Raman Signatures Of Monoclinic Distortion In (Ba1 XSrx .
Research articleReceived: 29 March 2017Revised: 28 April 2017Accepted: 23 May 2017Published online in Wiley Online Library: 18 July 2017(wileyonlinelibrary.com) DOI 10.1002/jrs.5195Raman signatures of monoclinic distortion in(Ba1 xSrx)3CaNb2O9 complex perovskitesJ. E. Rodrigues,a,b* D. M. Bezerra,c R. C. Costa,d,e P. S. Pizanidand A. C. HernandesaOctahedral tilting is the most common distortion process observed in centrosymmetric perovskite compounds. Indeed, crucialphysical properties of this oxide stem from the tilts of BO6 rigid octahedra. In microwave ceramics with perovskite-type structure,there is a close relation between the temperature coefficient of resonant frequency and the tilt system of the perovskite structure.However, in many cases, limited access facilities are needed to assign correctly the space group, including neutron scattering andtransmission electron microscopy. Here, we combine the Raman scattering and group theory calculations to probe the structural3distortion in the perovskite (Ba1 xSrx)3CaNb2O9 solid solution, which exhibits a structural phase transition at x 0.7, from D3d3trigonal to C2hmonoclinic cell. Both phases are related by an octahedral tilting distortion (a0b b in Glazer notation). Lowtemperature Raman spectra corroborate the group-theoretical predictions for Sr3CaNb2O9 compound because 36 modes detectedat 25 K agree well with the 42 (25Ag 17Bg) predicted ones. Copyright 2017 John Wiley & Sons, Ltd.Keywords: ordered perovskites; octahedral tilting; crystal structure; group theory; monoclinicIntroductionJ. Raman Spectrosc. 2017, 48, 1243–1249* Correspondence to: J. E. Rodrigues, Crystal Growth and Ceramic Materials Group,São Carlos Institute of Physics, University of São Paulo, CEP 13560-970 São Carlos,São Paulo, Brazil.E-mail: rodrigues.joaoelias@gmail.com; rodrigues.joaoelias@ursa.ifsc.usp.bra Crystal Growth and Ceramic Materials Group, São Carlos Institute of Physics,University of São Paulo, CEP 13560-970, São Carlos, SP, Brazilb Aix Marseille Université, CNRS, Université de Toulon, IM2NP UMR 7334, 13397,Marseille, Francec São Carlos Institute of Chemistry, University of São Paulo, CEP 13566-590, SãoCarlos, Brazild Department of Physics, Federal University of São Carlos, CEP 13565-905, SãoCarlos, Brazile Department of Environmental Engineering, Federal University of CampinaGrande, CEP 58840-000, Pombal, BrazilCopyright 2017 John Wiley & Sons, Ltd.1243It is well established that the compounds crystallizing in theperovskite structure (ABO3) display an enormous technologicaland scientific importance in solid-state physics and chemistry.[1,2]Due to their ability to accommodate a large variety of cations at Aand B sites, several physical properties may emerge from thisfeature, including ferroelectricity, piezoelectricity, superconductivity, ionic conductivity, half-metallic conductivity,multiferroicity, and ferromagnetism.[3,4] With a correct chemicaldesign, the parameters of each property can be tuned to improvetheir performance as sensors, transducers, capacitors, photovoltaiccells, electrolytes for fuel cell, magnetic memories, catalysts, ordielectric resonators.[5–7] From a structural point of view, thesubstitution at the A or B sites can provide two interestingphenomena in centrosymmetric perovskite structures, namely,octahedral tilting and B-site cation ordering.[8,9] In the first case,cooperative tilts of BO6 rigid octahedra should occur toaccommodate a smaller A site cation in the cuboctahedral cavity.[10]In the second one, two distinct cations, B0 and B″, yield an orderedpattern and then induce a superstructure formation. As aconsequence, these processes modify the overall symmetry of thecrystalline structure from the simple cubic perovskite (aristotypecell: Oh1 or Pm-3m). In this cell, the octahedral tilting leads to thehettotype structures including the most common orthorhombic16phase (D2hor Pnma).