Showing papers on "Tetragonal crystal system published in 1974"

Characterization and Stabilization of Metastable Tetragonal ZrO2

Abstract: Fine single-domain and polydomain particles of tetragonal ZrO2 were prepared by hydrothermal and heat treatment of ZrO2·nH2O. The particles were characterized by X-ray diffraction, electron microscopy, NMR, mass spectrometry, and ir spectroscopy. The main impurity in the samples was 1 to 2 wt% OH− ions, most of which were concentrated on the particle surfaces or at domain boundaries; some were also distributed in the lattice. Fine single-domain tetragonal particles were strain-free, but polydomain particles had large strains. The single-domain tetragonal particles were transformed much more easily than the polydomain particles by mechanical treatment. The stablization of metastable tetragonal ZrO2 cannot be explained adequately by the surface-energy theory. An explanation based on the concept of a martensitic transformation may be more reasonable.

Raman Scattering Study of the Crystallization and Phase Transformations of ZrO2

Abstract: X-ray diffraction, electron diffraction, and Raman scattering measurements are presented for dehydrated amorphous ZrO2. Although the material lacks sufficient crystallinity to diffract X rays, electron diffraction patterns indicate a microcrystallinity with a grain size of 15 to 30 A. Raman spectra characteristic of the metastable tetragonal polymorph were obtained from these materials. Heating the amorphous materials induces recrystallization into first a metastable tetragonal phase and then the stable monoclinic phase.

Structural Phase Transitions in CsPbBr 3

Abstract: Structural phase transitions in perovskite-type CsPbBr 3 have been investigated by neutron diffraction method. Phase transitions occur at 88°C and 130°C, which are respectively second and first order. The phase transition at 130°C is caused by condensation of the M 3 mode at the M point of the cubic Brillouin zone, while the one at 88°C results from condensation of the doubly degenerate R 25 -like mode ( Z 9 mode) at the Z point of the tetragonal Brillouin zone. Group theoretical considerations based on these results reveal that the crystal trans-forms from cubic perovskite structure (O h 1 - P m 3 m ) to tetragonal D 4h 5 - P 4/ m b m at 130°C and further to orthorhombic D 2h 16 - P m b n at 88°C. Possible atomic displacements induced at the phase transitions are obtained from the eigenvectors of the condensing modes.

Coupled modes with A 1 symmetry in tetragonal BaTi O 3

Abstract: The infrared properties of a system of first-order-coupled phonons are analyzed. The dielectric function and the Raman line shape of the polariton modes are derived. The parameters involved in the theory can be obtained from the Raman spectra of the TO and LO modes or from the Raman spectrum of the TO modes plus infrared-reflectivity measurements. It is shown that we can objectively distinguish real coupling from imaginary, contrary to the current belief. Numerical calculations are performed for the ${A}_{1}$-symmetry modes of tetragonal BaTi${\mathrm{O}}_{3}$, with good agreement for the polariton shapes and complete disagreement for the infrared reflectivity; damage at the crystal surface is pointed out as the probable cause of the discrepancy. In addition to the coupling between the lowest and the middle mode, previously known, a much larger coupling between the middle mode and the highest is shown to exist. Both couplings are shown to be real or nearly so. The discrepancy between the dielectric constant created by the resonant modes (electronic plus phonons) and the value obtained by electrical measurements is interpreted as a new indication that the crystal has a dynamical disorder; this disorder could also be cause of an anomalous broadening observed in the lowest polariton.

The structure of cubic ZrO 2 :YO 1.5 solid solutions by neutron scattering.

Abstract: The structure of solid solutions of ZrO2 containing 18%, 22% and 26% YO1.5 as well as 26% YbO1.5 have been examined by Bragg neutron diffraction and the diffuse scattering of long wavelength (7 AA) neutrons. The evidence points to a relaxation (0.36 AA) of the six nearest-neighbour oxygen ions surrounding an anion vacancy and there are probably similar but smaller relaxations of the four neighbouring cations which include two Y3+ (or Yb3+) ions to maintain local charge neutrality. At the highest trivalent ion concentration studied there are signs of preferential association of oxygen vacancies and at the lowest vacancy concentration the presence of some tetragonal phase is indicated, possibly as an intergrowth.

