Goto Takashi

Sato Mitsutaka

Tu Rong

MATERIALS TRANSACTIONS

Sato, Mitsutaka

Tu, Rong

Goto, Takashi

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

Preparation of Pyrochlore Ca2Ti2O6 byMetal-Organic Chemical Vapor Deposition著者 Sato Mitsutaka, Tu Rong, Goto Takashijournal orpublication titleMaterials Transactionsvolume 47number 10page range 2603-2605year 2006URL http://hdl.handle.net/10097/52363Preparation of Pyrochlore Ca2Ti2O6 by Metal-OrganicChemical Vapor DepositionMitsutaka Sato1, Rong Tu2 and Takashi Goto21Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8577, Japan2Institute for Materials Research, Tohoku University, Sendai 980-8577, JapanCa-Ti-O films were prepared by metal-organic chemical vapor deposition (MOCVD) using Ca(dpm)2 and Ti(O-i-Pr)2(dpm)2 precursors,and the effects of substrate temperature (Tsub) and Ca/Ti ratio (RCa/Ti) on the crystal structure and morphology were studied. Ca-Ti-O filmsconsisting of pyrochlore Ca2Ti2O6 and perovskite CaTiO3 phase were obtained at Tsub ¼ 1073K and 0:35 < RCa/Ti < 1. The content ofpyrochlore Ca2Ti2O6 increased with decreasing RCa/Ti. Pyrochlore Ca2Ti2O6 almost in a single phase was obtained at RCa/Ti ¼ 0:46. Themorphology of pyrochlore Ca2Ti2O6 was agglomerated fine grains about 50 nm in diameter having a columnar texture.[doi:10.2320/matertrans.47.2603](Received June 27, 2006; Accepted August 25, 2006; Published October 15, 2006)Keywords: metal-organic chemical vapor deposition, calcium titanate, pyrochlore, microstructure1. IntroductionSince a Ca-Ti-O system contains many useful materials, somany studies on the phase diagram and crystal structure ofcalcium titanates have been reported.1) Perovskite, CaTiO3,and several compounds such as Ca4Ti3O10 and Ca3Ti2O7have been known as stable phases in the Ca-Ti-O system.Savenko and Sakharov reported a cubic phase of Ca2Ti5O12having a lattice parameter of 0.862 nm.2) They prepared thiscompound by thermal decomposition of mixed hydroxides ofTi and Ca at 1023K. This compound partially transformed toperovskite CaTiO3 and rutile TiO2 at 1273K, and completelytransformed at 1373K. Ball and White re-indexed the XRDdata by Savenko and Sakharov, and concluded that themetastable phase should be pyrochlore Ca2Ti2O6 having alattice parameter of 0.995 nm.3) Since then, no paper on thepreparation of pyrochlore Ca2Ti2O6 has been published.Pyrochlore has a general composition formula of A2B2-X6Y, where A and B are metals, and X and Y are O, OH or F.Since pyrochlore oxides have unique properties such as giantmagnetoresistance (GMR) of Tl2Mn2O7,4) metal-insulatortransition of Tl2Ru2O75) and anomalous Hall effect of Mosystem pyrochlore,6) pyrochlore Ca2Ti2O6 would also haveinteresting properties. However, the thermal decompositionprocess can prepare only a powder form of pyrochloreCa2Ti2O6, and pyrochlore Ca2Ti2O6 bodies can not beobtained by sintering due to the transformation to perovskite.We have been studying metal-organic chemical vapordeposition (MOCVD) of Ca-Ti-O system, and firstly pre-pared pyrochlore Ca2Ti2O6. In this paper, the effects ofsubstrate temperature (Tsub) and Ca/Ti ratio (RCa/Ti) on