Phase transitions and ferroelectricity in NaSb3F10

2008 ◽  
Vol 42 (1) ◽  
pp. 58-62 ◽  
Author(s):  
R. J. Christie ◽  
P. K. Wu ◽  
P. Photinos ◽  
S. C. Abrahams
Keyword(s):  
Phase Transitions ◽  
Phase I ◽  
Thermal Hysteresis ◽  
Entropy Change ◽  
Pyroelectric Coefficient ◽  
Phase Iii ◽  
Space Group ◽  
Heating And Cooling ◽  
Order Of Magnitude ◽  
First Order Phase

Atomic coordinate analysis allows materials with appropriate but previously unrecognized dielectric properties to be predicted as new ferroelectrics if their crystal structure is known. An earlier such prediction that NaSb3F10is ferroelectric is confirmed herein without ambiguity. Its spontaneous polarizationPsis found to exhibit reproducible dielectric hysteresis at room temperature, withPs≃ 60 µC m−2, under the application of a field of 0.3 MV m−1or greater. The pyroelectric coefficient 〈p〉 = 17 (5) µC m−2 K−1at 298 K. NaSb3F10undergoes a phase transition atTC≃ 461 K, on correction for thermal hysteresis, with entropy change ΔS= 5.7 (3) J mol−1 K−1. The colorless crystals melt atTm ≃ 515 K and decompose above ∼600 K. The thermal hysteresis of ∼35 K inTC, on heating and cooling at 5–25 K min−1, is typical of first-order phase transitions. The space group in ferroelectric phase III isP63, and that in phase II is predicted to beP6322, a nonpolar supergroup ofP63; the supergroup expected in the prototypic nonferroic phase I isP63/mmc. The space group of phase III isnota direct subgroup of phase I. The dielectric permittivity ∊′ at 1 kHz increases over an order of magnitude between 300 K and a major inflection atTC, continuing to increase steadily thereafter toTm.

Structure–property correlation over five phases and four transitions in Pb5Al3F19

2003 ◽  
Vol 59 (5) ◽  
pp. 557-574 ◽  
Author(s):  
S. C. Abrahams ◽  
J. Ravez ◽  
H. Ritter ◽  
J. Ihringer
Keyword(s):  
Phase I ◽  
Phase Ii ◽  
Thermal Hysteresis ◽  
Ambient Pressure ◽  
Entropy Change ◽  
Phase Iii ◽  
Mode Analysis ◽  
First Order ◽  
Five Phases ◽  
Phase Iv

The calorimetric and dielectric properties of Pb5Al3F19 in the five phases stable under ambient pressure are correlated with structure for fuller characterization of each phase. The first-order transition between ferroelectric phase V and antiferroelectric phase IV at T V,IV = 260 (5) K exhibits a thermal hysteresis of 135 (5) K on heating, with a maximum atomic displacement Δ(xyz)max = 1.21 (6) Å; the transition from phase IV to ferroelastic phase III at 315 (5) K is also first order but with a thermal hysteresis of 10 (5) K and Δ(xyz)max = 0.92 (7)  Å; that from phase III to paraelastic phase II at 360 (5) K is second order without hysteresis and has Δ(xyz)max = 0.69 (4) Å; and the transition from phase II to paraelectric phase I at 670 (5) K is second or higher order, with Δ(xyz)max = 0.7 (4) Å. The measured entropy change ΔS at T V,IV agrees well with ΔS as derived from the increased configurational energy by Stirling's approximation. For all other phase transitions, 0.5 ≥ ΔS > 0 J mol−1 K−1 is consistent with an entropy change caused primarily by the changes in the vibrational energy. The structure of phase III is determined both by group theoretical/normal mode analysis and by consideration of the structures of phases II, IV and V reported previously; refinement is by simultaneous Rietveld analysis of the X-ray and neutron diffraction powder profiles. The structure of prototypic phase I is predicted on the basis of the atomic arrangement in phases II, III, IV and V. The introduction of 3d electrons into the Pb5Al3F19 lattice disturbs the structural equilibrium, the addition of 0.04% Cr3+ causing significant changes in atomic positions and increasing T IV,III by ∼15 K. Substitution of Al3+ by 20% or more Cr3+ eliminates the potential minima that otherwise stabilize phases IV, III and II.

scholarly journals Etude par resonance magnetique des mouvements moléculaires dans l'acrylonitrile solide

