Crystal structures and phase transformation in In2Se3 compound
semiconductor have been studied by electron
diffraction, high resolution electron microscopy and X-ray diffraction (XRD) together
with optical absorption measurements.
The time-temperature-transformation (TTT) diagram reveals that there
exist only two phases in In2Se3 and the transformation
temperature is 853 K. The transformation from the high temperature phase to the low
requires a long incubation time for crystal nucleation and
a relatively high temperature for crystal growth.
The low and high temperature phases are the vacancy ordered in screw form (VOSF)
phase and the layer structure phase, respectively. Both phases possess semiconducting
optical properties and are constructed on the basis of
a tetrahedral bonding structure.
The VOSF phase is of a defect wurtzite structure, in which vacancies
on 1/3 of the cation sites are ordered in screw form along the c-axis.
The space group is P61 or P65 with
a=7.14 Å, c=19.38 Å, Z=6.
The layer structure is constructed of five-layer Se–In–Se–In–Se sets and the sets
are linked by weak van der Waals' force with stacking sequence of ABC.
The space group is R3m with a = 4.00 Å,
c = 28.80 Å, Z=3 (indexed in hexagonal system).
In the layer structure, structure vacancies on 1/3 of the cation sites aggregate
to form a vacancy plane for every three In-planes.
The structural difference between the two phases is most clearly characterized
by the difference in coordination numbers of the Se atoms.
The crystal structure of CuIn3Se5 is investigated by complementary use of electron and X-ray diffractions. The lattice parameters are a=5.7539(3) and c=11.519(1) Å and the space group is uniquely determined to be I42m. The crystal is not constructed of a rigid unit cell but by three kinds of local Se tetrahedrons, such as CuIn2Se4, CuIn3Se4 and In3Se4, which minimize the numbers of electron excess and deficient bonds. The average unit cell is represented by Cu0.8In0.4In2Se4, where Se and some In atoms fully occupy the 8i site with x≈1/4 and z≈1/8, and the 4d site, respectively, and Cu and the remaining In atoms partially occupy 2a and 2b sites, respectively. Since the space group I42m is not a subgroup of I42d which is the space group of CuInSe2, CuIn3Se5 is not related to CuInSe2 from the viewpoint of crystal symmetry. It is neither a vacancy ordered compound nor of a defect chalcopyrite structure.
Great optical activity is realized by a vacancy ordered III2VI3 compound (Ga0.3In0.7)2Se3 with point group 6 which is based on wurtzite structure and characterized by the screw arrangement of cation atoms along the c axis. The transition of the fundamental absorption edge is direct and the band gap is estimated to be 2.05 eV. An anomalous optical rotatory dispersion around the absorption edge is observed and the maximum rotatory power of 125°/mm is obtained at λ=620 nm. The optical activity for red light is always above 60°/mm, that is 4–6 times as large as that of α quartz. (Ga0.3In0.7)2Se3 single crystal is very useful, especially for the He–Ne laser as an optically active substance; the rotatory power reaches 103°/mm, being more than 5 times of α quartz.
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