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[Masao Morishita](https://orcid.org/0000-0002-6330-3901), Hayate Miyoshi, Haruto Kawasaki, Hidefumi Yanagita

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[Stabilisation of solid-state cubic ammonia confined in a glass substance at ambient temperature under atmospheric pressure](https://mdr.nims.go.jp/datasets/63f28037-8fcf-4bcb-a527-f58eaf61761e)

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Stabilisation of solid-state cubic ammonia confinedin a glass substance at ambient temperature underatmospheric pressure†Masao Morishita, *a Hayate Miyoshi,b Haruto Kawasakib and Hidefumi YanagitacAmmonia, a widely available compound, exhibits structural transitions from solid to liquid to gas dependingon temperature, pressure, and chemical interactions with adjacent atoms, offering valuable insights intoplanetary science. It serves as a significant hydrogen storage medium in environmental science,mitigating carbon dioxide emissions from fossil fuels. However, its gaseous form, NH3(g), poses healthrisks, potentially leading to fatalities. The sublimation pressure (psub) of solid cubic ammonia, NH3(cr),below 195.5 K is minimal. In this study, we endeavoured to stabilise NH3(cr) at room temperature for thefirst time. Through confinement within a boric acid glass matrix, we successfully synthesised andstabilised cubic crystal NH3(cr) with a lattice constant of 0.5165 nm under atmospheric pressure.Thermodynamic simulations affirmed the stabilisation of NH3(cr), indicating its quasi-equilibrium statebased on the estimated standard Gibbs energy of formation, DfG�mðNH3ðcrÞ; 298:15 KÞ. Despite theseadvancements, the extraction of H2(g) from NH3(cr) within the boric acid glass matrix remainsunresolved. The quest for an external matrix with catalytic capabilities to decompose inner NH3(cr) intoH2(g) and N2(g) presents a promising avenue for future research. Achieving stability of the low-temperature phase at ambient conditions could significantly propel exploration in this field.IntroductionAmmonia, abundant throughout the cosmos, plays a crucialrole in various celestial bodies. Within the depths of Uranusand Neptune, the presence of “hot ice” is notable, comprised ofwater, methane, and ammonia, sustained under high temper-ature and pressure. This compound also manifests as stoi-chiometric ammonia hydrates in many of the icy moonsorbiting the outer planets.2 Moreover, in the atmospheric layersof Jupiter, ammonia functions as a signicant chemicalconstituent.3 This cosmic ubiquity of ammonia underscores itsrelevance to the origins of life, as it constitutes an essentialelement of the planetary environments. Notably, in practicalapplications on Earth, ammonia serves as a foundationalmaterial for the production of urea fertilisers via the Haber–Bosch process, a critical measure in averting food crises.4Furthermore, from an environmental science perspective,ammonia holds promise as a hydrogen storage medium formitigating carbon dioxide emissions stemming from fossil fuelutilisation.5–10 The foundational understanding of ammoniaspans disciplines such as space exploration, life sciences,material engineering, and environmental studies, primarilyrevolving around its stability across different phases: solid,liquid, and gas.11–19 Pressure–temperature diagrams detailingthese phases have been meticulously constructed, underscoringthe versatile nature of ammonia. For instance, its triple point,occurring at 195.5 K and 0.0609 bar, highlights the stability ofits gas phase even at ambient temperatures under atmosphericpressure. Notably, our recent advancements have led to thesynthesis of solid-state ammonia, adopting a cubic structurewithin boric acid through an innovative freeze-drying tech-nique. This breakthrough opens avenues for further explorationat the forefronts of diverse scientic domains, hinting atpromising implications and applications of cubic ammonia.1–19Shiing perspectives towards utilising ammonia asa hydrogen storage material prompts consideration of thechallenges associated with its storage and transport due to itsharmful effects on human health. These effects encompasspotential eye damage upon contact, respiratory distress uponinhalation, and the risk of impaired consciousness due toelevated blood ammonia levels, which can prove fatal in severecases. Thus, this study delves into investigating small-scale anddistributed mild-type ammonia production as a viablealternative.aNational Institute for Materials Science (NIMS) (Formerly Department of ChemicalEngineering and Materials Science), University of Hyogo, Japan. E-mail:MORISHITA.Masao@nims.go.jpbDepartment of Chemical Engineering and Materials Science, University of Hyogo,JapancSanalloy Industry Co., Ltd, Japan† Electronic supplementary information (ESI) available. See DOI:https://doi.org/10.1039/d4ra00229fCite this: RSC Adv., 2024, 14, 16128Received 2nd February 2024Accepted 8th May 2024DOI: 10.1039/d4ra00229frsc.li/rsc-advances16128 | RSC Adv., 2024, 14, 16128–16137 © 2024 The Author(s). Published by the Royal Society of ChemistryRSC AdvancesPAPERTo enhance the Harber–Boesch process,4 we explore the low-pressure synthesis of ammonia utilising