Non-cubic Structure Research Articles

Switching Lead for Tin in PbHfO3: Noncubic Structure of SnHfO3**

AbstractThe removal of lead from commercialized perovskite‐oxide‐based piezoceramics has been a recent major topic in materials research owing to legislation in many countries. In this regard, Sn(II)‐perovskite oxides have garnered keen interest due to their predicted large spontaneous electric polarizations and isoelectronic nature for substitution of Pb(II) cations. However, they have not been considered synthesizable owing to their high metastability. Herein, the perovskite lead hafnate, i.e., PbHfO3 in space group Pbam, is shown to react with SnClF at a low temperature of 300 °C, and resulting in the first complete Sn(II)‐for‐Pb(II) substitution, i.e. SnHfO3. During this topotactic transformation, a high purity and crystallinity is conserved with Pbam symmetry, as confirmed by X‐ray and electron diffraction, elemental analysis, and 119Sn Mössbauer spectroscopy. In situ diffraction shows SnHfO3 also possesses reversible phase transformations and is potentially polar between ≈130–200 °C. This so‐called ‘de‐leadification’ is thus shown to represent a highly useful strategy to fully remove lead from perovskite‐oxide‐based piezoceramics and opening the door to new explorations of polar and antipolar Sn(II)‐oxide materials.

Switching Lead for Tin in PbHfO3 : Noncubic Structure of SnHfO3.

The removal of lead from commercialized perovskite-oxide-based piezoceramics has been a recent major topic in materials research owing to legislation in many countries. In this regard, Sn(II)-perovskite oxides have garnered keen interest due to their predicted large spontaneous electric polarizations and isoelectronic nature for substitution of Pb(II) cations. However, they have not been considered synthesizable owing to their high metastability. Herein, the perovskite lead hafnate, i.e., PbHfO3 in space group Pbam, is shown to react with SnClF at a low temperature of 300 °C, and resulting in the first complete Sn(II)-for-Pb(II) substitution, i.e. SnHfO3 . During this topotactic transformation, a high purity and crystallinity is conserved with Pbam symmetry, as confirmed by X-ray and electron diffraction, elemental analysis, and 119 Sn Mössbauer spectroscopy. In situ diffraction shows SnHfO3 also possesses reversible phase transformations and is potentially polar between ≈130-200 °C. This so-called 'de-leadification' is thus shown to represent a highly useful strategy to fully remove lead from perovskite-oxide-based piezoceramics and opening the door to new explorations of polar and antipolar Sn(II)-oxide materials.

Towards synthetic L10-FeNi: Detecting the absence of cubic symmetry in Laser-Ablated Fe-Ni nanoparticles

The L10 crystal structure underlines an important class of chemically ordered alloys that exhibits uniaxial magnetocrystalline anisotropy. The near-equiatomic L10-FeNi extracted from meteorites has demonstrated intriguing magnetic properties for permanent magnet applications. However, the synthesis of this chemically ordered non-cubic structure has been a longstanding challenge. Here, we demonstrate the absence of cubic symmetry in near-equiatomic Fe-Ni nanoparticles synthesized by picosecond-pulsed laser ablation in liquids. The non-cubic phase detected in these particles can only be L10-FeNi or hexagonal close-packed (HCP) FeNi, and the absence of cubic symmetry was unequivocal. The orientation relationship between the non-cubic phase and the adjacent cubic phase was characterized by a series of transmission electron microscopy (TEM) techniques, which consistently suggests that the formation of the non-cubic phase involves a martensitic transformation process.

