Showing papers on "Tetragonal crystal system published in 2000"
TL;DR: High resolution x-ray powder diffraction measurements on poled PbZr1-xTixO3 (PZT) ceramic samples have shown that for both rhombohedral and tetragonal compositions the piezoelectric elongation of the unit cell does not occur along the polar directions but along those directions associated with the monoclinic distortion.
Abstract: High resolution x-ray powder diffraction measurements on poled PbZr1-xTixO3 (PZT) ceramic samples close to the rhombohedral-tetragonal phase boundary (the so-called morphotropic phase boundary) have shown that for both rhombohedral and tetragonal compositions the piezoelectric elongation of the unit cell does not occur along the polar directions but along those directions associated with the monoclinic distortion. This work provides the first direct evidence for the origin of the very high piezoelectricity in PZT.
921 citations
TL;DR: CeRhIn5 is a new heavy-electron material that crystallizes in a quasi-2D structure that can be viewed as alternating layers of CeIn3 and RhIn2 stacked sequentially along the tetragonal c axis that induces a first-order-like transition from an unconventional antiferromagnetic state to a superconducting state.
Abstract: CeRhIn5 is a new heavy-electron material that crystallizes in a quasi-2D structure that can be viewed as alternating layers of CeIn3 and RhIn2 stacked sequentially along the tetragonal c axis. Application of hydrostatic pressure induces a first-order-like transition from an unconventional antiferromagnetic state to a superconducting state with T(c) = 2.1 K.
748 citations
TL;DR: In this article, the stability region of all three phases has been established from high-resolution synchrotron x-ray powder-diffraction measurements on a series of highly homogeneous samples with $0.42l~xl~0.52.$ at 20 K.
Abstract: Recent structural studies of ferroelectric ${\mathrm{PbZr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{3}$ (PZT) with $x=0.48,$ have revealed a monoclinic phase in the vicinity of the morphotropic phase boundary (MPB), previously regarded as the boundary separating the rhombohedral and tetragonal regions of the PZT phase diagram. In the present paper, the stability region of all three phases has been established from high-resolution synchrotron x-ray powder-diffraction measurements on a series of highly homogeneous samples with $0.42l~xl~0.52.$ At 20 K, the monoclinic phase is stable in the range $0.46l~xl~0.51,$ and this range narrows as the temperature is increased. A first-order phase transition from tetragonal to rhombohedral symmetry is observed only for $x=0.45.$ The MPB, therefore, corresponds not to the tetragonal-rhombohedral phase boundary, but instead to the boundary between the tetragonal and monoclinic phases for $0.46l~xl~0.51.$ This result provides important insight into the close relationship between the monoclinic phase and the striking piezoelectric properties of PZT; in particular, investigations of poled samples have shown that the monoclinic distortion is the origin of the unusually high piezoelectric response of PZT.
501 citations
TL;DR: In this article, the relaxation time of BiFeO3 was estimated using the relaxation times of the intermediate compositions of the BaTiO3 mixture, and the authors showed that the capacitance observed is from the bulk of the sample.
Abstract: BiFeO3, when forming a solid solution with BaTiO3, shows structural transformations over the entire compositional range. Above 70 mole % of BiFeO3 the structure is rhombohedral and below 4 mole %, it is tetragonal. In between the structure is cubic. The ferroelectric TC decreases with increasing composition of BaTiO3 and a relatively small relaxation is observed. Impedance measurements showed a structural dependence and analysis of which has clearly shown that the capacitance observed is from the bulk of the sample. Relaxation time (τ) of BiFeO3 was estimated using the relaxation times of the intermediate compositions. Magnetization measurements showed field induced ferromagnetism. As the structure becomes cubic with increasing concentration of BaTiO3, paramagnetism sets in, as evidenced by the electron spin resonance spectra.
463 citations
TL;DR: A low-temperature monoclinic phase is confirmed to exist, and is demonstrated to act as a bridge between the well-known tetragonal and rhombohedral phases delimiting the morphotropic phase boundary (MPB).
