Synthesis, Structure and NH₃ Sorption Properties of Mixed Mg_1-xMn_x(NH₃)₆Cl₂ Ammines

Abstract

This paper describes the synthesis, crystal structure, and NH₃ sorption properties of Mg_1−xMn_x(NH₃)₆Cl₂ (x = 0–1) mixed metal halide ammines, with reversible NH₃ storage capacity in the temperature range 20–350 °C. The stoichiometry (x) dependent NH₃ desorption temperatures were monitored using in situ synchrotron radiation powder X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry. The thermal analyses reveal that the NH₃ release temperatures decrease in the mixed metal halide ammines in comparison to pure Mg(NH₃)₆Cl₂, approaching the values of Mn(NH₃)₆Cl₂. Desorption occurs in three steps of four, one and one NH₃ moles, with the corresponding activation energies of 54.8 kJ⋅mol⁻¹, 73.2 kJ⋅mol⁻¹ and 91.0 kJ⋅mol⁻¹ in Mg_0.5Mn_0.5(NH₃)₆Cl₂, which is significantly lower than the NH₃ release activation energies of Mg(NH₃)₆Cl₂ (E_a = 60.8 kJ⋅mol⁻¹, 74.8 kJ⋅mol⁻¹ and 91.8 kJ⋅mol⁻¹). This work shows that Mg_1−xMn_x(NH₃)_yCl₂ (x = 0 to 1, y = 0 to 6) is stable within the investigated temperature range (20–350 °C) and also upon NH₃ cycling.

1. Introduction

Energy storage materials and methods have gained high interest to ensure the transition to carbon-free future. Hydrogen, as a high-density energy carrier alternative to fossil fuels, is one of the promising solutions for energy storage systems via solid storage of hydrogen [1,2,3,4]. Several studies have highlighted the potential of ammonia for hydrogen-based energy systems [5,6,7,8].

Metal halide ammines have been studied as indirect hydrogen and ammonia storage materials [9,10,11]. Particularly Mg(NH₃)₆Cl₂ has received significant attention due to its high gravimetric NH₃ and H₂ capacity of 51.8 wt% and 9.2 wt%, respectively [12,13,14,15,16,17]. Mg(NH₃)₆Cl₂ crystallizes in the cubic space group Fm-3m with a K₂PtCl₆-structure type and a = 10.1899(4) Å [18]. NH₃ is thermally released in three steps at the temperatures of 142 °C (4 moles of NH₃), 230 °C (1 mole of NH₃) and 375 °C (1 mole of NH₃), respectively against an ammonia pressure of 1 bar [15]. The phase-formation and thermodynamic properties of Mg(NH₃)₆Cl₂, Mg(NH₃)₂Cl₂ and Mg(NH₃)Cl₂ have been thoroughly studied, and the high temperatures necessary to release the two last moles of NH₃ hampers the application of Mg(NH₃)₆Cl₂ as an effective energy storage system [19,20,21,22]. However, the desorption temperatures of NH₃ may be tailored toward lower NH₃ desorption temperatures via the formation of solid solutions, e.g., mixed cation metal halide ammines. Mn(NH₃)₆Cl₂ also exhibits a high gravimetric capacity of ammonia (44.8 wt%), and is isostructural to Mg(NH₃)₆Cl₂ with a slightly larger unit cell parameter: a = 10.249(3) Å [23]. Similar to Mg(NH₃)₆Cl₂, NH₃ is released from Mn(NH₃)₆Cl₂ in three steps with desorption temperatures of 80 °C, 180 °C, and 354 °C, respectively, and thus lower than the desorption temperatures of Mg(NH₃)₆Cl₂ [13,15]. However, only the sorption cyclability of the four first moles of NH₃ in Mn(NH₃)₆Cl₂ has been considered for ammonia storage applications, which is found to be reversible for at least 10 cycles [10].

