CERAMICS INTERNATIONAL, VoI 9. n. 2,1983 61

Inhibition Mechanism of the Anatase-Rutile Phase Transformation by Rare Earth Oxides

S. HISHITA, I. MUTOH*, K. KOUMOTO and H. YANAGIDA Department of Industrial Chemistry, Faculty of Engineering,

University of Tokyo, 7-3-1, Hongo, Bukyo-ku, Tokyo, 113, Japan *Department of Industrial Chemistry, Chiba Institute of Technology, 2-17-1, Tudanuma, Narashino-shi, 275, Japan

The effect of rare earth oxide additions on the anatase-rutile trasformation in doped Ti02 was investigated. Y203, La203, GeOz, Nd203, BmzOs, GdzO3, Tb40,, H0203, Dy203, Er203, Tm203 and Yb203, were found to suppress the transformation. The transformation process occurs in three stages. In the first stage, rare earth metal ions dissolve into interstitial sites in anatase. The resulting decrease in oxygen vacancies caused by the solid solution suppresses the nucleation of the transformation. In the second stage, the activation energy of the transformation was found to be 124 &3 Kcal/mol and 138 &4 Kcallmol for pure St’IStSSe and SnStSSe with 1 mOl% Dy203, reSp8CtiVdy. The transformation mechanism of this stage is not necessarily af- fected by additives. The final stage is affected by the amount of rare earth oxide-titania compound phase.

1 - INTRODUCTION

Titanium dioxide is known to have trhee crystalline polymorphs, anatase, brookite and rutile, although a fourth high pressure polymorph has recently been made and characterized’ ‘2. Most commercial processes, including precipitation from chloride or sulfate solutions, result in pro- duction of anatase, whereas rutile is required for many ap- plications. Additives which enhance the transformation of anatase to rutile are of industrial importance e.g. paint manifacture, ceramics and electronics. However anatase is under investigation for use as a catalyst for CO/Hz synthetic reactiona, alcohol dehydration 4, in a lithium/anatase cells, as an electrophoretic display materiale, etc.

The rate of the anatase-rutile transformation is very sen- sitive to reaction temperature, the amount and the type of ad- ditives, atmosphere, contact area of the powder, etc. The ef- fects of CuO, MnOz, Co0 and NiO on the transformation were investigated by lida and Ozaki7 and of Fez03 by Heald and WeissB. These additives accelerate transformation. Heald and Weiss proposed that acceleration was due to nuclei formed in Fe203. MacKanzie attributed the accelera- tion to an increase in the concentration of anion vacancies formed by reduction of foreign ionsg. The present authors have also suggested that the concentration of anoin vacan- cies was one of the most effective factors determining the rate of the transformationlo. Another mechanism, however, has been proposed by Suyama ad Katoll, who found that ad- dition of more than 20 mol% SiOl suppressed the transfor- mation remarkably; SiOz covered the surface of anatase par- ticles, inhibitmg nucleation of rutile with presumably took place at the contact between anatase particles.

The present study was performed in order to compare the relative effect of various rare earth oxide additives on the transformation and to discuss the mechanism of the transfor- mation, especially due to Dy203 addition.

2 - EXPERIMENTAL

Commercial anatase powder (Fuji Titanium Co., 99.98% purity) was used. Electron micrographs of the polyhedral powder indicated a mean diameter of 0.091 with a range of

0.05 to 0.14~. Samples were made by mixing anatase powders, in ethanol suspension, with 5 mol% of rare earth oxides shown in Table 1. These mechanically-mixed samples were dried in vacua.

To clarify the effect of the amount of additive, anatase samples with 0, 0.01, 0.1, 1 and 5 mol% Dy203 were prepared by adding nitric acid solution of Dy&O& to anatase. The resulting powders were decomposed by calcination at 500°C for 5 hours in air. These procedures did not affect the particle size of the anatase.

