Synthesis, crystal structure and high-temperature transport properties of the new cluster compound Rb2Mo15Se19

Abstract

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.

Thermoelectric Properties of Ternary and Quaternary Mo6 and Mo9 Cluster Selenides

“…Due to the character of Rb, the total MEC is equal to 54 in Rb2Mo15Se19 and should thus give rise to p-type metallic properties. Measurements of the thermoelectric properties at high temperatures have confirmed this prediction[35]. Further optimization of the thermoelectric properties of A2Mo15Se19 compounds implies the introduction of additional charges to bring the MEC closer to 60.…”

“…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 the intercluster spaces[30][31][32][33][34][35][36][37][38][39][40][41]. The largest thermal ellipsoids are observed for the Rb and Tl atoms in the Ag3RbMo9Se11 and Ag2Tl2Mo9Se11 compounds…”

“…Unlike the AgxMo9Se11 compounds, this prediction has been verified experimentally with an electricalresistivity following a semiconducting-like temperature dependence (seeFigure 5). These results suggest that the specific arrangement of the cluster unit can play an important role in determining the electronic properties of these compounds.The transport properties of compounds built up from an equal mixture of Mo6Se8 i Se6 a and Mo9Se11 i Se6 a cluster units have so far been investigated only on the ternary compound Rb2Mo15Se19[35] and on the quaternary compounds Ag3In2Mo15Se19 and Ag3Tl2Mo15Se19[33,41]. The optimal MEC for the Mo6Se8 and Mo9Se11 clusters are equal to 24 and 36, respectively, yielding an optimum MEC of 60 for the Mo15Se19 clusters[33].…”