Showing papers on "Perovskite (structure) published in 1972"

The structures of sodium niobate between 480° and 575°C, and their relevance to soft-phonon modes

Abstract: The structures of NaNb03 in the S phase (480°-520°c) and T1 phase (520°-575°c) have been determined by use of the octahedron tilting schemes of Glazer. Both phases are orthorhombic with unit cell edges approximately twice those of ideal perovskite; their space groups are Pnmm and Ccmm, respectively. An explanation of the T2 to T1 (575°c) transition is given in terms of a soft phonon mode at q = 1/2 1/2 1/2 A comparison is made with the low-temperature transitions in KMnF3.

Theory of Perovskite Ferroelectrics

Abstract: A model Hamiltonian has been constructed for the ferroelectric transition in the perovskite structure in terms of localized-strain and soft-normal-mode coordinates and temperature-independent model parameters. No anharmonic interactions higher than fourth order are considered. The free energy for tetragonal, orthorhombic, and trigonal distortions is calculated in a molecular-field approximation with the cubic structure as a reference configuration. From the free energy, the polarization and the strain distortions are determined. The soft-mode frequencies and the shifts in the acoustic-phonon frequencies are calculated from linearized equations of motion describing the fluctuations about these average values. The soft-mode frequencies in cubic structure vanish at the supercooling temperature, as usually assumed. However, in the distorted structure they remain finite at the stability limit determining the superheating temperature. The model Hamiltonian describes first- or second-order transitions depending on the strength of the coupling with the strain. For suitable choice of model parameters, the model allows for transition from the cubic to the tetragonal phase as in PhTi${\mathrm{O}}_{3}$, to the trigonal phase as in CsGe${\mathrm{Cl}}_{3}$ and various solid solutions, as well as a series of transitions from high- to low-symmetry structures, as in BaTi${\mathrm{O}}_{3}$.

Dielectric and Piezoelectric Properties in the Ternary System of Pb(Zn1/3Nb2/3)O3-Ba(Zn1/3Nb2/3)O3-PbTiO3

Abstract: Phase diagram of the ternary system of Pb(Zn1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3–PbTiO3 is determined from structural and dielectric measurements. In the vicinity of Pb(Zn1/3Nb2/3)O3, two phases of perovskite (ferroelectric) and pyrochlore (non-ferroelectric) types are coexistent. The piezoelectric active part is found near the morphotropic (rhombohedral-tetragonal) phase boundary. The most active one is obtained for the composition of 0.8Pb(Zn1/3Nb2/3)O3–0.2Pb0.7Ba0.3(Zn0.1Nb0.2Ti0.7)O3, where the planar coupling factor reaches 0.44.

Nuclear Magnetic Resonance of Ferromagnetic Mn 3 AlC and Mn 3 GaC

Abstract: The nuclear magnetic resonances in metallic perovskite compounds Mn 3 AlC and Mn 3 GaC have been studied. In Mn 3 AlC the Al 27 and Mn 55 resonances are observed at 78 and 140 MHz, respective at 4.2 K. The corresponding hyperfine fields at Mn 55 and Al 27 and 133 and 70 kOe at 0 K, respectively. In Mn 3 GaC three signals are observed only in the ferromagnetic phase. The resonances occur at 120, 156.5 and 199 MHz at 136 K, which are assigned to Mn 55 , Ga 69 and Ga 71 nuclei, respectively. The hyperfine fields at Mn 55 and Ga nuclei are 123 and 163 kOe at 0 K respectively. The temperature dependence of the frequencies and the echo intensity have been measured.

Optical Absorption of NaMnF3

Abstract: Optical absorption of a NaMnF3 single crystal has been studied at 300°K and 80°K. The scheme of fitting the band positions involves four parameters B, C, Dq and α. With α fixed at its free ion value (76 cm−1) the values of B, C, and Dq at 80°K are found to be 845, 3040, and 775 cm−1, respectively. Vibrational progressions are observed in the fine structure of some of the bands at 80°K from which approximate vibration frequencies of the crystal have been determined and compared with those of other perovskite fluorides. Some of the bands observed above 35 000 cm−1 are identified with the simultaneous electronic excitation of Mn2+ ion pairs.