1973 ◽  
Vol 51 (23) ◽  
pp. 3889-3900 ◽  
Author(s):  
Buu Ban ◽  
C. CHACHATY
Keyword(s):  
Phase Transitions ◽  
Phase I ◽  
Phase Ii ◽  
Phase Iii ◽  
Solid Matrix ◽  
Peroxy Radicals ◽  
Γ Irradiation ◽  
Rotational Oscillation ◽  
Résonance Magnétique ◽  
Oxygen Addition

Phase transitions and molecular motions in solid acrylonitrile and its deuterated homologue CH2=CDCN, have been studied between 100 and 191 °K (m.p.) by wide line n.m.r. and by T1 relaxation time measurements. Phase I (164 °K < T < 191 °K) is trapped and becomes metastable by quick cooling of acrylonitrile at 77 °K. It changes into the phase II, stable between 113 °K and 164 °K by a long duration annealing at 155–160 °K. The phase II → phase III transition occurs at 113 °K. It may be assumed that phase III, stable below this temperature, is rigid at T < 105 °K. Phase II may be characterized by a rotational oscillation of molecules around an axis defined by the N atom and the middle of the vinyl double bond. In phase I, acrylonitrile molecules undergo a binary reorientation motion around this axis with an activation energy of 4.2 kcal mol−1. The motion of peroxy radicals, trapped in acrylonitrile has been also studied by e.s.r. These radicals were produced by oxygen addition to free radicals previously formed by γ irradiation of acrylonitrile at 77 °K. The g anisotropy variation with temperature, shows no discontinuities at phase transitions, the activation of reorientation of peroxy radicals being 0.65 kcal mol−1. This result suggests that we are dealing in fact with macroradicals, the internal rotation of which is only observable in a solid matrix.

Isolated versus Condensed Anion Structure II; the Influence of the Cations (1,3-propanediammonium, 1,4-phenylendiammonium, and n-propylammonium) on Structures and Phase Transitions of CdBr2-4Salts A 79,81Br NQR and X-ray Structure Analysis

1996 ◽  
Vol 51 (12) ◽  
pp. 1216-1228 ◽  
Author(s):  
Hideta Ishihara ◽  
Shi-qi Dou ◽  
Keizo Horiuchi ◽  
V. G. Krishnan ◽  
Helmut Paulus ◽  
...  
Keyword(s):  
Phase Transitions ◽  
Phase I ◽  
Chain Structure ◽  
Phase Iii ◽  
X Ray ◽  
Dsc Measurements ◽  
Structure Determinations ◽  
A Chain ◽  
Successive Phase ◽  
Phase Iv

Abstract The influence of the cations on the condensation of anions CdBr42- in salts (A')CdBr4 (II) and (A)2CdBr4 (II) is studied by 79,81Br NQR and X-ray crystal structure determinations. (A')CdBr4 : A' = [H3N(CH2)3NH3]2+ (1) crystallizes with a layer-type anion structure at 298 K and A' = [1,4-(H3N)2C6H4]2+ (2) crystallizes with a chain-type anion structure at 298 K. (A)2 CdBr4 : A = [n-H3C(CH2)2NH3]+ (3) crystallizes with a layer-type anion structure at 293 K. (1) shows successive phase transitions at 328, 363, and 495 K according to the NQR and DSC measurements. Phase IV of (1): at 298 K orthorhombic, Pnma, Z = 4,a = 772.1 (4), b = 1905.4(9), c = 789.8(4) pm. 81Br NQR spectrum showed a doublet at 77 K (phase IV) with ν1= 61.177 and ν2 = 45.934 MHz and also a doublet at 350 K (phase III) with ν1= 57.581 and ν2 = 48.747 MHz. (2): at 295 K orthorhombic, Pnma, Z = 4, a = 802.5(3), b = 1775.1(6), c = 881.9(3) pm; the five-coordinated Cd atom and one-dimensional [CdBr4]2- anion chain structure was observed. This coordination and chain structure are very rare for (A')CdX4 (II) or (A)2CdX4 (II). Two 81Br NQR lines were observed at 77 K: ν1= 70.159 and ν3 = 40.056 MHz. One more line appeared at 85 K: ν2 = 53.622 MHz. A 81Br NQR triplet was observed at 273 K: ν1 = 67.919, ν2 = 56.317, and ν3 = 40.907 MHz. (3) shows successive phase transitions at 121, 135, 165, and 208 K according to the NQR, DSC, and DTA measurements. Phase I of (3): at 293 K orthorhombic, Cmca, Z = 4, a = 783.4(4), b = 2480.2(10), c = 806.5(4) pm. 81Br NQR doublet was observed at 77 K (phase V) and at 300 K (Phase I) with ν1 = 61.060 and ν2 = 54.098 MHz (77 K); v1 = 55.835 and ν2 = 55.964 MHz (373 K). No NQR line could be observed in phases II, III, and IV.

