The phase transitions in the host-guest composite structure of potassium have been determined between 19 and 54 GPa by a combination of x-ray powder and single-crystal diffraction. The guest component of the incommensurate composite phase K-III undergoes an "intraphase" transition at 30.0(7) GPa from a C-centered tetragonal structure to an A-centered orthorhombic form. At 39.7(5) GPa, the guest component undergoes a reentrant transition back to the C-centered tetragonal form. This is the first such reentrant transition observed in the composite structures of the elements. The transition in the guest at 30.0(7) GPa is preceded by a loss of the c-glide symmetry of the host component of the structure, and this is not recovered until some 3 GPa after the reentrant transition at 39.7 GPa.

A martensitic phase transition exhibiting shape memory, transformation-induced plasticity, or superelasticity typically involves a transformation between a high temperature, high symmetry phase and a low temperature, low symmetry phase. There have been numerous attempts using molecular dynamics to simulate the shape memory behavior, where the memory is stored in a twinned martensite and deformation occurs by motion of twin boundaries. However, the 3D case has always proved elusive, because suitable interatomic potentials to produce a unique low temperature phase are difficult to obtain. Here we present a study in which the binary Morse potential is tuned specifically to maximize the difference between L1 0 and B19 (Strukturbericht notation, spacegroups P4/mmm and Pmma) structures. The twinned structure of martensite has been induced by gradually cooling the sample below the transition temperature. A bar-shaped sample was plastically deformed in the martensite phase, and on reheating above the transition temperature the initial shape was recovered. The effect of the shear-induced phase transition on the nanostructure of resulting martensite has also been investigated. An unusual discovery is that of a hierarchy of twins: nanotwins accommodate the mismatch between austenite and martensite at the habit plane, while dynamically created macrotwins are responsible for the deformation behavior and shape memory.