Non-cubic Phases Research Articles

Observation of a rhombohedral phase change during the dehydration of zinc-exchanged zeolite A

Temperature dependent powder X-ray diffraction studies of zinc-exchanged zeolite A (LTA structure type; cubic at room temperature) have demonstrated the presence of a non-cubic phase between 150 °C and 600 °C. Analysis of the data indicated that a rhombohedral phase was formed with refined lattice parameters a = 17.182(3) Å and α = 59.147(9)° (after quenching from 150 °C to room temperature under vacuum). Heating above 600 °C produced a second cubic phase, which persisted upon cooling, and the rhombohedral phase only appeared for samples that had not been heated above 600 °C. The distortion from cubic symmetry may be linked to the formation of species such as Zn x ( H 2 O ) y m + or Zn x ( H 2 O ) y ( OH ) z n + during dehydration.

Rotational motion in the molecular crystals meta- and ortho-carborane studied by deuteron nuclear magnetic resonance

Spin-lattice and spin-spin-relaxation times, one- and two-dimensional spectra as well as two- and four-time correlation functions were measured for the molecular crystals ortho- and meta-carborane using deuteron nuclear magnetic resonance. It is found that in their noncubic phases these crystals exhibit highly anisotropic motions. In order to allow for a quantitative description of the motional geometry of the carboranes several stochastic models are formulated. By comparison of the model calculations with the experimental results it is found that the dynamics of these quasi-icosahedrally shaped molecules is governed by a composite reorientation process. Here the molecules perform threefold jumps around a molecule-fixed axis which itself can be tilted in four different directions with respect to a crystal-fixed axis. The tilt angle increases significantly with increasing temperature. On the basis of measurements of four-time stimulated-echo functions, implications for dynamic heterogeneity also in comparison with that of supercooled liquids are discussed.

Study on the magnetic properties and phase composition of Tb 1−xPr xFe 1.96 ( [formula omitted]) compounds

The magnetic properties and the phase structure of Tb 1− x Pr x Fe 1.96 ( x = 0 – 0.7 ) were studied. It is found that the saturation magnetization on easy axis is decreased with increasing of Pr content, from 77.24 Am 2 kg −1 for x = 0 , reduces to minimum value of 11.84 Am 2 kg −1 for x = 0.5 , and returns to 37.14 Am 2 kg −1 for x = 0.7 . The non-cubic phases appear beyond x = 0.2 and the matrix changes from TbFe 2 phase for x = 0 to (Tb, Pr)Fe 3 phase for x = 0.4 , at last to (Tb, Pr) 2Fe 17 phase for x = 0.7 . The compound structures turn more complex as the Pr content increased.

Phase transitions in BaTiO3: a high-pressure neutron diffraction study

Abstract The sequence of ferroelectric phase transitions (Pm3̅m—P4mm—Amm2) in BaTiO3 has been studied as a function of temperature (298 K–130 K) at a fixed pressure (3.2 GPa). The same sequence of transitions is observed as for ambient pressures, but the transition temperatures are reduced. The spontaneous strains of the non-cubic phases are determined. Whilst the structure of the tetragonal phase at high pressure is qualitatively similar to the ambient structure, both the spontaneous strain and the Ti displacement are smaller in the high pressure structure than the ambient structure.

A combined X-ray diffraction and Raman scattering study of the phase transitions in Sr 1−xCa xTiO 3 ( x=0.04, 0.06, and 0.12)

Results of powder X-ray diffraction and Raman scattering studies on the phase transitions in Sr 1− x Ca x TiO 3 (SCT) are presented for x = 0.04 , 0.06 and 0.12 in the temperature range 8–473 K. It is proposed that the space group of SCT in the composition range 0.06 ⩽ x ⩽ 0.35 is Imma with a 0 b - b - tilt system and not I 4 / mcm with a 0 a 0 c - tilt system, as assumed by earlier workers. The lowering of the crystal symmetry from I 4 / mcm to Imma is supported by the observation of additional Raman lines, in agreement with the factor group analysis for the Imma space group. The structural E g mode, characteristics of the non-cubic phase, is shown to be present even in the cubic phases of x = 0.06 and x = 0.12 but not of x = 0.04 indicating the change in the local structure of the cubic phase of SCT for x ⩾ 0.06 . The presence of symmetry forbidden TO 2 mode in the Raman spectra of the cubic phase of SCT for x < 0.06 and its absence for x ⩾ 0.06 provides yet another characteristic feature distinguishing the I 4 / mcm and Imma space groups. The implications of the change in the tilt system from a 0 a 0 c - to a 0 b - b - on the development of the polar order is also discussed.

