Members of the PbFCI-Type Family: Possible Candidates for Room-Temperature Photochemical Hole Burning

We report on crystal growth and about physico-chemical studies on SryBa1-yFClxBr1-x (y = 0, 0.5, and 1) compounds doped with Sm. Persistent spectral hole burning at 300 K is further reported on Sr0.5Ba0.5FCl0.5Br0.5:Sm single crystals

Etude structurale de composés solides en relation avec une transition de phase

Syntheses and structural studies of crystalline solids were performed on several groups of compounds which undergo one or more structural phase transitions. A combination of different and complementary experimental methods such as X-ray and neutron diffraction, chemical etching, polarized tight microscopy in transmission and reflection and thermal analysis, together with special syntheses techniques were used. The experiments were carried out at room temperature and at temperatures situated between 20K and 1300K

The absolute configuration of an as-grown and a mechanically switched single crystal of the perovskite BiFeO₃

Plate-shaped untwinned and twinned single crystals of the perovskite BiFeO 3 show different etch figures and etch figure arrangements on the surface when attacked with dilute nitric acid. These etch figures can be correlated with the orientation of the spontaneous polarization vector. With a view to correlating the direction of the polarization vector with the atomic positions in the crystal, the absolute configuration was determined from X-ray diffraction patterns at room temperature on a dendritic ferroelastic/ferroelectric asgrown single domain crystal. The typical etch figure arrangement close to the crystal edges was used as a reference system. The absolute structure shows that the part of the crystal corresponding to the positive end of the spontaneous polarization vector etches faster than the negative one. The structure refinement of a single crystal switched by uniaxial stress along the polar axis showed a complete reorientation of the (super)structure with rearrangement to an angle of 70.5° compared with the former polarization direction. As already suggested by the analysis of new etch figures close to 45°-inclined domain walls, no merohedral twinning occurs during this rearrangement. Despite atom displacements from 0.4 Å (for Fe) and 0.9 Å (for O) the crystal does not break during switching.</p

Powder diffraction refinement of cubic and rhombohedral iron iodine boracite, Fe₃B₇O₁₃I

Structural parameters of iron iodine boracite, Fe3B7O13I, [cubic with space group F[image omitted]3c at 373 K, lattice parameters acub = 12.22869(18) Å; rhombohedral with space group R3c at 10 K, lattice parameters ahex = 8.64534(21) and chex = 21.2035(8) Å] was refined from X-ray powder diffraction data; Rprofile = 2.4% (373 K) and 3.0% (10 K). For precise lattice parameter determination, the structure at low temperature was refined by using a sample with silicon as reference. The refinements show that the (averaged) difference between the two iron iodine distances increases from cubic → orthorhombic → rhombohedral structure from 0 Å → 0.20(1) ↠ 0.32(4) Å, respectively

Phosphorescent compounds

Phosphorescent composition comprises an alkaline earth metal silicate doped with europium fluoride and another lanthanide fluoride. Independent claims are also included for: (1) producing a composition as above by grinding a mixture of alkaline earth metal carbonate or acetate and silica, calcining the mixture at 1100-1500[deg] C, adding the dopants, calcinining the mixture in a nitrogen/hydrogen atmosphere at 900-1000[deg] C, and cooling, grinding and screening the product; (2) product comprising a composition as above applied to a support that makes it visible at night; (3) timepieces with at least one component incorporating a product as above

Structure of a ferroelectric and ferroelastic monodomain crystal of the perovskite BiFeO₃

A ferroelec. and ferroelastic monodomain single crystal of BiFeO3 at space group R3c is rhombohedral, with a 5.57874(16), c 13.8688(3) .ANG.; arh 5.6343 .ANG., arh 59.348 Deg; d. (calcd.) = 8.337 for Z = 6. Final R = 2.4, Rw = 1.9% for 422 reflections. The structure can be described as a rhombohedrally distorted simple cubic perovskite cell. The O octahedron is distorted with min. and max. O-O distances of 2.710(7) and 3.015(9) .ANG., resp., and rotated by about +- a = 13.8(3) Deg around the 3-fold axis. The Fe atom is shifted away from the center of the deformed octahedron by .apprx.0.134(7) .ANG. along the 3-fold axis. The Bi atom is shifted with respect to 2 neighboring octahedron centers by .apprx.0.540(7) .ANG. along the 3-fold axis. The ferroelec. poling mechanism was analyzed: atom displacements were calcd. for 60, 120 and 180 Deg switching of Ps and found to be 0.44, 0.62 and 0.82 .ANG. for Fe. Av. oxygen displacements for +a (-a) rotated octahedra were 0.87 (0.89) 1.13 (1.24) and 1.34 (1.53) .ANG.. Av. Bi shifts were <0.06 .ANG.. As the displacements increase in the order 60, 120 and 180 Deg switching of Ps, the 180 Deg reversal must have a very high activation energy and will be avoided