Showing papers on "Tetragonal crystal system published in 2007"

Crystal structures and elastic properties of superhard IrN2 and IrN3 from first principles

Abstract: First principles calculations were performed to investigate the structural, elastic, and electronic properties of IrN2 for various space groups: cubic Fm-3m and Pa-3, hexagonal P3(2)21, tetragonal P4(2)/mnm, orthorhombic Pmmn, Pnnm, and Pnn2, and monoclinic P2(1)/c. Our calculation indicates that the P2(1)/c phase with arsenopyrite-type structure is energetically more stable than the other phases. It is semiconducting (the remaining phases are metallic) and contains diatomic N-N with the bond distance of 1.414 A. These characters are consistent with the experimental facts that IrN2 is in lower symmetry and nonmetallic. Our conclusion is also in agreement with the recent theoretical studies that the most stable phase of IrN2 is monoclinic P2(1)/c. The calculated bulk modulus of 373 GPa is also the highest among the considered space groups. It matches the recent theoretical values of 357 GPa within 4.3% and of 402 GPa within 7.8%, but smaller than the experimental value of 428 GPa by 14.7%. Chemical bonding and potential displacive phase transitions are discussed for IrN2. For IrN3, cubic skutterudite structure (Im-3) was assumed.

Phase structures and electrical properties of new lead-free (Na0.5K0.5)NbO3–(Bi0.5Na0.5)TiO3 ceramics

Abstract: Highly dense (1−x)(Na0.5K0.5)NbO3–x(Bi0.5Na0.5)TiO3 (NKN-BST) solid solution piezoelectric ceramics have been fabricated by ordinary sintering. All compositions show pure perovskite structures, showing room-temperature symmetries of orthorhombic at x⩽0.02, of tetragonal at 0.03⩽x⩽0.09, of cubic at 0.09 0.20. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was identified in the composition range of 0.02

Phase transitional behavior in K0.5Na0.5NbO3–LiTaO3 ceramics

Abstract: In addition to x-ray diffraction, Raman spectrum measurements provide direct evidence of the tetragonal and orthorhombic phases coexistence in lead-free ceramics (1−x)K0.5Na0.5NbO3 (KNN)–xLiTaO3 when x equals 5mol%. This is caused by the phase transition temperature between tetragonal and orthorhombic decreasing to around room temperature due to the Li and Ta doping in KNN, and not by constituting a region of morphotropic phase boundary as presented by most published papers. The results indicate that although this kind of ceramic displays good properties, it needs further study to verify if it is suitable to be used in a varying temperature environment.

Transparent nanometric cubic and tetragonal zirconia obtained by high-pressure pulsed electric current sintering

Abstract: Transparent samples of cubic (8 mol % yttria) and tetragonal (3 mol % yttria) zirconia were prepared from nanometric powders by the pulsed electric current sintering process. The crystallite size of the resulting dense samples was about 50 nm in both cases. The consolidation pressure had a positive effect on the occurrence of transparency for both modifications. Transmittance in the near infrared for 1 mm thick samples is above the 60 % for the cubic (8 %YSZ) and above 50 % for the tetragonal (3 % YSZ) zirconia, representing between 70 and 80 % of the theoretical values of the two modifications. Samples had a yellowish-brown coloration which was attributed to the presence of color centers. Annealing in oxygen improved transmittance initially, but prolonged annealing resulted in translucent samples. The role of porosity in transmittance is analyzed.

Analysis of crystallographic evolution in (Na,K)NbO3-based lead-free piezoceramics by x-ray diffraction

Abstract: The crystallographic structure of (Na,K)NbO3-based compounds and phase transitional behavior in (1−x)(K0.5Na0.5)NbO3–xLiNbO3 (x=0, 3, 6, and 8mol%) ceramics were accurately determined using high resolution x-ray diffraction (XRD). The phase transition between orthorhombic and tetragonal was distinctly characterized focusing on the XRD peak changes in {222} and {400} diffraction planes, by which lattice parameters of the perovskite type subcell, a, b, and c as well as angle β can be calculated. The discontinuous change in angle β at x=6mol% is located at the phase transitional region, where enhanced piezoelectric constant d33=215pC∕N was obtained. The present study provides a precise as well as simple method to investigate crystallographic structures in (Na,K)NbO3-based ceramics.