The mixtures were heated in silica boats. To analyze quantitatively the anatase-rutile transformation, powder dif- fraction patterns were recorded with an automatic X-ray spectrometer using filtred copper radiation, and the amounts of each determined by measuring the relative intensities of the anatase (101) peak (20=25.35”, CuKa) and the rutile (110) peak (20=27.46O, Cuk&. The analytical error, in- cluding other experimental errors,was about &3%. The lat- tice constants of anatase and rutile were also measured fro*m diffraction patterns using magnesium oxide (a0 = 4.21134A) as an internal standard to an approximate accuracy of h2.5 x 10-3percent.

3 - RESULTS AND DISCUSSION

3.1 Effect of additives on the extent of transformation

The experimental results at 870°C are shown in Fig. 1 in which effects of known inhibitors, Nb205” and SiOz ’ are also shown for comparison. The results indicate that rare earth oxides are better inhibitors.

Data for rare earth oxides at 870% are shown in Figs. 2 and 3. Except for La203, these curves are of sigmoid shape, similar to that of undoped anatase.

Figure 4 shows the temperature dependence of the ex- tent of transformation. Regardless of the nature of the ad- ditive, the transformation is slowed up to 950°C.

TABLE 1 - Additives for mechanical mixing samples.

Oxide Cation ( +. 3) radius Particle size*

(A) GLm)

0.65- 1.06 0.68 - 1.23 1.06-1.97

y203 1.05” La203 1.061 Ce02 1.034 NdD3 0.995 Sm203 0.964 G&03 0.936 Tb407 0.923 Dy203 0.908 Ho203 0.894 Er203 0.681 Tm203 0.869 Yb203 0.858

* 95% of particles falls in this range. ?? ?? Its coordination number is 8, and others 6.

0.41 - 0.79 1.15-1.85 1.10-1.59 1.05-1.62 1.05-1.62 0.93 - 1.80 1.25-1.53 1.16-1.84 1.25 - 2.07

62 P. HISHITA. I. MUTOH. K. KOUMOTO and H. YANAGIDA

00% *o 2 4 6 8 10

TIME{ hr) FIGURE 1 - Effects of additives on the anatase rutile transforma- tion at 870°C

?? - pureanatase a - 5 mol% 902 mixed anatase 0 - 5 mol% NbzOs mixed anatase a - 5 mol% Ho203 mixed anatase

and 5 mol% Dy,O, mixed anatase

,

0.0 - - 0 10 20 30 40 50 60 70

TIMEthr)

FIGURE 2 - Effects of rare earth oxides addition on the anatase- rutile transformation at 870°C

0 - 5 mol% Ybz03 mixed anatase o - 5 mol% Tb407 mixed anatase ?? - 5 mol% Ho203 mixed anatase ??- 5 mol% Ndz03 mixed anatase

cf. A - pureanatase

Kinetic studies have been made by previous in- vestigators. Czandera et all3 found the reaction to be second order, while first-order reaction was reported by Raol4 and Sullivan and Colemanls. A contracting reaction front was observed to be a rate-determining step by Heald and Weiss* and by MacKenzieg. The Avrami equation was successfully applied to the reaction by Suzuki and Tukudal6. Shannon and PaskIT, however, reported that the models examined such as first-order, contracting reaction interface, rapid

0 10 20 30 40 50 60 70 TIME( hr)

FIGURE 3 - Effects of rare earth oxides addition on the anatase- rutile transformation at 870°C

0 - 5mol%Smz03 ??- 5 mol% Dyz03 ?? - 5 mol% YzOz ??- 5 mol% LazO,

1

850 900 950 1000 1050 TEMPERATUREtot,)

FIGURE 4 - Temperature dependence of the anatase-rutile transformation. Heating time is 1 hr.

A - pure anatase 0 - 5 mol% H0203 mixed anatase ??- 5 mol% Tm203 mixed anatase

@ - 5 mol% Er20p mixed anatase A - 5 mol% Nd2Os mixed anatase a - 5 mol% Sm20s mixed anatase ?? - 5 mol% 0~20~ mixed anatase

growth of nuclei and overlapping of nuclei models explained the kinetic data equally well.

