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Chang Q Sun

Publications and source records attributed to Chang Q Sun.

36 records · Page 2Linked to original sources
arXiv2014

A Supersolid Skin Covering both Water and Ice

The mysterious nature and functionality of water and ice skins remain baffling to the community since 1859 when Farady firstly proposed liquid skin lubricating ice. Here we show the presence of supersolid phase that covers both water and ice using Raman spectroscopy measurements and quantum calculations. In the skin of two molecular layers thick, molecular undercoordination shortens the H-O bond by ~16% and lengthens the OH nonbond by ~25% through repulsion between electron pairs on adjacent O atoms, which depresses the density from 0.92 for bulk ice to 0.75 gcm-3. The O:H-O cooperative relaxation stiffens the H-O stretching phonon from 3200/3150 cm-1 to the same value of 3450 cm-1 and raises the melting temperature of both skins by up to ~310 K. Numerical derivatives on the viscosity and charge accumulation suggests that the elastic, polarized, and thermally stable supersolid phase makes the ice frictionless and water skin hydrophobic and ice like at room temperature.

cond-mat.soft↗
arXiv2013

Mpemba Paradox Revisited -- Numerical Reinforcement

Inspired by responses to the work (arXiv:1310.6514), we solved the one-dimensional, nonlinear Fourier initial and boundary condition problem using the finite element method. Examination of all possible parameters reveals the following: 1. Hydrogen bond has memory effect to emit energy at a rate, or with a relaxation time, depending on initial energy storage. 2. Skin super-solidity creates gradients in thermal diffusion coefficient for heat conduction in liquid with the optimal skin-bulk ratio of 1.48. 3. Convection alone produces no such effect. 4. Mpemba effect happens only in the highly non-diabetic source-path-drain cycling system.

physics.chem-ph↗
arXiv2013

O:H-O Bond Anomalous Relaxation Resolving Mpemba Paradox

We demonstrate that the Mpemba paradox arises intrinsically from the release rate of energy initially stored in the covalent H-O part of the O:H-O bond in water albeit experimental conditions. Generally, heating raises the energy of a substance by lengthening and softening all bonds involved. However, the O:H nonbond in water follows actively the general rule of thermal expansion and drives the H-O covalent bond to relax oppositely in length and energy because of the inter-electron-electron pair coupling [J Phys Chem Lett 4, 2565 (2013); ibid 4, 3238 (2013)]. Heating stores energy into the H-O bond by shortening and stiffening it. Cooling the water as the source in a refrigerator as a drain, the H-O bond releases its energy at a rate that depends exponentially on the initially storage of energy, and therefore, Mpemba effect happens. This effect is formulated in terms of the relaxation time tau to represent all possible processes of energy loss. Consistency between predictions and measurements revealed that the tau drops exponentially intrinsically with the initial temperature of the water being cooled.

physics.chem-ph↗
arXiv2013

Mechano-freezing of the ambient water

Raman spectroscopy examination of the 25 deg-C water freezing under compression revealed transition from 1.35 GPa to 0.86 GPa upon ice being formed at continued volume change. The transition is associated with a slight blue shift of the high-frequency phonon (omiga_H ~ 3120 cm-1) and creation of the low-frequency phonons (Omiga_L ~ 200 cm-1). In the liquid and in the solid phase, the increased pressure softens the Omiga_H and stiffens the Omida_L, which indicates the presence of the inter-electron-pair repulsion in both liquid and solid water.

physics.chem-ph↗
arXiv2013

Skin supersolidity slipperizing ice

Consistency between theory predictions and measurements and calculations revealed that the skin of ice, containing water molecules with fewer than four neighbours, forms a supersolid phase that is highly polarized, elastic, hydrophobic, with ultra-low density and high thermal stability. The supersolidity of skin sliperizes ice.

cond-mat.soft↗
arXiv2013

Mediation of hydrogen-bond coupling interactions by programmable heating and salting

