The invention is concerned with an illumination system, comprising a radiation source and a fluorescent material comprising at least one phosphor capable of absorbing a part of light emitted by the radiation source and emitting light of wavelength different from that of the absorbed light; wherein said at least one phosphor is an oxido-nitrido-silicate of general formula EA2-zSi5-aBaN8-aOa:Lnz, wherein 0 < z ≤ 1 and 0 < a < 5. comprising at least one element EA selected from the group consisting of Mg, Ca, Sr, Ba and Zn and at least one element B selected from the group consisting of Al, Ga and In, and being activated by a lanthanide selected from the group consisting of cerium, europium, terbium, praseodymium and mixtures thereof. The invention is also concerned with a phosphor, which is an oxido-nitrido-silicate of general formula EA2-zSi5-aBaN8-aOa:Lnz, wherein 0<z≤ 1 and 0 < a < 5. comprising at least one element EA selected from the group consisting of Mg, Ca, Sr, Ba and Zn and at least one element B selected from the group consisting of Al, Ga and In, and being activated by a lanthanide selected from the group consisting of cerium, europium, terbium and mixtures thereof.

Illumination System Comprising a Radiation Source and a Fluorescent Material

Silica is the main component of the Earth's crust and is also of great relevance in many branches of materials science and technology. Its phase diagram is rather intricate and exhibits many different crystalline phases1,2,3,4,5,6. The reported propensity to amorphization and the strong influence on the outcome of the initial structure and of the pressurization protocol1,7 indicate the presence of metastability and large kinetic barriers. As a consequence, theory is also faced with great difficulties and our understanding of the complex transformation mechanisms is still very sketchy despite a large number of simulations8,9,10,11,12,13. Here, we introduce a substantial improvement of the metadynamics method14,15, which finally brings simulations in close agreement with experiments. We unveil the subtle and non-intuitive stepwise mechanism of the pressure-induced transformation of fourfold-coordinated α-quartz into sixfold-coordinated stishovite at room temperature. We also predict that on compression fourfold-coordinated coesite will transform into the post-stishovite α-PbO2-type phase. The new method is far more efficient than previous methods, and for the first time the study of complex structural phase transitions with many intermediates is within the reach of molecular dynamics simulations. This insight will help in designing new experimental protocols capable of steering the system towards the desired transition.

Crystal structure transformations in SiO2 from classical and ab initio metadynamics.

Structural stability of metal hydrides, alanates and borohydrides of alkali and alkali- earth elements: A review

Numerous experiments have been performed in the past few years on solid hydride deposition under beryllium-seeded plasma action or on energetic hydrogen implantation into metallic beryllium. This article reports on calculations carried out using first-principles density functional theory (DFT) and discusses the results with respect to thermal desorption experiments. The structures of amorphous beryllium hydride were investigated for various H/Be ratios. They were compared to the structure of the organized BeH2 crystal as a test for the validity of this model. The formation and reactivity of atomic vacancies were also investigated, together with atomic hydrogen trapping. Hints are proposed for hydrogen detrapping mechanisms.

Quantum Modeling of Hydrogen Retention in Beryllium Bulk and Vacancies

Pressure-induced metallization and potential superconductivity of BeH2 has been a topic of interest. In the present study, we extensively explored the crystal structures of BeH2 in a wide pressure range of 0–300 GPa using an unbiased structure searching method coupled with first-principles density functional calculations. A series of pressure-induced structural transformations are predicted for BeH2, as Ibam (α phase) → P-3m1 (phase II) → R-3m (phase III) → Cmcm (phase IV). Calculated pressures of phase transition are 25, 140, and 202 GPa, respectively. The phase II is isostructural to the well-known 1T structure of transition metal dichalcogenides, which is composed of covalent bonded BeH2 slabs stacked along the perpendicular direction by van der Waals forces. The phase III is constructed by the same BeH2 slabs, but differs from the phase II in the stacking sequence. The α phase, phase II, and phase III all have insulating electronic states while their band gaps decrease as pressure increases. We predic...

Metallization and superconductivity of BeH2 under high pressure

The isotypism of BeH2 and SiO2: an ab initio study

The structure and properties of two different modifications of moganite have been studied using density functional theory, and the results have been compared to quartz. It is shown that the enthalpy difference between quartz and moganite, whose structure can be understood as Brazil twinning of quartz on a unit cell length scale, is negligible. This explains the significant amount of moganite in fine-grained quartz samples, as well as the frequent occurrence of Brazilian twinning in quartz. The compression mechanism of moganite has been elucidated, and it is argued that moganite is significantly more compressible than quartz. Observed and calculated NMR spectra are compared, and it is found that the bonding in quartz and moganite is very similar, consistent with the results of a Mulliken population analysis. The elastic stiffness coefficients of moganite have been predicted, and it is shown that formal-charge shell model interatomic potentials appear to be more transferable from quartz to moganite than partial-charge rigid ion equivalents.

Theoretical investigation of moganite

Theoretical study of the structures and properties of SrSiAl2O3N2 and Ce4[Si4O4N6]O

The structure and properties of ${\mathrm{Na}}_{3}\mathrm{O}(\mathrm{C}\mathrm{N})$ have been studied using parameter-free model calculations based on density functional theory. The results suggest that, unlike in pure alkalicyanides, the CN anions are orientationally disordered in the ground-state structure. The calculations show that in the fully disordered structure, the global strain is minimized.

Ute Hantsch

Papers

The isotypism of BeH2 and SiO2: an ab initio study

Theoretical investigation of moganite

Theoretical study of the structures and properties of SrSiAl2O3N2 and Ce4[Si4O4N6]O

Na 3 O ( C N ) : Ab initio calculations on a multidomain structure