Abstract
The polarized infrared reflectivity spectra of NH4H2PO 4 are reported for a wide spectral range (20-1 500 cm-1) in the para- and antiferroelectric phases. Measurements were performed despite the shattered character of the sample in the low-temperature phase. The phonon spectra are obtained by fitting the factorized form of the dielectric function to the experimental spectra. The most marked effect on the experimental spectra at the phase transition is the disappearance of a highly-damped low-frequency mode upon cooling below TN, in both polarizations. This mode originates from intersite proton motions, and is coupled with the Brillouin zone-corner antiferroelectric instability. The other modes are only slightly affected by the phase transition, especially the NH4 internal modes. The results are discussed in the light of a previous work on ferroelectric KH2PO4-type compounds, which enables to assign the lattice modes, and to confirm the role of an internal PO4 mode in the mechanism of ferroelectricity in these compounds.
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