Proton mobility through a second order phase transition: theoretical and experimental study of LaNbO4

The gradual change in the crystal structure of the high temperature proton conductor LaNbO4 through a second order phase transition and its relation to the activation enthalpy of mobility of protons have been studied by means of first principles calculations and conductivity measurements. The computations have revealed that protons diffuse by an inter-tetrahedral mechanism where the activation enthalpies of mobility are 39 and 60 kJ mol−1 in tetragonal and monoclinic LaNbO4, respectively. The activation enthalpy of mobility of protons for tetragonal LaNbO4, determined from the conductivity curve, is 35 kJ mol−1. Below the transition temperature the conductivity curve bends; initially dropping off steeply, followed by a less steep decrease towards lower temperatures. The bend in the conductivity curve at the onset of the phase transition in LaNbO4 should not be given the traditional interpretation as an abrupt change in the activation enthalpy of mobility. After application of the proper analysis of the conductivity data, which takes the second order transition into account, the activation enthalpy of mobility of protons is found to continuously increase with increasing monoclinic angle at decreasing temperature, reaching ∼57 kJ mol−1 at 205 °C for the end monoclinic phase.