K2Fe2B2O7: A transparent nonlinear optical crystal with frustrated magnetism

A new noncentrosymmetric ferroborate crystal, K2Fe2B2O7, has been grown from high temperature melt. Structure solution from single crystal X-ray diffraction shows that the title compound crystallizes in a trigonal space group P321 with cell dimensions of a=8.7475(12) Å and c=8.5124(17) Å. In the structure, FeO4 tetrahedron shares its three basal oxygen atoms with BO3 triangles forming a two-dimensional layer in the ab plane and the layers are connected by the apical Fe–O bonds along the c direction. The crystal is transparent in the visible and near infrared region from 500 to 2000 nm with three pronounced absorption bands ascribed to d–d transitions of tetrahedrally coordinated Fe3+ ions. Though, structurally analog to K2Al2B2O7, the further twisting of the BO3 groups between adjacent layers reduces its optical nonlinearity to a second-harmonic generation intensity of about 0.4 times that of KDP. Spin-glass behavior is observed at 20 K which is probably due to geometrically magnetic frustration of the triangular Fe net in the ab plane.