《1. Introduction》

1. Introduction

The development and production of energetic compounds with high energy density and stability are incredibly important because they play vital roles in both military and civil fields [1–3]. In the past few decades, research on energetic materials has experienced rapid progress, and the main achievements include high nitrogencontaining heterocycles [4–8], cage-strained molecules with more energetic groups (–NO2, –NNO2, –N3, –C(NO2)3, etc.) [9], energetic salts [10–13], cocrystal explosives [14–19], and energetic coordination compounds [20–24]. However, the majority of energetic materials with excellent detonation performance tend to have low stability, complicated preparation methods, and high cost, which limit their practical application and make accessing advanced practicable high-energetic materials a great challenge.

Molecular perovskites with a general formula of ABX3 provide a unique architectural platform, and their components can be customized to meet specific requirements, such as those for energetic materials. Molecular perovskites have been extensively studied due to their rich physical properties and potential applications [25–31]. Recently, we were the first to employ the perovskite structure to produce a dense packing of A-site fuel cations (i.e., H2dabco2+ = 1,4-diazabicyclo[2.2.2]octane-1,4-diium) with triple X-site oxidative anions (i.e., ClO4– ), which alternate at the molecular level. This condensed structure enabled fast and effective explosive reactions while maintaining high stability and low cost, as demonstrated by four molecular perovskite high-energetic materials, (H2dabco)M(ClO4)3 (M = Na+ , K+ , Rb+ , and NH4+ for DAP-1, DAP2, DAP-3, and DAP-4, respectively) [32–34]. In addition, we also briefly investigated the influence of A-site fuel cations on the oxygen balance (OB) with another two metal-containing perovskite compounds (H2pz)Na(ClO4)3 (H2pz2+ = piperazine-1,4-diium; PAP-1) and (H2dabco-O)K(ClO4)3 (H2dabco-O2+ = 1-hydroxy-1,4- diazabicyclo [2.2.2]-octane-1,4-diium; DAP-O2) [35]. Among these six members, DAP-4, which is metal-free, shows particular promise in applications such as explosives and propellant components and thus has attracted considerable attention [36–38]. Nevertheless, the field of molecular perovskite high-energetic materials is still in its infancy and is thus replete with opportunities and challenges. As the detonation performance (typically the detonation pressure and the velocity) is positively correlated to the crystal density and formation enthalpy of energetic materials, increasing both of these parameters simultaneously in crystals is always one of the most important issues for designing advanced high-energetic materials [39,40]. Subsequently, to elucidate the relationship between the structural details and properties and obtain better overall performance than with DAP-4 [41], we designed and systematically investigated five metal-free compounds solely by changing the A-site organic cations in DAP-4, and found that improving the OB while keeping the spherical shape of A-site cations to match the anionic cages tends to yield a better overall detonation performance. However, it is difficult to increase the crystal density and formation enthalpy simultaneously, solely by adjusting the A-site cations; unfortunately, our previous results did not show a significant improvement in detonation performance.

The ABX3-type perovskites have three typical subclasses featuring different linkages of the BX6 octahedra [42]. In addition to the cubic structure (i.e., CaTiO3 prototype) consisting of corner-sharing BX6 octahedra, the hexagonal structure (i.e., BaNiO3 prototype) consisting of face-sharing BX6 octahedra is also a well-known and important model, and has the capacity to allow an alternate and dense packing of fuel and oxidative components at the molecular level. To explore new perovskite high-energetic compounds with a hexagonal structure, we focused on seeking derivatives of the NH4+ cation to serve as metal-free B-site cations. Compared with the NH4+ cation, the NH3OH+ and NH2NH3+ cations have higher formation enthalpies and could form more hydrogenbonding interactions in crystals; thus, they are favored for designing highly energetic materials [11,43,44]. With these cations, two new metal-free hexagonal perovskite high-energetic materials, (H2dabco)B(ClO4)3 (B = NH3OH+ and NH2NH3+ for DAP-6 and DAP-7, respectively, Fig. 1), were prepared with a one-step selfassembly process in aqueous solution under ambient conditions. To the best of our knowledge, DAP-6 and DAP-7 represent the first examples of perovskite compounds with NH3OH+ and NH2NH3+ , respectively, as B-site cations rather than A-site cations [45–49]. The structures, thermal stabilities, and energetic performances of DAP-6 and DAP-7 were studied experimentally and theoretically. DAP-6 and DAP-7 exhibit higher densities and formation enthalpies than the NH4+ analog ((H2dabco)(NH4)(ClO4)3; DAP-4) in the cubic structure and therefore possess higher detonation performances and excellent specific impulse (Isp), which enable them to be promising candidates for practical explosives and propellants.

