scholarly journals Crystal Chemistry of Six Grossular Garnet Samples from Different Well-Known Localities

Minerals ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 767
Author(s):  
Sytle M. Antao
Keyword(s):  
Ionic Radius ◽  
Rietveld Method ◽  
Cubic Phase ◽  
X Ray Diffraction ◽  
X Ray ◽  
Cubic Phases ◽  
Backscattered Electron ◽  
Grossular Garnet ◽  
Elemental Maps ◽  
Monochromatic Synchrotron

Two isotropic grossular (ideally Ca3Al2Si3O12) samples from (1) Canada and (2) Tanzania, three optically anisotropic grossular samples (3, 4, 5) from Mexico, and one (6) anisotropic sample from Italy were studied. The crystal structure of the six samples was refined in the cubic space group Ia3¯d, using monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and the Rietveld method. The compositions of the samples were obtained from electron microprobe analyses (EPMA). The HRPXRD traces show a single cubic phase for two isotropic samples, whereas the four anisotropic samples contain two different cubic phases that were also resolved using X-ray elemental line scans, backscattered electron (BSE) images, and elemental maps. Structural mismatch from two cubic phases intergrown in the birefringent samples gives rise to strain-induced optical anisotropy. Considering the garnet general formula, [8]X3[6]Y2[4]Z3[4]O12, the results of this study show that with increasing unit-cell parameter, the Y-O distance increases linearly and rather steeply, the average <X-O> distance increases just slightly in response to substitution mainly on the Y site, while the Z-O distance remains nearly constant. The X and Z sites in grossular contain Ca and Si atoms, respectively; both sites show insignificant substitutions by other atoms, which is supported by a constant Z-O distance and only a slight increase in the average <X-O> distance. The main cation exchange is realized in the Y site, where Fe3+ (ionic radius = 0.645 Å) replaces Al3+ (ionic radius = 0.545 Å), so the Y-O distance increases the most.

scholarly journals Crystal Chemistry of Birefringent Uvarovite Solid Solutions

Minerals ◽  
2019 ◽  
Vol 9 (7) ◽  
pp. 395 ◽  
Author(s):  
Sytle M. Antao ◽  
Jeffrey J. Salvador
Keyword(s):  
Crystal Chemistry ◽  
Solid Solutions ◽  
Rietveld Method ◽  
X Ray Diffraction ◽  
Scattered Electron ◽  
X Ray ◽  
Group I ◽  
Elemental Maps ◽  
Monochromatic Synchrotron ◽  
The Ideal

The crystal chemistry of five optically anisotropic uvarovite samples from different localities (California, Finland, Russia, and Switzerland) were studied with electron-probe microanalysis (EPMA) and the Rietveld method. Monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data were used, and Rietveld refinement was carried out with the cubic space group, I a 3 ¯ d . The general formula for garnet is [8]X3[6]Y2[4]Z3[4]O12. Uvarovite has the ideal formula, Ca3Cr2Si3O12, which may be written as Ca3{Cr,Al,Fe}Σ2[Si3O12] because of solid solutions. HRPXRD traces show multiple cubic garnet phases in each sample that has a heterogeneous chemical composition. The optical and back-scattered electron (BSE) images and elemental maps contain lamellar and concentric zoning as well as patchy intergrowths. With increasing a unit-cell parameter for uvarovite solid solutions, the Z–O distance remains constant, and the average <X–O> distance increases slightly in response to the Cr3+ ⇔ Al3+ cation substitution in the Y site. The Y–O distance increases most because Cr3+ (radius = 0.615 Å) is larger than Al3+ (radius = 0.545 Å) cations. The Fe3+ (radius = 0.645 Å) cation is also involved in this substitution. Structural mismatch between the cubic garnet phases in the samples gives rise to strain-induced optical anisotropy.

The mystery of birefringent garnet: is the symmetry lower than cubic?

