Dissolved Intermediates in Ammonothermal Crystal Growth: Stepwise

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Dissolved Intermediates in Ammonothermal GaN Crystal Growth: Stepwise Condensation of [Ga(NH)] towards GaN 2

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Jan Hertrampf, Eberhard Schluecker, Dietrich Gudat, and Rainer Niewa Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00815 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on August 7, 2017

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Crystal Growth & Design

Dissolved Intermediates in Ammonothermal Crystal Growth: Stepwise Condensation of [Ga(NH2)4]− towards GaN Jan Hertrampf,[a] Eberhard Schlücker,[b] Dietrich Gudat[a]* and Rainer Niewa[a]* [a] Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany [b] Lehrstuhl für Prozessmaschinen und Anlagentechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Cauerstraße 4, 91058 Erlangen, Germany ABSTRACT: 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 Cscontaining liquid initially contains an equilibrium mixture of tetraamidogallate ions [Ga(NH2)4]– and a dinuclear complex [(H2N)3Ga(µ-NH)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 towards 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 galliumcontaining species in saturated solutions of the solid intermediates Li[Ga(NH2)4] and Na2[Ga(NH2)4]NH2 in liquid ammonia at room temperature and autogenous pressure.

Introduction During the last two decades the ammonothermal synthesis gained increasing interest in research especially for GaN crystal growth. Generally, group-III nitrides are very promising materials for semiconductor industry for a wide range of applications in optoelectronic devices like blue lasers, blue LED’s, high power amplifiers or sensors.13 GaN, as example, shows a direct band-gap (~3.2 eV),3 high thermal conductivity (~ 2.3 W/cm∙K)4 and an electron mobility of ~1500 cm2/V∙s.4 These characteristics make GaN a particularly attractive material for applications in different device types. Usually, GaN layers are grown on substrates like SiC or Al2O3 by vapour phase epitaxy or similar methods.5 One disadvantage of such substrates is the comparably large lattice mismatch, for example of 13.9 % for sapphire,6 which is the reason for a significant misfit dislocation concentration in the GaN layer. The ideal alternative seems application of homoepitaxy, meaning the use of single crystalline GaN substrates, but these are difficult to grow. The ammonothermal synthesis belongs to the larger group of solvothermal methods and was adopted from the hydrothermal method by replacing water with ammonia.7 Characteristic for solvothermal synthesis techniques is the application of temperatures above the boiling point of the solvent at atmospheric pressure, which require conducting the reaction in a closed vessel under elevated pressure. In 1966, the first ammonothermal syntheses were carried out to obtain Be3N2 (the reported values for T = 673 K, p ~ 20 MPa exceed the critical data of ammonia of Tc = 405.2 K, pc = 11.3 MPa and imply a reaction in a

supercritical medium).8,9 In the following decades, a large number of nitrides, amides and imides were obtained by the ammonothermal method.10 For GaN technology, a breakthrough was achieved by Dwilinski et al., who obtained ammonothermal GaN as polycrystalline material utilizing LiNH2 or NaNH2 as mineralizers in 1995.11 Nowadays it is possible to grow large ammonothermal GaN crystals up to several inches,12,13 but still there is not much knowledge about the dissolved species and their function in GaN crystal growth. We are interested in the mineralization process and the intermediate species occurring during the ammonothermal process. Using the typical ammonobasic mineralizers (e. g. alkali metals or their amides) in ammonothermal GaN synthesis, ternary solid amidogallates are formed. These intermediates are speculated to represent the dissolved species, responsible for the mass transport and GaN crystal growth in the ammonobasic milieu.14 First investigations in liquid ammonia have led to ternary amides like NaGa(NH2)4,15 Na2Ga(NH2)516 and KGa(NH2)4.17 However, of all these compounds only Na[Ga(NH2)4] was distinctly characterized and reproduced, and its decomposition to h-GaN was proven.17,18 Under ammonothermal conditions, we have recently obtained Li[Ga(NH2)4] in two modifications,19 Na2[Ga(NH2)4]NH2,19 and 20,21 Ba[Ga(NH2)4]2 in three modifications from the reactions of Ga metal with the corresponding mineralizers. All these solid intermediates contain isolated [Ga(NH2)4]− ions. In addition to these intermediates, a liquid with the approximate composition KGa(NH2)n ∙ x NH3 according to acid-base-titration was reported.22 In an earlier work, the

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thermal decomposition of such a liquid indicated the presence of an amide-imide containing material.23 Furthermore, Na[Al(NH2)4] is known to form a liquid under gentle heating and concurrent release of ammonia.18 The formation of imidometalates as intermediates in such thermal decomposition processes was earlier postulated for several alkaline and alkaline-earth metal amidoaluminates,24-26 as well as for Na[Ga(NH2)4] via NaGa(NH)2,27 however, without unambiguous proof. Recent studies indicate a stepwise condensation during the thermal decomposition of Li[Al(NH2)4] to eventually form AlN and Li3[AlN2] via intermediate phases with general composition LiAl(NH2)x(NH)yNz,28 but follow-up investigations found no indication for the occurrence of imide species during this process.29 The present work reports on the identification of the solute species formed upon dissolution of mononuclear amidogallate intermediates in liquid ammonia. Additionally, we analyse the chemical nature and composition of liquid intermediates occurring during ammonothermal GaN synthesis using RbNH2 or CsNH2 as mineralizers and report on reactions of the Cs-containing liquid after purposeful addition and removal of ammonia, respectively. The characterization is derived from results of IR, Raman, and solution NMR spectroscopic measurements. Furthermore, DFT-calculations were carried out to aid in the interpretation of the spectral data. The results of these investigations permit for the first time a direct spectroscopic observation and constitutional assignment of the present gallium containing complexes and, thus, shed light on the chemical processes during ammonobasic GaN crystal growth.

