Structural and proton-dynamical effects in a proton-irradiated KH₂PO₄ single crystal

A KH₂PO₄ (KDP) crystal, irradiated by a 1 MeV hydrogen ion beam to a dose of 10¹⁵ ions∕cm², was studied by means of x-ray diffraction (XRD), ¹H nuclear magnetic resonance (NMR), and dielectric constant measurements. The XRD pattern for the a-cut KDP crystal revealed a decrease in the lattice constant along the a axis after the proton irradiation. According to the ¹H NMR spin-lattice relaxation rate measurements, the proton irradiation gave rise to reduction in the activation energy in the paraelectric phase, from 0.42 to 0.28 eV, in agreement with the temperature dependent second moment measurements indicating the proton motion is more activated after the proton irradiation. Besides, analysis of the temperature-dependent dielectric constants using a mean-field approximation revealed a change in the hydrogen bond induced by the proton irradiation.