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.

Ab initio exploration and prediction of AE-containing nitrido(litho/magneso)tetrelates (AE = Ca, Sr; Tt = Si, Ge) with [Si2N6]10- or [Ge2N6]10- units.

Recently, a number of different structurally related nitrides characterized by pairs of edge-sharing Si-N tetrahedra forming [Si2N6]10- units have emerged via different synthesis methods. Concurrently, upon doping with rare earth elements (e.g. Eu2+ and Ce3+), numerous applications in the field of luminescent materials were revealed, ranging from the visible spectrum to the near IR. This compound class in turn emphasizes the extraordinary large tuning range with respect to relative composition by formal cation exchange. In this contribution, we study the dynamical stabilities of the existing Si-based nitridotetrelates and hypothetical Ge analogues promising for future synthesis efforts of luminescent materials by means of extensive phonon calculations. Further calculations of electronic and mechanical properties corroborate the fundamental suitability of the predicted compounds for the applications of potential luminescent materials with regard to band gap (Eg) and Debye temperature (ΘD). Calculated enthalpies of the reaction provide further beneficial insights for future experimental attempts. Our study hence highlights a potential range of novel stable nitridogermanates with isotypic structures and suitable electronic properties for optoelectronic applications.

Orange-Emitting Li4Sr4[Si4O4N6]O:Eu2+-a Layered Lithium Oxonitridosilicate Oxide.

We report on the structure and properties of the lithium oxonitridosilicate oxide Li4Sr4[Si4O4N6]O:Eu2+ obtained from solid-state metathesis. The crystal structure was solved and refined from single-crystal X-ray data in the space group P42/ nmc (No. 137) [ Z = 2, a = 7.4833(6), c = 9.8365(9) Å, and R1(obs) = 0.0477]. The structure of Li4Sr4[Si4O4N6]O:Eu2+ is built up from a layered 2D network of SiN3O tetrahedra and exhibits stacking disorder. The results are supported by transmission electron microscopy and energy-dispersive X-ray spectroscopy as well as lattice energy, charge distribution, and density functional theory (DFT) calculations. Optical measurements suggest an indirect band gap of about 3.6 eV, while DFT calculations on a model free of stacking faults yield a theoretical electronic band gap of 4.4 eV. Samples doped with Eu2+ exhibit luminescence in the orange spectral range (λem ≈ 625 nm; full width at half-maximum ≈ 4164 cm-1; internal quantum efficiency at room temperature = 24%), extending the broad field of phosphor materials research toward the sparsely investigated materials class of lithium oxonitridosilicate oxides.

Ammonothermal Synthesis, Optical Properties, and DFT Calculations of Mg2 PN3 and Zn2 PN3.

The phosphorus nitrides, Mg2 PN3 and Zn2 PN3 , are wide band gap semiconductor materials with potential for application in (opto)electronics or photovoltaics. For the first time, both compounds were synthesized ammonothermally in custom-built high-temperature, high-pressure autoclaves starting from P3 N5 and the corresponding metals (Mg or Zn). Alkali amides (NaNH2 , KNH2 ) were employed as ammonobasic mineralizers to increase solubility of the starting materials in supercritical ammonia through formation of reactive intermediates. Single crystals of Mg2 PN3 , with length up to 30 µm, were synthesized at 1070 K and 140 MPa. Zn2 PN3 already decomposes at these conditions and was obtained as submicron-sized crystallites at 800 K and 200 MPa. Both compounds crystallize in a wurtzite-type superstructure in orthorhombic space group Cmc21 , which was confirmed by powder X-ray diffraction. In addition, single-crystal X-ray diffraction measurements of Mg2 PN3 were carried out for the first time. To our knowledge, this is the first single-crystal X-ray study of ternary nitrides synthesized by the ammonothermal method. The band gaps of both nitrides were estimated to be 5.0 eV for Mg2 PN3 and 3.7 eV for Zn2 PN3 by diffuse reflectance spectroscopy. DFT calculations were carried out to verify the experimental values. Furthermore, a dissolution experiment was conducted to obtain insights into the crystallization behavior of Mg2 PN3 .

Oxoberyllates SrBeO2 and Sr12 Be17 O29 as Novel Host Materials for Eu2+ Luminescence.

