Magnetoelastic tetragonal-to-orthorhombic distortion in ${\mathrm{ErNi}}_2{\mathrm{B}}_2$C

We have performed synchrotron x-ray scattering experiments on single crystals of ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$. Below 6.3 K, the $\mathrm{}(2,0,0)\mathrm{}$ Bragg peak splits, indicating a tetragonal-to-orthorhombic distortion. The mismatch between the $a$ and $b$ lattice parameters appears coincident with the onset of long-range antiferromagnetic order and increases continuously with decreasing temperature, reaching a value of $a/b-1\approx 0.2$% at 3.7 K.