Ag 3 V 2 (PO 4 ) 2 F 3 , a new compound obtained by Ag + /Na + ion exchange into the Na 3 V 2 (PO 4 ) 2 F 3 framework

Phosphate polyanionic compounds have been used for several technological applications and are especially widespread in battery research. Na 3 V 2 (PO 4 ) 2 F 3 is a material that holds great promise as a positive electrode for Na-ion batteries. We study here the Ag + /Na + ion exchange that is possible due to the high ionic mobility of Na + in this material. A nearly complete ion exchange was obtained and we report the crystal structure of the new orthorhombic phase Ag 3 V 2 (PO 4 ) 2 F 3 , determined by synchrotron X-ray and neutron powder diffraction. Silver occupies the same crystallographic sites as sodium and, except for the differences caused by steric effects, Ag 3 V 2 (PO 4 ) 2 F 3 preserves the symmetry constraints already present in the parent compound Na 3 V 2 (PO 4 ) 2 F 3 . We also followed the evolution of the crystal structure upon heating/cooling, to observe an order/disorder transition analogous to the one already reported for Na 3 V 2 (PO 4 ) 2 F 3 , but occurring at a significantly higher temperature.