Chemical composition, crystal structure and spontaneous polarization of swedenborgite

Abstract

The chemical composition of swedenborgite sample obtained from the type locality, Långban, Värmland, Sweden, was determined using scanning electron microscopy and energy-dispersive X-ray spectroscopy. It was observed that swedenbolgite crystals possess both Ca-free and Ca-containing zones. The crystal structures of swedenborgite [space group P6₃mc, a = 5.4402(10) Å c = 8.8690(9) Å, Z = 2] was refined to R1 = 0.012 using 1573 unique reflections. In addition, the threshold energy of the Sb K-edge XANES spectrum of swedenborgite was found to be higher than that of Sb₂O₃, but almost the same as that of Sb₂O₅. These results indicated that the oxidation state of Sb in swedenborgite was almost pentavalent, although the presence of a small amount of trivalent Sb is also suggested. It was therefore assumed that the following substitution relationship exists: 2Na⁺ + Sb⁵⁺ ⇄ 2Ca²⁺ + Sb³⁺, and that the charge balance of Ca occupation is achieved by reduction of some Sb⁵⁺ ions in the Ca-containing zone. The general formula of swedenborgite was therefore expressed as (Na_1−xCa_x)Be₄Sb⁵⁺_1−0.5xSb³⁺_0.5xO₇ (x = 0.0 or 0.05-0.07). The distortions of the Be-O distances along the c-axis and O-Be-O angles of BeO₄ trigonal pyramid in swedenborgite were significantly larger than those in BeO bromellite. Opposite coordinate shifts between cations and anions along the c-axis occur because of the asymmetric arrangement around the NaO₁₂ tetradecahedra with upper face sharing and lower edge sharing. The structure of swedenborgite therefore exhibits a biased arrangement of cations and anions parallel to the c-axis, which induces spontaneous polarization.