Effect of precursor solution on the formation of perovskite phase of

Thin Solid Films 410(2002)177–182

0040-6090/02/$-see front matter ?2002Elsevier Science B.V .All rights reserved.PII:S0040-6090?02.00269-9

Effect of precursor solution on the formation of perovskite phase of

Pb (Mg Nb )O thin films

1y 32y 33P .D.Spagnol *,J.A.Varela ,M.A.Z.Bertochi ,B.D.Stojanovic ,S.M.Tebcherani a ,a a b a

Chemistry Institute,Physics-Chemistry Department,UNESP ,C.P .355,CEP 14801-970Araraquara-SP ,Brazil

a

Center for Multidisciplinary Studies University of Belgrade,Belgrade,Yugoslavia

b

Received 6June 2001;received in revised form 4March 2002;accepted 13March 2002

Abstract

Precursor solutions for Pb (Mg Nb )O (PMN )synthesis were obtained by Pechini’s method.The influence of the 1y 32y 33concentration of organic materials on the phase formation has been studied.For this purpose,PMN solutions were prepared with different precursors and were characterized by thermogravimetric and differential thermal analysis.The obtained solutions were deposited onto a Si (100)substrate by dip coating and pre-treated in a hot plate at 3008C for 1h.The films were annealed at 600,700,800and 9008C for 1h and characterized by X-ray diffraction.The perovskite phase was formed after annealing at 600and 7008C when the solution of PMN was prepared with a lower amount of organic material and starting with niobium oxide.By increasing the temperature to 800or 9008C,only the formation of pyrochlore phase was observed.With the solution prepared from niobium ethoxide,only the presence of pyrochlore phase was observed independently of the annealing temperature.?2002Elsevier Science B.V .All rights reserved.

Keywords:Thin film;Lead magnesium niobate;Chemical method

1.Introduction

Ferroelectric thin films have attracted much attention for their potential applications including ferroelectric non-volatile memory,capacitors,optoelectronic,piezoe-lectric and pyroelectric devices due to their multifunc-tional properties such as polarization switching,high value of dielectric constant and pyroelectric,piezoelec-tric and electro-optic coefficients w 1x .The lead magne-sium niobate is the advanced dielectric material for capacitors and dynamic random access memory (DRAM )elements.The application of ceramic multilay-er capacitors is growing fast because of their large capacitance per unit volume.The perovskite phase Pb (Mg Nb )O ,from now on called PMN,is a 1y 32y 33ferroelectric relaxor with a diffuse phase transition with high dielectric constant ()20000)at room temperature and a relative low sintering temperature.A significant problem,however,exists in the preparation of this

*Corresponding author.Tel.:q 55-16201-6707;fax:q 5516-222-7932.

E-mail address:priscilaspagnol@https://www.360docs.net/doc/658218858.html, (P .D.Spagnol ).material,because it is difficult to prepare PMN with only perovskite https://www.360docs.net/doc/658218858.html,ually,an unwanted pyroch-lore phase also forms during the processing of PMN and this degrades the dielectric properties w 2x .The perovskite PMN phase is preferred since it has better properties than pyrochlore PMN phase.

PMN thin films have been obtained using different chemical and physical methods.The most often used physical processes are thermal evaporation w 3x ,diode sputtering,ion-beam sputtering w 4x ,magnetron sputter-ing w 5–7x and pulsed laser ablation w 8–12x .In the case of chemical preparation,the most commonly used meth-ods are chemical vapor deposition,sol–gel route w 13–16x and metal–organic citrate solution (Pechini’s method ).Pechini’s method w 17x presents some advan-tages in the preparation of thin films.By this method it is possible to obtain ceramics at low temperature and prepare multicomponents ceramics.Also it is easy to add dopants and seeds in any quantity.Pechini’s process allows the solution preparation in an aqueous solution that facilitate the viscosity adjust and finally it is a process with a global low cost.

178P .D.Spagnol et al./Thin Solid Films 410(2002)

177–182

Fig.1.Diagrams for the preparation of niobium Pechini’s solution.Route 1:(a )niobium precursor solution from oxide;Route 2:(b )niobium precursor solution from ethoxide.

Although Pechini’s method presents all of these advantages,there is nothing described in the literature about the preparation of PMN thin films using Pechini’s method.The objective of this work is to verify the behavior of Pechini’s method in the preparation of PMN thin films.For that,a thermal study of the different solutions was performed,as well as the study of for-mation of crystalline phases of PMN deposited onto Si (100)substrate.

