Chemistry of Ammonothermal Synthesis

Ammonothermal Crystal Growth of Functional Nitrides for Semiconductor Devices: Status and Potential

The state-of-the-art ammonothermal method for the growth of nitrides is reviewed here, with an emphasis on binary and ternary nitrides beyond GaN. A wide range of relevant aspects are covered, from fundamental autoclave technology, to reactivity and solubility of elements, to synthesized crystalline nitride materials and their properties. Initially, the potential of emerging and novel nitrides is discussed, motivating their synthesis in single crystal form. This is followed by a summary of our current understanding of the reactivity/solubility of species and the state-of-the-art single crystal synthesis for GaN, AlN, AlGaN, BN, InN, and, more generally, ternary and higher order nitrides. Investigation of the synthesized materials is presented, with a focus on point defects (impurities, native defects including hydrogenated vacancies) based on GaN and potential pathways for their mitigation or circumvention for achieving a wide range of controllable functional and structural material properties. Lastly, recent developments in autoclave technology are reviewed, based on GaN, with a focus on advances in development of in situ technologies, including in situ temperature measurements, optical absorption via UV/Vis spectroscopy, imaging of the solution and crystals via optical (visible, X-ray), along with use of X-ray computed tomography and diffraction. While time intensive to develop, these technologies are now capable of offering unprecedented insight into the autoclave and, hence, facilitating the rapid exploration of novel nitride synthesis using the ammonothermal method.

Ammonothermal Synthesis and Solid-State NMR Study of the Imidonitridosilicate Rb3Si6N5(NH)6

The imidonitridosilicate Rb3Si6N5(NH)6, being only the second representative of this compound class, was synthesized ammonothermally at 870 K and 230 MPa. Its crystal structure was solved from single-crystal X-ray diffraction data. The imidonitridosilicate crystallizes isotypically with the respective potassium compound in space group P4132 with the lattice parameter a=10.9422(4) Å forming a three-dimensional imidonitridosilicate tetrahedra network with voids for the rubidium ions. The structure model and the presence of the imide groups were verified by Fourier-Transform infrared (FTIR) and magic-angle spinning (MAS) NMR spectroscopy, using cross polarization 15N{1H} and 29Si{1H} MAS NMR experiments. Rb3Si6N5(NH)6 represents a possible intermediate during the ammonothermal synthesis of nitridosilicates. The characterization of such intermediates improves the understanding of the reaction pathway from ammonothermal solutions to nitrides. Thus, the ammonothermal synthesis is an alternative approach to the well-established high-temperature synthesis leading to the compound class of nitridosilicates.

Controlling Phase in Colloidal Synthesis

A fundamental precept of chemistry is that properties are manifestations of the elements present and their arrangement in space. Controlling the arrangement of atoms in nanocrystals is not well understood in nanocrystal synthesis, especially in the transition metal chalcogenides and pnictides, which have rich phase spaces. This Perspective will cover some of the recent advances and current challenges. The perspective includes introductions to challenges particular to chalcogenide and pnictide chemistry, the often-convoluted roles of bond dissociation energies and mechanisms by which precursors break down, using very organized methods to map the synthetic phase space, a discussion of polytype control, and challenges in characterization, especially for solving novel structures on the nanoscale and time-resolved studies.

BaLa5V2O3N7: a novel anti-perovskite oxynitride for electrode applications

The exploration of transition metal oxynitrides has garnered significant interest due to their intriguing property diversity. Herein, we present a promising new transition metal oxynitride BaLa5V2O3N7, which features an anti-perovskite structure type. This unique structural configuration endows the material with remarkable conductivity, particularly at low temperatures, paving the way for the material to be used in a wide range of technological applications.

