Crystal structures of two kinds of optically active semiconductors (Ga0.60I
n0.40)(2)Se-3 and (Ga0.40In0.60)(2)Se-3, both of which are defect wurtzite
structures but show different reduced c(w)/a(w) ratios, have been studied u
sing powder X-ray diffraction and Rietveld analysis. The final reliability
factors are as follows: R-wp = 10.00%, S = 2.4234, R-F = 4.19% for (Ga0.60I
n0.40)(2)Se-3 and R-wp = 10.48%, S = 2.4324, R-F = 2.78% for (Ga0.40In0.60)
(2)Se-3. The structure of (Ga0.60In0.40)(2)Se-3 with c(w)/a(w) = 1.633 is c
alled the ideal defect wurtzite structure; the bond lengths are nearly the
same and the bond angles are almost to be the ideal bond angle of tetrahedr
al bonding 109.47 degrees. Meanwhile, the structure of (Ga0.40In0.60)(2)Se-
3 with c(w)/a(w) = 1.565 is called the layered defect wurtzite structure; t
he nearest (001)(group III) and (001)(Se) layers are waved and very close t
o each other as if they merged into one atomic layer. The average bond leng
th along the c axis is 5.89% longer than that between the atoms in the merg
ed (001)(group III) and (001)(Se) layers; the average bond angle whose one
side is the bond along the c axis is 13.11% smaller than 109.47 degrees. Ba
sed on these structural results, the anomalous dependence of the optical ba
nd gap on composition is discussed from the view of bonding nature. The add
itional ionic bonding between the merged (001) layers in the layered defect
wurtzite structure makes the c parameter smaller but the band gap larger t
han those of the ideal defect wurtzite structure although the ionic bonding
nature is dominant for the both defect wurtzite structures.