A New Modification of Rb[Al(NH2)4] and Condensation in Solid State

A new modification of Rb[Al(NH2)4] in space group C2/c, which differs from the known structural modification in the way the [Al(NH2)4]−-tetrahedra are arranged in the surrounding area of the rubidium cation, was obtained from ammonothermal synthesis at 673 K and 680 bar. The crystal structure was determined by Rietveld refinements and further investigated by infrared and Raman spectroscopy. Thermal gravimetric investigations indicate two decomposition steps up to 450 °C, which can be assigned to ammonia leaving the material while the sample liquefies. During the third and final step, volatile rubidium amide is released, leaving nano-scaled cubic AlN behind. Investigating differently aged samples implies decomposition and condensation of amidoaluminate ions already at ambient temperature, which is supported by refinements of single crystal X-ray diffraction data, revealing lower nitrogen amounts than expected. The observed single crystal also exhibits a significantly smaller volume than the reported structures, further supporting the decomposition–condensation mechanism.


Introduction
Within the last decade, the continuing rise in the importance of semiconductors-for example, for LED technology-has also driven the need for new and improved semiconducting materials such as AlN [1], GaN [2], InN [3], InAs [4] or ZnO [5]. Mostly nitrides, the synthesis route for these materials often involves chemical vapor deposition (CVD) [6,7]. With CVD being a highly energy-consuming and elaborate method, ammonothermal synthesis-using supercritical ammonia as reaction medium (critical data T c = 134.5 • C and p c = 119 bar)-combines the scalability and production of freestanding superior high-quality nitride single crystals. The ammonothermal method derived from hydrothermal synthesis, the more commonly known solvothermal technique, was first utilized by Jacobs and Juza in 1966 [8]. It gained popularity as a superior method to obtain nitride crystals with high purity and low defect concentration. Although various groups have investigated the chemical properties of supercritical ammonia in synthesis over the years, there are still questions about the exact mechanisms during crystal growth in ammonothermal synthesis [9]. To determine and identify the existing species in supercritical state, so-called intermediates, which are typically solid products providing insight into the dissolved species, are synthesized at different temperatures and pressures and with the use of various mineralizers. Mineralizers are typically ammonoacids or -bases and serve to increase solubility. So far, in ammonothermal synthesis, mostly amidometalates have been found as intermediates when using ammonobasic mineralizers such as ANH 2 (A = Li − Cs) [9][10][11], while ammoniates are found when adding an ammonoacidic mineralizer like an ammonium halide (i.e., NH 4 Cl) to the reaction [12].
The mineralizers not only increase solubility but also determine the dissolution and crystallization zone in the autoclave, depending on the temperature dependence of the solubility. Having a temperature gradient when heated in a vertical furnace, ammonobasic mineralizers tend to result in crystallization in the zone of higher temperature; this means that, frequently, there is a retrograde solubility for ammonobasic systems [13,14]. For the ammonoacidic systems, both normal and retrograde solubility have been observed, being dependent on the administered temperature and used mineralizer [15,16].
While we have presented various solid intermediates in ammonoacidic [12] and particularly ammonobasic GaN crystal growth in recent years [17], we particularly obtained liquid phases upon using the amides of the heavier alkali metals K, Rb, and Cs [11]. The obtained ionic liquids can be characterized as losing ammonia even at ambient temperature, leading to partial condensation of amidogallate ions to form binuclear mixed amidoimidogallate ions. On the other hand, solid amidometalates appear to be prone to the formation of multiple modifications, as presented for various metals like aluminum, gallium or manganese in combination with alkali and alkaline earth metals [17][18][19]. The tetraamidoaluminate Rb[Al(NH 2 ) 4 ] was earlier found to crystallize in the tetragonal space group P4/n; however, the presence of a second monoclinic modification in P2/c was already indicated but not structurally characterized [18]. Here, we report a modification of Rb[Al(NH 2 ) 4 ] in C2/c and discuss data which indicate condensation of the amidoaluminate complex ions in solid state, as earlier observed in a similar manner in liquid for the corresponding Ga-containing system [11].

