Synthesis, Structure, and Electrochemical Properties of K-Based Sulfates K₂M₂(SO₄)₃ with M = Fe and Cu

Stabilizing new host structures through potassium extraction from K-based polyanionic materials has been proven to be an interesting approach to develop new Li + /Na + insertion materials. Pursuing the same trend, we here report the feasibility of preparing langbeinite “Fe 2 (SO 4 ) 3 ” via electrochemical and chemical oxidation of K 2 Fe 2 (SO 4 ) 3 . Additionally, we succeeded in stabilizing a new K 2 Cu 2 (SO 4 ) 3 phase via a solid-state synthesis approach. This novel compound crystallizes in a complex orthorhombic structure that differs from that of langbeinite as deduced from synchrotron X-ray and neutron powder diffraction. Electrochemically, the performance of this new phase is limited, which we explain in terms of sluggish diffusion kinetics. We further show that K 2 Cu 2 (SO 4 ) 3 decomposes into K 2 Cu 3 O(SO 4 ) 3 on heating, and we report for the first time the synthesis of fedotovite K 2 Cu 3 O(SO 4 ) 3 . Finally, the fundamental attractiveness of these S = 1 / 2 systems for physicists is examined by neutron magnetic diffraction, which reveals the absence of a long-range ordering of Cu 2+ magnetic moments down to 1.5 K.