2013 ◽  
Vol 28 (4) ◽  
pp. 281-288 ◽  
Author(s):  
Sytle M. Antao
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Rietveld Method ◽  
Chemical Formula ◽  
X Ray Diffraction ◽  
X Ray ◽  
Garnet Phase ◽  
Cubic Phases ◽  
General Chemical ◽  
Monochromatic Synchrotron

The cause of birefringence in several garnet-group minerals with general chemical formula, [8]X3[6]Y2[4]Z3[4]O12, which was observed over 100 years ago, is unknown, although many different reasons were proposed, including symmetry lower than cubic. In this study, electron microprobe analyses (EMPA) were obtained for a Ti-rich andradite, ideally Ca3(Fe23+)Si3O12, from Magnet Cove, Arkansas, USA, and the results show that the sample is inhomogeneous with two distinct compositions. The crystal structure was refined by the Rietveld method, cubic space group $Ia\overline 3 d$, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data, which shows a mixture of three distinct cubic phases that are intergrown together and cause birefringence because of strain arising from small structural mismatch. This mixture of three cubic phases was not observed by any other experimental technique. These results have many implications, including garnet phase transitions from cubic to lower symmetry in the mantle, which has important geophysical consequences.

scholarly journals X-Ray Powder Diffraction Studies of Mechanically Milled Cobalt

2012 ◽  
Vol 626 ◽  
pp. 913-917
Author(s):  
W.S. Yeo ◽  
Z. Nur Amirah ◽  
H.S.C. Metselaar ◽  
T.H. Ong
Keyword(s):  
Grain Size ◽  
Phase Transformation ◽  
Surface Energy ◽  
Phase Formation ◽  
Rietveld Method ◽  
High Energy ◽  
Cubic Phase ◽  
X Ray Diffraction ◽  
Structure Defects ◽  
X Ray

The allotropic phase transformation of cobalt powder prepared by high-energy ball milling was investigated as a function of milling time. Measurement of crystallite size and micro-strain in the powder systems milled for different times were conducted by X-ray diffractometry. The X-ray diffraction (XRD) peaks were analyzed using the Pearson VII profile function in conjunction with Rietveld method. X-ray diffraction line broadening revealed that allotropic transformation between face-centred-cubic phase (fcc) and hexagonal close-packed phase (hcp) in cobalt is grain size dependent and also on the accumulation of structure defects. The results showed that the phase formation of cobalt depends on the mill intensity that influences of both the grain size and the accumulation of structure defects. However, this theory alone is not adequate to explain the effects in this work. It was found that the total surface energy (Ω) theory satisfactorily explains the phase transformation behavior of cobalt. The smaller value of surface energy (Ω) of the fcc crystal than the hcp phase when size decreases may alter the qualitative aspects of the phase formation.

Diffraction Study of The Transformation of an Al-Cu-Fe Atomized Powder Upon Annealing at 500°C

2000 ◽  
Vol 643 ◽  
Author(s):  
P. Weisbecker ◽  
G. Bonhomme ◽  
A. Cael ◽  
L. Zhang ◽  
J.M. Dubois
Keyword(s):  
Diffraction Study ◽  
Initial Mixture ◽  
Heat Treatments ◽  
Cubic Phase ◽  
Nominal Composition ◽  
X Ray Diffraction ◽  
X Ray ◽  
Cubic Phases ◽  
Cscl Type ◽  
Oxygen Atoms

AbstractUsing powder X-ray diffraction, we have studied the transformation of an atomized powder of nominal composition Al62Cu25.5Fe12.5 upon annealing at 500°C in air or in vacuum. The initial mixture of icosahedral and B2 cubic phases transforms within 2 hours into a nearly pure icosahedral compound. We observe however that a small amount of residual cubic phase is still present after annealings as long as 65 hours. Furthermore, the initial cubic phase splits in two components, a CsCl-type and a disordered bcc cubic phase. By comparison of the heat treatments in air and in vacuum, we point out that oxygen atoms diffuse into the icosahedral lattice.

