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

The adaptation of the incommensurate structure modulation in Ca2CoSi2O7 (dicalcium cobalt disilicate) single crystals to decreasing temperature has been examined using in situ high‐resolution transmission electron microscopy and electron diffraction. The transition from the incommensurate to the commensurate lock‐in phase of Co‐åkermanite exhibits a pronounced hysteresis of a highly strained metastable state with a characteristic microdomain morphology. A network of domain walls surrounding single orientation domains develops out of the room‐temperature tartan pattern, the domains increase in size and their alignment changes from crystallographic to random. At 100 K the phase transition becomes almost complete. In parallel, the evolution of the modulation structure can be described by a change from a loose arrangement of octagonal tilings into a close‐packed configuration of overlapping octagons in the commensurate low‐temperature lock‐in phase. Thereby, the octagon represents the ordered distribution of low‐coordinated Ca clusters within a nanodomain extending over 4 × 4 subunits, on average [Riester et al. (2000). Z. Kristallogr.215, 102–109]. The modulation wavevector was found to change from q1,2 = 0.295 (a* ±b*) at 300 K to q1,2 = 0.320 (a* ±b*) at 100 K.