Comparison of different field tests to assess the physical capacity of post-COVID-19 patients.

BACKGROUND: In coronavirus disease (COVID-19), physical capacity is one of the most impaired sequelae. Due to their simplicity and low cost, field tests such as the six-minute walk test (6MWT) are widely used However, in many places it is difficult to perform them and alternatives can be used such as the 1 min sit-to-stand test (1min-STST) or the Chester step test (CST). Therefore, our objective was to compare the 6MWT, 1min-STST and the CST in post-COVID-19 patients. METHODS: We conducted a cross-sectional analysis in post-COVID-19 patients, compared with matched controls (CG). Demographic characteristics and comorbidities were collected. We analysed oxygen saturation (SpO2), heart rate (HR), and the modified Borg scale in the 6MWT, 1min-STST, and CST. Additionally, the correlations between tests were analysed. RESULTS: We recruited 27 post-COVID-19 patients and 27 matched controls. The median age was 48 (IQR 43-59) years old (44% female). The median distance walked in 6MWT was 461 (IQR 415-506) m in post-COVID-patients and 517 (IQR 461-560) m in CG (p = 0.001). In 1min-STST, the repetitions were 21.9 ± 6.7 and 28.3 ± 7.1 in the post-COVID-19 group and CG, respectively (p = 0.001). In the CST, the post-COVID-19 group performed 150 (86-204) steps vs the CG with 250 (250-250) steps (p < 0.001). We found correlations between the 6MWT with the 1min-STST in COVID-19 patients (r = 0.681, p < 0.001) and CG (r = 0.668, p < 0.001), and between the 6MWT and the CST in COVID-19 patients (r = 0.692, p < 0.001). CONCLUSION: The 1min-STST and the CST correlated significantly with the 6MWT in patients post-COVID-19 being alternatives if the 6MWT cannot be performed.

Local negative permittivity and topological phase transition in polar skyrmions.

Topological solitons such as magnetic skyrmions have drawn attention as stable quasi-particle-like objects. The recent discovery of polar vortices and skyrmions in ferroelectric oxide superlattices has opened up new vistas to explore topology, emergent phenomena and approaches for manipulating such features with electric fields. Using macroscopic dielectric measurements, coupled with direct scanning convergent beam electron diffraction imaging on the atomic scale, theoretical phase-field simulations and second-principles calculations, we demonstrate that polar skyrmions in (PbTiO3)n/(SrTiO3)n superlattices are distinguished by a sheath of negative permittivity at the periphery of each skyrmion. This enhances the effective dielectric permittivity compared with the individual SrTiO3 and PbTiO3 layers. Moreover, the response of these topologically protected structures to electric field and temperature shows a reversible phase transition from the skyrmion state to a trivial uniform ferroelectric state, accompanied by large tunability of the dielectric permittivity. Pulsed switching measurements show a time-dependent evolution and recovery of the skyrmion state (and macroscopic dielectric response). The interrelationship between topological and dielectric properties presents an opportunity to simultaneously manipulate both by a single, and easily controlled, stimulus, the applied electric field.

Global impact of diet and temperature over aquaculture of Octopus vulgaris paralarvae from a transcriptomic approach.

Common octopus, Octopus vulgaris, is an economically important cephalopod species. However, its rearing under captivity is currently challenged by massive mortalities previous to their juvenile stage due to nutritional and environmental factors. Dissecting the genetic basis and regulatory mechanism behind this mortality requires genomic background knowledge. A transcriptomic sequencing of 10 dph octopus paralarvae from different experimental conditions was constructed via RNA-seq. A total of 613,767,530 raw reads were filtered and de novo assembled into 363,527 contigs of which 82,513 were annotated in UniProt carrying also their GO and KEGG information. Differential gene expression analysis was carried out on paralarvae reared under different diet regimes and temperatures, also including wild paralarvae. Genes related to lipid metabolism exhibited higher transcriptional levels in individuals whose diet includes crustacean zoeas, which had an impact over their development and immune response capability. High temperature induces acclimation processes at the time that increase metabolic demands and oxidative stress. Wild individuals show an expression profile unexpectedly similar to Artemia fed individuals. Proteomic results support the hypothesis revealed by transcriptional analysis. The comparative study of the O. vulgaris transcriptomic profiles allowed the identification of genes that deserve to be further studied as candidates for biomarkers of development and health. The results obtained here on the transcriptional variations of genes caused by diet and temperature will provide new perspectives in understanding the molecular mechanisms behind nutritional and temperature requirements of common octopus that will open new opportunities to deepen in paralarvae rearing requirements.