Our studies show that in the range of fractional transfor- mation between 0.3 to 0.7, the data fit all of above expres- sions equally well exept at < 0.2 or >0.8.

The Avrami equation, cx = 1 - exp (- bt”), (Eq. [l]), where (Y is the extent of transformation in time, t; b and n are con- stants, was adopted to compare the relative effect of the ad- ditives. Differentiating Eq. [I], we obtain

INHIBITION MECHANISM OF THE ANATASE-RUTILE PHASE TRASFORMATION BY RARE EARTH OXIDES 63

1

0

-1 --2

I

E -2 I

f-3

-4

. 0 1 2 3 4 5

Ln(TIME) (hr) FIGURE 5 - Plots of In (-In (1 -a)) as function of In t for various anatase samples at 870°C.

A - pure anatase 0 - 5 mol% Ybz03 mixed anatase ??- 5 mol% Tb407 mixed anatase ?? - 5 mol% Ho203 mixed anatase ??- 5 mol% Ndz03 mixed anatase A - 5 mol% CeO2 mixed anatase v - 5 mol% Gd203 mixed anatase

da _ ,,bt’” Jr-

From Eqs. [1] and [2], we have

“(1 - o) PI

g = nb”“[ - /n( 1 - a)](“- I)“‘( 1 - (y)

= k[ - /n(l - a)]‘” - I)“‘(1 - o) 131

where k = nb”“, and k can be defined as the rate constant of Eq. [3]. From Eq. [l], we also have

/n[ - /n(l - a)] = nlnt + Inb [41

As shown in Figs. 5 and 6, Eq. [4] describes experimental data reasonably well in the range 0.05 5 (Y I 0.7. As far as the shape of the curve is concerned, there is not a large dif- ference among all samples exept for a La203-doped sam- ple. These results suggest that the transformation process consists of three stages:

First stage (o 5 0.05) /n[ - /n(l - a)] 5 - 3.0; This stage did not occur for pure anatase. Second stage: CY is from 0.05 to 0.62 for pure anatase i.e.

- 3.Os/n[ -/n(l -cY)]ZG -0.02.

Final stage: cr is more than 0.62 for anatase i.e.

/n[ - /n(l - a)] 2 - 0.02.

From the results of lattice constant measurements, it was found that ontly c axis of anatase changes with CW; both a and c axes of rutile produced from anatase by transformation were little changed, as shown in Fig. 7. The c axis of pure ana- tase expands linearly as (Y increases; least squares determi- nation fits these data to

C=2.0172.cr+9.5124(A) [51

The reason for c aXiS expansion in pure anatase is not

1

0

-1

z 0 I -2

+

i-3

-4

0 1 2 3 4 5 LntTIME) (hr)

FIGURE 6 - Plots of In (-In (1 - (Y)) as function of In t for various anatase samples at 870°C.

0 - 5 mol% SmzOl mixed anatase ??- 5 mol% DyzOl mixed anatase . - 5 mol% Yz03 mixed anatase ??- 5 mol% La203 mixed anatase A - 5 mol% Tmz03 mixed anatase T - 5 mol% Er203 mixed anatase

0 .

.

anatase C axis

1 T

.*0

rutile A axis 1

0.0 I anatase A axis 1

T 1

??.0 I Wile C axis i

T c I

9.0 0.1 a2 0.3 0.4 a5 0.6 0.7 EXTENT OF TRANSFORMATION ( m 1

FIGURE 7 - Lattice constant changes 87O’C. ?? - pureanatase 0 - 5 mol% YbzOj mixed anatase ??- 5 mol% HozOs mixed anatase A - 5 mol% Dy~0, mixed anatase v - 5 mol% LazOl mixed anatase

known: it may be attributed to the structural stabilization of anatase lattice. A similar behaviour of the anatase c axis was reported by Horn et a/18.

