Synthesis and Structure of Two New Layered Ternary Nitrides, SrZrN2 and SrHfN2

Tolerance Factor and Phase Stability of the KCoO2-Type AMN2 Nitrides

In order to help understand the structural stability of KCoO2-type ternary nitrides AMN2, referring to perovskite structure, a tolerance factor t is proposed to describe the size effect on the phase/symmetry options of the experimentally accessible AMN2 nitrides. This leads to a range of t values above 0.946 for structurally stable KCoO2-type AMN2 nitrides with t values around 0.970 for the orthorhombic and tetragonal phase boundary. In contrast, most of AMN2 nitrides exhibit α-NaFeO2-type structure with t ∼ 0.898-0.946 and cations ordered or disordered rocksalt structure while t below 0.898. Employing the proposed criterion, the structure formation for other ternary AMN2 compositions with lanthanum and alkaline earth cations for the A sites were predicted, which was testified through the synthesis attempts and complemented by formation energy evaluations. The efforts to synthesize the ternary Lanthanide and alkaline earth-based AMN2 nitrides were unsuccessful, which could associate the structural instability with the large formation energies of lanthanide nitrides LaMN2 and the greater tolerance factor of 1.048 for BaTiN2. The experimentally already synthesized AMN2 nitrides could be categorized into three types with different tolerance factors, and scarce AMN2 nitrides with lower formation energies would be accessible using different synthetic routes beyond the traditional solid-state synthesis method.

Mechanistically Guided Materials Chemistry: Synthesis of Ternary Nitrides, CaZrN2 and CaHfN2

Recent computational studies have predicted many new ternary nitrides, revealing synthetic opportunities in this underexplored phase space. However, synthesizing new ternary nitrides is difficult, in part because intermediate and product phases often have high cohesive energies that inhibit diffusion. Here, we report the synthesis of two new phases, calcium zirconium nitride (CaZrN2) and calcium hafnium nitride (CaHfN2), by solid state metathesis reactions between Ca3N2 and MCl4 (M = Zr, Hf). Although the reaction nominally proceeds to the target phases in a 1:1 ratio of the precursors via Ca3N2 + MCl4 → CaMN2 + 2 CaCl2, reactions prepared this way result in Ca-poor materials (CaxM2-xN2, x < 1). A small excess of Ca3N2 (ca. 20 mol %) is needed to yield stoichiometric CaMN2, as confirmed by high-resolution synchrotron powder X-ray diffraction. In situ synchrotron X-ray diffraction studies reveal that nominally stoichiometric reactions produce Zr3+ intermediates early in the reaction pathway, and the excess Ca3N2 is needed to reoxidize Zr3+ intermediates back to the Zr4+ oxidation state of CaZrN2. Analysis of computationally derived chemical potential diagrams rationalizes this synthetic approach and its contrast from the synthesis of MgZrN2. These findings additionally highlight the utility of in situ diffraction studies and computational thermochemistry to provide mechanistic guidance for synthesis.

Sampling the Materials Space for Conventional Superconducting Compounds

A large scale study of conventional superconducting materials using a machine-learning accelerated high-throughput workflow is performed, starting by creating a comprehensive dataset of around 7000 electron-phonon calculations performed with reasonable convergence parameters. This dataset is then used to train a robust machine learning model capable of predicting the electron-phonon and superconducting properties based on structural, compositional, and electronic ground-state properties. Using this machine, the transition temperatures (Tc ) of approximately 200 000 metallic compounds are evaluated, all of which are on the convex hull of thermodynamic stability (or close to it) to maximize the probability of synthesizability. Compounds predicted to have Tc values exceeding 5 K are further validated using density-functional perturbation theory. As a result, 541 compounds with Tc values surpassing 10 K, encompassing a variety of crystal structures and chemical compositions, are identified. This work is complemented with a detailed examination of several interesting materials, including nitrides, hydrides, and intermetallic compounds. Particularly noteworthy is LiMoN2 , which is predicted to be superconducting in the stoichiometric trigonal phase, with a Tc exceeding 38 K. LiMoN2 has previously been synthesized in this phase, further heightening its potential for practical applications.