High-Pressure Coupling Reactions to Produce a Spherical Bulk RexN/Fe3N Composite

We report a novel high-pressure coupling (HPC) reaction that couples the nitridation of Re with high-pressure solid-state metathesis (HPSSM) of Fe₃N to produce a spherical bulk Re_xN/Fe₃N composite. Compared with conventional methods, upon coupling of the HPSSM reactions, the synthetic pressure for Re nitridation was successfully reduced from 13 to 10 GPa (for Re₃N) and from 20 to 15 GPa (for Re₂N). The product Re_xN species would be surrounded by product Fe₃N, resulting in a spherical bulk Re_xN/Fe₃N composite (x = 2 or 3). The composite exhibits a soft magnetic behavior, and the content of nitrogen in Re_xN (x = 2 or 3) was controlled by adjusting the P-T conditions. The HPC reaction establishes a new approach for the bulk synthesis of 5d transition metal nitride.

Synthesis of Colloidal GaN and AlN Nanocrystals in Biphasic Molten Salt/Organic Solvent Mixtures under High-Pressure Ammonia

Group III nitrides are of great technological importance for electronic devices. These materials have been widely manufactured via high-temperature methods such as physical vapor transport (PVT), chemical vapor deposition (CVD), and hydride vapor phase epitaxy (HVPE). The preparation of group III nitrides by colloidal synthesis methods would provide significant advantages in the form of optical tunability via size and shape control and enable cost reductions through scalable solution-based device integration. Solution syntheses of III-nitride nanocrystals, however, have been scarce, and the quality of the synthesized products has been unsatisfactory for practical use. Here, we report that incorporating a molten salt phase in solution synthesis can provide a viable option for producing crystalline III-nitride nanomaterials. Crystalline GaN and AlN nanomaterials can be grown in a biphasic molten-salt/organic-solvent mixture under an ammonia atmosphere at moderate temperatures (less than 300 °C) and stabilized under ambient conditions by postsynthetic treatment with organic surface ligands. We suggest that microscopic reversibility of monomer attachment, which is essential for crystalline growth, can be achieved in molten salt during the nucleation and the growth of the III-nitride nanocrystals. We also show that increased ammonia pressure increases the size of the GaN nanocrystals produced. This work demonstrates that use of molten salt and high-pressure reactants significantly expands the chemical scope of solution synthesis of inorganic nanomaterials.

On the Role of Amides and Imides for Understanding GaN Syntheses from Ammonia Solution: Molecular Mechanics Models of Ammonia, Amide and Imide Interactions with Gallium Nitride

A key requisite to characterizing GaN precipitation from ammonia solution from molecular simulations is the availability of reliable molecular mechanics models for the interactions of gallium ions with NH₃, NH₂⁻, and NH²⁻ species, respectively. Here, we present a tailor‐made force field which is fully compatible to an earlier developed GaN model, thus bridging the analyses of Ga³⁺ ions in ammonia solution with the aggregation of [Ga_x(NH)_y(NH₂)_z]^+3x−2y−z precursors and the modelling of GaN crystals. For this, quantum mechanical characterization of a series of Ga‐coordination clusters is used for parameterization and benchmarking the generalized amber force field (GAFF2) and tailor‐made refinements needed to achieve good agreement of both structural features and formation energy, respectively. The perspectives of our models for larger scale molecular dynamics simulations are demonstrated by the analyses of amide and imide defects arrangement during the growth of GaN crystal faces.

Novel Fluoridoaluminates from Ammonothermal Synthesis: Two Modifications of K2AlF5 and the Elpasolite Rb2KAlF6

Two new modifications of the pentafluoridoaluminate K2AlF5 were obtained from ammonothermal synthesis at 753 K, 224 MPa and 773 K, 220 MPa, respectively. Both crystallize in the orthorhombic space group type Pbcn, with close metric relations and feature kinked chains of cis-vertex-connected AlF6 octahedra resulting in the Niggli formula ∞1{[AlF2/2eF4/1t]2−}. The differences lie in the number of octahedra necessary for repetition within the chains, which for K2AlF5-2 is realized after four and for K2AlF5-3 after eight octahedra. As a result, the orthorhombic unit cell for K2AlF5-3 is doubled in chain prolongation direction [001] as compared to K2AlF5-2 (1971.18(4) pm versus 988.45(3) pm, respectively), while the unit cell parameters within the other two directions are virtually identical. Moreover, the new elpasolite Rb2KAlF6 is reported, crystallizing in the cubic space group Fm3¯m with a = 868.9(1) pm and obtained under ammonothermal conditions at 723 K and 152 MPa.

