Goto Takashi

Tu Rong

Yue Xianyan

MATERIALS TRANSACTIONS

English

Yue, Xianyan

Tu, Rong

Goto, Takashi

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Dielectric properties of poly- andsingle-crystalline Ba1-xSrxTi2O5著者 Yue  Xianyan, Tu  Rong, Goto  Takashijournal orpublication titleMaterials Transactionsvolume 48number 5page range 984-989year 2007URL http://hdl.handle.net/10097/52207Dielectric Properties of Poly- and Single-Crystalline Ba1xSrxTi2O5XinYan Yue, Rong Tu and Takashi GotoInstitute for Materials Research, Tohoku University, Sendai 980-8577, JapanPoly- and single-crystalline SrO substituted BaTi2O5 (Ba1xSrxTi2O5) were prepared by arc-melting and ﬂoating-zone (FZ) melting,respectively. Both showed a strong b-axis orientation and had maximum permittivity ("max) at 1mol% SrO substitution. The "max values of poly-and single-crystalline Ba0:99Sr0:01Ti2O5 were 3300 and 42190, respectively. The Curie temperatures (Tc) of poly- and single-crystallineBa1xSrxTi2O5 at x ¼ 0:03 were 748 and 742K, respectively. The electrical conductivity () of single-crystalline Ba1xSrxTi2O5 was higherthan that of poly-crystalline Ba1xSrxTi2O5. Larger size of single-crystalline Ba1xSrxTi2O5 specimen than that of BaTi2O5 was obtained.[doi:10.2320/matertrans.48.984](Received November 1, 2006; Accepted February 23, 2007; Published April 25, 2007)Keywords: Ba1xSrxTi2O5, arc-melting, poly-crystal, ﬂoating-zone (FZ) melting, single-crystal, ac impedance spectroscopy1. IntroductionBarium titanate (BaTiO3, BT) is a well-known lead-freeferroelectric material with a sharp peak of permittivity at theCurie temperature (Tc) about 400K.1) Many attentions havebeen paid on the eﬀect of substitution for BT to modify the Tcand to ﬂatten the peak of permittivity.2–5) In particular,Ba1xSrxTiO3 (BST) has been much interested because of thewide-ranged substitution of SrO in BT.6–8) Although bothpoly- and single-polycrystalline BST exhibited the decreasein Tc and the increase in peak permittivity with increasingSrO substitution, single-crystalline BST showed more sig-niﬁcant eﬀect of substitution on dielectric property than poly-crystalline BST.9–14)We and Akishige et al. independently reported thatBaTi2O5 (BT2) is a ferroelectric compound of a high Curietemperature (Tc ¼ 750K), and the space group of BT2 wasdetermined as monoclinic C2 consisting with the ferroelec-tricity in the b-direction.15–20) However, the BT2 singlecrystal with a large size can be hardly prepared because of thecleavage parallel to the growth direction. BT2 may decom-pose to BT and Ba6Ti17O40 (B6T17) in a temperature rangebetween 1423 and 1585K.16,21) This may cause the diﬃcultyof obtaining large-size single-crystalline BT2. We haveprepared b-axis orientated Ba1xSrxTi2O5 (x ¼ 0:03 to 0.1)by arc-melting and reported that a small amout of SrOsubstitution can signiﬁcantly improve the dielectric propertyof Ba1xSrxTi2O5. In the present study, we have preparedb-axis oriented poly- and a large size single-crystallineBa1xSrxTi2O5 with SrO substitution of x < 0:03 by arc- andFZ (Floating Zone) melting, respectively, and studied theeﬀect of SrO substitution on the dielectric property of poly-and single-crystalline Ba1xSrxTi2O5.2. Experimental ProceduresTiO2, BaCO3, and SrCO3 (99.9% purity) powders weremixed in a molar ratio