The Isotope-Effect in the Phase Transition of KH2PO4: New Insights from Ab Initio Path-Integral Simulations
We investigate the quantum-mechanical localization of 1H and 2H isotopes in the symmetric low-barrier hydrogen-bonds of potassium dihydrogen phosphate (KDP) crystals in the paraelectric phase. The spatial density distributions of these hydrogen atoms are suspected to be responsible for the surprisingly large isotope effect observed for the ferroelectric phase transition in KDP. We employ ab initio path integral molecular dynamics simulations to obtain the nuclear real-space and momentum-space densities n(R) and n(k) of 1H and 2H, of which the latter densitites are compared to experimental neutron compton scattering data. Our results suggest a qualitative difference in the nature of the paraelectric phase in KDP between the two isotopes. Whereas both paraelectric states result from quantum delocalization, the essential difference is the change from a probably coherent to incoherent tunneling behavior of the hydrogen atoms across the hydrogen-bonds.