Abstract: A first-principles-derived approach is developed to study finite-temperature properties of Pb(Zr1-xTix)O3 (PZT) solid solutions near the morphotropic phase boundary (MPB). Structural and piezoelectric predictions are in excellent agreement with experimental data and direct first-principles results. A low-temperature monoclinic phase is confirmed to exist, and is demonstrated to act as a bridge between the well-known tetragonal and rhombohedral phases delimiting the MPB. A successful explanation for the large piezoelectricity found in PZT ceramics is also provided.
450 citations
TL;DR: In this article, the authors used time-of-flight neutron powder diffraction to determine the correct crystal structures for Sr4Fe4O11 (orthorhombic, space group Cmmm, a=10.974(1) A, b=7.702(1), and c=5.473(1)-A) and Sr8Fe8O23 (tetragonal, I4/mmm) through comparison of the goodness of fit for Rietveld refinements of candidate models and bond-length distributions.
409 citations
TL;DR: In this paper, the structural evolution between the end members Na0.5 Bi 0.5 (rhombohedral at 300 K) and K0.1 Bi 0.5 (tetragonal at 300 k) is investigated using X-ray diffraction (XRD) and Raman spectroscopy.
Abstract: The (Na1 - x Kx )0.5 Bi0.5 TiO3 perovskite solid solution is investigated using x-ray diffraction (XRD) and Raman spectroscopy in order to follow the structural evolution between the end members Na0.5 Bi0.5 TiO3 (rhombohedral at 300 K) and K0.5 Bi0.5 TiO3 (tetragonal at 300 K). The Raman spectra are analysed with special regard to the hard modes and suggest the existence of nano-sized Bi3+ TiO3 and (Na1 - 2x K2x )+ TiO3 clusters. The complementary use of XRD and Raman spectroscopy suggests, in contrast to previous reported results, that the rhombohedral tetragonal phase transition goes through an intermediate phase, located at 0.5 x 0.80. The structural character of the intermediate phase is discussed in the light of sub- and super-group relations.
347 citations
TL;DR: In this article, the phase transformation of diamond cubic silicon under nano-indentation with the aid of molecular dynamics analysis using the Tersoff potential was discussed. But, the authors did not consider the effect of nanoindentations on the transition from a tetrahedron structure to an amorphous phase.
Abstract: This paper discusses the phase transformation of diamond cubic silicon under nano-indentation with the aid of molecular dynamics analysis using the Tersoff potential. By monitoring the positions of atoms within the model, the microstructural changes as silicon transforms from its diamond cubic structure to other phases were identified. The simulation showed that diamond cubic silicon transforms into a body-centred tetragonal form (β-silicon) upon loading of the indentor. The change of structure is accomplished by the flattening of the tetrahedron structure in diamond cubic silicon. Upon unloading, the body-centred tetragonal form transforms into an amorphous phase accompanied by the loss of long-range order of the silicon atoms. By performing a second indentation on the amorphous zone, it was found that the body-centred-tetragonal-to-amorphous phase transformation could be a reversible process.
307 citations
TL;DR: The phase diagram of La1-xSrxMnO3, as a function of hole doping x and tetragonal distortion c/a, which consists of ferromagnetic, A-, C-, and G-type antiferromagnetic states, is obtained by the first-principles band structure calculations.
Abstract: The phase diagram of La1-xSrxMnO3, as a function of hole doping x and tetragonal distortion cia, which consists of ferromagnetic (FM), A-: C-, and G-type antiferromagnetic (AF) states, is obtained by the first-principles band structure calculations. Effec
279 citations
TL;DR: In this article, the influence of dopants on the crystal structure of barium titanate was studied at room temperature and at 250°C using X-ray diffraction, and the results of atomistic computer simulations gave indication on the preferential incorporation site of the dopant.
