RESUMEN
Zintl compounds often feature complex structural fragments and small band gaps, favoring promising thermoelectric properties. In this work, a new phase Ca2ZnSb2 is synthesized and characterized to be a LiGaGe-type structure. It is isotypic to Yb2MnSb2 with half vacancies at transition metal sites and undergoes a phase transition to Ca9Zn4+xSb9 after annealing. Interestingly, Ca2ZnSb2 and Yb2MnSb2 are amenable to diverse doping mechanisms at different sites. Here, by substituting smaller Li on cation sites, two novel layered compounds Ca1.84(1)Li0.16(1)Zn0.84(1)Sb2 and Yb1.82(1)Li0.18(1)Mn0.96(1)Sb2 with the P63/mmc space group are discovered, which can be viewed as derivatives of LiGaGe type. Despite having lower occupancy, the structural stability is improved compared with the prototype compounds owing to the reduced interlayered distances. Besides, the band structure analyses demonstrate that the bands near the Fermi level are mainly governed by the interlayered interaction. Due to the highly disordered structure, Yb1.82Li0.18Mn0.96Sb2 features ultralow thermal conductivity from 0.79 to 0.47 W·m-1·K-1 among the testing range; in addition, a remarkable Seebeck coefficient of 270.77 µV·K-1 at 723 K is observed. The discovery of the Ca2ZnSb2 phase enriches the 2-1-2 map, and the size effect induced by cations provides new ideas for material designing.
RESUMEN
A series of Mg/Mn mixed intermetallic compounds Mg1-xMn2+xAs2 (x = 0.17, 0.48, 0.69) were synthesized by using metal flux reactions. Single-crystal X-ray diffraction measurements indicated that CaAl2Si2-type phases with Mn and Mg atoms located on the cation sites (Wickoff site: 1a) were obtained. The special structure of these Mg1-xMn2+xAs2 compounds corresponded to unique magnetic behavior, which led to increased divergence between zero-field-cooling (ZFC) and field-cooling magnetic susceptibilities with decreasing temperature. The small magnetic hysteresis loop measured at 300 K for Mg0.31(2)Mn2.69As2 revealed its room-temperature ferromagnetism, and its ZFC exchange bias behavior at low temperatures indicated the existence of both ferromagnetic (FM) and antiferromagnetic (AFM) interactions. Spin-polarized density functional theory calculations were also performed to verify the magnetic ground state, and these were consistent with the experimental results.
RESUMEN
Zintl phases with nominal 9-4-9 formulas are very interesting for their potential applications as thermoelectric materials. However, the formation of such phases usually requires divalent transition metals, for example, Zn, Mn, and Cd, which are covalently bonded to the pnictogen atoms. In this report, for the first time, two Mg-containing compounds with such structures as Sr9Mg4.45(1)Bi9 and Sr9Mg4.42(1)Sb9 were synthesized and their structures were determined by the single-crystal X-ray diffraction method. Both title compounds crystallize in the orthorhombic space group Pnma and are isostructural with Ca9Mn4.41(1)Sb9, which features complex polyanion structures compared to the classical 9-4-9 phases. For Sr9Mg4.45(1)Bi9, its low thermal conductivity, combined with its high electrical conductivity and moderate Seebeck coefficient, leads to a decent figure of merit of 0.57 at 773 K, which obviously prevails in the unoptimized 9-4-9 phases. The discovery of such Mg-containing 9-4-9 phases is very significant, as the discovery not only enriches the structure map of the well-known 9-4-9 family but also provides very valuable thermoelectric candidates surely deserving of more in-depth investigation.
RESUMEN
Three new quaternary Zintl phases with the "9-4-9" formula, Ae9Mn4-xAlxSb9 (Ae = Ca, Yb, Eu), have been synthesized using Pb as the metal flux, and their crystal structures have been established by single-crystal X-ray diffraction. Both Ca9Mn2.91(4)Al1.09Sb9 and Yb9Mn3.59(6)Al0.41Sb9 are isostructural with Ca9Mn4Bi9, and they crystallize in the orthorhombic space group Pbam with unit cell dimensions of a = 12.4571(8), 12.2884(16) Å, b = 22.1352(16), 22.024(3) Å, and c = 4.6012(3), 4.6187(6) Å, respectively. Their anionic structures can be viewed as infinite ribbons based on corner-shared tetrahedrons. Also, Eu9Mn2.87(4)Al1.13Sb9 has the space group Cmca and a = 9.4883(7) Å, b = 23.6895(18) Å, and c = 24.4845(19) Å. The structural relationships between Ca9Mn2.91(4)Al1.09Sb9 and Eu9Mn2.87(4)Al1.13Sb9 are compared and discussed as well. The successful Al3+ substitution provides additional electrons to the compounds to achieve structural stability. Magnetic susceptibility and electrical resistivity measurements, performed on single crystals of Eu9Mn2.87(4)Al1.13Sb9, indicate complex magnetic properties and semiconductor behavior. The physical properties of Yb9Mn3.59(6)Al0.41Sb9 are similar to those observed for Yb9Mn4.18(2)Sb9.
RESUMEN
Developing active, stable, and low-cost electrocatalysts to generate hydrogen is a great challenge in the fields of chemistry and energy. Nonprecious metal catalysts comprised of inexpensive and earth-abundant transition metals are regarded as a promising substitute for noble metal catalysts used in hydrogen evolution reaction (HER), but are still practically unfeasible mainly due to unsatisfactory activity and durability. Here we report a facile two-step preparation method for WOx nanowires with high concentration of oxygen vacancies (OVs) via calcination of W-polydopamine compound precursors. The resulting hybrid material possesses a uniform and ultralong 1D nanowires structure and a rough and raised surface, which can effectively improve the specific surface area. The products exhibit excellent performance for H2 generation: the required overpotentials for 1 and 10 mA cm-2 are 18 and 108 mV, the Tafel slope is 46 mV/decade, and the electrochemically active surface area is estimated to be â¼77.0 m2 g-1. After 1000 cycles, the catalyst works well without significant current density drop. Our experimental results verified metallic transition metal oxides as superior non-Pt electrocatalysts for practical hydrogen evolution reactions.
