Li18 P6 N16 -A Lithium Nitridophosphate with Unprecedented Tricyclic [P6 N16 ]18- Ions

Trigonal Planar [PN3]4- Anion in the Nitridophosphate Oxide Ba3[PN3]O

Nitridophosphates, with their primary structural motif of isolated or condensed PN4 tetrahedra, meet many requirements for high performance materials. Their properties are associated with their structural diversity, which is mainly limited by this specific building block. Herein, we present the alkaline earth metal nitridophosphate oxide Ba3[PN3]O featuring a trigonal planar [PN3]4- anion. Ba3[PN3]O was obtained using a hot isostatic press by medium-pressure high-temperature synthesis (MP/HT) at 200 MPa and 880 °C. The crystal structure was solved and refined from single-crystal X-ray diffraction data in space group R3 ‾ ${\bar 3}$ c (no. 167) and confirmed by SEM-EDX, magic angle spinning (MAS) NMR, vibrational spectroscopy (Raman, IR) and low-cost crystallographic calculations (LCC). MP/HT synthesis reveals great potential by extending the structural chemistry of P to include trigonal planar [PN3]4- motifs.

Nitride Synthesis under High-Pressure, High-Temperature Conditions: Unprecedented In Situ Insight into the Reaction

High-pressure, high-temperature (HP/HT) syntheses are essential for modern high-performance materials. Phosphorus nitride, nitridophosphate, and more generally nitride syntheses benefit greatly from HP/HT conditions. In this contribution, we present the first systematic in situ investigation of a nitridophosphate HP/HT synthesis using the reaction of zinc nitride Zn3N2 and phosphorus(V) nitride P3N5 to the nitride semiconductor Zn2PN3 as a case study. At a pressure of 8 GPa and temperatures up to 1300 °C, the reaction was monitored by energy-dispersive powder X-ray diffraction (ED-PXRD) in a large-volume press at beamline P61B at DESY. The experiments investigate the general behavior of the starting materials under extreme conditions and give insight into the reaction. During cold compression and subsequent heating, the starting materials remain crystalline above their ambient-pressure decomposition points, until a sufficient minimum temperature is reached and the reaction starts. The reaction proceeds via ion diffusion at grain boundaries with an exponential decay in the reaction rate. Raising the temperature above the minimum required value quickly completes the reaction and initiates single-crystal growth. After cooling and decompression, which did not influence the resulting product, the recovered sample was analyzed by energy-dispersive X-ray (EDX) spectroscopy.

Please Mind the Gap: Highly Condensed P-N Networks in LiP4 N7 and Li3-x P6 N11-x (NH)x

Alkali nitridophosphates AP4 N7 and A3 P6 N11 (A=Na, K, Rb, Cs) have been known for decades. However, their Li homologues have remained elusive. In this work, the highly condensed lithium (imido)nitridophosphates LiP4 N7 and Li3-x P6 N11-x (NH)x (x=1.66(3)) were synthesized from LiPN2 and P3 N5 in the multianvil press at 10 GPa. They constitute the first lithium nitridophosphates with 3D networks exhibiting a degree of condensation larger than 0.5 and high thermal stability. LiP4 N7 crystallizes in the orthorhombic space group P21 21 21 with a=4.5846(6) Å, b=8.0094(11) Å, and c=13.252(2) Å (Z=4). Li3-x P6 N11-x (NH)x crystallizes in the triclinic space group P1 - ${\mathrel{\mathop{{\rm { 1}}}\limits^{{\rm -}}}}$ with Z=2, a=4.6911(11) Å, b=7.024(2) Å, c=12.736(3) Å, α=87.726(11), β=80.279(11), and γ=70.551(12)°. Both compounds are stable against hydrolysis in air.

Finding Order in Disorder: The Highly Disordered Lithium Oxonitridophosphate Double Salt Li8+x P3 O10-x N1+x (x=1.4(5))

The crystalline lithium oxonitridophosphate Li8+x P3 O10-x N1+x , was obtained in an ampoule synthesis from P3 N5 and Li2 O. The compound crystallizes in the triclinic space group P1 - ${\mathrel{\mathop{{\rm { 1}}}\limits^{{\rm -}}}}$ with a=5.125(2), b=9.888(5), c=10.217(5) Å, α=70.30(2), β=76.65(2), γ=77.89(2)°. Li8+x P3 O10-x N1+x is a double salt, the structure of which contains distinctive complex anion species, namely non-condensed P(O,N)4 tetrahedra, and P(O,N)7 double tetrahedra connected by one N atom. Additionally, there is mixed occupation of O/N positions, which enables further anionic species by variation of O/N occupancies. To characterize these motifs in detail, complementary analytical methods were applied. The double tetrahedron exhibits significant disorder in single-crystal X-ray diffraction. Furthermore, the title compound is a Li+ ion conductor with a total ionic conductivity of 1.2×10-7  S cm-1 at 25 °C, and a corresponding total activation energy of 0.47(2) eV.

