Structural Order-Disorder Transitions and Phonon Conductivity of Partially Filled Skutterudites

Abstract

Filled skutterudites are high-performance thermoelectric materials and we show how their phonon conductivity is greatly influenced by the topology of the filler species. We predict (ab initio) the phase diagram of Ba x Co 4 Sb 12 and find several stable configurations of Ba ordering over the intrinsic voids. The phonon conductivity predicted using molecular dynamics shows a minimum in the two-phase mixture regime, dominated by significantly reduced long-range acoustic phonon transport. Binary skutterudites are compounds with the general formula MX 3 (M ¼ Co, Rh, Ir and X ¼ P, As, Sb) having a crystal structure with a bcc lattice and belonging to the space group Im3. The structure consists of a periodic array of trigonally distorted and tilted MX 6 octahedrons. The metal M at the center is octahedrally coordinated by the pnicogen X [13]. The tilt of the MX 6 octahedrons gives rise to empty spaces (icosahedral voids) that form a bodycentered sublattice. As first shown in [14], the voids are large enough and can be filled by foreign species entering as cations; however, the filler solubility limit is less than a full filling of all available voids [4,15].Binary skutterudites possess very high charge carrier mobilities and the interest in these compounds as novel TE materials for power generation followed a suggestion [16] and observation [11] that their L can be dramatically reduced upon filling the structural voids. Recent research has resulted in the TE figure of merit (ZT) of n-type skutterudites approaching the value of 1.5 at 800 K [17]. While the presence of the filler species in the skutterudite matrix seems to be essential for achieving low L and hence high ZT, whether the filler acts as a rattling local vibration mode [1][2][3][4] or plays some other important role [7,[18][19][20] remains controversial. So far the only insight into the role of ordered phases on the L of partially-filled skutterudites has been the observation in [3] that the random distribution of filler ions scatter phonons more effectively than when most voids are filled. In this letter, we address the issue of ordering of the filler species, a topic that has not yet been explored and that has a great influence on phonon scattering. To this end we examine Ba x Co 4 Sb 12 , a skutterudite with a large filling fraction limit for Ba, using theoretical and computational treatments.We begin with a determination of a temperaturecomposition phase diagram of Ba x Co 4 Sb 12 by combining ab initio calculations with the cluster expansion (CE) method and the Monte Carlo (MC) simulations. Thereafter, using equilibrium molecular dynamics (MD) simulations and the Green-Kubo fluctuation-dissipation linear response theory and L decomposition, along with the phase diagram, we predict the L of partially filled Ba x Co 4 Sb 12 as a function of concentration and temperature.A prediction of phase stability at finite temperature requires the use of statistical mechanics to account for entropic contributions to the free energy. The most important source of entr...

Structure and Failure Mechanism of the Thermoelectric CoSb3/TiCoSb Interface

“…24 These calculated values are in good agreement with the previously published theoretical value of 9.14 Å for CoSb 3 and 5.89 Å for TiCoSb. [25][26][27] To determine the favorable surface slab, we calculated the surface energy, γ , from the following expression, 28 A is the surface area.…”

Configuring pnicogen rings in skutterudites for low phonon conductivity

“…40. Using this combined strategy we predict a further 33% reduction in κ L (much closer to the theoretical minimum, κ min , of an amorphous phase).…”

“…where n is the atomic number density, yields CT = 4.758 × 10 -16 s. 40 The parameter a 1 is the coefficient for the Rayleigh point-defect scattering rate, given by , 4 3 , ,…”

“…Here κ e = Lσ e T, where σ e is the measured electrical conductivity and L is the Lorenz number, determined from the experimental Seebeck coefficient by assuming a single parabolic band. 35 The experimental value of κ L obtained in this way, which In order to analyze the effect of pnicogen ring substitution on κ L , we use experimentally-and DFT-parameterized analytical models for phonon-phonon and point-defect scattering, 26,[36][37][38][39][40] as well as non-equilibrium ab initio molecular dynamics (NEAIMD) simulation. 41 Starting with κ L of CoSb 3 and CoGe 1.5 Te 1.5, which are dominated by phonon-phonon scattering, we add an analytical factor for point-defect scattering at intermediate alloy compositions.…”

“…41 Starting with κ L of CoSb 3 and CoGe 1.5 Te 1.5, which are dominated by phonon-phonon scattering, we add an analytical factor for point-defect scattering at intermediate alloy compositions. [37][38][39][40]42 Using the Matthiessen rule 42 , the overall κ L with the inclusion of the point…”