Atomic structure of the Mg3B7O13Cl unit cell projected on (001)cubic for the cubic phase (a) and (001)pseudocubic for the orthorhombic phase (b). (c) The shear strain associated with the transition is revealed by showing the relative position of oxygen atoms common to four unit cells in the cubic phase (schematized in grey) and for the orthorhombic phase (in red). The shear distortion is exaggerated by a factor of 300. Labels a and b correspond to the orthorhombic lattice parameters. Labelled angles correspond to the true values expected for the strained unit cell for Mg–Cl boracite. (d) Local topographical map of Cu–Cl boracite crystal surface as measured by atomic force microscopy. The scale bar measures 2 μm. (e) Corrected lateral-mode piezoresponse map (a.u.) for the area shown in d and scan obtained with relative cantilever orientation at 90°, (f). Grey motif indicates cantilever orientation. (g) Montage of piezoreponse maps in e,f. Coloured legend represents the four in-plane polarized domain states with orientations labelled by arrows.