scholarly journals Zur Polymorphie von TbCI3/ Polymorphism of TbCl3

1988 ◽  
Vol 43 (8) ◽  
pp. 1023-1028 ◽  
Author(s):  
Harald Gunsilius ◽  
Horst Borrmann ◽  
Arndt Simon ◽  
Werner Urland
Keyword(s):  
High Temperature ◽  
Metastable State ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
Space Group

Abstract3 different modifications of TbCl3 were synthesized. TbCl, (UCl3-type), probably in a metastable state, crystallizes in space group P63/m with a = 737.63(2) pm, c = 405.71(2) pm and Z - 2. TbCl3 (PuB3-type) crystallizes in space group Cmcm with a = 384.71(6) pm, b = 1177.37(7) pm. c = 851.77(4) pm and Z = 4. h-TbCl3, the high temperature phase being stable above 790 K. crystallizes in space group P42/mnm with a = 642.51(4) pm, c = 1177.14(18) pm and Z = 4.

Mode-crystallography analysis of the crystal structures and the low- and high-temperature phase transitions in Na0.5K0.5NbO3

2015 ◽  
Vol 48 (2) ◽  
pp. 318-333 ◽  
Author(s):  
B. Orayech ◽  
A. Faik ◽  
G. A. López ◽  
O. Fabelo ◽  
J. M. Igartua
Keyword(s):  
Phase Transitions ◽  
High Temperature ◽  
Crystal Structures ◽  
Degrees Of Freedom ◽  
High Temperature Phase ◽  
Structure Evolution ◽  
Internal Space ◽  
Temperature Phase ◽  
Conventional Solid State Reaction ◽  
Space Group

Na0.5K0.5NbO3has been synthesized by the conventional solid-state reaction process. The crystal structures and phase transitions, at low and high temperature, determined from the Rietveld refinements of X-ray and neutron powder diffraction data are reported. The structure evolution of Na0.5K0.5NbO3in the temperature range from 2 to 875 K shows the presence of three phase transitions. The first one, at ∼135 K, is discontinuous from the rhombohedralR3c(No. 161) space group to the room-temperature orthorhombicAmm2 (No. 38) space group; the second is discontinuous from the orthorhombic to the tetragonalP4mmspace group (No. 99) at ∼465 K, and the third is continuous from the tetragonal to the cubic Pm\overline{3}m space group (No. 221) at ∼700 K. The obtained phase-transition sequence isR3c→Amm2 →P4mm→Pm\overline{3}m. No previous studies at low temperature have been carried out on the material with composition Na0.5K0.5NbO3. In the course of the determination of the three experimentally found phases, a novel method of refinement is presented. This is a step forward in the use of the symmetry-adapted modes as degrees of freedom in the refinement process: the parameterization of a direction in the internal space of the, in this case, sole irreducible representation, GM4−, responsible for the symmetry breaking from the parent cubic space group to the polar distorted low-symmetry phases. Eventually, this procedure enables the calculation of the spontaneous polarization.

Irreversible single-crystal to polycrystal and reversible single-crystal to single-crystal phase transformations in cyanurates

2001 ◽  
Vol 57 (6) ◽  
pp. 791-799 ◽  
Author(s):  
Menahem Kaftory ◽  
Mark Botoshansky ◽  
Moshe Kapon ◽  
Vitaly Shteiman
Keyword(s):  
Phase Transformation ◽  
High Temperature ◽  
Low Temperature ◽  
Single Crystal ◽  
High Temperature Phase ◽  
Monoclinic Space Group ◽  
Temperature Phase ◽  
Space Group ◽  
Reversible Phase ◽  
Low Temperature Phase