Recent Advances for the Synthesis and Applications of 2-Dimensional Ternary Layered Materials

Layered materials with unique structures and symmetries have attracted tremendous interest for constructing 2-dimensional (2D) structures. The weak interlayer interaction renders them to be readily isolated into various ultrathin nanosheets with exotic properties and diverse applications. In order to enrich the library of 2D materials, extensive progress has been made in the field of ternary layered materials. Consequently, many brand-new materials are derived, which greatly extend the members of 2D realm. In this review, we emphasize the recent progress made in synthesis and exploration of ternary layered materials. We first classify them in terms of stoichiometric ratio and summarize their difference in interlayer interaction, which is of great importance to produce corresponding 2D materials. The compositional and structural characteristics of resultant 2D ternary materials are then discussed so as to realize desired structures and properties. As a new family of 2D materials, we overview the layer-dependent properties and related applications in the fields of electronics, optoelectronics, and energy storage and conversion. The review finally provides a perspective for this rapidly developing field.

Design, Synthesis, and Optoelectronic Properties of the High-Purity Phase in Layered AETMN2 (AE = Sr, Ba; TM = Ti, Zr, Hf) Semiconductors

We report the synthesis and optoelectronic properties of high phase-purity (>94 mol %) bulk polycrystals of KCoO₂-type layered nitrides AETMN₂ (AE = Sr, Ba; and TM = Ti, Zr, Hf), which are expected to exhibit unique electron transport properties originating from their natural two-dimensional (2D) electronic structure, but high-purity intrinsic samples have yet been reported. The bulks were synthesized using a solid-state reaction between AENH and TMN precursors with NaN₃ to achieve high N chemical potential during the reaction. The AETMN₂ bulks are n-type semiconductors with optical band gaps of 1.63 eV for SrTiN₂, 1.97 eV for BaZrN₂, and 2.17 eV for BaHfN₂. SrTiN₂ and BaZrN₂ bulks show degenerated electron conduction due to the natural high-density electron doping and paramagnetic behavior in all of the temperature ranges examined, while such unintentional carrier generation is largely suppressed in BaHfN₂, which exhibits nondegenerated electron conduction. The BaHfN₂ sample also exhibits weak ferromagnetic behavior at temperatures lower than 35 K. Density functional theory calculations suggest that the high-density electron carriers in SrTiN₂ come from oxygen impurity substitution at the N site (O_N) acting as a shallow donor even if the high-N chemical potential synthesis conditions are employed. On the other hand, the formation energy of O_N becomes larger in BaHfN₂ because of the stronger TM-N chemical bonds. Present results demonstrate that the easiness of impurity incorporation is designed by density functional calculations to produce a more intrinsic semiconductor in wider chemical conditions, opening a way to cultivating novel functional materials that are sensitive to atmospheric impurities and defects.

dz2 orbital character of polyhedra in complex solid-state transition-metal compounds

In transition-metal compounds, the character of the d orbitals often plays an important role in the development and enhancement of novel physical and chemical properties. Density functional theory calculations of the electronic structures of various d0- and d1-complex transition-metal compounds consisting of either face-sharing octahedra, edge-sharing octahedra, or edge-sharing trigonal prismatic layers were performed to investigate the nature of their d orbitals. The dz2 orbital of the transition metal was shown to make a significant contribution to the electronic structure near the Fermi level in nine different complex transition-metal compounds (oxides, nitrides, and sulfides), regardless of the type of polyhedral geometry and connectivity. The importance of controlling and designing the dz2 orbital character of transition metals near the Fermi level was shown to be important in developing novel physical and chemical properties in complex transition-metal compounds.

Elastic, electronic, chemical bonding and thermodynamic properties of the ternary nitride Ca4TiN4: Ab initio predictions

In order to shed light on the unexplored properties of the ternary nitride Ca₄TiN₄, we report for the first time the results of an ab initio study of its structural, electronic, elastic, chemical bonding and thermodynamic properties. Calculated equilibrium structural parameters are in excellent concordance with available experimental data. Electronic properties were explored through the calculation of the energy band dispersions and density of states. It is found that Ca₄TiN₄ has an indirect band gap (Z-Γ) of 1.625 (1.701) eV using LDA (GGA). Nature of the chemical bonding was studied via Mulliken population analysis and charge density distribution map. It is found that the Ca-N bond is dominantly ionic, whereas the Ti-N one is dominantly covalent. Elastic properties of both single-crystal and polycrystalline phases of the title compound were explored in details using the stain-stress approach. Analysis of the calculated elastic moduli reveals that the title compound is mechanically stable, ductile and elastically anisotropic. Temperature and pressure dependencies of the unit-cell volume, bulk modulus, heat capacities, volume thermal expansion coefficient, Grüneisen parameter and Debye temperature were investigated based on the quasiharmonic Debye model.

