NMR-NQR studies of structural phase transitions

Moving dislocations in II–VI semiconductors carry a large electric charge. This charge is not in thermal equilibrium, but is due to the sweeping up of electrons from point defects. Its movement produces a dislocation current during plastic deformation, and conversely, the application of an external field changes the flow stress. This paper reviews the structure and properties of these dislocations, the theory of their charge and the phenomena which are a consequence of the strong mutual interactions of the dislocation and electronic sub-systems in these crystals. The materials show a large photoplastic effect (a change in flow stress under illumination), and related effects due to the injection of electrons at an electrode. Deformation produces reversible changes in the conductivity, pulsed and continuous luminescence and the emission of electrons from the surface.     

On structural phase transitions from a hypothetical phase

Examples known so far of structural phase transitions which can be treated as an instability of a hypothetical parent phase are enumerated. Physical requirements for being reasonable to consider such hypothetical phase are pointed out. The possibility of using this concept for explanation of successive structural phase transitions in Rochelle salt and langbeinites is discussed.The author thanks Dr. V.Janovec for many valuable remarks to this paper.     

Theory for photoinduced structural phase transitions

A general concept for photoinduced structural phase transitions is developed in terms of the hidden multistability of the ground state and the proliferations of optically excited states. Taking the ionic→neutral (I - N) phase transition in the organic charge transfer crystal, TTF—CA, as a typical example for this type of transition, we, at first, theoretically show an adiabatic path of this transition, which starts from a single charge transfer exciton in the ionic phase, but finally reaches a neutral domain with a macroscopic size. In connection with this I—N transition, the concept of the initial condition sensitivity is also developed so as to clarify experimentally observed nonlinear characteristics of this material. In the next, using a more simplified model for the many-exciton system, we theoretically study the early time quantum dynamics of the exciton proliferation, which finally results in the domain formation of a large number of ex-citons. For this purpose, we derive a stepwise iterative equation to describe the exciton proliferation, and clarify the origin of the initial condition sensitivity.     

Acoustic dispersion near structural phase transitions

We show on various examples, that elastic constant measurements provide a valuable tool for studying the dynamics of solids near phase transitions. Many experimental methods used in the investigation of phase transitions (NMR, neutron scattering, etc.) are able to give information on the dynamics, but in different regions of q- and ω-space, thereby probing different dynamical processes.

A comparison of the dynamics obtained from the elastic measurements with other techniques (NMR) demonstrates this very clearly for the cases of KSCN and C_60.     

Anharmonic phonons and structural phase transitions

Exact results for the order parameter and the meansquare displacement as functions of temperature are given for a quantum interacting phonon Hamiltonian with quartic anharmonicities. Upper bounds for the transition temperature are also derived. Approximate theories including the mean field approximation, the random phase approximation (or quasiharmonic approximation) and the self consistent approach (using Blume-Hubbard scheme) are compared with our exact results. The mean field approximation for the meansquare displacement is found to violate our bounds.The classical value is shown to form a lower bound for the kinetic energy. Upper bounds for the kinetic energy are obtained showing the region of temperature in which the use of the high temperature expansion of the fluctuation-dissipation theorem is justified. Comparison of the Hamiltonian and our results with electron-paramagnetic-resonance measurements are discussed.     

Hierarchical microstructure in structural phase transitions

We consider a model in the context of martensitic materials in which hierarchical twinning near the habit plane (austenite-martensite interface) is a new and crucial ingredient. The model includes (1) a triple-well potential in local deviatoric (rectangular) strain, (2) strain gradient terms up to second order in strain and fourth order in gradient, and (3) all symmetry allowed compositional fluctuation-induced strain gradient terms. The last term favors branching of domain walls which enables communication between macroscopic and microscopic regions essential for shape memory. Below the transition temperature (T₀) we obtain the conditions under which branching of twins is energetically favorable. Above T₀ a hierarchy of branched domain walls also stabilizes tweed formation (criss-cross patterns of twins). External stress or pressure modulates (“patterns”) the spacing of domain walls. Results based on 2D time-dependent Ginzburg-Landau simulations are shown for twins, tweed and hierarchy formation.     

Lattice dynamics and structural phase transitions

Results of lattice dynamics, or atomic motions in a solid, explain many of the thermodynamic properties of solids. Inelastic neutron scattering conveniently explores the atomic motions, quantized as phonons. Of particular interest are materials that undergo structural phase transitions. The soft mode theory has been successful in relating anomalous phonon behavior to structural changes in solids. One such example is the ferromagnetic shape memory alloy, Ni₂MnGa, which undergoes a sequence of phase transitions leading to a magnetic, incommensurate modulated, tetragonal phase as the ground state. The experiments, coupled with first principles calculations, provide evidence that strong electron–phonon coupling is the driving mechanism of the phase transformation.     

Secondary order parameters in the structural phase transitions

In the Landau type phase transitions the primary order parameter is related with irreducible representation which subduces the high symmetry space groupG _o to the low symmetry space subgroupG andG _o G. We propose to define the secondary order parameters as irreducible representations which subduceG _o to the subgroupsH _l , whereH _l are simultaneously supergroups forG, i.e.G _o H _l G. From this statement general conclusions are drawn. In particular, for some phase transitions with elargement of the elementary cell, in addition to the superlattice reflections from the primary order parameter, the superlattice reflections from the secondary order parameters can arise in another non-equivalent high symmetry point of the reciprocal space.     

