Tunneling splitting of Jahn-Teller ions in oxides

The magnitude of tunneling splitting, 3?, that governs decoherence in quantum systems under symmetric potentials, is frequently unknown. Using first-principles calculations, we have obtained 3? for a number of E?e Jahn-Teller impurities in solids. Calculated values span 6 orders of magnitude on passing from KCl?Ag2+ to MgO?Cu2+. Values associated with Cu2+- and Ag2+-doped MgO are 2 orders of magnitude larger than those previously assumed and consistent with the existence of a dynamic Jahn-Teller effect. The origin and enhancement with pressure of these high 3? values is discussed in detail.The support by the Spanish Ministerio de Ciencia y Tecnología under Projects FIS2006-02261 and FIS2009- 07083 is acknowledged

Pressure-induced changes in Cr3+-doped elpasolites and LiCaAlF6: interpretation of macroscopic data

In the research of pressure effects on Cr3+-doped insulating lattices, it is crucial to understand the dependence of the 10Dq parameter on the sample volume, V. This problem is explored in the present work through ab initio calculations on Cr3+-doped K2NaScF6, Cs2NaYCl6, and Cs2NaYBr6 elpasolite lattices as a function of pressure in the 0?5 GPa range. From the calculated values of the lattice parameter and the Cr3+?X? (X=F, Cl, and Br) distance, R, it is found that R varies with the cell volume, vc, as v(1/3?)c where ? lies around 2.6. These results allow one to understand quantitatively the 10Dq dependence on V?m/3 for LiCaAlF 6:Cr3+ where the measured exponent m=2.3 is seemingly anomalous when compared to the values found for ruby (m=4.5) or NiO (m=5).The support by the Spanish Ministerio de Ciencia y Tecnología under Project No. FIS2009-07083 is acknowledged

Impurities bound to vacancies in insulators: electronic relaxation and physical properties of the Cr 3 + − V M model center in K M F 3 ( M = Mg, Zn)

This work is aimed at gaining a better insight into the influence of a close vacancy, V, on the properties of impurities in insulating materials. To achieve this goal the Cr3+−VM model center formed in KMF3 (M = Mg, Zn) fluoroperovskites has been explored in detail by means of ab initio calculations on clusters involving up to 87 ions. It is shown that the presence of the M2+ vacancy, VM, induces a significant structural relaxation on the CrF3−6 cubic complex which cannot be fully understood assuming that ions were rigid spheres that could not be polarized. Thus, although VM forces all the ligands to move away, the Cr3+−F− distance corresponding to the furthest ligand, Ffar, is found to be slightly higher than that for the closer equatorial ions. This unexpected fact is shown to be due to the electronic relaxation also induced by VM on the CrF3−6 complex, causing a charge of 0.2e to be transferred from the closest ligand to VM, Fnext, mainly to Ffar, and, to a lesser extent, to any equatorial ligand. This transfer of charge is mainly accomplished through orbitals lying in planes containing the C4 axis. In spite of these changes due to the vacancy, the 4A2g→4T2g optical transition is found to be weakly altered, a fact that concurs with available experimental data and whose origin is discussed. In contrast, electron paramagnetic resonance parameters such as gyromagnetic or superhyperfine tensors, which do depend on the electronic density around a point of the CrF3−6 complex, are shown to be particularly sensitive to the electronic relaxation induced by VM. In particular, the present study explains that the dominant component of the superhyperfine tensor for the Fnext ligand is clearly higher than that for Ffar, in agreement with experimental data. The relevance of the present results for understanding the electronic properties of other systems involving vacancies is also discussed. Some results on the Cr3+−Li+ center formed in KMgF3 are also discussed for comparison.The support by the Spanish Ministerio de Ciencia y Tecnología under Project FIS2009-07083 is acknowledge

Insulators containing CuCl4X22? (X = H2O, NH3) units: Origin of the orthorhombic distortion observed only for CuCl4(H2O)2 2?

