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9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

New Zintl Phase Yb 10 MgSb 9 with High Thermoelectric Performance

Abstract Yb 10 MgSb 9 is a new Zintl compound (with a composition closer to Yb 10.5 MgSb 9 ) and a promising thermoelectric material first reported in this work. Undoped Yb 10 MgSb 9 has an ultralow thermal conductivity due to crystallographic complexity and exhibits a relatively high peak p‐type Seebeck coefficient and high electrical resistivity. This is consistent with Zintl counting and density functional theory (DFT) calculations that the composition Yb 10.5 MgSb 9 should be a semiconductor. Na is found experimentally to be an effective p‐type dopant potentially due to the replacement of Na + for Yb 2+ , allowing for a significant decrease in electrical resistivity. With doping, a dramatic improvement of electrical conductivity is observed and the glass‐like thermal conductivity remains low, allowing for a significant enhancement of the thermoelectric figure of merit, zT . Doping increases the zT from 0.23 in undoped Yb 10 MgSb 9 to 1.06 in 7 at% Na‐doped Yb 10 MgSb 9 at 873K. This high thermoelectric performance found through Na‐doping places this material amongst the leading p‐type Zintl thermoelectrics, making it a promising candidate for future studies and high‐temperature thermoelectric applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ca(MgSb)2 by Materials Project

Ca(MgSb)2 crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. Ca2+ is bonded to six equivalent Sb3- atoms to form CaSb6 octahedra that share corners with twelve equivalent MgSb4 tetrahedra, edges with six equivalent CaSb6 octahedra, and edges with six equivalent MgSb4 tetrahedra. All Ca–Sb bond lengths are 3.28 Å. Mg2+ is bonded to four equivalent Sb3- atoms to form MgSb4 tetrahedra that share corners with six equivalent CaSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent CaSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 15–56°. There are three shorter (2.87 Å) and one longer (2.94 Å) Mg–Sb bond lengths. Sb3- is bonded to three equivalent Ca2+ and four equivalent Mg2+ atoms to form a mixture of distorted edge and corner-sharing SbCa3Mg4 pentagonal bipyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Sr(MgSb)2 by Materials Project

Sr(MgSb)2 crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. Sr2+ is bonded to six equivalent Sb3- atoms to form SrSb6 octahedra that share corners with twelve equivalent MgSb4 tetrahedra, edges with six equivalent SrSb6 octahedra, and edges with six equivalent MgSb4 tetrahedra. All Sr–Sb bond lengths are 3.41 Å. Mg2+ is bonded to four equivalent Sb3- atoms to form MgSb4 tetrahedra that share corners with six equivalent SrSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent SrSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 18–54°. There are three shorter (2.89 Å) and one longer (2.93 Å) Mg–Sb bond lengths. Sb3- is bonded to three equivalent Sr2+ and four equivalent Mg2+ atoms to form a mixture of distorted corner and edge-sharing SbSr3Mg4 pentagonal bipyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ba(MgSb)2 by Materials Project

Ba(MgSb)2 crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. Ba2+ is bonded to six equivalent Sb3- atoms to form BaSb6 octahedra that share corners with twelve equivalent MgSb4 tetrahedra, edges with six equivalent BaSb6 octahedra, and edges with six equivalent MgSb4 tetrahedra. All Ba–Sb bond lengths are 3.55 Å. Mg2+ is bonded to four equivalent Sb3- atoms to form MgSb4 tetrahedra that share corners with six equivalent BaSb6 octahedra, corners with six equivalent MgSb4 tetrahedra, edges with three equivalent BaSb6 octahedra, and edges with three equivalent MgSb4 tetrahedra. The corner-sharing octahedra tilt angles range from 21–52°. There are three shorter (2.92 Å) and one longer (2.93 Å) Mg–Sb bond lengths. Sb3- is bonded to three equivalent Ba2+ and four equivalent Mg2+ atoms to form a mixture of distorted edge and corner-sharing SbBa3Mg4 pentagonal bipyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on MgSb by Materials Project

MgSb is Tetraauricupride structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Mg2+ is bonded in a body-centered cubic geometry to eight equivalent Sb2- atoms. All Mg–Sb bond lengths are 3.17 Å. Sb2- is bonded in a body-centered cubic geometry to eight equivalent Mg2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Finding the order in complexity: The electronic structure of 14-1-11 zintl compounds

Yb 14 MnSb 11 and Yb 14 MgSb 11 have rapidly risen to prominence as high-performing p-type thermoelectric materials. However, the fairly complex crystal structure of A 14 MX 11 Zintl compounds renders the interpretation of the electronic band structure obscure, making it difficult to chemically guide band engineering and optimization efforts. In this work, we delineate the valence-balanced Zintl chemistry of A 14 MX 11 compounds using the molecular orbital theory. By analyzing the electronic band structures of Yb 14 MgSb 11 and Yb 14 AlSb 11 , we show that the conduction band minimum is composed of either an antibonding molecular orbital originating from the (Sb 3 ) 7– trimer or a mix of atomic orbitals of A, M, and X. The singly degenerate valence band is comprised of non-bonding Sb pz orbitals primarily from the Sb atoms in the (MSb 4 ) m– tetrahedra and of isolated Sb atoms distributed throughout the unit cell. Such a chemical understanding of the electronic structure enables strategies to engineer electronic properties (e.g., the bandgap) of A 14 MX 11 compounds.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Intermediate Yb valence in the Zintl phases Yb 14 MSb 11 (M=Zn,Mn,Mg): XANES, magnetism, and heat capacity