Beryllate structures are marginally investigated, but show intriguing structural relations to oxo- and nitridosilicates. A typical feature is the coordination of Be in both trigonal planar and tetrahedral coordination by O. A broad range of structures is accessible by variable combinations of both building units. Three novel ternary Sr-oxoberyllates are characterized by single-crystal X-ray diffraction analysis, density functional theory calculations and investigations on luminescence properties: α-SrBeO2 , ß-SrBeO2 and Sr12 Be17 O29 are obtained with oxoberyllate substructures, made up of either BeO3 triangles or BeO4 tetrahedra. The compounds can be described as chain- and layer-type beryllates. When excited with blue light of (In,Ga)N LED chips, ß-SrBeO2 and Sr12 Be17 O29 show visible emission in the yellow and orange spectral range, respectively, upon doping with Eu2+ . Exceptional narrow-band yellow emission (λem =564 nm, fwhm=55 nm) makes ß-SrBeO2 :Eu2+ a promising phosphor for application in phosphor-converted (pc-)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.

Reversible Polymerization of Adamantane-type [P4 N10 ]10- Anions to Honeycomb-type [P2 N5 ]5- Layers under High-Pressure.

The high-pressure polymorph Li5 P2 N5 of Li10 P4 N10 (="2 Li5 P2 N5 ") was synthesized by high-pressure/high-temperature reaction of LiPN2 and Li7 PN4 or ß-Li10 P4 N10 at 9 GPa, using the Li3 N self-flux method in a Walker-type multianvil assembly. Li5 P2 N5 is the first lithium nitridophosphate with a layered structure and is made up of corner sharing PN4 tetrahedra forming a corrugated honeycomb-type layer of linked sechser-rings in chair conformation. The arrangement of the P atoms is analogous to that of black phosphorus. The structure was elucidated from single-crystal X-ray data. To confirm the structure Rietveld refinement, 6 Li, 7 Li and 31 P solid-state NMR spectroscopy were conducted. To corroborate Li5 P2 N5 as the corresponding high-pressure polymorph of ß-Li10 P4 N10 DFT calculations and temperature dependent X-ray powder diffraction were carried out. DFT calculations estimated the transition pressure to 6.5 GPa (without accounting for temperature), which is in line with the synthesis pressure.

Ammonothermal Synthesis and Optical Properties of Ternary Nitride Semiconductors Mg-IV-N2 , Mn-IV-N2 and Li-IV2 -N3 (IV=Si, Ge).

Grimm-Sommerfeld analogous nitrides MgSiN2 , MgGeN2 , MnSiN2 , MnGeN2 , LiSi2 N3 and LiGe2 N3 (generally classified as II-IV-N2 and I-IV2 -N3 ) are promising semiconductor materials with great potential for application in (opto)electronics or photovoltaics. A new synthetic approach for these nitride materials was developed using supercritical ammonia as both solvent and nitride-forming agent. Syntheses were conducted in custom-built high-pressure autoclaves with alkali metal amides LiNH2 , NaNH2 or KNH2 as ammonobasic mineralizers, which accomplish an adequate solubility of the starting materials and promote the formation of reactive intermediate species. The reactions were performed at temperatures between 870 and 1070 K and pressures up to 230 MPa. All studied compounds crystallize in wurtzite-derived superstructures with orthorhombic space groups Pna21 (II-IV-N2 ) and Cmc21 (I-IV2 -N3 ), respectively, which was confirmed by powder X-ray diffraction. Optical bandgaps were estimated from diffuse reflectance spectra using the Kubelka-Munk function (MgSiN2 : 4.8 eV, MgGeN2 : 3.2 eV, MnSiN2 : 3.5 eV, MnGeN2 : 2.5 eV, LiSi2 N3 : 4.4 eV, LiGe2 N3 : 3.9 eV). Complementary DFT calculations were carried out to gain insight into the electronic band structures of these materials and to corroborate the optical measurements.

Li12 P3 N9 with Non-Condensed [P3 N9 ]12- -Rings and its High-Pressure Polymorph Li4 PN3 with Infinite Chains of PN4 -Tetrahedra.

Li12 P3 N9 was synthesized by solid-state reaction of Li3 N and P3 N5 at 790 °C. It is made up of non-condensed [P3 N9 ]12- dreier-rings of PN4 -tetrahedra. The corresponding high-pressure polymorph, Li4 PN3 , was synthesized under high-pressure/high-temperature conditions from Li12 P3 N9 or LiPN2 and Li7 PN4 at 6 or 7 GPa, respectively, using the multianvil technique. Li4 PN3 is the first lithium catena-nitridophosphate and contains PN3 zweier-chains of corner sharing PN4 -tetrahedra. To confirm the structure elucidated from single-crystal X-ray data, Rietveld refinement, 6 Li, 7 Li, and 31 P solid-state NMR spectroscopy, FTIR spectroscopy and EDX measurements were carried out. To examine the phase transition of Li12 P3 N9 to Li4 PN3 at 6 GPa and to corroborate the latter as the corresponding high-pressure polymorph, DFT calculations were conducted. Electronic band gap and electron localization function (ELF) calculations were carried out to elucidate the electronic properties and bonding behavior of both polymorphs.

