A photosensitive Cr3+ center in photorefractive Bi12SiO20 crystals co-doped with chromium and phosphorus

Abstract

ABSTRACT:A trigonal Cr 3+ (3d 3 ) defect in sillenite type Bi 12 SiO 20 crystals co-doped with chromium and phosphorus has been identified by means of multi-frequency X-band (9.4 GHz), Q-band (35 GHz) and W-band (94 GHz) electron paramagnetic resonance (EPR). A consistent analysis of the observed spectra and their angular dependence was reached using an S=3/2 spin Hamiltonian with axial symmetry around a <111> crystallographic direction, an isotropic g-value g = 1.983, and a zero field splitting parameter B 2 0 = 0.1950 cm -1 . The spectra are attributed to a chromium ion in the unusual Cr 3+ valence replacing a substitutional Si 4+ in tetrahedral oxygen coordination. Evidence is found that the symmetry lowering from tetrahedral to trigonal is not spontaneous but induced by an associated defect, for which a P 5+ ion in a nearest-neighbor Si 4+ site is the most plausible candidate. Optical excitation results in a reversible charge transfer process directly correlated with a photochromic effect: near-UV light leads to a strong reduction of the Cr 3+ EPR signals, and induces a broad band at 675 nm in the absorption spectrum, and both effects are reversed under red light excitation.

Crystal field parameters and energy levels scheme of trivalent chromium doped BSO

“…Since electrical charges of both ions are not equal is needed a charge compensation, obtained by substitution of Bi 3+ ions with P 5+ ions [8].…”

“…Nevertheless, it should be pointed out that reports on the consistent crystal field analysis of Cr 3+ doped BSO crystals with calculations of crystal field parameters (CFPs) from structural data, and energy levels scheme, are scarce. The electron paramagnetic resonance (EPR) identifies a chromium ion in the unusual Cr 3+ valence replacing by substitution Si 4+ ions in tetrahedral oxygen coordination [8]. Evidence is found that the site symmetry of chromium ion is lowering from tetrahedral symmetry to trigonal one.…”

“…To observed defect symmetry as a trigonal perturbation along a (111) direction could either result from a nearby associated defect, e.g., a vacancy or an impurity ion, or could occur spontaneously. Indeed, a Jahn-Teller type distortion degenerate ground state, which is the case of Cr 3+ ion, possessing a threefold degenerate 4 T 1 ground level in tetrahedral coordination [8].The second possible explanation of trigonal perturbation results from a nearby defect. The association with a P 5+ dopant ion provides a plausible explanation for the symmetry of the Cr 3+ center in BSO: Cr +P, because the strong donor character of the P-doping is very efficient conversion to trivalent chromium.…”

Crystal Interface Control at Low Thermal Gradient and Investigation of the Effect of Cr on the Crystal Structure and Optical Properties of Bismuth Silicate

“…In literature, there are reports on the growth of Cr-doped BSO crystal and investigations on the photorefractive effect, dielectric property, optical activity, Faraday effect, etc. [15][16][17][18][19][20][21][22][23][24] For the growth of the crystal, Czochralski technique has been used as BSO is a congruently melting compound. [25,26] It is known that the devices mentioned earlier require crystals with a low dislocation density and high optical homogeneity.…”

“…[7,8] It has been reported that the photorefractive property of BSO is enhanced significantly on doping with metals like Cr, Fe, Mg, Ru, etc. [8,[15][16][17][18][19][20][21][22][23][24][25][26][27] Though most of these dopants make material sensitive to photorefraction in the visible region, especially around 500 nm, that is, 2.5 eV, practical feasibility prefers the region near the infrared (1.5À2 eV), mainly because of the availability of compact laser diodes in this region. [28] Cr doping shifts the photorefractive sensitivity of BSO to the near-infrared region and therefore makes it a potential material for applications like optical image processing, holographic interferometry, optical storage, and phase conjugation.…”