Dolyniuk J, Whitfield P S, Lee K, Lebedev O I, Kovnir K
Department of Chemistry , University of California , Davis , One Shields Avenue , Davis , CA 95616 , USA . Email:
Chemical and Engineering Materials Division , Oak Ridge National Laboratory , Oak Ridge , Tennessee 37830 , USA.
Chem Sci. 2017 May 1;8(5):3650-3659. doi: 10.1039/c7sc00354d. Epub 2017 Feb 20.
Order-disorder-order phase transitions in the clathrate-I BaCuP were induced and controlled by aliovalent substitutions of Zn into the framework. Unaltered BaCuP crystallizes in an ordered orthorhombic () 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 superstructure is the avoidance of Cu-Cu bonds. This superstructure is preserved upon aliovalent substitution of Zn for Cu in BaCu Zn P with 0 < < 1.6 (10% Zn/M), but vanishes at greater substitution concentrations. Higher Zn concentrations (up to 35% Zn/M) resulted in the additional substitution of Zn for P in BaM P (M = Cu, Zn) with 0 ≤ ≤ 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 (3). 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 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 BaCuP.
通过在骨架中进行锌的异价取代，诱导并控制了笼型I BaCuP中的有序-无序-有序相变。未改变的BaCuP结晶为有序的正交（）笼型I超结构，在笼型骨架的23个不同晶体学位置上保持金属和磷原子的完全分离。形成这种超结构的驱动力是避免Cu-Cu键。在0 < < 1.6（10% Zn/M）的BaCu Zn P中，用锌异价取代铜时，这种超结构得以保留，但在更高的取代浓度下会消失。更高的锌浓度（高达35% Zn/M）导致在0 ≤ ≤ 1的BaM P（M = Cu，Zn）中锌进一步取代磷。这导致骨架中形成Cu-Zn键，导致正交超结构坍塌为笼型I更常见的立方亚晶胞（3）。在所得的立方相中，每个笼型骨架位置由三种不同元素共同占据：Cu、Zn和P。对Ba-Cu-Zn-P笼型I的详细结构表征，包括单晶X射线衍射、同步加速器X射线和中子粉末联合衍射、对分布函数分析、电子衍射和高分辨率电子显微镜，以及元素分析，表明立方笼型骨架中存在局部有序，暗示了Cu-Zn键的演变。对于锌含量最高的化合物，由于在笼型I骨架中形成了具有三角对称性和Cu-Zn键的另一种超结构，检测到无序-有序转变。结果表明，组成、合成和晶体结构的微小变化对锌取代的BaCuP的结构和输运性质有显著影响。
