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1.
Gustavo Sophia Christian Sarrazin Michel Rérat Roberto Dovesi 《Phase Transitions》2013,86(11):1069-1084
The pressure-induced giant dielectric anomaly at 0 K of ABO3 perovskites is investigated at the Hartree-Fock, density-functional theory and hybrid levels. Its mechanism is analyzed in terms of thermodynamic phase stability, structural and phonon contributions and Born effective charges. It is shown that the IR-active soft phonon is responsible for the anomaly. This mode always involves a displacement and a deformation of the oxygen octahedra, while the roles of A and B ions vary among the materials and between high- and low-pressure phase transitions. A sharp increase in the phonon amplitude near the phase transition gives rise to the dielectric anomaly. The use of hybrid functionals is required for agreement with experimental data. The calculations show that the dielectric anomaly in the pressure-induced phase transitions of these perovskites is a property of the bulk material. 相似文献
2.
Strong in‐plane bonding (covalent) and weak van der Waals (vdW) interplanar interactions characterize a number of layered solids, as epitomized by graphite. The advent of graphene (Gr), individual atomic two‐dimensional (2D) layers, isolated from mineral graphite via micromechanical exfoliation enabled the ability to pick, place and stack of arbitrary compositions. Moreover, this discovery implicated an access to other 2D vdW solids beyond graphene and artificially stacking atomic layers forming heterostructures/superlattices. Raman spectroscopy (RS) is a fast reliable non‐destructive analytical tool and an integral part for lattice dynamical structural characterization of crystalline solids at nanoscale, revealing not only the collective atomic/molecular motions but also localized vibrations/modes and specifically used to determine the number of graphene layers and of other 2D vdW solids. We present Raman spectroscopy in first‐, second‐ and higher‐order vibrational modes involving 3 and 4 phonons (overtones and combination) and mapping of graphene (mono‐, bi‐, tri‐ and few‐) layers, semiconducting transition metal dichalcogenides (TMDs) [molybdenum disulfide (MoS2) and tungsten disulfide (WS2)] and wide bandgap hexagonal boron nitride (h‐BN) dispersed monolayers, revealing various molecular vibrations and structural quality/disorder. First‐ and higher‐order phonon modes are observed and analyzed in terms of Raman intensity (spatial inhomogeneity or thickness variation), band position (intrinsic mechanical strain) and intensity ratio (structural disorder) as a function of graphene layer (n). An empirical relation for G band position with n is corroborated. All of the higher order modes are observed to upshift almost linearly with n, betraying the underlying interlayer vdW interactions. These findings exemplify the evolution of structural parameters in layered materials in changing from 3‐ to 2‐ or low‐dimensional regime. The results are presented in view of applications of graphene by itself and in combination that help better understanding of physical and electronic properties for nano‐/optoelectronics. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
3.
M. Hidaka N. Ohama A. Okazaki H. Sakashita S. Yamakawa 《Solid State Communications》1975,16(9):1121-1124
The structural and magnetic phase transitions in KMnF3 have been examined by means of neutron diffraction. The structural transition at 91 K accompanied with the M3 soft-mode phonon condensation shows typical second-order character. The transition temperature and characterization of other structural and magnetic transitions are also given. 相似文献
4.
Eeuwe S. Zijlstra Tobias Zier Bernd Bauerhenne Sergej Krylow Philipp M. Geiger Martin E. Garcia 《Applied Physics A: Materials Science & Processing》2014,114(1):1-9
By exciting or heating electrons, ultrashort laser pulses have a direct influence on bond strengths in two- and three-dimensional solids. Here, we present results of ab initio molecular dynamics simulations performed using our in-house Code for Highly-excIted Valence Electron Systems (CHIVES) for three systems, which each shows a distinctly different structural response to a femtosecond laser pulse. In solid silicon, we show that ultrafast laser-induced bond breaking leads to nonthermal melting, a process which occurs in three stages, involving subsequently superdiffusive, fractionally diffusive, and normally diffusive atomic motions. For TiO $_2$ , we find that the A $_{1g}$ phonon is coherently excited. At room temperature, we demonstrate that these oscillations are strongly coupled to other phonon modes. In graphene with a single Stone–Wales defect, we study the in-plane and out-of-plane laser-induced atomic motions and find bond breaking, which destroys the structure, when the electrons are heated to at least 31,000 K. 相似文献
5.
