  Anhydrous sodium hydroxide, a common and structurally simple compound, shows
spectacular isotope effects: NaOD undergoes a first-order transition, which is
absent in NaOH. By combining ab initio electronic structure calculations with
path integrals, we show that NaOH is an unusual example of a quantum
paraelectric: zero-point quantum fluctuations stretch the weak hydrogen bonds
(HBs) until they become unstable and break. By strengthening HBs via isotope
substitution or applied pressure, the system can be driven down to a
broken-symmetry antiferroelectric phase. We also provide a simple quantitative
criterion for HB breaking in layered crystals and show that nuclear quantum
effects are crucial in paraelectric to ferroelectric transitions in
hydrogen-bonded hydroxides.