《Fig. 1》

Fig. 1. The structure of metal-free hexagonal perovskite compounds (DAP-6 and DAP-7). For clarity, all X-site ClO4– anions except one are presented as small green spheres, while B-site cations (NH3OH+ and NH2NH3+ ) are presented as magenta spheres.

《2. Results and discussion》

2. Results and discussion

《2.1. Single-crystal structures》

2.1. Single-crystal structures

The single crystals of DAP-6 and DAP-7 were obtained by the slow evaporation of the source solution after several days. The single-crystal X-ray crystallography at 223 K showed that DAP-6 and DAP-7 crystallize in the monoclinic space groups P21 and P21/m, respectively (Table 1) and that both possess a hexagonal perovskite-type structure (prototype phase BaNiO3) with the formula ABX3, with H2dabco2+ as the A-site cation, NH3OH+ or NH2NH3+ as the B-site cation, and ClO4– as the X-site anion. The crystal structures contain infinite linear {B(ClO4)3}n 2n– chains consisting of face-sharing B(ClO4)6 octahedra and H2dabco2+ cations located in the interchain space (Fig. 1). As the effective radii of both NH3OH+ (216 pm) and NH2NH3+ (217 pm) are much larger than that of NH4+ (146 pm) [50], both DAP-6 and DAP-7 adopt a hexagonal packing structure rather than the cubic one adopted by DAP-4, such that each B(ClO4)6 octahedron shares two faces with the adjacent octahedra to fulfill the hydrogen-bonding interactions in crystals (vide infra). The hydrogen-bonding interactions between NH3OH+ and ClO4– in DAP-6 seem to be stronger than those between NH2NH3+ and ClO4– in DAP-7, as suggested by the fact that the shortest atomic distance d(N···O) is 2.88(5) Å (1 Å = 10–10 m) in DAP-6 and 3.020(5) Å in DAP-7 (Table 2). Accordingly, the linear {B(ClO4)3}n 2n– chains of DAP-6 can pack into the structure more compactly, resulting in a slightly higher crystal density (Dc) for DAP-6 (1.92 g·cm-3 ) than for DAP-7 (1.90 g·cm-3 ) calculated by single-crystal X-ray crystallography at 223 K. This fact was further confirmed by the density for DAP-6 (1.90 g·cm-3 ) and DAP-7 (1.87 g·cm-3 ) from Pawley refinements on capillary powder X-ray diffraction data collected at 298 K (Table S1 in Appendix A).

《Table 1》

Table 1 Crystallographic data and structural refinements for two metal-free hexagonal perovskite compounds.

T: temperature; k: wavelength; a, b, c: cell length;  β: cell angle; V: cell volume; Z: formula units; Dc: crystal density; Rint: merging residual value; R1: unweighted residual factor; wR2: weighted residual factor; I: intensity of reflection; σ(I): estimated standard uncertainty of the reflection; CCDC: Cambridge Crystallographic Data Centre.

a Rint = ; R1 = ; wR2 = ; where Fo and Fc are the experimental and calculated structural factors, respectively, and w is a weight factor.

《Table 2》

Table 2 Selected hydrogen-bond geometries for the B-site cations in DAP-6 and DAP-7 at 223 K.