XRD Stress Analysis in a TiAl Based Intermetallic Multicrystal

Under specific manufacturing processes and heat treatments, the gamma titanium Aluminide Ti- 47Al-2Cr-2Nb presents a nearly lamellar microstructure with coarse lamellar grains (with alternating g and a2 lamellae) and dispersed small g grains. The obtained microstructure gives a relatively coarse grain size (approximately 0.4 mm x 3 mm). Residual stresses can then be generated between g and a2 lamellae inside the lamellar grain and also between different grains because of the lattice mismatch between tetragonal structure of g phase and hexagonal structure of a2 phase. A specific single crystalline XRD method is applied on these two non-cubic phases to determine the local residual stresses in two grains on each side of a grain boundary.

Mössbauer and structural studies of Fe 0.7− x V x Si 0.3 alloy system

X-ray diffraction patterns for Fe0.7−xVxSi0.3 (x = 0, 0.05, 0.1, 0.15, 0.2, and 0.3) yield a single BCC-type phase for small values of x and a minor non-cubic phase starts to develop for large values of x. The lattice parameter is found increase with increasing vanadium concentration. Mossbauer experiments were made at room temperature and 20 K. Room temperature spectra show magnetic order for x = 0, 0.05, and 0.1, a central paramagnetic line of high intensity superimposed to the magnetically ordered component for x = 0.15, and a almost completely paramagnetic state for x = 0.2 and 0.3. A similar behavior prevails at 20 K, except that hyperfine splittings are larger and the magnetic ordering starts to show up for x = 0.2 and 0.3. In data analysis, each room-temperature and 20-K spectrum is fitted with a distribution of hyperfine sextets. The results clearly indicate the expected atomic disorder in the materials. The average magnetic hyperfine field is found to decrease with increasing x, reflecting the replacement of magnetic iron by nonmagnetic vanadium. The average isomer shift shows the same vanadium-concentration dependence. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Defects and hyperfine interactions in Ni–Y intermetallics(Y = Al, Ga, In,Ti) via 27Al, 47Ti, 61Ni, 69,71Ga and 115In nuclear resonance

Defect structures and nuclear electric hyperfine interactions have been investigated in a series ofbinary Ni–Al, Ni–Ga, Ni–In intermetallic compounds, and the shape memory compound NiTi using27Al,47,49Ti,61Ni,71Ga and115In nuclear resonance.Components of the 61Ni NMRspectrum for a series of cubic Ni1−xAlx and Ni1−xGax specimenson either side of x = 0.5 have been identified as due to Ni substitutions and Al (or Ga) vacancies. For stoichiometric NiAl,Ni2Al3 and NiAl3 the 61Ni lines are narrow and distinguished by well-separated Knight shifts, although the61Ni lineshape for the ordered vacancy compoundNi3Al4 indicates a substantial nuclear quadrupole interaction at the Ni site.The substitution for Al of the group III elements Ga and In,which have isoelectronic outer shells, acts to increase the61Ni Knight shiftsuch that 61K(Al)<61K(Ga)<61K(In). This trend is observed not only for cubic NiAl and NiGa, but also for trigonalNi2Al3,Ni2Ga3 and Ni2In3.In NiTi the sharp first-order transition between the (high-temperature) cubic phase andthe (low-temperature) monoclinic phase has been observed while monitoring the61Ni and 49,47Ti NMR lineshapes as a function of temperature.In the non-cubic phases of these materials (including hexagonalε-NiIn) the electric field gradient tensor componentsVzz and η at the atomic sites have been determined from the nuclear quadrupole perturbed27Al,47Ti,61Ni,71Ga NMR lineshapestogether with 69Ga and 115In NQR transition frequencies, and compared with values derived from ab initio calculation.

