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Synthesis and characterization of aurivillius phase Bi 5 Ti 3 FeO 15 layered perovskite for visible light photocatalysis

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Synthesis and characterization of aurivillius phase Bi 5 Ti 3 FeO 15 layered perovskite for visible light photocatalysis
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  Synthesis and Characterization of Aurivillius Phases in the Bi–Ag–Ti–OSystem Xing Hu, w Sre W o D. S ˇkapin, and Danilo Suvorov * Advanced Materials Department, Jo m ef Stefan Institute, 1000 Ljubljana, Slovenia New Aurivillius phases in the Bi–Ag–Ti–O system were inves-tigated by means of a solid-state reaction and X-ray diffraction.We found that the oxygen partial pressure has a significant in-fluence on the synthesis of the Aurivillius phases. The mixed-layer Aurivillius phase Ag 0.5 Bi 8.5 Ti 7 O 27  was observed afterfiring in an O 2  flow, but a single-phase material is difficult toobtain. A single-phase compound of the four-layer Aurivilliusphase Ag 0.5 Bi 4.5 Ti 4 O 15  was obtained on firing in an oxygenpartial pressure of 10 bar (1  10 6 Pa). The dielectric properties(at 1 MHz) of the Ag 0.5 Bi 4.5 Ti 4 O 15  compound were as follows: T  max 5 687 1 C,  e r 5 166 ( B 20 1 C), and tan  d 5 0.004 ( B 20 1 C).I. Introduction R ECENTLY , a great deal of attention has been given to theAurivillius phase compounds because of their potentialfor applications in ferroelectric non-volatile memories(Fe-RAM) and high-temperature piezoelectrics. 1,2 The Aurivil-lius phases are a family of layered bismuth oxides that have beenknown for 50 years. 3 The structural formula of these com-pounds is usually described as [Bi 2 O 2 ] 2 1 [A n  1 B n O 3 n 1 1 ] 2  , whichconsists of perovskite-like (A n  1 B n O 3 n 1 1 ) 2  layers interleavedwith [Bi 2 O 2 ] 2 1 layers along the  c -axis. The Aurivillius phasesalso occur as mixed-layer compounds with the general formula[Bi 2 O 2 ][A n  1 B n O 3 n 1 1 ][Bi 2 O 2 ][A m  1 B m O 3 m 1 1 ]. The structure ismade up of an intergrowth of one half of the unit cell of theusual bismuth layer-structured oxides Bi 2 A n  1 B n O 3 n 1 3  and onehalf of the unit cell of Bi 2 A m  1 B m O 3 m 1 3  along each  c -axis.The restriction placed on the choice of A and B ions inAurivillius phases is that electrical neutrality must be maintainedin the overall composition. Armstrong and Newnham 4 suggest-ed that the instability could be related to the stability of theperovskite layer and the size mismatch between the [Bi 2 O 2 ] 2 1 and the perovskite layers. The A site can be occupied by large12-fold-coordinated cations, such as Na 1 , K 1 , Ca 2 1 , Sr 2 1 ,Ba 2 1 , Pb 2 1 , Bi 3 1 , or Ln 3 1 , and the B site by sixfold-coordinat-ed cations, such as Fe 3 1 , Cr 3 1 , Ti 4 1 , Nb 5 1 , Ta 5 1 , or W 6 1 .Although Ag 1 has a radius similar to Na 1 and Sr 2 1 , Subbarao 5 failed to obtain Ag 0.5 Bi 4.5 Ti 4 O 15  and so suggested that theelectronic configuration or polarizability also played a role in thestabilityoftheAurivilliusphases.However,inreality,itisthepoorthermal stability of Ag 2 O that prevents the synthesis of Ag-based compounds via a conventional solid-state reactionmethod. To synthesize the perovskite compound Ag 1/2 Bi 1/2 TiO 3 ,high-temperature, high-pressure techniques were used byPark  et al  ., 6 and the oxidizing conditions generated by thedecomposition of Ag 2 O were used by Inaguma  et al  . 