FERROELASTIC PHASE-TRANSITIONS IN CALCIUM-CHLORIDE AND CALCIUM BROMIDE

The structural transitions of CaCl2, and CaBr2 at 491 K and 778 K, res pectively, represent clear examples of proper ferroelastic transformat ions of the optical type. In both cases, the instability of the high-t emperature rutile-type structure is caused by the softening of an opti cal phonon at the center of the Brillouin zone in combination with a s ymmetry-allowed, strong bilinear coupling of the eigenvector of that m ode with distinct strain components. The order parameter of B-1g symme try induces the orthorhombic CaCl2-type structure in the low-temperatu re phase. Static effects on the structural parameters and dynamical ef fects on the phonon spectra, as seen by Raman spectroscopy, are discus sed. Primary and secondary spontaneous strains, domain patterns, and i mplications to elastic properties are considered.