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Detailed comparative study of EPR, ENDOR and optical absorption spectra was carried out for congruent, nearly stoichiometric and regularly ordered lithium niobate (LN) crystals doped with various chromium concentrations.

It was found that the main axial Cr³⁺ center has the highest electron density on the Nb nuclei and consequently substitutes for Li (center No 1 with zero-field splitting D=0.764 cm^-1). Several satellite centers were registered in nonstoichiometric crystals (No 2-9, D»0.7–0.87 cm^-1). Since all satellite centers disappear in stoichiometric crystals, they were interpreted as complexes, composed of Cr³⁺_Li and intrinsic defect in the nearest or next nearest cation shells. New family of Cr³⁺ centers (No 10, D=0.043 cm^-1) was found in crystals grown from the melt with potassium (LN_(K)) and 1wt.% Cr. In this family Cr³⁺ substitutes for Nb. Several ¹H and additional ⁷Li lines were detected in ENDOR spectra. This shed a light on the mechanisms of the charge compensation of Cr³⁺_Nb: by H⁺ or by Li⁺ in structural vacancies. Different Cr_Nb centers have one or both of these compensating defects.

The estimation of D for low-symmetry exchange pairs showed that they consist of Cr³⁺_Li–Cr³⁺_Li centers “glued” by intrinsic defects (No 11), but not of self-compensated axial Cr³⁺_Li–Cr³⁺_Nb centers (No 12). The lasts are "EPR silent", however they were registered in the luminescence spectra. Charge compensation by the interstitial H⁺ can be realized also for other impurities substituting for Nb, especially in crystals with a decreased concentration of intrinsic defects (like LN_(K) or VTE LN). The appearance of new features in the nearly perfect material illustrates a possibility to tailor LN properties by variations of concentrations of both intrinsic defects and impurity. Accumulated knowledge about charge compensation mechanisms of different non-isovalent impurities allows us to proceed to more efficient intentional engineering of LN crystals with the required characteristics for advanced applications.

The results were published in:

V.Grachev, G.Malovichko. EPR, ENDOR, and optical absorption study of Cr³⁺ centers substituting for niobium in Li-rich lithium niobate crystals.- Physical Review, B62, 7779-7790 (2000).

G.Malovichko, V.Grachev, E.Kokanyan, O.Schirmer. EPR, NMR and ENDOR study of intrinsic and extrinsic defects in disordered and regularly ordered lithium niobate crystals. Ferroelectrics, 239, 357-366 (2000).

G.Malovichko, V.Grachev, E. Kokanyan, O. Schirmer. Point imperfections and clusters of intrinsic and extrinsic defects in non-stoichiometric and stoichiometric Lithium Niobate: The Regularly ordered crystal. - Radiation Effects and Defects in Solids, 150, No 2, 227-231 (619-623) (1999).

G.Malovichko, V.Grachev, E.Kokanyan, O.Schirmer. Axial and low-symmetry centers of trivalent impurities in lithium niobate. Chromium in congruent and stoichiometric crystals.- Physical Review, B59, 9113-9125 (1999).

V.Grachev, G.Malovichko, O.Schirmer. Models of axial and low-symmetry impurity centers in lithium niobate crystals derived from EPR and ENDOR data. - Radiation Effects and Defects in Solids, 150, No 2, 277-280 (669-672) (1999).

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