Multifunctional character of revived La-modified lithium titanate electrolyte: solar cell devices at a glance

The multi-functional material Li2LaFeTiO6 belonging to double perovskite family is expected to be non-cubic structure and interaction between La and Fe introduce octahedral B-site cation dis-order or anti-site effect. The material is prepared by conventional solid state reaction technique and its formation has been checked through X-ray diffraction (XRD). It is observed that material is orthorhombic crystal structure with Pbnm space group. Scanning electron microscope (SEM) picture of the surface uncovers distinctive shape and sizes of the grains are consistently appropriated all through the surface leaving not many voids. Energy dispersive spectroscopy (EDX) study indicates there is no impurities are seen in the present material. Fourier transform infrared (FTIR) spectroscopy revels the perovskite phase and different modes of vibration. The room temperature UV–Visible spectroscopic analysis reveals material is a potential candidate photovoltaic/solar cell device. The dielectric and electrical measurement is carried out using computer control LCR analyzer. The room temperature dielectric constant and loss tangent of the material is respectively > 400 and < 0.1 which reflects it can be used for multi-layer capacitor and micro-electronics.

Garnet, the archetypal cubic mineral, grows tetragonal

Garnet is the archetypal cubic mineral, occurring in a wide variety of rock types in Earth’s crust and upper mantle. Owing to its prevalence, durability and compositional diversity, garnet is used to investigate a broad range of geological processes. Although birefringence is a characteristic feature of rare Ca–Fe3+ garnet and Ca-rich hydrous garnet, the optical anisotropy that has occasionally been documented in common (that is, anhydrous Ca–Fe2+–Mg–Mn) garnet is generally attributed to internal strain of the cubic structure. Here we show that common garnet with a non-cubic (tetragonal) crystal structure is much more widespread than previously thought, occurring in low-temperature, high-pressure metamorphosed basalts (blueschists) from subduction zones and in low-grade metamorphosed mudstones (phyllites and schists) from orogenic belts. Indeed, a non-cubic symmetry appears to be typical of common garnet that forms at low temperatures (<450 °C), where it has a characteristic Fe–Ca-rich composition with very low Mg contents. We propose that, in most cases, garnet does not initially grow cubic. Our discovery indicates that the crystal chemistry and thermodynamic properties of garnet at low-temperature need to be re-assessed, with potential consequences for the application of garnet as an investigative tool in a broad range of geological environments.

Effect of Cr, C, B and Mo substitutions on the structure and magnetic properties of Zr-Co Rare-Earth-free magnetic alloy

Acquiring new permanent magnets without Rare-Earths is a new challenge in the field of magnetic materials and Co-rich Co-Zr alloys seem to be a solution to be pursued. The microalloying effects of different elements on the structure and stabilization of hard magnetic Zr2Co11 phase of Zr-Co alloy have been analysed in the past and it was shown that the Zr2Co11 non-cubic high anisotropy structure can be stabilized by the partial substitution of Co in the ZrCo alloy with elements with high atomic radius. In the present work, we made substitutions of up to 5% alloying elements such as Mo, C, B, Cr in order to highlight how the element with the small / large atomic radius stabilize the nano-hard magnetic phase. The structural and magnetic properties of the new Zr18Co77Cr3X2 (X = C, B, Mo) prepared bulk alloys have been analysed and related to those of the original Zr13Co87.

Aimsgb: An algorithm and open-source python library to generate periodic grain boundary structures

An algorithm implemented in an open-source python library was developed for building periodic grain boundary models in a universal fashion. The software framework, aimsgb, aims to generate tilt and twist grain boundaries from an input cubic or non-cubic crystal structure for ab-initio and classical atomistic simulation. It can output a coincidence site lattice (CSL) grain boundary for a cubic input structure and a non-CSL grain boundary for a non-cubic input structure. This framework has two useful features: (i) it can calculate all the CSL matrices for generating CSL from a given Sigma (Σ) value and rotation axis, allowing the users to build the specific CSL and grain boundary models; (ii) it provides a convenient command line tool to enable high-throughput generation of tilt and twist grain boundaries by assigning an input crystal structure, Σ value, rotation axis, and grain boundary plane. The developed algorithm in the open-source python library is expected to facilitate studies of grain boundary in materials science. The software framework is available on the website: aimsgb.org.