General Information
Since both the product and the starting materials are sensitive to air and moisture, all handling was done in a glove box filled with argon (MBraun, Garching, Germany, p(O 2 ) < 0.1 ppm). Synthesis experiments were conducted in custom-made autoclaves (98 mL volume, nickel-based alloy 718) [20]. For reaction purposes, one-side closed tubular furnaces were used (HTM Reetz GmbH, Berlin, Germany); this set-up causes a natural temperature gradient in the autoclave of around 100 K between the bottom and top of the reaction vessel, referred to as hot and cold zone. Any given temperature refers to what the furnace was brought up to, not the temperature inside the autoclave, which is lower depending on the temperature administered [21]. The maximum pressure inside the autoclave during the synthesis was observed by a digital pressure transmitter (P2VA1/5000 and DA2510, HBM GmbH Ismaning, Germany). After loading the autoclave with the starting materials, it was assembled, evacuated and flushed with ammonia once before using a dry-ice/ethanol cooling bath to condense the ammonia into the reaction vessel at approximately −72 • C. The NH 3 (99.999% anhydrous, Linde) was purified from H 2 O, CO 2 and O 2 to a grade less than one ppbV with the use of a MicroTorr MC400-720F gas purifier (Rainer Lammertz pure gas products, Hürth, Germany). To correctly determine the amount of ammonia condensed into the autoclave, a self-built tensi-eudiometer after Hüttig was used [22]. The following chemicals were purchased and used as received: aluminum powder (99.97 metal basis, Alpha Aesar, Landau, Germany), RbNH 2 was synthesized by reaction of Rb in NH 3 at 383 K and 50 bar in 200 mL steel autoclaves (Carl Roth GmbH + Co. KG, Karlsruhe, Germany). The product was confirmed to be single-phase RbNH 2 by powder X-ray diffraction.

Synthesis
The new modification of Rb[Al(NH 2 ) 4 ] was synthesized from the starting products with the following procedure: the autoclave was loaded with Al (204.04 mg, 7.413 mmol) and RbNH 2 (828.32 mg, 8.154 mmol), filled with ammonia (1.6886 mol) to a total of 43% and then heated to 673 K with a heating rate of 1.58 K min −1 , reaching a maximum pressure of 680 bar. The autoclave was kept at this temperature for a duration of 48 h and then cooled down to room temperature with a rate of 0.063 K min −1 . The product was collected in the form of colorless quadratic crystals next to microcrystalline powder from the hot zone of the autoclave. Powder diffraction of the product showed no traces of residue RbNH 2 .