lithium compoundcatalysts.5 Furthermore, alternative methods for ammoniaproduction are investigated, including the cleavage of strongN–N bonds in N2(g) molecules through electrolysis,6–8 dischargeprocesses,9 and surface plasmon resonance.10Conversely, ammonia can be transformed into H2(g) andN2(g) with the aid of appropriate catalysts such as Ni,20zeolites,21 and CaNH.22 The resulting H2(g) can then be sepa-rated using a membrane for efficient utilisation.23 Additionally,the solid-state cubic NH3(cr) with its low sublimation pressurepresents another potential avenue for hydrogen storage andsupply, particularly in small-scale and distributed mild-typeproduction scenarios. Notably, this study marks the rstdescription of the synthesis and thermodynamic analysis ofcubic ammonia.ExperimentalProcess designThe triple point of NH3 is characterised by specic temperature(195.5 K) and pressure (0.0609 bar) values.11–19 Below 195.5 Kand at pressures exceeding 0.0609 bar, NH3 exists in its solid-state cubic form.24,25 Given this, achieving the stabilisation ofsolid-state cubic-NH3(cr) at ambient temperature and atmo-spheric pressure presents signicant challenges. To addressthis obstacle, we developed a process design aimed at creatingan embedded structure. This structure effectively connedNH3(cr) within a glass matrix, thereby enabling the stabilisationof NH3(cr) at ambient temperatures and under atmosphericpressure.Fig. 1(a) depicts a unit cell of cubic-NH3(cr),24–26 comprisingfour molecules denoted by open circles. Fig. 1(b) presentsa schematic illustrating the process design. In this design,a boric glass shell serves as the matrix for conning the NH3(cr).The glass matrix, termed B2O3(gl)–B(OH)3(gl), is composed ofboron trioxide (B2O3(gl)) and orthoboric acid (B(OH)3(gl)), syn-thesised by the dehydration of an aqueous solution resultingfrom the mixing of B2O3(cr) and pure water. Importantly, thegeometric structure of NH3(cr) is conned within the B2O3(gl)–B(OH)3(gl) matrix.It is noteworthy that in the cubic boron nitride (cBN) crystals,the covalent bond between a boron and a nitrogen atom isexceptionally strong. Similarly, a strong B–N bond27 is formed atthe interface between the NH3(cr) and the B2O3(gl)–B(OH)3(gl)glass matrix. This robust interface is anticipated to stabilise thecubic structure of NH3(cr), ensuring stability under ambienttemperature and atmospheric pressure conditions.The freeze-drying technique was utilised to encapsulatecubic NH3(cr) within B2O3(gl)–B(OH)3(gl). Initially, a frozenmixture of ammonia and boric acid in water was prepared at 77K through liquid-nitrogen cooling, wherein ammonia wastrapped as frozen ammonium (NH4+ (cr)) ions within ice(H2O(cr)). Subsequently, NH4+(cr) needed to undergo a reactionwith hydroxide ions (OH−(cr)) within the ice to achieve chargeneutrality and enable sublimation into NH3(g). However,OH−(cr) mobility is restricted at low temperatures, resulting incondensation of NH4+(cr). Finally, the removal of the ice mole-cules (H2O(cr)) facilitated the containment of NH3(cr) withinB2O3(gl)–B(OH)3(gl).Sample preparationCommercial aqueous ammonia solution (29 mass% NH3(aq.),Kanto Chemical Co., Ltd, Tokyo) and boron trioxide powder(B2O3(cr), 99.9%, Kojundo Chemical Laboratory Co., Ltd, Sai-tama) served as the starting materials. To ensure the stability ofthe NH3(cr) within the embedded structure, crucial for theformation of B–N bonds27 (Fig. 2), a 1 : 1 relative ratio of B to Nwas targeted. Thus, 160 mg of B2O3(cr) powder and 0.3 mL of 29mass% NH3(aq.) were employed, equating to 4.6 × 10−3 molesfor both B and N. Initially, the required amount of B2O3(cr)powder was placed in a test tube, followed by addition ofNH3(aq.) to impregnate the B2O3(cr) powder. Subsequently, thetest tube containing NH3(aq.)-impregnated-B2O3(cr) powderwas sealed with a rubber cap, featuring a centrally punched hole(diameter 0.5 mm) for vacuum evacuation during the freeze-drying process.Fig. 2 depicts a schematic of the apparatus utilised for freeze-drying. A test tube containing NH3(aq.)-impregnated B2O3(cr)Fig. 1 (a) Unit cell of cubic NH3(cr)24–26 composed of four moleculesmarked with open circle; (b) schematic model for stabilising the solid-state cubic ammonia (NH3(cr)) at ambient temperature, which isotherwise only stable below 195.5 K under atmospheric pressure.Fig. 2 Schematic of freeze-drying apparatus and method for syn-thesising the solid-state cubic-NH3(cr).© 2024 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 16128–16137 | 16129Paper RSC Advancespowder, sealed with a rubber cap, was positioned withina custom-made copper reaction vessel. An exhaust port wasintegrated into the copper vessel to facilitate vacuum evacua-tion. Once the copper vessel, along with the test tube, wasplaced in a thermostatic bath, liquid nitrogen was employed tofreeze the moisture, ammonium ions (NH4+(aq.)), andhydroxide ions (OH−(aq.)). Subsequently, aer 1 h, frozenH2O(cr) was sublimated via vacuum evacuation for 2 h at 77 K,with NH4+(aq.) being condensed using a rotary vacuum pump.Following freeze-drying, the copper vessel was promptlyremoved to prevent moisture adsorption by the freeze-driedproducts (FD products) due to condensation. Evacuationusing