Electron small polarons and bipolarons in LiNbO(3)

Area of Science:

• Solid State Physics
• Materials Science
• Optical Materials

Background:

• Electron small polarons and bipolarons significantly influence the performance of lithium niobate (LiNbO3), a key optical material.
• Lattice coupling generally hinders polaron tunneling, leading to localization even with minor crystal irregularities.
• The optical absorption mechanisms of free and defect-bound polarons in LiNbO3 are complex and require a unified explanation.

Purpose of the Study:

• To provide an overview of electron small polaron and bipolaron properties in LiNbO3.
• To consistently attribute optical absorption properties to small polarons and bipolarons.
• To analyze optical absorption bands and defect binding energies using a phenomenological approach.

Main Methods:

• Phenomenological analysis of optical absorption bands (peak energies, lineshapes, widths).
• Analysis of defect binding energies induced by lattice distortion.
• Consistency checks with electrical conductivity and thermopower data.
• Review and critique of alternative models for absorption features.

Main Results:

• Optical properties of free and Nb(Li) antisite defect-bound electrons are consistently explained by small polarons.
• Electron pairs forming bipolarons bound to Nb(Li)-Nb(Nb) neighbors are also described.
• A universal criterion for identifying small polaron absorption bands in oxides is proposed.
• Bipolaron dissociation energy (0.27 eV) is consistent with polaron data and explains electrical conductivity and mobility activation energy.

Conclusions:

• The polaron/bipolaron model consistently explains optical absorption, electrical conductivity, and temperature dependence in LiNbO3.
• The model accounts for polaron concentrations under varying illumination and heating conditions.
• Alternative models fail to consistently explain the experimental findings.