Lithium niobate (LN) crystals remain of the interest for applications as well as for fundamental studies. The Cr³⁺ ions turned out to be a very good probe of that structure since they substitute for both Li or Nb ions in LN host. Several studies with use of EPR and ENDOR techniques have been performed that helped to identify several Cr³⁺ centers both in Li (Cr_Li) and Nb (Cr_Nb) sites. Also high-pressure low-temperature optical spectroscopy was very successful in identification of optically active Cr³⁺ centers. High-pressure application transforms low-strength crystal field of Cr³⁺ centers into high-strength crystal field, what increases spectral resolution of the measurements and allows for their easier characterization.
Doping of lithium niobate with trivalent ions requires some charge compensation. Recently on the basis of EPR and ENDOR it has been suggested that interstitial hydrogen ions or additional Li⁺ ions in structural vacancies in LiNbO₃ can provide required charge compensation for chromium ions in niobium sites (Cr_Nb). It is known that LN can accommodate large amount of OH^- molecules and their concentration can be controlled either by annealing crystals in water vapor or by proton-exchange in relatively thin surface layer of the crystal.
We studied influence of both annealing in water vapor and proton exchange on high-pressure low-temperature luminescence spectra of near stoichiometric LN:Cr,MgO crystals, containing both Cr_Li and Cr_Nb centers by using diamond-anvil cell. The observed changes of the spectra of Cr³⁺ ions are associated with the larger inhomogeneous broadening and increase of of the splitting of the ²E level in one of the Cr_Li center, so called center γ. This shows that neither increase of hydrogen concentration in LN by almost two orders of magnitude (from 10¹⁸ to almost 10²⁰ cm^-3) nor proton-exchange procedure do not create any new optically active Cr³⁺ centers in those crystals.

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