Phase Transformations in the Absence of an Externally Applied Stress

Phase Transformations in the Absence of an Externally Applied Stress

Form IV (distorted CsCl structure) and form III (NiAs structure type?) are both orthorhombic and the transformation between these forms are reconstructive. Proton magnetic resonance (PMR) studies indicate that, at room temperature and above, the NH₄ ions are undergoing random reorientations, while X-ray studies indicate that the NO₃ ions are in fixed orientations only in forms IV and III. On heating III transforms to II which is tetragonal and structurally similar to IV: consequently the III to II transformation is also reconstructive.

Fig. 4 Single crystal of Phase IV transforms on heating at about 32°C into phase III. A 'faceted' interface advances slowly but steadily from the bottom left corner.

Fig. 5 Transformation to III is now complete. Rotation of the specimen in the movie (crossed polarisers) shows it is an extremely fine-grained polycrystal..

Fig. 6 III transforms on heating at ~85°C to II. The movie shows an interface which has a complex shape and which advances from south to north.

Fig. 7 Phase II cooled quickly from melt (via phase I). Contrasting colours represent crystals of different orientations- this preparation route is ideal for experiments needing polycrstalline aggregates. The movie contains a sequence where several regions of the polycrystal are rotated between crossed polarisers - elongate subgrains exist inside many of the larger grains.

Fig. 8 In this movie the II-I phase boundary moves in response to heating and cooling. The first temperature change is heating and the mobile phase boundary takes on cuspate shapes indicating some local pinning. The polarisers here are slightly uncrossed so that bubbles, etc in the cubic phase I are visible. Note that this is a solid state transition, not melting. The last part of the movie shows cooling where the grains of phase II enlarge mainly by epitaxial growth of those grains that originally touched the I-II boundary. This leaves large columnar grains of II behind the southward-moving boundary.

In form II the NO₃ ions are undergoing rotational disorder in their plane and electrical conductivity and PMR studies indicate that the NH₄ ions are diffusing relatively freely compared with forms IV and III. However, the II to I transformation is displacive and the rotational disorder of the NO₃ ions in form I is no longer restricted to the plane of the ion. Thus both ions in I are essentially spherically symmetric and, consequently, I is cubic with the CsC1 structure. PMR evidence indicated a further, very significant, increase in the diffusion of the NH₄ ions at the II to I transformation, so that form I behaves like a superionic conductor. Forms II and IV are simply slight distortions of form I, brought about essentially by increasing rotational and/or translational order on cooling. The reconstructive transformations into III from either II (on cooling) or IV (on heating) occur only if some solvent for NH₄NO₃ is present, even in very small amounts. In perfectly dry specimens displacive transformations between IV, II and I only occur.

Although NH₄NO₃ is not a structural analogue of any important rock-forming mineral, it is, however, a very suitable material for studying the microstructural changes that take place during different types of solid-state phase transformations. The microstructural changes have been observed in thin films in the light microscope. Being reconstructive, the transformations involving form III usually lead to a significant reduction in grain size (Fig 5). However, the "massive" transformations between II and I lead to significant grain growth. The II to IV transformation is "martensitic" – small lenses of IV propagate rapidly across large grains of II, and then thicken steadily until all of the host grain is consumed (Fig 9).

Fig. 9 Transformation of II to IV on cooling through ~50°C. "Martensitic"- rapid but sporadic. In the movie sequence a remarkable series of banded microstructures reveal how this type of shear transformation proceeds. Crystallographically controlled but narrow lenses of phase IV initiate then expand through each crystal of II. In some regions the advance of the lenses is steady, other regions show "instantaneous" conversion. There must be local internal stresses influencing this process.

Fig. 10 Transformation of IV to II on heating through ~50°C. The banded microstructure of IV results from a process like that seen in Fig 9 and corresponds to domains of different crystallographic orientation. The movie sequence shows the bands 'fading' with heating as the complex region steadily transforms into a single orientation of phase II and this process is essentially complete by the movie's end.

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