Antiferroelectric Transitions in NH4H2PO4 and NH4H2AsO4 Studied by Infrared Absorption

The antiferroelectric phase transition of NH₄H₂PO₄ and NH₄H₂PO₄, as well as of their deuterated analogs, were studied by observing the changes in the infrared absorption spectra (250–4000 cm⁻¹) accompanying the phase transition. The behavior of the spectrum of NH₄H₂PO₄ and ND₄D₂PO₄ while approaching $T_c$ suggests that the same hydrogen‐dynamics mechanism is responsible for the phase transition in both cases, and that the fluctuating double‐minimum‐potential‐well theory for the hydrogens applies here too. The differences observed at temperatures below $T_c$ indicate a different final arrangement of the hydrogens in the antipolar phase. There are more PO₄ lines in the NH₄H₂PO₄ low‐temperature spectrum, indicating that the hydrogens are arranged asymmetrically along the $c$ axis, thus inducing an antiparallel dipole network. The ir absorption spectrum of a mixed crystal NH₄H₂(AsO₄)_0.95(PO₄)_0.05 was measured at different temperatures in order to check the validity of the site‐group assignments in these crystals. The gradual reduction in the correlation field splitting on increasing the deuterium percentage in the antipolar phases of ND₄D₂PO₄ compounds suggests a more distorted deuteron network. Possible assignment for the absorption peaks in both phases of NH₄H₂AsO₄ and ND₄D₂AsO₄ (60% D) are given.