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Photoinduced change in the electrical conductivity of polycrystalline silveriodide(!"#$%&'(!")*+),-./0 123(4567(89:;(Hokkaido Univ.) ,Rahima Khaton, Toshifumi Iimori, Nobuhiro Ohta

[Introduction] Silver iodide, a typical solid state ionic material, exhibits various phases and iswell known as a superionic conductor having the phase transition temperature at 147 0C.
Among the phases, thermodynamically stable hexagonal wurtzite type structure named as !AgI and meta-stable face centered cubic structure designated as "-AgI coexist with each other.
Furthermore, the !-AgI transforms to "-AgI by pressing or by grinding whereas "-AgI
transforms to !-AgI by aging or by heating involving interesting electrical properties [1]. In
the present study, the photoinduced change in the electrical properties of polycrystalline AgI in
its "-AgI and !-AgI structures (ionic system) has been reported.
[Experimental] The pellets of !-AgIhaving the pale yellow color and the pelletof greenish yellow colored "-AgI wereprepared by using commercially available
silver iodide powder. The samples werecharacterized by using X-ray diffraction
powdermethod,andthe

electricalconductivity measurements were carried
outbyusing

compleximpedance

spectroscopy (ac method) and steady state
photocurrent excitation spectroscopy (dcmethod) at room temperature.

Fig 1: XRD pattern of "-AgI (a) and !-AgI (b).

[Results and discussions] The X-ray diffraction patterns of prepared "-AgI and !-AgI pellets
are shown in Fig 1(a) and 1(b), respectively. The freshly prepared polycrystalline "-AgI pellet
contains almost 100 % "-component whereas the pale yellow colored !-AgI pellet contains
83% !-structure and 17 % "-structure. The broadening and disappearance of the diffraction
band at (h0l) position in the XRD of "-AgI pellet indicates the transformation of !-AgI to "-AgI. Fig 2(a) and 2(b) present the photoinduced change in the Cole-Cole plots ofpolycrystalline "-AgI and !-AgI, respectively. A significant shift of the bulk resistance to thelower resistance state is clearly observed with the photoirradiation at 450 nm in the case of !AgI. A small shift of the bulk resistance of "-AgI is also obtained as shown in the inset of Fig

Fig 2: Cole-Cole plots of polycrystalline "AgI (a) and#!-AgI (b) with (!) and without(") photoirradiation at 450 nm measured by
the ac method. The inset of (a) is theexpansion of Cole-Cole plots in the bulk
resistance region indicating the shift of theCole-Cole plots with photoirradiation.

Fig 3: Photocurrent excitation spectrumof polycrystalline "-AgI following thephotoirradiation at 300 nm and at 450nm indicating the red and blue shifts ofthe exciton band, respectively.

1(a). In Fig 3(a), the exciton band in the photocurrent excitation spectrum of "-AgI is red
shifted (30 nm) by the photoirradiation at 300 nm for 10 min and the red shifted exciton bandis again blue shifted by the photoirradiation at 450 nm for 10 min as shown in Fig 3(b).
Therefore, the reversible red and blue shifts of the exciton band in the photocurrent excitation
spectrum of polycrystalline "-AgI are obtained by alternate photoirradiation at 300 nm and at
450nm, respectively. The photoinduced enhancement of the electrical conductivity ofpolycrystalline AgI, the photocurrent excitation spectrum and its dependences on the excitation
light intensity as well as on the excitation time will be discussed.
[Reference] [1] R. Khaton, S-I. Khasiwagi, T. Iimori and N. Ohta, Appl. Phys. Lett. 93,234102 (2008)