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 (<b>1</b>) is a pure vanadate analogue of the coparsite mineral characterized by [O₂Cu₄]⁴⁺ 1<i>D</i> single rods, whereas β-Cu₄O₂(VO₄)Cl (<b>2</b>) adopts a new structure type with the [O₂Cu₄]⁴⁺ 2<i>D</i> layered topology. The thermal expansions of both <b>1</b> and <b>2</b> studied by high-temperature single-crystal X-ray diffraction are reported. Using <i>ab initio</i> calculations, we infer the presence of antiferromagnetic Cu1-Cu3 units with strong couplings on the order of 200-400 K forming chains (<b>1</b>) and layers (<b>2</b>). 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, <b>1</b> reveals an antiferromagnetic ordering below <i>T</i>_<i>N</i> = 24 K with a weak uncompensated magnetic moment.