Ternary nitride semiconductors in the rocksalt crystal structure

Inorganic nitrides with wurtzite crystal structures are well-known semiconductors used in optical and electronic devices. In contrast, rocksalt-structured nitrides are known for their superconducting and refractory properties. Breaking this dichotomy, here we report ternary nitride semiconductors with rocksalt crystal structures, remarkable electronic properties, and the general chemical formula Mgx TM 1-xN (TM = Ti, Zr, Hf, Nb). Our experiments show that these materials form over a broad metal composition range, and that Mg-rich compositions are nondegenerate semiconductors with visible-range optical absorption onsets (1.8 to 2.1 eV) and up to 100 cm2 V-1⋅s-1 electron mobility for MgZrN2 grown on MgO substrates. Complementary ab initio calculations reveal that these materials have disorder-tunable optical absorption, large dielectric constants, and electronic bandgaps that are relatively insensitive to disorder. These ternary Mgx TM 1-xN semiconductors are also structurally compatible both with binary TMN superconductors and main-group nitride semiconductors along certain crystallographic orientations. Overall, these results highlight Mgx TM 1-xN as a class of materials combining the semiconducting properties of main-group wurtzite nitrides and rocksalt structure of superconducting transition-metal nitrides.

Three-dimensionality of electronic structures and thermoelectric transport in SrZrN₂ and SrHfN₂ layered complex metal nitrides

Layered materials have several properties that make them suitable as high-performance thermoelectric materials. In this study, we focus on the complex metal nitrides SrZrN2 and SrHfN2, which have an α-NaFeO2 layered crystal structure. To determine their electronic band structure features and potential thermoelectric transport properties, we calculated the electronic band structures and electronic transport coefficients for SrZrN2 and SrHfN2 using density-functional theory and Boltzmann transport theory, respectively. Despite the layered crystal structure, SrZrN2 and SrHfN2 both had three-dimensional electronic structures and isotropic electronic transport because of the contribution of the Sr 4d(x(2)-y(2)) + 4d(xy) orbitals to the bottom of the conduction bands in addition to that of the Zr 4d(z)(2) (Hf 5d(z)(2)) orbital. The three-dimensional electronic structures predict the appearance of large Seebeck coefficients (-145 μV K(-1) at 300 K, -370 μV K(-1) at 1200 K) and large electronic thermoelectric figures of merit.

Theoretical study on the structural, electronic and physical properties of layered alkaline-earth-group-4 transition-metal nitrides AEMN2

Thermodynamic, structural, and electronic properties of the layered ternary nitrides AEMN₂ (AE = alkaline-earth; M = group 4 transition metal) both with the KCoO₂ and α-NaFeO₂ structure-types are examined within density-functional theory.

High-Tc Superconductivity in Electron-Doped Layer Structured Nitrides

▪ Abstract  A new series of superconductors based on layer structured nitrides has been developed. The general compositions of the nitrides are MNX (M = Zr, Hf; X = Cl, Br, I). The β-type polymorph consists of MN double layers sandwiched between close-packed halogen layers, which are characterized as semiconductors with a band gap of 3–4 eV. Electrons can be doped to the nitride layers by intercalation of alkali metals between the layers. Upon the intercalation, the compounds become superconductors with the transition temperatures (Tc s) as high as 13 and 25.5 K for β-ZrNCl and β-HfNCl systems, respectively. The Tc of the electron doped β-HfNCl is higher than that observed in any intermetallic compound and suggests that layered nitrides may exhibit Tc s comparable to those observed in layer structured complex copper oxide superconductors. The layer structured nitrides can be variously modified by the amounts of doping, the types of alkali metals, and the interlayer separation, which can be controlled by co-intercalation of organic molecules with alkali metals. This article dicusses topics including the synthesis and structure of the transition metal nitride halides, intercalation, superconductivity, and band structures.

Synthesis and Structure of the New Ternary Nitride SrTiN(2)

A new ternary nitride, SrTiN(2), has been synthesized by the solid-state reaction of Sr(2)N with TiN and characterized by powder X-ray diffraction. SrTiN(2) crystallizes in the tetragonal space group P4/nmm (a = 3.8799(2) Å, c = 7.6985(4) Å, Z = 2) and is isostructural with KCoO(2). Titanium is coordinated to five nitrogens in a distorted square-based pyramidal geometry, forming layers of edge-sharing pyramids which stack along the (001) direction. Strontium is situated between the Ti-N layers and is coordinated to five nitrogen atoms. The title compound is only the third example of a ternary titanium nitride.