Low-temperature Crystal Research Articles

Mechanism of pressure-induced martensitic phase transformations: A molecular-dynamics study.

The pressure-induced martensitic transformation from bcc to hcp iron has been studied in great detail with synchrotron radiation and diamond-anvil cells. The mechanism of the transformation has been postulated to be associated with a sliding of parallel bcc (110) planes combined with a contraction along the [001] direction; the (110) planes becoming the close-packed planes of the hcp structure. The synchrotron experiments support this picture of the transformation. We carry out the computer experiment of pressure-loading a bcc crystal to induce this transformation using molecular dynamics. We present the detailed microscopic motion of the particles during the transformation and show that the mechanism mentioned above for the transformation is confirmed for two different potentials and for two different particle numbers. The reverse transformation shows hysteresis, as is experimentally observed. For the two potentials which give the transformation we find that the final structure is often a close-packed polytype. We never observed a fcc crystal as the final structure. For another potential the final state was a defective crystal with no obvious resemblance to a close-packed structure. Most of our simulations were started by applying pressure to a low-temperature bcc crystal. A study of the sliding of the bcc (110) planes relative to each other suggests that the hcp structure is the preferred structure at low enough temperatures in pressure-induced bcc-to-close-packed structural transformations. For both potentials that give successful transformations the final structure is hcp at low enough temperature. In the phase diagram for iron the hcp structure is the low-temperature structure obtained by compressing bcc iron.

Molecular dynamics methods to study structural phase transformations in solids

Over the years, molecular dynamics calculations have used periodic boundary conditions in which the repeating parallelepiped containing the molecules remained fixed in size and shape. New methods have been developed which allow the shape and size of the parallelepiped to vary with time in such a way that the applied thermodynamics tension and enthalpy or temperature remain constant. A discussion of the theory along with a study of a stress induced structural phase transformations in a low temperature crystal is presented.

Picosecond vibrational cooling in mixed molecular crystals studied with a new coherent raman scattering technique

We demonstrate the pump-induced coherent Stokes Raman scattering (CSRS) technique by measuring vibrational cooling in low temperature crystals of pentacene in naphthalene following excitation of a vibration 747 cm −1 above the S 1 origin. Using picosecond photon echoes and a two-color pump-probe technique, we find that the initial state decays in 33 ps, and reappears at the origin 25 ps later. We show that pump-induced CSRS simultaneously measures the decay from the initial state and reappearance at the origin. This technique has many of the advantages of conventional coherent Raman (e.g. intense coherent signals), but is a direct measure of the population dynamics in the initial and final states.

Vibrational relaxation and vibrational cooling in low temperature molecular crystals

The processes of vibrational relaxation (VR) and vibrational cooling (VC) are investigated in low temperature crystals of complex molecules, specifically benzene, naphthalene, anthracene, and durene. In the VR process, a vibration is deexcited, while VC consists of many sequential and parallel VR steps which return the crystal to thermal equilibrium. A theoretical model is developed which relates the VR rate to the excess vibrational energy, the molecular structure, and the crystal structure. Specific relations are derived for the vibrational lifetime T1 in each of three regimes of excess vibrational energy. The regimes are the following: Low frequency regime I where VR occurs by emission of two phonons, intermediate frequency regime II where VR occurs by emission of one phonon and one vibration, and high frequency regime III where VR occurs by evolution into a dense bath of vibrational combinations. The VR rate in each regime depends on a particular multiphonon density of states and a few averaged anharmonic coefficients. The appropriate densities of states are calculated from spectroscopic data, and together with available VR data and new infrared and ps Raman data, the values of the anharmonic coefficients are determined for each material. The relationship between these parameters and the material properties is discussed. We then describe VC in a master equation formalism. The transition rate matrix for naphthalene is found using the empirically determined parameters of the above model, and the time dependent redistribution in each mode is calculated.

Proton spin-lattice relaxation time studies and atom–atom non-bonded potential calculations on ferrocenecarbaldehyde (η5-C5H5)Fe(η5-C5H4CHO)

The barriers to molecular rotations in the low temperature crystal phase of ferrocenecarbaldehyde, (η5-C5H5)Fe(η5-C5H4CHO), have been measured using proton spin-lattice relaxation methods and assigned to different motions, based on the results of non-bonded atom–atom potential calculations. The principal relaxation is interpreted in terms of a Cole–Davidson distribution of correlation times, and assigned to rotation of the unsubstituted ring, with an activation energy of 15.3 kJ mol−1. Other relaxation processes, observed via T1D measurements, include rotation of the aldehyde group and oscillations of the whole molecule, with barriers of 42.4 and 25.6 kJmol−1, respectively.