serve symmetric and asymmetric ν(NH2) stretching modes between 3500 cm−1 and 3000 cm−1, and a broad background signal from the silica glass of the sample container between 2500 cm−1 and 1000 cm−1. Bands with maxima near 550 cm−1 and 200 – 230 cm–1 are attributed to ν(GaN) and δ(GaN) modes, respectively. A comparison of the wavenumbers of the ν(GaN) stretching modes with those obtained for solid Li[Ga(NH2)4],19 Na[Ga(NH2)4],19 Na2[Ga(NH2)4]NH2,19 and Ba[Ga(NH2)4]220 from single crystal Raman data is shown in Table 1. The low intensities of the ν(GaN) bands of the lithium and sodium amidogallates (Figure 1) relate directly to the low solubility of these materials at ambient temperature. With the exception of solid Na2[Ga(NH2)4]NH2, the ν(GaN) frequencies of solid and dissolved samples do not deviate significantly, but are much broader for the solutions than for the solid materials. Our DFT calculations indicate that the observed bands represent a superposition of several normal modes with dominant ν(GaN) and some ρ(NH) character. Table 1. ν(GaN) valence modes of selected amidogallates. Substance

ν(GaN) / cm−1

Li[Ga(NH2)4]−P21/n

54419

Li[Ga(NH2)4]−P21

55619

Na[Ga(NH2)4]

From reactions of Ga metal with the amides of Rb and Cs under ammonothermal conditions (60 – 120 MPa) we obtain liquid products if the temperature is maintained at about 600 K, slightly lower than usually applied for ammonobasic GaN crystal growth. Above 773 K we observe the formation of GaN in the products with use of these mineralizers. The color of the liquid products may vary from brown to yellow and colorless, probably due to small amounts of already formed GaN. Chemical analysis of the Cs-containing liquid after synthesis and storage under atmospheric pressure results in slightly lower nitrogen and hydrogen contents than expected for a composition Cs[Ga(NH2)4], and small oxygen impurities (see experimental section). All liquids may spontaneously solidify to amorphous solids. This transition occurs typically within one week for the Rb-containing product, while the Cscontaining product generally stays liquid over much longer periods of time (up to 1.5 years) if stored under inert gas atmosphere. Exemplarily, we have characterized the Cs-containing liquid in detail.

Vibrational Spectroscopy The Raman spectra of the liquids containing rubidium (as solution diluted with ammonia) and cesium together with those of saturated solutions of Li[Ga(NH2)4] and Na2[Ga(NH2)4]NH2 are depicted in Figure 1. We can ob-

Solid

55030

Na2[Ga(NH2)4]NH2 Ba[Ga(NH2)4]2−ܲ4ത

60319

Li[Ga(NH2)4

556

Na2[Ga(NH2)4]NH2 Rb-liquid

Results and Discussion

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55020

Dissolved in NH3

556 554

Cs-liquid

553

Cs-liquid

552

The IR spectrum of the Cs-containing liquid (Figure 2) displays bands attributable to ν(NH2) (3320, 3270 cm–1), δ(HNH) (1554 cm–1) and ρ(NH2) modes ("wagging" mode at 663 cm–1 with a shoulder around 880 cm–1), which are also visible in the reported spectra of solid Na[Ga(NH2)4], ternary amidozincates like K2[Zn(NH2)4]30 or 30 Rb2[Zn(NH2)4], and even Be(NH2)2.31,32 In addition, we note a rather strong band at 995 cm–1, which is absent in all spectra of amidometalates with tetrahedral ions [M(NH2)4]n– known so far. Since the intense δs(NH3) (inversion) mode of ammonia occurs in this spectral region, we were first tempted to assign this band to residual NH3. However, because the band is still present after ammonia had been evaporated at 50 °C under reduced pressure, and the positions of both the 995 and 1554 cm–1 bands match neither the reported values of δ(NH3) modes for liquid ammonia (δas(NH3) 1630 cm–1, δs(NH3) 1050 – 1060 cm–1)33 nor for ammine complexes (δas(NH3) ∼1635 cm–1, δs(NH3) 1174 – 1040 cm–1),34 this hypothesis was ruled out. Gratifyingly, DFT-calculations on the vibrational spectra of amidogallates (see Fig. 2) suggested an alternative


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assignment of the 995 cm–1 band to an IR-active ρ(NH) mode of a µ-bridging imido moiety in a complex [(H2N)3Ga(µ-NH)Ga(NH2)3]2–. This finding provided a first direct hint to the existence of a previously postulated24-26 condensed imidometalate which was later confirmed by NMR studies (see below), and we interpret the IRspectrum of the Cs-containing liquid therefore as superposition of spectral bands arising from mononuclear and dinuclear imido-bridged amidogallates.