Two-Step Solid-State Synthesis of Ternary Nitride Materials

Ternary nitride materials hold promise for many optical, electronic, and refractory applications; yet, their preparation via solid-state synthesis remains challenging. Often, high pressures or reactive gases are used to manipulate the effective chemical potential of nitrogen, yet these strategies require specialized equipment. Here, we report on a simple two-step synthesis using ion-exchange reactions that yield rocksalt-derived MgZrN2 and Mg2NbN3, as well as layered MgMoN2. All three compounds show almost temperature-independent and weak paramagnetic responses to an applied magnetic field at cryogenic temperatures, indicating phase-pure products. The key to synthesizing these ternary materials is an initial low-temperature step (300-450 °C) to promote Mg-M-N nucleation. The intermediates then are annealed (800-900 °C) to grow crystalline domains of the ternary product. Calorimetry experiments reveal that initial reaction temperatures are determined by phase transitions of reaction precursors, whereas heating directly to high temperatures results in decomposition. These two-step reactions provide a rational guide to material discovery of other bulk ternary nitrides.

Single-Bonded Cubic AsN from High-Pressure and High-Temperature Chemical Reactivity of Arsenic and Nitrogen

Chemical reactivity between As and N₂, leading to the synthesis of crystalline arsenic nitride, is here reported under high pressure and high temperature conditions generated by laser heating in a diamond anvil cell. Single‐crystal synchrotron X‐ray diffraction at different pressures between 30 and 40 GPa provides evidence for the synthesis of a covalent compound of AsN stoichiometry, crystallizing in a cubic P2₁3 space group, in which each of the two elements is single‐bonded to three atoms of the other and hosts an electron lone pair, in a tetrahedral anisotropic coordination. The identification of characteristic structural motifs highlights the key role played by the directional repulsive interactions between non‐bonding electron lone pairs in the formation of the AsN structure. Additional data indicate the existence of AsN at room temperature from 9.8 up to 50 GPa. Implications concern fundamental aspects of pnictogens chemistry and the synthesis of innovative advanced materials.

Sodium rare earth metal amides Na3 RE(NH2)6 (RE = La–Nd, Er, Yb) from ammonothermal synthesis

The amides Na3 RE(NH2)6 have been obtained from the metals in supercritical ammonia under ammonobasic conditions at 573 K and 70 MPa for RE = La–Nd, and at 473 K and 40 MPa for RE = Er, Yb. All compounds are formed in the hot zone within a temperature gradient, indicating a retrograde solubility under the applied process conditions. These amides represent soluble intermediates in ammonothermal binary rare earth metal nitride synthesis. All compounds were obtained as microcrystalline powders, while single crystals of those amides containing the heavier rare earth metals could be isolated. The crystal structures were solved and refined from single-crystal and powder X-ray diffraction intensity data. The results of vibrational spectroscopy are reported. Thermal analysis measurements under inert gas atmosphere demonstrated a decomposition to the respective black binary rare earth metal nitrides REN1−δ .

Deprotonating Melamine to Gain Highly Interconnected Materials: Melaminate Salts of Potassium and Rubidium

A new class of materials, melaminate salts of potassium and rubidium, has been obtained by deprotonating molecular melamine in liquid ammonia. Potassium melaminate KC₃N₆H₅·NH₃ and rubidium melaminate RbC₃N₆H₅·1/2NH₃ were characterized by single-crystal XRD, showing that the melaminate anion is slightly distorted compared to the neutral molecule due to the ionic imine group, but it still forms extensive hydrogen bonding networks. The melaminate anion also displays an increased coordination ability of μ₄ and μ₆₊₁ (the maximum for melamine is μ₃). Thermal gravimetry coupled with mass spectrometry evidence a multistep decomposition with liberation of ammonia first and then cyanamide and larger fragments. A plausible decomposition mechanism is proposed. The infrared spectrum allows to identify the fingerprint of the melaminate vibrations such as to partially characterize the also synthesized amorphous sodium melaminate NaC₃N₆H₅·nNH₃ and the proposed tripotassium melaminate K₃C₃N₆H₃.

