Showing papers on "Tetragonal crystal system published in 1980"
TL;DR: A detailed structure for the tetragonal form of sodium hyaluronate has been obtained by analysing X-ray fibre diffraction data using new molecular modelling techniques, and no double-helix model has been found to be free of unacceptable non-bonded contacts or to fit the diffraction intensities as closely.
153 citations
TL;DR: In this article, backscattering and x-ray diffraction were used to analyze WSi2 and MoSi2 films produced by Asion implantation through the respective metallic films deposited on Si substrates.
Abstract: MoSi2 and WSi2 films produced by As‐ion implantation through the respective metallic films deposited on Si substrates were analyzed by backscattering and x‐ray diffraction. The backscattering results indicate that As atoms are snowplowed into Si during the formation of the silicides. Crystallographic observations on similar samples both before and after various heat treatments provide evidence for the existence at low temperatures of the hexagonal phase of WSi2, presumably unreported up to now, which is similar to the corresponding phase of MoSi2. In the case of the W disilicide, however, the temperature for the transition, hexagonal to tetragonal, is so low that the low‐temperature phase is unlikely to be obtained by the usual diffusion‐controlled mechanisms.
127 citations
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TL;DR: The metastable tetragonal ZrO 2 transforms to a stable monoclinic structure thus increasing the energy required for the crack to propagate and retarding its growth.
Abstract: Fracture toughness and strength of an Al 2 O 3 /ZrO 2 ceramic is increased by incorporating metastable grains of tetragonal ZrO 2 in the structure. During cracking, the metastable tetragonal ZrO 2 transforms to a stable monoclinic structure thus increasing the energy required for the crack to propagate and retarding its growth. The ZrO 2 occupies from 5 to 95% of the volume of the ceramic and has dissolved in it a rare earth oxide such as Y 2 O 3 , CeO 2 , La 2 O 3 and/or Er 2 O 3 to promote retention of the metastable tetragonal ZrO 2 .
124 citations
119 citations
TL;DR: In this article, the local symmetry of amorphous alloys was determined through NMR investigation of quadrupolar effects in some amorphized alloys (La75Ga25, Mo70B30, Mo48Ru32B20, Ni78P14B8).
Abstract: Through NMR investigation of quadrupolar effects in some amorphous alloys (La75Ga25, Mo70B30, Mo48Ru32B20, Ni78P14B8) the local symmetry is determined around the s-p element sites. A comparison with the corresponding crystalline compounds (La3Ga, Mo2B, Ni3B) which have typically different crystal structures (cubic, tetragonal, orthorhombic, respectively) gives conclusive evidence that the amorphous structure retains to a significant extent the local symmetry of the crystalline counterpart.
84 citations
TL;DR: In this article, the isothermal transformation behavior of the metallic glass Fe40Ni40P14B6 between 320 and 400°C is described and the orientation relationship between the two phases is described.
Abstract: The isothermal transformation behaviour of the metallic glass Fe40Ni40P14B6 between 320 and 400° C is described. Crystallization occurs by a eutectic mechanism to form Fe-Ni austenite and a body-centred tetragonal phase which is isomorphous with Fe3P and Ni3P. The eutectic crystals have a barrel shape such that the c-axis of the tetragonal phase is parallel to the barrel axis. The orientation relationship between the two phases is 〈1 1 0〉T ∥ 〈1 1 0〉γ and 〈0 0 1〉T ∥ 〈1 1 2〉γ. The austenite phase contains (1 1 1) twins.
67 citations
TL;DR: In this paper, a tetragonal distortion of the fcc lattice of γ-iron in a copper matrix has been investigated by X-ray diffraction in the temperature range from 300 to 6 K.
53 citations
TL;DR: In this paper, the structure of 9-methyladenine (C6HvNs) has been refined using 2983 reflections with a measured neutron wavelength of 0.9085 A (sin 0/2 _< 0.75 A-~).
Abstract: At 126 K, 9-methyladenine (C6HvNs) is monoclinic, space group P2~/e, with a = 7.506 (1), b = 12.285 (2), c = 8.483 (1) A, fl = 122.83 (1) °, and four molecules per unit cell. The structure has been refined using 2983 reflections with a measured neutron wavelength of 0.9085 A (sin 0/2 _< 0.87 A-~), and 373 reflections at a wavelength of 1.0474 A (sin 0/2 < 0.75 A-~). The latter consist mainly of 2h,2k,21 reflections which were remeasured when it was found that the shorter wavelength was contaminated with 22 neutrons produced by double Bragg diffraction at the Be monochromating crystal. The final R(F 2) value was 0.049. Bond lengths and angles have e.s.d.'s of 0.001 A, 0.08 ° for non-hydrogen atoms, otherwise 0.003 A, 0.2 ° . Values are in general agreement with those of the previously reported room-temperature X-ray refinement. The hydrogen bonding of the amino group appears to cause displacement of the H atoms from the molecular plane. The largest thermal motion in the structure involves the methyl H atoms which are librating about the N(9)--CHa bond with r.m.s. amplitudes 0.28, 0.29, 0.30 A. There is a short intermolecular distance of 2-52 A between a methyl H atom and N(3).