Figure 1. Raman spectra of the Cs- and Rb-containing liquids (pure and diluted with ammonia, respectively), saturated solutions of Na2[Ga(NH2)4]NH2 and Li[Ga(NH2)4] in liquid ammonia, and pure liquid ammonia (from top to bottom) recorded from samples sealed in fused silica ampoules. The asterisk denotes an unwanted signal from an external light source. The inlay shows vertical expansions of the spectra of Na2[Ga(NH2)4]NH2 and Li[Ga(NH2)4] (100-fold).

Figure 2. IR spectrum of the Cs-containing liquid (blue curve, bottom) and calculated IR spectra of 2– [(H2N)3Ga(NH)Ga(NH2)3] (green curve, middle) and – [Ga(NH2)4] (red curve, top). The calculated spectra were obtained from harmonic frequency analyses at the pcmωB97xD/def2-tzvp level and are unscaled.

NMR Spectroscopy Unambiguous deduction of the molecular structures of the gallium-containing species present in ammonia solutions of Li[Ga(NH2)4], Na2[Ga(NH2)4]NH2 and the aforementioned rubidium- and cesium-containing liquids, as well as in the pure cesium-containing liquid itself, was feasible from the results of multinuclear NMR studies (see Table 2). The 71Ga NMR spectra of all solutions in ammonia exhibit an identical resonance at 337.1±0.5 ppm, and the 1H NMR spectra show beside the solvent signal an additional resonance with a slightly variable chemical shift (0.09 to – 0.24 ppm). The 15N NMR signals of the attached nitrogen atoms are readily detected and assigned in 1H-detected two-dimensional NMR spectra (gs-HMBC) or 15N-detected one-dimensional 1H,15N INEPT spectra. These techniques yield also the magnitude of the 1JNH coupling constants (Table 2) and the signal multiplicities caused by 1H,15N spin coupling which prove that the 1H and 15N NMR signals are attributable to amido (NH2) groups. The observed 71 Ga NMR chemical shift is typical for a species with tetracoordinate gallium – reported values for complexes with a GaN4 framework are 304 ppm for a monomeric azagallatrane35 and 276 ppm for [Ga2(NMe2)6]36 – and the very low line width (0.20 to 0.26 kHz compared to 8.4 kHz for [Ga2(NMe2)6])36 reveals that the 71Ga nucleus occupies a site with high (i. e. tetrahedral) local symmetry. Altogether, these findings lead us to conclude that all solutions contain the isolated complex anion [Ga(NH2)4]– which had already been established as a constituent of the solid ternary amides (vide supra). In contrast to the solutions of Li[Ga(NH2)4] and Na2[Ga(NH2)4]NH2 in ammonia where [Ga(NH2)4]− constitutes the only detectable gallium-containing species, the spectra of solutions of the rubidium- and cesiumcontaining liquids display additional weak signals at lower chemical shifts (δ71Ga = 313.3, 291.5, 286.6, 262.5 ppm). Their number and relative intensities (3 – 13 % for the signal at 313.3 ppm and 0 – 2 % for the remaining ones) vary erratically between different sample batches and depend further on the handling of the samples, and we attribute these signals therefore to impurities which arise presumably from the presence of adventitious oxygen containing species (i. e. metal oxide impurities in the autoclave walls or traces of moisture introduced during the work-up procedure). The 71Ga NMR chemical shifts are still in the region of four-coordinate complexes, and the detection of 1H and 15N NMR signals attributable to the impurity with the highest abundance proves this species to be likewise an amido complex. Computational predictions suggest that the 71Ga chemical shift is typical for complexes with a N3O coordination sphere and, using the match between predicted and observed 71Ga chemical shifts as selection criterion, lead to a tentative assignment of this species as a dinuclear complex [(H2N)3Ga(µO)Ga(NH2)3]2−.



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Table 2. Multinuclear (1H, 15N, 71Ga) NMR data of identified mono- and dinuclear amidogallates a) Complex

Cation –

[Ga(NH2)4]

Li

[Ga(NH2)4]– –

+

Solv.

δ71Ga b)

δ1H

1

δ15N

JNH [Hz]

c)

NH3

337.7 (0.26)

0.00

–372.0

Na+

NH3

337.5 (0.20)

–0.07

–371.1 d)

59

[Ga(NH2)4]

+

Rb

NH3

336.6 (0.20)

0.09

–371.1

59

[Ga(NH2)4]–

Cs+

NH3

336.6 (0.21)

–0.24

–367.7

58.5

[Ga(NH2)4]–

Cs+

none

336.9 (1.15)

–1.4 to –1.7

–357.1 to –359.2

e)

[Ga2(NH)(NH2)6]2–

2 Cs+

none

f)

NH2 NH

–1.5 to –1.8 (12 H) –2.9 to –3.1 (1 H)

–353.8 to –354.3 –345.3 to –345.7

57 57

[Ga2(NH)2(NH2)4]2–

2 Cs+

none

f)

NH2 NH

–1.9 (8 H) –3.2 (2 H)

–353.4 –340.7

59 59

[Ga2(O)(NH2)6]2–

2 Cs+

NH3

313.7 (0.23)

NH2

–0.18

–364.0

55

2–

+

none

312.2 (1.39)

NH2

–1.38

–356.8

g)

[Ga2(O)(NH2)6] a)

