Materials Science and Engineering A304–306 (2001) 800–804

Soft chemical routes to the synthesis of extended solidsolutions of wurtzite ZnO–MO(M = Mg, Co, Ni)

Vikram Jayaram∗, B. Sirisha RaniDepartment of Metallurgy, Indian Institute of Science, Bangalore 560 012, India

Abstract

Spray pyrolysis and gel decomposition have been used to generate extended solid solutions of MO(M = Mg, Co and Ni) in thewurtzite modification of ZnO through kinetic limitations. Both routes yield a significantly greater extent of solution of CoO as comparedto MgO and NiO. Published lattice stabilities for the wurtzite modifications of MO and the rock-salt form of ZnO are used to developfree energy–composition curves to relate the ease of solution to the driving force for partitioning. Reasons for the absence of single-phaserock-salt-based solid solutions are discussed. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Wurtzite; Spray pyrolysis; Gel decomposition; Metastability

1. Introduction

The chemical route to the synthesis of non-equilibriumceramic phases [1] offers many advantages, such as low tem-peratures and process simplicity, over the traditional physicalmethods such as evaporation and melt quenching [2]. Theappearance of disorder has been traditionally rationalised forstoichiometric compounds through atomistic calculations,e.g. CoO [2], or qualitative arguments based on the disor-dered occupation of sites, e.g., in compounds such as spinelsor YTaO4 [3]. More recently, it has become clear that ther-modynamic analyses that are based on driving forces for nu-cleation can provide a framework in which the selection ofthe phase and the maximum extent of solid solution may beunderstood for multi-component solutions [4]. The use offree energy functions, often extrapolated from high temper-ature data to the lower temperatures of chemical synthesis,has provided valuable insight into the composition domainsof appearance of phases such as extended solid solutions ofcubic and tetragonal ZrO2–MgO [5], rock-salt and spinelin MgO–MgAl2O4 [6]. Qualitative interpretation of resultsthrough schematic free energy andT0 curves has led to theunderstanding of the nucleation of disordered fluorite phasesin Pb(Zr,Ti)O3 [7] and Y2O3 [8] and of extended solid solu-tions and glass formation in ZrO2–Al2O3 [9], ZrO2–Y2O3[8] and Fe2O3–Al2O3 [10].

Recently, it was shown by the spray decomposition of ac-etate solutions in ZnO–CoO [11] that the solid solubility in

∗ Corresponding author. Tel.:+91-80-3344411, ext: 2259;fax: +91-80-3341683.

the wurtzite phase could be extended from the equilibriumvalue of less than 0.5% to nearly 70% at 500◦C. This pairof compounds is a member of a wider class of combina-tions of oxides with simple structures which pose the fol-lowing interesting problem that is partially addressed in thepresent work: if a homogeneous precursor has the option ofcrystallising into one of the two structures of equal com-plexity (wurtzite–rock-salt), what governs the temperatureand composition ranges in which only a single-phase formsand in which partitioning to a two-phase mixture must oc-cur? In the case of ZnO–CoO, the equilibrium solubility ofZnO in rock-salt is, at 12% [12], at least 25 times the sol-ubility of CoO in wurtzite at 600◦C. Nevertheless, it is thewurtzite field that is metastably extended by spray pyroly-sis up to∼70% CoO, while single-phase rock-salt solutionsdo not form even in compositions (e.g., ZnO–90% CoO)which lie in the single-phase part of the phase diagram. Ad-ditional features of interest in ZnO-based systems are theirapplications in varistors, sensors and, most recently, in bluelaser sources [13], as substrates that possess a good latticematch with GaN semiconductors [14] and the possibilityof band-gap engineering by solute additions in the wurtzitephase [15,16].

The present study explores the synthesis of solid solutionsin three binary systems: ZnO–MO, where M= Mg, Ni, Co.Both spray pyrolysis as well as gel decomposition routesare employed. The retention of metastability by both routesfollows from the imposition of kinetic constraints. In spraypyrolysis, dehydration and the early stages of decomposi-tion must be traversed quickly to avoid possible segregationdue to fractional precipitation or sequential decomposition

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Fig. 1. Binary equilibria between ZnO-rich wurtzite solid solutions andMO-rich rock-salt solutions showing solubility limits at 900◦C (from[12,17]).

of salts. Thus, rapid heating of fine droplets becomes essen-tial. Gellation permits the formation of rigid cationic net-works at low temperatures and is particularly attractive forthe fabrication of thin films and also, incidentally, when con-trolled atmospheres are needed to avoid valence changes incationic species such as cobalt [11].

