-
[show abstract]
[hide abstract]
ABSTRACT: The effects of partial substitution of chlorine by bromine in incommensurate betaine calcium chloride di-hydrate (BCCD) have been investigated by FTIR reflectivity and elastic neutron scattering. Bromine distorts the H-bonds linking the Ca complexes and induces local symmetry breaking. The temperature dependence of b and c lattice parameters of the 4% and 8% brominated compounds reveals that this distortion has effects that are markedly anisotropic. This anisotropy may help to clarify why bromine progressively destabilizes the low temperature phases of BCCD.
Ferroelectrics 01/2004; 302:151-153. · 0.39 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report dielectric and optical birefringence measurements in 38 % brominated BCCD, which confirm the transition to the X-phase, at ambient pressure, around Tc =80 K. These results are compared with elastic neutron scattering data. A coexistence of the parent Pnma phase with the X-phase is observed in a large range of temperature centered at about Tc . It is found that the mosaic spread of the crystal varies strongly and irreversibly when a temperature cycle Pnma - X-phase - Pnma is performed. A 30% brominated BCCD has also been investigated by elastic neutron scattering. For this compound, an intermediate incommensurate phase occuring between T1 ≈ 111 K and Tc ≈50 K is observed.
Ferroelectrics 01/2000; 240:1435-1442. · 0.39 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report a combined experimental study by means of elastic neutron scattering and dielectric measurements of a partially
deuterated and x =4% brominated BCCD (Betaine Calcium Chloride Dihydrate) crystal. The lowest-temperature phase is one-dimensional
modulated and characterized by the coexistence of different commensurate domains (with ∂ = 1/4, 4/17, 2/9 and 1/5 on cooling),
but with a clear predominance of the five-fold phase. A huge global thermal hysteresis of the wave-vector of the modulation,
attaining values of about 9 K in the incommensurate phase and up to 15 K in the “harmless” low temperature part of the phase
diagram, is observed up to Ti. The role of lattice defects on this phenomenon is discussed. Similarly to the behaviour of the pure compound, the structural
modulation evolves on cooling towards a soliton regime (growth of third and fifth-order satellite peaks), probably with respect
to a non-stabilized non-modulated ferroelectric phase. The critical temperatures deduced from dielectric constant and pyroelectric
current measurements are in very good agreement with those obtained from neutron scattering. The dielectric anomaly observed
in eb(T)\varepsilon _b^(T) at T @ 85T \simeq 85 K, and known as the “Ts-anomaly”, could not be related with any special feature detected in the neutron data, and in particular no correlation between
this anomaly and the appearance of the soliton regime can be established.
PACS. 61.12.-q Neutron diffraction and scattering - 64.70.Rh Commensurate-incommensurate transitions - 77.22.Ch Permittivity
(dielectric function)
Physics of Condensed Matter 04/1999; 10(3):447-456. · 1.53 Impact Factor
