Neutron incoherent inelastic scattering from (NH₄)_xRb_1-xH₂PO₄

Abstract

Spectra of incoherently inelastically scattered neutrons from (NH₄)_xRb_1-xH₂PO₄ (x = 0.0, 0.35, 1.0) in the energy transfer region of 2 to 200 meV are reported. All the peaks in the measured spectra are identified and compared with optical data. The energies (≈ 126 and ≈ 160 meV) associated with the motion of acidic protons are the same in the three salts at 6 K inspite of the fact that RDP (x = 0.0) is a ferroelectric, ADP (x = 1.0) is an antiferroelectric, and ARDP (x = 0.35) a dipolar glass. In the region (2 to 32 meV) of lattice mode RDP and ARDP spectra are similar, but they are considerably different from the ADP spectrum. Spectra corresponding to librational modes (≈ 38 meV) of NH₄⁺ are also different in ADP and ARDP. Reasons for these differences are discussed. The effect of temperature on the above spectra is studied between 6 and 160 K.