Chemical Dynamic Behavior at Solid Surfaces

Abstract

We 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 TSCC, -rBJ in the whole wavenumber region below4000 cm-' and in a temperature interval 2 K S T S 170 K. A general survey of the Raman and infrared reflection spectra is given and their temperature and concentration dependences are discussed. We 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 temperaturedependent frequency renormalisation of the phonons on approaching T;.One of the soft modes is Raman-active with a negligible LGTO 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 lowtemperature phase. The contribution of the soft modes to the anomaly of the dielectric constant at low frequencies is calculated and it is found that the displacive behaviour is of minor importance as compared with the order-disorder contribution to the character of the structural phase transition in TSCC.A microscopic order parameter is proposed that supports the assumption of a pseudoproper phase transition in TSCC.