We show that programmable heating and salting share the same effect on the frequency shift of the O:H and the H-O stretching phonons of the O:H-O hydrogen bond, which revealed that both heating and salting lengthens and softens the O:H bond and shortens and stiffens the H-O bond due to the weakening of the Coulomb repulsion between electron pairs of adjacent oxygen atoms. Understanding provides possible mechanism for the Hofmeister series and the detergent effect on cloth cleaning.

cond-mat.soft↗
arXiv2013

Scaling relation for the bond length, mass density, and packing order of water ice

The packing order of molecules and the distance between adjacent oxygen atoms (dOO) in water and ice are most basic yet puzzling. Here we present a scaling solution for this purpose based only on the mass density (gcm-3), (Equation) where dL is the length (Å) of the O:H van der Waals bond and dH the H-O polar-covalent bond projecting on the O---O line. Validated by the measured proton symmetrization of compressed ice, dOO of water and ice, and dOO expansion at water surface, this solution confirms that the fluctuated, tetrahedrally-coordinated structure is unique for water ice.

physics.chem-ph↗
arXiv2013

Missing short-range interactions in the hydrogen bond of compressed ice

Combining the Lagrangian-Laplace mechanics and the known pressure dependence of the length-stiffness relaxation dynamics, we have determined the critical, yet often-overlooked, short-range interactions in the hydrogen bond of compressed ice. This approach has enabled determination of the force constant, cohesive energy, potential energy of the vdW and the covalent segment at each quasi-equilibrium state as well as their pressure dependence. Evidencing the essentiality of the inter-electron-pair Coulomb repulsion and the segmental strength disparity in determining the asymmetric H-bond relaxation dynamics and the anomalous properties of ice, results confirmed that compression shortens and stiffens the OH bond and meanwhile lengthens and softens the covalent bond.

cond-mat.mtrl-sci↗
arXiv2012

Hidden force opposing ice compression

Coulomb repulsion between the unevenly-bound bonding and nonbonding electron pairs in the O:H-O hydrogen-bond is shown to originate the anomalies of ice under compression. Consistency between experimental observations, density functional theory and molecular dynamics calculations confirmed that the resultant force of the compression, the repulsion, and the recovery of electron-pair dislocations differentiates ice from other materials in response to pressure. The compression shortens and strengthens the longer-and-softer intermolecular O:H lone-pair virtual-bond; the repulsion pushes the bonding electron pair away from the H+/p and hence lengthens and weakens the intramolecular H-O real-bond. The virtual-bond compression and the real-bond elongation symmetrize the O:H-O as observed at ~60 GPa and result in the abnormally low compressibility of ice. The virtual-bond stretching phonons (< 400 cm-1) are thus stiffened and the real-bond stretching phonons (> 3000 cm-1) softened upon compression. The cohesive energy of the real-bond dominates and its loss lowers the critical temperature for the VIII-VII phase transition. The polarization of the lone electron pairs and the entrapment of the bonding electron pairs by compression expand the band gap consequently. Findings should form striking impact to understanding the physical anomalies of H2O.

physics.chem-ph↗
arXiv2011

Hydrogen bond relaxation dynamics and the associated symmetric, volumetric, vibronic, and phase transitional anomalies of frozen H2O under compression

Coulomb repulsion between the unevenly-bounded bonding "-" and nonbonding ":" electron pairs in the "O2- : H+/p-O2-" hydrogen-bond is found to originate the anomalies of low-compressibility, phonon relaxation dynamics, proton symmetrization in the hydrogen-bond, and the change of the critical temperature for the VIII-VII phase transition of ice under compression. The resultant force of the compression, the repulsion, and the uneven binding strength of the electron pairs make the softer intermolecular "O2- : H+/p" nonbonding lone pair be highly compressed and stiffened but the stiffer intramolecular "H+/p-O2-" bond be elongated and softened. Consequently, the softer nonbond phonons (< 400 cm-1) are stiffened and the stiffer bond phonons (> 3000 cm-1) are softened upon compression. The nonbond compression and the real bond elongation results in the O2--H+/p : O2- symmetrization and the low compressibility of ice. Findings should form the starting point to unveil the physical anomalies of H2O under various stimuli.