Oxygen Vacancies in the Perovskite-type Ferrites. II. Mössbauer Effect in the SrFeO2.5 -LaFeO3 Solid-solution System

Abstract: The Mossbauer effect was studied for the solid-solution Sr1−xLaFeO2.5+x⁄2 system, prepared by firing in a vacuum. The intensity of the characteristic six absorption lines of the tetrahedral-iron site in the brownmilleritetype phase (0.0≤x≤0.02) was weakened with an increase in the La content because of an increasing incorporation of oxygen, while the Mossbauer parameters remained constant. SrFeO2.5 with a brownmillerite-type structure, completely changed into a cubic perovskite-type structure at the composition with x=0.30; however, the original absorption peaks for the tetrahedral-iron site still exist even if the intensity is considerably decreased. Moreover, new peaks came out, with Mossbauer parameters different from those for the octahedral- or the tetrahedraliron sites. On the basis of the values of their Mossbauer parameters, the origin of the new peak can be assigned to the presence of a five-coordinated iron site surrounded by a trigonal bipyramidal polyhedron of oxygen rather than a square pyram...

Soft modes and the structure of ferroelectric tetragonal potassium tantalate-niobate (I)

Abstract: The structure of ferroelectric tetragonal KTN (KTaxNb1 − xO3 with x = 0.56), which is isomorphous with BaTiO3, has been determined by neutron diffraction. The atomic displacements from the cubic perovskite positions are δ(K) = 0.012 ± 0.003, δ(Ta,Nb) = 0.0003, δ(O(1)) = −0.010 ± 0.001 and δ(O(2)) = δ(O(3)) = −0.011 ± 0.001 in fractions of the unit cell edge c = 4.006 A = 1.0033a. These displacements are in fair agreement with those expected from the soft optic mode identified by inelastic neutron scattering. The essential character of this mode at q = 0 is a vibration of the oxygen atoms against the remainder, with the heavy (Ta, Nb) atoms virtually fixed at the centre of mass.

固相反応におけるマグネシウムニオブ酸鉛Pb(Mg1/3Nb2/3)O3の生成過程

Abstract: 1. The processes in the solid state reaction on the formation of perovskite type Pb(Mg1/3Nb2/3)O3 from its component oxides have been investigated by a powder X-ray diffraction measurement. As a result, it is found that pyrochlore phase exists in three types, and that an interchange of these phases occurs at about 790°C, accompanied with the melting phenomena.2. In the formation of the perovskite type, it seems that there are two processes involving the solid reaction on the bases of 2PbO⋅Nb2O5 and 3PbO⋅Nb2O5, respectively, and the composition of the product from 3:1:1 mixture (PbO:Nb2O5:MgO in molecular ratio) is an intermediate state of these in the two processes. Lattice constant of rhombohedral pyrochlore phase gradually approaches that of cubic one as the amount of Pb substituted by Mg increases, and the transformation to cubic type is completed in the substituted amount about five mole per cent, and above this amount the substitution by Mg does not occur.3. When MgO contained in the mixture is in excess (e.g., 3:1:1.2), the amount of PbO volatilized decreases, and simultaneously the yield of perovskite increases. It is considered that the existence of excess MgO promotes the reactivity of MgO to the (NbO3) network and drives the free PbO into the stable lattice.4. By the hydrothermal treatment, cubic pyrochlore is obtained at relatively low temperature although no perovskite is formed.5. For tetragonal pyrochlore, two atomic configurations are hypothetically suggested. Based on these configurations, the calculated powder X-ray patterns are compared with the observed one. It is concluded that the network-vacant configuration is more probable.

X-ray and dielectric studies of irradiated perovskite type compounds.