Symmetry codes: a: 1 – x, y + 1/2, 1 – z; b: – x, y + 1/2, – z; c: 1 – x, y + 1/2, – z; d: 1– x, y – 1/2, – z; e: 2 – x, y + 1/2, 1 – z; f: x, 1/2 – y, z; g: 1– x, – y, 1– z.

a D, H, and A stands for donor atom, hydrogen atom, and acceptor atom related in the hydrogen-bonding interactions.

Few studies have investigated the non-quasi-spherical units that rarely act as B-site cations in perovskites. To further understand the weak interactions around B-site cations, Hirshfeld surface analyses were performed for the NH3OH+ cations in DAP-6 and the NH2NH3+ cations in DAP-7. The surfaces are mapped with normalized contact distance, dnorm (Fig. 2), in a red-white-blue scheme indicating the intermolecular contacts shorter (red), around (white), and longer (blue) than the van der Waals separation. The asymmetric unit of DAP-6 or DAP-7 includes four NH3OH+ or one half NH2NH3+ cations, respectively. As suggested by the numerous large red spots on Hirshfeld surfaces (Fig. 2), in both DAP-6 and DAP-7, very high percentages (average value of 82.4% for four NH3OH+ cations and 89.5% for one NH2NH3+ cation, respectively) of the surface area were found associating with H…O/N and O/N…H short-contacts, which denote electrostatically attractive hydrogen-bonding interactions between the B-site cations and the adjacent ClO4– anions and/or B-site cations. Similarly, Hirshfeld surface analyses for the A-site cations (H2dabco2+) in DAP-6 and DAP-7 indicated that each A-site cation also forms abundant hydrogen-bonding interactions with adjacent perchlorate anions, as suggested by the attractive H…O contacts associating with 86.1% and 82.2% of the surface area for DAP-6 and DAP-7, respectively. In short, together with the attractive Coulomb interactions between cations and anions, these abundant hydrogen-bonding interactions facilitate the close packing of the face-sharing B(ClO4)6 octahedra along the infinite linear {B(ClO4)3}n 2n– chains, which are further closely packed with the interchain H2dabco2+ cations and result in high crystal densities for both DAP-6 and DAP-7.

《Fig. 2》

Fig. 2. The Hirshfeld surfaces mapped with dnorm for (a) NH3OH+ cations in DAP-6 and (b) NH2NH3 + cations in DAP-7, where the red and blue spots represent the intermolecular contacts shorter and longer than van der Waals separations, respectively. Symmetry codes: a: 1 – x, y + 1/2, 1 – z; b: – x, y + 1/2, – z; c: 1 – x, y + 1/2, – z; d: 1– x, y – 1/2, – z; e: 2 – x, y + 1/2, 1 – z; f: x, 1/2 – y, z; g: 1– x, – y, 1– z; h: – x, – y, –z; i: –x, –1/2 + y, –z; j: x –1, +y, –1 + z.

《2.2. Thermal stability and long-term stability》

2.2. Thermal stability and long-term stability

The thermal behaviors of DAP-6 and DAP-7 were characterized by differential thermal analysis (DTA) with a heating rate of 5 °C·min-1 . As shown in Table 3 [3,10,32,41], the onset decomposition temperatures (Td) of DAP-6 and DAP-7 are 245.9 and 375.3 °C, respectively, which are higher than those of cyclotrimethylene trinitramine (RDX; 210.0 °C) and hexanitrohexaazaisowurtzitane (CL-20; 215.0 °C), [10] due to their strong intra-ionic covalent bonds, the inter-ionic attractive Coulombic interactions, and the aforementioned abundant hydrogen-bonding interactions. In addition, the powder samples of DAP-6 and DAP-7 had been stored at ambient conditions for three and five months, respectively, and their powder X-ray diffraction (PXRD) patterns are almost the same as those of the assynthesized samples (Figs. S1 and S2 in Appendix A), suggesting long-term stabilities under ambient conditions for both DAP-6 and DAP-7.

