Flux Synthesis, UV–vis Absorbance, and Magnetism of Cesium Copper Silicates with an Isolated Super-Super Exchange Spin Dimer in Cs6Cu2Si9O23

Single crystals of four new cesium copper silicates were grown from CsCl/CsF flux. [CsCs₄Cl][Cu₂Si₈O₂₀] is a salt-inclusion compound that crystallizes in space group P4/m with lattice parameters a = 12.2768(3) Å and c = 8.6470(2) Å; Cs₆Cu₂Si₉O₂₃ crystallizes in space group P2₁/n with lattice parameters a = 15.0763(9) Å, b = 6.9654(4) Å, c = 26.9511(17) Å, and β = 99.240(2)°; Cs₈Cu₃Si₁₄O₃₅ crystallizes in space group C2/c with a = 39.2236(13) Å, b = 6.9658(2) Å, c = 14.9115(5) Å, and β = 97.1950(10)°; and Cs₂CuSi₃O₈ is a member of the stuffed tridymite family and crystallizes in a monoclinic distortion of the CsAlSiO₄ structure type with space group C2/m and a = 12.8587(3) Å, b = 5.38510(10) Å, c = 9.0440(2) Å, and β = 133.2580(10)°. All four compounds contain CuO₄-flattened tetrahedra. The degree of flattening can be correlated with the UV-vis spectra. Cs₆Cu₂Si₉O₂₃ exhibits spin dimer magnetism that can be attributed to super-super-exchange between two Cu(II) ions connected by a silicate tetrahedron. The other three compounds all show paramagnetic behavior down to 2 K.