Si29 magic-angle-spinning NMR spectroscopy of the ferroelastic-to- incommensurate transition in Sr2SiO4

The soliton density of the incommensurate L phase of Sr2SiO4 has been determined by Si29 magic-angle-spinning NMR spectroscopy from Tc=83°C and 200°C. On heating, the soliton density increases rapidly from 0.06 at 83°C to 0.6 at 86°C but then increases only gradually to 0.76 at 200°C. On cooling through the transition, there is a 10°thermal hysteresis and the decrease in soliton density with temperature is more gradual than on heating. The temperature dependence of the soliton density is similar to that for the A2BX4 class of improper ferroelectrics such as Rb2ZnCl4, although the commensurate phase of Sr2SiO4 is ferroelastic rather than ferroelectric. In addition, the modulation wave appears to be pinned over this temperature range.