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R3.16.1

NMR study of proton beam irradiated TlH2PO4

Se Hun Kim, Kyu Won Lee, Jae Won Jang and Cheol Eui Lee* Department of Physics, Korea University, Seoul 136-701, Korea ABSTRACT We have investigated proton beam irradiation effects on TlH2PO4 (TDP) showing an antiferroelectric phase transition and a ferroelastic phase transition. The sample was irradiated by 1 MeV proton beams to a dose of 1015 ions/cm2 and studied by means of 1H NMR measurements. The NMR rotating-frame spin-lattice relaxation time was measured as a function of temperature, and analyzed in order to understand the proton motions and the order parameter reflecting the structural changes caused by the proton irradiation. INTRODUCTION TlH2PO4 (TDP) is closely related to the KH2PO4 (KDP)-type crystals, which are interesting hydrogen-bonded materials undergoing structural phase transitions accompanied by ferroelectricity or antiferroelectricity [1]. In these crystals, it is known that protons in double well potentials on the hydrogen bonds undergo a phase transition accompanied by displacements in the heavy atom (K,P,O) structure. TDP undergoes two major phase transitions and three more complex phase transitions [2-9]. The room temperature phase (phase II) is known to be paraelectric and ferroelastic while the low-temperature phase (phase III) is believed to be antiferroelectric. The high-temperature phase (phase I) is known to be paraelectric and paraelastic. TDP has a monoclinic crystal structure in phases II and III, and an orthorhombic structure in phase I [2-4,9,10]. In TDP the low temperature (II-III) antiferroelectric phase transition occurs at Tc = 230 K, and the high temperature (I-II) ferroelastic phase transition at Tc’ = 357 K. TDP has three different crystallographic hydrogen bonds as determined by x-ray and neutron scattering, and a crystal structure of TDP illustrating the three inequivalent H sites can be found in the literature. The two shorter bonds, 0.243 nm and 0.245 nm, respectively, are centrosymmetric and form zigzag chains along the c axis. Protons 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 [11].

R3.16.2

In the present work, we carried out 1H rotating-frame spin-lattice relaxation measurement of a virgin polycrystalline TDP sample and a H+ beam irradiated sample, respectively. The experimental results are discussed in view of the order-disorder dynamics above and below Tc, comparing a virgin TDP sample with a H+ irradiated one. EXPERIMENTAL DETAILS A polycrystalline TDP sample was irradiated with 1 Mev H+ to a dose of 1015 ions/cm2. A virgin and H+ treated samples are investigated in this work using a 200 MHz 1H pulsed NMR spectrometer. The rotating-frame spin-lattice relaxation data at the frequency of the rotating frame, ω1/2π = 55.6 kHz, were obtained in the temperature range 150 – 400K. D