The polymorphism of alkali metal formates. Part 4. A Raman study of the phase transition in KHCOO

1991 ◽  
Vol 69 (11) ◽  
pp. 1774-1780 ◽  
Author(s):  
A. M. Heyns ◽  
K.-J. Range ◽  
K. Müller
Keyword(s):  
Phase Transition ◽  
Phase I ◽  
Raman Spectra ◽  
Orthorhombic Phase ◽  
Fermi Resonance ◽  
Theoretical Treatment ◽  
Ambient Conditions ◽  
Definite Structure ◽  
Raman Study ◽  
Intramolecular Motions

KHCOO II is orthorhombic at ambient conditions and it is shown that traces of moisture affect the polymorphism of these very hygroscopic crystals. Dry KHCOO II transforms into phase I at 417 K (144 °C), and this phase can be supercooled to room temperature, remaining metastable for several days before transforming back to the orthorhombic phase II. The Raman spectra of phases I and II, as well as of supercooled phase I, are reported in the present study. The absence of some prominent translational modes in the Raman spectra of KHCOO II, compared to NaHCOO II, can be explained on the basis of a group-theoretical treatment. From the temperature dependence of the linewidths of various Raman-active librational and internal modes, activation energies are obtained for intramolecular motions of the formate ions. Fermi resonance occurs between the overtone of the bending mode 2ν5 and the C—H stretching mode ν1 in KHCOO II and the coupling constant W increases with temperature. The Raman and X-ray data show that KHCOO I is structurally different from NaCHOO I, but it is not possible to assign a definite structure to this phase on the basis of a Raman spectrum alone. Key words: Potassium formate, phase transition, Raman spectra.

scholarly journals Phase Transition and Polymerization of Acetonitrile under High Pressure

2014 ◽  
Vol 70 (a1) ◽  
pp. C763-C763
Author(s):  
Haiyan Zheng ◽  
Kuo Li ◽  
George Cody ◽  
Chris Tulk ◽  
Jamie Molaison ◽  
...  
Keyword(s):  
Phase Transition ◽  
High Pressure ◽  
Reaction Mechanism ◽  
Neutron Diffraction ◽  
Raman Spectra ◽  
Hydrogen Transfer ◽  
Layer Structure ◽  
Van Der Waals ◽  
Orthorhombic Phase ◽  
Organic Polymer

Successful application of high pressure on synthesis of organic polymer, including the conducting polymer and super hard materials depends on the knowledge of reaction mechanism. The evolution of crystal structure under high pressure especially the structure close to transition pressure is crucial to conclude the reaction mechanism. Nitriles represent a large class of interstellar molecules and are the potential source of amino acids. Understanding its behavior at extreme conditions has gained increasing attention recently. Acetonitrile (CH3CN), the simplest organic compound with C≡N triple bond, can act as a model system for studying the pressure induced polymerization. The phase transition of acetonitrile under high pressure has been studied extensively.[1-3] However, it is still controversial and there is no any detailed discussion about its polymerization mechanism under high pressure. Here, we report the in-situ high pressure Raman spectra and powder neutron diffraction results on CD3CN, which indicates a minor phase transition at 5 GPa. The neutron diffraction shows that CD3CN keeps the orthorhombic phase from 1.66 GPa to 20.58 GPa which is very close to the reaction pressure. The week hydrogen bonding CD...N arranges the molecule into 3-dimensional framework which can be treated as two sets of diamond like structures interpenetrating with each other. Interestingly, the observed N...D distance is 1.984 Å at 20.58 GPa, shorter than the van der Waals distance of N...H (2.75 Å) by 28%. The van der Waals separation is often taken as a reference distance for the molecular instability. Thus, a hydrogen transfer process during the polymerization can be concluded. This deduction is also supported by the solid state NMR and FTIR results of the recovered polymerized CH3CN (p-CH3CN) from high pressure. In addition, the atomic pair distribution function and Raman spectra indicate the p-CD3CN or p-CH3CN has a random packed layer structure with nano-graphene lattice.

scholarly journals Pressure-induced coherent sliding-layer transition in the excitonic insulator Ta2NiSe5

IUCrJ ◽  
2018 ◽  
Vol 5 (2) ◽  
pp. 158-165 ◽  
Author(s):  
Akitoshi Nakano ◽  
Kento Sugawara ◽  
Shinya Tamura ◽  
Naoyuki Katayama ◽  
Kazuyuki Matsubayashi ◽  
...  
Keyword(s):  
Phase Transition ◽  
Phase I ◽  
Phase Ii ◽  
Ambient Pressure ◽  
Structural Phase ◽  
Orthorhombic Phase ◽  
Phase Iii ◽  
Layer Transition ◽  
Coulombic Interactions ◽  
Excitonic Insulator

The crystal structure of the excitonic insulator Ta2NiSe5has been investigated under a range of pressures, as determined by the complementary analysis of both single-crystal and powder synchrotron X-ray diffraction measurements. The monoclinic ambient-pressure excitonic insulator phase II transforms upon warming or under a modest pressure to give the semiconductingC-centred orthorhombic phase I. At higher pressures (i.e.>3 GPa), transformation to the primitive orthorhombic semimetal phase III occurs. This transformation from phase I to phase III is a pressure-induced first-order phase transition, which takes place through coherent sliding between weakly coupled layers. This structural phase transition is significantly influenced by Coulombic interactions in the geometric arrangement between interlayer Se ions. Furthermore, upon cooling, phase III transforms into the monoclinic phase IV, which is analogous to the excitonic insulator phase II. Finally, the excitonic interactions appear to be retained despite the observed layer sliding transition.