7 It is possible that many compounds might exist, but have re-mained undetected because of their extremely low free energy of formation compared with neighboring compounds and the highnucleation barrier to complex-phase formation compared withthat of a simpler structure. Morgan PED 8 suggested that fast orslow firing of molecularly mixed or other unusual precursormixtures would be helpful in finding these compounds.In the present study, we tried to synthesize Aurivillius phasesin the Bi–Ag–Ti–O system with the nominal compositionsAg 0.5 Bi 8.5 Ti 7 O 27 , Ag 0.5 Bi 4.5 Ti 4 O 15 , and AgBi 5 Ti 5 O 18 , where n 5 3.5, 4, and 5 corresponds to [Bi 2 O 2 ][A n  1 B n O 3 n 1 1 ] and n 5 3.5 corresponds to the intergrowth of 3 1 4. The synthesestook place in an O 2  flow and in 10 bar of O 2  (1  10 6 Pa). Thesolid-state reaction method was used to prepare all the compo-sitions. The dielectric and ferroelectric properties of single-phaseAg 0.5 Bi 4.5 Ti 4 O 15  were also investigated. II. Experimental Procedure The starting materials used in this study were Bi 2 O 3  (99.975%),Ag 2 O ( 4 99%), and TiO 2  (anatase, 99.9%). The TiO 2  powdersweredriedat600 1 C.Thepowders,innominalcompositions,werehomogenized in ethanol by ball milling with 3-mm yttria-stabi-lizedzirconiaballs.Thepowderswereplacedinaluminacruciblesinatubefurnace,firedinanoxygenflowat10 1 C/minto900 1 Cfor12 h. Two more firings inan oxygen flow werealsoperformedonthe above reaction product with the nominal composition Ag 0.5- Bi 8.5 Ti 7 O 27 . One was heating at 10 1 C/min to 600 1 C, followed by0.1 1 C/min to 900 1 C and soaking for 6 h; the other was heating at10 1 C/minto900 1 Cfor12h.Eachtimethesamplesweretakenoutof the furnace, they were reground and re-milled. The powderswere also fired under an oxygen partial pressure  ð P O 2 Þ  of 10 bar(10 6 Pa)at1000 1 Cfor6h(heatingrate5 1 C/min).Thesingle-phaseAg 0.5 Bi 4.5 Ti 4 O 15  was sintered at 1000 1 C for 3 h in 10 bar of O 2 .The constituent phases were assessed using powder X-raydiffraction (XRD) studies (Bruker AXS D4 Endeavor diffracto-meter with Cu K  a  radiation, Bruker AXS, Karlsruhe, Germany).The unit-cell parameters were determined using Crysfire andTopas R software. The bulk density of the sintered samples wasevaluated by measuring the dimensions and the weight. The di-electric properties were measured with an impedance analyzer(Model HP 4284A LF, Hewlett-Packard, Palo Alto, CA). Forthe dielectric and ferroelectric property measurements, some sil-ver paste was fired on the samples at 750 1 C for 10 min. Thermalanalyses (DSC) were performed with a Netzsch STA 449C ther-mal analysis system (Netzsch, Selb, Germany) in an atmosphereof air. The DSC was carried out from room temperature to820 1 C at a heating rate of 2 1 C/min. The ferroelectric propertieswere examined with a hysteresis meter (Radiant Technologies,Precision LC, Albuquerque, NM). III. Results and Discussion The nominal compositions