at different frequencies. Fig. 7 shows the variation of relative dielectric constant (e0 ) for Sr0.99Er0.01SnO3d, as a function of frequency at 298, 323,373 and 533 K. The nature of the graphs indicates a dispersive behavior of the material at low frequencies reflecting blocking effects. Such dispersive behavior can be mainly due to two reasons: polarized structure of studied material and the associated mobile charge carriers [20,21]. The high observed value of e0 at low frequency of all the temperatures can be due to presence of different types of polarizations (viz. electronic, dipolar, interfacial, ionic orientation, etc.) [22,23]. Further, the decrease of the e0 with increasing frequency (Figs. 3 and 4) is a typical characteristic of dielectric materials and may be attributed to the fact that, at lower frequency region the permanent dipoles align themselves along the direction of the field and contribute to the total polarization of the dielectric material. On the other hand, at higher frequency the variation in field is too rapid for the dipoles to align themselves in the direction of field, i.e., dipoles can no longer follow the field, so their contribution to the total polarization and hence to the dielectric permittivity become negligible [24]. Therefore the dielectric constant (e0 ) decreases with increase in frequency. From Fig. 8, it is clear that dielectric constant increases with erbium substitution. The dielectric constant for the composition x = 0.03 is much higher than that of x = 0.00 and 0.01 samples. This indicates that the vacancies created due to inherent charge compensation are not
Fig. 3. Thermal variation of the real permittivity (e0 ) for Sr0.99Er0.01SnO3d compound.
Fig. 5. Dielectric loss tan d for Sr0.99Er0.01SnO3d compound.
3.3. Thermal analysis Phase transitions behavior in Er doped SrSnO3 was analyzed with a quantitative differential scanning calorimetry (DSC). These measurements were carried out at a rate of 10 K min1 in the temperature range 298–673 K using 77.7 mg of Sr0.99Er0.01SnO3d powder set in a platinum pan. Fig. 2 shows the DSC curves of Sr0.99Er0.01SnO3d. We note two anomalies of heat at 532 K and 647 K, where the slope of measurement change indicates two consecutive phase transitions, which are nearly the same as those reported in literatures of related compounds [6,19]. We use discussed only the results of Sr0.99Er0.01SnO3d because these two phase transitions were observed for x = 0.03 at same temperatures. At 532 K, a weak endothermic peak with no large thermal signature was difficult observed; it is shown in the inset (I) of Fig. 2. This anomaly is well manifested in dielectric data. At 647 K, DSC measurement shows a l-shaped anomaly, this reveals a new second-order, order–disorder transition, whereas the first transition can be indicated as the first-order phase transition. 3.4. Dielectric properties
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Fig. 8. Variation of real permittivity of Sr1xErxSnO3d (x = 0.01, 0.03) as a function of temperature. Fig. 6. Dielectric loss tan d for Sr0.97Er0.03SnO3d compound.
fully reoxidized and grain/grain boundary interface is giving boundary layer capacitance, thus making them suitable materials for barrier layer capacitor application [9]. For x = 0.01 and x = 0.03 a very broad peak is observed in the (tan d) vs. T plot at different frequency around 350 K. This indicates the presence of a relaxation process. Strong frequency dependence of the dielectric constant in these compositions shows that interfacial polarization contributes significantly to the polarization in these samples. Interfacial polarization arises whenever phases with different conductivities are present. In Sr1xErxSnO3d compounds, Sr2+/Er3+ ions are distributed in the 12-fold coordination sites, so the observed diffuse phase transition behavior at Tm can be attributed to the disordering of B-site cations. So, the non-stoichiometric defect states also influence the dielectric function of a material. The dielectric relaxation is observed in the present compositions at the temperature range of 510–610 K. On the other hand, it has been previously commented that for dielectric materials the charge carriers have to be taking into account in the dielectric relaxation mechanisms [24]. When an electric field is applied to the material, there is the known reorientation of the dipoles but also the displacement of the charge carriers. Therefore, the electrical conductivity behavior should be considered. We observed that the dielectric peak of erbium doped SrSnO3 broadens with increasing of erbium content. It suggests that Sr1xErxSnO3d (x = 0.01 and x = 0.03) materials show more diffuse phase transition with the increasing of erbium. It is already known from the literature that the broadening of maximum dielectric constant (em) can be attributed to the coalescence of the low
Fig. 7. Variation of (e0 ) as a function of frequency at 298, 323, 373 and 533 K.
