Distortion Of Local Symmetry Research Articles

Down-/Up-Conversion Emission Enhancement by Li Addition: Improved Crystallization or Local Structure Distortion?

Local symmetry distortion by Li addition is acknowledged as an effective strategy for enhancing the luminescence of lanthanide (Ln) doped into a wide range of lattice hosts. Despite extensive literature, direct evidence that supports Li-induced modification of the local crystal-field at the Ln sites is still missing. Herein, we show that the emission enhancement by Li addition in Ln,Li–Y2O3 is due to improved crystallization and not to local structure distortion. Our approach is based on the premise that any distortion/lowering of the local symmetry would reflect into the alteration of the emission shapes and shortening of the emission decays. To this aim, we have extensively investigated the evolution with Li addition and calcination temperature of down (optical and X-ray induced) and up-conversion (UPC) emission of Ln-Y2O3 measured across the visible to near-infrared range. First, a center to center (corresponding to Ln in the C2 and S6/C3i sites of the cubic Y2O3 lattice) as well as global comparison o...

Effect of cerium doping on optical and scintillation properties of transparent YAG ceramic

Abstract Cerium doped transparent YAG ceramics 0.15–2 at% cerium) were prepared using nano-powder technique and vacuum sintering. The effective solubility limit of cerium in YAG was found to lie between 1.0 and 1.5 at%. The PL intensity increases with the cerium content and attains a maximum for 1.0 at% doping. Lifetime of PL is not very sensitive to cerium content, however, a slight decrease in the life-time from 66 ns to 55 ns was observed with increase in cerium content from 0.15 to 2.0 at%. This decrease in PL intensity and life-time is attributed to concentration quenching for the YAG ceramics with cerium content higher than 1.0 at%. A red-shift in the PL peak position was observed and attributed to the local symmetry distortion in YAG matrix.

Enhancing Quantum Yield via Local Symmetry Distortion in Lanthanide-Based Upconverting Nanoparticles

Lanthanide-based upconverting nanoparticles exhibit significant promise for solar energy generation, biological imaging, and security technologies but have not seen widespread adoption due to the prohibitively low efficiencies of current materials. Weak transition dipole moments between 4f orbitals hinder both photon absorption and emission. Here, we introduce a novel way to increase the radiative transition rates in Yb,Er-based upconverting nanoparticles based on local symmetry distortion. Beginning from a host matrix of the well-studied hexagonal (β)-phase NaYF4, we incrementally remove Y3+ ions and cosubstitute for them a 1:1 mixture of Gd3+ and Lu3+. These two ions act to expand and contract the lattice, respectively, inducing local-level distortion while maintaining the average host structure. We synthesize a range of β-NaY0.8–2xGdxLuxF4:Yb0.18Er0.02 nanoparticles and experimentally confirm that particle size, phase, global structure, and Yb3+ and Er3+ concentrations remain constant as x is varied. U...

Pressure-enhanced Insulating State and Trigonal Distortion Relaxation in Geometrically Frustrated Pyrochlore Eu2Sn2O7

The geometrically frustrated pyrochlore Eu2Sn2O7 is an insulator with slight trigonal lattice distortion at ambient condition. High pressure is applied to this system to investigate the responses of structural evolution, optical emission and electrical transport properties. In situ high pressure synchrotron X-ray diffraction, Raman spectroscopy, and photoluminescence studies are performed in Eu2Sn2O7 up to 31.2 and 34.1 GPa, respectively. The abrupt change of the oxygen atomic position without breaking the crystal symmetry is accompanied by disappearing of Raman mode involving SnO6 octahedron distortion around 17.8 GPa. It indicates a pressure-induced second-order iso-structural transition, which suppresses the trigonal distortion in the SnO6 octahedron but enhances the local symmetry distortion of EuO8 hexahedron. Anomalous luminescence of the Eu3+ 4f–4f transition is observed, which confirms the enhancement of EuO8 hexahedral distortion at high pressure region. In situ high-pressure electrical transport...

