Showing papers on "Tetragonal crystal system published in 1999"
TL;DR: In this paper, highly crystalline monoclinic and tetragonal BiVO4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV3O8 and K3V5O14) with Bi(NO3)3 for 3 days in aqueous media at room temperature.
Abstract: BiVO4 photocatalysts for O2 evolution, which work under visible light irradiation, were prepared by an aqueous process. The BiVO4 photocatalysts were obtained by the reaction of layered potassium vanadate powder (KV3O8 and K3V5O14) with Bi(NO3)3 for 3 days in aqueous media at room temperature. Highly crystalline monoclinic and tetragonal BiVO4 were selectively synthesized by changing the ratio of vanadium to bismuth in the starting materials. X-ray diffraction and scanning electron microscopy measurements showed that the monoclinic BiVO4 was formed via a tetragonal BiVO4 intermediate. Tetragonal BiVO4 with a 2.9 eV band gap mainly possessed an ultraviolet absorption band while monoclinic BiVO4 with a 2.4 eV band gap had a characteristic visible light absorption band in addition to the UV band. The UV bands observed in the tetragonal and monoclinic BiVO4 were assigned to the band transition from O2p to V3d whereas the visible light absorption was due to the transition from a valence band formed by Bi6s or ...
1,761 citations
TL;DR: A previously unreported ferroelectric phase has been discovered in a highly homogeneous sample of PbZr0.52Ti0.48O3 by high-resolution synchrotron x-ray powder diffraction measurements as mentioned in this paper.
Abstract: A previously unreported ferroelectric phase has been discovered in a highly homogeneous sample of PbZr0.52Ti0.48O3 by high-resolution synchrotron x-ray powder diffraction measurements. At ambient temperature the sample has tetragonal symmetry (at=4.037 A, ct=4.138 A), and transforms below ∼250 K into a phase which, unexpectedly, has monoclinic symmetry (am=5.717 A, bm=5.703 A, cm=4.143 A, β=90.53°, at 20 K). The intensity data strongly indicate that the polar axis lies in the monoclinic ac plane close to the pseudocubic [111] direction, which would be an example of the species m3m(12)A2Fm predicted on symmetry grounds by Shuvalov.
899 citations
TL;DR: In this paper, a constitutive model for the non-linear switching of ferroelectric polycrystals under a combination of mechanical stress and electric field is developed for nonlinear switching, where the switching event, which converts one crystal variant into another, gives rise to a progressive change in remanent strain and polarisation.
Abstract: A constitutive model is developed for the non-linear switching of ferroelectric polycrystals under a combination of mechanical stress and electric field. It is envisaged that the polycrystal consists of a set of bonded crystals and that each crystal comprises a set of distinct crystal variants. Within each crystal the switching event, which converts one crystal variant into another, gives rise to a progressive change in remanent strain and polarisation and to a change in the average linear electromechanical properties. It is further assumed that switching is resisted by the dissipative motion of domain walls. The constitutive model for the progressive switching of each crystal draws upon elastic–plastic crystal plasticity theory, and a prescription is given for the tangent moduli of the crystal, for any assumed set of potentially active transformation systems. A self-consistent analysis is used to estimate the macroscopic response of tetragonal crystals (representative of lead titanate) under a variety of loading paths. Also, the evolution of the switching surface in stress-electric field space is calculated. Many of the qualitative features of ferroelectric switching, such as butterfly hysteresis loops, are predicted by the analysis.
388 citations
TL;DR: In this paper, the authors investigated the piezoelectric properties of barium titanate single crystals at room temperature as a function of crystallographic orientation, and showed that electric field exposure below 6 kV/cm resulted in a high d33 of 203 pC/N and a hysteresis-free strain vs electric-field behavior, which suggested the formation of an engineered domain configuration.