The c axes of mixed anatases are larger, within accuran- cy of the experiment, at the earlier stage of the transforma-

64

Tl T2

TIME (arbitrary) FIGURE 8 - Schematical diagram for new phase growth in 5 mol% NdzOz mixed anatase sample

0.8

0.6

0.4

0.2 .

““a _._ 0 10 20 30 40 50

TIME(hr) FIGURE 9 - Effects of 0~203 amount on the anatase-rutile transformation at 870°C. Additives were added by a decomposi- tion method.

0 - 0 mol% DyzO, mixed anatase ??- 0.01 mol% Dyz03 mixed anatase ?? - 0.1 mol% DyzO, mixed anatase ??- 1 mol% Dyz03 mixed anatase and 5 mol% Dyz03 mixed anatase

tion. ((~<0.2), than that of pure anatase, which indicates that rare earth ions diffuse into the anatase structure in the first stage of transformation.

After an induction period, rare earth titanate phases were found in most of the samples. The formation of compound phases is schematically shown in Fig. 8 for Nd203 - doped samples, where NdTiO3 and Nd2Ti207 developed. In a La203 - doped sample, La2Ti20, and/or La203 - 3Ti01.9 were formed in 3 hr at 870%. This rapid formation of a compound phase and the exceptional shape of kinetic data may be caused by the tendency of La203 to form La(OH)3 in air, which is stable up to 450% and then form a metastable phaselQ: the dehydration of La(OH)S or the reorganization of the metastable phase may accelerate the diffusion of La3’ ions into anatase. In a Yb203 -doped sample, traces of a compound - possibly Yb2Ti20, - were found when heated for 51 hrs at 870%. The inhibition effect may be partly related to the ease of forming compounds.

P. HISHITA, I. MUTOH, K. KOUMOTO and H. YANAGIDA

0 1 2 3 4 5 Ln(TIME) (hr) .

FIGURE 10 - Plots of In (- In (1 -a)) as a function of In t for sam- ple with various content of Dy203 at 870°C.

0 - 0 mol% DvzOl mixed anatase •I - 0.01 mol6D&03 mixed anatase ?? - 0.1 mol% DvzOq mixed anatase s - 1 mol% Dy& mixed anatase and 5 mol% Dy203 mixed anatase

3.2 Effect of the amount of Dy20j on the transformation

The results are shown in Fig. 9. The inhibition effect was saturated with more than 1 mol% addition. It was also found that Dy20~ added by the decomposition method were more suppressive than that added by mechanical mixing. The transformation mechanism, however, seems not to be changed by the difference in the methods of addition, since the extent of transformation analyzed by Eq. [4] showed similar results in both cases, as shown in Fig. 10.

The particle size of Dy20S powder obtained by the decomposition method, calculated according to the Scher- rer’s equation, was about 0.9pm. The large inhibition effects in this sample may be due to fineness of the 0~203 particles. A compound, Dy2Ti207, occured at an earlier stage of the transformation than in the mechanically mixed sample.

The temperature dependence of the transformation is shown in Fig. 11. Figure 12 shows the results of lattice cos- tant measurements. The a axis of anatase and both a and c axes of rutile were not changed. The c axis of anatase changed similary to that for the mechanically mixed sample; its behaviour does not depend on the temperature or amount of i&03, but upon the extent of transformation.

Rate constants for the second step, shown in Table 2, were used to construct Arrehenius plots (Fig. 13) for pure anatase and for a decomposed sample with 1 mol% Dy203. The activation energy was 124 f 3 KcaVmol for pure anatase and 138+4 Kcallmol for 1 mol% 0~20~ - mixed sample, in agreement with those reported by previous in- vestigators*,l3,14,16,17. From their similar behavior judged by lattice constant changes and activation energies, pure and 1 mol% Dy203 - doped anatase are considered to have the same transformation mechanisms at the second stage.

