The interpretation of X-ray diffraction data from thin epitaxial lead-tin telluride crystals—I

Calculations are made of the expected strength of X-ray diffraction peaks from (100) PbTe and Pb_1−xSn_xTe crystals. The calculations show that in practice the form of the 800 diffraction peak can be related to the epitaxial layer thickness of heterostructures and also indicates the degree of tin diffusion across the heterojunction. The calculations are illustrated in graphical form.The growth system used for the structures examined is shown to produce tin diffusion across a stratum of crystal comparable in thickness to the thickness of the deposited PbTe surface layers.     

The interpretation of X-ray diffraction data from thin epitaxial lead tin telluride crystals—II

The difusion of tin across heterojunctions formed between thin epitaxial layers of lead telluride, or lead tin telluride grown on each other, has been examined by comparing the experimental X-ray diffraction data with theoretical diffraction envelopes, calculated by convoluting diffraction data with idealized tin diffusion profiles. The results show that, for lead telluride grown on lead tin telluride, the tin diffusion is best described by an error function, and for lead tin telluride grown on lead telluride. an exponential function provides the best fit between experimental and calculated data.     

Simultaneous electrical and X-ray diffraction studies on neodymium metal to 152 GPa

Using designer diamond anvils and angle dispersive X-ray diffraction technique at a synchrotron source, we have performed simultaneous electrical and structural studies on neodymium metal to 152 GPa in a diamond anvil cell. Four-probe electrical resistance measurement shows a 38% decrease in the electrical resistivity, associated with the delocalization of the 4f-shell electrons, starting at 100 GPa up to a final pressure of 152 GPa. The continuous decrease in electrical resistivity is consistent with the observation of a gradual phase transition to α-U structure in this pressure range. The (1 1 1) diffraction peak of α-U structure first appears at 100 GPa and increases in intensity with increasing pressure to 152 GPa. This increase in intensity is attributed to an increasing volume fraction of α-U phase and an increase in structural y-parameter from 0.07 at 118 GPa to 0.095 at 152 GPa. In contrast to the abrupt pressure-induced f-electron transition seen in cerium and praseodymium, the continuous evolution of α-U structure and electrical resistivity in neodymium confirms the gradual nature of 4f delocalization process in this element.     

Brief report: Volume dependence of Grüneisen parameter for solids under extreme compression

The Nie expression is amended in such a way that the expression follows the infinite pressure behaviour, i.e., \(P\to \infty \) or V → 0. A new empirical relationship is developed to predict the values of volume dependence of Grüneisen parameter. NaCl and ε-Fe have been employed to test the suitability of the expression. The results obtained reveal that the relationship is reliable as there is a good agreement between the calculated and the experimental data.     

X-ray diffraction from the ripple structures created by femtosecond laser pulses

In this paper, we present the investigation and characterization of the laser-induced surface structure on an asymmetrically cut InSb crystal. We describe diffraction from the ripple surface and present a theoretical model that can be used to simulate X-ray energy scans. The asymmetrically cut InSb sample was irradiated with short-pulse radiation centred at 800 nm, with fluences ranging from 10 to 80 mJ/cm². The irradiated sample surface profile was investigated using optical and atomic force microscopy. We have investigated how laser-induced ripples influence the possibility of studying repetitive melting of solids using X-ray diffraction. The main effects arise from variations in local asymmetry angles, which reduce the attenuation length and increase the X-ray diffraction efficiency.     

Accelerated mechanosynthesis of high-nitrogen stainless steel: Mössbauer and X-ray diffraction studies

A completely austenitic structure has been obtained ten times faster via the mechanical alloying (MA) of high-nitrogen 25Cr10Mn1N alloy from metal components and manganese nitride than during the MA of similar steels in a nitrogen atmosphere. A mechanism for the bcc → fcc phase transformation that occurs during MA, and where deformation stacking faults of the layer fault type on the {211} atomic planes of the bcc phase play a key role, is considered and proposed.     

