Lattice dynamics and structural phase transitions in the chain compounds TMMC and TMCC. II. Raman scattering and ultrasonic measurements

For pt.I see ibid., vol.2, p.8209 (1990). The structural phase transitions occurring in the chain compounds (CH 3 ) 4 NMnCl 3 (TMMC) and (CH 3 ) 4 NCdCl 3 (TMCC) are studied by means of Raman scattering and ultrasonic measurements. The Raman spectra recorded in the different structural modifications are fully understood on the basis of the structural data previously established in paper I. In addition, these data provide useful information on the transition mechanisms, which are of complex nature. Generally, these mechanisms involve order-disorder processes due to the reorientations of the (CH 3 ) 4 N + groups (TMA) coupled with a displacive contribution coming from the MCl 3 octahedra chains (rotatory and translatory soft modes). The ultrasonic measurements clearly show that the I to or from III phase transition (P6 3 /m(Z=2) to or from P2 1 /m(Z=2)) is pseudo-proper ferroelastic, as well as the I to or from II (P6 3 /m(Z=2) to or from P2 1 /b(Z=4)) transition; this latter should be of the trigger type, in order to account for the unit-cell doubling.