C4v Site Research Articles

Site-selective time resolved laser spectroscopy and DFT studies of Nd3+ optical centers in BaF2 doped crystals

Site-selective time-resolved laser spectroscopy techniques, which provide high spectral resolution at low temperatures (4.2 K and 6.5 K), were used to clarify the formation mechanism, spectral and kinetic properties of the Nd3+ ion single-site optical centers formed in the fluorite-type BaF2 single crystals doped with Nd3+. This formation occurs by heterovalent substitution of Ba2+ host cations with additional charge compensation by F− anions. To determine the lifetimes of the Stark 4F3/2(1) levels of the Nd3+ impurity centers, the fluorescence decay kinetics was measured using the gated photon counting technique. The data from spectroscopic experiments were combined with density functional theory (DFT) calculations to identify and analyze the local structure of Nd3+ cations in the Nd3+ ions doped BaF2 crystal. Our studies have shown that several types of single-site Nd3+ optical centers can be distinguished in BaF2:Nd3+. We confirmed the existence of an optical center with high C4v site symmetry and a long radiative lifetime of the 4F3/2(1) state τ = 14.3 ms. We also describe two other configurations of centers with local structure characterized by low site symmetry of Cs, which were identified and described for the first time. The decrease in the tetragonal site symmetry from C4v to Cs was explained by the fact that the Nd3+ dopants, distributed over the equivalent cationic positions of the fluorite-type crystal lattice, coordinate to anionic motifs that essentially differ in the position of the additional charge compensator F−. This structural variability induced in the local coordination environment of the optical center affects its spectroscopic properties through a shift in the energy of the 4F3/2(1) Stark level of the Nd3+ ion and results in a different radiative lifetime τ.

Analysis of site symmetries of Er3+ doped CaF2 and BaF2 crystals by high resolution photoluminescence spectroscopy

The understanding of complex relationships between luminescent properties, local symmetry of an emitting center, and the host crystal structure provides a better insight into optical properties of materials. In this work, the alkaline earths CaF2 and BaF2 fluoride crystals doped with 0.1 mol% ErF3 were investigated. The crystals structure has been studied using a synchrotron and laboratory X-ray diffraction. The C3v and C4v sites symmetry were determined using erbium probed high resolution emission spectroscopy (HRPL) at low temperature (LT) of 10 K. The considerable difference in room temperature (RT) optical properties for CaF2 compared to BaF2 crystals was observed. Such difference in absorption intensity of 4.7 times of erbium 4G11/2 manifold in UV, and 7.5 times in green emission from 4S3/2 manifold, could be due to the distinction in the host crystals cationic radius (ΔrCa,Ba) and the dopant-host ionic radius (ΔrCa-Er, ΔrBa-Er). Those Δr differences influence the structure and lead to the following symmetry formation: In CaF2, the C4v and C3v isolated centers were identified, with the determined Er3+- F−i bond lengths of 2.734 Å and 4.735 Å respectively; In BaF2, only C3v isolated centers were identified with the determined Er3+- F−i bond lengths of 5.380 Å. The present work is the first study that takes into account correlations of optical properties, the local symmetry and the structure in mentioned fluorides crystals, and it could be a step forward in the lanthanide doped optical materials systematics.

Fluorescence of Nd3+ optical centers close to cubic symmetry in a calcium fluoride crystal co-doped with Na+