Observation of room-temperature polar skyrmions.

Complex topological configurations are fertile ground for exploring emergent phenomena and exotic phases in condensed-matter physics. For example, the recent discovery of polarization vortices and their associated complex-phase coexistence and response under applied electric fields in superlattices of (PbTiO3)n/(SrTiO3)n suggests the presence of a complex, multi-dimensional system capable of interesting physical responses, such as chirality, negative capacitance and large piezo-electric responses1-3. Here, by varying epitaxial constraints, we discover room-temperature polar-skyrmion bubbles in a lead titanate layer confined by strontium titanate layers, which are imaged by atomic-resolution scanning transmission electron microscopy. Phase-field modelling and second-principles calculations reveal that the polar-skyrmion bubbles have a skyrmion number of +1, and resonant soft-X-ray diffraction experiments show circular dichroism, confirming chirality. Such nanometre-scale polar-skyrmion bubbles are the electric analogues of magnetic skyrmions, and could contribute to the advancement of ferroelectrics towards functionalities incorporating emergent chirality and electrically controllable negative capacitance.

Phase coexistence and electric-field control of toroidal order in oxide superlattices.

Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO3/SrTiO3 superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a1/a2 phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotropic electrotoroidal compounds. Finally, application of electric fields to this mixed-phase system permits interconversion between the vortex and the ferroelectric phases concomitant with order-of-magnitude changes in piezoelectric and nonlinear optical responses. Our findings suggest new cross-coupled functionalities.

Selection of reliable reference genes for RT-qPCR studies in Octopus vulgaris paralarvae during development and immune-stimulation.

The common octopus, Octopus vulgaris is a new candidate species for aquaculture. However, rearing of octopus paralarvae is hampered by high mortality and poor growth rates that impede its entire culture. The study of genes involved in the octopus development and immune response capability could help to understand the key of paralarvae survival and thus, to complete the octopus life cycle. Quantitative real-time PCR (RT-qPCR) is the most frequently tool used to quantify the gene expression because of specificity and sensitivity. However, reliability of RT-qPCR requires the selection of appropriate normalization genes whose expression must be stable across the different experimental conditions of the study. Hence, the aim of the present work is to evaluate the stability of six candidate genes: ß-actin (ACT), elongation factor 1-α (EF), ubiquitin (UBI), ß-tubulin (TUB), glyceraldehyde 3-phosphate dehydrogenase (GADPH) and ribosomal RNA 18 (18S) in order to select the best reference gene. The stability of gene expression was analyzed using geNorm, NormFinder and Bestkeeper, in octopus paralarvae of seven developmental stages (embryo, paralarvae of 0, 10, 15, 20, 30 and 34days) and paralarvae of 20days after challenge with Vibrio lentus and Vibrio splendidus. The results were validated by measuring the expression of PGRP, a stimuli-specific gene. Our results showed UBI, EF and 18S as the most suitable reference genes during development of octopus paralarvae, and UBI, ACT and 18S for bacterial infection. These results provide a basis for further studies exploring molecular mechanism of their development and innate immune defense.

Challenges in assignment of orbital populations in a high spin manganese(III) complex.

Magnetic, structural and computational data of four complex salts with the same mononuclear high spin octahedral Mn(iii) complex cation are reported. The manifestation of Jahn-Teller-like distortions in the Mn(iii) cation is dependent on the nature of the charge-balancing anion, with small anions yielding a planar elongation and large anions freezing out a preferential axial elongation along one of the amine-Mn-imine directions within that same plane. Modulation of the lattice by changing the charge balancing anion results in mixing of the orbital symmetry due to vibrational perturbation.

Risk factors for tibial tuberosity fracture after tibial tuberosity advancement in dogs.

