Full-potential electronic structure of Hf2AlC and Hf2AlN

The MAX phase materials are a class of ternary compounds with formula unit M_n+1AX_n (MAX), where n = 1, 2 or 3, M is an early transition metal, A is an A-group (mostly IIIA and IVA) element, and X is either C and/or N. These ternary carbides and nitrides have an unusual combination of the properties of both metals and ceramics. They exhibit high hardness, but fully reversible plasticity and negligible thermoelectric power. In this work, we report the electronic structure of nanolaminated Hf₂AlX (X = C and N) by means of a first-principles method, the “full-potential linearized plane-wave method” (including spin–orbit interaction) based on the density functional theory. We have investigated the lattice parameters, bulk moduli, band structures, total and partial densities of states, and charge densities. We hope that this work will inspire future experimental research on these Hf-based ternary materials.     

An investigation of the Al–Ni–Rh phase diagram between 50 and 100 at% Al

The Al-rich part of Al–Ni–Rh was studied between 800 and 1080 °C. The Al₉Rh₂ phase was found to contain up to 8 at% Ni. The orthorhombic Al–Rh ɛ₆-phase extends up to 17.5 at% Ni, high-temperature cubic C-Al₅Rh₂ up to about 10 at% Ni while low-temperature hexagonal H-Al₅Rh₂ extends up to 4 at% Ni. The Al₇Rh₃ phases contained up to 3 at% Ni. The solubility of Rh in Al₃Ni is up to 3 at% and in Al₃Ni₂ up to 5 at%. The isostructural binary AlNi and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. A ternary hexagonal phase similar to Al₂₈Ir₉ (a = 1.213 and c = 2.626 nm) was found to be formed between Al₇₆Ni₄Rh₂₀ and Al₇₆Ni₁₃Rh₁₁. The formation of the high-temperature stable decagonal phase was confirmed. Another ternary phase, whose structure is not yet clarified, was revealed around Al₇₀Ni₁₁Rh₁₉. Partial 1080, 1000, 900 and 800 °C isothermal sections of the Al–Ni–Rh phase diagram are presented.     

Consideration of the validity of the 14 valence electron rule for semiconducting chimney-ladder phase compounds

Electronic structure calculations for compounds known as Nowotny chimney-ladder (CL) phases have been performed to ascertain an empirical rule that CL compounds with a valence electron concentration (VEC) of 14 are a semiconductors. RuGa₂ and RuAl₂ which have a TiSi₂-type structure, a prototype of the CL phase, with a VEC value of 14 are indirect-gap semiconductors with estimated band gaps of 0.235 and 0.20 eV, respectively. Ru₂Si₃ and Ru₂Ge₃ with VEC=14 are predicted to be direct-gap semiconductors but the band gap decrease in the heavier elements results in closure of the gap in Ru₂Sn₃. Ir₃Ga₅ and Ir₄Ge₅ will be metallic or semimetallic though their VEC values are 14. The Fermi level of Mn₁₁Si₁₉, whose VEC is slightly smaller than 14, is located just before the gap and seems not to be inconsistent with p-type semiconducting behavior. The Fermi levels of Rh₁₀Ga₁₇ and Rh₁₇Ge₂₂ whose VECs exceed 14 are located past the gap. Cr₁₁Gei₁₉ and Mo₁₃Ge₂₃, whose VECs are smaller than 14 would be metallic. These results show that the above rule is a rather good criterion for exploration of semiconducting chimney-ladder phase compounds but compounds with VEC=14 are not always semiconductors.     