The ammonia release temperatures are associated with the binding energy of NH₃ with its surrounding ions, which depends on the elements and crystal structures of the metal halides as elucidated in a recent study [24]. Formation of solid solutions has been suggested as an approach to tailor the NH₃ desorption temperatures and kinetics [25,26,27]. The NH₃ binding energies were investigated in SrCl₂-CaCl₂ solid solutions, and they were found to be intermediate those of the two precursors [28]. This led to studies on solid solutions of Sr_1−xBa_x(NH₃)₈Cl₂ and Sr_1−xCa_x(NH₃)₈Cl₂, and their respective NH₃ release properties. Sr_1−xBa_x(NH₃)₈Cl₂ solid solutions showed that varying the relative ratios of metal allowed tuning of the desorption temperature of ammonia. The gradual effect on the ammonia release temperature was observed with the optimal mixing condition of 37.5 % of BaCl₂ in Sr_1−xBa_x(NH₃)₈Cl₂ showing the full release of ammonia at temperature T < 100 °C of the final mixed metal halide ammine [25]. Similarly, it was demonstrated for Sr_1−xCa_xCl₂ solid solutions and the corresponding Sr_1−xCa_x(NH₃)₈Cl₂ ammines that the NH₃ absorption and desorption properties could be enhanced by tuning the mixing ratio [26,27]. Additionally, the ammonia storage properties and crystal structures of the CaCl₂-CaBr₂, SrCl₂-SrBr₂ and SrCl₂-SrI₂ solid solutions have also been investigated, and intermediate ammonia storage properties of the mixed anion metal halides were observed [28,29,30]. These studies show the possibility of forming mixed metal halides with tunable ammonia sorption properties. Solid solutions of borohydride-based ammines have also been investigated as potential solid-state hydrogen storage materials. The solid solutions of Mg_1−xMn_x(BH₄)₂⋅6NH₃ and structural similarities of Mg(BH₄)₂ and Mn(BH₄)₂ and their corresponding ammines were studied [31,32]. Similar to the present study it revealed temperature changes for ammonia release when compared to those of the pristine samples.

Inspired by the structural similarities between Mg(NH₃)₆Cl₂ and Mn(NH₃)₆Cl₂, this work addresses an investigation of solid solutions of Mg_1−xMn_x(NH₃)₆Cl₂. Here we show the synthesis of these novel series of mixed metal halide ammines with tunable properties for the NH₃ desorption. We present the Mg_1−xMn_x(NH₃)₆Cl₂ (x = 0.025, 0.05, 0.1, 0.3 and 0.5) solid solutions obtained by mechanical mixing of MgCl₂ and MnCl₂, followed by annealing and subsequent exposure to anhydrous NH₃ gas. The mixed metal halide ammines were systematically investigated with in situ powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and volumetric Sieverts techniques. The thermally induced ammonia release for the mixed metal halide ammines is discussed: three NH₃ desorption events are observed and the crystal structures of the intermediate ammine phases are identified and structurally characterized. The kinetics, absorption, and desorption properties of NH₃ are studied. The results presented in this work show that by changing the relative Mg/Mn ratio the NH₃ sorption properties can be tuned and optimized depending on the application.

2. Materials and Methods

2.1. Sample Preparation

Anhydrous MgCl₂ and MnCl₂ powders with a purity of 99.999% were purchased from Alfa Aesar and Sigma-Aldrich, respectively. Mg_1−xMn_xCl₂ solid solutions (x = 0.025, 0.05, 0.1, 0.3 and 0.5) were obtained using a SPEX SamplePrep 8000D Dual Mixer high-energy ball mill. The powders were placed in a 25 mL hardened steel vial together with hardened steel balls (10 mm diameter) in a ball-to-powder mass ratio of 16:1 and sealed in an Ar-filled glove box (<1 ppm of O₂ and H₂O). The ball milling program was for one hour.

The as-milled powders were annealed to increase the crystallinity. Batches of ~0.5 g of the as-milled powders were sealed in a stainless-steel cylinder inside a glove box, and subsequently heated to 350 °C with a heating rate of 1 °C⋅sec⁻¹ and kept isothermal at 350 °C for 24 h. These samples are denoted “as-synthesized” samples. Subsequently, the as-synthesized samples were placed in a high temperature stainless-steel cylinder and connected to an in-house built Sieverts apparatus. The samples were then exposed to an NH₃ gas pressure of 2.5 bar at room temperature (RT) for at least 3 h. MnCl₂ was ammoniated for 3 h at T = −20 °C and 1 bar NH₃. It was then stored in a glovebox freezer at −34 °C prior to the experiments, due to instability of the Mn(NH₃)₆Cl₂ at ambient conditions. These samples are denoted “ammoniated” samples.

