Abstract:Using the Rietveld profile method, the atomic coordinates and anisotropic temperature factors of KCaF 3 were refined. At room temperature, KCaF 3 crystallizes in monoclinic B2 l /m symmetry, with the lattice parameters: a = 8.754(2) A, fc = 8.765(4) A, c = 8.760 (5) A, £=90.48(3)°, V=672.1(3) A 3 , Z=8. The refinement procedure was stopped when R B -0.05 and the Durbin-Watson statistic factor=0.85 had been reached. The structure determined is related to the tilting of CaF 6 octahedra of the ab + c~ type, which… Show more
“…In this crystal the CaÛF distances are 2.21 and 2.16(9) C (Ratuszna et al 1997) and the emission band of Ce 3+ is at 408 nm (3.02 eV) (Mazurak et al 1999). Because D oct = -8/9 D cubic , for the eightfold coordination and CaÛF distances as for the fluoroperovskite, the measured emission line should be at 363 nm and, thus, the energy gap should be smaller than for the fluorite.…”
Abstract. Natural samples of fluorite and apatite from granites, pegmatites, carbonatites and andesitic tuffs were investigated by steady-time spectroscopy to characterize the luminescence properties of Ce 3+ and Eu 2+ . The luminescence of Ce 3+ has been clearly seen in fluorite as 320 and 337 or 343 nm bands. In apatites, two distinct bands for two different Ca crystal sites were obtained: 340-380 nm for Ca(1) and 420-450 nm for Ca(2). The luminescence spectra of Eu 2+ in the fluorite crystals were measured even at for low concentration of this element (0.11 ppm). For Ce 3+ , it has been showed that the crystal field strength depends more on the nature of the ligand than on the Me-ligand distances.
“…In this crystal the CaÛF distances are 2.21 and 2.16(9) C (Ratuszna et al 1997) and the emission band of Ce 3+ is at 408 nm (3.02 eV) (Mazurak et al 1999). Because D oct = -8/9 D cubic , for the eightfold coordination and CaÛF distances as for the fluoroperovskite, the measured emission line should be at 363 nm and, thus, the energy gap should be smaller than for the fluorite.…”
Abstract. Natural samples of fluorite and apatite from granites, pegmatites, carbonatites and andesitic tuffs were investigated by steady-time spectroscopy to characterize the luminescence properties of Ce 3+ and Eu 2+ . The luminescence of Ce 3+ has been clearly seen in fluorite as 320 and 337 or 343 nm bands. In apatites, two distinct bands for two different Ca crystal sites were obtained: 340-380 nm for Ca(1) and 420-450 nm for Ca(2). The luminescence spectra of Eu 2+ in the fluorite crystals were measured even at for low concentration of this element (0.11 ppm). For Ce 3+ , it has been showed that the crystal field strength depends more on the nature of the ligand than on the Me-ligand distances.
“…Moreover, the influence on the calculated shielding of CdF 2 is the strongest, in accordance with its strongest covalent character. 36 There are four kinds of fluorine atoms in the crystal with a ratio of 1 : 1 : 2 : 2. All of them have similar coordination environments to the other AMF 3 .…”
using the DFT/GIAO method. The aug-cc-pVTZ basis set was used for the fluorine atom under investigation and LanL2DZ for the remaining fluorine atoms. The 3-21G(2d) basis set was used for the aluminum atom and CRENBL for the other metal atoms. When appropriate cluster models were employed, the theoretical results obtained from the B3LYP/GIAO calculations are in good agreement with experimental measurements and may be better than those obtained from empirical calculations. The correlation coefficient and the slope of the fitting line between our theoretical predictions and the experimental observations are close to unity.
“…Ratuszna et al (1997) provided evidence for their monoclinic metric from a pattern decomposition of the pseudocubic 200 multiplet shown in Figure 2(a) of their paper. However the pattern decomposition is inconsistent with their unit cell metric, with the calculatedBragg angle for the pseudocubic 002 for Cu K α 1 being 41.187° 2 Θ and not located at the peak position marked on the diagram ∼40.94° 2 Θ .…”
Section: Structural Crystallography Of Kcaf3mentioning
confidence: 97%
“…The space group at room temperature was described as D 2 h 16 , i.e., space group P n m a , P b n m in the setting used, but had been given Glazer symbol a − b − c + which corresponds to space group P 11 2 1 ∕ m (Glazer, 1972; Howard and Stokes, 1998). More recently Ratuszna et al (1995, 1997) have studied the solid solution Rb 1− x K x CaF 3 using X-ray powder diffractometry with filtered Cu K α radiation and concluded that the end member phase KCaF 3 (Ratuszna et al , 1997) was in fact monoclinic with space group B 2 1 ∕ m , a nonstandard setting of P 2 1 ∕ m . Parts of this solid solution were later studied at significantly Figure 1.The relationship between the orthorhombic unit cell in space group Pnma , viewed down the orthorhombic b axis between y =0 and 0.5, and the pseudocubic unit cell (four cells shown).…”
Section: Structural Crystallography Of Kcaf3mentioning
The crystal structure of the perovskite phase KCaF3 has been redetermined at 4.2 and 300 K using powder neutron diffraction collected at the highest resolution. At both temperatures the phase was found to be orthorhombic in space group Pnma, with lattice parameters a=0.622 879(5) nm, b=0.870 031(7) nm, c=0.611 210(5) nm at 4.2 K, and a=0.621 488(6) nm, b=0.876 360(8) nm, c=0.616 481(6) nm at 300 K. The CaF6 octahedron is regular at both temperatures with octahedral rotations of 9.6° and 13.2° for the in-phase and anti-phase tilts, respectively, at 4.2 K. No evidence was found to support the recent revision of the space group from Pnma to the monoclinic space group B21∕m.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
