Assembling the Puzzle of Coparsite Polymorphism: Synthesis, Thermal Expansion, and Quantum Magnetism of α- and β-Cu4O2(VO4)Cl

Two new dimorphic spin–1/2 quantum magnets, α- and β-Cu₄O₂(VO₄)Cl, were synthesized via a chemical vapor transport method that emulates mineral formation in volcanic fumaroles. α-Cu₄O₂(VO₄)Cl (1) is a pure vanadate analogue of the coparsite mineral characterized by [O₂Cu₄]⁴⁺ 1D single rods, whereas β-Cu₄O₂(VO₄)Cl (2) adopts a new structure type with the [O₂Cu₄]⁴⁺ 2D layered topology. The thermal expansions of both 1 and 2 studied by high-temperature single-crystal X-ray diffraction are reported. Using ab initio calculations, we infer the presence of antiferromagnetic Cu1–Cu3 units with strong couplings on the order of 200–400 K forming chains (1) and layers (2). The Cu2 atoms are weakly coupled to such units. Magnetic susceptibility measurements corroborate this scenario by showing deviations from the paramagnetic behavior even at 300 K. Moreover, 1 reveals an antiferromagnetic ordering below T_N = 24 K with a weak uncompensated magnetic moment.