A Novel Mineral-like Copper Phosphate Chloride with a Disordered Guest Structure: Crystal Chemistry and Magnetic Properties.

Novel copper phosphate chloride has been obtained under middle-temperature hydrothermal conditions. Its crystal structure was established based on the low-temperature X-ray diffraction data: Na 2 Li 0.75 (Cs,K) 0.5 [Cu 5 (PO 4 ) 4 Cl]·3.5(H 2 O,OH), sp. gr. C 2/ m , a = 19.3951(8) Å, b = 9.7627(3) Å, c = 9.7383(4) Å, β = 99.329(4)°, T = 150 K, Mo K α (λ = 0.71073 Å), R = 0.049. The crystal structure includes tetrameric copper clusters as the main building blocks, which are built of four CuO 4 Cl pyramids sharing apical Cl vertices. The clusters are combined through phosphate groups and additional copper-centered polyhedra to form two mostly ordered periodic layers. Between the layers and inside the framework channels, alkali ions, H 2 O molecules, or OH groups are statistically distributed. Na 2 Li 0.75 (Cs,K) 0.5 [Cu 5 (PO 4 ) 4 Cl]·3.5(H 2 O,OH) is a synthetic modification of a sampleite-polymorph of the lavendulan mineral group and represents a new member in a mero-plesiotype series of copper phosphates and arsenates, for which the crystal structures contain two-periodic [Cu 4 X ( T O 4 ) 4 ] ∞ modules ( T = As, P; X = Cl, O). Magnetically, this phase exhibits the phase transition at T C = 6.5 K, below which it possesses a weak ferromagnetic moment.