Paraelectric phenomena

Bridges, F.

doi:10.1080/10408437508243475

Cited by 145 publications

(23 citation statements)

References 142 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…They are covered by some detailed review articles (LOTY, 1967;NARAYANAMURTI and POHL, 1970;BRIDGES, 1975;PRESS, 1981), so we do not enter this field any further in this article. g) Internal vibrations of molecular defects.…”

Section: R---~----~---'-----r----r---~--~mentioning

confidence: 98%

See 1 more Smart Citation

Vibrational Infrared and Raman Spectra of Non-Metals

Bilz¹,

Strauch²,

Wehner³

1984

Light and Matter Id / Licht Und Materie Id

View full text Add to dashboard Cite

The vibrational properties of crystals determine the photon infrared absorption, inelastic neutron scattering and, to a large extent, inelastic photon scattering by phonons, i.e., Raman scattering. The interpretation of infrared and Raman spectra requires, therefore, an understanding of the basic features of lattice dynamics. The quantum theory of solids describes the crystal properties in terms of elementary excitations and their mutual interactions. Dynamic properties are represented by phonons (lattice vibrations) and their interactions mainly with other phonons (anharmonicity), electrons (electron-phonon coupling), and photons (interaction with radiation). This characterizes the scope of the article. Its emphasis is on the interrelation between theory and experiment, i.e., on the microscopic or model interpretation of experimental spectra.Work on infrared absorption and Raman scattering of non-metallic solids was summarized twenty years ago in two review articles in this Encyclopedia by LECOMTE (1958) and by MIZUSHIMA (1958). Since then, many important new experimental data have become available, thanks to very refined techniques for measuring spectra of pure and imperfect single crystals. Furthermore, the theory of the interactions of phonons and photons has been much improved, stimulated by the remarkable development of the theory of lattice vibrations and the mathematical techniques of many-body physics.In this article we describe the dispersion and absorption of infrared radiation and Raman scattering in non-metallic crystals, both perfect and containing point defects. At present, quantitative calculations are usually restricted to diatomic crystals, especially those with simple structures such as the alkali halides or germanium and its homologues, since our knowledge of lattice vibrations is still rather poor for polyatomic and low-symmetry crystals. The theory of lattice vibrations, as reviewed by COCHRAN and COWLEY (1967) in Vol. XXV /2a of this Encyclopedia, constitutes the background and the natural starting point for our investigations. There are other recent reviews by LUDWIG (1967), COCHRAN (1971), MARADUDIN et al. (1971), SINHA (1973), and several articles in: HORTON and MARADUDIN (eds., 1974). A summary of the developments during the last few years is given in Chap. B.An important part of the theory of lattice vibrations is the construction of models. A good example is the so-called shell model for phonons (see Sect. 4) which describes the adiabatic linear electron-ion interaction in terms of localized charges and coupling constants. This provides a natural explanation of some long-range ion-ion forces in insulators in terms of induced dipole forces. Anharmonic extensions of this shell model and its modifications seem very desirable, and first attempts in this direction are discussed.Models can often give a qualitatively and sometimes quantitatively correct description of certain processes in terms of a few parameters. They provide, therefore, an orientation for microscopic approaches. Furthermo...

show abstract

Section: R---~----~---'-----r----r---~--~mentioning

confidence: 98%

“…Electric-field effects have been investigated mainly for electronic transitions and for paraelectric impurities (for the latter see the review by BRIDGES, 1975). Electric-field-induced frequency shifts of localized modes were first observed by HAYES et al (1965b) andMACDONALD (1967).…”

Section: Llrr(wr +W+»llrr(wr -W_)mentioning

confidence: 98%

Vibrational Infrared and Raman Spectra of Non-Metals

Bilz¹,

Strauch²,

Wehner³

1984

Light and Matter Id / Licht Und Materie Id

View full text Add to dashboard Cite

show abstract

“…We have studied one of the simplest systems, namely, KC1 doped with lithium. From many previous experiments it is well known that the small lithium ions replacing the cations of the host crystal are located in off-center positions in the (111) directions [1,2]. At low temperatures the Li+ ions tunnel between these sites and form eight-level systems.…”

