Covalency and Deformability of Ag + ‐Ions in the Lattice Dynamics of Silver Halides

The dispersion relation for the normal modes of vibration in AgBr have been determined a t 80 and 295 K along symmetry directions. The neutron inelastic scattering experiments have been performed on a triple axis spectrometer a t the "Diorit" reactor. The phonon frequencies show a strong temperature dependence. The dispersion curves can be well fitted by an eleven-parameter model with deformable electronic shells of the silver ions.In Silberbromid wurden die Frequenzen der Normalschwingungen bei 80 und 295 K entlang der Symmetrierichtungen durch unelastische Neutronenstreuung bestimmt. Die Experimente wurden mit einem Dreiachsen-Spektrometer am Ileaktor ,,Diorit" durchgefiihrt. Die Dispersionskurven zeigen eine sehr starke Temperaturabhangigkeit. Die gemessenen E'requenzen konnen durch ein Elf-Parameter-Model1 mit deformierbaren Elektronenschalen der Silber-Ionen gut beschrieben werden.

Section: (3)mentioning

confidence: 85%

Crystal dynamics of silver bromide at 80 and 295 K

Bührer

1975

“…The effect of the quadrupolar deformation on longitudinal modes is very small. Other workers such as Uolonin and Tolpygo [14] and Fischer et al [15] working on quadrupolar deformation effects have also found a significant contribution to TO frequencies in the (111) direction for crystals with NaCl structure.…”

Section: Application and Resultsmentioning

confidence: 90%

Extended Deformable Shell Model Calculation of the Lattice Statics and Dynamics of LiH and LiD Crystals

Ghosh

Basu

Sengupta

1981

An attempt is made for a unified study of the lattice statics and dynamics of the LiH-LiD crystals. So far it has not been possible to obtain a unified description of the different properties of this crystal on the basis of a single model and with a single set of model parameters. All the previous calculations suffer from this inadequacy and their results show that there are certain problems regarding the stability of the static lattice structure, the cohesive energy, and a consistent description of the dielectric properties and the dispersion of phonons for this crystal. The present calculation based on the extended deformable shell model removes the earlier difficulties and provides a good overall description of the lattice mechanical properties of this crystal with a single set of parameters. Another salient feature of the present investigation is to indicate the effect of the quadrupolar distortion of the charge cloud, in particular, of the hydrogen ion which has been neglected in all the earlier investigations of the lattice mechanics of these crystals. The calculation clearly points out that without including this effect it is not possible to obtain a coherent description of the lattice mechanics in all its totality. Some qualitative justification for this effect being significant for the crystals under consideration is also presented.Es wird versucht, die Gitterstatik und -dynamik von LiH-LiD-Kristallen von einem einheitlichen Standpunkt aus zu untersuchen. Bisher war es nicht moglich, eine einheitliche Beschreibung der verschiedenen Eigenschaften dieses Kristalls auf der Grundlage eines einheitlichen Modells und eines einheitlichen Satzes von Modellparametern zu erhalten. Alle friiheren Berechnungen leiden darunter und ihre Ergebnisse zeigen, daB bestimmte Probleme beziiglich der Stabilitiit der statischen Gitterstruktur, der Kohiisionsenergie und einer konsistenten Beschreibung der dielektrischen Eigenschaften und der Phononendispersion fur diesen Kristall existieren. Die dargelegte Rechnung auf der Grundlage des erweiterten Modells deformierbarer Schalen beseitigt die friiheren Schwierigkeiten und liefert eine gute Gesamtbeschreibung der gittermechanischen Eigenschaften dieses Kristalls mit einem einzigen Parametersatz. Ein anderes herausragendes Merkmal der Untersuchung ist das Aufzeigen der Quadrupolverzerrung der Ladungswolke insbesondere des Wasserstoffions, die bisher in allen friiheren Untersuchungen der Gittermechanik dieser Kristalle vernachlassigt wurde. Die Rechnung zeigt klar, daS es ohne Einbeziehung dieses Effekts nicht moglich ist, eine koharente Beschreibung der Gittermechanik in ihrer Gesamtheit zu erzielen. Eine qualitative Rechtfertigung fur diesen Effekt, der fur die betrachteten Kristalle von Bedeutung ist, wird ebenfalls dargelegt.

“…While the model treatment assumes an ionic bonding with tetrahedral symmetry the silver halides possess a considerable degree of covalency. Fischer et al [14] considered the deformability of the d-shell of the Ag+ ion and its covalent coupling to the neighbours. In particular, they introduced a quadrupolar and a rotational degree of freedom into their extended shell model to compute the phonon dispersion of these materials.…”

Section: Resultsmentioning

confidence: 99%

Long‐Wavelength Optical Phonons of AgBr_xCl_1−x Mixed Crystals

Bootz

Osten

Uhle

1974

Infrared reflectivity and second-order Raman scattering in the mixed crystal system AgBr,C11-, were studied a t 77 K. The T O ( r ) and L O ( r ) phonon frequencies obtained by means of a Kramers-Kronig analysis shift continuously between their energetic positions in the pure components indicating one-mode behaviour of the system. This result is discussed in view of a recent model that describes the optical modes of a mixed crystal.The LO(r) phonon frequencies taken from the Raman spectra are found to be slightly lower. A resonant enhancement of the 2 L 0 scattering process that occurs for z = 0.42 with shorter-wavelength excitation is used to determine the longitudinal optical phonon frequency. Infrarot-Reflexionsvermogen und Ramanstreuung zweiter Ordnung werden im