X-ray study of the α–β transformation of berlinite (AlPO4)

1976 ◽  
Vol 54 (6) ◽  
pp. 638-647 ◽  
Author(s):  
H. N. Ng ◽  
C. Calvo
Keyword(s):  
Room Temperature ◽  
Translational Motion ◽  
Fold Axis ◽  
X Ray Diffraction ◽  
Temperature Dependent ◽  
X Ray ◽  
Atomic Displacements ◽  
Β Phase ◽  
Four Corners ◽  
Dauphiné Twinning

The α–β transformation of berlinite (AlPO4) at 586 °C was studied by X-ray diffraction. Atomic displacements were obtained from results of least-squares refinement of data taken between room temperature and 600 °C using reflections whose intensity is unaffected by Dauphiné twinning. The results suggest a rotational motion of the PO4 and AlO4 tetrahedra around the two-fold axis together with a translational motion along the same axis as the transition is approached from below. The vibrational amplitudes of the atoms increase with temperature and have exceeded half of the separation between Dauphiné twin-related configurations at 500 °C. The final β-phase configuration is not achieved by this twinning due to the mismatch of the two configurational potential minima in the a direction. Analysis of the intensity vs. temperature data favours a single minimum model for the β phase configuration over an order–disorder model. The β-AlPO4 structure consists of alternate PO4 and AlO4 tetrahedra sharing all four corners with P—O and Al—O distances 1.505 and 1.694 Å respectively. The results are correlated with those obtained from temperature dependent studies by Raman scattering and by EPR on Fe3+-doped AlPO4.

Mechanical Alloying Behavior in the Nb-Si, Ta-Si, and Nb-Ta-Si Systems

1988 ◽  
Vol 133 ◽  
Author(s):  
K. S. Kumar ◽  
S. K. Mannan
Keyword(s):  
High Temperature ◽  
Mechanical Alloying ◽  
Mechanical Milling ◽  
Room Temperature ◽  
Crystalline Compound ◽  
X Ray Diffraction ◽  
X Ray ◽  
Β Phase ◽  
Α Phase

ABSTRACTThe mechanical alloying behavior of elemental powders in the Nb-Si, Ta-Si, and Nb-Ta-Si systems was examined via X-ray diffraction. The line compounds NbSi2 and TaSi2 form as crystalline compounds rather than amorphous products, but Nb5Si3 and Ta5Si3, although chemically analogous, respond very differently to mechanical milling. The Ta5Si3 composition goes directly from elemental powders to an amorphous product, whereas Nb5Si3 forms as a crystalline compound. The Nb5Si3 compound consists of both the tetragonal room-temperature α phase (c/a = 1.8) and the tetragonal high-temperature β phase (c/a = 0.5). Substituting increasing amounts of Ta for Nb in Nb5Si3 initially stabilizes the α-Nb5Si3 structure preferentially, and subsequently inhibits the formation of a crystalline compound.

Temperature-induced structural phase transitions in RERhSn (RE = Y, Gd-Tm, Lu)

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Simon Engelbert ◽  
Rolf-Dieter Hoffmann ◽  
Jutta Kösters ◽  
Steffen Klenner ◽  
Rainer Pöttgen
Keyword(s):  
Rare Earth ◽  
Phase Transitions ◽  
Driving Force ◽  
Room Temperature ◽  
Structural Phase ◽  
Structural Phase Transitions ◽  
Line Broadening ◽  
X Ray Diffraction ◽  
Temperature Dependent ◽  
X Ray

Abstract The structures of the equiatomic stannides RERhSn with the smaller rare earth elements Y, Gd-Tm and Lu were reinvestigated on the basis of temperature-dependent single crystal X-ray diffraction data. GdRhSn crystallizes with the aristotype ZrNiAl at 293 and 90 K. For RE = Y, Tb, Ho and Er the HP-CeRuSn type (approximant with space group R3m) is already formed at room temperature, while DyRhSn adopts the HP-CeRuSn type below 280 K. TmRhSn and LuRhSn show incommensurate modulated variants with superspace groups P31m(1/3; 1/3; γ) 000 (No. 157.1.23.1) (γ = 3/8 for TmRhSn and γ = 2/5 for LuRhSn). The driving force for superstructure formation (modulation) is a strengthening of Rh–Sn bonding. The modulation is expressed in a 119Sn Mössbauer spectrum of DyRhSn at 78 K through line broadening.

scholarly journals Temperature-dependent structural change of 1D ice, water nanopipe, in crystal