Raman spectral studies of solutions at elevated temperatures and pressures.13. Sodium formate – water

1993 ◽  
Vol 71 (10) ◽  
pp. 1728-1733 ◽  
Author(s):  
Richard J. Bartholomew ◽  
Donald E. Irish
Keyword(s):  
Raman Spectra ◽  
Elevated Temperatures ◽  
Sodium Formate ◽  
Fermi Resonance ◽  
Spectral Studies ◽  
Coupling Constant ◽  
Ambient Conditions ◽  
Modes Of Vibration ◽  
Resonance Doublet ◽  
Raman Spectral

Raman spectra of the formate anion in water (H2O and D2O) have been measured for four concentrations under ambient conditions and for two concentrations at temperatures ranging from 49 to 239 °C and a pressure of 10 MPa. Five of the six fundamental modes of vibration are polarized. This result is inconsistent with C2ν symmetry. The Fermi resonance doublet clearly results from the interaction of 2ν5 and ν1. The latter mode decreases in frequency as the temperature rises, thus increasing the coupling and hence the intensity of the 2ν5 component. The coupling constant, W, and the positions of the unperturbed bands [Formula: see text] and [Formula: see text] have been calculated. No evidence to support a bifurcated structure for the solvated anion was found.

Raman studies of cyanate: Fermi resonance, hydration and hydrolysis to urea

1993 ◽  
Vol 71 (10) ◽  
pp. 1764-1773 ◽  
Author(s):  
Murray H. Brooker ◽  
Nanping Wen
Keyword(s):  
Hydrogen Bond ◽  
Raman Spectra ◽  
Room Temperature ◽  
Solid Phase ◽  
Equilibrium Mixture ◽  
Fermi Resonance ◽  
Ambient Conditions ◽  
Parallel Reactions ◽  
Associated Pair ◽  
Hydrolysis Of

Raman spectra were measured for potassium cyanate in the solid phase and as aqueous solutions in H2O and D2O for freshly prepared and for aged solutions. The results indicated that the assignment of the Fermi doublet, ν1 and 2ν2, for solid potassium cyanate was reversed from the assignment for the aqueous solution. The Fermi doublet has an associated pair of hot bands at 1191 and 1315 cm−1 which originate from the 638 cm−1 ν2 state, 010. Assignment of the hot bands was confirmed by studies of solid potassium cyanate at liquid-N2 temperature, room temperature, and at 473 K. Raman spectra of aged aqueous potassium cyanate revealed that the cyanate ion hydrolyzed slowly and spontaneously at room temperature (even without added ammonium) to produce urea and a carbamate, carbonate equilibrium mixture in parallel reactions. Hydrolysis of cyanate in aqueous ammonium chloride solution resulted in almost total conversion of cyanate to urea. The reaction was not reversible under ambient conditions. Differences in peak frequencies and half-widths were observed for the cyanate dissolved in H2O compared to solutions in D2O. The results provide evidence for strong hydrogen bonding of cyanate to water and are consistent with greater structure in the D2O solution. Theoretical ab initio calculations indicated that the water molecules hydrogen bond well at both the oxygen and nitrogen atoms of cyanate although the hydrogen bond to nitrogen was found to be slightly stronger.

Vibrational Spectra of SiH4 and SiD4–SiH4 Mixtures in the Condensed States

1972 ◽  
Vol 50 (1) ◽  
pp. 35-42 ◽  
Author(s):  
R. P. Fournier ◽  
R. Savoie ◽  
Nguyen Dinh The ◽  
R. Belzile ◽  
A. Cabana
Keyword(s):  
Phase Transition ◽  
Low Temperature ◽  
Phase I ◽  
Raman Spectra ◽  
Vibrational Spectra ◽  
Crystalline Phase ◽  
Solid Phase ◽  
Transition Phase ◽  
Solid Phase Transition

The i.r. and Raman spectra of liquid and crystalline SiH4 and SiD4–SiH4 mixtures have been recorded. The spectra show striking changes when the crystal undergoes the solid–solid phase transition. Phase I is disordered. Possible site, and factor groups for the low temperature crystalline phase are proposed.

Single Crystal Raman Spectra of the Phase Transition in KTCNQ

1982 ◽  
Vol 85 (1) ◽  
pp. 297-303 ◽  
Author(s):  
A. D. Bandrauk ◽  
K. D. Truong ◽  
S. Jandl
Keyword(s):  
Phase Transition ◽  
Single Crystal ◽  
Raman Spectra

Dynamics of the commensurate-incommensurate phase transition in C2O4D ND4, 1/2 D2O: a polarized Raman study under pressure

1993 ◽  
Vol 3 (4) ◽  
pp. 1007-1029 ◽  
Author(s):  
M. Krauzman ◽  
A. Colline ◽  
D. Kirin ◽  
R. M. Pick ◽  
N. Toupry
Keyword(s):  
Phase Transition ◽  
Incommensurate Phase ◽  
Raman Study ◽  
Polarized Raman
Sign in / Sign up

Export Citation Format

Share Document