Peter I Macfarlane

Characterization of Cr3+ centres in LiNbO3 using fluorescence line narrowing

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.

Substitutional disorder and the optical spectroscopy of gallogermanate crystals

Results of optical absorption, photoluminescence, fluorescence lifetime and fluorescence-line-narrowing studies on several -doped crystals with the gallogermanate crystal structure are presented. Such crystals are unusual in that most fluoresce via both the broad-band transition and the sharper transition (R line), both of which are considerably broadened by the effects of substitutional disorder. An established adiabatic model is used to reproduce the selectively excited broad-band photoluminescence spectra and this shows that the substitutional disorder creates a distribution in the strength of both the octahedral crystal field and the electron - phonon coupling. The variations in electron - phonon coupling are attributed to variations in the strength of the non-octahedral crystal fields that determine the magnitudes of the coupling to non-symmetric phonon modes. This effect is shown to dominate the broadening of the band. The analysis of results of fluorescence-line-narrowing measurements on the R lines shows that these lines are also broadened by a distribution of both octahedral and non-octahedral crystal fields. In the case of crystals that contain a substitutionally disordered site in the close vicinity of the ion, the strong-field sites that give rise to R-line emission are shown to comprise an independent distribution and the broadening is dominated by the effects of the non-octahedral component of the crystal field. When the substitutionally disordered sites are further from the ion, the broad-band and R-line fluorescence derive from a single distribution of crystal fields. In this case the effect of the variation of the crystal field is again shown to be dominated by the non-octahedral contribution though the R-line fluorescence shows little evidence of this due to the strong-field sites being the extreme of the overall distribution.

A study of substitutional disorder in : I. Fluorescence line narrowing

A high-resolution study of the optical properties of -doped is presented. Excitation into the usual broad absorption bands produces fluorescence from a very broad line with a peak at at 15 K. The line is observed at an energy higher at temperatures above 100 K or by using excitation spectroscopy. The large broadening of the excited state is attributed to substitutional disorder at the and sites on the basis of site selective excitation and fluorescence line narrowing measurements. Estimates of the distributions of octahedral and non-octahedral contributions to the crystal field are made and it is shown that the non-octahedral crystal field is dominant.

The influence of substitutional disorder on non-radiative transitions in -doped gallogermanate crystals

The substitutionally disordered nature of gallogermanates creates a distribution of crystal-field strengths and symmetries for Cr 3+ dopant ions. This causes the Huang-Rhys factor of the 4 T 2 → 4 A 2 transition to vary from site to site, leading to a strongly broadened luminescence band. This luminescence is blue-shifted and strongly quenched with increasing temperature. Calculations show that this behaviour is due to large variations in the internal conversion rates for ions in different parts of the crystal-field distribution. It is shown that a confinement potential rather than a harmonic potential is required to account for the data.

Substitutional disorder and the ground state spectroscopy of gallogermanate crystals

Electron spin resonance (ESR) spectra at X-band microwave frequencies are reported for dopant ions in (CGGO), (SGGO), (LGGO) and (LGS). The ESR measurements identify two distinct sites for in CGGO and SGGO and a single site in LGGO and LGS characterized by spin S = 3/2 for ions in the ground state. In CGGO a sharp line ESR spectrum, fitted to an axially symmetric spin Hamiltonian with , and , is assigned to substituents at trigonally distorted octahedral sites with a regular array of and ions in second-nearest-neighbour sites. A second spectrum of broader lines, due to ions in orthorhombically distorted sites associated with random occupation of the second-nearest-neighbour sites by and ions, has slightly different spin Hamiltonian parameters; , and . In this spectrum the principal axis is rotated by about from the c-axis towards the a-axis. In SGGO there are also ESR spectra from ions in trigonal and orthorhombic symmetry sites with almost identical spin Hamiltonian parameters. The ground state splittings of the trigonal centres measured optically by fluorescence line narrowing gives for both crystals. In contrast, only the trigonal centres are identified in LGGO and LGS, their ESR spectra being very similar to those of the trigonal centres in CGGO and SGGO. These results are discussed in terms of the substitutional disorder in these crystals and the nature of the site environment of substituents. The spectra correspond to ions in strong and weak crystal field sites, respectively, in agreement with earlier optical studies.

Nephelauxetic inhomogeneous broadening of optical transitions

Abstract A highly monochromatic laser has been used to probe the inhomogeneous broadening of the R-lines in Cr 3+ :LiNbO 3 . Fluorescence line narrowing (FLN) and radiative lifetime studies reveal that in LiNbO 3 the broadening is caused by disorder which results in a distribution in the values of the electron-electron Racah parameters. Such nephelauxetic broadening in Cr 3+ :LiNbO 3 is in contrast to the broadening in Cr 3+ -doped glasses and disordered oxides, where distributions in the crystal field terms are dominant.

A study of substitutional disorder in : II. Electron spin resonance and polarization of optical spectra

Electron spin-resonance (ESR) studies of in (CYA) reveal spectra consisting of fine-structure pairs of narrow symmetric lines and fairly broad asymmetric lines, corresponding to ions in sites with ordered and disordered configurations of the second-nearest-neighbour ions, respectively. The polarized intensities of the absorption band and the -line fluorescence, measured with the E-vector of the radiation parallel and perpendicular to the c-axis, are in the ratio . These polarization ratios suggest that the octahedra undergo an angular distortion such that the magnetic -axis of the spectrum is rotated away from the crystal c-axis. The microstructure of the surrounding is discussed in terms of the ESR and polarization results.

A study of exchange interaction in -doped

The broadening of the lines of impurity ions in the substitutionally disordered crystals (x=0, 0.1, 0.5, 1) has been studied. Fluorescence line narrowing removes the inhomogeneous broadening created by variation in the crystal field and results in instrument-limited spectral features for x = 0 and 0.1. Exchange interactions between ions in octahedral positions and ions at second-nearest-neighbour sites cause significant broadening of the narrowed line when x = 0.5 and 1. Comparisons of experimental and simulated spectra have been used to estimate the magnitude of the exchange-coupling constant, - . Such simulations also show that the selection rule for the transition is broken by the spin in the - exchange system.

Crystal field distribution and non-radiative transitions in Cr3+-doped gallogermanates

The large broadening of the fluorescence transitions in Cr3+-doped gallogermanates is attributed to variations in the distortions to the octahedron of nearest-neighbour oxygen ions. For the weak field sites that fluoresce via the 4T2 → 4A2 transition, this causes the Huang-Rhys factor to vary from site to site. Such a crystal field distribution results in strong broadening of the emission band, and also enhances non-radiative decay processes. The non-radiative decay is described in terms of an internal conversion process and modelling shows a confinement potential to reproduce the experimental data most accurately.

EPR and optical spectroscopy of Cr3+ doped CaYAIO4

EPR and optical measurements have been carried out on Cr 3+ doped CaYAlO 4 at 1.6 K, 16 K and 300 K. Q-band measurements revealed a strong, asymmetric EPR line close to g≃2 for B//c, together with three weak fine-structure lines corresponding to tetragonal symmetry. The R 1 line of Cr 3+ has an inhomogeneous broadened linewidth of about 100 cm -1 at 16 K but a homogeneous width of less than 10 cm -1 , the measurement of which was monochromator slit width limited. The large inhomogeneous broadening of the optical and EPR spectra are attributed to the compositional disorder of Ca 2+ and Y 3+ in the host lattice.