4,6-Dimethoxy-3-methyldihydrotriazine-2-one (1) undergoes a single-crystal to single-crystal reversible phase transformation at 319 K. The low-temperature phase crystallizes in monoclinic space group P21/n with two crystallographically independent molecules in the asymmetric unit. The high-temperature phase is obtained by heating a single crystal of the low-temperature phase. This phase is orthorhombic, space group Pnma, with the molecules occupying a crystallographic mirror plane. The enthalpy of the transformation is 1.34 kJ mol−1. The small energy difference between the two phases and the minimal atomic movement facilitate the single-crystal to single-crystal reversible phase transformation with no destruction of the crystal lattice. On further heating, the high-temperature phase undergoes methyl rearrangement in the solid state. 2,4,6-Trimethoxy-1,3,5-triazine (3), on the other hand, undergoes an irreversible phase transformation from single-crystal to polycrystalline material at 340 K with an enthalpy of 3.9 kJ mol−1; upon further heating it melts and methyl rearrangement takes place.

scholarly journals On the high temperature phase transition in Ba(Zr0.20Ti0.80)O3 ceramic

2017 ◽  
Vol 07 (04) ◽  
pp. 1750025 ◽  
Author(s):  
K. P. Chandra ◽  
A. R. Kulkarni ◽  
K. Prasad
Keyword(s):  
Phase Transition ◽  
High Temperature ◽  
Lattice Structure ◽  
High Temperature Phase ◽  
Monoclinic Space Group ◽  
Temperature Phase ◽  
Potential Candidate ◽  
Space Group ◽  
Structure Changes ◽  
Temperature Phase Transition

Temperature dependent X-ray diffraction (XRD) and dielectric properties of perovskite Ba(Zr[Formula: see text]Ti[Formula: see text]O3 ceramic prepared using a standard solid-state reaction process is presented. Along with phase transitions at low temperature, a new phase transition at high temperature (873[Formula: see text]C at 20[Formula: see text]Hz), diffusive in character has been found where the lattice structure changes from monoclinic (space group: [Formula: see text] to hexagonal (space group: [Formula: see text]). This result places present ceramic in the list of potential candidate for intended high temperature applications. The AC conductivity data followed hopping type charge conduction and supports jump relaxation model. The experimental value of [Formula: see text][Formula: see text]pC/N was found. The dependence of polarization and strain on electric field at room temperature suggested that lead-free Ba(Zr[Formula: see text]Ti[Formula: see text]O3 is a promising material for electrostrictive applications.

35Cl NQR and Structural Studies of Chloroacetanilides C6H3Cl2NHCOCH3-xClx, 1 ≤ x ≤ 3

1992 ◽  
Vol 47 (1-2) ◽  
pp. 160-170
Author(s):  
Dirk Groke ◽  
Shi-Qi Dou ◽  
Alarich Weiss
Keyword(s):  
High Temperature ◽  
Low Temperature ◽  
Temperature Dependence ◽  
Phenyl Group ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
X Ray Diffraction ◽  
Chlorine Atoms ◽  
Space Group ◽  
Low Temperature Phase

AbstractThe temperature dependence of 35Cl NQR frequencies and the phase transition behaviour of chloroacetanilides (N-[2,6-dichlorophenyl]-2-chloroacetamide, -2,2-dichloroacetamide, -2,2,2-trichloroacetamide) were investigated. The crystal structure determination of N-[2,6-dichlorophenyl]- 2-chloroacetamide leads to the following: a = 1893.8 pm, b = 1110.7 pm, c = 472.1 pm, space group P212121 = D24 with Z = 4 molecules per unit cell. The arrangement of the molecules and their geometry is comparable to the high temperature phase of the acetyl compound N-[2,6-dichlorophenyl]- acetamide. For N-[2,6-diclorophenyl]-2,2,2-trichloroacetamide it was found: a = 1016.6 pm, b = 1194.3 pm, c = 1006.7 pm, ß= 101.79°, space group P21/c = C52h, Z = 4. The structure is similar to the low temperature phase of N-[2,6-dichlorophenyl]-acetamide. Parallelism between the temperature dependence of the 35C1 NQR lines of the CCl3 group and the X-ray diffraction results concerning the different behaviour of the chlorine atoms was observed. The structures of the compounds show intermolecular hydrogen bonding of the N - H • • • O - C type. The phenyl group and the HNCO function are nearly planar. A bleaching out of several 35Cl NQR lines at a temperature far below the melting point of the substances was observed. The different types of chlorine atoms (aromatic, chloromethyl) can be distinguished by their temperature coefficients of the 35Cl NQR frequencies. All the resonances found show normal "Bayer" temperature behaviour. N-[2,6-dichlorophenyl]-2,2-diehloroacetamide shows several solid phases. One stable low temperature phase and an instable high temperature phase (at room temperature) were observed. The different phases were detected by means of 35Cl NQR spectroscopy and thermal analysis

