AlB2 Type Research Articles

Ba3Mg4Au4 – a ternary auride composed of BaAu2- and BaMg2Au-related slabs

Abstract The ternary auride Ba3Mg4Au4 was synthesized from the elements in a sealed tantalum ampoule. The Ba3Mg4Au4 structure was refined from single-crystal X-ray diffractometer data: Gd3Cu4Ge4 type, space group Immm, a = 447.95(10), b = 843.07(18), c = 1564.2(5) pm, wR2 = 0.0935, 680 F 2 values, 23 variables. Ba3Mg4Au4 is a 1:2 intergrowth structure of BaAu2-(AlB2 type) and BaMg2Au-(MgCuAl2 type) related slabs. The two crystallographically independent gold atoms both have tricapped trigonal prismatic coordination, i.e. Au1@Mg6Ba3 and Au2@Mg2Ba6Au. The Au–Mg (284–303 pm) and Ba–Au (331–349 pm) distances cover small ranges that are close to the sums of the covalent radii. The magnesium atoms in the MgCuAl2-related slab show Mg–Mg distances of 320–332 pm. The different coloring variants of the Gd3Cu4Ge4 type are briefly discussed.

Diborides of Multielement Transition Metals: Methods for Calculating Physical and Mechanical Characteristics

From the first principles simulation (using the method of “a priori pseudopotential” and the “quasi-harmonic approximation” method- author’s developments), the basic characteristics of diborides and diborides of multielement transition metals (DMTMs) with an AlB2 type structure were calculated. For both diborides and DMTMs, the linear coefficients of thermal expansion (LCTE) along the axial axes differ little from each other, i.e., transition metal diborides and hexagonal lattice DMTMs are quasi-isotropic. Quasi-isotropy makes it possible to estimate the LCTE using an analytical formula that depends on the melting temperature. In the absence of experimental data on the melting point of DMTMs, a method for calculating it from first principles is presented. The theoretical hardness values of transition metal diborides and DMTMs with averaged parameters were calculated from the first principles. The hardness of both bulk and nano-sized DMTMs was assessed using a hybrid method. There is agreement between the calculated and available experimental data.

Crystal and electronic structure of the ternary Zintl bismuthide BaLiBi

AbstractReported is the accurate refinement of the structure of the ternary bismuthide BaLiBi, based on single‐crystal X‐ray diffraction data. This compound crystallizes with the ZrBeSi structure type with the space group P63/mmc (no. 194), a=4.9917(6) Å, c=9.079(2) Å, V=195.92(7) Å3 with two formula units per unit cell. In addition to being a colored ternary variant of the AlB2 type, the crystal structure of BaLiBi can be also viewed as a “stuffed” variant of the NiAs structure, where the Bi atoms form a hexagonal close packing, the Ba atoms occupy the octahedral voids in this packing, and the Li atoms are located between adjacent tetrahedral voids on their common triangular faces. In the absence of direct Bi–Bi interactions, the BaLiBi crystal structure rationalized according to the notation (Ba2+)(Li+)(Bi3−), suggesting an electron‐balanced composition, i. e., a Zintl phase. In line with this notation, scalar‐relativistic first‐principle calculations with the LMTO code reveal a semiconducting ground state, with a bandgap of about 0.6 eV. Fully relativistic electronic structure calculations predict a semimetallic ground state.

Electromagnetic properties and structure of superconducting materials based on AHP-FCE model

Abstract In this paper, the electromagnetic properties and structural characteristics of superconducting materials are studied based on AHP hierarchical analysis and FCE fuzzy comprehensive evaluation method. A sub-analytical model is established using hierarchical analysis and a weight set is established using FCE fuzzy comprehensive evaluation method, respectively, and a comprehensive evaluation model is established to study the average magnetization intensity and vortex of samples with different pore size types and sizes under the same conditions. Then the superconducting current situation at special locations is studied, focusing on summarizing the structural characteristics of most typical superconductors. The number of vortices in the superconducting square tube gradually decreases as the pore size increases. The maximum absolute value of its average magnetization intensity decreases with the increase of the aperture diameter, while its penetrating magnetic field increases subsequently. The superconducting square tube remains in the superconducting state until the magnetic field reaches a certain value. Most of the superconducting material structures are hierarchical crystal structures, such as MgB2, a simple binary and substance of simple hexagonal crystal system, and AlB2 type simple hexagonal structure with theoretical density of 2.605g/cm3 and dot constants of a=30.84nm and c=35.24nm, which belong to the P6/mmm space group. Superconductors not only have zero resistance properties at critical temperature, but also have electromagnetic properties that conventional conductors do not have at all under certain conditions, so it is important to study the properties of superconducting materials.