2 Cs

b)

53

c)

d)

spectra recorded at 296(2) K if not stated otherwise; numbers in parentheses denote half width in kHz; at 243 K; at 223 f) g) K; not detectable due to unfavourable relaxation parameters; not assignable because of overlap of unresolved lines; not determined. e)

Besides yielding structural information, the NMR spectra of the ammonia solutions provide also qualitative insight into dynamic processes. A major difference in the 1 H NMR spectra of the Li[Ga(NH2)4] and Na2[Ga(NH2)4]NH2 solutions refers to the line shape of the solvent signal, which appears in the first case as a partially collapsed 1:1:1 triplet reflecting the scalar coupling between the 1H and 14N (I = 1, natural abundance 99.64 %) spins and in the second case as a broad singlet. The coalescence of the individual multiplet components is attributable to the fact that proton exchange between ammonia and the free amide ions in Na2[Ga(NH2)4]NH2 efficiently accelerates the intermolecular proton scrambling between ammonia molecules that is responsible for quenching the scalar coupling.37 This exchange also induces a coalescence of the signals of NH3 and free NH2–, which is confirmed independently by the 1H and 15N NMR spectra of an ammonia solution of cesium amide; both spectra display single lines representing the population weighted dynamic average of the signals of the individual species. The loss of the coupling fine structure in the solvent signal is also observed for solutions of the rubidium- and cesium-containing liquids, even if the reasons are in this cases not immediately evident. Recording of two-dimensional 1H-EXSY spectra reveals further that in all solutions intermolecular proton exchange between ammonia and the [Ga(NH2)4]– anions of the solute takes place. However, the observation of separate solvent and solute signals in the 1H and 15N NMR spectra of an ammonia solution of Na2[Ga(NH2)4]NH2 and the low intensities of the appropriate exchange crosspeaks suggest that the rates of exchange processes involving the amidogallate ion are by several orders of magnitude lower than those governing the proton scrambling between amide ions and ammonia. The spectral data allow currently no decision whether the exchange pro-

ceeds by intramolecular scrambling of intact amide ions or a sequence of proton transfer steps (which we consider more likely). The 1H and 15N NMR spectra of the pure cesiumcontaining liquid contain besides the signals of [Ga(NH2)4]– and [(H2N)3GaOGa(NH2)3]2− the resonances of two new NHn moieties (see Table 2). Analysis of multiplicity edited 1H,15N INEPT spectra (Fig. 3) and twodimensional 1H,15N HSQC correlations allows us to assign one set of signals to a further amido (NH2) and the remaining one to an imido (NH) unit, thus confirming the evidence already obtained from the IR-data. The intensity ratio of 12 (NH2) : 1 (NH) determined from spectral deconvolution of the 1H NMR signals is invariant towards changes in the overall composition of the sample (vide infra), which suggests that both fragments belong to a single species. The corresponding 71Ga NMR signal is obviously obscured by the more intense resonance of [Ga(NH2)4]–, which is significantly broadened as a consequence of the high viscosity of the liquid. Combining all evidence, we assign the new species as a dinuclear complex [(H2N)3Ga(µ-NH)Ga(NH2)3]2– whose molecular structure is based on a pair of corner-sharing GaN4 tetrahedra. The compatibility of this assignment with the observed spectral data is corroborated by computational studies (vide infra). Two-dimensional 1H-EXSY spectra reveal that the components in the pure liquid undergo a similar intermolecular proton exchange on a second timescale as had been observed for the ammonia solutions.


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Crystal Growth & Design tion [(H2N)2Ga(µ-NH)2Ga(NH2)2]2– featuring a structural framework consisting of two corner-sharing GaN4 tetrahedra. Formation of this species by cyclization of [(H2N)3Ga(µ-NH)Ga(NH2)3]2– under cleavage of one molecule of NH3 is reversible (see equation 2), but the determination of an equilibrium constant is in this case unfeasible due to the failure to achieve a reliable quantification of all species present.

a)

b)

2 2–

c)

[(H2N)3Ga(µ-NH)Ga(NH2)3] [(H2N)2Ga(µ-NH)2Ga(NH2)2] 1 3

3

2

-340

-345

15

1

-350

4

-355

1

-360

ppm

15

Figure 3. (a) N{ H} and (b) refocussed H, N INEPT spectrum of the pure cesium-containing amidogallate liquid, and 1 15 (c) refocussed H, N INEPT spectrum of a sample obtained after evaporation of ammonia at slightly elevated temperature. Positive and negative signals in the INEPT spectra indicate resonances of NH2 and NH units, respectively. Signal – 2− assignment: 1 – [Ga(NH2)4] , 2 – [(H2N)3Ga(NH)Ga(NH2)3] , 2− 2− 3 – [(H2N)2Ga(NH)2Ga(NH2)2] , 4 – [(H2N)3GaOGa(NH2)3] . The broadening of the signal of 1 in a) is due to the effects of relaxation or chemical exchange which cause also the quenching of this signal in the INEPT spectrum shown in trace b).