The origin of the difference in structure between compounds containing CuCl4X22? (X=H2O, NH3) units is analyzed by means of first-principles calculations. While NH3-containing compounds display tetragonal symmetry, H2O-containing ones display an orthorhombic distortion at low temperature where the equatorial Cl? ions are no longer equivalent. Our simulations of optical and vibrational transitions show good agreement with all available experimental optical absorption and Raman data. As a salient feature, the value of the force constant for the B1g mode, K(B1g), driving the orthorhombic distortion in the CuCl4(H2O)22? unit is found to be four times smaller than that calculated for CuCl4(NH3)22?, stressing that CuCl4(H2O)22? is in the verge of the D4h?D2h instability. The analysis of results obtained for different values of the distortion coordinate, Q(B1g), clearly shows that the softening undergone by K(B1g) in CuCl4(H2O)22? comes mainly from the vibronic admixture of the antibonding a?1g(?3z2-r2) orbital with the bb1g bonding (or charge transfer) level. This mechanism is thus similar to that responsible for distortions observed in some fluoroperovskites and oxoperovskites. The present results, quantifying the importance of vibronic effects in structural instabilities, clearly demonstrate that, contrary to what was suggested by several authors, the instability in CuCl4(H2O)22? is not related to the Jahn-Teller effect and that the orthorhombic distortion observed in the pure compound Rb2CuCl4(H2O)2 has a local origin.The support by the Spanish Ministerio de Ciencia y Tecnología under Project No. FIS2009-07083 is acknowledged

Internal electric fields and color shift in Cr3+-based gemstones

Seeking to better understand the origin of the different colors of emerald and ruby, both ab initio periodic and cluster calculations have been carried out. The calculations reproduce the interatomic distances measured for pure Be3Si6Al2O18 and Al2O3 as well as the Cr3+?O2? distances in emerald and ruby. The mean Cr3+?O2? distance for Be3Si6Al2O18:Cr3+ and Al2O3:Cr3+ is found to be practically equal to 1.97 Å, in agreement with recent experimental values. The present calculations confirm that the variations of optical properties due to Cr3+ impurities along the series of ionic oxides can be understood merely through the CrO69? unit but subject to the electric field due to the rest of the lattice ions. As a salient feature it is proved that changes in electronic density and covalency due to the internal field are not the cause of the color shift. Therefore, the red color of ruby is not due to the polarization of the electronic cloud around chromium as a result of the C3 local symmetry. The present study also demonstrates that the variation of the ligand field splitting parameter, 10Dq, induced by the internal electric field comes mainly from the contributions of first shells of ions around the CrO69? unit. As a consequence, 10Dq in emerald is not influenced by the internal field, as the contribution from Be2+ first neighbors is practically compensated by that of Si4+ second neighbors. In contrast, in ruby the t2g levels are shifted by the internal field 0.24 eV more than the eg ones, so explaining the color shift in this gemstone in comparison with emerald. This result is shown to arise partially from the asymmetric form of the internal electrostatic potential along the C3 axis in Al2O3.Support from the Spanish Ministerio de Ciencia y Tecnología under Project No. FIS2009-07083 is acknowledge

High magnetic anisotropy of Fe+ ions in KTaO3 and SrCl2

The zero-field splitting constant, D, and the gyromagnetic tensor of the off-center system KTaO3:Fe+ and SrCl2:Fe+ have been explored by means of calculations based on the density-functional theory at the C4v local equilibrium geometry. The calculated D values for KTaO3:Fe+ (9?cm?1) and SrCl2:Fe+ (53 cm?1) are found to be much higher than typical figures measured for insulating compounds containing common 3d Kramers ions with a spin S>1/2 in the ground state. This result together with the calculated g? and g? values concur with available experimental information. The high magnetic anisotropy derived for Fe+ in KTaO3 and SrCl2 is shown to be strongly related to the existence of a 4E excited state lying only at about 3000 and 600?cm?1, respectively, above the ground state. Implications of present findings in the search of new molecular magnets with high values of the magnetic anisotropy are discussed in some detail.The support by the Spanish Ministerio de Ciencia y Tecnología under Projects No. FIS2006-02261 and No. FIS2009-07083 is acknowledged.The support by the Spanish Ministerio de Ciencia y Tecnología under Projects No. FIS2006-02261 and No. FIS2009-07083 is acknowledged

Surface modification of Ti6Al4V by laser peenimng: Implications on the in vitro ion release, bacterial adhesion and biocompatibility

Trabajo presentado en el 25th European Conference on Biomaterials (ESB 2013), celebrado en Madrid (España), del 8 al12 de Septiembre 2013Fatigue is considered the main cause of mechanical failure of Ti6Al4V implants. Therefore, significant benefits might be obtained by surface treatments aimed to develop subsurface compressive residual stresses, hence enhancing fatigue strength. Laser peening (LP) of metallic biomaterials has recently emerged as a useful technique due to its capability to promote deeper compressive stresses than conventional grit blasting processesPeer reviewe

VI jornadas de intercambio de experiencias educativas

Se recogen las ponencias presentadas en las VI jornadas de intercambio de experiencias educativas.AsturiasUniversidad de Oviedo. Facultad de Ciencias de la Educación; Calle Aniceto Sela s. n.; 33005 Oviedo; +34985103215; +34985103214;ES