Comparative study of optical and magneto‐optical properties of normal, disordered, and inverse spinel‐type oxides

Abstract, , , , and thin films were fabricated by pulsed laser deposition at high and low temperatures resulting in crystalline single‐phase normal, inverse, as well as disordered spinel oxide thin films with smooth surface morphology. The dielectric function, determined by spectroscopic ellipsometry in a wide spectral range from 0.5 to 8.5 eV, is compared with the magneto‐optical response of the dielectric tensor, investigated by magneto‐optical Kerr effect spectroscopy in the spectral range from 1.7 to 5.5 eV with an applied magnetic field of 1.7 T. Crystal field, inter‐valence, and inter‐sublattice charge transfer transitions, and transitions from to metal cation 3d or 4s bands are identified in both the principal diagonal elements and the magneto‐optically active off‐diagonal elements of the dielectric tensor. Depending on the degree of cation disorder, resulting in local symmetry distortion, the magneto‐optical response is found to be strongest for high crystal quality inverse spinels and for disordered normal spinel structure, contrary to the first principle studies of and . The results presented provide a basis for deeper understanding of light–matter interaction in this material system that is of vital importance for device‐related phenomena and engineering.

Size effect on magnetic susceptibility of titanium monoxide nanocrystals

For the first time a magnetic quantum‐size effect is found in paramagnetic titanium monoxide nanocrystals with the sizes down to 30 nm. In situ measurements in the temperature range from 300 to 1200 K showed that the magnetic susceptibility of superstoichiometric titanium monoxide nanocrystals TiOy (y > 1) depends not only on the structural state of the sample, but also on the size of nanocrystals. The data analysis both for the ordered and disordered TiOy showed that the reason for inversely proportional dependence of Van‐Vleck paramagnetism on the size of titanium monoxide nanocrystals is distortion of local symmetry of titanium and oxygen atoms near the nanocrystal surface. Van‐Vleck contribution induced by the atomic‐vacancy disordering in titanium monoxide is proportional to the deviation of the long‐order parameter from its maximal value.

Simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF4:Yb3+/Er3+ microcrystals by simply tuning the KF dosage.

A strategy has been adopted for simultaneous morphology manipulation and upconversion luminescence enhancement of β-NaYF4:Yb3+/Er3+ microcrystals by simply tuning the KF dosage. X-ray power diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectra (PL) were used to characterize the samples. The influence of molar ratio of KF to Y3+ on the crystal phase and morphology has been systematically investigated and discussed. It is found that the molar ratio of KF to Y3+ can strongly control the morphology of the as-synthesized β-NaYF4 samples because of the different capping effect of F− ions on the different crystal faces. The possible formation mechanism has been proposed on the basis of a series of time-dependent experiments. More importantly, the upconversion luminescence of β-NaYF4:Yb3+/Er3+ was greatly enhanced by increasing the molar ratio of KF to RE3+ (RE = Y, Yb, Er), which is attributed to the distortion of local crystal field symmetry around lanthanide ions through K+ ions doping. This synthetic methodology is expected to provide a new strategy for simultaneous morphology control and remarkable upconversion luminescence enhancement of yttrium fluorides, which may be applicable for other rare earth fluorides.

Influence of Cr3+ ions doping on growth and upconversion luminescence properties of β-NaYF4:Yb3+/Er3+ microcrystals

β-NaYF4:Yb3+/Er3+ microcrystals codoped with Cr3+ ions were prepared for the first time via a facile hydrothermal method. The influence of doping concentration of Cr3+ ions on growth and upconversion (UC) luminescence properties of β-NaYF4:Yb3+/Er3+ microcrystals were investigated in detail. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra and photoluminescence (PL) spectra were used for characterization. The results demonstrated that the morphology and size of β-NaYF4:Yb3+/Er3+ microcrystals were influenced by doping concentration of Cr3+ ions, the possible mechanism was proposed. The UC intensity of β-NaYF4:Yb3+/Er3+ microcrystals was significantly enhanced by doping of Cr3+ ions. In contrast to β-NaYF4:Yb3+/Er3+ microcrystals without doping of Cr3+ ions, the green and red emission intensities of the microcrystals codoped with 15 mol% Cr3+ ions were enhanced by 16 and 7 times, respectively. The enhancement of the UC intensities should be ascribed to the distortion of local symmetry around Er3+ ions, the UC mechanism was discussed. This work will be helpful for improving the UC intensity of other lanthanide-doped UC nano- and micro-crystals.

