Raman scattering investigation of ferroelastic Sb5O7I crystals

Abstract

Abstract. Sb5071 undergoes a displacive phase transition at 481 K where the symmetry is changed from CZ6h to C~h. In the low temperature monoclinic phase the crystal is ferroelastic. The polarized Raman spectra of SbsOTI ha9e been measured at various temperatures below and above the phase transition. The frequencies and symmetries of most of the theoretically expected Raman active phonons in the ferroelastic phase have been determined. The observation of a soft mode in the ferroelastic phase which disappears above the phase transition together with the fact that the unit cell of the ferroelastic phase is twice as large as that of the paraelastic structure permits the conclusion that the phase transition results from a phonon instability at the Brillouin zone boundary M-point of the hexagonal phase, The temperature dependent splittings and intensity changes of several Raman lines are discussed with respect to the ferroelastic property of the crystal and the phase transition.

The elastic behaviour of ferroelastic Sb5O7I polytype 2MC studied by ultrasonic experiments and Brillouin scattering

“…Starting from the hexagonal phase the ordering coordinate transforms like the irreducible representation M 2 of the factor group of the wavevector qM of C6Zh [4]. The three components of the ordering coordinate Qj (j = 1, 2, 3) belong to the three equivalent wavevectors of the star of qM.…”

“…One face was metallized with evaporated aluminium and an x-or y-cut quartz transducer having 15 or 30 MHz fundamental resonance frequency was bonded to it by epoxy resin. By the puls-echo overlap method we tried to measure all the possible longitudinal, transverse, quasi-longitudinal, and quasi-transverse waves along the main axes a, b, and c* as a function of temperature in the range 100 K to 520 K. Using a constant mass density p = 5550 kg/m 3 [3] we converted the measured veloc- (4) for e22, to (5) for e(a,a) and a superposition of (4) and (5) for e(c*,c*). The nomenclature is according to Table 1 ities into the corresponding elastic stiffnesses.…”

“…The fact that both types of spontaneous strain, (e6) and (el-ez) (in hexagonal notation) appear [4] indicates that more than one order-parameter component is involved in the ordering. The isotropic fourth-order coefficient in the Landau expansion is denoted by a4.…”

“…Consequently the spontaneous strain is proportional to the square of the order parameter, rendering this material an improper ferroelastic. In addition 2 MC-SbsOTI was studied intensively by Raman scattering [4,5]. Special attention has been paid to the intensity of phonon modes forbidden in the hexagonal phase.…”

Raman scattering investigation and symmetry analysis of ferroelectric/ferroelastic Sb5O7I polytype 2MA

“…The acentric crystals are both ferroelastic and ferroelectric. In previous publications we reported on Raman scattering measurements and the development of a phenomenological theory to account for the phase transitions of the purely ferroelastic, centric two-slab polytype 2MC, which was initially called c~-SbsOTI [5,6]. This crystal, the structure of which is the basis of the centric polytypes, is found to be most abundant in the crystal growing process.…”