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K3.4.1

NMR and impedance study of H+-ion irradiated TlH2PO4 Se Hun Kim1, Kyu Won Lee1, Jae Won Jang1, Cheol Eui Lee1* and S. J. Noh2 1 Department of Physics, Korea University, Seoul 136-713, Korea 2 Department of Applied Physics, Dankook University, Seoul 140-714, Korea ABSTRACT The H+-ion treatment effect on TlH2PO4, a KH2PO4 (KDP)-type ferroelectrics, was studied by nuclear magnetic resonance (NMR) and AC dielectric measurements. A sample of TlH2PO4 was irradiated by 1-MeV H+ ion beams to a dose of 1015 ions/cm2. The irradiation changed the hydrogen-bond geometry, presumably affecting the order-disorder proton dynamics. The deformation of the PO4 tetrahedra was identified by the isotropic chemical shift and the full width at half maximum (FWHM) of the high-resolution 31P NMR spectra. A prominent decrease in the dielectric constant was also observed after the irradiation. The macroscopic and microscopic changes due to the irradiation are discussed in the light of the proton dynamics. INTRODUCTION TDP undergoes two major phase transitions: antiferroelectric phase transition at Tc = 230 K and ferroelastic phase transition at Tc’ = 357 K [1-4]. The room temperature phase is paraelectric and ferroelastic, whereas the low-temperature phase is known to be antiferroelectric and the high-temperature phase to be paraelectric and paraelastic. TDP has a monoclinic primitive cell at room temperature with a=14.308 Å, b=4.518 Å, c=6.516 Å, and β=91.76o [5,6]. TDP has three different crystallographic hydrogen bonds as determined by x-ray and neutron diffraction, and the crystal structure of TDP illustrating three inequivalent H sites can be found in the literature [2,6]. The two shorter bonds, 0.243 nm and 0.245 nm, respectively, are centrosymmetric and form zigzag chains along the c-axis. Hydrogens of these bonds are at special positions at a center of inversion and undergo an order-disorder phase transition through the phase transition temperature Tc. The longest bond, 0.25 nm, is asymmetric along the b-axis and the protons are at a general position both above and below Tc [2]. The tunneling model including some variants, as a traditional model for the ferroelectric phase transition in the hydrogen-bonded ferroelectrics, may be severely challenged by several experimental and theoretical works [7-11]. A promising candidate is the theory of the so-called geometric isotopic effect, where the shift in the transition temperature with deuteration is attributed not to the decrease in tunneling but to the changes in the hydrogen bond geometry [10,11]. There are two characteristic lengths in a O-H---O hydrogen bond, one being the oxygen separation R (hydrogen bond length) and the other being the separation δ between the two

K3.4.2

possible hydrogen sites. Several works were reported on the characteristic length dependence of the transition temperature [7,9]. The deuteron substitution retaining the lengths was also reported not to shift the transition temperature [8]. In view of the hydrogen bond, TDP has very short bond lengths R=0.