Nanocrystalline buildup, relaxor behavior, and polarization characteristic in PZT–PNZN quaternary ferroelectrics

AbstractThe Pb(Zr0.47Ti0.53)O3–Pb[(Ni0.6Zn0.4)1/3Nb2/3]O3 (PZT–PNZN) quaternary ferroelectrics presenting the high piezoelectric coefficients are the active element in a range of piezoelectric devices. However, the buildup of nanoceramics with stable perovskite structure is still a big challenge, which prohibits the miniaturization of piezoelectric devices. In this work, the high dense PZT–PNZN nanoceramics with average grain size of 130 nm were prepared by a combination of high‐energy ball milling (HEBM) and spark plasma sintering (SPS). The domain configuration and relaxor behavior in this nanoceramic are reported for the first time. Although the noncubic crystal structure and dielectric anomaly suggest a ferroelectric behavior, only very slim polarization–electric (P–E) hysteresis loops are observed. The suppression of macroscopic ferroelectric polarization mainly originates from the decrease in crystal tetragonality and the enhanced clamping effect on domain wall motion. The piezoelectric properties, d33=65 pC/N and kp=17%, raise the possibility of PZT–PNZN nanoceramics for application in multilayer piezoelectric devices.

Controlling the microstructure and associated magnetic properties of Ni0.2Mn3.2Ga0.6 melt-spun ribbons by annealing

Here we report on the structural and magnetic properties of Ni0.2Mn3.2Ga0.6 melt-spun ribbons. The as-spun ribbons were found to exhibit mixed cubic phases that transform to non-cubic structure upon annealing. Additionally, an amorphous phase was found to co-exist in all ribbons. The SEM images show that minor grain formation occurs on the as-spun ribbons. However, the formation of extensive nano-grains was observed on the surfaces of the annealed ribbons. While the as-spun ribbons exhibit predominantly paramagnetic behavior, the ribbons annealed under various thermal conditions were found to be ferromagnetic with a Curie temperature of about 380 K. The ribbons annealed at 450 °C for 30 minutes exhibit a large coercive field of about 2500 Oe. The experimental results show that the microstructure and associated magnetic properties of the ribbons can be controlled by annealing techniques. The coercive fields and the shape of the magnetic hysteresis loops vary significantly with annealing conditions. Exchange bias effects have also been observed in the annealed ribbons.

Phenomenological theory of bulk diffusion in metal oxides

Phenomenological description of bulk diffusion in oxide ceramics has been proposed. Variants of vacancy and vacancy-free diffusion models have been considered. In the vacancy models, ion migration is described as a fluctuation with the formation of a “liquid corridor,” along which the diffusion ion transport in a “melt” is performed, or, as a fluctuation with the formation of an “empty corridor,” in which the ion motion proceeds without activation. The vacancy-free model considers a fluctuation with the formation of a spherical liquid region whose sizes correspond to the first coordination sphere. It has been shown that both the vacancy models work in cubic metal oxides and the vacancy-free model is effective for describing diffusion in oxides having a noncubic structure. Detailed comparison of the models developed has been performed. It has been shown that the values of the activation energies for diffusion of metal and oxygen ions agree with the published data on bulk diffusion in stoichiometric oxide ceramics.

Transparent conducting properties of SrSnO3 and ZnSnO3

We report optical properties of doped n-type SrSnO3 and ZnSnO3 in relation to potential application as transparent conductors. We find that the orthorhombic distortion of the perovskite structure in SrSnO3 leads to absorption in the visible as the doping level is increased. This arises from interband transitions. We find that strain tuning could modify this absorption, but does not eliminate it. On the other hand, we find that ZnSnO3 although also having a non-cubic structure, can retain excellent transparency when doped, making it a good candidate transparent conductor.

