C. GLYCOTHERMAL SYNTHESIS OF MIXED METAL OXIDES

Solvothermal Synthesis



47



by the glycothermal method. GdAG (metastable phase) has been prepared by

many researchers, but synthesis of SmAG and RuAG has never been reported by

any other methods.

Hydrothermal synthesis of single-phase REAG requires higher temperatures (350 to 600°C) and pressures (70–175 MPa),153,154 although Mill' reported

that the lower temperature limit for the formation of YAG was 280°C.153 He

also reported that with an increase in the ionic size of the RE element, the

lower temperature limit increased. The REAG with the largest RE ion size

that has been thus far prepared hydrothermally is TbAG,153 and it was reported

that 420 to 450°C was required for the formation of this garnet. Therefore

there seems to be no possibility that SmAG and EuAG can be prepared by the

hydrothermal method, because the ionic size of these elements is much larger

than that of Tb.

2. Rare-Earth (Nd-Lu) Gallium Garnets

The reaction of RE (Nd-Lu) acetates with Ga(acac)3 in 1,4-BG at 300°C yielded

the corresponding RE gallium garnets (REGGs).21,156 The garnet phases were

reported to be thermodynamically stable for all the RE elements from samarium

to lutetium,157 and all of the stable REGGs were prepared by the glycothermal

method. The reaction at 270°C also gave phase pure REGGs for Sm-Lu, but

the reactions at 250°C gave amorphous products except for the reaction of

yttrium, which gave yttrium gallium garnet (YGG). Hydrothermal reaction of

Ga(acac)3 with RE acetate under conditions identical to the glycothermal reaction (except reaction medium) gave -Ga2O3 together with a small amount of

the garnet phase.21

The reaction of praseodymium and cerium acetates with Ga(acac)3 gave

RECO3(OH) as the sole crystalline product. When the reaction was carried out

in the presence of gadolinium gallium garnet (GGG) seed crystals, garnets were

crystallized in spite of the fact that unit cell parameters of these garnets are much

larger than those of GGG.21 These results suggest that under glycothermal conditions, nucleation is the most difficult process, and that once nucleation takes

place, crystal growth proceeds easily.

The particles of gallium garnets with large RE ions are spherical (0.5–2 µm)

and the surface of the particles is smooth. On the other hand, the surface of the

particles (0.1–0.3 µm) of the gallium garnets of terbium and RE elements having

ionic sizes smaller than terbium is rough, with apparent polycrystalline outlines.

However, high-resolution images of the latter type of particles show that a whole

particle is covered with a single lattice fringe, indicating that each particle is a

single crystal grown from only one nucleus. The authors concluded that the latter

type of morphology is formed by quite rapid crystal growth.21

Monodispersed particles are formed for garnet with smaller RE ions.21 Monodispersed particles can be prepared if a burst of nuclei is formed at the early

stage of the reaction and if nucleation does not take place during the crystal

growth stage.158 Once nucleation occurs in the glycothermal synthesis of gallium

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Chemical Processing of Ceramics, Second Edition



garnets, the concentration level decreases, which is determined by the balance

between the dissolution rate and rate of consumption of the reactants in the

solution by crystal growth. For the small RE ions, crystals grow rapidly, and

therefore the concentration of the reactants becomes low, prohibiting nucleation

during the crystal growth stage.

3. Metastable Hexagonal REFeO3

The reactions of RE acetates with Fe(acac)3 yielded three different types of the

product, depending on the ionic size of the RE element. For Nd-Gd, the product

was RE(CH3COO)O, and no binary oxide was formed. The product obtained

from Er-Lu acetate was a novel phase of REFeO3 having a hexagonal crystal

system (ao = 6.06, co = 11.74).159 The hexagonal phase was also formed in the

reaction at 220°C, but the product had quite a low crystallinity. The product is

isomorphous to the hexagonal YMnO3 (ao = 6.125, co = 11.41, P63cm).160 In

the structure, iron atoms are in trigonal bipyramidal coordination surrounded

by five oxygen atoms. Hydrothermal reaction of a mixture of Fe(acac)3 with

RE acetate yields Fe2O3 (hematite) together with an amorphous RE species.159,161 In the glycothermal reaction, metal alkoxide (glycoxide) or acetylacetonate is a starting material or may be formed as an intermediate, and the reaction

proceeds by thermal decomposition of these alkoxides (glycoxides) instead of

the hydrolysis of the alkoxide process, which usually yields amorphous products.