NaNbO3 and (Sr,Ca)TiO3 exhibit an unusual complex sequence of temperature- and pressure-driven structural phase transitions. We have carried out
lattice dynamical studies to understand the phonon modes responsible for these phase transitions. Inelastic neutron scattering
measurements using powder samples were carried out at the Dhruva reactor, which provide the phonon density of states. Lattice
dynamical models have been developed for SrTiO3 and CaTiO3 which have been fruitfully employed to study the phonon spectra and vibrational properties of the solid solution (Sr,Ca)TiO3.
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6.
7.
ABSTRACTSemimetal bismuth (Bi) is known to possess a wide range of peculiar properties, owing to its unique electronic band structure. Its electronic band can easily be distorted by structural changes, and thereby undergo transitions between semimetal to either semiconductor or metal states. Utilising a focused laser beam, one can easily introduce structural defects, along with phase changes, oxidation, and morphological modifications. Confocal Raman microscopy indicated that the as-fabricated Bi droplets inhibit the Raman signal from the underlying silicon (Si) substrate. After a laser flash heating step, the intensity of Si optical phonons was strongly enhanced at the positions of Bi droplets, and exceeding the intensity from the bare Si substrate. Thus, such laser irradiating step on the Bi droplets induces an optical phase change. The optical phase change was detected as going from inhibition to strong enhancement of the underlying Si substrate Raman signal. From the observed Bi optical phonon modes (Eg and A1g), alterations in the Raman peaks due to laser exposure indicated that the ordered crystallinity in pristine Bi droplets became deteriorated. The effects of atomic displacements and loss of structural order in Bi droplets impacts its dielectric response. The observed Si Raman signal enhancement is similar to the surface-enhanced Raman scattering effect typically known for noble metals. 相似文献
8.
Y. Lépine 《Solid State Communications》1982,43(5):375-378
The Peierls and spin-Peierls phase transitions are studied in solids in which a structural instability is already present. It is found that the presence of this intrinsic mode can increase considerably the critical temperature. For small values of the critical temperature, the transition is of the BCS-type, like the Peierls (or spin-Peierls) phase transition, but with an effective electron (or spin)-phonon coupling constant renormalized by the anharmonicity and by the instability of the phonon. Numerical results are also presented for larger critical temperatures. Then the BCS behaviour is no longer observed. 相似文献
9.
We investigate temperature and concentration driven phase transitions (structural and reentrant phase transitions included) in magnetic and superconducting systems with the use of a wide class of model Hamiltonians applied to rare earth (Re) based compounds and alloys (integer and fluctuating valence systems). Studying the temperature or concentration dependence of the chemical potential we observe small but distinct and well localized kinks at all critical points as evidence for phase transitions. For systems with, at least, two kinds of interacting electrons the kinks at critical temperatures or concentrations occur also in the electronic average occupation numbers (critical electron redistribution). These observations suggest a direct and universal experimental application of the chemical potential as a detector of phase transitions for temperature and concentration driven phase transitions, as well as, for pressure-or external field-induced transitions in solids. The agreement between the calculated critical temperature behaviour of the chemical potential, presented in this paper, and experimental measurements for high-temperature superconductor YBa2Cu3O7-δ entirely supports these general observations. 相似文献
10.
I. A. Damaskin S. L. Pyshkin S. I. Radautsan V. E. Tazlavan 《Optical and Quantum Electronics》1973,5(5):405-410
A study has been made of photoconductivity in Cdln2S4 induced by 1.17 eV photons from a Q-switched neodymium laser. Both three-photon absorption, and two-photon absorption involving the emission of a phonon, have been investigated. The experimental data are in satisfactory agreement with the theory of multi-quantum transitions which takes into account the peculiarities of crystal zone structure.The investigation of competing multi-quantum transitions is a new method which permits one to obtain information on the fundamental optical constants of solids. 相似文献
11.
Raman spectra of Cs2ZnI4 single crystal have been measured in different scattering orientations covering the successive phase transitions down to 68 K. Two second order anomalies at around ice temperature (270 K) and at ~ 92 K are observed. The anomaly around ice temperature exhibits the features of a normal-incommensurate transition evidenced by softening of a low-lying lattice phonon whose frequency decreases as the transition temperature is approached from below whereas the latter one appears to arise from usual structural distortions. 相似文献
12.
