Multiple Fluorine-Substituted Phosphate Germanium Fluorides and Their Thermal Stabilities

Anhydrous compounds
are crucially important for many technological applications, such
as achieving high performance in lithium/sodium cells, but are often
challenging to synthesize under hydrothermal conditions. Herein we
report that a modified solvo-/hydro-fluorothermal method with fluoride-rich
and water-deficient condition is highly effective for synthesizing
anhydrous compounds by the replacement of hydroxyl groups and water
molecules with fluorine. Two anhydrous phosphate germanium fluorides,
namely, Na3[GeF4(PO4)] and K4[Ge2F9(PO4)], with chainlike
structures involving multiple fluorine substitutions, were synthesized
using the modified solvo-/hydro-fluorothermal method. The crystal
structure of Na3[GeF4(PO4)] is constructed
by the common single chains ∞1{[GeF4(PO4)]3–} built from alternating
GeO2F4 octahedra and PO4 tetrahedra.
For K4[Ge2F9(PO4)], it
takes the same single chain in Na3[GeF4(PO4)] as the backbone but has additional flanking GeOF5 octahedra via an O-corner of the PO4 groups, resulting
in a dendrite zigzag single chain ∞1{[Ge2F9(PO4)]4–}. The multiple
fluorine substitutions in these compounds not only force them to adopt
the low-dimensional structures because of the “tailor effect”
but also improve their thermal stabilities. The thermal behavior of
Na3[GeF4(PO4)] was investigated by
an in situ powder X-ray diffraction experiment from room temperature
to 700 °C. The modified solvo-/hydro-fluorothermal method is
also shown to be effective in producing the most germanium-rich compounds
in the germanophosphate system.