  The Fe-based langasites are the first reported compounds presenting a
magnetic ordering in this rich family, besides being well known for
piezo-electric properties and optical activity. The structural, magnetic and
dielectric properties of the Fe langasite compounds, with various substitution
of non-magnetic cations, have been studied with x-ray and neutron diffraction,
magnetostatic measurements, M\"ossbauer spectroscopy, and dielectric
measurements. The title compounds (trigonal, space group P321) display a
helical magnetic order with signatures of frustration below TN ~24- 35 K where
an anomaly of the dielectric permittivity is observed. The influence of the
cationic substitutions and the nature of the magnetoelectric coupling is
hereafter addressed.