Raman scattering studies on nanocrystalline BaTiO3 Part I—isolated particles and aggregates

Abstract: BaTiO3 powders with various crystallite sizes, which were prepared through microemulsion-mediated synthesis, were thoroughly studied by Raman spectroscopy. Clear evidence for the presence of the tetragonal phase was found for ultrafine powders with an average crystallite size above 30 nm. The lifetime of phonons that are specifically assigned to the tetragonal phase decreased with decreasing crystallite size below a critical size of 100 nm. In particles as fine as 100 nm, the short mean free path of phonons, mainly due to internal pressure, causes decoupling of the coupled A1 (TO) phonons and a diffuse phase transition behaviour (TC = 115 °C). Coupled A1(TO) phonons, which give a spectral dip at around 180 cm−1 and a lesser extent of diffuseness, were revealed for powders consisting of particles as large as 0.17 µm (TC = 123 °C). Further coarsening upon annealing induced the formation of aggregates, resulting in the shift of phase transition points to higher temperatures for the rhombohedral to orthorhombic and the orthorhombic to tetragonal transitions and to lower temperatures for the tetragonal to cubic transition, respectively. Phase stability in powders is discussed by considering possible factors such as internal pressure in isolated particles and internal stress in aggregates. Copyright © 2007 John Wiley & Sons, Ltd.

Structural, Optical, and Magnetic Properties of Co-doped SnO2 Powders Synthesized by the Coprecipitation Technique

Abstract: We have used the coprecipitation technique to synthesize polycrystalline Co-doped SnO2 diluted magnetic semiconductors with Co concentrations of 0, 0.5, and 2.0%. X-ray diffraction patterns showed for all samples the expected SnO2 tetragonal structure with no additional peaks corresponding to parasitic phases. Transmission electron microscopy (TEM) did not indicate the presence of magnetic parasitic phases and confirmed that Co ions are uniformly distributed inside the samples. Optical absorption measurements showed an energy band gap which decreases when increasing the Co concentration. Raman spectroscopy shows an intensity loss of classical cassiterite SnO2 vibration lines, which is an indication of significant structural modifications and disorder of the SnO2 lattice. Magnetization measurements revealed a mixture of paramagnetic and antiferromagnetic behavior for Co-doped SnO2 with no sign of ferromagnetism.

Structural distortion and magnetism of BiFeO3 epitaxial thin films: A Raman spectroscopy and neutron diffraction study

Abstract: A previous study of the growth conditions has shown that single-phase BiFeO3 thin films can only be obtained in a narrow pressure–temperature window and that these films display a weak magnetic moment. Here, we study in more detail the structure and the magnetism of single-phase BiFeO3 films by means of reciprocal space mapping, Raman spectroscopy and neutron diffraction. X-ray and Raman data suggest that the BiFeO3 structure is tetragonal for 70 nm thick films and changes to monoclinic for 240 nm thick films, thus remaining different from that of the bulk (rhombohedral) structure. In the 240 nm monoclinically distorted film, neutron diffraction experiments allow the observation of a G-type antiferromagnetic order as in bulk single crystals. However, the satellite peaks associated with the long-wavelength cycloid present in bulk BiFeO3 are not observed. The relevance of these findings for the exploitation of the magnetoelectric properties of BiFeO3 is discussed.

Structural and electrical properties of Ca2+-modified PZT electroceramics

Abstract: A perovskite ceramic oxide having the general formula Pb0.97Ca0.03Zr0.05Ti0.95O3 (PCZT) was prepared using precursor solution method. The compound belongs to the tetragonal system. TEM micrographs show that the particles are more or less spherical in shape. Complex impedance analysis indicates the presence of grains separated by grain boundaries, which are also evident from the SEM micrographs. The sample impedance behavior has been explained in terms of the bricklayer model. The relaxation process taking place is short range. The conductivity variation shows a typical Arrhenius-type behavior having linear dependence on logarithm of conductivity.

