Ab-initio perspective on the Mollwo-Ivey relation for F-centers in alkali halides

TL;DR

This paper uses advanced ab-initio calculations to analyze the Mollwo-Ivey relation for F-centers in alkali halides, revealing that lattice constant scaling predicts an exponent of 2, while ion-size effects reduce it to about 1.8.

Contribution

It provides a detailed ab-initio analysis separating lattice constant effects from ion-size effects on the Mollwo-Ivey relation.

Findings

Lattice constant scaling alone predicts an exponent n=2.

Ion-size effects account for the observed n≈1.8.

State-of-the-art calculations accurately reproduce experimental trends.

Abstract

We revisit the well-known Mollwo-Ivey relation that describes the "universal" dependence of the absorption energies of F-type color centers on the lattice constant $a$ of the alkali-halide crystals, $E_{\mbox{abs}}\propto a^{-n}.$ We perform both state-of-the-art ab-initio Quantum Chemistry and post-DFT calculations of F-center absorption spectra. By "tuning" independently the lattice constant and the atomic species we show that the scaling of the lattice constant alone (keeping the elements fixed) would yield $n=2$ in agreement with the "particle-in-the-box" model. Keeping the lattice constant fixed and changing the atomic species enables us to quantify the ion-size effects which are shown to be responsible for the exponent $n \approx 1.8$.