a vacuum pump persisted for 1 h at 295 K. Finally, the FDproducts were collected.Throughout the freeze-drying process, NH3(g)28 and BHO(g)29were generated. These gaseous molecules were captured in anaqueous boric acid solution using a trap, as illustrated in Fig. 3.The trapped species could subsequently be utilised in therecycling process to produce FD products.The phases present in the collected FD products were iden-tied through a combination of techniques. X-ray diffraction(XRD) analysis was conducted using a Rigaku, Ultima IVinstrument (Tokyo) while laser Raman spectroscopy was per-formed with a Nano Photon RAMAN Touch system (Osaka).To determine the boron content within the FD products,inductively coupled plasma emission spectroscopy (ICP, Shi-madzu, ICPS-8100, Kyoto) was employed.The hydrogen and nitrogen contents were assessed utilisingcombustion thermal conductivity analysis (Elementar, VarioVMACRO, Yokohama).For evaluating the upper storage temperature of NH3(cr),thermogravimetric analysis (TG, Rigaku, Thermoplus, TG 8110,Tokyo) was employed.To analyse the contents of H2 and NH3 gases that evolvedfrom the sample during thermal decomposition, gas and ionanalyses were performed. Gas analysis was conducted using a J-Science lab GC7100 instrument (Kyoto), while ion analysis uti-lised a Thermo Fisher Scientic Integrion RFIC (Tokyo) system.The penetration depth of X-rays, approximately 30 mm,allows for effective analysis of the structure of NH3(cr)embedded within the inner layer using XRD.30 However, thelimitations of laser Raman spectroscopy lie in its restrictedpenetration depths, largely conned to the shallow layers nearthe surface due to the wavelength constraints of the laser.Consequently, the intensity of the spectrum originating fromthe inner layer tends to be weak. To supplement Raman spec-troscopy, thermodynamic simulations were conducted to vali-date the formation of cubic ammonia.31–36Results and discussionsFig. 3 presents the FD product, appearing as a white powder at297 K under atmospheric pressure. Analysis of XRD patterns(Fig. 4) conrmed that the FD product consisted predominantlyof solid-state cubic NH3(cr). In contrast to the commercialaqueous ammonia solution, which emits a pungent odour dueto NH3(g) vaporisation, the odourless nature of the FD product,composed of NH3(cr), was noteworthy.Fig. 4(a) depicts the XRD pattern of the FD product recorded at297 K, revealing its main constituent as NH3(cr). The prominentpeak observed at a 2q value of 30.18° in this FD product alignedconsistently with that of the solid-state cubic NH3(cr) detected at171,24,25 160,26 and 77 K (ref. 24 and 25) via in situ XRD conductedby Olovson and Templeton,24,25 as well as by Boese et al.26 Notably,Fig. 3 FD product obtained through freeze-drying, comprising solid-state cubic NH3(cr) as the main constituent.Fig. 4 XRD patterns of reaction products comprising the solid-statecubic ammonia (NH3(cr)) as main product, ammonium pentaborate(NH4B5O8$4H2O(cr)) (open circle), and ammonia borane (NH3BH3(cr))(open square) as the sub-products.Fig. 5 Lattice constant as function of the diffraction degree, q, for thesolid-state cubic ammonia (NH3(cr)) at 297 K.16130 | RSC Adv., 2024, 14, 16128–16137 © 2024 The Author(s). Published by the Royal Society of ChemistryRSC Advances Paperthe NH3(cr), which is naturally stable below 195.5 K under atmo-spheric pressure, was maintained at 297 K.The lattice constant of the NH3(cr) in the FD product wasdetermined through extrapolation of the diffraction data at a 2qof 90.00°.30 Subsequently, utilising eqn (1), the lattice constantswere computed for the (111), (210), and (211) planes at variousdiffraction angles denoted as 2q°.Sin2q ¼ l24a2�h2 þ k2 þ l2�; (1)In eqn (1) the Cu Ka-ray wavelength is 0.154056 nm, and h, k,and l represent the plane indices. As shown in Fig. 5, the latticeconstants derived from the (111), (210), and (211) planes wereplotted against 1/2(cos2 q/sin q + cos2 q/q), employing the leastsquares method, the lattice constant was extrapolated at a 2q of90.00° to ascertain its precise value.Table 1 presents a comparative analysis of cubic latticeconstants, denoted as ‘a’, for NH3(cr) as determined in thisstudy alongside values obtained at various temperatures,namely 171,24,25 160,26 and 77 K,24,25 through in situ XRD con-ducted by Olovson and Templeton,24,25 as well as by Boese et al.26The lattice constant of 0.5165 nm identied in our investigationaligned closely with 0.5138,24,25 0.51305,26 and 0.508424,25 nmrecorded at 171,24,25 160,26 and 77 24,25 K, respectively.To assess thermal expansion, the coefficient a was computedby differentiating the data between 297 K in our study and 171 Kas per Olovson and Templeton.24,25 This was then compared withthe ndings reported by Olovson and Templeton24,25 and Boeseet al.26 for the temperature range spanning from 160 (ref. 26) to 77K,24,25 as detailed in Table 1. The resulting coefficients weredetermined to be 1.26 × 10−4 and 3.34 × 10−4, respectively.These values are intricately linked to variations in the latticevibration modes relative to temperature. Notably, larger coeffi-cients are discernible within the lower temperature spectrumcompared to those approximated around