Section: Isotope Effect Of Pairs Of Coupled Tunneling Systemsmentioning

confidence: 99%

Isotope Effect of Pairs of Coupled Tunneling Systems

et al. 1995

View full text Add to dashboard Cite

Using dielectric rotary echo experiments we have studied the coherent tunneling of strongly coupled Li+ ions in KC1 crystals. Since our experiments were performed at resonance frequencies of about 1 GHz, we have investigated tunneling systems with energy splittings E/ke ( 50 mK. Measurements of the Rabi frequency for samples doped with different lithium isotopes give a conclusive proof that the low-energy states arise from pairs of coupled Li+ ions. We discuss the tunnel splitting and the microscopic arrangement of such pairs. PACS numbers: 77.22.d, 61.72.Ji The low temperature dynamical behavior of dielectric solids is determined to a large extent by the presence of defects. At high temperatures the motion of defects is thermally activated, but at low temperatures tunneling is the dominating mechanism. Therefore tunneling systems (TSs) formed by defects are responsible for most elastic, dielectric, and thermal properties of dielectrics. Particularly, alkali halides containing a small concentration of point defects have been studied extensively, and their properties are well understood on a microscopic basis: Tunneling of individual impurities between several equivalent sites or orientations give rise to an excitation spectrum with energies typically of the order of F/ks --1 K. Together with selection rules due to the symmetry of the host crystal their low temperature dielectric behavior can be described quantitatively [1 -4]. With rising concentration, interaction between the defects becomes important. This is clearly demonstrated by many experiments on crystals like KBr:CN containing a higher amount of impurities or on glasses where TSs again are responsible for their unusual properties. In all these cases the consequences of the interaction are hardly understood. At concentrations low enough that most of the TSs can still be considered as isolated the properties interacting with TSs have been described by taking into account the statistical occurrence of coupled pairs [5,6]. The complex many body problem at high concentrations has been treated only very recently using a mode coupling approach [7]. On the experimental side the influence of interactions between substitutional molecules, such as OH in KC1, on their vibrational spectrum has been investigated in detail [8]. Moreover, evidence of a common tunneling motion of defects coupled in pairs has been found in several experiments (e.g., Refs. [9 -11]). However, a microscopic understanding of the energy spectrum and the dynamics of these coupled tunneling systems is lacking. We have studied one of the simplest systems, namely, KC1 doped with lithium. From many previous experiments it is well known that the small lithium ions replacing the cations of the host crystal are located in off-center positions in the (111) directions [1,2]. At low temperatures the Li+ ions tunnel between these sites and form eight-level systems. Because of the high symmetry of the host crystal, the eigenstates are partially degenerate. The resulting four levels are equally spaced with ...

show abstract

“…Nevertheless harmonic models have also failed to provide any reliable interpretation to important experimental observations for off-centre ions, the most obvious of which perhaps are the anomalous isotope effects and the peculiarities of lower-symmetry F A centres. Earlier work on para-electric off-centre impurities has been reviewed by Bridges [3], while that on FA centres is described at length by Luty [4]. The lack of any convincing theory has resulted in numerous attempts to address the matter in terms of phenomenological ,(rigid-sphere ion models,.…”

Section: -Introductionmentioning

confidence: 99%

Revisited vibronic model for Li+ ion and F A (Li) centre in alkali halides

et al. 1991

View full text Add to dashboard Cite

(ricevuto fl 23 Maggio 1991; approvato fl 17 Giugno 1991)Summary. --The controversial far-infrared optical absorption in Li-doped alkali halides is now attributed to transitions between the low-lying energy levels of an anharmonic oscillator composed of three halogen-ion pairs centred at the cation site which vibrate in the Tlu ungerade mode. This oscillator is electronically coupled to the impurity dragging it into rotation over < 111) sites. Several experimental features derive simply from our model such as the absence of any clear-cut lithium-isotope effect and the low-frequency range. The capital effect of an F centre at a (001 > nearest-neighbour (nn) site is to reduce the dimensionality of vibration forcing the impurity to rotate in the perpendicular (001)-plane. This upgrades the optical frequency in proportion to (3/2) 1/2 and shrinks the off-centre displacement. We finally show that recent computer-simulation data on Li-doped alkali halides provide a solid ground for regarding the Li off-centredness as a pseudo-Jahn-Teller phenomenon.PACS 78.50 -Impurity and defect absorption in solids.

show abstract

Paraelectric phenomena

Cited by 145 publications

References 142 publications

Vibrational Infrared and Raman Spectra of Non-Metals

Vibrational Infrared and Raman Spectra of Non-Metals

Isotope Effect of Pairs of Coupled Tunneling Systems

Revisited vibronic model for Li+ ion and F A (Li) centre in alkali halides

Contact Info

Product

Resources

About