The Phosphidosilicates AE2 Li4 SiP4 (AE=Ca, Sr, Eu) Ba4 Li16 Si3 P12.

The quaternary phosphidosilicates AE2 Li4 SiP4 (AE=Ca, Sr, Eu) and Ba4 Li16 Si3 P12 were synthesized by heating the elements and Li3 P under argon atmosphere. Their crystal structures were determined by single crystal X-ray diffraction. AE2 Li4 SiP4 crystallize in a new layered structure type (P21 /m, Z=2) with CdI2 -analoguos layers. Edge sharing CaP6 octahedra are separated by layers of vertex-sharing SiP4 and LiP4 tetrahedra, which contain additional chains of LiP6 octahedra. Ba4 Li16 Si3 P12 forms likewise a new structure type (P21 /c, Z=16) with a three-dimensional network of SiP4 , Si2 P6 and LiP4 entities as well as one phosphorus site not bonded to silicon. Barium is located in capped trigonal prisms of phosphorus which form strongly corrugated layers. 31 P and 29 Si solid-state NMR spectra confirm the crystal structures of the compounds AE2 Li4 SiP4 . 7 Li spectra show only one signal in spite of quite different crystallographic positions, which indicate possible Li+ mobility. However, this signal is much broader compared to the known Li+ conducting phosphidosilicates. Accordingly, electrochemical impedance measurements show low Li+ conductivities.

From Framework to Layers Driven by Pressure - The Monophyllo-Oxonitridophosphate ß-MgSrP3 N5 O2 and Comparison to its α-Polymorph.

Oxonitridophosphates exhibit the potential for broad structural diversity, making them promising host-compounds in phosphor-converted light-emitting diode applications. The novel monophyllo-oxonitridophosphate ß-MgSrP3 N5 O2 was obtained by using the high-pressure multianvil technique. The crystal structure was solved and refined based on single-crystal X-ray diffraction data and confirmed by powder X-ray diffraction. ß-MgSrP3 N5 O2 crystallizes in the orthorhombic space group Cmme (no. 67, a=8.8109(6), b=12.8096(6), c=4.9065(3) Å, Z=4) and has a structure related to that of Ba2 CuSi2 O7 . DFT calculations were performed to investigate the phase transition from α- to ß-MgSrP3 N5 O2 and to confirm the latter as the corresponding high-pressure polymorph. Furthermore, the luminescence properties of Eu2+ doped samples of both polymorphs were investigated and discussed, showing blue and cyan emission, respectively (α-MgSrP3 N5 O2 ; λmax =438 nm, fwhm=46 nm/2396 cm-1 ; ß-MgSrP3 N5 O2 ; λmax =502 nm, fwhm=42 nm/1670 cm-1 ).

Discovery of Two Polymorphs of TiP4 N8 Synthesized from Binary Nitrides.

TiP4 N8 was obtained from the binary nitrides TiN and P3 N5 upon addition of NH4 F as a mineralizer at 8 GPa and 1400 °C. An intricate interplay of disorder and polymorphism was elucidated by in situ temperature-dependent single-crystal X-ray diffraction, STEM-HAADF, and the investigation of annealed samples. This revealed two polymorphs, which consist of dense networks of PN4 tetrahedra (degree of condensation κ=0.5) and either augmented triangular TiN7 prisms or triangular TiN6 prisms for α- and ß-TiP4 N8 , respectively. The structures of TiP4 N8 exhibit body-centered tetragonal (bct) framework topology. DFT calculations confirm the measured band gaps of α- and ß-TiP4 N8 (1.6-1.8 eV) and predict the thermochemistry of the polymorphs in agreement with the experiments.

Heterobimetallic uranyl(VI) alkoxides of lanthanoids: formation through simple ligand exchange.

Lanthanoid and actinoid silylamides are versatile starting materials. Herein we show how a simple ligand exchange with tert-butanol leads to the formation of the first trimeric heterobimetallic uranyl(VI)-lanthanoid(III) alkoxide complexes. The µ3 coordination of the endogenous uranyl oxo atom results in a significant elongation of the bond length and a significant deviation from the linear uranyl arrangement.

The First High-Pressure Chromium Oxonitridoborate CrB4 O6 N-an Unexpected Link to Nitridosilicate Chemistry.