Abstract: 2014 As a result of irradiating polycrystalline samples of BaTiO3, SrTiO3, PbTiO3, NaNbO3 and PbZrO3 by fast electrons, and by gamma-radiation of Co60, qualitatively identical reversible changes in the dielectric constant have been revealed. X-ray evidence of the crystal structure transformation into an amorphous state, followed by the formation of a 03B2-PbO2 phase, have been found in PbTiO3 as a result of in-pile irradiation by fast neutron integrated fluxes 03A6 > 1020 cm-2. Lattice parameter measurements in PbTiO3 indicate a tendency toward the radiation induced phase transformation, analogous to that of BaTiO3 and PbZrO3. LE JOURNAL DE PHYSIQUE TOME 33, AVRIL 1972, Classification : Physics Abstracts 17.29 Experimental data obtained in the last years have made it possible to outline the general features of processes which take place in perovskite type materials under the effect of various irradiation conditions [1]. Nevertheless additional information is necessary to solve a number of problems concerning the nature of reversible and irreversible radiation effects in various materials of this type. This work deals with a study of the experiments pertaining to two aspects of the problem mentioned above. 1. The effect of électron and gamma irradiation on the dielectric constant of some ferro-, antiferro-, and paraelectric perovskites. Changes of low field dielectric constant (e) for polycrystal SrTi03, BaTi03, PbZr03, NaNb03, PbTi03 which occur because of gamma and fast electron irradiation are shown in figures 1 a and 1 b respectively (also see [2]). Radiation stability of SrTi03 appears to be maximum but that of PbTi03-minimum though qualitative changes of a are similar for all the compounds under study. Herewith electron energy (1.14 and 9.0 MeV) and also dose rate (100 and 400 rd/s) changes do not affect the nature of the changes of 8. Crystal lattice parameters of all the irradiated materials remain unchanged within + 0.001 À. Radiation induced changes of 8 vanish as a result of subsequent annealing at 150 °C for about an hour. Therefore one may consider these changes to be reversible [1], [3], [4] and they may be treated as radiation induced ageing found earlier on the s values measured both in high [5] and low electric fields [6]. In the course of irradiation the accumulation of reversible defects are accompanied by the annealing both of FIG. 1. Changes of low field dielectric constant of polycrystal BaTiO 3, PbTiO 3, PbZrO 3, NaNbO 3, SrTiO3 measured at room temperature vs integrated dose of irradiation : a) by fast electrons (E = 1.14 MeV ; flux density = 5 x 1012 cm-2. s-1) ; b) by gamma-rays of Co6o (dose rate 100-400 r/s). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01972003304044300

Matériauxcrystal chemistry of perovskite-like layer-type ferroelectrics

Abstract: The number of monooctahedral layers in the perovskite-like package m depends on the value of tolerance factor t. The lower and upper criteria of layer structure formation are t1 ≈ 0.87 and tu ≈ 0.99. In [Bi2O2]2+ layers only one of the Bi3+ ions may be replaced, and only by trivalent cation ; that leads to increasing of Tc. Depending upon the polarizability of A and O2-ions increasing of m causes both raising and lowering of Tc. Bi2WO6 is the first one-layer ferroelectric with Tc ≈ 950 °C. Crystals Bi2NbO5F and Bi2TaO5F are also one-layer ferroelectrics with Tc ≈ 30 °C and 10 °C.

Oxidation of Reduced Perovskite-type Ferrites

Abstract: The oxidation of the reduced solid solution Sr1-xMxFeO2.5+x/2(MY3+ and La3+) was studied by means of the high temperature X-ray diffractometry and the thermogravimetry. The end member SrFeO2.5, which is isostructural with the brownmillerite having orthorhombic symmetry, changes to the cubic perovskite-type structure when Sr is substituted for Y in the composition range of 0.15

Interpretation of Mossbauer spectra of Mn3GaC doped with 57Fe

Abstract: The number of components in Mn3GaC paramagnetic spectrum at 300 K is related to the carbon content of this perovskite carbide. From the ferromagnetic spectrum at 77 K analysed on the basis of the paramagnetic spectrum decomposition, the spin direction is shown to be parallel to the (101) direction.

Energy Bands for Transition Metals and Compounds

Abstract: The present status of band‐structure calculations for transition metals and compounds is reviewed by considering a few selected examples of both superconducting and nonsuperconducting materials. Similarities and differences in the band structures of metals and compounds are illustrated by comparing results for the 3d metals with those for the monoxides. Analogous results are described for 4d and 5d metals, including bcc Nb, Ta, and W, and hcp Re. Finally, energy‐band results are presented for several superconducting compounds, including materials with the rocksalt (NbN), β‐tungsten (V3Ga), and perovskite (SrTiO3) structures.