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

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.

High-Pressure High-Temperature Synthesis of Mixed Nitridosilicatephosphates and Luminescence of AESiP3 N7 :Eu2+ (AE=Sr, Ba)

Tetrahedra-based nitrides with network structures have emerged as versatile materials with a broad spectrum of properties and applications. Both nitridosilicates and nitridophosphates are well-known examples of such nitrides that upon doping with Eu²⁺ exhibit intriguing luminescence properties, which makes them attractive for applications. Nitridosilicates and nitridophosphates show manifold structural variability; however, no mixed nitridosilicatephosphates except SiPN₃ and SiP2N4NH have been described so far. The compounds AESiP₃ N₇ (AE=Sr, Ba) were synthesized by a high-pressure high-temperature approach using the multianvil technique (8 GPa, 1400-1700 °C) starting from the respective alkaline earth azides and the binary nitrides P₃ N₅ and Si₃ N₄ . The latter were activated by NH₄ F, probably acting as a mineralizing agent. SrSiP₃ N₇ and BaSiP₃ N₇ were obtained as single crystals. They crystallized in the barylite-1O (M=Sr) and barylite-2O structure types (M=Ba), respectively, with P and Si being occupationally disordered. Cation disorder was further supported by solid-state NMR spectroscopy and energy-dispersive X-ray spectroscopy (EDX) mapping of BaSiP₃ N₇ with atomic resolution. Upon doping with Eu²⁺ , both compounds showed blue emission under UV excitation.

Post-Synthetic Modification: Systematic Study on a Simple Access to Nitridophosphates

Nitridophosphates are a well‐studied class of nitrides with diverse materials properties, such as luminescence or ion conductivity. Despite the growing interest in this compound class, their synthesis mostly works through direct combination of starting materials. Herein, we present a systematic study on a promising method for post‐synthetic modification by treating pre‐synthesized nitridophosphates with halides under elevated pressures and temperatures. Herein, we focus on the applicability of this approach to P/N compounds with different degrees of condensation. Accordingly, BaP₂N₄, Ba₃P₅N₁₀Br, SrH₄P₆N₁₂, CaP₈N₁₄, and Ca₂PN₃ are investigated as model compounds for framework‐, layer‐, and chain‐type nitridophosphates. The formation of structurally related, as well as, completely unrelated compounds, compared to the starting materials, shows the great potential of the approach, which increases the synthetic possibilities for nitridophosphates significantly.

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.

The Long-Periodic Loop-Branched Chain Structure of the Oxonitridophosphate La21 P40 O46 N57 , Elucidated by a Combination of TEM and Microfocused Synchrotron Radiation

The lanthanum oxonitridophosphate La21 P40 O46 N57 was synthesized by high-pressure metathesis from partially hydrolysed LiPN2 and LaCl3 at 750-950 °C and 7-9 GPa. The combination of transmission electron microscopy (TEM) and diffraction using microfocused synchrotron radiation revealed a monoclinic crystal structure (space group P21 /n, a=14.042(4), b=7.084(3), c=41.404(10) Å, β=97.73(3)° and Z=2), which is characterized by loop-branched 21 member single chains of P(O,N)4 tetrahedra that extend along [2 0 1]. These chains are related to the loop-branched dreier single chains with dreier-ring loops in stillwellite (CeBSiO5 ). In La21 P40 O46 N57 , these chains are characterized by a complex long-periodic conformation and exhibit disorder that involves La/N and P split positions. This is an extraordinarily long periodicity with respect to branched single chains of tetrahedra. La21 P40 O46 N57 constitutes the first rare-earth oxonitridophosphate exhibiting a chain structure. Single-crystal data are consistent with electron and powder X-ray diffraction.

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.

SrP3 N5 NH: A Framework-Type Imidonitridophosphate Featuring Structure-Directing Hydrogen Bonds

Nitridophosphates and imidonitridophosphates show intriguing structural diversity, including unprecedented structure types. Highly condensed strontium imidonitridophosphate SrP₃ N₅ NH has been synthesized at 8 GPa and 1100 °C using a high-pressure high-temperature approach starting from stoichiometric amounts of Sr(N₃ )₂ , P₃ N₅ and NH₄ Cl. Herein, NH₄ Cl was used as a hydrogen source and as a precursor for in situ formation of SrCl₂ , which acts as mineralizer and facilitates growth of single-crystals with a diameter of ≤30 μm. SrP₃ N₅ NH (P2₁ /c (no. 14), a=5.01774(2), b=8.16912(4), c=12.70193(5) Å, β=101.7848(3)°, Z=4) adopts an unprecedented network structure, represented by the point symbol (3.4.5.6.7² )(3.4.5.7² .8)(3.6.7³ .8). This unique three nodal P/N(H) network is stabilized by moderately strong hydrogen bonds causing a structure-directing effect, which has not yet been reported for imidonitridophosphates.

LiPr2P4N7O3: Structural Diversity of Oxonitridophosphates Accessed by High-Pressure Metathesis

The structural diversity of tetrahedra networks of phosphates can greatly be enhanced by introduction of mixed N/O anion positions. LiPr₂P₄N₇O₃ exemplifies the benefits of N/O mixed anion positions as it is the first rare-earth (oxo)nitridophosphate with a single-layered structure and a degree of condensation (atomic ratio of tetrahedra centers (P) to tetrahedra corners (N/O atoms)) of 2/5. The compound was prepared through high-pressure metathesis starting from PrF₃, LiPN₂, Li₂O, and PON using a hydraulic 1000t press and the multianvil technique. LiPr₂P₄N₇O₃ crystallizes as pale-green single-crystals, from which its structure was determined (space group P2₁/ c (no. 14), a = 4.927(1), b = 7.848(2), c = 10.122(2) Å, β = 91.55(3)°, Z = 2, R₁ = 0.020, wR₂ = 0.045). The structure consists of single-layers of vertex-sharing Q³-type P(N/O)₄ tetrahedra forming four- and eight-membered rings arranged in the fashion of the Archimedean fes net. UV-vis spectroscopy revealed the typical Pr³⁺ f -f transitions, leading to a pale-green color of the crystals. Moreover, the optical band gap was determined to 4.1(1) eV, assuming a direct transition. High-temperature powder X-ray diffraction showed the beginning of a gradual decomposition starting at ca. 500 °C.

Accessing Tetravalent Transition-Metal Nitridophosphates through High-Pressure Metathesis

Advancing the attainable composition space of a compound class can lead to fascinating materials. The first tetravalent metal nitridophosphate, namely Hf_9-x P₂₄ N_52-4x O_4x (x≈1.84), was prepared by high-pressure metathesis. The Group 4 nitridophosphates are now an accessible class of compounds. The high-pressure metathesis reaction using a multianvil setup yielded single crystals that were suitable for structure analysis. Magnetic properties of the compound indicate Hf in oxidation state +IV. Optical measurements show a band gap in the UV region. The presented route unlocks the new class of Group 4 nitridophosphates by significantly improving the understanding of this nitride chemistry. Hf_9-x P₂₄ N_52-4x O_4x (x≈1.84) is a model system and its preparation is the first step towards a systematic exploration of the transition-metal nitridophosphates.