CrB4 O6 N crystallizes in the non-centrosymmetric space group P63 mc (no. 186) with the lattice parameters a=5.1036(1), c=8.3519(3) Å, and a volume of 188.40(1) Å3 . It was synthesized in a high-pressure/high-temperature experiment at 7 GPa and 1673 K and represents the first high-pressure oxonitridoborate. It is built up of starlike-shaped entities of four BO3 N tetrahedra, connected via one common nitrogen atom that resembles the fourfold-coordinated nitrogen atoms in the homeotypic nitridosilicates MYbSi4 N7 (M=Sr, Ba). Building up a network with channels that contain the Cr3+ ions, CrB4 O6 N contains for the first time a tetrahedral building unit in contrast to trigonal planar B(O/N)3 entities in all other known oxonitridoborates. The structural relations as well as the results of spectroscopic measurements and calculations on the chromium oxonitridoborate are discussed.

Supertetrahedral anions in the phosphidosilicates Na1.25Ba0.875Si3P5 and Na31Ba5Si52P83.

Solid ionic conductors are one key component of all-solid-state batteries, and recent studies with lithium, sodium and potassium phosphidosilicates revealed remarkable ion conduction capabilities in these compounds. We report the synthesis and crystal structures of two quaternary phosphidosilicates with sodium and barium, which crystallize in new structure types. Na1.25Ba0.875Si3P5 contains layers of T3 supertetrahedra, while Na31Ba5Si52P83 forms defect T5 entities and contains Si-Si bonds and P3 trimers. Though T1-relaxometry data indicate a relatively low activation energy for Na+ migration of 0.16 eV, the crystal structures lack sufficient three-dimensional migration paths necessary for fast sodium ion conductvity.

Polymorphism and Fast Potassium-Ion Conduction in the T5 Supertetrahedral Phosphidosilicate KSi2 P3.

The all-solid-state battery (ASSB) is a promising candidate for electrochemical energy storage. In view of the limited availability of lithium, however, alternative systems based on earth-abundant and inexpensive elements are urgently sought. Besides well-studied sodium compounds, potassium-based systems offer the advantage of low cost and a large electrochemical window, but are hardly explored. Here we report the synthesis and crystal structure of K-ion conducting T5 KSi2 P3 inspired by recent discoveries of fast ion conductors in alkaline phosphidosilicates. KSi2 P3 is composed of SiP4 tetrahedra forming interpenetrating networks of large T5 supertetrahedra. The compound passes through a reconstructive phase transition from the known T3 to the new tetragonal T5 polymorph at 1020 °C with enantiotropic displacive phase transitions upon cooling at about 155 °C and 80 °C. The potassium ions are located in large channels between the T5 supertetrahedral networks and show facile movement through the structure. The bulk ionic conductivity is up to 2.6×10-4  S cm-1 at 25 °C with an average activation energy of 0.20 eV. This is remarkably high for a potassium ion conductor at room temperature, and marks KSi2 P3 as the first non-oxide solid potassium ion conductor.

Electrochemical Synthesis and Crystal Structure of the Organic Ion Intercalated Superconductor (TMA)0.5Fe2Se2 with Tc = 43 K.

Intercalation of organic cations in superconducting iron selenide can significantly increase the critical temperature (Tc). We present an electrochemical method using ß-FeSe crystals (Tc ≈ 8 K) floating on a mercury cathode to intercalate tetramethylammonium ions (TMA+) quantitatively to obtain bulk samples of (TMA)0.5Fe2Se2 with Tc = 43 K. The layered crystal structure is closely related to the ThCr2Si2-type with disordered TMA+ ions between the FeSe layers. Although the organic ions are not detectable by X-ray diffraction, packing requirements as well as first-principle density functional theory calculations constrain the specified structure. Our synthetic route enables electrochemical intercalations of other organic cations with high yields to greatly optimize the superconducting properties and to expand this class of high-Tc materials.

BaGe8As14: a semiconducting sodalite-type compound.

A new sodalite-type compound, namely BaGe8As14 was synthesized via solid-state reactions and structurally characterized with single crystal X-ray diffraction (space group I4[combining macron]3m). Vertex-sharing GeAs4-tetrahedra form ß-cages with additional Ge/As-mixed sites located slightly above or below the six-membered rings. The structure is similar to the borate mineral rhodizite. Barium atoms are disordered due to a slight shift off the centers of large ß-cages. This partially disordered structure together with a narrow bandgap of 0.43 eV in line with low resistivity (2 × 10-2Ω cm), and a high carrier concentration (1.6 × 1020 cm-3) at 300 K qualifies BaGe8As14 as a potential thermoelectric material.

