Neutron diffraction characterization and magnetic properties of the scheelite-type ErCrO4 polymorph

The new scheelite-type ErCrO4 polymorph has been obtained from the zircon-type ErCrO4 phase by a treatment at 4 GPa and 823 K. X-ray and neutron diffraction studies reveal that this scheelite-type polymorph crystallizes with tetragonal symmetry, space group I41/a being the lattice parameters a = 4.9989(10) ? and c = 11.2332(23) ?. Although bisdisphenoids [ErO8] and tetrahedra [CrO4] polyhedra are present in both compounds, dramatic changes have been observed in the magnetic properties of both polymorphs of ErCrO4. The zircon one is ferromagnetic with TC? 15 K; while the corresponding scheelite-type polymorph behaves as antiferromagnetic with TN? 21 K. The observed change of sign in these compounds could be explained by considering the differences found in the superexchange Er?O?Cr bond angles through which the magnetic interactions take place in both polymorphs.