2. Experimental procedure

The solid state portions of the phase diagrams which areschematically illustrated in Fig. 1 indicate significantly lowersolubility of MO in wurtzite than of ZnO in MO as shown inFig. 1 [12,17]. At 500◦C, the solubilities of MO in wurtziteare less than 0.5%, while they can be as high as 10 and25% in CoO and NiO. Compositions of ZnO–MO (M=Mg, Ni, Co) as shown in Table 1 were prepared by spray

Table 1Domains of phase separation

Composi-tion (mol%)

Temperature (◦C)

300 400 500 800

MgO20 Single-phase Single-phase Single-phase Two-phase25 Single-phase Single-phase Single-phase Two-phase30 Single-phase Two-phase Two-phase Two-phase35 Two-phase Two-phase Two-phase Two-phase80 Two-phase Two-phase Two-phase Two-phase90 Two-phase Two-phase Two-phase Two-phase

NiO20 Single-phase Single-phase Single-phase Two-phase25 Single-phase Single-phase Two-phase Two-phase30 Two-phase Two-phase Two-phase Two-phase

CoO40 Single-phase Single-phase Single-phase Two-phase60 Single-phase Single-phase Single-phase Two-phase70 Single-phase Two-phase Two-phase Two-phase90 Two-phase Two-phase Two-phase Two-phase

pyrolysis and gel decomposition with increasing amounts ofMO until phase separation was observed upon synthesis. Toexamine phase formation at the MO-rich end, compositionsof MgO and CoO with 10 and 20 mol% ZnO were synthe-sised by spray pyrolysis alone.

2.1. Spray pyrolysis

Aqueous solutions of metal acetates (100 g/l) weresprayed using compressed air (in the case of Mg and Ni)and nitrogen (for Co-containing compositions) on to ateflon-coated aluminium pan that was maintained at 300◦Cfor Mg and Ni acetates and at 200◦C for Co acetates. Thepan surface temperature was monitored by a thermocoupleand spraying was stopped when the temperature droppedby more than 20◦C since early experiments indicated thelikelihood of segregation when decomposition took place atlow temperatures.

2.2. Gelling

The citrate gelling route was employed in the follow-ing manner. Solutions of metal acetates to generate the re-quired oxide compositions were prepared to a concentrationof 100 g/l. Citric acid of 2 N was added drop by drop to thesolution of acetates till the pH came to 3, whereupon the so-lution temperature was raised to 80◦C at which it was heldfor 12 h to induce gel formation.

Heat treatment of the sprayed powders and the gelswas carried out at 300–800◦C in either air in the case ofZnO–MgO and ZnO–NiO, or in vacuum with Co metal act-ing as an oxygen getter in the case of ZnO–CoO to preventoxidation of Co to+3. The decomposition of the sprayedpowders and gels were studied in selected compositionsby thermogravimetric analysis (TGA) and infrared (IR) ab-sorption spectroscopy. The evolution of crystalline phaseswas followed by X-ray diffraction (XRD). The detection ofphase separation in ZnO–MgO poses particular problemsbecause of the low atomic scattering factor of Mg comparedto that of Zn. Calibration samples of mechanical mixturesof MgO and ZnO were used to establish the minimum de-tectability limit for the MgO-rich rock-salt solid solution inthe presence of wurtzite.

3. Results

The TGA of selected compositions is shown in Fig. 2. Thesprayed powders lose 8% of their weight by 350◦C with lit-tle weight loss beyond 500◦C. The gels contain significantlymore volatile matter and shed nearly 52% of their weightduring heat treatment up to 800◦C. IR spectra (Fig. 3) in-dicate three peaks at 1555, 1609 and 1425 cm−1 (Fig. 3(a))from the as-gelled sample of ZnO–20% MgO, which areknown to arise from acetates due to the stretching vibrationsof C–H and the coupling of stretching vibrations of C=O and

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Fig. 2. Weight loss by thermogravimetry of: (a) sprayed ZnO–20% MgO;(b) sprayed ZnO–20% NiO; (c) gelled ZnO–20% MgO. Heating rate was10◦C/min in nitrogen. Cobalt acetate precursors behave similarly to MgOand lose less than 1% beyond∼400◦C.