In the alkoxide process, part of the free energy of the starting materials is

consumed in the hydrolysis stage, whereas in the glycothermal reaction, instability of the intermediate phase gives a large driving force to product formation.

Therefore crystal growth proceeds rapidly and metastable phases can be formed

by the glycothermal reaction.

4. Other Mixed Oxides

Glycothermal reaction of two starting materials gives various crystalline mixed

oxides, and some of the mixed oxides synthesized by the glycothermal method

are summarized in Table 2.3.8,99,142,144,159,162,163 According to our working hypothesis, the reaction between metaoxo anion and metal cation is expected. In other

words, the glycothermal synthesis of mixed oxides can be regarded as an acidbase reaction. Metal ions of highly electronegative elements form acidic oxides,

and the hydroxyl groups in the coordination sites of these metal ions can be

easily deprotonated, yielding metaoxo ions. On the other hand, metal cations

with elements that are less electronegative form basic oxide, and the hydroxyl

groups in the coordination sites of these metal ions can be liberated, yielding

hydroxide ion and metal cation. As shown in Table 2.3, combining these two

types of elements gives crystalline oxides. Although combining titanium and

RE ions resulted in the formation of amorphous products, the glycothermal

reaction of each starting material yields a crystalline product. Therefore, in the



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Solvothermal Synthesis



49



TABLE 2.3

Mixed Oxide Formed by Glycothermal Reactions

Metal Cation that Forms Basic Oxide

Acidic

Oxide

P2O5

Nb2O5

Al2O3

Ga2O3

Fe2O3

TiO2

Ta2O5



RE3+



Zn2+



REPO4

RE3NbO7

RE3Al5O12

RE3Ga5O12

REFeO3

Amorphous

Unknown



Zn3(PO4)2

ZnNb2O6

ZnAl2O4

ZnGa2O4

ZnFe2O4

Zn2TiO4

Amorphous



Ca2+

Ca5(OH)(PO4)3

CaNb2O6



Li+



Ba2+



LiNbO3



LiTi2O4



BaTiO3



Ca3Ta2O7



amorphous product, titanium and RE ions somehow strongly influenced each

other.138

For another possible reaction mechanism for the formation of mixed oxides,

binary alkoxide (glycoxide) may be formed prior to the formation of the mixed

oxide. In fact, various binary glycoxides have been prepared and their crystal

structures have been elucidated.164–166 Formation of BaTiO3 by the glycothermal

reaction may be explained by this mechanism,167 because Ba-Ti binary alkoxide

is well established.166,168 However, formation of garnet phases (Sections III.C.1

through III.C.3) cannot be explained by this mechanism because addition of seed

crystals, in some cases, gives a product with a chemical composition completely

different from that of the product obtained without the addition of the seed

crystals.

Besides acid-base chemistry, combining two elements that have similar properties sometimes gives mixed oxides. An example is formation of -Al2O3-Ga2O3

by the reaction of aluminum alkoxide and gallium acetylacetonate.169 A linear

relationship between the lattice parameter of the solid solution and the composition of the two starting materials is observed.169,170 Since the products have

particle sizes in nano scale with quite large surface areas, reaction mechanisms

are not yet fully elucidated, but statistical decomposition of the intermediates

seems to be most plausible.

Some other important properties for the formation of mixed oxides are summarized below:

•

•



In most cases, the particles of the glycothermally prepared mixed oxide

are spherical and each particle is a single crystal grown from a nucleus.

The nucleation step is the most difficult process; addition of seed

crystals has a significant effect on product formation. When nucleation

takes place, crystal growth proceeds quite rapidly.161



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Chemical Processing of Ceramics, Second Edition



•



•



•

•

•

•



Only one mixed oxide is crystallized for the system where many oxide

phases are present in the phase diagram, and the mixed oxide can be

crystallized from a starting composition far from the stoichiometric

one.

Addition of a small amount of water (up to 5% by volume) usually

facilitates the reaction, but larger amounts of water may disturb the

formation of single-phase product by the formation of stable intermediates.

The use of ethylene glycol usually yields amorphous products (see

Section III.C.5).171,172

The particle size of the mixed oxide is usually larger than simple oxide

prepared by the glycothermal method.

When product with a large crystal size is formed, the product particles

contain a significant number of crystal defects.

The surface of the product particles is covered with organic moieties

attached through covalent bonding. The organic moieties can be eliminated by heat treatment at 250 to 300°C in an air flow.