[11,12]Particularly, the octahedral tilting and B-site cation ordering arepromising in the case of the dielectric resonator devices formicrowave applications.[13,14] In the last decades, such devices havebeen applied successfully in the telecommunication fields,including millimeter waves, base station, and mobile phones.[15]For high performance, the microwave dielectric ceramics shouldpresent a high dielectric permittivity (ε0 ), low dielectric loss (tanδ 10 4), and near zero-temperature coefficient of resonantfrequency (τ f). By using the chemical design, for instance, it ispossible to improve the dielectric loss at microwave by controllingthe B-site cation ordering at long range.[16] On the other hand, atemperature-stable device can be achieved by inducing theoctahedral tilting.[17,18] Both approaches are frequently employedto obtain desired microwave parameters in the 1:2 orderedperovskite, with the chemical formula A32 B0 2 B″25 O9 (A Ba, Sr,Ca; B0 Ca, Mg, Co, Zn; B″ Ta, Nb). In its undistorted state, such3a structure adopts a trigonal unit cell belonging to the D3dspacegroup, with an alternate distribution of the B-site cations alongthe ‹111›c cubic cell direction.[9]Recently, we have reported in the Ba3CaNb2O9 compound thatthe dielectric loss is related to the 1:2 ordered domain amount inthe crystallite. Also, we have argued that the origin of its high τfvalue stems from the structural instabilities associated with theonset of an octahedral tilt transition.[19] In this sense, Sr2 isselected as the A-site substituent cation for the Ba3CaNb2O9perovskite. Thus, on the basis of ionic radii,[20] the strontium
J. E. Rodrigues et al.ion may introduce distortions in the crystalline structure. In thispaper, we synthesized the (Ba1 xSrx)3CaNb2O9 solid solution toprobe possible distortion induced by the octahedral tilting(CaO6 and NbO6 units). As far as we know, there are no availablereports on the structural features concerning this system.Furthermore, Raman spectroscopy is applied to detect the localstructure disturbance induced by symmetry lowering. Asreported by many authors,[21–26] the combination of Ramanscattering and factor-group analysis is an important tool toinvestigate the structural changes in perovskites and relatedcompounds through the optical phonon behavior at the firstBrillouin zone (q 0). This approach will be used here to assignour Raman results.Experimental detailsAll polycrystalline samples were prepared by using theconventional solid-state synthesis method under air atmosphere.We aimed to synthesize the solid solution investigated here(hereafter: BCN–SCN system). Stoichiometric amounts of BaCO3(Alfa Aesar: 99.80%), SrCO3 (Sigma Aldrich: 99.90%), CaCO3 (AlfaAesar: 99.95%), and Nb2O5 (Alfa Aesar: 99.90%) were weighed andhomogenized in a nylon jar containing isopropyl alcohol andzirconia cylinders for 24 h. After a drying procedure at 373 K, themixture was thermally treated at 1573 K for 2 h and then groundto obtain fine powders.X-ray powder diffraction patterns (XRPDs) were acquired by aRigaku Rotaflex RU-200B diffractometer (Bragg–Brentano θ–2θgeometry; Cu-Kα radiation λ 1.5406 Å; 50 kV and 100 mA) overa 2θ range from 10 to 100 with a step size of 0.02 (step time of1 s). Raman spectra were collected in a backscattering geometryby a Jobin-Yvon T64000 triple monochromator with an LN2 cooledcharge-coupled device camera and a 1800 grooves mm 1holographic grating (resolution 1 cm 1). An Olympus microscopeis attached to the spectrometer, and the Raman signals wereexcited by a 514 nm line of an argon laser (Coherent, Innova 70C).The excitation source was kept below 2 mW and recorded afterthe 50 objective. For low-temperature measurements (25 up to300 0.5 K), a helium closed cycle cryostat (with a temperaturecontroller LakeShore model 330) was employed. All spectra werefurther corrected by the Bose–Einstein thermal factor prior to thefitting procedure by using the Lorentzian line shape, including abaseline subtraction.Glazer notation and group theory analysis1244In this work, we will describe the octahedral tilting process inperovskite by using a notation designed by Glazer.