Luminescence spectroscopy of the earth

The first attempt to consider global radiation luminescence fields (spheres) of the Earth has been undertaken. In general four types of luminescent fields characterize the superficial geospheres. (1) Luminescence of organic and bioorganic substances comprising the atmosphere, the hydrosphere, the lithosphere and the technosphere, being a unique distinguishing property of our planet in the Universe. (2) Recombination luminescence of key rock-forming minerals (feldspar, quartz, carbonates) that is characteristic of the granite and sedimentary layers occupying some 30% of the Earth's crust. (3) Photoluminescence of dozens of ironless minerals in so-called fluorescence anomalies formed with the help of fluids ascending along fractures and faults to the surface of the Earth. While ascending they accumulate many rare metal elements including transition ones. Entering crystal lattices of ironless mineral-dielectrics these metals form centres of luminescence. (4) Artificial luminescent materials created in the technosphere in the 20th century. The main types have origins in natural luminophors. So, in every field of application of photo-, thermo, cathodo-, electro-, X-ray-luminophors and laser crystals they have many analogues among the luminescent minerals.     

Luminescence of Eu<Superscript>2+</Superscript> ions in alkaline earth dilithiosilicates

Spectral-luminescent properties of Eu²⁺ ions in alkaline earth dilithiosilicates of composition MLi₂SiO₄ (M = Ca, Sr, Ba) have been studied at 77 K. The reasons for the different positions of the Eu²⁺ 4f ⁶5 d → 4f ⁷ emission band maximum in spectra of MLi₂SiO₄ were found based on the obtained results. It was shown that the increase in the decay time of the Eu²⁺ emission on going from CaLi₂SiO₄ (0.41μs) through BaLi₂SiO₄ (0.64 μs) to Sr-silicate (1.11μs) correlates with the shift of the emission maximum to longer wavelengths.     

Unified study of alkaline earth metal

A comprehensive unified study of fcc alkaline earth metal Ca, Sr and the bcc phase of Sr and Ba has been made. Properties studied include equilibrium lattice parameter, total crystal energy, second order elastic constants, pressure derivative of the second order elastic constants and phonon spectrum in the symmetry direction. The results obtained show an overall agreement with experiment. The results partially reproduced the experimentally observed phonon-crossover in Ba, and this has been found as the many-body effect in lattice dynamics.     

Fluorescence of Eu-activated alkaline earth halosilicates

Three luminescent materials, each with a distinct and novel crystalline structure have been found in the alkaline earth halosilicate series. Their emission is in the blue/green part of the spectrum, the peaks falling at 500 nm for the Sr/Cl compound, 442 nm for the Ca/Cl compound and 514 nm for the Ca/Br compound. The strontium chlorsilicate, Sr₂Si₃O₈ 2SrCl₂, is of particular interest showing good temperature dependence characteristics (365 nm excitation) and also exhibiting an acicular crystal habit. Other alkaline earth/halide combinations and the binary alkaline earth systems were examined.     

Luminescence of Ce in hydrated rare earth bromides

Intense photoluminescence is reported for hydrated bromides of Gd, La and Y (LaBr₃·7H₂O, CeBr₃·7H₂O, YBr₃·8H₂O and GdBr₃·n H₂O) prepared by the wet chemical method and activated with Ce³⁺. Intensities are comparable to that for a commercial phosphor (SrB₄O₇:Eu²⁺—Sylvania 2052). Luminescence in hydrated salt is usually quenched. The observation of intense luminescence in hydrated bromides is remarkable.     

Dielectric behaviour of alkaline earth oxides and sulphides

Dielectric behaviour of MgO, CaO, SrO, BaO, SrS and BaS has been studied on the basis of the shell model and the exchange charge shell model. Values of polarizabilities and effective charge parameter calculated from the two models are compared with experimental data.     

Structural stability of alkaline earth oxide crystals

Using recemtly developed interionic potentials we have studied the structural stability problem of MgO, CaO, SrO and BaO crystals. The observed crystal structure for the alkaline earth oxides viz. NaCl structure is found to be stable only when Lundqvist's three body potential is taken into account along with Madelung's potential, overlap repulsive potential and van der Waals potentials.     