Elastic constants of amorphous and single-crystal Pd40Cu40P20

We have measured the adiabatic second-order elastic constants of amorphous Pd₄₀Cu₄₀P₂₀ (isotropic, two independent elastic constants) and single-crystal Pd₄₀Cu₄₀P₂₀ (tetragonal, six independent elastic constants) over the range 3.9 < T < 300 K. Below ∼20 K, the elastic constants of single-crystal Pd₄₀Cu₄₀P₂₀ vary as C(T) = C(0)[1 − bT² − dT⁴], which is the same temperature dependence found in metallic elements. In contrast, below ∼20 K the elastic constants of amorphous Pd₄₀Cu₄₀P₂₀ vary as C(T) = C(0¹)[1 − aT]. The bulk modulus of amorphous Pd₄₀Cu₄₀P₂₀ is approximately 2% lower than that of crystalline Pd₄₀Cu₄₀P₂₀, whereas the shear modulus of the glass is 13–36% lower than the four independent shear moduli of the tetragonal crystal. The lower moduli of the glass are best explained by additional atomic displacements, beyond those experienced by the atoms in a crystal, that occur because the atoms in glasses do not occupy centers of symmetry.     

Pure orthorhombic zirconia islands grown on single-crystal sapphire substrates

This study reports on the occurrence of pure orthorhombic zirconia obtained for the first time at ambient pressure. Thin films of pure zirconia are composed of isolated islands which generally present heteroepitaxial relationships with the underlying sapphire substrate. Epitaxial growth develops at high temperature in the stability domain of the tetragonal phase of zirconia. On cooling, most of the tetragonal islands transform into monoclinic, but numerous islands present an orthorhombic structure with Pbc2₁ and Pbca space groups. It is first suggested that these two orthorhombic phases are formed because the tetragonal → monoclinic reconstructive phase transition was impeded due to the pre-existing heteroepitaxial relationships that developed between the tetragonal zirconia phase and the substrate. However, the orthorhombic phases are much better interpreted as alternative phases that arise as a consequence of some constraints produced by the substrate. Thus, these phases represent metastable structures when appropriate external anisotropic constraints are applied.     

Electronic properties and crystal structures of RE3Rh2Ga2 and RE3Rh3Si2 (RE = La,Ce)

Polycrystalline samples of Ce₃Rh₂Ga₂, Ce₃Rh₃Si₂ and their La-based isostructural analogues were studied by means of magnetic susceptibility, magnetization and electrical resistivity measurements. The crystal structure of La₃Rh₃Si₂ (Ce₃Rh₃Si₂ type) was investigated by single-crystal X-ray diffraction. The cerium gallide was found to order ferromagnetically at T_C = 3.5 K, whereas for the silicide more complex magnetic behaviour was established with antiferromagnetic order setting at T_N = 6.5 K and subsequent change in the magnetic structure occurring at T_t = 5.5 K. Both cerium compounds exhibit weak Kondo effect. The L_III-edge XAS data indicated rather stable 4f¹ character of cerium in both compounds. The electronic band structures of Ce₃Rh₂Ga₂ and Ce₃Rh₃Si₂ were calculated by the LMTO method and compared with those of Ce₃Rh₂Ge₂ and La₃Rh₂Ge₂. For each Ce-based compound the total electronic DOS is dominated by a peak of Ce 4f states near the Fermi level and a Rh 4d band located about 2 eV below the Fermi energy.     

Solid state amorphization and crystallization in Zr66.7Pd33.3 metallic glass

We observed quasicrystal-to-amorphous-to-crystal (Q–A–C) transition in Zr_66.7Pd_33.3 metallic glass. The unique disordering–ordering phase transition was induced by 2 MeV electron irradiation at 298 K. Electron irradiation can induce not only the solid-state amorphization but also crystallization of a glassy structure under the same irradiation conditions. The Q–A–C transition can be explained by change in the phase stability of crystal, quasicrystal and amorphous phases by electron irradiation induced atomic displacement.     

