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 “Fe2(SO4)3” via electrochemical and chemical oxidation of K2Fe2(SO4)3. Additionally, we succeeded in stabilizing a new K2Cu2(SO4)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 K2Cu2(SO4)3 decomposes into K2Cu3­O­(SO4)3 on heating, and we report for the first time the synthesis of fedotovite K2Cu3­O­(SO4)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 Cu2+ magnetic moments down to 1.5 K.