R. Borromei

Electronic spectrum of the manganate(V) ion in different host lattices

A comparative account is given of the 4.2 K linearly polarised absorption spectra of Sr5(PO4)3 Cl and Ca2PO4Cl crystals containing MnO3–4, with emphasis on the differing site-group perturbations. With the inclusion of a previously undetected shoulder at 11 500 cm–1 in the Ca2PO4Cl case, it is possible to account for the energies and polarisations of all the bands using similar values of Δ and B(respectively 11 000–11 500 and 500–550 cm–1) but different parameters describing the tetragonal and trigonal site perturbations. It is confirmed that all bands in the visible are of ligand-field type and that the lowest charge-transfer band is at 32 000–33 000 cm–1.

Absorption spectra of zircon crystals doped with Cr(IV): ZrSiO4:Cr4+☆

Abstract Crystals of ZrSiO 4 ‘doped’ with Cr 4+ have been prepared from oxide fluxes. The polarized absorption spectra (α,σ,π) of single crystals have been recorded in the range 6000–25 000 cm −1 at various temperatures from 10 K to room temperature. The polarized absorption spectra present three main wide bands in the regions 9600–14 000 (π), 15 000–19 500 (α, σ) and 20 500–24 000 (α, σ) cm −1 . It is shown that with these spectra only the hypothesis of a replacement of Si 4+ with Cr 4+ is acceptable. An explanation and assignment of the bands are proposed by considering their polarizations and the site symmetry perturbation related to the distortion T d → D 2 d of the oxygen coordination. In particular the first band is assigned to the transition 3 A 2 ( 3 T 1 , et 2 )← 3 B 1 and the next two bands to the transitions 3 E ( 3 T 1 , et 2 )← 3 B 1 and 3 E ( 3 T 1 , t 2 )← 3 B 1 . This result agrees with a recent investigation on Y 3 Al 5 O 12 :Cr 4+ .

The luminescence of VO43− ions in Ca2PO4Cl

Abstract The emission spectra of both single crystals and polycrystalline samples of Ca2PO4Cl “doped” with VO43− have been measured at several temperatures from 10 K to room temperature. In both a wide emission band has been observed within the range 16000–28000 cm−1. This band shows two components, which behave differently with temperature. The deconvolution of the decay curves, expressed in terms of a linear combination of exponentials, requires three exponentials below 250 K. The relative lifetimes are 3700 μs, 1800 μs and 250 μs at 10 K. The emission wide band is assigned to a triplet → singlet transition connected to a transfer of an electron from a molecular orbital mainly localized on one oxygen atom (2pπ type) to a molecular orbital mainly localized on the vanadium atom (de, or di2 type). The lifetimes of the single crystal and of the polycrystalline samples are rather close. However there is evidence of a somewhat different behaviour of the two kinds of samples with temperature.

Electronic spectrum of NiII in MgWO4, ZnWO4 and CdWO4 at 4.2 K

The ligand-field spectrum of NiII has been measured in crystals of MWO4(M = Mg, Zn, Cd) from 5000 to 30 000 cm–1. Spectra were measured with light incident on the (100) and (010) faces with the electric vector parallel to all three crystallographic axes at temperatures from 300 to 4.2 K. The splittings and intensities of the bands in different polarizations are interpreted in terms of a major C2v and a minor C2 perturbation of the octahedral site. Most of the observed intensity is derived from the static low-symmetry distortion but the effect of vibronic interactions is also discussed.

Low-temperature absorption spectrum of the VO43− ion in single crystals of Ca2PO4CI

Abstract Polarized absorption spectra of single crystals of Ca 2 PO 4 Cl doped with V(V) ions have been measured at liquid-helium temperature. Only one broad and very intense band is observed from 30000 to 45000 cm −1 . In T d symmetry it is assigned as a charge-transfer transition 1 T 2 ← 1 A 1 . Under theT d → D 2d site perturbation, the excited state 1 T 2 is split into 1 B 2 and 1 E.

Low-temperature absorption spectrum of the nickel(II) ion in single crystals of the columbite NiNb2O6

The ligand-field spectrum of NiII in NiNb2O6 crystals has been investigated in the range 5000–30 000 cm–1 at temperatures from 300 to 4.2 K. The spectra were measured using polarized light with the electric vector parallel to all three crystallographic axes. The spilittings and intensities of the bands are interpreted in terms of a major C2ν and a minor C2 perturbation of the octahedral site. Most of the observed intensity is derived from the static low-symmetry distortion, but the effects of spin–orbit coupling and of vibronic interactions are also taken into account. The spectrum is remarkably similar to that of NiII in the structurally related wolframites MWO4(M = Mg, Zn, Cd).

The electronic spectrum of NiTa2O6

Abstract The polarized absorption spectrum of single crystals of NiTa2O6 has been measured from 6000 to 28 000 cm−1 at various temperatures from 300 to 10 K. All the observed absorption regions are due to electronic transitions mainly localized on the Ni2+ ions. The first band system in the region 6000–10500 cm−1 corresponds to a transition which occurs mainly by magnetic dipole mechanism. The band systems in the other regions are due to vibronic transitions. By applying our method of analysis of the site geometry based on symmetry descent, we deduce that the Oh→D2h distortion of NiO6n− can be decomposed into two contributions related to the steps Oh→D4h and D4h→D2h and that the second contribution is much larger than the first. Successively we give an explanation for most of the features of the spectra by considering only the D4h→D2h perturbation and the vibronic interactions. The effect of the spin-orbit coupling has also been considered.

Electronic spectrum of cobalt(II) in ZnWO4 and MgWO4 at 4.2 K

The ligand-field spectrum of CoII has been measured in crystals of MWO4(M = Zn, Mg) from 5000 to 30 000 cm–1. Spectra were measured with light incident on the (100) and (010) faces with the electric vector parallel to all three crystallographic axes at 300, 77 and 4.2 K. The splittings and intensities of the bands in different polarizations are interpreted in terms of a major C2v and a minor C2 perturbation of the octahedral site. Most of the observed intensities are derived from the static low-symmetry distortion but the effects of vibronic interaction and of spin–orbit coupling are also discussed.

Dynamic jahn-teller effect in a ligand-field excited state of MnO3−4 in Sr5(PO4)3Cl

Abstract The 3A2 → 3E (3T2) band system of MnO3−4 in Sr5(PO4)3Cl at 4.2 K between 10000 and 12000 cm−1 consists of a single progression with 18 components (average frequence 94.5 cm−1) and a double-humped distribution of Franck-Condon factors. as predicted for the vibronic interaction of an E electronic state with an e vibrational mode.

Low-temperature absorption spectra of the MoO2−4 ion in Cs2SO4 and of the ReO−4 ion in RbClO4 and in (C2H5)4NClO4

Abstract Absorption spectra of single crystals of Cs 2 SO 4 doped with MoO 2− 4 and of RbClO 4 and (C 2 H 5 ) 4 HClO 4 doped with ReO − 4 have been measured at the liquid-helium temperature. All spectra show two band systems with pronounced vibrational structures. In T d symmetry they must correspond to 1 T 2 - 1 A 1 charge-transfer electornic transitions. It is likely that in the two band systems there are more than two electronic transitions.