Transition from the incommensurately modulated structure to the lock-in phase in Co-akermanite

The adaptation of the incommensurate structure modulation in Ca2COSi2O7 (di calcium cobalt disilicate) single crystals to decreasing temperature has be en examined using in situ high-resolution transmission electron microscopy and electron diffraction. The transition from the incommensurate to the com mensurate lock-in phase of Co-(a) over circle ermanite exhibits a pronounce d hysteresis of a highly strained metastable state with a characteristic mi crodomain morphology. A network of domain walls surrounding single orientat ion domains develops out of the room-temperature tartan pattern, the domain s increase in size and their alignment changes from crystallographic to ran dom. At 100 K the phase transition becomes almost complete. In parallel, th e evolution of the modulation structure can be described by a change from a loose arrangement of octagonal tilings into a close-packed configuration o f overlapping octagons in the commensurate low-temperature lock-in phase. T hereby, the octagon represents the ordered distribution of low-coordinated Ca clusters within a nanodomain extending over 4 x 4 subunits, on average [ Riester et al. (2000). Z. Kristallogr. 215, 102-109]. The modulation waveve ctor was found to change from q(1,2) = 0.295 (a* +/- b*) at 300 K to q(1,2) = 0.320 (a* + b*) at 100 K.