MAGNETOELASTIC TETRAGONAL-TO-ORTHORHOMBIC DISTORTION IN ERNI2B2C

We have performed synchrotron x-ray scattering experiments on single c rystals of ErNi2B2C. Below 6.3 K, the (2,0,0) Bragg peak splits, indic ating a tetragonal-to-orthorhombic distortion. The mismatch between th e a and b lattice parameters appears coincident with the onset of long -range antiferromagnetic order and increases continuously with decreas ing temperature, reaching a value of a/b-1 approximate to 0.2% at 3.7 K.