Abstract: The influence of dopants (Cr, Co, Fe, Ni, Y, Er, Tb, Gd, Pr and La) on the crystal structure of barium titanate was studied at room temperature and at 250°C using X-ray diffraction. Fine BaTiO3 powders (Ba/Ti=1.02, size ≈30 nm) have been doped with 1 at% of the foreign element and annealed for 14 and 62 h at two different temperatures: 950 and 1350°C. The room temperature structure of doped BaTiO3 was in any case tetragonal with c/a ratio lower than in the undoped perovskite, but dependent on dopant nature and particle size. For powders calcined at 1350°C, the particle size was in the range 1–5 μm and the decrease in tetragonality was mainly determined by dopant incorporation. Powders treated at 950°C had particles more than one order of magnitude finer (0.1–0.2 μm) and a systematic lowering of the c/a ratio in comparison to the samples annealed at higher temperature was observed. Comparison of the experimental variations of the unit cell edge of cubic BaTiO3 (250°C) with the results of atomistic computer simulations gives indication on the preferential incorporation site of the dopant. In particular, La3+ and Pr3+ prefer to substitute at the Ba site, whereas Tb3+ and Gd3+ give partial substitution at the Ti site. For Er3+ and Y3+ preferential substitution at the Ti site is predicted. For transition metal ions, substitution at the Ti site with oxygen vacancy compensation is confirmed, although their behaviour is less accurately reproduced.
270 citations
TL;DR: In this paper, the aging of the NC 19 Fe Nb alloy (Inconel 718), previously quenched from 990°C, is characterized by X-ray diffraction, transmission electron microscopy and micro-hardness investigation.
TL;DR: In this paper, two powdered batches of undoped zirconia, prepared by spray pyrolysis and characterized by 6.9 and 13.2 nm average crystallite size were annealed for different temperatures and for different times.
TL;DR: In this paper, a zirconia catalyst was used for the selective synthesis of dimethyl carbonate from methanol and CO 2 and the ratio of the tetragonal phaseto the monoclinic phase was higher on this catalyst than on other catalysts prepared at different calcination temperatures.
TL;DR: In this paper, room temperature x-ray diffraction, magnetization and neutron diffraction measurements up to 500 K were carried out on polycrystalline bulk double perovskites AA'FeMoO6 (AA' = Ba2, BaSr, Sr2, Ca2) in order to determine and correlate their structural and magnetic properties.
Abstract: Room temperature x-ray diffraction, magnetization and neutron diffraction measurements up to 500 K have been carried out on polycrystalline bulk double perovskites AA'FeMoO6 (AA' = Ba2, BaSr, Sr2, Ca2) in order to determine and correlate their structural and magnetic properties. As the average ionic radius is diminished, the crystallographic structure evolves from cubic (for AA' = Ba2, BaSr) to tetragonal (Sr2) and finally to monoclinic (Ca2). In the case of AA' = Sr2, a novel crossover from a high temperature paramagnetic cubic state to a low temperature ferrimagnetic tetragonal state has been observed. For all the studied compounds, neutron diffraction patterns and magnetization measurements are consistent with a ferrimagnetic ordering of the Fe and Mo sublattices. A remarkable correlation is found between the Curie temperature and the electronic bandwidth, which is controlled by structural parameters.
TL;DR: In this paper, a series of experiments in the system Fe-FeS at a pressure of 21 GPa and temperatures between 950 and 1400 °C were performed, and two new iron-excess iron-sulfur compounds, Fe3S and Fe2S, formed at subsolidus temperatures.
Abstract: We performed a series of experiments in the system Fe-FeS at a pressure of 21 GPa and temperatures between 950 and 1400 °C, and we found two new iron-excess iron-sulfur compounds, Fe3S and Fe2S, formed at subsolidus temperatures. Powder X-ray diffraction data revealed that Fe3S has a tetragonal cell, isostructural with Fe3P (space group I 4). The tetragonal unit-cell dimensions for Fe3S are a = 9.144(2) A and c = 4.509(2) A, with a zero-pressure density of 7.033 g/cm3. Static compression experiments on Fe3S were carried out in a diamond-anvil cell, using synchrotron X-ray diffraction technique. A least-squares fit to the experimental data at room temperature yielded bulk modulus K = 170 ± 8 GPa with a corresponding pressure derivative K ′ = 2.6 ± 0.5 or K = 150 ± 2 GPa with fixed K ′ = 4.