High lithium ionic conductivity in the lithium halide hydrates Li3-n(OHn)Cl (0.83 < or = n < or = 2) and Li3-n(OHn)Br (1 < or = n < or = 2) at ambient temperatures.

Lithium ionic conductivity and phase transitions in a series of lithium halides hydrates and hydroxides with general formula Li3-n(OHn)X (0.83 < or = n < or = 2; X = Cl,Br) were studied using impedance measurements and 1H and 7Li NMR spectroscopy. All compounds studied in this work crystallize in the antiperovskite structure or are closely related to this structure type. With the exception of LiCl. H2O, all compounds with integer lithium content exhibit good lithium ionic conductivity in their high temperature cubic phases above T = 33 degrees C. Lithium doping of samples LiX.H2O and Li2(OH)X leads to a suppression of the phase transition into the noncubic phases and the good ionic conductivity is extended down to lower temperatures (T < 0 degree C). Thus, lithium doping of the lithium halide hydrates provides a promising tool for tailoring the ionic conductivity at ambient temperatures to its optimum value.

TEM and XAS Characterization of Hard Magnetic Phase in Nd-Fe Alloys

ABSTRACTNd80Fe20 alloys were prepared by copper mold casting and directional solidification (DS) to obtain a coarse microstructure, which can be identified unambigously. The mold-cast samples exhibit a fine globular eutectic structure and show high room temperature coercivity (∼ 5 kOe). Transmission electron microscopic (TEM) investigations reveal the presence of fcc (a = 5.32 Å) Nd-rich and nanocrystalline non-cubic Fe-rich phases. The directionally solidified (DS) samples exhibit large grains (composition close to NdFe2). The NdFe2 grains (formed in the DS samples) show clear magnetic domains with uniaxial anisotropy indicating that the NdFe2-type phase observed in the present samples is not a cubic phase. X-ray absorption spectroscopy (XAS) was used to analyze the local configuration around Fe atoms in the mold-cast samples. The XAS patterns do not match with the bcc and fcc standards, which supports the presence of NdFe2 phase with uniaxial symmetry in the DS samples.

Force constant calculation from Raman and IR spectra in the three non‐cubic phases of BaCeO3

AbstractRaman spectroscopic data for BaCeO3 were modeled with the use of a small set of force constants in its three phases (Pnma, Imma and R3c) using experimental IR and Raman data. An IR reflectance spectrum of BaCeO3 in its Pnma phase is provided for the first time. Raman band symmetry assignments in the Pnma phase were re‐examined and the assignments of Genet et al. were confirmed. The distribution of the Raman bands of the Pnma phase between the zone center and X point in Imma is provided and band assignments were extended to the Imma and R3c phases. Calculated values for both Raman‐ and IR‐active modes are provided in the three phases. The variations in force constants throughout the phase transitions are briefly discussed. Copyright © 2001 John Wiley &amp; Sons, Ltd.

137Ba and 47,49Ti NMR: electric field gradients in the non-cubic phases of BaTiO 3

Electric field gradients have been measured at the Ba, and Ti sites in BaTiO 3 from 137Ba and 47,49Ti NMR spectra of a recrystallized, strain-free polycrystalline specimen in the ferroelectric tetragonal, orthorhombic and rhombohedral phases. The strain-free nature of the recrystallized material has been established with convergent beam electron diffraction patterns and electron micrographs. The electric field gradients at the Ba and Ti sites in the rhombohedral phase are compared with recent theoretical predictions.