Phase transitions in adamantane derivatives: 2-chloroadamantane

1988 ◽  
Vol 66 (8) ◽  
pp. 1973-1978 ◽  
Author(s):  
Ralph M. Paroli ◽  
Nancy T. Kawai ◽  
Ian S. Butler ◽  
Denis F. R. Gilson
Keyword(s):  
Phase Transitions ◽  
Phase Ii ◽  
Phase Iii ◽  
Adamantane Derivatives ◽  
Second Phase ◽  
Scanning Calorimetry ◽  
Ft Ir ◽  
First Order Phase ◽  
Ir And Raman ◽  
Transition Behaviour

The phase transition behaviour of 2-chloroadamantane, 2-C10H15Cl, has been investigated by differential scanning calorimetry (DSC), and FT-IR and Raman spectroscopy. Two transitions were detected by both DSC and vibrational spectroscopy at 231 and 178 K, on cooling, and at 242 and 227 K, on heating. The measured enthalpies were 8.3 kJ mol−1 for the first transition (phase I → phase II), and 0.47 kJ mol−1 for the second (phase II → phase III). The entropies were 35 and 2.3 J K−1 mol−1, respectively. These are similar to those observed for other 2-substituted adamantanes, but significantly different from those for 1-substituted derivatives. The large hystereses observed for the two transitions are independent of the DSC scanning rate and are characteristic of first-order phase transitions. The dramatic differences observed in the vibrational spectra of phases I and II provide clear evidence of an order–disorder transition at about 235 K.

scholarly journals Effect of Drying Temperature on Iron Fischer-Tropsch Catalysts Prepared by Solvent Deficient Precipitation

2017 ◽  
Vol 2017 ◽  
pp. 1-11 ◽  
Author(s):  
Michael K. Albretsen ◽  
Baiyu Huang ◽  
Kamyar Keyvanloo ◽  
Brian F. Woodfield ◽  
Calvin H. Bartholomew ◽  
...  
Keyword(s):  
Phase Transitions ◽  
Phase I ◽  
Pore Structure ◽  
Catalytic Properties ◽  
Catalytic Performance ◽  
Phase Iii ◽  
Fischer Tropsch ◽  
Two Phase ◽  
Methane Selectivity ◽  
Physical Changes

A novel solvent deficient precipitation (SDP) method to produce nanoparticles was studied for its potential in Fischer-Tropsch synthesis (FTS) catalysis. Using Fe(NO3)3·9H2O as the iron-containing precursor, this method produces ferrihydrite particles which are then dried, calcined, reduced, and carbidized to form the active catalytic phase for FTS. Six different drying profiles, including final drying temperatures ranging between 80 and 150°C, were used to investigate the effect of ammonium nitrate (AN), a major by-product of reaction between Fe(NO3)3·9H2O and NH4HCO3 in the SDP method. Since AN has two phase-transitions within this range of drying temperatures, three different AN phases can exist during the drying of the catalyst precursors. These AN phases, along with physical changes occurring during the phase transitions, may affect the pore structure and the agglomeration of ferrihydrite crystallites, suggesting possible reasons for the observed differences in catalytic performance. Catalysts dried at 130°C showed the highest FTS rate and the lowest methane selectivity. In general, better catalytic performance is related to the AN phase present during drying as follows: phase III > phase II > phase I. However, within each AN phase, lower drying temperatures led to better catalytic properties.

scholarly journals First- and second-order phase transitions in RE6Co2Ga (RE = Ho, Dy or Gd) cryogenic magnetocaloric materials

2021 ◽  
Author(s):  
Dan Guo ◽  
Luis M. Moreno-Ramírez ◽  
Carlos Romero-Muñiz ◽  
Yikun Zhang ◽  
Jia-Yan Law ◽  
...  
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Order Phase Transition ◽  
Density Functional ◽  
Magnetic Ordering ◽  
Density Functional Theory Calculations ◽  
Entropy Change ◽  
Second Order ◽  
First Order Phase Transition ◽  
First Order Phase

