Zintl Phase Material Research Articles

Single-crystal growth, structure and thermal transport properties of the metallic antiferromagnet Zintl-phase β-EuIn2As2.

Zintl-phase materials have attracted significant research interest owing to the interplay of magnetism and strong spin-orbit coupling, providing a prominent material platform for axion electrodynamics. Here, we report the single-crystal growth, structure, magnetic and electrical/thermal transport properties of the antiferromagnet layer Zintl-phase compound β-EuIn2As2. Importantly, the new layered structure of β-EuIn2As2, in rhombohedral (R3̄m) symmetry, contains triangular layers of Eu2+ ions. The in-plane resistivity ρ(H, T) measurements reveal metal behavior with an antiferromagnetic (AFM) transition (TN ∼ 23.5 K), which is consistent with the heat capacity Cp(H, T) and magnetic susceptibility χ(H, T) measurements. Negative MR was observed in the temperature range from 2 K to 20 K with a maximum MR ratio of 0.06. Unique 4f7J = S = 7/2 Eu2+ spins were supposed magnetically order along the c-axis. The Seebeck coefficient shows a maximum thermopower |Smax| of about 40 μV K-1. The kink around 23 K in the Seebeck coefficient originates from the effect of the antiferromagnetic phase on the electron band structure, while the pronounced thermal conductivity peak at around 10 K is attributed to the phonon-phonon Umklapp scattering. The results suggest that the Eu2+ spin arrangement plays an important role in the magnetic, electrical, and thermal transport properties in β-EuIn2As2, which might be helpful for future potential technical applications.

Enhancing thermoelectric performance in P-type Mg3Sb2-based Zintls through optimization of band gap structure and nanostructuring

P-type Mg3Sb2-based Zintls have attracted considerable interest in the thermoelectric (TE) field due to their environmental friendliness and low cost. However, compared to their n-type counterparts, they show relatively low TE performance, limiting their application in TE devices. In this work, we simultaneously introduce Bi alloying at Sb sites and Ag doping at Mg sites into the Mg3Sb2 to cooperatively optimize the electrical and thermal properties for the first time, acquiring the highest ZT value of ∼0.85 at 723 K and a high average ZT of 0.39 in the temperature range of 323–723 K in sample Mg2.94Ag0.06Sb1.9Bi0.1. The first-principle calculations show that the co-doping of Ag and Bi can shift the Fermi level into the valence band and narrow the band gap, resulting in the increased carrier concentration from 3.50 × 1017 cm−3 in the reference Mg3Sb0.9Bi0.1 to ∼7.88 × 1019 cm−3 in sample Mg2.94Ag0.06Sb0.9Bi0.1. As a result, a remarkable power factor of ∼778.9 µW m−1 K−2 at 723 K is achieved in sample Mg2.94Ag0.06Sb0.9Bi0.1. Meanwhile, a low lattice thermal conductivity of ∼0.48 W m−1 K−1 at 723 K is also obtained with the help of phonon scattering at the distorted lattice, point defects, and nano-precipitates in sample Mg2.94Ag0.06Sb0.9Bi0.1. The synergistic effect of using the multi-element co-doping/-alloying to optimize electrical properties in Mg3Sb2 holds promise for further improving the TE performance of Zintl phase materials or even others.

Emergence of 1/3 magnetization plateau and successive magnetic transitions in Zintl phase Eu3InAs3

Single crystals of ${\mathrm{Eu}}_{3}{\mathrm{InAs}}_{3}$ have been successfully synthesized by indium flux reactions. ${\mathrm{Eu}}_{3}{\mathrm{InAs}}_{3}$ crystallizes in the ${\mathrm{Ca}}_{3}\mathrm{Al}{\mathrm{As}}_{3}$-type structure with an orthorhombic space-group $Pnma$. Theoretical calculations indicate that ${\mathrm{Eu}}_{3}{\mathrm{InAs}}_{3}$ is a semiconductor with a direct band gap of 0.3 eV. ${\mathrm{Eu}}_{3}{\mathrm{InAs}}_{3}$ has large negative Seebeck coefficients in the range of 300--450 \ensuremath{\mu}V/K at room temperature. ${\mathrm{Eu}}_{3}{\mathrm{InAs}}_{3}$ displays two antiferromagnetic transitions (${T}_{N1}=13$ and ${T}_{N2}=10$ K) for both $H\ensuremath{\parallel}b$ and $H\ensuremath{\perp}b$ which can be suppressed by magnetic fields. At 2 K, field-induced ferromagnetic states are reached for both $H\ensuremath{\parallel}b$ and $H\ensuremath{\perp}b$. Particularly, the $H\ensuremath{\parallel}b$ magnetization curves show a plateau at 1/3 of the saturated magnetization and an anomaly at 2/3 of the saturated magnetization. ${\mathrm{Eu}}_{3}{\mathrm{InAs}}_{3}$ is a Zintl phase material showing a 1/3 magnetization plateau here.