Mixed Valence and Unusual Germanium Coordination in SrGe8As10, BaGe8As10, and BaGe7P12.

The new Zintl-compounds AEGe8As10 (AE = Sr, Ba) and BaGe7P12 were synthesized via solid-state reactions, and their structures were determined by single crystal and powder X-ray diffraction. SrGe8As10 and BaGe8As10 crystallize in the space group Cmce and show complex 3D networks composed of three different Ge-As motifs and As-As bonds with mixed valence of germanium in the oxidation states +2, + 3, and +4. Mixed valences of germanium +3 and +4 occur in BaGe7P12, which crystallizes in the space group R3̅ with a 3D network built up of Ge2P6 dumbbells and P-P bonds. An exceptional 6-fold coordinated germanium resides in the center of a GeP6 trigonal antiprism. High temperature X-ray diffraction shows thermal stabilities up to 923-953 K. UV-Vis and resistivity measurements reveal a semiconducting nature with small indirect band gaps between 0.02 and 1.6 eV. Electronic band structure calculations confirm the semiconducting state and indicate covalent bonds within the Ge-Pn polyanions.

High-pressure synthesis and crystal structure of SrGa4As4.

Strontium tetra-gallate(II,III) tetra-arsenide, SrGa4As4, was synthesized in a Walker-type multianvil apparatus under high-pressure/high-temperature conditions of 8 GPa and 1573 K. The com-pound crystallizes in a new structure type (P3221, Z = 3) as a three-dimensional (3D) framework of corner-sharing SrAs8 quadratic anti-prisms with strontium situated on a twofold rotation axis (Wyckoff position 3b). This arrangement is surrounded by a 3D framework which can be described as alternately stacked layers of either condensed GaIIIAs4 tetra-hedra or honeycomb-like layers built up from distorted ethane-like GaII 2As6 units com-prising Ga-Ga bonds.

Solid Solutions of Grimm-Sommerfeld Analogous Nitride Semiconductors II-IV-N2 (II=Mg, Mn, Zn; IV=Si, Ge): Ammonothermal Synthesis and DFT Calculations.

Grimm-Sommerfeld analogous II-IV-N2 nitrides such as ZnSiN2 , ZnGeN2 , and MgGeN2 are promising semiconductor materials for substitution of commonly used (Al,Ga,In)N. Herein, the ammonothermal synthesis of solid solutions of II-IV-N2 compounds (II=Mg, Mn, Zn; IV=Si, Ge) having the general formula (IIa 1-x IIb x )-IV-N2 with x≈0.5 and ab initio DFT calculations of their electronic and optical properties are presented. The ammonothermal reactions were conducted in custom-built, high-temperature, high-pressure autoclaves by using the corresponding elements as starting materials. NaNH2 and KNH2 act as ammonobasic mineralizers that increase the solubility of the reactants in supercritical ammonia. Temperatures between 870 and 1070 K and pressures up to 200 MPa were chosen as reaction conditions. All solid solutions crystallize in wurtzite-type superstructures with space group Pna21 (no. 33), confirmed by powder XRD. The chemical compositions were analyzed by energy-dispersive X-ray spectroscopy. Diffuse reflectance spectroscopy was used for estimation of optical bandgaps of all compounds, which ranged from 2.6 to 3.5 eV (Ge compounds) and from 3.6 to 4.4 eV (Si compounds), and thus demonstrated bandgap tunability between the respective boundary phases. Experimental findings were corroborated by DFT calculations of the electronic structure of pseudorelaxed mixed-occupancy structures by using the KKR+CPA approach.

Supertetrahedral Layers Based on GaAs or InAs.

