Polymorphism and polymorph-dependent luminescence properties of the first lithium oxonitridolithosilicate Li3SiNO2:Eu2.

Building on studies of monoclinic Li3SiNO2, a polymorph, ß-Li3SiNO2, with a previously unknown structure type was synthesized. The ß-phase crystallizes in the orthorhombic space group Pbca (no. 61) with lattice parameters of a = 18.736(2), b = 11.1267(5), c = 5.0897(3) Å, and a cell volume of V = 1057.2(1) Å3. Using high-temperature solid-state reactions in sealed tantalum tubes, it was possible to obtain high purity samples (<5 wt% of side phase LiSi2N3 according to Rietveld analysis) containing exclusively one or the other polymorph, depending solely on the cooling rate. In contrast to the monoclinic phase, orthorhombic ß-Li3SiNO2 additionally contains a third layer and shows a layer-sequence of the type ABCB. Doped with the activator ion Eu2+, the new polymorph exhibits an intense yellow emission (λmax = 586 nm, fwhm = 89 nm, 0.33 eV, 2650 cm-1) under irradiation with UV to blue light. Hence, the structural difference between the two polymorphs goes along with a significant blue-shift of 16 nm. The results from single-crystal diffraction and single-grain luminescence measurements were confirmed by Rietveld analysis of bulk samples and powder luminescence data.

Modular Principle for Complex Disordered Tetrahedral Frameworks in Quenched High-Pressure Phases of Phosphorus Oxide Nitrides.

Invited for the cover of this issue are the groups of Oliver Oeckler at Leipzig University and Wolfgang Schnick at University of Munich (LMU). The image background depicts a diffraction pattern of an intergrown crystal containing P40 O31 N46 as a night sky. P40 O31 N46 and P74 O59 N84 form complex disordered frameworks, a cutout of which symbolizes the earth's surface (which mainly contains silicates with related building blocks). The structures are built up from chain-like building units, which fall like rain, symbolizing the modular building scheme. Read the full text of the article at 10.1002/chem.202203892.

Modular Principle for Complex Disordered Tetrahedral Frameworks in Quenched High-Pressure Phases of Phosphorus Oxide Nitrides.

The crystal structures of the new phosphorus oxide nitrides P40 O31 N46 and P74 O59 N84 , which were synthesized from amorphous phosphorus oxide nitride imide, exhibit complex frameworks built up from P(O,N)4 tetrahedra. The latter form various chain-like building units with various degrees of branching. These modular units can be combined and arranged in different ways, which leads to closely related structures and several disordered configurations in each compound. As the material was obtained by high-pressure high-temperature synthesis, the disorder is most likely a consequence of quenching a high-pressure phase with P(O,N)5 trigonal bipyramids. Under ambient conditions, P atoms are expected to relax by moving to the centers of the face-sharing tetrahedra that constitute the bipyramid. Diffraction patterns acquired with microfocused synchrotron radiation reveal that domains of both compounds are intergrown with H3 P8 O8 N9 , whose tetrahedral framework represents a cutout of the structures of both P40 O31 N46 and P74 O59 N84 . Powder diffraction patterns do not indicate any further phases.

Li2 Ba4 Al2 Ta2 N8 O, the First Barium Nitridoalumotantalate with BCT-Zeolite Type Structure.

Single-crystals of Li2 Ba4 Al2 Ta2 N8 O:Eu2+ were synthesized from Ba3 N2 , Al2 O3 , Li3 N, Eu2 O3 , and lithium metal by a high-temperature solid-state reaction in a weld shut tantalum ampule. The crystal structure of Li2 Ba4 Al2 Ta2 N8 O was determined by single-crystal X-ray diffraction and it crystallizes in the orthorhombic space group Pnnm (no. 58) with the lattice parameters a=1006.71(3), b=1026.58(3), c=607.10(2) pm, and a volume of V=0.62742(3) nm3 . The compound is built up from AlN4 and TaN4 tetrahedra, which form a three-dimensional network corresponding to the BCT-zeolite type structure. Li2 Ba4 Al2 Ta2 N8 O is homeotypic to Li2 Sr4 Si4 N8 O and Li2 Sr4 Al2 Ta2 N8 O but, additionally, it could be successfully doped with the activator ion Eu2+ and hence features an experimental observed overall emission at λmax =565 nm (fwhm=89 nm) consisting of a superposition of two adjusted emission bands at λmax =557 nm (fwhm=69 nm) and at λmax =604 nm (fwhm=102 nm).

