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Showing papers on "Tetragonal crystal system published in 1972"



Journal ArticleDOI
TL;DR: In this paper, the authors present a review of the state-of-the-art work in the field of thermal Neutron Diffraction, focusing on the use of the Aeta Cryst.
Abstract: ABRAHAMS, S. C. 8~ PRINCE, E. (1962). J. Chem. Phys. 36, 50. BACON, G. E. (1969). Acta Cryst. A25, 391. BINAS, H. (1966). Z. anorg, allg. Chem. 347, 133. BUSING, W. R. & LEVY, H. A. (1964). Acta Cryst. 17, 142. CALLERI, M. t~ FERRARIS, G. (1967). Per. Mineral, 36, 1. CASSIEN, M., HERPIN, P. & PERMINGEAT, F. (1966). Bull. Soc. franc. Min~r. Crist. 89, 18. CHIDAMBARAM, P., SEQUEIRA, A. • SIKKA, S. K. (1964). J. Chem. Phys. 41, 3616. COOPER, M. J. (1969). Acta Cryst. A25, 488. COPPENS, P. (1970). Chap. 6 in Thermal Neutron Diffraction. Edited by B. T. M. WILLIS. Oxford Univ. Press. COULSON, C. A. (1970). Chap. 5 in Thermal Neutron Diffraction. Edited by B. T. M. Wn.Lm. Oxford Univ. Press. CURRY, N. A., DENNE, W. A. & JONES, D. W. (1968). Bull. Soc. Chim. France, p. 1748. DENNE, W. A. & JONES, D. W. (1969). Aeta Cryst. A25, S 125. FERRARIS, G., JONES, D. W. & YERKESS, J. (1971a). Acta Cryst. B27, 349. FERRARIS, G., JONES, D. W. 8¢. YERKESS, J. (1971b). Acta Cryst. B27, 354. FISCHER, E. (1960). Chem. der Erde, 20, 162. GUI~RIN, H. (1941). Ann. Chim. 16, 101. MARTIN, C., DURIF, A. & AVERBUCH-POUCHOT, M.-T. (1970). Bull. Soc. fi'anf. MinOr. Crist. 93, 397.

116 citations


Journal ArticleDOI
TL;DR: The structure of NaNbO3, phase T2 (immediately below the cubic phase), has been determined from lattice parameters and difference reflections as discussed by the authors, and the anomalous changes in mean square values of atomic displacements have been derived from changes in lattice parameter.
Abstract: The structure of NaNbO3, phase T2 (immediately below the cubic phase), has been determined from lattice parameters and difference reflections. It is tetragonal C-face-centred, with a= 7·8698(8) A, c=3·9428(4) A at 600°c. It corresponds to condensation of an M3 soft-phonon mode from the cubic phase, in contrast to SrTiO3 where the condensing mode is Γ25. The anomalous changes in mean square values of atomic displacements have been derived from changes in lattice parameters. Hence the intensities of x-ray scattering are calculated and compared with direct measurements of diffuse scattering by other workers, with satisfactory agreement. A mechanism for the transition is postulated.

93 citations


Journal ArticleDOI
TL;DR: In this article, the crystal structure of zirconium molybdate was determined from integrated precession photographs using MoKα radiation, and a three-dimensional network of cross-linked chains built-up of quite regular [ZrO 3 eq (OH 2 eq O 2 ax ]-pentagonal bipyramids and distorted cis -MoO 4 (OH)(H 2 O-octahedra).

85 citations


Journal ArticleDOI
TL;DR: In this article, the authors used the Landau-Devonshire theory of perovskite ferroelectrics to explain the morphotropic phase transition in the ceramic solid solutions Pb(Zr, Ti) O3.

