J. MATER. CHEM., 1994, 4(8), 1271-1274 Preparation of Single-phase Pb(Mg,,,Nb,,,)O, Samples utilizing Information from Solubility Relationships in the Pb-Mg-Nb-Citric Acid-H,O System Jin-Ho Choy,*" Yang-Su Han? Seung-Wan Songa and Soon-Ho Changb a Department of Chemistry, Seoul National University, Seoul 151-742, Korea Nectronics and Telecommunications Research Institute, P.0. Box 8, Daeduk Science Town, Daejeon, Korea The optimum pH for preparing single-phase Pb(Mg,,3Nb2i3)03 (PMN) powder can be estimated from the solubility vs. pH diagrams for the corresponding metal ions in hydroxide, carbonate and citrate media. The homogeneous and stoichiometric citrate PMN precursor could be prepared from the system Pb-Mg-Nb-citric acid-H,O by adjusting the pH of the solution to 6.Ultrafine (0.05-0.3 pm) and stoichiometric PMN powder could be obtained through the thermal decomposition of the citrate precursor at the relatively low temperature of 900 "C. A ferroelectric relaxer material with the chemical composition Pb( Mgli3Nb2,,)O3 (PMN) was first synthesized in the late 1950s.' Since that time, PMN has been extensively studied because of its high relative permittivity and high electrostric- tive However, difficulty has been experienced in preparing X-ray monophasic samples of PMN.7-9 Depending upon the synthetic conditions, the thermodynamically stable pyrochlore phase can be formed as a second phase during the initial stages of reaction and consequently reduces the relative permittivity of the final material.In addition, the processing temperature should be kept as low as possible in order to reduce the evaporation of Pb0.l' In the recent literature, many attempts have been made to prepare single-phase PMN: ( 1) the conventional ceramic route, which requires repeated calcination at high temperature (>900 "C) and a long reaction period (ca. 24 h);' (2) two-step synthesis, i.e. reaction PbO with the oxide precursor MgNb206;7 and (3) sol-gel processes using the alkoxide" and citrate as precursors." The first two methods require a high reaction temperature and a long reaction period and consequently suffer from a lack of reproducibility of physical properties and of control of PbO content and particle size. The third method, however, can potentially overcome these difficulties.Since the sol-gel method can lead to highly pure, homogeneous and stoichiometric PMN powders with finer particle size, which can be processed at comparatively low temperature, they may offer significant advantages over the conventionally processed powders. In addition, the lower processing temperature in the sol-gel method enables minim- ization of PbO evaporation upon heat treatment of the sol-gel-processed material. However, in the conventional sol-gel method using citric acid," a polyhydroxy alcohol (ethylene glycol) was used as the esterification agent, this causes the formation of a dense and rigid intermediate of citric acid and ethylene glycol. In this case, additional extensive post-calcination grinding is usually required to break down the hard agglomerates formed during the calcination.Thus, in order to obtain a soft and porous intermediate, our attention was focused on the devel- opment of an improved citrate gel process utilizing only the chelating ability of citric acid without using any esterification agent. As previously pointed 0ut,I2-l5 the chelating ability and the complex formation of citric acid with various metal ions are highly dependent upon the pH of the solution. It is, therefore, necessary to study the behaviour of ionic species present in an aqueous solution. For this purpose, an attempt was made to draw the solubility diagrams based on the theoretical calculation of solubilities of individual metal hydroxides, carbonates and citrates.Theoretical Calculation of Solubilities It is necessary to predict theoretically the fc jrmation and dissolution of chemical species present in the Pb-Mg-Nb-citric acid-H,O system with respect to pH and metal-ion concentrations. Since the solubility of each ionic species for hydroxides, carbonates and citrates is given directly as a function of pH, the solubility diagram can ciasily be obtained by plotting the logarithmic molar concentration calculated from the values of the