Ionic Conductivity in Ordered and Disordered Phases of Plastic Crystals

Abstract

Pentaglycerine (PG) and tris-hydroxy-aminomethane (Tris) are potential thermal energy storage materials which undergo energetic solid-state phase transformations. The low temperature a-phase structure of PG is body-centered tetragonal and that of Tris is orthorhombic. The high-temperature phase structure of both compounds is face-centered cubic. Polymorphic changes in the structure occur at 89~ for PG and 135~ for Tris. These compounds are dielectrics at low temperature with conductivities on the order of i0 -8 ~-~ cm -~ (at 21~ and I0 -9 ~-~ cm ~ (at 25~ for PG and Tris, respectively. Heating samples from room temperature leads to an initial decrease in a-phase conductivities followed by an increase for both of these compounds as a result of proton hopping. Continuous increases in the conductivity were observed at the transition temperature, with discontinuity evident in the case of PG. The conductivity variations in the a-and ~-phases have been found to be thermally activated. The activation energies are nearly equal suggesting similar conduction mechanisms and charge carriers. From the relaxation time data, an equation has been obtained for the charge carrier self-diffusion coefficient for the ~-phase of PG, just above the transition temperature; D (~) = 0.085e ~7,1~7~T. The conductivity in the ~]-phases are independent of the frequency in the range of i0 to i03 Hz. The temperature and frequency effects on the conductivities, relaxation times, and diffusional parameters of PG and Tris are presented.Molecular "plastic crystals" which undergo solid-state transitions are potential candidates for thermal energy storage applications.

Fast ion conduction in an acid doped pentaglycerine plastic crystal

“…Even in the pure sample, the shape of the transition is asymmetric [22]. Chandra et al, and Smith et al, based on a study of pentaerythritol, have proposed that the α → γ order-disorder transition is responsible for this thermal event [20,21]. This transition apparently involves the rearrangement of the -OH groups via rotation about the C-C axis.…”

“…1) contains several hydroxyl groups which one would envisage could lead to better compatibility with acid dopants and potentially offers alternative sites for the proton to reside and migrate between. Pure pentaglycerine has been studied by Chandra et al [20] and was shown to have a solid-solid phase transition from an ordered crystalline phase to a disordered plastic crystal phase at 85°C and a melting point at approximately 200°C. In the pure state, the ionic conductivity should be negligible, although Chandra was able to measure a value of 1.34 × 10 − 6 S/cm, at 105°C, apparently in the plastic phase, which may be attributed to impurities or alternatively, H + transport via the hydroxyl groups.…”

Atomistic simulation of structure and dynamics of the plastic crystal diethyl(methyl)(isobutyl)phosphonium hexafluorophosphate

“…The conformational characteristics of plastic crystals lead to many interesting physical and chemical properties. These properties may be potentially exploited in applications such as thermal energy storage 2,5,6 and as solid electrolytes. 1,[7][8][9][10] Organic ionic plastic crystals (OIPCs) are a more recently developed subclass of plastic crystal that display specific fast-ion transport capability, where dopant ions (such as Li + ) are highly mobile in an OIPC matrix.…”

Plastic crystal phases with high proton conductivity

“…These materials typically display one or more solid-solid phase transitions, from a fully ordered crystalline phase at low temperatures to a disordered, plastic phase. 16,17 One source of the disorder at the molecular level is the presence of rotational motion of the molecule on its lattice site, giving rise to what are known as rotator phases. 18,19 In the ionic compounds, the rotator motions can involve either the cation or the anion or both, and it is these that become progressively activated in the higher temperature phases.…”