2–

+ NH3 (2)

The 133Cs NMR spectra of all samples studied contain a single sharp resonance with a shift that decreases from a value of 240.2 ppm in the pure ionic liquid to 166.9 ppm in the most dilute ammonia solution studied. The variation in δ133Cs exhibits a linear correlation with the molar fraction of NH3 in the mixtures and reflects the change that occurs when the amidogallate anions which solvate the cation in the ionic liquid are to an increasing extent replaced by ammonia molecules. Shifts in δ133Cs on a similar scale arising from the pairing of cesium cations with different anions have previously been observed for methylamine solutions of some simple inorganic cesium salts.38 Similar, although much smaller, solvation induced changes are also evident for the 1H and 15N chemical shifts of the anions of cesium amidogallates (Table 2).

The formation of the dinuclear complex can be considered to arise from condensation of two monomeric amidogallate ions according to equation (1). 2 [Ga(NH2)4]– [(H2N)3Ga(µ-NH)Ga(NH2)3]2– + NH3

(1)

The feasibility of this reaction is directly proven by a control experiment in which the pure liquid was first stepwise diluted with several small portions of ammonia which were then eventually removed again by evaporation under reduced pressure. Evaluation of the 1H NMR spectra recorded after each step (Fig. 4) confirms that the relative amounts of the individual reactants adapt to the changed overall composition, and that the samples may thus be considered as dynamically equilibrating multicomponent mixtures. Calculation of the dimensionless equilibrium constant Kc for (1) from the relative signal intensities yields Kc = 1.20(4)∙10-2 and a free enthalpy ∆Gcondens = 10.8(1) kJ mol-1 at a temperature of 295±1 K, which confirms the condensation to be endergonic. Inspection of the 1H and two-dimensional 1H,15N HSQC NMR of samples obtained by forced evaporation ammonia at slightly elevated temperatures (up to 323 K, Fig. 4) revealed the appearance of two new signal sets which eventually grow in intensity when the removal of ammonia is continued and are, as in [(H2N)3Ga(µNH)Ga(NH2)3]2–, assigned to the nuclei of another pair of amido and imido units. The intensity ratio of 8 (NH2) : 2 (NH) suggests the presence of a cyclic dimer of composi-

1

Figure 4. Expansions of the H NMR spectra of the cesiumcontaining liquids with different amounts of ammonia showing the signals of NH3 and NH2 (left) and NH groups (right; 8-fold vertical expansion), respectively. To the original solution (trace (a)), small portions of ammonia were first stepwise added (traces (b), (c)), and then evaporated again, first at ambient temperature (traces (c) – (e)) and finally under gentle heating up to 323 K (traces (g), (h)). Signal assign– 2− ment: 1 – [Ga(NH2)4] , 2 – [(H2N)3Ga(NH)Ga(NH2)3] , 3 – 2− [(H2N)2Ga(NH)2Ga(NH2)2] .



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and observed 71Ga chemical shifts for [Ga(NH2)4]– and a set of reference compounds ([Ga(H2O)6]3+ and amidocomplexes featuring coordination of the metal atom by four to six nitrogen donor atoms, cf. Table 3) exhibit an excellent linear correlation (δ71Gacalcd = 1.11∙δ71Gaobs – 0.90 with R2 = 0.999, see Fig. 5), which confirms that the computational model gives a realistic description of the influence of structural changes on the magnetic shielding.

Last, but not least, we want to mention that the NMR measurements provide also some insight into differential solubilities of alkali metal amidogallates in ammonia. Since the solutions of Li[Ga(NH2)4] and Na2[Ga(NH2)4]NH2 are in contact with undissolved solids, they may be considered as saturated, and integration of the 1H NMR spectra allows then a straightforward determination of the saturation concentration. Accordingly, we determined the molar fraction χ of dissolved gallate complexes in saturated solutions in liquid ammonia at 295(1) °C as 1.4(2)∙10–3 for Li[Ga(NH2)4] and 2.6(2)∙10–3 for Na2[Ga(NH2)4]NH2 and the molalities of the saturated solutions as 8.4(5)∙10–2 mol/kg for Li[Ga(NH2)4] and 0.15(9) mol/kg for Na2[Ga(NH2)4]NH2, respectively. Considering that the rubidium and cesium amidogallate ionic liquids display no miscibility gap upon mixing with ammonia, these results seem to indicate that the solubilities of alkali metal amidogallates in liquid ammonia at room temperature and autogenous pressure increase with the size of the alkali metal counter ion.

Ion pairing as in [(H2N)2Ga(µ-NH2)2Li(NH3)2] and formation of dinuclear complexes like [(H2N)3Ga(µ-NH)Ga(NH2)3]2– and [(H2N)2Ga(µNH)2Ga(NH2)2]2– – the latter exists as a pair of stereoisomers with cis- or trans-arrangement of the NH-moieties which have very similar energies (the cis-isomer is at the PCM-ωB97xD/def2-tzvp level of theory by 0.5 kJ mol-1 more stable) and NMR parameters – induce a subtle shielding contribution (see Table 3). The range of chemical shift changes (∆δ71Ga = –2.6 to ‒6.5 ppm) is of similar magnitude than the viscosity-induced line broadening effects observed for the cesium-containing ionic liquid. This result confirms the earlier conjecture that the effect of a condensation of monomeric amidogallates on 71Ga chemical shifts is practically invisible and thwarts an assignment of the observed minor signals in the 71Ga NMR spectrum to condensation products of [Ga(NH2)4]–.