Low- and high-temperature structures of neopentylglycol plastic crystal

1993 ◽  
Vol 8 (2) ◽  
pp. 109-117 ◽  
Author(s):  
Dhanesh Chandra ◽  
Cynthia S. Day ◽  
Charles S. Barrett
Keyword(s):  
High Temperature ◽  
Phase Structure ◽  
Binary Systems ◽  
Structural Parameters ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
Space Group ◽  
Plastic Crystals ◽  
Cell Dimensions ◽  
Unit Cell Dimensions

Plastic crystals, such as neopentylglycol, 2, 2-dimethyl-1,3-propanediol, that exhibit polymorphic behavior are emerging materials for thermal energy storage. The energy is stored isothermally in the γ phase, FCC, during solid-state phase transformations. This γ phase of NPG has been determined as an orientational disordered phase. The low temperature α phase structure, which is of great significance in the evaluation of lattice expansions and other parameters, was first determined in 1961. However, the reported unit cell dimensions and the intensities of the reflections led to erroneous indexing of the powder patterns in binary systems. The α phase structure is redetermined here as monoclinic, M= 104.15 amu, space group P21/n (an alternate setting of , space group No. 14), a = 5.979(1)Å, b= 10.876(2)Å, c=10.099(2)Å, β=99.78(1)°, V=647.2(2)Å3 at 20°(± 1)C, Dx= 1.069 g cm s−3 for Z=4. In this paper the redetermined structure of the α phase of NPG is presented in projections of the atomic positions, in tables, and in calculated powder pattern and these results are compared with those reported by others. The powder patterns obtained from the Bragg–Brentano diffractometer are compared with our calculated pattern from the single crystal data. The structural parameters of the high temperature phase of NPG as determined by a Guinier diffraction system are also reported.

Polymorphism of Li2Zn3

2012 ◽  
Vol 68 (1) ◽  
pp. 34-39 ◽  
Author(s):  
Volodymyr Pavlyuk ◽  
Ihor Chumak ◽  
Helmut Ehrenberg
Keyword(s):  
High Temperature ◽  
Low Temperature ◽  
Tight Binding ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
X Ray Diffraction ◽  
Space Group ◽  
Temperature Modification ◽  
Rhombic Dodecahedra ◽  
Low Temperature Phase

Crystal structures of low- and high-temperature modifications of the binary phase Li2Zn3 were determined by single-crystal X-ray diffraction techniques. The low-temperature modification is a disordered variant of Li5Sn2, space group R\bar 3m (No. 166). The high-temperature modification crystallizes as an anti-type to Li5Ga4, space group P\bar 3m1 (No. 164). Two polymorphs can be described as derivative structures to binary Li5Ga4, Li5Sn2, Li13Sn5, Li8Pb3, CeCd2 and CdI2 phases which belong to class 2 with the parent W-type in Krypyakevich's classification. All atoms in both polymorphs are coordinated by rhombic dodecahedra (coordination number CN = 14) like atoms in related structures. The Li2Zn2.76 (for the low-temperature phase) and Li2Zn2.82 (for the high-temperature phase) compositions were obtained after structure refinements. According to electronic structure calculations using the tight-binding–linear muffin-tin orbital–atomic spheres approximations (TB–LMTO–ASA) method, strong covalent Sn—Sn and Ga—Ga interactions were established in Li5Sn2 and Li5Ga4, but no similar Zn—Zn interactions were observed in Li2Zn3.