2014 ◽  
Vol 70 (a1) ◽  
pp. C565-C565
Author(s):  
Akio Wakahara ◽  
Yasuko In
Keyword(s):  
Structural Change ◽  
Water Molecules ◽  
Fold Axis ◽  
X Ray Diffraction ◽  
Temperature Dependent ◽  
Crystal Axis ◽  
One Dimensional ◽  
X Ray ◽  
Ice Water ◽  
Cluster Unit

One-dimensional ice (1D ice) is formed in the single crystal prepared by mixing tryptophan and pyridoxal-5-phosphate in aqueous solution. This ice (diameter=1.649nm at 90K) consists of a hollow-type nanowire, viz., water nanopipe, and the structure is constructed by piling up the cluster unit of 15 water molecules, five independent waters (W1,--W5) of which are arranged around a 3-fold axis. As can be seen from the side view of this nanopipe, a tape structure is made of the continuous chair-type six-membered rings running parallel to the longest crystal axis, and its three tapes arranged around 3-fold axis are linked together via two kinds of hydrogen bonds of W5 (W5-W2 and W5-W4). Thus, the overall structure of this 1D ice could be described as three-square nanometric column. In order to investigate the temperature-dependent structural change of 1D ice, the crystal structure was refined using the X-ray diffraction data measured at different temperature between 90K and 293K. From these results, it was confirmed that the nanopipe structure is stable under 200K, but W5 and W4 disappear at 220K and 273K, respectively, indicating the importance of W5 for the structural stability of 1D ice.

A temperature-dependent structure study of gem-quality hibonite from Myanmar

2010 ◽  
Vol 74 (5) ◽  
pp. 871-885 ◽  
Author(s):  
M. Nagashima ◽  
T. Armbruster ◽  
T. Hainschwang
Keyword(s):  
Room Temperature ◽  
Structure Study ◽  
X Ray Diffraction ◽  
Temperature Dependent ◽  
X Ray ◽  
Thermal Expansion Coefficients ◽  
Trigonal Bipyramidal ◽  
Expansion Coefficients ◽  
Increasing Temperature ◽  
Length Distortion

AbstractThe structure of hibonite from Myanmar (space group P63/mmc, Z = 2, at room temperature a = 5.5909(1), c = 21.9893(4) Å), with simplified formula CaAl12O19 and composition (Ca0.99Na0.01)Σ1.00 was investigated between temperatures of 100 K and 923 K by single-crystal X-ray diffraction methods. Structure refinements have been performed at 100, 296, 473 and 923 K. In hibonite from Myanmar, Ti substitutes for Al mainly at the octahedral Al4 site and, to a lesser degree, at the trigonal bipyramidal site, Al2. The Al4 octahedra build face-sharing dimers. If Ti4+ substitutes at Al4, adjacent cations repulse each other for electrostatic reasons, leading to off-centre cation displacement associated with significant bond-length distortion compared to synthetic (Ti-free) CaAl12O19. Most Mg and smaller proportions of Zn and Si are assigned to the tetrahedral Al3 site. 12-coordinated Ca in hibonite replaces oxygen in a closest-packed layer. However, Ca is actually too small for this site and engages in a ‘rattling-type’ motion with increasing temperature. For this reason, Ca does not significantly increase thermal expansion coefficients of hibonite. The expansion of natural Ti,Mg-rich hibonite between 296 and 923 K along the x and the z axes is αa = 7.64×10–6 K–1 and αc = 11.19×10–6 K–1, respectively, and is thus very similar to isotypic, synthetic CaAl12O19 and LaMgAl11O19 (LMA).

scholarly journals Polar and non-polar structures of NH4TiOF3

2019 ◽  
Vol 52 (1) ◽  
pp. 23-26
Author(s):  
O. Boytsova ◽  
I. Dovgaliuk ◽  
D. Chernyshov ◽  
A. Eliseev ◽  
P. O'Brien ◽  
...  
Keyword(s):  
Room Temperature ◽  
Phase 1 ◽  
Structural Features ◽  
X Ray Diffraction ◽  
X Ray ◽  
Β Phase ◽  
Α Phase ◽  
Starting Compound ◽  
Different Temperatures ◽  
Fourfold Axis

Ammonium oxofluorotitanate, NH4TiOF3, is probably the best known precursor for the synthesis of anatase mesocrystals. Transformation of NH4TiOF3 into TiO2 through thermal decomposition, accompanied by hydrolysis, preserves some structural features of the precursor. Currently, any discussion of the mechanism of this transformation is difficult, as the exact crystal structure of the starting compound is not available and no intermediate structures are known. This article describes the outcome of single-crystal and powder X-ray diffraction studies, revealing the existence of two polymorphs of the parent NH4TiOF3 at different temperatures. A second-order phase transition from the polar Pca21 α phase (1), stable at room temperature, to the Pma2 β phase (2) above ∼433 K has been demonstrated. The direction of the pseudo-fourfold axis in NH4TiOF3 coincides with the orientation of the fourfold axis of anatase mesocrystals, consistent with a topotactical transformation.

