Stabilization Of Cubic Phase Research Articles

Synthesis and Characterization of Zirconium Oxide Systems with Yttrium Rich Rare Earth Concentrate Additives

In this work, the yttrium rich rare earth concentrate (Re2(CO3)3) was used as additive aiming stabilization of cubic an tetragonal phases at commercial zirconium oxide with 3% mol of yttrium oxide. The use of high purity rare earth oxide as additive is being commercially used and this work aims to demonstrate the potential use of lower cost additives to produce solid electrolyte for oxygen sensors and fuel cell applications. The powders for the additive production were synthesized by the controlled precipitation method. After synthesis, the powders were de-agglomerated using mechanical grinding and mixed to commercial zirconia to produce the compositions ZrO2:3% Mol Y2O3:ƞ % Mol Re2O3 (ƞ=3,4,5,6), followed by uniaxial press and sintering at 1500 0C in two hours. The obtained sintered densities were above 96% of theoretical. X-Ray diffractometric analysis and Rietweld refinement demonstrated the stabilization of cubic and tetragonal phases for all samples with yttrium rich rare earth concentrate additives. Finally the electric behavior of the evaluated samples was carried out with complex impedance spectroscopy, showing conductivity improvement for samples with the chosen additive. At 500 0C the sample A-9% had a conductivity of 1,11E-3Ω-1.cm-1, well above of the sample without additive with conductivity 5,88E-4Ω1.cm-1, indicative that use of yttrium rich rare earth concentrate as additive increases considerably the ionic conductivity of comercial zirconium oxide. Key words: rare earth concentrate, controlled precipitation, ionic conductivity

Oxygen concentration-related impedance spectroscopic studies of La2Mo2O9 oxide ion conductors

The frequency dispersion of impedance, relaxation, and modulus formalism under oxygen, air, and nitrogen atmospheres are performed on La2Mo2−x W x O9 (x = 0, 0.3, 0.5, 0.7) phases from 800 °C to room temperature. Thermal analyses on a dilatometer demonstrate the stabilization of the high-temperature cubic phase when the W content is increased. The whole frequency-dependent relaxations from the bulk effect including different oxide ion diffusion to an interfacial contribution are discussed. The oxygen concentration variation (Po2) has a noticeable influence on the interfacial relaxation of La2Mo2O9 (LMO). A more resistive LMO in the low frequency range with nitrogen treatment is observed differing from the previous conclusions. Besides, an ion dynamic study has been performed and the two sets of overlapping Z″ and M″ peaks in the intermediate temperature range indicate an additional oxide ion jump path in W-doped LMO ceramics.

Stability and structure prediction of cubic phase in as cast high entropy alloys

According to the experimental dataset on the as cast high entropy alloys with cubic structure, the relationship between the stability and physical parameters of cubic structure has been investigated from the view of the atomic size difference, mixing enthalpy, electronegativity difference and valence electron concentration in the present work to reveal the stability and structure formation in as cast high entropy alloys. The results indicated that the mixing enthalpy is the effective parameter that can predict the stability of solid solutions in as cast high entropy alloys. The atomic size difference, electronegativity difference and valence electron concentration also play important roles in the formation of body centred cubic and face centred cubic crystals. Moreover, the back propagation artificial neural network was established using data collected from the calculated solid solution physical parameters and the structure characteristic of as cast high entropy alloys, and the structure of alloys was predicted using this network. The results showed that this model can be used to predict the structure of as cast high entropy alloy accurately, and it can also serve as a guide for the design and application of as cast high entropy alloy.

Ab initiophase stability at high temperatures and pressures in the V-Cr system

The phase stability of vanadium metal and vanadium-chromium alloys at high temperatures and pressures is explored by means of first-principles electronic-structure calculations. Utilizing the self-consistent ab initio lattice dynamics approach in conjunction with density-functional theory, we show that pressure-induced mechanical instability of body-centered cubic vanadium metal, which results in formation of a rhombohedral phase at around 60--70 GPa at room temperature, will survive significant heating and compression. Furthermore, alloying with chromium decreases the temperature at which stabilization of the body-centered cubic phase occurs at elevated pressure.