OBJECTIVE: To retrospectively identify factors that predispose to tibial tuberosity (TT) fracture after tibial tuberosity advancement (TTA) in dogs. METHODS: The medical records and radiographs of a group of control dogs (n = 212) that had TTA surgery (n = 241 procedures) and did not sustain a fracture between 2008 and 2013, and those of 12 dogs that did sustain a fracture (n = 13 procedures) between 2008 and 2013 at two veterinary teaching hospitals were evaluated to determine the effect of signalment, body weight and surgical inaccuracies on TT fracture. Multivariable logistic regression was performed with the occurrence of TT fracture as the outcome variable of interest. RESULTS: Signalment and body weight were not found to be associated with TT fracture. Of the surgical inaccuracies, osteotomy shape (p = 0.003), plate position (p = 0.009), and cage position (p = 0.039) were factors significantly associated with TT fracture. CLINICAL SIGNIFICANCE: This study provides data to support the hypothesis that poor plate position, poor cage position, and narrow distal osteotomy width are associated with TT fracture after TTA. We conclude that it is of paramount importance to pay careful attention to surgical technique in order to reduce this risk.

Surgical management and follow-up of triceps tendon avulsion after repeated local infiltration of steroids: two cases.

Disruption of the triceps tendon insertion is an uncommon injury, with only a few reports of traumatic triceps tendon avulsion in dogs or cats present in the veterinary literature. Although this injury has been previously described in two dogs that had received a local injection of corticosteroids for the treatment of soft tissue injuries, reports with long term clinical follow-up and outcome in working dogs are lacking. This report describes two surgically treated cases of complete triceps tendon avulsion rupture in Malinois police dogs. These two dogs had a history of repeated local infiltration of a corticosteroid preparation used to treat adventitial bursitis. The clinical features, surgical treatment, long-term management and outcome are reported. A modified triple locking loop pattern was used to suture the tendons. The long-term outcome was considered excellent with both dogs resuming work. Although it is an uncommon condition in small animals, avulsion of the triceps tendon should remain on the differential diagnosis list for acute onset forelimb lameness, particularly if there is a history of local infiltration with corticosteroids. The diagnosis should be based on clinical, radiographic and ultrasonographic examination.

Quantifying local and cooperative components in the ferroelectric distortion of BaTiO(3): learning from the off-center motion in the MnCl(6)(5-) complex formed in KCl:Mn(+).

The delicate balance between cooperative and local contributions in the ferroelectric distortions of BaTiO3 is explored by means of ab initio calculations. As a salient feature, it is found that a single Ti(4+) ion in BaTiO3 is not allowed to move off-center at ambient pressure, while this is no longer true if the lattice is expanded by only ∼5%, stressing the high sensitivity of the local contribution to chemical and hydrostatic pressures. In order to further understand the effect of local contributions on the phase transition mechanism of ferroelectrics, we have investigated the surprising C3v → C4v → Oh local transformations occurring in the 10-50 K temperature range for the MnCl6(5-) complex formed in KCl:Mn(+) that mimic the behavior of BaTiO3. From Boltzmann analysis of the vibronic levels derived from ab initio calculations and considering decoherence introduced by random strains, the present calculations reproduce the experimental phase sequence and transition temperatures. Furthermore, our calculations show that the off-center instability in KCl:Mn(+) would be suppressed by reducing by only 1% the lattice parameter, a situation that then becomes comparable to that found for BaTiO3 at ambient pressure. The present results thus stress the deep link between the structural phase transitions of ferroelectric materials and local phase transitions displayed by transition-metal impurities in insulators.

Sharp lines due to Cr³âº and Mn²âº impurities in insulators: going beyond the usual Tanabe-Sugano approach.

