On the Order‐Disorder Phase Transition of Anilinium Halides. Crystal Structure of the High and Low Temperature Phase of Anilinium Iodide, C6H5NHI⊖ (C6D5NHI−). X‐Ray and Neutron Diffraction Studies

Abstract

AbstractThe crystal structure of the high temperature phase I of anilinium iodide, C6H5NHI⊖, was determined by X‐ray and neutron diffraction at T = 294 K: D‐Pnaa, Z = 4, a = 1738.2(10) pm, b = 636.3(5) pm, c = 703.9(5) pm, ϱcalc = 1.886 Mgm−3. At Tc = 241 K, a ferroelastic phase transition occurs. The crystal structure of the low temperature phase II was studied on C6D5NHI⊖ by neutron diffraction at T = 140K: C52h‐P21/a, Z = 4, a = 1753.8(10) pm, b = 614.2(5) pm, c = 706.5(5) pm, β = 90.79(1)°, ϱcalc = 1.973 Mgm−3. Phase I is disordered with respect to four different positions of the −NH group, whereas phase II is ordered. The present results are compared with the crystal structures and the phase transition of anilinium bromide, C6H5NHBr⊖. The mechanism of the phase transition is discussed.