Ba4Al7Li28.08O26.92N1.08, the Barium Oxonitridolithoaluminate with a Highly Condensed LiO4 Tetrahedra Framework.

The new compound Ba4Al7Li28.08O26.92N1.08 consists of AlO4/AlO3N tetrahedra, 10-fold coordinated Ba2+ cations, and a highly condensed edge- and corner-sharing LiO4 tetrahedra framework, which leads to a degree of condensation greater than 1. The first barium oxonitridolithoaluminate was synthesized by a high-temperature solid-state reaction in a weld-shut tantalum ampoule and the crystal structure has been determined by single-crystal X-ray diffraction. Ba4Al7Li28.08O26.92N1.08 crystallizes in the monoclinic space group P21/m (no. 11) with the lattice parameters a = 1052.41(3), b = 615.93(2), c = 1088.45(4) pm, ß = 98.86(1)°, and a volume of V = 0.69712(4) nm3. In addition, Ba4Al7Li28.08O26.92N1.08 doped with the activator ion Eu2+, exhibits a broad band emission with a maximum at λmax = 524 nm (2.34 eV) with a fwhm of 112 nm (4373 cm-1/0.54 eV), which can be described by a superposition of two adjusted emission bands at λmax = 515 nm (2.41 eV) with a fwhm of 70 nm (2704 cm-1/0.34 eV), and at λmax = 574 nm (2.18 eV) with a fwhm of 127 nm (4127 cm-1/0.51 eV).

The crystal structure and luminescence properties of the first lithium oxonitridolithosilicate Li3SiNO2:Eu2.

The compound Li3SiNO2:Eu2+ was synthesized in high temperature solid-state reactions in weld shut tantalum ampules and the crystal structure of Li3SiNO2 has been determined by single-crystal X-ray diffraction. It crystallizes in the monoclinic space group C2/c (no. 15) with the lattice parameters a = 1049.01(3), b = 1103.42(3), c = 511.86(2) pm, ß = 116.14(1)°, and a volume of V = 0.53187(2) nm3. This compound is built up from two different layers, which are arranged alternately along the crystallographic a-axis. The results from single-crystal diffraction were confirmed by the Rietveld analysis of bulk samples. Moreover, Li3SiNO2 could be successfully doped with the activator ion Eu2+ and the luminescence spectroscopy of single-crystals revealed broad band emission at λmax = 601 nm (fwhm = 90 nm).

Rb[Li5 Si2 O7 ] - A Latecomer in the Family of Alkali Lithosilicates Hiding a Green-Emitting Lithosilicate.

In order to expand the field of alkali lithosilicates, a new representative of the substance class with a previously unknown structure type was found based on solid-state synthesis. The novel compound with the sum formula Rb[Li5 Si2 O7 ] crystallizes in the orthorhombic space group Pbcm (no. 57) with a=7.6269(3), b=9.5415(4), and c=9.4095(3) Šby means of single-crystal X-ray diffraction. The structure consists of a highly condensed lithosilicate framework, built up of corner- and edge-linked [LiO4 ]-tetrahedra and [Si2 O7 ]-units, and the rubidium ions aligned in channels. Suitable crystals of the material were obtained using sealed tantalum ampoules as reaction tube at a temperature of 750 °C. The new compound was further characterized via powder diffraction, Rietveld analysis, and EDX measurements. At first glance, Eu2+ -doped Rb[Li5 Si2 O7 ] reveals an intense green luminescence. In-depth crystal analysis shows that a core-shell formation is present even for apparently high quality single-crystals. As a minority phase, the known green phosphor RbLi[Li3 SiO4 ]2 :Eu2+ is the origin of the luminescence, representing a tiny core inside of the particles surrounded by a large matrix of transparent Rb[Li5 Si2 O7 ] dominating the single-crystal diffraction pattern.