cond-mat.other↗
arXiv2010

Purified graphite surface and vacancy states: undercoordination-induced quantum trap depression and lone pi-electron polarization

We present a simple approach for purifying graphite surface and vacancy states using an angle-resolved x-ray photoelectron residual spectroscopy (XPS). Complementing the discoveries of Ugeda et al [Phys Rev Lett 104, 096804 (2010)], outcomes conform the BOLS theory [Sun, Prog Solid State Chem 35, 1-159 (2007)] expectation and the recent findings that the shorter and stronger bonds between undercoordinated atoms induce local strain and quantum potential depression with an association of local densification of energy and core electrons. The shorter atomic distance and the densely and deeply trapped bonding and core charges polarize in turn the unpaired pi-electrons nearby vacancy, giving rise to the high protrusions and the Dirac EF states as observed. The quantum trap depression and the screening due to the polarized EF states split the crystal potential and hence the extra XPRS C 1s states.

cond-mat.mes-hall↗
arXiv2008

Size dependence of the pressure-induced phase transition in nanocrystals

Extending the recently-developed bond-order-length-strength (BOLS) correlation mechanism [Sun CQ, Prog Solid State Chem 2007, 35, 1-159] to the pressure domain has led to atomistic insight into the phase stability of nanostructures under the varied stimuli of pressure and solid size. It turns out that the competition between the pressure-induced overheating (TC elevation) and the size-induced undercooling (TC depression) dominates the measured size trends of the pressure-induced phase transition. Reproduction of the measured size and pressure dependence of the phase stability for CdSe, Fe2O3, and SnO2 nanocrystals evidences the validity of the solution derived from the perspective of atomic cohesive energy and its response to the external stimulus.

cond-mat.mes-hall↗
arXiv2008

The thermo-mechanical behavior of low-dimensional materials

Consistently atomistic understanding of the mechanism behind the intriguing behavior of low-dimensional systems including monatomic chains, hollow tubes, surface skins, nanocavities, nanowires, and nanograins has long been a high challenge. This article reports recent progress in this regard. A survey is presented and then is followed by analytical approaches in terms of local bond average (LBA) from the perspective of bonding energetics and its functional dependence on external stimuli of coordination environment and temperature change. It is shown that the measurable quantities of a specimen can be functionally correlated to the identities of the representative bonds and their responses to the external stimuli. It is understood that the shortened and strengthened bonds between the under-coordinated atoms and the associated local strain and energy trapping dictate intrinsically the mechanical behavior of systems with large portion of under-coordinated atoms. The thermal softening of a substance arises from thermally-induced bond expansion and lattice vibration that weakens the bonds through the internal energy increase. The competition between the energy-density-gain and the residual atomic cohesive-energy in the relaxed surface skin determines intrinsically the mechanical performance of a mesoscopic specimen, whereas competition between the activation and inhibition of atomic dislocations dominates extrinsically the yield strength of the specimen in plastic deformation.

cond-mat.mtrl-sci↗
arXiv2005

Revealing of the new freedom of size: atomic energy levels, screening and charge transporting

Miniaturization of a solid forms a new freedom that is fascinating, which allows us not only to tune the physical properties of a solid but also enables us to gain information about the energy levels of an isolated atom and the effect of screening and charge transporting in reaction on the energy level shift, a calibration of crystal binding. Incorporating the recent bond order-length-strength correlation mechanism [Sun, Phys. Rev. B 69, 045105 (2004)] to the size dependence of Auger photoelectron coincidence spectra of Cu nanoparticles with and without being passivated has enabled us to gain quantitative information about the 2p and 3d level energies of an isolated Cu atom and their shift upon bulk formation. The developed approach also enabled us to discriminate the effect of crystal-field screening from the effect of valence recharging (due to surface passivation and substrate-particle interaction) on the binding energies to the electrons at different energy levels of a specimen.