of Ag 0.5 Bi 8.5 Ti 7 O 27 , Ag 0.5 Bi 4.5 Ti 4 O 15 ,and AgBi 5 Ti 5 O 18  were heated in an oxygen flow at 10 1 C/minto 900 1 C and soaked for 12 h. The XRD analysis of thereaction mixtures revealed the presence of multiple phases D. Damjanovic—contributing editorThis work was financially supported by the Science and Education Foundation of theRepublic of Slovenia under contract No. OK-92P10SK. * Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: huxingmse@yahoo.com.cnManuscript No. 22222. Received September 7, 2006; approved February 14, 2007.  Journal J. Am. Ceram. Soc.,  90  [8] 2363–2366 (2007)DOI: 10.1111/j.1551-2916.2007.01770.x r 2007 The American Ceramic Society 2363  (shown in Fig. 1 and Table I). An Aurivillius phase, Bi 4 Ti 3 O 12 (PDF72-1019),andapyrochlorephase,Bi 2 Ti 2 O 7 (PDF89-4732),were observed in all the compounds. Because the strongest peakoftheAgphaseoverlappedwiththepeakoftheBi 4 Ti 3 O 12 phase,it was difficult to be certain from the XRD results whether Ag ispresent or not. The existence of Ag was expected in all the com-poundsbecauseofthedarkcolorofthesamples.Additionallinesin the 2 y  ranges 16.8 1  17 1 , 30.3 1  30.4 1 , and 39 1  39.3 1  werefound in all the compounds. Because the additional lines may beassociated with the Na 0.5 Bi 8.5 Ti 7 O 27  phase (PDF 32-1044), thenew phase was assumed to be Ag 0.5 Bi 8.5 Ti 7 O 27 .The amounts of Ag 0.5 Bi 8.5 Ti 7 O 27  phase can be estimatedfrom the strongest peak of each phase using the followingequation:%Ag 0 : 5 Bi 8 : 5 Ti 7 O 27  ¼  100  I  118 ð Ag 0 : 5 Bi 8 : 5 Ti 7 O 27 Þ = ½ I  118 ð Ag 0 : 5 Bi 8 : 5 Ti 7 O 27 Þ þ I  117 ð Bi 4 Ti 3 O 12 Þ þ I  222 ð Bi 2 Ti 2 O 7 Þ  A similar equation is frequently used for calculating theamounts of pyrochlore phase during the fabrication of Pb(Mg 1/3 Nb 2/3 )O 3 . 9 To increase the amount of the Ag 0.5 Bi 8.5- Ti 7 O 27  phase, the above reaction product with the nominalcomposition Ag 0.5 Bi 8.5 Ti 7 O 27  was heated at 10 1 C/min to600 1 C, followed by 0.1 1 C/min to 900 1 C, and soaked for 6 h,the so-called slow-heating method. 8 The amount of Ag 0.5 Bi 8.5- Ti 7 O 27  increased from 20% to 75% and the pyrochlore phasedisappeared, as shown in Fig. 2. Next, after the slow-heatingmethod, the reaction product was heated at 10 1 C/min to 900 1 Cand soaked for 12 h. The amount of Ag 0.5 Bi 8.5 Ti 7 O 27  increasedfrom 75% to 85%.It is well-known that the presence of a pyrochlore phase isalways unfavorable for the synthesis of a perovskite phase suchas Pb(Mg 1/3 Nb 2/3 )O 3 . 9 It was initially thought that firing a mix-ture of Bi 4 Ti 3 O 12 , TiO 2 , and Ag 2 O corresponding to the nom-inal composition of Ag 0.5 Bi 8.5 Ti 7 O 27  might aid the formation of the Ag 0.5 Bi 8.5 Ti 7 O 27  phase. Disappointingly, however, only trac-es of the Ag 0.5 Bi 8.5 Ti 7 O 27  phase were observed. Using the aboveslow-heating method for the corresponding reaction product,the amount of Ag 0.5 Bi 8.5 Ti 7 O 27  phase could be increased from5% to 45%.It is possible that the poor thermal stability of Ag 2 O mayprevent the synthesis of Ag-based compounds such as Ag 0.5- Bi 0.5 TiO 3  via a conventional solid-state reaction method. 