Showing papers on "Tetragonal crystal system published in 1968"
TL;DR: The Ferroelectric Ba0.27Sr0.75Nb2O5.78 as discussed by the authors is a tungsten bronze-type structure crystallizing in the tetragonal system, with lattice constants a = 12.43024
Abstract: Ferroelectric Ba0.27Sr0.75Nb2O5.78, with Tc = 348° ± 15°K, is a tungsten bronze‐type structure crystallizing in the tetragonal system, with lattice constants a = 12.43024 ± 0.00002 and c = 3.91341 ± 0.00001 A at 298°K, space group P4bm, and five formulas in the unit cell. The integrated intensities of 6781 structure factors were measured with PEXRAD, 875 symmetry‐independent structure factors being significantly above background. The metal‐atom positions were determined from the three‐dimensional Patterson function and the oxygen atoms from subsequent Fourier series. The final agreement factor between measured and calculated structure factors is 0.0508. The structure consists of close‐packed slightly puckered layers of oxygen atoms separated by nearly c / 2. The Nb atoms are slightly displaced from one layer, the Ba and Sr atoms from the other and in the same sense. The oxygen atoms in the Ba and Sr layer are disordered. Neither of the two independent sites occupied by the Ba and Sr atoms is fully filled....
570 citations
TL;DR: The structure of SrTiO3 has been the subject of X-ray, infrared, [2] ESR, [3-5] neutron, [6] and Raman [7-10] spectroscopic investigations since 1962 as discussed by the authors.
Abstract: The structure of SrTiO3 has been the subject of X-ray, [1] infrared, [2] ESR, [3–5] neutron, [6] and Raman [7–10] spectroscopic investigations since 1962. While the high-temperature phase is acknowledged to be simple cubic perovskite 0 h 1 , with one formula group per unit cell, there have been, until recently, apparent inconsistencies in the diverse data and a lack of agreement concerning the symmetry of the crystal at low temperatures. Following discovery of anomalies in the sound velocity of SrTiO3 near 110°K [11], ESR studies [3] showed the presence of a cubic to tetragonal phase transition. This was verified by means of X-ray analysis [1]; in the latter study the c/a ratio was determined to be 1.000 56, and two other phase transitions were inferred to be at 65 and 35°K. Infrared and neutron studies [2, 6] supported the X-ray symmetry assignment of structure at 77 °K: the crystal was viewed as having domain structure with one of the equivalent (100) axes in each domain slightly elongated in the tetragonal phase; below 110°K the crystal was thought to retain a single formula group per unit cell and all atoms at inversion centers. The distortion from cubic symmetry was apparently so slight, in fact, that even the predicted F1u → Eu + A2u doubling below 110°K of each of the Oh -phase IR-active vibrations was unobservable in infrared studies. [2]
526 citations
TL;DR: Among the different phases of BaTiO 3 and KNbO 3, only the rhombohedral form is completely ordered as mentioned in this paper, while the orthorhombic, tetragonal and cubic phases are only partially ordered: the structure as determined by classical diffraction data is an average structure.
501 citations
TL;DR: In this paper, the electronic properties and i.r.s. spectra of a series of bis(ethylenediamine)copper(II) complexes are reported, and an infrared criterion is established for detecting the presence of weakly co-ordinated polyanions in the long tetragonal positions above and below the [Cu en2]2+ cation.
Abstract: The electronic properties and i.r. spectra of a series of bis(ethylenediamine)copper(II) complexes are reported. From a gaussian analysis of the electronic spectra, measured at room temperature and at the temperature of liquid nitrogen, of the seven complexes of known crystal structure, and three transitions 2A1gâ†�2B1g, 2B2gâ†�2B1g, and 2Egâ†�2B1g in D4h symmetry are tentatively assigned. From the e.s.r. spectra of polycrystalline samples of the complexes, which yield tetragonal spectra, the values of (g‖– 2)/(g⊥– 2) have been determined. Where the ratio is 4·0 the observed g-values are considered to reflect approximately the local copper(II) ion g-values. In the latter case the g-values have been combined with the appropriate electronic transitions and the orbital reduction factors k⊥ and k‖ evaluated. From the i.r. spectra of the polyanions, an infrared criterion is established for recognising the presence of weakly co-ordinated polyanions in the long tetragonal positions above and below the [Cu en2]2+ cation. The term semi-co-ordination is introduced to describe this structural situation.
