We have employed Raman spectroscopy in order to study the mechanism of the significantly increased ferroelectric phase transition temperatures in proton-irradiated K(H1-x D-x)(2)PO4 crystals. Prominent changes occurred in the asymmetric stretching vibration mode frequency of D2PO4 molecules in the proton-irradiated crystals, thus stating the role of the lattice coupled to H/D. (C) 2016 Elsevier B.V. All rights reserved.