We report that a new 2D 3d-4f phosphonate [(CoLaIII)-La-III(notpH)(H2O)(6)]ClO4 center dot 5H(2)O (CoLa-II) can undergo a phase transition above 45 degrees C and 93% relative humidity, resulting in [H3O][CoLa(notp)-(H2O)(4)]ClO4 center dot 3H(2)O (CoLa-III). The transition is accompanied by the release of the proton from intralayer to interlayer, and thus the proton conductivity of the material is increased by 1 order of magnitude.