53 citations
TL;DR: In this paper, an extension scheme for the calculation of InGaAsP material parameters is described and experimental data on alloy composition, bandgap, lattice parameter and refractive index is presented and compared with calculated values.
Abstract: Extrapolation schemes for the calculation of InGaAsP material parameters are described. Experimental data on alloy composition, bandgap, lattice parameter and refractive index are presented and compared with calculated values. Refractive index steps between InGaAsP and InP were found to vary significantly with lattice mismatch. Errors in alloy composition of mismatched epitaxial layers, deduced from bandgap and lattice parameter measurements, can be caused by a tetragonal distortion of the crystal lattice.
52 citations
TL;DR: The crystal structure of trimethyltin glycinate has been determined by Patterson and Fourier techniques to a final "R"-factor of 0.066 for 1103 unique reflections as mentioned in this paper.
50 citations
TL;DR: In this article, the basic theory of the magnetoelectric effect in antiferromagnets is rederived in this article and extended to situations in which there is an external magnetic field including the spin-flop phase.
Abstract: Magnetoelectric experiments provide information on spin configurations, critical indices and microscopic ionic interactions. The basic theory of the magnetoelectric effect in antiferromagnets is rederived in this article and extended to situations in which there is an external magnetic field including the spin-flop phase. It is found that measurements of the magnetoelectric tensor α as a function of field in the antiferromagnetic and spin-flop phases of an antiferromagnet can provide more complete information in all three categories above. Certain higher-order terms are examined and it is found that it is possible to measure the staggered susceptibility directly in magnetoelectric materials. It is shown that a non-zero value of α causes a tetragonal antiferromagnet to be optically biaxial. A pedagogical discussion of the origins of magnetoelectric effects is given in the Appendix.
TL;DR: In this article, it was shown that the crystal structures of RbH2PO4 and KK 2PO4 are closely similar, but some small but important differences are revealed.
Abstract: The crystal structure if RbH2PO4 has been determined by neutron diffraction at room temperature and Tc+5K in the tetragonal (paraelectric) phase using single-crystal samples. The techniques of constrained refinement and statistical testing have been used to examine carefully the significance of some pertinent structural features. The crystal structure has also been determined at 77K in the orthorhombic (ferroelectric) phase using neutron powder-diffraction methods. The results for both phases are compared with those obtained from similar studies of KH2PO4. Whilst it is shown that the crystal structures of RbH2PO4 and KH2PO4 (and the way they change through the transition) are closely similar, some small but important differences are revealed. In particular, the separation of the 'disordered' proton sites in the tetragonal phase is significantly larger in RbH2PO4 (by approximately 0.05 AA); yet the hydrogen-bond lengths differ by only 0.003(2) AA, and the transition temperatures by only approximately 20K.
TL;DR: The boundary between ZrO/sub 2-x/ (cubic+tetragonal) was studied by mass spectroscopy at <1770/sup 0/K as mentioned in this paper.
Abstract: The boundary between ZrO/sub 2-x/ (cubic) and ZrO/sub 2-x/ (cubic+tetragonal) was studied by mass spectroscopy at <1770/sup 0/K. The temperature dependence of the ion currents of ZrO/sup +/ and ZrO/sub 2//sup +/ over ZrO/sub 1/./sub 98/ and over ZrO/sub 1/./sub 96/ was found.
TL;DR: In this paper, the accurate measurement of optical birefringence at simultaneous high pressures and temperatures using a diamond-anvil pressure cell is described using mean-field behavior.
Abstract: The accurate measurement of optical birefringence at simultaneous high pressures and temperatures using a diamond-anvil pressure cell is described Data are presented for BiVO4 which show that the optical birefringence may be described by mean-field behaviour An applied hydrostatic pressure of only 8 × 108 Pa reduces the mean-field temperature of the ferroelastic monoclinic tetragonal phase transition from 5197 to 393 K A birefringence–pressure–temperature phase diagram is presented
TL;DR: An X-ray single-crystal structure determination (R = 0.0397) shows that CsMnF4 crystallizes in the CsFeF4-type layer structure (tetragonal, space groupP4nmm, a = 794.40(6)pm, c = 633.76(9)pm), Z = 4).
