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Gd3Ga5O12and Y3Al5O12nanocrystalline powders codoped with Yb3+and Tb3+or Eu3+have been prepared using the sol−gel Pechini method. Nanocrystals obtained by this technique exhibit a cubic phase with the Ia3d̅space group. Particles show a broad distribution of sizes and shapes, ranging from elongated to quasi-spherical particles of tens of nanometers. Optical properties such as photoluminescence and excitation spectra or lifetime measurements have been studied on these samples. Green/blue or red visible upconversion luminescence from Tb3+ 5D3, 5D4→ 7FJ, or Eu3+ 5D0→ 7FJtransitions, respectively, has been observed upon Yb3+ 2F7/2→ 2F5/2excitation at 975 nm. A detailed investigation on the spectroscopy and excited state dynamics of these systems is extremely important in order to understand the upconversion processes.

Crystals of Yb³⁺ doped Rb2MnCl4 exhibit very intense yellow-orange Mn^{2+ 4}T1g → ⁶A1g upconversion luminescence under near-infrared ²F7/2 → ²F5/2 Yb³⁺ excitation around 1 μm at 15 K. Excitation with 10 ns pulses indicates that the upconversion process consists of a sequence of ground-state and excited-state absorption steps. The upconversion mechanism is cooperative, involving both Yb³⁺ and Mn²⁺ ions in the crystal, and we postulate an exchange mechanism between adjacent Yb³⁺ and Mn²⁺ ions to account for the unusually high efficiency of the process. The efficiency of the upconversion process is dependent on the nature of the Yb³⁺−Mn²⁺ connectivity. The upconversion in Rb2MnCl4:Yb³⁺, with a linear Yb³⁺−Cl^-−Mn²⁺ arrangement, is 3 orders of magnitude more efficient than in the Yb³⁺−(Br^-)3−Mn²⁺ arrangement of face-sharing octahedra in CsMnBr3:Yb³⁺.

The present work is focused on the effects produced by hydrostatic pressure on Cu 2+ (<100 ppm) doped (C 2 H 5 NH 3 ) 2 CdCl 4 single crystal. The energy E u ( ~ ) of the e u ( ~ ) M b 1 g (∼ x 2 m y 2 ) charge-transfer transition of (C 2 H 5 NH 3 ) 2 CdCl 4 : Cu 2+ undergoes a red shift of 1400 cm m 1 on passing from ambient pressure to 40 kbar. To understand this fact theoretical calculations of the {\rm CuCl}^{4-}_{6} complex using four different methods have been carried out. Based on these calculations, it is concluded that the experimental red shift undergone by E u ( ~ ) can only be reasonably explained through a decrease of ∼25 pm in R ax and an increase of ∼7 pm in R eq leading to a decrement of the {\rm CuCl}^{4-}_{6} volume. Variations of any metal-halide distances down to 0.1 pm can be well detected through the shifts of the charge-transfer band energy upon pressure.

The variations experienced by the energy Eu(π) of the eu(π)→b1g (~x²–y²) charge-transfer transition of (C2H5NH3)2CdCl4:Cu²⁺ upon pressure in the 0- to 40-kbar range have been measured at room temperature by means of a sapphire anvil cell. These data reveal that Eu(π) undergoes a red shift of 1400 cm⁻¹ on passing from ambient pressure to 40 kbars. To understand this puzzling result theoretical calculations of ∂Eu(π)/∂Req and ∂Eu(π)/∂Rax have been performed where Req and Rax mean the equatorial and axial Cu²⁺–Cl⁻ distances of the elongated CuCl6⁴⁻ complex, respectively. All results indicate that ∂Eu(π)/∂Req and ∂Eu(π)/∂Rax for Req=228 pm and Rax=297 pm are indeed negative. Moreover ab initio complete active space self-consistent field (CASSCF/CASPT2) and density functional calculations lead to ∂Eu(π)/∂Rax values, which are about 10 times smaller than those of ∂Eu(π)/∂Req. From the ensemble of experimental and theoretical results, it is concluded that a pressure of 40 kbars gives rise to a decrement of ≈25 pm of the axial distance and at the same time to an increase of ≈7 pm of the equatorial one. It is stressed that the present study on a diluted Jahn–Teller impurity lies far beyond the current possibilities of X-ray absorption structure techniques. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

This work investigates the effect of hydrostatic pressure on the excitation, emission and lifetime of Ti 3+ - doped Al 2 O 3 in the 0-110 kbar range. The application of pressure induces band shifts that are correlated with the corresponding local structural changes undergone by the TiO 6 complex. The increase of the Stokes-shift and the reduction of the Jahn-Teller (JT) splittings under pressure are analysed in terms of a simple model based on linear electron-phonon couplings to the a_{1_{\rm g}} and the JT e g vibrational modes.

