EPR study on deuteration-enhanced thermal stability of radiation-induced point defects in KDP and DKDP crystals

Background Potassium dihydrogen phosphate (KDP) and its deuterated analog (DKDP) are critical frequency conversion crystals in inertial confinement fusion (ICF) laser systems. However, DKDP exhibits inferior UV damage resistance compared to KDP, yet systematic experimental comparisons of radiation-induced point defects under identical conditions remain lacking.

Purpose This study aims to elucidate the influence of deuteration on the thermal stability and evolution dynamics of radiation-induced intrinsic point defects in KDP/DKDP crystals, providing microscopic experimental evidence for understanding their differential UV damage resistance.

Methods Single crystals of KDP and DKDP (70% deuteration) were irradiated with X-rays for 2.5 h at 77 K. Electron paramagnetic resonance (EPR) spectra were acquired at 35 K using a Bruker ELEXYS E500 spectrometer. The samples underwent isothermal annealing from 80 K to 295 K with 10 min holds at each step. Signal intensities were normalized after double integration using EasySpin software.

Results （1）Deuteration dramatically enhances self-trapped hole thermal stability: D₂PO₄⁰ retains 49.5% intensity after 80 K annealing, while H₂PO₄⁰ vanishes completely (<2%), representing a 25-fold stability difference. (2) The DPO₄⁻ concentration in DKDP increases continuously by 72.8%±5.1% over 80–140 K, forming a 60 K-wide "defect accumulation window," whereas HPO₄⁻ in KDP increases only 15.1%±1.8% before decaying. (3) Interstitial D⁰ electron centers exhibit higher survival (72%±4%) than H⁰ (55%±3%) after 100 K annealing.

Conclusions The isotope mass effect reduces zero-point vibrational energy (ΔE₀≈0.057 eV) and suppresses proton transport, confirming that deuteration selectively prolongs hole center lifetimes. This leads to elevated transient defect concentrations in DKDP at room temperature, providing microscopic experimental evidence for its inferior UV damage resistance and suggesting deuteration level is a key parameter for controlling damage resistance in ICF applications.