Three phase transitions (face centered to simple cubic, surface rearrangement from (2×2) to (1×1) and glass transition) in fullerene thin films and the effect of 150 MeV Ti ion irradiation on these transitions have been studied, using temperature dependence of electrical resistivity measurements. The structural properties of the C60 thin films are studied by X-ray diffraction, atomic force microscopy and UV-vis spectroscopy. It is observed that defect creation by ion irradiation in the films lead to quenching and broadening of structural phase transitions but the transition temperatures did not show significant shifting under ion irradiation. 相似文献
13.
Theoretical progress in the research of photoinduced phase transitions is reviewed with closely related experiments. After a brief introduction of stochastic evolution in statistical systems and domino effects in localized electron systems, we treat photoinduced dynamics in itinerant-electron systems. Relevant interactions are required in the models to describe the fast and ultrafast charge-lattice-coupled dynamics after photoexcitations. First, we discuss neutral-ionic transitions in the mixed-stack charge-transfer complex, TTF-CA. When induced by intrachain charge-transfer photoexcitations, the dynamics of the ionic-to-neutral transition are characterized by a threshold behavior, while those of the neutral-to-ionic transition by an almost linear behavior. The difference originates from the different electron correlations in the neutral and ionic phases. Second, we deal with halogen-bridged metal complexes, which show metal, Mott insulator, charge-density-wave, and charge–polarization phases. The latter two phases have different broken symmetries. The charge-density-wave to charge–polarization transition is much more easily achieved than the reverse transition. This is clarified by considering microscopic charge-transfer processes. The transition from the charge-density-wave to Mott insulator phases and that from the Mott insulator to metal phases proceed much faster than those between the low-symmetry phases. Next, we discuss ultrafast, inverse spin-Peierls transitions in an organic radical crystal and alkali-TCNQ from the viewpoint of intradimer and interdimer charge-transfer excitations. Then, we study photogenerated electrons in the quantum paraelectric perovskite, SrTiO3, which are assumed to couple differently with soft-anharmonic phonons and breathing-type high-energy phonons. The different electron–phonon couplings result in two types of polarons, a “super-paraelectric large polaron” with a quasi-global parity violation, and an “off-center-type self-trapped polaron” with only a local parity violation. The former is equivalent to a charged and conductive ferroelectric domain, which greatly enhances both the quasi-static electric susceptibility and the electric conductivity. Finally, we outline the development of time-resolved X-ray diffraction experiments, which directly accesses the dynamics of electronic, atomic and molecular motions in photoexcited materials. They are extremely useful when a three-dimensional structural long-range order is established and changes the symmetry. 相似文献
14.
The triple-axis neutron scattering technique has been used to observe the soft-phonon modes associated with the second-order structural phase transitions in the cubic antifluorite K2ReCl6. The transverse (ReCl6)?2 rotary mode has been found to be soft all along the [ζ00] direction, with the structural distortions being triggered by a condensation first at the zone center and then at the [100] zone boundary. In addition to the soft phonon modes there is a temperature dependent central component to the spectrum along the [ζ00] direction. 相似文献
15.
S. Yakovlev C.-H. Solterbeck E. Skou M. Es-Souni 《Applied Physics A: Materials Science & Processing》2006,82(4):727-731
Er substituted lead titanate thin films were processed via chemical solution deposition. Their structural properties were
investigated by means of atomic force microscopy, X-ray diffractometry and micro-Raman spectroscopy. The Curie temperature,
TC, was obtained from dielectric measurements. The results show that the grain size, tetragonality factor, c/a, E(1TO) transverse
optical phonon frequency and the ferroelectric-to-paraelectric phase transition are affected by Er doping. Particularly the
lattice tetragonality and Curie temperature of Er substituted films are found to be substantially lower in comparison to undoped
specimen. In contrast, the E(1TO) phonon frequency is only slightly reduced by Er doping. The Results obtained for pure and
Er substituted specimens are discussed in terms of combined effects of grain size, stress development and doping on the structural
characteristics.
PACS 68.55.-a; 77.80.Bh; 78.30.Hv 相似文献
16.