Na2La4(NH2)14·NH3, a lanthanum-rich intermediate in the ammonothermal synthesis of LaN and the effect of ammonia loss on the crystal structure

Single crystals of Na2La4(NH2)14·NH3 were obtained from supercritical ammonia under ammonobasic conditions at a temperature of 573 K and 120 MPa pressure. It represents a lanthanum-rich intermediate in the ammonothermal synthesis of LaN. Upon aging, the title compound loses the crystal ammonia, resulting in pale crystals of Na2La4(NH2)14, the original space group P212121 being retained in a very similar unit cell. However, the crystal structure reacts to subtle changes in the composition as well as to the modified coordination of particularly the sodium cations interconnecting lanthanum amide layers within a third dimension. Results of Raman spectroscopic studies are reported. The observations of thermal analysis measurements indicating the formation of lanthanum nitride, in combination with the observed retrograde solubility in liquid ammonia, contribute to the knowledge of the ammonothermal crystal growth of lanthanum nitride.

Numerical Simulation of Ammonothermal Crystal Growth of GaN—Current State, Challenges, and Prospects

Numerical simulations are a valuable tool for the design and optimization of crystal growth processes because experimental investigations are expensive and access to internal parameters is limited. These technical limitations are particularly large for ammonothermal growth of bulk GaN, an important semiconductor material. This review presents an overview of the literature on simulations targeting ammonothermal growth of GaN. Approaches for validation are also reviewed, and an overview of available methods and data is given. Fluid flow is likely in the transitional range between laminar and turbulent; however, the time-averaged flow patterns likely tend to be stable. Thermal boundary conditions both in experimental and numerical research deserve more detailed evaluation, especially when designing numerical or physical models of the ammonothermal growth system. A key source of uncertainty for calculations is fluid properties under the specific conditions. This originates from their importance not only in numerical simulations but also in designing similar physical model systems and in guiding the selection of the flow model. Due to the various sources of uncertainty, a closer integration of numerical modeling, physical modeling, and the use of measurements under ammonothermal process conditions appear to be necessary for developing numerical models of defined accuracy.

Pyridine-based Liquid-Phase Synthesis of Crystalline TiN and ZnSiN2 Nanoparticles

TiN and ZnSiN2 nanoparticles are obtained via a novel pyridine-based synthesis route. This one-pot liquid-phase route strictly avoids all oxygen sources (including starting materials, surface functionalization, solvents), which is highly relevant in regard of the material purity and material properties. Colloidally stable suspensions of crystalline, small-sized TiN (5.4±0.4 nm) and ZnSiN2 (5.2±1.1 nm) are instantaneously available from the liquid phase. Elemental analysis and electron energy loss spectroscopy confirm the purity of the compounds and specifically the absence of oxygen. The as-prepared ZnSiN2 show yellowish emission (500-700 nm) already at room temperature with its maximum at 570 nm.

Approaching Dissolved Species in Ammonoacidic GaN Crystal Growth: A Combined Solution NMR and Computational Study

Solutions of gallium trihalides GaX₃ (X=F, Cl, Br, I) and their ammoniates in liquid ammonia were studied at ambient temperature under autogenous pressure by multinuclear (⁷¹ Ga, ³⁵ Cl, ⁸¹ Br) NMR spectroscopy. To unravel the role of pH, the analyses were done both in absence and in presence of ammonium halides, which are employed as mineralizers during ammonoacidic gallium nitride crystal growth. While gallium trifluoride and its ammoniate were found to be too sparingly soluble to give rise to a NMR signal, the spectra of solutions of the heavier halides reveal the presence of a single gallium-containing species in all cases. DFT calculations and molecular dynamics simulations suggest the identification of this species as consisting of a [Ga(NH₃ )₆ ]³⁺ cation and up to six surrounding halide anions, resulting in an overall trend towards negative complex charge. Quantitative ⁷¹ Ga NMR studies on saturated solutions of GaCl₃ containing various amounts of additional NH₄ Cl revealed a near linear increase of GaCl₃ solubility with mineralizer concentration of about 0.023 mol GaCl₃ per mol NH₄ Cl at room temperature. These findings reflect the importance of Coulombic shielding for the inhibition of oligomerization and precipitation processes and help to rationalize both the low solubility of gallium halides in neutral ammonia solution and, in turn, the proliferating effect of the mineralizer during ammonoacidic gallium nitride formation.