Observation of delocalized states of the proton in carboxylic acid dimers in condensed matter

We report fluorescence line-narrowing experiments on low-temperature molecular crystals composed of hydrogen bonded dimers of benzoic acid. The results indicate that the acid protons are distinctly delocalized in nearly symmetric double-well potentials. The tunneling frequency is 4.8 × 10 9 s −1, a quantity not previously measured for acid protons. This is the first observation of delocalized protons of acid dimers in the condensed phase.

ELECTRON THEORY OF THE SUPERCONDUCTIVITY AND CRITICAL TRANSITION POINT Tc OF Ba2YCu309−d(d=2) BY THE APPLICATION OF THE EMPIRICAL ELECTRON THEORY OF SOLIDS AND MOLECULES

Application of “the empirical electron theory of solids and molecules” to the analysis of crystal structure obtained by Siegrist and his colleagues of Ba2YCu3O9−d(d≈2) directly yields the covalent bond electron structure of the crystal. The interpretation of the half oxygen vacancy in Cu-O4-O5 layers gives the necessity of the existence of empty thoroughfares for the superconductivity at low temperature crystal. Thermal agitation together with the fluctuation of the fractional covalent bonds render Ba atoms to rotate and to throw 04, 05 atoms to the thoroughfares to destroy the superconductivity. The calculated Tc agrees with the experimental value rather satisfactorily. The twinning observed by R. Beyers and his colleagues at IBM research centers further gives evidence for the thoroughfares existence.

Phase transition phenomena induced by the successive appearances of new types of aggregation states of water molecules in the “ l-dipalmitoylphosphatidylcholine-water” system

Thermal analysis of the aggregation state of water molecules m the “ L-di-palmitoylphosphatidylcholine (DPPC)-water system revealed that in the low-temperature crystal phase below the sub-transition temperature, there are three kinds of interlamellar water, i.e., tightly, loosely, and more loosely bound water, which are incorporated between the bilayers of l-DPCC molecules, while the Lβ' gel phase lacks the third bound water molecule. Furthermore, we found that both the sub- and pre-transitions below the main transition temperature appear, for the first time, at water contents of around 11 and 18 g%, respectively, just at which point the new types of water structure occur. These findings indicate that the condition necessary for the appearance of these two kinds of phase transition is the existence of the characterized aggregation state of water molecules, that is, more loosely bound water for the sub-transition and bulk free water coexisting with the Lβ' gel phase for the pre-transition.

Infrared band-shape analysis and hydrogen-bonding dynamics of the HCOOH low-temperature crystal

The infrared OH-stretching band shape of crystalline formic acid (HCOOH) at low temperature is analyzed on the ground of a recent theory involving a semiclassical approximation for the evolution operator. The adiabatic potential function for the slow O⋯O stretching mode ( N coordinate) is computed from the observed spectrum for the excited state of the fast ν OH vibration ( n coordinate) and is compared with that derived from a time-independent approach. Both theories are consistent with Morse-type potentials and a large shortening of the hydrogen bond in the excited state. The dipole moment operator appears to be N independent. An additional coupling between n and an external O⋯O bending coordinate of the hydrogen bond which manifests itself as a Franck—Condon progression of the ν OH mode is introduced to account for the observed band manifold. The structure and the dynamics of formic acid in the ground and upper states of the ν OH vibration are discussed and compared with those previously obtained for similar moderately strong hydrogen-bonded systems.

The vibrational spectra and force constant calculations of ethynylcyclohexane

The i.r. spectra of ethynylcyclohexane as a liquid and as amorphous and crystalline solids at 90 K were recorded between 4000 and 50 cm −1. High pressure i.r. spectra of the liquid (1–20 kbar) and crystalline sample were recorded. Raman spectra of the liquid, including semiquantitative polarization measurements, were obtained and the amorphous and crystalline solids studied at 90 K. The compound exists as an equilibrium of equatorial and axial conformers in the liquid and in the amorphous crystals while the molecule takes the e-conformer in the low temperature and high pressure anisotropic crystals. Under pressure the equilibrium is shifted towards the a-conformer in the liquid. In ethynylcyclohexane thiourea clathrate the e/ a ratio is approximately eight times smaller than in the liquid, but the clathrate decomposes with time at ca 330 K. A normal coordinate analysis was carried out, using a valence force field transferred from the halogenated cyclohexanes. It was extended with parameters for the side chain and fitted to the assigned fundamentals of the e and a conformers in cyano-, isocyano-, ethynyl- and trans-1, 4-dicyanocyclohexane. The fundamentals of ethynylcyclohexane were correlated with those of cyano- and isocyanocyclohexane.