Lihong NiYong Liu Chenlu SongWei Wang Gaorong HanYankai Ge 《Physica B: Condensed Matter》2011,406(21):4145-4149
First-principle calculations were carried out to investigate the epitaxial strain dependence of the structural stability and related properties of SrTiO3. The P4mm-to-P4/mmm-to-Amm2 phase transitions in SrTiO3 with the epitaxial strain changing from compressive to expansive have been identified by analysis of the soft-mode eigendisplacement. The zero-temperature critical strains for the P4mm-to-P4/mmm and P4/mmm-to-Amm2 phase transitions are −0.72% and 0.83%, respectively, which is consistent with the previous predictions based on the phenomenological thermodynamic model and the parameterized total-energy model. The influences of the epitaxial strain on the dielectric tensor and spontaneous polarization along the phase transition path are discussed in terms of charge transfer. Further investigations on the vibration modes show unusual softening behaviors of the A1 and B2 modes in the Amm2 phase, which may be associated with the soft phonon modes away from Γ point and possibly result in other phase transitions unreported by any previous studies. 相似文献
17.
《Solid State Communications》1986,60(9):763-766
We report hiogh resolution thermal expansion measurements between 1.2 and 300 K along the c1-axis in the organic conductor β-(BEDT-TTF)2I3 where the superconducting properties are strongly dependent on the way the system is prepared in the pressure-temperature diagram. We clearly observed a strong anomaly at 172 K related to the structural phase transition and the resistivity anomalies reported by several groups. The temperature analysis in the low temperature regime is coherent with a phonon dominant contribution. Finally, the presence of small hysteresis in a large region around the transitions seem to indicate why both the pressure and the temperature cycling may stabilize small regions of the high-Tc phase of the system, as observed. 相似文献
18.
E. Bucher J. P. Maita G. W. Hull Jr. L. D. Longinotti B. Lüthi P. S. Wang 《Zeitschrift für Physik B Condensed Matter》1976,25(1):41-48
We have investigated elastic, magnetic, thermal and structural properties of systems which exhibit induced magnetic behavior (TbP, PrSn3, Pr3Se4, Pr3Te4). Particular emphasis is given to phonon effects due to the crystalline electric field, phase transitions, and magnetic fields. TbP has a first order magnetic phase transition and both TbP, PrSn3 show a temperature dependence of the elastic constants, which can be explained with the known crystal field level scheme. The compounds Pr3S4, La3S4, Pr3Se4 exhibit structural instabilities at much higher temperatures than the magnetic or superconducting transition temperatures. Pr3Te4 does not exhibit a structural transition, but a strong elastic softening in a magnetic field. This effect is due to strong field dependent quadrupole transitions. All these systems are analyzed with respect to typical induced moment parameters. 相似文献
19.
Yizhak Yacoby Yakov Girshberg Edward A. Stern 《Zeitschrift für Physik B Condensed Matter》1997,104(4):725-731
Xray Absorption Fine Structure (XAFS) measurements of the local atomic structure of perovskite crystals undergoing various structural phase transitions are summarized and discussed. The results show that the local structure of crystals undergoing ferroelectric antiferroelectric and antiferrodistortive transitions is distorted in a disordered fashion far above the transition to the high symmetry phase. The size of the distortions is a large fraction of the distortion at temperatures far below T c. Based on these results we propose a model of ferroelectricity which accounts quantitatively for the temperature dependence of the dielectric function the soft mode frequency, the imaginary part of the dielectric constant and the central peak in PbTiO3 and KNbO3. 相似文献
20.
Varghese Swamy Leonid S. Dubrovinsky Falko Langenhorst Michael Drakopoulos Hans-Peter Weber 《Solid State Communications》2005,134(8):541-546
High-pressure structural transitions in nanocrystalline systems are of significant interest as models of first-order phase transitions. We demonstrate size-induced lattice expansion and significant atomic rearrangements in the crystal structure of nanocrystalline high-pressure baddeleyite-TiO2. The α-PbO2 structured TiO2 recovered after dozens of pressure cycles in the α-PbO2-baddeleyite pressure field displayed elongate 25-35 nm crystallites, compared to starting 34-nm anatase crystallites, suggesting crystallite coherency across anatase, baddeleyite, and α-PbO2 structures and ‘single structural domain’ behavior of the nanocrystalline system. 相似文献