| dc.description.abstract | The study of molecular dynamics in selenates and plumbates of the R?MX? family has revealed a few interesting features, like the small?angle torsional oscillations of the whole ion, a number of phase transitions and the presence of inequivalent ions, besides the normally expected reorientation mechanisms in these compounds. Systematic change in the molecular dynamics is seen with the methyl / polymethyl substitution. The salient features observed in these compounds are given in Table C1 and C2.
Besides the normally expected relaxation due to the general reorientation of the NH?? ion, ammonium hexabromo selenate has shown a number of phase transitions in the temperature range studied (400 K – 77 K). Spin?rotation interaction at high temperatures, hysteresis in the T? values and the nearly constant value of T? observed at lower temperatures are the special features observed.
The literature available on the study of molecular dynamics of the methyl ammonium hexahalo metallates has revealed that the relaxation is due to the correlated motion of the NH? and CH? groups at higher temperatures, while it is due to the uncorrelated motion and the methyl group reorientations at lower temperatures. Besides these general features, the methyl ammonium hexabromo selenate and methyl ammonium hexachloro plumbate have revealed the presence of the small?angle torsion of the methyl ammonium cation in the intermediate range of temperatures (around 150 K). At lower temperatures (<150 K) selenate shows the relaxation due to the uncorrelated motion of NH? and CH? groups while in the plumbate, uncorrelated motion is the cause of relaxation in one phase and in the other, the inequivalent methyl groups contribute.
Dimethyl ammonium ion has lower symmetry than the other compounds. The flip motion of the DMA ion about the diad axis is observed at higher temperatures (> 300 K) in a few compounds like the hexahalo platinate, stannate and tellurates and the methyl?group reorientation dominates at low temperatures. In DMA halides, the relaxation is observed to be only due to the reorientation of the methyl groups (below 350 K). However, DMA hexabromo selenate has revealed diffusion at higher temperatures (> 350 K) followed by the spin?rotation as well as the flip motion of the DMA ion. Around 150 K, the unusually shorter T? values observed have indicated the presence of small?angle torsion of the DMA ion. As expected, below 150 K, the relaxation is due to the dynamics of the methyl groups.
The substitution of three methyl groups restores the three?fold symmetry. The general observation in TrMA compounds is the proximity of the correlation times for the reorientation of TrMA ions and the methyl groups due to which the corresponding T? minima are barely resolved. Trimethyl ammonium hexabromo selenate shows a high?temperature T? minimum twice longer than the one expected for the motion of TrMA ion and a broad T? minimum around 150 K followed by another low?temperature minimum. The observed results indicate the presence of inequivalent TrMA ions and inequivalent methyl groups. Below 130 K, the small?angle torsion of the methyl groups becomes important. The correlation times for the TrMA and methyl?group motions are very close to each other in the plumbate analogue and hence the minima due to these motions are hardly resolved. The trimethyl ammonium hexachloro plumbate has shown the rarely encountered phase transitions in TrMA compounds, one around 290 K and the other around 119 K.
Tetramethyl ammonium ion is symmetric and exhibits both TMA tumbling as well as the methyl?group reorientation. Tetramethyl ammonium hexabromo selenate shows a high?temperature phase transition at 370 K (discontinuous jump in T?) and another at low temperature (slope change around 150 K). The analysis shows the relaxation to be due to the dynamics of inequivalent TMA ions at high temperatures which changes over to small?angle torsion around 150 K and slows down below this temperature. In the low temperatures the methyl reorientation becomes the dominant relaxation mechanism. A comparative study of TMA compounds shows three prominent phase transitions in these compounds. The one observed around 370 K in TMA hexabromo tellurates is identified as a cubic Fm3m to cubic Fd3c transition. Low?temperature phase transitions are also observed in the optical studies by Berg, Berg et al. and van der Ohe which they ascribe to the change in the motional modes of the methyl groups.
The low activation energy in the double salts indicates greater freedom for the cation in all the compounds studied in the present investigation. In the plumbates, the substitution of the metal atom by the heavier lead appears to have influenced the activation energies more. The T? behaviour observed at lower temperatures indicates the possibility of tunnelling reorientations in these compounds at low temperatures (below ~ 50 K). | |
