Controlling superstructural ordering in the clathrate-I Ba8M16P30 (M = Cu, Zn) through the formation of metal–metal bonds. Issue 5 (14th March 2017)

• Record Type: Journal Article
• Authors: Dolyniuk, J.; Whitfield, P. S.; Lee, K.; Lebedev, O. I.; Kovnir, K.
• Is Part Of: Chemical science. Volume 8:Issue 5(2017)
• Page Start: 3650
• Page End: 3659
• Publication Date: 2017-03-14
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c7sc00354d
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : Formation and avoidance of M–M bonds in the clathrate framework result in extreme structural complexity. Abstract : Order–disorder–order phase transitions in the clathrate-I Ba8 Cu16 P30 were induced and controlled by aliovalent substitutions of Zn into the framework. Unaltered Ba8 Cu16 P30 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 Ba8 Cu16− 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 Ba8 M16+ 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 ( Pm 3̄ n ). 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 Ba8 Cu16 P30 .