This work investigates the X-ray absorption spectra of the Cu K-edge in the layered compounds (C, H;?, +, NH&CuC14 (n=1-3) and their variation with pressure. The XANES spectra show a three peaked structure around 8.98 KeV at zero pressure which is characteristic of Cu2+ complexes in D4., symmetry. Application of hydrostatic pressure induces a progressive reduction of the energy separation between the second and third peaks that has been interpreted in terms of a progressive disappearance of the antiferrodistortive structure exhibited by the Cu2+ complexes in this compound series. The results are compared with recent pressure experiments on Raman and Charge Transfer spectroscopy as well as with Multiple Scattering calculations performed on these layered systems.

AbstractThis work investigates the dependence of the Cl−→ Cu2+Charge Transfer (CT) bands with the local structure around Cu2+in the Jahn-Teller CuCl4-6complexes formed in layer chloroperovskites with the K2NiF4-type structure. We present experimental results on the variation of the first CT band of CuCl4-6with hydrostatic pressure in the 0–80 kbar range. The study has been carried out in CuCl4-6formed either as impurity in Cu2+-doped crystals or in pure A2CuCl4compounds in order to establish structural correlations. A noteworthy feature is the redshift undergone by the first CT band at about 26000 cm−1, upon pressure in all the investigated systems. This redshift is interpreted in terms of anisotropic distortions of the octahedral elongated CuCl4-6complex. MS-Xα and Extended Hückel calculations of the electronic structure around the D4hequilibrium geometry allow us to establish correlations between the energy shift and the structural distortions of the complex.

AbstractThe aim of this work is to investigate the Charge Transfer (CT) spectra of CuCl2-4and CuBr2-4complexes formed in Cu2+-doped A2MX4crystals and their dependence with chemical pressure when Cu2+replaces the divlent cation at the MCl2-4tetrahedral site. A salient feature of this study is the weak sensitivity exhibited by the CT bands to structural changes of the complex. A similar phenomenon has also been observed for pure chlorocuprates. The analysis carried out on the available family of chlorocuprates indicates that the CT band-shift induced by structural distortions in CuCl2-4is smaller than that of the corresponding Crystal Field (CF) bands. We present a perturbative model for explaining the weak sensitivity of the CT bands. We present a perturbative model for explaining the weak sensitivity of the CT bands. in comparison to the CF ones. The estimates of the model accounts for the spectroscopic results observed along the A2MCl4: Cu2+series.

This work investigates the charge transfer spectra of CuCl64- complexes formed in the (RNH3)2CdxMn1-xCl4 crystal series. For x = 1, it is demonstrated that Cu complexes display an elongated D4h symmetry, forming an antiferrodistortive type structure which is similar to that found in the pure copper crystals. The presence of Mn induces an important enhancement of the charge transfer bands as well as significant redshifts. The results are interpreted in terms of distortions of the CuCl64- geometry which changes from D4h elongated (x = 1) to nearly D4h compressed (x = 0). The transition energy, polarization and assignment of the charge transfer bands are analysed.

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Three image analysis methods were studied and evaluated to solve the problem of removing long stems attached to mechanically harvested oranges: colour segmentation based on linear discriminant analysis, contour curvature analysis, and a thinning process which involves iterating until the stem becomes a skeleton. These techniques are able to determine the presence or absence of a stem with certainty, to locate the stems from random views with more than 90% accuracy and from profile images with an accuracy ranging from 92·4% to 100% depending on the method used. Finally, determination of the length and cutting point of the stem is achieved with only 3·8 % of failures.

SummaryA double-blind, placebo-controlled trial was carried out to assess the effectiveness of a new synthetic bioflavonoid, hidrosmin, in patients with chronic venous insufficiency of the lower limbs. Fifty-seven patients, showing varicose veins and ankle swelling and suffering from local pain and heaviness of the legs, were allocated at random to receive treatment for 45 days with 1 capsule 3-times daily of either 200 mg hidrosmin (30 patients) or placebo (27 patients). Pain and heavy legs were assessed using rating scales; swelling was assessed by a photographic method. The results showed that hidrosmin produced a significant clinical improvement in all of the parameters evaluated; compared with placebo, there was a marked reduction in the main subjective symptoms accompanied by a 10% reduction in swelling. Apart from 1 patient who complained of epigastric pain, there were no reports of adverse events during the study period.