Results and Discussion
When heating aluminum metal and RbNH 2 in an autoclave in supercritical ammonia to a furnace temperature of 673 K with a maximum pressure of 680 bar, a colorless substance is obtained from the hot area at the bottom of the autoclave. A typical temperature gradient within the autoclave is around 100 K for our autoclave setup [20]. The location of product formation indicates the retrograde temperature dependence of the solubility of the product, as is frequently observed for alkali metal amidometalates [13,14]. Powder X-ray diffraction results in a monoclinic unit cell with very similar unit cell parameters, as reported for a modification of Rb[Al(NH 2 ) 4 ] of unknown crystal structure [18], however, with C-centering rather than primitive Bravais lattice (see Table 1). Interestingly, single crystals with the same unit cell relations and space group assignment but significantly smaller volume were obtained from the samples. A full structure refinement based on single crystal X-ray diffraction intensity data was prohibitive at first due to twinning and disorder phenomena; nevertheless, this led us to the initial structure model of Rb[Al(NH 2 ) 4 ] in C2/c for Rietveld refinements of the powder X-ray diffraction data.
According to Rietveld refinements, the new monoclinic modification of Rb[Al(NH 2 ) 4 ] crystallizes in the space group C2/c with a = 1048.48(7) pm, b = 539.39(3) pm, c = 1046.56(7) pm and β = 90.342(2) • . Additional information on crystal structure determination is given in Table 1; fractional position and displacement parameters are gathered in Table 2. Figure 1 shows a graphical representation of the refinement. A comparison of the experimental powder pattern with the simulated patterns for the crystal structure in C2/c and the known modification in P4/n already indicates strong similarities, which can be traced back to very similar arrangements of Rb and Al atoms in both structures, but also reveals additional prominent reflections ( Figure 2).
It is obvious that the monoclinic structure emerges from the tetragonal modification as the diagonal between a tet and b tet becomes the new a mon and c mon axes, with the corresponding transformation matrix:  Table 1. Comparison of crystal structure data from PXRD and scXRD for the new monoclinic modification and results from literature [18]. For more information on the relation between structures obtained from powder and single crystal diffraction, compare main text body.         The main structural features of both modifications, the reported tetragonal and the new monoclinic form, are very similar: both contain tetrahedral tetraamidoaluminate ions, [Al(NH 2 ) 4 ] − , as well as a cuboctahedral coordination of Rb + . The distances between the aluminum and nitrogen atoms, however, differ slightly when compared, as a list of selected distances and angles (shown in Table 3) supports. We observe distorted tetrahedral coordination of Al in the monoclinic structure, while the tetragonal form shows four identical distances to N of the coordinating amido-ligands. When comparing the coordination environments of Rb, the distances are rather similar. In both, the cuboctahedral coordination is distorted, with four significantly shorter and eight longer distances, with the monoclinic modification exhibiting more strongly distorted cuboctahedra, however, with a nearly identical average distance. In both structures, these cuboctahedra are connected by face-sharing of opposite square faces forming strands, along the c-axis for the tetragonal modification and along the b-axis for the monoclinic one (compare Figure 3). Cuboctahedra of neighboring strands share edges, leading to the same motif for both modifications, only with tilting of the cuboctahedra of neighboring strands relative to each other in the monoclinic form ( Figure 4).

Angles This Work, PXRD This Work, scXRD Jacobs et al. [18]
Crystals 2020, 10, x FOR PEER REVIEW 6 of 12     (1) and Rb (right). The structural motif of the cuboctahedra is identical in the tetragonal structure.

