Sensitivity of Mild Hydrothermal Synthesis to the Reaction Conditions: Targeting Mixed-Metal Hexagonal Tungsten Bronze Fluorides A x M2+ x M3+ (1–x)F3 to Investigate Their Magnetic Behavior

The synthesis of five quaternary hexagonal tungsten bronze (HTB) fluorides, A x M2+ x M3+ (1–x)F3, (A = Cs and Rb; M2+ = Co2+, Ni2+, and Zn2+; and M 3+ = V3+) via a mild hydrothermal route is reported. The crystal structures and compositions were determined using a combination of single-crystal X-ray diffraction, Rietveld refinement of powder X-ray diffraction data, and inductively coupled plasma optical emission spectroscopy analysis. This study highlights the sensitivity of the mild hydrothermal method on the reaction temperature, solvent system, and quantity of starting reagents that directly influence the selective synthesis of kinetically stabilized fluoride materials, including hydrated fluorides, β-pyrochlores, and HTB. The magnetic susceptibility and isothermal magnetization data for all five compounds were collected, which revealed the existence of a strong antiferromagnetic component in these phases. The presence of the Kagome layer in the structure results in geometrical frustration and leads to frustration indexes ranging from 7 to 13 for these compounds.