Chemical ordering as a precursor to formation of ordered δ-UZr2 phase: a theoretical and experimental study

A combination of special quasi-random structure (SQS) analysis, density functional theory (DFT) based simulations and experimental techniques are employed in determining the transformation pathway for the disordered γ-(U, Zr) phase (bcc structure) to transform into the chemically ordered δ-UZr2 phase (C32, AlB2 type structure). A novel Monte-Carlo based strategy is developed to generate SQS structures to study the β → ω displacive phase transformation in A1−x B x binary random alloy. Structures generated with this strategy and using DFT calculations, it is determined that (222)bcc plane collapse mechanism is energetically unfavorable in chemically disordered environment at UZr2 composition. A mechanically and dynamically stable 24 atom SQS structure is derived which serves as a structural model of chemically ordered δ-UZr2 structure. Finally, a thermodynamic basis for the mechanism of the γ to δ transformation has been established which ensures chemical ordering is a precursor to the subsequent displacive transformation to form chemically ordered δ-UZr2 structure.

Structure and properties of MgB2 bulks: ab-initio simulations compared to experiment

Analysis of XRD patterns by Rietveld refinement has been shown that the main phase of superconducting MgB2-based bulk materials (with high level of superconducting characteristics) has AlB2 type structure and near MgB1.8-1.68O0.2-0.32 stoichiometry. The materials demonstrated the critical current densities up to 0.9 – 0.4 MA/cm2 j c (at 0 - 1 T, 20 K); up to 15 T B c2 (at 22.5 K) and Birr (at 18 K). The ab-initio simulation confirmed (1) benefits in binding energy and enthalpy of formation if stoichiometry of the solid solution is near MgB1.75O0.25; (2) energetic advantage in case if impurity oxygen present only in each second boron plane of MgB2 cell while the first boron plane of the same cell stays pristine and location of substituted oxygen atoms in the nearby positions. Besides, the results of ab-initio modeling allow explanation of the tendency towards segregation of O-impurity in MgB2 structure during synthesis or sintering, and formation of Mg-B-O inclusions or nanolayers (with MgO type of structure) which effect pinning. Calculated transition temperatures, Tc, for MgB1.75O0.25 occurred to be 23.3 K, while for MgB2 it was 21.13 K only. Experimental Tc of the bulk materials was 35.7-38.2 K.

Compressibility and thermal expansion study of δ-UZr2 at high pressure and high temperature

UZr2 has been synthesized using the arc-melting technique. At ambient the compound is found to crystallize in a modified C32, AlB2 type structure, called δ-structure and it remains stable up to 20 GPa. However, an anomalous decrease of c/a ratio has been observed in different pressure intervals at ambient temperature. At elevated temperature c/a ratio, shows a marginal increase with pressure. The bulk modulus and its pressure derivative at ambient temperature are found to be 108.3 GPa and 5.0, respectively. High-pressure and high-temperature studies reveal a softening in the material at higher temperatures. The thermal expansion coefficients are found to decrease from 4.6 × 10−5 K−1 at 1.2 GPa to 2.7 × 10−5 K−1 at 6.6 GPa in the temperature range 300 K–473 K.