[10] Each tiltsystem depends on the BO6 octahedral rotations about the axesof the so-called aristotype unit cell (an undistorted structure).Such a notation has the following representation: a#b#c#, wherethe letters represent the tilt magnitude about the x, y, and zorthogonal axes, respectively. The # symbol denotes no tilt(# 0), antiphase (# ), and in-phase octahedral tilting(# ) in neighboring layers. Therefore, the aristotypeperovskite is designated by a0a0a0 in Glazer notation. In the1:2 ordering, the undistorted structure belongs to the trigonal3D3dspace group (P-3m1; #164), for which 9 (4A1g 5Eg) Ramanactive phonons are expected. Table 1 summarizes the factorgroup analysis for the crystal structures discussed in this paper,where letters A and B denote the nondegenerate modes, whilewileyonlinelibrary.com/journal/jrsletter E represents the doubly degenerate ones. The subscriptsg (gerade) and u (ungerade) are the parity under inversionoperation in centrosymmetric crystals. Particularly, we areinterested in distorted structure with 1:2 ordering at the B sitederived from antiphase tilting process.[9] Table 1 also depictedthe site symmetries (Wyckoff positions) and the correspondingvibrational modes for the crystalline structures addressedhere.[27]Results and discussionThe XRPD patterns of the (Ba1 xSrx)3CaNb2O9 (x 0.0, 0.3, 0.5, 0.7,0.9, and 1.0) system are illustrated in Fig. 1(a). Within the detectionlimit of the X-ray diffraction technique, no phase segregation wasnoticed in the full-range composition. The end-members of theBCN–SCN system were indexed in accordance with theICSD#162758 and ICDD#17-0174 cards, respectively, for the BCNand SCN systems. Those cards are related to a trigonal crystal3structure belonging to the D3dspace group (P-3m1; #164) withone chemical formula per cell, in which the Ca2 and Nb5 ionsfeature an ordering behavior, occupying the 1a and 2d Wyckoffsites, respectively. The earlier structure is derived from thedisordered cubic cell within the Oh1 space group, as aconsequence of rhombohedral distortion along the ‹111›c cubicdirection,[28] which provides the following lattice parameters forthe resulting trigonal cell: ah 2ac and ch 3ac (ac is the cubiclattice constant). Because the Sr2 ion has a lower ionic radiusthan that for the Ba2 ion, one can expect the d-spacing shiftingbehavior. Indeed, a shifting can be noticed with increasingstrontium content in Fig. 1(b), resulting in a decrease of the unitcell volume, as shown in Fig. 1(c).Some diffraction peaks at the 2θ region from 10 to 25 start toappear for x 0.7. At the same time, an extra peak at 2θ–36.5 becomes more intense, being indexed as ½(3 1 1)c reflection fromthe simple cubic ABO3 perovskite. As reported for similarsystems,[29,30] a phase transition induced by the oxygen octahedratilting may cause the appearance of this peak in the patterns ofFig. 1(a). The atomic substitution for smaller ions at the A site leadsto the crystal symmetry lowering due to the A O bond decrementalong with the octahedral tilting, in which the BO6 octahedra aretilted in order to preserve the connectivity of their corners.[10,12]Such a behavior increases the number of the diffraction peaks asa result of the symmetry lowering process.[31,32] Fig. 1(c) compilesthe strontium content dependence of the cell volume andtheoretical density of the (Ba1 xSrx)3CaNb2O9 compositions. Here,the theoretical density was calculated from the XRPD data.[33]The linear trend of both curves can be regarded as a signatureof the solid solution formation in the system under investigation.Also, there is a good agreement between the earlier reporteddensities (for BCN and SCN systems) and those obtained in thiswork.Nagai et al. proposed that the antiphase tilting of both MgO6and TaO6 octahedra changed the space group of the3) to monoclinic(Ba1 xSrx)3MgTa2O9 compounds from trigonal (D3d6[29](C2h; A2/n or C2/c; #15). Another evidence of this phase transitioncomes from the (2 0 2)h peak splitting into two counterpartsindexed as (2–4 2) and (2 4 2) reflections from the monoclinicphase.[30,31] This behavior was not noticed here for the (2 0 2)h peak[Fig. 1(b)], although a slight increase in its intensity occurred forx 0.9. According to Goldschmidt,[34] the distortion and structuralstability of the perovskite structures can be predicted by using theCopyright 2017 John Wiley & Sons, Ltd.J. Raman Spectrosc. 2017, 48, 1243–1249
