Showing papers on "Tetragonal crystal system published in 1970"

Raman Spectra of Polycrystalline Solids; Application to the Pb Ti 1 − x Zr x O 3 System

Abstract: It is shown that Raman scattering from polycrystalline piezoelectric solids contains all of the essential features of Raman single-crystal spectra even when the crystal spectrum is quite anisotropic As an illustration of the Raman powder technique, the ceramic ferroelectric-antiferroelectric system PbTi${\mathrm{O}}_{3}$-PbZr${\mathrm{O}}_{3}$ has been investigated and the "soft" $E(\mathrm{TO})$ optical phonon modes measured in the tetragonal ferroelectric region

X-ray and neutron diffraction study of tetragonal barium titanate

Abstract: An attempt is made to differentiate between the three previously proposed models of the structure of tetragonal BaTiO3. In the new model, the serious parameter-interaction problem was avoided by the use of both neutron and X-ray diffraction data collected from c-domain single crystals. The result of the refinement gave the displacement of the Ti atom from its cubic position as 0.0135 ± 0.0004 A; the R value was 0.027 and 0.024 for the neutron and X-ray data respectively. The analysis also showed that the temperature parameters, B33, along the c axis are approximately independent of mass; B33 = 0.32 ± 0.03 A2 from X-ray and 0.33 ± 0.04 A2 from neutron diffraction. The average value obtained for the B11 temperature parameters, B11 = 0.50 A2, is considerably higher than the B33 value, which thus clearly indicates the existence of low frequency optic modes at room temperature.

Crystal Structures of the Three Modifications of Nitrogen 14 and Nitrogen 15 at High Pressure

Abstract: We have determined from x‐ray diffraction photographs that high‐pressure γ N2 is tetragonal with two molecules per unit cell in special position f of space group P42 / mnm. At an average pressure and temperature of 4015 atm and 20.5°K, respectively, the unit cell dimensions are a = 3.957 A and c = 5.109 A, giving a molar volume in good agreement with that from p–V–T measurements. The β N2 solid modification, which is contiguous with the melting curve, remains hexagonal up to at least 4125 atm and 49°K where the unit cell constants are a = 3.861 A and c = 6.265 A. Over the pressure range investigated, the c / a ratio is very close to the ideal value for closest packing of hard spheres. The atomic positions in hexagonal (β) N2, which are known to be highly disordered at low pressure, show no evidence of ordering at the highest pressures studied. The third allotrope, α N2, is cubic at 3785 atm and 19.6°K with four molecules in a unit cell 5.433 A on a side. Both the Pa3 and P213 space groups, which have been...

Polymorphic Behavior of Thin Evaporated Films of Zirconium and Hafnium Oxides

Abstract: Polymorphism in thin evaporated films of zirconium and hafnium oxides was investigated from 100° to 1500°C by electron diffraction and transmission electron microscopy. The films have metastable cubic structures at room temperature and at moderate temperatures. Zirconium oxide, depending on temperature, exists in cubic, tetragonal, and monoclinic forms, whereas hafnium oxide transforms directly from the cubic to the monoclinic structure. The transformation temperatures depend on the oxygen partial pressure. Air annealing of thin films of ZrO2 and HfO2 lowered the temperature of transformation of the tetragonal and the cubic structure into the monoclinic structure by about 150° and 100°C, respectively. The cubic/tetragonal transformation of ZrO2 is monotropic, whereas the tetragonal monoclinic transformation occurs by the typical nucleation and growth mechanism. Determination of grain size in ZrO2 at the tetragonal/monoclinic transformation temperature showed that the transformation occurs when a constant grain size of about 800 A is reached. The oxygen partial pressure, grain size, and temperatures at which the metastable phases exist were correlated. The rate of grain growth is enhanced by increase in oxygen partial pressure. The accelerated transformation in air is attributed to rapid attainment of the critical size; grain boundary energy is an important controlling factor in transformation.

The crystal structure of CaO.2Al2O3

Abstract: The possible use of 0.1% Nd 3+ in CaO. 2A1203 as a solid-state laser has made it necessary to determine the structure of this compound accurately and eliminate the uncertainty in previous publications. A full X-ray analysis has been performed on a single crystal using a four-circle diffractometer and the results have shown the crystal to have a space group C2/c. A comparison with the published structure of the supposed isomorphous strontium compound gave an initial structure which was found to refine satisfactorily to give a reliability index, R= 0-064, from which the atomic parameters were obtained. The local symmetries of the calcium and aluminum sites are then discussed with respect to the effect on the crystalline electric fields at substituted Nd3+ ions.

Monoclinic–tetragonal phase transition in zirconia: mechanism, pretransformation and coexistence

Abstract: The high temperature X-ray diffraction study of the monoclinic tetragonal phase transition in ZrO2 showed that it is spread over a temperature range 930–1220°C. Anomalous intensity changes are observed in the pretransformation region 930–1100°C. Coexistence of phases through hybrid crystal formation in the region 1100–1220 °C and the mechanism of transition are discussed. The orientation relationship between the monoclinic (m) and the tetragonal (t) crystal structures consists in the parallelism of the (100)m plane to (110)t and of the bm axis to the ct axis. A drastic change in a small temperature range during the tetragonal–monoclinic transition is interpreted as a cooperative change in both short and long range interactions. The large thermal hysteresis is attributed to the difference in the mechanism of transition during heating and cooling.

