The determination of the effective rotational potential in the plastic phase of C₂Cl₆from neutron Bragg scattering data

Abstract

The effective rotational potential V(to) in the plastic phase of C~C16 is computed from neutron diffraction Bragg data. The formalism used applies the Boltzmann factor exp (-flV(to)) in order to describe the orientational pair distribution function of the smeared nuclear scattering density. A model potential calculated from atom-atom interactions in an (8+1) cluster of molecules reproduces the potential gexp(ta) determined from the experiment fairly well. From this we conclude that gexp(Oa) is due to the time independent crystalline potential rather than due to the averaged random rotational torque. The lowest rotational barrier height is approximately 1000 K.

Orientational disorder in nickel(II)hexammine nitrate

“…Up to now only neutron diffraction data had been used for the determination of potential parameters from diffraction measurements, and only soft potentials with maxima around 1000 to 2000 K (NH 4 in (NH4)2SnCI 6 [22,26]; C2C16 in the plastic state [27]) had been found. Because of the additional damping of the X-ray intensities by the atomic form factor, X-ray data were considered as less applicable for this purpose.…”

Competing structures within the first shell of liquid C2Cl6: A molecular dynamics study

“…The potential parameters were chosen from the Gromos53a6 [23] force field, those values are presented in table 1. Although we have not found reported simulations on the liquid state of C 2 Cl 6 , there are previous works on the crystalline and plastic phases using MC [16,24] and MD [19] methods, which we use as a check for the potential in an auxiliary simulation on the known BCC structure before proceeding. The simulation was made on a 2000 molecules system using the NPT ensemble at the thermodynamics conditions of the liquid [25], namely T = 458K and P = 1atm.…”

Short-range-order correlations in the orientationally disordered phase of hexachloroethane. I. Diffuse X-ray scattering

“…These fluctuations are experimentally observable in the diffuse X-ray and coherent neutron scattering as stated in (1). The Bragg scattering expressed in (4), on the other hand, is proportional to the mean value of (1~) and can be used to determine in an analogous way an effective (rotational) potential acting on an individual molecule (Gerlach, Prandl & Vogt, 1984). The orientational fluctuations derived in (14) diverge for wave vector qo and temperature Tc at which…”

“…This leads to four distinguishable discrete orientations for each molecule, which are equally occupied. The interaction energies for the 16 possible nearest-neighbour configurations were calculated with the use of semiempirical atom-atom potentials (Gerlach, Prandl & Vogt, 1984). The Fourier-transformed interaction matrices Jq [equation (12)], together with a schematic drawing of these configurations, are given in Table 1.…”