Dielectric properties of the hydrogen-bonded ferroelectric crystal KH₂PO₄ (KDP) differ significantly from those of KD₂PO₄ (DKDP). It is well established that deuteration affects the interplay of hydrogen-bond switches and heavy ion displacements that underlie the emergence of macroscopic polarization, but a detailed microscopic model is missing. We show that all-atom path integral molecular dynamics simulations can predict the isotope effects, revealing the microscopic mechanism that differentiates KDP and DKDP. Proton tunneling generates phosphate configurations that do not contribute to the polarization. At low temperatures, these quantum dipolar defects are substantial in KDP but negligible in DKDP. These intrinsic defects explain why KDP has lower spontaneous polarization and transition entropy than DKDP. The prominent role of quantum fluctuations in KDP is related to the unusual strength of the hydrogen bonds and should be equally important in other crystals of the KDP family, which exhibit similar isotope effects.

氢键铁电晶体 KH₂PO₄ （KDP） 的介电性能与 KD₂PO₄ （DKDP） 的介电性能明显不同。众所周知，氘化会影响氢键开关和重离子位移的相互作用，这是宏观极化出现的基础，但缺少详细的微观模型。我们表明，全原子路径积分分子动力学模拟可以预测同位素效应，揭示了区分 KDP 和 DKDP 的微观机制。质子隧穿产生的磷酸盐构型对极化没有贡献。在低温下，这些量子偶极缺陷在 KDP 中很大，但在 DKDP 中可以忽略不计。这些内在缺陷解释了为什么 KDP 的自发极化和过渡熵比 DKDP 低。量子涨落在 KDP 中的突出作用与氢键的不寻常强度有关，并且在 KDP 家族的其他晶体中应该同样重要，它们表现出类似的同位素效应。