Isotypism and low-dimensional physical properties of CuAs_2/3H_2/3O₃ (theoparacelsite), the spin-Peierls phase CuGeO₃ and CuSiO₃

Abstract

Theoparacelsite is a recently discovered orthorhombic copper arsenate hydroxide mineral with the ideal chemical formula Cu₃As₂O₇(OH)₂, but because of disorder only two smaller formula units CuAs_2/3H_2/3O₃ are established in the orthorhombic unit cell with lattice parameters a = 4.6644(5) Å, b = 8.3212(8) Å and c = 2.9377(3) Å, and space group Pbmm. We show that this rare mineral is iso-typic to the unique inorganic spin-Peierls phase CuGeO₃ (T_SP = 14.3 K) and to the recently discovered copper polysilicate, CuSiO₃, which is also a quasi-one-dimensional S = 1/2 anti-ferromagnetic chain compound (T_N = 7.9 K). We discuss in detail the previously reported crystallographic data of the phases at room temperature in view of its ability to attain similar low-dimensional physical properties and conclude that the arsenate with its S = 1/2 spin chains of edge-sharing CuO₄₊₂ octahedra may be a further inorganic compound that could show the spin-Peierls effect. Although the Cu-O(2)-Cu angle, which is considered to be important for the magnetic interaction and ground state properties, has been found for the arsenate (98.8°) almost identical to that of the germanate (99.1°), the modified inter-chain coupling of the arsenate as a defect variant of the CuGeO₃ structure type may suppress the spin-Peierls effect and favour other states of magnetic order (chiral spin chains, AF order). With respect to the rarity of the mineral some remarks on synthesis routes are made.