Pressure-induced phase-transition sequence in CoF2: An experimental and first-principles study on the crystal, vibrational, and electronic properties

We report a complete structural study of CoF${}_{2}$ under pressure. Its crystal structure and vibrational and electronic properties have been studied both theoretically and experimentally using first-principles density functional theory (DFT) methods, x-ray diffraction, x-ray absorption at Co $K$-edge experiments, Raman spectroscopy, and optical absorption in the 0--80 GPa range. We have determined the structural phase-transition sequence in CoF${}_{2}$ and corresponding transition pressures. The results are similar to other transition-metal difluorides such as FeF${}_{2}$ but different to ZnF${}_{2}$ and MgF${}_{2}$, despite that the Co${}^{2+}$ size (ionic radius) is similar to Zn${}^{2+}$ and Mg${}^{2+}$. We found that the complete phase-transition sequence is tetragonal rutile ($P{4}_{2}/mnm$) $\ensuremath{\rightarrow}$ CaCl${}_{2}$ type (orthorhombic $Pnnm$) $\ensuremath{\rightarrow}$ distorted PdF${}_{2}$ (orthorhombic $Pbca$)+PdF${}_{2}$ (cubic $Pa\overline{3}$) in coexistence $\ensuremath{\rightarrow}$ fluorite (cubic $Fm\overline{3}m$) $\ensuremath{\rightarrow}$ cotunnite (orthorhombic $Pnma$). It was observed that the structural phase transition to the fluorite at 15 GPa involves a drastic change of coordination from sixfold octahedral to eightfold cubic with important modifications in the vibrational and electronic properties. We show that the stabilization of this high-pressure cubic phase is possible under nonhydrostatic conditions since ideal hydrostaticity would stabilize the distorted-fluorite structure (tetragonal $I4/mmm$) instead. Although the first rutile $\ensuremath{\rightarrow}$ CaCl${}_{2}$-type second-order phase transition is subtle by Raman spectroscopy, it was possible to define it through the broadening of the ${E}_{g}$ Raman mode which is split in the CaCl${}_{2}$-type phase. First-principles DFT calculations are in fair agreement with the experimental Raman mode frequencies, thus providing an accurate description for all vibrational modes and elastic properties of CoF${}_{2}$ as a function of pressure.

First-principles study of the structural stability of cubic, tetragonal and hexagonal phases in Mn3Z (Z=Ga, Sn and Ge) Heusler compounds

We investigate the structural stability and magnetic properties of the cubic, tetragonal and hexagonal phases of Mn3Z (Z=Ga, Sn and Ge) Heusler compounds using first-principles density-functional theory. We propose that the cubic phase plays an important role as an intermediate state in the phase transition from the hexagonal to the tetragonal phases. Consequently, Mn3Ga and Mn3Ge behave differently from Mn3Sn, because the relative energies of the cubic and hexagonal phases are different. This result agrees with experimental observations for these three compounds. The weak ferromagnetism of the hexagonal phase and the perpendicular magnetocrystalline anisotropy of the tetragonal phase obtained in our calculations are also consistent with experiment.

Nanosize stabilization of cubic and tetragonal phases in reactive plasma synthesized zirconia powders

Pure zirconium oxide powders with particle size 2–33 nm are synthesized by reactive plasma processing. Transmission electron microscopy investigation of these particles revealed size dependent behavior for their phase stabilization. The monoclinic phase is found to be stable when particle size is ≥20 nm; Tetragonal is found to be stabilized in the range of 7–20 nm and as the particle size decreases to 6 nm and less, the cubic phase is stabilized.