Yb 14 Mn Sb 11 is a magnetic Zintl compound as well as being one of the best high temperature p -type thermoelectric materials. According to the Zintl formalism, which defines intermetallic phases where cations and anions are valence satisfied, this structure type is nominally made up of 14 Yb 2 + , 1 MnSb 4 9 - , 1 Sb 3 7 - , and 4 Sb 3 - atoms. When Mn is replaced by Mg or Zn, the Zintl defined motifs become 13 Yb 2 + , 1 Yb 3 + , 1 (Mg, Zn) Sb 4 10 - , 1 Sb 3 7 - , and 4 Sb 3 - . The predicted existence of Yb 3 + based on simple electron counting rules of the Zintl formalism calls the Yb valence of these compounds into question. X-ray absorption near-edge structure, magnetic susceptibility, and specific heat measurements on single crystals of the three analogs show signatures of intermediate valence Yb behavior and in particular, reveal the heavy fermion nature of Yb 14 MgSb 11 . In these isostructural compounds, Yb can exhibit a variety of electronic configurations from intermediate ( M = Zn ), mostly 2+ ( M = Mn ), to 3+ ( M = Mg ). In all cases, there is a small amount of intermediate valency at the lowest temperatures. The amount of intermediate valency is constant for M = Mn , Mg and temperature dependent for M = Zn . The evolution of the Yb valence correlated to the transport properties of these phases is highlighted. The presence of Yb in this structure type allows for fine tuning of the carrier concentration and thereby the possibility of optimized thermoelectric properties along with unique magnetic phenomena.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Violation of the T –1 Relationship in the Lattice Thermal Conductivity of Mg 3 Sb 2 with Locally Asymmetric Vibrations

Most crystalline materials follow the guidelines of T –1 temperature-dependent lattice thermal conductivity (κ L ) at elevated temperatures. Here, we observe a weak temperature dependence of κ L in Mg 3 Sb 2 , T –0.48 from theory and T –0.57 from measurements, based on a comprehensive study combining ab initio molecular dynamics calculations and experimental measurements on single crystal Mg 3 Sb 2 . These results can be understood in terms of the so-called “phonon renormalization” effects due to the strong temperature dependence of the interatomic force constants (IFCs). The increasing temperature leads to the frequency upshifting for those low-frequency phonons dominating heat transport, and more importantly, the phonon-phonon interactions are weakened. In-depth analysis reveals that the phenomenon is closely related to the temperature-induced asymmetric movements of Mg atoms within MgSb 4 tetrahedron. With increasing temperature, these Mg atoms tend to locate at the areas with relatively low force in the force profile, leading to reduced effective 3 rd -order IFCs. The locally asymmetrical atomic movements at elevated temperatures can be further treated as an indicator of temperature-induced variations of IFCs and thus relatively strong phonon renormalization. The present work sheds light on the fundamental origins of anomalous temperature dependence of κ L in thermoelectrics.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Chapmanite [Fe 2 Sb(Si 2 O 5 )O 3 (OH)]: thermodynamic properties and formation in low-temperature environments

Abstract. of synthetic Sb 2 O 5 , MgSb 2 O 6 (analogue of the mineral byströmite), Mg[Sb(OH) 6 ] 2 ∙6H 2 O (brandholzite), and natural chapmanite [(Fe 1.88 Al 0.12 )Sb(Si 2 O 5 )O 3 (OH)]. Enthalpies of reactions, including formation enthalpies, were evaluated using reference compounds Sb, Sb 2 O 3 , Sb 2 O 5 , and other phases, with high-temperature oxide melt solution calorimetry in lead borate and sodium molybdate solvents. Heat capacity and entropy were determined by relaxation and differential scanning calorimetry. The best set of Δ f H o (kJ mol -1 ) and S o (J mol -1 K -1 ) is byströmite -1733.0±3.6, 139.3±1.0; brandholzite -5243.1±3.6, 571.0±4.0; and chapmanite -3164.9±4.7, 305.1±2.1. The data for chapmanite give Δ f G o of -2973.6±4.7 kJ mol -1 and log K=-17.10 for the dissolution reaction (Fe 1.88 Al 0.12 )Sb(Si 2 O 5 )O 3 (OH) + 6H + → 1.88Fe 3+ + 0.12Al 3+ + 2SiO$_2^0$ + Sb(OH)$_3^0$ + 2H 2 O. Analysis of the data showed that chapmanite is finely balanced in terms of its stability with schafarzikite (FeSb 2 O 4 ) and tripuhyite (FeSbO 4 ) under a specific, narrow range of conditions when both aqueous Fe(III) and Sb(III) are abundant. In such a model, chapmanite is metastable by a narrow margin but could be stabilized by high SiO$_2^0$(aq) activities. Natural assemblages of chapmanite commonly contain abundant amorphous silica, suggesting that this mechanism may be indeed responsible for the formation of chapmanite. Chapmanite probably forms during low-temperature hydrothermal overprint of pre-existing Sb ores under moderately reducing conditions; the slightly elevated temperatures may help to overcome the kinetic barrier for its crystallization. During weathering, sheet silicates may adsorb Sb 3+ in tridentate hexanuclear fashion, thus exposing their chapmanite-like surfaces to the surrounding aqueous environment. Formation of chapmanite, as many other sheet silicates, under ambient conditions, is unlikely.

58 GEOSCIENCES↗