Monoclinic distortion in (Ba1 xSrx)3CaNb2O9 complex perovskitesTable 1. Factor group analysis for the crystal structures addressed in this work (ΓT ΓAc ΓSi ΓIR ΓR). Abbreviations: Ac, acoustic; Si, silent; IR,infrared; R, RamanIon2 Ba2 Ba2 Ca5 Nb2O2OΓTΓAcΓSiΓIRΓR2 Sr2 Sr2 Ca5 Nb2O2OΓTΓAcΓSiΓIRΓR2 Sr2 Sr2 Sr2 Ca2 Ca5 Nb5 Nb2O2O2O2O2O2OΓTΓAcΓSiΓIRΓRWyckoff siteSymmetry3D3d;Irreducible representation0 0 0Trigonal:P-3m1; #164; a a a1bD3d2dC3v1aD3d2dC3v3fC2h6iCS4A1g A2g 5Eg 2A1u 8A2u 10EuA2u EuA2g 2A1u7A2u 9Eu4A1g 5Eg4Trigonal: D3d; P-3c1; #165; a a a2aD34dC32bS64dC36fC212gC16A1g 8A2g 14Eg 7A1u 9A2u 16EuA2u Eu8A2g 7A1u8A2u 15Eu6A1g 14Eg03Monoclinic: C2h; A2/m; #12; a b 18jC125Ag 17Bg 19Au 29BuAu 2Bu018Au 27Bu25Ag 17Bgtolerance factor (t). For the BCN–SCN system, the tolerance factorcan be obtained as follows:1 ½ð1 x ÞRBa þ xRSr þ ROt ¼ pffiffi 1;2 3 ðRCa þ 2RNb Þ þ RO(1)J. Raman Spectrosc. 2017, 48, 1243–1249A2g A2u Eg EuA1g A1u A2g A2u 2Eg 2EuA1u A2u 2EuA1g A1u A2g A2u 2Eg 2EuA1g A1u 2A2g 2A2u 3Eg 3Eu3A1g 3A1u 3A2g 3A2u 6Eg 6Eu2Ag Au Bg 2Bu2Ag Au Bg 2Bu2Ag Au Bg 2BuAu 2BuAu 2Bu2Ag Au Bg 2Bu2Ag Au Bg 2Bu2Ag Au Bg 2Bu2Ag Au Bg 2Bu2Ag Au Bg 2Bu3Ag 3Au 3Bg 3Bu3Ag 3Au 3Bg 3Bu3Ag 3Au 3Bg 3BuHence, one can expect a distorted 1:2 structure for SCN rather thanthe undistorted D3d cell for BCN.[31] To investigate those structuralchanges in more detail, Raman scattering measurements werecollected in all samples of the BCN–SCN solid solution at roomtemperature, as discussed next.Raman spectra of the (Ba1 xSrx)3CaNb2O9 solid solution aredisplayed in Fig. 2(a). In the spectrum of BCN (x 0.0), the peaksat 85 and 90 cm 1 are assigned to Eg and A1g external modes,respectively. The bands centered at 413 and 820 cm 1 denotethe internal modes concerning the stretching and breathingmotions of NbO6 octahedra. The twisting breath mode of oxygenoctahedron occurs at 355 cm 1 with Eg symmetry. The peaksCopyright 2017 John Wiley & Sons, Ltd.wileyonlinelibrary.com/journal/jrs1245so that RBa, RSr, RCa, RNb, and RO are the ionic radii of Ba (1.61 Å;CN 12), Sr (1.44 Å; CN 12), Ca (1.00 Å; CN 6), Nb (0.64 Å; CN 6),and O (1.35 Å; CN 2), taken from the Shannon’s table.[20] Thecalculated t value was around 0.992 for BCN, which decreaseslinearly until 0.935 for SCN. It means that the BCN–SCN mayexperience a phase transition as the Sr2 substitution increases.A2u EuA1g A2u Eg EuA2u EuA1g A2u Eg EuA1u 2A2u 3Eu2A1g A1u A2g 2A2u 3Eg 3Eu
J. E. Rodrigues et al.Figure 1. Crystal structure probed by XRPD technique: (a) X-ray diffraction pattern for the (Ba1 xSrx)3CaNb2O9 powder sample, at room temperature,indicating the d-spacing shifting behavior. (b) See the (2 0 2)h peak. (c) The unit cell volume and theoretical density values are shown to illustrate theirlinear dependence with strontium content. Although the BCN and SCN structures were indexed in agreement with the D3d trigonal structure, the indices5of planes are labeled following the pseudo-cubic setting (space group Oh). [Colour figure can be viewed at wileyonlinelibrary.com]Figure 2. Raman crystallography: (a) Raman spectra for the (Ba1 xSrx)3CaNb2O9 powder sample, indicating the changes in low wavenumber region in Sr-richsamples (x 0.7). (b) Strontium content dependence of the peak parameters (center and normalized area) ascribed to the 1:1 and 1:2 ordered regions. Detailson the spectral decomposition can be found in Fig. S1. [Colour figure can be viewed at wileyonlinelibrary.com]1246appearing at approximately 135, 250, 280, and 610 cm 1 can beattributed to the D3d trigonal structure.