The onset of quadrupole ordering at the structural phase transition in DyB2C2

X-ray diffraction and heat capacity data for DyB2C2 are used to derive quantitative information about the structural phase transition at 24.7 K. A successful interpretation is achieved in terms of a continuous transition to a crystal structure belonging to the space group P42/mnm (No 136) that supports a motif of Dy 4f quadrupoles which belong to the Ag representation of the point group C2h for the Dy site in the distorted structure. There is evidence in favour of the Ising universality class for the transition, including a critical exponent β = 0.30 ± 0.02 and a near-logarithmic divergence of the heat capacity. (letter to the editor)