The solid solutions of ${\text{CaBaFe}}_{4-x}{\text{Li}}_x{\text{O}}_7$ $\left({\text{CBFLi}}_x\right)$ with the “114” hexagonal symmetry $\left(P{6}_{3}mc\right)$ could be synthesized for $0\le x\le 0.4$. The dc magnetization study of these oxides shows that they are ferrimagnetic (FiM). The FiM to paramagnetic transition temperature, $T_C$, decreases from 265.6 K for $x=0$ to 97.5 K for $x=0.4$ in agreement with the introduction of the diamagnetic ${\text{Li}}^{1+}$ cation at the Fe sublattice. No magnetic saturation is observed and the coercive field $H_C$ is very large, ranging from 0.7 to 0.9 T at 5 K. Importantly, the ac magnetic susceptibility, ${\chi }_{\text{ac}}\left(T\right)$, indicates a frequency-dependent freezing temperature, $T_f$, increasing with frequency and satisfying the conventional power law, $\tau /{\tau }_0={\left(T_f/T_{\text{SG}}-1\right)}^{-z\upsilon }$. For the undoped ${\text{CaBaFe}}_4{\text{O}}_7$ (CBFO) $\left(x=0\right)$, the best fitting parameters are characteristic of a spin glass with a transition temperature $T_{\text{SG}}=176\text{K}$, spin-relaxation time ${\tau }_0=4.9\times {10}^{-12}\text{s}$, and critical exponent $\left(z\upsilon \right)$ of 6.4. In contrast, for ${\text{CBFLi}}_x$ $\left(0.1\le x\le 0.4\right)$ series the ${\tau }_0$ increases from $1.3\times {10}^{-14}$ to $8\times {10}^{-16}\text{s}$ as $x$ is increased; the values of $z\upsilon$ are in the range of 12–13, which lies much closer to that of cluster glasses (${\tau }_0={10}^{-14}–{10}^{-16}\text{sec}$; $z\upsilon =12–14$). The resistivity behavior is consistent with small polaronic hopping which changes to variable range hopping at low temperatures. The measurements under $H\sim 7\text{T}$ show that only CBFO (undoped) exhibits magnetoresistance, with a maximum value of 4.3% at 264 K.

Derivatives of the natural mineral colusite with general formula Cu₂₆A₂E₆S₃₂ (A = V, Nb, Ta, Cr, Mo, W; E = Ge, Sn, As, Sb) are an emerging class of excellent thermoelectric materials. Within just a few years, they have been promoted as serious candidates for cost-efficient, environmentally-friendly thermoelectric power generation, reaching performances comparable to tetrahedrites and other advanced thermoelectric sulphides. From ultralow thermal conductivity to the record-high power factor, this review gives an extensive description of the current state of the art regarding the crystal chemistry and electrical and thermal transport properties of colusites.

The discovery of the monoclinic Cu₅Sn₂S₇phase adds a high mobility member to the Cu–Sn–S diagram and demonstrates how the crystal structure can be advantageously manipulated to design novel compositions for energy-oriented applications.