A study of the phase diagram of (K,Na,Li)NbO3 determined by dielectric and piezoelectric measurements, and Raman spectroscopy

Abstract: A composition-temperature phase diagram of the system (1−x)(K0.5Na0.5)NbO3–xLiNbO3 is presented for 0⩽x⩽0.1. Using dielectric and piezoelectric resonance measurements, and Raman spectroscopy, ceramic samples containing 2%–10% LiNbO3 were studied over a temperature range of 7–770K showing a complex sequence of phase transitions. Analysis of the different Raman, piezoelectric, and dielectric data shows distinct transitions from cubic to tetragonal to orthorhombic to rhombohedral phase for x=0.02–0.05. The symmetries of the phases were assigned using analogy to phase diagram of (K0.5Na0.5)NbO3 single crystals and ceramics (x=0). At x>0.07 only one transition between ferroelectric phases occurs where tetragonal phase transforms to another phase, possibly of rhombohedral, orthorhombic, or monoclinic symmetry. In the region between x=0.05 and x=0.08, the phase transition sequence is more complex. Below 100K this phase with unidentified symmetry creates a vertical boundary with the rhombohedral phase present nea...

Phase Transformation and Photoluminescence Properties of Nanocrystalline ZrO2 Powders Prepared via the Pechini-type Sol−Gel Process

Abstract: Nanocrystalline ZrO2 fine powders were prepared via the Pechini-type sol−gel process followed by annealing from 500 to 1000 °C. The obtained ZrO2 samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR), and photoluminescence spectra (PL), respectively. The phase transition process from tetragonal (T) to monoclinic (M) was observed for the nanocrystalline ZrO2 powders in the annealing process, accompanied by the change of their photoluminescence properties. The 500 °C annealed ZrO2 powder with tetragonal structure shows an intense whitish blue emission (λmax = 425 nm) with a wide range of excitation (230−400 nm). This emission decreased in intensity after being annealed at 600 °C (T + M-ZrO2) and disappeared at 700 (T + M-ZrO2), 800 (T + M-ZrO2), and 900 °C (M-ZrO2). After further annealing at 1000 °C (M-ZrO2), a strong blue-green emission appeared again (λmax = 470 nm). Based o...

Structural distortion and magnetism of BiFeO3 epitaxial thin films: a Raman spectroscopy and neutron diffraction study

Abstract: A previous study of the growth conditions has shown that single-phase BiFeO3 thin films can only be obtained in a narrow pressure-temperature window and that these films display a weak magnetic moment. Here we study in more detail the structure and the magnetism of single-phase BiFeO3 films by means of reciprocal space mapping, Raman spectroscopy and neutron diffraction. X-ray and Raman data suggest that the BiFeO3 structure is tetragonal for 70 nm-thick films and changes to monoclinic for 240 nm-thick films, thus remaining different from that of the bulk (rhombohedral) structure. In the 240 nm monoclinically distorted film neutron diffraction experiments allow the observation of a G-type antiferromagnetic order as in bulk single crystals. However, the satellite peaks associated with the long-wavelength cycloid present in bulk BiFeO3 are not observed. The relevance of these findings for the exploitation of the magnetoelectric properties of BiFeO3 is discussed.

Structure and electrical properties of K0.5Na0.5NbO3-LiSbO3 lead-free piezoelectric ceramics

Abstract: Lead-free piezoelectricceramics ( 1 − x ) K 05 Na 05 Nb O 3 – x Li Sb O 3 have been fabricated by a conventional ceramicsintering technique The results of x-ray diffraction suggest that Li + and Sb 5 + diffuse into the K 05 Na 05 Nb O 3 lattices to form a solid solution with a perovskite structure The ceramics can be well sintered at 1070 – 1110 ° C The introduction of Li Sb O 3 into the Na 05 K 05 Nb O 3 solid solution decreases slightly the paraelectric cubic-ferroelectric tetragonal phase transition temperature ( T c ) , but greatly shifts the ferroelectric tetragonal-ferroelectric orthorhombic phase transition ( T O – F ) to room temperature Coexistence of the orthorhombic and tetragonal phases is formed at 005 < x < 007 at room temperature, leading to a significant enhancement of the piezoelectric properties For the ceramic with x = 006 , the piezoelectric properties become optimum: piezoelectric constant d 33 = 212 pC ∕ N , planar and thickness electromechanical coupling factors k P = 46 % and k t = 47 % , respectively, remanent polarization P r = 150 μ C ∕ cm 2 , coercive field E c = 174 kV ∕ mm , and Curie temperature T C = 358 ° C

Phase Evolution in Yttria-Stabilized Zirconia Thermal Barrier Coatings Studied by Rietveld Refinement of X-Ray Powder Diffraction Patterns