X-ray diffraction and Mössbauer spectroscopy studies of cementite dissolution in cold-drawn pearlitic steel

Cementite dissolution in cold-drawn pearlitic steel (0.8 wt.% carbon) wires has been studied by quantitative X-ray diffraction (XRD) and Mössbauer spectroscopy up to drawing strain 1.4. Quantification of cementite-phase fraction by Rietveld analysis has confirmed more than 50% dissolution of cementite phase at drawing strain 1.4. It is found that the lattice parameter of the ferrite phase determined by Rietveld refinement procedure remains nearly unchanged even after cementite dissolution. This confirms that the carbon atoms released after cementite dissolution do not dissolve in the ferrite lattice as Fe-C interstitial solid solution. Detailed analysis of broadening of XRD line profiles for the ferrite phase shows high density of dislocations (～10¹⁵/m²) in the ferrite matrix at drawing strain 1.4. The results suggest a dominant role of ?1?1?1? screw dislocations in the cementite dissolution process. Post-deformation heat treatment leads to partial annihilation of dislocations and restoration of cementite phase. Based on these experimental observations, further supplemented by TEM studies, we have suggested an alternative thermodynamic mechanism of the dissolution process.     

X-ray diffraction and optical studies of CdS1−x Sex solid solutions with stacking faults

The existence in CdS_1?x Se_x crystals (with x=0.10–0.50) of crystalline regions with stacking faults (SF) was first demonstrated by x-ray diffraction and optical methods. X-ray diffraction studies showed SF to be present in all the samples investigated, but in different concentrations. The effect of SF present in CdS_1?x Se_x solid solutions of different compositions on their exciton reflectance and photoluminescence (PL) spectra has been studied. Crystals with high SF concentrations were found to exhibit new exciton bands, which are manifested in reflectance and PL spectra. In the CdS_1?x Se_x phase with SF, resonant emission due to free excitons and the corresponding phonon replicas have been observed. The effect of reabsorption, which can bring about a change in the zero-phonon emission line shape (doublet formation), as well as affect the intensity ratios of the zero-phonon line to the phonon replicas, has been analyzed. It is pointed out that the variation with temperature of the shape of the SF-induced PL exciton line indicates its complex structure, with the constituents of this structure varying in intensity with increasing temperature.     

Phase evolution studies of sol�Cgel derived lead zirconate titanate (PZT) nanopowder using X-ray diffraction and X-ray photoelectron spectroscopy

We report here the formation of single phase lead zirconate titanate (PZT) nanopowder with composition Pb(Zr_0.52Ti_0.48)O₃ and average crystallite size 12?C20?nm, synthesized by sol?Cgel process. The phase evolution of PZT gel powder, heat treated at temperatures 550, 650 and 800°C was monitored by X-ray diffraction (XRD) and X-ray photo-electron spectroscopy (XPS). The high resolution XPS spectra of Pb4f, Zr3d, Ti2p and O1s show that PZT with pure perovskite structure is obtained at 800°C while at lower temperatures pyrochlore phase co-exists with perovskite phase. The XRD results also support this analysis. We have also identified the pyrochlore phase using XPS by analyzing the corresponding variations in the FWHM values, peak positions and the separation between the spin doublets of Pb, Zr and Ti associated with it. The composition of the final powder obtained with pure perovskite structure is calculated and is close to the designed value.     

Mössbauer,X-ray diffraction and AC susceptibility studies on nanoparticles of zinc substituted magnesium ferrite

Nanoparticles of zinc substituted Mg-ferrite with compositions Mg_(1-x)Zn _x Fe₂O₄ (x = 0.15, 0.30 and 0.50) having particle sizes in the range 6.4 nm to 21.4 nm prepared by the co-precipitation method were characterized by ⁵⁷Fe Mössbauer spectroscopy, X-ray diffratometry and AC magnetic susceptibility measurements. Mössbauer measurements at room temperature and down to 20 K clearly indicate presence of superparamagnetic particles in all the samples. AC magnetic susceptibility data show lowering of blocking temperature with decrease of particle size. Superparamagnetic relaxation was observed for larger particle size in samples with higher Zn content, which is attributed to the weakening of A-B exchange interaction in ferrite lattice due to replacement of Fe^{3 +} in tetrahedral site by Zn^{2 +} ions.Received: 16 April 2004, Published online: 23 July 2004PACS: 75.50.Tt Fine-particle systems; nanocrystalline materials - 76.80. + y Mössbauer effect; other gamma-ray spectroscopy - 75.30.Cr Saturation moments and magnetic susceptibilitiesS. Das: Present address: Department of Physics, Jadavpur University, Kolkata - 700032, India     

X-ray diffraction and Mössbauer studies of X20Cr13 steel subjected to ball milling

The powders of X20Cr13 steel were subjected to ball milling process in a planetary ball mill. X-ray diffraction and Mössbauer spectroscopy studies revealed the refinement of the structure of this steel down to a nanocrystalline range practically without any phase transformations. Both techniques allowed to detect the alloyed ferrite as well as residual content of iron containing M₂₃C₆-type carbide, which was dissolved into the ferrite during milling. Hyperfine magnetic fields in ball milled steel samples did not differ significantly from those for the bulk steel disc.     