Crystal Structure of Luminescent ZnSiP2

Abstract: ZnSiP2 crystallizes with the chalcopyrite structure: there is no detectable difference in x‐ray scattering between luminescent and nonluminescent crystals. The level of impurity, either in the form of chemical dopant, physical defect, or a departure of less than about 1% from complete order at the Zn and Si sites hence determines if the crystal luminesces or not on excitation by an electron beam. The lattice constants of this tetragonal crystal at 298°K are a = 5.399 ± 0.001 and c = 10.435 ± 0.002 A. The space group is I42d, and there are four formulas in the unit cell. A total of 2023 reflections were measured with the luminescent crystal, and 1600 with the nonluminescent crystal, using pexrad. The final agreement factor, based on a combination of 197 symmetry‐independent reflections from the first crystal and 176 from the second, is 0.049. Each Zn is tetrahedrally surrounded by 4 P atoms at 2.375 ± 0.001 A distance, and each Si by 4 P atoms at 2.254 ± 0.001 A. All phosphorus tetrahedra have four P–P ed...

Cubic FeS, a Metastable Iron Sulfide

Abstract: Studies of the corrosion products of metallic iron formed in the absence of air oxidation in solutions of hydrogen sulfide have revealed the existence of a new phase, cubic FeS, associated with tetragonal iron sulfide, Fe1 + xS. This new phase is metastable and has a sphalerite-like structure.

Study of Order‐Disorder in Rock‐Salt‐Related Structures by Infrared Spectroscopy

Abstract: Infrared spectra were determined for a series of oxides related by ordering and distortion to the rock-salt structure. The expected spectrum of the tetragonal 1:1 ordering (space group D4h19) contains 5 ir-active and 8 Raman-active bands. For the trigonally distorted structure (space group D3d5) factor group calculations yield 4 ir and 2 Raman modes. The expected additional structure appears in the spectra of the tetragonal materials but is broadened and washed out by the long-range polarization field. The spectra of the trigonal materials contain twice the predicted number of modes. The broadened bands of the 6-coordinated structures are compared with the sharp spectra of 4-coordinated γ-LiAlO2.

Elastic Anomaly of Ba2NaNb5O15

Abstract: The temperature characteristics of elastic, piezoelectric, dielectric, and optical constants of Ba2NaNb5O15 have been measured from room temperature up to the Curie point. The shear elastic compliance related to the tetragonal principal axes (TS66) is found to become large near the transition point from tetragonal to orthorhombic phase. A phenomenological approach is tried, in which the free energy is expanded in power series of the polarization and strain, and it becomes clear that this tetragonal‐orthorhombic transition is little dependent of the behavior of spontaneous polarization but that elastic instability is the main cause of this phase transition. The elastic, piezoelectric, dielectric, and optical constants in the orthorhombic phase are derived from the constants in the tetragonal phase by the biased effect of the spontaneous shear strain to the elastic higher‐order terms in the free energy.

Synthese und Kristallstruktur von Tetramethylammonium‐Gismondin

Abstract: A new synthetic zeolite, (CH3)4NA1Si3O8, H2O, has been synthesized and shown to be an isotype of the mineral gismondine, CaAl2Si2O8,4H2O. Sodium and other cations can be introduced by ion exchange after thermal decomposition of the organic cation. A continuous structural change to «cubic» NaP has thereby been recorded, which indicates that the latter is also based on a gismondine-type aluminosilicate framework. The crystal structure of tetramethylammonium-gismondine has been determined using X-ray powder data supplemented by electron diffraction. The crystals are tetragonal, a = 9.84 and c = 10.02 A, with 4 formula units per unit cell. The apparent symmetry of the framework structure is I41/amd, however, this is violated by the organic cation. Two of the methyl groups are pointing to oxygen atoms of the framework and the short methyl-oxygen distances indicate CH O interaction.

Phase Separation by Spinodal Decomposition in the Tetragonal System

Abstract: Coherent spinodal curves for the [001] and [100] directions were calculated for the system TiO2-SnO2. Inside the [001] spinodal curve, the crystalline solutions decomposed by a modulated phase composed of TiO2-rich and SnO2-rich plates perpendicular to the [001] direction. No modulation of structure was observed at compositions and temperatures between the [001] coherent spinodal and the equilibrium curve. The kinetics of the decomposition showed that the wavelength of the modulation decreased in the early stages of the separation, reaching minimum observable values of approximately 110 A at 925°C, 170 A at 1000°C, and 240 A at 1150°C. In the second stage when the coherency was lost, the wavelength increased. Theoretical calculations to find the elastic free energy/unit volume to cause fluctuation of composition in the spinodal process were worked out for the tetragonal system. The calculations were done for an arbitrary direction and involved elastic constants. The theoretical prediction and the experimental results for the system TiO2-SnO2 agreed well.