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.     

Ferroic transitions in the multiferroic (1 − x)Pb(Fe1/2Nb1/2)O3–xPbTiO3 system and its phase diagram

The results of the first comprehensive study of ferroic phase transitions as a function of temperature and compositions in the mixed multiferroic (1 ? x)Pb(Fe_1/2Nb_1/2)O₃–xPbTiO₃ (PFN–xPT) system are reported. Temperature-dependent powder synchrotron X-ray diffraction studies confirm unambiguously an interferroelectric transition between monoclinic and tetragonal phases for x ? 0.10. The tetragonal phase of PFN–xPT for x ? 0.10 and x > 0.10 is found to transform to the cubic phase on heating above room temperature. All these transitions are accompanied by distinct anomalies in the temperature dependence of dielectric constant. Our magnetization studies reveal that the nature of the magnetic phase transition changes across the morphotropic phase boundary (MPB) composition of PFN–xPT such that for tetragonal compositions with x > 0.08 there is only one magnetic transition, whereas two antiferromagnetic transitions are observed for monoclinic compositions with x < 0.08. These studies have enabled us to construct a phase diagram of PFN–xPT for the first time.     

Structural dependence of the piezoelectric properties of KNbO3 nanowires synthesized by the hydrothermal method

Because of the presence of OH^? and H₂O in the KN unit cell, tetragonal KNbO₃ (KN) nanowires were formed when the synthesis was carried out at 120 °C for 48 h. However, when the fabrication was conducted at high temperatures (?150 °C) or at 120 °C for a long period of time (?72 h), orthorhombic KN nanowires were formed. Moreover, the KN nanowires synthesized at 120 °C for 60 h showed a morphotropic phase boundary (MPB) structure in which both tetragonal and orthorhombic structures coexisted. Tetragonal, orthorhombic and MPB KN nanowires were also grown on the Nb⁵⁺-doped SrTiO₃ substrate, and their d₃₃ values were measured for the first time. A tetragonal KN nanowire exhibited a d₃₃ value of 23.5 pm V^?1, which is larger than that of the orthorhombic KN nanowire (11.6 pm V^?1), probably because of the softening effect of the metal vacancies. The MPB KN nanowires exhibited a larger d₃₃ value of 40.0 pm V^?1. The d₃₃ values of KN nanowires increased to 104.5, 137.1 and 146.0 pm V^?1 for the orthorhombic, tetragonal and MPB KN nanowires, respectively, after the KN nanowires were poled along the [1 0 0] direction by application of a DC voltage of 10 V.     

Fracture mechanism of scandia-doped zirconia

The bending strength and bending modulus of 5, 9 and 12 mol.% scandia-doped zirconia were investigated as a function of temperature up to 1273 K. The fracture mechanisms of various crystal phases were discussed based on the determined strengths and also the fractographic investigation. The monoclinic phase of the scandia-doped zirconia has a small grain size, which strengthens the material by fracturing in the intergranular mode up to 1273 K. The tetragonal phase fractures transgranularly at room temperature accompanied by a stress-induced transformation, which greatly contributes to a strengthening the material, whereas the strengthening effect disappears at elevated temperature and the fracture mode becomes intergranular. The rhombohedral phase, which only exists below 873 K, also fractures completely transgranularly, leaving patterns of its herringbone structure on the fracture surface, which could be of a rather brittle nature and disappears at higher temperature. The fracture mode of the cubic phase is also transgranular at room temperature, with a relatively smooth fracture surface. It has no particular strengthening effect, but is partially intergranular at high temperature.     

A TEM study on the crystallization behavior of an yttrium-doped Zr-based bulk metallic glass

Transmission electron microscopy (TEM) is used to conduct the systematic study of the annealing induced crystallization, both continuously and isothermally, of a Zr-based metallic glass. Through detailed microstructure analysis, it is found that the crystallization of this metallic glass is initialized by a nano-scaled primary crystallization process with a tetragonal structured crystal (a = 0.96 nm and c = 2.82 nm) as the primary phase. A eutectic crystallization is followed afterwards with two types of crystalline phases as the crystallization products, one of which is determined to be a metastable orthorhombic phase (a = 0.69 nm, b = 0.75 nm and c = 0.74 nm). Upon annealing at a raised temperature or isothermal treatments, a solid state phase transformation takes place and the orthorhombic metastable phase transforms into two types of tetragonal crystalline phases. The whole crystallization process of this metallic glass is in turn realized, and the thermal stability and nano-crystallization mechanism are discussed based on the microstructure and thermal analyses.     