TL;DR: The structure of the tetragonal phase of the A-site-substituted perovskite sodium bismuth titanate has been determined by neutron powder diffraction at 698 K and exhibits an unusual combination of in-phase tilts and antiparallel cation displacements along the polar c axis, which results in a new variant of the perovkite structure.
Abstract: The structure of the tetragonal phase of the A-site-substituted perovskite sodium bismuth titanate, Na0.5Bi0.5TiO3, has been determined by neutron powder diffraction at 698 K. The structure was refined in space group P4bm with a (= b) = 5.5191 (1), c = 3.9085 (1) A, V = 119.055 (5) A3, Z = 2 and Dx = 5.91 Mg m−3. The structure exhibits an unusual combination of in-phase (a0a0c+) tilts and antiparallel cation displacements along the polar c axis, which results in a new variant of the perovskite structure.
TL;DR: The tentative assignment of the vibrational spectra of different phases has shown that they could be perfectly described by the molecular symmetry of the polymers which are the structure-forming elements of crystalline polymerized phases as mentioned in this paper.
Abstract: Comparative studies of the infrared (IR) and Raman spectra of the orthorhombic, tetragonal, and rhombohedral pressure-polymerized phases of ${\mathrm{C}}_{60}$ as well as the pressure-dimerized state have been performed at ambient conditions. The tentative assignment of the vibrational spectra of different phases has shown that they could be perfectly described by the molecular symmetry of the ${\mathrm{C}}_{60}$ polymers which are the structure-forming elements of crystalline polymerized phases. This allowed one to consider the IR and Raman spectra of the crystalline polymerized phases as characteristic of those of the appropriate one- and two-dimensional ${\mathrm{C}}_{60}$ polymers and to use them for molecular fractional analysis of polymerization products.
TL;DR: In this article, a room temperature free shear strain of 5.7% is reported in a single crystal of Ni-Mn-Ga having a composition close to the Heusler alloy Ni2MnGa.
Abstract: A room temperature free shear strain of 5.7% is reported in a single crystal of Ni–Mn–Ga having a composition close to the Heusler alloy Ni2MnGa. A twin boundary was created in a 2 mm×2 mm×25 mm single crystal using a permanent magnet with surface field strength of about 320 000 A/m. A sharp 6.5° bend occurs in the sample at the twin boundary. The surface magnetization changes abruptly across this boundary. By moving the sample relative to the edge of the magnet, we were able to sweep the boundary back and forth along the crystal length. Surface magnetization was measured using a Hall probe and the results confirm that the easy axis is the tetragonal c axis. Powder x-ray diffraction shows that the fcc to body-centered-tetragonal bct martensitic transition of this material involved a 6% reduction of the bct cell c/a ratio, from √ to about 1.33. The maximum achievable strain is thus estimated to be 6.2%. The twin planes in the system are the {112}bct and were observed to lie almost normal to the long axis of the sample tested.
TL;DR: In this article, it has been concluded that SrZrO3, orthorhombic in space group Pnma at room temperature, changes by a continuous transition to a pseudo-tetragonal structure in Imma at about 750 °C, then by a discontinuous transformation to tetragonal I4/mcm at about 840   cm, and finally, continuously, to cubic at 1070  c.
Abstract: The structures of SrZrO3 have been studied in fine temperature steps, from room temperature to 1230 °C, by very high resolution neutron powder diffraction. It has been concluded that SrZrO3, orthorhombic in space group Pnma at room temperature, changes by a continuous transition to a pseudo-tetragonal structure in Imma at about 750 °C, then by a discontinuous transformation to tetragonal I4/mcm at about 840 °C, and finally, continuously, to cubic at 1070 °C. The sequence of structures is rationalised by reference to a recent group theoretical analysis (Howard C J and Stokes H T 1998 Acta Crystallogr. B 54 782).
