Below $T_C$, the single-crystal study of CaBaCo${}_4$O${}_7$, a noncollinear ferrimagnet ($T_C=64$ K) with a polar orthorhombic space group (Pbn2${}_1$) between 4 and 293 K, shows a large change in the electric polarization along its $\vec{c}$ axis reaching $\Delta P=17$ mC/m${}^2$ at 10 K. At 62.5 K, a magnetic-field-driven giant variation in polarization $P$(9 T) − $P$(0 T) $=$ 8 mC/m${}^2$ is observed. Moreover, the present magnetoelectric measurements are fully consistent with the $m$′$m$2′ magnetic point group. This ferrimagnetic oxide, which belongs to the “114” structural family, opens an avenue for the search for new magnetoelectrics.

The structural and physical properties of $\mathrm{Yb}\mathrm{Ba}{\mathrm{Co}}_4{\mathrm{O}}_{7+\delta }$ have been studied. The polycrystalline samples of this “114” phase of composition ${\mathrm{Yb}}_{0.95}\mathrm{Ba}{\mathrm{Co}}_4{\mathrm{O}}_{7+\delta }$ have been prepared in air. By postannealing, two samples corresponding to $\delta \sim 0$ and $\delta \sim 1$ have been obtained. The physical properties of the compound “${\mathrm{O}}_7$” $(\delta \sim 0)$ exhibit a hysteretic transition at $T_{\mathrm{S}}=160\mathrm{K}–180\mathrm{K}$ as a function of $T$ as shown by resistivity, magnetization, and thermoelectric power measurements. These measurements indicate a hole-doped electrical conduction compatible with an activated regime for the hopping of high spin ${\mathrm{Co}}^{3+}$ holes in a matrix of high spin ${\mathrm{Co}}^{2+}$. By increasing the oxygen content, i.e., the cobalt oxidation state from 2.25 for “${\mathrm{O}}_7$” to 2.75 for “${\mathrm{O}}_8$,” the signatures of this transition are suppressed. This shows that below the structural transition at $T_{\mathrm{S}}$, the release of the magnetic frustration helps the development of antiferromagnetic order. The latter in turn creates the charge localization at $T_{\mathrm{S}}$.

Dielectric and polarization measurements of the noncentrosymmetric “114” cobaltite CaBaCo${}_4$O${}_7$ were carried out on polycrystalline pellets. Our results show that this material exhibits a clear dielectric peak at $T_C$ and polarization in the ferrimagnetic state below $T_C$ = 70 K. The ferroelectric nature of this phase is supported by pyroelectric measurements. It is shown that this multiferroic material exhibits a magnetodielectric effect of 16$\%$ under 14 T just below $T_C$. In the absence of any structural transition in the whole temperature range (300–4 K), the emergence of polarization at $T_C$ suggests that the ferrimagnetic ordering induces or assists the ferroelectricity. The symmetry analysis shows that in the magnetic phase the Heesch-Schubnikov point group is $m^{\prime }m{2}^{\prime }$, which offers a unique potential for the possible coexistence of ferroelectricity, ferromagnetism, and ferrotoroidicity in this compound along three directions oriented at 90°.

Thermoelectric materials that are efficient well above ambient temperature are needed to convert waste-heat into electricity. Many thermoelectric oxides were investigated for this purpose, but their power factor (PF) values were too small (∼10−4 W m−1 K−2) to yield a satisfactory figure of merit zT. Changing the anions from O2− to S2− and then to Se2− is a way to increase the covalency. In this review, some examples of sulfides (binary Cr–S or derived from layered TiS2) and an example of selenides, AgCrSe2, have been selected to illustrate the characteristic features of their physical properties. The comparison of the only two semiconducting binary chromium sulfides and of a layered AgCrSe2 selenide shows that the PF values are also in the same order of magnitude as those of transition metal oxides. In contrast, the PF values of the layered sulfides TiS2 and Cu0.1TiS2 are higher, reaching ∼10−3 W m−1 K−2. Apparently the magnetism related to the Cr–S network is detrimental for the PF when compared ...

The structural, magnetic, and electrical properties of a ${\mathrm{Fe}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ polycrystalline sample have been characterized. It is found that this compound crystallizes in the $P\overline{3}c1$ space group of the $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ structure and is thus isostructural to ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ and ${\mathrm{Mn}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$, two linear magnetoelectric oxides. But in marked contrast, its ${\varepsilon }^{\prime }\left(T\right)$ curve reveals two broad transitions at $T_{N1}\cong 90\mathrm{K}$ and $T_{N2}\cong 77\mathrm{K}$, the former corresponding to the antiferromagnetic ordering temperature. Below $T_{N1}$, the M$\left(H\right)$ magnetization curves reveal the existence of spin flop at about 6 T. In this temperature region, a H-induced electric polarization for ${\mu }_{0}H>6$ T is evidenced by both sets of $I_p{\left(T\right)}_H$ and $P{\left(H\right)}_T$ curves. All these results point towards ${\mathrm{Fe}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ being a magnetoelectric member of the $A_4{B}_2{\mathrm{O}}_9$ family ($A=\mathrm{Mn}$, Fe, Co and $B=\mathrm{Nb}$, Ta).