Earlier, it was shown by the EPR method that at a very low concentration of the Nd3+ ion in the Nd3+:CaF2 crystal, highly symmetric cubic centers with the Oh symmetry are formed. In the present study, the optical centers of the Nd3+ ion with an anomalously long radiative lifetime τR = 13.6 ms of the 4F3/2(1) crystal-field (CF) level in the Nd3+ (0.05 at.%): Na+ (0.2 at.%): CaF2 single crystal at T = 7 K were detected by the method of time-resolved site-selective fluorescence laser spectroscopy. Its fluorescence excitation at the 4I9/2(1) → 4G5/2(1) transition of Nd3+ is in a longer wavelength spectral range compared to optical tetragonal (C4v) L-, orthorhombic (Cs) pair M-, and quartet N- centers. New optical centers exhibit inhomogeneous splitting and can be attributed to nearly cubic sites with symmetry close to Oh. Also numerous Nd3+ optical sites in the Nd3+ (0.03 at.%): CaF2 and Nd3+ (0.05 at.%): Na+ (0.2 at.%): CaF2 crystals at T = 7 K with τR = 240 μs ‒ 8.5 ms of the 4F3/2(1) CF level of Nd3+ were detected in a shorter wavelength spectral range compared to the nearly cubic sites. Among them, the sites with the longest τR = 8.5 ms except for the nearly cubic sites, which were identified as C2v symmetry sites. Their radiative lifetime is about six times that of the C4v site (τR = 1.4 ms). This means that, for optical sites without a center of inversion, the radiative lifetime of the excited state at sites with higher symmetry can be almost an order of magnitude less than at sites with lower symmetry. We explained this result by the different distance between the Nd3+ ion and the ion, which compensates for the excess of its charge. Thus, we have demonstrated the use of the Judd-Ofelt approach to explain the difference in the rates of emission of the excited state in different optical sites of the same rare-earth ion in the same crystal matrix.

F−-Eu3+ charge transfer energy and local crystal environment in Eu3+ doped calcium fluoride

A series of CaF2:xEu3+ (x = 0.01–0.05) phosphors were synthesized by coprecipitation and high temperature solid-state method. Three different F--Eu3+ charge transfer (CT) bands in CaF2:xEu3+ (x = 0.01–0.05) were detected through the PL spectra. Three local crystal environments (deformed Oh octahedron, C3V, C4V) were proposed in Eu3+ doped CaF2. The broad peak at 277 nm is F- → Eu3+ CT peak, in which Eu3+ ions located deformed Oh octahedron sites, the broad peak at 312 nm is due to Eu3+ ions situating at C4v sites, also, Eu3+ ions with C3v sites can make the F- → Eu3+ CT produce a 38-nm redshift to 315 nm. Density functional theory (DFT) was performed to calculate the energy band gap(Eg) according to the three models. The calculation results consist with the experiment. The excitation peaks can be tuned in the same host through changing the local structure of Eu3+.

Ab-initio calculations of Judd–Ofelt intensity parameters for transitions between crystal-field levels

Wavefunction-based ab-initio calculations of the electric-dipole moments of 4fN–4fN transitions of lanthanide ions are performed to extract the Atpλ intensity parameters. The extraction method is an extension of our earlier calculations of crystal-field (CF) parameters for lanthanide ions in crystals. The CASSCF/RASSI-SO (Complete-Active-Space Self-Consistent-Field/Restricted-Active-Space State-Interaction Spin-Orbit) calculations have been carried out on the chosen model system of CaF2: Ce3+ with an interstitial fluoride ion (Fi−) on z-axis (Ce3+ ion occupying the C4v symmetry site). In consideration of the site symmetry and the coordination situation of Ce3+ ion at C4v site in CaF2 as well as the superposition model (SM), the calculated intensity parameters for Ce3+ ion can be classified into three categories, and detailed discussions are then given.

Optical properties of the Dy3+ ion in DyOCl

A detailed analysis of the energy level scheme of the Dy3+ ion with the 4f9 electron configuration was carried out in the DyOCl matrix. The UV–vis-NIR absorption spectra of the tetragonal DyOCl were measured at selected temperatures between 9 and 300 K. The emission spectra of the Dy3+ ion in the LaOCl and GdOCl matrices were obtained at 77 K and room temperature. The experimental energy level scheme of 179 c.f. components representing 57 SLJ states was successfully simulated according to the C4v site symmetry by a phenomenological model using 14 free ion and five c.f. parameters with an RMS deviation of 21 cm−1. The results for DyOCl are consistent with those for other RE3+ ions in RE oxychloride hosts.

