Synthesis and Crystal Structure of the High-pressure Iron Borate β-FeB2O4

Abstract

The iron borate β -FeB2O4 was synthesized under high-pressure / high-temperature conditions of 8 GPa and 1030 ◦C. The structure of β -FeB2O4 is isotypic to HP-NiB2O4, representing the second example of a borate in which every BO4 tetrahedron shares a common edge with a second one. β - FeB2O4 crystallizes in the space group C2/c (Z = 4) with the parameters a = 950.0(2), b = 562.9(2), c = 443.7(1) pm, β = 108.50(3)◦, V = 0.22495(8) nm3, R1 = 0.0293, and wR2 = 0.0647 (all data). The structure consists of layers of BO4 tetrahedra, connected via strings of edge-sharing FeO6 octahedra. A ligand field splitting of Δo ≈8860 cm−1 is estimated from polarized single-crystal electronic absorption spectra of β -FeB2O4. The tetragonal distortion of the ligand field in the [FeIIO6] chromophore amounts to −(8/3)dσ ≈2900 cm−1. In the range of 16000 cm−1 ≤ṽ ≤24000 cm−1, rather strong spin-forbidden transitions within the [FeIIO6] chromophore are observed