Structure–property relations governing isotope-dependent polarization and Curie temperature in KDP from first principles

We investigate isotope-dependent ferroelectricity in KH 2 PO 4 (KDP) using first-principles (FP) methods. The spontaneous polarization ( P s ) scales linearly with the proton off-centering distance δ along the O–H ⋯ O hydrogen bond, establishing a clear structure–property relationship. By incorporating nuclear quantum effects through path-integral Monte Carlo simulations with a first-principles-derived effective model, we quantitatively reproduce the experimental dependence of P s on deuteration, showing that isotope substitution tunes ferroelectricity through changes in proton/deuteron delocalization and associated geometric effects. Using the same FP-derived model under pressure, we further determine the linear dependence of the Curie temperature ( T c ) on δ , in agreement with neutron diffraction experiments, revealing a complementary T c – δ correlation that provides direct insight into how structural tuning controls the ferroelectric transition. The combined P s – δ and T c – δ relations rationalize the giant isotope effect in KDP and suggest strategies to expand the polarization and the accessible T c range through deuteration and control of hydrogen-bond geometry. Ab initio molecular dynamics simulations further show that proton transfer at the transition is collective and assisted by hydrogen-bond-length fluctuations. These results provide a unified microscopic picture of isotope-controlled ferroelectricity in KDP, and identify structure–property relations that may be useful for investigating related hydrogen-bonded ferroelectrics.