Abstract: Piezoelectric properties of barium titanate single crystals were investigated at room temperature as a function of crystallographic orientation. When a unipolar electric field was applied along [001], its strain vs electric-field curve showed a large hysteresis, and finally barium titanate crystal became to single-domain state with piezoelectric constant d33 of 125 pC/N over 20 kV/cm. On the other hand, electric-field exposure below 6 kV/cm along [111] resulted in a high d33 of 203 pC/N and a hysteresis-free strain vs electric-field behavior, which suggested the formation of an engineered domain configuration in a tetragonal barium titanate crystal. Moreover, when an electric field over 6 kV/cm was applied along [111], two discontinuous changes were observed in its strain vs electric-field curve. In situ domain observation and Raman measurement under an electric field suggested an electric-field-induced phase transition from tetragonal to monoclinic at around 10 kV/cm, and that from monoclinic to rhombohedral at around 30 kV/cm. Moreover, in a monoclinic barium titanate crystal, electric-field exposure along [111] resulted in the formation of another new engineered domain configuration with d33 of 295 pC/N.
351 citations
TL;DR: The single phase La-doped BaTiO3 with the formula Ba 1−xLaxTi1−x/4O3: 0.20 was prepared by solid state reaction of oxide mixtures at 1350°C, 3 days, in O2.
Abstract: Single phase La-doped BaTiO3 with the formula Ba1−xLaxTi1−x/4O3: 0⩽x⩽0.20 was prepared by solid state reaction of oxide mixtures at 1350 °C, 3 days, in O2. The tetragonal distortion in undoped BaTiO3 decreased with x and samples were cubic for x⩾0.05. Both the tetragonal/cubic and orthorhombic/tetragonal transition temperatures decreased with x, but at different rates, and appeared to coalesce at x∼0.08. The value of the permittivity maximum at the tetragonal/cubic phase transition in ceramic samples increased from ∼10 000 for x=0 at 130 °C to ∼25 000 for x=0.06 at ∼−9 °C. At larger x, the permittivity maximum broadened, showed “relaxor”-type frequency dependent permittivity characteristics and continued to move to lower temperatures. Samples fired in O2 were insulating and showed no signs of donor doping whereas air-fired samples were semiconducting, attributable to oxygen loss.
331 citations
TL;DR: The structure of CaTiO3 has been studied at high temperatures by powder neutron diffraction methods as mentioned in this paper, and two phase transitions are evident, with an intermediate tetragonal (I/mcm) structure forming near 1500 K and a primitive cubic structure above 1580 K. Detailed Rietveld analyses of the data suggest there may also be a phase transition from the room temperature Pbnm structure to an orthorhombic Cmcm structure around 1380 K, which is a remarkable feature of the results.
Abstract: The structure of CaTiO3 has been studied at high temperatures by powder neutron diffraction methods. From inspection of the diffraction data two phase transitions are evident, with an intermediate tetragonal (I/mcm) structure forming near 1500 K and a primitive cubic structure above 1580 K. Detailed Rietveld analyses of the data suggest there may also be a phase transition from the room temperature Pbnm structure to an orthorhombic Cmcm structure around 1380 K. A remarkable feature of the results is the regular variation in the out-of-phase octahedral tilt angle over the entire temperature range.
277 citations
TL;DR: In this article, the deformation of single crystal Ni and NiCu random alloy nanowires subjected to uniform strain rates but kept at 300 K was studied. And they found that at high strain rates, the Ni nanowire is elastic up to 7.5% strain with a yield stress of 5.5 GPa.
Abstract: Using molecular dynamics simulations with a many-body force field, we studied the deformation of single crystal Ni and NiCu random alloy nanowires subjected to uniform strain rates but kept at 300 K. For all strain rates, the Ni nanowire is elastic up to 7.5% strain with a yield stress of 5.5 GPa, far above that of bulk Ni. At high strain rates, we find that for both systems the crystalline phase transforms continuously to an amorphous phase, exhibiting a dramatic change in atomic short-range order and a near vanishing of the tetragonal shear elastic constant perpendicular to the tensile direction. This amorphization which occurs directly from the homogeneous, elastically deformed system with no chemical or structural inhomogeneities exhibits a new mode of amorphization.
276 citations
TL;DR: The effective piezoelectric transverse coefficient e(31,f) was measured on various lead zirconate-titanate (PZT) and aluminum nitride thin films.
