A. Vanderlee et al., RESOLUTION OF THE 66-FOLD SUPERSTRUCTURE OF DYSE1.84 BY X-RAY-DIFFRACTION AND 2ND-MOMENT SCALED HUCKEL CALCULATIONS, Chemistry of materials, 9(1), 1997, pp. 218-226
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.