Improvement of thermoelectric properties of alkaline-earth hexaborides

Electronic and Structural Properties of MPtxB6-2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

Two new ternary platinum borides, YPtxB6-2x and YbPtxB6-2x, were obtained by argon-arc melting of the elements followed by annealing at 780 °C (750 °C). The structures of these compounds combine the fragments of CaB6- and AuCu3-type structures [space group Pm3̅m; x = 1.15, a = 4.0550(4) Å and x = 1.34, a = 4.0449(2) Å for YPtxB6-2x and YbPtxB6-2x, respectively; single-crystal X-ray diffraction]. Two possible variants of B/Pt ordering (space group P4/mmm) were created via a group-subgroup approach targeting the derived stoichiometry. The architecture of the type-I YPtxB6-2x structure model (a' = a, b' = b, c' = c) combines the 4.82 boron nets alternating with the layers of Y and Pt; the type-II YPtxB6-2x structure model (a' = 2a, b' = 2b, c' = c) exhibits columns of linked [B24] truncated cubes filled with Y running along the c axis. The striking features of both structural models are [B4Pt2] octahedra. The structural similarities with hitherto reported structures (YB2C2, M2Ni21B20, MNi21B20, and ErNiB4) were drawn supporting the verity of these models. A chemical bonding analysis for type-I and type-II YPtxB6-2x based on electron localization function distribution revealed a two-center interaction forming the 4.82 boron nets for type-I YPtxB6-2x and a covalent bonding within [B4Pt2] octahedra as well as a two-center interaction for B-B intraoctahedral bonds for type-II YPtxB6-2x. Analysis of Bader charges revealed the cationic character of the yttrium atoms. The interactions for nondistorted areas of the structures agree well with the bonding picture calculated for constituent building structures, YB6 and YPt3. Electronic structure calculations predict YPtxB6-2x to be a metal with the density of states of around N(EF) = 1 states eV-1 f.u.-1. The exploration of the Y-Pt-B system in the relevant concentration range elucidated the homogeneity field of YPtxB6-2x (0.90 ≤ x ≤ 1.40) and revealed the existence of three more ternary phases at 780 °C: YPt2B (space group P6222), YPt3B (space group P4mm), and YPt5B2 (space group C2/m).

Electric Transport Properties of NaAlB14 with Covalent Frameworks

A synthetic protocol was developed for obtaining a single phase of polycrystalline NaAlB₁₄ with strongly connected intergrain boundaries. NaAlB₁₄ has a unique crystal structure with a tunnel-like covalent framework of B that traps monovalent Na and trivalent Al ions. Owing to the atmospheric instability and volatility of Na, the synthesis of polycrystalline NaAlB₁₄ and its physical properties have not been reported yet. This study employed a two-step process to achieve single-phase polycrystalline NaAlB₁₄. As a first step, a mixture of Al and B with excess Al was sintered in the Na vapor atmosphere followed by HCl treatment to remove excess Al as a second step. For obtaining bulk samples with strong grain connection, vacuum or high-pressure (HP) annealing was employed. HP annealing promoted bandgap shrinkage due to the crystal strain and defect levels and suppressed intergranular resistance. As a result, the HP-annealed sample achieved superior transport properties (0.1 kΩ cm at 300 K) to the vacuum-annealed sample (260 kΩ cm). Furthermore, from the viewpoint of its crystal structure and DFT calculations, the most probable site for the defect was suggested to be the Na site.

New Synthesis Route for Complex Borides; Rapid Synthesis of Thermoelectric Yttrium Aluminoboride via Liquid-Phase Assisted Reactive Spark Plasma Sintering

Y_xAl_yB₁₄ ceramics are of high interest as high temperature thermoelectric materials with excellent p, n control. In this study, direct synthesis of dense polycrystalline Y_xAl_yB₁₄ (x ~0.64, 0.52 ≤ y ≤ 0.67) ceramics was successfully carried out by spark plasma sintering using commercially available precursors. YB₄, AlB₂ and B powders were reactively sintered with an additive AlF₃ at 1773 K for 5–60 min in reduced Ar atmosphere. The sinterability was remarkably enhanced by liquid phase sintering comparing to conventional synthesis techniques. Phase composition analysis by X-ray diffraction showed that main peaks belong to Y_xAl_yB₁₄ with the MgAlB₁₄ structure type and no peaks of AlF₃ were detected. The thermoelectric behavior was changed from p-type to n-type with increasing Al occupancy. Power factor and ZT values measured in this study were found to be in the same range as the best values previously reported. This original synthesis process is found to be less precursor-consuming as compared to previous synthesis processes, and strikingly, less time-consuming, as the synthesis time, is shortened from 8 h to 5 min for p-type and to 1 h for n-type. The total process time is shortened from ≥3 days to ~4–5 h. This discovery opens the door for more accessible synthesis of complex borides.