Formation of δ phase and its effects on magnetic properties and magnetostriction of Tb 0.5Pr 0.5(Fe 0.4+ xCo 0.6− x) 1.9 (0⩽ x⩽0.6)

Structure, magnetic properties and magnetostriction of Tb 0.5Pr 0.5(Fe 0.4+ x Co 0.6− x ) 1.9 (0⩽ x⩽0.6) alloys are investigated. The existence of the δ phase for 50% Tb and 50% Pr is beneficial to the formation of the MgCu 2-type Laves phase. The composition dependence of Curie temperature has the same trend as the Slater–Pauling curve for the moment of Fe–Co alloys. The saturation magnetization M s and the remanence M R decrease, whereas the coercivity i H c increases with decreasing Co content, due to the increased amount of the non-cubic PuNi 3-type phase. The polycrystalline magnetostriction | λ ∥− λ| of the alloys at room temperature increases with increasing magnetic field and does not achieve saturation up to 796 kA/m.

Synthesis and characterization of cubic boron nitride thin films using the ME-ARE method

Cubic boron nitride (cBN) films were deposited by a magnetically enhanced activated reactive evaporation (ME-ARE) technique. Pulsed DC instead of r.f. power was used to bias the substrate. The effect of deposition parameters such as substrate bias voltage, plasma discharge current and gas flow ratio on the formation of cBN was investigated. BN films were characterized by FTIR and TEM. CBN films with a high content of cubic phase were successfully synthesized. It was found that formation of cBN film requires the bombardment of ions with both high flux and high energy. TEM observation showed that the cBN film had grain sizes of 15-50 nm and that a non-cubic phase BN, 10-15 nm was initially grown.

Structure and magnetic properties of melt-spinning Pr(Fe0.6Co0.4)2 alloys

Pr(Fe 0.6 Co 0.4 ) 2 ribbons were prepared by melt spinning with different wheel speeds from 35 to 45 m/s. Their structure, magnetic properties, and thermal stability are investigated. At a wheel speed of 35 m/s, the ribbon consists of a mixture of Pr(Fe,Co)2 cubic Laves phase and some noncubic phases. An almost Pr(Fe,Co)2 nanocrystalline single phase with a Curie temperature of 305 °C is obtained at a wheel speed of 40 m/s. Except for Pr(FeCo)2 phase a small amount of amorphous phase is observed with increasing wheel speeds to 45 m/s. Pr(Fe,Co)2 phase becomes unstable and decomposes above 770 °C. The resin-bonded Pr(Fe,Co)2 nanocrystalline phase which is obtained at a wheel speed of 40 m/s combines high magnetostriction (λ∥−λ⊥=140 ppm), with significant coercivity, Hci=5 kOe.

Anomalous metallic lithium phases: Identification by ESR, ENDOR, and the bistable Overhauser effect

Metallic lithium particles precipitated in lithium hydride by UV irradiation are investigated by electron spin resonance (ESR), electron-nuclear double resonance (ENDOR), and ESR at low temperatures under conditions of bistable Overhauser effect [Phys. Rev. B 47, 15 023 (1993)]. Although the conventional ESR spectroscopy gives only a single structureless line for conduction electrons, ENDOR clearly shows the existence of two well-defined populations of lithium particles with different crystallographic structures. The presence of a quadrupole structure in the ENDOR spectrum indicates that one of the two populations belongs to the $9R$ close-packed phase of metallic lithium, normally stable below 80 K. The other population belongs to a cubic phase. At low temperatures the saturation of the ESR line produces a bistable Overhauser effect which gives a hysteresis of the ESR trace. This bistable conduction electron spin resonance clearly confirms the existence of two different populations of lithium particles. The presence of a broad hysteresis of the resonance at 4 K is the consequence of a very long ${T}_{2},$ which amounts to $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{s}$ and ${10}^{\ensuremath{-}6}\mathrm{s}$ in the cubic and the noncubic phases, respectively. These values are much larger than those deduced from the linewidth of the unsaturated ESR line. It is concluded that ENDOR combined with bistable ESR could provide very selective methods for the investigation of the lithium anode in lithium batteries.

