Showing papers on "Perovskite (structure) published in 1974"

Silicate perovskite from phase transformations of pyrope‐garnet at high pressure and temperature

Abstract: Both natural and synthetic pyrope have been compressed to pressures above 300 kbar in a diamond-anvil press and heated to temperatures above 800°C by a continuous YAG laser. After quenching and release of pressure, X-ray diffraction shows that the natural pyrope transforms to a single phase with perovskite-like structure whereas the synthetic pyrope disproportinates into a mixture of MgSiO3 (perovskite) plus Al2O3 (corundum). The orthorhombic perovskite lattice parameters for the natural pyrope are ao = 4.816 ± 0.004 A, bo = 4.973 ± 0.004 A, and co = 6.997 ± 0.006 A with Z = 4. The orthorhombic cell dimensions for MgSiO3 (perovskite) are ao = 4.818 ± 0.005 A, bo = 4.869 ± 0.005 A, and co = 6.906 ± 0.007 A with Z=4. The calculated density of MgSiO3 perovskite is thus 4.12 g/cm³, or 3.7% denser than the isochemical mixed oxides. These experiments are the first demonstration of static high pressure phase transformations in silicate garnets and the first examples of a pure silicate existing in the perovskite structure.

Polycritical Points and Floplike Displacive Transitions in Perovskites

Abstract: The critical behavior occurring at displacive phase transitions in anisotropically stressed perovskite crystals is examined. It is shown that, under various conditions, the asymptotic critical behavior may be of Ising, $\mathrm{XY}$-model, or Heisenberg type. The existence of a "spin-flop"-like transition (at zero stress) between two differently ordered phases is predicted. An explanation of the discrepancy between the measured exponents and those predicted theoretically is proposed, and several new experiments are suggested.

Crystallographic, Electric and Thermochemical Properties of the Perovskite-Type Ln1-xSrxCoO3 (Ln: Lanthanoid Element)

Abstract: The perovskite-type compounds Ln1-xSrxCoO3 (Ln=lanthanoid element) were synthesized, and their crystallographic, electric and thermochemical properties were investigated. The region of the perovskite formation was determined. At the lower Sr content, lanthanoid ions, La to Sm except Ce, form the perovskite-type cobalt oxides. From Eu to Er, the lower boundary of the perovskite formation increases as the atomic number increases. The relaxation of a rhombohedral distortion of La1-xSrxCoO3, as Sr content is changed, is quantitatively described. At certain levels of Sr content, the compound shows that its temperature coefficient of resistivity is small in a wide temperature range. The resistivity and the non-stoichiometry of the compounds have a close relation in that the resistivity increases as the oxygen-deficiency is increased.

Neutron powder profile refinement of ferroelectric and antiferroelectric crystal structures: Sodium niobate at 22°C

Abstract: The profile refinement of neutron powder patterns permits a rapid and precise determination of structural transitions. For example, the room temperature structure of the rather complex distorted perovskite NaNbO3has been obtained independently in good agreement with the X-ray measurements of Sakowski-Cowley, Lukaszewicz and Megaw. The speed and ease of neutron powder data collection means that structures can be compared for many different conditions of temperature, pressure and chemical composition.

CRYSTAL SYSTEMS AND LATTICE CONSTANTS OF PEROVSKITE-TYPE COMPOUNDS BaThO3 AND Ba2(BaU)O6

Abstract: BaThO3 had an orthorhombic lattice (a=6.345A, b=6.376A, c=8.992A) with four pseudomonoclinic perovskite units(a′=c′=4.498A, b′=4.496A, β=90°17′). Ba2(BaU)O6 was a tetragonal, ordered perovskite(a=b=6.286A, c=8.972A). Ahrens′ radius of octahedral Ba2+ in Ba2(BaU)O6 was evaluated to be 1.16A via a lattice parameter variation versus radius of M4+ ion in perovskites BaMO3.

Static pseudo Jahn--Teller effect at point defects in irradiated ferroelectric crystals (perovskite structures). II. Theoretical

Abstract: The pyroelectric current spectrum (P.C.S.) measured in iron-doped barium titanate shows new features which can be explained taking into account a static pseudo Jahn-Teller effect both at trapped-electron defect centres (T.E.D.C.) and at photoexcited electron defect centres (P.E.D.C). Furthermore it is necessary to introduce a polaronic exchange between T.E.D.C. and P.E.D.C. (The role of the dynamic part of the Jahn-Teller effect does not have to be considered although it is not necessarily weak.) The static part of this effect is introduced by the interaction of a single phonon mode and a two-electronic level system. The contribution of this effect is found greater at T.E.D.C. than at P.E.D.C. The model differs from the latest ones proposed for barium titanate (Fridkin) and for lithium niobate (Staebler) so far as they do not include the Jahn-Teller effect but rather macroscopic internal fields. Our model does not exclude this last possibility.