6,7 Toreduce the influence of the poor thermal stability of the Ag 2 Ocompound, an oxygen partial pressure of 10 bar was used duringthe formation of the present compounds. The XRD results of three nominal compositions fired at 1000 1 C for 6 h in 10 bar of O 2  are shown in Fig. 3. For the nominal composition Ag 0.5 Bi 4.5- Ti 4 O 15 , all the reflection lines can be matched to the SrBi 4 Ti 4 O 15 phase (PDF 43-0973). In Ag 0.5 Bi 4.5 Ti 4 O 15 , no silver metal wasobtained after the calcinations and sintering, and the powders orpellets remained yellow. For the nominal composition Ag 0.5- Bi 8.5 Ti 7 O 27 , the single phase that we expected was not obtained.The dominant phases were the two Aurivillius compounds:Bi 4 Ti 3 O 12  and Ag 0.5 Bi 4.5 Ti 4 O 15 ; we observed only the shouldersof some peaks related to Ag 0.5 Bi 8.5 Ti 7 O 27 , as shown in Fig. 3.For the nominal composition AgBi 5 Ti 5 O 18 , which is on the linebetween Ag 0.5 Bi 4.5 Ti 4 O 15  and Ag 0.5 Bi 0.5 TiO 3 , the followingthree phases were detected: Ag 0.5 Bi 4.5 Ti 4 O 15 , Bi 2 Ti 2 O 7 , andsilver metal.The phase constituents of the nominal compositions fired un-der different conditions are shown in Table I. In the presentstudy, there was no evidence for the existence of AgBi 5 Ti 5 O 18 ( n 5 5 in [Bi 2 O 2 ][A n  1 B n O 3 n 1 1 ]). An attempt to synthesize Na-Bi 5 Ti 5 O 18  and KBi 5 Ti 5 O 18  by Subbarao 5 and Uchida and Kiku-chi 10 was also unsuccessful. The maximum value of   n  in theAurivillius phase is supposed to be 4 when the A site is occupiedby a monovalent ion and a Bi ion. The existence of the mixed-layer Aurivillius phase Ag 0.5 Bi 8.5 Ti 7 O 27  was confirmed by theXRD results when the nominal compositions were fired in anoxygen flow. It was difficult to obtain the single-phase mixed-layer structure of Ag 0.5 Bi 8.5 Ti 7 O 27  because of the presence of thesimpler three-layer structure of Bi 4 Ti 3 O 12 . It is believed that asmall concentration of Ag can be incorporated into the Bi 4- Ti 3 O 12  phase, and the Bi 4 Ti 3 O 12  phase with dissolved Ag has acomposition close to Ag 0.5 Bi 8.5 Ti 7 O 27 . A slow heating rate or aprolonged dwelling time should be used to enhance the forma-tion of the Ag 0.5 Bi 8.5 Ti 7 O 27  phase because of the higher freeenergy of formation of Ag 0.5 Bi 8.5 Ti 7 O 27  compared with that of Bi 4 Ti 3 O 12 . The structure of the Ag 0.5 Bi 8.5 Ti 7 O 27  phase wasrefined in the orthorhombic space group I2 cm, suggested byBoullay  et al  ., 11 Z 5 1, and the lattice parameters are  a 5 5.451(1)A ˚,  b 5 5.419(1) A ˚, and  c 5 36.751(8) A ˚. 10 20 30 40 50 60 CBA Ag 0.5 Bi 8.5 Ti 7 O 27 Ag 0.5 Bi 8.5 Ti 7 O 27 Bi 2 Ti 2 O 7 Bi 2 Ti 2 O 7 Bi 4 Ti 3 O 12 Bi 4 Ti 3 O 12 15 16 17 18 29 30 31 CBA    R  e   l  a   t   i  v  e   I  n   t  e  n  s   i   t  y   R  e   l  a   t   i  v  e   I  n   t  e  n  s   i   t  y 2 θ (degree)2 θ (degree) (a)(b) Fig.1.  