84 citations



Journal ArticleDOI
TL;DR: In this paper, the formation of solid solution in the Co3−xMnxO4 system in atmospheres of oxygen, air and argon was examined at a constant temperature of 1000 °C.
Abstract: The formation of solid solution in the Co3−xMnxO4 system in atmospheres of oxygen, air and argon was examined at a constant temperature of 1000 °C. In oxygen, a small amount of the NaCl-type compound was found to co-exist with the cubic spinel in the composition rangex≤0.1. A single phase of the cubic spinel was found in the range 0.1 to 1.3 and the tetragonal spinel above 1.9. In the rangex=1.3 to 1.9 where the cubic and tetragonal spinels co-exist they both have very broadened diffraction line profiles. In air, the identified phases and the changes in their lattice constants with composition were very similar to those in oxygen, except that the NaCl-type compound and the cubic spinel co-existed over a larger range. In argon, the cubic spinel was not observed over any of the composition range and the NaCI-type compound and the tetragonal spinel co-existed in the wide range of 1.1 to 2.3. The experimental results are discussed with regard to the cation distribution in the spinel and also to the relative stability of Co3+- and Mn3+-ions under the low partial pressure of oxygen.

73 citations





Journal ArticleDOI
E. Pytte1
TL;DR: In this article, a model Hamiltonian was constructed for the ferroelectric transition in the perovskite structure in terms of localized-strain and soft-normal-mode coordinates and temperature-independent model parameters.
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${\mathrm{O}}_{3}$.

Journal ArticleDOI
TL;DR: In this article, the EPR spectra of Er3+, Yb3+, and Gd3+ have been observed in the host single crystals ZrSiO4, HfSiO 4, and Th SiO4 which were grown by a flux-gradient technique.
Abstract: The EPR spectra of Er3+, Yb3+, and Gd3+ have been observed in the host single crystals ZrSiO4, HfSiO4, and ThSiO4 which were grown by a flux‐gradient technique. For most systems, spectra were observed which exhibited both tetragonal and orthorhombic symmetry. The tetragonal spectra were characterized by a principal axis system which coincided with the principal crystallographic axes. The observed orthorhombic spectra were of two types. For Gd3+ in ZrSiO4 and HfSiO4, orthorhombic spectra due to eight inequivalent sites were observed and had principal axis systems which were not simply related to the host crystallographic axes. All other orthorhombic spectra resulted from four inequivalent sites whose z and x axes lay in {110} planes with equal angles between the crystal tetragonal symmetry axis (c axis) and each z axis. For both types of orthorhombic spectra, the corresponding EPR transitions from the different sites were super‐imposed with the applied magnetic field parallel to the c axis. Spin‐Hamiltonian parameters have been determined for the observed tetragonal and orthorhombic spectra. Powder pattern analysis was employed for the Gd3+ orthorhombic spectra, due to the complexity which resulted from the large number of inequivalent sites. The implications of these results regarding the nature of the various sites are examined in conjunction with the results obtained previously by others for impurities in zircon‐structure and scheelite‐structure hosts.

Journal ArticleDOI
TL;DR: In this article, the V1−xNbxO2 system has been studied by X-ray diffraction as function of temperature and composition; as in earlier works, three phases are shown to exist: a monoclinic phase which has the low temperature VO2 structure, and two tetragonal phases, one having the rutile structure and the other having low temperature NbO2 structure.

Journal ArticleDOI
TL;DR: In this paper, the authors used X-ray and electron metallographic techniques, electron microscope micro-analysis and differential thermal analysis to study splat-cooled aluminium-germanium alloys containing between 15 and 50 at.c.
Abstract: X-ray and electron metallographic techniques, electron microscope micro-analysis and differential thermal analysis have been used to study splat-cooled aluminium-germanium alloys containing between 15 and 50 at. % germanium. The observed microstructure and constitution have been correlated with the solidification process at cooling rates in the range 102–109 K/sec. At very high cooling rates the retention of almost all the germanium in f.c.c. solid solution has been shown possible. At slightly lower cooling rates two metastable intermediate compounds were produced; δ1, which was tetragonal with α=6.59 A and c=12.01 A with the possible stoichiometric composition Al5,Go3, and S2, also tetragonal, with a=6.25 A and c=9.44 A, and having a composition AlGe or AlGe2. On thermal decomposition the two compounds yielded a further metastable compound δ3, which was hexagonal with a=2.88 A and c=4.58 A. On further heating, this compound decomposed to the equilibrium aluminium and germanium solid solutions. D...