of ðBaþ SrÞ=Ti ¼ 1=2, and the molarfraction of SrCO3 was changed from 0 to 0.03. The mixedpowders were pressed into pellets with 20mm in diameter at10MPa, and calcined at 1223K for 43 ks in air. The pelletswere arc-melted on a water-cooled copper plate in an Aratmosphere. The mixed powders were also isostaticallypressed in a latex tube with 10mm in diameter, and sinteredat 1503K for 43 ks in air. The sintered rods were melted anddirectionally solidiﬁed by an FZ method at a rate of 5:6106 ms1 in a ﬂowing Ar-O2 gas. The crystal orientationwas identiﬁed by X-ray diﬀraction (CuK). The specimenswere cut perpendicularly to (010) plane and the dielectricproperties were measured using an AC impedance analyzer(Hewlett Parckard 4194A) at frequencies (f) from 102 to107 Hz and at temperatures from 293 to 1073K in air.Composition of specimens was analyzed by an electron probemicroanalysis (EPMA).3. Results and DiscussionFigure 1 shows the appearance of specimens prepared byFZ. The single-crystalline Ba1xSrxTi2O5 had a cleavageplane of (001), same as that of BT2. Single-crystallineBa0:99Sr0:01Ti2O5 4mm in diameter and 16mm in length wascut. This size was larger than that of single-crystalline BT2without SrO substitution.15,19) Kojima et al. reported that aFig. 1 Specimens obtained by FZ melting.Materials Transactions, Vol. 48, No. 5 (2007) pp. 984 to 989#2007 The Japan Institute of Metalssmall amount of SrO substitution was eﬀective to preparelarge-size single-crystalline BT.14,22) The SrO substitution inBT2 might enable one to obtain large-size single-crystallineBa1xSrxTi2O5. The SrO distribution in the as-grown crystalsprepared by FZ melting was almost uniform (0:02%)except the initial and ﬁnal growth regions. Figure 2 demon-strates the relationship between the nominal starting compo-sition of Ba1xSrxTi2O5 sintered bodies and the ﬁnalcomposition of poly- and single-crystalline Ba1xSrxTi2O5prepared by arc-melting and FZ-melting, respectively. The Srconcentration in the as-grown crystals was slightly lowerthan that in the Ba1xSrxTi2O5 sintered bodies. The fastcooling speed in present FZ method would cause a non-equilibrium solidiﬁcation. Nevertheless the solidiﬁcation hasreached to a steady state with the eﬀective distributioncoeﬃcient being less than unity. The detail of the distributioncoeﬃcients can be available in literatures.23) Kojima et al.reported that the Sr concentration in as-grown BST crystalsalmost agreed with those of starting composition at x < 0:05in an FZ method.14) Since BaTiO3 is a simple perovskite,Ba2þ ions could be easily substituted by Sr2þ ions. However,BT2 has a monoclinic crystal structure with complicatedcorner and edge shared TiO6 octahedra, and therefore Sr2þsubstitution in BT2 might be more diﬃcult than that in BT. Inthe following, the compositions of Ba1xSrxTi2O5 in bothpoly- and single-crystalline specimens were represented asthe nominal starting composition of Ba1xSrxTi2O5 sinteredbodies.Figure 3 shows the XRD patterns of poly- and single-crystalline Ba1xSrxTi2O5. The powder XRD pattern showedthat poly-crystalline Ba0:99Sr0:01Ti2O5 was in a single phaseof BT2 (Fig. 3(a)). We have already reported that thesolubility limit of SrO in BT2 would be about 10mol%.The XRD pattern of poly-crystalline Ba1xSrxTi2O5 plateshowed a signiﬁcant (020) orientation (Fig. 3(b)). Columnargrains perpendicular to the water-cooled copper plate wereobserved in the cross section of specimens by SEM. TheXRD pattern of single-crystalline Ba0:995Sr0:005Ti2O5 plateperpendicular and parallel to the growth direction showedonly the (020) and (001) peaks, corresponding to a strong b-axis orientation as depicted in Fig. 3(c) and (d), respectively.Figure 4 demonstrates the eﬀect of SrO content on thelattice parameter of poly- and single-crystalline Ba1xSrx-Ti2O5. The lattice parameter decreased with increasing x forboth poly- and single-crystalline Ba1xSrxTi2O5. However,the decrease in lattice parameter of the poly-crystallinespecimens was slightly more signiﬁcant than that of single-crystalline specimens. This may be caused of the higher Srcontent in the poly-crystalline specimens as shown in Fig. 2;Fig. 2 Relationship between the starting composition of Ba1xSrxTi2O5sintered bodies and the ﬁnal composition of poly- and single-crystallineBa1xSrxTi2O5 prepared by arc-melting and FZ-melting.Fig. 3 XRD patterns of poly- and single-crystalline Ba1xSrxTi2O5.Fig. 4 Eﬀect of SrO concentration on lattice parameters of poly- andsingle-crystalline Ba1xSrxTi2O5.Dielectric Properties of Poly- and Single-Crystalline Ba1xSrxTi2O5 985the higher Sr concentration in Ba1xSrxTi2O5, the lowerlattice parameters.Figure 5 shows the Cole-Cole plots of poly- and single-crystalline Ba1xSrxTi2O5. An equivalent electrical circuitwith series combination of parallel R (resistance) and C(capacitance) elements can be employed to analyse theelectrical characteristics of material. The number and shapeof semicircles in the Cole-Cole plot would indicate compo-nents in the material. In the Cole-Cole plot of poly-crystallineBa0:99Sr0:01Ti2O5, one depressed semicircle was observed.That may be deconvoluted into two semicircles implying abulk and additional component. Although a second phase wasnot clearly detected in the poly-crystalline Ba0:99Sr0:01Ti2O5by XRD, a small amount of BT and Ba6Ti17O40 (B6T17) dueto the partial decomposition of BT2 was identiﬁed by electronprobe microanalysis (EPMA). The main circle in a higherfrequency range and the small circle in a lower frequencyrange might be contributed by the BT2 bulk and the secondphases due to associate capacitance values of 1:31 1010 Fand 5:7 108 F,24,25) respectively (Fig. 5(a)). The Cole-Cole plot of single-crystalline Ba0:99Sr0:01Ti2O5 may becontributed by only single semicircle by the BT2 bulkresponse due to the small capacitance value of 2:4 1010 F(Fig. 5(b)).Figure 6 depicts the imaginary parts of complex impe-dance (Z00) and modulus (M00) of poly- and single-crystallineBa1xSrxTi2O5 as a function of frequency (f) at 921K.The plots of Z00 and M00 vs. f of the poly-crystalline Ba0:99-Sr0:01Ti2O5 showed a diﬀerence of peak frequencies. Thesepeak frequencies were almost in agreement with thefrequency of the top of the ﬁrst semicircle shown in Fig. 5(a)consisting with the bulk response. Since the peak frequencyof the Z00 vs. f and M00 vs. f plots can be calculated from theequation of 2 fRC ¼ 1, they should have been coincided intheory. The discrepancy of these peak frequencies has beenoften reported.26,27) Some defect structure by the rapidquenching or the second phase by the partial decompositionmay cause the discrepancy of the peak frequencies. On theother hand, the single crystalline Ba0:99Sr0:01Ti2O5 had thesame peak frequency in the Z00 and M00 vs. f plots as shown inFig. 