KCaEr₂CuS₅: A New Pentanary Rare-Earth Layered Chalcogenide without Substitutional Disorder

A new pentanary rare-earth chalcogenide without substitutional disorder, KCaEr2CuS5, was obtained from reaction
of the Er−Ca−Cu−S precursor with KBr flux by either a two-step flux method or a direct reaction of the mixture of
the binary components (1Cu2S:2Er2S3:3CaS) with excess KBr as flux. KCaEr2CuS5 crystallizes in the orthorhombic
space group Cmcm (No. 63) with a = 3.9327(5) Å, b = 13.410(2) Å, c = 17.042(2) Å, V = 898.8(3) Å3, Z = 4,
R = 0.0632, and Rw = 0.0627. The KCaEr2CuS5 structure consists of four different building units:  ErS6 octahedra,
CaS6 octahedra, CuS4 tetrahedra, and KS6 trigonal prisms; it is characterized by

[Er2CuS5]3- layers that are
formed by the interconnection of double ErS6 octahedral chains with CuS4 tetrahedral chains in the a−c plane. K+
and Ca2+ cations are located in the cavities defined by S2- anions between the

[Er2CuS5]3- layers. KCaEr2CuS5
is a semiconductor with an estimated band gap of 2.4 eV and shows a Curie−Weiss paramagnetic behavior in the
5−300 K range.