2.Experimental procedure

Lead acetate (Merck ),magnesium carbonate (Chem-ical Kinetics ),niobium oxide (Aldrich ),niobium ethox-ide (Inorgtech ),citric acid (Mallinckrodt )and ethylene glycol (Mallinckrodt )were used as raw materials.The solutions were prepared according to the method devel-oped by Pechini w 17x .The niobium citrate was prepared in the proportion 1:3:12(metal y citric acid y ethylene glycol ),respectively,regarding the oxide (route 1)or the ethoxide (route 2)of niobium as described in the flowcharts presented in Fig.1.The solution of magne-sium citrate was prepared following the proportion 1:3:12(metal y citric acid y ethylene glycol ).The citric acid was added in ethylene glycol during magnetic stirring at 808C,followed by the addition of magnesium carbonate.The lead citrate was prepared by the same way as the magnesium citrate keeping the proportion 1:5:16(metal y citric acid y ethylene glycol ).

The PMN solution was obtained varying the precursor solution of each metal and consequently the amount of organic material.The Pechini’s solution of niobium was nominated by the number 1or 2,depending on the route used (Fig.1).The letters A,Bor C indicate the form how each solution of PMN was prepared (Fig.2).In solution A,all metals were added as organic solution with an overall proportion of 11:40(citric acid:ethylene glycol ).

In solution B,the niobium and magnesium were added in the organic citrate solution and the lead was

179

P .D.Spagnol et al./Thin Solid Films 410(2002)

177–182Fig.2.Different ways of preparation of Pechini’s

solutions.

Fig.3.Typical citrate ion metal complexes.

added as acetate in the solution with an overall propor-tion of 6:24(citric acid:ethylene glycol ).Finally,solution C,only niobium was added as organic citrate solution.Magnesium and lead were added as carbonate and acetate with an overall proportion of 3:12(citric acid y ethylene glycol ).In this way,the concentration of organic material decreases from solution A to solution C.

The molar concentration and viscosity of solutions A,Band C were adjusted,respectively,approximately 0.1M and 40cP .The viscosity was adjusted with water using a Brookfield viscosimeter (TC-500)with a spindle 18at 258C.Differential thermal analysis (DTA )and thermogravimetric analysis (TGA )(SDT2960)of the solutions were determined by using a heating rate of 2.58C min ,with a -Al O as the standard,and synthetic y 123air as gas carrier.

Only one film layer was deposited onto Si (100)substrate (Nova Electronic Materials INC )by the dip-coating process (MQCTL2000MP ).The substrate with-draw speed was adjusted at 10mm min .For y 1polyesterification and elimination of water,the solutions were pre-annealed at 3008C for 1h in a hot plate.After the pre-annealing,the films were treated in the range of 600–9008C for 1h in a box furnace.After annealing,the films were characterized by X-ray diffraction (Sie-mens D-500:40kV ,30mA,208F 2u G 608,Du s 0.028,l Cu ka monochromatized by a graphite crystal,diver-

gence slit s 2mm,reception slit s 6mm,step time:10s )to characterize crystalline phases.3.Results and discussion

Certain hydroxycarboxylic acids,such as citric,tar-taric and glycolic,form polybasic acid chelates with metal ions.When compared with the majority of the acids,citric is more widely used in the Pechini process-ing because of its high stability.The typical citrate ion metal complex tends to be fairly stable due to the strong coordination of the citrate ion to the metal involving two carboxyl and one hydroxyl group,as shown in Fig.3.

The desired metal ions are provided through various inorganic or organic constituents.The by-product from the reaction between the salt that has the metallic cation and citric acid is water,and between metal ethoxide and citric acid is an alcohol.This is the case for route 1and 2,respectively.

The presence of a polyhydroxyl alcohol,such as ethylene glycol,allows the formation of an organic ester with the acid chelate.A condensation reaction occurs with the formation of a water molecule.The hydroxyl group in the formed water arises from the carboxylic acid and the protons from the alcohol w 18x .

In our case,the general reaction sequence that occurs during the polymerization process is described below.The metals chelate to citric acid and form a polybasic acid chelate.Addition of ethylene glycol to the mixture leads to the formation of an ester.When heated,polym-erization takes place,thereby forming an organic matrix throughout the solution.Finally,the organics are removed during the annealing treatment at temperatures higher than 4008C.