Journal ArticleDOI
TL;DR: The X-band and K-band ESR spectra of transplanar copper(II) complexes of amino acids, whose crystal structures are now known, were measured in a polycrystalline state at room temperature and in frozen aqueous-methanolic solutions at the temperature of liquid nitrogen.
Abstract: The X-band and K-band ESR spectra of the trans-planar copper(II) complexes of amino acids, whose crystal structures are now known, were measured in a polycrystalline state at room temperature and in frozen aqueous-methanolic solutions at the temperature of liquid nitrogen. The polycrystalline samples showed a variety of ESR line shapes, most of which had three different principal g values; g1=2.04–2.07, g2=2.08–2.12, and g3=2.16–2.29. On the other hand, all the ESR line shapes observed for the frozen solutions were quite similar, and they indicate a normal tetragonal copper (II)-ion environment. There was, moreover, a remarkable difference in the g values between the complexes in the two states. The correlation among the g values, the d-d band energies, the bonding parameters (a measure of the degree of covalency of the copper-ligand bondings), and the geometrical configurations of the coordinating atoms for the complexes in a crystal is discussed in some detail.


Journal ArticleDOI
TL;DR: In this article, the optical spectra of tetragonal HgI 2 single crystals were investigated at 4.2 K and 79 K in 2∼6 eV region of photon energy.
Abstract: The optical spectra of tetragonal HgI 2 single crystals were investigated at 4.2 K and 79 K in 2∼6 eV region of photon energy. The main part of the dichroic optical spectra is well described by the optical transition from three p -like valence bands, which are split due to spin orbit interaction and crystal field perturbation, to an s -like conduction band. The optical structures, which seem to be due to exciton series, were observed in the fundamental absorption edge for an ordinary light. The tail part of the absorption edge was well expressed by Urbach Rule.


Journal ArticleDOI
TL;DR: In this article, the positional parameters of oxygen atoms in the tetragonal rare earth vanadates have been refined on the basis of neutron diffraction powder patterns, and the results obtained for DyVO 4, TbVO 4, HoVO 4 and YbVO4 are compared with theoretical predictions of Baglio and Sovers.

Journal ArticleDOI
TL;DR: In this paper, the stiffness tensor of a 10 at. % thallium alloy is compared with those of indium itself, and a sensitive indication of the impending phase change is found in the linear compressibilities: indium and its tetragonal alloys with thalium show a marked inclination to adopt a cubic structure.
Abstract: Ultrasonic wave velocities and attenuation have been measured by the pulse echo method in both the cubic and tetragonal forms of indium + thallium alloys in the vicinity of the martensitic phase transformation. Results lead to insight into the lattice stability of the alloys and the microscopic nature of the transition; anomalously small values of the modulus ½( C 11 – C 12 ) found in both crystallographic modifications, coupled with marked increases in ultrasound absorption near the transition, attest to lattice mode softening as the basis of the transformation mechanism. Detailed measurements of the components of the stiffness tensor of a 10 at. % thallium alloy are presented and compared with those of indium itself. The lower crystal stability of the alloy relative to that of indium is graphically demonstrated by sections of velocity surfaces. A sensitive indication of the impending phase change is found in the linear compressibilities: indium and its tetragonal alloys with thallium show a marked inclination to adopt a cubic structure.

Journal ArticleDOI
TL;DR: In this article, the structures of AlB 2 and Mn 5 Si 3 -type hexagonal and Cr 5 B 3 and ThCr 2 Si 2 type tetragonal rare-earth silicides have been investigated by a high-temperature X-ray diffraction method in the temperature range 20 °-1200 °C.
Abstract: The structures of AlB 2 and Mn 5 Si 3 -type hexagonal and Cr 5 B 3 and ThCr 2 Si 2 type tetragonal rare-earth silicides have been investigated by a high-temperature X-ray diffraction method in the temperature range 20 °–1200 °C. Thermal expansion coefficients and transition products of the different compounds are given.

