**Acknowledgements**

The authors are grateful to Yurii I. Terekhin for preparing all CaF<sup>2</sup> samples, Tatiana S. Semenova, Lyudmila F. Koriakina, and Marina A. Petrova for additive coloring the samples, and also Evgenii B. Verkhovskii and Anatolii K. Kupchikov for a technical assistance (all of them, ITMO University).

The paper is dedicated to the memory of our colleague, Aleksandr S. Shcheulin (1955–2016), who was an active participant of this study.

Research was funded by Russian Science Foundation (Agreement #14‐23‐00136).
