Release of Tetrahydrofuran, Structural Phase Transitions and Dynamic Relaxation Processes in Ca (BH4)2

2012 ◽  
Vol 184 ◽  
pp. 24-32 ◽  
Author(s):  
Annalisa Paolone ◽  
O. Palumbo ◽  
P. Rispoli ◽  
Rosario Cantelli ◽  
E. Rönnebro ◽  
...  
Keyword(s):  
Phase Transitions ◽  
Room Temperature ◽  
Structural Phase ◽  
Young Modulus ◽  
Relaxation Processes ◽  
Structural Phase Transitions ◽  
Exponential Factor ◽  
Β Phase ◽  
Α Phase ◽  
Calcium Borohydride

Various calcium borohydride samples were investigated by means of combined measurements of thermogravimetry and mass spectrometry, and anelastic spectroscopy. On heating, the release of 2-5% tetrahydrofuran (THF) is detected in all the samples at temperatures below ~480 K, even in those which were previously thermally treated, according to procedures known from the literature, in order to remove the solvent. Dehydrogenation takes place above 480 K. Above room temperature the temperature dependence of the Young modulus of Ca (BH4)2clearly monitors the release of THF and two irreversible structural phase transitions: from the α to the α’ phase around 460 K and from the α’ to the β phase, nearly completely evolved around 590 K. Moreover, the coefficient of elastic energy dissipation presents two dynamic processes below room temperature; a peak around 120 K characterized by an activation energy of 0.20 eV and a pre-exponential factor typical of atom-cluster relaxations, that we attributed to the dynamics of THF molecules retained in the borohydride lattice, and a peak around 200 K, possibly due to the relaxation of H vacancies.

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.

Structural phase transitions, and Br...N and Br...Br interactions in 1-phenyl-2-methyl-4-nitro-5-bromoimidazole

2004 ◽  
Vol 60 (3) ◽  
pp. 333-342 ◽  
Author(s):  
Maciej Kubicki
Keyword(s):  
Phase Transitions ◽  
Crystal Packing ◽  
Structural Phase ◽  
Molecular Geometry ◽  
Second Phase ◽  
Triclinic Space Group ◽  
Space Group ◽  
Β Phase ◽  
Α Phase ◽  
Second Order Transition

Crystals of C10H8N3O2Br undergo two reversible phase transitions between 295 and 100 K. The first, of an order–disorder nature, is a second-order transition and takes place continuously over a wide temperature range. This transition is connected with the doubling of the length of the c axis of the unit cell and with the change of the space group from P21/m with Z′ = 1/2 (room-temperature α-phase) to P21/c, Z′ = 1 (β-phase, 200–120 K). During this transition the molecule loses the C s symmetry of the α-phase. The second transition takes place between 118 and 115 K, and is accompanied by a change of the crystal symmetry to the triclinic space group P\bar 1 (low-temperature γ-phase). This second phase transition is accompanied by the twinning of the crystal. Neither the molecular geometry nor the crystal packing shows any dramatic changes during these phase transitions. Halogen bonds C—Br...N and dihalogen interactions Br...Br play a crucial role in determining the crystal packing and compete successfully with other kinds of weak intermolecular interactions.

Structural phase transitions and adduct release in calcium borohydride

2011 ◽  
Vol 509 ◽  
pp. S691-S693 ◽  
Author(s):  
A. Paolone ◽  
O. Palumbo ◽  
P. Rispoli ◽  
A. Miriametro ◽  
R. Cantelli ◽  
...  
Keyword(s):  
Phase Transitions ◽  
Structural Phase ◽  
Structural Phase Transitions ◽  
Calcium Borohydride

Phase and Structural Behaviour in the NdAlO3-EuAlO3 System

2013 ◽  
Vol 200 ◽  
pp. 93-99 ◽  
Author(s):  
Natalia Ohon ◽  
Leonid Vasylechko ◽  
Yurii Prots ◽  
Marcus Schmidt ◽  
Caroline Curfs
Keyword(s):  
Phase Diagram ◽  
Phase Transitions ◽  
Binary System ◽  
Solid Solutions ◽  
Room Temperature ◽  
Structural Phase ◽  
Structural Phase Transitions ◽  
Structural Behaviour ◽  
First Order

Phase and structural behaviour in the NdAlO3–EuAlO3 system has been studied in the whole concentration range. Depending on x two kinds of solid solutions Nd1‑xEuxAlO3 exist at room temperature: one with rhombohedral (x < 0.15) and one with orthorhombic (x≈ 0.15–0.20, where the co-existence of both phases was observed. First-order structural phase transitions Pbnm↔Rc has been detected in Nd1-xEuxAlO3 with x = 0.3, 0.4, 0.6 at 520 K, 627 K and 988 K, respectively. Based on the experimental and literature data, the phase diagram of the pseudo-binary system NdAlO3–EuAlO3 has been constructed.

Tunability of τf in perovskites and related compounds

2002 ◽  
Vol 17 (8) ◽  
pp. 2033-2040 ◽  
Author(s):  
P. L. Wise ◽  
I. M. Reaney ◽  
W. E. Lee ◽  
D. M. Iddles ◽  
D. S. Cannell ◽  
...  
Keyword(s):  
Phase Transition ◽  
Phase Transitions ◽  
Resonant Frequency ◽  
Room Temperature ◽  
Structural Phase ◽  
Structural Phase Transitions ◽  
Ionic Polarizability ◽  
Microwave Dielectrics ◽  
Related Compounds ◽  
Range Of Values

Tuning the temperature coefficient of resonant frequency (τf) in microwave dielectrics has been attributed to two main mechanisms: (i) dilution of the average ionic polarizability; (ii) the onset of an octahedral tilt transition above room temperature. The contributions of each mechanism have been isolated using ceramics in the Srn+1TinO3n+1, SrxCa1−x)3Ti2O7, and (SrxCa1−x)TiO3 series. In the Srn+1TinO3n+1 series, relative permittivity (εr) and τf are linearly proportional over a broad range of values, 100–37 and 800–140 ppm/°C, n = 4 and 1, respectively. No structural phase transitions occur on cooling from the prototype symmetry, and the mechanism of tuning is attributed solely to dilution of the average ionic polarizability as the SrO:SrTiO3 ratio increases. Exchanging Ca for Sr in the (SrxCa1−x)3Ti2O7 series resulted in an 80% reduction in the magnitude of τf from +320 to +50 ppm/°C but only 21% in permittivity (58 to 46). The effect was nonlinear and attributed primarily to the onset of a phase transition involving rotations of the octahedra on cooling. Superlattice reflections associated with the octahedral tilt transition have been identified.

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