Distances
Crystals 2020, 10, x FOR PEER REVIEW 6 of 12    four tetrahedra provide edges and four tetrahedra vertices. While in the tetragonal modification, all vertex-sharing tetrahedra are located on one side of the cuboctahedron, in the monoclinic form, they are more evenly distributed around the central rubidium ion ( Figure 5).
Crystals 2020, 10, x FOR PEER REVIEW 7 of 12 formed by amide ions by aluminum. Locally, this leads to different arrangements of [Al(NH2)4] −tetrahedra in the surrounding of rubidium ions. In order to form the cuboctahedral surrounding, in both forms, four tetrahedra provide edges and four tetrahedra vertices. While in the tetragonal modification, all vertex-sharing tetrahedra are located on one side of the cuboctahedron, in the monoclinic form, they are more evenly distributed around the central rubidium ion ( Figure 5). Within the tetragonal structure, the cuboctahedra around rubidium are connected by facesharing parallel to the c-axis, therefore forming strands. The already mentioned [Al(NH2)4] −tetrahedra are found in layers parallel (110), alternating in edge-and corner-sharing with the cuboctahedra. In the monoclinic modification, in contrast, there are layers composed of two cornersharing tetrahedra as well as two edge-sharing with the cuboctahedra which extend parallel to the baxis in (101) ( Figure 5). The cuboctahedra surrounding rubidium show the same face-sharing as in the P4/n modification, although parallel to the b-axis (compare Figure 3). Furthermore, they are connected via edge-sharing orthogonal to the b-axis, similar to the tetragonal structure.
Similar cases of polymorphs of amidometalates differing solely in arrangements of amidometalate tetrahedra and coordination polyhedra of alkali or alkaline earth metal ions turn out to be rather frequent, with recent examples being the three forms of Ba[Ga(NH2)4]2 [17] or the low and high temperature phases of Rb2[Mn(NH2)4] [19].
The known tetragonal modification was obtained under similar conditions at 393-443 K and 800-1200 bar, thus at lower temperatures and higher pressures than the new monoclinic form. The slightly smaller volume of Vtetra = 147.69 × 10 6 pm 3 compared to Vmono = 147.97 × 10 6 pm 3 per formula unit does not give a simple clue about phase relations between both phases, since the exact composition may play a decisive role for the unit cell volume. This is also expressed in the significantly larger volume of the reported powder X-ray diffraction data of an earlier observed monoclinic modification of unknown crystal structure (P2/c, see Table 1) [18]. Additionally, we have observed single crystals with generally the same symmetry but significantly smaller monoclinic unit cell parameters and thus volume for the new modification in C2/c (see Table 1). Refinements of the crystal structure resulted in the same monoclinic structure as described above; however, a close inspection revealed additional electron density in further tetrahedral voids formed by amide ions, while the nominal aluminum and nitrogen sites are only occupied to less than unity. Upon occupation of the additional sites with further Al atoms (Figure 6), the reliability factors dropped from R1 = 0.1005 and wR2 = 0.2805 to 0.0647 and 0.1670, respectively. The observed apparent disorder we interpret in analogy to that observed for the respective liquid amidogallate system [11]: already at ambient temperature, the material loses ammonia, leading to condensation of the tetraamidoaluminate tetrahedra to form edge-sharing binuclear units in solid state according to: Within the tetragonal structure, the cuboctahedra around rubidium are connected by face-sharing parallel to the c-axis, therefore forming strands. The already mentioned [Al(NH 2 ) 4 ] − -tetrahedra are found in layers parallel (110), alternating in edge-and corner-sharing with the cuboctahedra. In the monoclinic modification, in contrast, there are layers composed of two corner-sharing tetrahedra as well as two edge-sharing with the cuboctahedra which extend parallel to the b-axis in (101) ( Figure 5). The cuboctahedra surrounding rubidium show the same face-sharing as in the P4/n modification, although parallel to the b-axis (compare Figure 3). Furthermore, they are connected via edge-sharing orthogonal to the b-axis, similar to the tetragonal structure.
Similar cases of polymorphs of amidometalates differing solely in arrangements of amidometalate tetrahedra and coordination polyhedra of alkali or alkaline earth metal ions turn out to be rather frequent, with recent examples being the three forms of Ba[Ga(NH 2 ) 4 ] 2 [17] or the low and high temperature phases of Rb 2 [Mn(NH 2 ) 4 ] [19].
The known tetragonal modification was obtained under similar conditions at 393-443 K and 800-1200 bar, thus at lower temperatures and higher pressures than the new monoclinic form. The slightly smaller volume of V tetra = 147.69 × 10 6 pm 3 compared to V mono = 147.97 × 10 6 pm 3 per formula unit does not give a simple clue about phase relations between both phases, since the exact composition may play a decisive role for the unit cell volume. This is also expressed in the significantly larger volume of the reported powder X-ray diffraction data of an earlier observed monoclinic modification of unknown crystal structure (P2/c, see Table 1) [18]. Additionally, we have observed single crystals with generally the same symmetry but significantly smaller monoclinic unit cell parameters and thus volume for the new modification in C2/c (see Table 1). Refinements of the crystal structure resulted in the same monoclinic structure as described above; however, a close inspection revealed additional electron density in further tetrahedral voids formed by amide ions, while the nominal aluminum and nitrogen sites are only occupied to less than unity. Upon occupation of the additional sites with further Al atoms (Figure 6), the reliability factors dropped from R1 = 0.1005 and wR2 = 0.2805 to 0.0647 and 0.1670, respectively. The observed apparent disorder we interpret in analogy to that observed for the respective liquid amidogallate system [11]: already at ambient temperature, the material loses ammonia, leading to condensation of the tetraamidoaluminate tetrahedra to form edge-sharing binuclear units in solid state according to: The slow loss of ammonia over time at even ambient temperature from the material can be observed as a corrosive gas escaping, while the formerly whitish-grey powder seems to get darker, starting to liquefy and become viscous, comparable to honey. Elemental analysis of differently aged samples showed lower nitrogen amounts than expected, averaging at around 90% of the expected percentage (see Table 2). Additionally, the ammonia loss even at ambient temperature can be observed in the results of thermogravimetric measurements as discussed below. These facts can explain the reduced volume and occupation of the nitrogen site, while the two occupied aluminum positions in the structure refinements add up to the expected aluminum content.
Structurally, the nominally empty tetrahedral holes are equivalent to those fully occupied by Al(1) in the ideal structure of the tetraamidoaluminate. When including the disordered aluminum positions, the resulting [Al(NH2)4] − -tetrahedra form edge-sharing strands parallel to the b-axis. Occupation of a nominally empty tetrahedral void by Al(2) locally leads to formation of a [(NH2)2Al(NH)2Al(NH2)2] 2− condensed ion, if the neighboring voids within the same strand are empty, explaining the reduced occupation of the Al(1) site. Resulting distances between aluminum atoms within the resulting binuclear amidoimidoaluminate ions of around 252(1) pm (see Table 3) are similar to those observed in nitridoaluminates Sr3[Al2N6] and Ba3[Al2N6], with 254 pm and 256 pm, respectively [25,26]. The latter compounds contain similar edge-sharing units [N2AlN2AlN2] 6− ; thus, the observed deprotonation and condensation in solid state may be interpreted as early steps in nitridoaluminate formation. We have earlier observed analogous behavior of the liquid intermediates in the system Cs[Ga(NH2)4]−Cs2[(NH2)2Ga(NH)2Ga(NH2)2], which, close to ambient temperature, can variably take up or release amounts of ammonia depending on the applied pressure and temperature conditions [11].