2.2. Thermal Analysis

Combined thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the monometallic and mixed metal halide ammines were measured using a Netzsch STA 449 F3 Jupiter apparatus. The powders (~40 mg) were placed in an alumina crucible with a pierced lid under protective Ar atmosphere in a glove box. The alumina crucibles were shortly exposed to air (ca. 1 min) during mounting in the TGA-DSC apparatus. The powders were heated from RT to 455 °C with a heating rate of 5 °C⋅min⁻¹ in an Ar flow of 50 mL⋅min⁻¹. Additionally, the batches of Mg(NH₃)₆Cl₂, Mg_0.5Mn_0.5(NH₃)₆Cl₂ and Mn(NH₃)₆Cl₂ powder (~10 mg) were measured at six different heating rates of 1, 2, 5, 10, 20 and 40 °C⋅min⁻¹ for Kissinger analyses [33].

2.3. Synchrotron Radiation Powder X-ray Diffraction

High resolution in situ temperature varied synchrotron radiation powder X-ray diffraction (SR-PXD) experiments were performed at the Swiss Light Source (SLS), Switzerland and at the Diamond Light Source, Oxford, UK. At SLS, data were obtained at the Material Science powder diffraction beamline X04SA [34] using a monochromatic beam in Debye–Scherrer geometry with a Mythen microstrip detector with a wavelength of λ = 0.709396 Å. At Diamond, data were obtained at the I11 beamline [35] using a wide angle position sensitive detector and a wavelength of λ = 0.82646 Å. In both cases, the samples were loaded into 0.5 mm borosilicate glass capillaries in an Ar-filled glove box (<1 ppm of O₂ and H₂O), sealed with grease and rotated during data acquisition. The samples were heated at 5 °C⋅min⁻¹ from RT to 500 °C using a heat blower. The temperature was calibrated using a NaCl standard prior to diffraction runs [36]. The powder diffraction data were normalized and reduced, then modeled and refined according to the Rietveld method as implemented in the TOPAS software [37].

The structural models of Mg(NH₃)₆Cl₂, Mg(NH₃)₂Cl₂ and Ni(NH₃)Cl₂ were used as starting points for Rietveld refinements of the hexammine, diammine, and monoammine phases of the mixed metal halide ammines, respectively. The diffraction peaks were modeled by a Thompson-Cox-Hastings pseudo-Voigt function. The scale factor, zero-shift, unit cell parameters, atomic positions and background were refined. The N-H and H-H distances were restrained using soft restraints function during the Rietveld refinements.

2.4. Sorption Kinetics and Cycling

The two pristine materials MgCl₂ and MnCl₂ and the mixed metal halides Mg_0.9Mn_0.1Cl₂ and Mg_0.5Mn_0.5Cl₂ (m ~0.1 g) were studied with regards to their NH₃ absorption and desorption kinetics. The absorption process was conducted under 2.5 bar of NH₃ at RT, while the desorption reaction was achieved by heating the samples up to 350 °C with a heating rate of 2 °C⋅min⁻¹ under 1 bar of NH₃. A calibrated volume consisting of a reference volume (V = 482.9 mL) and a sample chamber (V = 23.25 mL) was used during the experiments and the moles of absorbed and desorbed NH₃ were calculated according ideal gas law using the formula below:

$$\mathsf{\Delta }n=\frac{\mathsf{\Delta }PV}{RT},$$

(1)

where Δn is the number of NH₃ moles absorbed or desorbed, ΔP is the pressure change in the system occurring due to absorption or desorption of NH₃, V is the volume, R is the gas constant and T is the temperature. In all cases, the number of absorbed or desorbed moles was normalized by the molar weight of the corresponding compound. Each NH₃ desorption was followed by evacuation of the released NH₃ from the cylinder to avoid reabsorption of the NH₃ gas during cooling to RT. The NH₃ desorption/absorption was cycled four times for each sample.

3. Results and Discussion

3.1. Structural Characterization of the As-Synthesized and Ammoniated Samples at RT

SR-PXD data were collected for the pristine samples, MgCl₂ and MnCl₂, and for the as-synthesized Mg_1−xMn_xCl₂, (x = 0.025, 0.05, 0.1, 0.3 and 0.5) samples. The unit cell parameters are presented in Table S1 in the Supporting Information. The diffraction patterns in Figure 1 confirm the formation of a MgCl₂-MnCl₂ solid solutions, as only a single set of Bragg diffraction peaks belonging to Mg_1−xMn_xCl₂ are observed, which is positioned in between that of MgCl₂ and MnCl₂. MgCl₂ and MnCl₂ are isostructural and crystallize in the trigonal CdCl₂-type structure with space group symmetry R-3m [38,39].