High‐Density Frenkel Defects as Origin of N‐Type Thermoelectric Performance and Low Thermal Conductivity in Mg3Sb2‐Based Materials

AbstractMg3Sb2‐based intermetallic compounds with exceptionally high thermoelectric performance exhibit unconventional n‐type dopability and anomalously low thermal conductivity, attracting much attention to the underlying mechanisms. To date, investigations have been limited to first‐principle calculations and thermodynamic analysis of defect formation, and detailed experimental analysis on crystal structure and phonon modes has not been achieved. Here, a synchrotron X‐ray diffraction study clarifies that, against a previous view of a simple crystal structure with a small unit cell, Mg3Sb2 is inherently a heavily disordered material with Frenkel defects, charge‐neutral defect complexes of cation vacancies and interstitials. Ionic charge neutrality preserved in Mg3Sb2 is responsible for exotic n‐type dopability, which is unachievable for other Zintl phase materials. The thermal conductivity of Mg3Sb2 exhibits deviation from the standard T−1 temperature dependency with strongly limited phonon transport due to a strain field. Inelastic X‐ray scattering measurement reveals enhanced phonon scattering induced by disorder. The results will draw renewed attention to crystal defects and disorder as means to explore new high‐performance thermoelectric materials.

Research progress of two-dimensional covalent bond substructure Zintl phase thermoelectric materials

Thermoelectric materials can realize the direct conversion between thermal energy and electrical energy, and thus having important applications in semiconductor refrigeration and heat recovery. Zintl phase is composed of highly electronegative cations and anions, which accords with the concept of “phonon glass, electron crystal” (PGEC). Thermoelectric properties of Zintl phase have attracted extensive interest, among which the two-dimensional (2D) covalent bond structure featured Zintl phases have received more attention for their outstanding electrical properties. In this review, Zintl phase materials with two-dimensional covalent bond substructures are reviewed, including 1-2-2-type, 9–4+<i>x</i>–9-type, 2-1-2-type and 1-1-1-type Zintl phase. The 1-2-2-type Zintl phase is currently the most widely studied and best-performing Zintl material. It is worth mentioning that the maximum <i>ZT</i> value for the Mg<sub>3</sub>Sb<sub>2</sub>-based n-type Zintl material with the CaAl<sub>2</sub>Si<sub>2</sub> structure has been reported to reach 1.85, and the average <i>ZT</i> value near room temperature area also reaches 1.4. The 9–4+<i>x</i>–9-type Zintl material with a mass of atoms in unit cell contributes to lower thermal conductivity thus relatively high <i>ZT</i> value. The 2-1-2-type Zintl material has extremely low thermal conductivity due to the intrinsic vacancies, which has been developing in recent years. The 1-1-1-type Zintl material with the same ZrBeSi structure as the 2-1-2-type Zintl material, shows better electrical transport performance. In sum, this review summarizes the recent progress and optimization methods of those typical Zintl phases above. Meanwhile, the future optimization and development of Zintl phase with two-dimensional covalent bond substructures are also prospected.

Realizing a High ZT of 1.6 in N-Type Mg3Sb2-Based Zintl Compounds through Mn and Se Codoping.

Mg3Sb2-based compounds by virtue of nontoxicity and low-cost have become a promising class of candidates for midtemperature thermoelectric power generation. Here, we successfully fabricated n-type Mg3Sb2-based materials using an inexpensive and efficient approach of one-step ball milling and spark plasma sintering, and demonstrate that a complementary and favorable effect of multiple elements coalloying/-doping leads to an excellent thermoelectric performance. The intrinsic p-type conducting behavior for Mg3Sb2 could be changed to n-type through Bi and Se coalloying on Sb sublattices with excess Mg, resulting from the suppression of Mg vacancies and the formation of Mg interstitial. Furthermore, Mn doping on Mg sublattices could soften the chemical bonds, leading to the increase of carrier mobility and concentration simultaneously. Additionally, multielement coalloying/-doping could significantly increase the lattice disorder, which undoubtedly strengthens the phonon scattering and readily results in a suppressed lattice thermal conductivity. As a result, a highest ZT value of 1.6 at 723 K and an average ZT value up to 1.1 were obtained in the temperature range of 323-723 K in the Mg3.18Mn0.02Sb1.5Bi0.49Se0.01 sample, which is one of the highest values among the Te free Mg3Sb2. This work could give guidance for improving the thermoelectric performance of Zintl phase materials or even others using the multielement codoping/-alloying strategy.