On thermal and vibrational properties of LaGaO3 single crystals

The thermal expansion and vibrational properties of [1 0 0] and [0 0 1] LaGaO₃ single crystals have been studied by thermal mechanical analysis and micro-Raman spectroscopy. A first-order orthorhombic to rhombohedral phase transition has been confirmed by both techniques, as well as by in situ heating using optical microscopy. The appearance of a metastable intermediate phase, tentatively assigned as monoclinic, has been detected both by optical microscopy and Raman spectroscopy upon heating of the [1 0 0] and [0 0 1] LaGaO₃ single crystals. Not only temperature, but the stress-induced orthorhombic to rhombohedral phase transition has also been detected by Raman mapping of the residual impression made by Vickers indentation. The position map of bands belonging to the lower-temperature/pressure orthorhombic and the higher-temperature/pressure rhombohedral phase show that the rhombohedral phase is located inside the impression, where the applied indentation stresses are the highest, whereas no rhombohedral phase is detected outside the impression, where the surface has not been altered by contact stresses.     

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.     

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.     

In situ observation of the reaction of scandium and carbon by neutron diffraction

The formation of scandium carbides by reaction of the elements has been investigated by in situ neutron diffraction up to 1823 K. On heating, the recrystallization of α-Sc occurs between 1000 and 1223 K. The formation of Sc₂C and ScC (NaCl-B1 type structure) phases has been detected at 1323 and 1373 K, respectively. The formation of a new orthorhombic scandium carbide phase was observed at 1473(50) K. Once the scandium carbides are formed they are stable upon heating or cooling. No other phases were detected in the present study, in which the system was always carbon saturated. The thermal expansion coefficients of all phases have been determined, they are constant throughout the temperature interval studied.     

TEM investigation of interfaces during cuprous island growth

The geometry and epitaxial relationships of interfaces generated during the early-stage oxidation of Cu(1 0 0) surfaces were studied using transmission electron microscopy. The predominant orientation relationship between Cu₂O islands and the Cu substrate is cube-on-cube growth, whereby equivalent planes and directions of oxide islands and the metal substrate are matched across the interface, while other epitaxies are occasionally observed. A 6 × 7 coincidence site lattice configuration is observed at the Cu–Cu₂O interface for the cube-on-cube epitaxy. The geometry of Cu₂O–Cu interfaces is found to depend on the specific epitaxial orientations of Cu₂O islands with the Cu substrate: wedge-shaped interfaces are developed for cube-on-cube growth, and edge-on interfaces are formed for other epitaxies. These growth features are attributed to the minimization of the interface energy via the competing factors among the coincidence lattice misfit, misfit dislocations and the metal–oxide interface area.     

A fast method for determining favourable orientation relationships and interface planes: Application to titanium–titanium hydrides transformations

The transformation of titanium into several hydride phases has been studied by transmission electron microscopy at the surface of an acid-etched titanium substrate designed for biomedical applications. Four different orientation relationships have been found between face-centred cubic TiH_2−ε (ε ≪ 1), face-centred tetragonal TiH and hexagonal close-packed Ti. A fast method based on the alignment of diffraction spots perpendicular to interfaces (Δg condition) is proposed, to account for the experimental orientation relationships and interface planes. Different paths leading to the formation of the two above-mentioned hydride phases have been thus identified. A mechanism of misfit relaxation has been observed and described.     

