Thermal evolution of tetramethylammonium tetrafluoborate and perchlorate investigated through dielectric and IR spectroscopy

An investigation of the permittivity and ac conductivity was performed for the first time on the isostructural tetrasubstituted alkylammonium (CH 3)4NBF4 and (CH3) 4NClO4 single crystals, in the wide temperature range 12-300 K. For both the explored compounds, an orientational order/disorder phase transition was brought out as a result of the overlapping of two distinct transitions occurring with different dynamics. The first one is the settlement of a stable array, at low temperature, of the anionic groups (orientationally disordered at RT) with a first order character; the second one is the correspondent cationic sublattice rearrangement with the characteristics of second order. The critical temperatures and limits of the transitions were defined. Different conductivity processes above and below the transition were detected and analyzed. At lower temperatures the intra- and inter-molecular motions of the cationic group were brought out through their contribution to the dielectric constant. The temperature values of the freezing out of the vibrational motions were experimentally established for both the observed ionic crystals. Furthermore, the IR spectroscopy has provided spectra of the vibrational modes in the different phases below and above the phase transition, in excellent agreement with the dielectric data. In a general perspective, the obtained results clearly suggest that the thermal evolution displayed by the two investigated compounds is characteristic of the isostructural tetramethylammonium salts; the expected variations for different anionic groups are discussed.