Infrared spectroscopy of Kh₂Po₄-type ferroelectrics

Lattice dynamics, phase transitions and hydrogen effective charges of betaine phosphite: a comparison with betaine phosphate and their deuterated analogues

“…Spectra have been recorded at five intermediate temperatures. All reflectivity spectra have been fitted with (1) and (2). Examples of agreement between model and experiment are shown in figures 2 and 3.…”

“…The factorized form of the dielectric function was successfully used to fit the spectra of compounds displaying parent properties, e.g. RbDP [1][2][3], and BP [12]. For the case of BPI too, the model (1) appears to be the simplest one which is able to fit the data satisfactorily.…”

“…This is just the situation in which the factorized form of the dielectric function (1) is the most relevant because it does not implicitly assume the same damping for two neighbour TO and LO modes as the more usual summation model does, as detailed in [19]. In particular, this situation may give rise to asymmetric phonon lines that result from coupling of a narrow line with a broad continuum, and the factorized form (1) is able to describe the line profiles satisfactorily in this case, as shown in [1][2][3] and [12] for example.…”

“…However, the relation is not straightforward. For example, it was shown in RbH 2 PO 4 that the coupling of PO 4 vibrations with those of the O-H • • • O bonds is responsible for the spontaneous polarization found oriented in a direction perpendicular to that of the O-H • • • O bonds [1][2][3]. It is interesting, therefore, to check whether such a coupling takes place in betaine compounds in which the symmetry of the inorganic pyramidal groups HPO 3 is modified with respect to the tetrahedra which are present in all KDP compounds and BP too.…”

“…Similar spectral species were observed in the spectra of the parent RADP family, viz. NH 4 H 2 PO 4 , RbH 2 PO 4 and mixed crystals Rb 1−x (NH 4 ) x H 2 PO 4 [1][2][3]. By comparison with the RADP family, the study of BP [12] has exemplified the problems related to generalized mode-mode coupling.…”