351 citations
TL;DR: In this paper, the thermal behavior of amorphous hydrated zirconium oxide and its coprecipitates with other oxides was studied and a relation with the high-temperature solid solution was established.
Abstract: The thermal behavior of amorphous hydrated zirconium oxide and its coprecipitates with other oxides was studied. Solid solution with a limit of solubility seems to exist in the amorphous coprecipitates. A relation with the high-temperature solid solution was established. The structure of the amorphous zirconia was studied by X-ray and neutron diffraction; a thin plate model related to the structure of tetragonal zirconia is proposed. Heating the precipitates in an oxygen-free atmosphere produced a black nonstoichiometric oxide and stabilization of the tetragonal form.
316 citations
210 citations
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131 citations
TL;DR: In this paper, the formation of the ordered phase and the possible magnetic structures of ordered manganese alloys were discussed and the dependence of lattice parameters on concentration and temperature was determined.
Abstract: Investigations of the magnetic properties in the ordered CuAu‐I‐type manganese alloys are reviewed. The formation of the ordered phase and the possible magnetic structures are discussed. New experimental results for the Mn‐Ni, Mn‐Pd, and Mn‐Pt systems are reported.Neutron diffraction shows the basic antiferromagnetic structure to be the same for all the alloys investigated. In MnPt a magnetic‐structure transformation has been observed. Below 800°K the magnetic moments turn from the basal plane gradually to the tetragonal axis. The variation of the transition temperature in the Mn‐Pt system as well as that of the Neel temperature in the Mn‐Ni, Mn‐Pd, and Mn‐Pt systems with concentration were measured and are theoretically analyzed.The dependence of lattice parameters on concentration and temperature was determined. The lattice parameters suffer a significant but continuous change near the Neel temperature which is attributed to Mn‐Mn exchange interaction.Magnetic susceptibility measurements support the neu...
126 citations
TL;DR: In this article, the characteristics of a number of the more useful materials and particularly Ba 2 NaNb 5 O 15, Sr 2 NaNiB 5 O15, and K 3 Li 2 Nb 5O 15 are considered in some detail.
115 citations
TL;DR: In this article, the room temperature lattice spacings and magnetic susceptibilities of some PdIn alloys, and the room-temperature lattice spacing of some PtIn and PtSn alloys were examined.
Abstract: The room temperature lattice spacings and magnetic susceptibilities of some PdIn alloys, and the room temperature lattice spacings of some PtIn and PtSn alloys, have been examined in the present work. Extensive primary solid-solubility of indium and tin in α-palladium and α-platinum has been found and whereas indium expands the lattice of α-palladium to a slightly greater extent than tin, in the α-platinum solid solutions, larger expansions were obtained in the case of the tin alloys. The phases Pt3Sn and Pt3In have the ordered L12-type structure whereas Pd3In has a face-centred tetragonal (f.c.t.) structure but is face-centred cubic (f.c.c.) in the deformed (as-crushed) state. It was not possible to determine whether Pd3In is ordered because of the closely similar scattering factors of palladium and indium. The axial ratio of tetragonal Pd3In decreases with increasing temperature. The magnetic susceptibilities of the α PdIn alloys have been correlated with the effective valency of the alloys and deviations from the behaviour expected from the simple rigid band model occur at indium contents in excess of 8 at.%∗. The PdIn B2-type phase exists over an appreciable composition range (~16 at.%), and the variation of the lattice spacings over this range indicated that constitutional vacancies are probably formed at indium contents in excess of 50 at.%. The structures and lattice spacings of the phases Pd2In3, PdIn3, PtSn and PtSn2 have also been determined in the present work.
TL;DR: In this article, it is argued that such a relationship exists only when the ligands can be considered to be either pure π-acceptors or pure -donors.
Abstract: Mossbauer spectra of compounds having the general formulae bis(1, 2-cyclohexanedione dioxime) iron (II) XY and phthalocyanineiron(II)(Z)2, where X, Y, Z represent ligands at the axial positions, have been measured at 77°K and in some cases at room temperature. The spectra consist of quadrupole-split doublets, from which two parameters have been obtained : the isomer shift δ and the quadrupole splitting, ΔEQ. These parameters appear to be linearly related to each other within both series of complexes. It is argued that such a relationship exists only when the ligands can be considered to be either pure π-acceptors or pure π-donors.
TL;DR: In this article, the authors investigated the crystal and magnetic structures and their transformations by X-ray and neutron diffraction as well as electrical resistivity measurements on samples of Mn0·91Ni1·09, MnNi and Mn1·07Ni0·93 composition.
