Ferroelastic phase transition in: A neutron diffraction study

Crystalline (Formula presented) with the (Formula presented) symmetry at room temperature was studied by Laue and four-circle neutron diffractometry from room temperature down to 50 K. At (Formula presented) the crystal undergoes a second-order proper ferroelastic phase transition to a polydomain structure with a nonprimitive monoclinic (Formula presented) space group. Satellites were not found below (Formula presented) indicating that the continuous distribution of sites observed in previous (Formula presented) nuclear quadrupole resonance experiments is due either to an undetected incommensurate phase characterized by a very small displacement amplitude, or due to the fraction of the crystal volume occupied by domain walls. We argue that thick domain walls are expected in the present structure due to the absence of mechanical compatibility between domains.