Abstract
We report the results of a combined neutron diffraction and total scattering study of the orientational order-disorder phase transitions in potassium cyanide, KCN. The diffraction data are analysed in terms of the spontaneous strains that accompany the phase transitions. The total scattering data are analysed using the Reverse Monte Carlo method, which gives direct access to the distribution of atomic positions and hence molecular orientations in each phase. Incorporating information from diffuse scattering in this way provides a means to measure the coefficients of the orientation distribution function up to almost arbitrarily high orders, and furthermore has the advantage that this function is naturally positive everywhere. The results for the cubic phase show that the distribution of orientations never exceeds 25% difference from an isotropic distribution.
Highlights
The methylammonium includes a C–N bond, a direct analogue of the cyanide ion in KCN, for example, and a recent study by molecular dynamics simulation of MAPbI3 compared the ordering to that of KCN from singlecrystal diffraction, suggesting that the methylammonium ions are somewhat more isotropic, again the spherical harmonic expansion was terminated at very low order [22]
In order to understand the strain behaviour associated with the phase transition, we obtained the form of the Landau expansion of the free energy associated with the
The degree of orientational order/disorder described by the orientation distribution function of each phases is highlighted by calculating the three-dimensional representations as shown in Figure 13, together with views of representative Reverse Monte Carlo (RMC) configurations
Summary
Orientational disorder is a common phenomenon in molecular crystals or inorganic crystals.‡ Often, at least in the first phases found on cooling below the melting point, the. The methylammonium includes a C–N bond, a direct analogue of the cyanide ion in KCN, for example, and a recent study by molecular dynamics simulation of MAPbI3 compared the ordering to that of KCN from singlecrystal diffraction, suggesting that the methylammonium ions are somewhat more isotropic, again the spherical harmonic expansion was terminated at very low order [22] For these reasons it is timely to return to the example of KCN. We take this material as a case study in the use of the neutron total scattering method to provide information about the distribution of orientations – and of electric dipole and quadrupole moments – in an orientationally disordered or partially ordered crystal
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
