Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science1B, 4B2/2004P010

Evolution of Crystal and Electronic Structures in (Ag,In)xZn2-2xS2 solid solutions
Valery PETRYKIN*1, Masato KAKIHANA1, Issei TSUJI2, Hideki KATO2, Akihiko KUDO2,3Yusuke WAKABAYASHI4, Hiroshi SAWA4, Masahiko TANAKA4
1
IMRAM, Tohoku Univ., Katahira 2-1-1, Aoba-ku, Sendai 980-8577 Japan
2
Tokyo Univ. of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
3CREST, JST, 4-1-8 Honcho, Kawaguchi-shi, Saitama 332-0012, Japan4Photon Factory, High Energy Research Organization, Oho, Tsukuba, Ibaraki 305-0801, JapanIntroduction
AgxInxZn2-2xS2
solidsolutions
exhibithighphotocatalytic activity for hydrogen production undervisible light in the presence of sacrificial reagents [1]. The
activity in this series demonstrates a pronounced
maximum vs. x (Ag and In concentration) in the range of0.15-0.5. It means that electronic band structure of thesemiconducting samples undergoes a significant change
in the investigated Ag and In concentration range and it
has a remarkable impact on the photocatalytic properties.
Structural characterization of these materials is difficult
when conventional laboratory XRD equipment is used - itprovides information only about 50 reflections, and eventhe simplest refinement procedure is quite unstable. In++3addition, Ag and In have similar X-ray scatteringfactors at CuKα and it is not possible to refine theirpositions separately. On the other hand, computations ofelectronic structures rely on accurate structural
information. The goal of this proposal was to obtainaccurate structural parameters for these solid solutions
using synchrotron radiation. The obtained data will be
used for computations of electronic band structures.

parameters of the hexagonal unit cell show linear trend vs.
composition. The site symmetry of metal and sulfur
positions allows variation of z parameters for metal ionsand sulfur. However only the relative value of zS - zM
(metal-sulfur distance) should be considered as anindependent parameter. Therefore during the last stage of
the Rietveld refinement, all z parameters except for Zn
were allowed to vary. Fig.1 summarizes two types ofmetal-sulfur distances in the distorted metal-sulfurtetrahedron. One can see that the distortion of M-S4
tetrahedron decreases with increasing x and the distances
tend towards usual In-S and Ag-S distances in AgInS2compound (2.468Å). The release and refinement of theother z coordinates yields the sets of Ag-S, In-S and Zn-S
distances that are clustering around “average” points inFig.1. However for x=0.18 unreasonable distances of2.63Å for Ag-S, and 2.28 Å for Ag-S and In-S wereobtained during independent refinement of z, which could
be due to either intergrowth of sphalerite structure (thiscomposition x=0.18 is close to cubic-hexagonaltransition) or uncertainty introduced by the complex
shape of background from the capillary combined withlow concentration of Ag and In.

Experiment
Six specimens of AgxInxZn2-2xS2 solid solutions (x=0.18,0.22, 0.28, 0.33, 0.4, 0.5) were carefully ground andpacked into the 0.3 mm capillary tubes. Powder
diffraction data for all samples were collected at room
temperature using Debye-Scherrer camera equipped withimaging plate detector at BL-1B beam station and in theflat plate geometry using Multi Detector System at BL4B2 beam station for four samples (x=0.18, 0.28, 0.33,0.5). At BL-1B two datasets were acquired using
1.28843Å (Zn-K edge) and 0.69801Å wavelength of theincident X-ray beam, while at BL-4B2 only 1.306348 Å
wavelength was used. The obtained diffraction patterns
were combined with the patterns collected at SPring-8
near Ag-K and In-K absorption edges and used for joint
refinement of in total four/five diffraction patterns foreach sample by GSAS package [2].

Fig. 1. Interatomic distances in the metal – sulfur
tetrahedra vs. composition x.

Results
It was found that all solid solutions in the system
AgxInxZn2-2xS2 (x=0.18, 0.22, 0.28, 0.33, 0.4, 0.5) possess
wurtzite structure. No superstructural reflections havebeen detected in the diffraction patterns. The lattice

References[1] I. Tsuji et al., J.Am.Ceram.Soc. 126, 13406 (2004)[2] A.C. Larson and R.B. Von Dreele Los Alamos
National Laboratory Report LAUR 86-748 (2000).
* valery@tagen.tohoku.ac.jp

Users' Report191