Ammonothermal Conditions Research Articles

Ammonothermal Synthesis and Crystal Structure of the Ternary Amide Na2Ba(NH2)4

AbstractThe ternary amide Na2Ba(NH2)4 was synthesized at ammonothermal conditions (870 K, 135 MPa) in custom‐built high‐pressure autoclaves. The compound was structurally characterized using X‐ray diffraction and crystallizes in space group Pccn (no. 56) with lattice parameters a=10.6492(2), b=7.8064(2) and c=8.1046(2) Å. To the best of our knowledge, the structure type has not yet been observed in any ternary amide before and can be described as a defective variant of the NaCl structure type. The presence of amide ions in the compound is verified by Fourier‐transform infrared (FTIR) spectroscopy and the experimental spectrum is compared to the theoretical spectrum obtained through density functional theory (DFT) calculations. Na2Ba(NH2)4 complements the range of reported ternary alkali metal alkaline‐earth metal amides with the smallest Shannon radius ratio of rA/AE=0.76. The influence of this ratio on the formation of the new structure type is discussed as well. The characterization of intermediate species such as this ternary amide extends the understanding of the ammonothermal synthesis and can be useful for synthetic control in the formation of nitrides at ammonothermal conditions.

(Invited) Progress in Elucidating the Growth Process for Basic Ammonothermal GaN

Gallium nitride (GaN) is an attractive materials platform for the pursuit of high-power, high-efficiency (opto-)electronic devices. Key to allowing this wide band gap material system to achieve its full potential is the availability of low-cost, large-area, high-crystal-quality native substrates. While important advances have been made to this effect, continued progress and breakthroughs are required to truly meet this objective.The ammonothermal method, which utilizes supercritical ammonia to preferentially dissolve and crystalize GaN on a native seed, offers a true bulk crystal growth process to yield large, inexpensive GaN substrates. To further enhance the growth rate and quality of GaN, much still needs to be learned and gained from an improved understanding of the chemical processes and growth mechanisms occurring in the ammonothermal environment. A key challenge to achieving this, is the difficulty in obtaining in-situ information during growth of GaN due to the use of thick-walled autoclaves containing the corrosive environment at temperatures up to ~ 650 °C and pressures up to ~ 2500 atm.This talk will present the progress made in our understanding of the underpinnings of the basic ammonothermal growth of GaN. Specifically, it will discuss advances made to allow us to understand the thermodynamic foundation of the solvent, for the first time. This is achieved by the creation and application of a new, simple equation of state for supercritical ammonia under ammonothermal conditions. Additionally, by combining this solvent knowledge with in-situ temperature, total system measurements, and GaN single crystal growth, various aspects of the crystal growth process and solute behaviors are elucidated and will be presented. Consequences towards our ability to enhance the growth rate of basic ammonothermal GaN will then be discussed.

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.

Synthesis and Characterization of the Amidomanganates Rb2[Mn(NH2)4] and Cs2[Mn(NH2)4

We report the successful synthesis of Rb2[Mn(NH2)4] and Cs2[Mn(NH2)4] from ammonothermal conditions at 723 K and pressures above 850 bar. Both compounds were obtained single phase according to powder X-ray diffraction. The crystal structures were determined by single crystal X-ray diffraction. For Rb2[Mn(NH2)4] we have obtained the high-temperature phase. The structures are analyzed with respect to the earlier reported alkali metal amidomanganates. Upon heating in inert atmosphere Cs2[Mn(NH2)4] decomposes to manganese nitrides. IR spectroscopic results are reported.

Two Intermediates in Ammonothermal InN Crystal Growth: [In(NH3)5Cl]Cl2 and InF2(NH2)

AbstractThe ammonothermal synthesis is a promising instrument for synthesis and crystal growth of the metastable semiconductor InN. Two novel intermediates within the ammonothermal InN crystal growth have been synthesized, namely pentaamminechloridoindium dichloride, [In(NH3)5Cl]Cl2, and indium amide difluoride, InF2(NH2). Both intermediates were obtained under ammonothermal conditions (Tmax=663 K, pmax=118.3 MPa, and Tmax=753 K, respectively) in a near‐ammononeutral milieu as established for InN crystal growth, using the respective indium trihalide together with potassium amide as mineralizer system. The crystal structure of [In(NH3)5Cl]Cl2 shows strong structural relations to the corresponding intermediate [Ga(NH3)5Cl]Cl2 within ammonoacidic GaN crystal growth. At ambient pressure, it decomposes above 450 K under release of ammonia. InF2(NH2) shows attributes of a classical MX3 crystal structure, realizing the motif of a hexagonal close packing of (NH)2− together with F− ions and indium occupying one third of the octahedral holes between every layer of the packing. Vibrational spectroscopy for both solid intermediates is reported.

