The dependence of 10Dq upon the metal–ligand distance,R, for transition-metal complexes. What is its microscopic origin?

Experimental results on 3d O_h complexes in insulators reveal that 10Dq α R^?n, where R is the metal–ligand distance and n is close to five. This strong dependence determines the Huang–Rhys factor, S(A_1g), associated to the symmetric A_1g mode of the first excited state of complexes like MnX and CrX (X = halide) and makes it possible to measure R changes down to ∽ 10^?3 Å. This work is devoted to understanding, within a molecular orbital framework, the microscopic origin of such a dependence, which is related to the corresponding one displayed by the transferred spin densities fσ, f_s, and fπ. The analysis is focused on MnF. As a main result, it is shown that though fσ ? f_s the interaction between d(e_g) orbitals and 2s orbitals of F^? is not only primarily responsible for the R dependence of 10Dq but also makes a significant contribution to the 10Dq value itself. The present work thus shows that the significant dependence of 10Dq upon R is ultimately related to the strong dependence of f_s and the isotropic superhyperfine constant A_s upon R displayed by the experimental results of several 3d impurities. © John Wiley & Sons, Inc.     

Impurities in noncubic crystals: stabilization mechanisms for Jahn-Teller ions in layered perovskites

Mechanisms responsible for the local geometry around Jahn-Teller impurities in K2NiF4 type lattices are shown to be different from those generating the warping in cubic crystals. The present density functional theory calculations reveal that the elastic anisotropy of the host lattice (visible for closed shell impurities) and the electric field created by the rest of lattice ions upon active electrons make it possible to have d(9) ions in an elongated geometry but with a A(1g) ground state. The puzzling difference between equilibrium geometries for Cu2+ and Ni+ in layered perovskites can reasonably be understood.     

C-terminal processing of yeast Spt7 occurs in the absence of functional SAGA complex

Background  

Spt7 is an integral component of the multi-subunit SAGA complex that is required for the expression of ~10% of yeast genes. Two forms of Spt7 have been identified, the second of which is truncated at its C-terminus and found in the SAGA-like (SLIK) complex.     

Study of the spin-Hamiltonian of [MX4]2- Jahn-Teller complexes on the basis of M.O. theory

First order calculations of the spin Hamiltonian parameters of [MX₄]^2- Jahn-Teller complexes on the basis of M.O. theory are shown. The M.O.-coefficients and the d-d transitions (in agreement with the experimental values) are estimated for Cu²⁺: LiCl from the analysis of the EPR results.     

Pressure effects on CrCl6(3-) embedded in cubic Cs2NaMCl6 (M=Sc,Y) lattices: study through periodic and cluster calculations

The structural, elastic, vibrational, and optical properties of cubic elpasolites Cs2NaMCl6 (M=Sc,Y) containing CrCl6(3-) complexes have been investigated by means of both periodic and cluster calculations as a function of pressure in the framework of density functional theory. Aside from calculating the host lattice bulk modulus BH and the local modulus B1 associated with the CrCl6(3-), complex particular attention is paid to the pressure dependence of Huang-Rhys factors, Sa and Se (related to local a1g and eg modes), and the Stokes shift associated with the first electronic excited state 4T2g (t2g 2eg) of CrCl6(3-). The present calculations provide a big difference between BH=231 kbars and B1=676 kbars derived for Cs2NaScCl6:Cr3+ at zero pressure which plays a key role for a right interpretation of pressure effects on vibration frequencies and optical parameters due to CrCl6(3-). The significant decrease of Huang-Rhys factors, Sa and Se, due to the pressure observed experimentally is well accounted for by the present work which supports that partial differential Sa/ partial differential P is determined by the Gruneisen constant gamma a of the a1g local mode (whose frequency is nu a) and the dependence of 10Dq on the metal-ligand distance. At the same time, the present results point out that the Stokes shift would be little pressure dependent in the range of 0-50 kbars. Accordingly the Ham effect in the 4T2g (t2g 2eg) state of CrCl6(3-) in the cubic elpasolites would also happen for a pressure up to 50 kbars but the spin-orbit constant would increase with respect to that at zero pressure. From the analysis carried out in this work it is also concluded that the figures d nu a/dP=0.55 cm(-1)kbar and dSa/dP=-7.2 x 10(-3) kbar(-1) extracted from the complex emission band of Cs2NaScCl6:Cr3+ are hardly compatible. This fact underlines the usefulness of ab initio calculations for helping in the analysis of complex experimental findings. Finally, as the CrCl6(3-) unit is found to be to a good extent elastically decoupled from the rest of the elpasolite lattice, a model is shown to lead to an approximate relation between the pressure derivative of the local modulus and the Gruneisen constant gamma a.     

Transition metal impurities in wide gap materials: are the electronic properties well described through the ligand field theory?

In the traditional ligand field theory the electronic properties due to a transition metal (TM) impurity, M, in an insulator are explained only in terms of the MX _N complex formed with the N nearest anions. This work is aimed at emphasizing the role played by the electrostatic potential, V _R (r), exerted by the rest of lattice ions upon the localized electrons of the TM complex. This potential, neglected in the traditional ligand field theory, is shown to play a key role when comparing the electronic properties of the same TM complex but embedded in two lattices which are not isomorphous even if both are cubic. As a relevant example it is shown that the different 10 Dq values exhibited by in the normal perovskite KMgF₃ (10 Dq = 14,100 cm⁻¹) and in the inverted perovskite BaLiF₃ (10 Dq = 16,720 cm⁻¹) can hardly be understood only through a distinct Cr³⁺–F⁻ distance. In contrast such a difference is shown to come mainly from the different shape of V _R (r) in these two cubic lattices. the importance of this internal electric field is pointed out to grow when comparing two host lattices with the same ligand and coordination number but very different symmetry. This situation is found in the case of ruby (Al₂O₃ : Cr³⁺) and emerald (Be₃Si₆Al₂O₁₈ : Cr³⁺) where V _R (r) is behind the different colours exhibited by such gemstones. For the sake of clarity a brief discussion on the foundations of the ligand field theory is reported in the present work as well. Contribution to the Serafin Fraga Memorial Issue.     

The transfer matrix: A geometrical perspective

We present a comprehensive and self-contained discussion of the use of the transfer matrix to study propagation in one-dimensional lossless systems, including a variety of examples, such as superlattices, photonic crystals, and optical resonators. In all these cases, the transfer matrix has the same algebraic properties as the Lorentz group in a (2+1)-dimensional spacetime, as well as the group of unimodular real matrices underlying the structure of the abcd$abcd$ law, which explains many subtle details. We elaborate on the geometrical interpretation of the transfer-matrix action as a mapping on the unit disk and apply a simple trace criterion to classify the systems into three types with very different geometrical and physical properties. This approach is applied to some practical examples and, in particular, an alternative framework to deal with periodic (and quasiperiodic) systems is proposed.     

General unit-disk representation for periodic multilayers

We suggest a geometrical framework in which to discuss periodic layered structures in the unit disk. Bandgaps appear when the point representing the system approaches the unit circle. We show that the trace of the matrix describing the basic period allows for a classification in three families of orbits with quite different properties. The laws of convergence of the iterates to the unit circle can then be considered universal features of the reflection.