theformation of pyrochlore Ca2Ti2O6 were reported.2. Experimental ProceduresA vertical cold-wall type CVD apparatus was used toprepare Ca-Ti-O films. Source precursors of Ca(dpm)2 (bis-dipivaloylmethanato-calcium) and Ti(O-i-Pr)2(dpm)2 (bis-isopropoxy-bis-dipivaloylmethanato-titanium) powders wereheated at 523 to 573 and 393 to 453K, respectively. Thesource vapors were carried into the CVD reactor with Ar gas.O2 gas was separately introduced by using a double tubenozzle, and mixed with the precursor vapors above a sub-strate holder. The total gas flow rate (FRtot ¼ FRAr þ FRO2 þFRsource vapor) was fixed at 3:33 106 m3 s1. The totalpressure (Ptot) in the CVD reactor was kept at 0.8 kPa, and thesubstrate temperature (Tsub) was changed from 873 to1073K. Detailed experimental set up and the experimentalprocedure were reported elsewhere.7) The deposition con-ditions are summarized in Table 1. Fused quartz glass plates(10 15 0:5mm) were used as substrates. The crystalstructure was identified by X-ray diffraction (XRD). Themicrostructure was observed by scanning electron microsco-py (SEM) and transmission electron microscopy (TEM).3. Results and DiscussionFigure 1 shows that the XRD patterns of the Ca-Ti-O filmsprepared at Tsub ¼ 1073K and Ptot ¼ 0:8 kPa. The Ca-Ti-Ofilms consisted of perovskite CaTiO3, pyrochlore Ca2Ti2O6and anatase TiO2. A small amount of Ca2Ti2O6 phase wasdetected at RCa/Ti ¼ 0:95 (Fig. 1(c)), and the intensity ofCa2Ti2O6 increased with decreasing RCa/Ti, and the Ca2-Ti2O6 phase became as a main phase at RCa/Ti ¼ 0:34(Fig. 1(a)). Pyrochlore Ca2Ti2O6 is a face-centered cubicTable 1 Deposition condition of Ca-Ti-O film.Precursor Temperature, TprecCa(dpm)2 : 323{573KTi(OiPr)2(dpm)2 : 193{453KTotal gas flow rate, FRtot : 3:33 106 m3 s1Carrier Gas : ArCa(dpm)2 : 0:83 106 m3 s1Ti(OiPr)2(dpm)2 : 0:83 106 m3 s1O2 gas glow rate, FRO2 : 1:2 106 m3 s1Total pressure, Ptot : 0.8 kPaDeposition temperature, Tdep : 873{1073KDeposition time : 0:3{0:9 ksSubstrate : fused quartz glassMaterials Transactions, Vol. 47, No. 10 (2006) pp. 2603 to 2605#2006 The Japan Institute of Metals RAPID PUBLICATIONstructure whose lattice parameter could be 0.9953 nm.3) TheCa-Ti-O film showed in Fig. 1(a) was identified as a mixtureof CaTiO3, Ca2Ti2O6 and a small amount of anatase TiO2.The lattice parameter of Ca2Ti2O6 was calculated as a ¼0:999 nm that was close to that of JCPDS data of pyrochloreCa2Ti2O6.3) Ca(OH)2 peaks in Fig. 1(b) must be formed bythe reaction of CaO and moisture in air after deposition.Mixed phases of CaTiO3, anatase TiO2 and/or CaO wereobtained but no Ca2Ti2O6 phase was identified at Tsub ¼ 873and 973K.Figure 2 shows the electron diffraction pattern of theCa-Ti-O film prepared at Tsub ¼ 1073K, Ptot ¼ 0:8 kPa andRCa/Ti ¼ 0:34. The film was mainly Ca2Ti2O6 where theincident zone axis was [001] and every electron diffractionspots were indexed as pyrochlore Ca2Ti2O6.Figure 3 shows the surface and cross-sectional morphol-ogy of the Ca-Ti-O film prepared at Tsub ¼ 1073K, Ptot ¼0:8 kPa and RCa/Ti ¼ 0:34. The film had a columnar textureas shown in Fig. 3(b). The surface had a granular micro-structure with 300 nm in