A Ni-free Zr-based bulk metallic glass with remarkable plasticity

The effect of Nb and Pd combination on the glass forming ability (GFA) and mechanical properties of Zr₅₃Cu₃₀Nb_xPd_9?xAl₈ (x = 3.5–6.0) bulk metallic glasses (BMGs) were systematically investigated by X-ray diffractometry (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and compression test. TEM observation revealed that a nanocrystalline phase embeds in the amorphous matrix of the as-cast Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ alloy. A tiny nano-crystalline phase (with size about 5–20 nm) embedded uniformly in the amorphous matrix of the Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ alloy was observed and identified to be the tetragonal structured NbPd₃ phase based on the analyses of nano beam electron diffraction. According to the results of thermal analyses, the composition of Zr₅₃Cu₃₀Nb₅Pd₄Al₈ and Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ present the optimum GFA as well as thermal stability in the Zr₅₃Cu₃₀Nb_xPd_9?xAl₈ (x = 3.5–6.0) alloy system. In addition, the result of compression test shows that the yield strength significantly increases from 1700 MPa (Zr₅₃Cu₃₀Nb₅Pd₄Al₈) to 1900 MPa (Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈). A remarkable compression plastic strain (11.2%) occurs at Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ BMG rod with 2 mm in diameter. This significant increase in plasticity is presumably due to the restriction on shear banding by the nano-size second phase.     

The crystal structure of high temperature phase Ta2O5

The high temperature phase of Ta₂O₅ and its variants were maintained in a pure Ta₂O₅ specimen using the conventional solid-state reaction method and the advanced laser irradiation technique. The structure of the high temperature phase is determined to be tetragonal with lattice parameters of a = 3.86 Å and c = 36.18 Å and space group of I4₁/amd. Compared to the previously reported crystal structure, the number of the total basis atoms was reduced to be 6 from 11. The modified crystal structure interprets well the recorded high-resolution electron microscopy images and the selected area electron diffraction patterns. Based on the basic tetragonal structure, the crystal lattice structures of the orthorhombic and monoclinic variants were determined and the corresponding space groups were analyzed according to the variation of the symmetrical elements. In addition, crystal atomic structures of these two variants were proposed based on the tetragonal structure.     

XPS study on the surface films of a newly designed Ni-free Ti-based bulk metallic glass

The electrochemical and corrosion behavior of a newly designed Ni-free Ti-based bulk metallic glass (BMG) was investigated. The Ti₄₅Zr₁₀Pd₁₀Cu₃₁Sn₄ BMG exhibits excellent corrosion resistance after immersion in 3 mass% NaCl, 1 N H₂SO₄ and 1 N H₂SO₄ + 0.01 N NaCl solutions. X-ray photoelectron spectroscopy measurements were used to analyze the changes of the elements on the alloy surface before and after immersion in various solutions. The formation of Ti- and Zr-enriched highly protective thin surface films is responsible for the high corrosion resistance of this alloy in corrosive solutions.     

An investigation of the Al–Pd–Rh phase diagram between 50 and 100 at% Al

Partial 1100, 1000, 900 and 790 °C isothermal sections of the Al–Pd–Rh phase diagram were studied. The isostructural binary AlPd and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. The Al–Pd and Al–Rh ε-phases form another continuous range. The C–Al₅Rh₂ phase dissolves up to 13 at% Pd, Al₉Rh₂ and Al₇Rh₃ are extended up to 3 at% Pd. Two ternary phases: cubic C₂ (a=1.5483 nm) and hexagonal C₃ (a=1.09159, c=1.3386 nm) were revealed. The former extends along about 65 at% Al from 4 to 27 at% Pd.     

Directional thermoelectric properties of Ru2Si3

The orthorhombic compound Ru₂Si₃ is currently of interest as a high-temperature thermoelectric material. In order to clarify the effects of crystal orientation on the thermoelectric properties of Ru₂Si₃, we have examined the microstructure, Seebeck coefficient, electrical resistivity, and thermal conductivity of Ru₂Si₃ along the three principal axes, using these measured quantities to describe the relative thermoelectric performance as a property of crystal orientation. Ru₂Si₃ undergoes a high temperature (HT)→low temperature (LT) phase change and polycrystalline Si platelet precipitation during cooling, both of which are expected to effect the thermoelectric properties. The HT tetragonal→LT orthorhombic phase transformation results in a [010]//[010], [100]//[001] two-domain structure, while polycrystalline Si precipitation occurs on the (100)_LT and (001)_LT planes. The [010] orientation is found to posses superior thermoelectric properties (with the dimensionless figure of merit, ZT_[010]/ZT_[100]>4 at 900 K), due principally to the larger Seebeck coefficient along the [010] direction. The effect of the domain structure on the thermoelectric properties is discussed.     

