Phonon-induced quadrupolar ordering of the magnetic superconductor   TmNi$_2$B$_2$C
  We present synchrotron x-ray diffraction studies revealing that the lattice of thulium borocarbide is distorted below T_Q = 13.5 K at zero field. T_Q increases and the amplitude of the displacements is drastically enhanced, by a factor of 10 at 60 kOe, when a magnetic field is applied along [100]. The distortion occurs at the same wave vector as the antiferromagnetic ordering induced by the a-axis field. A model is presented that accounts for the properties of the quadrupolar phase and explains the peculiar behavior of the antiferromagnetic ordering previously observed in this compound. 
N. H. Andersen, J. Jensen, T. B. S. Jensen, M. v. Zimmermann, R.   Pinholt, A. B. Abrahamsen, K. Noegaard Toft, P. Hedegard,...