New insights into the chemical structure of Y2Ti2O7−δ nanoparticles in oxide dispersion-strengthened steels designed for sodium fast reactors by electron energy-loss spectroscopy

In this paper we study by high resolution scanning transmission electron microscopy coupled with electron energy-loss spectroscopy (STEM-EELS) an oxide dispersion-strengthened (ODS) steel with the nominal composition Fe–14Cr–1W–0.3TiH2–0.3Y2O3 (wt.%) designed to withstand the extreme conditions met in Gen. IV nuclear reactors. After denoising via principal component analysis (PCA) the data are analyzed using independent component analysis (ICA) which is useful in the investigation of the physical properties and chemical structure of the material by separating the individual spectral responses. The Y–Ti–O nanoparticles are found to be homogeneously distributed in the ferritic matrix, sized from 1 to 20nm and match a non-stoichiometric pyrochlore-Y2Ti2O7−δ structure for sizes greater than 5nm. We show that they adopt a (Y–Ti–O)–Cr core–shell structure and that Cr also segregates at the matrix grain boundaries, which may slightly modify the corrosion properties of the steel. Using Ti-L2,3 and O-K fine structure (ELNES) the Ti oxidation state is shown to vary from the center of the nanoparticles to their periphery, from Ti4+ in distorted Oh symmetry to a valency often lower than 3+. The sensitivity of the Ti “white lines” ELNES to local symmetry distortions is also shown to be useful when investigating the strain induced in the nanoparticles by the surrounding matrix. The Cr-shell and the variation of the Ti valence state highlight a complex nanoparticle–matrix interface.

Tunable Optical and Photocatalytic Performance Promoted by Nonstoichiometric Control and Site-Selective Codoping of Trivalent Ions in NaTaO3

The present work explores a solid state route to synthesis of trivalent ions (Eu3+, La3+, etc.) doped NaTaO3 with controlled nonstoichiometric chemistry and lattice parameters with an aim to exploring electronic structure and photocatalytic performance. All samples were fully characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic absorption spectrophotometry, UV–vis diffuse reflectance spectroscopy, and photoluminescence measurement. By employing Eu3+ as a model trivalent ion doped in NaTaO3 lattice, the effects of site-selective doping and nonstoichiometric chemistry on the lattice parameters, band gap structure, photocatalytic activity toward methylene blue solution, and photocatalytic hydrogen evolution were systematically investigated. A nonstoichiometric Na/Ta molar ratio led to site-selective occupation of Eu3+ ions which was changed from sole substitution to dual substitutions. Meanwhile, the nonstoichiometric Na/Ta molar ratio and site-selective occupation of Eu3+ resulted in a monotonous lattice expansion and local symmetry distortion. Lattice variation, doping effects, and its relevant defect chemistry had a great impact on the ν3 mode vibration of the O–Ta bond, which became asymmetric and shifted toward higher wavenumbers. Moreover, contrary to theoretical predictions, Eu3+-doped NaTaO3 nanocrystals showed an abnormal narrowing of the band gap energies and weak visible light absorption with variation of Na/Ta molar ratios, which is thought to be related to doping effects, defect chemistry, and variation of lattice parameters. With well-defined lattice structure and defect centers and electronic structure via nonstoichiometric control and trivalent ions doping, the photocatalytic activity of trivalent ions-doped NaTaO3 can be well regulated and optimized.

Dielectric spectra in molecular single crystals

Abstract A theoretical description of the frequency-dependent, complex dielectric permittivity of molecular single crystals is given within the framework of the relaxation polarization mechanism. The extended angular jump model, accounting for the local symmetry distortion, simulates the hindered molecular motion. Both the real and imaginary parts of dielectric permittivity are anisotropic in all cases of point symmetry groups of molecule motion, except for those of the cubic crystallographic system. The three-dimensional graphs of dielectric spectra, which can be helpful in practical application, illustrate the predicted spectra.

Static and dynamic magnetic properties of an[Fe13]cluster

The static and dynamic magnetic properties of an $[{\mathrm{Fe}}_{13}]$ cluster were investigated using several experimental techniques. The cluster crystallizes in a cubic space group, but careful investigation of the crystallographic data revealed that the symmetry is distorted locally. dc magnetic susceptibility measurements showed the presence of competing antiferromagnetic isotropic exchange interactions leading to a high-spin ground state and many low-lying excited spin states, which was confirmed by inelastic neutron scattering measurements. From high-field electron paramagnetic resonance measurements a small zero-field splitting of the spin ground state was inferred, which supports the local symmetry distortion found in the crystallographic studies. The spin dynamics slows down at sub-kelvin temperatures, where ac susceptibility measurements indicated that part of the sample shows superparamagnetic-like behavior and the other part relaxes through magnetization tunneling. The M\"ossbauer data confirmed the slowing down of the spin dynamics, indicating that this occurs mainly in the peripheral spins.