scholarly journals Analysis of LiKSO4 crystals in the temperature range from 573 to 943 K

2000 ◽  
Vol 56 (4) ◽  
pp. 607-617 ◽  
Author(s):  
Carlos Basílio Pinheiro ◽  
Marcos Assunção Pimenta ◽  
Gervais Chapuis ◽  
Nivaldo Lúcio Speziali
Keyword(s):  
Single Crystal ◽  
Phase Ii ◽  
Room Temperature ◽  
Structure Model ◽  
Structural Studies ◽  
X Ray Diffraction ◽  
X Ray ◽  
Atomic Displacements ◽  
Disordered Models ◽  
Different Temperatures

The structural phases observed in LiKSO_4 crystals due to thermotropic transitions have been studied for more than a century. Nowadays many different phases are referenced, but some of the results are still controversial. Structural studies by single-crystal X-ray diffraction from room temperature to 803 K are presented here. Phase II (708 \lt T \lt 943 K) is extensively discussed on the basis of ordered and disordered models, using harmonic and anharmonic atomic displacements, and considering a twinned crystal composed of three orthorhombic domains. Analyses of the same phase at different temperatures determine the best structure model.

X-Ray Diffraction Study of the Electron Density and Anharmonicity in K2PtCl6

1993 ◽  
Vol 48 (1-2) ◽  
pp. 12-20 ◽  
Author(s):  
Renzo Restori ◽  
Dieter Schwarzenbach
Keyword(s):  
Charge Density ◽  
Room Temperature ◽  
Heavy Atom ◽  
X Ray Diffraction ◽  
Large Core ◽  
Electron Densities ◽  
Core Electron ◽  
Data Set ◽  
X Ray ◽  
Atomic Displacements

Abstract X-ray diffraction data in heavy-atom compounds may be sensitive to anharmonic atomic displacements, since the large core electron densities result in appreciable scattering amplitudes at large reciprocal distances. Since bonding electron densities may also exhibit sharp features affecting high-order reflections, they may be difficult to distinguish from anharmonic effects. We have analyzed an accurate room-temperature single-crystal X-ray data set of K2 PtCl6 using least-squares anharmonic displacement and charge density formalisms. The Hirshfeld charge density formalism, which has successfully been applied to many light-atom structures, fails to parametrize satisfactorily the data, whereas the electron densities at K and CI are easily accounted for by an anharmonic Gram-Charlier expansion to 4th order. Densities around Pt are parametrized only by a combination of anharmonicity and charge density formalisms. If economical parametrizations of the experimental data are preferred to more complicated ones, anharmonicity may be conjectured to play an important rôle while the main bonding feature consists of a preferential occupation of the 5d-orbitals of Pt with t2g symmetry.

Untersuchungen zur Polymorphie der Cäsium-Dodekahalogeno-closo-Dodekaborate Cs2[B12X12] (X = Cl–I)

2020 ◽  
Vol 75 (8) ◽  
pp. 777-790
Author(s):  
Ioannis Tiritiris ◽  
Kevin U. Bareiß ◽  
Thomas Schleid
Keyword(s):  
Phase Transitions ◽  
Defect Structure ◽  
Room Temperature ◽  
Solid Phase ◽  
X Ray Diffraction ◽  
Temperature Dependent ◽  
Space Group ◽  
X Ray ◽  
Cubic System ◽  
Cesium Salts

AbstractThermoanalytic DSC and temperature-dependent X-ray diffraction investigations on the cesium dodecahalogeno-closo-dodecaborates Cs2[B12X12] (X = Cl–I) have revealed solid-solid phase transitions from their trigonal room-temperature α-forms (e.g. α-Cs2[B12Cl12]: a = 959.67(3) pm, c = 4564.2(2) pm, Z = 6, space group R$\overline{3}$) into cubic high-temperature modifications. The isotypic title compounds crystallize in the space group Pm$\overline{3}$n (e.g. β-Cs2[B12Cl12]: a = 1051.98(6) pm, Z = 2) with a W3O-type defect structure. The statistic occupation of six possible positions with only four Cs+ cations results in a cation-deficient A2B arrangement for Cs2[B12X12]. Upon cooling the β-phase, a third polymorph was observed, which also crystallizes in the cubic system, but now in the space group Ia$\overline{3}$d (e.g. γ-Cs2[B12Cl12]: a = 2102.2(3) pm, Z = 16), and has to be regarded as a phase with only a partially disordered cation substructure. In this crystal structure the [B12X12]2− anions exhibit a NaTl-type arrangement, in which the Cs+ cations occupy suitable interstices. The phase transitions of the differently halogenated cesium salts follow no specific trend as the transition from the trigonal α- to the cubic β-form occurs at 178 °C for the chlorinated, at 270 °C for the iodinated and at 325 °C for the brominated examples. On further heating however, β-Cs2[B12I12] starts to decompose at 945 °C first, followed by β-Cs2[B12Br12] and β-Cs2[B12Cl12] at 959 °C and 983 °C, respectively.