thermodynamic equilibrium constants for hydroxides, carbonates and citrates.12-' Solubility of Metal Hydroxides Fig. 1 shows the log ci (log molar concentration of metal ions) dependence of lead hydroxide as a function pH at cbf 25°C.The slope of this solubility line is determined by rearranging and taking logarithms to the hydrolysis reactions of the possible lead species [eqn. (1)-(4) in Tablt: 11. As shown in Fig. 1, the precipitation of lead hydroxide can be expected to begin at ca. pH 7 and reaches to a maximum at ca. pH 10 (when [Pb2+]=0.1 mol dm-3). The log c, us. pH diagram (25 "C) of magnesium hydroxide derived fr( )m eqn. 0 2 4 6 81012 PH Fig. 1 Solubility diagram for the lead hydroxide system at 25 "C J. MATER. CHEM.. 1994, VOL. 4 Table 1 Reactions and equilibrium constants used in the solubility calculations eqn. reaction symbol for constants log K ref. ~~~ ~ Pb2'+H20=PbOH'+H' -7.7 16-18 Pb2+ +2H20 =Pb(0H),'(aq)+2Ht -17.1 Pb2' +3H20=Pb(OH),-+3H+ -28.1 Pb(OH),(s)+2H+ =Pb2+ +2H20 12.7 Mg2++H20=MgOH++H+ -11.4 16,18 Mg(OH),(s) +H' =MgOH' +H20 5.4 Nb5+ +4H20=Nb(OH)4f +4H' -0.9 16,18 Nb5++6H20 =Nb(OH),- +6H' -7.6 Nb(OH),(s)=Nb5+ +50H--71.0 COZ(g)+ HZO=H2C03 -1.5 19 H2C03*=H++HC03--6.3 HC0,-=H+ +CO,'--10.2 PbC02(s)=Pb2' +C032--13.1 MgC03(s)=Mg2++C0,'--7.5 -3.1 18 -4.8 -6.4 16.9 -0.4 Pb2++Cl-=PbCl' 1.6 17 Pb2' +2C1-=PbCl,"(aq) 1.3 Pb2' +3C1- =PbC13- 1.7 Pb2+ + 4C1-=PbC142-1.4 PbCl,(s) =PbC12(aq) -4.8 P*, overall stability constants representing the protolytic equilibria; P, overall stability constant; K, normal stability; KH, Henry's constant; K,,, solubility constant.I(5) and (6) in Table 1 is shown in Fig. 2. The precipitation of magnesium hydroxide can occur only in basic condition (pH>9, when [Mg2'] =0.1 mol dm-3).Fig. 3 shows the solubility curve of niobium hydroxide, which was derived from eqn. (7)-(9) in Table 1. Niobium hydroxide can be formed in the pH range <8.5 ([Nb"] =0.1 mol dm-3) and is most stable at ca. pH 3. Solubility of Metal Carbonates From the equilibrium constants for characterizing the solu- bility and the equilibria of carbonates [eqn. (10)-(14)], the log ci us. pH diagrams of metal carbonates are plotted in Fig. 4. Thermodynamic data for niobium carbonate are not 0 2 4 6 8 10 12 PH Fig. 3 Solubility diagram for the niobium hydroxide system at 25 "C available in the literature because of its low formation con- stant, which is, therefore, neglected in this work.The solubility curves of lead and magnesium carbonates are shown in Fig. 4. Solubility of Metal Citrates Taking into account the dissociation constants for citric acid and the pertinent equilibria [eqn. (15)-(19) in Table 11, the log ci us. pH diagrams of metal citrates are plotted in Fig. 5 and 6. The solubility of niobium citrate is not considered 0 2 4 6 8 1012 here, because no solubility product (Ksojis available probably because of strong tendency of this citrate to form soluble PH species, i.e. it has a high solubility. Fig. 5 and 6 show that the Fig. 2 Solubility diagram for the magnesium hydroxide system at optimum pH domains for the complex formation of lead and 25 "C magnesium citrates are pH 6-7 and pH >6, respectively.J. MATER. CHEM., 1994, VOL. 4 / 0 2 46 81012 PH Fig. 4 Solubility diagram for the lead carbonate (-) and magnesiumcarbonate (---) systems at 25 "C 11111,l IIIII 0 2 4 6 8 10 12 PH Fig. 5 Solubility diagram for the lead citrate system at 25 "C 0 2 4 6 8 1012 PH Fig. 6 Solubility diagram for the magnesium citrate system at 25 "C Solubility of Lead Chloride Fig. 7 shows the predominance area diagram of the Pb2+-C1--H20 system as a function of pH and log molar concentration of chloride at 25"C, as derived from eqn. m r"9. I)Q 12 Fig. 7 Predominance area diagram for the Pb2+-CI--H:0 systemat 25°C (20)-(24) in Table 1. Lead hydroxy species are predominant at pH>9, but lead chloride species predominate at pH<9 (when [Cl-] =1.0 mol dm-3).Also, the difference in equilib- rium constants between PbC1, (s) and other chloride species is so large that rapid formation of PbCl, (s) is expected below ca. pH 9 Experimenta1 The starting reagents were NbCl,, Pb(N03), and Mg(NO3),-6H,O with high purity. First, niobium chloride was dissolved in 30% H202 aqueous solution to prevent the hydrolytic precipitation of