Fluorescence and phosphorescence of Cr3+ in cubic hosts

Structural and Optical Properties of Single- and Few-Layer Magnetic Semiconductor CrPS4

Atomically thin binary two-dimensional (2D) semiconductors exhibit diverse physical properties depending on their composition, structure, and thickness. By adding another element in these materials, which will lead to formation of ternary 2D materials, the property and structure would greatly change and significantly expanded applications could be explored. In this work, we report structural and optical properties of atomically thin chromium thiophosphate (CrPS₄), a ternary antiferromagnetic semiconductor. Its structural details were revealed by X-ray and electron diffraction. Transmission electron microscopy showed that preferentially cleaved edges are parallel to diagonal Cr atom rows, which readily identified their crystallographic orientations. Strong in-plane optical anisotropy induced birefringence that also enabled efficient determination of crystallographic orientation using polarized microscopy. The lattice vibrations were probed by Raman spectroscopy and exhibited significant dependence on thickness of crystals exfoliated down to a single layer. Optical absorption determined by reflectance contrast was dominated by d-d-type transitions localized at Cr³⁺ ions, which was also responsible for the major photoluminescence peak at 1.31 eV. The spectral features in the absorption and emission spectra exhibited noticeable thickness dependence and hinted at a high photochemical activity for single-layer CrPS₄. The current structural and optical investigation will provide a firm basis for future study and application of this kind of atomically thin magnetic semiconductors.

Large Differences in the Optical Spectrum Associated with the Same Complex: The Effect of the Anisotropy of the Embedding Lattice

Transition-metal complexes with a well-defined geometry are usually considered to display almost the same properties independently of the system where they are embedded. Here we show that the above statement is not true depending on the anisotropy of the host lattice, which is revealed in the form of the electric field created by the rest of lattice ions over the complex. To illustrate this concept we analyze the origin of the surprisingly large differences in the d-d optical transitions of two systems containing square-planar CuF₄^2- complexes, CaCuF₄, and center II in Cu²⁺-doped Ba₂ZnF₆, even though the Cu²⁺-F^-distance difference is just found to be 1%. Using a minimalist first-principles model we show that the different morphology of the host lattices creates an anisotropic field that red-shifts the in vacuo complex transitions to the 1.25-1.70 eV range in CaCuF₄, while it blue-shifts them to the 1.70-3.0 eV region in Ba₂ZnF₆:Cu²⁺. This particular example shows how the lattice anisotropy strongly alters the optical properties of a given transition-metal complex. This knowledge opens a new path to tune the spectra of this large family of systems.

Structural insights intoM2O–Al2O3–WO3(M= Na, K) system by electron diffraction tomography

TheM2O–Al2O3–WO3(M= alkaline metals) system has attracted the attention of the scientific community because some of its members showed potential applications as single crystalline media for tunable solid-state lasers. These materials behave as promising laser host materials due to their high and continuous transparency in the wide range of the near-IR region. A systematic investigation of these phases is nonetheless hampered because it is impossible to produce large crystals and only in a few cases a pure synthetic product can be achieved. Despite substantial advances in X-ray powder diffraction methods, structure investigation on nanoscale is still challenging, especially when the sample is polycrystalline and the structures are affected by pseudo-symmetry. Electron diffraction has the advantage of collecting data from single nanoscopic crystals, but it is frequently limited by incompleteness and dynamical effects. Automated diffraction tomography (ADT) recently emerged as an alternative approach able to collect more complete three-dimensional electron diffraction data and at the same time to significantly reduce dynamical scattering. ADT data have been shown to be suitable forabinitiostructure solution of phases with large cell parameters, and for detecting pseudo-symmetry that was undetected in X-ray powder data. In this work we present the structure investigation of two hitherto undetermined compounds, K5Al(W3O11)2and NaAl(WO4)2, by a combination of electron diffraction tomography and precession electron diffraction. We also stress how electron diffraction tomography can be used to obtain direct information about symmetry and pseudo-symmetry for nanocrystalline phases, even when available only in polyphasic mixtures.

Synthesis, characterization and spectroscopic investigation of Cr3+ doped wollastonite nanophosphor

This work explores the preparation of nanocrystalline Cr(3+) (1-5 mol%) doped CaSiO3 phosphors by solution combustion process and study of its photoluminescence (PL) behavior. The nanopowders are well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infra-red (FTIR) spectroscopy. PXRD results confirm monoclinic phase upon calcination at 950°C for 3h. SEM micrographs indicates that the powder is highly porous and agglomerated. The TEM images show the powder to consist of spherical shaped particles of size ∼30-60 nm. Upon 323 nm excitation, the emission profile of CaSiO3:Cr(3+) exhibits a narrow red emission peak at 641nm due to (2)E→(4)A2 transition and broad band at 722 nm due to (4)T2g→(4)A2g. It is observed that PL intensity increases with increase in Cr(3+) concentration and highest PL intensity is observed for 3 mol% doped sample. The PL intensity decreases with further increase in Cr(3+) doping. This decrease in PL intensity beyond 3 mol% is ascribed to concentration quenching. Racah parameters are calculated to describe the effects of electron-electron repulsion within the crystal lattice. The parameters show 21% reduction in the Racah parameter of free ion and the complex, indicating the moderate nephelauxetic effect in the lattice.