Interaction potentials for modelling GaN precipitation and solid state polymorphism

We outline a molecular mechanics model for the interaction of gallium and nitride ions ranging from small complexes to nanoparticles and bulk crystals. While the current GaN force fields allow the modelling of either bulk crystals or single ions dispersed in solution, our model covers both and hence paves the way to describing aggregate formation and crystal growth processes from molecular simulations. The key to this is the use of formal  +3 and  -3 charges on the gallium and nitride ions, whilst accounting for the charge transfer in GaN crystals by means of additional potential energy terms. The latter are fitted against experimental data of GaN in the wurtzite structure and benchmarked for the zinc-blende and rock-salt polymorphs. Comparison to quantum chemical references and experiment shows reasonable agreement of structures and formation energy of [GaN] _n aggregates, elastic properties of the bulk crystal, the transition pressure of the wurtzite to rock-salt transformation and intrinsic point defects. Furthermore, we demonstrate force field transferability towards the modelling of GaN nanoparticles from simulated annealing runs.

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.

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 (II^a_1−xII^b_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.

High-pressure synthesis of ultraincompressible hard rhenium nitride pernitride Re2(N2)(N)2 stable at ambient conditions

High-pressure synthesis in diamond anvil cells can yield unique compounds with advanced properties, but often they are either unrecoverable at ambient conditions or produced in quantity insufficient for properties characterization. Here we report the synthesis of metallic, ultraincompressible (K₀ = 428(10) GPa), and very hard (nanoindentation hardness 36.7(8) GPa) rhenium nitride pernitride Re₂(N₂)(N)₂. Unlike known transition metals pernitrides Re₂(N₂)(N)₂ contains both pernitride N₂^4- and discrete N^3- anions, which explains its exceptional properties. Re₂(N₂)(N)₂ can be obtained via a reaction between rhenium and nitrogen in a diamond anvil cell at pressures from 40 to 90 GPa and is recoverable at ambient conditions. We develop a route to scale up its synthesis through a reaction between rhenium and ammonium azide, NH₄N₃, in a large-volume press at 33 GPa. Although metallic bonding is typically seen incompatible with intrinsic hardness, Re₂(N₂)(N)₂ turned to be at a threshold for superhard materials.

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.

Ammonothermal Synthesis of Nitrides: Recent Developments and Future Perspectives

Nitrides represent an intriguing class of functional materials with a broad range of application fields. Within the past decade, the ammonothermal method became increasingly attractive for the synthesis and crystal growth of nitride materials. The ammonothermal approach proved to be eminently suitable for the growth of bulk III-nitride semiconductors like GaN, and furthermore provided access to numerous ternary and multinary nitrides and oxonitrides with promising optical and electronic properties. In this minireview, we will shed light on the latest research findings covering the synthesis of nitrides by this method. An overview of synthesis strategies for binary, ternary, and multinary nitrides and oxonitrides, as well as their properties and potential applications will be given. The recent development of autoclave technologies for syntheses at high temperatures and pressures, in situ methods for investigations of crystallization processes, and solubility measurements by ultrasonic velocity experiments is briefly reviewed as well. In conclusion, challenges and future perspectives regarding the synthesis and crystal growth of novel nitrides, as well as the advancement of autoclave techniques are discussed.

