Crystal Distortion of Dy2Ti2O7 at the Spin Ice Transition Temperature

Dy2Ti2O7 with the pyrochlore structure shows a spin ice transition at about I K. The Dy3+ ion in this compound has an effective spin S-eff= 1/2. The Dy ions reside on the vertices of corner-linked tetrahedra. Due to a strong single-ion anisotropy, the ground state of Dy3+ is well expressed by an Ising doublet with local < 111 > quantization axes. We measured the x-ray diffraction for a Dy2Ti2O7 powder sample between 0.15 K and 20 K. The lattice spacing increases drastically between 1.5 K and about 1 K, with decreasing temperature. The lattice spacing of the (222) diffraction is considerably larger than the other directions.