ambient conditions,aligning with both theoretical predictions31 and empiricalevidence.31–36 The diffraction peaks from other planes of NH3(cr)were difficult to detect, being likely that X-ray intensity wasdecreased during penetrating GM.However, the determined latticeconstant and thermal expansion constant is concluded to bereasonable.Ammonium pentaborate tetrahydrate (NH4B5O8$4H2O(cr))37and ammonia borane (NH3BH3(cr))38–43 are shown in Fig. 4(a).These include impurity phases stemming from the sub rawmaterial B2O3(cr).Shore and Böddeker39 demonstrated the preparation process ofNH3BH3(cr), which involves several steps: (A) dispersing diborane(B2H6(g)) in tetrahydrofuran (THF) at 195 K to yield THF$BH3; (B)distilling liquid NH3(l) onto the THF solution followed by stirring;(C) distilling away NH3(g) and THF, subsequently extracting NH3-BH3(cr) either from the remaining solid mixture of NH3BH3(cr) ordiammoniate of diborane H3B(NH3)2+BH4−. Remarkably, thecurrent process for producing the FD product closely mirrored theaforementioned synthesis of NH3BH3 at low temperatures byShore and Böddeker.39Fig. 6 displays the Raman spectrum of NH3(cr) within the FDproduct, compared with one measured by the spectrum obtainedvia in situ Raman spectroscopy conducted by Nye and Medina at93.6 K.12 The peaks A, B, and C observed in NH3(cr) within the FDproduct aligned consistently with the peaks a, b, and c detected inthe low-temperature in situ measurements by Nye and Medina at93.6 K.12 These peaks correspond to the Raman scattering spectraoriginating from the intermolecular vibrations of the ammoniamolecules within the cubic unit cell, as illustrated in Fig. 1(a).Notably, the peak intensity of NH3(cr) was relatively low owing toconnement within the glass matrix, which limited the penetra-tion of laser-induced Raman scattering.Fig. 4(b) depicts the XRD pattern of the sample aer main-taining the FD product following a duration of 4 h at 363 K.Notably, the disappearance of peaks corresponding to NH3(cr)signied sublimation into NH3(g), as described by reaction (I) inTable 2. Consequently, solely the peaks attributable to theremaining NH4B5O8$4H2O(cr) remained observable. Thisconversion of stored NH3(cr) into NH3(g) was notable for itslower energy consumption.Fig. 4(c) illustrates the XRD pattern of the sample aer sub-jecting the FD product at 793 K for 2 h. The absence of peaksassociated with NH4B5O8$4H2O(cr) suggested its thermal decom-position according to reaction (II) in Table 2. Subsequently,a broad pattern emerged, revealing the presence of the remainingTable 1 Comparison of lattice constant, a, and the thermal expansioncoefficient, a, for the synthesised solid-state cubic ammonia NH3(cr) at297 K with values obtained at 77,24,25 160,26 and 171 24,25 K, as reportedby Olovson and Templeton24,25 and Boese et al.,26 and a estimatedfrom these dataSubstance a/nm a T/K RemarksNH3(cr) 0.5165 — 297 Present studyNH3(cr) 0.5138 — 171 24 and 25NH3(cr) 0.51305 160 26NH3(cr) 0.5084 — 77 24 and 25NH3(cr) — 1.26 × 10−4 171–297 Present studyNH3(cr) — 3.34 × 10−4 77–160 24 and 26Fig. 6 Raman spectrum of the cubic ammonia (NH3(cr)) confined inthe glass matrix, compared with one measured by the in situ Ramanspectroscopy by Nye and Medina at 93.6 K.12© 2024 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 16128–16137 | 16131Paper RSC Advancesglass matrix (GM) containing the B2O3(gl) and B(OH)3(gl)components.The thermal decomposition process of orthoboric acidB(OH)3(cr) unfolds in a series of steps.44 First, at 373 K,orthoboric acid B(OH)3(cr) undergoes decomposition to formmetabolic acid HBO2(cr).27 Subsequently, at 413 K, metabolicacid HBO2(cr) further decomposes into tetra boric acidH2B4O7(cr).44 Finally, at 573 K, H2B4O7(cr) undergoes decom-position to yield boron trioxide B2O3(cr). Consequently,orthoboric acid B(OH)3(cr) transitions into B2O3(cr), as illus-trated in reaction (III) in Table 2.In a similar vein, a fraction of B(OH)3(gl) within the GMtransforms into B2O3(gl), as depicted in reaction (IV) in Table 2,a transformation supported by the B, H, and N elementalanalyses. Furthermore, interactions between B2O3(gl) andB(OH)3(gl) lead to the formation of gaseous BHO(g) and 3/2O2(g), a phenomenon also conrmed by elemental analyses.Table 3 presents the elemental analysis results for B, deter-mined using ICP, and for H and N, analysed via the combustionthermal conductivity. These assessments were conducted for boththe FD product and the GM resulting from the thermal decom-position of the FD product at 793 K for 2 h. The values areexpressed as mol%, with the O content derived by subtracting thesum of the B, H, and N contents from 100%. Notably, the FDproduct comprised 47.8 mol% H. In accordance with the samplepreparation outlined earlier, 4.6× 10−3 mol of both B and N wereutilised, alongside 160 mg of B2O3(cr) powder placed in a testtube, followed by 29 mass% NH3(aq.). Consequently, 205 mg ofthe FD product was collected. Based on the chemical compositiondepicted in Table 2, the quantities of N and B in the 205 mg FDproduct were determined to be 1.6 × 10−3 and 3.0 × 10−3,respectively. It is noteworthy to consider that portions of N and Bmay have been lost as sublimed