Abstract: One failure mechanism of thermal barrier coatings composed of yttria-stabilized zirconia (YSZ) has been proposed to be caused, in part, by the transformation of the tetragonal phase of YSZ into its monoclinic phase. Normally, studies of phase evolution are performed by X-ray diffraction (XRD) and by evaluating the intensities of a few diffraction peaks for each phase. However, this method misses some important information that can be obtained with the Rietveld method. Using Rietveld's refinement of XRD patterns, we observed, upon annealing of YSZ coatings, an increase of cubic phase content, a reduction in as-deposited tetragonal phase content, and the appearance of a new tetragonal phase having a lower yttria content that coexists with the as-deposited tetragonal phase of YSZ.

Effects of K/Na ratio on the phase structure and electrical properties of (KxNa0.96-xLi0.04)(Nb0.91Ta0.05Sb0.04)O3 lead-free ceramics

Abstract: (KxNa096−xLi004)(Nb091Ta005Sb004)O3 (x=032–056) lead-free piezoelectric ceramics were prepared by conventional sintering The effects of K∕Na ratio on the phase structure and electrical properties of the ceramics were studied These results indicate that the phase structure undergoes a transition from orthorhombic to tetragonal phase with increase of x The ceramics with x=038 exhibit enhanced electrical properties (d33∼306pC∕N; kp∼48%; kt∼49%; Tc∼337°C; ϵr∼1327; tanδ∼25%; Pr∼349μC∕cm2; Ec∼113kV∕cm) Enhanced electrical properties of the ceramics should be attributed to the polymorphic phase transition near room temperature These results show that the ceramic with x=038 is a promising lead-free piezoelectric material

Reorientation of magnetic anisotropy in epitaxial cobalt ferrite thin films

Abstract: Spin reorientation has been observed in CoFe2O4 thin single crystalline films epitaxially grown on (100) MgO substrate upon varying the film thickness. The critical thickness for such a spin-reorientation transition was estimated to be 300 nm. The reorientation is driven by a structural transition in the film from a tetragonal to cubic symmetry. At low thickness, the in-plane tensile stress induces a tetragonal distortion of the lattice that generates a perpendicular anisotropy, large enough to overcome the shape anisotropy and to stabilize the magnetization easy axis out of plane. However, in thicker films, the lattice relaxation toward the cubic structure of the bulk allows the shape anisotropy to force the magnetization to be in plane aligned.

Influence of CuO on the Structure and Piezoelectric Properties of the Alkaline Niobate‐Based Lead‐Free Ceramics

Abstract: Recently, it was reported that the alkaline niobate-based (K,Na,Li)(Nb,Ta,Sb)O3 (LF4) perovskite ceramics showed excellent piezoelectric properties and have been regarded as a new candidate of lead-free piezoelectric materials. However, the effects of additives on the piezoelectric property of LF4 have not been studied intensively so far. In this study, the effect of CuO addition to the LF4 was investigated. The sample showed a tetragonal phase at room temperature and also a second phase with low melt point. With 0.05 wt% CuO added, the Cu2+ firstly incorporated into the A site, while higher amounts of Cu2+ substituted B-site ions. The CuO-doping changed the LF4 to “hard” piezoelectric ceramics with improvement of mechanical quality factor from 26 to 137 but the piezoelectric d31 constant and the electromechanical coupling constant reduced drastically at the same time.

One-dimensional Bi2O3 nanohooks: synthesis, characterization and optical properties

Abstract: We report the synthesis of one-dimensional (1D) Bi2O3 nanohooks by the oxidative metal vapor phase deposition technique. Surface morphology observations confirm the formation of 1D nanohooks with nanoparticles at their tips. Structural analysis by x-ray diffraction (XRD) and transmission electron microscopy (TEM) reveals the crystalline nature of the 1D nanostructure. Elemental analysis confirms that the 1D nanohooks consist of only elements Bi and O. The XRD study suggests that the synthesized product is of two phases (α- and β-Bi2O3) with monoclinic and tetragonal crystal structures respectively. The phonon vibration modes corresponding to Bi2O3 are determined by Raman scattering. A broadband visible photoluminescence (PL) is observed in the wavelength region 500–900 nm, also indicating the extension of luminescence into the near-infrared region. The existence of broadband visible emission can be attributed to the existence of defect/impurity states induced by oxygen vacancies.

