Phase Transitions in the Brominated Ferroelectric Tris‐Sarcosine Calcium Chloride
paraelectrics SrTiO 3 and KTaO 3 . [ 9 ] However, the ultra-weak nature of the dipole moment in TSCC referred to below may reduce the effects of such long-range dipole interactions to a narrow temperature region around the critical point. This would mean that more conventional classical or quantum critical behaviour would be expected over a wider range of temperatures and tuning parameters. TSCC is hydrogen bonded and was characterized in the past as order-disorder albeit with no direct supporting evidence. [ 16,17 ] Since the time of these studies TSCC has come to be a prototypical displacive ferroelectrics, [ 18 ] as demonstrated by the existence of an under-damped soft mode. This mode can be followed into the GHz frequency regime [ 19 ] from highT values of ca 630 GHz = 21 cm −1 . Deuteration [ 20 ] produces little change in T C , implying that the transition is not controlled by the N-Cl-hydrogen bonds, a result compatible with the soft-mode mechanism operating in TSCC. [ 2 ]
Study of ferroelectric properties of deuterated tris-sarcosine calcium chloride
By means of permittivity, spontaneous polarisation and polarisation reversal measurements the authors studied the influence of the substitution of deuterium atoms for hydrogen on the ferroelectric properties of tris-sarcosine calcium chloride (TSCC). Although most of the protons in the hydrogen bonds were substituted by deuterium (as checked by means of multi-pulse NMR techniques for solids and by infrared spectroscopic measurements) there is no change of the phase transition temperature. By combining density and NMR measurements it could also be shown that the volume of the unit cell did not alter for the various deuterated crystals. The absence of the deuteration effect on Tc and on the parameters of the Landau expansion of Ps suggests that a proton ordering in the hydrogen bond of the type N-H. . .Cl does not occur in practice. This behavior can be understood because the hydrogen bond is rather asymmetric so that the protons occupy predominantly equilibrium positions near the nitrogen atoms. For an interpretation of the results presented the theory of Chaudhury (1982) and co-workers may be applied.
Spectroscopic investigation of the ferroelectric phase transition in tris-sarcosine calcium chloride: an order-disorder system with displacive features. I. Experimental results
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.