Ab initio structure determination of the high-temperature phase of anhydrous caffeine by X-ray powder diffraction

2005 ◽  
Vol 61 (3) ◽  
pp. 329-334 ◽  
Author(s):  
Patrick Derollez ◽  
Natália T. Correia ◽  
Florence Danède ◽  
Frédéric Capet ◽  
Frédéric Affouard ◽  
...  
Keyword(s):  
High Temperature ◽  
Phase I ◽  
Metastable State ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
X Ray Diffraction ◽  
Rietveld Refinements ◽  
X Ray ◽  
Ab Initio Structure

The high-temperature phase I of anhydrous caffeine was obtained by heating and annealing the purified commercial form II at 450 K. This phase I can be maintained at low temperature in a metastable state. A powder X-ray diffraction pattern was recorded at 278 K with a laboratory diffractometer equipped with an INEL curved position-sensitive detector CPS120. Phase I is dynamically orientationally disordered (the so-called plastic phase). The Rietveld refinements were achieved with rigid-body constraints. It was assumed that on each site, a molecule can adopt three preferential orientations with equal occupation probability. Under a deep undercooling of phase I, below 250 K, the metastable state enters in a glassy crystal state.

Monoclinic-to-orthorhombic phase transition of the hexamethylenetetramine–2-methylbenzoic acid (1/2) cocrystal with temperature-dependent dynamic molecular disorder

2016 ◽  
Vol 72 (12) ◽  
pp. 971-980 ◽  
Author(s):  
Tze Shyang Chia ◽  
Ching Kheng Quah
Keyword(s):  
Phase Transition ◽  
High Temperature ◽  
Low Temperature ◽  
Structural Phase ◽  
Point Group ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
Temperature Dependent ◽  
Space Group ◽  
Low Temperature Phase

As a function of temperature, the hexamethylenetetramine–2-methylbenzoic acid (1/2) cocrystal, C6H12N4·2C8H8O2, undergoes a reversible structural phase transition. The orthorhombic high-temperature phase in the space groupPccnhas been studied in the temperature range between 165 and 300 K. At 164 K, at2phase transition to the monoclinic subgroupP21/cspace group occurs; the resulting twinned low-temperature phase was investigated in the temperature range between 164 and 100 K. The domains in the pseudomerohedral twin are related by a twofold rotation corresponding to the matrix (100/0-10/00-1. Systematic absence violations represent a sensitive criterium for the decision about the correct space-group assignment at each temperature. The fractional volume contributions of the minor twin domain in the low-temperature phase increases in the order 0.259 (2) → 0.318 (2) → 0.336 (2) → 0.341 (3) as the temperature increases in the order 150 → 160 → 163 → 164 K. The transformation occurs between the nonpolar point groupmmmand the nonpolar point group 2/m, and corresponds to a ferroelastic transition or to at2structural phase transition. The asymmetric unit of the low-temperature phase consists of two hexamethylenetetramine molecules and four molecules of 2-methylbenzoic acid; it is smaller by a factor of 2 in the high-temperature phase and contains two half molecules of hexamethylenetetramine, which sit across twofold axes, and two molecules of the organic acid. In both phases, the hexamethylenetetramine residue and two benzoic acid molecules form a three-molecule aggregate; the low-temperature phase contains two of these aggregates in general positions, whereas they are situated on a crystallographic twofold axis in the high-temperature phase. In both phases, one of these three-molecule aggregates is disordered. For this disordered unit, the ratio between the major and minor conformer increases upon cooling from 0.567 (7):0.433 (7) at 170 Kvia0.674 (6):0.326 (6) and 0.808 (5):0.192 (5) at 160 K to 0.803 (6):0.197 (6) and 0.900 (4):0.100 (4) at 150 K, indicating temperature-dependent dynamic molecular disorder. Even upon further cooling to 100 K, the disorder is retained in principle, albeit with very low site occupancies for the minor conformer.