Effect of Microstructure on Transmission Properties of Polycrystalline Transparent Ceramics

The transmission properties of polycrystalline transparent ceramics are influenced by the chemical composition and the microstructure of the material. The fundamental optical mechanism of transparent ceramics and the influencing factors on transmission properties were discussed in this paper. The Mie theory for light scattering is applied to calculate scattering coefficients of residual pores and optical birefringence. The in-line transmission curves of the transparent alumina were calculated as a function of pore size, porosity and grain size. The results show that scattering by the residual pores is the dominant influencing factors on transmission properties. The scattering by the residual pores increases with an increase in porosity and the maximum of the scattering was observed when the pore size close to the optical wavelength. Optical birefringence in the transparent ceramics with non-cubic crystal structure has an important effect on the in-line transmission. The in-line transmission increases with a decrease in grain size. Keywords: transparent ceramics; microstructure; light scattering; birefringence

Local and Average Structures and Magnetic Properties of Sr2FeMnO5+y, y = 0.0, 0.5. Comparisons with Ca2FeMnO5 and the Effect of the A-Site Cation

Sr(2)FeMnO(5+y) was synthesized under two different conditions, in air and in argon, both of which resulted in a cubic, Pm ̅3m, structure with no long-range ordering of oxygen vacancies. The unit cell constants were found to be a(0) = 3.89328(1) Å for argon (y = 0.0) and a(0) = 3.83075(3) Å for air (y = 0.5). In contrast, Ca(2)FeMnO(5) retains long-range brownmillerite oxygen vacancy ordering for either air or argon synthesis. Remarkably, Sr(2)FeMnO(5.0) oxidizes spontaneously in air at room temperature. A neutron pair distribution function (NPDF) study of Sr(2)FeMnO(5.0)(Ar) showed evidence for local, brownmillerite-like ordering of oxygen vacancies for short distances up to 5 Å. Mössbauer spectroscopy results indicate more than one Fe site for Sr(2)FeMnO(5+y)(Ar and air), consistent with the noncubic local structure found by NPDF analysis. The isomer shifts and quadrupole splittings in both air- and argon-synthesized materials are consistent with the 3+ oxidation state for Fe in sites with coordination number four or five. This is confirmed by an L-edge XANES study. Mn is almost entirely in the 3+ state for Sr(2)FeMnO(5.0)(Ar), whereas Mn(4+) is predominantly present for Sr(2)FeMnO(5.5)(air). Magnetic susceptibility data show zero-field-cooled/field-cooled (ZFC/FC) divergences near 50 K for the Ar sample and 25 K for the air sample, whereas Ca(2)FeMnO(5) is long-range G-type antiferromagnetically ordered at 407(2) K. Hyperfine magnetic splitting, observed in temperature-dependent Mössbauer measurements, indicates short-range magnetic correlations that persist up to 150 K for Sr(2)FeMnO(5.0)(Ar) and 100 K for Sr(2)FeMnO(5.5)(air), well above the ZFC/FC divergence temperatures. Neutron diffraction data confirm the absence of long-range magnetic ordering at room temperature and 4 K for Sr(2)FeMnO(5.0)(Ar) but indicate the presence of domains with short-range G-type order at 4 K with an average dimension of ∼50 Å (y = 0); thus, this material is actually a superparamagnet rather than a true spin glass. In sharp contrast, corresponding data for Sr(2)FeMnO(5.5)(air) show mainly a very weak magnetic Bragg peak, indicating that ∼4% of the sample has G-type antiferromagnetic ordering at 4 K.

Yttrium doped BSCF membranes for oxygen separation

This work investigates the partial substitution of yttrium in place of iron in BSCF to form Ba 0.5Sr 0.5Co 0.8Fe 0.2− x Y x O 3− δ , where x varied between 0 and 0.2. X-ray diffraction patterns showed the formation of a perovskite cubic phase structure up to x = 0.15, whilst the full substitution of yttrium ( x = 0.2) for iron resulted in a non-cubic crystal structure that did not exhibit oxygen permeation. The yttrium partial substitution in BSCFY discs (1.2 mm thick) delivered best results for x = 0.025, as oxygen fluxes reached 2.05 ml cm −2 min −1 at 900 °C, an increase of 160% as compared to a blank BSCF ( x = 0) membrane. This was attributed to the combined effect of the formation of a greater number of oxygen vacancies, together with improved ion mobility, associated with the beneficial yttrium substitution into the BSCF perovskite structure which stemmed from the crystal lattice expansion.