TL;DR: In this article, Barium titanate (BaTiO 3 : BT) nanocrystals were tailored by hydrothermal method in the presence of polyoxyethylene (20) sorbitan monooleate (Tweent 80) as a polymeric surface modi"er at 2303C for 0.2 h.
TL;DR: In this article, Li signals with different quadrupole constants are detected in 7Li NMR spectra of orthorhombic/tetragonal perovskites, which have been associated with two crystallographic sites.
TL;DR: The elastic, piezoelectric, and dielectric properties of a 0.955Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PZN-4.5%PT) multi-domain single crystal have been determined experimentally using combined resonance and ultrasonic methods.
Abstract: The elastic, piezoelectric, and dielectric properties of a 0.955Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/-0.045PbTiO/sub 3/ (PZN-4.5%PT) multi-domain single crystal, poled along [001] of the original cubic direction, have been determined experimentally using combined resonance and ultrasonic methods. At room temperature, the PZN-4.5%PT single crystal has rhombohedral symmetry. After being poled along [001], four degenerate states still remain. Statistically, such a domain-engineered crystal may be treated as having an average tetragonal symmetry, and its material constants were determined based on 4 mm symmetry. It was confirmed that the electromechanical coupling coefficient k/sub 33/ for the domain-engineered samples is >90%, and the piezoelectric constant d/sub 33/ is >2000 pC/N. A soft shear mode with a velocity of 700 m/s was found in the [110] direction. From the measured experimental data, the orientational dependence of phase velocities and electromechanical coupling coefficients was calculated. The results showed that the transverse and longitudinal coupling coefficients, k/sub 31/ and k/sub 33/, reach their maximum along [110] and [001], respectively.
TL;DR: The isostructural compounds of general formula Cu2Te2O5X2 (X = Cl, Br) have been synthesized by chemical transport reaction in the tetragonal system as mentioned in this paper.
Abstract: The isostructural compounds of general formula Cu2Te2O5X2 (X = Cl, Br) have been synthesized by chemical transport reaction. They crystallize in the tetragonal system, space group P4 with a = 7.62...
TL;DR: Diffraction data for Sb-II reveal that it too can be fitted with the same composite structure, and the structures of these two high-pressure phases of Bi and Sb are similar to those reported recently in the alkaline-earth metals Ba and Sr.
Abstract: The long-unknown crystal structure of Bi-III has been solved. It comprises a body-centered-tetragonal (bct) ``host'' and a bct ``guest'' component made up of chains that lie in channels in the host; the guest is incommensurate with the host along the tetragonal $c$ axis. Diffraction data for Sb-II reveal that it too can be fitted with the same composite structure. The structures of these two high-pressure phases of Bi and Sb are similar to those reported recently in the alkaline-earth metals Ba and Sr.
TL;DR: In this paper, the phase composition of plasma-sprayed yttria stabilized zirconia (YSZ), ZrO2 with 8% by mass Y2O3, was studied using neutron and X-ray diffraction.
Abstract: The phase composition of plasma-sprayed yttria stabilized zirconia (YSZ), ZrO2 with 8% by mass Y2O3, was studied using neutron and X-ray diffraction. Comparison shows that neutron diffraction is superior for analysis of the phase composition as well as for the analysis of the yttria content of the tetragonal phase. The presence of large amounts of the cubic phase is probably often neglected or underestimated in standard XRD analysis due to scattering-related limitations and the inherent difficulty of the analysis. The importance of this fact needs to be addressed in future studies. The amount of monoclinic, tetragonal, and cubic phases was determined using neutron Rietveld refinement for feedstock powders, as-sprayed deposits and for samples annealed for 1 h at temperatures of 1100, 1200, 1300, and 1400°C. The two studied feedstock materials were manufactured by different production methods, and contained various amounts of the monoclinic phase depending on manufacturing method. While the tetragonal phase dominated, both feedstock powders also contained significant amounts of the cubic phase. The as-sprayed deposits were composed of mostly tetragonal phase, with only traces of the monoclinic phase; the amount of the cubic phase was reduced with respect to the feedstock. For one of the materials the cubic phase content remained significant (approx. 25% by mass), and for the other the cubic phase content was significantly lower (approx. 6% by mass). The cubic phase content after annealing at 1400°C was in both cases similar — approximately 40% by mass. There was no significant change in monoclinic phase content observed in this experiment. The yttria fraction within the tetragonal and cubic phases was followed and changes are discussed.