Low Temperature Crystal Growth and Ceramic Sintering of BaTiO3–Type Oxyfluorides

The interest of LiF as additive to BaTiO3 has been pointed out for sintering dielectric oxyfluoride ceramics at low temperature (tsint=930°C). The understanding of the corresponding sintering process leads us to characterize ceramics and single crystals prepared at t<1000°C. Ceramic grains and crystals have a composition Ba(Ti1-xLix)O3-3xF3x. The whole composition of the ceramics (grain and grain boundaries) is compared to that of crystals. This study per formed on many samples allows to prove the existence of a grain boundary phase, with low melting temperature, facilitating the grain coalescence. The diffuse character of the phase transition and the grain composition gradient induce a large maximum of the ceramic permittivity at Tc, result of great interest for dielectric capacitors.

Molecular dynamics simulations on a parallel computer plastic crystals and related systems

Four molecular dynamics projects concerning dynamic molecular disorder are discussed, all having in common the fact that they are being done on a computer which is an array of parallel processors. The computer architecture is ideal for these systems as the molecules do not undergo translational diffusion.The first system is SF6 both in the plastic phase and below, where two phase transitions have been identified. In the plastic phase orientational disorder is apparent, the molecules arranging themselves so as to minimise the effect of the frustration between the nearest neighbour attraction and the next-nearest neighbour repulsion. The phase transitions represent the compromises enforced by this frustration as the temperature is lowered. A time correlation analysis shows that it is unlikely that phonon-like modes can exist in the plastic phase.The second system is naphthalene very near to melting, where the predominant molecular reorientation is about the axis of greatest inertia. This is contrasted with experimental results which suggest that the reorientation about the axis of least inertia is associated with melting, the other reorientations not being catastrophic for the crystallinity of the system. Arguments are presented that the system does not have a plastic phase just below the melting point.The third system is butane, where the molecule itself has internal degrees of freedom which are incorporated in the model. A number of crystal phases have been discovered, most of which must be metastable. On warming the stable triclinic phase to a temperature above the plastic transition in the real system, the model system takes on an orthorhombic structure where the symmetry is a time average.The last system discussed is an invented two-dimensional system of molecules we call quaternane, as they have four atoms arranged in a chain. Each molecule has two internal degrees of freedom both of which carry a double-well potential. The system develops domain walls when warmed from a low-temperature crystal phase, and it is apparent that the structure within the walls shows smectic liquid-crystal-like behaviour. This model could well prove to be a starting point for liquid-crystal simulations at the atomic level.

ChemInform Abstract: LOW‐TEMPERATURE CRYSTAL STRUCTURE OF A FIVE‐COORDINATE IRON(II) COMPLEX EXHIBITING SINGLET‐TRIPLET SPIN EQUILIBRIUM

Chemischer InformationsdienstVolume 15, Issue 50 Physical Organic Chemistry ChemInform Abstract: LOW-TEMPERATURE CRYSTAL STRUCTURE OF A FIVE-COORDINATE IRON(II) COMPLEX EXHIBITING SINGLET-TRIPLET SPIN EQUILIBRIUM M. BACCI, M. BACCISearch for more papers by this authorC. A. GHILARDI, C. A. GHILARDISearch for more papers by this authorA. ORLANDINI, A. ORLANDINISearch for more papers by this author M. BACCI, M. BACCISearch for more papers by this authorC. A. GHILARDI, C. A. GHILARDISearch for more papers by this authorA. ORLANDINI, A. ORLANDINISearch for more papers by this author First published: December 11, 1984 https://doi.org/10.1002/chin.198450052AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume15, Issue50December 11, 1984 RelatedInformation

Laser-induced fluorescence spectrometry of methylnaphthalene derivatives prepared in a low-temperature aromatic crystal

Experimental results show that durene is a suitable matrix for the analysis of a mixture of methylnaphthalenes (MN) and for the analysis of MN in shale oil. MNs are an example of the type of compounds for which a suitable alkane matrix could not be found; other examples occur whenever the linear dimensions of polycyclic aromatic hydrocarbons and n-alkane cannot be closely matched. 16 references, 2 figures, 1 table.

High—pressure Raman study of liquid and crystalline CS2 up to 45 kbar at 300 K

The Raman spectra of liquid and crystalline CS2 have been measured at pressures up to 45 kbar and 300 K. The pressure dependences of the internal modes and the four librational lattice modes are reported: For the internal modes, the frequency of the symmetrical stretching modes v1 increases hardly with pressure, because of the very strong S=C=S bonding. The overtones 2v2 of the bending modes in the liquid state shifted to lower frequencies a little with increasing pressure, and splitted by 18 cm-1 into two major peaks at the solidification point (about 13 kbar). In the pressure-induced solid phase above 13 kbar at 300 K, the four external (librational) modes were observed, which have been predicted by the symmetry consideration. Comparison of the spectrum of high-pressure crystals at 300 K with that of the low-temperature crystals at 1 bar indicates that the structure of each crystal resembles mutually.

