On the order-disorder phase transition of anilinium halides. 127I-NQR investigation of the anilinium iodides C6H5NH3+Idirectdifference, C6H5ND3+Idirectdifference; C6D5ND3+Idirectdifference. Dilatometric studies on anilinium bromide and anilinium iodide single crystals

127I-NQR measurements in the range 77≤T/K < or approx.520 are reported for anilinium iodides C6H5NH3+Idirectdifference, C6H5ND3+Idirectdifference, C6D5NH3+Idirectdifference, and C6D5ND3+Idirectdifference. The compound undergoes a ferroelastic phase transition at 240.3 K (C6H5NH3+Idirectdifference). The transition temperature changes to 233.6 K (C6H5ND3+Idirectdifference) and to 233.4 K (C6D5ND3+Idirectdifference) by deuteration. The NQR parameters are thereby influenced too. At 293 K the -NH3+ group deuteration influences e2ΦzzQh-1 and η much stronger than the ring deuteration, whereas at 77 K these effects are of comparable magnitude. - The thermal expansion coefficients were measured for C6H5NH3+Xdirectdifference, X=Br, I, from 120 K to 300 K by dilatometry and by X-ray diffraction. In C6H5NH3+Idirectdifference α[010] increases and both, α[100] and α]001], decrease with increasing temperature. On hand of the known crystal structures the ferroelastic phase transition in the anilinium halides is discussed. The basic mechanism of the transition is the same for C6H5NH3+Idirectdifference and C6H5NH3+Brdirectdifference. Freezing in of the -NH3+ group hindered rotation is the main process governing the phase transition, but motions of the phenyl ring are of influence, too. (orig.)