First-principles study of the mechanisms of the pressure-induced dielectric anomalies in ferroelectric perovskites

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.     

Multiphonon Raman spectroscopy properties and Raman mapping of 2D van der Waals solids: graphene and beyond

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.     

Femtosecond-laser-induced bond breaking and structural modifications in silicon, TiO_2, and defective graphene: an ab initio molecular dynamics study

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.     

A comment on the phase transitions in KMnF3

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.     

Inelastic neutron scattering and lattice dynamics of NaNbO<Subscript>3</Subscript> and Sr<Subscript>0.70</Subscript>Ca<Subscript>0.30</Subscript>TiO<Subscript>3</Subscript>

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.      

Optical phase change in bismuth through structural distortions induced by laser irradiation

ABSTRACT

Semimetal 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 (E_g and A_1g), 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.     

Peierls and spin-Peierls phase transitions in structurally unstable quasi-one-dimensional solids

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.     

On the new universal possibility to detect phase transitions in correlated electron systems

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 YBa₂Cu₃O_7-δ entirely supports these general observations.     

Multi-quantum photoconductivity in Cdln2S4

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.     

Raman scattering study of the phase sequence in A2BX4 halides

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.     

Electrical transport study of structural phase transitions in C60 films and the effect of swift heavy ion irradiation

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.     

Theory of photoinduced phase transitions in itinerant electron systems

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.     

Structural and dielectric properties of Er substituted sol-gel fabricated PbTiO3 thin films

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, T_C, 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     

Soft rotary mode and structural phase transitions in K2ReCl6

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.     

First-principle study of strain-driven phase transition in incipient ferroelectric SrTiO3

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.     

Lattice effects in singlet ground state systems

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.     

自旋弛豫时间测定对固体代谢物分子力学性质的研究

自旋弛豫时间测定是研究固体物质分子动力学性质的有效手段,已广泛应用于植物糖、氨基酸、维生素、肾上腺素以及睾丸激素等多类代谢物分子动力学性质的研究中. 近来上述方法也被用于其他代谢物分子动力学性质与其多晶型相变的关系以及对代谢物结构依赖性的研究. 该文将在介绍自旋弛豫时间及其测定方法的基础上,着重对自旋弛豫时间测定研究植物糖、氨基酸及其衍生物分子动力学性质的应用工作进行讨论.     

Spontaneous local distortions and structural phase transitions

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.     

Lattice vibrations in ordered and disordered thiourea

Raman scattering and infrared reflection spectra of single crystals of thiourea were recorded at several temperatures between 2 K and 240 K in the spectral region (03500) cm^–1. The intention was to search for changes in frequency, linewidth, and intensity of optical phonon modes near the various phase transitions which are known to exist in this material. All observed phonon modes have been tabulated according to a group-theoretical enumeration, and they are attributed to internal motions of the molecular groups and external lattice vibrations. A survey of the temperature dependence of all observed lines is given.Some modes are in fact influenced by the phase transitions in a characteristic manner. The observed temperature dependences can be interpreted by a simple pseudospinphonon coupling model (following contribution II).