X-ray powder diffraction investigation of new oxynitride precursors: rare earth oxide compounds of fluorite- and sheelite-type structures in the Yb-(Zr,W)-O system

2007 ◽  
Vol 22 (4) ◽  
pp. 344-351 ◽  
Author(s):  
Pascal Maillard ◽  
Patricia Bénard-Rocherullé ◽  
Thierry Roisnel ◽  
Franck Tessier
Keyword(s):  
Rietveld Method ◽  
Cubic Phase ◽  
Second Phase ◽  
Line Broadening ◽  
X Ray Diffraction ◽  
Solid State Method ◽  
Space Group ◽  
X Ray ◽  
Profile Matching ◽  
T Phase

Nanocrystallized oxide precursors of colored (oxy)nitrides related to the system Yb-(Zr-W)-O have been successfully prepared using a chimie douce process—the amorphous citrate route. The process involves first a formation of fine and homogeneous powdered solids obtained by calcination at 600 °C, a temperature much lower than that of the conventional solid-state method. At this stage, the X-ray diffraction patterns exhibit large line broadening effects. Finally, two well-crystallized and pure quaternary oxides have been readily obtained by heating and under annealing conditions at 850 and 900 °C for 12 h. For one of the patterns, all the X-ray diffraction lines can be easily indexed to a cubic phase with the fluorite structure conforming to the Fm3m space group [Yb2Zr1.21W0.41O6.65◻1.35 called C-phase: a=5.1864(2) Å]. The second phase adopts the sheelite-type structure [Yb2ZrWO8 called T-phase: space group I41/a, a=5.1584(5), and c=10.8246(6) Å]. By taking into account the present compositions determined by EDS measurements, Rietveld structure refinements produce final RB factors of 0.015 and 0.044, and Rwp factors of 0.069 and 0.089, respectively. In order to characterize the microstructure of the materials (crystallite size and lattice distortion) at the nanometer scale, a study based on diffraction line broadening analysis applying the whole pattern refinement method was also undertaken with confidence. The results show smooth angular variations of the values of FWHMs, indicating that the microstructural properties are isotropic for the cubic and tetragonal oxides. More precisely, the results indicate that whatever the profile fitting approach used (“profile matching” procedure and Rietveld method), the reliability factors Rwp are systematically better with a combined size strain than with zero strain considerations. The strain magnitudes observed for the C-phase-850 °C as well as for the T-phase-900 °C should be viewed as realistic strain.

Structural analysis of lead magnesium niobate using synchrotron powder X-ray diffraction and the Rietveld method

2016 ◽  
Vol 72 (3) ◽  
pp. 404-409 ◽  
Author(s):  
Ashok Bhakar ◽  
Adityanarayan H. Pandey ◽  
M. N. Singh ◽  
Anuj Upadhyay ◽  
A. K. Sinha ◽  
...  
Keyword(s):  
Cell Volume ◽  
Room Temperature ◽  
Unit Cell Volume ◽  
Rietveld Method ◽  
Unit Cell ◽  
Lead Magnesium Niobate ◽  
Cubic Phase ◽  
X Ray Diffraction ◽  
X Ray ◽  
Lead Magnesium

The room-temperature synchrotron powder X-ray diffraction pattern of the single phase perovskite lead magnesium niobate (PMN) has shown significant broadening in theqrange ∼ 5–7 Å−1compared with standard LaB6synchrotron powder X-ray diffraction data, taken under similar conditions. This broadening/asymmetry lies mainly towards the lower 2θ side of the Bragg peaks. Attempts to fit this data with the paraelectric cubic phase (Pm\bar 3m) and the local rhombohedral phase (R3m) corresponding to polar nanoregions (PNRs) are made using the Rietveld method. Rietveld refinements show that neither cubic (Pm\bar 3m) nor rhombohedral (R3m) symmetry can fit this XRD pattern satisfactorily. The two-phase refinement fits the experimental data satisfactorily and suggests that the weight percentage of the PNRs is approximately 12–16% at room temperature. The unit-cell volume of these rhombohedral PNRs is approximately 0.15% larger than that of the unit cell volume of the paraelectric cubic phase.