Pyridine-based low-temperature synthesis of CoN, Ni3N and Cu3N nanoparticles

CoN, Ni₃N and Cu₃N nanoparticles were prepared via low-temperature, oxygen-free liquid-phase synthesis in refluxing pyridine. This approach, leading to high-purity, narrow-size (3–5 nm) nitrides, can be generally very promising for obtaining nanosized nitrides and to address their material properties.

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.

Ammonothermal Synthesis of Crystalline Oxonitride Perovskites LnTaON2 (Ln=La, Ce, Pr, Nd, Sm, Gd)

The perovskite type oxonitridotantalates LnTaON₂ with Ln=La, Ce, Pr, Nd, Sm, and Gd were synthesized by the ammonothermal method employing custom-built autoclaves made of nickel-based superalloy. Metal powders were reacted with NaOH and NaN₃ as mineralizers under supercritical conditions with purified ammonia at temperature range of 870-1070 K and pressure values of 150-300 MPa. Crystal structures and the space groups were determined by using powder X-ray diffraction and refined by the Rietveld method. The refined lattice parameters are for LaTaON₂ (a=5.7156(1), b=8.0675(1), c=5.7465(1) Å, R_wp =0.0471), CeTaON₂ (a=5.6761(11), b=8.0386(16), c=5.7891(12) Å, R_wp =0.183), PrTaON₂ (a=5.6920(1), b=8.0197(1), c=5.6804(1) Å, R_wp =0.0349), NdTaON₂ (a=5.6884(1), b=8.0037(2), c=5.6554(1) Å, R_wp =0.026), SmTaON₂ (a=5.6827(1), b=7.9656(2), c=5.6103(1) Å, R_wp =0.042), GdTaON₂ (a=5.6160(10), b=7.9359(12), c=5.5962(10) Å, R_wp =0.118). LaTaON₂ crystallizes in space group Imma (no. 74) and the other compounds LnTaON₂ with Ln=Pr, Nd, Sm, Gd in Pnma (no. 62). SEM measurements were performed to investigate the elemental composition and morphology of the oxonitride perovskites. The bandgap values of the oxonitrides (LaTaON₂ 1.8 eV, CeTaON₂ 1.7 eV; PrTaON₂ 1.9 eV, NdTaON₂ 2.0 eV, SmTaON₂ 2.0 eV, GdTaON₂ 1.8 eV) were estimated by using UV/Vis measurements and the Kubelka-Munk function.

Ammonothermal Synthesis of Earth-Abundant Nitride Semiconductors ZnSiN2 and ZnGeN2 and Dissolution Monitoring by In Situ X-ray Imaging

In this contribution, first synthesis of semiconducting ZnSiN₂ and ZnGeN₂ from solution is reported with supercritical ammonia as solvent and KNH₂ as ammonobasic mineralizer. The reactions were conducted in custom-built high-pressure autoclaves made of nickel-based superalloy. The nitrides were characterized by powder X-ray diffraction and their crystal structures were refined by the Rietveld method. ZnSiN₂ (a=5.24637(4), b=6.28025(5), c=5.02228(4) Å, Z=4, R_wp =0.0556) and isotypic ZnGeN₂ (a=5.46677(10), b=6.44640(12), c=5.19080(10) Å, Z=4, R_wp =0.0494) crystallize in the orthorhombic space group Pna2₁ (no. 33). The morphology and elemental composition of the nitrides were examined by electron microscopy and energy-dispersive X-ray spectroscopy (EDX). Well-defined single crystals with a diameter up to 7 μm were grown by ammonothermal synthesis at temperatures between 870 and 1070 K and pressures up to 230 MPa. Optical properties have been analyzed with diffuse reflectance measurements. The band gaps of ZnSiN₂ and ZnGeN₂ were determined to be 3.7 and 3.2 eV at room temperature, respectively. In situ X-ray measurements were performed to exemplarily investigate the crystallization mechanism of ZnGeN₂ . Dissolution in ammonobasic supercritical ammonia between 570 and 670 K was observed which is quite promising for the crystal growth of ternary nitrides under ammonothermal conditions.