A molecular dynamics simulation study of the plastic crystalline phase of sulphur hexafluoride

A molecular dynamics simulation study of the plastic crystalline phase of SF6 is reported. Various equilibrium properties have been measured and, where possible, compared with corresponding experimental measurements. The form of the molecular orientational distribution function has been obtained and is found to be in good agreement with that determined in a recent neutron scattering study. Information concerning the single-molecule dynamics has been obtained from calculations of linear and angular velocity autocorrelation functions, and it has been shown that the SF6 molecules undergo hindered rotation. The existence of coupling between the translational and rotational motions has been demonstrated, and it is shown how the behaviour of the plastic crystal is related to the form of the intermolecular potential. In addition, a transition to a trigonal low-temperature crystal phase at 80K has been observed, and the structure of this phase has been identified and found to be in good agree with experimental results. The calculations have been performed using a large parallel-array processor, and features of the simulation algorithms that reflect the architecture of such a computer are discussed.

High-pressure Raman study of liquid and molecular crystal at room temperature. Carbon disulphide

The Raman spectra of liquid and crystalline CS 2 were measured at pressures up to 45 kbar and room temperature. The pressure dependences of the internal modes (ν 1 and 2ν 2) and the four librational lattice modes are reported. Comparison of the spectrum of high-pressure crystals at 300 K with that of the low-temperature crystals at 1 bar indicates that the structure of each crystal resembles mutually.

Statistical thermodynamics of alkyl chain conformations in lipid bilayers

An examination is made of the conformations and packing of the chains in bulk n-alkane in low temperature crystals, the intermediate α-crystal state and in the liquid state. Flory's ‘random-coil model’ of liquid alkane is examined. The phase behaviour of the fully hydrated dipalmitoyl lecithin (DPPC) bilayer is analysed and comparisons are made with n-alkanes. The state of the DPPC chains above the gel/liquid-crystal phase transition is discussed in detail and it is argued that the chains exist as slightly perturbed random-coils. An examination is made of the changes in chain packing which occur when the lipid chains are unsaturated, and when cholesterol or protein are introduced into the membrane. The possibility of rigid-body re-orientation of the lipid chains around the normal to the bilayer is examined. It is argued that such motion is not sufficiently collective to invalidate the perturbed random-coil model. With the introduction of a substantial mole fraction of cholesterol, however, rigid-body tilting becomes the dominant form of disorder in the hydrophobic region of the bilayer.

Vibrational spectra and dynamics of conformation and hydrogen bonding of n-methylacetamide. I. Conformational dynamics of the CH 3CONHCH 3 molecule and NH out of plane band splitting

The infrared spectra of CH 3CONHCH 3 and CH 3CONDCH 3 have been investigated as low temperature crystals, pure liquids and solutions in various solvents in the 400-800 cm −1 range. A new assignment of the bands associated with the NH group is given. Multiplets of γ NH and γ ND fundamentals have generally been observed and have been interpreted in terms of a double minimum potential function of the γ NH mode with a tunnelling between two minima. The potential functions of γ NH and τ CN modes are similar and can be combined to give a potential surface with four minima corresponding to four molecular conformations. The influence of the hydrogen bonding on the γ NH splitting and barrier height is discussed.

ChemInform Abstract: CONFORMATION AND VIBRATIONAL SPECTRA OF 1,5‐HEXADIYNE (BIPROPARGYL) AND 1,5‐HEXADIYNE‐1,6‐D2

Abstract The infrared spectra of 1,5-hexadiyne (bipropargyl) and 1,5-hexadiyne-1,6- d 2 as vapours, liquids, as solutes in various solvents and as crystalline solids at low temperatures and at high pressures have been recorded. Raman spectra were obtained for the liquids, including semiquantitative polarization measurements, and for the low temperature crystals. The data were interpreted in terms of two conformers, anti and gauche , in the vapour and liquid state and one, the anti, in the crystalline forms. A phase transition for 1,5-hexadiyne was observed at ca. 240 K. Both the high and low temperature crystals had molecules in the anti conformer. Interpretation of the spectra in terms of conformational equilibria was facilitated by a thorough vapour phase band contour analysis. With a few exceptions, all the vibrational fundamentals for both conformers were assigned and found to be in good agreement with results from normal coordinate calculations.