Anisotropic thermal expansion in yttrium silicate

2003 ◽  
Vol 18 (7) ◽  
pp. 1715-1722 ◽  
Author(s):  
Koichiro Fukuda ◽  
Hiroyuki Matsubara
Keyword(s):  
Thermal Expansion ◽  
High Temperature ◽  
Phase Space ◽  
Linear Thermal Expansion ◽  
High Temperature Phase ◽  
Acute Angle ◽  
Temperature Phase ◽  
Space Group ◽  
Anisotropic Thermal Expansion ◽  
Increasing Temperature

In this study, crystals of Y2SiO5 were examined by high-temperature powder x-ray diffractometry to determine the changes in unit-cell dimensions with temperatures up to 1273 K for the X1 phase (the low-temperature phase, space group P121/c1) and 1473 K for the X2 phase (the high-temperature phase, space group I12/a1). The lattice deformations of both phases induced by thermal expansion were investigated by matrix algebra analysis to determine the directions and magnitudes of the principal distortions (λi, i = 1, 2, and 3). For the X1 phase, λ1 and λ2 invariably showed a positive thermal expansion. On the other hand, λ3 showed a negative thermal expansion below 1173 K; the maximum contraction of 0.10(4)% occurred at 685 K. The λ2 axis invariably coincides with the crystallographic b axis. The directions of λ1 and λ3, defined by the acute angle λ3 ^ c changed between 53(3)° (T = 394 K) and 45(1)° (T = 788 K). For the X2 phase, all of the principal distortions steadily increased with increasing temperature. The angle λ3 ^ c steadily decreased from 71(2)° to 62.1(1)° with increasing temperature. The mean linear thermal expansion coefficients were, when compared at the same temperatures, necessarily higher for the X1 phase than for the X2 phase. The lattice change of X1–RE2SiO5 (RE = Y and Yb–La), which was induced by the substitution of rare-earth (RE) ions, showed a striking resemblance with the lattice deformation of X1-Y2SiO5, which was caused by the thermal expansion. Because the lattice change of the former must be caused by the isotropic expansion of the RE sites, the anisotropic thermal expansion of the latter would be essentially attributable to the isotropic thermal expansion of the YO9 and YO7 polyhedron.

An X-Ray, Raman and IR Study of α-CsReO4, the High-Temperature Modification of Cesium Perrhenate

1992 ◽  
Vol 47 (11) ◽  
pp. 1513-1520 ◽  
Author(s):  
Klaus-Jürgen Range ◽  
Peter Rögner ◽  
Heyns Anton M. ◽  
Prinsloo Linda C.
Keyword(s):  
High Temperature ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
X Ray Diffraction ◽  
Data Set ◽  
Space Group ◽  
X Ray ◽  
Reversible Phase Transition ◽  
Ir Study ◽  
Reversible Phase

Cesium perrhenate undergoes a reversible phase transition from the orthorhombic room-temperature phase (β-CsReO4, space group Pnma) to a tetragonal high-temperature phase (α-CsReO4). The space group of α-CsReO4 was unambiguously confirmed by high-temperature X-ray diffraction to be I41/amd in contrast to the earlier literature, in which space group I41/a (and hence, a scheelite type structure) was assumed for α-CsReO4. From a data set obtained at 468 K the crystal structure of α-CsReO4 (a = 5.9607(4), c = 14.446(1)Å, Z = 4) was refined to R = 0.032, Rww = 0.029 using anisotropic displacement factors.In the vibrational spectra not all of the predicted com ponents for β-CsReO4 could be confirmed by the experiments, but for α-CsReO4 very good agreement was obtained between the predicted and observed bands. The fact that v2 at ~ 300 cm–1 is not observed in the IR spectra of α-CsReO4 confirms the space group I41/amd, since, under I41/a symmetry, this band is allowed.The structure of α-CsReO4 consists of isolated ReO4 tetrahedra which are linked together by cesium atoms. The relationships between the scheelite, α-CsReO4 and β-CsReO4 structural types are discussed.

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