RESOLUTION OF THE 66-FOLD SUPERSTRUCTURE OF DYSE1.84 BY X-RAY-DIFFRACTION AND 2ND-MOMENT SCALED HUCKEL CALCULATIONS

We report the superlattice of the modulated structure DySe1.84. The mo dulation is of the charge density wave type, associated with electroni c states on selenium atoms arranged in two-dimensional square nets. Th e complete modulated structure can be described in the superspace grou p formalism in a 3 + 2 dimensional space group C-cmm1(Pm21n). The basi c cell observed by single-crystal X-ray diffraction is orthorhombic wi th a = 3.9912(3) Angstrom, b = 3.9863(1) Angstrom, and c = 8.206(1) An gstrom. The modulation wave vectors q(1) = alpha a + beta b* + 1/2c* and q(2) = alpha a - beta b* + 1/2c*, where alpha = 0.33338(16) and b eta = 0.27284(6). Structure refinement based on first- and second-orde r satellites as well as main lattice reflections produced solutions in four different space groups that could not be differentiated on the b asis of agreement factors. A best solution was sought by use of second -moment scaled, tight-binding band calculations. The lowest energy mod el found was similar to but not identical with two of those refined fo r X-ray data. This lowest energy structure can be rationalized by anal ysis of second-nearest-neighbor interactions within the selenium squar e nets.