P. J. Alonso

Smectic Cu(II), Pd(II) and VO(IV) complexes: synthesis, EPR and thermogravimetric studies

The octadecanoate of N(-(4-hexyloxyphenyl)salicylaldimine and the corresponding Cu(II), Pd(II) and VO(IV) complexes were synthesized and studied. The compounds were characterized by IR, 1H NMR, elemental and thermal analysis and polarized light microscopy. Thermogravimetric analysis and electronic spectra of the complexes are reported. Electron paramagnetic resonance (EPR) measurements were performed.

A CW-EPR and ESEEM spectroscopic study of the dithiadiazolyl radicals p-XC6F4CNSSN (X = CN, Br)

Two dithiadiazolyl radicals, p-NCC6F4CNSSN and p-BrC6F4CNSSN, have recently been found to be paramagnetic in the solid state. While the β-phase of the first one exhibits spontaneous magnetization below 36 K, the second one shows a paramagnetic character in the solid state. The spin density distribution in these radicals is examined through continuous-wave electron paramagnetic resonance and electron spin echo envelope modulation spectroscopies. Hyperfine correlation sublevel spectroscopy provides information about the interaction of the unpaired spin with F and N nuclei. A signal coming from the interaction with Br nucleus is also detected. The superhyperfme coupling constants of the unpaired electron with the magnetic nuclei are obtained and values of the corresponding spin densities, ƒs and ƒσ- ƒπ, can be estimated in the isolated radicals. Spin density distribution has also been calculated in both molecules with density functional theory, being in excellent agreement with those determined from the spectra. The spin density is mainly concentrated in the dithiadiazolyl ring, but some spin density is observed on the fluorinated aromatic rings. They also provide a strong basis to understand the differences of the magnetic behavior of both molecules in terms of their respective packing in the solid state.

Magnetic exchange effects in nematogenic Schiff's base Cu(II) complexes. An EPR study

Abstract Two new nematogens, copper complexes derived from Schiffs bases, bis(N-n-pentyl-4-[(4′-decyloxy)benzoyloxy]salicylaldiminate) copper(II) (labelled as Cu5) and bis(N-(4″-n-pentoxyphenyl)-4-[(4′-decyloxy)benzoyloxy]salicylaldiminate) copper(II) (CuO5) have been studied by electron paramagnetic resonance spectroscopy at different temperatures in their different mesophases. Both compounds show a nematic phase and CuO5 also presents a smectic C mesophase at lower temperatures. The copper coordination geometry in frozen solutions in toluene and in concentrated samples is square planar, while in solutions in their analogous zinc complexes, a twist of the N-Cu-O coordination planes is found. In the stable solid phases, the spectra reveal the existence of intermolecular magnetic exchange coupling. The fluid phases of Cu5 can be frozen forming different structures that depend on the freezing process rate. In Cu5 the exchange interaction is strongly reduced in the nematic phase because of the loss of posit...

Tetragonally distorted Cr2+ ions in BaF2 and SrCl2 studied by ESEEM spectroscopy

Electron spin echoes on tetragonally distorted non-Kramers Cr2+ ions in chromium doped BaF2 and SrCl2 single crystals have been observed. The superhyperfine interaction (shf) with the nearest halide ions has been studied using the electron spin echo envelope modulation (ESEEM) signal. For Cr2+ in BaF2 the nearest coordination shell consists of four equivalent fluorine ions in a plane perpendicular to the defect tetragonal axis, the Cr2+ ion being off-centre displaced towards the fluorine plane. The Cr2+ shf tensor for BaF2 has been determined. For SrCl2:Cr ESEEM signals associated to the shf interaction with nearest chloride ions were observed but the measurements did not allow us to give a definitive defect model in this case.

Non-cubic Mn2+-centers in BaF2

Abstract The EPR spectra of thermally treated BaF2: Mn samples is reported. After thermal annealing at 900 K a trigonal Mn2+ center with g=2.000±0.005, |D|=2725±40MHz, |A|=265±10MHz, D A >0 , is observed. Annealing at 1200 K produces an orthorhombic Mn2+ center with g=2.00±0.01, |D|=2430±40MHz, |E|=570±20MHz, |A|=265±10MHz, D A . The superhyperfine (SHF) structures due to interactions with the neighbouring fluorines indicates that the trigonal manganese interacts with four fluorines, three of them equivalent. The orthorhombic Mn2+ shows interaction with four equivalent fluorine nuclei.

Radiation effects in ZnF2CdF2 based glasses

Radiation damage induced by X-irradiation in glasses with the composition (in mol%) 32 ZnF2, 28 CdF2, 20 BaF2, 11 LiF, 5 AlF3, 4 LaF3 has been investigated by optical absorption and electron paramagnetic resonance (EPR). Some new absorption bands in the 250–700 nm region are produced by liquid nitrogen temperature (LNT) X-irradiation. Irradiation at room temperature (RT) produces limited absorption with the maximum at about 275 nm. A strong EPR signal at g = 2.014 likely due to hole trapped centers appears after RT irradiation. Samples irradiated at LNT also show another signal due to VK centers and some new lines in the g = 2 region. Some correlations have been found between EPR and optical signals but a definitive assignment is not yet possible.

EPR of V2+ in SrCl2

The X-band EPR spectrum of SrCl2:V has been measured at liquid nitrogen temperature. A signal associated with V2+ in a site of trigonal symmetry is observed. The EPR data have been explained using the spin hamiltonian = μβHgS + D[S2z − 13S(SH)] + SAI, with D ⪢ hv, g∥ = 1.957 ± 0.004, g‖ = 1.954 ± 0.004, A∥ = 230 ± 5 MHz, A‖ = 235 ± 5 MHz. This V2+ defect is similar to those previously reported in fluoride crystals with the fluorite structure.

A mn+ center in SrCl2

Abstract When SrCb: Mn is irradiated at liquid nitrogen temperature Mn+ defects are created. Their EPR spectrum consists of a six-sextet isotropic signal which can be explained using an isotropic spin-Hamiltonian with a g-value of 2,000±0.005 and hyperfine constant of 700±5 MHz. A superhyperfine structure due to the interaction with the eight surrounding Cl− ions can be also seen when the magnetic field is along the directions. A model consisting of a Mn+ ion in the center of a cube of chlorines is proposed for the Mn+ defects. An absorption band that appears after irradiation at about 390 nm can be tentatively assigned to the 7S(3d54s) → 7P(3d54p) electronic transition of cubic Mn+.