Non-linear and non-local behaviour in spontaneously electrical solids

Abstract

Using reflection-absorption infrared spectroscopy (RAIRS), we show that solids displaying spontaneous dipole orientation possess quite general non-local and non-linear characteristics, exemplified through their internal electric fields. The most graphic illustration of this, uncovered originally through electron beam studies, may be found in films of cis-methyl formate (cis-MF), for which data demonstrated the counter-intuitive property that the degree of dipole order in the film does not monotonically decrease as the temperature of deposition rises, but rather increases sharply above ∼77 K. Here we show how RAIRS provides independent evidence to support this conclusion. These new data confirm (i) that the behaviour of spontelectrics is governed by an expression for the degree of dipole orientation, which is continuous in temperature, but with a discontinuity in the derivative, and (ii) that the temperature of deposition associated with this discontinuity matches the temperature above which dipole order switches from the expected reduction with temperature to an increase with temperature.

Assigning a structural motif using spontaneous molecular dipole orientation in thin films

“…The current work complements the work reported in reference [7] and demonstrates, for the first time, the clear potential of spontelectric measurements in helping to reveal the structural motif inherent in a solid phase. While the IR spectroscopy of thin films of methyl formate is relatively well known [16,17], the absence of any structural data has made interpretation of the spectra as a function of deposition temperature more complex.…”

“…This intriguing behaviour, which remains without explanation, indicates that, by some means, the material is aware of the presence of the 90 underlying layers, which collectively cause the character of thicker films to change [3 -5]. The nonlinearity inherent in spontelectrics, see below, is best illustrated by studies of cis-methyl formate [6,7], the subject of the present work.…”

“…ammonia [12]), of substantial band origin shifts with deposition temperature through the impact of the electronic Stark Effect on Wannier-Mott excitons within a spontelectric solid. The impact of the spontelectric effect on star formation and its potential for engineering electric field structures has also been explored [3 -5] A detailed infrared (IR) study of cis-methyl formate has also revealed this bi-modal behaviour with temperature below and above 77 K and is entirely consistent with the non-linear and non-local growth of spontelectric materials [7].…”

“…The asymmetric and symmetric modes become split by 10, 14, and 13 cm -1 in D1, D2, D3, respectively. been given [7]. In this work, we focus on the observation that above 90 K, no spontelectric field can be measured.…”

Low temperature aging in a molecular glass: the case of cis-methyl formate

“…Values of the dipole moment in the solid, are strongly reduced compared to the gas phase, due to mutual depolarization in the solid. We use the standard expression m = m 0 /(1 + ak/s 3 ), 13,20 where s is the average spacing between successive layers, a is the molecular polarizability, k = 11.034 and m 0 is the gas phase dipole moment of the molecules involved. For cis-MF, s = 0.244 nm, a = 5.25 Â 10 À30 m 3 , m 0 = 1.766 D, giving m = 0.354 D. The model embodied in eqn ( 1) and (2) has been shown to reproduce the values and temperature dependence of observed spontelectric fields in solid films of nitrous oxide, nitrous oxide diluted in xenon, cis-MF and CCl x F y (x + y = 4).…”