AbstractRare-earth (RE) rich intermetallics crystallizing in orthorhombic Ho6Co2Ga-type crystal structure exhibit peculiar magnetic properties that are not widely reported for their magnetic ordering, order of magnetic phase transition, and related magnetocaloric behavior. By tuning the type of RE element in RE6Co2Ga (RE = Ho, Dy or Gd) compounds, metamagnetic anti-to-paramagnetic (AF to PM) phase transitions could be tuned to ferro-to-paramagnetic (FM to PM) phase transitions. Furthermore, the FM ground state for Gd6Co2Ga is confirmed by density functional theory calculations in addition to experimental observations. The field dependence magnetocaloric and Banerjee’s criteria demonstrate that Ho6Co2Ga and Dy6Co2Ga undergo a first-order phase transition in addition to a second-order phase transition, whereas only the latter is observed for Gd6Co2Ga. The two extreme alloys of the series, Ho6Co2Ga and Gd6Co2Ga, show maximum isothermal entropy change (∣ΔS iso max (5 T)∣) of 10.1 and 9.1 J kg−1K−1 at 26 and 75 K, close to H2 and N2 liquefaction, respectively. This outstanding magnetocaloric effect performance makes the RE6Co2Ga series of potential for cryogenic magnetic refrigeration applications.

Model for the crystal packing and conformational changes of biphenyl in incommensurate phase transitions

2004 ◽  
Vol 60 (2) ◽  
pp. 228-237 ◽  
Author(s):  
Alexander Dzyabchenko ◽  
Harold A. Scheraga
Keyword(s):  
Phase Transitions ◽  
High Temperature ◽  
Phase I ◽  
Crystal Packing ◽  
Twist Angle ◽  
Basic Structure ◽  
Incommensurate Phase ◽  
Phase Iii ◽  
Temperature Structure ◽  
High Temperature Structure

Standard atom–atom potentials for hydrocarbons and a torsional potential to account for the π-electron conjugation energy were used to model the crystal structures and phase transitions of biphenyl. The model describes the high-temperature phase (I) with its planar molecule as a stationary point of the energy hypersurface. Phase I represents a low-energy barrier between the symmetry minima of the ground state (phase III), in which the molecule is twisted with torsion angles of opposite sign. Global-energy minimization was carried out by considering both regular structures, with one or two independent molecules, and quasi-one-dimensional superstructures built of N cells (N up to 16) of the high-temperature structure. The various energy-minimized biphenyl structures demonstrate remarkable similarity in their crystal packing; in particular, there are characteristic rows of cooperatively twisted molecules parallel to the superstructure dimension b. The structures built of centrosymmetric rows (P\bar 1, Z = 4 and 8) are almost as low in energy as the basic structure (an N = 2 superstructure, Pa, Z = 4); moreover, one of them is isostructural with the low-temperature p-quaterphenyl structure. With N > 8, structures of lower energy than that of the basic structure (N = 2) were found; their common feature is an M-fold modulation of the twist angle over the supercell period, with M smaller than N and generally not a simple fraction of it. The global minimum was found to conform to the ratio k = M/N = 6/14, which is close to the experimentally observed k = 6/13 in the incommensurate phase III. Enthalpy minimization showed an overall decrease in the magnitude of the twist angle down to τ ≃ 0°, as well as the evolution of the modulated structures towards the high-temperature structure with increasing pressure, in agreement with evidence for the high-pressure limit of the incommensurate biphenyl phases.

X-ray studies of the phase transitions of bis(guanidinium)zirconium bis(nitrilotriacetate) hydrate, (C(NH2)3)2Zr(N(CH2COO)3)2 · H2O

1999 ◽  
Vol 214 (3) ◽  
Author(s):  
J. Schreuer ◽  
E. Haussühl
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Phase I ◽  
Phase Iii ◽  
Group Symmetry ◽  
Translational Symmetry ◽  
X Ray Diffraction ◽  
Space Group Symmetry ◽  
X Ray ◽  
Structural Differences

AbstractThe structural differences of phase I (at 193 K) and phase III (293 K) of bis(guanidinium)zirconium bis(nitrilotriacetate) hydrate were investigated by means of X-ray diffraction. The phase transition III → I is characterised by a loss of translational symmetry as it is indicated by the change of space group symmetry from

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