R8-xRu3In7+x, R3Ru1-xIn3, R39Ru12-xIn35, and R16Ru5In14 (R=La-Nd, Sm, Gd-Er, Lu) - New ternary indides with 2D intergrowth of CsCl- and AlB2-related slabs

Twelve rare-earth ruthenium indides were synthesized by a two-step reaction and their crystal structures were determined by single-crystal X-ray diffraction. R3Ru1-xIn3 (Pbam, oP28) and R16Ru5In14 (P2/m, mP35) crystallize with new structure types whereas R39Ru12-xIn35 (Pbam, oP172) and R8-xRu3In7+x (Pmma, oP36) present occupation variants of Nd39Ir10.98In36.02 and Y5Cu5Mg8 structures, respectively. The new indides are built of two different layers alternatively stacking along the shortest unit cell axis. The first layer is formed exclusively of rare-earth atoms while the second layer is built of ruthenium and indium thus displaying RIn and RRu fragments of CsCl type and RIn2 and RRu2 fragments of AlB2 type.

Magnetic and Transport Properties of the Kondo Lattice Compound YbPtAs**Supported by the National Basic Research Program of China under Grant No 2015CB921303, the National Key Research Program of China under Grant No 2016YFA0300604, the Strategic Priority Research Program (B) of Chinese Academy of Sciences under Grant No XDB07020100, and the National Natural Science Foundation of China under Grant No 11404175.

We report the electrical resistivity, magnetic susceptibility, and heat capacity studies on a new intermetallic compound YbPtAs, which crystallizes in a modified AlB2 type structure. The Yb ions in YbPtAs are in a trivalent state and order antiferromagnetically around Néel temperatures TN1 = 6.5 K and TN2 = 2.2 K, respectively, deduced both from the magnetic susceptibility χ(T) and heat capacity C(T) measurements. The magnetic contribution in resistivity, ρm(T), exhibits a broad maximum at around 100 K and a logarithmic temperature dependence in the high-temperature region, indicative of the presence of the Kondo effect in YbPtAs.

First principles studies of hydrogen insertion effects on magnetic properties, bonding and structure reordering of UZr2

In the context of the need to recycle depleted uranium coming from nuclear waste, hydrogen absorption in uranium and mainly its alloys is an important solution. We here consider hydrogen insertion in UZr2 intermetallic leading to experimentally observed composition UZr2H6. Drastic changes are shown from density functional calculations to induce atom reordering within the metal substructures in the ground state. Particularly among different starting hypotheses for H sites, energy minimization is observed when the hydrogenated intermetallic undergoes metal substructures reordering with unmixed occupancy of Al (U) and B (Zr) sites in AlB2 type structure. Hydrogen is identified as covalently bonded within the plane of Zr metal, illustrated by anionic charged H−0.55 derived from Bader analysis of charge density. Hydrogen uptake leads to prevailing volume expansion effects versus chemical bonding ones, involving strong localization of uranium 5f states and increased magnetization versus the pristine intermetallic. Both the intermetallic and the ternary hydride are found ferromagnetic in the ground state.

Phonon dispersion models for MgB2 with application of pressure

We evaluate, via the Local Density and the Generalised Gradient Approximations to the Density Functional Theory (DFT), the change in form and extent of the E2g phonon anomaly of MgB2 with increase in applied pressure up to 20GPa. Ab initio DFT calculations on the phonon dispersion (PD) for MgB2 show a phonon anomaly symmetrically displaced around Γ, the reciprocal lattice origin. This anomaly is related to nesting between diametrically opposite sides of tubular elements of Fermi surfaces, which correspond to sigma bonding and run approximately parallel to the Γ–A reciprocal space direction. The anomaly is parallel to Γ–A and along Γ–M and Γ–K. The extent of the E2g phonon anomaly, δ, along Γ–M and Γ–K is a measure of the thermal energy, Tδ, that matches within error the experimental onset superconducting transition temperature, Tc. Ab initio DFT calculations with pressure for −5GPa<P< 20GPa show a linear reduction in Tδ that closely matches experimental Tc values for MgB2. For phonon-mediated superconductors with AlB2–type structures, the thermal energy of the phonon anomaly, Tδ, is a reliable predictor of Tc.