Pressure-jump X-ray studies of liquid crystal transitions in lipids

2006 ◽  
Vol 364 (1847) ◽  
pp. 2635-2655 ◽  
Author(s):  
John M Seddon ◽  
Adam M Squires ◽  
Charlotte E Conn ◽  
Oscar Ces ◽  
Andrew J Heron ◽  
...  
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Lipid Membranes ◽  
Cubic Phase ◽  
Lipid Phase ◽  
Pressure Jump ◽  
X Ray Diffraction ◽  
X Ray ◽  
Lipid Phase Transitions ◽  
Cubic Phases

In this paper, we give an overview of our studies by static and time-resolved X-ray diffraction of inverse cubic phases and phase transitions in lipids. In §1 , we briefly discuss the lyotropic phase behaviour of lipids, focusing attention on non-lamellar structures, and their geometric/topological relationship to fusion processes in lipid membranes. Possible pathways for transitions between different cubic phases are also outlined. In §2 , we discuss the effects of hydrostatic pressure on lipid membranes and lipid phase transitions, and describe how the parameters required to predict the pressure dependence of lipid phase transition temperatures can be conveniently measured. We review some earlier results of inverse bicontinuous cubic phases from our laboratory, showing effects such as pressure-induced formation and swelling. In §3 , we describe the technique of pressure-jump synchrotron X-ray diffraction. We present results that have been obtained from the lipid system 1 : 2 dilauroylphosphatidylcholine/lauric acid for cubic–inverse hexagonal, cubic–cubic and lamellar–cubic transitions. The rate of transition was found to increase with the amplitude of the pressure-jump and with increasing temperature. Evidence for intermediate structures occurring transiently during the transitions was also obtained. In §4 , we describe an IDL-based ‘ AXcess ’ software package being developed in our laboratory to permit batch processing and analysis of the large X-ray datasets produced by pressure-jump synchrotron experiments. In §5 , we present some recent results on the fluid lamellar– Pn 3 m cubic phase transition of the single-chain lipid 1-monoelaidin, which we have studied both by pressure-jump and temperature-jump X-ray diffraction. Finally, in §6 , we give a few indicators of future directions of this research. We anticipate that the most useful technical advance will be the development of pressure-jump apparatus on the microsecond time-scale, which will involve the use of a stack of piezoelectric pressure actuators. The pressure-jump technique is not restricted to lipid phase transitions, but can be used to study a wide range of soft matter transitions, ranging from protein unfolding and DNA unwinding and transitions, to phase transitions in thermotropic liquid crystals, surfactants and block copolymers.

scholarly journals Crystal chemistry of birefringent spessartine

2014 ◽  
Vol 29 (3) ◽  
pp. 233-240 ◽  
Author(s):  
Sytle M. Antao ◽  
Stephanie A. Round
Keyword(s):  
Rietveld Method ◽  
Structural Parameters ◽  
Phase 1 ◽  
Site Occupancy ◽  
Weight Fraction ◽  
Cubic Phase ◽  
X Ray Diffraction ◽  
Two Phase ◽  
End Members ◽  
Monochromatic Synchrotron