5. Reaction in Ethylene Glycol

The reaction of metal alkoxides in ethylene glycol at 250 to 300°C usually yields

amorphous products.171,172 This is because cleavage of the C–O bond in the

glycoxide is difficult because of the inductive effect of the intramolecular

hydroxyl group. Metal cations are fixed in the networks of the gel structure of

ethylene glycol molecules. From the amorphous product, obtained by the glycothermal reaction of two suitable starting materials, mixed oxide is crystallized at

low temperature.171,172 For example, the reaction of aluminum isopropoxide with

yttrium acetate (Y/Al = 2) yields an amorphous product. By calcination of the

product, single-phase monoclinic yttrium aluminum oxide (Y4Al2O9) was crystallized at relatively low temperature.172 From a mixture of the same starting

materials with stoichiometric composition for garnet (Y/Al = 3/5), YAG was

crystallized at 920°C through the intermediate formation of hexagonal

YAlO3.171,172 This crystallization behavior is essentially identical with that of the

gel derived by hydrolysis of yttrium aluminum double alkoxide.173

Although solvothermal reaction in ethylene glycol usually yields amorphous

products, addition of a small amount of water may give crystalline product. Thus

Kominami et al.174 reported that solvothermal treatment of TiO(acac)2 in ethylene

glycol in the presence of sodium laurate and a small amount of water at 300°C

yielded microcrystalline brookite having an average size of 14 nm × 67 nm

without contamination of other TiO2 phases.



D. CRYSTALLIZATION



OF



AMORPHOUS STARTING MATERIALS



The starting materials are prepared by the sol-gel method175–181 or the (co)precipitation method,182–186 and the precursors are solvothermally treated to crystallize



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Solvothermal Synthesis



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the products. Since hydrous gel is highly hydrophilic, solvents miscible with

water are usually favored, and lower alcohols and glycols are usually used.

Ethylenediamine was also used.181 Inert organic solvents together with a surfactant

to disperse the precursor gel may also work,183 and may control the morphology

of the agglomerated particles. Solvothermal crystallization of sonochemically

prepared hydrous yttrium-stabilized zirconia (YSZ) colloid was also reported.187

A semicontinuous process for BaTiO3 synthesis was reported by Bocquet et al.176

The first step is the hydrolysis of the alkoxide BaTi(O-iPr)6 in isopropanol at 100

to 200°C. The second step is a thermal treatment of the formed solids under the

supercritical state of the solvent.

An important point is that the precursor gels thus prepared contain significant amounts of water, even if the gels are dried by some suitable method.

One of the weak points of solvothermal crystallization may be difficulty in

controlling the water content in the precursor gel. Water facilitates hydrothermal

crystallization of the precursor gel, and therefore the essential chemistry here

may be hydrothermal. Actually, solvothermal crystallization usually requires

higher temperatures and more prolonged reaction times than hydrothermal

crystallization.175 Moreover, in the solvothermal crystallization of stabilized

zirconia, the presence of an adequate amount of water was reported to be so

critical in dissolving the oxide powder that no crystallization occurred in absolute alcohol.184

Solvothermal crystallization usually produces products with smaller sizes

than those obtained by the hydrothermal method. The formation of polymorphs that cannot be obtained by the hydrothermal method is frequently

reported: PbTiO 3 with pyrochlore structure/perovskite PbTiO 3, 175 cubic

BaTiO3/tetragonal-phase BaTiO3,176,178 cubic ZrO2/tetragonal ZrO2.177 Formation of these phases may be due to the smaller crystallite size of the solvothermal products.

For the hydrothermal crystallization of amorphous gel, the dissolutionrecrystallization mechanism usually, although not always, takes place in which

dissolution of amorphous particles occurs followed by nucleation of the product

in the solution (homogeneous nucleation) and crystal growth. Ostwald ripening

occurs when the product has enough solubility. For solvothermal crystallization,

similar dissolution-recrystallization mechanisms are frequently reported to

occur.175,178,183,184 However, since a limited amount of water is present in the

reaction system, microscopic chemistry may differ from that of hydrothermal

chemistry. Water is adsorbed on the surface of the amorphous particles and

dissolves a part of the surface having higher energies. Water molecules transfer

the solute species to other parts of the particle surface that have lower energies,

which can act as the nucleation site of the product (heterogeneous crystallization). Crystal growth takes place by diffusion of the component with the aid

of adsorbed water, finally converting whole amorphous particles into crystals.

This mechanism is proposed because solvothermal crystallization of the amorphous precursors usually leads to nanocrystals, in spite of the fact that nucleation



© 2005 by Taylor & Francis Group, LLC