[19] Two remaining bandsare due to the 1:1 domain boundary in the crystalline structure, aselucidated by Blasse et al.[35] Such bands are allowed when thecrystallites exhibit the accumulation of antisite defects, definingthe antiphase boundary defects.[5] It is clear that this phenomenonoccurred in the full-range composition. Recently, Ma et al. analyzedthe boundary regions of 1:2 ordered domain in Ba3(Co, Zn, Mg)Nb2O9 perovskites by using high-resolution transmission electronmicroscopy technique, detecting an extra stabilized orderedstructure in these regions.[36] In practice, it means that there aretwo well-defined zones in the crystallites containing 1:1 (domainboundary) and 1:2 (domain) orders at B site. Therefore, we shouldexpect the fingerprints of each region in the Raman spectra at Fig. 2(b) illustrates the effect of strontium content on the peakparameters ascribed to the 1:1 and 1:2 regions, respectively. Onecan note the redshift behavior of the intense oxygen breathingA1g mode ranging from 820 (x 0.0) to 811 cm 1 (x 1.0). The1:2 peak area has a minimum value at x 0.5, corresponding to amaximum one for the 1:1 peak area. Such a trend can be bettervisualized by the contour map in Fig. 3. As a whole, thisphenomenon can be interpreted as follows: the Sr2 incorporationleads to an increase in the domain boundary distribution untilx 0.5. From this value, a decrease in this boundary region wasnoticed. In a recent work, we found out that there is a dependencebetween the 1:1 peak parameters and the dielectric performance ofthe Ba3CaNb2O9-based microwave ceramics. In particular, adecrease in the 1:1 peak intensity led to an improved qualityfactor.[19] Azough et al. also reported that the high-quality factorCopyright 2017 John Wiley & Sons, Ltd.J. Raman Spectrosc. 2017, 48, 1243–1249
Monoclinic distortion in (Ba1 xSrx)3CaNb2O9 complex perovskitesFigure 3. 1:1 and 1:2 ordered regions: The contour plot of the Sr content2 dependence of the Raman spectra in high wavenumber region. The Srincorporation leads to an increase in the domain boundary distributionuntil x 0.5. The vertical dashed line is a guide for the eye. [Colour figurecan be viewed at wileyonlinelibrary.com](Q f) depends on the removal of domain boundaries in singledomain grains (or crystallites) of Ba3MgTa2O9 ceramics.[37]Therefore, we can predict that the Sr2 cations at the A site modifynot only the thermal stability (i.e., τf parameter) but also thedielectric loss at microwave.Unlike the BCN sample, there are an increased number of Ramanbands in the SCN spectrum, as depicted in Figs 4(a) and 4(b). Ourresults are in accordance with those reported for Sr3MgTa2O9perovskites.[32] Dias et al. showed that the Sr3MgNb2O9 system3can be indexed as a monoclinic cell belonging to the C2hspacegroup (A2/m; #12). This assumption is based on the Howard andStokes seminal work describing the group–subgroup relationshipsfor A3B0 B″2O9 possible phases.[9,30] From the untitled trigonal phase3(D3d; a0a0a0 in Glazer notation), five derived structures can be1obtained directly from the octahedral tilting process, namely, C3i35 0 (P-3; #147; a a a ), C2h (C2/m; #12; a b b ), C2h (P21/c; #14;34(A2/m; #12; a0b b ), and D3d(P-3c1; #165; a a a ).a0a0c ), C2hAs one can see, only two of them are ascribed as an antiphase(out-of-phase) octahedral rotation with a negative superscript.An antiphase rotation usually appears in perovskites with anegative value for the temperature coefficient of resonancefrequency (τf).
Raman signatures of monoclinic distortion in (Ba1 xSr x)3CaNb2O9 complex perovskites J. E. Rodrigues,a,b* D. M. Bezerra,c R. C. Costa,d,e P. S. Pizanid and A. C. Hernandesa Octahedral tilting is the most common distortion process observed in centrosymmetric perovskite compounds.
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