species, NH(g),27 and BHO(g),29 asillustrated in Fig. 3, which were captured as an aqueous ammo-nium–boric acid solution using a trap positioned between the FDvessel and vacuum pump.Table 4 presents the molar percentage data detailing thephase constituents of both the FD and GM products, followinga temperature hold at 793 K for 2 h. These estimates werederived from the elemental compositions outlined in Table 3. Inthe FD product, the molar percentages were determined asfollows: 37mol% NH3(cr), 5 mol%NH4B5O8$4H2O(cr), 10 mol%B2O3(gl), and 48 mol% B(OH)3(gl), labelled as calc. FD. Withinthis, the molar ratio of B2O3(gl) to B(OH)3(gl) was calculated as0.17 : 0.83. Utilising mass percentage data, the compositionrevealed 11 mass% NH3(cr), 12 mass% NH4B5O8$4H2O(cr), 24mass% B2O3(gl), and 53 mass% B(OH)3(gl), with a at B2O3(gl) toB(OH)3(gl) ratio of 0.31 : 0.69. Notably, B2O3(gl) acted as a glass-forming component, while B(OH)3(gl) dissolved within B2O3(gl),resulting in the formation of a glass structure.Elemental compositions were deduced from the molarpercentage data, consistent with the calculated values for theFD product listed in Table 3. The composition of the calc. FDproduct aligned with the FD product, affirming the reason-ableness of the molar percentage (Table 4). According to XRD(Fig. 4(a–c)) and TG (Fig. 8) analyses, NH3(cr), comprising37 mol% of the FD product, sublimated into NH3(g) between325 and 373 K, as expressed in reaction (I) in Table 2. NH4B5-O8$4H2O(cr), constituting 5 mol% of the FD product, under-went thermal decomposition to NH3(g) above 373 K, asindicated in reaction (II) in Table 2. The resulting B2O3(gl) andB(OH)3(gl) were absorbed into the GM during the decomposi-tion process. In the FD product, the molar ratio of B2O3(gl) toB(OH)3(gl) was 0.17 : 0.83. Therefore, the shi from a 0.83 ratioof B(OH)3(gl) to B2O3(gl) to a 0.43 ratio resulted in a phasecomposition similar to that of the GM consisting of 50 mol%B2O3(gl)–50 mol% B(OH)3(gl) aer thermal decomposition at793 K for 4 h. Nitrogen was detected in the elemental compo-sition of GM aer decomposition (Table 3). This residual Nlikely originated from the B–N bonding. However, the details ofthese coordination states remain unknown.The quantities of the gaseous components were determinedthrough the sublimation and thermal decomposition reactionsoutlined in reactions (I–IV) in Table 2. To calculate, we summedTable 2 Thermodynamic reactions for the related substancesNH3(cr) # NH3(g) (I)NH4B5O8$4H2O(cr) # NH3(g) + 4H2O(g) + 2B2O3(gl) + BHO(g) + 1/2O2(g) (II)B(OH)3(gl) # (1/2)B2O3(gl) + (3/2)H2O(g) (III)B2O3(gl) + B(OH)3(gl) # 3BHO(g) + 3/2O2(g) (IV)Table 3 Chemical compositions of elements for the freeze-driedproduct (FD product) composed of solid-state cubic ammoniaNH3(cr), glass matrix confining NH3(cr) (GM), calc. FD product, andcalc. GMN/mol% H/mol% B/mol% O/mol% RemarksFD 6.1 47.8 11.9 34.2 ICP & com.GM 0.3 26.8 26.1 46.8 ICP & com.Calc. FD 6.3 46.5 13.4 33.8 EstimationCalc. GM 0 25.0 25.0 50.0 EstimationTable 4 Estimated molar percentage data of constituents of freeze-dried product (calc. FD) comprising the solid-state cubic ammoniaNH3(cr) and the components for GM after holding at 793 K for 2 h (calc.GM glass)Calc. FD/mol% Calc. GM/mol%NH3(cr) in FD 37 —NH4B5O8$4H2O(cr) in FD 5 —B2O3(gl) in FD 10 —B(OH)3(gl) in FD 48 —B2O3(gl) at 793 K for 2 h — 50B(OH)3(gl) at 793 K for 2 h — 5016132 | RSC Adv., 2024, 14, 16128–16137 © 2024 The Author(s). Published by the Royal Society of ChemistryRSC Advances Paperthe masses of NH3(g) in reaction (I), H2O(g), BHO(g), and O2(g)in reaction (II), as well as H2O(g) in reaction (III), and BHO(g)and O2(g) in reaction (IV). The masses of the gaseous species inreactions (I–III) were assessed based on the molar percentagesof the phase constituents. Upon adding the masses of BHO(g)and O2(g) in reaction (V), assuming an equivalent amount of theentire B2O3(gl) formed in the decomposition of the FD productwas added to the sum of the masses of the gaseous species inreactions (I–III), we evaluated that 40.8 mass% of the initial FDproduct underwent gasication. Subsequently, the gasicationmass loss during TG, aer the completion of thermal decom-position, was determined to be 44.7%. Despite the complexity ofthe reactions involved, the calculated sum of mass loss duringgasication aligned with the mass loss observed in TG. Thisconsistency indicates an efficient conversion of NH3(cr) storedin the FD product (37 mol%) into NH3(g) within thetemperature range of 325–373 K, with reduced energyconsumption. H2(g) was separated using a hydrogen separationmembrane. The solid-state cubic NH3(cr) synthesised in thisstudy holds promise as an excellent hydrogen storage material.Future investigation will focus on determining the relative ratioof NH3(cr) to GM.The phase stability of the NH3(cr) was investigated bycomputing its standard Gibbs energy of formation DfG�m, with“standard” referring to thermodynamic values under 1 bar.Initially, the Gibbs energy of formation, DfGm, for NH3(g) at195.5 K under 0.0609 bar—representing the temperature andpressure of the triple point—was calculated.11–19 Given thatNH3(cr) is equilibrated with NH3(g) at the triple point, the DfGmdatum of