Molecular perovskites have attracted growing attentions for high-performance solar cells, photo-detectors, and ferroelectric materials. This sort of material is rather hot recently. "What else can molecular perovskite do, we have been thinking and exploring." introduced by Profs. Wei-Xiong Zhang and Xiao-Ming Chen, from Sun Yat-Sen University, China, whose group recently reported a new sort of high-energetic materials produced by inorganic-organic perovskites, published on Science China Materials, 2018, doi:10.1007/s40843-017-9219-9.

"As the simplest high-symmetry structure for ternary compounds, perovskite structure is very charming." said Prof. Zhang. Molecular perovskites topologically mimic the structure of inorganic perovskites, but have at least one molecular component.

"We believe that, the diverse molecular components enable them to host many interesting phenomena and functionalities waiting to be discovered. Last year, we designed a molecular perovskite to host a new 'bond-switching' ferroelectric mechanism, and this time, we discovered that the molecular perovskites could be one sort of excellent energetic materials."

"Different from traditional strategy that the oxidizer and fuel groups are combined into a single molecule, we employed perovskite structure to incorporate the molecular oxidizer and fuel components into a ternary crystal for new energetic materials with high performance, high stability and low cost." said Prof. Zhang.

Since the black powder, the first known explosive, was discovered by ancient Chinese in the seventh century, people never stop finding more powerful, stable, reliable and low-cost energetic materials for military devices and civil industry.

Although their explosive performances are escalated with increasing the number of nitro-groups and the structural tension of carbon skeleton, the classic organic energetic compounds, such as trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), cyclotetramethylene tetranitramine (HMX), hexanitrohexaazaisowurtzitane (CL-20) and octanitrocubane (ONC), generally become increasingly instable, sensitive and costly along with the generation.

"It is frustrated that the best practicable explosive is yet HMX, invented during the Second World War, despite its medium performance and cost." said Prof. Chen.

"The perovskite structure allows each fuel cation to be tightly surrounded by twelve oxidative anions and each oxidative anion surrounded by four fuel cations. Such compact and alternative arrangement of fuel and oxidizer components in a proper ratio is essential to achieve sufficient combustion in a very-short time and then a rapid detonation.

"In addition, we found that the perovskite structure leads to obvious structural tension in the cage whose size is almost not changed by alkaline metal ions with different size. It indicates that organic cations in the cage take steric effect on the anionic frameworks.", explained by Zhang.

Based on the calculation by professional software EXPLO5, the explosive performances, such as detonation heat, detonation velocity, detonation pressure, are improved.

"Beside the classical explanation that energy release is due to the breaking and recombination of chemical bonds during the denotation reaction, the release of the structural tension in the frameworks also make a great contribution to the explosive performances." emphasized by Zhang.

"The perovskite structure is favorable for the stability." added by Zhang, "The new explosive materials have rather high thermal stability and low impact sensitivity, which is good for their storage and transportation."

Strategically, Zhang highlighted "Different from traditional design focusing on the intra-molecular functional groups, we emphasize the inter-molecular assembly in the specified crystal." As he said, low-cost fuel and oxidizer are integrated into highly-symmetric ternary crystals. "This design is rather flexible. Molecular components with suitable shape and size can assemble into such kind of ternary crystals to optimize the oxygen balance, crystal density and so on. Better explosive performances are expected", said Zhang and Chen.

Research Report: "Molecular perovskite high-energetic materials,"

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Science China Press
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