Bi-doping effects on the structure and oxygen permeation properties of BaSc 0.1Co 0.9O 3− δ perovskite membranes

In this work, we investigate the effect of partial substitution of bismuth oxide on BaSc 0.1Co 0.9O 3− δ perovskite membranes. Doping was carried out in the A-site (Ba 1− x Bi x Sc 0.1Co 0.9O 3− δ ) and B-site (BaBi x Sc 0.1Co 0.9− x O 3− δ ). Bi doping at or below 10% ( x ≤ 0.10) in both sites resulted in significant increase of oxygen flux, up to two orders of magnitude at the low to medium temperature range (650–850 °C) as compared to non-doped disk membranes. Among all compositions, B-site doped x = 0.05 showed the highest oxygen fluxes, reaching 2.17 ml min −1 cm −2 at 950 °C. These results suggest that Bi doping at or below 10% conferred superior ionic oxygen diffusion transport, owing to the formation of cubic structures. Further increases in the Bi-doping amount formed non-cubic structure delivering very low oxygen fluxes. The structure transition phenomena from non-cubic to cubic crystal lattices occurred for non-doped and A-site doped membrane with 20 and 30% mole bismuth oxide in excess of 800 °C, concomitantly with increases in oxygen fluxes and sharp rise of electrical conductivity. A-site and B-site doped x = 0.05 compounds exposed to nitrogen atmosphere at 850 °C for 7 days showed reasonable stable structure.

On screening for Special Nuclear Materials (SNMs) with X-ray diffraction

A novel detection technique employing X-ray diffraction (XRD) to screen for Special Nuclear Materials (SNMs), in particular for uranium, has been recently proposed. It is based on the interesting fact that uranium (and incidentally, plutonium) has a non-cubic lattice structure, in contrast to all other non-SNM, high-density elements of the Periodic Table. The principle of this screening technique is briefly elucidated by comparing the XRD lines of uranium with those of lead, a material of high atomic number ( Z) commonly found in container traffic. Several physical conditions that must be satisfied to enable XRD for SNM screening are considered. To achieve adequate penetration, both of suspicious high- Z materials and their containers, photon energies of 1 MeV and above must be employed. Implications from partial coherence theory for the XRD measurement geometry at such photon energies are presented. The question of multiple scatter degradation of the coherent scatter signal is addressed. Technological considerations relevant to performing XRD at 1 MeV, particularly regarding the radiation source and detector, are discussed. A novel secondary aperture scheme permitting high energy XRD is presented. It is concluded that the importance of the application and the prospect of its feasibility are sufficient to warrant experimental verification.

The Synthesis and Structures of Elpasolite Halide Scintillators

AbstractLow-cost, high-performance gamma-ray spectrometers are urgently needed for nonproliferation and homeland security applications. Available scintillation materials fall short of the requirements for energy resolution and sensitivity at room temperature. The emerging lanthanide halide based materials, while having the desired luminosity and proportionality, have proven difficult to produce in the large sizes and low cost required due to highly anisotropic properties caused by the non-cubic crystal structure. New cubic materials, such as the recently discovered elpasolite family (A2BLnX6; Ln-lanthanide and X-halogen), hold promise for scintillator materials due to their high light output, proportionality, and toughness. The isotropic nature of the cubic elpasolites leads to minimal thermomechanical stresses during single-crystal solidification, and eliminates the problematic light scattering at the grain boundaries. Therefore, it may be possible to produce these materials in large sizes as either single crystals or transparent ceramics with high production yield and reduced costs. In this study, we investigated the “cubic” elpasolite halide synthesis and studied the structural variations of four different compounds, including Cs2NaLaBr6, Cs2LiLaBr6, Cs2NaLaI6, and Cs2LiLaI6. Attempts to produce a large-area detector by a hot forging technique were explored.

