Spectroscopic investigation of the ferroelectric phase transition in tris-sarcosine calcium chloride: an order-disorder system with displacive features. I. Experimental results

Abstract

The authors report infrared and Raman spectroscopic investigations of single crystals of tris-sarcosine calcium chloride (TSCC), of the isomorphous tris-sarcosine calcium bromide (TSCB) and of mixed crystals with different chlorine-bromine concentration ratios x (for TSCC1-xBx) in the whole wavenumber region below 4000 cm-1 and in a temperature interval 2K<or=T<or=170 K. A general survey of the Raman and infrared reflection spectra is given and their temperature and concentration dependences are discussed. They concentrate on three different soft modes observed only in the ordered phase and on several 'hard-core' modes, which can be found in different spectral regions and which demonstrate a temperature-dependent frequency renormalization of the phonons on approaching Tc -. One of the soft modes is Raman-active with a negligible LO-TO splitting, the other two soft modes appear in the infrared spectrum with a small LO-TO splitting. Contrary to the usual behaviour of polar soft modes the LO frequencies also soften. The temperature and concentration dependences of these three modes are presented in detail. They are correlated with the presence of three different lattice sites for the sarcosine molecules in the low-temperature phase. The contribution of the soft modes to the anomaly of the dielectric constant at low frequencies is calculated. A microscopic order parameter is proposed that supports the assumption of a pseudo-proper phase transition in TSCC.