TL;DR: The linear trinuclear compound Co3(dpa)4Cl2 (1; dpa = di(2-pyridyl)amide anion) crystallizes from CH2Cl2 solution in two forms simultaneously as mentioned in this paper.
Abstract: The linear trinuclear compound Co3(dpa)4Cl2 (1; dpa = di(2-pyridyl)amide anion) crystallizes from CH2Cl2 solution in two forms simultaneously, namely, an orthorhombic form 1·CH2Cl2 and a tetragonal form 1·2CH2Cl2. The three linearly arranged cobalt atoms in 1 are supported by four dpa ligands in a spiral configuration. The chain of cobalt atoms is symmetrical in 1·CH2Cl2, but unsymmetrical in 1·2CH2Cl2. Both crystal structures have been studied at various temperatures. A reversible second-order phase transition (165 K) from orthorhombic (Pnn2) to monoclinic (Pn) symmetry for the crystal of 1·CH2Cl2 has been documented by X-ray studies at 296, 168, and 109 K as well as a neutron diffraction study at 20 K. The linear tricobalt unit in 1·CH2Cl2 becomes slightly unsymmetrical at low temperature although the two Co−Co bonds remain statistically equivalent (Co−Co ≈ 2.32 A) throughout the experimental temperature range. No phase transition was observed for the tetragonal form 1·2CH2Cl2 at low temperature, but th...
TL;DR: In this article, the frequencies of the softest polar optic phonons of tetragonal SrTiO3 were accurately determined using high-resolution hyper-Raman spectroscopy on oriented-domain crystals.
Abstract: The frequencies of the softest polar optic phonons of tetragonal SrTiO3 are accurately determined using high-resolution hyper-Raman spectroscopy on oriented-domain crystals. The splitting of the polar soft mode in the tetragonal phase is quantitatively related to the 105 K structural transition, i.e., to the rotation of the oxygen octahedra and to the associated macroscopic strains. In addition to quantum fluctuations, these tetragonal distortions are found to be essential in preventing the ferroelectric transition.
TL;DR: In this paper, the structure of nanocrystallized zirconia has been investigated in the 1-40-GPa pressure range at ambient temperature, and a continuous transformation towards the ideal fluorite cubic structure was observed from 8 GPa to 30 GPa.
Abstract: The structure of nanocrystallized zirconia has been investigated in the 1--40-GPa pressure range at ambient temperature. A continuous transformation towards the ideal fluorite cubic structure is observed from 8 GPa. The complete transformation seems to be achieved at 30 GPa. The compressibility of the tetragonal and cubic forms are derived and discussed.
TL;DR: In this paper, the lattice constants are obtained from the electron diffraction patterns for various orientations of single particles in the size range of 15-250 nm in diameter and the origin of the expansion is discussed on the basis of x-ray photoelectron spectroscopic analyses and a computer simulation.
Abstract: Nanocrystalline barium titanate particles are prepared by the alkoxide method. The lattice constants are obtained from the electron diffraction patterns for various orientations of single particles in the size range of 15--250 nm in diameter. The present result indicates that the structural change from a tetragonal (ferroelectric) phase to a cubic (paraelectric) one occurs around 80 nm in diameter, which is in good agreement with a critical diameter recently reported. Large lattice expansions of more than 2.5% are detected in the particles down to 15 nm in diameter. The origin of the expansion is discussed on the basis of x-ray photoelectron spectroscopic analyses and a computer simulation.
TL;DR: In this paper, two alloys, having different oxidation behavior (Zy4 and Zr-1NbO), have been investigated during oxidation at high temperature (743 K) and low oxygen pressure (10 kPa) by in situ X-ray diffraction (XRD).