TL;DR: In this article, the polarised single-crystal electronic spectra and e.s. spectra of six tetra-amminecopper(II) complexes of known crystal structure have been examined.
Abstract: The polarised single-crystal electronic spectra and e.s.r. spectra of six tetra-amminecopper(II) complexes of known crystal structure have been examined. In the two tetragonal octahedral complexes the order of the one-electron orbital energy levels is established as dx2–y2 > dz2 > dxy > dxz,dyz. In the strictly square coplanar complex Na4 Cu(NH3)4[Cu(S2O3)2], the order of the dxy and dxz,dyz levels may be inverted. The energy of the dz2â†�dx2–y2 transition can be used as a measure of the relative tetragonal distortion present and is correlated with the in-plane copper–nitrogen bond lengths. From the single-crystal e.s.r. spectra and the electronic energy levels the orbital reduction parameters are estimated.
TL;DR: In this paper, an extensive solid solution was observed in the system Pb(Sc1/2/,Nb 1/2,1-x,Tix,O3) consisting of the rhombohedral phase with a small amount of the tetragonal phase present.
Abstract: Extensive solid solution was observed in the system Pb(Sc1/2/,Nb1/2,)1-x,Tix,O3. In the range 0 ≤ x ≤ 0.425 a rhombohedral ferroelectric phase was stable at 25° C. In the range 0.45 ≤ x ≤ 1.00 a tetragonal ferroelectric phase was stable at this temperature. The phase diagram of the system below 500° C strongly resembles that of PbZrO3−PbTiO3. The compound Pb(Sc1/2Nb1/2)O3 exhibited rhombohedral perovskite cell symmetry below the ferroelectric ↔ paraelectric transition temperature, and the angle a was acute. The radial coupling coefficient was 0.46 for the composition Sc1/2Nb1/2)0.575Ti0.4250O3. At 25°C this composition consisted primarily of the rhombohedral phase with a small amount of the tetragonal phase present. The ferroelectric ↔ paraelectric transition occurred over a temperature range in the rhombohedral phase field. The spontaneous polarization was finite at temperatures considerably above the temperature of the permittivity maximum for a given rhombohedral solid solution.
TL;DR: In this paper, the optical spectra of Os4+ (5d4) in single cubic crystals of K2PtCl6, Cs2ZrCl6 and Rb2ZRCl6 at 4.2°K has been studied.
Abstract: The optical spectra of Os4+ (5d4) in single cubic crystals of K2PtCl6, Cs2ZrCl6, Cs2HfCl6, and Rb2ZrCl6 at 4.2°K has been studied. The spectrum is characterized by very narrow lines in the region from 10 000–36 000 cm−1. Vibrational structure is observed which may be assigned to the ungerade modes of the OsCl62− complex in the lattice. A crystal‐field analysis within the d4 manifold fits the observed ten electronic, 0–0 transitions with an rms deviation of 228 cm−1 for the parameters A = 0, B = 741, C = 1296, V = 22490, and λ5d = 2416 cm−1. The calculated molar magnetic susceptibility agrees with that found for concentrated crystals such as K2OsCl6 and Cs2OsCl6. The analysis of the electronic states was dependent upon a calculation of intensities for the t2g4 → t2g3eg transitions. The calculated results are given for a tetragonal distortion of the cubic field and are shown to qualitatively agree with the linewidth of the transitions. The calculated g values for all observed transitions are tabulated. Fina...
TL;DR: In this article, the formation of the modulated structure was verified by electron microscopy, which showed alternately dark and light lamellae composed of SnO2-rich and TiO 2-rich plates perpendicular to [001].
Abstract: Tetragonal crystalline solutions in the system TiO2–SnO2 were annealed at 850°C and 1000°C to study the mechanism of separation within the exsolution dome. X-ray diffraction results indicated a unidirectional composition variation along [001]. The formation of the modulated structure was verified by electron microscopy, which showed alternately dark and light lamellae composed of SnO2-rich and TiO2-rich plates perpendicular to [001]. The sum of the thicknesses of the dark and light lamellae (‘wavelength') decreased in the early stages of the separation and then increased at longer annealing times, the thickness varying between 180 and 400 A. It is proposed that the precipitation proceeds as follows: crystalline solution→intermediate modulated phase → equilibrium phases. The first step shows all the characteritics of the spinodal type of mechanism. It is shown theoretically that a one-directional fluctuation of composition parallel to [001] can be expected in the system TiO2-SnO2.
