Three
new salt inclusion compounds (CsX)Cu5O2(PO4)2 (X = Cl, Br,
I), phosphate analogues of the kagomé mineral averievite, are
reported. Their crystal structures are composed of trigonal networks
of corner-sharing OCu4 anion-centered tetrahedra, forming
capped-kagomé planes, which can also be regarded as two-dimensional
slices along the [111] direction of a pyrochlore lattice. Magnetization
and heat capacity measurements reveal strong geometric frustration
of this network and complex magnetic behavior. X-ray and neutron diffraction
studies show that all three compounds undergo a trigonal-to-monoclinic
phase transition upon cooling, with a first-order phase transition
seen in CsBr and CsI analogues. Along with the previously reported
(CsCl)Cu5O2(VO4)2, these
three new compounds belong to a large family of OCu4-based
networks, which are a playground for studying frustrated quantum magnetism.