Fig. 3. IR spectra from gel-derived ZnO–20% MgO: (a) as gelled; (b)heated to 280◦C; (c) heated to 500◦C. The sample is still amorphous at280◦C. A spectrum from pure ZnO from acetate heated to 500◦C is shownin (d). Acetate groups (∗) in the as-gelled sample (a) are still present at280◦C in (b), but disappear by 500◦C (c). The Mg–O stretching peakdevelops at 1485 cm−1 (c), while additional peaks at 423 and 532 cm−1

correspond to Zn–O as well as Mg–O.

Fig. 4. XRD patterns for ZnO–20% MgO at: (a) 300◦C, (b) 500◦C, and(c) 800◦C for sprayed (2) and gelled (1) powders. The peak designatedR at ∼21◦ corresponds to 2 0 0 rock-salt and is used as a signature forphase separation.

C=C groups. Fig. 3(b) and (c) shows that acetate groups arestill present at 280◦C but disappear by 500◦C, while an addi-tional peak corresponding to the Mg–O stretching vibrationappears at 1485 cm−1. The peaks at 423 and 532 cm−1 canbe assigned to the stretching vibrations of Zn–O as also seenin the pure ZnO standard in Fig. 3(d). As-sprayed powdersof ZnO–MgO display a similar trend.

The XRD scans of heat treated oxides in ZnO–MgO areshown in Figs. 4 and 5. The appearance of the rock-salt2 0 0 peak at∼21◦ indicates the onset of phase separation.Table 1 summarises the single- and two-phase domains thatwere determined from similar XRD patterns in all three sys-tems. The wurtzite lattice parameters are known to be onlyweakly dependent on the dissolved rock-salt oxides [18].Coupled with the broad, low angle diffraction maxima thatare available, this poor sensitivity precluded a reliable deter-mination of any lattice parameter variation in the wurtzite

Fig. 5. XRD patterns for ZnO–30% MgO at: (a) 300◦C, (b) 400◦C, andfor ZnO–35% MgO at: (c) 300◦C. The 30% composition shows phaseseparation at 400◦C, while the 35% is phase separated at the sprayingtemperature of 300◦C. (2): Sprayed, (1): gelled.

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Table 2Intensity ratios for selected peaks for MgO–ZnO mixturesa

MgO (mol%) 30 20 10 5I200,R/I1010,W 0.31 0.20 0.14 0.09

a Signal is not detectable for the rock-salt phase below∼5%.

phase. Table 2, which summarises the intensity ratios ofselected diffraction maxima from mechanical mixtures ofMgO and ZnO, reveals that it is not possible to reliably de-tect<5 mol% MgO in ZnO.

4. Discussion

The formation of single-phase solid solutions displaystwo interesting features. The first is the large difference insolubility in ZnO–wurtzite (W) between CoO, on the onehand, and NiO/MgO, on the other. The second feature is theabsence of single-phase solid solutions based on rock-salt,despite the fact that some of the MO-rich compositions areclose to the equilibrium solubility limit, e.g., MgO–10%ZnO, i.e., even where the supersaturation is small. In systemsthat contain mixed oxide compounds of complex structures,it has been argued that the kinetics of growth and the inter-facial energy between crystal and precursor, both favour thenucleation and growth of disordered solutions, even whenthe complex phase is thermodynamically preferred. Thus,MgO with up to 40 mol% alumina forms as a disorderedrock-salt solid solution despite the lower free energy ofspinel [6]. Similar arguments may be used to explain theappearance of a defective fluorite structure in Pb(Zr,Ti)O3[7] and tantalates of yttrium and niobium [3] whose stableforms are cubic (perovskite) and monoclinic, respectively,or the preference for fluorite or bixbyite instead of crystalli-sation of the phase Y4Zr3O12 in Y2O3–ZrO2 [8], or eventhe formation ofg-alumina in preference toa-alumina. Inthe present case, however, the competing structures are bothsimple in their arrangements of both cations as well as an-ions and it is difficult to introduce structural complexity asa contributing factor.

