Showing papers on "Tetragonal crystal system published in 1976"

Lattice Distortion and Elastic Properties of Antiferromagnetic γ Mn-Ni Alloys

Abstract: Lattice parameters and elastic moduli of polycrystalline γ Mn alloys containing 12–40 at% Ni have been measured in a temperature range between –180° and 250°C. The face-centred cubic lattice undergoes a tetragonal distortion, either c / a 1, or an orthorhombic distortion at low temperatures. Both Young's modulus and shear modulus exhibit a step-type change at the Neel point and a broad minimum near the transition temperature of lattice distortion. The elastic behaviour is discussed on the basis of the so-called Δ E effect due to the displacement of antiferromagnetic domain walls. This effect may possibly be enhanced by the lattice softening in the γ Mn–Ni alloys.

The Crystal and Molecular Structure of Chloro-α,β,γ,δ-tetraphenylporphinatocobalt(III)

Abstract: The structure of the title compound, C44H28N4CoCl, has been determined by single-crystal X-ray diffraction. The crystals are body-centered tetragonal, with a=b=13.693(2), c=9.701(2) A, and Z=2. The space group was determined to be I4 from the measurement of the anomalous dispersion effect. The structure was predicted prior to the X-ray analysis from the calculation of the intermolecular van der Waals energy; the result agreed quite well with the experimentally determined structure. A tetragonal pyramidal five coordination with the chlorine atom at the apex was found for the cobalt atom. The cobalt-chlorine distance is 2.149 (6) A, about 0.1 A shorter than those found in ordinary octahedral coordinations.

Influence of the electronic structure on the titanium-vanadium-hydrogen phase diagram☆☆☆

Abstract: Calculations have been carried out for cubic TiH 2 and VH 2 and the tetragonally-distorted TiH 2 in an attempt to explain the phase diagram of the TiVH system. The calculations demonstrate that the distortion is caused by the existence of degenerate states at the Fermi level, degeneracy of which is lifted by the tetragonal distortion- -an example of the Jahn-Teller effect in a metal. The effect of varying vanadium concentration and varying hydrogen concentration on the position of the phase boundaries is discussed in terms of the calculated results.

Electron spin resonance of axially symmetric Cr3+ centres in KMgF3 and KZnF3

Abstract: The ESR spectra of five Cr3+ centres in KMgF3 and KZnF3 are reported. All are axially symmetric and can be described by the spin-Hamiltonian H=g/sub /// beta BzSz+gperpendicular to beta (BxSx+BySy)+D(Sz2-1/3S(S+1)) with S=3/2. In KMgF3 one finds spectra with D=-0.1528 cm-1 (trigonal), D=-0.2167 cm-1 (tetragonal) and D=-2.4 cm-1 (tetragonal). In KZnF3, centres are found with D=-0.1613 cm-1 (trigonal) and D=-0.0522 cm-1 (tetragonal). The trigonal spectra are ascribed to Cr3+ ions associated with K+ vacancies. The tetragonal spectra are attributed to Cr3+ ions with divalent cation vacancies with O2 ions or with other impurities on the tetragonal axis.

Proton- 14 N double resonance study of the structural phase transitions in the perovskite type layer compound (CH 3 NH 3 ) 2 CdCl 4 .

Abstract: Using a proton-nitrogen double resonance technique we have determined the quadrupole coupling of14N in the room temperature orthorhombic (Cmca), the low temperature tetragonal (P42/ncm), and the monoclinic low temperature (P21/c) phases of (CH3NH3)2CdCl4. In all these phases all nitrogens are chemically equivalent demonstrating that the disorder in the orientations and H-bonding arrangements of the CH3-NH3 groups in theC m c a andP42/ncm phases is indeed dynamic and not static. In the monoclinic phase the14N quadrupole coupling constant equalse2qQ/h=880 kHz and the asymmetry parameter isη=0.20, wherease2qQ/h=790 kHz,η=0.1 in the tetragonal low temperature phase ande2qQ/h=726 kHz,η=0.21 in the room temperature orthorhombic phase. The observed increase in the14N quadrupole coupling constant on going from the orthorhombic phase to the tetragonal low temperature phase which is coupled with a simultaneous decrease in the asymmetry parameter can be understood in terms of a partial freezing in of the dynamic disorder in the C-N bond directions whereas the14N quadrupole coupling tensor in the monoclinic phase is characteristic of a frozen in C-N bond in a deformed lattice, where the N-H — Cl bonds are of different length.