NH3(cr) equals that of NH3(g). Subsequently, the DfG�mdatum at 298.15 K under 1 bar for cubic NH3(cr) was estimated.The DfG�m datum for NH3(g) at 195.5 K under 1 bar wasdetermined using eqn (2),where DfG�m ðNH3ðgÞ; 298:15 KÞ was taken from the thermo-dynamic state base (TDB) edited by Barlin (−16.41 kJ (mol ofcompound)−1),27 and the C�p;m data for NH3(g),45 N2(g),29 andH2(g)29 were adopted from the TDB reported by Wagman andCox,45 and Chase et al.29Similarly, the DfG datum of NH3(g) at 195.5 K under 0.0609bar was calculated using eqn (3):At the triple point, NH3(cr) is in equilibrium with NH3(g),establishing the equality of their DfGm datum, as dened ineqn (4):DfG(NH3(cr), 195.5 K, 0.0609 bar) = DfG(NH3(g), 195.5 K,0.0609 bar) kJ (mol of compound)−1 (4)The DfG�m datum for NH3(cr) at 195.5 K under 1 bar wasdetermined using eqn (5),DfG�mðNH3ðgÞ; 195:5 K; 1 barÞ ¼ DfG�mðNH3ðgÞ; 298:15 KÞ þð195:5298:15C�p;mðNH3ðgÞÞdT� 12ð195:5298:15C�p;mðN2ðgÞÞdT � 32ð195:5298:15C�p;mðH2ðgÞÞdT�T(ð195:5298:15C�p;mðNH3ðgÞÞTdT � 12ð195:5298:15C�p;mðN2ðgÞÞTdT � 32ð195:5298:15C�p;mðH2ðgÞÞTdT)¼ � 26:25 kJ ðmol of compoundÞ�1(2)DfG�mðNH3ðcrÞ; 195:5 K; 1 barÞ ¼ DfGðNH3ðcrÞ; 195:5 K; 0:0609 barÞ þ VmD0:06091pðNH3ðcrÞÞ� 12RTð10:0609pðN2Þp�dpðN2Þ � 32RTð10:0609pðH2Þp�dpðH2Þ¼ �30:80 kJ ðmol of compoundÞ�1 (5)DfGðNH3ðgÞ; 95:5 K; 0:0609 barÞ ¼ DfG�mðNH3ðgÞ; 195:5 K; 1 barÞ þRTð0:06091pðNH3Þp�dpðNH3Þ�12RTð0:06091pðN2Þp� dpðN2Þ � 32RTð0:06091pðH2Þp�dpðH2Þ¼ �21:70 kJ ðmol of compoundÞ�1(3)© 2024 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 16128–16137 | 16133Paper RSC Advanceswhere the second term, VmD0.06091p(NH3(cr)), is the increase ofmolar Gibbs energy of NH3(cr) with change in pressure. Becausethis term is negligible,46 it was not considered in thecalculations.Finally, the DfG�m datum of the cubic NH3(cr) at 298.15 Kunder 1 bar was calculated from eqn (6),with estimated C�p;mðNH3ðcrÞÞ data due to lack of directmeasurement. The resulting value ofDfG�mðNH3ðcrÞ; 298:15 K; 1 barÞ datum, determined as−12.03 kJ (mol of compound)−1, presents a novel nding,potentially impacting general science, owing to the quasi-equilibrium phase of cubic solid-state ammonia. The thermo-dynamic data were summarized in Table 5.Furthermore, the standard Gibbs energy of transition fromthe gas to cubic state DtrsG�m, at 298.15 K under 1 bar wascalculated using eqn (7),DtrsG�mð298:15 K; 1 barÞ ¼ DfG�mðNH3ðcrÞ; 298:15 K; 1 barÞ�DfG�mðNH3ðgÞ; 298:15 K; 1 barÞ¼ þ4:38 kJ ðmol of compoundÞ�1(7)Hence, to stabilise NH3(cr) at 298.15 K, supplying the DtrsG�mdatum is essential. A potential candidate is the phase boundaryenergy between NH3(cr) and GM, suggesting a strong N–B bondformation, whichmay stabilize the NH3(cr) structure at ambientpressure. Table S1 (see ESI)† shows the standard Gibbs energiesof formation ðDfG�mÞ for BN(cr),27 CrN(cr),27 and TiN(cr)27 relatedto the B–N bond along phase boundary between NH3(cr) andGM. Note that cubic BN(cr) having diamond like hardness isquasi equilibrium phase. Therefore, the DfG�m datum27 of BN(cr)was determined for hexagonal structure as the equilibriumphase. The DfG�m data for CrN(cr)27 and TiN(cr)27 are deeplynegative consistent with their high hardness due to strongcovalency. As a result, they are used as coating materials to givewear resistance. The DfG�m datum of BN(cr)27 is intermediatevalue of ones of CrN(cr)27 and TiN(cr),27 meaning that the B–Ncovalent bond in its crystal is robust. From analogicalreasoning, the B–N bonds along the phase boundary betweenNH3(cr) and GM appear to be robust. Another candidate is thatthe Gibbs energy of mixing, DmixG,47,48 indicating deeper equi-librium among NH3(cr), GM, and NH4B5O8$4H2O(cr) than withNH3(g). Additionally, the pressure from the embedded struc-ture, particularly the compression pressure from the outer GM,appears to maintain the cubic structure of NH3(cr). Nonethe-less, further investigation is warranted. The B–N bond statesalong the phase boundary between NH3(cr) and GM are likely tobe further investigated by rst principles calculation.49 Theternary quasi phase equilibria among NH3(cr)–GM–B5O8$4H2-O(cr) is further investigated by the CALPHAD phase diagramcalculation.47,48The C�p;mðNH3ðcrÞÞ data were derived by extrapolating fromthe data of solid-state hydrogen, H(cr),50 and nitrogen, N(cr),50utilising the Neumann–Kopp law. At exceedingly low tempera-tures, the isochoric heat capacity, C�V;m can be closely approxi-mated by eqn (8), as nearly all phonons predominantly occupynear-ground states, where ‘n’ represents the number of atomsin the formula unit.C�V;m ¼ n12p4R5�TQD�3(8)In eqn (8), R is the gas constant (=8.3145 (J K−1 mol−1)) andQD is the Debye temperature. Consequently, at low tempera-tures, QD should be independent of T.QD ¼�12p45�1=3n1=3 C�V;mR!