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. Methylarnrnonium lead(I1) halides, CH3NH3PbX3 (X = C1, Br, I), have been investigated by single-crystal X-ray diffraction, 2~ and ' 4~ nnu, 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 (10) at 162.7 and 326.6 K. The respective entropies of transition (J K-' mol-I) are 11 .O and 5.1 for CL; 8.7,3.4, and 5.3 for BR; and 16.1 and 1.9 for 10. The highest-temperature phase, phaseI, of each halide is of the cubic (Pm3m) perovskite type. The cation in CL(4 and BR(4 could not be localized in the electron density maps; the thermal motion of the halogen atom is highly anisotropic. The In T,(~H) vs. T' plots (N-deuterated samples as well as CD3NH3PbC13) show significant departures from linearity: the temperature variation of T,(~H) in BR(II) and IO(Il) can be represented by functions of the type In T1(H) = ko -k 2 r 2 , which give adequate analytical representations of T~(~H ) a n d~, ( '~N ) 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,, -T)", i.e. they appear to exhibit critical behaviour with respect to T. The I4N nrnr 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(II4, and IO(II4. The temperature dependence of the activation energy E, 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 ' H nrnr results on (CH3ND3)2TeBr6. The electrical conductivity, between 0 and 95"C, of CH3NH3Pb13 increases with temperature and exhibits no discontinuity at T,, = 326.6 K; the activation energy for the conduction process is estimated as -0.4 eV.

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In light of the intense recent interest in the methylammonium lead halides, CHNHPbX (X = Cl, Br, and I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temperature via H,N, and Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudoisotropic tumbling (picosecond time scale). For example, the correlation time, τ, for the C-N axis of the iodide salt is 0.85 ± 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic ↔ tetragonal phase transition; however, H andN NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temperature dependence of the cation ordering is rationalized using a six-site model, with two equivalent sites along the c-axis and four equivalent sites either perpendicular or approximately perpendicular to this axis. As the cubic ↔ tetragonal phase transition temperature is approached, the six sites are nearly equally populated. Below the tetragonal ↔ orthorhombic phase transition, H NMR line shapes indicate that the C-N axis is essentially frozen.