The MgCl₂-MnCl₂ solid solution follows Vegard’s law approximately, as the volume is a function of the relative content of cations and in between that of the two pristine compounds, see Figure 2a. The larger ionic radius of Mn²⁺ (0.83 Å) as compared to Mg²⁺ (0.72 Å) results in an increase of the unit cell volume [40]. The deviation from Vegard’s law might be due to a localized strain field caused by difference in Mg and Mn sizes, as well as to the different outer electronic structures of the mixing components (Mg and Mn in our case) [41]. The solid solution is maintained after ammoniation and the deviation from Vegard’s law remains. It should be noted that a negative deviation from Vegard’s law should be also expected if contamination from iron contained in the milling media results in the form Mg_1−x−yMn_xFe_yCl₂. Indeed, the ionic radius for Fe²⁺ (0.63 Å) is smaller than the radii of both Mg²⁺ and Mn²⁺ [40]. However, even if slight contaminations from the milling media and metallic Fe cannot be ruled out, they are expected to be very limited due to the relatively short milling time (1 h) used in this work. Additionally, metallic Fe must be oxidized to Fe²⁺ in order to substitute Mg²⁺/Mn²⁺ and form Mg_1−x−yMn_xFe_yCl₂. Therefore, it is most likely that the deviation from Vegard’s law observed here might be due to the different outer electronic structures of Mg and Mn. Rietveld refinement and structural characterization of the ammoniated samples confirm the cubic Mg(NH₃)₆Cl₂ structure for all mixed cation hexammines (Supporting Information, Figure S1–S5). Figure 2b illustrate Vegard’s law (blue dotted line) for Mg_1−xMn_x(NH₃)₆Cl₂. Surprisingly, the unit cell volume for samples with x < 0.05 are lower than that of Mg(NH₃)₆Cl₂.

The atomic positions of the monometallic and mixed hexammines obtained from Rietveld refinement are presented in

Table 1. Atomic positions of the monometallic and mixed hexammines: Mg_1−xMn_x (x = 0 to 1) in (0, 0, 0), Cl in (1/4, 1/4, 1/4) and N in (x, 0, 0)*.

Compound	N (x-coordinate)
Mg(NH3)6Cl2	0.21150(7), 0, 0
Mg0.975Mn0.025(NH3)6Cl2	0.21268(15), 0, 0
Mg0.95Mn0.05(NH3)6Cl2	0.2132(2), 0, 0
Mg0.9Mn0.1(NH3)6Cl2	0.21363(8), 0, 0
Mg0.7Mn0.3(NH3)6Cl2	0.21269(9), 0, 0
Mg0.5Mn0.5(NH3)6Cl2	0.21267(10), 0, 0
Mn(NH3)6Cl2	0.21540(14), 0, 0

* The data are obtained at RT.

. During the Rietveld refinements Mg and Cl atoms are fixed in the 4a and 8c positions, respectively, while the x-coordinate of N atom (24e position) is refined. N-H and H-H distances are restrained at ~1.107 Å and ~1.345 Å, respectively. Mg-N and Mn-N bond distances for the monometallic hexammines are 2.1564(7) Å and 2.2100(14) Å and thus similar to the previously reported values (2.197(5) [18] and 2.270(15) [23], respectively). Mg_1−xMn_x-N bond distances are intermediate between the Mg-N and Mn-N bond distances. The unit cell parameters of the monometallic Mg(NH₃)₆Cl₂ and Mn(NH₃)₆Cl₂ obtained in this study are a = 10.19579(9) Å and a = 10.26017(2) Å which correspond to the values reported previously [18,23]. The unit cell parameters for Mg_1−xMn_x(NH₃)₆Cl₂ are also intermediate between the unit cell parameters of the monometallic hexammines.

3.2. Thermal Analysis

Figure 3 shows the TGA-DSC measurements performed on Mg(NH₃)₆Cl₂, Mn(NH₃)₆Cl₂ and Mg_0.5Mn_0.5(NH₃)₆Cl₂. The DSC measurements for the other mixed ammines are shown in the Supporting Information (Figure S6). All the hexammine compounds are relatively stable at RT, except for Mn(NH₃)₆Cl₂, which slowly releases NH₃ in the glove box at RT. Thus, Mn(NH₃)₆Cl₂ was kept at T = −34 °C in a glovebox freezer prior to the TGA-DSC measurements.