Nanostructuring of Strontium Hexaboride via Lithiation

We describe the top-down nanostructuring of a metal boride using SrB₆ as an example. To accomplish this transformation, we demonstrate (1) the direct lithiation of a metal boride using n-butyllithium and then (2) the reactive disassembly of Li-SrB₆ into nanoparticles using water. The identity of the Li-SrB₆ intermediate, a mixture of Li₂B₆, Li_xSr_1-2xB₆, and SrB₆ phases, was established by powder X-ray diffraction (PXRD), solid-state ¹¹B and ⁷Li NMR, transmission electron microscopy, selected-area electron diffraction, and scanning electron microscopy. The necessary 2Li⁺/Sr²⁺ substitution is enabled by cation mobility within the hexaboride lattice. The subsequent reaction with water results in Li₂B₆ decomposition and the release of <100 nm SrB₆ nanoparticles, which were characterized by PXRD, solid-state ¹¹B and ⁷Li NMR, and high-resolution TEM. This chemistry opens new solution-based modification and processing options for metal borides.

Effect of Current on Diffusivity in Metal Hexaborides: A Spark Plasma Sintering Study

We present the effect of pulsed direct current on metal ion diffusion in CaB₆-SrB₆ diffusion couples, showing that the diffusivity of Ca²⁺ and Sr²⁺ across the diffusion couple interface is higher toward the positive electrode when subjected to a current flow of 2.2 kA at a temperature of 2007 K. We attribute this enhanced mobility to the movement of negatively charged metal vacancies toward the positive electrode in the system. Energy-dispersive spectroscopy is used to map the concentration of Ca²⁺ and Sr²⁺ in the region near the interface, and diffusion profiles are fitted with error functions. The concentration curves display concentration-dependent Boltzmann-Matano diffusivity. Total dopant values (Q) have been used to numerically compare the differences between Ca²⁺ diffusion in SrB₆ and Sr²⁺ diffusion in CaB₆. We determine an enhancement of 3.8× for Ca²⁺ into SrB₆ versus an enhancement of 1.8× for Sr²⁺ into CaB₆. No new phases are formed at the interface between CaB₆ and SrB₆, since hexaboride compounds readily form solid solutions. The results elucidate the role of pulsed direct current on the diffusion of metal ions in hexaboride compounds.

Synthesis, and magnetic and optical properties of nanocrystalline alkaline-earth hexaborides

Cubic-shaped ultrafine alkaline-earth hexaborides (MB₆, M = Ca, Ba, Sr) have been synthesized via a solid-state reaction of MO with NaBH₄ at 1150 °C.

New route toward nanosized crystalline metal borides with tuneable stoichiometry and variable morphologies

Herein we highlight for the first time the ability to tune the stoichiometry of metal boride nanocrystals through nanoparticle synthesis in thermally stable inorganic molten salts. Two metal–boron systems are chosen as case studies: boron-poor nickel borides and boron-rich yttrium borides. We show that NiB, Ni₄B₃, Ni₂B, Ni₃B, and YB₆ particles can be obtained as crystalline phases with good selectivity. Anisotropic crystallization is observed in two cases: the first boron-rich YB₄ nanorods are reported, while boron-poor NiB nanoparticles show a peculiar crystal habit, as they are obtained as spheres with uniaxial defects related to the crystal structure. Crystallization mechanisms are proposed to account for the appearance of these two kinds of anisotropy at the nanoscale.

The stability of B6 octahedron in BaB6 under high pressure

We have performed in situ synchrotron X-ray diffraction and first-principles calculations to explore the compression behavior of barium hexaboride (BaB₆) under high pressure.

Synthesis and characterization of single-crystal strontium hexaboride nanowires

Catalyst-assisted growth of single-crystal strontium hexaboride (SrB6) nanowires was achieved by pyrolysis of diborane (B2H6) over SrO powders at 760-800 degrees C and 400 mTorr in a quartz tube furnace. Raman spectra demonstrate that the nanowires are SrB6, and transmission electron microscopy along with selected area diffraction indicate that the nanowires consist of single crystals with a preferred [001] growth direction. Electron energy loss data combined with the TEM images indicate that the nanowires consist of crystalline SrB 6 cores with a thin (1 to 2 nm) amorphous oxide shell. The nanowires have diameters of 10-50 nm and lengths of 1-10 microm.