DETERMINATION OF THE COMMENSURATELY MODULATED STRUCTURE OF NBGE2 5TE2FROM TWINNED-CRYSTAL DATA/

The commensurate modulation of the NbGe2/5Te2 structure has been deter mined from X-ray diffraction data collected on a twinned crystal. The (3 + 1)D superspace group is P2(1)/n(0betagamma), beta = 0.0, gamma = 0.4. The basic unit cell dimensions are a = 6.4437(9), b = 16.004(2), c = 3.8936(4) angstrom, alpha = 119.10(1)degrees, V = 350.85(7) angstr om3, and Z = 4. Final R factors are 0.051, 0.122, 0.187, and 0.110 for 742 main reflections, 1573 first-order superreflections, 927 second-o rder superreflections, and the combined set, respectively. Twinning is taken into account by an extra basic vector; the intensities of all r eflections, overlapping as well as non-overlapping, are used in the fi nal refinement. The twin volume ratio is refined to nu = 0.418(1). NbG e2/5Te2 is a layered compound consisting of [Te/Nb, Ge/Te] sandwiches with a close to orthorhombic basic structure modulated by strong monoc linic, commensurate occupational and displacive waves. Bond distances in the structure of NbGe2/5Te2 compare very well with those in the thr ee- and seven-fold orthorhombic superstructures of NbGe1/3Te2 and NbGe 3/7Te2, respectively. It is shown that all three structures are built from the same intrasandwich subunits, and that the stacking, and accor dingly the 3D symmetry, is determined by the subunit geometry. The pin ning of two sandwiches on each other is related to a few short intersa ndwich Te-Te contacts that in turn can be correlated to the electronic stabilization of the compound. (C) 1994 Academic Press, Inc.