Structural distortions in antiferromagnetic TbB4 and ErB4

Correlations between low temperature structural distortions and the antiferromagnetic ordering of the rare earth tetraborides TbB4 and ErB4 have been investigated by high precision X‐ray diffraction measurements in conjunction with peak profile analysis. For TbB4, we have to state two separate phase transitions: a crystallographic lattice distortion from tetragonal to orthorhombic symmetry at T=80K, which is followed by a magnetic ordering transition at a definitely lower temperature at TN=43K. The difference between a and b axes is 0.020(4)A at 4.2K; the relative volume change is less than 10−3. For ErB4, we observe a tetragonal to orthorhombic lattice distortion simultaneously with the antiferromagnetic ordering at TN=13K, which is attributed to magnetostrictive effects.