Computational Studies Computational studies were carried out in order to validate our NMR based constitutional assignments by a comparison of observed and predicted 71Ga and 15N chemical shifts. To this end, we first localized energy optimized equilibrium structures of the target amidogallate complexes and suitable reference compounds, and then calculated the chemical shifts at the final geometries. Solvent effects were included in both stages by the use of a polarizable continuum model (PCM). A listing of the calculated chemical shifts is compiled in Table 3. The computed

Table 3. Predicted 71Ga and 15N chemical shifts (in ppm) of amidogallate complexes and selected model compounds obtained at the PCM-ω ωB97xD/def2-tzvpp//PCM-ω ωB97xD//def2-tzvp level. Conversion of computed magnetic shieldings into chemical shifts was done as described in the experimental section. Compound

PCM-Solvent

δ71Ga a)

δ15N a) NH2

NH

δ71Gaexp b)

[Ga(H2O)6]3+

H2O

Ga2(NMe2)6

C6H6

299.5

--

--

276.036

toluene

332.4

--

--

304.035

toluene

283.4

--

--

255.035

[Ga(NH2)4]

NH3

377.5

–375.6

--

[(H2N)2Ga(µ-NH2)2Li(NH3)2]

NH3

375.0

–373.0

--

[(H2N)3Ga(µ-NH)Ga(NH2)3]2–

NH3

375.2

–370.7

–364.0

NH3

371.1

–372.2

–356.3

Aza-gallatrane monomer c) Aza-gallatrane dimer

d)

–

trans-[(H2N)2Ga(µ-NH)2Ga(NH2)2]2– 2–

0.0

cis-[(H2N)2Ga(µ-NH)2Ga(NH2)2]

NH3

372.2

–371.5

–351.5

[Ga(OH)(NH2)3]–

NH3

359.7

–377.0

--

[(H2N)3GaOGa(NH2)3]2–

NH3

349.7

–372.1

--

[Ga(OH)2(NH2)2]–

NH3

333.2

–377.9

a)

average values in case of the presence of several identical units of the given type; c) d) erence set; N(CH2CH2NMe)3Ga; {N(CH2CH2NMe)3Ga}2.


–372.0

-b)

experimental chemical shifts for the ref-

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Crystal Growth & Design ferent approach including a molecular dynamics simulation. Nonetheless, the experimentally observed trends in chemical shifts for NH and NH2 moieties in the same compound or between different species in the same environment are quite well reproduced by the computations. In this respect, the 15N chemical shift calculations further corroborate the structural assignment of [(H2N)3GaOGa(NH2)3]2– as the predicted deshielding of the 15N NMR signal of this species with respect to [Ga(NH2)4]– matches the experimental observations while an alternative assignment as monomeric [Ga(OH)(NH2)3]– would imply a contradiction between predicted and observed trends in δ15N.

Conclusion

Figure 5. Plot of calculated (at the PCM-ωB97xD/def2tzvpp//PCM-ωB97xD/def2-tzvp level of theory) vs. observed 71 Ga NMR chemical shifts. Blue diamonds denote the entries – for [Ga(NH2)4] and the reference compounds listed in Table 3, and the blue trend line was determined from linear regres71 sion analysis using these data which gave δ Gacalcd = 71 2 1.11∙δ Gaobs – 0.90 with R = 0.999. Filled and open green 2– diamonds represent the entries for [(H2N)3GaOGa(NH2)3] – and [Ga(OH)(NH2)3] , and red symbols those for [(H2N)2Ga(µ-NH2)2Li(NH3)2] and 2– [(H2N)2Ga(µ-NH)2Ga(NH2)2] , respectively. The red horizontal bars symbolize the uncertainty in the chemical shift of the two latter complexes that is caused by the failure to locate the exact position of the resonance in the presence of the – broad signal of [Ga(NH2)4] .

Larger changes in computed 71Ga chemical shifts result, as expected, from variations in the first ligation sphere (∆δ71Ga = –17.9 to –44.3 ppm for [Ga(OH)n(NH2)4–n]–, n = 1, 2). From a rough comparison with the computational results, the metal atom in the main impurity in the cesium-containing ionic liquid can be safely assigned a N3O ligation sphere. The better coincidence with the correlation between observed and predicted chemical shifts (Fig. 5) pushes the balance to assign the constitution of this species as a dinuclear complex [(H2N)3GaOGa(NH2)3]2– rather than mononuclear [Ga(OH)(NH2)3]–. Inspection of the δ71Ga values of the minor impurities suggests that complexes with N2O2 coordination are also present, but the available data is currently to scarce for a more precise structural assignment. The computed 15N chemical shift for [Ga(NH2)4]– is in very close agreement with the observed value in dilute solutions of lithium and sodium tetraamidogallates, and a comparison of the δ15N values for [Ga(NH2)4]– and [(H2N)2Ga(µ-NH2)2Li(NH3)2]2– confirms that the computational model gives also a qualitatively correct prediction of the deshielding influence of ion pairing. A quantitative representation of the solvation induced changes in more concentrate solutions or the pure ionic liquid is, however, out of reach and would presumably require a totally dif-

