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We report the first direct observation of the influence of high magnetic fields on the Jahn-Teller (JT) transition in TbVO(4). Contrary to spectroscopic and magnetic methods, x-ray diffraction directly measures the JT distortion; the splitting between the (311)/(131) and (202)/(022) pairs of Bragg reflections is proportional to the order parameter. Our experimental results are compared to mean-field calculations, taking into account all possible orientations of the grains relative to the applied field, and qualitative agreement is obtained.

The occurrence of a quadrupolar ordering within the γ orthorhombic symmetry at 7 K in TmAu 2 (MoSi 2 -type tetragonal structure) was recently discovered with properties closely reminiscent of the ones observed in isomorphous TmAg 2 . Both the magnetoelastic couplings and the quadrupolar pair interactions are here determined by means of third-order magnetic susceptibility and parastriction measurements. The existence of an antiferromagnetic ordering around 3 K in the quadrupolar phase leads to complex magnetic phase diagrams in the (H, T ) plane, which have been established along the main crystallographic directions of the tetragonal cell and appear to result from the balance between magnetic and quadrupolar interactions.

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