A Double-Band Emitter with Ultranarrow-Band Blue and Narrow-Band Green Luminescence.

Understanding the origin and mechanisms of luminescence is a crucial point when it comes to the development of new phosphors with targeted luminescence properties. Herein, a new phosphor belonging to the substance class of alkali metal lithosilicates with the generalized sum formula Cs4-x-y-z Rbx Nay Liz [Li3 SiO4 ]4 :Eu2+ is reported. Single crystals of the cyan-emitting UCr4 C4 -type phosphor show a peculiar double-band luminescence with one ultranarrow emission band at 473 nm and a narrow emission band at 531 nm under excitation with UV light (λexc =408 nm). Regarding occupation of the channels by the light metal ions, investigations of single-crystal XRD data led to the assumption that domain formation with distinct lithium- and sodium-filled channels occurs. Depending on which of these channels hosts the activator ion Eu2+ , a green or blue emission results. The herein-presented results shed new light on the luminescence process in the well-studied UCr4 C4 -type alkali metal lithosilicate phosphors.

SrAl2-xLi2+xO2+2xN2-2x:Eu2+ (0.12 ≤ x ≤ 0.66)-Tunable Luminescence in an Oxonitride Phosphor.

Starting from the recently published narrow band red phosphor SALON, a tunable oxonitride-phosphor can be derived by introducing disorder into the structure. To achieve this, the oxygen content of the reaction mixture is increased, thereby prohibiting the oxygen/nitrogen ordering observed in SALON. The resulting compound is isotypic to UCr4C4 and exhibits mixed oxygen/nitrogen and lithium/aluminum sites. Further variation of the oxygen/nitrogen ratio revealed that the structure remains stable over a wide range of compositions. The compound can therefore be described by the general sum formula SrAl2-xLi2+xO2+2xN2-2x with x ranging between 0.12 and 0.66. When doped with Eu2+, the title compound exhibits an intense luminescence upon excitation with blue light. The maximum of this emission varies depending on the oxygen content and can be tuned to values between 581 nm (x = 0.66) and 672 nm (x = 0.12).

Sr[Li2Al2O2N2]:Eu2+-A high performance red phosphor to brighten the future.

Innovative materials for phosphor converted white light-emitting diodes are in high demand owing to the huge potential of the light-emitting diode technology to reduce energy consumption worldwide. As the primary blue diode is already highly optimized, the conversion phosphors are of crucial importance for any further improvements. We report on the discovery of the high performance red phosphor Sr[Li2Al2O2N2]:Eu2+ meeting all requirements for a phosphor's optical properties. It combines the optimal spectral position for a red phosphor, as defined in the 2016 Research & Development-plan of the United States government, with an exceptionally small spectral full width at half maximum and excellent thermal stability. A white mid-power phosphor-converted light-emitting diode prototype utilising Sr[Li2Al2O2N2]:Eu2+ shows an increase of 16% in luminous efficacy compared to currently available commercial high colour-rendering phosphor-converted light-emitting diodes, while retaining excellent high colour rendition. This phosphor enables a big leap in energy efficiency of white emitting phosphor-converted light-emitting-diodes.

HF-Free Synthesis of Li2SiF6:Mn4+: A Red-Emitting Phosphor.

Li2SiF6:Mn4+ was synthesized via a new HF-free synthesis route by a high-pressure/high-temperature doping experiment at 5.5 GPa and 750 °C. It is proven that the phosphor cannot be synthesized by the common wet-chemical precipitation route in aqueous HF. The sample was characterized by powder X-ray diffraction, EDX, and luminescence spectroscopy. At room temperature, Li2SiF6:Mn4+ exhibits seven emission lines with the strongest line at λmax ≈ 630 nm and a dominant wavelength of λdom ≈ 618 nm. The CIE coordinates are 0.688 and 0.312 for x and y, respectively. The compound shows a luminous efficacy of radiation (LER) of 218 lm Wopt-1, which exceeds the LER of current state-of-the-art red LED phosphor K2SiF6:Mn4+ by 7% due to a blue-shift of the emission. It reveals excellent thermal quenching behavior up to 125 °C.