Free energy curves of the relevant phases offer a route tounderstand the observed variations. In Fig. 6, the free en-ergies of the stable phases (R–MO and W–ZnO) are set tozero, while the values for the wurtzite forms of MO(M =Ni, Mg, Co) are taken from published thermodynamic anal-yses that are based on the measurements of the activity ofMO in equilibrium with those of the corresponding spinel,MAl 2O4 in which the M+2 ion occupies a tetrahedral site[18]. Direct measurement of the enthalpy of transformationfrom W to R has been reported only for CoO [19] and pro-vides a value similar to that obtained from activity measure-ments. For ZnO, high pressure data on the equilibrium be-tween W and R as a function of temperature [20] is used toestimate the free energy of the rock-salt form. First princi-ples calculations of the lattice energies using pair potentials

with added contributions from the octahedral site prefer-ence energy indicate a comparable trend for the enthalpy oftransformation in CoO and NiO (unpublished research) [2].Since the free energy curves are used only to identify the in-tersection compositions of W and R (e.g.,c1 for CoO–ZnOin Fig. 6) and since the addition of ideal configurational en-tropy of mixing does not alter these compositions, the solu-tion free energy is interpolated as straight lines between theend components. Thus, non-ideal contributions to the freeenergy of mixing are ignored.

One may visualise the formation of the oxide solid solu-tion as taking place from a higher free energy transient pre-cursor state (not shown). In systems such as Al2O3–ZrO2[9], this state may be identified with a glass. In Fig. 6,c1 represents the maximum extent of solution of CoO inwurtzite–ZnO if bulk free energy change is the factor thatdetermines the choice of a single-phase solid solution. Com-positions richer thanci should then crystallise as rock-salt.The variousci for either set of data clearly distinguish be-tween CoO and NiO/MgO: a simple consequence of thepoorer stability of the tetrahedral forms of MgO and NiO.(The activity data do not distinguish between the sphaleriteand wurtzite forms.)

The use of onlyci as an indicator of preference for Wor R is not entirely justified because experimental evidencein ZnO–MgO and ZnO–CoO reveals that the competition isnot between single-phase solid solutions, W and R, but be-tween W and a phase separated mixture of R and W. How-ever, the driving force for phase separation at a particularcomposition,1G∗ (Fig. 6) also increases asci decreases.Thus, even a competition between W and a phase separatedmixture of W and R (according to the common tangent ruleof equal chemical potentials in both phases) would also pre-dict a similar trend in the experimentally observed maxi-mum solubilities. Implicit in the above comparison is thatthe kinetics of phase separation is not substantially different

Fig. 6. Free energy–composition curves for W and R solid solutionsbetween ZnO and MO. The lines represent linear interpolations betweenend values that are obtained from the literature (see text). The free energiesof the stable phases are set to zero. The cross-over compositionsci (shownasc1 for CoO) do not change if ideal configurational entropy is included.1G∗ represents the driving force for partitioning from W to a two-phasemixture of W and R as prescribed by the common tangent rule.

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in the three binary systems and that the trend may be relatedlargely to the change in driving force.

The question still remains: why do the MO-rich composi-tions not crystallise as single-phase rock-salt even when thedriving forces are large? Or alternatively, if MO-rich com-positions display enough mobility (and lack of kinetic con-straint) to partition, why do the ZnO-rich compositions notbehave likewise. A hint of a possible answer comes fromthe strong preference for tetrahedral coordination that stemsfrom the precursor itself. It is known that the decompositionof the acetates of Co and Zn proceed via an intermediateoxy-acetate, Zn4O(CH3,CO2)6, in which Zn (or Co) is four-fold coordinated to oxygen [21–23]. Under exceptional cir-cumstances, even pure CoO may be synthesised as wurtzite[2,24], though we were unable to achieve this result either byspray pyrolysis or by the decomposition of the salt in a get-tered high vacuum. This prior tetrahedral coordination actsin a manner similar to a low liquid–solid interfacial energyin solidification and ensures that a Zn-rich wurtzite solidsolution always crystallises preferentially. The dependenceof coordination in the oxide on that in the precursor is wellknown from the various transformation sequences that yieldhexagonal or cubic anion packing in alumina from precur-sors such as boehmite, gibbsite, etc. The similarity of the re-sults in both methods of preparation is not too surprising inthat decomposition of an acetate is likely to be the final stepin both cases prior to the formation of the oxide. There hasbeen an earlier report of the formation of extended solutionsof MgO in W–ZnO by laser ablation [15]. XRD was used toinfer a maximum solubility limit of 35% MgO at∼600◦C.While this value is not far from the present one of 25%,our work does show that even 30% MgO solutions phaseseparate at 400◦C. It must be pointed out that these authorsused the relatively weak (12%) 2 2 2 peak from rock-salt asa signature of phase separation. The strongest peak, 2 0 0,was not available owing to the epitaxy between the film andthe (0 0 0 1)sapphire substrate. Our own results show thatthe detection of small amounts of R–MgO (<5%) in a ma-trix of W–ZnO is not easy even when the 2 0 0 reflection isused. It is therefore possible that the solubility of MgO wasoverestimated in the earlier work.