Growth and characterization of low yttria-doped fully cubic stabilized zirconia-based single crystals

For the first time, fully cubic stabilized zirconia-based single crystals with large nitrogen contents and optimized anion vacancy concentrations, interesting for new electrochemical devices such as nitrogen sensors, were prepared by a two-step approach. This two-step approach combines growth of nitrogen-free zirconia single crystals via the skull melting method and their subsequent direct nitridation to form cubic nitrogen doped zirconia-based single crystals. In the initial step, zirconia crystals with various yttria contents (2, 3, 4 and 6mol%) were prepared by the skull-melting technique. As determined by X-ray powder diffraction, tetragonal stabilized crystals were obtained, containing monoclinic segregations, when the yttria content of the grinded crystals was below 4mol%. Subsequently, the crystals were nitrided with nitrogen gas at 1500°C using different reaction times. Nitrogen content and crystallographic identity of the nitrided samples were investigated, revealing successful nitrogen incorporation and, hence, full cubic stabilization of the initial nitrogen-free samples also for the lowest yttria content. The degree of deviation from the cubic fluorite-type structure is discussed with respect to crystallographic parameters (‘pseudo-cubic factor’ etc.). The stabilization of cubic or tetragonal phases to ambient temperature is related to the anion vacancy concentration with respect to different impacts of the anion vacancies generated either by yttria or nitrogen incorporation (effective vacancy concentration [Veff]).

Comparative study of electronic structure of cubic and hexagonal phases of Re3W as non-centrosymmetric and centrosymmetric low-TC superconductors

The comparative study of structural properties, stability and electronic structure of cubic and hexagonal phases of the intermetallic Re3W, which have attracted recently interest as non-centrosymmetric and centrosymmetric low-TC superconductors, was performed by means of the first-principles calculations. The optimized lattice constants, cell volumes, theoretical density, formation energies and cohesive energies, as well as electronic densities of states are obtained and analyzed. We have found that the lowering of TC (as going from h-Re3W with TC ∼ 9.4 K to c-Re3W with TC ∼ 7.8 K) correlates with lowering of the near-Fermi density of states (of the mixed W 5d + Re 5d type with admixtures of (Re,W) 6s, 6p states) owing to lowering of contributions of W 5d and Re 6p states. Our estimations of average electron-phonon coupling constants λ show that these phases are near to the weak coupling limit, with some growth of constant λ for h-Re3W (λ ∼ 0.60) as compared with c-Re3W (λ ∼ 0.42).

First-principles study of hexagonal tungsten trioxide: Nature of lattice distortions and effect of potassium doping

A density functional theory study is reported on pure and potassium doped tungsten trioxide. The nature of lattice distortions in the hexagonal phase is analyzed and a new symmetry group is proposed. The structure and stability of cubic, monoclinic, and hexagonal phases is studied as a function of potassium doping and an approximate phase diagram is derived. K${}_{x}$WO${}_{3}$ undergoes a monoclinic to hexagonal phase transition at $x\ensuremath{\sim}3%$.

Effect of Pressure on Structural, Electronic and Elastic Properties of Cubic (Pm3m) SnTiO<sub>3</sub> Using First Principle Calculation

The electronic band structure, density of state and elastic properties of lead-free perovskite oxide SnTiO3 (ST) were investigated by employing first principles calculation using the Density Functional Theory (DFT) within local density approximation (LDA). The energy band gap was calculated from the separation between the Ti 3d (conduction band) and the maximum of O 2p (valence band). This gives an indirect band gap of 2.36 eV. The elastic constants and their pressure dependence were calculated up to 30 GPa and the independent elastic constants (C11, C12, and C44), bulk modules, B were obtained and analyzed. The results showed that SnTiO3 have a mechanical stability in cubic phase (Pm3m).