�1=3T (9)Because C�V;m and C�p;m exhibit striking similarity at extremelylow temperatures,33,34,36 QD was derived from C�p;m values under5 K and T, aligning with the relationship dened by eqn (9). QDTable 5 Estimated thermodynamic data (TD) for NH3(cr) and NH3(g).Standard states are N2(g) and H2(g)PhaseTD/kJ(mol of compd)−1 T/K p/barGas DfG�ma −16.41 298.15 1Gas DfG�m −26.25 195.5 1Gas DfGm −21.70 195.5 0.0609Solid DfGm −21.70 195.5 0.0609Solid DfG�m −30.80 195.5 1Solid DfG�m −12.03 298.15 1— DtrsG�mb +4.38 298.15 1a Ref. 27. b DtrsG�m ¼ DfG�mðNH3ðcrÞÞ � DfG�mðNH3ðgÞÞ.DfG�mðNH3ðcrÞ; 298:15 K; 1 barÞ ¼ DfG�mðNH3ðcrÞ; 195:5 K; 1 barÞ þð298:15195:5C�p;mðNH3ðcrÞÞdT� 12ð298:15195:5C�p;mðN2ðgÞÞdT � 32ð298:15195:5C�p;mðH2ðgÞÞdT�T(ð298:15195:5C�p;mðNH3ðcrÞÞTdT � 12ð298:15195:5C�p;mðN2ðgÞÞTdT � 32ð298:15195:5C�p;mðH2ðgÞÞTdT)¼ �12:03 kJ ðmol of compoundÞ�1(6)16134 | RSC Adv., 2024, 14, 16128–16137 © 2024 The Author(s). Published by the Royal Society of ChemistryRSC Advances Papervalues for H(cr) and N(cr) were subsequently estimated as 141 Kand 101 K, respectively.To extrapolate C�p;mðNH3ðcrÞÞ across the 0–300 K range,a composite average of C�p;m data of H(cr) and N(cr) was employed,in accordance with the Neumann-Kopp law as shown in Fig. 7.Subsequent evaluation of the C�p;mðNH3ðcrÞÞ data involved utilis-ing recently developed formulas,32–36 detailed in the ESI (TableS2).† An optimised tting function was then obtained andsubstituted into eqn (6) to ascertain DfG�mðNH3ðcrÞ; 298:15 KÞ.Fig. 8 shows the TG results of the FD product used toinvestigate the sublimation of NH3(cr). No mass loss wasobserved below 325 K, indicating the possibility of the stablestorage of the NH3(cr). At 325–750 K, mass loss occurredfollowing reaction (I–IV) in Table 2 due to the sublimation ofNH3(cr), the thermal decompositions of NH4B5O8$4H2O andB(OH)3 in GM, and forming BHO gas. The results of the TG dataof B(OH)3 and B2O3 as GM forming elements were shown inFig. 8. In TG of B(OH)3, mass loss occurred due to the thermaldecomposition to form B2O3 and forming BHO gas. Since B2O3is hygroscopic, its part is inevitably changed to form B(OH)3absorbing atmospheric moisture. Therefore, the mass lossresulted from forming B(OH)3. However, the mole percents ofthe constituents in FD product was conrmed from B, H and Nanalyses by ICP spectroscopy and combustion thermalconductivity analysis. Considering hygroscopicity, utilization ofB(OH)3 as starting substance is likely to be advantageous. Thisproblem should be further investigated.Finally, the investigation delved into the potential separationof hydrogen gas (H2(g)) from NH3(cr) conned in GM at 343 K.The process of sublimation posed challenges for the penetra-tion of ammonia gas molecules through the GM layer at thistemperature. Consequently, it is conjectured that NH3(cr) likelydecomposes into hydrogen and nitrogen atoms, with subse-quent penetration of H atoms through the GM layer, resultingin the formation of H2(g) on the surface. Conversely, N atomsare inferred to undergo a reaction with the B atoms in GM,leading to the formation of NH4B5O8$4H2O(cr).To validate this hypothesis, a sample weighing 0.5 g of FDproduct was enclosed within a stainless-steel container andsubjected to heating in a thermostatic bath at 343 K for 5 min,followed by holding for 6 h. Subsequently, the gas present in thestainless-steel container was collected in a gas bag post-coolingto room temperature. Analysis of the gas composition involvedmeasuring the H2(g) content using a gas chromatograph. Theresidual gas was absorbed into a boric acid aqueous solution,and the content of NH4+(aq.) was determined using an ionchromatograph to quantify the presence of NH3(g).Table 6 illustrates the quantities of H2(g) and NH3(g)following the FD product's exposure at 343 K for 6 h. Unfortu-nately, direct collection of H2(g) from the FD product was notfeasible. However, NH3(g) was successfully detected, witha measured quantity of 8.9 mg per gram of FD product. Thisdiscovery underscores the efficient collection of NH3(g) at rela-tively low temperatures, signifying a promising avenue forenergy conservation.Nevertheless, a substantial amount of NH3(cr) is anticipatedto remain sequestered within the GM, likely destined forconversion into NH4B5O8$4H2O(cr). This process indicates thepotential for exploring alternative matrices with catalytic prop-erties, such as zeolites21 and CaNH22 to facilitate the decom-position of internal NH3(cr) into H2(g) and N2(g). Thus,investigating outer matrices with catalytic capabilities repre-sents an intriguing frontier for future research endeavours.ConclusionRenewable energy sources such as solar photovoltaics and windpower are heavily reliant on weather and geography, necessi-tating complementary energy storage technologies. AmmoniaTable 6 Amount of H2(g) and NH3(g) after holding the FD product at343 K for 6 hSubstance Amount/mg (g-FD product)−1H2(g) NoneNH3(g) 8.9Fig. 7 C�p;m data from thermodynamic simulation for solid state cubicNH3(cr) (Table S2†).Fig. 8 TG analyses of the samples prepared from freeze-drying (FDproduct), commercial B(OH)3(cr) and B2O3(cr), respectively. The initialmasses of FD product, commercial B(OH)3(cr) and B2O3(cr) were 19.2,20.6 and 13.7 mg, respectively.