Evidence for a subtle structural symmetry breaking in EuB6

This work presents a systematic Raman scattering study and first-principles calculations for the EuB6 system. Evidence for the presence of an incipient (∼1 x 10-4 A) tetragonal symmetry break of its crystalline structure was found. Forbidden Raman modes at ωfRm(1)∼1170 cm-1, ωfRm(2)∼1400 cm-1, and ωfRm(3)∼1500 cm-1 were observed. The tetragonal symmetry of ωfRm(2) and ωfRm(3) together with spin-polarized first-principles simulations of the structural and magnetic properties fully support such a break of symmetry. Our data and calculations explain the occurrence of ferromagnetism in Eu hexaborides, previously reported.