Local Structural Origins of the Distinct Electronic Properties of Nb-SubstitutedSrTiO3andBaTiO3

The perovskite SrTiO3 becomes metallic with 0.03% to 0.1% Nb substitution on the Ti site, while BaTiO3 remains insulating above 10% Nb substitution. Given the nearly identical structure and electron counts of the two materials, the distinct ground states for low substitution have been a long-standing puzzle. Here we find from neutron studies of average and local structure the subtle yet critical difference that we believe underpins the distinct electronic properties in these fascinating materials. While SrTi0.875Nb0.125O3 possesses a distorted noncubic structure at 15 K, (Nb/Ti)O6 octahedra in the structure are regular. BaTi0.875Nb0.125O3, on the other hand, shows evidence for local cation off centering while retaining a cubic structure.

Ab initiostudy of the physical properties ofγ-Al2O3: Lattice dynamics, bulk properties, electronic structure, bonding, optical properties, and ELNES/XANES spectra

Based on the most recently determined noncubic structure for $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ by Menendez-Proupin and Gutierrez, a comprehensive list of physical properties is investigated theoretically. These include lattice dynamics and phonon spectra, elastic constants and bulk structural parameters, electronic structure and interatomic bonding, optical properties, and x-ray absorption near-edge structure (XANES) spectra. Compared to similar calculations of $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$, we find a smaller lowest zone-center vibrational mode at $97.6\text{ }{\text{cm}}^{\ensuremath{-}1}$, a lower heat capacity, a smaller bulk modulus, and a much larger thermal-expansion coefficient. The threefold bonded O ions introduce highly localized vibrational modes near $751\text{ }{\text{cm}}^{\ensuremath{-}1}$. The calculated thermal Gr\"uneisen parameter indicates a strong anharmonicity in $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$. The elastic tensor and the elastic wave velocities are also evaluated showing the longitudinal wave to be nearly isotropic. For the electronic structure, we find that $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ has a smaller band gap but a refractive index similar to $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$. Highly localized states at the top of the valence band originating from threefold bonded O in the more covalently bonded ${\text{AlO}}_{4}$ tetrahedra are identified. The calculated Mulliken effective charges and bond order values indicate that the structural model for $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ has a high degree of disorder. The octahedral unit $({\text{AlO}}_{6})$ is a stronger polyhedron than the tetrahedral unit $({\text{AlO}}_{4})$ although the latter has stronger Al--O bonds. The calculated $\text{Al-}K$, $\text{Al-}{L}_{3}$, and $\text{O-}K$ edges for Al and O in $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ show strong dependence on their local coordination and environments. These results are in good agreement with available experimental data but the effect of the $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ samples' porosity should be properly assessed. It is argued that the traditional view that stoichiometric $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ is a defective spinel with cation vacancies (or its variations) should be modified. $\ensuremath{\gamma}{\text{-Al}}_{2}{\text{O}}_{3}$ is better described as an amorphous networklike structure such that the ratio of tetrahedrally coordinated Al to octahedrally coordinated Al is close to 0.6; and the O ions are bonded to Al in either a threefold or fourfold configurations in about equal proportion.

A Self-Consistent Substrate Thermal Profile Estimation Technique for Nanoscale ICs—Part I: Electrothermal Couplings and Full-Chip Package Thermal Model

As CMOS technology scales to nanometer regime, power dissipation issues and associated thermal problems have emerged as critical design concerns in most high-performance integrated circuits (ICs) including microprocessors. In this scenario, accurate estimation of the silicon junction (substrate or die) temperature is crucial for various performance analyses and chip-level thermal management. This paper introduces the notion of self-consistency in the junction temperature estimation by taking into account various electrothermal couplings between chip power, average junction temperature, operating frequency, and supply voltage. The self-consistent solutions of the average junction temperature are shown to have significant implications for various chip-level power, performance, reliability, and cooling cost tradeoffs. Moreover, a realistic package thermal model is introduced that comprehends different packaging layers and noncubic structure of the package, which are not accounted for in traditional analyses. The model is subsequently incorporated in the self-consistent substrate thermal profile estimation, which is discussed in Part II with implications for power estimation and thermal management in nanometer-scale CMOS technologies.