《Table 3》

Table 3 Detonation properties of three classic organic explosives, DAP-4, DAP-O4, DAP-6, and DAP-7.

ρ: crystal density; Q: detonation heat; D: detonation velocity; P: detonation pressure; IS: impact sensitivity; FS: friction sensitivity; HMX: cyclotetramethylene tetranitramine; DAP-O4: (H2dabco-O)(NH4)(ClO4)3 (H2dabco-O2+ = 1-hydroxy-1,4-diazabicyclo[2.2.2]-octane-1,4-diium).

a The crystal densities estimated from capillary powder X-ray diffraction (PXRD) data collected at room temperature.

b The onset decomposition temperatures evaluated from DTA (5 °C·min-1 ).

c Oxygen balance based on CO2 for CaHbNcCldOe: OB = 1600[e –2a – (b – d)/2]/MW, where MW is molecular weight.

《2.3. Detonation parameters》

2.3. Detonation parameters

The detonation parameters of DAP-6 and DAP-7 were calculated using the density function theory (DFT) and the extended Kamlet– Jacobs (K–J) equation, and the results are shown in Table 3. The results suggested that DAP-7 has higher detonation heat, detonation velocity, and detonation pressure than that of RDX, while DAP-6 has a better detonation performance than that of cyclotetramethylene tetranitramine (HMX). In particular, DAP-6 possesses a remarkably high detonation heat (6.35 kJ·g-1 ), which is superior to that of all previously reported perovskite energetic materials and even that of CL-20 (6.23 kJ·g-1 ).

To further reveal the effects of the molecular components on the detonation performance for DAP-6 and DAP-7, a previously reported compound, (H2dabco-O)(NH4)(ClO4)3 (H2dabco-O2+ = 1- hydroxy-1,4-diazabicyclo[2.2.2]-octane-1,4-diium; DAP-O4), which has the highest detonation performance among the six previously known metal-free perovskite energetic compounds [32,41], was compared with DAP-6 and DAP-7. As shown in Fig. 3, in view of the molecular components, DAP-6 could be regarded as a modified version of DAP-O4 by moving the oxygen atom from the A-site H2dabco-O2+ cation to the B-site NH4+ cation; thus, DAP-6 and DAP-O4 are isomers with a same empirical chemical formula and the same OB (–23.3%). However, all detonation parameters of DAP-6 are higher than those of DAP-O4, presenting a new record for perovskite energetic compounds. Such an improvement on the detonation performances from DAP-O4 to DAP-6 mainly comes from the increase of both formation enthalpy and crystal density. Specifically, the formation enthalpies of both NH3OH+ (669.5 kJ·mol-1 ) and H2dabco2+ (1657.5 kJ·mol-1 ) cations in DAP6 are higher than those of the corresponding NH4+ (626.4 kJ·mol-1 ) and H2dabco-O2+ (1626.3 kJ·mol-1 ) cations in DAP-O4 (see Table S2 and Fig. S3 in Appendix A). Additionally, DAP-6 has a higher crystal density (1.90 g·cm-3 ) than that of DAP-O4 (1.85 g·cm-3 ), likely due to its hexagonal dense packing model, which further contributes to its improved detonation performance. Similarly, compared with DAP-4, although DAP-7 has the similar crystal density (1.87 g·cm-3 ) and even a slightly lower OB (–27.9% for DAP-4 vs 28.7% for DAP-7), the detonation parameters of DAP-7 are slightly higher than those of DAP-4, because of the higher formation enthalpy of the NH2NH3+ cation (770.0 kJ·mol-1 ) in DAP-7 than that of the NH4+ cation in DAP-4 (see Table S2).

《Fig. 3》

Fig. 3. Isomeric relationship between DAP-6 in the hexagonal perovskite structure and DAP-O4 in the cubic perovskite structure.