C/N/O centred metal clusters: super valence bonding and magic structure with 26 valence electrons

By using genetic algorithm combined with B3LYP and QCISD methods, this paper investigates the stabilities and electronic structures of Al6OM m (M = Na, K; m = 2,4,6) and a few other Al n XNa m (X = C, N, O) clusters. The results show that the nonmetal doped metal clusters with 26 valence electrons have enhanced stabilities and large energy gaps. This paper extends the Jellium model for the application to the nonmetal doped metal clusters and explains the electronic origin of this strong magic structure. The nonmetal X atom is situated in the centre of the magic clusters. The 2s/2p orbitals of the central atom interact strongly with the superatomic 1S/1P orbitals and form bonding and antibonding orbitals. The bonding orbitals make the C/N/O atoms form s2p6 shell closure, and the antibonding orbitals make the metal moieties form closed 2S22P6 shells. The 26 valence electrons form closed s2p6S2P6D10 shells, and this electronic configuration can be taken as the combination of the octet rule and 18-electron rule. The octahedral Al6O2− core is a superatomic anion with great stability, and it can be used as building blocks to assemble Zintl phase materials by interaction with alkali metals.

Growth of uniform CaGe2 films by alternating layer molecular beam epitaxy

Layered Zintl phase van der Waals (vdW) materials are of interest due to their strong spin-orbit coupling and potential for high mobility. Here, we report the successful growth of large area CaGe2 films, as a model of layered Zintl phase materials, on atomically flat Ge(111) substrates by molecular beam epitaxy (MBE) using an alternating layer growth (ALG) protocol. Reflection high energy electron diffraction (RHEED) patterns of the Ge buffer layer and CaGe2 indicate high quality two dimensional surfaces, which is further confirmed by atomic force microscopy (AFM), showing atomically flat and uniform CaGe2 films. The appearance of Laue oscillations in X-ray diffraction (XRD) and Kiessig fringes in the X-ray reflectivity (XRR), which are absent in co-deposited CaGe2, confirms the uniformity of the CaGe2 film and the smoothness of the interface. These results demonstrate a novel method of deposition of CaGe2 that could be also applied to other layered Zintl phase vdW materials. Also, the high quality of the CaGe2 film is promising for the exploration of novel properties of germanane.

Structural and thermoelectric properties of zintl-phase CaLiPn (Pn=As, Sb, Bi)

First-principles calculations were carried out to study the structural, mechanical, dynamical and transport properties of zintl phase materials CaLiPn (Pn=As, Sb and Bi). We have used two different approaches to solve the system based on density functional theory. The plane wave pseudopotential approach has been used to study the structural and dynamical properties whereas, full potential linear augment plane wave method is used to examine the electronic structure, mechanical and thermoelectric properties. The calculated ground-state properties agree quite well with experimental values. The computed electronic structure shows the investigated compounds to be direct band gap semiconductors. Further, we have calculated the thermoelectric properties of all the investigated compounds for both the carriers at various temperatures. We found a high thermopower for both the carriers, especially n-type doping to be more favourable, which enabled us to predict that CaLiPn might have promising applications as a good thermoelectric material. Further, the phonon dispersion curves of the investigated compounds showed flat phonon modes and we also find lower optical and acoustic modes to cut each other at the lower frequency range, which further indicate the investigated compounds to possess reasonably low thermal conductivity. We have also analysed the low value of the thermal conductivity through the empirical relations and discussions are presented here.