An entropy driven phase transformation in a Au43.3Cu31.8Al24.9 shape-memory alloy

The 18-karat Au_43.3Cu_31.8Al_24.9 alloy displays shape memory effect. The parent phase has L2₁ structure which transforms to B2 at high temperature. The B2 phase is retained in metastable state by quenching from the B2 field. It then transforms irreversibly with an endothermic peak in Differential Scanning Calorimetry (DSC) to the stable L2₁ structure in the temperature range between 407 K and 435 K as a function of heating rate. The transformation is, therefore, entropy-driven. Details are reported on the enthalpy of transformation and its temperature dependence. The transformation is discussed with reference to the entropy content of phases as a function of temperature which counteracts austenite stability. The role of vacancy motion is outlined. A comparison is made with analogous transitions occurring in molecular substances and metallic materials.     

Square net distortion engineering in the ternary variants of titanium antimonide,Ti2−δMδSb (M = Zr,Hf)

The binary antimonide Ti₂Sb crystallizes in a unique structure type, which may be considered as a distorted variant of the La₂Sb structure type. The new ternary variants, Ti_2−δM_δSb (M = Zr, Hf) with δ < 0.85, were prepared by arc melting the elements in stoichiometric ratios. The ternaries crystallize in the body centered tetragonal space group I4/mmm, with lattice dimensions varying with δ and M, e.g. a = 3.977(1) Å and c = 15.011(2) Å for Ti_1.5Zr_0.5Sb (Z = 4). Single crystal structure investigations as well as powder X-ray profile refinements employing the Rietveld method and pair distribution function technique revealed, that with increasing δ the distortions lessen and finally disappear, as was predicted based on the Kleinke–Harbrecht structure map.     

Mechanical instabilities and structural phase transitions: The cubic to tetragonal transformation

A variety of technologically important crystalline phases are predicted by first-principles electronic structure methods to be mechanically unstable at 0 K, raising fundamental questions about the finite temperature excitations that stabilize these phases at high-temperature. Here, we show that anharmonic vibrational degrees of freedom can stabilize a cubic phase that is mechanically unstable at 0 K with respect to a tetragonal distortion. We develop an effective anharmonic strain Hamiltonian for a cubic lattice and parameterize its coefficients to first-principles calculations of the energy surface of TiH₂, a compound that undergoes a cubic to tetragonal transformation around 300 K and for which the cubic phase is predicted to be mechanically unstable. Monte Carlo simulations applied to the effective Hamiltonian predict a cubic to tetragonal phase transition and provide insight about the true nature of the high-temperature cubic phase.     

Deposition and characterization of superhard biphasic ruthenium boride films

Recently, the superhardness of rhenium diboride films was reported. In this study the first successful preparation and characterization of ruthenium boride films is presented. The morphology, topography, microstructure and hardness of films, prepared by pulsed laser deposition, were investigated. The films, which are 0.7 μm thick, have a dense grain texture, and are composed of two phases Ru₂B₃ (main phase, 65% volume fraction) and RuB₂ (35%). The RuB₂ phase does not show any preferred orientation, while Ru₂B₃ is textured preferentially along the (1 1 4) and (1 0 5) directions, with crystallite growth parallel within 1.9° of average mismatch. The composite Vickers microhardness of the film–substrate systems was measured, and the intrinsic hardness of the films was separated using an area law-of-mixtures approach. The obtained films were found to be superhard, the intrinsic film hardness value (49 GPa) being much higher than that for the RuB₂ bulk used as the target for film deposition and than that for the Ru₂B₃ bulk.     