TGA-DSC data shows the desorption process of the monometallic and mixed chloride ammines which consists of three events. With a heating rate of 5 °C⋅min⁻¹, the onset temperatures of the initial ammonia desorption of 4 NH₃ moles from Mg(NH₃)₆Cl₂ and Mn(NH₃)₆Cl₂ are observed at 121 °C and 79 °C, respectively. For the solid solution Mg_0.5Mn_0.5(NH₃)₆Cl₂, the onset temperature for the first desorption is 116 °C. The onset temperature for the next NH₃ release is at 179 °C for Mg_0.5Mn_0.5(NH₃)₂Cl₂, significantly lower as compared to 211 °C in Mg(NH₃)₂Cl₂ and similar to 179 °C in Mn(NH₃)₂Cl₂. The onset temperature of the last NH₃ desorption of Mg_0.5Mn_0.5(NH₃)Cl₂ is 276 °C, which is lower than the last desorption event onset temperature of Mg(NH₃)Cl₂ – 289 °C. Mn(NH₃)Cl₂ starts desorbing the last mole of NH₃ at 257 °C.

Each NH₃ desorption step is followed by mass loss. In the first desorption step, 4 NH₃ moles are released and Mg_0.5Mn_0.5(NH₃)₆Cl₂ experience a 30.2% mass loss, while the next two desorption events reduce the mass of the sample by 8.3% and 8.1%, respectively. The mass loss ratio 4:1:1 of the monometallic and mixed hexammines (Supporting Information, Table S2) corresponds to the moles of NH₃ desorbed in each desorption event, i.e., four moles of NH₃ released in the first desorption step, and 1 mole of NH₃ released in the second and third step, respectively, and agrees well with the theoretical weight loss expected from the NH₃ desorption. The gravimetric NH₃ capacities for the monometallic and the mixed cation hexammines are presented in Table S3. SR-PXD data measured of Mg_1−xMn_x(NH₃)₆Cl₂, (x = 0, 0.025, 0.05, 0.1, 0.3 and 0.5) after the TGA-DSC measurements confirm the reformation of Mg_1−xMn_xCl₂ after full NH₃ release, thus confirming the stability of the solid solution (Supporting Information, Figure S7).

Kissinger analysis was performed on the DSC heat flow signals for the three desorption events measured for Mg(NH₃)₆Cl₂, Mn(NH₃)₆Cl₂ and Mg_0.5Mn_0.5(NH₃)₆Cl₂ to determine the activation energy and investigate their NH₃ desorption kinetics. Kissinger plots for the three endothermic events with the release of 4, 1 and 1 moles of NH₃ are shown in Figure 4a–c. The corresponding activation energies were calculated for each desorption event and are presented in Figure 4d–f. The activation energy of the first four moles of NH₃ desorption from Mg_0.5Mn_0.5(NH₃)₆Cl₂ is 54.8 kJ⋅mol⁻¹, which is approximately in between that of Mg(NH₃)₆Cl₂ (E_a = 60.8 kJ⋅mol⁻¹) and Mn(NH₃)₆Cl₂ (E_a = 43.5 kJ⋅mol⁻¹). For the second NH₃ desorption step, the activation energy for Mg_0.5Mn_0.5(NH₃)₂Cl₂ is 73.2 kJ⋅mol⁻¹, in between that of Mg(NH₃)₂Cl₂ (E_a = 74.8 kJ⋅mol⁻¹) and Mn(NH₃)₂Cl₂ (E_a = 67.7 kJ⋅mol⁻¹). The final desorption event of Mg_0.5Mn_0.5(NH₃)Cl₂ has an activation energy of 91.0 kJ⋅mol⁻¹, as compared to Mg(NH₃)Cl₂ (E_a = 91.8 kJ⋅mol⁻¹) and Mn(NH₃)Cl₂ (E_a = 90.9 kJ⋅mol⁻¹).

The activation energies for Mg_0.5Mn_0.5(NH₃)₆Cl₂ in all three desorption events are significantly lower as compared to monometallic Mg(NH₃)₆Cl₂. Therefore, by obtaining the mixed metal halide ammines, it is possible to tailor the desorption temperature and kinetics of the mixed metal halide ammines compared to the monometallic halide ammines.