Structure Determination of the Crystalline LiPON Model Structure Li5+x P2 O6-x N1+x with x≈0.9

Non-crystalline lithium oxonitridophosphate (LiPON) is used as solid electrolyte in all-solid-state batteries. Crystalline lithium oxonitridophosphates are important model structures to retrieve analytical information that can be used to understand amorphous phases better. The new crystalline lithium oxonitridophosphate Li_5+x P₂ O_6-x N_1+x was synthesized as an off-white powder by ampoule synthesis at 750-800 °C under Ar atmosphere. It crystallizes in the monoclinic space group P2₁ /c with a=15.13087(11) Å, b=9.70682(9) Å, c=8.88681(7) Å, and β=106.8653(8)°. Two P(O,N)₄ tetrahedra connected by an N atom form the structural motif [P₂ O_6-x N_1+x ]^(5+x)- . The structure was elucidated from X-ray diffraction data and the model corroborated by NMR and infrared spectroscopy, and elemental analyses. Measurements of ionic conductivity show a total ionic conductivity of 6.8×10^-7  S cm^-1 at 75 °C with an activation energy of 0.52±0.01 eV.

Synthesis of Nitride Zeolites in a Hot Isostatic Press

The recently introduced nitridophosphate synthesis in a hot isostatic press (HIP) enabled simple access to large‐scale product quantities starting from exclusively commercially available starting materials. Herein, we show that this method is suitable for the synthesis of highly condensed functional nitridophosphates, as well. Hence, the syntheses of the nitridophosphate zeolites Ba₃P₅N₁₀X (X=Cl, Br) are presented as proof of concept for this innovative access. Furthermore, samples of unprecedented Sr₃P₅N₁₀X (X=Cl, Br) were prepared and characterized to demonstrate the advantages of this synthetic approach over commonly used methods. Luminescence investigations on Eu²⁺‐doped samples of AE₃P₅N₁₀X (AE=Sr, Ba; X=Cl, Br) were carried out and characteristics of observed emission bands are discussed.

HIP to be Square: Simplifying Nitridophosphate Synthesis in a Hot Isostatic Press

(Oxo)Nitridophosphates have recently been identified as a promising compound class for application in the field of solid-state lighting. Especially, the latest medium-pressure syntheses under ammonothermal conditions draw attention of the semiconductor and lighting industry on nitridophosphates. In this contribution, we introduce hot isostatic presses as a new type of medium-pressure synthetic tool, further simplifying nitridophosphate synthesis. In a second step, phosphorus nitride was replaced as starting material by red phosphorus, enabling the synthesis of Ca₂ PN₃ as model compound, starting only from readily available compounds. Moreover, first luminescence investigations on Eu²⁺ -doped samples reveal Ca₂ PN₃ :Eu²⁺ as a promising broad-band red-emitter (λ_em =650 nm; fwhm=1972 cm^-1 ). Besides simple handling, the presented synthetic method offers access to large sample volumes, and the underlying reaction conditions facilitate single-crystal growth, required for excellent optical properties.

Sr3 P3 N7 : Complementary Approach by Ammonothermal and High-Pressure Syntheses

Nitridophosphates exhibit an intriguing structural diversity with different structural motifs, for example, chains, layers or frameworks. In this contribution the novel nitridophosphate Sr3 P3 N7 with unprecedented dreier double chains is presented. Crystalline powders were synthesized using the ammonothermal method, while single crystals were obtained by a high-pressure multianvil technique. The crystal structure of Sr3 P3 N7 was solved and refined from single-crystal X-ray diffraction and confirmed by powder X-ray methods. Sr3 P3 N7 crystallizes in monoclinic space group P2/c. Energy-dispersive X-ray and Fourier-transformed infrared spectroscopy were conducted to confirm the chemical composition, as well as the absence of NHx functionality. The optical band gap was estimated to be 4.4 eV using diffuse reflectance UV/Vis spectroscopy. Upon doping with Eu2+ , Sr3 P3 N7 shows a broad deep-red to infrared emission (λem =681 nm, fwhm≈3402 cm-1 ) with an internal quantum efficiency of 42 %.

Crystalline Nitridophosphates by Ammonothermal Synthesis

Nitridophosphates are a well-studied class of compounds with high structural diversity. However, their synthesis is quite challenging, particularly due to the limited thermal stability of starting materials like P₃ N₅ . Typically, it requires even high-pressure techniques (e.g. multianvil) in most cases. Herein, we establish the ammonothermal method as a versatile synthetic tool to access nitridophosphates with different degrees of condensation. α-Li₁₀ P₄ N₁₀ , β-Li₁₀ P₄ N₁₀ , Li₁₈ P₆ N₁₆ , Ca₂ PN₃ , SrP₈ N₁₄ , and LiPN₂ were synthesized in supercritical NH₃ at temperatures and pressures up to 1070 K and 200 MPa employing ammonobasic conditions. The products were analyzed by powder X-ray diffraction, energy dispersive X-ray spectroscopy, and FTIR spectroscopy. Moreover, we established red phosphorus as a starting material for nitridophosphate synthesis instead of commonly used and not readily available precursors, such as P₃ N₅ . This opens a promising preparative access to the emerging compound class of nitridophosphates.