This work is aimed at understanding the different behavior of optical sharp lines (corresponding to 10Dq-independent transitions) of Mn(2+) and Cr(3+) in normal and inverted perovskites that cannot be explained within the usual Tanabe-Sugano approach. In particular, we want to clarify why on passing from KMgF3:M to LiBaF3:M (M = Mn(2+), Cr(3+)) the energy, E((6)A1 → (4)A1), for Mn(2+) decreases by Δ = 1100 cm(-1), while Δ < 100 cm(-1) for the energy E((2)E →( 4)A2) corresponding to Cr(3+). The origin of this surprising difference in these model systems is clarified by writing the transition energies of MF6 complexes through the ten Coulomb and exchange integrals consistent with the cubic symmetry and not considered in the usual Tanabe-Sugano approach. It is shown that E((6)A1 → (4)A1) depends on exchange integrals K(3z(2) - r(2), xy) and K(x(2) - y(2), xy), while E((2)E → (4)A2) depends on K(xz, yz) where the two involved electrons display a π character. These exchange integrals have been calculated just considering a MF6 unit subject to the internal electric field due to the rest of the lattice ions. In addition to a reasonably reproduction of the main trends observed experimentally for the model systems, the present calculations prove that the exchange integrals are not related in a simple way to the covalency of involved orbitals. Particular attention is also paid to explain why the transitions, which are 10Dq-independent are less sensitive to the host lattice change than those which do depend on 10Dq. The present work shows that K(xz, yz) for Cr(3+) is particularly insensitive to the host lattice change and thus sheds light on the origin of the near independence of E((2)E → (4)A2) along the series of oxides doped with such an impurity .

Tibial tuberosity fracture as a complication of tibial tuberosity advancement.

OBJECTIVES: To retrospectively compare two clinical subsets of dogs suffering tibial tuberosity (TT) fracture (incidental finding or sudden onset severe lameness) as a complication of tibial tuberosity advancement (TTA) and to report the surgical management and outcome of TT fracture as a complication of TTA. MATERIAL AND METHODS: The medical records of 10 dogs with eleven TT fractures or crest fractures after TTA were reviewed. The outcome and complications were determined from clinical and radiographic follow-up examinations. Limb function was evaluated between six and 12 weeks postoperatively. Owners were contacted by phone for long-term follow-up at least six months after the last examination. RESULTS: Four dogs required surgical stabilization and six dogs had conservative management. In the surgical group, every case experienced a sudden non-weight-bearing lameness after the initial TTA surgery. In three of the four cases an attempt was made to stabilize the TT and crest fracture while maintaining the TT advancement. Postoperative complications were encountered in three of the four surgically treated cases. Functional outcome was considered excellent in seven cases and good in the other three. CLINICAL SIGNIFICANCE: Tibial tuberosity fracture is a complication of TTA that seems to have a favourable prognosis, although it can result in significant morbidity and in some cases revision surgery may be required.

Development and validation of the Spanish Hazard Perception Test.

OBJECTIVE: The aim of the current study is to develop and obtain valid evidence for a hazard perception test suitable for the Spanish driving population. To obtain valid evidence to support the use of the test, the effect of hazardous and quasi-hazardous situations on the participants' hazard prediction is analyzed and the pattern of results for drivers with different driving experience--that is, learner, novice, and expert drivers and reoffender vs. nonoffender drivers--is compared. Potentially hazardous situations are those that develop without involving any real hazard (i.e., the driver did not actually have to decelerate or make any evasive maneuver to avoid a potential collision). The current study analyzed repeat offender drivers attending compulsory reeducation programs as a result of reaching the maximum number of penalty points on their driving license due to repeated violations of traffic laws. METHOD: A new video-based hazard perception test was developed, using a total of 20 hazardous situation videos plus 8 quasi-hazardous situation videos. They were selected from 167 recordings of natural hazards in real Spanish driving settings. RESULTS: The test showed adequate psychometric properties and evidence of validity, distinguishing between different types of drivers. Psychometric results confirm a final version of the hazard perception test composed of 11 video clips of hazards and 6 video clips of quasi-hazards, for which an overall Cronbach's alpha coefficient of.77 was obtained. A lack of ability to detect quasi-hazards and distinguish them from hazardous situations was also found for learner, novice, and reoffender drivers. Learner drivers obtained lower average scores than novice and experienced drivers with the hazardous situation videos, and learner drivers obtained lower average scores than experienced drivers with the quasi-hazardous situation videos, suggesting that the ability to correctly identify hazardous traffic situations may develop early by accumulating initial driving experience. However, the ability to correctly identify quasi-hazardous situations may develop later with the accumulation of further driving experience. Developing this ability is also difficult for reoffender drivers. CONCLUSION: The test has adequate psychometric properties and is useful in distinguishing between learner, novice, and expert drivers. In addition, it is useful in that it analyzes the performance of both safe and unsafe drivers (reoffenders who have already lost their driving license).