TL;DR: In this article, magnetization and Mossbauer effect studies of the body centered tetragonal (I4/mmm) systems RM 2 Si 2-x Ge x and RM 4+ x Al 8-x (R = 4f, M = nd element) are presented.
TL;DR: In this article, a neutron diffraction study of cubic CsPbX3 (X = Cl or Br) has been carried out over the temperature ranges 325-623 K for CspbCl3 and 408-673 K for PbBr3 and the potential parameters were then obtained using a numerical integration method to analyse the temperature dependence of the temperature factors.
Abstract: A neutron diffraction study of cubic CsPbX3 (X = Cl or Br) has been carried out over the temperature ranges 325-623 K for CsPbCl3 and 408-673 K for CsPbBr3. The temperature factors for the perovskite structure were derived following the method of Matsubara [Prog. Theor. Phys. (1975), 53, 1210-1211] which includes the use of cumulant coefficients to characterize anharmonic components for an Einstein model. The potential parameters were then obtained using a numerical integration method to analyse the temperature dependence of the temperature factors. It was found that the anharmonic components in the potential were very large for the Cs and X atoms which undergo displacements on passing through the phase transitions at lower temperatures (321 K for CsPbCl3 and 403 K for CsPbBr3). On the other hand, a harmonic potential is quite adequate to describe the thermal vibration of the Pb atoms, which are not displaced at the phase transitions. Thus, the existence of the anharmonicity in the cubic phase seems to be anticipating the atomic displacement through the successive phase transitions for these substances. In addition to this anharmonicity, the temperature factors of the X atoms parallel to the (100) plane show an anomalous behaviour near the cubic to tetragonal phase-transition temperature, which should be connected with the softening of phonon mode at this phase transition.
TL;DR: In this article, the normal-spinel domain of homogeneity between the compositions InS 1.50−e and InS1.35 has been investigated for the spinel lattice.
TL;DR: In this paper, the magnetism between 4.2 and 300 K of the uranates (V) Li7UO6, Li3UO4, LiUO3, NaUO 3, KUO2, Na2NpO4 and the plutanate (VII) Li5PuO6 are reported.
TL;DR: In this article, phase relations between Y2O3 and WO3-WO3 were studied at >1400°C and the eutectic between 3Y 2 O3 and 3Y O 3 WO 3 was 2334°C at ∼90 mol% Y 2O.
Abstract: Phase relations in the system Y2O3-Y2O3·WO3 were studied at >1400°C. Yttrium oxide with the rare-earth C-type structure transformed into a hexagonal structure at 2360° and melted at 2400°C. The eutectic between Y2O3 and 3Y2O3·WO3 was 2334°C at ∼90 mol% Y2O. 3Y2O3·WO3 with a rhombohedral structure changed into a cubic phase at 1765° and melted at 2360°; 5Y2O3·2WO3 with a tetragonal structure changed into the cubic phase at 1740°C. This cubic phase was stable at >1740° and had a solid-solution range of 69 to 75 mol% Y2O3 at 1800°C. 7Y2O3·4WO3 melted incongruently at 2107°C. Y2O3·WO3 with a monoclinic structure (α’phase) transformed into an orthorhombic structure (e phase) at 1470°, then into a tetragonal structure (β phase) at 1600° to 1700°C. This compound melted incongruently at 1706°C. A phase diagram is given.
TL;DR: In this paper, the authors used elastic neutron scattering to investigate fluorite-forbidden reflections which arise from what previous authors have called the ''ordered' structure, showing that the extra reflections are due to precipitation of tetragonal ZrO2 in both systems.
Abstract: Single crystals of ZrO2–15 mol % CaO and ZrO2–9 mol % Y2O3, annealed for either 400 h at 1273 K or 2.5 h at 1673 K and then quenched, were examined by electron diffraction and dark-field transmission electron microscopy. The thermal treatments duplicated those of Faber, Mueller & Cooper [Phys. Rev. B (1978), 17, 4884–4888], who used elastic neutron scattering to investigate fluorite-forbidden reflections which arise from what previous authors have called the `ordered' structure. The present results show that the extra reflections are due to precipitation of tetragonal ZrO2 in both systems. Imaging with the diffuse-scattered intensity also present reveals small domains, presumably associated with oxygen-vacancy ordering, in the cubic solid-solution matrix. The diffuse intensity is observed whether or not tetragonal ZrO2 precipitates are present.
TL;DR: The polarization of the three stretching vibration bands of hydrogen or deuterium impurities in the tetragonal low temperature phase of monodomain SrTiO3 and their splitting under applied electric fields allow these modes to be assigned to energetically different positions - one in the (Ti-O)-plane and two with components in c-direction along the oxygen octahedra as discussed by the authors.
