Understanding the underlying mechanisms that suppress thermal conduction in solids is of paramount importance for the targeted design of materials for thermal management and thermoelectric energy conversion applications. Bismuth copper oxychalcogenides, BiOCuQ (Q = Se, Te), are highly crystalline thermoelectric materials with an unusually low lattice thermal conductivity of ∼0.5 Wm(-1) K(-1), a value normally found in amorphous materials. Here we unveil the origin of the unusual thermal transport properties of these phases. First principles calculations of the vibrational properties combined with analysis of in-situ neutron diffraction data, demonstrate that weak bonding of copper atoms within the structure leads to an unexpected vibrational mode at low frequencies, which is likely to be a major contributor to the low thermal conductivity of these materials. In addition, we show that anharmonicity and the large Grüneisen parameter in these oxychalcogenides are mainly related to the low frequency copper vibrations, rather than to the Bi(3+) lone pairs.

a b s t r a c tBi 2 O 2 Te was synthesised from a stoichiometric mixture of Bi, Bi 2 O 3 and Te by a solid state reaction. Analysis of powder X-ray diffraction data indicates that this material crystallises in the anti-ThCr 2 Si 2 structure type (space group I4/mmm), with lattice parameters a¼ 3.98025(4) and c¼ 12.70391(16) Å. The electrical and thermal transport properties of Bi 2 O 2 Te were investigated as a function of temperature over the temperature range 300 rT (K)r 665. These measurements indicate that Bi 2 O 2 Te is an n-type semiconductor, with a band gap of 0.23 eV. The thermal conductivity of Bi 2 O 2 Te is remarkably low for a crystalline material, with a value of only 0.91 W m À 1 K À 1 at room temperature.

The effect of Pb 2+ doping on the structure and thermoelectric properties of BiOCuSe (also known as BiCuSeO or BiCuOSe) is described. With increasing Pb 2+ content, the expansion of the unit cell results in a weakening of the bonding between the [Bi 2(1-x) Pb 2x O 2 ] 2(1-x)+ and the [Cu 2 Se 2 ] 2(1-x)layers. The electrical resistivity and Seebeck coefficient decrease in a systematic way with growing Pb 2+ levels. The thermal conductivity rises due to the increase of the electronic contribution with doping. The power factor of materials with a 4-5% Pb 2+ content takes values of ca. 8 W cm-1 K-2 over a wide temperature range. ZT at 673 K is enhanced by ca. 50% when compared to values found for other dopants, such as Sr 2+ or Mg 2+ .

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Searching for novel low-cost and eco-friendly materials for energy conversion is a good way to provide widespread utilization of thermoelectric technologies. Herein, we report the thermal behavior, phase equilibria data, and thermoelectric properties for the promising argyrodite-based Cu 7 P-(S x Se 1−x ) 6 thermoelectrics. Alloying of Cu 7 PSe 6 with Cu 7 PS 6 provides a continuous solid solution over the whole compositional range, as shown in the proposed phase diagram for the Cu 7 PS 6 − Cu 7 PSe 6 system. As a member of liquid-like materials, the investigated Cu 7 P(S x Se 1−x ) 6 solid solutions possess a dramatically low lattice thermal conductivity, as low as ∼0.2−0.3 W m −1 K −1 , over the entire temperature range. Engineering the configurational entropy of the material by introducing more elements stabilizes the thermoelectrically beneficial high-symmetry γ-phase and promotes the multivalley electronic structure of the valence band. As a result, a remarkable improvement of the Seebeck coefficient and a reduction of electrical resistivity were observed for the investigated alloys. The combined effect of the extremely low lattice thermal conductivity and enhanced power factor leads to the significant enhancement of the thermoelectric figure of merit ZT up to ∼0.75 at 673 K for the Cu 7 P(S x Se 1−x ) 6 (x = 0.5) sample with the highest configurational entropy, which is around twice higher compared with the pure selenide and almost four times higher than sulfide. This work not only demonstrates the large potential of Cu 7 P(S x Se 1−x ) 6 materials for energy conversion but also promotes sulfide argyrodites as earth-abundant and environmentally friendly materials for energy conversion.

Municipal and industrial wastewater can be a potential source of magnesium. Therefore, the development of magnesium recovery technology can both release the burden of wastewater treatment and help recycle the metal, which is in highmarket demand. Also, the recovery of magnesium in the form of magnesium carbonate has an implication on carbon capture and storage (CCS). In this study, fluidized bed homogeneous crystallization (FBHC) was employed for the recovery of magnesium from actual industrial effluent. The optimal conditions for the operation of FBHC were pH, 11.3; [Mg 2+ ]/[CO 32− ], 1.2; surface loading rate, 1.8 kg/m 2 h; and upflow velocity, 15.5 m/h, where the total recovery (TR) and crystallization (CR) efficiencies reached 88.5 and 85.4%, respectively. The recovered products were of high purity (93.5%) and in the form of nesquehonite (MgCO 3 •3H 2 O) pellets (size 1.2 mm), which could be further reused easily. From the scanning electron microscopy analysis, it was observed that they possessed a round shape and a smooth surface. In summary, FBHC is a promising recovery technology for magnesium-rich wastewater, where carbon capture and storage can be simultaneously integrated.

Doping of BiOCuSe at the copper site with divalent cadmium and zinc cations has been investigated. Analysis of the powder X-ray diffraction data indicates that the ZrCuSiAs structure of BiOCuSe is retained up to substitution levels of 10 and 5 at.% for Cd 2+ and Zn 2+ , respectively. Substitution of monovalent Cu + with divalent Cd 2+ or Zn 2+ leads to an increase in the magnitude of the electrical resistivity and the Seebeck coefficient. All synthesized materials behave as p-type semiconductors.

In 4 Se 3 is an attractive n-type thermoelectric material for mid-range waste heat recovery, owing to its low thermal conductivity (~ 0.9 W•m -1 K -1 at 300 K). Here, we explore the relationship between the elastic properties, thermal conductivity and structure of In 4 Se 3 . The experimentallydetermined average sound velocity (2010 m s -1 ), Young's modulus (47 GPa), and Debye temperature (198 K) of In 4 Se 3 are rather low, indicating considerable lattice softening. This behavior, which is consistent with low thermal conductivity, can be related to the complex bonding found in this material, in which strong covalent In-In and In-Se bonds coexist with weaker electrostatic interactions. Phonon dispersion calculations show that Einstein-like modes occur at  30 cm -1 . These Einstein-like modes can be ascribed to weakly bonded In + cations located between strongly-bonded [(In 3 ) 5+ (Se 2-) 3 ] -layers. The Grüneisen parameter for the softbonded In + at the frequencies of the Einstein-like modes is large, indicating a high degree of bond anharmonicity and hence increased phonon scattering. The calculated thermal conductivity and elastic properties are in good agreement with experimental results.