Structural phase transition in the spin gap system YbAl₃C ₃

The unresolved phase transition at 80K in YbAl₃C₃ has been studied by X-ray diffraction. It has been confirmed that the transition is a structural one with atomic displacement. Superlattice reflections that appear below 80 K show that the crystal structure changes from hexagonal to orthorhombic. From the reflection conditions, we conclude that the space group in the low temperature phase is Pbca, where all the atoms occupy the general 8c site. By fitting the observed intensities, we present two possible models of atomic displacement. The displacement results in the deformation of the triangular lattice of Yb spins, which is considered to be responsible for the formation of a spin-singlet ground state at low temperatures.