Solar Cells and Energy Materials by Takeo Oku

Author:Takeo Oku
Language: eng
Format: epub
Publisher: De Gruyter
Published: 2017-04-17T04:00:00+00:00

6.2Crystal structures of CH3NH3PbX3 (X = Cl, Br, or I)

The crystals of methylammonium trihalogenoplumbates(II) (CH3NH3PbX3, X = Cl, Br, or I) have perovskite structures and undergo structural transitions upon heating [27–29]. The crystal systems and transition temperatures are summarized in Table 6.1, as reported in previous works [27, 29]. Atomic sites were indicated from the space group table [30]. Although the CH3NH3PbX3 perovskite crystals have cubic symmetry for the highest temperature phase, the CH3NH3 ion is polar and has C3v symmetry, which should result in a disordered cubic phase [31]. In addition to the disordering of the CH3NH3 ion, the halogen ions were also disordered in the cubic phase, as shown in Fig. 6.1(a) and Table 6.2 [31]. Site occupancies were set to 1/4 for I and 1/12 for C and N. The CH3NH3 ion occupies 12 equivalent orientations of the C2 axis, and hydrogen atoms have two kinds of configurations on the C2 axis. The total degree of freedom is 24 [29].

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