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Wide ranges of absorbance spectra were measured to elucidate a difference in the antiferro-electric (AF) ordering mechanisms below 50 and 168 K in Cs3H(SeO4)2 and Cs3D(SeO4)2, respectively. Collective excitations due to deuterons successfully observed at 610 cm−1 exhibit a remarkable isotope effect. This indicates that the transfer state in the dimer of Cs3D(SeO4)2 is dominated by a deuteron hopping in contrast to Cs3H(SeO4)2, where a proton hopping makes a tiny contribution compared to a phonon-assisted proton tunneling (PAPT) associated with 440-cm−1 defbend. The fluctuation relevant to the AF ordering in Cs3D(SeO4)2 is not driven by the conventional deuteron hopping but by the phonon-assisted deuteron hopping associated with 310-cm−1 defbend. Consequently, Cs3D(SeO4)2 has a distinct ordering mechanism from Cs3H(SeO4)2, in which quantum fluctuations toward the AF ordering are enhanced through the PAPT associated with the in-phase libration. [ABSTRACT FROM AUTHOR]

In phases III and IV of Cs3H(SeO4)2, the vibrational state and intrabond transfer of the proton in the dimeric selenates are systematically studied with a wide range of absorbance spectra, a spin–lattice relaxation rate of 1H-NMR ( T 1 − 1 ), and DFT calculations. The OH stretching vibrations have extremely broad absorption at around 2350 (B band) and 3050 cm−1 (A band), which originate from the 0–1 and 0–2 transitions in the asymmetric double minimum potential, respectively. The anharmonic-coupling calculation makes clear that the A band couples not only to the libration but also to the OH bending band. The vibrational state (nano-second order) is observed as the response of the proton basically localized in either of the two equivalent sites. The intrabond transfer between those sites (pico-second order) yields the protonic fluctuation reflected in T 1 − 1 . Together with the anomalous absorption [νp2 phonon, libration, tetrahedral deformation (δ440), and 610-cm−1 band], we have demonstrated that the intrabond transfer above 70 K is dominated by the thermal hopping that is collectively excited at 610 cm−1 and the phonon-assisted proton tunneling (PAPT) relevant to the tetrahedral deformation [PAPT(def)]. Below 70 K, T 1 − 1 is largely enhanced toward the antiferroelectric ordering and the distinct splitting emerges in the libration, which dynamically modulates the O(2)–O′(2) distance of the dimer. The PAPT(lib) associated with the libration is confirmed to be a driving force of the AF ordering. [ABSTRACT FROM AUTHOR]