6(b), due to minimal defect or second phase in thesingle-crystalline specimen.Figure 7 shows the temperature dependence of permittiv-ity of poly- and single-crystalline Ba1xSrxTi2O5. Thepeak permittivity at the Tc changed depending on x. Thepermittivity of poly-crystalline Ba1xSrxTi2O5 showed themaximum permittivity at x ¼ 0:01 (Fig. 7(a)). The permit-tivity of the single-crystalline Ba1xSrxTi2O5 showed themuch sharper and higher peaks than poly-crystalline speci-mens at the Tc. The lower permittivity of poly-crystallinespecimens may be caused of the imperfectness of b-axisorientation and crystal defects (Fig. 7(b)). Since the poly-crystalline Ba1xSrxTi2O5 was prepared by arc-melting in anAr atmosphere, as-grown specimens were black in colorsuggesting oxide vacancies. Although the poly-crystallinespecimens were heat-treated in air at 1323K for 43 ks tocompensate oxide vacancies, some defects could be remainedFig. 5 Cole-Cole plots of Ba1xSrxTi2O5: poly-crystalline (a) and single-crystalline (b).Fig. 6 M00 and Z00 of Ba1xSrxTi2O5 as a function of frequency: poly-crystalline (a) and single-crystalline (b).986 X. Yue, R. Tu and T. Gotoin the specimens. Akishige et al. reported that the single-crystalline BT2 grown in a reducing atmosphere showed abroad peak around Tc, implying that the oxide vacancieswould associate with the dielectric dispersion.18) The diﬀer-ence of lattice parameter between poly- and single-crystallineBa1xSrxTi2O5 could suggest the local segregation of Sr2þ.This might also cause the decrease in the permittivity of poly-crystalline Ba1xSrxTi2O5.Figure 8 demonstrates the eﬀect of x on the Tc andthe peak permittivity ("max) of poly- and single-crystallineBa1xSrxTi2O5 at the Tc. The Tc of poly-crystallineBa1xSrxTi2O5 decreased from 752 to 748K with increasingx from 0 to 0.03. The Tc of single-crystalline Ba1xSrxTi2O5decreased from 748 to 742K with increasing x from 0 to 0.03(Fig. 8(a)). The Tc of BST decreased linearly from 405 to392K with increasing x from 0 to 0.03, implying moresensitive to the Sr2þ substitution compared to that ofBa1xSrxTi2O5.28,29) The solubility limit of Sr2þ in BT2 wasmuch lower than that in BT because of the more complicatedcrystal structure of BT2. The smaller solubility of Sr2þ in BT2may result in the smaller change of Tc than that in BT. Themaximum permittivity of poly- and single crystallineBa1xSrxTi2O5 were 3300 and 42190 at x ¼ 0:01, respec-tively (Fig. 8(b)).Figure 9 shows the temperature dependence of reciprocalpermittivity ("1) of poly- and single-crystalline Ba1x-SrxTi2O5. The "1 of both specimens obeyed the Curie-Weisslaw at temperatures higher than the Tc. The Curie-Weisstemperature (T0) of poly-crystalline Ba0:99Sr0:01Ti2O5, 718K, was lower than the Tc, while that of single-crystallineBa0:99Sr0:01Ti2O5 was almost in agreement with the Tc. In thecase of BST, the diﬀerence between T0 and Tc was 5 to 10Kfor single-crystalline30) whereas 15 to 30K for polycrystal-line specimens.31) It is commonly understood that the T0 isalways lower than Tc, which suggests the ﬁrst order trans-formation. In the present study, the diﬀerence between