Abstract: The polarization of the three stretching vibration bands of hydrogen or deuterium impurities in the tetragonal low temperature phase of monodomain SrTiO3 and their splitting under applied electric fields allow these modes to be assigned to energetically different positions - one in the (Ti-O)-plane and two with components in c-direction along the oxygen octahedra.
TL;DR: In this paper, the sequence of soft mode condensation is the reverse of that in RbCaF3 1, where the M3 and X' modes condense together at 560 K to give an ortho-rhombic Bbmm cell with tilts (a-b+c°).
Abstract: In KCaF3 the sequence of soft mode condensation is the reverse of that in RbCaF3 1). The M3 and X' modes condense together at 560 K to give an ortho-rhombic Bbmm cell with tilts (a-b+c°). TheΤ25 mode then condenses at 551 K to give the usual Puma structure with tilts (a-b+a-). The intermediate phase is not then uniaxial (tetragonal) as in RbCaF3, which explains the different linear birefringence results. At room temperature the tilt angles are large and equal, as in RbCaF3, indicating ordering on well defined potential minima.
TL;DR: The existence of an intermetallic compound of the type RGa6 (R rare earth) on the gallium-rich side of the Nd-Ga and Ce-Ga systems was established as discussed by the authors.
Abstract: The existence of an intermetallic compound of the type RGa6 (R rare earth) on the gallium-rich side of the Nd-Ga and Ce-Ga systems was established. The compounds NdGa6 and CeGa6 have a primitive tetragonal structure with the following lattice parameters: NdGa6 a = 5.996 ± 0.001 A b = 7.620 ± 0.001 A CeGa6 a = 6.03 A b = 7.632 A A large solid solubility range exists between the RGa2-type compound and Nd-80at.%Ga or Ce-78at.%Ga. This phase has a hexagonal structure, the lattice parameters of which increase with the concentration of gallium.
TL;DR: In this article, it is argued that the experimentally observed reorientation of the NH4+ ions at the transition temperature is a consequence of the structural transition, which itself is driven by the instability of the cubic phase as expressed by the Goldschmidt tolerance factor.
Abstract: The compounds NH4MF3 (M=Zn, Co, Mn) exhibit structural phase transitions at Tc=115.1, 124.5 and 181K respectively. The crystal structures have been determined at 4.2 and 300K by neutron diffraction using powdered samples. At 300K the compounds have the ideal cubic perovskite structure, while below Tc they are tetragonal (c/a>1). In the Co and Mn compounds an antiferromagnetic spin structure of type G has been observed at 4.2K. No evidence could be found for the weak ferromagnetic moment in the Co compound as has been detected by susceptibility and magnetisation measurements. The change in cell dimensions with temperature has been followed using X-rays. The discontinuity of the cell dimensions at Tc indicates a first-order character of the structural transitions, in agreement with the hysteresis in the specific heat measurements. It is argued that the experimentally observed reorientation of the NH4+ ions at the transition temperature is a consequence of the structural transition, which itself is driven by the instability of the cubic phase as expressed by the Goldschmidt tolerance factor.
TL;DR: Amorphous electrodeposited thin films of nickel phosphides, containing 20 and 22 at. % P were studied by electron microscopic methods as discussed by the authors, and the results showed that the amorphous phase transformed into Ni12P5 and several other new crystalline phases with or without additional metallic nickel.
Abstract: Amorphous electrodeposited thin films of nickel phosphides, containing 20 and 22 at. % P were studied by electron microscopic methods. Upon beam heating the amorphous phase transformed into Ni12P5 and several other new crystalline phases, with or without additional metallic nickel. The following crystallizations from the electrodeposited Ni‐P films were observed: (1a) Amorphous Ni‐P→NixPy(Ni)+Ni. The probable values of x and y are 5 and 2, respectively. The lattice parameters are a=6.61 and c=12.31 A. Upon further beam heating (1a) transforms to: (1b) NixPy(Ni)+Ni→Ni3P+Ni. The body‐centered tetragonal unit cell dimensions of Ni3P are a=8.93 and c=4.39 A. (2) Amorphous Ni‐P→Ni12P5. The unit cell of Ni12P5 is again body‐centered tetragonal, with a=8.64 and c=5.07 A. (3) Amorphous Ni‐P→Ni3P. The hexagonal unit cell parameters are a=5.00 and c=8.66 A. (4) Amorphous Ni‐P→Ni3P+Ni. The unit cell is again body‐centered tetragonal, like that of (1b). However, the dimensions are a=6.10 and c=5.04 A. (5) Amorphous N...
TL;DR: The boundaries between the paramagnetic, superconducting and magnetically ordered phases in the tetragonal pseudoternary system Ho(IrxRh1-x)4B4 have been established by means of magnetic susceptibility measurements down to 1.2 K.