Electronic transport and thermoelectricity in selenospinel Cu$_{6-x}$Fe$_{4+x}$Sn$_{12}$Se$_{32}$

TL;DR

This study investigates the electronic transport and thermoelectric properties of Cu$_{6-x}$Fe$_{4+x}$Sn$_{12}$Se$_{32}$ single crystals, revealing polaronic conduction, variable-range hopping, and structural glassy features at low temperatures.

Contribution

It provides new insights into the conduction mechanisms and low-temperature behavior of selenospinel thermoelectric materials with varying Fe content.

Findings

Polaronic transport mechanism identified between 350 and 50 K.

Transition to variable-range hopping conduction at lower temperatures.

Observation of structural glassy features in heat capacity measurements.

Abstract

We report a study of selenospinel Cu$_{6-x}$Fe$_{4+x}$Sn$_{12}$Se$_{32}$ ($x$ = 0, 1, 2) single crystals, which crystalize in a cubic structure with the $Fd\overline{3}m$ space group, and show typical semiconducting behavior. The large discrepancy between the activation energy for electrical conductivity $E_\rho$ (32.3 $\sim$ 69.8 meV), and for thermopower $E_\textrm{S}$ (3.2 $\sim$ 11.5 meV), indicates a polaronic transport mechanism between 350 and 50 K. With decreasing temperature, it evolves into variable-range hopping conduction. Furthermore, the heat capacity shows a hump around 25(5) K and diverges from the Debye $T^3$ law at low temperatures, indicating the observation of structural glassy features in these crystalline solids.