The effect of atmosphere on the transformation is shown in Table 3. These results, showing the transformation of Dy20~ - doped sample to be suppressed in argon relative to oxygen, are not in agreement with the results for pure anatase obtained by Shannon 20 and MacKenzie*‘. However, for pure anatase it has been well known that the transforma- tion is enhanced in a reducing atmosphere because of for- matiqn of anion vacancies, which probably serve as nuclea-

lNHlSlTlON MECHANISM OF THE ANATASE-RUTILE PHASE TRASFORMATION BY RARE EARTH OXIDES 65

1

0

-1 z=z 3

k-2 + I

=-3

-4

0 1 2 3 4 5

Ln(TIME 1 (hr)

FIGURE 11 - Plots of In (- In (1 -a)) as a function of In t for pure and 1 mol% DyzOs mixed anatase samples at various temperatu- re. pure anatase: 1 mot% Dy203 mixed anatase: v - at830°C A - at 87OOC A - at 850°C a - at900°C ?? - at870°C ?? - at985OC 0 - at905OC

TABLE 2 - Values of n, band k at different temperatures.

sample constant 830

Temperature (OC) 850 870 905

pure :

0.93 1.08 1.19 1.56 anatase 0.024 0.034 0.125 0.211

k( = nblln) 0.017 0.046 0.207 0.572

sample constant Temperature (“C)

670 900 965 1 mol%

DY203 “b

1.20 1.19 1.67 3.29x 10-%.40x 10-45.79x 10-z

z& k(=nbl/n) 1.46x10~33.47x10-34.05x10-’

TABLE 3 - Effect of atmosphere on the anatase-Wile transfor- mation. Conditions: 913OC for 48hrs., gas flow 200ml/min., 1 mol% Dy203 mixed anatase by decomposition.

atmosphere

Ar ambient

air 02

extent of transformation (a)

0.054

0.066 0.116

tion sites for the transformation*v9v*0~*l. The formation of ox- ygen vacancies is expressed as:

00*=V;+1/202uj+2e I81

where Oo’denotes O2 - ion at normal oxygen site, V;; is an ox- ygen vacancy and e’ is a conduction electron.

In the system Ti02 - Dy203, the solution behaviour may be expressed by the following equations,

(a) --- substitutional solid solution

Dy203 + l/2 02 (g) + 2 e’ = 2 Dy+i + 4 0;

b) --- interstitiali solid solution

[71

9.54 [

_ 4.60 “4

4.59 Am.& ti . .

z 4 G

4.50

8

I

s

!j 3.79

3.66 i

c4PYIm.e u+ 00 3.76

2.97

2.96 I Ama* e . .

2.95 -I-

. . anatase C axis

. .

Wile A axis

??? ?

anatase A axis

00

Mile C axis

0.0 0.l 0.2 0.3 0.4 0.5 0.6 0.7 EXTENT OF TRANSFORMATION (0 )

FIGURE 12 - Lattice constant changes for various samples. ?? - pure anatase at 870% A - 0.1 mol% Dy~03 mixed anatase at 87OOC ??- 1 mol% DyzO3 mixed anatase at 87OOC v - 5 mol% Dy20, mixed anatase at 870°C 0 - 1 IIIOl% Dy203 mixed anataSe at 9oo”c

0

-1

-2

-3 r f

-4

-5

-6

-7;

100 950 900 650 (“C)

8.0 0.2 0.4 6.6 6.6 9.0 9. 10000/T (K)

2

Figure 13 - Arrhenius plots for pure and 1 mol% DyzOs mixed sample

?? - pure anatase 0 - 1 md% DyZo3 mixed anatase

Dy203 = 2 Dyi.. + A 02 (g) + 6 e’ 2

PI

where Dyii denotes Dy3+ site, and Dyi.. is Dy3+ ion at inter- stitial site. lf doping of Dy203 forms Dy+i according to Eq. [7], the electrical conductivity of the sample should decrease due to the decrease in the concentration of electrons. If do-

66 P. HISHITA. I. MUTOH, K. KOUMOTO and H. YANAGIDA

ping of Dy203 forms Dy;., according to Eq [S], the conductivi- ty would increase due to an increase in the concentration of introduced electrons.