Scandia-stabilized zirconia doped with yttria: Synthesis, properties, and ageing behavior

Synthesis, ionic conductivity, and ageing behavior of [x Y2O3–(10−x) Sc2O3]–90 ZrO2 (x=2, 3, 4, and 5mol%) as well as 10–11mol% Sc2O3–ZrO2 electrolytes are presented. High dense homogeneous ceramic samples with fine microstructure were prepared from laser synthesized mixed nanopowders 10YSZ and 10ScSZ using low sintering temperatures of 1010–1300°C. Stabilization of cubic fluorite-type phase has been achieved for Y-doped ScSZ composites at these temperatures. It is shown that the DC conductivity of the Y-doped ScSZs decreases proportionally to the yttria content throughout the temperature range studied, from 500 to 900 °C. Ageing behavior of the electrolytes was studied at 850°C for 1200–2700h. Low resistivity degradation of Y-doped ScSZ not exceeded 10% during extended annealing for 2700h has been obtained for the samples sintered below 1100°C. High and stable conductivity has been obtained for 10ScSZ and 11ScSZ. Any decrease in their conductivity during extended annealing was not detected.

High-temperature phonon stabilization ofγ-uranium from relativistic first-principles theory

A microscopic explanation for temperature stabilization of the body-centered cubic (bcc) phase in the actinide metals is proposed. We show that for a prototype actinide, uranium, phonon-phonon interaction promotes bcc $\ensuremath{\gamma}$-U when heated, even though at low temperatures, it is mechanically a strongly unstable phase. Utilizing the recently developed self-consistent ab initio lattice dynamics (SCAILD) scheme in conjunction with highly accurate and fully relativistic density functional theory we obtain phonon dispersion and density of states that compare well with data acquired from inelastic neutron-scattering experiments. The investigation thus establishes that high-temperature lattice dynamics can be modeled from ab initio theory even for complex materials with substantial electron correlation including the actinides.

Interplay between Size, Composition, and Phase Transition of Nanocrystalline Cr3+-Doped BaTiO3 as a Path to Multiferroism in Perovskite-Type Oxides

Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr(3+)-doped BaTiO(3) nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium(III) doping was determined to be a dominant factor for destabilization of the tetragonal phase. A combination of magnetic and magneto-optical measurements revealed that nanocrystalline films prepared from as-synthesized paramagnetic Cr(3+)-doped BaTiO(3) nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 μ(B)/Cr(3+)), similarly to magnetically doped transparent conducting oxides. The observed ferromagnetism increases with decreasing constituent nanocrystal size because of an enhancement in the interfacial defect concentration with increasing surface-to-volume ratio. Element-specific XMCD spectra measured by scanning transmission X-ray microscopy (STXM) confirmed with high spatial resolution that magnetic ordering arises from Cr(3+) dopant exchange interactions. The results of this work suggest an approach to the design and preparation of multiferroic perovskite materials that retain the ferroelectric phase and exhibit long-range magnetic ordering by using doped colloidal nanocrystals with optimized composition and size as functional building blocks.

Thermal stability of the cubic phase in Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 - δ (BSCF)1

Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3 - δ (BSCF) is a material with excellent oxygen ionic and electronic transport properties reported by many research groups. In its cubic phase, this mixed ionic-electronic conducting (MIEC) perovskite is a promising candidate for oxygen permeation membranes. For this application, its long-term stability under operating conditions (especially temperature and oxygen partial pressure) is of crucial importance. The present work is focused on the thermal stability of the BSCF cubic phase in the targeted temperature range for applications (700…900 °C) in light of previous studies in literature reporting a reversible transition to a hexagonal phase somewhere below 900 °C. To this end, single phase cubic BSCF powders were annealed at different temperatures over varying periods of time. Phase composition was subsequently analysed by X-ray diffractometry (XRD) in order to determine both the temperature limit and the time-scale for the formation of the hexagonal phase. Additionally, the long-term behaviour of the electrical conductivity was examined on bulk samples at 700 °C, 800 °C and 900 °C over several hundreds of hours, showing a prolonged decrease at 800 °C. The decrease in electrical conductivity at this temperature was also examined on bulk samples with different grain sizes, showing a more pronounced decrease the smaller the average grain size. Coexistence of both phases (cubic and hexagonal) could also be shown for 700 °C, however with a different phase equilibrium than at 800 °C.