The first principle studies of the structural and vibrational properties of solid β-HMX under compression

The structural, vibrational and electronic properties of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β-HMX) crystal under high pressure up to 40 GPa have been studied using density functional theory (DFT). The pressure dependences on the cell volume, lattice constants, and molecular geometry of solid β-HMX are presented and discussed. It is found that the N–N bonds are significantly reduced under compression, which may be of importance for initial decomposition. Based on the optimized crystal structures, the vibrational frequencies for the internal and lattice modes of the β-HMX crystal at ambient and high pressures are computed, and the pressure-induced frequency shifts of these modes are discussed.     

Synchrotron X-ray diffraction studies on the phase transitions in the spinel LixMn3−xO4 intercalation compounds

Detailed investigations have been undertaken of the lithium for manganese substitution effect on the LiMn₂O₄, in the system Li_xMn_3−xO₄, for 0.95≤x≤1.05, that is for the nearly stoichiometric lithium content. Synchrotron X-ray measurements have been performed in the temperature range 10–300 K. The diffraction experiments were carried out at the DESY-HASYLAB high-resolution powder diffractometer (beamline B2), equipped with a closed-cycle He-cryostat. Very small changes in the lithium content influence clearly the low-temperature crystal structure of Li_xMn_3−xO₄, spinels and the nature of phase transitions. It was found that for x=0.95 the sample remains tetragonal in the whole 10–300 K temperature range. The stoichiometric LiMn₂O₄ transforms from cubic to orthorhombic at about 280 K. For x=1.0125 the temperature of phase transition from cubic to orthorhombic decreases down to about 260 K, whereas for x=1.025 the transformation goes from cubic to tetragonal phase, at the temperature 220 K. No phase transition has been observed for the cubic sample with x=1.0375. These results partly explain the divergences in recent reports on the low-temperature structure and phase transformations of lithium manganese oxides.     

Characterization of Maghsail meteorite from Oman by Mössbauer spectroscopy, X-ray diffraction and petrographic microscopy

The meteorite found at Maghsail (16 55 70 N–53 46 69 E) west of Salalah Oman, has been studied by ⁵⁷Fe Mössbauer spectroscopy, X-diffractometry and petrographic microscopy. In the polished section the meteorite exhibits a porphyritic texture consisting of pyroxene and olivine phenocrysts in a fine to medium grained ground mass in addition to minor phases possibly skeletal chromite, troilite and minute amount of iron oxides. X-ray diffraction supports the existence of these compounds. The Mössbauer spectra of powdered material from the core of the rock at 298 K and 78 K exhibit a mixture of magnetic and paramagnetic components. The paramagnetic components are assigned to the silicate minerals olivine and pyroxene. On the other hand, the magnetic spectra reveal the presence of troilite and iron oxides. The petrographic analyses indicate that the iron oxides are terrestrial alteration products.     

Conversion electron and X-ray Mössbauer studies of boronized low-carbon steel under corrosion and oxidation conditions

Iron-boride layers on low-carbon steel were produced by thermochemical diffusion process. The surface interaction products: Fe₂B, FeB, FeB_x (x>1) and a solid solution of iron in boron were identified by surface Mössbauer spectroscopy (CEMS and XMS). Samples of original and boronized steel were subjected to corrosion process by immersion in HCl (0.1 N) solution for 150 h. While the steel sample was strongly corroded, none corrosion product was found on the boronized sample surface. However, significant changes in relative percentages of the various iron boride phases were detected. Also, samples of original and boronized steel were subjected to oxidation process by heat-treatment in air at 300°C for 8 h and 500°C for 4 h. At 300°C, while bulk Fe₃O₄ and -Fe₂O₃ were formed on the steel surface, none iron oxide was detected on the boronized surface. At 500° C, while only pure bulk -Fe₂O₃ was detected on the steel surface, a particle size distribution of-Fe₂O₃, with particle size of about 100 Å, was probably formed on the boronized surface, as evidenced by CEMS.     

