Martensitic transformation cubic ? rhombohedral in rubidium nitrate

Orientations and twinning in the structural transformation aragonite-type to calcite-like in potassium nitrate crystals

“…That a structure having dynamically disordered anions can convert by a lattice deformation with tilt of the anions is shown by the transformation RbNO3 I --, II, which is from cubic to KNOa I type. This transformation is martensitic and produces fine lamellae, themselves internally twinned (Kennedy, 1970;Kennedy & Kriven, 1976). If, as has been argued here, the oxygen atoms can be unlocked from their cation coordination either by thermal motion or by displacements consequent upon the structural deformation, then the array of ion centres can be transformed homogeneously.…”

“…One would, however, expect a martensitic subtexture based on the twinning to be more complex, as in rubidium nitrate (Kennedy & Kriven, 1976). In so far as these results are relevant also to calcium carbonate, they imply that structural transformation should be regarded as a potential additional source of twinning in naturally occurring calcite coexisting with aragonite.…”

Structural relationships between the polymorphs of silver nitrate

“…Although the structures of M(XO3) crystals are due in part to the mutual coordination of O atoms and cations (Fraser, Kennedy & Snow, 1975), this coordination can be unlocked in other transformations by a reconstructive activated step without disruption of the whole framework, which can deform homogeneously to produce the new lattice. This has been shown experimentally by the mode of conversion of TINO 3 III to the RbNO 3 III structure (Kennedy & Patterson, 1965); and especially of cubic RbNO 3 III to the NaNO3-1ike RbNO 3 II, in which the planes of the anions are quite differently oriented (Kennedy & Kriven, 1976). KNO3 II also transforms to the high-NaNO 3 structure in nonrandom though complex ways (Kennedy & Kriven, 1972;Kennedy & Odlyha, 1977).…”