High-resolution X-ray diffraction study of the cubic-to-tetragonal transition in BaTiO3

Solid State Communications, Printed in Great Britain.

HIGH-RESOLUTION

Vol.62,No.lO,

X-RAY DIFFRACTION

pp.691-693,

0038-1098/87 $3,00 + .00 © ]987 Pergamon Journals

STUDY OF THE CUBIC-TO-TETRAGONAL

K.Irie #, M.Shiono * , H.Nakemura, Department

1987

of Physics, ( Received

N.Ohnlshl

Kyushu University,

TRANSITION

IN BaTiO 3

and A.Okazski

Fukuoka 812, Japan

24 January 1987 by H.Kamimura

)

A drastic change is found in the diffraction pattern of a single-crystal specimen of B~TiO 3. At and Just below the transition temperature, the original single peak of 004 of the tetragonal phase splits or collapses into a group of small peaks. They appear around the positions of 004 and 400, and probably correspond to different orientations of grains and domains. The positions and relative intensities of the peaks randomly vary with time and temperature. These features are observed for both heating and cooling processes.

The cubic-to-tetragonal transition in BaTiO 3 is a paraelectric-to-ferroelectric transition of first order, and has been extensively studied since 1940's. It is commonly known that the structure, and hence the lattice constant, changes discontinuously at the transition temperature T c. The temperature range of the twophase coexistence and the width of the thermal hysteresis have been reported by Kay I , Shebanov 2 and others; however, their results do not agree with one another. Moreover, X-ray diffraction patterns of single-crystal specimens as a function of temperature around T c have not been reported in a systematic manner. One of the present authors and the colleagues 3'~ have developed the high-angle doublecrystal X-ray diffractometry (HADOX) and applied to the study of the structural phase transitions in SrTi035'6 and KMnF37'8. These transitions are second order or nearly second order; the HADOX has been useful with its high sensitivity to a change in the lattice constant. In the transition in BaTiOj, the change is so large that the conventional X-ray diffraction technique might have an enough resolution to observe the detail of the transition. By the present authors' experience, however, the high sensitivity and resolution of HADOX have always offered unexpected new information. The present experiment was undertaken in these circumstances to trace the first-order structural phase transition in BaTiO 3 for the first time with a high resolution. The HADOX diffractometer was coupled with a microcomputer. Monochromatic FeKe I beams were obtained with 004, at @ =82.5 ° , of SrTiO 3 at 300.00 ± 0.01 K. The specimen crystal was a butterfly BaTiO 3 crystal (Semi Elements Co., USA) with a size of 4 x 5 x 0 . 5 mmJ; the largest surface was (001). The penetration depth, the inverse of the linear absorption coefficient, of FeK~ I to the specimen crystal was about 2.1 ~m. The specimen was mounted in a thermostat; the temperature was raised by electrically

heating. A series of measurements at constant temperatures, with a stability better than 0.01 K, were ~ade in a temperature range 290 to 420 K, for several thermal cycles, for 004 and 400 at O ~ 74 ° . The angle is not satisfactorily high, but highest for easily available characteristic X-rays for 00£ and bOO; measurements at higher angles can be made with white X-rays, but only at the expense of the intensity. The diffractometer is designed to measure a Bragg peak in the (+,-) arrangement. A change In the lattice constant can be deduced from a c h ~ e in the peak position m only when the direction of the scattering vector is kept unchanged in the specimen. For a flrst-order transition, this may not perfectly be guaranteed. Nevertheless, we can discuss a change in the profile of Bragg peaks. Figure i shows general features of the diffraction pattern around Tc: a) for heating and b) for cooling. In the tetragonal phase of BaTi03, there are, in general, the a and c domains, in which the e axis is parallel and perpendicular to the surface, respectively. Since, in the present case, the specimen in the tetragonal phase is of a monodomain, namely of the c domain, only the 004 peak is observed at the lowest temperature. About 0.3 K below Te, the peak breaks into fragments with larger widths; at the same time, another group of small peaks, which are marked by hatching, appear around the position of the 400 peak. The appearance of the two groups around the 004 and 400 positions indicates that there are the a domains as well at the c domains; it can be seen from the total intensity of each group that the total volume of each domain is comparable. From the lattice symmetrical point of view, the positions of not all of the peaks can be related to the lattice spacing: that is, there are too many peaks to be assigned. A plausible explanation of the peak splitting is that they correspond to grains, or crystallites, with orientations slightly different from each other. The separation in ~ is of

Now returned to Kurume College of Technology, Kurume 830, Japan. Present address : Department of Physics, University of York, York YOI 5DD, England. 691

692

CUBIC-TO-TETRAGONAL

~

a)

TRANSITION

OOcu b

IN BaTiO 3

Vol. 62, No.

b)

,,

400cub

Tc+2K

x3

Tc-2K I

I

0

2

Fig,l.

