Ferroelectric–paraelectric phase transitions with no group–supergroup relation between the space groups of both phases?

2001 ◽  
Vol 57 (4) ◽  
pp. 599-601 ◽  
Author(s):  
E. Kroumova ◽  
M. I. Aroyo ◽  
J. M. Pérez-Mato ◽  
R. Hundt
Keyword(s):  
Phase Transitions ◽  
Room Temperature ◽  
Paraelectric Phase ◽  
Polar Space ◽  
Space Group ◽  
Space Groups ◽  
Actual Space

The structures of Sr3(FeF6)2, β-NbO2, TlBO2 and CrOF3, previously reported as possible ferroelectrics with no group–supergroup relation between the ferroelectric and the paraelectric symmetries, have been carefully studied. We could not confirm any structural pseudosymmetry with respect to a space group which is not a supergroup of their room-temperature polar space group. In all cases, pseudosymmetry was indeed detected, but only for non-polar supergroups of the actual space groups of the structures. In this sense, the four compounds are possible ferroelectrics, but fulfilling the usual group–supergroup relation between the phase symmetries.

Phase transitions and twinned low-temperature structures of tetraethylammonium tetrachloridoferrate(III)

2014 ◽  
Vol 70 (5) ◽  
pp. 470-476 ◽  
Author(s):  
Martin Lutz ◽  
Yuxing Huang ◽  
Marc-Etienne Moret ◽  
Robertus J. M. Klein Gebbink
Keyword(s):  
Phase Transitions ◽  
Room Temperature ◽  
Polar Axis ◽  
Temperature Structure ◽  
Orthorhombic Space Group ◽  
Space Group ◽  
Space Groups ◽  
Disordered Structure ◽  
Compound C ◽  
Three Space

The title compound, [(C2H5)4N][FeCl4], has at room temperature a disordered structure in the high-hexagonal space group P63 mc. At 230 K, the structure is merohedrally twinned in the low-hexagonal space group P63. The volume has increased by a factor of 9 with respect to the room-temperature structure. At 170 and 110 K, the structure is identical in the orthorhombic space group Pca21 and twinned by reticular pseudomerohedry. The volume has doubled with respect to the room-temperature structure. All three space groups, viz. P63 mc, P63 and Pca21, are polar and the direction of the polar axis is not affected by the twinning. In the P63 and Pca21 structures, all cations and anions are well ordered.

Structures and phase transitions in barium sodium niobate tungsten bronze (BNN)

2021 ◽  
Vol 77 (6) ◽  
Author(s):  
Thomas A. Whittle ◽  
Christopher J. Howard ◽  
Siegbert Schmid
Keyword(s):  
Phase Transitions ◽  
Room Temperature ◽  
Tungsten Bronze ◽  
Standard Setting ◽  
Experimental Results ◽  
Temperature Structure ◽  
Sodium Niobate ◽  
Tetragonal Tungsten Bronze ◽  
Space Group ◽  
The Subject

The room-temperature structure of the filled tetragonal tungsten bronze, Ba2NaNb5O15 (BNN), has been the subject of a number of studies, and these studies have given an almost corresponding number of different results. From a group theoretical examination of the different possibilities and a review of the published experimental results we conclude that the room-temperature structure is that proposed by Labbé et al. [J. Phys. Condens. Matter (1989), 2, 25–43] in the space group Bbm2 (Ama2 in standard setting) on a 2\sqrt{2}a × \sqrt{2}a × 2c cell. Upon heating, the structure remains ferroelectric but becomes tetragonal (space group P4bm) at 550 K, then paraelectric (space group P4/mbm) at and above 860 K.

The structures and phase transitions in 4-aminopyridinium tetraaquabis(sulfato)iron(III), (C5H7N2)[FeIII(H2O)4(SO4)2]

2019 ◽  
Vol 75 (6) ◽  
pp. 1144-1151
Author(s):  
Tamara J. Bednarchuk ◽  
Wolfgang Hornfeck ◽  
Vasyl Kinzhybalo ◽  
Zhengyang Zhou ◽  
Michal Dušek ◽  
...  
Keyword(s):  
Phase Transitions ◽  
Single Crystal ◽  
Room Temperature ◽  
Variable Temperature ◽  
Temperature Phase ◽  
X Ray Diffraction ◽  
Scanning Calorimetry ◽  
Hybrid Compound ◽  
Space Group ◽  
X Ray

