J. Derouet

Chemical syntheses by means of microwave digestion as a focused open-vessel system: structural properties of oxides and hydroxides as powders

Abstract By means of a focused microwave (monomode system) apparatus, it is possible to carry out some chemical syntheses in very short times. Various kinds of oxides and hydroxides have been irradiated by this method. Different transition phases and different reduction and oxidation reactions have been observed in the course of the heating treatments. The most important results are provided by black powder, which absorbs the microwave energy and quickly becomes incandescent red at low temperature. However, the absorptance is a function of the dielectric constant and dielectric loss values, which vary from one oxide to another. Compared with some previous results obtained using multimode closed-vessel domestic ovens, the focused system allows us to homogenize the heating inside the powder and to reach higher temperatures in the cases of a few absorbent materials (particularly white powders).

Structural and spectroscopic characterisation of Cr:Li2MgSiO4(γ0)

To understand the exceptionally long fluorescence lifetime of fourfold coordinated CrIV in γ0 Li2MgSiO4 orthosilicate, the structure of this matrix has been refined from neutron diffraction data. Here we also report the crystal growth of Cr:Li2MgSiO4 (low temperature form) by a flux method, and the results of EXAFS and optical investigations of chromium in Li2MgSiO4 host lattice.

Magnetic splitting factors for Stark levels in neodymium oxychloride

Abstract From the determination of crystal field parameters for the 4f3 configuration at the C4v site of Nd3+ in tetragonal NdOCl, one obtains wave vectors for each Stark level whose composition yields computed anisotropic g (magnetic splitting factors). The calculated factors are compared with the results of two series of optical absorption experiments, one at 12 K with single crystals under a magnetic field up to 1.8 T, the other at 4.2 K under a magnetic field up to 6 T. The parallel (C4 axis) Zeeman splittings were determined for 28 Stark levels. The agreement between calculation and experiment is generally good. Deviations are discussed in terms of crystal field parameters fluctuations for individual groups of levels.

Spectroscopic characterization of chromium (IV, V, VI) in Cr:Li2MSiO4 (M=Mg,Zn)

To understand the exceptionally long fluorescence lifetime of CrIV in Li2MSiO4 (M=Mg,Zn), several spectroscopic investigations are performed on Cr:Li2MSiO4 (M=Mg,Zn) crystals. X-ray absorption near-edge structure investigations attest that chromium is localized in tetrahedral sites and that CrVI is the major species while CrIV is the minor one in both compounds. Electron paramagnetic resonance studies confirm the occurrence of CrV in elongated tetrahedral environment (dx2−y2 ground state), corresponding probably to the silicon site, and suggest the presence of several charge compensation schemes. Fluorescence and fluorescence dynamics of CrIV:Li2MSiO4 are reported. The very long CrIV excited state lifetime previously observed on powders samples is confirmed for the crystals (117 μs at room temperature, 305 μs at 30 K for Cr:Li2MgSiO4) and is explained by combined contributions to the emission from both 1E level and the lowest component of the 3T2 level, in thermal equilibrium. Contrary to the situation fo...

Simultaneous simulation of optical and magnetic properties of neodymium compounds

Abstract In this paper we review the calculation of magnetic properties (paramagnetic susceptibility and its evolution vs. temperature and g values) of a series of neodymium compounds in single-crystal form by using the wavefunctions deduced from previous simulations on experimental energy level schemes.

The correlation between the optical absorption spectrum and paramagnetic properties of neodymium trifluoride

The 4f3 configuration of Nd3+ in NdF3 has been revisited. The real point symmetry of the crystalline matrix-C2-has been considered for the simulation, which means 15 crystal field parameters (CFPs). The simulation has been conducted by considering together the energy level scheme derived from the optical absorption spectra and the paramagnetic susceptibility and its variation with the temperature and the g values of the ground level. It is shown that only a single set of CFPs reproduces this information correctly, whereas various sets are found if only the energy level scheme is retained in the simulation process.