Abstract: The effective piezoelectric transverse coefficient e(31,f) was measured on various lead zirconate-titanate (PZT) and aluminum nitride thin films. The measurement set-up is based on the collection of the electric charges created by the forced deflection of a Si cantilever coated with a piezoelectric material. The maximum value was obtained from a tetragonal composition of PZT (45/55 Zr/Ti ratio) and was equal to 8.3 C/m(2). The ALN layer exhibited 97% of the theoretical value calculated from single crystal data, i.e., e(31,f)- = 1.02 C/m(2) (C) 1999 Elsevier Science S.A. All rights reserved.
269 citations
TL;DR: The high-temperature phase transitions of cubic perovskites have been studied using powder neutron diffraction and the Rietveld method as discussed by the authors, and the transition from the tetragonal to cubic phase is consistent with a continuous phase transition.
Abstract: The high-temperature phase transitions of ${\mathrm{SrZrO}}_{3}$ have been studied using powder neutron diffraction and the Rietveld method. The material is tetragonal $(I4/mcm)$ between \ensuremath{\sim}1020 and 1360 K. At higher temperatures (g1360 K) the structure was found to be the ideal cubic perovskite $(Pm3\ifmmode\bar\else\textasciimacron\fi{}m).$ From the neutron-diffraction data and the space-group assignments, the transition from the tetragonal to cubic phase is consistent with a continuous phase transition. The angle of tilt of the oxygen octahedron in the tetragonal phase is taken to be the order parameter and its temperature variation is typical of a tricritical phase transition.
232 citations
TL;DR: In this paper, it has been suggested that the structural phase transition is driven by a band Jahn-Teller distortion involving redistribution of electrons between 3d sub-bands of different symmetries.
Abstract: Polarized neutron scattering has been used to determine the changes in the distribution of unpaired electrons which take place in the martensitic transition in Ni2MnGa. Ni2MnGa is a ferromagnetic Heusler alloy which undergoes a reversible transition at about 220 K from a high temperature cubic phase to a low temperature tetragonal one. It has been suggested, on the basis of band structure calculations, that the structural phase transition is driven by a band Jahn-Teller distortion involving redistribution of electrons between 3d sub-bands of different symmetries. The results of the neutron scattering experiments show that the transition from the cubic to the tetragonal phase is accompanied by a transfer of magnetic moment from Mn to Ni. The unpaired electrons in the cubic phase have overall eg symmetry. In the tetragonal phase, the degeneracy of the eg and t2g bands is raised and the unpaired electrons are redistributed in such a way that the sub-bands based on orbitals extending towards the c-axis are preferentially occupied. Although the experimental moments differ in detail from those expected from band structure calculations, the change in symmetry of the magnetization distribution is consistent with a band Jahn-Teller origin for the phase transition.
203 citations
TL;DR: The ferromagnetic shape memory alloy Ni2MnGaGa has been grown on GaAs by molecular beam epitaxy as mentioned in this paper, which has a Curie temperature of ∼320 K.
Abstract: The ferromagnetic shape memory alloy Ni2MnGa has been grown on GaAs by molecular beam epitaxy. In situ reflection high energy electron diffraction, ex situ x-ray diffraction, and transmission electron microscopy selective area electron diffraction indicate the single crystal growth of a pseudomorphic tetragonal phase of Ni2MnGa on (001) GaAs. Both vibrating sample magnetometry and superconducting quantum interference device magnetometry measurements show that the Ni2MnGa film is ferromagnetic with in-plane magnetization and has a Curie temperature of ∼320 K.
Patent•
28 Oct 1999
TL;DR: In this article, a zirconia sol comprising an aqueous phase having dispersed a plurality of single crystal ZIRconia particles having an average primary particle size less than about 20 nm is described.
Abstract: A zirconia sol comprising an aqueous phase having dispersed therein a plurality of single crystal zirconia particles having an average primary particle size less than about 20 nm. The zirconia sols are substantially non associated having a dispersion index ranging from about 1-3 and are highly crystalline exhibiting a crystallinity index of about 0.65 or greater. Of the crystalline phase, about 70% or greater exists in combined cubic and tetragonal crystal lattice structures without a crystal phase stabilizer. Also described is a hydrothermal method of making zirconia sols having substantially non-associated zirconia particles and composite materials made from the zirconia sols.
