Operando X-ray scattering study of thermoelectric β-Zn₄Sb₃

Abstract

The application of thermoelectrics for energy harvesting depends strongly on operational reliability and it is therefore desirable to investigate the structural integrity of materials under operating conditions. We have developed an operando setup capable of simultaneously measuring X-ray scattering data and electrical resistance on pellets subjected to electrical current. Here, operando investigations of β-Zn4Sb3 are reported at current densities of 0.5, 1.14 and 2.3 A mm−2. At 0.5 A mm−2 no sample decomposition is observed, but Rietveld refinements reveal increased zinc occupancy from the anode to the cathode demonstrating zinc migration under applied current. At 1.14 A mm−2 β-Zn4Sb3 decomposes into ZnSb, but pair distribution function analysis shows that Zn2Sb2 units are preserved during the decomposition. This identifies the mobile zinc in β-Zn4Sb3 as the linkers between the Zn2Sb2 units. At 2.3 A mm−2 severe Joule heating triggers transition into the γ-Zn4Sb3 phase, which eventually decomposes into ZnSb, demonstrating Zn ion mobility also in γ-Zn4Sb3 under electrical current.

Operando structural investigations of thermoelectric materials

“…This structural complexity is believed to be the main origin of the low thermal conductivity found in this system, which is highly desirable for thermoelectric applications (Nylé n et al, 2004;Cargnoni et al, 2004;Snyder et al, 2004). When placed in a potential gradient, the Zn ions have been shown to migrate along the current direction, which ultimately results in an irreversible decomposition of -Zn 4 Sb 3 into ZnSb in the Zn-deficient region, and elemental Zn is observed in the Zn-rich region of the sample (Yin et al, 2012(Yin et al, , 2014Jørgensen et al, 2020;Hung et al, 2017). It was generally assumed that the migrating Zn ions were positioned on the interstitial sites, but pair distribution func-tion and powder X-ray diffraction (PXRD) data measured using our initial operando setup suggested Zn atoms on the main lattice site, Zn1, as the migrating species (Jørgensen et al, 2020).…”

“…When placed in a potential gradient, the Zn ions have been shown to migrate along the current direction, which ultimately results in an irreversible decomposition of -Zn 4 Sb 3 into ZnSb in the Zn-deficient region, and elemental Zn is observed in the Zn-rich region of the sample (Yin et al, 2012(Yin et al, , 2014Jørgensen et al, 2020;Hung et al, 2017). It was generally assumed that the migrating Zn ions were positioned on the interstitial sites, but pair distribution func-tion and powder X-ray diffraction (PXRD) data measured using our initial operando setup suggested Zn atoms on the main lattice site, Zn1, as the migrating species (Jørgensen et al, 2020). Here, we utilize the unique capabilities of ATOS to investigate the atomic structure throughout the entire sample under realistic conditions.…”

“…We primarily ascribe this to the varying sample temperature, as the diffusion constant of Zn in -Zn 4 Sb 3 has been shown to be highly temperature dependent (Chalfin et al, 2007). The sample temperature at the different points along the sample can be approximated from the initial increase in unit-cell size, as described by Jørgensen et al (2020), and it is shown together with the compositional change in Fig. 4(b).…”

“…Appropriate stability tests should be performed on samples that resemble what would be used in a working module. Some studies have considered the effect of current and Seebeck potential on bulk samples (Reardon et al, 2017;Yin et al, 2012;Qiu et al, 2018), which is particularly important in highperformance materials containing mobile ions such as Cu 2 Se, Cu 2 S and Zn 4 Sb 3 (Qiu et al, 2018;Liu et al, 2012;Roth & Iversen, 2019;Jørgensen et al, 2020;Brown et al, 2013;He et al, 2014). In these systems, irreversible compositional changes have been observed through ex situ characterization methods; however, although ex situ characterization provides valuable information, insights into the reaction dynamics and mechanisms are inherently inaccessible.…”

“…We recently introduced the first version of a TE operando setup (Jørgensen et al, 2020). In that setup, densified barshaped samples could be exposed to direct current while the electrical resistance and X-ray diffraction data were measured.…”