The organic–inorganic hybrid compound 4-aminopyridinium tetraaquabis(sulfato)iron(III), (C5H7N2)[FeIII(H2O)4(SO4)2] (4apFeS), was obtained by slow evaporation of the solvent at room temperature and characterized by single-crystal X-ray diffraction in the temperature range from 290 to 80 K. Differential scanning calorimetry revealed that the title compound undergoes a sequence of three reversible phase transitions, which has been verified by variable-temperature X-ray diffraction analysis during cooling–heating cycles over the temperature ranges 290–100–290 K. In the room-temperature phase (I), space group C2/c, oxygen atoms from the closest Fe-atom environment (octahedral) were disordered over two equivalent positions around a twofold axis. Two intermediate phases (II), (III) were solved and refined as incommensurately modulated structures, employing the superspace formalism applied to single-crystal X-ray diffraction data. Both structures can be described in the (3+1)-dimensional monoclinic X2/c(α,0,γ)0s superspace group (where X is ½, ½, 0, ½) with modulation wavevectors q = (0.2943, 0, 0.5640) and q = (0.3366, 0, 0.5544) for phases (II) and (III), respectively. The completely ordered low-temperature phase (IV) was refined with the twinning model in the triclinic P{\overline 1} space group, revealing the existence of two domains. The dynamics of the disordered anionic substructure in the 4apFeS crystal seems to play an essential role in the phase transition mechanisms. The discrete organic moieties were found to be fully ordered even at room temperature.

A structural study of the (Na1−xKx)0.5Bi0.5TiO3 perovskite series as a function of substitution (x) and temperature

2002 ◽  
Vol 17 (4) ◽  
pp. 301-319 ◽  
Author(s):  
G. O. Jones ◽  
J. Kreisel ◽  
P. A. Thomas
Keyword(s):  
Phase Transitions ◽  
Phase Space ◽  
Room Temperature ◽  
Profile Analysis ◽  
Rhombohedral Phase ◽  
Polar Axis ◽  
Detailed Characterization ◽  
Space Group ◽  
Structural Modifications

Rietveld neutron powder profile analysis of the (Na1−xKx)0.5Bi0.5TiO3 (NKBT) series (x=0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) is reported over the temperature range 293–993 K. A detailed characterization of the structures and phase transitions occurring across this series as a function of temperature has been made. Room-temperature refinements have revealed a rhombohedral phase, space group R3c for x=0, 0.2, and 0.4, which exhibits an antiphase, a−a−a− oxygen tilt system with parallel cation displacements along [111]p. An intermediate zero-tilt rhombohedral phase, space group R3m possessing cation displacements along [111]p, has been established for x=0.5 and 0.6. At the potassium-rich end of the series at x=0.8 and 1.0, a tetragonal phase, space group P4mm is observed possessing cation displacements along [001]. At the sodium-rich end of the series for x=0.2, the unusual tetragonal structure with space group P4bm is seen for Na0.5Bi0.5TiO3 which possesses a combination of in-phase a0a0c+ tilts and antiparallel cation displacements along the polar axis. Temperature-induced phase transitions are reported and structural modifications are discussed.

scholarly journals From patterns to space groups and the eigensymmetry of crystallographic orbits: a reinterpretation of some symmetry diagrams in IUCr Teaching Pamphlet No. 14

2012 ◽  
Vol 45 (4) ◽  
pp. 834-837
Author(s):  
Leopoldo Suescun ◽  
Massimo Nespolo
Keyword(s):  
Alternative Interpretation ◽  
Polar Space ◽  
Space Group ◽  
Space Groups ◽  
General Orbit

The space group of a crystal pattern is the intersection group of the eigensymmetries of the crystallographic orbits corresponding to the occupied Wyckoff positions. Polar space groups without symmetry elements with glide or screw components smaller than 1/2 do not contain characteristic orbits and cannot be realized in patterns (structures) made by only one crystallographic type of object (atom). The space-group diagram of the general orbit for this type of group has an eigensymmetry that corresponds to a special orbit in a centrosymmetric supergroup of the generating group. This fact is often overlooked, as shown in the proposed solution for Plates (i)–(vi) of IUCr Teaching Pamphlet No. 14, and an alternative interpretation is given.

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 Structural Systematics of Nitrogen Base Adducts of Group 2 Salts. IX. Some Complexes of Group 2 Metal Halides With 2,2':6',2"-Terpyridine

1996 ◽  
Vol 49 (1) ◽  
pp. 137 ◽  
Author(s):  
BW Skelton ◽  
AF Waters ◽  
AH White
Keyword(s):  
Single Crystal ◽  
Room Temperature ◽  
Nitrogen Base ◽  
Space Group ◽  
Trigonal Symmetry ◽  
X Ray ◽  
Space Groups ◽  
Structure Determinations ◽  
Group 2 ◽  
A Site

