We report on the synthesis, crystal structure and high-temperature transport properties of Rb 2 Mo 15 Se 19 , a new member of the large family of M 2 Mo 15 Se 19 (M = In, Tl, K, Ba) cluster compounds. Polycrystalline samples and single crystals of the ternary selenide Rb 2 Mo 15 Se 19 were obtained by solid-state reactions. The trigonal crystal structure, successfully refined in space group (No. 167) with unit-cell parameters a = 9.7618(1) Å, c = 58.254(1) Å and Z = 6, was determined by single-crystal X-ray diffraction. The crystal structure contains Mo 6 Se 8 i Se 6 a and Mo 9 Se 11 i Se 6 a cluster units in equal proportion and separated from each other by large voids, which are filled up by Rb atoms. Measurements of the electrical resistivity, thermopower and thermal conductivity revealed that Rb 2 Mo 15 Se 19 behaves as a p-type metal with relatively low electrical resistivity and thermopower. Despite its complex crystal structure, the lattice thermal conductivity that amounts to 1.3 W.m-1 .K-1 at 300 K is significantly higher than in Ag-filled analogues, which provides possible guidelines to enhance its thermoelectric properties.

Mo-based cluster compounds are an emerging class of potential candidates for thermoelectric applications due to several key crystallographic characteristics that drive their ability to transport heat close to that of amorphous systems. Here, we report on a detailed investigation of the crystal structure and high-temperature transport properties of the cluster compound K2Mo15Se19 and of its Ag-filled variant Ag3K2Mo15Se19. Single-crystal X-ray diffraction confirm that both compounds crystallize with a hexagonal crystal structure, successfully solved in the space group 3 ̅ , built by octahedral Mo6Se8 and bioctahedral Mo9Se11 clusters with K and Ag cations filling the large inter-cluster voids. In agreement with electronic band structure calculations, the electron transfer that occurs from the filling cations to the cluster subnetwork provides a simple guiding rule to predict the evolution of the transport properties upon filling 2 with Ag. The metallic state observed in K2Mo15Se19 is turned into a more heavily-doped semiconducting character in the presence of Ag resulting in enhanced thermopower values in Ag3K2Mo15Se19. The beneficial influence of Ag on the thermoelectric properties is also reflected by lowered lattice thermal conductivity values that reach 0.45 W m -1 K -1 at 750 K.These results indicate that inserting an additional element in the ternaries M2Mo15Se19 is an interesting route to improving their thermoelectric properties.

The search for new multifunctional materials displaying proton-conducting properties is of paramount necessity for the development of electrochromic devices and supercapacitors as well as for energy conversion and storage. In the present study, proton conductivity is reported for the first time in three molybdenum cluster-based materials: (H)[MoBrS(OH)]-12HO and (H)[MoX(OH)]-12HO (X = Cl, Br). We show that the self-assembling of the luminescent [MoL(OH)] cluster units leads to both luminescence and proton conductivity (σ = 1.4 × 10 S·cm in (H)[MoCl(OH)]-12HO under wet conditions) in the three materials. The latter property results from the strong hydrogen-bond network that develops between the clusters and the water molecules and is magnified by the presence of protons that are statistically shared by apical hydroxyl groups between adjacent clusters. Their role in the proton conduction is highlighted at the molecular scale by ab initio molecular dynamics simulations that demonstrate that concerted proton transfers through the hydrogen-bond network are possible. Furthermore, thermogravimetric analysis also shows the ability of the compounds to accommodate more or less water molecules, which highlights that vehicular (or diffusion) transport probably occurs within the materials. An infrared fingerprint of the mobile protons is finally proposed based on both theoretical and experimental proofs. The present study relies on a synergic computational/experimental approach that can be extended to other proton-conducting materials. It thus paves the way to the design and understanding of new multifunctional proton-conducting materials displaying original and exciting properties.