High pressure structural phase transitions of PbPo

First-principles calculations have been performed to investigate the high pressure phase transitions and dynamical properties of the less known lead polonium compound. The calculated ground state parameters for the NaCl phase show good agreement with the experimental data. The obtained results show that the intermediate phase transition for this compound is the orthorhombic Pnma phase. The PbPo undergoes from the rocksalt to Pnma phase at 4.20 GPa. Further structural phase transition from intermediate to CsCl phase has been found at 8.5 GPa. In addition, phonon dispersion spectra were derived from linear-response to density functional theory. In particular, we show that the dynamical properties of PbPo exhibit some peculiar features compared to other III–V compounds. Finally, thermodynamics properties have been also addressed from quasiharmonic approximation.     

Molecular dynamics investigation of structural phase transitions

A two dimensional model exhibiting a structural phase transition is described, and results of molecular dynamics calculations on the model are presented. The static properties indicate a second order phase transition and show the growth of large ordered regions as the transition is approached. The spectral function for the order parameter autocorrelation function has an intense central peak near the transition.     

Nematic phase transitions in mixtures of thin and thick colloidal rods

We report experimental measurements of the phase behavior of mixtures of thin (charged semiflexible fd virus) and thick (fd-PEG, fd virus covalently coated with polyethylene glycol) rods with diameter ratio varying from 3.7 to 1.1. The phase diagrams of the rod mixtures reveal isotropic-nematic, isotropic-nematic-nematic, and nematic-nematic coexisting phases with increasing concentration. In stark contrast to predictions from earlier theoretical work, we observe a nematic-nematic coexistence region bound by a lower critical point. Moreover, we show that a rescaled Onsager-type theory for binary hard-rod mixtures qualitatively describes the observed phase behavior.     

Reconstructive structural phase transitions in dense Mg

The question raised recently about whether the high-pressure phase transitions of Mg follow a hexagonal close-packed (hcp) → body centered cubic (bcc) or hcp → double hexagonal close-packed (dhcp) → bcc sequence at room temperature is examined by the use of first principles density functional methods. Enthalpy calculations show that the bcc structure replaces the hcp structure to become the most stable structure near 48 GPa, whereas the dhcp structure is never the most stable structure in the pressure range of interest. The characterized phase-transition mechanisms indicate that the hcp → dhcp transition is also associated with a higher enthalpy barrier. At room temperature, the structural sequence hcp → bcc is therefore more energetically favorable for Mg. The same conclusion is also reached from the simulations of the phase transitions using metadynamics methods. At room temperature, the metadynamics simulations predict the onset of a hcp → bcc transition at 40 GPa and the transition becomes more prominent upon further compression. At high temperatures, the metadynamics simulations reveal a structural fluctuation among the hcp, dhcp, and bcc structures at 15 GPa. With increasing pressure, the structural evolution at high temperatures becomes more unambiguous and eventually settles to a bcc structure once sufficient pressure is applied.     

Light scattering studies on structural phase transitions

A method of analyzing the overdamped polariton spectra have been developed. Polariton dispersion curves of the soft E-mode in BaTiO₃ have been obtained. By observing polariton peaks of KDP, low-frequency B₂(z) response has been concluded to be oscillatory. Pure E-TO plateau response of KDP has been attributed to the proton motion of E-symmetry in x-y plane. The phase transition in LiNH₄C₄H₄O₆·H₂O has been identified to be the intrinsic ferroelastic transition, by observing a soft acoustic phonon and by comparing with KDP. High pressure polarized Raman spectra of a single domain BNN crystal have been observed using a diamond anvil cell.     

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.     

Propagation of sound at continuous structural phase transitions

Structural phase transitions of second order can be divided into two groups: (i) distortive phase transitions, with a soft (ultimately overdamped) optic mode, and (ii) elastic phase transitions, with an acoustic soft mode or no soft phonon for shear or isostructural transitions, respectively. The propagation of sound shows significantly different features in these two cases. We consider the theory of the critical variation of the velocity of ultrasonic modes as well as the damping and dispersion near transitions of second order.Talk given at the Conference on Transport and Propagation in Nonlinear Systems, Los Alamos, May 21–25, 1984.     

Jahn-Teller solitons, structural phase transitions, and phase separation

It is demonstrated that under common conditions a molecular solid subject to Jahn-Teller interactions supports stable Q-ball-like nontopological solitons. Such solitons represent a localized lump of excess electric charge in periodic motion accompanied by a time-dependent shape distortion of a set of adjacent molecules. The motion of the distortion can correspond to a true rotation or to a pseudorotation about the symmetric shape configuration. These solitons are stable for Jahn-Teller coupling strengths below a critical value; however, as the Jahn-Teller coupling approaches this critical value, the size of the soliton diverges signaling an incipient structural phase transition. The soliton phase mimics features commonly attributed to phase separation in complex solids.