Thermal Analysis
To further study the decomposition behavior of the amidoaluminate under increased temperature, thermal gravimetric experiments (TG) were conducted on the obtained microcrystalline powder. When using a heating rate of 10 K/min up to 1000 °C, three steps of decomposition are observed (Figure 7). The first two steps occur up to 200 and 350 °C, respectively, with a loss of mass around 8% each. Here, most likely, ammonia is leaving the compound, resulting in condensation products like Rb2[Al2(NH)2(NH2)4] first, followed by the formation of AlN and RbNH2. To further investigate these steps, another part of the sample was heated in a glass tube under argon atmosphere so as to visually observe the transitions. When reaching temperatures above 300 °C, liquefying was detected, which continued until around 350 °C, at which, visually, a liquid in the glass aperture could be observed. Additionally, the color changed, gradually getting darker to brown and later even to a The slow loss of ammonia over time at even ambient temperature from the material can be observed as a corrosive gas escaping, while the formerly whitish-grey powder seems to get darker, starting to liquefy and become viscous, comparable to honey. Elemental analysis of differently aged samples showed lower nitrogen amounts than expected, averaging at around 90% of the expected percentage (see Table 2). Additionally, the ammonia loss even at ambient temperature can be observed in the results of thermogravimetric measurements as discussed below. These facts can explain the reduced volume and occupation of the nitrogen site, while the two occupied aluminum positions in the structure refinements add up to the expected aluminum content.
Structurally, the nominally empty tetrahedral holes are equivalent to those fully occupied by Al(1) in the ideal structure of the tetraamidoaluminate. When including the disordered aluminum positions, the resulting [Al(NH 2 ) 4 ] − -tetrahedra form edge-sharing strands parallel to the b-axis. Occupation of a nominally empty tetrahedral void by Al(2) locally leads to formation of a [(NH 2 ) 2 Al(NH) 2 Al(NH 2 ) 2 ] 2− condensed ion, if the neighboring voids within the same strand are empty, explaining the reduced occupation of the Al(1) site. Resulting distances between aluminum atoms within the resulting binuclear amidoimidoaluminate ions of around 252(1) pm (see Table 3) are similar to those observed in nitridoaluminates Sr 3 [Al 2 N 6 ] and Ba 3 [Al 2 N 6 ], with 254 pm and 256 pm, respectively [25,26]. The latter compounds contain similar edge-sharing units [N 2 AlN 2 AlN 2 ] 6− ; thus, the observed deprotonation and condensation in solid state may be interpreted as early steps in nitridoaluminate formation. We have earlier observed analogous behavior of the liquid intermediates in the system Cs[Ga(NH 2 ) 4 ]-Cs 2 [(NH 2 ) 2 Ga(NH) 2 Ga(NH 2 ) 2 ], which, close to ambient temperature, can variably take up or release amounts of ammonia depending on the applied pressure and temperature conditions [11].