Anharmonic Stabilization of the High-Pressure Simple Cubic Phase of Calcium

The phonon spectrum of the high-pressure simple cubic phase of calcium, in the harmonic approximation, shows imaginary branches that make it mechanically unstable. In this Letter, the phonon spectrum is recalculated by using density-functional theory ab initio methods fully including anharmonic effects up to fourth order at 50 GPa. Considering that the perturbation theory cannot be employed with imaginary harmonic frequencies, a variational procedure based on the Gibbs-Bogoliubov inequality is used to estimate the renormalized phonon frequencies. The results show that strong quantum anharmonic effects make the imaginary phonons become positive even at zero temperature so that the simple cubic phase becomes mechanically stable, as experiments suggest. Moreover, our calculations find a superconducting T(c) in agreement with experiments and predict an anomalous behavior of the specific heat.

Role of oxygen vacancy and grain boundary energy in stability of tetragonal and cubic pure zirconia powders

The mechanical treatment of monoclinic zirconia in high energy planetary ball mill reduces the grain size, increases microstrain, causes phase transition to metastable nanostructured tetragonal or cubic phases and produce considerable amounts of amorphous phase. Annealing of ball milled zirconia in chlorine atmosphere causes amorphous phase to be crystallised and change into cubic and tetragonal phases and also produces oxygen vacancy in zirconia (ZrO2‐x). Transmission electron microscopy observations suggest that the cubic phase has 19 nm diameter at least and reveal that zirconia particles have plenty of grain boundaries although the surface area measurements (Brunauer–Emmett–Teller) of cubic phase showed a lower critical surface area. Under this condition, the grain boundary energy and oxygen vacancy have an important role in the stability of tetragonal and cubic phases. The chlorine atmosphere increases the stability temperature of cubic and tetragonal phases up to 800 and 1000°C respectively.

Alloying-Driven Phase Stability in Group-VB Transition Metals under Compression

ABSTRACTThe change in phase stability of Group-VB (V, Nb, and Ta) transition metals due to pressure and alloying is explored by means of first-principles electronic-structure calculations. It is shown that under compression stabilization or destabilization of the ground-state body-centered cubic (bcc) phase of the metal is mainly dictated by the band-structure energy that correlates well with the position of the Kohn anomaly in the transverse acoustic phonon mode. The predicted position of the Kohn anomaly in V, Nb, and Ta is found to be in a good agreement with data from the inelastic x-ray or neutron scattering measurements. In the case of alloying the change in phase stability is defined by the interplay between the band-structure and Madelung energies. We show that band-structure effects determine phase stability when a particular Group-VB metal is alloyed with its nearest neighbors within the same d-transition series: the neighbor with less and more d electrons destabilize and stabilize the bcc phase, respectively. When V is alloyed with neighbors of a higher (4d- or 5d-) transition series, both electrostatic Madelung and band-structure energies stabilize the body-centered-cubic phase. The opposite effect (destabilization) happens when Nb or Ta is alloyed with neighbors of the 3d-transition series.

Extended phase diagram of the three-dimensional phase field crystal model

We determine the phase diagram of the phase field crystal model in three dimensions byusing numerical free energy minimization methods. Previously published results,based on single mode approximations, have indicated that in addition to theuniform (liquid) phase, there would be regions of stability of body-centered cubic,hexagonal and stripe phases. We find that in addition to these, there are also regionsof stability of face-centered cubic and hexagonal close packed structures in thismodel.

Pressure-dependent stability of cubic and wurtzite phases within the TiN–AlN and CrN–AlN systems

Ab initio calculations are employed to demonstrate a strong pressure dependence of the maximum AlN mole fraction preserving the cubic phase in Ti1−xAlxN and Cr1−xAlxN hard coatings before transforming to the wurtzite structure. Under a compression of 4 GPa an increase by ∼0.1 AlN mole fraction is obtained for both systems. Consequently, stress-related effects cannot be neglected in the discussion of the wide spread of experimentally obtained stability ranges.