New Manganese-Bearing Antimonides and Bismuthides with Complex Structures. Synthesis, Structural Characterization, and Electronic Properties of Yb₉Mn_4+xPn₉ (Pn = Sb or Bi)

Abstract

Reported are structure and property studies on a series of isostructural compounds with general formulas Yb 9 Mn 4þx Sb 9 (x ≈ 0.2), A 9 Mn 4þx Bi 9 (A = Ca and Yb; x ≈ 0), and the Zn-substituted derivatives Yb 9 (Mn,Zn) 4þx Sb 9 (0 < x < 0.4). They have been synthesized from the corresponding elements by high-temperature reactions, and their structures have been established by single-crystal X-ray diffraction. Despite being nearly stoichiometric phases, whose average crystallographic arrangement can be described with the orthorhombic Ca 9 Mn 4 Bi 9 type structure (space group Pbam, No. 55), these ternary materials exist within narrow homogeneity ranges, whereupon an interstitial site is partially occupied by the transition metal. Electronic structure calculations confirm that the "empty" A 9 Mn 4 Sb 9 or A 9 Mn 4 Bi 9 structures are electron deficient, making them suitable hosts for small dopant atoms. The expected poorly metallic behavior is evidenced from temperature-dependent resistivity data gathered from single-crystals. Magnetic susceptibility measurements in the range 2-350 K reveal complex antiferromagnetically coupled structures. These experimental results are further corroborated by theoretical calculations based on the linear muffin-tin orbital (LMTO) and the full potential linearized augmented plane wave (FP-LAPW) methods. †

Glass-like lattice thermal conductivity and high thermoelectric efficiency in Yb9Mn4.2Sb9

“…This suggests to us that this phase has a narrow composition range near Yb 9 Mn 4.2 Sb 9 , consistent with the initial report by Bobev et al 19 For high temperature transport measurements, two separately prepared samples (referred to as sample A and B) with compositions of approximately Yb 9 Mn 4.2 Sb 9 were used, both of which had similar, low concentrations of impurity phases ($1%) as veried by EPMA.…”

“…However, the limitations of this model leave some uncertainty in the optimal value of n, and the ability to control or reduce n may lead to further optimization of zT. Since the interstitial Mn content in Yb 9 Mn 4.2 Sb 9 appears to have a very narrow range, varying the Mn content is not a likely route to controlling n. In contrast, in the isostructural Zn-analogue (Yb 9 Zn 4+x Sb 9 ), a wider composition range may be possible, 19,21 suggesting that a solid solution between these may be a potential route to controlling n and increasing the performance of this system.…”

Thermoelectric properties of the Yb9Mn4.2−xZnxSb9solid solutions

“…The Mn analogue, Yb 9 Mn 4.2 Sb 9 , is a line compound with an electron decient composition, resulting in a carrier concentration of 9 were calculated from XRD results using Rietveld fits. As the amount of Zn increases, each parameter changes linearly from the Mn end member side to the Zn end member and this follows a reported trend 37,38 Composition (Fig. 3c) decreases with temperature as expected when acoustic phonons are the primary scattering source.…”

“…1 The Yb 9 M 4+x Sb 9 structure contains infinite M 4 Sb 9 "ribbons" made up of corner sharing MSb 4 tetrahedra, where M is a 2+ transition metal that can be either Mn, Cd or Zn. 37,38 Partially occupied Minterstitial sites (yellow spheres) link neighboring M 4 Sb 9 ribbons into a pseudo two-dimensional framework. 9) determined from renement of single crystal X-ray diffraction data.…”

Structural Variability versus Structural Flexibility. A Case Study of Eu9Cd4+xSb9 and Ca9Mn4+xSb9 (x ≈ 1/2)

“…7−12 Arguably, the best known and frequently studied compound in this series is Yb 14 MnSb 11 , which has become the benchmark thermoelectric material in the high-temperature range. 7,8 Not long ago, our team reported another Zintl phase with similar chemical makeup, Yb 9 Mn 4+x Sb 9 (x ≈ 0.2), 13 which subsequently was also shown to have high thermoelectric efficiency. 14,15 For Yb 9 Mn 4+x Sb39, a noteworthy feature is the very low, glasslike lattice thermal conductivity, which can be correlated with the complex structure of this material.…”