The nature of additional (redundant from the point of view of selection rules) optical phonons observed in most crystals with ion—covalent bonds between atoms, in particular, in their solid solutions, is discussed. These 'redundant' phonons are located in the frequency region of the longitudinal—transverse splitting of fundamental phonons, where the real part of the crystal permittivity is negative. They are also split by a crystal field into transverse and longitudinal phonons, the frequencies of redundant longitudinal phonons being lower than those of redundant transverse phonons, and the oscillator strength of these phonons proves to be negative.

Typically, an English fulltext is available in about 3 months from the date of publication of the original article.

Keywords: optical phonons, local vibrations, solid solutions, oscillator strengths, inverted oscillator, effective ion charge, infrared spectroscopy PACS: 63.50.−x, 78.30.−j (all) DOI: 10.3367/UFNe.2020.01.038719 URL: https://ufn.ru/en/articles/2020/8/d/ Citation: Vinogradov E A "Optical phonons with a negative oscillator strength" Phys. Usp. 63 (8) (2020) Received: 27th, November 2019, revised: 30th, December 2019, accepted: 15th, January 2020 Оригинал: Виноградов Е А «Оптические фононы с отрицательной силой осциллятора» УФН 190 829–835 (2020); DOI: 10.3367/UFNr.2020.01.038719