{101^-4} twin boundary in calcite: Structure and physical properties

Calcite, a CaCO3 polymorph, is one of the most significant materials in nature. In addition to its fundamental role in the global carbon cycle, it fills an unparalleled range of applications, both as a natural biomineral and more directly engineered materials. One of the most notable characteristics of calcite is its tendency to form twins, the boundaries of which are expected to determine key attributes of the host mineral at mesoscopic scales. Using a classical molecular dynamics simulation performed with a thermodynamically consistent rigid-ion force field, we show that the walls between the two twin domains aligned along the hexagonal (101^-4) plane of calcite have some unexpected properties: (1) rather than monoatomic planes, these twin walls are composed of two layers with fundamentally different distortions, (2) atomic shifts within the twin walls create strong polarity while the bulk remains centrosymmetric, and (3) the temperature evolution of the structural order parameter is mostly, but not completely, related to the tilt and rotation of CO₃^2- molecules. Molecular dynamics simulations using thermodynamically accurate interatomic potentials demonstrate that the temperature evolution of twin walls is different from that of the bulk material. We show that kinks in twin walls have a low energy and generate high structural disorder near the kink position. Our methods are applicable to other molecular systems and emphasize the dominance of twin walls in materials.