Electron Microscopic Study of Long Period Modulated Structures with Continuously Variable Periodicity
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ELECTRON MICROSCOPIC STUDY OF LONG PERIOD MODULATED STRUCTURES WITH CONTINUOUJSLY VARIABLE PERIODICITY CAII,*
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D. CLISD. VAN DYCK, C. CONDE-AMIANO, and S. AH•ELINCKX ' * ER 210 CNRS, 1. P1. A. Briand F-92190 Heudon, France; **RUCA, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium; **Departemento de Optica, Facultad de Fisica , E-Sevilla, Espana; ****Also at SCK/CEN, B-2400 Mol, Belgium.
ABSTRACT Some systems show continuous and reversible phase transitions which are characterised by the appearance of irrational sunerlattice reflections with a position that shifts continuously and reversibly with temnerature. This diffraction feature is not necessarily caused by a deformation modulation but can also originate from the reneated occurrence of olanar defects with a variable "average" neriodicity. The planar defects can be of different type (e.g. planes of different composition, antiphase boundaries, twin planes) as shown for the systems Ni 3 +xTe2, Cu 3 _xTe 2 , Cu2._xS, mPbS-nBi 2 S3 (• > 2) and Cu 0 . 7 5 VS2 , using electron-mnicroscopy and electron diffraction.
INTRODUCTION Diffraction patterns showing incommensurate suoerlattice reflections are often interpreted in terms of a continuously varying structure. In this way, it was originally suggested without detailed analysis [1], that the hightemperature phases in the Ni 3 +xTe 2 and CU3_xTe2 systems should contain a so called occupation wave. Similarly, the incommensurate reflections observed in Cu-intercalated VS2 (Cu 0 . 7 5 VS2 ) could be attributed to the formation of a charge density wave [2], since evidence for such a COW was found in the cubic spinel structure Cu 0 . 5 VS2 [3], which is remarkable since COW formation is usually associated with anisotropic structures. In this work, we will show that, in both examples, an interoretation in terms of a repeated stacking of defect planes with a quasi irrational average periodicity can be more justified. In Ni 3 +xTe 2 and CU3_xTe 2 , [4,5,6,7]; the defect planes are planes of different composition whereas in Cu 0 . 7 5 VS2 [8], the defect planes are antiphase boundaries in the intercalate layer. This latter system is investigated in more detail. Some other examoles are shown such as the system galena-bismuthinite mPbS-nBi 2 S3 , n/in > 2 [9] and digenite CU2_xS [10,11] in which the pseudo-incommensurability is caused by the stacking of microtwin planes resp. copper layers.
ELECTRON DIFFRACTION The electron diffraction patterns of all the above mentioned systems are characterised by the presence of two major satellites at both sides of each Bragg reflection of the unmodulated basic structure. The satellite nositions shift continuously and reversibly as a function of temperature and/or comnosition, between two extremal values often corresnonding with commensurate superstructures. The diffraction pattern of Cu 0 . 7 5 V5 2 is shown in Fig. 1 as a tynical example. Here the distance between the satellites and the basic reflections increases with decreasing temperature (below room temperature), th
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