Effect of epitaxial strain on ferroelectric polarization in multiferroic BiFeO3 films

Abstract: Multiferroic BiFeO3 epitaxial films with thickness ranging from 40 nm to 960 nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. X-ray characterization shows that the structure evolves from angularly-distorted tetragonal with c/a ~ 1.04 to more bulk-like distorted rhombohedral (c/a ~ 1.01) as the strain relaxes with increasing thickness. Despite this significant structural evolution, the ferroelectric polarization along the body diagonal of the distorted pseudo-cubic unit cells, as calculated from measurements along the normal direction, barely changes.

Higher permittivity rare earth doped HfO2 for sub-45-nm metal-insulator-semiconductor devices

Abstract: Rare earth (RE) doping (Gd, Er, Dy) of HfO2 reduces leakage current by three orders of magnitude compared with pure HfO2. The key to reducing HfO2 leakage current and equivalent oxide thickness (EOT) is stabilization of the higher permittivity tetragonal phase. RE doping of 10–20at.% stabilizes tetragonal HfO2 and increases permittivity. The maximum permittivity achieved for HfREOx is 28. The maximum permittivity for ZrREO is 32. HfGdO metal-insulator-semiconductor capacitors with EOT=1.93nm and leakage current <1×10−8A∕cm2 after 1070°C have been demonstrated.

Structurally Heterogeneous Model of Extrinsic Magnetostriction for Fe-Ga and Similar Magnetic Alloys: Part I. Decomposition and Confined Displacive Transformation

Abstract: A compositionally and structurally heterogeneous model for ferromagnetic alloys has been developed and applied to Fe-Ga alloys that exhibit giant magnetostriction. Our model assumes that the Fe-Ga bcc solid solution near its solubility limit is a coarsening-resistant nanodispersion of a DO3 phase that is formed due to coherency lifting by excess vacancies, as is known to be the case for chemically similar Fe-Al alloys. This compositionally heterogeneous state undergoes a cubic → tetragonal displacive transformation that brings the structure closer to an equilibrium fcc-based L12 ordered phase. The predictions of our theory are in agreement with existing electron microscopy and X-ray diffraction observations.

Direct observation of the structural component of the metal-insulator phase transition and growth habits of epitaxially grown VO2 nanowires.

Abstract: We have grown epitaxially orientation-controlled monoclinic VO2 nanowires without employing catalysts by a vapor-phase transport process. Electron microscopy results reveal that single crystalline VO2 nanowires having a [100] growth direction grow laterally on the basal c plane and out of the basal r and a planes of sapphire, exhibiting triangular and rectangular cross sections, respectively. In addition, we have directly observed the structural phase transition of single crystalline VO2 nanowires between the monoclinic and tetragonal phases which exhibit insulating and metallic properties, respectively, and clearly analyzed their corresponding relationships using in situ transmission electron microscopy.

Microstructure, phase transition, and electrical properties of (K0.5Na0.5)1−xLix(Nb1−yTay)O3 lead-free piezoelectric ceramics

Abstract: Lead-free ceramics (K0.5Na0.5)1−xLix(Nb1−yTay)O3 have been prepared by an ordinary sintering technique. Our results reveal that Li+ and Ta5+ diffuse into the K0.5Na0.5NbO3 lattices to form a solid solution with a perovskite structure. The substitution of Li+ induces an increase in the Curie temperature (TC) and a decrease in the ferroelectric tetragonal–ferroelectric orthorhombic phase transition temperature (TO-T). On the other hand, both TC and TO-T decrease after the substitution of Ta5+. A coexistence of the orthorhombic and tetragonal phases is formed at 0.03

Orbital ordering and magnetic field effect in MnV2O4.

Abstract: We studied the structural properties of an orbital-spin-coupled spinel oxide, MnV2O4, mainly by single-crystal x-ray diffraction measurement. It was found that a structural phase transition from cubic to tetragonal and ferrimagnetic ordering occur at the same temperature (Ts,TN=57 K). The structural phase transition was induced also by magnetic field above Ts. In addition, magnetic-field-induced alignment of tetragonal domains results in large magnetostriction below Ts. We also found that the structural phase transition is caused by the antiferro-type ordering of the V t2g orbitals.