Synthesis and characterization of electric and magnetic properties of the new intermetallic compounds M 3(GaTe 3) 2 (M = Cr, Fe, Co)

The new amorphous intermetallic materials, M 3(GaTe 3) 2 (M = Cr, Fe, Co) are prepared by using a rapid precipitation metathesis reaction between the Zintl phase material K 3GaTe 3 and the divalent transition metal halides in aqueous solution. The d.c. specific resistivity measurements of these materials exhibit metallic (M = Fe, Co) and semiconductor (M = Cr) behavior. The magnetic and electrical properties for these materials are examined as a function of temperature, Cr 3(GaTe 3) 2 exhibits antiferromagnetic coupling between localized magnetic moments and highly temperature dependent semiconductivity. Two materials undergo a transition to a spin glass state at low temperature (for M = Fe, T f = 22 K; for M = Co, T f = 2.2 K). Magnetization data are also reported as both thermal remanent magnetization (TRM) and isothermal remanent magnetization (IRM) as a function of magnetizing field and temperature. At a magnetization field of 1 kG and a temperature of 6 K, the Fe 3(GaTe 3) 2 exhibit a photomagnetic response consistent with a disruption of the spin-glass state that results from a series of pulses of ultraviolet radiation.

Crystal and Electronic Structure of a Quaternary Layered Compound from the K–In–Ge–Sb System: K9In9GeSb22

The quaternary Zintl phase material, K9In9GeSb22(1), has been prepared at 650°C and its structure determined by single crystal X-ray diffraction: triclinic, space groupP1witha= 11.62 (1) Å,b= 13.04 (3) Å,c= 11.27 (2) Å, α = 113.5 (2)°, β = 91.7 (1)°, γ = 112.3 (1)°,V= 1415 (4) Å3, andDcalcd= 4.851 g/cm3forZ= 2 andR(Rw) = 7.9% (8.8%). Antimonide 1 contains layers comprised of indium, germanium, and arsenic separated by K+cations. The In–Ge–Sb layers in 1 contain In(2−x)GexSb6(x≅ 1) dimers with In(Ge)–In(Ge) bonds, InSb4tetrahedra, and Sb3trimers, as well as infinite chains with Sb–Sb bonds. There is an unusual metal–metal bonded site in 1, that contains contains a solid solution of 1:1 In and Ge, with an In(Ge)–In(Ge) separation of 2.659 (1) Å. The nature of this experimentally determined solid solution at theM–Mbonded site, and its effects on the electronic structure of K9In9GeSb22, was investigated via tight binding calculations.

Selective removal of potassium from K4In4Sb6 via laser ablation/ionization

Results are reported on the Zintl phase material, K4In4Sb6, with respect to laser ablation and subsequent laser ionization/removal processes. A 308 nm laser pulse is used to ablate the Zintl compound, while a second laser ionizes ejected species within the extraction region of a time-of-flight mass spectrometer. With the second laser operating at 248 nm, selective ionization and removal of the potassium is clearly demonstrated. Such a strategy takes advantage of the different ionization potentials of K, In, and Sb, and implications for possible applications of this research to film growth are discussed.

Synthesis and crystal structure of a novel Zintl phase: K 3SbTe 3

A ternary Zintl phase material of the formula K 3SbTe 3 has been prepared following a high temperature procedure. The material consists of an ionic lattice of potassium ions and discrete [SbTe 3] 3− ions. K 3SbTe 3 is a poor semiconductor (ϱ = 2 × 10 5Ω-cm) and dissolves in several polar solvents (e.g., H 2O, DMF, DMSO). Crystal data for K 3SbTe 3 include the following: cubic, space group P2 13 (No. 198), a = 10.306(1) Å, V = 1094.6(2) Å 3, Z = 4, D calc = 3.773 g cm −3 at T = 110(5) K, Mo K radiation, λ ( K) = 0.71073 Å, μ = 114.7 cm −1, M r = 621.86, F(000) = 1056, and R = 0.023 for 574 unique observed reflections.

Synthesis and crystal structures of the layered I–III–V Zintl phases, K4In4X6, where X = As, Sb

Ternary Zintl phase materials of the formula K4In4X6, where X = As or Sb, have been prepared following a high-temperature procedure. The crystal structure of K4In4As6 has been determined at low temperature and consists of [In4As64–]n sheets of ca. 5 A thickness insulated by a layer of potassium ions of ca. 3 A thickness. The crystal structure of K4In4Sb6 has been determined at room temperature and is isomorphous with the K4In4Sb6 compound. For K4In4As6, a= 14.323(3)A, b= 7.106(2)A, c= 15.795(3)A, β= 90.30(2)°, space group P21/c, Z= 4, ρc= 4.44 g cm–3 at T= 100(5) K, R= 0.035 for 2688 unique observed reflections. For K4In4Sb6, a= 15.282(5)A, b= 7.544(1)A, c= 16.788(3)A, β= 90.52(2)°, space group P21/c, Z= 4, ρc= 4.62 g cm–3 at T= 299 K, R= 0.042 for 3001 unique observed reflections.