Structural and Electronic Properties of the Noncubic Superconducting FulleridesA4′C60(A′=Ba,Sr)

The absence of superconductivity in noncubic alkali fullerides, ${A}_{4}{\mathrm{C}}_{60}$ $(A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{K},\mathrm{Rb},\mathrm{Cs})$ has been associated with the effects of electron correlation and electron-phonon interaction on the narrow ${t}_{1u}$-derived conduction band. We find that the structurally related alkaline earth fullerides, ${A}_{4}^{\ensuremath{'}}{\mathrm{C}}_{60}$ $({A}^{\ensuremath{'}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{Sr},\mathrm{Ba})$, are superconducting with small values of the densities of states at the Fermi level, ${N}_{{\ensuremath{\epsilon}}_{F}}$. Close contacts between ${\mathrm{Ba}}^{2+}$ ions and neighboring ${\mathrm{C}}_{60}$ units in ${\mathrm{Ba}}_{4}{\mathrm{C}}_{60}$ imply a strong orbital hybridization, which is evidently responsible for stabilizing the noncubic superconducting phase, in contrast to insulating ${A}_{4}{\mathrm{C}}_{60}$.

On the lattice parameters of sodium niobate at room temperature and above

Sodium niobate undergoes a complicated sequence of phase transitions on heating from room temperature, before becoming cubic above 641°C. In all five non-cubic phases exhibited, the octahedra are tilted about <1 0 0> pseudocubic directions. The phases stable between −80°C and 360°C, and between 360°C and 480°C are antiferroelectric with the niobiums displaced from the centres of the octahedra. The combination of octahedral tilts and cation displacements leads to the edge lengths of the psedocubic cell of each phase becoming a multiple of the cubic cell found above 641°C. We have re-examined these phases using HRPD at ISIS, Rutherford Appleton Laboratory, UK, and find that the multiplicities of the pseudocubic cells describing the phases stable in the ranges 360–480°C and 480–520°C are both 2×4×6, and not 2×2×6 and 2×2×2 as reported in the literature. Furthermore, the high resolution used in the scattering experiment has revealed that the phase stable at room temperature has symmetry which is lower than orthorhombic.

Mössbauer spectroscopy of Al substituted Fe in holmium iron garnet

A Mossbauer spectroscopy study was made on Ho3Fe5-xAlxO12 (x=0.0, 0.05, 0.7). X‐ray diffraction patterns indicate that the samples with x=0.0 and 0.05 have the garnet structure, while the sample with x=0.7 has an additional noncubic structural phase. The room temperature spectrum for samples with x=0.0 and 0.05 consists of two magnetic components corresponding to the octahedral and tetrahedral sites with hyperfine magnetic fields (Bhf) of 50 T and 40 T, respectively. For x=0.7 we observe a new magnetic component with Bhf= 45 T, a reduction in the intensity and broadening of the tetrahedral component, and the evolution of a nonmagnetic central component. These variations are evidently due to the addition of aluminium to the system. At liquid nitrogen temperature the samples with x=0.0 and 0.05 are nearly identical. It was also observed that the increase in Bhf for the octahedral site is smaller than that for the tetrahedral site as the temperature is lowered to 80 K.

Preparation of BN films in the BNF system

Abstract Low-pressure diamond synthesis is today well established, while low-pressure cubic boron nitride (BN) synthesis is not, as yet, successful. For realizing a c-BN CVD process, effective selective etching reactions have to be formulated, enabling the removal of all non-cubic BN phases formed. Preliminary tests showed clearly that crystalline h-BN is etched preferentially in comparison to c-BN by a BF 3 /Ar gas mixture. Deposition experiments revealed that BN deposits could not be obtained from a BF 3 -N 2 -Ar gas mixture activated by means of a microwave plasma CVD process. A comparison with thermodynamic calculations confirmed this observation. However, BN-films could be deposited from BF 3 -N 2 -Ar-H 2 gas mixtures. Thereby white-brownish deposits were obtained on CVD-diamond substrates. Variations of the gas phase compositions and substrate temperatures (500–700°C) influenced morphology and phase composition of the BN layers. The deposits obtained were characterized by scanning electron microscopy (SEM) and infra-red (IR)-spectroscopy. So far it has not been possible to identify clearly any c-BN deposits in our experiments.