temperature phase transformations. For all frequencies measured we find a subtle but unambiguous anomalies in the dielectric constant around the temperature 643 K for x = 0.01. For x = 0.03 a very broad peak is observed in the dielectric permittivity at T = 633 K indicating a diffuseness of this phase transition, which can be regarded as discontinuous by comparing with those of SrSnO3 [6]. The introduction of Er3+ strengthens the diffuseness of the phase transition and then induces more relaxor diffuse phase transitions with x = 0.03. The first phase transition which was detected at T = 543 K in compound with x = 0.01 cannot be observed in compound with x = 0.03 due to diffuseness. This transition could be detected when measuring at low frequency. For x = 0.01, the first transition at 543 K is clearly characterized by the dielectric losses in the rage of frequencies 1–100 kHz while the second transition was observed at 643 K for the low-frequency variations, of the dielectric constant (Figs. 3 and 4). The anomaly almost disappears for the higher frequencies, showing that it could be related to a low frequency relaxation process. Thus, this anomaly could be correlated with a low-frequency relaxation process due to oxygen vacancies. This fact suggests that this phase transition can be strongly related to the tilting of SnO6 octahedra. One can conclude that Sr1xErxSnO3d materials show more diffuse phase transition with the increasing of Er content [13]. Furthermore, we note that for all composition of erbium (x = 0.01 and 0.03) the dielectric losses remain low (tan d < 1) and show an obvious anomaly at the transition temperature. The dielectric loss increases with temperature and the increase are almost
Fig. 9. Arrhenius relations of Ln(sT3/2) vs. 1000/T for the Sr0.99Er0.01SnO3d.
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exponential. This shows that the conductivity contribution is mainly due to hopping process [9,19]. This mechanism was already mentioned in our previous work [13], where transport properties in the system Sr1xErxSnO3d, x = 0.03, were investigated at high temperature. The Sr0.97Er0.03SnO3d compound showed semiconductive behavior and the electrical transport mechanism agrees with the non-adiabatic small polaron hopping model between nominal states Sn4+/Sn2+ in the temperature ranges 350–525 and 525–693 K separately [13]. Also, in Fig. 9 we demonstrate the electric conductivity of Sr0.99Er0.01SnO3d as a function of 1000/T, which can be characterized by the two phase transitions that occurs at T = 543 K and T = 643 K. These compounds were found to undergo a phase transition with increasing temperature. The low dielectric losses and high temperature stability of these materials should perform new ceramic resonators with quality factor high and low size. This should give rise to more selective filters with reduced insertion losses and with a particularly small size compared to the cavity filters and conventional microstrip line filters. 4. Conclusion Raman studies under ambient conditions along with dielectric properties were carried out on a series of nanocrystalline erbium stannate strontium. Our analysis of Raman spectroscopy shows that the full width at half height (FWHH 10–16 cm1) of peak at 224 cm1 increases with the increase of Er content, indicates to a strong short-range disorder in agreement with the presence of vacancies and/or related to the Sr/Er substitutional disorder, as usually observed in Raman spectra of perovskites. Our analysis confirms ‘Glazer’s law’ in this material that transitions due to the oxygen’s octahedron tilt is always displacive in perovskites. The present study shows that the inherent charge neutrality created by vacancies in the erbium doped stannates makes them important materials from sensor application. These can be used as barrier layer capacitor. Also, the dielectric constant increases with increase of erbium content and could be useful gas sensing material. Phase transitions in Sr1xErxSnO3d at 543 K and 643 K for x = 0.01 have been detected by DCS analysis. These transitions were confirmed by the study of the dielectric properties. The diffuseness of the phase transition of Sr1xErxSnO3 enhances with the increasing of erbium content. The dielectric loss increases with
