Operando X-ray scattering study of thermoelectric β-Zn₄Sb₃

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.