ADENINE TAUTOMER ELECTRONIC STRUCTURAL SIGNATURES STUDIED USING DUAL SPACE ANALYSIS

Electronic structural signatures of the adenine-7H tautomer in its ground state ( X1A′) have been identified using dual space analysis. The RHF/TZVP and B3LYP/TZVP calculations employed in the present work have shown that proton transfer from N(9)to N(7)makes the purine rings of adenine-7H geometrically closer to purine rather than adenine-9H, at the cost distortion of the amino group local symmetry. The shifts in ring lengths in [Formula: see text] and [Formula: see text], which are proposed in this work as the summation of the bond lengths in the hex- and pentagon-rings, respectively, are [Formula: see text] and [Formula: see text] for adenine-9H and [Formula: see text] and [Formula: see text] for adenine-7H, respectively, from the corresponding rings in purine. We also demonstrate that energetic properties such as the orbital energies are insensitive to tautomerism. However, the N(9)to N(7)proton transfer significantly changes the dipole moments: a large increase from 2.44D (adenine-9H) to 7.49D (adenine-7H) is observed. Finally, four molecular orbitals, 27a′, 25a′, 24a′ and 23a′ are identified in momentum space as bearing the most important structural signatures of the adenine-7H tautomer.

Topotactic Transformations of Sodalite Cages: Synthesis and NMR Study of Mixed Salt‐Free and Salt‐Bearing Sodalites.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Topotactic transformations of sodalite cages: synthesis and NMR study of mixed salt-free and salt-bearing sodalites.

A series of mixed sodalite samples, Na(8)[Al(6)Si(6)O(24)]Br(x).(H(3)O(2))(2-x), with the unit cell stoichiometries varying in the 0 < x <2 region, was made by hydrothermal synthesis and subsequently transformed into Na(6+x)[Al(6)Si(6)O(24)]Br(x).(4H(2)O)(2-x) and Na(6+x)[Al(6)Si(6)O(24)]Br(x).circle(2-x) sodalites. Here, circle refers to an empty sodalite cage. The three series, referred hereafter to as the Br/basic, Br/hydro, and Br/dry series, were characterized by powder diffraction X-ray and by (23)Na, (27)Al, and (81)Br magic angle spinning (MAS) NMR and high-resolution triple quantum (TQ) MAS NMR spectroscopy. We determined that incorporation of Br(-) anions is 130 times more preferred than incorporation of H(3)O(2)(-) anions during the formation of sodalite cages, which permitted precise control of the halide content in the solid. Monotonic trends in chemical shifts were observed as a function of cage occupancy, reflecting continuous changes in structural parameters. A linear correlation between (81)Br chemical shift and lattice constant with a slope of -86 ppm/A was observed for all three series. Likewise, (23)Na chemical shifts for Na(+) cations in salt-bearing sodalite cages correlate linearly with the lattice constant. Both results indicate a universal dependence of the (23)Na and (81)Br chemical shifts on the Na-Br distance. The (27)Al chemical shifts of Br/basic and Br/hydro sodalites obey an established relation between delta(cs) and the average T-O-T bond angle of 0.72 ppm/degrees. Br/dry sodalites show two aluminum resonances, characterized by significantly different chemical shifts and quadrupolar interaction parameters. In that series, local symmetry distortions are evident from strong quadrupolar perturbations in the NMR spectra. P(Q) values for (27)Al vary between 0.8 MHz in Br/basic sodalites and 4.4 MHz in the Br/dry series caused by deviations from the tetrahedral symmetry of the salt-free sodalite cages. For (23)Na, P(Q) values of 0.8, 0.8, 2.0, and 5.7 MHz were found for sodium in bromo, basic, hydro, and dry cages, respectively. In addition, both (23)Na and (81)Br spectra offer some evidence that the Br(-) anions in the Br/dry sodalite are displaced from the center of the expanded sodalite cage. For all three series, the spectral deconvolution of the (23)Na NMR line shapes permits an accurate determination of the mixed sodalite stoichiometry.

31P MAS-NMR study of R′ xR″ yP 6O 18.nH 2O cyclohexaphosphates

Several alkali/alkaline-earth R x +R y 2+P 6O 18.nH 2O cyclohexaphosphates, with R +=Li, Na, K, Cs, NH 4, Tl, DBA; R 2+=Ca, Zn, Cd, have been analyzed by 31P MAS-NMR spectroscopy. From their NMR spectra, the number and the multiplicity of crystallographic sites have been analyzed and compared with those deduced by symmetry considerations from X-ray diffraction data. A linear dependence of the isotropic chemical shift values of NMR components on the POP angles of cycles has been obtained. However, some influence of the charge and polarizing strength of alkali and alkaline-earth cations has also been detected. Finally, principal values of the chemical shift tensor of P atoms have been analysed in terms of local symmetry and tetrahedral ring distortions.