temperature indicating that conduction and dielectric relaxation are attributed to hopping of electrons among Sn2+ and Sn4+ ions. Acknowledgement The authors gratefully acknowledge the ICDD for financial support (Grant number: 09-04). References [1] P.A. Cox, Transition Metal Oxides: An Introduction to Their Electronic Structure and Properties, Clarendon Press, Oxford, 1992. [2] A.M. Azad, L.L.W. Shyan, M.A. Alim, J. Mater. Sci. 34 (1999) 1175–1187. [3] E.H. Mountstevens, J.P. Attfield, A.T.S. Redfern, J. Phys. Condens. Matter 15 (2003) 8315–8326. [4] A. Vegas, Acta Crystallogr. B 42 (1986) 167–172. [5] M.A. Green, K. Prassides, P. Day, D.A. Neumann, Int. J. Inorg. Mater. 2 (2000) 35–41. [6] M.K. Singh, N.K. Karan, R.S. Katiyar, J.F. Scott, H.M. Jang, J. Phys. Condens. Matter 20 (2008) 55210–55215. [7] M.K. Singh, J.W. Hong, N.K. Karan, H.M. Jang, R.S. Katiyar, S.A.T. Redfern, J.F. Scott, J. Phys. Condens. Matter 22 (2010) 095901–095905. [8] M. Spark, M. Ku, N. Aichi, Dielectric material with low temperature coefficient and high unloaded quality, process for producing the same, and single/multilayer circuit board containing the same, EP 0832 859 A1 (1998). [9] P.K. Bajpai, K. Ratre, M. Pastor, T.P. Sinha, Bull. Mater. Sci. 26 (2003) 461–464. [10] W. Cai, Y. Fan, J. Gao, C. Fu, X. Deng, J. Mater. Sci. Mater. Electron. 22 (2011) 265–272. [11] S. Upadhyay, O. Parkash, D. Kumar, J. Phys. D: Appl. Phys. 37 (2004) 1483–1491. [12] O. Parkash, K.D. Mandal, C.C. Christopher, M.S. Sastry, D. Kumar, J. Mater. Sci. 31 (1996) 4705–4708. [13] S. Ouni, S. Nouri, J. Rohlicek, R. Ben Hassen, J. Solid State Chem. 192 (2012) 132–138. [14] M.E. Lines, A.M. Glass, Principle and Application of Ferroelectrics and Related Materials, Clarendon Press, Oxford, 1977. [15] E. Moreira, J.M. Henriques, D.L. Azevedo, E.W.S. Caetano, V.N. Freire, E.L. Albuquerque, J. Solid State Chem. 184 (2011) 921–928. [16] F. Genet, S. Loridan, G. Lucazeau, J. Raman Spectrosc. 28 (1997) 255–276. [17] P. Colomban, F. Romain, A. Neiman, I. Animitsa, Solid State Ionics 145 (2001) 339– 347. [18] P.S. Dobal, S.B. Majumber, S. Bhaskar, R.S. Katiyar, J. Raman Spectrosc. 30 (1999) 567–572. [19] J.F. Scott, R. Palai, A. Kumar, M.K. Singh, N.M. Murari, N.K. Karan, R.S. Katiyar, J. Am. Chem. Soc. 91 (2008) 1762–1768. [20] L. Bucio, E. Orozco, A.H. Tera, J. Phys. Chem. Solids 67 (2006) 651–658. [21] A.K. Singh, S.K. Barik, R.N.P. Choudhary, P.K. Mahapatra, J. Alloys Compd. 479 (2009) 39–42. [22] S. Saha, T.P. Sinha, Phys. Rev. B 65 (2002) 134103–1341010. [23] B. Behera, P. Nayak, R.N.P. Choudhary, Mater. Lett. 61 (2007) 3859–3862. [24] P. Kumar, A.K. Sharma, B.P. Singh, T.P. Sinha, N.K. Singh, Mater. Sci. Appl. 3 (2012) 369–376.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Phase transitions, dielectric properties, and vibrational study of stannates perovskites Sr1xErxSnO3d S. Ouni a, S. Nouri a, H. Khemakhem b, R. Ben Hassen a,* a Unite´ de recherche´ de chimie des mate´riaux et de l’environnement UR11ES25, ISSBAT, Universite´ de Tunis ElManar 9, Avenue Dr. Zoheir Safi, 1006 Tunis, Tunisia b Laboratoire des mate´riaux ferroe´lectriques (LFM), De´partement de Physique, Faculte´ des Sciences de Sfax, Sfax, Tunisia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 June 2013 Received in revised form 13 November 2013 Accepted 5 December 2013 Available online 12 December 2013
