  The crystal structures and magnetic properties of three previously unreported
A2B2F7 pyrochlore materials, NaSrMn2F7, NaCaFe2F7, and NaSrFe2F7 are presented.
In these compounds, either S=2 Fe2+ or S=5/2 Mn2+ is on the B site, while
nonmagnetic Na and Ca (Na and Sr) are disordered on the A site. The materials,
which were grown as crystals via the floating zone method, display high
effective magnetic moments and large Curie-Weiss thetas. Despite these
characteristics, freezing of the magnetic spins, characterized by peaks in the
susceptibility or specific heat, is not observed until low temperatures. The
empirical frustration index, f=- {\theta}cw/Tf, for the materials are 36
(NaSrMn2F7), 27 (NaSrFe2F7), and 19 (NaCaFe2F7). AC susceptibility, DC
susceptibility, and heat capacity measurements are used to characterize the
observed spin glass behavior. The results suggest that the compounds are
frustrated pyrochlore antiferromagnets with weak bond disorder. The magnetic
phenomena that these fluoride pyrochlores exhibit, in addition to their
availability as relatively large single crystals, make them promising
candidates for the study of geometric magnetic frustration.