Phonon dispersion anomalies and superconductivity in metal substituted MgB2

We have calculated the extent of the E2g phonon anomaly for Mg1−xMxB2 where M=Sc, Ti, Cd and Ba for 0<x<1 using ab initio DFT models with the LDA and GGA functionals. Using superlattice models along the c axis to represent metal substitution in MgB2, we show that phonon dispersion (PD) plots vary significantly with x, in particular, the nature and extent of the phonon anomaly around the origin, G, of the reciprocal lattice. Measurement of this phonon anomaly along the G-K and G-M directions provides an estimate of the thermal energy, Tδ, of the anomaly, which approximates experimentally determined Tc within standard error for Sc and Ti substitution. We demonstrate that substitutions of Cd and Ba in MgB2 show a higher calculated Tδ than MgB2 by more than 20K. Syntheses of these Cd and Ba compositions are not extant and may not be possible given the limited solubility of metals in MgB2. Nevertheless, ab initio DFT models of phonon behaviour in AlB2–type structures provide an effective tool for prediction of physical properties and for design of new materials.

Preparation, microstructure of B film and its applications in MgB2 superconducting Josephson junction

Magnesium diboride is a binary compound with a simple AlB2 type crystal structure and a high-Tc (nearly 40 K) superconductor. The rather high Tc value and the specific properties make it a potential material for electronic applications. The key structure for the application is a Josephson junction. The growth of tri-layer structure consisting of MgB2 film and tunneling barrier layer is a key technology for a Josephson junction. Boron is a kind of good insulating medium. Preparation of MgB2/B/MgB2 tri-layer structures by chemical vapor deposition (CVD) method is investigated. The experimental results indicate that the depositing temperature will influence the microstructure of boron film significantly and different crystal structures of boron films are obtained at different temperatures.The boron film is an amorphous film while the deposition temperature is lower than 500 ℃, and the amorphous B film can be transformed into MgB2 superconducting film by annealing in Mg vapor. For precursor B films deposited at 470 ℃ and 500 ℃, the critical temperatures of the relevant MgB2 films are 39.8 K and 38.5 K, respectively. As the deposition temperature is higher than 550 ℃, the boron film becomes crystallized, and increasing deposition temperature will increase the crystallinity of the B film as can be seen from the samples deposited at 550 ℃, 600 ℃, 650 ℃ and 680 ℃. The boron film turns out to be of -phase crystalline texture, which is verified by X-ray diffraction and scanning electron microscope. What is more, the crystalline boron film is a kind of inert film, and it does not react with Mg in Mg vapor, thus it cannot be transformed into superconducting film in the subsequent annealing steps. By utilizing the property of the crystallized boron film, a square-shaped Josephson junction with a size 100 m100 m of MgB2/B/MgB2 structure is prepared. The thickness of boron dielectric layer is about 10 nm, and the DC Josephson effect is observed by the I-V measurement of the junction. Compared with other tri-layer structure based on MgB2 material, such as the MgB2/MgO/MgB2, the structure in which B film serves as a barrier layer eliminates the oxygen and can be fabricated in-situ easily by CVD method, and reliable Josephson junctions can be expected by such a technology.

Influence of boron vacancies on phase stability, bonding and structure of MB2 (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) with AlB2 type structure

Transition metal diborides in hexagonal AlB2 type structure typically form stable MB2 phases for group IV elements (M = Ti, Zr, Hf). For group V (M = V, Nb, Ta) and group VI (M = Cr, Mo, W) the stability is reduced and an alternative hexagonal rhombohedral MB2 structure becomes more stable. In this work we investigate the effect of vacancies on the B-site in hexagonal MB2 and its influence on the phase stability and the structure for TiB2, ZrB2, HfB2, VB2, NbB2, TaB2, CrB2, MoB2, and WB2 using first-principles calculations. Selected phases are also analyzed with respect to electronic and bonding properties. We identify trends showing that MB2 with M from group V and IV are stabilized when introducing B-vacancies, consistent with a decrease in the number of states at the Fermi level and by strengthening of the B–M interaction. The stabilization upon vacancy formation also increases when going from M in period 4 to period 6. For TiB2, ZrB2, and HfB2, introduction of B-vacancies have a destabilizing effect due to occupation of B–B antibonding orbitals close to the Fermi level and an increase in states at the Fermi level.

