Flux Synthesis, UV-vis Absorbance, and Magnetism of Cesium Copper Silicates with an Isolated Super-Super Exchange Spin Dimer in Cs 6 Cu 2 Si 9 O 23 .

Single crystals of four new cesium copper silicates were grown from CsCl/CsF flux. [CsCs 4 Cl][Cu 2 Si 8 O 20 ] is a salt-inclusion compound that crystallizes in space group P 4/ m with lattice parameters a = 12.2768(3) Å and c = 8.6470(2) Å; Cs 6 Cu 2 Si 9 O 23 crystallizes in space group P 2 1 / n with lattice parameters a = 15.0763(9) Å, b = 6.9654(4) Å, c = 26.9511(17) Å, and β = 99.240(2)°; Cs 8 Cu 3 Si 14 O 35 crystallizes in space group C 2/ c with a = 39.2236(13) Å, b = 6.9658(2) Å, c = 14.9115(5) Å, and β = 97.1950(10)°; and Cs 2 CuSi 3 O 8 is a member of the stuffed tridymite family and crystallizes in a monoclinic distortion of the CsAlSiO 4 structure type with space group C 2/ 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 CuO 4 -flattened tetrahedra. The degree of flattening can be correlated with the UV-vis spectra. Cs 6 Cu 2 Si 9 O 23 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.