Mild Hydrothermal Synthesis of Zirconium-Containing Fluorides: K ₂ [ <i>M</i> (H ₂ O) ₆ ][Zr ₂ F ₁₂ ] ( <i>M</i> = Fe, Co), Rb ₂ [ <i>M</i> (H ₂ O) ₆ ][Zr <sub>2<

High-quality single crystals of the formula K 2 [ M (H 2 O) 6 ][Zr 2 F 12 ] ( M = Fe, Co), Rb 2 [ M (H 2 O) 6 ][Zr 2 F 12 ] ( M = Fe, Co, Ni, Cu, Zn), Cs 2 [ M (H 2 O) 6 ][Zr 2 F 12 ] ( M = Fe, Co, Ni), and Cs 2 Zr 3 Mn 3 F 20 were obtained through a mild hydrothermal synthesis. The new materials crystallize in the monoclinic space group P 2 1 / n . The crystal structures and chemical compositions were determined using single crystal and powder X-ray diffraction. The thermal and optical properties of these materials are reported. UV–vis diffuse reflectance data identifies d–d electronic transitions of the Co-, Ni-, and Cu-containing compounds. Each hydrated compound (except the Fe containing compounds) exhibits similar thermal decomposition, fully dehydrating by 200 °C based on thermogravimetric analysis measurements. Magnetic measurements indicate that the new compositions are paramagnetic down to 2 K.