TABLE 4 - Electrical conductivity of anatase at 840% in ambient air.

sample conductivit Y relative density apparent density (Q-‘-cm- ) PM (g-cm _ ‘)

pure anatase 2.62x 1om4 66.1 2.579

1 mol% Dy203 mixed anatase 4.79x 10-j 55.0 2.145

cf. pure DyzOj

5.62~10-~ 51.2 4.18

Table 4 shows the electrical conductivity measured by DC four-probe method at 840% in air. The sample mixed with 1 mol% DyzOj has a conductivity one order of magnitude larger than that of pure sample. This suggests that the behaviour of DyzOj in anatase is expressed by Eq. [8]. The effects of DyzOJ in anatase is expressed by Eq. [8]. The effects of Dy,03 on the transformation can be explained by assuming Eq. [8]: the concentration of electrons in- creases with increasing Dyi.. concentration (Eq. [8]), so the equilibrium of Eq. [6] shifts to left and the concentration of oxygen vacancies, which are nucleation sites for the transformation, decreases. In oxygen rich atmospheres, this equilibrium shifts to left and the concentration of Dyi’. con- seguently decreases. Thus the effect of inhibition is small in oxygen relative to argon.

3.3 Inhibition mechanism of the anatase-rutile transfor- mation

It has been reported that the transformation proceeds from the surface towards the centeP. From the present ki- netic studies, the transformation process can be divided into three stages, and the inhibition mechanism explained as fol- lows:

In the first stage, rare earth oxides are dissolved in the anatase lattice (Eq. 181). An increase in the concentration of conduction electrons decreases the concentration of oxygen vacancies. Thus the number of nucleation sites is decreas- ed, and the transformation inhibited.

The cation radius of the solute (shows in Table I) on the solid solution formation process does not appear from Figs. 2 and 3 or Figs. 5 and 6 to be important. Differences in behaviour may be due to the different size of solute particles.

In the second stage, the growth of nuclei - both rutile and rare earth titanate - occur from the surface of anatase particles: The latter depends on the concentration of dopant. The,rate of the nucleation and growth depends on the con- centration of oxygen vacancies. The activation energy and the way in which the lattice constant changes are, however, the same as for pure anatase. Thus, in this stage the transformation proceeds by the same mechanism as for pure anat se.

lathe final stage the number of nucleation sites at the surface of the remaining anatase particles decreases because of the accumulation of product phases; The rate of transformation is increasingly governed by the nucleation and growth at other sites, possibly in the interior of anatase particles. For pure anatase, accumulating rutile decreases the number of nucleation sites, but for samples doped with rare earth oxides, rutile and rare earth titanates should decrease it. The fact that the extent of conversion at the tran- sition between the second and final stages is smaller in the doped sample than in the pure sample, supports the above scheme. The difference in the extent of transtormation at the transition may in part be due to differences in the rate of for- ming titanate compounds.

4 - CONCLUSIONS

1. The anatase-rutile transformation is suppressed by the addition of rare earth oxides - YZ03, La203, Ce02, Nd203, SmzOj, Gd203, Ho203, Er203, Tm203, YbzOs. 2. The transformation process can be divided into three stages. In the first, or nucleation, stage rare earth ions dissolve, occupying interstitial sites of anatase lattice, and thereby decreasing the concentration of anion vacancies which act as sites for nucleation. 3. In the second stage, the activation energy is 124&3 Kcal/mol and 138 f 4 Kcallmol for pure anatase and 1 mol% Dy20s mixed anatase, respectively. The transformation mechanism for doped samples is the same as for undoped anatase. 4. In the final stage, the rate of the transformation decreases. The decrease in the transformation rate is pro- bably due to a change in nucleation sites from surface to in- ternal nucleation and also depends on the amount of rutile and compound phases formed.

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Received July 9,1962; final text received November 15, 1962.