Photoelastic response of alkaline earth aluminosilicate glasses

Understanding the structural origins of the photoelastic response in oxide glasses is important for discovering new families of zero-stress optic glasses and for developing a predictive physical model. In this Letter, we have investigated the composition dependence of the stress optic coefficient C of 32 sodium aluminosilicate glasses with different types of alkaline earth oxides (MgO, CaO, SrO, and BaO). We find that most of the composition dependence of the stress optic response can be captured by a linear regression model and that the individual contributions from the alkaline earths to C depend on the alkaline earth-oxygen bond metallicity. High bond metallicity is required to allow bonds to be distorted along both the bonding direction and perpendicular to it. These findings are valuable for understanding the photoelastic response of oxide glasses.     

High pressure behavior of alkaline earth tellurides

We have predicted high pressure structural behavior and elastic properties of alkaline earth tellurides (AETe; AE = Ca, Sr, Ba) by using two body interionic potential approach with modified ionic charge (Z _m e). This method has been found quite satisfactory in case of the rare earth compounds. The equation of state curve, structural phase transition pressure from NaCl (B1) to CsCl (B2) phase and associated volume collapse at transition pressure of alkaline earth tellurides (AETe) obtained from this approach, so have been compared with experimentally measured data reveal good agreement. We have also investigated bulk modulus, second and third order elastic constants and pressure derivatives of second order elastic constants at ambient pressure which shows predominantly ionic nature of these compounds. First time, we have calculated the Poisson ratio, Young and Shear modulus of these compounds.      

Optical properties of alkaline earth oxide crystals

Abstract

Single crystals have been obtained of undoped calcium, strontium and barium oxides. Recent work on the F-band in these crystals will be reviewed, quoting values for the spin-orbit interaction constants for the first excited states of F-centres. A zero-phonon line has been found in undoped calcium oxide, and there has been a controversy whether or not this line is due to the F-centre. Another zero-phonon line occurs in bismuth-doped calcium oxide. The properties of both zero-phonon lines have been studied by applying uniaxial stress or a magnetic field to the crystal at liquid helium temperature. Both lines are found to be due to electric dipole transitions at centres of cubic or terahedral symmetry.     

Single-stage sub-Doppler cooling of alkaline earth atoms

We report the first experimental study of sub-Doppler cooling in alkaline earth atoms (87Sr) enabled by the presence of nuclear spin-originated magnetic degeneracy in the atomic ground state. Sub-Doppler cooling in a sigma(+)-sigma(-) configuration is achieved despite the presence of multiple, closely spaced excited states. This surprising result is confirmed by an expanded multilevel theory of the radiative cooling force. Detailed investigations of system performance have shed new insights into (sigma(+)-sigma(-)) cooling dynamics and will likely play an important role in the future development of neutral atom-based optical frequency standards.     

Cohesive and elastic properties of alkaline earth chalcogenide crystals

In the present study an analysis of the cohesive and elastic properties has been performed in fourteen alkali chalcogenide crystals using the composite potential forms based on the Drude oscillator model. The short-range overlap repulsion has been considered up to the second nearest neighbour interactions. The van der Waals coefficients have been estimated from the Kirkwood-Muller formulae. Cohesive energies, bulk-modulus and its pressure derivatives calculated for the entire family of crystals under study are found to present a good agreement with available experimental data, better than those obtained by previous workers using the traditional Born potential model.On leave from theDepartment of Physics, G. B. Pant University, Pantnagar-263 145, India.Authors are thankful to Mr. Saleh O. Harraga for the computational help.     

Lattice energy and phase transition of alkaline earth chalcogenide

In this study, we have investigated the high-pressure structural phase transition of alkaline earth's chalcogenides within the framework of three-body potentials. We are considering short-range repulsive interactions up to the second nearest neighbors. The structural phase transition from the low-pressure NaCl (B1) to the high-pressure CsCl (B2) structure is estimated by Gibbs free energy calculations. The results are satisfactory and in agreement with the available experimental and other theoretical results.     

Oxygen incorporation in the alkaline earth fluorides

We consider the thermodynamics and kinetics of oxygen incorporation in the alkaline earth fluor ides, basing our discussion on the results of calculated energies of formation of a range of defects and defect clusters. Our results strongly suggest that oxygen may be incorporated only by hydrolysis and not by oxidation, and that the reaction will only occur in doped crystals. We propose a mechanism for the reaction, involving proton exchange at the surface of the crystals followed by diffusion of hydroxyl ions into the crystal; there they react with dopant-interstitial complexes to produce dopant-oxide complexes of types that have been detected in spin resonance studies.