A polycrystalline stannates perovskites Sr1xErxSnO3d (x = 0.00, 0.01 and 0.03) were synthesized by sol– gel method and their vibrational properties were investigated using Raman scattering. The substitution of Er in the Sr site, results in a slight change of the position of the Raman spectrum bands. The differential scanning calorimetry (DSC) shows two phase transitions at 532 K and 634 K for Sr0.99Er0.01SnO3d. The dielectric behavior of each solid solution (x = 0.01 and x = 0.03) has been studied as a function of temperature and frequency and has confirmed the observed phase transitions. The diffuseness of the phase transitions of these materials enhances with the increasing of erbium content. The dielectric constant showed a strong increase near the phase transitions temperature which value depends with frequency. This phenomenon is usually observed in relaxor materials. The conduction and the dielectric relaxation are attributed to hopping of electrons among Sn2+ and Sn4+ ions. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Sol–gel chemistry C. Raman spectroscopy D. Dielectric properties D. Phase transition
1. Introduction SrSnO3 is the most common ferroelectric oxide in the perovskite ABO3 structure, which is used as various electronic devices such as capacitors, thermistors, transducers and non-volatile memories in semiconductor industries because of its dielectric and ferroelectric properties [1]. Indeed, the alkaline earth stannates, ASnO3 (A = Ca, Sr and Ba), have recently been investigated as potential capacitor components with a small temperature coefficient of capacitance [2]. The interesting properties of the perovskites structure are known to be strongly linked to subtle structural variations. For many perovskites, the distortions from the ideally cubic structure are found to be due to the tilting of the octahedra. A good example of this is that varying the degree of octahedral tilting in a SrSnO3 perovskite changes the extent of orbital overlap through the BO6 octahedral network there by affecting electronic properties such as conductivity, magnetism and dielectric properties [1]. The alkaline earth stannates therefore give good insights into the evolution of the perovskite structure by octahedral tilting and deformation on substitution at the A site [3]. At room temperature, SrSnO3 adopts the Pbnm orthorhombic structure but with only a very small distortion from the cubic form,
* Corresponding author. Tel.: +216 98692745; fax: +216 71573526. E-mail address: [email protected] (R. Ben Hassen). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.12.012
the typical laboratory of X-ray diffraction does not resolve the lower symmetry distortion [4,5]. Mountstevens et al. [3] have shown that SrSnO3 undergoes two structural phase transitions, as follows: Pnma–Imma transition at 909 K, and interpreted as a continuous order–disorder transition, and a first-order transition to a tetragonal I4/mcm phase at 1173 K. And they have emphasized that this would be unique for an octahedron-tilting transition in oxide perovskite, as they have always been found to be displacive (‘Glazer’s rule’). Further, Singh et al. [6] have reported a continuous or nearly continuous order–disorder transition from orthorhombic (pseudo-tetragonal) to orthorhombic structure on heating in SrSnO3 at 650 K, and a discontinuous transition near 533 K. The transition at 650 K is manifested in an order of magnitude increase in Raman linewidths and in a l-shaped anomaly in the specific heat. Their work on Raman scattering and differential thermal analysis showed that the disorder sets in at much lower temperature 650 K, so that the 909 K transition is a disorder– disorder or a displacive transition within an already highly disordered structure. Hence it may not be as unusual as had been claimed, and Glazer’s rule remains inviolate. The discontinuous transition observed at 533 K is manifest primarily in the Raman spectroscopy and verified by dielectric measurements. The dielectric behavior of stannate perovskites has been studied as a function of temperature and frequency. The frequency dependence of dielectric constant and dielectric loss in these materials indicates that space charge polarization contributes
