From NaZn₄Sb₃ to HT-Na_1–xZn_4–ySb₃: Panoramic Hydride Synthesis, Structural Diversity, and Thermoelectric Properties

Abstract

Two new sodium zinc antimonides NaZn4Sb3 and HT-Na1-xZn4-ySb3 were synthesized by using reactive sodium hydride, NaH, as a precursor. The hydride route provides uniform mixing and comprehensive control over the composition, facilitating fast reactions and high-purity samples, whereas traditional synthesis using sodium metal results in inhomogeneous samples with a significant fraction of the more stable NaZnSb compound. NaZn4Sb3 crystalizes in the hexagonal P63/mmc space group (No. 194, Z = 2, a = 4.43579(4) Å, c = 23.41553(9) Å), and is stable upon heating in vacuum up to 736 K. The layered crystal structure of NaZn4Sb3 is related to the structure of the well-studied thermoelectric antimonides AeZn2Sb2 (Ae = Ca, Sr, Eu). Upon heating in vacuum NaZn4Sb3 transforms to HT-Na1-xZn4-ySb3 (x = 0.047(3), y = 0.135(1)) due to partial Na/Zn evaporation/elimination, as was determined from hightemperature in-situ synchrotron powder X-ray diffraction. HT-Na1-xZn4-ySb3 has a complex monoclinic structure with considerable degrees of structural disorder (P21/c (No. 14, Z = 32), a = 19.5366(7) Å, b = 14.7410(5) Å, c = 20.7808(7) Å, β = 90.317(2)°) and is stable exclusively in a narrow temperature range of 736 − 885 K. Further heating of HT-Na1-xZn4-ySb3 leads to a reversible transformation to NaZnSb above 883 K. Both compounds exhibit similarly low thermal conductivity at room temperature (0.9 W•m −1 K −1 ) and positive Seebeck coefficients (38-52 µV/K) indicative of holes as the main charge carriers. However, resistivities of the two phases differ by two orders of magnitude.Here, we have explored the ternary Na-Zn-Sb system and discovered two new compositionally similar, but structurally different ternary antimonides, both are featuring new structure types. Using the fast hydride route coupled with in-situ high-temperature powder X-ray diffraction experiments, compositional and temperature screening allowed for synthesis of two new ternary phases: NaZn4Sb3 phase and what at first appeared to be its polymorph, but in fact it is a different compound with slightly Na/Zn depleted composition HT-Na1-xZn4-ySb3, stable in narrow temperature range. The hydride route yields single phase samples of both antimonides, allowing for the experimental access to their transport properties. The crystal structures, synthesis, structural transformations, and transport properties of the NaZn4Sb3 and HT-Na1-xZn4-ySb3 are discussed herein.

Lithium nickel borides: evolution of [NiB] layers driven by Li pressure

“…It is slightly positively shifted compared to the peak at ~75 ppm for RT-LiNiB phase but within the range of 7 Li chemical shifts in other Li-containing intermetallics. 10,11 The slight increase in the 7 Li chemical shift for RT-Li 1+x NiB as compared to RT-LiNiB can likely be attributed to the subtle differences in the Li coordination environment in the two compounds. The increased width of the 7 Li peak may be a result of the increased complexity of RT-Li 1+x NiB structure, where extra Li atoms are incorporated between the [NiB] layers, leading to a variety of local 7 Li environments and distribution of shifts.…”

“…Previously we showed 10,11 the hydride route is feasible for preparation of ternary lithium nickel borides providing fast reactions and accurate compositional control. The experimentally determined, LiH:Ni:B = 1.3:1:1.15, molar ratio results in phase-pure samples of LiNiB phases with minimal excess of LiH and B needed.…”

“…The hydride synthesis route, where salt-like alkali metal hydrides are used as a source of alkali metals, has been successfully utilized for the preparation of alkali/alkali-earth antimonides, arsenides, germanides, and silicides. [4][5][6][7][8][9][10] Application of this method for the Li-Ni-B system significantly shortened the synthesis time of ternary phases. For instance, LiNi 3 B 1.8 can be synthesized in a span of ~24 hours compared with the previously reported 3-month annealing for the reaction of elements.…”

Synthesis, Crystal and Electronic Structure of Layered AMSb Compounds (A = Rb, Cs; M = Zn, Cd)

“…These include binary, ternary, and multinary structures that range from simple 0D to complex 3D motifs. [1][2][3][4][5][6] Because of this, metal antimonides have found themselves at the forefront of many applications, such as thermoelectrics, photodetectors, LEDs, integrated circuits, and others. [7][8][9][10][11] A notable example is the well-known Yb 14 MnSb 11 , which has been identified as an efficient high-temperature p-type leg for thermoelectric generators.…”

First-principles predictions of low lattice thermal conductivity and high thermoelectric performance of AZnSb (A = Rb, Cs)

“…[4][5][6][7][8][9] Some of the antimony-based compounds exhibit topological behavior and its combination with other elements can crystallize in a wide range of structures, from zero to 3-dimensional motifs. [9][10][11][12][13][14][15] These structures can be either too simple or too complex. The Sb-based compounds have thus a wide range of applications as thermoelectrics, photovoltaic, and other electronic components.…”