RESUMEN
Two new chiral Zintl compounds, Sr14Sn3As12 and Eu14Sn3As12, were synthesized from tin-flux reactions, and the structures were determined by using single-crystal X-ray diffraction. Both compounds crystallize in the trigonal space group R3 (No. 146, Z = 3) with the anion structures containing various units: dumbbell-shaped [Sn2As6](12-) dimers, [SnAs3](7-) triangular pyramids, and isolated As(3-) anions. Very interestingly, these two compounds exhibit opposite chirality in the observed crystal structures, resembling enantiomorphs. Detailed structure analyses suggest possible steric effects among the anion clusters, and on the basis of the calculated electronic structures, substantial electron lone pairs exist on the anions of both compounds, which may provide a hint to understanding the origination of chirality in these intermetallic compounds.
RESUMEN
The focus of this article is on the synthesis and structural characterization of the new ternary antimonides Eu(9)Cd(4+x)Sb(9) and Ca(9)Mn(4+x)Sb(9) (x ≈ (1)/2). Although these compounds have analogous chemical makeup and formulas, which may suggest isotypism, they actually belong to two different structure types. Eu(9)Cd(4.45(1))Sb(9) is isostructural with the previously reported Eu(9)Zn(4.5)Sb(9) (Pbam), and its structure has unit cell parameters a = 12.9178(11) Å, b = 23.025(2) Å, and c = 4.7767(4) Å. Ca(9)Mn(4.41(1))Sb(9) crystallizes in the orthorhombic space group Pnma with unit cell dimensions a = 12.490(2) Å, b = 4.6292(8) Å, and c = 44.197(8) Å and constitutes a new structure type. The two structures are compared and contrasted, and the structural relationships are discussed. Exploratory work aimed at the arsenic-based analogues of either type led to the identification of Ca(9)Zn(4.46(1))As(9), forming with the latter structure [a = 11.855(2) Å, b = 4.2747(8) Å, and c = 41.440(8) Å]. Differential thermal analysis and electrical resistivity measurements, performed on single crystals of Ca(9)Zn(4+x)As(9), indicate high thermal stability and semiconducting behavior. Magnetic susceptibility measurements on Eu(9)Cd(4+x)Sb(9) samples confirm the expected Eu(2+) ([Xe]4f(7)) ground state.
RESUMEN
Mg3Bi2-based materials are a very promising substitute for current commercial Bi2Te3 thermoelectric alloys. The successful growth of Mg3Bi2-based single crystals with high room-temperature performance is especially significant for practical applications. Previous studies indicated that the effective suppression of Mg defects in Mg3Bi2-based materials was crucial for high performance, which was usually realized by applying excessive Mg during syntheses. However, utilization of excessive Mg generates Mg-rich phases between the crystalline boundaries and is unfavorable for the long-term stability of the materials. Here, bulk single crystals with a low-content Mg component such as Mg3.1Bi1.49Sb0.5Te0.01 were successfully grown. For compensating Mg defects, Li was chosen as the additional electron dopant. The results indicate that Li is a very effective electron compensator when low-concentration doping is applied. For high-concentration doping, Mg atoms in the lattice are substituted by Li, leading to decreased electron concentration again. This strategy is very significant for improving the room-temperature performance of Mg3Bi2-based materials. As a result, a record-high figure of merit of 1.05 at 300 K is achieved for Mg3+xLi0.003Bi1.49Sb0.5Te0.01 single crystals.
RESUMEN
In this study, a trisodium citrate (TSC)-assisted hydrothermal method is utilized to prepare three-dimensional hydroxyapatite (3D HA). Understanding the role of TSC in the preparation of 3D HA crystals may provide valuable methods to design advanced biomaterials. As one of the indexes of solution supersaturation, the initial pH (ipH) value can not only directly affect the nucleation rate, but also affect the growth of HA crystals. In this work, the effect of the ipH on the microstructure, particle size distribution, and specific surface area of the 3D HA is explored. Results showed that the morphology of 3D HA transformed from a bundle to a dumbbell ball and then a dumbbell with an increase in the ipH. A corresponding mechanism of such a structural evolution was proposed, providing inspiration for the fabrication of innovative 3D HA structures with enhanced biological functionality and performance.
RESUMEN
Calcium phosphates (Ca-P) represent a significant class of biological minerals found in natural hard tissues. Crystallization through phase transformation of a metastable precursor is an effective strategy to guide the growth of crystalline Ca-P with exceptional functionality. Despite extensive research on Ca-P, the exact process during the crystallization of amorphous particles to hydroxyapatite (HA) remains elusive. Herein, pure HA microspheres with a core-shell structure are crystallized via dissolution and re-crystallization of smooth amorphous calcium phosphate (ACP) microspheres. The transformation is initiated with the increase of the hydrothermal treatment time in the presence of sodium trimetaphosphate and l-glutamic. The underlying mechanisms along with the kinetics of such transformation are explored. Nanocrystalline areas are formed on the smooth ACP microspheres and crystallization advances via nanometre-sized clusters formed by directional arrangement of nanocrystalline whiskers. Our findings shed light on a crucial but unclear stage in the genesis of HA crystals, specifically under the conditions of hydrothermal synthesis.