TL;DR: In this article, single-crystal platelets (3×3×0.1 mm) of both tetragonal and orthorhombic lead oxide have been grown from dilute NaOH solutions at high pressures using conventional hydrothermal techniques.
Abstract: Single‐crystal platelets (3×3×0.1 mm) of both tetragonal and orthorhombic lead oxide have been grown from dilute NaOH solutions at high pressures using conventional hydrothermal techniques. The predominant yield of the two phases was varied in accord with the P‐T phase diagram of PbO. Electrical measurements include the Hall coefficient and electrical conductivity as a function of temperature, while the corresponding photoeffects and optical absorption with monochromatic radiation were also studied. ``As‐grown'' tetragonal crystals were highly conducting n‐type with σ(300°K) ∼10−3 to 1.0Ω−1 cm−1, and had Hall mobilities of about 100 cm2V−1 sec−1. In constrast the ``as‐grown'' orthorhombic crystals were rather insulating with σ(300°K) ∼10−5Ω−1 cm−1 and a Hall mobility μe∼50 cm2V−1 sec−1. Heat treatment in air altered both the electrical and optical properties. Photoconductivity was observed in both crystalline forms. The optical measurements indicate band transitions at 1.90 eV and 2.75 eV for the tetragon...
TL;DR: In this paper, Young's modulus, resistivity and susceptibility measurements of manganese-rich γ-phase alloys have been investigated with x-ray and neutron diffraction.
Abstract: Manganese-rich γ-phase alloys of manganese-palladium and manganese-nickel have been investigated with x-ray and neutron diffraction, and the Neel and structure-transformation temperatures fixed with the aid of Young's modulus, resistivity and susceptibility measurements. High manganese content alloys distort from a face-centred cubic to a face-centred tetragonal structure at the Neel temperature. Alloys containing more than 12 at.% Pd and 15 at.% Ni are cubic below the Neel temperature (1 gt-or-equal, slanted c/a greater-than sign R: 0.999), and do not distort till a much lower temperature. In the more solute-rich alloys the structure transformation is suppressed entirely. A mechanism for the distortion is suggested, and it is pointed out that the resistivity and susceptibility results are not consistent with a localized model of the d electrons in these alloys.
TL;DR: In this article, a mechanism of energy dissipation due to strain-induced reorientation of tetragonality in binary manganese-copper alloys containing 60-90 wt.-% Mn is described.
Abstract: The results are described of some studies of a mechanism of energy dissipation due to strain-induced reorientation of tetragonality in binary manganese-copper alloys containing 60–90 wt.-% Mn. The dependence of the structure upon the progress of ageing and the temperature of observation is reported. A high damping capacity is associated with the presence of a tetragonal structure, and the damping capacity varies approximately as (strain)n, where n lies between 0.15 and 0.44. The reorientation of tetragonal domains has been investigated by X-ray techniques, and, the corresponding movement of domain boundaries has been observed and recorded by optical microscopy.
TL;DR: Tetragonal ZrO2 can be formed directly from the mono-clinic form at 25°C by applying pressures greater than 37 kbar as discussed by the authors, but the transition is reversible and cannot be retained in a metastable condition at ambient conditions.
Abstract: Tetragonal ZrO2 can be formed directly from the mono-clinic form at 25° C by applying pressures greater than 37 kbars. The transition is reversible, and the tetragonal phase cannot be retained in a metastable condition at ambient conditions.
TL;DR: Welin this paper determined the structure of a related mineral, thaumasite (Ca6H4(SiO4)2(SO4) 2(CO3)2.26H2O), which he found based on chains of Ca2+ and SiO44− ions with the remaining ions and water molecules in between.
Abstract: ETTRINGITE (Ca6[Al(OH)6]2(SO4)3.26H2O) occurs as a natural mineral, and is technically important as a hydration product of Portland and supersulphated cements and in its use, as satin white, for coating paper. It forms hexagonal, prismatic crystals which in synthetic material are often highly elongated. Bannister, Hey and Bernal1 obtained unit-cell data (hexagonal: a 11.26, c 21.48 A, space group P63/mmc, Z = 2). No crystal structure determination based on single crystal evidence has been reported, but Welin2 determined the structure of a related mineral, thaumasite (Ca6H4(SiO4)2(SO4)2(CO3)2.26H2O), which he found to be based on chains of Ca2+ and SiO44− ions with the remaining ions and water molecules in between.