Maximum Entropy Method Visualization of Disorder and Ion Migration in Thermoelectric Cu2‐δSe

• Published in: Advanced theory and simulations 2018-09, Vol.1 (9), p.n/a
• Main Authors: Dalgaard, Kirstine J., Eikeland, Espen Z., Sist, Mattia, Iversen, Bo B.
• Format: Article
• Language: English
• Subjects: Cu2Se; ion migration; maximum entropy method; phase transitions; thermoelectrics
• ISSN: 2513-0390; 2513-0390
• DOI: 10.1002/adts.201800068

Cu2‐δSe is a cheap, nontoxic high performance thermoelectric material with extraordinary properties such as liquid‐like phonons or a large enhancement of the thermopower at the phase transition between the low temperature β‐phase and the super‐ion conducting high temperature α‐phase. Here, the nuclear‐weighted X‐ray maximum entropy method (NXMEM) is used to study disorder and ion migration in both the β‐ and the α‐phase based on the analysis of single crystal X‐ray diffraction data. The NXMEM density calculated at different temperatures very convincingly shows an unbiased view of ion migration from copper‐rich to copper‐deficient layers eventually leading to the equal distribution in the cubic high temperature phase. This directly confirms that copper mobility and disordering is the driving force behind the peculiar phase transition. In the super‐ionic phase, no density is observed in the octahedral holes even though it is the point of lowest density in the Se anion procrystal. Thus, Cu ions neither occupy this site at equilibrium nor use this site for migration. The NXMEM density suggests an ion migration pathway between 32f sites skirting around the octahedral cavity. In Cu2Se, the strong cation–cation repulsion leads to cooperative effects with highly complex equilibrium disorder and ion migration paths. Thermoelectric Cu2‐δSe exhibits a peculiar thermopower enhancement prior to a super‐ionic phase transition around 400 K, presumably related to structural changes in the low temperature crystal structure when approaching the critical temperature. The main structural changes and characteristics of the super‐ionic phase are mapped using the nuclear‐weighted X‐ray maximum entropy method (NXMEM).