First-principles and experimental characterization of the electronic properties of CaGaSiN3 and CaAlSiN3: the impact of chemical disorder.

We report a detailed investigation of the electronic, mechanical and optical properties of the recently discovered nitridogallosilicate CaGaSiN3 which has potential as a LED-phosphor host material. We focus on chemical disorder effects, originating from the Ga/Si site, and compared them to those of isostructural CaAlSiN3. We calculate the elastic moduli and the Debye temperature in terms of quasi harmonical approximation. Spectral properties like the joint density of states (JDOS) are evaluated and the absorption, reflectance and energy loss function are obtained from the dielectric function. The optical band gap of CaGaSiN3 from experiment is compared to the electronic band gap in terms of electronic DOS and band structure calculations. All properties are evaluated for different ordering models of Ga/Si while the experimentally observed substitutional disorder is accounted for by utilizing the Coherent Potential Approximation (CPA). We conclude a shrinking of the band gap for both CaGaSiN3 and CaAlSiN3 due to atomic disorder, which is unfavorable for potential phosphor applications. This study contributes to materials design considerations, and provides a close look on the electronic impact of substitutional disorder. Moreover, we open the scope for future investigations on solid solutions and phosphor host materials with low doping concentrations.

Ammonothermal Synthesis of Novel Nitrides: Case Study on CaGaSiN3.

The first gallium-containing nitridosilicate CaGaSiN3 was synthesized in newly developed high-pressure autoclaves using supercritical ammonia as solvent and nitriding agent. The reaction was conducted in an ammonobasic environment starting from intermetallic CaGaSi with NaN3 as a mineralizer. At 770 K, intermediate compounds were obtained, which were subsequently converted to the crystalline nitride at temperatures up to 1070 K (70-150 MPa). The impact of other mineralizers (e.g., LiN3 , KN3 , and CsN3 ) on the product formation was investigated as well. The crystal structure of CaGaSiN3 was analyzed by powder X-ray diffraction and refined by the Rietveld method. The structural results were further corroborated by transmission electron microscopy, 29 Si MAS-NMR, and first-principle DFT calculations. CaGaSiN3 crystallizes in the orthorhombic space group Cmc21 (no. 36) with lattice parameters a=9.8855(11), b=5.6595(1), c=5.0810(1) Å, (Z=4, Rwp =0.0326), and is isostructural with CaAlSiN3 (CASN). Eu2+ doped samples exhibit red luminescence with an emission maximum of 620 nm and FWHM of 90 nm. Thus, CaGaSiN3 :Eu2+ also represents an interesting candidate as a red-emitting material in phosphor-converted light-emitting diodes (pc-LEDs). In addition to the already known substitution of alkaline-earth metals in (Ca,Sr)AlSiN3 :Eu2+ , inclusion of Ga is a further and promising perspective for luminescence tuning of widely used red-emitting CASN type materials.

High-Pressure Synthesis of Melilite-type Rare-Earth Nitridophosphates RE2P3N7 and a Ba2Cu[Si2O7]-type Polymorph.

High-pressure metathesis was proposed to be a gateway to the elusive class of rare-earth nitridophosphates. With this method the first ternary compounds of this class with sum formula RE2P3N7 were prepared, a melilite-type with RE = Pr, Nd, Sm, Eu, Ho, Yb (Ho2P3N7: P4̅21m, a = 7.3589(2), c = 4.9986(2) Å, Z = 2) and a Ba2Cu[Si2O7] structure type with RE = La, Ce, Pr (Pr2P3N7: monoclinic, C2/c, a = 7.8006(3), b = 10.2221(3), c = 7.7798(3) Å, ß = 111.299(1)°, Z = 4). The phase relation between the two structure types was prior unknown and is here evidenced by experimental data as well as density functional theory calculations performed for the Pr2P3N7 compounds. Adequate classification of both structures types with regard to Liebau nomenclature, vertex symbol, and point symbol is made. Additionally, the tiling patterns of the monolayered structures are deducted. We demonstrate that high-pressure metathesis offers a systematic access to rare-earth nitridophosphates with an atomic ratio of P/N between 1/2 and 1/4.

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.