Tc andT0 of Ba1xSrxTi2O5 specimens was almost the same as thatof BT, about 5 and 25K for single-crystalline and poly-crystalline specimens, respectively. The phase transforma-tion mechanism of BT2 has been studied by Yashima et al.,and reported that BT2 would transform from C2 to C2=mlikely to be the ﬁrst order transformation.32)Figure 10 shows the temperature dependence of electricalconductivities () of poly- and single-crystalline Ba1xSrx-Ti2O5. The  of both specimens increased with the increasingFig. 7 Temperature dependence of permittivity of Ba1xSrxTi2O5 at f ¼105 Hz: poly-crystalline (a) and single-crystalline (b).Fig. 8 Eﬀect of SrO content on "max and Tc for Ba1xSrxTi2O5 at f ¼105 Hz: "max  x plot (a) and Tc  x plot (b).Dielectric Properties of Poly- and Single-Crystalline Ba1xSrxTi2O5 987temperature. The  of single-crystalline Ba1xSrxTi2O5 washigher than that of poly-crystalline Ba1xSrxTi2O5. The of poly-crystalline Ba1xSrxTi2O5 was lower than that ofpoly-crystalline BT2, while the  of single-crystallineBa1xSrxTi2O5 was almost same with single-crystallineBT2. For single-crystalline BT2, the  in the b directionwas higher than those in the a and c directions.33) The  ofpoly-crystalline Ba1xSrxTi2O5 was lower than that of single-crystalline specimens. This may be caused of the imperfect-ness of b-direction orientation and the scattering at grainboundary and local segregation. The activation energy (Ea) ofpoly-crystalline Ba1xSrxTi2O5 was about 1.62 eV, while theEa of single-crystalline Ba1xSrxTi2O5 was about 1.01 eV.These values were independent of SrO content.Figure 11 depicts the temperature dependence of dielectricloss (tan ) of poly- and single-crystalline Ba1xSrxTi2O5 atf ¼ 105 Hz. At lower temperature range (T < 400K) thetan  of poly-crystalline Ba1xSrxTi2O5 was higher than thatof single-crystalline Ba1xSrxTi2O5. At the higher temper-ature range (T > Tc) both of them showed a sharp increasebecause of the increase in electric conduction. The peaks oftan  near the Tc could be caused by the general Kramers-Kronig relationship; the higher "0, the higher "00.4. ConclusionsStrongly b-axis oriented poly- and single-crystallineBa1xSrxTi2O5 were prepared by arc-melting and FZ, re-spectively. A single-crystalline Ba0:99Sr0:01Ti2O5 of 4mm indiameter and 16mm in length was obtained. The permittivityof poly- and single-crystalline Ba1xSrxTi2O5 showed themaximum at x ¼ 0:01. The highest permittivities of poly-and single-crystalline Ba1xSrxTi2O5 were of 3300 and42190, respectively. The Tc of poly- and single-crystallineBa1xSrxTi2O5 decreased from 752 to 748K and from 748 to742K, respectively, with increasing x from 0 to 0.03. The of single-crystalline Ba1xSrxTi2O5 was higher than that ofpoly-crystalline Ba1xSrxTi2O5. The tan  of single-crystal-line Ba1xSrxTi2O5 was smaller than that of poly-crystallinespecimen.AcknowledgementsThe study was ﬁnancially supported by the Grant-in-Aidsfor Exploratory Research (17656209) of the Ministry ofEducation Culture Sports, Science and Technology (MEXT)and by the JSPS-KOSEF Asian CORE University Program ofthe Japan Society for the Promotion of Science (JSPS).Fig. 9 Temperature dependence of "1 of poly- and single-crystallineBa1xSrxTi2O5 perpendicular to (010) plane at f ¼ 105 Hz.Fig. 10 Temperature dependence of  of poly- and