Characterization of Cr³⁺centres in LiNbO₃using fluorescence line narrowing

Abstract

The optical spectroscopic properties of Cr3+ in LiNbO3 have been investigated using optical absorption, fluorescence, fluorescence line narrowing (FLN), selective excitation and radiative lifetime measurements. The present results are compared with data obtained using other techniques including electron spin resonance (ESR) and electron nuclear double resonance (ENDOR), and shown to be consistent with there being five Cr3+ centres. The predominant centre is due to substitution at the Li+ site, which provides the weak crystal field in which Cr3+ ions emit into a broad 4T2 to 4A2 band. Using FLN, two centres experiencing a stronger crystal field are shown to be due to substitution at this same site, probably perturbed by the location of Nb5+ antisites and Nb5+ vacancies in the next-nearest-neighbour cation shell. The addition of Mg2+ ions to LiNbO3 appears not to create new Cr3+ complexes: rather it modifies the nature of the disorder, thereby changing the relative concentrations of the different centres. To this extent it becomes possible to identify two other lightly occupied Cr3+ centres where substitution is at the Nb5+ site. This total of five Cr3+ centres in LiNbO3 gives a consistent interpretation of all the available spectroscopic evidence, including ESR and ENDOR, while pointing to the difficulties of theoretical work in sites where the differences in the energy levels of Cr3+ ions are too small to be distinguished by the available models.

Dominant Cr3+ centers in LiNbO3 under hydrostatic pressure

“…In the rhombohedral (space group R3c) lattice of LiNbO 3 the cations Li + and Nb 5+ are surrounded by 6 oxygen ions forming trigonally distorted octahedrons. Present interpretation (see for example [4]) based on ESR data [10-12 and Nb ) in the LiNbO 3 lattice, which was confirmed by ESR and ENDOR studies [13][14][15]. Above that Mg concentration threshold the samples change their color from green to pink due to a strong red shift of broad band transitions 4 [4,15].…”

“…These lines manifest themselves also in absorption and photoexcitation spectra of Cr 3+ : LiNbO 3 at ambient pressure [2][3][4]20] and are interpreted as lines of dominant low field Cr…”

“…LiNbO 3 crystals doped with Cr 3+ ions have being investigated intensively since 1968, and considerable progress was made recently in the understanding of the complicated optical spectra of the material [1][2][3][4][5][6][7][8][9]. In the rhombohedral (space group R3c) lattice of LiNbO 3 the cations Li + and Nb 5+ are surrounded by 6 oxygen ions forming trigonally distorted octahedrons.…”

“…Present interpretation (see for example [4]) based on ESR data [10-12 and Nb ) in the LiNbO 3 lattice, which was confirmed by ESR and ENDOR studies [13][14][15]. Above that Mg concentration threshold the samples change their color from green to pink due to a strong red shift of broad band transitions 4 [4,15]. As it was shown earlier [16] the effect of high hydrostatic pressure on spectroscopic properties of crystals with octahedrally coordinated impurity Cr 3+ ions is a powerful tool for studying of the electronic structure of impurity centers.…”

“…According to the crystal field theory for Cr 3+ (3d 3 ) ions in octahedral field the increase of Dq strongly shifts up energies of 4 T 2 and 4 T 1 states while energy of the 2 E state depends on the distance from ligands much less [17]. Therefore the applied pressure drastically changes the distance between low lying 4 Nb centers). High hydrostatic pressure measurements were performed using a diamond-anvil cell.…”