A theoretical investigation of mixing thermodynamics, age-hardening potential and electronic structure of ternary M11–xM2xB2 alloys with AlB2 type structure

Transition metal diborides are ceramic materials with potential applications as hard protective thin films and electrical contact materials. We investigate the possibility to obtain age hardening through isostructural clustering, including spinodal decomposition, or ordering-induced precipitation in ternary diboride alloys. By means of first-principles mixing thermodynamics calculations, 45 ternary M11–xM2xB2 alloys comprising MiB2 (Mi = Mg, Al, Sc, Y, Ti, Zr, Hf, V, Nb, Ta) with AlB2 type structure are studied. In particular Al1–xTixB2 is found to be of interest for coherent isostructural decomposition with a strong driving force for phase separation, while having almost concentration independent a and c lattice parameters. The results are explained by revealing the nature of the electronic structure in these alloys, and in particular, the origin of the pseudogap at EF in TiB2, ZrB2, and HfB2.

Phase Relations and Crystal Structures in the Ternary Systems Sr‐{Ag, Au}‐{Si, Ge}

AbstractPhase equilibria in the isothermal sections of the ternary systems Sr‐{Ag, Au}‐{Si, Ge} were determined in the (Si) or (Ge) rich part up to 33.3 at % Sr after annealing at 700 °C (Sr‐{Ag, Au}‐Ge) or 800 °C (Sr‐{Ag, Au}‐Si). Tentative liquidus projections were constructed for the Si, Ge‐rich part of all the four phase diagrams. These systems are characterized by a series of ternary compounds, exhibiting in some cases extended homogeneity regions at a constant Sr content. Compounds in the Si, Ge‐rich region essentially form along two sections: at 33.3 at % Sr (AlB2‐family) and at 20 at % Sr (BaAl4‐family of structure types). The crystal structures of the novel equilibrium phases were derived by X‐ray single crystal diffraction: Sr2Ag1+xSi3–x–y□y [Sr2LiSi3 type; x = 0.17, y = 0.43: a = 8.4488(2), b = 14.6376(2), c = 18.4018(2) Å], SrAg2–xSi2+x [ThCr2Si2 type; x = 0.1: a = 4.37664(6), c = 10.4517(2) Å], Sr(Au1–y□y)(Si1–xAux)3 [BaNiSn3 type; x = 0.59, y = 0.64: a = 4.4594(2), c = 10.1013(6) Å] and SrAu5–x□xSi2 [BaAu5Si2 type; x = 0.7: a = 8.7557(3), b = 6.9945(2), c = 9.8873(3) Å]. The crystal structures of Sr(AgxGe1–x)2 [AlB2 type; x = 0.25: a = 4.3431(2), c = 4.5938(1) Å], SrAg2–xGe2+x [ThCr2Si2 type; x = 0.2: a = 4.4476(2), c = 10.822(1) Å] and Sr(AuxGe1–x)2 [AlB2 type; x = 0.25: a = 4.3263(2), c = 4.5852(7) Å] were evaluated by X‐ray powder Rietveld analysis. Composition dependent polymorphism was observed among the BaAl4‐type derivative structures for SrAu2–xGe2+x: CaBe2Ge2 type [x = 0.45: a = 4.4796(2), c = 10.6315(5) Å], BaCu2Sb2 type [x = 0.35: a = 4.4866(2), c = 31.808(1) Å] and ThCr2Si2 type [x = 0.2: a = 4.5214(4), c = 10.336(1) Å]. Similarly, Sr(AuxSi1–x)2 exhibits the Ca2AgSi3 structure type [x = 0.25: a = 8.3407(2), b = 14.4465(2) and c = 9.2664(3) Å] and the AlB2 type [x = 0.375: a = 4.2241(2), c = 4.5799(3) Å]. The structural chemistry of the BaAl4 derivative structures within the sections SrT2–xX2+x is discussed.