cond-mat.mes-hall↗
arXiv2005

Size-induced acoustic hardening and optic softening of Ramn phonons

It has been puzzling that the Raman optical modes shift to lower frequency (or termed as optical mode softening) associated with creation of Raman acoustic modes that shift to higher energy (or called as acoustic hardening) upon nanosolid formation and size reduction. Understandings of the mechanism behind the size-induced acoustic hardening and optic softening have been quite controversial. On the basis of the recent bond order-length-strength (BOLS) correlation [Phys. Rev. B 69 045105 (2004)], we show that the optical softening arises from atomic cohesive energy weakening of surface atoms and the acoustic mode hardening is predominated by intergrain interaction. Agreement between predictions and observations has been reached for Si, CdS, InP, TiO2, CeO2, and SnO2 nanostructures with elucidation of vibration frequency of the corresponding isolated dimers. Findings further evidence the impact of bond order loss to low-dimensional systems and the essentiality of the BOLS correlation in describing the behavior of nanostructures.

cond-mat.mtrl-sci↗
arXiv2005

Mechanical strength of atomic chains, surface skins, and nanograins

This report deals with the correlation between the mechanical strength and thermal stability of systems extending from monatomic chains to surface skins and solids over the whole range of sizes with emphasis on the significance of atomic coordination imperfection. Derived solutions show that a competition between the bond order loss and the associated bond strength gain of the lower coordinated atoms dictate the thermo-mechanics of the low dimensional systems. Bond order loss lowers the atomic cohesive energy that determines the temperature of melting (Tm), or the activation energy for atomic dislocation, whereas bond strength gain enhances the energy density, or mechanical strength, in the surface skin. Therefore, the surface is harder at T << Tm whereas the surface becomes softer when the T approaches the surface Tm that is lower than the bulk due to bond order loss. Hence, the strained nanostructures are usually stiffer at low T whereas the harder skins melt easier. Quantitative information has been obtained about the bonding identities in metallic monatomic chains and carbon nanotubes. Solutions also enable us to reproduce the inverse Hall-Petch relationship with clarification of factors dominating the transition from hardening to softening in the nanometer regime.

cond-mat.mtrl-sci↗
arXiv2005

Size and confinement effect on nanostructures

This report presents a systematic understanding of the nature behind the unusual behavior of a nanosolid, and a surface as well, in mechanics, thermodynamics, acoustics, optoelectronics, magnetism, dielectrics, atomic diffusivity and chemical reactivity towards predictable design and controllable growth of nanostructured materials. A bond order-length-strength (BOLS) correlation mechanism has been developed, which has enabled the tunability of a variety of properties of a nanosolid in connection with surface to be consistently predicted and experimentally verified. The BOLS correlation indicates that the coordination number (CN) imperfection of an atom at site surrounding a defect or in the surface skin causes the remaining bonds of the lower-coordinated atom to contract spontaneously. The spontaneous bond contraction is associated with bond-strength gain or atomic potential well deppression, which localize electrons and enhance the density of charge, mass, and energy in the relaxed region. The enhancement of energy density in the relaxed region perturbs the Hamiltonian and the associated properties such as the band-gap width, core-level energy, Stokes shift (electron-phonon interaction), and dielectric susceptibility. On the other hand, bond order loss lowers generally the cohesive energy of the lower-coordinated atom from the value of a fully-coordinated atom, which dictates the thermodynamic process such as self-assembly growth, atomic vibration, thermal stability, and activation energies for atomic dislocation and diffusion. Consistency between predictions and observations evidences the enormous impact of atomic CN imperfection and the validity and essentiality of the BOLS correlation.

cond-mat.mtrl-sci↗