Nitridophosphates: A Success Story of Nitride Synthesis

Nitridophosphates and phosphorus nitrides are thoroughly investigated classes of nitrides. During thirty years of research, the methods for their synthesis evolved from the condensation of molecular precursors at moderate temperatures and ambient pressures to state-of-the-art high-pressure and high-temperature processes. Landmark breakthroughs made in recent years led to a comprehension-based proficiency in nitridophosphate synthesis that is illustrated by the large compositional and structural diversity of the nitridophosphates known today. Herein, we review the advances made in synthesis with regard to the prevalent problem of nitride synthesis: the susceptibility of nitride ions to oxidation.

SrH4 P6 N12 and SrP8 N14 : Insights into the Condensation Mechanism of Nitridophosphates under High Pressure

The (imido)nitridophosphates SrH₄ P₆ N₁₂ and SrP₈ N₁₄ were synthesized as colorless crystals by high-pressure/high-temperature reactions using the multianvil technique (5 GPa, ca. 1075 °C). Stoichiometric amounts of Sr(N₃ )_2, P₃ N₅ , and amorphous HPN₂ were used as starting materials. Whereas the crystal structure of SrH₄ P₆ N₁₂ was solved and refined from single-crystal X-ray diffraction data and confirmed by Rietveld refinement, the structure of SrP₈ N₁₄ was determined from powder diffraction data. In order to confirm the structures, ¹ H and ³¹ P solid-state NMR spectroscopy and FTIR spectroscopy were carried out. The chemical composition was confirmed with EDX measurements. Both compounds show unprecedented layered network structure types, built up from all-side vertex-sharing PN₄ tetrahedra which are structurally related. The structural comparison of both compounds gives first insights into the hitherto unknown condensation mechanism of nitridophosphates under high pressure.

Li+ Ion Conductors with Adamantane-Type Nitridophosphate Anions β-Li10 P4 N10 and Li13 P4 N10 X3 with X=Cl, Br

β-Li₁₀ P₄ N₁₀ and Li₁₃ P₄ N₁₀ X₃ with X=Cl, Br have been synthesized from mixtures of P₃ N₅ , Li₃ N, LiX, LiPN₂ , and Li₇ PN₄ at temperatures below 850 °C. β-Li₁₀ P₄ N₁₀ is the low-temperature polymorph of α-Li₁₀ P₄ N₁₀ and crystallizes in the trigonal space group R3. It is made up of non-condensed [P₄ N₁₀ ]^10- T2 supertetrahedra, which are arranged in sphalerite-analogous packing. Li₁₃ P₄ N₁₀ X₃ (X=Cl, Br) crystallizes in the cubic space group Fm3‾m . Both isomorphic compounds comprise adamantane-type [P₄ N₁₀ ]^10- , Li⁺ ions, and halides, which form octahedra. These octahedra build up a face-centered cubic packing, whose tetrahedral voids are occupied by the [P₄ N₁₀ ]^10- ions. The crystal structures have been elucidated from X-ray powder diffraction data and corroborated by EDX measurements, solid-state NMR, and FTIR spectroscopy. Furthermore, we have examined the phase transition between α- and β-Li₁₀ P₄ N₁₀ . To confirm the ionic character, the migration pathways of the Li⁺ ions have been evaluated and the ion conductivity and its temperature dependence have been determined by impedance spectroscopy. XPS measurements have been carried out to analyze the stability with respect to Li metal.

Strategies Based on Nitride Materials Chemistry to Stabilize Li Metal Anode

Lithium metal battery is a promising candidate for high-energy-density energy storage. Unfortunately, the strongly reducing nature of lithium metal has been an outstanding challenge causing poor stability and low coulombic efficiency in lithium batteries. For decades, there are significant research efforts to stabilize lithium metal anode. However, such efforts are greatly impeded by the lack of knowledge about lithium-stable materials chemistry. So far, only a few materials are known to be stable against Li metal. To resolve this outstanding challenge, lithium-stable materials have been uncovered out of chemistry across the periodic table using first-principles calculations based on large materials database. It is found that most oxides, sulfides, and halides, commonly studied as protection materials, are reduced by lithium metal due to the reduction of metal cations. It is discovered that nitride anion chemistry exhibits unique stability against Li metal, which is either thermodynamically intrinsic or a result of stable passivation. The results here establish essential guidelines for selecting, designing, and discovering materials for lithium metal protection, and propose multiple novel strategies of using nitride materials and high nitrogen doping to form stable solid-electrolyte-interphase for lithium metal anode, paving the way for high-energy rechargeable lithium batteries.