Journal ArticleDOI
TL;DR: In this article, the authors describe the thermal expansion coefficient of a triclinic crystal with orthorhombic symmetry, and the direction of measurement of the coefficient of linear thermal expansion is indicated by a subscript giving the crystal face normal to the directions of measurement.
Abstract: T h e thermal expansion coefficient is a second-order symmetric tensor and hence contains six independent constants for a triclinic crystal. This number is reduced for crystals with higher symmetry. Hexahydro-l,3,5-trinitro-s-triazine (RDX, C3H6N606) crystallizes with orthorhombic symmetry, and its thermal expansion is expressed by three coefficients of linear expansion measured along the principal crystallographic axes. Pentaerythritol tetranitrate (PETN, CSHsK4012) is tetragonal, and two coefficients, one along the unique crystallographic axis and one perpendicular to it, describe the thermal expansion, The direction of measurement of the coefficient of linear thermal expansion is indicated by a subscript giving the crystal face normal to the direction of measurement-e.g., C U ( ~ Z O \ is measured in a direction perpendicular to the (120) crystal face. It happens, for these materials, that a(1oo) is measured parallel to the a crystallographic axis, and similarly ~ ~ ( 0 1 0 ) and

Journal ArticleDOI
TL;DR: In this paper, the experimental results on the crystal structure, transport properties, magnetic susceptibility and NMR properties of the mixed oxide V 1− x Nb x O 2 system are discussed in terms of existing theories for VO 2.

Journal ArticleDOI
TL;DR: In this article, the Tanabe-Sugano strong field was used to analyze the optical spectra of the tetrahedral complexes (CH3)4N]2MnCl4 and [(CH3]4N], 2MnBr4) for low-temperature single-crystal absorption, emission and magnetic studies.
Abstract: Low-temperature single-crystal absorption, emission and magnetic studies are reported on the tetrahedral complexes [(CH3)4N]2MnCl4 and [(CH3)4N]2MnBr4. All nine d-d bands predicted by crystal field theory are observed in the optical spectra, many with detailed fine structure. The general features of the spectra are analysed using the Tanabe-Sugano strong-field scheme, including the Tree's correction, with excellent results. The finer spectral details are accounted for by spin-orbit interaction via a complete 3d 5 crystal field-spin orbit calculation. The effects of tetragonal distortions are considered and shown to be small. Several ‘mystery’ bands are observed at low temperatures in thick crystals and discussed in terms of possible sextet-doublet transitions.

Journal ArticleDOI
TL;DR: In this paper, the crystal structure of Cd(en)Ni(CN) 4 · 2C 6 H 6 was determined by the single crystal X-ray diffraction method and the structure was solved by the Fourier method and refined by the block-diagonal least squares method to an R -value of 0.089 for 619 reflection data.




Journal ArticleDOI
TL;DR: In this article, a high-pressure tetragonal polymorph of arsenic is reported for which a = 8·691, c = 6·363 and c/a = 0·732.

Journal ArticleDOI
TL;DR: In this paper, the behavior of the rhombohedral LnOF and the tetragonal Ln4O3F6 at high temperatures was investigated by differential thermal analysis and the high-temperature X-ray diffraction technique.
Abstract: The behavior of the rhombohedral LnOF and the tetragonal Ln4O3F6 at high temperatures was investigated by differential thermal analysis and the high-temperature X-ray diffraction technique. In the case of the rare earth oxyfluorides, LnOF (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y), the high-temperature transition rhombohedral\ightleftarrowscubic was observed by means of DTA and a high-temperature X-ray camera. The heat of transformation from rhombohedral to cubic was obtained by means of DTA. The transition temperatures and the heat of transformation, ΔH, shift to higher temperatures and to greater values respectively as the atomic numbers of the rare earth elements increase, although there are some anomalies. One endothermic peak was observed for the rare earth oxyfluorides, Ln4O3F6, from Sm to Lu. On the other hand, no peak was observed up to 1300°C for La, Pr, Nd, and Y. The behavior of the peak temperature of DTA against the atomic number for Ln4O3F6 is similar to that for LnF3; th...