High pressure structural behavior of YGa2: A combined experimental and theoretical study

High pressure structural stability studies were carried out on YGa2 (AlB2 type structure at NTP, space group P6/mmm) up to a pressure of ~35GPa using both laboratory based rotating anode and synchrotron X-ray sources. An isostructural transition with reduced c/a ratio, was observed at ~6GPa and above ~17.5GPa, the compound transformed to orthorhombic structure. Bulk modulus B0 for the parent and high pressure phases were estimated using Birch–Murnaghan and modified Birch–Murnaghan equation of state. Electronic structure calculations based on projector augmented wave method confirms the experimentally observed two high pressure structural transitions. The calculations also reveal that the ‘Ga’ networks remains as two dimensional in the high pressure isostructural phase, whereas the orthorhombic phase involves three dimensional networks of ‘Ga’ atoms interconnected by strong covalent bonds.

Structural and magnetic properties of Nd2NiGe3

Structural analysis of the room temperature diffraction data of Nd2NiGe3 shows that it exhibits AlB2 type structure with space group P6/mmm. The crystal structure of Nd2NiGe3 consists of two dimensional Ni/Ge hexagonal units sandwiching the neodymium atoms between them. Temperature dependent magnetic susceptibility data follows Curie–Weiss law in the temperature range 20–300K yielding an effective magnetic moment of 3.83μB/Nd and paramagnetic Curie temperature −2.1K indicating trivalent Nd and weak antiferromagnetic interactions, respectively. Low field magnetic susceptibility measurement show a weak antiferromagnetic like ordering around 3K. Temperature dependent neutron diffraction measurements rules out long range antiferromagnetic ordering in this compound. The electrical resistivity measurement shows metallic nature of Nd2NiGe3, which exhibits sudden drop in resistivity below 3K.

Deformation mechanisms in a precipitation-strengthened ferritic superalloy revealed by in situ neutron diffraction studies at elevated temperatures

The ferritic superalloy Fe–10Ni–6.5Al–10Cr–3.4Mo strengthened by ordered (Ni,Fe)Al B2-type precipitates is a candidate material for ultra-supercritical steam turbine applications above 923K. Despite earlier success in improving its room-temperature ductility, the creep resistance of this material at high temperatures needs to be further improved, which requires a fundamental understanding of the high-temperature deformation mechanisms at the scales of individual phases and grains. In situ neutron diffraction has been utilized to investigate the lattice strain evolution and the microscopic load-sharing mechanisms during tensile deformation of this ferritic superalloy at elevated temperatures. Finite-element simulations based on the crystal plasticity theory are employed and compared with the experimental results, both qualitatively and quantitatively. Based on these interphase and intergranular load-partitioning studies, it is found that the deformation mechanisms change from dislocation slip to those related to dislocation climb, diffusional flow and possibly grain boundary sliding, below and above 873K, respectively. Insights into microstructural design for enhancing creep resistance are also discussed.

Structural and magnetic properties in the polymorphs of CeRh0.5Ge1.5

We investigate the structural and magnetic properties in the polymorphs of a new compound CeRh0.5Ge1.5. Depending upon the starting materials, and the slightly different synthesis method, we find that CeRh0.5Ge1.5 compound exists in two different space groups. The first compound, α-CeRh0.5Ge1.5 crystallizes in tetragonal α-ThSi2 structure type in space group I41/amd with lattice parameters, a=4.2034(6)Å and c=14.770(3)Å. In this structure, the cerium atoms occupy the position between the Rh/Ge tetrahedral layers. On the other hand, the second compound, namely β-CeRh0.5Ge1.5 crystallizes in the AlB2 type hexagonal structure in space group P6/mmm, with lattice parameters, a=4.2615(7)Å and c=4.1813(9)Å. The crystal structure of β-CeRh0.5Ge1.5 consists of two dimensional Rh/Ge hexagonal units and the cerium atoms are sandwiched between them. Magnetization studies exhibit magnetic ordering, as evident from a sharp peak in the plot of magnetic susceptibility measured as a function of temperature in a fixed magnetic field, in α-CeRh0.5Ge1.5 and β-CeRh0.5Ge1.5 at 3.6K and 12K, respectively. Structural and magnetic properties of both compounds are presented and discussed here.