TL;DR: In this paper, a single-crystalline DyB 2 C 2 compound with the tetragonal structure was investigated and the magnetic properties were investigated on a single crystalized DyB2C 2 compound, and the results were well interpreted by postulating that the phase between T C and T Q is an antiferroquadrupolar ordered one.
Abstract: Magnetic properties were investigated on a single-crystalline DyB 2 C 2 compound with the tetragonal structure. Spontaneous magnetizations appear along the a- and [1 1 0]-directions below T C =15.3 K. A very small shoulder is observed only along the c -axis at T Q =24.7 K, although large λ-type anomalous specific heats are observed at T C and T Q . There exists a large magnetic anisotropy in the tetragonal basal plane which is observed in magnetization processes below T C . Neutron powder diffraction experiments reveal that magnetic reflections are observed only below T C . In the magnetically ordered phase, the Dy moments are arranged to be perpendicular to neighbors along the c -axis. The results are well interpreted by postulating that the phase between T C and T Q is an antiferroquadrupolar ordered one. DyB 2 C 2 is a novel compound with a high antiferroquadrupolar ordering temperature of 24.7 K.
TL;DR: In this article, a series of new compounds, Ba 1−x R 2x/3 Nb 2 O 6 (RNd, Eu, Gd, Tb, Dy, Ho and Er) with tetragonal tungsten bronze (TTB) structure were synthesized.
Abstract: A series of new compounds, Ba 1−x R 2x/3 Nb 2 O 6 (RNd, Eu, Gd, Tb, Dy, Ho and Er) with tetragonal tungsten bronze (TTB) structure were synthesized. For Ba–R–Nb–O system, single phase TTB was obtained when X =0·25. To understand the formation condition of these TTB type compounds, two tolerance factors for TTB, t A1 and t A2 , were proposed against the coordination polyhedra around A1 and A2 site cations, respectively. One of the factors was at least a requisition to stabilize crystal structure for all TTB type compounds. Ba 1− x R 2 x /3 Nb 2 O 6 showed ferroelectric characteristics and the Curie temperature, T c , increased as the decrease of the ionic radius of rare earth cations. These new compounds have potential to be candidates to apply ferroelectric random access memory (FRAM).
TL;DR: In this article, a new nonlinear optical crystal of coordination complex, zinc cadmium tetrathiocyanate (ZnCd(SCN)4), was successfully synthesized through several reactions in water.
TL;DR: The structure of phase IV of barium has been solved from single-crystal and powder diffraction x-ray data as discussed by the authors, which is composed of a tetragonal host, with guest chains in channels along the $c$ axis of the host.
Abstract: The structure of phase IV of barium has been solved from single-crystal and powder diffraction x-ray data. It is composed of a tetragonal ``host,'' with ``guest'' chains in channels along the $c$ axis of the host. These chains form two different structures, one well crystallized and the other highly disordered and giving rise to strong diffuse scattering. The guest structures are incommensurate with the host.
TL;DR: In this paper, the crystal structure of SrHfO{sub 3} has been studied at high temperatures using powder neutron diffraction and the Rietveld method and it has been shown that at higher temperatures the structure is the ideal cubic perovskite (Pm3{sup -}m).
Abstract: The crystal structure of SrHfO{sub 3} has been studied at high temperatures using powder neutron diffraction and the Rietveld method. From 300 K to approximately 670 K the structure of SrHfO{sub 3} is orthorhombic (Pnma). By 870 K the material adopts a second orthorhombic structure (Cmcm). The material then undergoes a further phase transition and is tetragonal (/4/mcm) from {approx}1000 to 1353 K. At higher temperatures (>1360 K) the structure is the ideal cubic perovskite (Pm3{sup -}m). The angle of rotation of the oxygen octahedron in the tetragonal phase is taken as the order parameter and its temperature variation is consistent with a second order phase transition.
TL;DR: In this article, a model for the tetragonal compound Ni2MnGa and the non-stoichiometric phases was developed with the help of transmission electron microscope (TEM) data, X-ray powder diffraction measurements, and a computational simulation.