Synthesis and Characterization of Nickel Manganite Ceramics by Polymeric Precursors Method

2016 ◽  
Vol 881 ◽  
pp. 123-127 ◽  
Author(s):  
A.C.B. de Oliveira ◽  
D.M.S. Ribeiro ◽  
C.G.P. Moraes ◽  
R.S. Silva ◽  
Nilson Santos Ferreira ◽  
...  
Keyword(s):  
Room Temperature ◽  
Sintering Temperature ◽  
Synthesis And Characterization ◽  
X Ray Diffraction ◽  
Temperature Dependent ◽  
Precursor Method ◽  
Spinel Type ◽  
X Ray ◽  
Lower Resistivity

This work presents the synthesis and characterization of NTC ceramic (Negative coefficient Temperature) based on nickel manganite (NiMn2O4) produced by the polymeric precursor method. NiMn2O4 were sintered at 900-1200 °C during 3h to produce the ceramics samples. The effect of sintering temperature on microstructure and electric properties of the NiMn2O4 ceramics was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and temperature dependent resistance R(T) measurements. The XRD measurement indicated formation of cubic spinel-type structure of NiMn2O4. The crystallite size (as confirmed by XRD) and the particle size (as confirmed by SEM) increased as the sintering temperature increased from around 18nm (900 °C) to 100nm (1200 °C). All samples showed NTC behavior and, among the studied ceramics, that one sintered at 1200 °C showed lower resistivity value (~103Ω.cm) at room temperature.

scholarly journals Studies of Optical, Dielectric, Ferroelectric, and Structural Phase Transitions in 0.9[KNbO3]-0.1 [BaNi1/2Nb1/2O3−δ]

Crystals ◽  
2021 ◽  
Vol 12 (1) ◽  
pp. 35
Author(s):  
Blanca Yamile Rosas ◽  
Alvaro A. Instan ◽  
Karuna Kara Mishra ◽  
S. Nagabhusan Achary ◽  
Ram S. Katiyar
Keyword(s):  
Phase Transitions ◽  
Room Temperature ◽  
Elevated Temperatures ◽  
Structural Phase ◽  
Cubic Phase ◽  
Structural Phase Transitions ◽  
X Ray Diffraction ◽  
Temperature Dependent ◽  
X Ray ◽  
Wide Range

The compound 0.9[KNbO3]-0.1[(BaNi1/2Nb1/2O3−δ] (KBNNO), a robust eco-friendly (lead-free) ferroelectric perovskite, has diverse applications in electronic and photonic devices. In this work, we report the dielectric, ferroelectric, and structural phase transitions behavior in the KBNNO compound using dielectric, X-ray diffraction, and Raman studies at ambient and as a function of temperature. Analyses of X-ray diffraction (XRD) data at room temperature (rtp) revealed the orthorhombic phase (sp. Gr. Amm2) of the compound with a minor secondary NiO cubic phase (sp. Gr. Fm3m). A direct optical band gap Eg of 1.66 eV was estimated at rtp from the UV–Vis reflectance spectrum analysis. Observation of non-saturated electric polarization loops were attributed to leakage current effects pertaining to oxygen vacancies in the compound. Magnetization studies showed ferromagnetism at room temperature (300 K) in this material. XRD studies on KBNNO at elevated temperatures revealed orthorhombic-to-tetragonal and tetragonal-to-cubic phase transitions at 523 and 713 K, respectively. Temperature-dependent dielectric response, being leaky, did not reveal any phase transition. Electrical conductivity data as a function of temperature obeyed Jonscher power law and satisfied the correlated barrier-hopping model, indicating dominance of the hopping conduction mechanism. Temperature-dependent Raman spectroscopic studies over a wide range of temperature (82–673 K) inferred the rhombohedral-to-orthorhombic and orthorhombic-to-tetragonal phase transitions at ~260, and 533 K, respectively. Several Raman bands were found to disappear, while a few Raman modes such as at 225, 270, 289, and 831 cm−1 exhibited discontinuity across the phase transitions at ~260 and 533 K.

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