Transition temperatures and crystal structures of single‐crystal and polycrystalline NbNx films

Abstract: Single‐crystal cubic NbNx films with thicknesses of a few thousand angstroms were epitaxially grown on cleaved (100) planes of single‐crystal MgO plates by the vapor phase growth technique. By heat treatment of some of these films in NH3+H2 or in H2 alone, films having the superlattice structures of Nb4N3, NbN, or Nb4N5 were prepared. The maximum Tc was observed in cubic NbNx. Both Nb4N3 and Nb4N5, the tetragonal phases with long‐range‐ordered arrangement of vacancies, exhibited superconductivity. NbN1.0 and Nb5N6, both with hexagonal structure, did not exhibit superconductivity down to 1.77 K. As the composition of cubic NbNx became close to stoichiometric, its Tc increased. However, the maximum Tc of cubic NbNx with nearly stoichiometric composition was limited by the lattice instability of cubic NbNx and by the resulting cubic NbNx to hexagonal NbN transformation. It might be expected that a higher Tc for compounds of the NbN family could be obtained if a NaCl‐type crystal with less vacancies could exist as a stable or metastable phase at ordinary temperatures.

Piezoelectric nonlinear optic CuGaSe2 and CdGeAs2: Crystal structure, chalcopyrite microhardness, and sublattice distortion

Abstract: CuGaSe2 and CdGeAs2 crystallize in the chalcopyrite‐type structure. The lattice constants of tetragonal CuGaSe2 are a = 5.5963 ± 0.0001 and c = 11.0036 ± 0.0002 A at 298 °K, those of CdGeAs2 are a = 5.9432 ± 0.0001 and c = 11.2163 ± 0.0003 A at 298°K. The space group is I42d with four formulas in a unit cell. 1747 reflections were measured with PEXRAD for CuGaSe2 and 1892 for CdGeAs2 using MoKα x radiation. Least squares refinement of the structural parameters using 149 symmetry‐independent CuGaSe2 and 300 CdGeAs2 structure factors led to final agreement factors of 0.035 and 0.038, respectively. CuGaSe2 is slightly Cu deficient: the value of the x coordinate in the final model is 0.2431 ± 0.0002, with considerable vibrational anisotropy. CdGeAs2 is stoichiometric: the final model has x = 0.2785 ± 0.0002, also with significant vibrational anisotropy, but less than that in CuGaSe2. The two compounds confirm the pattern of sublattice distortion in AIBIIIC2VI and AIIBIVC2V chalcopyrites previously found. The...

Antiferromagnetism in γ-Phase Mn-Ir Alloys

Abstract: Crystallographic and magnetic properties were studied in the disordered γ-phase Mn 1- x Ir x (0.05 1 occurs at a temperature below their Neel temperature, T N . The tetragonal distortion temperature, T d in the antiferromagnetic ordered state decreases rapidly with increasing Ir and the alloys with x >0.15 remain cubic down to low temperatures. On the other hand, T N increases with alloying Ir at a rate of about 9 K/at%Ir up to 730 K at x =0.25 and it is extrapolated to 500±20 K for fcc Mn. It was concluded that the anomalous rise of T N is not mainly attributed to atomic order nor to the enhancement of exchange interaction due to lattice expansion, but that the perturbation of band structure due to alloying leads to stronger magnetic interaction. A Cu 3 Au type atomic ordering in the vicinity of x =0.25 was found to raise the T N by about 200 K. The origin of tetragonal distortion is discussed.

Structural studies by high‐resolution electron microscopy: tetragonal tungsten bronze‐type structures in the system Nb2O5–WO3

Abstract: The contrast in many-beam lattice images from very thin crystals of 4Nb2O5. 9WO3 is shown to be directly related to its known structure. On the basis of this correlation, the structure of 2Nb2O5.7WO3 is derived from the observed image contrast; it is an ordered intergrowth of the ReO3 and tetragonal tungsten bronze structural types. The structures of typical fault boundaries and disordered intergrowths are described, and those of the reported compounds 6Nb2O3. 11WO3 and 3Nb2O5.8WO3 are discussed.