Syntheses and room-temperature single-crystal X-ray structure determinations are recorded for a number of adducts of MX2.ntpy stoichiometry of salts of Group 2 metals, MX2, with 2,2':6',2"-terpyridine ( tpy ). ' Homoleptic' adducts, [M( tpy )3] X2, are found in space groups with trigonal symmetry, with M located on a site of at least 3 symmetry, the structures being derivative of the P 62c array found in [ Pb ( tpy )3] (ClO4)2, but with variations in anion and cation stackings . [M( tpy )3] I2.~1.7MeOH, M = Ca (1), Sr (2), are trigonal , P3c1, a ≈ 13, c ≈ 15.3 Ǻ, Z = 2 f.u., conventional R on ׀ F ׀ being 0.050, 0.086 for No 894, 865 independent 'observed' (I > 3σ(I)) reflections respectively. [ Sr ( tpy )3] Br2.2MeOH (3), although similar, is modelled in space group P3c1, a 13.040(4), c 15.13(1) Ǻ, Z = 2 f.u ., R 0.041 for No 447. [( tpy )2BaI2] (4) is monoclinic, P 21/c, a 10.812(4), b 16.740(5), c 17.458(4) Ǻ, β 109.39(2)°, Z = 4 f.u ., R 0.042 for No 3256. [( tpy )2Ba(O2ClO2)2] (5) is triclinic, Pī , a 14.220(4), b 11.212(2), c 10.511(2) Ǻ, α 65.66(2), β 87.32(2), γ 88.92(2)°, Z = 2 f.u ., R 0.029 for No 4676.

Matching selenium-atom peak positions with a different hand or origin

2002 ◽  
Vol 35 (3) ◽  
pp. 368-370 ◽  
Author(s):  
G. David Smith
Keyword(s):  
Computer Program ◽  
Iterative Procedure ◽  
Polar Axis ◽  
Polar Space ◽  
Selenium Atom ◽  
Space Group ◽  
Space Groups ◽  
Fortran 90 ◽  
Symmetry Operations

An algorithm is described for matching and correlating two or more sets of peaks or atoms. The procedure is particularly useful for matching putative selenium atoms from a selenium-atom substructure as obtained fromEmaps from two or more random-atom trials. The algorithm will work for any space group exceptP1. For non-polar space groups, the procedure is relatively straightforward. For polar space groups, the calculation is performed in projection along the polar axis in order to identify potential matching peaks, and an iterative procedure is used to eliminate incorrect peaks and to calculate the displacement along the polar axis. The algorithm has been incorporated into a computer program,NANTMRF, written in Fortran 90. Less than 0.5 s are required to match 27 peaks in space groupP21, and the output lists the correct origin, enantiomorph, symmetry operations, and provides the relative displacements between pairs of matching peaks.

TEM of Phase Transitions in Tridymite and Cristobalite Based Materials.

2000 ◽  
Vol 6 (S2) ◽  
pp. 358-359
Author(s):  
Gustaaf Van Tendeloo
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
High Temperature ◽  
Solid State ◽  
Wide Temperature Range ◽  
Room Temperature ◽  
Paraelectric Phase ◽  
High Temperature Phase ◽  
Temperature Phase ◽  
Extended Defects

We have determined the structure of the paraelectric phase of BaAl2O4, which is a stuffed tridymite, by different TEM techniques and we will describe the phase transition between the ferroelectric room temperature phase and the paraelectric high temperature phase. We have also obtained HREM images of the higly radiation sensitive acristobalite phase of (Si0,9 Ge0,1)O2 and analysed the extended defects in this material.The stuffed tridymite BaAl2O4 is ferroelectric at room temperature and undergoes a paraelectric-ferroelectric (PEFE) phase transition. The transition is reversible, takes place over a wide temperature range (400K-670K) and has a dynamical character. BaAl2O4 is easily obtained by solid state reaction of BaCO3, and A12O4,. The stoichiometric amounts of the initial reagents were mixed, grinded in an agate mortar under acetone and pressed into a pellet. The pellet was annealed in alumina crucibles at 1000 °C and 1300 °C for 40 h in air and furnace cooled.

Solving crystal structures inP1: an automated procedure for finding an allowed origin in the correct space group

2000 ◽  
Vol 33 (2) ◽  
pp. 307-311 ◽  
Author(s):  
Maria Cristina Burla ◽  
Benedetta Carrozzini ◽  
Giovanni Luca Cascarano ◽  
Carmelo Giacovazzo ◽  
Giampiero Polidori
Keyword(s):  
Crystal Structure ◽  
Crystal Structures ◽  
Structural Model ◽  
Structure Refinement ◽  
Human Intervention ◽  
Space Group ◽  
Space Groups ◽  
Structure Solution ◽  
Actual Space ◽  
Complex Crystal

Crystal structure solution inP1 may be particularly suitable for complex crystal structures crystallizing in other space groups. However, additional efforts and human intervention are often necessary to locate correctly the structural model so obtained with respect to an allowed origin of the actual space group. An automatic procedure is described which is able to perform such a task, allowing the routine passage to the correct space group and the subsequent structure refinement. Some tests are presented proving the effectiveness of the procedure.

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