Phase transition behavior and electrical properties of [(K0.50Na0.50)1−xAgx](Nb1−xTax)O3 lead-free ceramics

The effect of AgTaO₃ on the electrical properties of (K_0.5Na_0.5)NbO₃ lead-free ceramics was systematically investigated, and the phase transition behavior of the ceramics was also studied in terms of high temperature X-ray diffraction. The experimental results show that Ag⁺ and Ta⁵⁺ ions diffuse into the (K_0.5Na_0.5)NbO₃ lattices to form a stable solid solution with orthorhombic structure, and also lead to the decrease in the orthorhombic to the tetragonal phase transition temperature and the Curie temperature. The 0.92(K_0.5Na_0.5)NbO₃–0.08AgTaO₃ ceramics exhibit optimum electrical properties (d₃₃ = 183 pC/N, k_p = 41%, T_c = 356 °C, T_o–t = 158 °C, ?_r ～ 683, and tan δ ～ 3.3%) and good thermal-depoling behavior. The piezoelectric properties of (1 ? x)(K_0.5Na_0.5)NbO₃–xAgTaO₃ ceramics are much superior to pure (K_0.5Na_0.5)NbO₃ ceramics. X-ray diffraction patterns for the 0.92(K_0.5Na_0.5)NbO₃–0.08AgTaO₃ ceramic at different temperatures indicated a pure perovskite phase with an orthorhombic structure at below 160 °C, a tetragonal structure at 160–350 °C, and a cubic structure at above 360 °C. As a result, the (1 ? x)(K_0.5Na_0.5)NbO₃–xAgTaO₃ ceramic is one of the promising candidate materials for lead-free piezoelectric ceramics.     

Effect of the nanoindentation rate on the shear band formation in an Au-based bulk metallic glass

This study investigated the nanoindentation behavior of Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 bulk metallic glass samples at loading rates ranging from 0.03 to 300 mN s⁻¹. Notable shear band pop-in events were observed. The pop-in size was observed to increase linearly with the load and decreased exponentially with the strain rate. A free-volume mechanism was proposed for interpreting these observations quantitatively. The results and analyses also shed light on the shear band nucleation and evolution processes in bulk metallic glasses.     

Excellent mechanical properties of metastable c-WN fabricated at high pressure and high temperature

Tungsten nitrides (WNs) are promising functional materials with high hardness, but the greatest challenge is to synthesize stoichiometric and bulk materials. In this paper, bulk tungsten mononitride (c-WN) with sodium chloride structure, which is a metastable phase, has been successfully synthesized at high pressure and high temperature (HPHT) using W₃N₄ as precursor. It is found that synergistic effect of pressure and temperature was useful to control the complete decomposition of W₃N₄ and to suppress further decomposing of as-synthesized c-WN. The compression ability and Vickers hardness were investigated by in situ high pressure X-ray diffraction (XRD) and Vickers microhardness tests, respectively. It is worth noting that the bulk modulus of c-WN is 422.9 ± 6.7 GPa, which is comparable to diamond. The Vickers hardness, 29 GPa obtained under an applied load of 0.49 N, is nearly 45% higher than that of TiN which is widely used as hard wear protective coatings. The excellent mechanical properties of c-WN may be ascribed to strong pd hybridization which has been further proved by XPS.     

Influence of crystallinity on the structural and hydrogenation properties of Mg2X phases (X=Ni,Si, Ge,Sn)

The hydrogen storage properties of nanocrystallized Mg₂Ni prepared by ball-milling have been studied. Hydrogenation at 200 °C leads to the formation of the monoclinic low-temperature phase Mg₂NiH₄ containing a much larger amount of microtwinning than the hydride obtained by hydrogenation of well-crystallized Mg₂Ni at the same temperature. The large amount of defects and dislocations present in the ball-milled alloys seems to favour the formation of microtwinning upon hydrogenation. Additionally, Mg₂X compounds (X=Si, Ge, Sn) have been prepared by powder metallurgy and ball-milling. Cubic phases with antifluorite structure are obtained, except for Mg₂Sn, for which a metastable rhombohedral phase is synthesized by ball-milling. Mg₂Si decomposes into MgH₂ and Si when ball-milled under hydrogen atmosphere, whereas Mg₂Ge and Mg₂Sn do not absorb hydrogen and are not decomposed.