Concerning the theoretical discussion mentioned above,the thermal analysis was used to confirm the behavior of the different solutions.The weight losses for the solutions 1A,1Band 1C were,respectively,approximately 98,85and 84%(Fig.4b ).The first stage (Fig.4a )(160–2008C )is related to water elimination,which is produced during the chelation process.The

180P .D.Spagnol et al./Thin Solid Films 410(2002)

177–182

Fig.4.(a,c )DTA and (b,d )DTG for the material prepared from the solutions 1A,1B,1C,2A,2B and 2C.

second stage (Fig.4a )(420–5108C )relates to the pyrolysis of organic compound,nucleation and crystal-lization of PMN.It is not easy to separate the process of the formation of pyrochlore and perovskite phase in Pechini’s method,like is possible for sol–gel method w 19x ,because the energy produced in situ is very high and both phases occur almost simultaneously.Thus,broadening of signal in this range is related to both processes;the first part of the signal is related to the pyrochlore formation and the second part is related to the perovskite.It is easier to distinguish the temperature of each phase formation in solution C,because in solutions A and Bthis behavior occurs at the same temperature.This behavior could be associated with the interaction between the by-product ethanol or ethylene glycol,since both are an alcohol,with the chelate during the esterification process,changing the size and the stabilization of the organic chain that complexed with the metallic cation w 20,21x .

Analyzing Fig.4d,the weight losses for solutions 2A,2Band 2C are,respectively,approximately 86,80and 79%.An initial endothermic effect observed at the DTA curve (Fig.4c )at approximately 1808C was attributed to the elimination of the excess of alcohol and by-products (ethanol )of the niobium ethoxide.The second signal at approximately 460–5108C could be related to the pyrochlore and perovskite phase formation.It can be noticed that this DTA curve is different compared to the DTA curve of route 1.The main reason for this is that the energy involved in the two processes is different,and the energy for the second one is smaller.In solution B,these differences are less expressed and it was very difficult to separate the two processes.Thus,it is possible to say that the inorganic constituent that pro-vides the metal ions is very important in the stabilization of organic acid chelate and it is interfering directly on temperature of perovskite nucleation and crystallization.According to the PMN thin film preparation,the influence of annealing temperature was discussed below.Concerned the previous results obtained for PMN pow-der formation,only solutions 1C and 2C were consid-ered.The formation of the crystalline phases after annealing at 600,700,800and 9008C for 1h was analyzed by XRD (Fig.5).Related to the obtained diffractograms,S indicates the phase related to the substrate;Pe to the perovskite and Pi to the pyrochlore.Obviously,the highest amount of perovskite phase was obtained for the film annealed at 6008C for solution 1C.The amount of perovskite phase decreases,after annealing at 7008C and it disappears completely when the film is annealed at 800and 9008C.It can be assumed that this behavior is related to the diffusion of

181

P .D.Spagnol et al./Thin Solid Films 410(2002)

177–182Fig.5.X-Ray diffraction for the solutions (a )1C and (b )2C,treated in different

temperatures.

Fig.6.X-Ray diffraction for the different solutions annealed at 6008C for 1h,for the solutions (a )1A,1B,1C and (b )2A,2B,2C.

Pb into the surface.Because the surface area of the layer is very high,it is possible to say that after 7008C the volatilization of lead occurs even though the volatil-ization temperature for lead is 8808C.When this volatilization occurs,the deviation on the stoichiometry favors the formation of pyrochlore phase.

In the films obtained from niobium ethoxide only the pyrochlore phase was formed independent of the tem-perature (Fig.5b ).In this case,the formation of perov-skite phase was suppressed due to the presence of excess of alcohol during the reaction of the niobium ethoxide with citric acid and ethylene glycol.This alcohol inhibits the complexation of niobium cation,and since it is free of the polyester chain,it reacted with lead to form predominantly pyrochlore phase.This is in accordance with the previous observation that different energies were required for the formation of pyrochlore and perovskite phases shown by DTA curves for solutions 1C and 2C.The different stabilities of organic chains could explain the formation of only the pyrochlore phase in solution 2.The excess of alcohol is responsible for the alteration of the chelation process of the metals,changing the size and the way that the metals react with the organic chains.These characteristics promote the fast liberation of niobium and lead.Together with a small amount of magnesium,the niobium and lead will react leading to the formation of pyrochlore (Pb MgNb O ).This is a magnesium deficient phase.6622The incorporation of magnesium in the pyrochlore struc-ture to transform into perovskite will occur only at temperatures at which the lead is already volatized.For this,the transformation of pyrochlore into perovskite is not favored.

The heating rate during the thermal treatment has no influence on the PMN phase formation.The same results are obtained with heating rates of 10and 208C min .y 1Previous results show that the best condition for anneal-ing the precursor routes are 6008C for 1h.Only solution C produces the perovskite phase.To verify the influence of the amount of organic material on the PMN phase formation,the films prepared from the solutions A,Band C were analyzed by XRD (Fig.6).