X-ray diffraction studies of specific features in the atomic structure of Fe-Si alloys in the α area of the phase diagram

The atomic structure of Fe-Si alloys with a silicon concentration of 5–8 at % (α-area of the phase diagram) was studied using X-ray diffraction. The effect of quenching after annealing at a disordering temperature of 850°C on the structural state of the alloys was elucidated. It is shown that the quenched samples are characterized by a short-range ordering; namely, there is a local B2-type order at a concentration of 5–6 at % Si and, in addition, DO₃-phase clusters are formed at 8 at % Si. The atomic structure of B2 clusters and their nearest surroundings is established.     

High pressure X-ray diffraction and Raman spectroscopic studies of the phase change of D2O ice VII at approximately 11 GPa

High pressure experiments were performed on D₂O ice VII using a diamond anvil cell in a pressure range of 2.0–60 GPa at room temperature. In situ X-ray diffractometry revealed that the structure changed from cubic to a low symmetry phase at approximately 11 GPa, based on the observed splitting of the cubic structure's diffraction lines. Heating treatments were added for the samples to reduce the effect of non-hydrostatic stress. After heating, splitting diffraction lines became sharp and the splitting was clearly retained. Although symmetry and structure of the transformed phase have not been determined, change in volumes vs. pressure was calculated, assuming that the low-symmetry phase had a tetragonal structure. The bulk modulus calculated for the low-symmetry phase was slightly larger than that for the cubic structure. In Raman spectroscopy, the squared vibrational frequencies of ν₁ (A_1g), as a function of pressure, showed a clear change in the slope at 11–13 GPa. The full width at half maxima of the O-D modes decreased with increasing pressure, reaching a minimum at approximately 11 GPa, and increased again above 11 GPa. These results evidently support the existence of phase change at approximately 11 GPa for D₂O ice VII.     

X-ray diffraction studies of heterostructures based on solid solutions Al x Ga1 − x As y P1 − y : Si

The growth of MOCVD-hydride epitaxial heterostructures based on ternary solid solutions Al _x Ga_1?x As heavily doped with phosphorus and silicon has been studied using high-resolution X-ray diffraction and X-ray microanalysis. The prepared epitaxial films are five-component solid solutions (As _x Ga_1?x As _y P_{1 ? y} )_{1 ? z} Si _z .     

In situ X-ray diffraction studies of phase transition in Pb1− x La x Zr0.40Ti0.60O3 ferroelectric ceramics

The temperature dependence of the crystalline structure and the lattice parameters of Pb_{1? x} La _x Zr_0.40Ti_0.60O₃ ferroelectric ceramic system with 0.00 ≤ x ≤ 0.21 was determined. The samples with x ≤ 0.11 show a cubic-to-tetragonal phase transition at the maximum dielectric permittivity, T _max. Above this amount and especially for the x = 0.12 sample, a spontaneous phase transition from a relaxor ferroelectric state (cubic phase) to a ferroelectric state (tetragonal phase) is observed upon cooling below the T _max. Unlike what has been reported in other studies, the x = 0.13, 0.14, and 0.15 samples, which present a more pronounced relaxor behavior, also presents a spontaneous normal-to-relaxor transition, indicated by a cubic to tetragonal symmetry below the T _max. The origin of this anomaly has been associated with an increase in the degree of tetragonality, confirmed by the measurements of the X-ray diffraction patterns. The differential thermal analysis (DSC) measurements also confirm the existence of these phase transitions.     

Photoluminescence, FTIR and X-ray diffraction studies on undoped and Al-doped ZnO thin films grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis

Undoped and aluminum-doped zinc oxide (ZnO) thin films have been grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis (USP) technique using zinc acetate dihydrate and aluminum chloride hexahydrate (Al source) dissolved in methanol, ethanol and deionized water. A number of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and photoluminescence (PL) were used to characterize the obtained ZnO thin films. It was seen that the orientation changed with increase in substrate temperature. During the ZnO deposition Zn source reacted with polycrystalline α-Al₂O₃ substrate to form an intermediate ZnAl₂O₄ spinel layer. It has been interestingly found that the intensity of green emission at 2.48 eV remarkably increased when the obtained ZnO:Al films were deposited at 380 °C. The FTIR absorbance intensity of spectroscopic band at 447±6 cm⁻¹ is very sensitive to oxygen sublattice disorder resulting from non-stoichiometry, which is consistent with the result of PL characterization.