Controlling superstructural ordering in the clathrate-I Ba8M16P30 (M = Cu, Zn) through the formation of metal-metal bonds.

Order-disorder-order phase transitions in the clathrate-I Ba8Cu16P30 were induced and controlled by aliovalent substitutions of Zn into the framework. Unaltered Ba8Cu16P30 crystallizes in an ordered orthorhombic (Pbcn) clathrate-I superstructure that maintains complete segregation of metal and phosphorus atoms over 23 different crystallographic positions in the clathrate framework. The driving force for the formation of this Pbcn superstructure is the avoidance of Cu-Cu bonds. This superstructure is preserved upon aliovalent substitution of Zn for Cu in Ba8Cu16-x Zn x P30 with 0 < x < 1.6 (10% Zn/Mtotal), but vanishes at greater substitution concentrations. Higher Zn concentrations (up to 35% Zn/Mtotal) resulted in the additional substitution of Zn for P in Ba8M16+y P30-y (M = Cu, Zn) with 0 ≤ y ≤ 1. This causes the formation of Cu-Zn bonds in the framework, leading to a collapse of the orthorhombic superstructure into the more common cubic subcell of clathrate-I (Pm3n). In the resulting cubic phases, each clathrate framework position is jointly occupied by three different elements: Cu, Zn, and P. Detailed structural characterization of the Ba-Cu-Zn-P clathrates-I via single crystal X-ray diffraction, joint synchrotron X-ray and neutron powder diffractions, pair distribution function analysis, electron diffraction and high-resolution electron microscopy, along with elemental analysis, indicates that local ordering is present in the cubic clathrate framework, suggesting the evolution of Cu-Zn bonds. For the compounds with the highest Zn content, a disorder-order transformation is detected due to the formation of another superstructure with trigonal symmetry and Cu-Zn bonds in the clathrate-I framework. It is shown that small changes in the composition, synthesis, and crystal structure have significant impacts on the structural and transport properties of Zn-substituted Ba8Cu16P30.

Structures, Phase Transitions and Tricritical Behavior of the Hybrid Perovskite Methyl Ammonium Lead Iodide.

We have examined the crystal structures and structural phase transitions of the deuterated, partially deuterated and hydrogenous organic-inorganic hybrid perovskite methyl ammonium lead iodide (MAPbI3) using time-of-flight neutron and synchrotron X-ray powder diffraction. Near 330 K the high temperature cubic phases transformed to a body-centered tetragonal phase. The variation of the order parameter Q for this transition scaled with temperature T as Q ∼ (Tc-T)ß, where Tc is the critical temperature and the exponent ß was close to », as predicted for a tricritical phase transition. However, we also observed coexistence of the cubic and tetragonal phases over a range of temperature in all cases, demonstrating that the phase transition was in fact first-order, although still very close to tricritical. Upon cooling further, all the tetragonal phases transformed into a low temperature orthorhombic phase around 160 K, again via a first-order phase transition. Based upon these results, we discuss the impact of the structural phase transitions upon photovoltaic performance of MAPbI3 based solar cells.