Phase relationships and thermodynamics of refractory metal pnictides: The metal-rich tantalum arsenides

Abstract: The metal-rich tantalum-arsenic system has been reinvestigated. X-ray powder diffraction techniques were used in conjunction with controlled thermal decomposition experiments in an effusion cell connected to a vacuum electrobalance. Monophase polycrystalline samples of Ta3As, Ta2As, Ta5As4 and TaAs have been prepared. Single crystals of each of these phases have been obtained by halogen vapour transport. Single-crystal precession camera studies indicated that Ta3As was of a new structure type with a monoclinic lattice and space group B2 b , a = 14.683(2), b = 14.558(1), c = 5.098(1) A , γ = 90.562° (3). The structure of TaAs was refined by using single-crystal diffractometer data. The results confirm the structure assigned earlier on the basis of powder data. In addition, a modification of the TaAs phase has been identified but not, as yet, fully characterized. Powder and single-crystal data indicate a primitive tetragonal cell with structure and lattice parameters very similar to the body-centered form. Refined lattice parameters have been determined for all the phases. The difficulties of constructing a phase diagram for the system are discussed.

A neutron diffraction study of structure and thermal motion in several monovalent metal azides

Abstract: The crystal structures of the monovalent metal azides β‐NaN3, KN3, TlN3, and RbN3 were reinvestigated by means of neutron diffraction. All except β‐NaN3 were studied at low temperature as well as at room temperature. These compounds form ionic crystals with spherical metal cations and linear azide anions. Because the azide ions in each case occupy centrosymmetric sites, their thermal motions can be treated, except for the small amplitude N–N stretching and the internal bending motions, as the motions of rigid bodies without screw correlations. All structures were, therefore, refined using rigid‐body constraints, leading to final R values, both weighted and unweighted, ranging from 0.020 to 0.045. KN3, TlN3, and RbN3 all crystallize in the tetragonal space group I4/mcm, while β‐NaN3 crystallizes in the space group R3m. The uncorrected N–N distances range from 1.159 to 1.183 A, but when corrections for thermal motions are applied all distances except for β‐NaN3 lie in the range 1.181–1.187 A, with a weight...

EPR and optical absorption of Fe + , Fe 2 + , Fe 3 + , and Fe 4 + on tetragonal sites in CdSi P 2

Abstract: An EPR investigation of CdSi${\mathrm{P}}_{2}$:Fe reveals the presence of substitutional, photosensitive ${\mathrm{Fe}}^{+}$, ${\mathrm{Fe}}^{2+}$, ${\mathrm{Fe}}^{3+}$, and ${\mathrm{Fe}}^{4+}$ impurities, each charge state being associated with a highly anisotropic EPR spectrum. The non-Kramers ions ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{4+}$ are still observable at 300 K. At 20 K all Fe centers, except ${\mathrm{Fe}}^{+}$, exhibit a well-resolved phosphorus ligand hyperfine splitting. The anisotropy of this splitting is used to demonstrate the magnetic inequivalence of the two Si sites in CdSi${\mathrm{P}}_{2}$. For ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{4+}$ a complete analysis of the $S=2$ spin Hamiltonian is possible. Near-ir absorption bands due to ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{4+}$ are observed. A correlation with the EPR results allows the determination of the crystal-field parameters $\mathrm{Dq}$, $\mathrm{Ds}$, and $\mathrm{Dt}$. Static crystal-field theory, supplemented to include covalency effects, according to the model of Vallin and Watkins, accounts satisfactorily for all experimental results, except for the EPR data of ${\mathrm{Fe}}^{4+}$. It is suggested that this discrepancy is due to covalent effects beyond those explicitly considered.

Indexing program for powder patterns especially suitable for triclinic, monoclinic and orthorhombic lattices

Abstract: A fast computer program written in Fortran is presented. It calculates with a trial-and-error method the unit-cell dimensions from the d values obtained from powder patterns. The special techniques used in this program to keep the computation time short are described. The program itself is designed primarily for triclinic and orthorhombic lattices. Monoclinic lattices are treated like triclinic ones: for tetragonal, hexagonal and rhombohedral structures orthorhombic cells can be calculated.