The crystal structure of one isotropic [(1) Brazil] and three birefringent spessartine samples [(2) California, (3) Tanzania, and (4) Colorado] were refined using the Rietveld method, cubic space group$Ia\overline 3 d$, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. The results of electron-microprobe analysis (EMPA) indicate homogeneous compositions, in terms of end-members, as follows: (1) Sps54Alm43, (2) Sps90Alm8, (3) Sps64Prp27Grs3, and (4) Sps73Alm19. Their crystal structures were modeled well as indicated by the Rietveld refinement statistics where the reducedχ2and overallR(F2) values for each sample are: (1) 1.395 and 0.0329, (2) 1.082 and 0.0354, (3) 1.025 and 0.0347, and (4) 1.016 and 0.0413. Two cubic phases occur in samples 2–4, and a single cubic phase occurs in sample-1. The dominant cubic phase-1 with locality, weight fraction (%), unit-cell parameter (Å), distances (Å), and site occupancy factors (sofs) are as follows: (1) Brazil: 100%,a = 11.581 54 (1), average <Mn–O> = 2.3156, Al–O = 1.8949 (3), Si–O = 1.6376 (3) Å, Mn(sof) = 0.961(1), Al(sof) = 0.945(1), and Si(sof) = 0.936(1); (2) California: 96.67(7)%,a = 11.613 32(1), average <Mn–O> = 2.3249, Al–O = 1.8956 (4), Si–O = 1.6416 (4) Å, Mn(sof) = 0.951(1), Al(sof) = 0.946(1), and Si(sof) = 0.927(1); (3) Tanzania: 69.46(6)%,a = 11.598 45(1), average <Mn–O> = 2.3199, Al–O = 1.8964 (5), Si–O = 1.6398 (5) Å, Mn(sof) = 0.808(1), Al(sof) = 0.942(1), and Si(sof) = 0.922(1); and (4) Colorado: 98.58(6)%,a = 11.606 89(1), average <Mn–O> = 2.3204, Al-O = 1.8948 (6), Si–O = 1.6450 (6) Å, Mn(sof) = 0.949(1), Al(sof) = 0.967(2), and Si(sof) = 0.913(2). The two-phase intergrowth causes strain that arises from mismatch of the structural parameters and gives rise to strain-induced birefringence.

HIGH FREQUENCY RAMAN MODES IN NANOCRYSTALLINE LEAD (II) FLUORIDE

2006 ◽  
Vol 05 (04n05) ◽  
pp. 585-591
Author(s):  
P. THANGADURAI ◽  
S. RAMASAMY ◽  
R. KESAVAMOORTHY
Keyword(s):  
Cubic Phase ◽  
Vibrational Modes ◽  
X Ray Diffraction ◽  
High Frequencies ◽  
X Ray ◽  
Gas Condensation ◽  
Cubic Phases ◽  
Raman Modes ◽  
Experimental Values ◽  
First Time

Nanocrystalline PbF 2 was prepared by inert gas condensation technique. Structural studies using X-ray diffraction showed that as-prepared and annealed samples contained both orthorhombic and cubic phases of PbF 2. The annealed samples contain dominantly the cubic phase. For the first time Raman scattering and PL experiments were carried out on these nano- PbF 2. In addition to regular lattice vibrational modes such as T2g for cubic phase and Ag and B1g for orthorhombic phases, a few new Raman modes have been observed at high frequencies. PL studies confirmed that the new Raman modes are due to the presence of electronic centers. The regular modes agree well with the theoretically calculated values and previously reported experimental values.

Texture measurements in Al2O3 using BEKP

1994 ◽  
Vol 52 ◽  
pp. 608-609
Author(s):  
M. D. Vaudin ◽  
J. P. Cline
Keyword(s):  
Neutron Diffraction ◽  
Rapid Method ◽  
Crystallographic Orientation ◽  
Specimen Surface ◽  
X Ray Diffraction ◽  
Anisotropic Crystal ◽  
X Ray ◽  
Verification Method ◽  
Crystal Properties ◽  
Backscattered Electron

The study of preferred crystallographic orientation (texture) in ceramics is assuming greater importance as their anisotropic crystal properties are being used to advantage in an increasing number of applications. The quantification of texture by a reliable and rapid method is required. Analysis of backscattered electron Kikuchi patterns (BEKPs) can be used to provide the crystallographic orientation of as many grains as time and resources allow. The technique is relatively slow, particularly for noncubic materials, but the data are more accurate than any comparable technique when a sufficient number of grains are analyzed. Thus, BEKP is well-suited as a verification method for data obtained in faster ways, such as x-ray or neutron diffraction. We have compared texture data obtained using BEKP, x-ray diffraction and neutron diffraction. Alumina specimens displaying differing levels of axisymmetric (0001) texture normal to the specimen surface were investigated.BEKP patterns were obtained from about a hundred grains selected at random in each specimen.

Sign in / Sign up

Export Citation Format

Share Document