single-crystallineBa1xSrxTi2O5.Fig. 11 Temperature dependence of tan  of poly- and single-crystallineBa1xSrxTi2O5.988 X. Yue, R. Tu and T. GotoREFERENCES1) F. D. Morrison, D. C. Sinclair and A. R. West: J. Appl. Phys. 86 (1999)6355–6366.2) A. D. Hilton and B. W. Ricketts: J. Phys. D: Appl. Phys. 29 (1996)1321–1325.3) D. Hennings and A. Schnell: J. Am. Ceram. Soc. 65 (1982) 539–544.4) N. Maso, H. Beltran, E. Cordoncillo, A. A. Flores, P. Escribano, D. C.Sinclair and A. R. West: J. Mater. Chem. 16 (2006) 3114–3119.5) N. Maso, H. Beltran, E. Cordoncillo, P. Escribano and A. R. West:J. Mater. Chem. 16 (2006) 1626–1633.6) H. Abdelkeﬁ, H. Khemakhem, G. Velu, J. C. Carru and R. V. Muhll:J. Alloys and Compounds 399 (2005) 1–6.7) B. Su, J. E. Holmes, B. L. Cheng and T. W. Button: J. Electroceramics.9 (2002) 111–116.8) L. Szymczak, Z. Ujma, J. Handerek and J. Kapusta: CeramicsInternational 30 (2004) 1003–1008.9) X. Wei and X. Yao: Mater. Sci. Eng. B99 (2003) 74–78.10) L. Zhou, P. M. Vilarinho and J. L. Baptista: J. Euro. Cera. Soci. 19(1999) 2015–2020.11) M. Valant and D. Suvorov: J. Am. Ceram. Soc. 87 (2004) 1222–1226.12) F. Brown and W. H. Todtf: J. Appl. Phys. 35 (1964) 1594–1598.13) K. Bethe and F. Welz: Mat. Res. Bull. 6 (1971) 209–218.14) H. Kojima, M. Watanabe and I. Tanaka: J. Crystal Growth 155 (1995)70–74.15) T. Kimura, T. Goto, H. Yamane, H. Iwata, T. Kajiwara and T. Akashi:Acta. Cryst. C59 (2003) i128–i130.16) T. Akashi, H. Iwata and T. Goto: Mater. Trans. 44 (2003) 802–804.17) T. Akashi, H. Iwata and T. Goto: Mater. Trans. 44 (2003) 1644–1646.18) Y. Akishige, K. Fukano and H. Shigematsu: J. Electroceramics. 13(2004) 561–565.19) Y. Akishige: Jpn. J. Appl. Phys. 44 (2005) 7144–7147.20) Y. Akishige, K. Fukano and H. Shigematsu: Jpn. J. Appl. Phys. 42(2003) L946–L948.21) G. Pfaﬀ: J. Mater. Sci. Lett. 9 (1990) 1145–1147.22) J. Furukawa and T. Tsukamoto: Jpn. J. Appl. Phys. 30 (1991) 2391–2393.23) W. G. Pfann: Zone Melting, (John Wiley & Sons, Inc.) 7–22.24) J. T. S. Irvine, D. C. Sinclair and A. R. West: Advanced Materials 2(1990) 132–138.25) T. J. Hwang and G. M. Choi: Sensors and Actuators B40 (1997) 187–191.26) R. Gerhardt: J. Phys. Chem. Solids. 55 (1994) 1491–1506.27) M. A. L. Nobre and S. Lanfredi: J. Appl. Phys. 93 (2003) 5576–5582.28) D. Rytz, B. A. Wechsler, K. W. Kirby and C. C. Nelson: Jpn. J. Appl.Phys., Suppl. 24-2 (1985) 622–624.29) B. A. Wechsler and K. W. Kirby: J. Am. Ceram. Soc. 75 (1992) 981–984.30) M. E. Drougard and D. R. Young: Phys. Rev. 95 (1954) 1152–1153.31) H. Abdelkeﬁ, H. Khemakhem, G. Velu, J. C. Carru and R. Von derMuhll: J. Alloys Compounds 399 (2005) 1–6.32) M. Yashima, R. Tu, Takashi Goto and H. Yamane: Appl. Phys. Lett. 87(2005) 101909-1-101909-3.33) R. Tu and T. Goto: Mater. Trans. 47 (2006) 2898–2903.Dielectric Properties of Poly- and Single-Crystalline Ba1xSrxTi2O5 989

Dielectric properties of poly- and single-crystalline Ba1-xSrxTi2O5

Institutional Repositories DataBase (IRDB)

https://tohoku.repo.nii.ac.jp/?action=repository_action_common_download&item_id=44135&item_no=1&attribute_id=18&file_no=1

Dielectric properties of poly- and single-crystalline Ba1-xSrxTi2O5

Abstract

Similar works

Full text

Available Versions

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Institutional Repositories DataBase (IRDB)