The solid-state compounds M15Tr22As32 and M3Ga6As8 (M = Sr, Eu; Tr = Ga, In) were synthesized by heating the elements, and their crystal structures were determined by single-crystal and powder X-ray diffraction (space group C2/c). The structures are hierarchical variants of the HgI2 type and consist of layers of polymeric T5 (M15Tr22As32) or T6 supertetrahedra (M3Ga6As8), separated by strontium or europium cations. These compounds constitute hitherto unknown GaAs- or InAs-based supertetrahedral structures and represent the first binary vacancy-free T5 and T6 supertetrahedra. Vacancies or mixed-metal strategies for charge compensation, as known from related chalcogenides, are not required for supertetrahedra based on charge-neutral GaAs or InAs. Optical band gap, resistivity, and Hall-effect measurements together with DFT calculations reveal that the supertetrahedral compounds are direct band gap semiconductors similar to binary GaAs or InAs. Magnetic susceptibility measurements confirm Eu2+ in Eu15Ga22As32, Eu15In22As32, and Eu3Ga6As8 and indicate antiferromagnetic ordering below 10 K.

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.

Temperature induced valence phase transition in intermediate-valent YbPd2Al3.

A temperature induced valence phase transition from Yb3+ at higher temperatures to Yb2+ at lower temperatures was observed at T = 110(1) K for intermetallic YbPd2Al3. The title compound has been prepared from the elements in sealed tantalum ampoules. The structure was refined from single-crystal data and the title compound was found to crystallize in the hexagonal YNi2Al3 type structure with space group P6/mmm and lattice parameters of a = 929.56(7) and c = 420.16(3) pm (300 K data). Full ordering of the Pd and Al atoms within the [Pd2Al3] δ- polyanion was observed. Magnetic measurements revealed an anomaly in the dc susceptibility data and intermediate valent Yb at higher temperature, as observed from the effective magnetic moment. The proposed valence phase transition was also observed as a λ-type anomaly in heat capacity measurements (T = 108.4(1) K), however, no systematic shift of the λ-peak was observed in field dependent heat capacity measurements. An antiferromagnetic ordering at this temperature, however, could be excluded, based on field-dependent susceptibility measurements and magnetization isotherms. No dynamic phenomenon was observed in ac susceptibility measurements, excluding e.g. spin-glass behavior. Subsequent temperature dependent single-crystal and powder X-ray diffraction experiments indicated a steep increase in the length of the c axis around T = 110 K upon cooling. However, no structural phase transition was found via single-crystal diffraction experiments conducted at 90 K. The anomaly was also observed in other physical measurements of e.g. the electrical resistivity, indicating a clear change in the electronic structure of the material. X-ray photoelectron spectroscopy conducted at room temperature shows the presence of both, Yb2+ and Yb3+, underlining the mixed-valent state. Members of the solid solution Yb1-x Ca x Pd2Al3 (x = 0.33, 0.67, 1) were finally used to further study the charge ordering and the present temperature induced valence phase transition.

Abrupt Europium Valence Change in Eu2Pt6Al15 around 45 K.

Eu2Pt6Al15 has been prepared from the elements via arc-melting and subsequent temperature treatment; the structure was refined from single crystal X-ray diffraction data. The compound crystallizes in an orthorhombic (3 + 1)D commensurately modulated structure (Sc2Pt6Al15 type) with space group Cmcm(α,0,0)0 s0 (α = 2/3). Full ordering of the Pt and Al atoms within the [Pt6Al15]δ- polyanion was observed. Magnetic measurements revealed an anomaly in the susceptibility data at T = 41.6(1) K, which was also observed as λ-type anomaly in heat capacity measurements ( T = 40.7(1) K). Temperature dependent powder X-ray diffraction experiments indicated a drastic shortening of the c axis (-18 pm, -1.1%) around 45 K, while the a axis nearly remains the same (-1 pm, -0.2%). Measurements of the electrical resistivity verified the anomaly, indicating a clear change in the electronic structure of the material. The observed anomalies in the physical measurements can be explained by a temperature driven first order valence change from Eu2+ at higher temperatures (>55 K) to Eu3+ at low temperatures. This valence change was proven by temperature dependent 151Eu Mössbauer spectroscopic investigations. Isostructural Eu2Pt6Ga15 was prepared in comparison, and it shows divalent Eu atoms down to 2.5 K along with antiferromagnetic ordering at TN = 13.1(1) K.

Fast Sodium-Ion Conductivity in Supertetrahedral Phosphidosilicates.