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significantly to their observed dielectric parameters [6–8]. Moreover, the electric and dielectric properties of SrSnO3 can be modified by doping with various isovalent cations on both A and B sites [9–12]. The isovalent A site dopants such as Sr2+ are effective in displacing or shifting the Curie temperature but do not have a dramatic effect on dielectric maximum. In this work, vibrational property by Raman scattering is studied and dielectric properties of Sr1xErxSnO3d system with different erbium contents are investigated in the frequency range 1 kHz to 1 MHz for temperatures ranging from 300 to 800 K. 2. Experimental The solid solutions Sr1xErxSnO3d (x = 0.00, 0.01 and 0.03) were prepared by the sol–gel method. To examine the structure of the Sr1xErxSnO3d powder, an X-ray diffraction (XRD) study was carried out; the details of the preparation method and crystal structure have been described in detail elsewhere [13]. Raman scattering data were obtained using a Lab Ram HR 800 spectrometer (Horiba-Jobin-Yvon) with an excitation wavelength of 633 nm. A differential scanning calorimetry (DSC) analysis was carried out by Setaram DSC 131 evo equipment in flowing protective nitrogen gas at a heating rate of 10 K min1 up to 673 K. Series of experiments were performed using 77.7 mg sample size. To measure the dielectric properties, silver electrodes were formed on both surfaces of sintered disk. The relative dielectric permittivity (e) and dielectric loss (tan d) were measured as parametric functions of frequency vs. temperature in the range 298–800 K and frequency from 1 kHz to 1 MHz using an Agilent 4284A component analyzer. The electrical direct current resistivity measurements were carried out on sintered pellet using a Lucas Labs 302 fourpoint probe with a Keithley 2400 digital Source Meter (Keithley Instruments Inc., Cleveland, OH) in the temperature range 350– 713 K. 3. Results and discussion 3.1. Crystal structure Erbium-substituted strontium stannate having a general formula Sr1xErxSnO3d (0 = 0, 0.01 and 0.03) has been prepared by sol–gel method. In order to identify the phase composition and to refine the unit cell parameters of Sr1xErxSnO3d, the samples were examined by X-ray diffraction as discussed in our earlier publication [13]. The distribution of Er3+ in the crystal structure has been studied. The XRD patterns of these solid solution revealed extra lines of reflections of cassiterite phase SnO2. At room temperature, Rietveld refinement of the data revealed that the crystal structure of the representative compound (Sr0.97Er0.03SnO3d) is orthorhombic perovskite with point group mmm (D2h) and space group Pbnm. The cell dimensions are: a = 5.7152(1) A˚, b = 5.7092(1) A˚ and c = 8.0710(2) A˚. The Er doped SrSnO3 oxide contains some SnO2 minor impurity phase (0.13%). Hence the solubility limit of erbium in strontium stannate is up to 3 mol%. Tilting of the SnO6 octahedra generally occurs depending on the amount of erbium added [13]. 3.2. Raman spectroscopy In order to study the structural instability due to octahedral tilting and its effect on the optical phonons of the Sr1xErxSnO3d (x = 0.00, 0.01 and 0.03), we performed Raman scattering measurements as discussed below. Fig. 1 shows the Raman spectra obtained for x = 0.00, 0.01 and x = 0.03. X-ray diffraction structural determination indicates that the Er-doping did not change the orthorhombic structure of the SrSnO3 material. At room temperature
Fig. 1. Raman spectra of Sr1xErxSnO3 (x = 0.00, 0.01 and 0.03) at the room temperature.