3.3. In Situ SR-PXD

The in situ SR-PXD data for Mg_0.5Mn_0.5(NH₃)₆Cl₂ in the temperature range RT to 402 °C, with a heating rate of 5 °C⋅min⁻¹, are shown in Figure 5a, while Figure 5b shows diffraction patterns at selected temperatures for each of the ammoniated compounds observed during heating. Rietveld refinements of the mixed metal hexammines are presented in the supporting material (Figures S1–S5). The in situ SR-PXD data from RT for monometallic Mg(NH₃)₆Cl₂ is shown in the supporting material (Figure S6). SR-PXD data at RT contain Bragg peaks from Mg_0.5Mn_0.5(NH₃)₆Cl₂ (97.6(6) wt%) and Mg_0.5Mn_0.5(NH₃)₂Cl₂ (2.4(5) wt%). Upon heating, the Bragg peaks corresponding to Mg_0.5Mn_0.5(NH₃)₆Cl₂ disappear between 115 and 125 °C, while peaks corresponding to Mg_0.5Mn_0.5(NH₃)₂Cl₂ increase significantly in intensity. Upon further heating, the Bragg peaks corresponding to Mg_0.5Mn_0.5(NH₃)₂Cl₂ decrease in intensity from ~230 °C and peaks from Mg_0.5Mn_0.5(NH₃)Cl₂ appear at ~246 °C. The peaks from Mg_0.5Mn_0.5(NH₃)Cl₂ disappear at 325 °C and some Bragg peaks from an unknown compound appear at 328 °C (Figure 5b, yellow diffraction pattern) before the full desorption of NH₃ and formation of the Mg_0.5Mn_0.5Cl₂ solid solution. The appearance of unknown diffraction peaks might be due to a non-stoichiometric transition from the Mg_0.5Mn_0.5(NH₃)Cl₂ monoammine to the Mg_0.5Mn_0.5Cl₂ chloride. Such behavior was previously reported for Mn(NH₃)Cl₂, which was observed to release the last NH₃ via two or more desorption events [13,42]. This is also observed in our data from the Sieverts measurements and will be discussed later in Section 3.4.

In situ SR-PXD data for Mn(NH₃)₆Cl₂ in the temperature range RT to 406 °C, with a heating rate of 5 °C⋅min⁻¹ confirms the presence of such intermediate phase at 307 °C (Figure S9). However, due to the fast heating rate and, therefore, dominant peaks from Mn(NH₃)Cl₂ and MnCl₂ phases in the diffraction pattern, it was challenging to index it and extract the unit cell parameters for Mn(NH₃)_1-δCl₂. The same applies for the diffraction pattern of possible Mg_0.5Mn_0.5(NH₃)_1-δCl₂ phase at 328 °C from the in situ data for Mg_0.5Mn_0.5(NH₃)₆Cl₂ (Figure 5), where the dominant diffraction peaks from Mg_0.5Mn_0.5(NH₃)Cl₂ make the indexing challenging. Observation of these intermediate diffraction patterns suggests that the transition of Mn(NH₃)Cl₂ to MnCl₂ and Mg_0.5Mn_0.5(NH₃)Cl₂ to Mg_0.5Mn_0.5Cl₂, which was described as non-stoichiometric process [42], in our study undergoes by stoichiometric NH₃ releases of δ moles.

The NH₃ desorption temperatures are decreased significantly for Mg_0.5Mn_0.5(NH₃)₆Cl₂ as compared to monometallic Mg(NH₃)₆Cl₂ (Figure S8), confirming the results from TGA-DSC analysis. The first NH₃ desorption step of 4NH₃ moles and transformation from Mg_0.5Mn_0.5(NH₃)₆Cl₂ to Mg_0.5Mn_0.5(NH₃)₂Cl₂ (T = 125 °C) is 20 °C lower than observed for Mg(NH₃)₆Cl₂ (T = 146 °C). All NH₃ is desorbed from Mg_0.5Mn_0.5(NH₃)₆Cl₂ at T = 337 °C, significantly lower than reported for Mg(NH₃)Cl₂ (T = 375 °C) [15]. The first desorption step of Mg(NH₃)₆Cl₂ at 146 °C is similar to the temperature reported in the literature (T = 142 °C) [15].

Rietveld refinements of the hexa-, di-, monoammine and chloride are shown in Figure 6, and

Table 2. Structural parameters for the present Mg_0.5Mn_0.5(NH₃)_yCl₂ (y = 6, 2, 1 and 0) phases investigated in this study.

Chemical Formula	Mg0.5Mn0.5(NH3)6Cl2	Mg0.5Mn0.5(NH3)2Cl2	Mg0.5Mn0.5(NH3)Cl2	Mg0.5Mn0.5Cl2
T (°C)	RT	214	280	402
Crystal system	Cubic	Orthorhombic	Monoclinic	Trigonal *
Space group	Fm-3m	Cmmm	I2/m	R-3m
a (Å)	10.22037(8)	8.258(5)	15.575(1)	3.69494(3) Å
b (Å)	-	8.290(4)	3.756(1)	-
c (Å)	-	3.812(2)	14.453(1)	17.8698(4)
β (°)	-	-	106.55(3)	-
V (Å3)	1067.58(3)	261.0(2)	810.6(9)	211.28(6)
Z	4	2	8	3

* Hexagonal parameters are used in this work.

summarizes their structural information.