CP/MAS NMR and X-ray crystallographic characterization of trans-PdX2(PPh2vinyl)2 (X = Cl, I); UV and Et2O·BF3 reaction studies, including the formation of [Pd(µ-Cl)(PPh2vinyl)2]2[BF4]2

The complexes PdCl2(PPh2vinyl)2 and PdI2(PPh2vinyl)2 have been prepared and crystallographically characterized as their trans square planar isomers. Trans-PdCl2(PPh2vinyl)2 exists in a centrosymmetric structure with a planar PdCl2P2 core whereas trans-PdI2(PPh2vinyl)2 shows no local Pd-centred symmetry and significant distortion of the PdI2P2 core from planarity. Crystallographic data is in accord with the 31P{1H} CP/MAS NMR spectra of the two complexes, which display a single resonance for the chloro-complex and two resonances for the iodo-complex. UV irradiation of CH2Cl2 solutions of PdCl2(PPh2vinyl)2 followed by re-dissolution in CDCl3 indicates no permanent chemical change. However, post-irradiation CP/MAS 31P{1H} NMR spectroscopy demonstrates the presence of trans-PdCl2(PPh2vinyl)2 in two solid state structures, plus cis-PdCl2(PPh2vinyl)2. PdI2(PPh2vinyl)2 exists only in its trans form in both solid state and solution. Irradiation results in phosphine displacement and the formation of sym-[PdI2(PPh2vinyl)]2 and free phosphine, the latter being characterized as Ph2P(O)vinyl following aerobic oxidation. PdCl2(PPh2vinyl)2 reacts in the presence of Et2O·BF3 to afford [Pd(µ-Cl)(PPh2vinyl)2]2[BF4]2 whereas PdI2(PPh2vinyl)2 is recovered in 95% yield, with the remaining material undergoing phosphine abstraction to form sym-[PdI2(PPh2vinyl)]2.Key words: palladium, phosphine, alkene, isomerization, CP/MAS.

Polyoxometallates as models for oxide catalysts: Part II. Theoretical semi-empirical approach to the influence of the inner and outer Mo coordination spheres on the electronic levels of polyoxomolybdates

In complement to Part I, theoretical MO calculations (EHMO level) show that the main factor which influences the electronic energy levels of polyoxomolybdates is not always the local Mo symmetry (tetrahedral or octahedral). The effect of protonation and/or grafting on a surface oxide is weak but slightly larger for tetrahedral than for octahedral species and could lead to wrong attributions. The condensation of several sites in polyoxomolybdates has a greater effect since it involves a MO delocalization over the whole compound, and therefore a loss of the individuality of each octahedral or tetrahedral unit. It leads to a decrease in the HOMO-LUMO gap yielding to an absorption red shift. The effect of distortion of local symmetry by bond elongation tends to decrease the HOMO-LUMO gap in an isolated species. In a condensed system, this distortion lowers the MoMo interactions and moderates the red shift due to the effect of condensation. This latter effect, however, remains predominant.

Conformational distortions from local symmetry in cyclic and acyclic compounds: Crystallographic data analysis of C 3CZCC 3 fragments (Z = C, P, Si, N, S, O).

The local conformational symmetry distortions of C 3CZCC 3 fragments versus standard reference are analyzed in a conformational φ 1, φ 2 mean torsion angle space. Local distortions induced by strain are interpreted as resulting from a conrotatory (φ 1 x φ 2 > 0 quadrants 1 and 3) or a disrotatory relaxation process (φ 1 x φ 2 < 0 quadrants 2 and 4). Impressive sectorization of acyclic fragments in quadrants 1 and 3 and cyclic ones (6 atom cycles) in quadrants 2 and 4 is reported.

Impurities and Charge Compensation in Some Molybdenium Oxides

AbstractThe role of impurities in the defect formation in MoO3 and Na2Mo2O7 crystals is discussed. The EPR spectra of the X‐ray and uv‐irradiation centres are investigated. The constants of the appropriate spin‐Hamiltonians are obtained. It is shown that the localization radius of an electron (or a hole) exceeds the interatomic distances, therefore the EPR spectra have a low symmetry with noncoincident axes of the A‐ and g‐tensors. The unpaired density distribution depends on the characteristics of the lattice and impurity cations such as the electronegativity, the coordination number, and the degree of the local environment symmetry distortion.