Superconductivity and phase stability of Nb3Ge

Abstract: The A-15 phase of chemically vapor-deposited Nb3Ge has been investigated by annealing techniques in the temperature range 700–1450°C and by irradiation with high-energy neutrons (E>1 MeV). By x-ray diffraction analysis the samples were found to contain about 55 wt % of the A-15 phase, 35 wt % of tetragonal Nb5Ge3, and minor amounts of NbO2, NbO, and hexagonal Nb5Ge3. The superconducting transition temperatureTc and lattice parametera0 of the A-15 phase have been measured as a function of annealing temperature and neutron irradiation.Tc for the unirradiated sample remained above 19 K for long-term anneals in the temperature range 700–1000°C but decreased rapidly above 1000°C, reaching a limiting value of about 6 K at 1250°C, compared to the as-deposited value of about 20 K. The decrease inTc was accompanied by an increase ina0 and an increase in the amount of tetragonal Nb5Ge3 to about 50 wt %. The observed decrease inTc was irreversible in that subsequent annealing at 1100°C did not result in any increase inTc or decrease ina0. Large depressions inTc were also observed in a sample irradiated with high-energy neutrons. The sample having aTc of about 21 K was found to exhibit no superconductivity to 1.5 K after exposure to a fluence of ∼5×1019 n/cm2. An increase ina0 was also found as a function of neutron irradiation. However, in this case the effects were reversible in thatTc anda0 of the irradiated sample could be restored to close to their original values by annealing in the 800–900°C range. The results are discussed in terms of stability of the A-15 phase and the effect of site-exchange disorder and composition on the superconducting properties and lattice parameters. It is concluded that the stoichiometric composition is metastable and upon annealing transforms to a stable, Nb-rich A-15 phase containing about 83 at % Nb with aTc of ∼6 K accompanied by precipitation of tetragonal Nb5Ge3.

Lattice Softening and Martensitic Transformation in Cu–Ni–Zn β Phase Alloys

Abstract: Cu 50- x Ni x Zn 50 β phase alloys undergo a martensitic transformation from the CsCl-type ordered cubic (B2 structure) to the AuCuI-type tetragonal (Li 0 structure). The martensite L1 0 phase appears at room temperature when Ni composition x becomes larger than 32.5 at.%, i.e. when electron atom ratio ( e / a ) becomes smaller than 1.175. In order to clarify the origin of B2–Li 0 transition, composition and temperature dependence of the elastic constants in Cu 50- x Ni x Zn 50 alloys were measured. The elastic constant C '=( C 11 - C 12 )/2 decreases with increase in x and d C '/d T is positive (i.e. lattice softening) in the cubic phase, which are suggested to have an important role on the martensitic transformation in these alloys.

Magnetic Form Factor of Cu2+ in K2CuF4

Abstract: Magnetic form factor of Cu 2+ in the ferromagnet K 2 CuF 4 has been measured by means of the polarized neutron diffraction technique. The observed form factor is in good agreement with the calculated one based on the orbital ordered wave functions in the tetragonal crystalline field. In the calculation the effects of covalency and of spin-orbit interaction are taken into account.

The ligand hyperfine interaction with rare earth ions. I. A revised covalent model

Abstract: It is shown that a reformulation of the covalent model can explain the 19F hyperfine interactions observed for Yb3+ and Tm2+ in cubic and tetragonal sites of alkaline earth fluorides. The same model does not explain data for Ce3+, Gd3+, or Eu2+ indicating that the spin polarization model is the dominant mechanism in the first part of the rare earth series but that the covalent model becomes dominant in the latter half of the series. The analysis shows that the distortion from cubic symmetry in the tetragonal site is not as severe as an earlier NMR analysis suggested but is still large. It further suggests that the interstitial ion is closer to the rare earth ion than most observers have assumed.

The crystal structure of 2,5-dimercaptothiadiazole

Abstract: compounds be discussed but also the deviations of the structures from the cubic symmetry found with inorganic cations. With the amino acid, Te atoms are at the apex of an A face-centred monoclinic lattice. The contacts between Br atoms in different anions range from 3.943 to 4.134 A, which are larger than van der Waals contacts. With the succinamide, Te atoms are 0-515 A away from the apex of the C face-centred lattice. The van der Waals contacts between Br atoms in different TeBr6 aions are grouped into four short contacts of 3.705 and two longer contacts of 3.953 A. A larger organic cation tends to distort the octahedra network but does not deform it fundamentally. In the two structures, the cohesion is governed by hydrogen bonds and Br . . -Br intermolecular van der Waals contacts. The packing of the molecules approaches close packing, with the octahedra in contact, and the cavities occupied by the organic cations.