The transformation of CsCl type ⇋ NaCl type. Part II: Orientation relations in phase transformations CsCl type --> NaCl type in ammonium halides
The polymorphic transformation CsCl type → NaCl type in ammonium chloride, bromide, and iodide is shown to be topotaxial and orientation relations derived by X-ray diffraction are reported for NH 4Cl and NH 4Br. These orientation relations are similar to those maintained in the reverse transformation NaCl type → CsCl type. At least the commonest relation can result from cooperative displacements. Transformed crystals of NH 4I and NH 4Br store stresses and imperfections sufficient to modify significantly the ideal thermodynamics of transformation.