diameter, and the agglomeratedgrains consisted of smaller grains about 50 nm in diameter(Fig. 3(a)).Figure 4 shows the relationship between RCa/Ti and thefraction of Ca2Ti2O6 phase (F) in the Ca-Ti-O films preparedat Tsub ¼ 1073K and Ptot ¼ 0:8 kPa. The fraction of pyro-chlore Ca2Ti2O6 phase can be calculated from eq. (1).F ¼ICa2Ti2O6ICa2Ti2O6 þ ICaTiO3 þ ITiO2ð1Þwhere, I is the sum of all peaks for each phase in the Ca-Ti-Ofilms. The Ca2Ti2O6 phase formed in a Ti-rich region of0:3 < RCa/Ti < 1. The fraction of Ca2Ti2O6 phase increasedwith decreasing RCa/Ti, and the content of Ca2Ti2O6 phasereached 82% at RCa/Ti ¼ 0:46. Savenko and Sakharovreported that the metastable Ca2Ti5O12 phase formed in aTi-rich region of RCa/Ti ¼ 0:2{0:4. An excess TiO2 may benecessary to stabilize the pyrochlore Ca2Ti2O6 phase.10° 20° 30° 40° 50° 60° 70° 80°Intensity (arb. units)2θ (CuKα)(a)(b)(c)222400440622444800Ca2Ti2O6CaTiO3Ca(OH)2TiO2 (anatase)Fig. 1 XRD patterns of Ca-Ti-O films prepared at Tsub ¼ 1073K andPtot ¼ 0:8 kPa. (a) RCa/Ti ¼ 0:34, (b) RCa/Ti ¼ 0:66 and (c) RCa/Ti ¼ 0:95.(400)(440)_(220)_(040)_(220)_(220)(040)Fig. 2 Electron diffraction pattern of mainly pyrochlore Ca2Ti2O6 phaseprepared at Tsub ¼ 1073K, Ptot ¼ 0:8 kPa and RCa/Ti ¼ 0:34.1 µm(b)5 µm(a)Fig. 3 Surface and cross-sectional morphologies of mainly pyrochlore Ca2Ti2O6 phase prepared at Tsub ¼ 1073K, Ptot ¼ 0:8 kPaand RCa/Ti ¼ 0:34.2604 M. Sato, R. Tu and T. Goto4. ConclusionsPyrochlore Ca2Ti2O6 almost in a single phase was firstlyprepared by MOCVD using Ca(dpm)2 and Ti(O-i-Pr)2(dpm)2precursors at Tsub ¼ 1073K, Ptot ¼ 0:8 kPa and 0:3 <RCa/Ti < 1. The content of Ca2Ti2O6 phase increased withdecreasing RCa/Ti, and reached 82% at RCa/Ti ¼ 0:46. Thepyrochlore Ca2Ti2O6 showed a columnar texture consistingof agglomerated grains of 50 nm in diameter.AcknowledgementsThis research was partially supported by the Japan societyfor the Promotion of Science (JSPS), Grant-in-Aids forScientific Research (B), 18360310, and the JSPS-KOSEFAsian Core University program.REFERENCES1) L. Kaufman: Physica B + C 150 (1988) 99–114.2) V. G. Savenko and V. V. Sakharov: J. Inor. Chem. 24 (1979) 1389–1391;through JCPDS Card No. 33-0315.3) C. J. Ball and T. J. White: J. Solid State Chem. 65 (1986) 148–150;through JCPDS Card No. 04-0103.4) C. N. R. Rao: Chem. Eur. J. 2 (1996) 1499–1504.5) T. Takeda, M. Nagata, H. Kobayashi, R. Kannno and Y. Kawamoto:J. Solid State Chem. 140 (1998) 182–193.6) T. Katsufuji, H. Y. Hwang and S.-W. Cheong: Phys. Rev. Lett. 84 (2000)1988–2001.7) M. Sato, R. Tu and T. Goto: Mater. Trans. 47 (2006) 1386–1390.0204060801000.1 1 3Molar ratio of Ca and Ti precursors, RCa/TiFraction of Ca 2Ti 2O6, F  (%)Fig. 4 Effect of RCa/Ti on the peak intensity ratio of Ca2Ti2O6 in the filmsprepared at Tsub ¼ 1073K and Ptot ¼ 0:8 kPa.Preparation of Pyrochlore Ca2Ti2O6 by Metal-Organic Chemical Vapor Deposition 2605

Preparation of Pyrochlore Ca2Ti2O6 by Metal-Organic Chemical Vapor Deposition

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Preparation of Pyrochlore Ca2Ti2O6 by Metal-Organic Chemical Vapor Deposition

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