Short-Range Antiferromagnetic Ordering of Netlike S = 1/2 Linear Trimeric Units in the Copper Germanate K2Cu3Ge4O12.

K2Cu3Ge4O12 was synthesized via a solid-state reaction in a high-temperature experiment at 1073 K. Crystal structure analysis provided the following data: space group Cmcm (no. 63), a = 1407.9(2), b = 578.0(1), c = 1389.2(1) pm, V = 1.1305 nm3, and Z = 4. The structure consists of alternating layers of netlike arranged trimeric [Cu3O8]10- units and layers of four-membered rings of GeO4 tetrahedra. The potassium cations connect the different structural moieties. Although both structural motifs are well-known, the way they are connected in K2Cu3Ge4O12 is unique. K2Cu3Ge4O12 was further characterized via vibrational spectroscopy and SEM-EDX measurements. Magnetic measurements exhibit an antiferromagnetic behavior at low temperatures along with an unusual pseudo-2D coupling.

Alkali Lithosilicates: Renaissance of a Reputable Substance Class with Surprising Luminescence Properties.

A hitherto unknown synthetic access to alkali lithosilicates, a substance class first described by Hoppe in the 1980s, is reported. With the synthesis and characterization of NaK7 [Li3 SiO4 ]8 , a new representative has been discovered, expanding the family of known alkali lithosilicates. Astonishingly, NaK7 [Li3 SiO4 ]8 and the already established alkali lithosilicates Na[Li3 SiO4 ] as well as K[Li3 SiO4 ] display unforeseen luminescence properties, when doped with Eu2+ . Na[Li3 SiO4 ]:Eu2+ exhibits an ultra-narrow blue, K[Li3 SiO4 ]:Eu2+ a broadband, and NaK7 [Li3 SiO4 ]8 :Eu2+ a yellow-green double emission upon excitation with near-UV to blue light. Consequently, all of the investigated substances of this class of compounds are highly interesting phosphors for application in phosphor converted LEDs.

Stishovite's Relative: A Post-Coesite Form of Phosphorus Oxonitride.

Phosphorus oxonitride (PON) is isoelectronic with SiO2 and may exhibit a similar broad spectrum of intriguing properties as silica. However, PON has only been sparsely investigated under high-pressure conditions and there has been no evidence on a PON polymorph with a coordination number of P greater than 4. Herein, we report a post-coesite (pc) PON polymorph exhibiting a stishovite-related structure with P in a (5+1) coordination. The pc-PON was synthesized using the multianvil technique and characterized by powder X-ray diffraction, solid-state NMR spectroscopy, TEM measurements and in situ synchrotron X-ray diffraction in diamond anvil cells. The structure model was verified by single-crystal X-ray diffraction at 1.8 GPa and the isothermal bulk modulus of pc-PON was determined to K0 =163(2) GPa. Moreover, an orthorhombic PON polymorph (o-PON) was observed under high-pressure conditions and corroborated as the stable modification at pressures above 17 GPa by DFT calculations.

Crystal structures of cristobalite-type and coesite-type PON redetermined on the basis of single-crystal X-ray diffraction data.

Hitherto, phospho-rus oxonitride (PON) could not be obtained in the form of single crystals and only powder diffraction experiments were feasible for structure studies. In the present work we have synthesized two polymorphs of phospho-rus oxonitride, cristobalite-type (cri-PON) and coesite-type (coe-PON), in the form of single crystals and reinvestigated their crystal structures by means of in house and synchrotron single-crystal X-ray diffraction. The crystal structures of cri-PON and coe-PON are built from PO2N2 tetra-hedral units, each with a statistical distribution of oxygen and nitro-gen atoms. The crystal structure of the coe-PON phase has the space group C2/c with seven atomic sites in the asymmetric unit [two P and three (N,O) sites on general positions, one (N,O) site on an inversion centre and one (N,O) site on a twofold rotation axis], while the cri-PON phase possesses tetra-gonal I-42d symmetry with two independent atoms in the asymmetric unit [the P atom on a fourfold inversion axis and the (N,O) site on a twofold rotation axis]. In comparison with previous structure determinations from powder data, all atoms were refined with anisotropic displacement parameters, leading to higher precision in terms of bond lengths and angles.