The capability of hibonite elongated grains to influence physical,microstructural, and mechanical properties of zirconia toughened alumina–CeO2–MgO ceramics

This work investigated the capability of hibonite (CaAl₁₂O₁₉) phase on structure and microstructure of zirconia toughened alumina–CeO₂–MgO ceramics. Three different additives (CeO₂, MgO and CaCO₃) were introduced into zirconia toughened alumina ceramics prepared by solid state reaction. X-ray diffraction and FESEM analyses were employed to observe the role of secondary phases especially hibonite and its influence on the microstructural features and other properties. Among the secondary phases present, EDX analysis revealed that hibonite phase contributed to elongated grains. Vickers indentations hinted a strong difference in the efficiency of compositional adjustment among the composites. The excellent Vickers hardness and fracture toughness results obtained for 3 wt.% CaCO₃ additions showed the appearance of 5.9% hibonite with the value of 1485 HV and 7.10 MPa·√m, respectively.     

Structural,electronic and elastic properties of noble metal sub-nitrides M2N (M = Ru,Rh, Pd)

Ab initio calculations are performed to investigate the structural stability, electronic structure and elastic properties of noble metal sub-nitrides M₂N (M = Ru, Rh, Pd). The metal nitrides, Ru₂N and Rh₂N are found to be stable in anti-fluorite structure, whereas Pd₂N is stable in tetragonal structure. In Ru₂N and Rh₂N, structural phase transition is predicted from antifluorite to orthorhombic phase whereas tetragonal to antifluorite and then antifluorite to pyrite phase transitions are predicted in Pd₂N under high pressure. The electronic structure reveals that these nitrides are metallic. The calculated elastic constants indicate that these materials are mechanically stable at ambient condition. The Debye temperature values are reported for all the phases of these nitrides. The high bulk modulus indicates that these materials are super hard materials. They are ductile in nature at normal and high pressures. The bonding nature of these materials is found to be a mixture of covalent, metallic and ionic attribution at ambient condition.     

Low-temperature transformation kinetics of electron-beam deposited 5 wt.% yttria-stabilized zirconia

The transformation kinetics of ZrO₂ coatings stabilized with 5.6 mol% YO_1.5 (5YSZ), and deposited by electron-beam physical vapor deposition, were studied between room temperature and 600 °C using in situ Raman spectroscopy, and are described in the form of a transformation-time–temperature diagram. The coatings were found to be monoclinic (m) at temperatures below 375 °C, while above 400 °C they transformed to the tetragonal (t) phase. On cooling, the coatings transformed back to monoclinic below ∼375 °C. Between 375 and 400 °C, the transformation rates approached zero, indicating that the thermodynamic driving force for the transformation also approaches zero in this temperature range. This provides a direct measurement of the T₀(m–t) temperature for the 5YSZ composition.     

Low thermal conductivity without oxygen vacancies in equimolar YO1.5 + TaO2.5- and YbO1.5 + TaO2.5-stabilized tetragonal zirconia ceramics

A narrow range of composition exists along both the ZrO₂–YTaO₄ and ZrO₂–YbTaO₄ quasi-binaries over which the tetragonal zirconia phase can be retained on cooling. Unlike other stabilized zirconia materials which have low thermal conductivity as a result of phonon scattering by oxygen vacancies, these compositions do not contain oxygen vacancies and yet an equimolar YO_1.5 + TaO_2.5 composition has been reported to also exhibit low thermal conductivity [1]. We find that zirconia compositions along the quasi-binaries have low and temperature-independent thermal conductivities, and that the thermal conductivities and their temperature dependence are consistent with a defect scattering model that takes into account a minimum phonon mean free path due to the inter-atomic spacing. Furthermore, the conductivities of the Yb and Y trivalent-doped compositions scale in a predictable manner with atomic site disorder effects on the cation sub-lattice associated with the lighter Y³⁺ ions and the heavier Yb³⁺ and Ta⁵⁺ ions. The lowest thermal conductivity measured was ～1.4 W mK^–1 at 900 °C. The low thermal conductivity and phase stability makes these systems promising candidates for low conductivity applications, such as thermal barrier coatings.     

The transformation sequences in the cubic → tetragonal decomposition

The decomposition of a generic supersaturated binary cubic solid solution into a mixture of cubic and tetragonal phases is investigated by phase field microelasticity modeling and simulations. It is shown that the decomposition in such a system is not necessarily developed by conventional nucleation and growth of the tetragonal phase. There are three temperature and composition ranges where the sequences of transient structures formed are different. The transformation pathways are predicted and the corresponding thermodynamic parameters are identified. In particular, the simulations reveal unusual transformation sequences occurring in the process of isostructural decomposition followed by cubic → tetragonal MT confined within one of the decomposed cubic phases. Mechanisms for the formation of the stress-accommodating multi-domain aggregates of the tetragonal phase and the checkerboard-like structures comprised of parallel rods of cubic and tetragonal phases are discussed.     