Thermal Analysis
To further study the decomposition behavior of the amidoaluminate under increased temperature, thermal gravimetric experiments (TG) were conducted on the obtained microcrystalline powder. When using a heating rate of 10 K/min up to 1000 • C, three steps of decomposition are observed (Figure 7). The first two steps occur up to 200 and 350 • C, respectively, with a loss of mass around 8% each. Here, most likely, ammonia is leaving the compound, resulting in condensation products like Rb 2 [Al 2 (NH) 2 (NH 2 ) 4 ] first, followed by the formation of AlN and RbNH 2 . To further investigate these steps, another part of the sample was heated in a glass tube under argon atmosphere so as to visually observe the transitions. When reaching temperatures above 300 • C, liquefying was detected, which continued until around 350 • C, at which, visually, a liquid in the glass aperture could be observed. Additionally, the color changed, gradually getting darker to brown and later even to a black substance. When heating above 350 • C, the liquid starts to release gas, indicted by small bubbles, and solidifies again, with a concomitant significant decrease in volume. Similar liquefying under the release of ammonia was earlier reported for Na[Al(NH 2 ) 4 ] [27]. Intermediates in the ammonobasic GaN synthesis with amides of the heavier alkali metals K-Cs typically are obtained as liquids with variable ammonia content and the tendency to solidify after time [11].
Crystals 2020, 10, x FOR PEER REVIEW 9 of 12 black substance. When heating above 350 °C, the liquid starts to release gas, indicted by small bubbles, and solidifies again, with a concomitant significant decrease in volume. Similar liquefying under the release of ammonia was earlier reported for Na[Al(NH2)4] [27]. Intermediates in the ammonobasic GaN synthesis with amides of the heavier alkali metals K-Cs typically are obtained as liquids with variable ammonia content and the tendency to solidify after time [11]. The TG curve reveals that the loss of ammonia already slowly starts at or below ambient temperature. Furthermore, loss of one molecule ammonia per formula unit Rb[Al(NH2)4] for the first two steps should result in steps of 9.7%, significantly larger than particularly the first step of 7.3% weight loss, also indicating an earlier partial loss of ammonia.
The eventual formation of AlN and RbNH2 after the second step in the TG (thermal gravimetry) is supported by rubidium amide then leaving the mixture at 450 °C onwards, with a mass loss of 58.9%, to leave aluminum nitride in badly crystallized cubic structure in the crucible. For comparison, the decomposition of pure RbNH2 is added to Figure 7. The thermal decomposition seems to result in nano-crystalline AlN, as a powder diffraction of the residue after DTA (differential thermal analysis) shows very broad reflection fitting to cubic sphalerite-type AlN [28]. The stable modification of AlN is hexagonal wurtzite-type; however, sphalerite-type AlN has been reported to form from carbothermal nitridation of Al2O3 [29]. Consulting the Scherrer equation, the broadened peaks result in a mean particle size of approximately 2.5 nm.