TL;DR: In this paper, a large cell tetragonal structure in space group I41/amd was proposed for the spinel LiMn2O4 at 10 K and 333 K. While complete segregation of the Mn3+ and Mn4+ ions is not possible in this space group, bond-valence analysis indicates that the distribution of Mn3 + and Mn 4 + ion is not random and that there is a degree of charge segregation.
Abstract: Powder neutron diffraction has been used to study the nature of the structural transition away from the Fd3m cubic structure upon cooling below ∼285 K in the spinel LiMn2O4. We report powder data taken between 10 K and 333 K and propose a large cell tetragonal structure in space group I41/amd for the material at 100 K. While complete segregation of the Mn3+ and Mn4+ ions is not possible in this space group, bond-valence analysis indicates that the distribution of Mn3+ and Mn4+ ions is not random and that there is a degree of charge segregation. Further, LiMn2O4 is also of interest because it is an example of a geometrically frustrated antiferromagnet. Direct current magnetic susceptibility measurements show field-cooled, zero-field-cooled irreversibility at ∼65 K and a maximum in zero-field-cooled data at ∼40 K. Neutron diffraction shows magnetic scattering in the form of a broad peak assigned to short-range order which develops above 100 K. Upon cooling to 60 K additional Bragg peaks are seen, signaling ...
TL;DR: Park et al. as discussed by the authors used in situ x-ray diffraction to measure strain curves of single crystals of 92% Pb(Zn1/3Nb2/3)O3 and 8% pbTiO3 poled in the 〈001〉 direction, and observed shifts in the (002) and (330) peaks as functions of applied voltage.
Abstract: We have used in situ x-ray diffraction to measure strain curves of single crystals of 92% Pb(Zn1/3Nb2/3)O3–8% PbTiO3 poled in the 〈001〉 direction. We observed shifts in the (002) and (330) peaks as functions of applied voltage (in the 〈001〉 direction), thereby measuring the crystallographic strain. Our measurements confirm that the high strains observed using macroscopic methods reflect microscopic strains in the crystal lattice. By watching the disappearance of a rhombohedral splitting in the (330) peak, we also find direct evidence that the field-induced phase transition reported by S.-E. Park and T. R. Shrout [J. Appl. Phys. 82, 1804 (1997)] occurs between a pseudorhombohedral and tetragonal crystal structure.
TL;DR: In this paper, the structural phase transitions in perovskite-like NaTaO3, from room temperature to 933 K, have been examined, and the structure in orthorhombic Cmcm is characterized by simultaneous tilting of the oxygen atom octahedron about two of its tetrad axes.
Abstract: Neutron powder diffraction has been used to examine the structural phase transitions in the perovskite-like NaTaO3, from room temperature to 933 K. The room temperature orthorhombic structure (Pbnm, a = 5.4768(1), b = 5.5212(1), c = 7.7890(2)) transforms to orthorhombic Cmcm at around 700 K, then to tetragonal P4/mbm at 835 K, and finally to cubic Pmm above 890 K. The structure in orthorhombic Cmcm is characterized by simultaneous tilting of the oxygen atom octahedron about two of its tetrad axes, the tilting of successive octahedra being out of phase along the b-axis, and in phase along the c-axis. The two tilt angles are comparable just above 700 K, but the out-of-phase tilt angle falls smoothly to zero as the transition to tetragonal is approached, in the manner suggestive of a tricritical transition. This results in an unusual variation of lattice parameters with temperature in the orthorhombic Cmcm phase. In the tetragonal phase the lattice parameters vary smoothly; however near the transition to cubic the in-phase tilt angle changes more rapidly with temperature than might be expected in a continuous phase transition.
TL;DR: This study investigated the effects of supersaturation, temperature, and pH on the nucleation rate of tetragonal lysozyme crystals.
TL;DR: In this article, the authors used neutron diffraction data to determine the crystal structure of a series of oxygen-deficient barium titanate powders, which were prepared by reduction of stoichiometric, tetragonal BaTiO============ 3======
Abstract: Rietveld refinements using neutron diffraction data have been used to determine the crystal structure of a series of oxygen-deficient barium titanate powders, BaTi
IV
1–x
Ti
III
x
O
3–x/2
(0
TL;DR: In this paper, the thermal expansion of Mo5Si3, a tetragonal structure with I4/mcm symmetry, is strongly anisotropic along the a and c directions with αc/αa=2.2.
