Crystal structure and vibrational properties of new luminescent hosts K3YF6 and K3GdF6

The luminescence hosts K₃YF₆ and K₃GdF₆ were obtained in a single-crystal form. Their crystal structure was determined from single-crystal X-ray diffraction data. Both crystals adopt monoclinic system with space group P2₁/n, Z=2. Lattice parameters for K₃YF₆ are refined to the following values , , , β=90.65(3) and for K₃GdF₆, , , β=90.80(3). The vibrational analysis, IR and Raman spectroscopy at room temperature, was applied to these compounds in order to study the site symmetry of Y³⁺ and Gd³⁺ ions.     

Tetragonal low-temperature structure of LiAl

The cubic face-centered structure of LiAl (, at ) transforms into a tetragonal body-centered structure (I4₁/amd, , at ). This first-order phase transition at about during heating is probably the reason for the so-called “ anomalies” in some physical properties like specific heat, electrical resistivity and nuclear-spin lattice relaxation. This transition seems to be correlated with the composition Li:Al of the alloy and the amount of Li vacancies.     

The composite structure of Cu2.33−xV4O11

The copper vanadium oxide bronze Cu_2.33−xV₄O₁₁ exhibits a three part composite structure refined on the basis of XRD low-temperature studies. It crystallizes in the triclinic system with the non-centric superspace group X1 and cell parameters ; ; ; α=90.0°; β=101.95(3)°; γ=90.0° with a modulation q-vector equal to (0,0.11,0). The three different parts of this composite structure differ by their b-unit cell repeat defined as b₁ ; () and (). These parts are respectively associated to the V₄O₁₁ substructure and to each of the two different copper sites. Such refinement allows us to describe the structure using only one and fully occupied crystallographic site for each of the Cu ions. The maximum composition (x=0) is then achieved. Bond valence sum calculations on the basis of such composite structure is in agreement with electronic structure calculation made using the average one and allows us to attribute the proper valence state to each Cu ions. Then, the calculated ratio appears, contrary to the average structure, in prefect agreement with the one deduced from XPS experiment.