Spectroscopy
As expected, the infrared (IR) spectroscopy conducted on the new modification of rubidium tetraamidoaluminate shows little difference to the reported data for the tetragonal modification in P4/n [18]. This is not surprising considering that the only difference lies in the alignment of the tetrahedra, while the main structure motifs are unchanged but for distortion. The observed signals of IR and Raman spectroscopy have been summarized in Table 4 in comparison to the literature data for the tetragonal modification. Furthermore, a graphic representation of both the measured IR spectrum as well as segments from the Raman spectrum is shown in Figure 8. The TG curve reveals that the loss of ammonia already slowly starts at or below ambient temperature. Furthermore, loss of one molecule ammonia per formula unit Rb[Al(NH 2 ) 4 ] for the first two steps should result in steps of 9.7%, significantly larger than particularly the first step of 7.3% weight loss, also indicating an earlier partial loss of ammonia.
The eventual formation of AlN and RbNH 2 after the second step in the TG (thermal gravimetry) is supported by rubidium amide then leaving the mixture at 450 • C onwards, with a mass loss of 58.9%, to leave aluminum nitride in badly crystallized cubic structure in the crucible. For comparison, the decomposition of pure RbNH 2 is added to Figure 7. The thermal decomposition seems to result in nano-crystalline AlN, as a powder diffraction of the residue after DTA (differential thermal analysis) shows very broad reflection fitting to cubic sphalerite-type AlN [28]. The stable modification of AlN is hexagonal wurtzite-type; however, sphalerite-type AlN has been reported to form from carbothermal nitridation of Al 2 O 3 [29]. Consulting the Scherrer equation, the broadened peaks result in a mean particle size of approximately 2.5 nm.

Spectroscopy
As expected, the infrared (IR) spectroscopy conducted on the new modification of rubidium tetraamidoaluminate shows little difference to the reported data for the tetragonal modification in P4/n [18]. This is not surprising considering that the only difference lies in the alignment of the tetrahedra, while the main structure motifs are unchanged but for distortion. The observed signals of IR and Raman spectroscopy have been summarized in Table 4 in comparison to the literature data for the tetragonal modification. Furthermore, a graphic representation of both the measured IR spectrum as well as segments from the Raman spectrum is shown in Figure 8. Since the amide ion displays a C2v symmetry, we find several vibration modes, both symmetric and asymmetric, in IR and Raman spectroscopy. For both the symmetric and asymmetric valence vibrations, we find doublets at 3400 and 3250 cm −1 alike. In the Raman spectrum, we can still observe the asymmetric valence mode, while the symmetric one does not appear. At 1544 and 1241 cm −1 , we can observe two signals for the bending deformation of the amide ion. When compared to the measurement of the tetragonal structure, we noticed that the second signal is shifted to a lower wave number, although still within range for bending vibrations. At the fingerprint section, the Al-N lattice vibrations can be found. In the Raman spectrum, we could also observe at least one lattice vibration mode, although possible additional signals could not be found due to noise in the spectrum. Since the amide ion displays a C 2v symmetry, we find several vibration modes, both symmetric and asymmetric, in IR and Raman spectroscopy. For both the symmetric and asymmetric valence vibrations, we find doublets at 3400 and 3250 cm −1 alike. In the Raman spectrum, we can still observe the asymmetric valence mode, while the symmetric one does not appear.

Conclusions
At 1544 and 1241 cm −1 , we can observe two signals for the bending deformation of the amide ion. When compared to the measurement of the tetragonal structure, we noticed that the second signal is shifted to a lower wave number, although still within range for bending vibrations. At the fingerprint section, the Al-N lattice vibrations can be found. In the Raman spectrum, we could also observe at least one lattice vibration mode, although possible additional signals could not be found due to noise in the spectrum.

Conclusions
A new modification of Rb[Al(NH 2 ) 4 ] in C2/c was obtained and the crystal structure determined by powder X-ray diffraction. Thermal analysis indicates decomposition in three steps via a liquid phase with approximate composition Rb 2 [Al 2 (NH) 2 (NH 2 ) 4 ] prior to the formation of cubic nano-AlN. However, decomposition and condensation of amidoaluminate ions apparently already starts at ambient temperature within solid state, as indicated by thermal and elemental analysis as well as the results of single crystal X-ray diffraction performed on an aged crystal.