5. Conclusions

1. Large extensions of solid solubility have been producedin wurtzite–ZnO with additions of MgO (25%), NiO(20%) and CoO (70%) by low temperature decomposi-tion of acetate-derived precursors.

2. The relatively greater extent of stability of metastablesolid solutions containing CoO can be correlated with themagnitude of the driving force for phase separation andfollows from the greater stability of the wurtzite form ofCoO compared to that of NiO and MgO.

3. The absence of single-phase rock-salt solid solutions evenin ZnO-deficient compositions could be related to thestrong bias for tetrahedral coordination that is devel-oped during decomposition, possibly in an intermediateoxy-acetate phase.

Acknowledgements

Financial support for this work was provided by theIndo-US programme through the National Science Foun-dation — International Programmes via grant number9633039.

References

[1] F.F. Lange, Liquid precursors for ceramics: kinetically limitedcrystallization, spherical particles, fibres and thin films, in:L.L. Hench, J.K. West, D.R. Ulrich (Eds.), Proceedings of theUltrastructure Processing of Ceramics, Glasses, Composites, OrderedPolymers and Advanced Optical Materials, Orlando, FL, Wiley, NewYork, 1992, p. 611.

[2] R.W. Grimes, K.P.D. Lagerlof, J. Am. Ceram. Soc. 74 (1991) 270.[3] S.A. Mather, P.K. Davies, J. Am. Ceram. Soc. 78 (1995) 2737.[4] C.G. Levi, Acta Mater. 46 (1998) 787.[5] M.-L. Balmer, F.F. Lange, C.G. Levi, J. Am. Ceram. Soc. 75 (1992)

946.[6] T. Bhatia, K. Chattopadhyay, V. Jayaram, Mater. Sci. Eng. A 226–228

(1997) 930.[7] A.D. Polli, F.F. Lange, C.G. Levi, J. Am. Ceram. Soc. 83 (2000) 873.[8] V. Jayaram, M. DeGraef, C.G. Levi, Acta Metall. Mater. 42 (1994)

1829.[9] M.-L. Balmer, F.F. Lange, C.G. Levi, J. Am. Ceram. Soc. 77 (1994)

2069.[10] P.K. Narwankar, F.F. Lange, C.G. Levi, J. Am. Ceram. Soc. 80

(1997) 1684.[11] V. Jayaram, J. Rajkumar, B. Sirisha Rani, J. Am. Ceram. Soc. 82

(1999) 473.[12] C.H. Bates, W.B. White, R. Roy, J. Inorg. Nucl. Chem. 28 (1966)

397.[13] A. Ohtomo, M. Kawasaki, Y. Sakurai, I. Ohkubo, R. Shiroki, Y.

Yoshida, T. Yasuda, Y. Segawa, H. Koinuma, Mater. Sci. Eng. B 56(1998) 263.

[14] X.W. Sun, R.F. Xiao, H.S. Kwok, J. Appl. Phys. 84 (1998) 5776.[15] A. Ohtomo, M. Kawasaki, T. Koida, K. Masabuchi, H. Koinuma,

Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72(1998) 2466.

[16] A. Ohtomo, Y. Sakurai, I. Ohkubo, R. Shiroki, Y. Yoshida, T.Yasuda, Y. Segawa, M. Kawasaki, H. Koinuma, Presented at theSecond Symposium of Atomic Scale Surface and Interface Dynamics,Gakushuin University, Tokyo, Japan, 1998.

[17] S. Raghavan, J.P. Hajra, G.N.K. Iyengar, K.P. Abraham, Thermochim.Acta 89 (1991) 151.

[18] A. Navrotsky, A. Muan, J. Inorg. Nucl. Chem. 33 (1971) 35.[19] J. DiCarlo, A. Navrotsky, J. Am. Ceram. Soc. 76 (1993) 2465.[20] C.H. Bates, W.B. White, R. Roy, Science 137 (1962) 993.[21] H. Koyama, Y. Saito, Bull. Chem. Soc. Jpn. 24 (1954) 112.[22] J. Charalambous, R.G. Copperthwaite, S.W. Jeffs, D.E. Shaw, Inorg.

Chem. Acta 14 (1975) 53.[23] R.W. Grimes, A.N. Fitch, J. Mater. Chem. 1 (1991) 461.[24] M.J. Redman, E.G. Steward, Nature (London) 193 (1962) 867.