Sr1xErxSnO3d (x = 0.01 and 0.03) were characterized [13] by orthorhombic symmetry with the point group mmm (D2h) and the space group Pbnm (D162h). The distribution of ions in crystallographic positions can be summarized using the Wyckoff notation: Sr2+/Er3+ ions in 4(c), Sn4+ ions in 4(b), and O2 ions in 4(c) and 8(d). The B-site Sn4+ ion is surrounded with the six neighboring O2 ions in a SnO6 octahedron. The orientation of the corner-sharing SnO6 octahedra in Sr1xErxSnO3d (x = 0.01 and 0.03) can be described by using the tilting Glazer system [14] a+ac+ with in-phase tilting about the direction of the c-axis. The Raman active modes for this Pbnm structure are GRaman = 7Ag + 5B1g + 7B2g + 5B3g. These can be classified as two symmetric and four antisymmetric octahedral stretching modes, four bending modes, and six rotation or tilt modes of the octahedra. The other eight modes are associated with the strontium/erbium cations. Our study is based by comparing the spectra, mainly in the range between 50 and 300 cm1. The peaks of SrSnO3 compound which appear at around 91, 117, 155, 173, 223 and 260 cm1 were well detailed by Moreira et al. [15]. In Sr1xErxSnO3d (x = 0.01 and 0.03), all these modes are conserved except the one at 155 cm1 which disappears completely. The intense peak at 224 cm1 (for x = 0.01 and x = 0.03) can be assigned to an Ag mode corresponding to the scissors movement of Sn–O–Sn groups along the c-axis. The peak at 92 cm1 is due to an Ag mode which may correspond to strong librational character of the SnO6 octahedra, more or less coupled to Sr/Er translations, as observed in perovskite structure calculations [15,16]. Because we considered, in our description of X-ray diffraction and in Infrared studies [13] that the SnO6 octahedra, the units with the more covalent bonds, to be the vibrational unit, no internal modes could exist for the distorted alkali-earth/rare earth cations-oxygen cage. The modes observed at 117 and 173 cm1 (for x = 0.00 spectrum) are directly related to the thermally induced disorder [17]. These two peaks begin to disappear in the spectrum with x = 0.01 and vanished totally in spectrum with x = 0.03. So, we can conclude that the substitution Sr/Er in the alkaline earth cage modifies this thermally induced disorder. The peaks at 261 cm1 (for x = 0.01), and 264 cm1 (for x = 0.03) are related to O–Sn–O bending within the ab plane and Sn–O–Sn scissoring perpendicular to the c-axis [15]. On the other hand, the mode observed at 224 cm1 should be very sensitive to any distortion and disorder in the oxygen atom sublattice. The full width at half height (FWHH 10–16 cm1) of this peak increases with the increase of Er content indicates to a strong short-range disorder, in agreement with the presence of vacancies and/or related to the Sr/Er substitutional disorder, as usually observed in Raman spectra of perovskites [16,18].
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Fig. 2. Differential thermal analysis (DSC) of Sr0.99Er0.01SnO3d at elevated temperatures.
Fig. 4. Thermal variation of the real permittivity (e0 ) for Sr0.97Er0.03SnO3d compound.