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

Abstract(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 Ca2PN3 as model compound, starting only from readily available compounds. Moreover, first luminescence investigations on Eu2+‐doped samples reveal Ca2PN3:Eu2+ as a promising broad‐band red‐emitter (λem=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.

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 Ca2PN3 as model compound, starting only from readily available compounds. Moreover, first luminescence investigations on Eu2+‐doped samples reveal Ca2PN3: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.

Cs2Cd(C2H)2(C2): A Crystalline Acetylide with Bridging C2 Units.

Cs2Cd(C2H)2(C2) was synthesized by heating known Cs2Cd(C2H)4 either in a dry argon atmosphere at 200 °C or under ammonothermal conditions (130 °C, ∼ 100 bar). The crystal structure of the resulting dark orange-brown microcrystalline material was solved and refined from synchrotron powder diffraction data (Cmcm, Z = 4). Cs2Cd(C2H)2(C2) is composed of Cd2+ cations tetrahedrally coordinated end-on by four acetylide groups. Two of them are terminating C2H- groups, whereas the other two positions are occupied by bridging C22- anions. Thus, a polymeric ∞1[Cd(C2H)2(C2)2/22-] chain-like anion results and these chains are separated by Cs+ cations. So obviously Cs2Cd(C2H)2(C2) is formed from Cs2Cd(C2H)4 by a condensation reaction of two of its four C2H- groups under the release of one acetylene (C2H2) molecule. This reaction mechanism is supported by DSC/TGA measurements, and the crystal structure of Cs2Cd(C2H)2(C2) is further supported by IR spectroscopic investigations.

In situ investigation of decomposing ammonia and ammonobasic solutions under supercritical conditions via UV/vis and Raman Spectroscopy

An optical cell was used in this work, which allows spectroscopic measurements of fluids up to 600 °C and 300 MPa. The maximum pressure reached in this work was 254 MPa and the internal cell temperature was a maximum of 563 °C at a heating temperature of 617 °C. In the following, a combination of Raman and UV/vis spectroscopy was used for in situ investigation of the breakdown of sodium azide above 300 °C combined with the formation of sodium amide. It was verified that the breakdown of NaN3 occurs before the formation of NaNH2 starts.In a second step the decomposition of supercritical ammonia under ammonothermal conditions was investigated solely via Raman spectroscopy. The study comprises four different fluid compositions: pure ammonia, ammonia with a ruthenium catalyst, ammonia and mineralizer (sodium azide) and last ammonia, mineralizer and gallium nitride (GaN). By this, it was possible to proof that with pure ammonia and without a proper catalyst the decomposition of ammonia stops quite far before chemical equilibrium is reached.

Ammonothermal Synthesis of Alkali‐Alkaline Earth Metal and Alkali‐Rare Earth Metal Carbodiimides: K5–xMx(CN2)2+x(HCN2)1–x(M= Sr, Eu) and Na4.32Sr0.68(CN2)2.68(HCN2)0.32

Alkali‐alkaline earth metal and alkali‐rare earth metal carbodiimides, namely K5–xMx(CN2)2+x(HCN2)1–x(x= 0 – 1) (M= Sr, Eu) and Na4.32Sr0.68(CN2)2.68(HCN2)0.32, were synthesized under ammonothermal conditions in high‐pressure autoclaves. The structures of the three compounds can be derived from homeotypic K5H(CN2)3and Na5H(CN2)3by partial substitution of K+or Na+by Sr2+or Eu2+. The reactions were carried out in two step syntheses (T1= 673 K,T2= 823 K) starting from sodium or potassium azide, dicyandiamide and strontium or Eu(NH2)2, respectively. The crystal structures were solved and refined from single‐crystal X‐ray diffraction data [K4.16Sr0.84(CN2)2.84(HCN2)0.16: space groupIm3m(no. 229),a= 7.8304(5) Å,Z= 2,R1= 0.024,wR2= 0.052; K4.40Eu0.60(CN2)2.60(HCN2)0.40: space groupIm3m(no. 229),a= 7.8502(6) Å,Z= 2,R1= 0.022,wR2= 0.049]. In contrast to the potassium carbodiimides, the sodium‐strontium carbodiimide was only synthesized as microcrystalline powder. The crystal structure was determined by powder X‐ray diffraction and refined by the Rietveld method [Na4.32Sr0.68(CN2)2.68(HCN2)0.32: space groupIm3m(no. 229),a= 7.2412(1) Å,Z= 2,Rwp= 0.050]. The presence of hydrogencyanamide units ([HNCN]–) next to carbodiimide units ([CN2]2–) in all compounds was confirmed by FT‐IR spectroscopy.