The films obtained from the niobium oxide and annealed at 6008C for 1h,independent of the amount of organic material added,show the presence of perov-skite phase.Looking at the diffractograms,it can be noticed that the addition of a greater amount of organic material leads to the formation of films with less amount of perovskite phase.Otherwise,films obtained from niobium ethoxide do not show the presence of the perovskite phase in any condition of annealing or amount of organic material.

182P.D.Spagnol et al./Thin Solid Films410(2002)177–182

This way,it is necessary to reduce the amount of organic material to the minimum during the preparation of thin films by Pechini’s method.Besides,this organic material proportion is strongly related to the formation of the perovskite phase and avoidance of the pyrochlore. Thus,the best condition for obtaining a film with a higher amount of perovskite phase is a solution prepared from the niobium oxide,with smaller amount of organic material(3:12,citric acid y ethylene glycol)and where the annealing temperature should not be higher than600 8C.

4.Summary

It has been shown that the form of precursor solution presented is a very important parameter in the prepara-tion of thin films of PMN by Pechini’s method.In this case,it has been noticed that the Nb–Pechini’s solution prepared with niobium oxide is better than the niobium ethoxide.The stability and the size of the organic chains are different in each case,mainly because the by-product of the reaction between the niobium ethoxide with citric acid,which was shown in the TG-DTA curves. The annealing temperature affects the formation of perovskite phase.An increase in the annealing temper-ature promotes the diffusion of lead on the surface.For temperatures higher than7008C lead will volatize. When this volatilization occurs,the deviation on the stoichiometry favors the formation of pyrochlore phase. The overall conclusion is that the annealing temperature can not be higher than6008C.

It was noticed that,by the Pechini’s method,the phase formation is strongly related to the amount of organic material.Increasing the amount of organic mate-rial promotes the reduction of perovskite phase.

Acknowledgments

The authors acknowledge Sao Paulo State Research

?

Foundation(FAPESP),Brazilian National Research Council(CNPq),and Financing of Studies and Projects (FINEP y PRONEX)for the financial support of this work.

References

w1x V.Trt?k,M.Jel?nek,E.B.Kluenkov,J.Phys.D:Appl.Phys.

′′

27(1994)1544.

w2x P.Ravindranathan,S.Komarneni,R.Roy,J.Am.Ceram.Soc.

73(1990)1024.

w3x N.Wakiya,S.Nagata,O.Sakurai,H.Funakubo,K.Shinozaki, N.Mizutani,J.Korean Phys.Soc.32(1998)S1490.

w4x S.Yamamichi,T.Sakuma,K.Takemura,Y.Miyasaka,Jpn.J.

Appl.Phys.30(1991)2193.

w5x J.Kelly,C.Tantigate,A.Safari,J.Am.Ceram.Soc.80(1997) 957.

w6x M.C.Jiang,T.B.Wu,J.Mater.Res.9(1994)1879.

w7x M.C.Jiang,T.J.Hong,T.B.Wu,Jpn.J.Appl.Phys.33(1994) 6301.

w8x C.Tantigate,J.Lee,A.Safari,Appl.Phys.Lett.66(1995) 1611.

w9x J.P.Maria,W.Hackenberger,S.Trolier-McKinstry,J.Appl.

Phys.84(1998)5147.

w10x T.Nakamura,A.Masuda,A.Morimoto,T.Shimizu,Jpn.J.

Appl.Phys.35(1996)4750.

w11x Y.Matsumuro,Y.Oishi,M.Okuyama,J.Korean Phys.Soc.

32(1998)S1625.

w12x C.Tantigate,A.Safari,Microelectron.Eng.29(1995)115.

w13x S.Nagakari,K.Kamigaki,S.Nambu,Jpn.J.Appl.Phys.35 (1996)4933.

w14x K.Okuwada,S.C.Nakamura,M.Imai,K.Kakuno,Jpn.J.

Appl.Phys.30(1991)L1052.

w15x L.F.Francis,D.A.Payne,J.Am.Ceram.Soc.74(1991)3000. w16x K.Y.Y oon,J.H.Park,D.H.Kang,J.Am.Ceram.Soc.78 (1995)2267.

w17x M.P.Pechini,US Patent No.3330697,1967.

w18x H.U.Anderson,M.J.Pennell,J.P.Guha,Adv.Ceram.21 (1987)91.

w19x K.Okuwada,S.Nakamura,M.Imai,K.Kakuno,Jpn.J.Appl.

Phys.26(1990)1153.

w20x F.Fairbrother,J.B.Taylor,J.Chem.Soc.(1956)4946.

w21x V.R.L.Constantino,Ph.D.Thesis,Chemistry Institute,Sao?Paulo Stadual University,Brazil,1989.