In order to improve the dielectric and sensing behavior of strontium stannate, it is worthwhile to study the effect of donor doping on the dielectric behavior of the SrSnO3 in basing on the literatures [9,11,19]. Figs. 3–6 show the temperature dependences
of dielectric constant and losses of the Sr1xErxSnO3d materials with different erbium contents at different frequencies. Fig. 7 shows the variation of relative dielectric constant (e0 ) for Sr0.99Er0.01SnO3d, as a function of frequency at 298, 323,373 and 533 K. The nature of the graphs indicates a dispersive behavior of the material at low frequencies reflecting blocking effects. Such dispersive behavior can be mainly due to two reasons: polarized structure of studied material and the associated mobile charge carriers [20,21]. The high observed value of e0 at low frequency of all the temperatures can be due to presence of different types of polarizations (viz. electronic, dipolar, interfacial, ionic orientation, etc.) [22,23]. Further, the decrease of the e0 with increasing frequency (Figs. 3 and 4) is a typical characteristic of dielectric materials and may be attributed to the fact that, at lower frequency region the permanent dipoles align themselves along the direction of the field and contribute to the total polarization of the dielectric material. On the other hand, at higher frequency the variation in field is too rapid for the dipoles to align themselves in the direction of field, i.e., dipoles can no longer follow the field, so their contribution to the total polarization and hence to the dielectric permittivity become negligible [24]. Therefore the dielectric constant (e0 ) decreases with increase in frequency. From Fig. 8, it is clear that dielectric constant increases with erbium substitution. The dielectric constant for the composition x = 0.03 is much higher than that of x = 0.00 and 0.01 samples. This indicates that the vacancies created due to inherent charge compensation are not
Fig. 3. Thermal variation of the real permittivity (e0 ) for Sr0.99Er0.01SnO3d compound.
Fig. 5. Dielectric loss tan d for Sr0.99Er0.01SnO3d compound.
3.3. Thermal analysis Phase transitions behavior in Er doped SrSnO3 was analyzed with a quantitative differential scanning calorimetry (DSC). These measurements were carried out at a rate of 10 K min1 in the temperature range 298–673 K using 77.7 mg of Sr0.99Er0.01SnO3d powder set in a platinum pan. Fig. 2 shows the DSC curves of Sr0.99Er0.01SnO3d. We note two anomalies of heat at 532 K and 647 K, where the slope of measurement change indicates two consecutive phase transitions, which are nearly the same as those reported in literatures of related compounds [6,19]. We use discussed only the results of Sr0.99Er0.01SnO3d because these two phase transitions were observed for x = 0.03 at same temperatures. At 532 K, a weak endothermic peak with no large thermal signature was difficult observed; it is shown in the inset (I) of Fig. 2. This anomaly is well manifested in dielectric data. At 647 K, DSC measurement shows a l-shaped anomaly, this reveals a new second-order, order–disorder transition, whereas the first transition can be indicated as the first-order phase transition. 3.4. Dielectric properties
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Fig. 8. Variation of real permittivity of Sr1xErxSnO3d (x = 0.01, 0.03) as a function of temperature. Fig. 6. Dielectric loss tan d for Sr0.97Er0.03SnO3d compound.
fully reoxidized and grain/grain boundary interface is giving boundary layer capacitance, thus making them suitable materials for barrier layer capacitor application [9]. For x = 0.01 and x = 0.03 a very broad peak is observed in the (tan d) vs. T plot at different frequency around 350 K. This indicates the presence of a relaxation process. Strong frequency dependence of the dielectric constant in these compositions shows that interfacial polarization contributes significantly to the polarization in these samples. Interfacial polarization arises whenever phases with different conductivities are present. In Sr1xErxSnO3d compounds, Sr2+/Er3+ ions are distributed in the 12-fold coordination sites, so the observed diffuse phase transition behavior at Tm can be attributed to the disordering of B-site cations. So, the non-stoichiometric defect states also influence the dielectric function of a material. The dielectric relaxation is observed in the present compositions at the temperature range of 510–610 K. On the other hand, it has been previously commented that for dielectric materials the charge carriers have to be taking into account in the dielectric relaxation mechanisms [24]. When an electric field is applied to the material, there is the known reorientation of the dipoles but also the displacement of the charge carriers. Therefore, the electrical conductivity behavior should be considered. We observed that the dielectric peak of erbium doped SrSnO3 broadens with increasing of erbium content. It suggests that Sr1xErxSnO3d (x = 0.01 and x = 0.03) materials show more diffuse phase transition with the increasing of erbium. It is already known from the literature that the broadening of maximum dielectric constant (em) can be attributed to the coalescence of the low
Fig. 7. Variation of (e0 ) as a function of frequency at 298, 323, 373 and 533 K.
