The efficient green phosphor Ba(3)Si(6)O(12)N(2):Eu(2+) and its solid-solution series Ba(3-x)Sr(x)Si(6)O(12)N(2) (with x approximately = 0.4 and 1) were synthesized in a radio-frequency furnace under nitrogen atmosphere at temperatures up to 1425 degrees C. The crystal structure (Ba(3)Si(6)O(12)N(2), space group P3 (no. 147), a = 7.5218(1), c = 6.4684(1) A, wR2 = 0.048, Z = 1) has been solved and refined on the basis of both single-crystal and powder X-ray diffraction data. Ba(3)Si(6)O(12)N(2):Eu(2+) is a layer-like oxonitridosilicate and consists of vertex-sharing SiO(3)N-tetrahedra forming 6er- and 4er-rings as fundamental building units (FBU). The nitrogen atoms are connected to three silicon atoms (N3), while the oxygen atoms are either terminally bound (O1) or bridge two silicon atoms (O2) (numbers in superscripted square brackets after atoms indicate the coordination number of the atom in question). Two crystallographically independent Ba(2+) sites are situated between the silicate layers. Luminescence investigations have shown that Ba(3)Si(6)O(12)N(2):Eu(2+) exhibits excellent luminescence properties (emission maximum at approximately 527 nm, full width at half maximum (FWHM) of approximately 65 nm, low thermal quenching), which provides potential for industrial application in phosphor-converted light-emitting diodes (pc-LEDs). In-situ high-pressure and high-temperature investigations with synchrotron X-ray diffraction indicate decomposition of Ba(3)Si(6)O(12)N(2) under these conditions. The band gap of Ba(3)Si(6)O(12)N(2):Eu(2+) was measured to be 7.05+/-0.25 eV by means of X-ray emission spectroscopy (XES) and X-ray absorption near edge spectroscopy (XANES). This agrees well with calculated band gap of 6.93 eV using the mBJ-GGA potential. Bonding to the Ba atoms is highly ionic with only the 4p(3/2) orbitals participating in covalent bonds. The valence band consists primarily of N and O p states and the conduction band contains primarily Ba d and f states with a small contribution from the N and O p states.

Nitridophosphates have emerged as advanced materials due to their structural variability and broad technical applicability. Their binary parent compound P3 N5 , a polymeric network of corner- and edge-sharing PN4 tetrahedra with N and N sites, is a particularly interesting example. We present a study of the band gap and electronic structure of α-P3 N5 by using soft X-ray spectroscopy measurements and DFT calculations. The band gap, which is crucial for all applications, is measured to be 5.87±0.20 eV. This agrees well with the calculated, indirect band gap of 5.21 eV. The density of states are found to show dramatic variation between the nonequivalent N sites and a high degree of covalency. Coupled to these results is what is, to our knowledge, the largest core hole shift reported to date for a soft X-ray absorption spectrum. We propose an intuitive bonding scheme for α-P3 N5 that explains the observed band gap and unique density of states, while providing a framework for predicting these properties in known and yet to be discovered PN compounds. We briefly consider the implications of these results for new low-dimensional P and PN materials, which alongside graphene, could become important materials for nanoelectronics.

HP-Ca2Si5N8 was obtained by means of high-pressure high-temperature synthesis utilizing the multianvil technique (6 to 12 GPa, 900 to 1200 degrees C) starting from the ambient-pressure phase Ca2Si5N8. HP-Ca2Si5N8 crystallizes in the orthorhombic crystal system (Pbca (no. 61), a=1058.4(2), b=965.2(2), c=1366.3(3) pm, V=1395.7(7)x10(6) pm3, Z=8, R1=0.1191). The HP-Ca2Si5N8 structure is built up by a three-dimensional, highly condensed nitridosilicate framework with N[2] as well as N[3] bridging. Corrugated layers of corner-sharing SiN4 tetrahedra are interconnected by further SiN4 units. The Ca2+ ions are situated between these layers with coordination numbers 6+1 and 7+1, respectively. HP-Ca2Si5N8 as well as hypothetical orthorhombic o-Ca2Si5N8 (isostructural to the ambient-pressure modifications of Sr2Si5N8 and Ba2Si5N8) were studied as high-pressure phases of Ca2Si5N8 up to 100 GPa by using density functional calculations. The transition pressure into HP-Ca2Si5N8 was calculated to 1.7 GPa, whereas o-Ca2Si5N8 will not be adopted as a high-pressure phase. Two different decomposition pathways of Ca2Si5N8 (into Ca3N2 and Si3N4 or into CaSiN2 and Si3N4) and their pressure dependence were examined. It was found that a pressure-induced decomposition of Ca2Si5N8 into CaSiN2 and Si3N4 is preferred and that Ca2Si5N8 is no longer thermodynamically stable under pressures exceeding 15 GPa. Luminescence investigations (excitation at 365 nm) of HP-Ca2Si5N8:Eu2+ reveal a broadband emission peaking at 627 nm (FWHM=97 nm), similar to the ambient-pressure phase Ca2Si5N8:Eu2+.

We present a study of MgSiN2 using soft X‐ray absorption and emission spectroscopy which directly probe the partial density of states. MgSiN2 is new as a host lattice for luminescence materials and used in phosphor‐converted light‐emitting diodes. We compare our measurements to our full potential, all electron density funtional theory calculations. We find excellent agreement between experiment and theory and the band gap of MgSiN2 is measured to be 5.6 ± 0.2 eV in agreement with our calculated value of 5.72 eV. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)