© 2024 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 16128–16137 | 16135Paper RSC Advancesserves as a crucial hydrogen storage substance, yet its gaseousform poses signicant health risks, including potential fatality.Thus, safe ammonia storage systems must be developed. Ourprocess design aimed to establish an embedded structurehousing solid-state cubic ammonia (NH3(cr)) within a boric acidglass matrix. This innovative approach culminated in thesuccessful synthesis of stable NH3(cr), preserved within theglass matrix under ambient temperature and pressure, facili-tated by a freeze-drying process. Thermodynamic simulationsvalidated the stabilisation of NH3(cr), with estimated standardGibbs energy of formation affirming its stability in a quasi-equilibrium state. The pursuit of an outer matrix with cata-lytic capabilities, such as zeolites or CaNH, to facilitate thedecomposition of inner NH3(cr) into H2(g) and N2(g), emergesas the next frontier in research.Author contributionsM. M. conceived the idea and wrote the paper; H. M. and H. K.synthesized the samples and analysed their constituents andcompositions; and H. Y. rendered helpful discussions.Conflicts of interestThere are no conicts to declare.AcknowledgementsThis work was supported in part by the Japan Society for thePromotion of Science (JSPS) under Grant-in-Aid for scienticresearch 24K08127, an academic research grant from the HyogoScience and Technology Association, and special research grantfor hydrogen energy from the University of Hyogo.References1 W. B. Hubbard, Interior of the Giant Planet, Science, 1981,214, 145–149.2 J. S. Kargel, Ammonia-water Volcanism on Icy Satellites:Phase Relations at 1 atmosphere, Icarus, 1992, 100, 556–574.3 P. G. J. 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Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 16128–16137 | 16137Paper RSC Advances1Supplementary materialTable S1 The standard Gibbs energies of formation ( ) for BN(cr)27, CrN(cr) 27, ∆fG°mand TiN(cr) 27 related to the B−N bond along the phase boundary between NH3(cr) and the B2O3 (gl)–B(OH)3 (gl) glass matrix (GM). Substance /(kJ (mol of compd.)-1)∆fG°mBN(cr)27, *1 -228.501CrN(cr) 27 -92.703TiN(cr) 27 -309.155*1Hexagonal The  values for the solid state cubic NH3(cr) were evaluated by the recently developed 𝐶 °𝑝,  𝑚formulas: 32-34, 46,47 a semi-empirical power law function composed of the electronic and lattice vibration terms at 0 < T < 5-6 K; an empirical power law temperature function at 5-6 < T < 40 − 50 K; the Debye-Einstein (DE) at 40 − 50 < T < 300 K, expressed as    0 < T < 5 – 6 K                                         (S1)𝐶 ○𝑝,𝑚 =  𝛾𝑇 +  ∑𝑗 = 3,5,7,9𝐴𝑖𝑇𝑖            5– 6 K < T < 40 –50 K                                     (S2)𝐶 ○𝑝,𝑚 =6∑𝑗 = 0𝐵𝑗𝑇𝑗 = 3 R    40 –50 K < T <300 K   (S3) 𝐶 °𝑝,  𝑚{ 𝑚𝐷( Θ𝐷𝑇 ) +  𝑛𝐸( Θ𝐸1𝑇 ) + 𝑙𝐸( Θ𝐸2𝑇 )}where, for NH3(cr), the fitting functions were divided in three temperature ranges: at 0.5 – 5.20 K; 5.20 – 41.36 K; 41.36 – 300 K, as shown in Tables S1-S3.  At 0 – 5.20 K (eq.(S1)),  is the 𝛾coefficient of the electronic term contributing to . 31-36 For insulating substances, it results 𝐶 ○𝑝,𝑚from vacancies32 Ai are the coefficients for the lattice vibration term contributing to . 29-34 At 𝐶 ○𝑝,𝑚5.20 – 41.36 K (eq.(S2)), Bi are the adjustable coefficients for providing sufficient overlap to the measured  data and for connecting smoothly near the inflection points around 5.02 K and 𝐶 ○𝑝,𝑚Electronic Supplementary Material (ESI) for RSC Advances.This journal is © The Royal Society of Chemistry 2024241.36 K. At 41.36 – 300 K (eq.(S3)), ,  and  are Debye and Einstein functions; ( Θ𝐷𝑇 ) ( Θ𝐸1𝑇 ) ( Θ𝐸2𝑇 ),  and  are the Debye and Einstein temperatures used for adjustable parameter; m, n and Θ𝐷Θ𝐸1Θ𝐸2l are the adjustable parameters and the sum of m, n and l should be approximately closed to the number of atoms in the formula unit. 33-36.   Tables S1-S3 shows the adjustable parameters. Six  digits numbers for , , , m, n and l are necessary to reproduce the experimental  data Θ𝐷Θ𝐸1Θ𝐸2 𝐶 °𝑝,  𝑚as five digits numbers.The  data at 298.15 K of metal elements are about 25 (J K-1 (mol of atoms)-1) following 𝐶 °𝑝,  𝑚the Dulong-Petit law, and the Debye and Einstein functions are the theories(29) to satisfy the Dulong-Petit law. However, the compounds composed of non-metallic element do not follow the Dulong-Petit law, and their  data are empirically half values, e.g. the  data at 298.15 K 𝐶 °𝑝,  𝑚 𝐶 °𝑝,  𝑚of B2O3(cr) and B(OH)3 are 62.761 (J K-1 (mol of compd.)-1), i.e. , 12.552  (J K-1 (mol of atoms)1), and 86.060 (J K-1 (mol of compd.)-1), i.e. , 12.294 (J K-1 (mol of atoms)-1, respectively. Therefore, the   data for NH3(cr) were adopted as the half values calculated from the parameters 𝐶 °𝑝,  𝑚summarized in Table S2. Table S2 Parameters for the fitting functions used to fits the  data for the solid state cubic 𝐶 ○𝑝,𝑚NH3(cr) from Eqs.S1-S3. The notation E  kl indicates the power of 10. ±Temp. range: 0.5 – 5.20 K Coefficientγ(J K-2 (mol of compd.)-1 8.07997E-08A3(J K-4 (mol of compd.)-1 1.98334E-03A5(J K-6 (mol of compd.)-1 2.67852E-08A7(J K-8 (mol of compd.)-1 -3.48652E-09A9(J K-10 (mol of compd.)-1 1.93042E-10A11(J K-12 (mol of compd.)-1 -3.81599E-12Temp. range:  5.20 – 44.29 K CoefficientB0/(J K-1 (mol of comd.)-1) -2.49757E-01B1/(J K-2 (mol of comd.)-1) 2.18229E-01B2/(J K-3 (mol of comd.)-1) -6.30746E-023B3/(J K-4 (mol of comd.)-1) 9.77004E-03B4/(J K-5 (mol of comd.)-1) -4.12224E-04B5/(J K-6 (mol of comd.)-1) 7.37473E-06B6/(J K-7 (mol of comd.)-1) -4.92858E-08Temp. range:   44.29 – 300 K CoefficientΘD/K 1.10961.E+02m 6.20324E-01ΘE1/K 5.32855E+03n 1.37968E+00ΘE2/K 1.31707E+02l 2.00000E+00