Determination of the structure of IIIa-ZnIn2S4 using convergent-beam electron diffraction and single-crystal x-ray diffraction

The crystal structure of the ternary semiconductor IIIa-ZnIn2S4 was investigated using convergent-beam electron diffraction (CBED) and single-crystal x-ray diffraction to elucidate whether the structure is fully ordered, as described by the acentric R3m space group, or has some degree of disorder as described by the centric R (3) over barm space group. The CBED technique, based upon dynamical diffraction, permitted the unique identification of the true structural symmetry, while x-ray diffraction could not discriminate between the two space groups. CBED patterns perpendicular to the [001] crystal direction showed the 6mm symmetry characteristic of the R (3) over barm space group. This result agrees with early Raman and photoluminescence spectroscopy measurements, The structure is best described by the R (3) over barm space group, with cell parameters a = 3.8728(6) Angstrom and c = 37.0664(1) Angstrom and unit-cell volume V = 481.3(1) Angstrom (3). The length of the c axis can be described by the expression c = N(3.086 +/- 0.003) Angstrom, where N is the number of sulfur layers in the unit cell and Z = N/4 the number of formula units: for our structure N = 12 and Z = 3, The crystal structure consists of a close-packed arrangement of S atoms, with Zn and half of the In atoms distributed in a disorderly fashion in tetrahedral sites, and the other half of the In atoms located in octahedral sites. The atomic positions and an isotropic temperature factor are reported.