DISORDER AND POLYMORPHISM IN TETRAMETHYLAMMONIUM METAPERIODATE

An improved synthesis is reported for N(CH3)(4)+IO4-, and its structur e, low-temperature phase transition, polymorphism, and rotational diso rder were studied by vibrational spectroscopy, differential scanning c alorimetry, and single crystal X-ray diffraction. Its room-temperature structure (phase II) is isomorphic with N(CH3)(4)ClO4 and N(CH3)(4)BF 4 and belongs to the tetragonal space group P4/nmm with Z = 2 and unit cell dimensions a 8.749(2) and c = 6.054(2) Angstrom. The structure w as determined with higher precision (R = 0.036) than those of N(CH3)(4 )ClO4 (R = 0.134) and N(CH3)(4)BF4 (R = 0.089) and consists of ordered N(CH3)(4)(+) cations and disordered IO4- anions which undergo free ro tation about one of the I-O bonds and thereby satisfy the space group requirement for a 4-fold axis at their special (1/4, 1/4, Z) sites. As shown by its variable-temperature Raman spectra, polycrystallinic N(C H3)(4)IO4 undergoes a phase change between +10 and -10 degrees C. This phase II-phase III transition is of low energy and very broad and cou ld not be detected by DSC. The crystal structure of phase III was dete rmined at -30 degrees C (orthorhombic, Pbcm, Z = 4, a = 5.969(2) Angst rom, b = 12.214(3), c = 12.447(2) Angstrom, R = 0.038) and is characte rized by ordered N(CH3)(4)(+) cations and ordered IO4- anions; i.e., t he phase II-phase III transition involves the freezing out of the IO4- ion rotation. The vibrational spectra of phases II and III were recor ded and subjected to factor group analyses. The expected high-temperat ure, phase II-phase I transition, presumably due to the onset of rotat ional disorder of the cations and found for N(CH3)(4)BF4 at 328 degree s C and N(CH3)(4)ClO4 at 340 degrees C, could not be observed for N(CH 3)(4)IO4 because the latter compound explodes violently at about 247 d egrees C before reaching this transition. The results of this study co nfirm the interpretation, proposed in 1930 by Pauling and repeatedly c hallenged since then, that the phase transitions in these compounds ar e due to the onset of ion rotation rather than positional disorder of the rotational oscillation axes.