X-ray diffraction results of the A–C nominal compositionsfired at 900 1 C in an O 2  flow: (a) in the 2 y  range 10 1  –60 1 ; (b) in the 2 y ranges 15 1  –19 1  and 29 1  –31 1 . (A, Ag 0.5 Bi 8.5 Ti 7 O 27 ; B, Ag 0.5 Bi 4.5 Ti 4 O 15 ;C, AgBi 5 Ti 5 O 18 .) Table I. Starting Nominal Compositions of Aurivillius Phases and Phases Detected by XRD Nominal composition Fired in O 2  flow (at 900 1 C for 12 h) Fired in 10 6 Pa O 2  (at 1000 1 C for 6 h) Ag 0.5 Bi 8.5 Ti 7 O 27  Bi 4 Ti 3 O 12 1 Bi 8.5 Ag 0.5 Ti 7 O 27 1 Bi 2 Ti 2 O 7 1 Ag Bi 4 Ti 3 O 12 1 Ag 0.5 Bi 4.5 Ti 4 O 15 1 Ag 0.5 Bi 8.5 Ti 7 O 27 Ag 0.5 Bi 4.5 Ti 4 O 15  Bi 4 Ti 3 O 12 1 Bi 2 Ti 2 O 7 1 Ag 0.5 Bi 8.5 Ti 7 O 27 1 Ag Ag 0.5 Bi 4.5 Ti 4 O 15 AgBi 5 Ti 5 O 18  Bi 4 Ti 3 O 12 1 Ag 0.5 Bi 8.5 Ti 7 O 27 1 Bi 2 Ti 2 O 7 1 Ag Bi 2 Ti 2 O 7 1 Ag 0.5 Bi 4.5 Ti 4 O 15 1 Ag XRD, X-ray diffraction. 2364  Journal of the American Ceramic Society—Hu et al.  Vol. 90, No. 8  When fired under a pressure of oxygen, single-phase Ag 0.5- Bi 4.5 Ti 4 O 15  ( n 5 4) could be obtained. The structure of theAg 0.5 Bi 4.5 Ti 4 O 15  phase was refined in the orthorhombic spacegroup 12 A2 1 am,  Z  5 2, and the cell parameters are  a 5 5.4586(4)A ˚,  b 5 5.4293(7) A ˚, and  c 5 40.758(3) A ˚( R wp 5 0.1634)(Fig. 4(b)). We also heated the single-phase Ag 0.5 Bi 4.5 Ti 4 O 15 in flowing oxygen at 900 1 C for 6 h. The dominant phase wasstill Ag 0.5 Bi 4.5 Ti 4 O 15 , but the reflection of Ag 0.5 Bi 8.5 Ti 7 O 27 appears at the shoulder of some peaks, and the pyrochlorephase is observed, as shown in Fig. 4(a). We concluded thatAg 0.5 Bi 4.5 Ti 4 O 15  is not stable when fired in flowing oxygen.Dense ceramics ( 4 94% of the theoretical density) wereobtained when sintering Ag 0.5 Bi 4.5 Ti 4 O 15  at 1000 1 C for 3 h inpressurized O 2 . After sintering, the ceramic shows the sameyellow color as the powders after calcinations. A dielectric con-stant,  e r , of 166 and a dielectric loss, tan d , of 0.004 wereobtained at 1 MHz at room temperature. From room temper-ature to 300 1 C, the dielectric constant was found to be almosttemperature independent, as shown in Fig. 5. A sharp peak wasfound at 687 1 C, which coincides with the minimum of the di-electric loss. No significant difference was found for the value of  T  max , whether the temperature-dependence measurement was 15 16 17 18 29 30 31a+b+aa+b    R  e   l  a   t   i  v  e   I  n   t  e  n  s   i   t  y 2 θ (degree)a Ag 0.5 Bi 8.5 Ti 7 O 27 Bi 2 Ti 2 O 7 Bi 4 Ti 3 O 12 Fig.2.  X-ray diffraction results of the nominal compositionAg 0.5 Bi 8.5 Ti 7 O 27  in the 2 y  ranges 15 1  –19 1  and 29 1  –31 1 . (a, firing at10 1 C/min to 900 1 C for 12 h; b, 10 1 C/min to 600 1 C, followed by0.1 1 C/min to 900 1 C and soaked for 6 h). 10 20 30 40 50 60BCA2 θ (degree)2 θ (degree)15 16 17 18 29 30 31ABC    R  e   l  a   t   i  v  e   I  n   t  e  n  s   i   t  y   R  e   l  a   t   i  v  e   I  n   t  e  n  s   i   t  y Ag 0.5 Bi 8.5 Ti 7 O 27 Ag 0.5 Bi 4.5 Ti 4 O 15 AgBi 2 Ti 2 O 7 Bi 4 Ti 3 O 12 Ag 0.5 Bi 8.5 Ti 7 O 27 Ag 0.5 Bi 4.5 Ti 4 O 15 Bi 2 Ti 2 O 7 Bi 4 Ti 3 O 12 (a)(b) Fig.3.  X-ray diffraction results of the A–C nominal compositions firedat P O2  with 10 7 Pa: (a) in the 2 y  range 10–60 1 ; (b) in the 2 y  ranges15 1  –19 1  and 29 1  –31 1 . (A, Ag 0.5 Bi 8.5 Ti 7 O 27 ; B, Ag 0.5 Bi 4.5 Ti 4 O 15 ; C,AgBi 5 Ti 5 O 18 .) 10 20 30 40 50 60B    R  e   l  a   t   i  v  e   I  n   t  e  n  s   i   t  y 2 θ (degree)A (a)(b) Fig.4.  (a) X-ray diffraction results of Ag 0.5 Bi 4.5 Ti 4 O 15  (A, single phase;B, reheating single phase in an O 2  flow at 900 1 C for 6 h); (b) Rietveldrefinement results of Ag 0.5 Bi 4.5 Ti 4 O 15 . (The upper graph shows the fitbetween the experimental and calculated patterns, while the lower graphshows the difference between these two patterns.) 0 100 200 300 400 500 600 700020040060080010001200T max =687 ° CTemperature ( ° C)      ε   r 0.00.51.01.52.02.53.0 650 700 750 8000.00.10.20.30.40.5    D   S   C   (      µ    V   /  m  g   ) Temperature ( ° C) (at 1MHz)  t     an  δ    Fig.5.  Temperature dependence of the dielectric properties of theAg 0.5 Bi 4.5 Ti 4 O 15  ceramic. (The inset shows DSC analysis results;  T  max ,the temperature of   e max ). August 2007  Synthesis and Characterization of Aurivillius Phases  2365  performed at a heating rate of 5 1  or 2 1 C/min. Usually,  T  max  canbe approximated as  T  C . The DSC results shown in the insetgraph of Fig. 5 suggest that the phase transition at the  T  C  of Ag 0.5 Bi 4.5 Ti 4 O 15  is not of the first order, but of the second order.For bismuth layer-structured ferroelectrics, the  T  C  is stronglyinfluenced by the r i   of the A-site cations, and bismuth layer-structured ferroelectrics with smaller A-site cations tend to showa higher  T  C . The r i   of Ag 1 is larger than that of Na 1 , but the  T  C (687 1 C) of the Ag 0.5 Bi 4.5 Ti 4 O 15  is higher than that (662 1 C inAhn  et al  . 13 and 655 1 C in Ben Jannet  et al  . 14 ) of the Na 0.5 Bi 4.5- Ti 4 O 15 . The large electronegativity of the Ag 1 (1.93) may play arole in the influence on the Curie temperature (the electron-egativity of Na 1 is 0.93). The  T  C  of both the Ag 0.5 Bi 4.5 Ti 4 O 15 and the Na 0.5 Bi 4.5 Ti 4 O 15  is higher than that of the SrBi 4.5 Ti 4 O 15 (520 1 C) 15 and lower than that of the CaBi 4 Ti 4 O 15  (820 1 C). 16 The  P  –  E   hysteresis loops of the Ag 0.5 Bi 4.5 Ti 4 O 15  ceramicare shown in Fig. 6. The distorted loops indicate a lossy non-linear capacitor behavior. Because both the polar orthorhombicstructure and the sharp peak from the dielectric constantmeasurement suggest a ferroelectric phase at room temperaturein Ag 0.5 Bi 4.5 Ti 4 O 15 , we believe that the loops are the resultof both the weak ferroelectric properties and the leakagecurrent. The leakage–current density measured at 10 kV/cmwas 8  10  7 A/cm 2 . When the soaking time was increased,a larger  P r  and a round curve were obtained because of theleakage current. IV. Conclusions The nominal compositions of Aurivillius phases (Ag 0.5 Bi 8.5- Ti 7 O 27 , Ag 0.5 Bi 4.5 Ti 4 O 15 , and AgBi 5 Ti 5 O 18 ) in the Bi–Ag–Ti–Osystem were investigated by means of a solid-state reaction andXRD. The mixed-layer Aurivillius phase Ag 0.5 Bi 8.5 Ti 7 O 27  wasfound after firing in an O 2  flow. A slow heating rate and a pro-longed dwelling time can increase the amount of the Ag 0.5 Bi 8.5- Ti 7 O 27  phase. On firing in an oxygen partial pressure of 10 bar(1  10 6 Pa), a single-phase compound of the four-layer Aur-ivillius phase Ag 0.5 Bi 4.5 Ti 4 O 15  was obtained. There was no ev-idence for the existence of AgBi 5 Ti 5 O 18 . The dielectric properties(at 1 MHz) of the Ag 0.5 Bi 4.5 Ti 4 O 15  phase were as follows: T  max 5 687 1 C,  e r 5 166 ( B 20 1 C), and tan d 5 0.004 ( B 20 1 C). References 1 B. H. Park, B. S. Kang, S. D. Bu, T. W. Noh, J. Lee, and W. Jo, ‘‘Lanthanum-Substituted Bismuth Titanate for Use in Non-Volatile Memories,’’  Nature ,  401 ,682–4 (1999). 