S T R U C T U R E O F M A T E R I A L S

S C I E N T I F I C H I G H L I G H T S

1 3 0 H I G H L I G H T S 2 0 2 2 I

By combining mechanical alloying and fast densification techniques, a novel, highly pure Cu-deficient aikinite- type sulfide sample with the composition CuPbBi5S9 was synthesised. Synchrotron X-ray powder diffraction (SXRPD) patterns recorded at room temperature at beamline ID22 were perfectly indexed with an orthorhombic unit cell of the Pnma space group and refined parameters a = 11.4420(1) Å, b = 4.0187(1) Å and c = 11.2271(1) Å (Figure 123a). Despite the chemical composition of

Fig. 123: a) Rietveld refinement of the XRPD data recorded for CuPbBi5S9 sample. Comparison

between the crystal structure of (b) aikinite and (c) gladite, with the aikinite-like (A), bismuthinite- like (B) and krupkaite-like (K) ribbons highlighted.

Key: copper, orange spheres; bismuth, green spheres; lead, blue spheres; sulfur, yellow spheres.

d-g) SAED patterns (Kikuchi patterns as inset) and the related HRTEM images

(inset: FFT patterns).

A disordered synthetic mineral with ultralow thermal conductivity

Diffraction data collected at beamline ID22 reveal the disordered character of synthetic copper- deficient aikinite compound Cu1/3□2/3Pb1/3Bi5/3S3. Experiments and lattice dynamics calculations show that the ultralow thermal conductivity of this material is a result of combined structural disorder induced by the processing method and very low- energy vibrational optical modes.

From the discovery of the thermoelectric effect two centuries ago, thermoelectricity has evolved to become a new potential source of renewable electricity, as well as a novel and versatile technology for thermal energy recovery and cooling devices. Robust solid-state thermoelectric devices can be used as green energy power generators by converting a temperature difference into a useful electrical current. Alternatively, thermoelectric devices can be used as reliable, long-lived heaters or coolers for specific applications [1]. The design and optimisation of thermoelectric materials relies on the intricate balance between thermopower (S), electrical resistivity (r) and thermal conductivity (k) to optimise the figure of merit ZT=S2T/rk; perfecting such a balance is key to improving the efficiency of energy recovery systems and thermoelectric cooling devices. The challenge is to synthesise a material in which the electrical and thermal properties are decoupled. In short, how to develop an electrical conductor that conducts very little heat?

One strategy is to find a crystal structure in which a 1D, 2D or 3D conductive network can generate high electrical conductivity while the structural complexity, order/disorder and/or rattling phenomena can favour the scattering of the neutral heat carriers without compromising the electronic properties. Scattering phenomena introduce dissipation that stops the diffusion of heat due to lattice vibrations. Due to their very rich crystal chemistry, sulfide minerals such as bismuth- based sulfides include promising families of n-type compounds with high ZT value [1-3].