I &

)

l

6 0 crystal orientation uJ / 10 3 s HADOX b)

patterns

for

patterns

of

cooling. are

In

BaTiO 3 around the

expanded

the order of magnitude of 400 s of arc at 8 ~ 74°; with the conventional X-ray diffraction technique, the splitting will therefore be hardly observed. At Tc, another peak, which is 400 of the cubic phase, appears and develops, while the peaks of the tetragonal phase diminish. It is to be noted that the process is not monotonic: that is, the relative intensities of the split peaks and consequently the relative volumes of t h e grains vary with temperature, and with time a s shown below. When the transition finishes, t h e 400 peak is asymmetric and broad. After annealing, for several hours at T c + 2 K for instance, the peak recovers the symmetry of the profile and becomes narrower. On cooling, as shown in Fig.l b), the 400 peak of the cubic phase diminishes at Tc, where a group of small peaks appear around the 004 a n d 400 positions of the tetragonal phase. These peaks remain down to about T c - 0.3 K ; at lower temperatures, all the intensities concentrate to a single 004 peak whlch is asymmetric and broad before annealing. Thus the process is reversible as seen in the figure. Although the detail of the pattern, for example the number of split peaks and the relative intensity, is not the same, the process is reproducible in a sense: that is, between the single-crystal states of the tetragonal and cubic phases, there always appears a kind of buffer or transient state where the tetragonal phase is in a multidomain and multl-grain state. This is a new observation on the structural phase transition of first order, though the mechanism of the grain formation is not known. The reproducibillty has been confirmed in the measurements of about twenty thermal cycles.

second in

the

Tc and

I

I

T

2

4

6

: a) third

vertical

for

heating

rows,

and

the

direction.

From Fig.l, we see that the temperature range of the two-phase coexistence is narrow and about i/i0 of those given in literature1'2; a difference in the values of T c for heating and cooling, i.e. the temperature hysteresis, was also about i/i0 of those of the previous ones. The fact that the values are different in all the experiments suggests that the results depend on the condition under which measurements were made: the homogeneity and the stability of the specimen temperature, the rate of temperature variation, the quality of specimen crystal etc. The results thus implies that measurements under more ideal conditions will give even smaller values of these quantities. In Fig.2, a series of the diffraction pattern at T c are shown to demonstrate the time dependence, beginning from the bottom. The range of ~ covers only a part of that in Fig.l in order to repeat the scanning more quickly; it took about two hours for each instead of about five hours in the case of Fig.l. Once the transition starts, the pattern changes in a monotonlcal manner as seen in the upper half of Fig.2. In contrast, just before the start of the transition, or just below Tc, the pattern changes in a random manner as seen in the lower half: although the positions of the peaks show a slde-to-slde movement, in other words the movement is in phase to some degree, the relative intensities of the peaks vary. Moreover, in some cases, the peak separations are significantly different from others. This suggests that the reorientation of grains occurs after the scanning of the first peak but before that of the second of the same run. This kind of changes in the HADOX peak profile have not been observed in the phase transl-

l0

Vol. 62, No. 10

CUBIC-TO-TETRAGONAL TRANSITION IN BaTiO 3

693

I

I

I

I

0

0.5

1

1.5

/103s

Fig.2.

Partial

HADOX

of e v o l u t i o n

patterns

of B a T i O 3 at T c.

of the c u b i c

the

tetragonal

the

lower

half,

phase

are

a random

phase seen

The

and of d i m i n u t i o n

in the u p p e r

nature

processes

of ~ r a i n

half.

ef In

distribution

is shown.

tlons so far investigated; namely those in SrTIO3, KMnF3, K2SeO 4 etc. The only change in the profile was an increase of about i0 % in the peak width in the case of SrTiO39 at the transition temperature. That is a rather modest change in comparison with the present one. In the previous works on BaTIO 3 by means of conventional X-ray diffraction, the data were available only at quite few temperatures, and no anomalies in the peak profile are reported. The only exception is reported by K~nzlg I° , where an anomaly in the profile of 002 around T c is presented. The experiment was, however, of lower resolution, and has been referred to in

few papers. Nevertheless, the authors think that the observation is directly connected with the present results. The high resolution in the reciprocal space and in the temperature attained by HADOX makes it possible to trace the firstorder transition in a continuous manner. Full details of the transition described here together with the time-dependent aspects will be published in due course. Acknowledgement - The authors are grateful to Dr.N.Ohama, Dr.Y.Soejlma and Mr.H.Ohe for helpful discussions and for assistance in the X-ray experiment.

REFERENCES i. 2. 3. 4. 5. 6. 7. 8. 9. i0.

KAY, H.F., Aeta Cryst. _i, 229 (1948). SHEBANOV, L.A., phys.stat.sol, (a)65, 321 (1981). OKAZAKI, A. and OHAMA, N., J.Appl.Cryst. 12, 450 (1979). OHAMA, N., SAKASHITA, H. and OKAZAKI, A., J.Appl.Cryst. 12, 455 (1979). OHAMA, N., SAKASHITA, H. and OKAZAKI, A., Phase Transitions 4, 81 (1984). SATO, M., SOEJIMA, Y., OHAMA, N., OKAZAKI, A., SCHEEL, H.J. and M~LLER, K.A., Phase Transitions 5--, 207 (1985). SAKASHITA, H., OHAMA, N. and OKAZAKI, A., J.Phys.Soc.Jpn. 50, 4013 (1981). SAKASHITA, H. and OHAMA, N., Phase Transitions 2, 263 (1982). OKAZAKI, A., SOEJIMA, Y., OHAMA, N. and M~LLER,--K.A., Jap.J.Appl.Phys.Suppl. 24-2, 257 (1985). K~NZIG, W., Helv.Phys.Acta 24, 175 (1951).