Fast sodium-ion conductors are key components of Na-based all-solid-state batteries which hold promise for large-scale storage of electrical power. We report the synthesis, crystal-structure determination, and Na+ -ion conductivities of six new Na-ion conductors, the phosphidosilicates Na19 Si13 P25 , Na23 Si19 P33 , Na23 Si28 P45 , Na23 Si37 P57 , LT-NaSi2 P3 and HT-NaSi2 P3 , based entirely on earth-abundant elements. They have SiP4 tetrahedra assembled interpenetrating networks of T3 to T5 supertetrahedral clusters and can be hierarchically assigned to sphalerite- or diamond-type structures. 23 Na solid-state NMR spectra and geometrical pathway analysis show Na+ -ion mobility between the supertetrahedral cluster networks. Electrochemical impedance spectroscopy shows Na+ -ion conductivities up to σ (Na+ )=4×10-4  S cm-1 . The conductivities increase with the size of the supertetrahedral clusters through dilution of Na+ -ions as the charge density of the anionic networks decreases.

Flux synthesis, crystal structures, and physical properties of new lanthanum vanadium oxyselenides.

The new lanthanum vanadium oxyselenides LaVSe2O, La5V3Se6O7, La5V3Se7O5, La7VSe5O7, and La13V7Se16O15 were synthesized in eutectic NaI/KI fluxes, and their crystal structures were determined using single-crystal and powder X-ray diffraction experiments. LaVSe2O and La5V3Se6O7 adopt known structure types, whereas La5V3Se7O5, La7VSe5O7, and La13V7Se16O15 crystallize in hitherto unknown structure types. The main building blocks of these compounds are chains of edge-sharing VSe6, VSe5O, and/or VSe4O2 octahedra, linked together by edge-sharing OLa4 and/or OLa3V tetrahedra forming fluorite-like ribbons. LaVSe2O, La5V3Se7O5, and La7VSe5O7 contain only V(iii) ions, whereby La5V3Se6O7 and La13V7Se16O15 contain mixtures of either V(iii)/V(iv) or V(iii)/V(v) cations. Magnetic measurements indicate Curie-Weiss paramagnetism and magnetic ordering of the vanadium moments at low temperatures. More precisely, we observe antiferromagnetism for La5V3Se6O7, metamagnetism for La5V3Se7O5, ferromagnetism for La7VSe5O7 and a complex magnetic structure for La13V7Se16O15.

Mo2 B4 O9 -Connecting Borate and Metal-Cluster Chemistry.

We report on the first thoroughly characterized molybdenum borate, which was synthesized in a high-pressure/high-temperature experiment at 12.3 GPa/1300 °C using a Walker-type multianvil apparatus. Mo2 B4 O9 incorporates tetrahedral molybdenum clusters into an anionic borate crystal structure-a structural motif that has never been observed before in the wide field of borate crystal chemistry. The six bonding molecular orbitals of the [Mo4 ] tetrahedron are completely filled with 12 electrons, which are fully delocalized over the four molybdenum atoms. This finding is in agreement with the results of the magnetic measurements, which confirmed the diamagnetic character of Mo2 B4 O9 . The two four-coordinated boron sites can be differentiated in the 11 B MAS-NMR spectrum because of the strongly different degrees of local distortions. Experimentally obtained IR and Raman bands were assigned to vibrational modes based on DFT calculations.

Flux Synthesis, Crystal Structures, and Magnetic Ordering of the Rare-Earth Chromium(II) Oxyselenides RE2CrSe2O2 (RE = La-Nd).

The rare-earth chromium(II) oxyselenides RE2CrSe2O2 (RE = La-Nd) were synthesized in eutectic NaI/KI fluxes, and their crystal structures were determined by single-crystal and powder X-ray diffraction (Pb2HgCl2O2-type, C2/m, Z = 2). The magnetic structure of La2CrSe2O2 was solved and refined from neutron powder diffraction data. Main building blocks are chains of edge-sharing CrSe4O2 octahedra linked together by two edge-sharing ORE3Cr tetrahedra forming infinite ribbons. The Jahn-Teller instability of divalent Cr2+ (d4) leads to structural phase transitions at 200 and 130 K in La2CrSe2O2 and Ce2CrSe2O2, respectively. RE2CrSe2O2 are Curie-Weiss paramagnetic above TN ≈ 14-17 K. Neutron powder diffraction reveals anti-ferromagnetic ordering of the Cr2+ moments in La2CrSe2O2 below TN = 12.7(3) K with an average ordered moment of 3.40(4) µB/Cr2+ at 4 K, which was confirmed by muon spin rotation experiments.