Spectroscopic properties of KY3F10:Er3+

Absorption, luminescence and MCD spectra of KY3F10:Er3+ have been recorded at different temperatures between liquid helium and room temperature. Crystal-field and intensity calculations are presented. Special attention is paid to the three mutually perpendicular C4v sites in the KY3F10 host crystal and to the consequences for the parameter sets.

Crystal fields in (La1−xGdx)OCl:Eu3+ solid solutions

The formation of cation solid solution in the (La1−xGdx)OCl:Eu3+ (0≤xGd≤1; ΔxGd=0.1) series was studied by photoluminescence spectroscopy. The luminescence from the 5D0–2 to the 7F0–4 levels of the Eu3+ ion in the (La1−xGdx)OCl series was recorded at 77 K by using argon ion laser excitation (457.9 nm). The interpretation of the spectra according to the C4v site symmetry of the Eu3+ ion in the tetragonal PbFCl-type structure yielded nearly complete sets (18 to 19 levels) for the 7F0–4 levels. Simulations of the Stark level schemes were carried out with the aid of a phenomenological c.f. theory utilizing the five non-zero c.f. Bqk parameters (B02, B04, B44, B06 and B46) allowed for the C4v site symmetry. By using the calculated c.f. parameter sets a quantitative measure was obtained to monitor the formation of cation solid solutions. The strength of the c.f. effect was estimated with the c.f. strength parameters S and Sk (k=2, 4 and 6). The c.f. parameter sets reproduced the experimental 7FJ (J=0–4) energy level schemes with the rms deviations between 4 and 11 cm−1. The individual parameters as well as the c.f. strength parameters were found to evolve in a smooth manner indicating complete solid solubility in the (La1−xGdx)OCl series. Some local distortions from the C4v symmetry — probably of long range—leading to the splitting of the 7F1 doubly degenerate E level were observed, however.

Optical and EPR studies of VO2+ and Cu2+ in Rb2Cd(SO4)2.6H2O

The optical absorption and electron spin resonance spectra of vanadyl-and copper-doped rubidium cadmium sulphate hexahydrate are investigated at room and liquid nitrogen temperatures. From the analysis, the spectra are ascribed C4v site symmetry in the case of VO2+ ions and D2h site symmetry in the case of Cu2+ ions.

The transverse Zeeman effect in calcium fluoride crystals containing erbium

A detailed study has been made of the optical transverse Zeeman effect for trivalent erbium ions at sites of tetragonal C4v symmetry in crystals of calcium fluoride with magnetic fields of up to 6T. Interpretation of the optical absorption spectrum is complicated by the presence of magnetically non-equivalent C4v sites and Er3+ sites of other point symmetries, but selective excitation of fluorescence with a tunable dye laser has been used to isolate the spectrum of ions in the required transverse C4v sites. Selective laser excitation spectroscopy has also been used to locate several previously undetected Er3+ crystal-field energy levels for the C4v site. A crystal-field analysis has successfully accounted for the observed transverse Zeeman patterns, including the variation in these patterns as a function of orientation of the magnetic field in the (100) crystal plane and as a function of magnetic field strength.

Effect of concentration on the absorption peaks in the lowest 4f to 5d band of Ce3+in CaF2crystals

The effect of the concentration of Ce3+ on the absorption peaks in the lowest 4f to 5d band of Ce3+ in CaF2 crystals has been measured at Ce3+ concentrations between 0.005 and 5% by both wavelength modulation spectroscopy and conventional ultraviolet absorption spectroscopy. The data show that absorption of the C4v sites known to contribute to the spectrum at low Ce3+ concentrations is overtaken by absorption attributable to other centres at Ce3+ concentrations higher than about 0.1%.