niobium hydroxide, and lead and magnesium nitrates were dissolved in HNO, solution. Citric acid was then added to the nitrate mixture, and finally niobium chloride solution to this mixed (nitrate and citrate) solution. At this stage the pH of the solution was 1.5, which could be adjusted up to 6 by adding NH40H solution.Evaporating the water slowly from the solution at SO'C, a colourless colloidal suspension was first obtained, followed by bulky gel with high viscosity. The gel product was prefired at 500 "C for 2 h, ground and reheated at 900 "C for 40 min by the thermal shock technique to minimize the form,ition of the undesired pyrochlore phase and to diminish the PbO vaporization by a short reaction time.20 Results and Discussion In order to determine the optimum pH for preparing the homogeneous sol, it is necessary to simply overlap each solubility diagram for hydroxide, carbonate and citrate. As shown in Fig. 1-3, the hydroxide precipitation of Pb2+ and Mg2+ takes place in the pH range 8-12, while niobium hydroxide precipitation occurs at pH 1-6, implying that homogeneous and stoichiometric PMN precursors cannot be obtained by hydroxide coprecipitation in any pH domain.The carbonate coprecipitation method is also unsuitable due to the unstability of niobium carbonate. If we overlap Fig. 1, 4 and 5, we find that the optimum pH for the formation of the lead citrate complex is 6. where the solubility of citrates is minimized, indicating that the citrate complex is more stable than the hydroxide or carbon- ate. From the predominance area diagram for the system Pb2+-C1--H20 (Fig. 7), PbC1, precipitation would be expected below pH 9. In our experiment, however, lead dichloride was not formed at ca. pH 6 in the presence of citric acid. This result can be explained by the retarding eEect of I L 20 30 40 50 2Bldegrees Fig.8 X-Ray diffraction patterns of the PMN powders: (a)precursor, (b)prepared at 500 "C, (c)prepared at 900 "C. a,PMN; 'I,pyrochlore Fig. 9 Scanning electron micrograph of the PMN powders prepared at 900 "C (40 min) nucleation and crystal growth of lead dichloride in the pres- ence of a complex-forming agent such as citric acid in aqueous solution.21 Similarly, the optimum pH for Mg2+ solution is determined to be 6-8 from Fig. 2, 4 and 6. According to the solubility curve for Nb(OH)5 (Fig. 3) in the absence of citrate, niobium hydroxide precipitation would be very likely in the pH range 3-4. However, in the presence of citric acid as a chelating agent,22.23 the niobium ions would form soluble complexes such as Nb(OH)3(C6H,0,)-and Nb(OH),(C6H40,)-in this pH range, which retard the formation of niobium hydroxide.According to the solubility diagrams considered so far, it could be concluded that the optimum pH is ca. 6 for the formation of metal citrate complexes without forming any secondary phases such as hydroxide, carbonate or chloride. This theoretical consideration was confirmed experimentally, since we could obtain the stoichiometric PMN powder suc- cessfully through the thermal decomposition of a citrate precursor, which was prepared from the improved citrate gel J. MATER. CHEM., 1994, VOL. 4 process by adjusting the pH of the solution to the optimum pH of 6. Fig. 8 shows the diffraction patterns of samples prepared by the thermal shock technique at the given temperatures.The pyrochlore phase is formed less as the reaction tempera- ture is increased from 500 t? 900 "C. The lattice constant was determined to be a=4.039 A by the least-squares method. At 900 "C no pyrochlore phase was observed in the XRD pattern. The particle size and its distribution in the PMN powder, as measured by scanning electron microscopy, is shown in Fig. 9. The powder consists of nearly homogeneous particles with a size of 0.05-0.3 pm. From the results of electron probe microanalysis, the chemical composition of the sample synthe- sized at 900°C for 40min could be formulated as Pb0.93Mg0.36Nb0.6402.89. This research was supported in part by the Korean Science and Engineering Foundation (92-00-25-02).References 1 G. A. Smolenskii and A. T. Agranovskya, Sov. Pltys. Tech. Phys., 1958,3, 1380. 2 V. A. Bokov and I. E. Mylinikova, Sou. Phys. Solid State (Engl. 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