TL;DR: In this article, e-field induced phase transformation and associated changes in dielectric/piezoelectric properties of rhombohedral 0.92Pb(Zn1/3Nb2/3)O3-0.08PbTiO3 crystals were investigated.
Abstract: E-field induced phase transformation and associated changes in dielectric/piezoelectric properties of 〈001〉-oriented rhombohedral 0.92Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 crystals were investigated. The longitudinal strain level was found to abruptly increase at 20 kV/cm, corresponding to that where an induced phase appears within a multidomain matrix. Decreases in the dielectric constant (K3T∼4000 to 500), transverse coupling (k31∼70% to 50%) and transverse piezoelectric coefficient (d31∼−1100 to −200 pC/N) associated with the induced phase were the result of increased crystal anisotropy. By contrast, the thickness coupling (kT) increased from 53% at 0 kV/cm to 64% at 45 kV/cm, associated with this phase transition. The measured dielectric and piezoelectric properties found for the induced phase state were nearly identical to those of 〈001〉 poled tetragonal (1−x)PZN−xPT (x>0.1) crystals. Based on these results, it is evident that the symmetry of induced phase is tetragonal, 4 mm.
TL;DR: In this paper, the A-site-deficient perovskite solid solution (ADPESS) La0.67−xLi3xTiO3 (x=0.11) underwent an order-disorder transition reversibly with a reversible change in ionic conductivity.
TL;DR: In this paper, an extensive investigation of the temperature dependence of Raman spectra has been carried out on WO 3 powders from room temperature to 800°C, in particular the orthorhombic-to-tetragonal phase transition occurring at about 740°C has been studied.
TL;DR: In this article, thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited on LaAlO3 and SrTiO3 (001) substrates.
Abstract: Thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited on LaAlO3 and SrTiO3 (001) substrates. Comparison of their transport and magnetic properties with those of polycrystalline ceramic samples shows a metallic versus semiconductor temperature dependence and a saturation magnetization Ms at 10 K of 3.2 μB/f.u. in the film as against 3.0 for a tetragonal polycrystalline sample. However, the Curie temperature TC≈389 K is reduced from 415 K found for the tetragonal ceramic, which lowers Ms at 300 K in the thin films to 2.0 μB/f.u. compared to 2.2 μB/f.u. in the ceramics. A Wheatstone bridge arrangement straddling a bicrystal boundary has been used to verify that spin-dependent electron transfer through a grain boundary is responsible for the low-field magnetoresistance found in polycrystalline samples below TC.
TL;DR: In this article, a detailed structural study of ultrathin Ni films on Cu~100! was performed by applying quantitative low-energy electron diffraction, and the analysis of films with different thickness showed that the film growth is pseudomorphic throughout the coverage regime investigated, 1−11 monolayers.
Abstract: We report on a detailed structural study of ultrathin Ni films on Cu~100! applying quantitative low-energy electron diffraction. The analysis of films with different thickness shows that the film growth is pseudomorphic throughout the coverage regime investigated, 1‐11 monolayers. The vertical spacings of the Ni layers, which are unreconstructed, are largely independent of both film thickness and depth. They are by about 3.2% contracted compared to the bulk spacings of Ni ~5% compared to the bulk spacings of Cu!, so that all films correspond to homogeneous tetragonal phases. Due to the tetragonal distortion, the volume of Ni atoms in the films is practically the same as in bulk Ni. The layer spacing at the Ni-Cu interface is slightly contracted compared to the value resulting from a hard sphere model, and the same holds for the top Cu layer distance. In particular, within the accuracy limits ~1‐2% or less! we detect no structural differences for the change from in-plane to vertical magnetization with varying coverage or temperature or for the change from ferro- to paramagnetism at the Curie temperature. @S0163-1829~99!03719-4#
TL;DR: The crystal structure of MgC 2 was solved from powder X-ray and neutron diffraction data as discussed by the authors, and the crystal structure contains C 2 groups, with a triple-bond length of 1.215(6) A and a weak interaction between Mg and the triple bond of the crossing chains above and below.