Transition metal complexes coupled to vacancies in oxides: origin of different properties of Cr3+ in MgO bounded to a <100> or <110> Mg2+ vacancy.

Despite the importance of vacancies over the properties of insulating oxides its influence on neighboring transition metal ions is far from being understood. This work is devoted to find the origin of various up to now unexplained properties of chromium bounded either to a <100> or a <110> Mg(2+) vacancy in MgO. In these model systems particular attention is paid to understand, by means of ab initio calculations, why the cubic field splitting parameter, 10Dq, is surprisingly 1600 cm(-1) higher for a <100> than for a <110> vacancy, a fact behind the suppression of the sharp (2)E → (4)A2 luminescence in the latter case. Our calculations, which reproduce the main experimental facts, prove that the average Cr(3+)-O(2-) distance is the same within 0.8% for both systems, and thus, the low 10Dq value for a <110> vacancy is shown to be due mainly to the electrostatic potential from the missing Mg(2+) ion, which increases the energy of antibonding t(2g) (∼xy, xz, yz) levels. By contrast, for a <100> Mg(2+) vacancy that potential provides a supplementary increase of the e(g) (∼x(2) - y(2), 3z(2 )- r(2)) level energy and thus of 10Dq. The existence of the (2)E → (4)A2 luminescence for Cr(3+)-doped MgO under perfect cubic symmetry or with a <100> vacancy is shown to be greatly helped by the internal electric field created by the rest of the lattice ions on the CrO6(9-) unit, whose importance is usually ignored. The present results underline the role of ab initio calculations for unveiling the subtle effects induced by a close vacancy on the properties of transition metal ions in oxides. At the same time they stress the failure of the empirical superposition model for deriving the equilibrium geometry of C4v and C2v centers in MgO:Cr(3+).

Deviations from Born-Oppenheimer theory in structural chemistry: Jahn-Teller, pseudo Jahn-Teller, and hidden pseudo Jahn-Teller effects in C3H3 and C3H3(-).

The electronic structure and vibronic coupling in two similar molecular systems, radical C3H3 and anion C3H3(-), in ground and excited states, are investigated in detail to show how their equilibrium structures, in deviation from the Born-Oppenheimer approximation, originate from the vibronic mixing of at least two electronic states, producing the Jahn-Teller (JT), pseudo JT (PJT), and hidden PJT effects. Starting with the high-symmetry geometry D3h of C3H3, we evaluated its 2-fold degenerate ground electronic state (2)E″ and two lowest excited states (2)A1' and (2)E' and found that all of them contribute to the distortion of the ground state via the JT vibronic coupling problem E″ ⊗ e' and two PJT problems (E″ + A1') ⊗ e″ and (E″ + E') ⊗ (a2″ + e″); all the three active normal modes e'(1335 cm(-1)), e″(1030 cm(-1)), and a2″(778 cm(-1)) are imaginary, meaning that all the three vibronic couplings are sufficiently strong to cause instability, albeit in different directions. The first of them, the ground state JT effect, enhances one of the C-C bonds (toward an ethylenic form with C2v symmetry), while the two PJT effects produce, respectively, cis (a2″ toward C3v symmetry) and trans (e″) puckering of the hydrogen atoms. As a result, C3H3 has two coexisting equilibrium configurations with different geometry. In the C3H3(-) anion, the ground electronic state in D3h symmetry is an orbitally nondegenerate spin triplet (3)A2' with a group of close in energy singlet and triplet excited states in the order of (1)A1', (3)A1″, (1)E″, (3)E″, and (1)E'. This shows that two PJT couplings, ((3)A2' + (3)A1″) ⊗ a2″ and ((3)A2' + (3)E″) ⊗ e″, may influence the geometry of the equilibrium structure in the (3)A2' state. Indeed, both vibrational modes, a2″(1034 cm(-1)) and e″(1284 cm(-1)), are imaginary in this state. Similar to the radical case, they produce, respectively, cis (a2″) and trans (e″) puckering of the hydrogen atoms, but no e' distortion of the basic C3 triangle; the equilibrium configuration with Cs symmetry occurs along the stronger e″ distortions. Another higher-in-energy triplet-state minimum with C2v symmetry emerges as a result of a strong JTE in the excited (3)E″ electronic state. In addition to these APES minima with spin-triplet electronic states, the system has a coexisting minimum with a spin-singlet electronic state, which is shown to be due to the hidden PJT effect that couples two singlet excited states. The two lowest equilibrium configurations of the C3H3(-) anion with different geometry and spin realize a (common to all electronic e(2) configurations) magnetic and structural bistability accompanied by a spin crossover. Some general spectroscopic consequences are also noted. As a whole, this article is intended to demonstrate the efficiency of the vibronic coupling approach in rationalizing the origin of complicated structural features of molecular systems as due to a combination of nonadiabatic JT effects.