High energy storage lasers employing perovskites containing rare earths

Abstract: New lasers employ a halide crystal including alkali metals and rare earths with a very high concentration of the active ions that are arranged to have largest possible separation between themselves. Such laser crystals provide very high energy storage that is finally released in the form of optical radiation. The high energy storage is attributable to a relatively low gain per unit length. The halide crystal is of the perovskite type and typically includes two alkali metals, one of which, for example, sodium, has a much smaller ionic charge than the rare earths and, therefore, will be ordered on the octahedral sites of the perovskite crystal with respect to the rare earth atoms. The stoichiometry of the materials facilitates the high concentration of the rare earth active ions; and the ordering or strict sequencing of alkali and rare earth ions facilitates the large separation of the rare earth active ions which is necessary to minimize non-radiative decay via rare earth pair interaction. If other rare earths are employed other than the active ion, they can serve as diluents or energy transferring elements but are typically not ordered with respect to the active ions, even though on the octahedral sites, unless they are sufficiently different in size from the active ion. In addition, transition metal ions such as chromium can be included and will appear on the octahedral sites and serve to transfer energy to the active ion. When erbium (Er), ytterbium (Yb) or thulium (Tm) are included in suitable combinations, energy conversion of the emission from the infrared to the visible can be obtained. A typical crystal is dicesium sodium neodymium hexachloride (Cs 2 NaNdCl 6 ), in which Nd 3 + is the active ion. It is likely that the alkali metals may be replaced by other monovalent ions such as thallium (T1 + ), (Cu + ), and silver (Ag + ).

Defects and diffusion paths in perovskite-like structures ABO3+x

Abstract: A new mechanism is proposed for the formation and the migration of planar defects in ABO3+x, perovskite-type compounds.

Phase transitions in (CnH2n 1NH2)3Mnc14

Abstract: (CnH2n+1NH3)2MnCl4 compounds consist of perovskite type layers sandwiched between hydrocarbon radical layers and can be regarded as two-dimensional perovskites. New phase transitions were detected in compounds with n ≥ 3. All compounds show at room temperature an orthorhombic modification with twinning similar to domain patterns in BaTiO3. At higher temperatures the walls disappear at a second order transition. No evidence of ferroelectricity could be found for the room temperature phase of the n = 1 compound. The transitions are shifted by deuteration and a change in the chain length of the alky1 changes also the phase sequence.

Über Perowskitphasen in den Systemen AO–SE2O3–UO2,x mit A = Erdalkali und SE = Seltene Erden, La sowie Y. VII. Die Systeme Sr2(SE1‐3xSr2xU vx) UvO6 und Sr2,67‐yCayU v1,33O6

Abstract: Im System Sr2(SE1−3xSr2xUx)UO6 reicht die Phasengrenze nicht bis zur Zusammensetzung Sr2,67U1,33O6 (x = 0,33). Statt dessen bilden sich SrO-armere Perowskitphasen. Ein Einbau von Ca2+ ist in der Reihe Sr267-yCayU1,33O6 fur y = 0,67 und 1,33 moglich; fur grosere y-Werte treten erdalkalioxidarmere Verbindungen mit Lucken im Kationen- und Anionengitter auf. On Perovskite Phases in the Systems AO–SE23–UO2,x with A = Alkaline EarthMetal and SE = Rare Earths, La, a n d Y. VII. The Systems Sr2(SE1−3xSr2xU)UVO6 and Sr2,67−yCayUO6 In the Sr2(SETubingen1−3xSr2xUx)UO6 system the phase boundary do not reach the composition Sr267U1.33O6 (x = 0.33). Perovskite phases with lower SrO contents are built up instead. In the Sr2,67-yCayU1,33O6 serie an incorporation of Ca2+ is possible for y = 0.67 and 1.33. For greater y values compounds with lower alkaline earkh oxide contents are formed. They contain cationic and anionic vacancies.

Mossbauer effect in perovskite-type ferroelectrics: Lattice dynamics and chemical shift data

Abstract: Mossbauer study of polycrystallic BaTi03 (I), PbTi03 (11) and PbZr03 (111), doped with 1 at.% of radioactive Sn1 19m was performed. The Mossbauer fraction f and the chemical shift δ of the resonance γ-rays were measured in the temperature range 77-1000°K. In the vicinity of the phase transitions the data were taken in great detail.