Rietveld refinement of Mg_0.5Mn_0.5(NH₃)₆Cl₂ at RT is performed using the structural model of Mg(NH₃)₆Cl₂ (Figure 6a). Two phases are present in the sample, which are identified as Mg_0.5Mn_0.5(NH₃)₆Cl₂ and Mg_0.5Mn_0.5(NH₃)₂Cl₂ with the refined phase fractions to 97.6(6) wt% and 2.4(5) wt%, respectively, which might be resulted from partial NH₃ release at RT. Mg_0.5Mn_0.5(NH₃)₆Cl₂ crystallizes in a cubic unit cell, a = 10.22037(8) Å at RT with space group Fm-3m and is isostructural to Mg(NH₃)₆Cl₂. Octahedral Mg_0.5Mn_0.5(NH₃)₆ complexes are contained in a cubic lattice of Cl atoms, with each Mg_0.5Mn_0.5 atom octahedrally coordinated by six N atoms.

Mg_0.5Mn_0.5(NH₃)₂Cl₂ was refined using the Mg(NH₃)₂Cl₂ structure as a starting point. Figure 6b shows the Rietveld refinement of the SR-PXD data for Mg_0.5Mn_0.5(NH₃)₂Cl₂ at 214 °C. For the diammine Mg_0.5Mn_0.5(NH₃)₂Cl₂, space group Cmmm [43], each Cl atom is shared by two neighboring Mg_0.5Mn_0.5 atoms in edge-sharing octahedral chains.

The monoammine Mg_0.5Mn_0.5(NH₃)Cl₂ is isostructural to Ni(NH₃)Cl₂, which crystallizes in a monoclinic unit cell with space group I2/m [44] where each Cl atom is shared by three Mg_0.5Mn_0.5 atoms in edge-sharing double octahedral chains. Both broad and narrow diffraction peaks are observed for the monoammine phase (see Figure 6c), indicating the presence of structural disorder or stacking faults in the structure, resulting in a marked difference between the experimental and calculated patterns.

The Rietveld refinement of the fully desorbed Mg_0.5Mn_0.5Cl₂ at 402 °C is shown in Figure 6d. Mg_0.5Mn_0.5Cl₂ structure, space group R-3m, is formed by the octahedra of Cl atoms with central Mg_0.5Mn_0.5 atoms sharing half of their edges, and thus resulting in layers of Mg_0.5Mn_0.5Cl₂.

3.4. NH₃ Cycling and Kinetics

Studies of the NH₃ sorption kinetics and cyclability of the pristine and mixed metal halides with the lower and higher Mn contents (Mg_0.9Mn_0.1Cl₂ and Mg_0.5Mn_0.5Cl₂) were performed using a Sieverts apparatus. The results from the desorption cycles performed on Mg_0.9Mn_0.1(NH₃)₆Cl₂ are presented in the supporting information (Figure S10). The fifth NH₃ absorption process (after four cycles) of pristine MgCl₂, MnCl₂ and Mg_0.5Mn_0.5Cl₂ are presented in Figure 7. For absorption, the applied NH₃ gas pressure was ~2.5 bar and the processes were conducted at RT. The moles of absorbed NH₃ were calculated (see Equation (1)), where Δn is calculated from the pressure drop, ΔP, occurring during NH₃ absorption. The observed pressure drop was ΔP = 0.33 bar and the final pressures at the end of absorption were 2.21 and 2.22 bar for MgCl₂ and Mg_0.5Mn_0.5Cl₂, respectively. MnCl₂ only absorbed 5.5 moles of NH₃, which corresponds to a pressure drop of only ΔP = 0.29 bar reaching a final pressure p = 2.23 bar. This indicated that not all the MnCl₂ powder had reacted with NH₃, despite the still relatively high value of the final pressure of absorption. Indeed, due to the large volume expansion of the metal chloride during ammonia absorption, some clogging may occur and prevent ammonia from reaching all the salt crystals [16]. On the other, it cannot be excluded that that the absorption reaction stops because the equilibrium pressure for Mn(NH₃)₆Cl₂ at RT is higher than the final pressure reached during absorption (p = 2.23 bar). A more detailed thermodynamic study using pressure-composition-isotherms is needed to clarify this in detail. MgCl₂ absorbed 6 moles of NH₃ in less than 1000 s, while Mg_0.5Mn_0.5Cl₂ absorbed 6 moles of NH₃ in 6000 s. Similarly, the NH₃ absorption rate for the pristine halides are very different: Four moles of NH₃ is absorbed in MgCl₂ in about 200 s, while it took about 800 s to absorb the similar amount of NH₃ in MnCl₂. The rate of absorption for Mg_0.5Mn_0.5Cl₂ is similar to that of MnCl₂, indicating that Mn plays a predominant role for governing the kinetics of the hexammine formation.