Origin of small barriers in Jahn-Teller systems: quantifying the role of 3d-4s hybridization in the model system NaCl:Ni+.

Despite its relevance, the microscopic origin of the energy barrier, B, between the compressed and elongated geometries of Jahn-Teller (JT) systems is not well understood yet because of a lack of quantitative data about its various contributions. Seeking to clear up this matter, we have carried out both periodic and cluster ab initio calculations on the model system NaCl:Ni(+). This system is particularly puzzling because, according to experimental data, its barrier is much smaller than that for other d(9) and d(7) ions in similar lattices. All calculations performed on the model system lead, in fact, to values |B| ≤ 160 cm(-1), which are certainly smaller than B = 500 cm(-1) derived for NaCl:M(2+) (M = Ag, Rh) or B = 1024 cm(-1) obtained for KCl:Ag(2+). As a salient feature, analysis of calculations carried out as a function of the Qθ (∼3z(2) - r(2)) coordinate unveils the microscopic origin of the barrier. It is quantitatively proven that the elongated geometry observed for NaCl:Ni(+) is due to the 3d-4s vibronic admixture, which is slightly larger than the anharmonicity in the eg JT mode that favors a compressed geometry. The existence of these two competing mechanisms explains the low value of B for the model system, contrary to cases where the complex formed by d(9) or d(7) ions is elastically decoupled from the host lattice. Although the magnitude of B for NaCl:Ni(+) is particularly small, the tunneling splitting, 3Γ, is estimated to be below 9 cm(-1), thus explaining why the coherence is easily destroyed by random strains and thus a static JT effect is observed experimentally. As a main conclusion, the barrier in JT systems cannot be understood neglecting the tiny changes of the electronic density involved in small distortions. The present calculations reasonably explain the experimental g tensor of NaCl:Ni(+), pointing out that the d-d transitions in NiCl6(5-) are much smaller than those for CuCl6(4-) and the optical electronegativity of Ni(+) is only around 1.

Cu2+ in layered compounds: origin of the compressed geometry in the model system K2ZnF4:Cu2+.

Many relevant properties (including superconductivity and colossal magnetoresistance) of layered materials containing Cu(2+), Ag(2+), or Mn(3+) ions are commonly related to the Jahn-Teller instability. Along this line, the properties of the CuF6(4-) complex in the K2ZnF4 layered perovskite have recently been analyzed using a parametrized Jahn-Teller model with an imposed strain [Reinen, D. Inorg. Chem.2012, 51, 4458]. Here, we present results of ab initio periodic supercell and cluster calculations on K2ZnF4:Cu(2+), showing unequivocally that the actual origin of the unusual compressed geometry of the CuF6(4-) complex along the crystal c axis in that tetragonal lattice is due to the presence of an electric field due to the crystal surrounding the impurity. Our calculations closely reproduce the experimental optical spectrum. The calculated values of the equilibrium equatorial and axial Cu(2+)-F(-) distances are, respectively, R(ax) = 193 pm and R(eq) = 204 pm, and so the calculated distortion R(ax) - R(eq) = 11 pm is three times smaller than the estimated through the parametrized Jahn-Teller model. As a salient feature, we find that if the CuF6(4-) complex would assume a perfect octahedral geometry (R(ax) = R(eq) = 203 pm) the antibonding a(1g)*(∼3z(2) - r(2)) orbital is placed above b(1g)*(∼x(2) - y(2)) with a transition energy E((2)A(1g) → (2)B(1g)) = 0.34 eV. This surprising fact stresses that about half the experimental value E((2)A(1g) → (2)B(1g)) = 0.70 eV is not due to the small shortening of the axial Cu(2+)-F(-) distance, but it comes from the electric field, E(R)(r), created by the rest of the lattice ions on the CuF6(4-) complex. This internal field, displaying tetragonal symmetry, is thus responsible for the compressed geometry in K2ZnF4:Cu(2+) and the lack of symmetry breaking behind the ligand relaxation. Moreover, we show that the electronic energy gain in this process comes from bonding orbitals and not from antibonding ones. The present results underline the key role played by ab initio calculations for unveiling all the complexity behind the properties of the model system K2ZnF4:Cu(2+), opening at the same time a window for improving our knowledge on d(9), d(7), or d(4) ions in other layered compounds.

