A normal to incommensurate phase transition in malayaite, CaSnOSiO4

Malayaite, the tin analogue of titanite, has been shown to develop an incommensurately modulated phase with a modulation vector of q=0.26b* at a temperature of 20K [1]. The occurence of the modulated phase confirms density functional perturbation calculations of the phonon dispersion, indicating phonon softening in this particular range of k-space. Single crystal X-ray diffraction experiments at beamline P24 of PETRAIII/Desy at variable temperatures between 20 and 100K indicate a transition temperature of 48±3K for the corresponding normal to incommensurate phase transition. On approaching the phase transition temperature from below, the modulation vector remains almost constant, while the intensity of the satellite diffration maxima decreases. The crystal structure transforms from the C2/c(0b0)s0 superspace group to C2/c. The phase transition is discussed in relation to further anomalies observed between 90K and 400K by single crystal X-ray diffraction and Raman spectroscopy and in particular in relation to mode softening of a low frequency Bg-mode involving Ca-motion parallel to [001]. This mode softening is observable over the entire temperature range between 400K and 90K and culminates in the modulated low temperature structure of malayaite, as evidenced by the corresponding instability observed in the calculated phonon dispersion.

[1] Malcherek T., Paulenz B., Fischer M., Paulmann C. (2020) Acta Cryst. B76, 316-321