A high-pressure polymorph of phosphorus oxonitride with the coesite structure.

The chemical and physical properties of phosphorus oxonitride (PON) closely resemble those of silica, to which it is isosteric. A new high-pressure phase of PON is reported herein. This polymorph, synthesized by using the multianvil technique, crystallizes in the coesite structure. This represents the first occurrence of this very dense network structure outside of SiO2 . Phase-pure coesite PON (coe-PON) can be synthesized in bulk at pressures above 15 GPa. This compound was thoroughly characterized by means of powder X-ray diffraction, DFT calculations, and FTIR and MAS NMR spectroscopy, as well as temperature-dependent diffraction. These results represent a major step towards the exploration of the phase diagram of PON at very high pressures and the possibly synthesis of a stishovite-type PON containing hexacoordinate phosphorus.

Pentacoordinate phosphorus in a high-pressure polymorph of phosphorus nitride imide P4N6(NH).

Coordination numbers higher than usual are often associated with superior mechanical properties. In this contribution we report on the synthesis of the high-pressure polymorph of highly condensed phosphorus nitride imide P4 N6 (NH) representing a new framework topology. This is the first example of phosphorus in trigonal-bipyramidal coordination being observed in an inorganic network structure. We were able to obtain single crystals and bulk samples of the compound employing the multi-anvil technique. γ-P4 N6 (NH) has been thoroughly characterized using X-ray diffraction, solid-state NMR and FTIR spectroscopy. The synthesis of γ-P4 N6 (NH) gives new insights into the coordination chemistry of phosphorus at high pressures. The synthesis of further high-pressure phases with higher coordination numbers exhibiting intriguing physical properties seems within reach.

High-pressure polymorph of phosphorus nitride imide HP4N7 representing a new framework topology.

A new polymorph of phosphorus nitride imide HP4N7 has been synthesized under high-pressure/high-temperature conditions from P3N5 and NH4Cl at 6 GPa and temperatures between 800 and 1300 °C. Its crystal structure was elucidated using single-crystal X-ray diffraction data. ß-HP4N7 (space group C2/c, no. 15, Z = 4, a = 12.873(2) Å, b = 4.6587(4) Å, c = 8.3222(8) Å, ß = 102.351(3)°, R1 = 0.0485, wR2 = 0.1083) crystallizes in a new framework structure type that is made up of all-side vertex-sharing PN4 tetrahedra. The topology of the network is represented by the point symbol (3(2).4(2).5(2).6(3).7)(3(4).4(4).5(4).6(3)), and it has not been identified in other compounds so far. Structural differences between the two polymorphs of HP4N7 as well as the topological relationship to the recently discovered high-pressure polymorph ß-HPN2 are discussed. Additionally, FTIR and solid-state NMR spectroscopy are used to corroborate the results of the structure determination.

Homoleptic imidazolate frameworks [Sr(1-x)Eu(x)(Im)2]--hybrid materials with efficient and tuneable luminescence.

Homoleptic frameworks of the formula [Sr(1-x)Eu(x)(Im)(2)] (1) (x = 0.01-1.0; Im(-) = imidazolate anion, C(3)H(3)N(2)(-)) are hybrid materials that exhibit an intensive green luminescence. Tuning of both emission wavelength and quantum yield is achieved by europium/strontium substitution so that a QE of 80% is reached at a Eu content of 5%. Even 100% pure europium imidazolate still shows 60% absolute quantum efficiency. Substitution of Sr/Eu shows that doping with metal cations can also be utilized for coordination compounds to optimize materials properties. The emission is finely tuneable in the region 495-508 nm via variation of the europium content. The series of frameworks [Sr(1-x)Eu(x)(Im)(2)] presents dense MOFs with the highest quantum yields reported for MOFs so far.