Colour due to Cr3+ ions in oxides: a study of the model system MgO:Cr3+.

Seeking to understand why the cubic centre in MgO:Cr(3+) has the same 10Dq value as emerald, ab initio cluster and periodic supercell calculations have been performed. It is found that the equilibrium Cr(3+)-O(2-) distance, R, in MgO:Cr(3+) is equal to 2.03 Å and thus 0.06 Å higher than that measured for the emerald. Calculations carried out on the isolated CrO(6)(9-) complex at R = 2.03 Å give 10Dq = 14,510 cm(-1), which is 10% smaller than the experimental figure for MgO:Cr(3+). Nevertheless, when the internal electric field, ER(r), due to the rest of the lattice ions is also taken into account, the calculated 10Dq = 16,210 cm(-1) coincides with the experimental value. Accordingly, the colour shift for different oxides doped with Cr(3+) can be well understood on the basis of this extrinsic contribution to 10Dq usually ignored in a ligand field description. The calculated electrostatic potential, VR(r), related to ER(r), is found to be attractive when the electronic density is lying along <110> directions and |r| > 1 Å driven by the first shell of twelve Mg(2+) ions. The action of VR(r) upon the CrO(6)(9-) complex slightly decreases the energy of t2g(xy,xz,yz) orbitals with respect to that for eg(3z(2) - r(2),x(2) - y(2)) orbitals, thus enhancing the 10Dq value by 0.2 eV. However, the addition of VR(r) induces very small changes in the electronic density, a relevant fact that is related to the (2)E(t(2g)(3)) −> (4)A(2)(t(2g)(3)) emission energy being nearly independent of the host lattice along the series of Cr(3+)-doped oxides.

Cr3+-doped fluorides and oxides: role of internal fields and limitations of the Tanabe-Sugano approach.

This work is aimed at clarifying the changes on optical spectra of Cr(3+) impurities due to either a host lattice variation or a hydrostatic pressure, which can hardly be understood by means of the usual Tanabe-Sugano (TS) approach assuming that the Racah parameter, B, grows when covalency decreases. For achieving this goal, the optical properties of Cr(3+)-doped LiBaF(3) and KMgF(3) model systems have been explored by means of high level ab initio calculations on CrF(6)(3-) units subject to the electric field, E(R)(r), created by the rest of the lattice ions. These calculations, which reproduce available experimental data, indicate that the energy, E((2)E), of the (2)E(t(2g)(3)) → (4)A(2)(t(2g)(3)) emission transition is nearly independent of the host lattice. By contrast, the energy difference corresponding to (4)A(2)(t(2g)(3)) → (4)T(1)(t(2g)(2)e(g)(1)) and (4)A(2)(t(2g)(3)) → (4)T(2)(t(2g)(2)e(g)(1)) excitations, Δ((4)T(1); (4)T(2)), is shown to increase on passing from the normal to the inverted perovskite host lattice despite the increase in covalency, a fact which cannot be accounted for through the usual TS model. Similarly, when the Cr(3+)-F(-) distance, R, is reduced both Δ((4)T(1); (4)T(2)) and the covalency are found to increase. By analyzing the limitations of the usual model, we found surprising results that are shown to arise from the deformation of both 3d(Cr) and ligand orbitals in the antibonding e(g) orbital, which has a σ character and is more extended than the π t(2g) orbital. By contrast, because of the higher stiffness of the t(2g) orbital, the dependence of E((2)E) with R basically follows the corresponding variation of covalency in that level. Bearing in mind the similarities of the optical properties displayed by Cr(3+) impurities in oxides and fluorides, the present results can be useful for understanding experimental data on Cr(3+)-based gemstones where the local symmetry is lower than cubic.