Alkylammonium lead halides. Part 2. CH3NH3PbX3 (X = Cl, Br, I) perovskites: cuboctahedral halide cages with isotropic cation reorientation

Abstract

Methylammonium lead(II) halides, CH₃NH₃PbX₃ (X = Cl, Br, I), have been investigated by single-crystal X-ray diffraction, ²H and ¹⁴N nmr, adiabatic calorimetry, and other methods. The chloride (CL) has transitions at 171.5 and 177.4 K, the bromide (BR) at 148.4, 154.2, and 235.1 K, and the iodide (IO) at 162.7 and 326.6 K. The respective entropies of transition (J K⁻¹ mol⁻¹) are 11.0 and 5.1 for CL; 8.7, 3.4, and 5.3 for BR; and 16.1 and 1.9 for IO. The highest-temperature phase, phase I, of each halide is of the cubic (Pm3m) perovskite type. The cation in CL(I) and BR(I) could not be localized in the electron density maps; the thermal motion of the halogen atom is highly anisotropic. The ln T₁(²H) vs. T⁻¹ plots (N-deuterated samples as well as CD₃NH₃PbCl₃) show significant departures from linearity: the temperature variation of T₁(²H) in BR(II) and IO(II) can be represented by functions of the type ln T₁(H) = k₀ − k₂T⁻², which give adequate analytical representations of T₁(²H) and T₁(¹⁴N) in phase I as well. On cooling, BR(II) and IO(II) exhibit small quadrupole splittings QS(²H), which can be represented to a high degree of correlation by QS(²H) = k(T_tr − T)ⁿ, i.e. they appear to exhibit critical behaviour with respect to T. The ¹⁴N nmr results indicate that the C—N bond in phase I reorientates in an isotropic potential at a rate approaching that of the freely rotating methylammonium ion. Below phase I this motion takes place in an increasingly anisotropic potential in BR(II) and IO(II) and is essentially arrested in CL(II), BR(III), and IO(III). The temperature dependence of the activation energy E_a for the cation reorientation and other aspects of the non-Arrhenius behaviour are discussed, and the CH₃NH₃PbX₃ perovskites are compared with the corresponding (CH₃NH₃)₂TeX₆ halides, utilizing preliminary ²H nmr results on (CH₃ND₃)₂TeBr₆. The electrical conductivity, between 0 and 95 °C, of CH₃NH₃PbI₃ increases with temperature and exhibits no discontinuity at T_tr = 326.6 K; the activation energy for the conduction process is estimated as ~0.4 eV. Keywords: crystal structure, MeNH₃PbCl₃ and MeNH₃PbBr₃; heat capacity, MeNH₃PbX₃ (X = Cl, Br, I); methylammonium lead halides; solid-state NMR, ²H and ¹⁴N; spin-lattice relaxation, non-Arrhenius behaviour.