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Co²⁺-Cu²⁺ substitution in bieberite solid-solution series, (Co_1–xCu_x)SO₄·7H₂O, 0.00 x 0.46: Synthesis, single-crystal structure analysis, and optical spectroscopy

¹ Division of Mineralogy, Department of Material Science, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria
² Institute of Crystallography, RWTH—Aachen University, Jägerstr. 17/19, D-52056 Aachen, Germany

Correspondence: ^* E-mail: guenther.redhammer{at}sbg.ac.at

Single crystals of Cu-substituted bieberite (Co_1–xCu_x)SO₄·7 H₂O have been synthesized from aqueous solution by the evaporation method. It is shown that the solubility of Cu²⁺ in bieberite is limited to 0.46 atoms per formula unit (apfu). Chalcantite CuSO₄·5H₂O only accommodates a very small amount of ~0.03 Co²⁺ atoms. Structure analysis by X-ray diffraction of single crystals along the join CoSO₄·7H₂O–CuSO₄·5H₂O showed that the hepta-hydrate phases are monoclinic with P2₁/c symmetry at room temperature, whereas the penta-hydrate phases are triclinic, P1. Within the Co_1–xCu_xSO₄·7H₂O series (0 x 0.46), lattice parameters are distinctly altered by the Cu²⁺ substitution, however, the observed changes cannot be ascribed to different ionic radii of Co²⁺ and Cu²⁺ but are due to an increasing distortion of the lattice by increasing Cu²⁺ content. Cu²⁺ is not distributed randomly over the two possible crystallographic metal sites but exclusively enters the M2 site. While all the oxygen atoms coordinating the M1 site act as donors of hydrogen bonding to the sulfate tetrahedron, one oxygen atom at the M2 site is a hydrogen bond acceptor. This may cause the slight tetragonal elongation of the M2 coordination polyhedron, which favors the preferential occupation of this site by Cu²⁺. Optical spectra show typical features of both the Co²⁺ and the Cu²⁺ absorption signatures. While in Co²⁺-rich hepta-hydrate phases Co²⁺ absorption bands arising from both M1 and M2 sites are detected, there are only bands arising from the M1 site in Co_0.54Cu_0.46SO₄·7H₂O. This supports the observation of selective substitution of Cu²⁺ at the elongated M2 octahedral site. Cu²⁺ absorption bands can be assigned on the basis of D_4h symmetry with increasing splitting of the Cu²⁺, ²T_2g, and ²E_g energy levels with increasing Cu²⁺ content.

Key Words: Bieberite • crystal structure • optical spectroscopy • metal sulfate • synthesis