Crystal structure and magnetic properties of the new intermetallic Ce2Pt9Al16

The crystal structure of the ternary intermetallic Ce₂Pt₉Al₁₆ has been determined from single crystal X-ray data. It crystallizes in a new structure type in the orthorhombic system, with cell parameters a = 4.1597(9), b = 11.910(3), c = 18.186(4) Å, space group Immm, Z = 2, Pearson code oI54. Cerium atom has a hexagonal prismatic coordination, tetragonal prisms are typical for coordination environment of platinum and aluminium atoms. The structure is described as a stacking of Ce-filled hexagonal prisms and two types of empty tetragonal prisms. Magnetic susceptibility measurements reveal antiferromagnetic ordering below T_N = 3 K.     

Phase stability and transformations in NiTi from density functional theory calculations

We used density functional theory to characterize various crystalline phases of NiTi alloys: (i) high-temperature austenite phase B2; (ii) orthorhombic B19; (iii) the monoclinic martensite phase B19′; and (iv) a body-centered orthorhombic phase (BCO), theoretically predicted to be the ground state. We also investigated possible transition pathways between the various phases and the energetics involved. We found B19 to be metastable with a 1 meV energy barrier separating it from B19′. Interestingly, we predicted a new phase of NiTi, denoted B19′′, that is involved in the transition between B19′ and BCO. B19′′ is monoclinic and can exhibit shape memory; furthermore, its presence reduces the internal stress required to stabilize the experimentally observed B19′ structure, and it consequently plays a key role in NiTi’s properties.     

An investigation on hot corrosion behavior of YSZ-Ta2O5 in Na2SO4 + V2O5 salt at 1100 °C

This study compares the hot corrosion performance of yttria stabilized zirconia (YSZ), and YSZ-Ta₂O₅ (TaYSZ) composite samples in the presence of molten mixture of Na₂SO₄ + V₂O₅ at 1100 °C. For YSZ, the reaction between NaVO₃ and Y₂O₃ produces YVO₄ and leads to the transformation of tetragonal ZrO₂ to monoclinic ZrO₂. For TaYSZ, minor amounts of NaTaO₃, TaVO₅ and Ta₉VO₂₅ are formed as the hot corrosion products with only traceable amounts of YVO₄. Due to the synergic effect of doping of zirconia with both Y₂O₃ and Ta₂O₅, the TaYSZ sample has a much better hot corrosion resistance than YSZ.     

Quantification of hydrothermal degradation in zirconia by nanoindentation

The ageing of zirconia under hydrothermal conditions is a key issue in many applications, as the properties and thickness of the degraded layer influence the reliability of the material. This paper presents a study by nanoindentation of the hydrothermal degradation of tetragonal zirconia polycrystals doped with yttria with a tetragonal grain size of 0.3 μm and of the same material after heat-treatment at high temperature (1650 °C) in order to develop a duplex cubic-tetragonal microstructure with a large tetragonal grain size. Nanoindentation is used to evaluate the Young’s modulus and hardness of the degraded surfaces as a function of indenter penetration depth. The mechanisms of hydrothermal degradation are also studied. Using Raman spectroscopy, atomic force microscopy and focused ion beam (FIB) cross-sections, we show that hydrothermal degradation induces t–m phase transformation and microcracking under the surface. Microcracks in degraded zirconia, clearly observed in FIB cross-sections, are associated with the decrease in hardness and Young’s modulus of the degraded surface. Nanoindentation is used to detect and quantify the degradation of zirconia by calculating the decrease in hardness and Young’s modulus with penetration depth. Using a thin-film model, the depth of the degraded layer and its mechanical properties are also extracted. As expected, we demonstrate that duplex zirconia with a large tetragonal grain size is more sensitive to hydrothermal degradation than monophasic small-grain tetragonal zirconia.