Kris Sabbe

Magnetic resonance study of Rh complexes in AgCl microcrystals

X-ray or UV irradiation at room temperature of Rh3+ doped AgCl emulsion powders leads to the production of three paramagnetic Rh2+ related centres, labeled R4, R5 and R6. A combined X and Q band electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) study allowed the determination of a nearly complete structural model for these centres. In the X band ENDOR spectra of R4 and R5 interactions of the unpaired electron with nearby protons have been identified, indicating that for these centres Cl- ligands have been exchanged by H2O or OH-. The R6 centre, identified as a (RhCl6)4- complex, has been found to be fundamentally different from the dominant centre in large Rh2+ doped AgCl single crystals grown from the melt. The results are compared with recent work by other researchers in the same field.

Electron paramagnetic resonance and electron nuclear double resonance of NaCl:Rh2+: A comparison between similarly doped Bridgman- and solution-grown single crystals

In a recent electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) study several new rhodium related centres in a Bridgman-grown crystal were reported. For two centres a detailed microscopic model was established. The primary Rh2+ centre was identified as a [RhCl6]4− complex with a single charge compensating Na+ vacancy in a next nearest neighbour position. A second centre was identified as a rhodium dimer. Two other Rh2+ centres, with apparent axial symmetry, remained unidentified. In the present study a detailed comparison between Rh2+ defects in Bridgman- and solution-grown NaCl crystals is presented. One of the previously unidentified Rh2+ centres is shown not to be axial but to have orthorhombic-I symmetry and is identified as a cis-[RhCl6]4− complex with two next nearest neighbour Na+ vacancies. For the primary centre additional ENDOR interactions confirm the model mentioned above. Finally a comparison is made between the temperature stability of the primary and orthorhombic-I centre in Bridgman-grown and similarly doped solution-grown NaCl crystals.

X- and Q-band magnetic resonance study of iridium-doped silver chloride microcrystals

Irradiation at low temperature (T<120 K) of iridium-doped AgCl microcrystals with x-rays leads to the formation of a single Ir2+ complex. The comparison between X- and Q-band EPR (electron paramagnetic resonance) spectra reveals that the defect has a small orthorhombic distortion. Pulse annealing to temperatures higher than 170 K converts the complex into a second Ir2+ complex with axial symmetry. This conversion is monitored with X-band ENDOR (electron-nuclear double resonance), allowing a consistent spectrum decomposition and interpretation. For each defect a model is proposed.

Detection of charge compensating cation vacancies near Rh2+ complexes in AgCl and NaCl using Q-band ENDOR.

The effect of a charge compensating next nearest neighbour (nnn) cation vacancy on the electron nuclear double resonance (ENDOR) spectrum of a [RhCl6]4− complex in AgCl and NaCl single crystals is studied at Q-band. The Rh2+ ion has a low spin 4d7 ground state configuration and at low temperature, the complex undergoes an elongation along a crystal [001] direction (gz axis). The unpaired electron strongly interacts with two equivalent Cl− ions along the elongation axis. The electron nuclear double resonance spectrum of these axial Cl− ions is clearly separated from all other interactions. The Q-band ENDOR analysis reveals a tilt in the (010) plane of the superhyperfine and quadrupole tensors for this interaction, due to the presence of the nnn cation vacancy on the axis. The tilt can be directly observed in the spectra through a splitting of the ENDOR transitions recorded in this plane and may be regarded as a fingerprint for the presence of the vacancy.

Electron paramagnetic resonance and electron nuclear double resonance investigation of NACl: Ir : a model system for similarly doped silver halides

Iridium-doped Bridgman-grown NACl single crystals were studied with electron paramagnetic resonance and electron nuclear double resonance after irradiation at 77 K. An orthorhombic-I (IRCl 6 ) 4- complex, charge compensated by two next-nearest neighbour vacancies was detected. After annealing at 180 K this complex decays by either vacancy diffusion or an electronic process. The Ir 2+ complex is compared with the Rh 2+ complex formed in the same conditions in NACl crystals grown from solution.

Study of strongly overlapping Rh2+ EPR spectra by high-resolution magnetic resonance techniques

Recent EPR and ENDOR publications demonstrated that several closely related Rh2+ complexes may be simultaneously present in rhodium doped NaCl and AgCl. These complexes only differ with respect to vacancy compensation. They exhibit strongly overlapping X-band EPR spectra with g⊥ ≈ 2.5 and g‖ ≈ 2. In the present study, solution- and melt-grown NaCl single crystals doped with Rh3+ were subjected to several treatments, such as X-irradiation, heating and thermal pulse annealing. While X-band EPR alone has insufficient resolution to resolve the spectra of the various rhodium complexes, a combined X-band ENDOR and W-band EPR study allows a consistent spectrum decomposition and interpretation. The X- and W-band Rh2+ spectra of both solution- and melt-grown NaCl single crystals are discussed in terms of the same [RhCl6]2−·nvac (n = 0, 1, 2) complexes.

Single Crystal Magnetic Resonance Study of a Rh2☎ Dimer Centre in NaCl at Q-Band

Electron nuclear double resonance spectroscopy at 34 GHz is applied to determine the signs of the principal values and the directions of the corresponding principal axes of the zero field splitting tensor for a Rh2☎ dimer centre in NaC1. The results are in good agreement with the hypotheses concerning the spin Hamiltonian parameters deduced from the X- and W-band EPR analysis and support the model proposed for this centre consisting of two Rh2☎ ions on cation positions and two Na☎ vacancies.