Ultrasonic study on structural instability of Bi2Sr2Ca1Cu2O8 single crystal

PhysicaC 162-164 (1989) 454--455 North-Holland

ULTRASONIC STUDY ON S T R U C T U R A L INSTABILITY OF Bi2Sr2CalCu208 SINGLE CRYSTAL Ye-Ning WANG, Jin WU, Jin-Song ZHU, Hui-Min SHEN, *Yi-Feng YAN, and *Zhong-Xian Z H A O Institute of Solid State Physics, Nanjing University, Nanjing, P.R. China *Institute of Physics, Chinese Academy of Sciences, Beijing, P.R. China The velocity and attenuation vs temperature of ultrasonic longitudinal wave propagated along two directions in the c-plane at 10" to the a and b axes were measured for a Bi2Sr2CA1Cu208 single crystaL The anisotropicelasticity in the c-plane ismanifested. One direction shows obvious softening minimum around 250 K, the other reveals only monotonic stiffening down to Tc. The origin of the overall trend of elastic stiffening and some discrepancies appearing in the measurements so far for high Tc materials are analyzed or clarified. Attenuation peaks at 95K, 150K, and 250K are attributed to phase-like transition of first order. 1. I N T R O D U C T I O N Most elastic modulus measurements show stiffening with the temperature down to Tc in YBCO, ESCCX) and "rBcc_£), instead of softening as in the case of A-15 alloys and even of LSCO. There sometimes occurs a kink or inflection, instead of softening minimum at the phase-like transition temperatures 120 K and 240 K 1 in

directions at about 10" from the a and b axis, which are referred as A- direction (Fig. 1) axes and B-direction (Fig. 2). The pulse-echo technique was used to measure the sound velocity (V) and attenuation (a) at 7.5 MHz. Fig. 1 shows V and a along the A-direction as a function of temperature. A broad a peak appears

YBCO, although the ferroelastic stress-stain curve

~iround 250K, and a corresponding velocity curve reveals a pronounced softening minimum in the same

showed softening in YBCO 2. In addition, thermal

temperature range as the peak. This behavior is

hysteresis between heating and cooling curves was

characteristic of displacive phase transition. In the B-

sometimes but not always observed in YBCO. The reported results of measurements for sintered bulk

direction, as shown in Fig. 2, the velocity increases

materials vary significantly. Thus, it is important to work with single crystals to clarify the discrepancies. M. Saint-Paul et al. 3 have measured the sound velocity in the basal a-b plane of YBCO single crystal. No amsotrophy and no softening were observed because

monotonously with decreasing temperature, similar to the results in ceramic samples. We have not distinguished the a-axis and b- axis in the single crystal because their lattice parameters a(5.40/~) and b(5.42/~) are nearly the same. However, the elastic anisotrophy in c-plane is quite noticeable for these two directions. This

twins exist in the orthorhombie phase. In this paper, Bi2Sr2CaCu20$ single crystal were studied, in which no

is consistent with the fact that incommensurated modulation appears along the b-axis only4. A search for

twins were detected by TEM.

the exact soft mode is being carried on. There are two smaller peaks at 95K and 150K (Fig. 1) and two corresponding shallow velocity minimum can be observed as well. X-ray diffraction experiments reveal

2.

EXPERIMENTS AND RESULTS

Single crystals of Bi2Sr2CalCu208 (To = 84.5K) were grown by the flux method. The specimen had dimensions of 8.0x7.0xl.7 mm 3, with the c-plane parallel to the largest area. Longitudinal sound waves propagates in the basal plane along two waves 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

that lattice parameter jumps occur at the peak temperatures including 250 K as well, similar with YBCO. Because no structural symmetry change was discovered, we refer to it as phase-like transitions 1.

Y.-N. Wang et al. / Structural instability of Bi~rzCalCu20s single crystal

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HGURE 1 Velocity and attenuation vs temperature with longitudinal wave along A-direction for Bi2Sr2CaCu208 single crystal.

From Fig. 2, it can be seen that thermal hysteresis exists between heating and cooling curves, characteristic

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FIGURE 2 Velocity vs temperature with longitudinal wave along B-direction (perpendicular to A) measured on cooling and heating.

In this point of view, it is easy to understand why stiffening of sound velocity in the orthorhombic phase

of first order phase transitions. The hysteresis around

of YBa2Cu306.9 below 200 K is faster than that in

250 K is more pronounced than the others.

tetragonal phase of YBa2CU306.26. It is also why the hysteresis is larger above 200K than below 150K and

3. DISCUSSION It has been noted that the overall trend of stiffening

why the superconducting transition occurs near the phase-like transition at 120K instead of near 250K for

of the elastic modulus for YBCO and BSCCO is faster

YBCO.

than in the normal case, especially above 200 K. This

REFERENCES 1. Y.N. Wang, H.M. Shen, et al., J. Phys. C 20 (1987) L665. 2. H.M. Sben, Y.N. Wang, et al., J. Phys. C 20 (1987) L889. 3. M. Saint-Paul, et al., Solid State Commun. 66 (1988) 641. 4. R.M. Hazen, et aL, Phys. Rev. Lett. 60 (1988) 1174. 5. L.H. Sun, Y.N. Wang, et al., Phys. Rev. B 38 (1988) 5114. 6. M. Suzuki, Y. Okudeb et al., Jpn. J. Appl. Phys. Part 2. 27 (1988) t,308.

may be attn'buted to a decrease of lattice parameter faster than expected from normal thermal expansion. The variation of lattice parameter b vs temperature for YBa2Cu3Ox with various x have been measured. 5 The average slopes~ b/~t T (= (b300-bs0)/(300K-80K) of curves for x--7.0 an 6.9 are much greater than that of curves for x=6.5 and 6.2. Obviously, this mainly comes from the larger number of jumps of lattice parameter on the curves for x=7.0 and 6.9 than on other two, instead of being due to a large difference of thermal expansion. Because of the abnormal large stiffening background, softening is not easy to observe in polycrystal materials, but can be observed in suitable directions of single crystals (Fig. 1).