It has been known that the usual ammonobasic mineralizers LiNH2 and NaNH2 and gallium sources under the conditions of ammonothermal GaN crystal growth form solid intermediates Li[Ga(NH2)4], Na[Ga(NH2)4] and Na2[Ga(NH2)4]NH2, respectively.19 Even if these solid intermediates exhibit comparably low solubilities in ammonia at room temperature, it was now proven that the same complex tetraamidogallate ion [Ga(NH2)4]– as present in the solids constitutes also the only detectable Gacontaining solute species. In contrast, reactions of RbNH2 or CsNH2 with Gasources under ammonothermal conditions produce liquids, which exhibit complete miscibility with liquid ammonia at room temperature. The Cs-containing product recovered after the ammonothermal synthesis and venting ammonia at atmospheric pressure represents an ionic liquid, which contains cesium cations and both monomeric tetraamidogallate ions [Ga(NH2)4]– and dinuclear µimido-bridged dianions [(H2N)3Ga(µ-NH)Ga(NH2)3]2–. Both anions exist in a dynamic equilibrium. In dilute solutions in ammonia, exclusively [Ga(NH2)4]– can be detected by NMR spectroscopy, whereas the equilibrium can be shifted further to the side of the dinuclear complexes upon removing ammonia under reduced pressure. Eventually, the increasingly viscous liquid produces a second condensation product [(H2N)2Ga(µNH2)2Ga(NH2)2]2–, likely to be present in cis- and transconfiguration. Small oxygen impurities lead to formation of the µ-O-bridged dinuclear complex [(H2N)3GaOGa(NH2)3]2–. Scheme 1 depicts these interrelations. With this information, we shed not only light on the nature of the liquids obtained during ammonobasic GaN crystal growth in the presence of the amides of the heavier alkali metals as mineralizers, but additionally pinpoint [Ga(NH2)4]– as exclusive dissolved species in liquid ammonia at room temperature. Furthermore, we identify two species which are likely to represent the products of the initial condensation steps on the way to nitride formation. Whether the tetraamidogallate ion survives the higher pressure and temperature conditions of the ammonothermal GaN growth process of approximately 1 – 3 kbar and 400 – 600 °C3 and thus represents the dominating transport active species is subject of ongoing research.



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Scheme 1. Reactions under ammonothermal conditions with different alkali metal amides serving as mineralizer and occurring equilibria of Ga-containing complex species. Double arrows for GaN formation represent possible multi-step processes.

Experimental Synthesis Preparation and handling of the starting materials and reaction products were performed in an argon-filled glove box (MBraun, Garching, Germany, p(O2) < 0.1 ppb). All reactions were carried out in a customized 97 ml autoclave manufactured of Inconel 718®.39 The autoclave was heated in a tubular furnace (HTM Reetz GmbH, Berlin, Germany). One side of the furnace is closed, leading to a natural temperature gradient of about 100 K between hottest and coldest spot inside the autoclave. The given synthesis temperatures refer to the temperature of the furnace. Our experimental setup provides a temperature difference of about 150 K between furnace temperature and the interior of the reaction vessel.40 For pressure monitoring, a digital pressure transducer (P2VA1/5000bar and DA2510, HBM GmbH Ismaning, Germany) was used. The autoclave was loaded with the starting material and then evacuated. For condensing the ammonia gas, the vessel was cooled by a dry ice/ethanol bath (about 200 K). Ammonia (99.999% anhydrous, Linde) was purified to a final grade of < 1 ppbV of H2O, O2 and CO2 using a MicroTorr MC400-720F gas purifier (Rainer Lammertz pure gas products, Hürth, Germany), and its amount determined using a tensieudiometer after Hüttig.41 Gallium pellets (99.9999% metal basis, Alpha Aesar) were used as delivered. Rubidium and cesium amide were synthesized by heating the metal up to 397 K in liquid ammonia within an autoclave.42,43 Maximum pressures reached values between 60 MPa and 120 MPa depending on filling grade. For the synthesis of the Cs-containing liquid, the reaction vessel was loaded with CsNH2 (51.60 mmol) and Ga metal (51.60 mmol), evacuated, filled with ammonia (2.37 mol) and then placed in a vertically orientated tubular furnace. It was heated with a heating rate of 1.33 K∙min−1

from room temperature to 623 K, held at this temperature for 30 h, and then cooled with a cooling rate of 1.33 K∙min−1 to room temperature. The excess ammonia was then vented in a controlled manner through an oil filled bubbler, and the reaction product was collected as a viscous brownish to slightly yellow/colorless liquid. This liquid is very well reproducible, but may eventually solidify to an amorphous solid. For the synthesis of the liquid containing Rb, the autoclave was loaded with RbNH2 and Ga metal in a molar ratio of 1:1, filled with ammonia (ca. 2 mol), and heated up to 653 K for a duration of 30 h. The samples of Li[Ga(NH2)4] (mixture of two modifications) and Na2[Ga(NH2)4]NH2 were obtained according to literature procedures.19,40