temperature phase transformations. For all frequencies measured we find a subtle but unambiguous anomalies in the dielectric constant around the temperature 643 K for x = 0.01. For x = 0.03 a very broad peak is observed in the dielectric permittivity at T = 633 K indicating a diffuseness of this phase transition, which can be regarded as discontinuous by comparing with those of SrSnO3 [6]. The introduction of Er3+ strengthens the diffuseness of the phase transition and then induces more relaxor diffuse phase transitions with x = 0.03. The first phase transition which was detected at T = 543 K in compound with x = 0.01 cannot be observed in compound with x = 0.03 due to diffuseness. This transition could be detected when measuring at low frequency. For x = 0.01, the first transition at 543 K is clearly characterized by the dielectric losses in the rage of frequencies 1–100 kHz while the second transition was observed at 643 K for the low-frequency variations, of the dielectric constant (Figs. 3 and 4). The anomaly almost disappears for the higher frequencies, showing that it could be related to a low frequency relaxation process. Thus, this anomaly could be correlated with a low-frequency relaxation process due to oxygen vacancies. This fact suggests that this phase transition can be strongly related to the tilting of SnO6 octahedra. One can conclude that Sr1xErxSnO3d materials show more diffuse phase transition with the increasing of Er content [13]. Furthermore, we note that for all composition of erbium (x = 0.01 and 0.03) the dielectric losses remain low (tan d < 1) and show an obvious anomaly at the transition temperature. The dielectric loss increases with temperature and the increase are almost
Fig. 9. Arrhenius relations of Ln(sT3/2) vs. 1000/T for the Sr0.99Er0.01SnO3d.
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exponential. This shows that the conductivity contribution is mainly due to hopping process [9,19]. This mechanism was already mentioned in our previous work [13], where transport properties in the system Sr1xErxSnO3d, x = 0.03, were investigated at high temperature. The Sr0.97Er0.03SnO3d compound showed semiconductive behavior and the electrical transport mechanism agrees with the non-adiabatic small polaron hopping model between nominal states Sn4+/Sn2+ in the temperature ranges 350–525 and 525–693 K separately [13]. Also, in Fig. 9 we demonstrate the electric conductivity of Sr0.99Er0.01SnO3d as a function of 1000/T, which can be characterized by the two phase transitions that occurs at T = 543 K and T = 643 K. These compounds were found to undergo a phase transition with increasing temperature. The low dielectric losses and high temperature stability of these materials should perform new ceramic resonators with quality factor high and low size. This should give rise to more selective filters with reduced insertion losses and with a particularly small size compared to the cavity filters and conventional microstrip line filters. 4. Conclusion Raman studies under ambient conditions along with dielectric properties were carried out on a series of nanocrystalline erbium stannate strontium. Our analysis of Raman spectroscopy shows that the full width at half height (FWHH 10–16 cm1) of peak at 224 cm1 increases with the increase of Er content, indicates to a strong short-range disorder in agreement with the presence of vacancies and/or related to the Sr/Er substitutional disorder, as usually observed in Raman spectra of perovskites. Our analysis confirms ‘Glazer’s law’ in this material that transitions due to the oxygen’s octahedron tilt is always displacive in perovskites. The present study shows that the inherent charge neutrality created by vacancies in the erbium doped stannates makes them important materials from sensor application. These can be used as barrier layer capacitor. Also, the dielectric constant increases with increase of erbium content and could be useful gas sensing material. Phase transitions in Sr1xErxSnO3d at 543 K and 643 K for x = 0.01 have been detected by DCS analysis. These transitions were confirmed by the study of the dielectric properties. The diffuseness of the phase transition of Sr1xErxSnO3 enhances with the increasing of erbium content. The dielectric loss increases with
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