Simple overlap model and crystal field analysis of Cs2ZnCl4:Co2+. Correlation with optical and magnetic properties

Abstract The optical (energy level scheme) and magnetic properties (paramagnetic susceptibility as a function of temperature and g values) of Co 2+ in a Cs 2 ZnCl 4 single crystal were reproduced simultaneously according to the crystal field theory involving a set of free ions and B k q crystal field parameters. The Co 2+ are placed in a weak tetrahedral ligand field. The crystal field parameters calculated from the structure are in good agreement with the experimental values. The correlation between the g values and B k q parameters is discussed.

Correlation between the structure and paramagnetic susceptibility of NdOF

Abstract The high-resolution X-ray powder diffraction patterns on the polycrystalline NdOF were measured at selected temperatures between 6 and 295 K. The hexagonal unit cell constants a and c first decrease as a function of temperature as anticipated down to ca. 50 K but then suddenly a increases and c decreases strongly. The cell volume remains constant, however. The compression of the lattice in the direction parallel to the c-axis is due to the high anisotropy of the NdOF lattice. The rigid NdO + complex cation prevents the compression in the plane perpendicular to the c-axis and results in a slight dilatation in the plane. The experimental paramagnetic susceptibility measured between 3 and 850 K shows also an anomalous behavior around 50 K indicating that the structural changes influence the magnetic properties as well. The evolution of the paramagnetic susceptibility as a function of temperature was well simulated below 50 K by wave functions and energy level values obtained from inelastic neutron scattering measurements at 2.5 K. Above 50 K the simulation fails because of structural changes. The high anisotropy of the NdOF lattice was concluded to be responsible also for the anomalous behavior of the paramagnetic susceptibility as a function of temperature.

SPECTROSCOPIC PROPERTIES OF CS2ZNCL4:CO2+

Abstract The absorption and luminescence spectra of a Cs 2 ZnCl 4 single crystal doped with Co 2+ ions are reported. Two strong, broad peaks in the absorption spectra are assigned to 4 A 2 → 4 T 1 ( F ) and 4 A 2 → 4 T 1 ( P ) transitions, respectively. Other bands belong to transitions from ground level 4 A 2 to levels arising from split 2 G, 2 H, 2 P, 2 D, 2 F terms. The absorption spectra were dependent on polarisation. The crystal field parameter Dq and the Racah parameters were calculated as Dq = −325 cm −1 , B = 728 cm −1 , C = 3349 cm −1 . The spin-orbit coupling constant λ was found equal to 162 cm −1 . A broad emission band centred at 11 800 cm −1 was assigned to 4 T 1 ( P ) → 4 A 2 ( F ) transition. Decay curves of the observed emission were non-exponential, its short decay time, about 50 ns, indicating the electric dipole character of the observed emission and the strong influence of non-radiative processes.

Simulation of optical and magnetic properties of neodymium (4f3) in the solid state: extreme sensitivity to the wave vector’s composition

Abstract The optical absorption spectra of neodymium at 4 K are reproduced with a very good agreement between calculation and experiment (15 −1 ). The wavefunctions which are obtained as a result of the energy calculation can be used to compute other physical properties. Among them we consider the paramagnetic susceptibility and the electron paramagnetic resonance. We report the results of these calculations concerning six compounds (Nd 2 O 2 S, A-Nd 2 O 3 , NdOCl, NdOF, Nd 3+ :LaCl 3 and NdF 3 ) for which the optical absorption spectra were thoroughly interpreted. We notice that these properties, depending on the point symmetry of the neodymium, are very sensitive to the values of the coefficients of the kets in the wavefunctions. These wavefunctions are extremely dependent on the crystal field parameters (cfps). A regard on the magnetic properties is very useful to determine if a set of crystal field parameters which fits the overall Stark levels is also convenient for the ground state ( 4 I 9/2 ). A comparison is also made between cfps determined from atomic positions in the structure (Simple Overlap Model) and phenomenological cfps. We also consider the effect of the spin-correlated crystal field (SCCF) through the action of its radial contribution represented by the c k parameters ( k =2,4,6).