Elemental Analysis The oxygen content of the Cs-containing liquid was estimated by the hot gas extraction technique with an ONH836 Analyser (Leco Corporation, St. Joseph, Michigan, USA). Nitrogen and hydrogen contents were determined using a Vario Micro Cube (Elementar, Langenselbold, Germany). Found (wt.%): N 19.94, H 2.66, O 0.42. Calcd. for Cs[Ga(NH2)4]: N 21.01, H 3.02; for Cs2[Ga2(NH2)6(NH)]: N 18.98, H 2.54; for Cs2[Ga2(NH2)4(NH)2]: N 16.89, H 2.02; for Cs2[Ga2(NH2)6O]: N 16.24, H 2.34, O 3.09). Spectroscopy Raman and NMR spectra were collected from the pure Cs-containing liquid, samples of the Rb- and Cscontaining liquids diluted with liquid ammonia, and from saturated solutions of Li[Ga(NH2)4] and Na2[Ga(NH2)4]NH2, and were sealed in NMR tubes. For the preparation of the samples, the solid or liquid analyte was placed in a medium walled 5 mm NMR tube. Ammonia was condensed in (except for the sample of the pure Cs-containing liquid), and the tube was flame sealed. The


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samples were then warmed to ambient temperature, agitated to induce dissolution of the analyte, and stored for several hours to allow equilibration. To monitor the condensation reaction of the cesium amidogallate, a sample was prepared by placing the pure liquid in a 5 mm NMR tube equipped with a Young valve. A small amount of ammonia was condensed in and the resulting solution analyzed by NMR spectroscopy. After having repeated this process several times, the reverse reaction was monitored by repeatedly evaporating small fractions of ammonia and continuing the NMR measurements after each step. Raman spectra were collected at ambient temperature on a DXR-SmartRaman-Spektrometer (Thermo Fischer Scientific, Waltham, USA). For excitation, a laser beam with 780 nm wavelength was used. NMR spectra were recorded on a Bruker Avance AV 400 spectrometer (1H 400.1 MHz, 71Ga 122.0 MHz, 133Cs 52.5 MHz, 15N 40.5 MHz, 14N 28.9 MHz) at ambient temperature (296 – 299 K) in unlocked mode if not stated otherwise. Chemical shifts were calibrated using the 14N signal of liquid ammonia (δ = –380.2 at 298 K) as external standard and are referenced to external TMS (1H, Ξ = 100.000000 MHz) using 1.1 M Ga(NO3)3 in D2O (71Ga, Ξ = 30.496704 MHz), 0.1 M CsNO3 in D2O (133Cs, Ξ = 13.116142 MHz) and MeNO2 (15N, Ξ = 10.136767 MHz; 14N, Ξ = 7.226317 MHz) as secondary references. Coupling constants are given as absolute values. Pulse programs for one-dimensional (INEPT, INEPT+) and gradient selected two-dimensional experiments (HMQC, NOESY) were used from the standard Bruker pulse program library. Density functional studies were carried out with the Gaussian0944 suite of programs, and MOLDEN45 was used for visualization. For the calculations, the ωB97xD functional by Head-Gordon46 and basis sets from the def2family47Error! Reference source not found. by Weigend and Ahlrichs were employed. Numerical integrations were performed on an ultrafine grid, and solvent effects were included by using a PCM model as implemented in the Gaussian package and employing the same solvent parameters for ammonia as described elsewhere.48 Energyoptimization of molecular structures and harmonic vibrational analyses were carried out at the PCMωB97xD/def2-tzvp level without symmetry constraints, and magnetic shieldings were then calculated for the final geometries at the PCM-ωB97xD/def2-tzvpp level. The values of several equivalent fragments in a single molecule were averaged, and chemical shifts were finally determined as δs = (σref – σs)/(1 − σref) relative to [Ga(H2O)6]3+ for 71Ga and as δs = (σref – σs – 380.2) relative to MeNO2 for 15 N using the magnetic shielding constants of [Ga(H2O)6]3+ (σref = 1824.2 ppm) and liquid NH3 (σref = 266.6 ppm) calculated at the same computational level as the reference.

ASSOCIATED CONTENT

133

Supporting Information. Plot of Cs NMR shift vs. ammonia content, list of computed energies, number of imaginary frequencies, zero point energies and atomic coordinates.

AUTHOR INFORMATION Corresponding Author * [email protected], [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT We thank Dr. C. Ney, Dr. S. Nemrava and B. Förtsch for elemental analysis, M. Gediga for IR measurements. This work was carried out within the project “Chemie und Technologie der Ammonothermal-Synthese von Nitriden” (FOR1600) and funded by the German Research Foundation (DFG). The computational studies were supported by the bwHPC initiative and the bwHPC-C5 project provided through the associated compute services of the JUSTUS HPC facility at the University of Ulm. bwHPC and bwHPC-C5 (http://www.bwhpc-c5.de) are funded by the state of Baden-Württemberg and the DFG (grant no. INST 40/467-1 FUGG).

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For Table of Contents Use Only, Dissolved Intermediates in Ammonothermal GaN Crystal Growth: Stepwise Condensation of [Ga(NH2)4]– towards GaN Jan Hertrampf, Eberhard Schlücker, Dietrich Gudat, Rainer Niewa

Synopsis. Ammonothermal synthesis currently provides the best GaN crystals. Upon use of ammonobasic mineralizers, NMR reveals [Ga(NH2)4]– ions to represent the exclusive observable galliumcontaining dissolved species in diluted ammonia solutions at room temperature and under autogenous pressure. With RbNH2 or CsNH2 viscous liquids fully miscible with ammonia occur, containing different imido-condensed species depending on concentration, while oxide-impurities lead to oxo-bridged dimers.


Dissolved Intermediates in Ammonothermal Crystal Growth: Stepwise

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