《2.4. Specific impulse》

2.4. Specific impulse

The Isp, an important parameter indicating the performance of solid propellants, was calculated for each material using EXPLO5TM v6.04.02 code based on the heat of formation back-calculated from the assumed detonation reactions (see Tables S3–S5 in Appendix A). As shown in Table 3, DAP-7 has a calculated specific impulse (256.9 s) higher than that of DAP-4 (253.6 s) and HMX (250.8 s), while DAP-6 has an even higher calculated specific impulse of 265.3 s, which is not only higher than that of its isomer, DAP-O4 (262.5 s), but is even comparable to that of CL-20 (264.8 s). Such high performances of the new solid propellants, DAP-6 and DAP7, could be ascribed to their high decomposition heats, which benefit from the aforementioned high formation enthalpies of the cations. Notably, DAP-6 and DAP-7 have much higher hydrogen contents (4.03% and 4.26%, respectively) than that of CL-20 (1.4%). They, therefore, can yield more water vapor among the combustion products, which then have a lower average molecular weight, making an additional contribution to their high specific impulses.

《2.5. Sensitivities》

2.5. Sensitivities

The impact and friction sensitivities were tested on a BFH-10 BAM impact tester (OZM Research S.R.O., Czech Republic) and an FSKM-10 BAM friction apparatus (OZM Research S.R.O., Czech Republic), respectively. As listed in Table 3, the impact sensitivities of DAP-6 and DAP-7 were 12.0 and 27.5 J, respectively, suggesting they are more insensitive to impact than the typical high explosives such as RDX (7.5 J), HMX (7.0 J), and CL-20 (4.0 J). In contrast, similar to other molecular perovskite energetic materials, DAP-6 and DAP-7 seem to be sensitive to friction (FS ≤ 5 N), which is probably associated with the relatively rigid perovskite structure and its perchlorate component.

《3. Conclusions》

3. Conclusions

In summary, by elaborately designing and choosing the molecular components, two new metal-free hexagonal perovskite highenergetic materials, namely DAP-6 and DAP-7, were successfully fabricated for the first time using NH3OH+ and NH2NH3+ cations, respectively, as the B-site cations. The calculated detonation performances of DAP-6 and DAP-7 are better than that of the NH4+ analog (DAP-4), which has a cubic perovskite structure, due to their molecular assembly in denser hexagonally close-packed structures and cations with higher formation enthalpies. Because of the good thermal stability (Td = 245.9 and 375.3 °C) and detonation performance (D = 9.123 and 8.883 km·s-1 , P = 38.1 and 35.8 GPa), DAP-6 and DAP-7 are promising candidates for practical usage as explosives and propellants. In particular, DAP-6 has higher crystal density and formation enthalpy than isomeric DAP-O4, which has a cubic perovskite structure, and it exhibits a new record of detonation performance metrics among perovskite energetic compounds, particularly a remarkably high detonation heat (Q = 6.35 kJg1 ) and a specific impulse (Isp = 265.3 s) superior to that of CL-20 (Q = 6.23 kJ·g-1 , Isp = 264.8 s). The dense molecular arrangements of DAP-6 and DAP-7, together with the resulting high detonation parameters and specific impulses, show that the hexagonal perovskite structure may serve as a new promising model to tune the crystal density, OB, formation enthalpy, and eventually, the energetic performance for the development of advanced high-energetic materials in the future.

《Acknowledgements》

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21722107 and 21821003), the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01C161), and the Natural Science Foundation of Guangdong Province of China (2020A1515010460).

《Compliance with ethics guidelines》

Compliance with ethics guidelines

Yu Shang, Zhi-Hong Yu, Rui-Kang Huang, Shao-Li Chen, DeXuan Liu, Xiao-Xian Chen, Wei-Xiong Zhang, and Xiao-Ming Chen declare that they have no conflict of interest or financial conflicts to disclose.

《Appendix A. Supplementary data》

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.eng.2020.05.018.