IN SITU HIGH TEMPERATURE ELECTRON DIFFRACTION STUDIES ON THE MODULATED STRUCTURES IN (Bi, Pb)-BASED SUPERCONDUCTORS. Michael Czank*, Wulf Depmeier* and Nicolai D. Zhigadlo*, *Mineralogisches Institut, Univ. Kiel, D-24098 Kiel and **Inst. of Solid State and Semicond. Physics, Belorussian Acad. of Sciences, Minsk 220072
In addition to the known incommensurate modulations and phase transitions in the superconductor phases Bi2Sr2CaCu2O8-x (2212 phase) and (Bi2-yPby) Sr2Ca2Cu3O10-x (2223 phase) [e.g. 1-3], we observed new modulations over a range of temperatures (293-800K).
The 2212 phase is incommensurately modulated (q1=0.21b*) at room temperature. With in situ heating additional satellites with q2=0.46a* and q3=0.46b* to all h+k=2n reflections gradually appear above (630K. The new phase is quenchable, but q2 and q3 have additional very small components in b* and a*, respectively. The 2212 phase decomposes above (700K. No twinning, as reported [1 and 3] was observed.
In the 2223 phase two different crystal types are present: a) Grains having only one modulation (q=0.137b*), where the satellites are attached to all h,k reflections. b) Grains with two modulations (q1=0.21b* and q2=0.137b*), where q1 is mainly associated with h=2n and k=2n, while q2 has the strongest satellite intensities with h+k=2n and h,k=2n+1. In all crystals above (630K the 2ax2b superstructure gradually appears. The modulations coexist until the phase decomposes above (720K. We found twinning on (010) and on (100) at room temperature. Dark-field images show twin lamellae. Twins disappear gradually above (630K. Diffuse streaks along [110] and [1-10] are present in some crystals at room temperature and appear in others at higher temperature. Their intensities increase with temperature and finally new satellites with q=0.26a* + 0.26b* develop. These modulations are also quenchable.
Heating experiments in air support the assumption that the various modulations are connected to distortions and/or ordering within the different structural layers.
References:
[1] Yang et al., Phys.Rev.B, 48, 16873 (1993).
[2] Zandbergen and Groen, Physica C, 166, 282 (1990).
[3] Zhang and Van Tendeloo, Phil.Mag.A, 70, 549 (1994).