Dissolved Intermediates in Ammonothermal Crystal Growth: Stepwise Condensation of [Ga(NH2)4]− toward GaN

Reactions of Ga or GaN with RbNH2 or CsNH2 under ammonothermal conditions result in liquids representing intermediate compounds in the ammonobasic GaN crystal growth. These liquids are fully miscible with liquid ammonia at room temperature and under autogenous pressure, and may eventually solidify to amorphous solids. The Cs-containing liquid initially contains an equilibrium mixture of tetraamidogallate ions [Ga(NH2)4]− and a dinuclear complex [(H2N)3Ga(μNH)Ga(NH2)3]2–. Oxygen impurities induce the formation of a μ-O-bridged dinuclear complex [(H2N)3GaOGa(NH2)3]2–. Mononuclear tetraamidogallate ions are the dominating species directly after synthesis, but the equilibrium shifts gradually toward the imido complex when ammonia is removed, and eventually a second condensation step to [(H2N)2Ga(μ-NH)2Ga(NH2)2]2– occurs. Addition of excess liquid ammonia under ambient conditions causes quantitative conversion of the dinuclear species to [Ga(NH2)4]−. The [Ga(NH2)4]− ions are also the only observable gallium-co...

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, Rwp =0.0556) and isotypic ZnGeN2 (a=5.46677(10), b=6.44640(12), c=5.19080(10) Å, Z=4, Rwp =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.

Three Solid Modifications of Ba[Ga(NH2)4]2: A Soluble Intermediate in Ammonothermal GaN Crystal Growth

Ammonothermally grown GaN crystals are highly demanded as superior substrates for manufacturing high‐performance blue and white LEDs. Ba or Ba(NH2)2 can act as effective mineralizers under ammonothermal conditions (523 K, 110–150 MPa) and lead to the formation of barium bis(tetramidogallate), Ba[Ga(NH2)4]2, a new intermediate in h‐GaN synthesis, which was obtained in three solid forms. Single crystals of all three modifications were characterized by X‐ray diffraction techniques. Like earlier characterized intermediates in ammonobasic GaN crystal growth, all forms of Ba[Ga(NH2)4]2 contain isolated [Ga(NH2)4]– units. Differences between the forms originate in the coordination environments of Ba, leading to various frameworks with the [Ga(NH2)4]– complex ions. Raman and IR spectroscopic data are discussed. Ba[Ga(NH2)4]2 in an inert gas thermally decomposes in several steps to finally yield the intermetallic compound BaGa2.

Ammonothermal Synthesis and Crystal Structures of Diamminetriamidodizinc Chloride [Zn2(NH3)2(NH2)3]Cl and Diamminemonoamidozinc Bromide [Zn(NH3)2(NH2)]Br

The treatment of excess zinc in the presence of ammonium chloride under ammonothermal conditions of 873 K and 97 MPa leads to diamminetriamidodizinc chloride [Zn2(NH3)2(NH2)3]Cl with a two-dimensionally μ-amido-interconnected substructure. Similar reaction conditions using ammonium bromide instead of the chloride (773 K, 230 MPa) result in diamminemonoamidozinc bromide [Zn(NH3)2(NH2)]Br with one-dimensional infinite μ-amido-bridged chains. Both compounds were obtained as colorless, very moisture sensitive crystals. Crystal structures and hydrogen bond schemes are analyzed. Raman spectroscopic data of the chloride are reported.

ChemInform Abstract: Ammonothermal Synthesis and Characterization of Cs2[Zn(NH2)4

AbstractSingle crystals of the title amidozincate of several centimeters in dimensions are grown under ammonothermal conditions in the cold temperature zone of an autoclave using Zn powder and CsNH2 in a 1:2 molar ratio (stainless steel autoclave, 523→≈423 K, 155 MPa, 12 h).

Ammonothermal Synthesis and Characterization of Cs2[Zn(NH2)4

Cs2[Zn(NH2)4] was synthesized under ammonothermal conditions (sc‐NH3, 523 K, 155 MPa) from CsNH2 and Zn. Growth of cm‐sized crystals succeeded upon application of a temperature gradient. The crystal structure is based on the motif of a hexagonal closed packing of [Zn(NH2)4]2– ions with occurrence of no significant hydrogen bridges according to distances and vibrational spectroscopy. Cs+ ions are located within octahedral and tetrahedral holes of the packing.