The NH₃ desorption processes during cycling were carried out upon heating with a constant heating rate of 2 °C⋅min⁻¹ from RT to 350 °C under an initial NH₃ pressure of 1 bar, see Figure 8_. The moles of desorbed NH₃ were calculated using Equation (1) and the pressure increase, ΔP = 0.32 bar, due to NH₃ release. The three desorption steps of NH₃ were observed as a pressure increase and Δn was calculated. For Mg_0.5Mn_0.5(NH₃)₂Cl₂, the first 4 moles of NH₃ starts desorbing at around 100 °C and are fully released at 166 °C. The resulting Mg_0.5Mn_0.5(NH₃)₂Cl₂ desorbs one mole of NH₃ in the temperature range from 240 °C to 260 °C, forming Mg_0.5Mn_0.5(NH₃)Cl₂. The final NH₃ desorption step occurs above 300 °C. However, the transformation from monoammine to fully desorbed mixed metal chloride, Mg_0.5Mn_0.5Cl₂, does not proceed via a single step as for the previous desorption. Instead it undergoes through two discrete steps, consistent with the observations of a different crystalline phase in the in situ SR-PXD experiments.

For some hexammines, M(NH₃)₆Cl₂ (M = Mg, Ni), the desorption consists of three events [13]. In contrast, Mn(NH₃)₆Cl₂ was reported to undergo non-stoichiometric NH₃ release in the last desorption step [13,42]. In our study, we observe two separate steps, and from the number of desorbed NH₃ moles calculated from ΔP in Sieverts studies, δ seems to be equal to 0.5. Furthermore, the NH₃ desorption studies of Mg_0.9Mn_0.1(NH₃)₆Cl₂ (Figure S10) suggest that this phenomenon does not occur in Mg_1−xMn_x(NH₃)₆Cl₂ with low Mn content, as the last decomposition step occurs as a single event. This indicates that sufficiently high amount of Mn in Mg_1−xMn_x(NH₃)₆Cl₂ results in a change in the physical behavior to be similar to that of Mn(NH₃)₆Cl₂.

These results suggest that by changing the relative Mg:Mn ratio in Mg_1−xMn_x(NH₃)₆Cl₂ the NH₃ sorption propertied can be tuned and optimized. For instance, substituting Mg in Mn(NH₃)₆Cl₂ increases its stability, avoiding NH₃ desorption at RT. Furthermore, due to the low weight of Mg, the gravimetric capacity increases, with increasing Mg content. Finally, increasing the relative content of magnesium can be beneficial if cost reduction is desirable.

A thorough investigation of the NH₃ desorption reaction enthalpies is planned for further thermodynamic studies of the Mg_1−xMn_x(NH₃)₆Cl₂ hexammines by applying pressure composition isotherm (PCI) studies.

4. Conclusions

A series of novel mixed metal halide ammines, Mg_1−xMn_x(NH₃)₆Cl₂, with a usable ammonia capacity in the temperature range 20–350 °C were synthesized and characterized. The crystal structures of the different ammine phases are identified and investigated by in situ SR-PXD. All Mg_1−xMn_x(NH₃)₆Cl₂ solid solutions crystallize in a cubic unit cell with space group symmetry Fm-3m and unit cell parameters intermediate that of the two monometallic materials, Mg(NH₃)₆Cl₂ and Mn(NH₃)₆Cl₂. DSC analysis reveal a decrease in the onset temperature for NH₃ desorption for the solid solutions as compared to the monometallic Mg(NH₃)₆Cl₂. Activation energies for each desorption step are calculated and show the possibility of tailoring the activation energies for the NH₃ release in mixed metal chloride hexammines. The lower activation energies for NH₃ desorption in Mn(NH₃)₆Cl₂ resulted in a lowering of the activation energies for the solid solution Mg_0.5Mn_0.5(NH₃)₆Cl₂. Finally, NH₃ reversibility measurements reveal that the solid solution has a high stability, thus making them promising candidates for solid-state NH₃ storage systems.