Summary: 974 Chem. Mater. 1993,5, 974-978
Commensurate and Incommensurate Lattice Distortions in
Brendan Foran,? Stephen Lee,*'$and Meigan C. Aronsoni
Willard H.Dow Chemistry and Randall Physics Laboratories, University of Michigan,
Ann Arbor, Michigan 48109-1055
Received February 2,1993. Revised Manuscript Received April 19,1993
We report the synthesis and structure of two new compounds, DySe1.u and Rb0.33DySe2.67.
Both structures are new structural variants of the Cu2Sb and NdTe3 structure types. DySe1.u
has an incommensurate lattice distortion, ahd Rb0.33DySe2.67contains a site occupancy wave.
The substructure of DySe1.u is tetragonal with P4/nmm symmetry with a = 4.0124(5) A and
c = 8.261(2) A with final RIR, = 2.0/1.8%. The substructure of Rbo.33DySe2.6, is orthorhombic
with space group Cmcm and a = 4.0579(6) A, b = 26.47(1)A and c = 3.8909(6) A with finalRIR,
= 3.7/1.9%. In both structures there is a clear shift of Se atoms away from high symmetry
positions. We alsoreport extendedHuckelband calculationswhich suggestthe incommensurate
superlattice found in DySe1.u is of charge density wave type.
Transition-metalchalcogenideshave played an impor-
tant role in solid-state and materials chemistry for a
number of years. Much of the interest in these materials