Ammonothermal synthesis of GaN using Ba(NH2)2 as mineralizer

It is demonstrated that hexagonal GaN can be obtained under ammonothermal conditions (125MPa and 723K) using Ba(NH2)2 as mineralizer. The hexagonal wurtzite-type GaN crystallites are several µm in diameter, as examined by scanning electron microscopy. This is to our knowledge the first successful ammonothermal GaN synthesis using an alkaline-earth metal as mineralizer. Ba[Ga(NH2)4]2 was identified as intermediate species in the ammonothermal synthesis process. The formation of h-GaN using Sr(NH2)2 as mineralizer was indicated only at higher temperatures above 1000K.

Acidic ammonothermal growth of gallium nitride in a liner-free molybdenum alloy autoclave

This paper discusses promising materials for use as internal, non-load bearing components as well as molybdenum-based alloys for autoclave structural components for an ammonothermal autoclave. An autoclave was constructed from the commercial titanium–zirconium–molybdenum (TZM) alloy and was found to be chemically inert and mechanically stable under acidic ammonothermal conditions. Preliminary seeded growth of GaN was demonstrated with negligible incorporation of transition metals (including molybdenum) into the grown material (<1017cm−3). Molybdenum and TZM were exposed to a basic ammonothermal environment, leading to slight degradation through formation of molybdenum nitride powders on their surface at elevated temperatures (T>560°C). The possibility of a ‘universal’, inexpensive, liner-free ammonothermal autoclave capable of exposure to basic and acidic chemistry is demonstrated.

Trigonal-Bipyramidal Coordination in First Ammoniates of ZnF2: ZnF2(NH3)3 and ZnF2(NH3)2.

Single crystals of ZnF2(NH3)3 and ZnF2(NH3)2 were obtained under ammonothermal conditions (250 °C, 196 MPa and 500 °C, 136 MPa). Upon thermal decomposition of both ZnF2(NH3)3 and ZnF2(NH3)2, a microcrystalline powder of ZnF2(NH3) was obtained. ZnF2(NH3)3 and ZnF2(NH3)2 represent probable intermediates in a conceivable ammonothermal synthesis of the semiconductor Zn3N2 and manifest a rare trigonal-bipyramidal coordination of F(-) and NH3 ligands around Zn(2+) according to single-crystal X-ray diffraction. Thermal analysis of all three compounds showed not only ZnF2(NH3) but also ZnF2(NH3)2 to be decomposition intermediates of ZnF2(NH3)3 prior to the formation of ZnF2. All three compounds demonstrate hydrogen bonds, as indicated by the intensities and half-widths of the bands in the vibrational spectra and by short N-H···F distances in the crystal structures of ZnF2(NH3)3 and ZnF2(NH3)2. With ZnF2(NH3)3, ZnF2(NH3)2, and ZnF2(NH3), we present the first ammoniates of ZnF2.

Decomposition of supercritical ammonia and modeling of supercritical ammonia–nitrogen–hydrogen solutions with applicability toward ammonothermal conditions

Ammonothermal growth of nitrides occurs at temperatures in excess of 800K and pressures greater than 150MPa. For this region, no experimentally verified equation of state (EOS) exists for ammonia, nor is there any accurate description available for the equilibrium constant of the ammonia decomposition reaction for pressures in excess of 100MPa. To fill this void, experimental P-v-T data was collected for pure ammonia at T>700K and compared to extrapolated data from the reference EOS for ammonia. For the extrapolated region, the reference EOS provided excellent agreement to within error of the collected experimental data. A simplified EOS based on the Beattie–Bridgeman (BB) EOS was derived and fit to calculated and, for ammonia, extrapolated reference EOS data for ammonia, hydrogen, and nitrogen (T<1000K, P<300MPa). By applying mixing rules with separated contributions for polar and non-polar interactions, an EOS was derived for NH3–N2–H2 mixtures. With these expressions, an accurate description for the equilibrium constant for the ammonia decomposition reaction as a function of pressure and temperature was derived and verified against experimental data determined for total system pressures of 92, 151 and 210MPa at T ∼810K. Coupling of the EOSs with the equilibrium constant permitted accurate modeling of a sealed autoclave filled with pure ammonia, after incorporating corrections for the expansion of the internal free volume due to thermal expansion and elastic strain response of the autoclave walls due to internal pressure buildup. Calculated total system pressure and equilibrium ammonia density at various temperatures and initial ammonia fill densities are in very good to excellent agreement with experimental data. This paper thus provides a simple EOS for ammonia, hydrogen, nitrogen and NH3–N2–H2 mixtures accurate to within approximately 1–2% in pressure for temperatures greater than 700K. The equilibrium constant for the ammonia decomposition reaction includes non-ideal mixing contributions from the second virial coefficient with a resulting accuracy in the equilibrium mole fraction of ammonia of approximately 2% for T<850K.