Synthesis, Crystal Structure, and Transport Properties of the Hexagonal Mo₉ Cluster Compound Ag₃RbMo₉Se₁₁

Abstract

International audienceMo-based cluster compounds are promising candidates for thermoelectric applications at high temperatures due to their very low lattice thermal conductivity values. Here, we report on a detailed investigation of the crystal structure and transport properties measured in a wide range of temperatures (2-800 K) of polycrystalline Ag3RbMo9Se11. Single-crystal X-ray diffraction shows that this compound crystallizes in the hexagonal space group P63/m. The crystal structure is formed by stacked Mo9Se11 units leaving channels that are randomly filled by Rb(+) cations, while Ag(+) cations are located between the Mo9Se11 units. The high disorder in the unit cell induced by these atoms and their large anisotropic thermal displacement parameters are two key characteristics that lead to very low lattice thermal conductivity as low as 0.6 W m(-1) K(-1) at 800 K. The combination of semiconducting-like electrical properties and low ability to transport heat leads to a maximum dimensionless thermoelectric figure of merit ZT of 0.4 at 800 K

Thermoelectric Properties of Ternary and Quaternary Mo6 and Mo9 Cluster Selenides

“…Mo6Se8Se6 cluster units ( Figure 1b) similar to those found in the Chevrel phases [42]. [32,38], respectively. Low-temperature single-crystal X-ray diffraction measurements performed on Ag2Tl2Mo9Se11 evidenced that the ADP values of the Tl atoms exhibit a nearly temperature-independent behavior down to 85 K [32].…”

“…One important aspect of the transport in these compounds is the presence of weaklydispersive valence bands that give rise to electron-like and hole-like contributions to the transport [30][31][32][33][34][35][36][37][38][39][40][41]. The contributions to the thermopower of these two types of carriers are of opposite sign and thus usually tend to lower the measured values.…”

“…These voids are significantly larger than those present in the Chevrel phases, often leading to very large, anisotropic thermal displacement parameters for the cations [18][19][20][21][22][23][24][25][26][27][28][29]. The strong disorder induced by the presence of these intercluster atoms is an important ingredient that helps to limit the propagation of heat-carrying acoustic phonons [30][31][32][33][34][35][36][37][38][39][40][41].…”

“…In some cases, a quaternary phase can be obtained by direct reaction of a ternary cluster compound with an excess of another element during long-term annealing at high temperatures. This kind of synthesis has been successfully applied to Tl2Mo9Se11 in the presence of Ag powders which led to the formation of the novel quaternary compound The growth of single crystals of these various compounds could be realized by either a vapor-transport technique or by a recrystallization process by subjecting the polycrystalline pellets to an annealing treatment at very high temperature [31][32][33]38,41]. The former technique has been considered to grow single crystals of AgxMo9Se11 using AgCl as the transport agent in a sealed silica tube [31].…”

“…A temperature gradient of 30°C has been applied between both ends of the tubes kept at 1030 and 1060°C during 10 days. The later route has been used to obtain single crystals of the cluster compounds Ag3RbMo9Se11 [38],…”