2 D. Damjanovic, ‘‘Materials for High Temperature Piezoelectric Transducers,’’ Curr. Opin. Solid State Mater. Sci. ,  3  [5] 469–73 (1998). 3 B. Aurivillius, ‘‘Mixed Bismuth Oxides With Layer Lattices: I. The StructureType of CaNb 2 Bi 2 O 9 ,’’  Ark. Kemi  ,  1  [54] 463–80 (1949). 4 R. A. Armstrong and R. E. Newnham, ‘‘Bismuth Titanate Solid Solutions,’’ Mater. Res. Bull. ,  7 , 1025–34 (1972). 5 E. C. Subbarao, ‘‘Crystal Chemistry of Mixed Bismuth Oxides With Layer-Type Structure,’’  J. Am. Ceram. Soc. ,  45  [4] 166–9 (1962). 6 J. H. Park, P. M. Woodward, J. B. Parise, R. J. Reeder, I. Lubomirsky, and O.Stafsudd, ‘‘Synthesis, Structure and Dielectric Properties of (Bi 1/2 Ag 1/2 )TiO 3 ,’’ Chem. Mater. ,  11  [1] 177–83 (1999). 7 Y. Inaguma, T. Katsumata, R. P. Wang, K. Kobashi, M. Itoh, Y. J. Shan, and T.Nakamura, ‘‘Synthesis and Dielectric Properties of a Perovskite Bi 1/2 Ag 1/2 TiO 3 ,’’ Ferroelectrics ,  264 , 1785–90 (2001). 8 P. E. D. Morgan, ‘‘Preparing New Extremely Difficult-to-Form Crystal Struc-tures,’’  Mater. Res. Bull. ,  19  [3] 369–76 (1984). 9 S. L. Swartz and T. R. Shrout, ‘‘Fabrication of Perovskite Lead MagnesiumNiobate,’’  Mater. Res. Bull. ,  17  [10] 1245–50 (1982). 10 K. Uchida and T. Kikuchi, ‘‘Subsolidus Phase-Equilibria in System Na 2 O– Bi 2 O 3  –TiO 2  at 1000 Degrees,’’  J. Am. Ceram. Soc. ,  61  [1–2] 5–8 (1978). 11 Ph. Boullay, G. Trolliard, D. Mercurio, J. M. Perez-Mato, and L. Elcoro,‘‘Toward A Unified Approach to the Crystal Chemistry of Aurivillius-Type Com-pounds. I. The Structural Model,’’  J. Solid State Chem. ,  164  [2] 252–60 (2002). 12 C. H. Hervoches, A. Snedden, R. Riggs, S. H. Kilcoyne, P. Manuel, andP. Lightfoot, ‘‘Structural Behavior of the Four-Layer Aurivillius-Phase Ferroelec-trics SrBi 4 Ti 4 O 15  and Bi 5 Ti 3 FeO 15 ,’’  J. Solid State Chem. ,  164  [2] 280–91 (2002). 13 C. W. Ahn, I. W. Kim, M. S. Ha, W. K. Seo, J. S. Lee, and S. S. Yi,‘‘Dielectric and Piezoelectric Properties of Lead-Free Na 0.5 Bi 4.5  x La x Ti 4 O 15  andNa 0.5 Bi 4.5  x Nd x Ti 4 O 15  Ceramics,’’  Ferroelectrics ,  273 , 2639–44 (2002). 14 D. Ben Jannet, M. El Maaoui, and J. P. Mercurio, ‘‘FerroelectricVersus Relaxor Behaviour in Na 0.5 Bi 4.5 Ti 4 O 15 -BaBi 4 Ti 4 O 15  Solid Solutions,’’ J. Electroceram. ,  11  [1–2] 101–6 (2003). 15 Y. Noguchi, M. Miyayama, and T. Kudo, ‘‘Ferroelectric Properties of Inter-growth Bi 4 Ti 3 O 12  –SrBi 4 Ti 4 O 15  Ceramics,’’  Appl. Phy. Lett. ,  77  [27] 3639–41 (2000). 16 M. Demartin maeder, D. Damjanovic, and N. Setter, ‘‘Lead Free PiezoelectricMaterials,’’  J. Electroceram. ,  13 , 385–92 (2004).  & − 80 − 40 0 40 80 − 8 − 6 − 4 − 202468      P  o   l  a  r   i  z  a   t   i  o  n   (      µ   c   /  c  m    2    ) Electric Field (kV/cm) Fig.6.  P  –  E   hysteresis loops of Ag 0.5 Bi 4.5 Ti 4 O 15  ceramic with increasingelectric field (at 100 Hz). 2366  Journal of the American Ceramic Society—Hu et al.  Vol. 90, No. 8
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