Sharp lines due to Cr³⁺ and Mn²⁺ 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 .

Theoretical study of local environment of Mn(2+) in two different tetragonal sites in Cs(2)NaLaCl(6) elpasolite

A theoretical method for investigating the inter-relation between the electronic and the molecular structures of a 3d(5) ion in a tetragonal ligand-field has been established on the basis of a 252×252 complete energy matrix. By means of this method, the local lattice structures of Mn(2+) in two different tetragonal sites in Cs2NaLaCl6 elpasolite are determined by the experimental EPR spectra. The Mn(2+)-Cl(-) distances have been obtained as: R1=3.2803Å, R2=2.6495Å for (MnCl6)La cluster, and R1=3.4558Å, R2=2.5111Å for (MnCl6)Na cluster in the Cs2NaLaCl6:Mn(2+) system. And our results show R1>R2 for each cluster, which exhibits an elongation distortion relative to the regular octahedron. And the elongation magnitude of a tetragonal (MnCl6)Na cluster is larger than that of a (MnCl6)La cluster in the Cs2NaLaCl6:Mn(2+) system. Using this method, our calculation values of EPR parameters are well accordant with the experimental values.

Time-resolved spectroscopy in LiCaAlF(6) doped with Cr(3+): dynamical Jahn-Teller effect and thermal shifts associated with the (4)T(2) excited state

This work investigates the centre distribution of the Cr(3+) impurity, the dynamical Jahn-Teller effect in the first (4)T(2) excited state and the thermal shifts of the absorption and emission peaks in LiCaAlF(6):Cr(3+) by means of time-resolved emission spectroscopy. The electronic and vibrational fine structure observed in both the absorption and emission spectra at low temperature are assigned according to the vibrational modes of the internal (CrF(6))(3-) complex and the lattice modes. Zero-phonon lines associated with (4)T(2) --> (4)A(2) and (2)E --> (4)A(2) transitions were detected and assigned on the basis of available high pressure data in LiCaAlF(6):Cr(3+). We have identified the vibrational coupled modes responsible for the vibrational structure of the low temperature emission spectrum and the reduction of the zero-phonon line (ZPL) splitting caused by the dynamical Jahn-Teller effect in the (4)T(2) excited state (Huang-Rhys factor, S(e) = 0.92). In addition, from the temperature variation of the emission intensity I(T), transition energy E(T) and bandwidth H(T), we obtained the vibrational modes that are coupled to the emitting state. We have evaluated the two main contributions to the photoluminescence thermal shift through thermal expansion and high pressure measurements: the implicit contribution induced by changes of thermal population and the explicit contribution induced by thermal expansion effects--40% and 60% of the total shift, respectively.

Microscopic insight into properties and electronic instabilities of impurities in cubic and lower symmetry insulators: the influence of pressure

This article reviews the microscopic origin of properties due to transition-metal (TM) impurities, M, in insulator materials. Particular attention is paid to the influence of pressure upon impurity properties. Basic concepts such as the electronic localization in an MX(N) complex, the electrostatic potential, V(R), arising from the rest of the lattice ions or the elastic coupling of the complex to the host lattice are initially exposed. The dependence of optical and magnetic parameters on the impurity-ligand distance, R, in cubic lattices is discussed in a first step. Emphasis is put on the actual origin of the R dependence of 10Dq. Examples revealing that laws for strict cubic symmetry cannot in general be transferred to lower symmetries are later given. This relevant fact is shown to come from allowed hybridizations like nd-(n+1)s as well as the influence of V(R). As a salient feature the different colour in ruby and emerald is stressed to arise from distinct V(R) potentials in Al(2)O(3) and Be(3)Si(6)Al(2)O(18). The last part of this review deals with electronic instabilities. The phenomena associated with the Jahn-Teller (JT) effect in cubic lattices, the origin of the energy barrier among equivalent minima and the existence of coherent tunnelling in systems like MgO:Cu(2+) are discussed. An increase of elastic coupling is pointed out to favour a transition from an elongated to a compressed equilibrium conformation. Interestingly the equilibrium geometry of JT ions in non-cubic lattices is shown to be controlled by mechanisms different to those in cubic systems, V(R) playing again a key role. The relevance of first principles calculations for clarifying the subtle mechanisms behind off-centre instabilities is also pointed out. Examples concern monovalent and divalent TM impurities in lattices with the CaF(2) structure. The instability due to the transition from the ground to an excited state is finally considered. For complexes with significant elastic coupling vibrational frequencies and the Stokes shift are expected to undergo bigger changes through a chemical rather than a hydrostatic pressure. The reduction of Huang-Rhys factors upon increasing the pressure is discussed as well.