A neutron diffraction study of the hydrides of the early lanthanide elements at room temperature

Abstract: The order structure of CeD2.29 at room temperature has been determined by profile analysis of powder neutron diffraction data. A non-stoichiometric tetragonal superstructure is adopted; space group I41md, lattice parameters a=5.538(1), c=11.046(2)AA. The ideal composition of the superstructure is CeD2.5 but the fluorite phase is not ordered at this composition because of a high level of intrinsic disorder. It is presumed that the superstructure exists over a range of composition up to approximately CeD2.4 and identical order structures have been determined in this range for LaD2.30 and PrD2.37.

Cooperative Jahn-Teller effects in the mixed crystals TbpGd1-pVO4 and DypY1-pVO4

Abstract: The structural phase change arising from the cooperative Jahn-Teller effect has been studied by Raman scattering and optical birefringence methods in the mixed crystals TbpGd1-pVO4 and DypY1-pVO4 with p varying from one to zero. The introduction of Gd and Y dilutes the active components causing the phase change, resulting in a reduction of the transition temperature TD(p). Varying p varies the splitting of Eg phonons below Tp(p) and also the energy of low-lying collective electronic excitations. The Tb system shows unique behaviour with p=0.365; a transition from tetragonal to orthorhombic symmetry occurs at 8.5K but the crystal reverts to tetragonal symmetry below 2.3K. The effect in the Tb system can be explained by mean-field theory. The Dy compounds are explained using series expansion and modified mean-field methods.

The preparation and crystal structure of gold monochloride, AuCl

Abstract: Large single crystals of AuCl were prepared by a vapour-transport method. AuCl is tetragonal, a = 6.734, c = 8.674 A , space group 141/amd, Z = 8. The structure consists of zig-zag chains of Au and Cl; Au is linearly coordinated by Cl at distances of 2.36 A, the bond angles Au-Cl-Au are 92 °.

Nature of Intergranular Phase in Nonohmic ZnO Ceramics Containing 0.5 Mol% Bi2O3

Abstract: The intergranular phase obtained by sintering a binary mixture of ZnO + 0.5 mol% Bi2O3 was isolated by using a dilute solution of HCIO4, which etches ZnO preferentially. The combined results of selected-area electron diffraction and microscopy, microprobe analysis, and X-ray diffraction strongly indicate that the intergranular material is a polycrystalline phase of tetragonal β-Bi2O3 (P421c), rather than the amorphous ZnO-Bi2O3 phase reported earlier. It appears that the nonohmic behavior in this prototype metal-oxide varistor must be an interfacial property associated with the semiconducting ZnO grains separated by thin layers of high-resistivity Bi2O3.

Phase Relations in the System Bi2O3‐WO3

Abstract: Phase relations in the system Bi2O3-WO3 were studied from 500° to 1100°C. Four intermediate phases, 7Bi2O3· WO3, 7Bi2O3· 2WO3, Bi2O3· WO3, and Bi2O3· 2WO3, were found. The 7B2O · WO3 phase is tetragonal with a0= 5.52 A and c0= 17.39 A and transforms to the fcc structure at 784°C; 7Bi2O3· 2WO3 has the fcc structure and forms an extensive range of solid solutions in the system. Both Bi2O3· WO3 and Bi2O3· 2WO3 are orthorhombic with (in A)a0= 5.45,b0=5.46, c0= 16.42 and a0= 5.42, b0= 5.41, c0= 23.7, respectively. Two eutectic points and one peritectic exist in the system at, respectively, 905°± 3°C and 64 mol% WO3, 907°± 3°C and 70 mol% WO3, and 965°± 5°C and 10 mol% WO3.

Single-crystal vibrational spectra of beryl and dioptase

Abstract: Using oriented single crystals of beryl and dioptase, almost the full number of bands predicted by factor-group analysis has been observed in each case. The same [Si6O18]12– ring occurs in both minerals; in dioptase, slight distortions caused by the need to provide Cu2+ with a tetragonal environment have a profound effect on the spectrum which bears little resemblance to that of beryl. The bonding in both crystals is discussed.