A mistake in the order in which a crystal’s layers are stacked – one layer shifted or turned from where the pattern says it should sit, like one misaligned sheet in a neatly squared stack of paper. Layered crystals get them easily, because the weak bonds between layers barely care. But the stacking can change a material’s magnetism, symmetry or electronic behaviour, so faults can make one crystal behave unlike the next.
Going deeper
A stacking fault leaves every layer intact and breaks only the order in which the layers sit. Diffraction shows it as streaks between the spots of a perfect crystal.
Order in the stack
A layered crystal is defined by two things: what one layer looks like, and how each layer sits on the one below. MoS2 is the same sheet in its and 3R forms; only the stacking differs, and with it the symmetry. A stacking fault is a place where the sequence breaks – one layer shifted sideways by a fraction of the lattice, or rotated, so the patterns below and above it no longer match.
In a layered crystal such mistakes cost very little energy, because the only thing holding the layers in register is the weak attraction. They form while crystals grow, when they cool through a change of structure, and when they are bent, pressed or cleaved.
Why they matter
The layers themselves are untouched, but many properties depend on how they sit. In α-RuCl3, well-stacked crystals order magnetically near 7 K; faulted regions order near 14 K, so a second transition in the heat capacity is the usual sign of a faulted sample, and some early results turned out to depend on it. In CrI3 the stacking decides whether neighbouring layers couple ferromagnetically or antiferromagnetically. Stacking also sets symmetry, so a fault can switch effects such as , or the of some stacks, on or off locally.
Finding them
reveals stacking faults as streaks and broadened peaks along the direction perpendicular to the layers, where a perfect crystal gives sharp spots, and the amount of streaking estimates how often they occur. of a cross-section shows them layer by layer. In thin , second-harmonic generation and the low-frequency modes in which whole layers slide against each other are sensitive to stacking and can be mapped across a sample. Growers reduce faults by growing and cooling slowly and by avoiding when handling crystals.
For specialists
A planar defect in which the stacking sequence is interrupted – for example …ABCABABC… in place of …ABCABCABC… – without a change in the layers themselves. In van der Waals crystals the energy cost is small, so faults form during growth, on cooling through structural transitions, and during cleaving and handling. They produce diffuse streaks along c* in diffraction and can alter , symmetry-allowed responses and , as in α-RuCl3, where faulted regions order near 14 K rather than 7 K.
Where this comes from
Introduction to Dislocations (5th edition)Hull and Bacon · Butterworth-Heinemann, Oxford (2011)