Polymerization of nitrogen in cesium azide under modest pressure

Abstract

Alkali metal azides can be used as starting materials in the synthesis of polymeric nitrogen, a potential high-energy-density material. The structural evolutionary behaviors of nitrogen in CsN3 have been studied up to 200 GPa using particle swarm optimization structure search combining with density functional theory. Three stable new phases with C2/m, P21/m, and P-1 structure at pressure of 6, 13, and 51 GPa are identified for the first time. The phase transition to chain like structure (P-1 phase) occurs at a modest pressure 51 GPa, the azide ions N3 (-) (linear chains of three N atoms with covalent bonds and interact weakly with each other) begin to show remarkable polymeric N properties in the CsN3 system. Throughout the stable pressure range, the structure is metallic and consists of N atoms in sp(2) hybridizations. Our study completes the structural evolution of CsN3 under pressure and reveals that the introduced Cs atoms are responsible for the decreased synthesis pressure comparing to pure molecular nitrogen under compression.

Phase transitions of cesium azide at pressures up to 30 GPa studied using in situ Raman spectroscopy

“…The structure with partial orientational disordering of azide anions in Phase III would explain the number of observed Raman-active modes. Furthermore, the structural model with partial orientational disordering of the azide anions accounts for the discrepancy between the pressures of the phase transitions in CsN 3 observed experimentally 23 and theoretically calculated 38 because ab initio calculations 38 are limited to ordered structures.…”

“…The structure of Phase IV might be related to the C2/m structure found to be stable at pressures above 6 GPa in the recent ab initio calculations. 38 Very recently, rubidium azide (RbN 3 ), which is closely related to CsN 3 , was studied at high pressures at room temperature by means of in situ X-ray diffraction. 39 It was found that the first pressure-induced phase transition in RbN 3 occurs at a pressure of 6.5 GPa (significantly higher than the Phase II to Phase III but close to the Phase III to Phase IV transitions in CsN 3 ) in the high-pressure c-phase assigned to the C2/m space group.…”

“…This bonding change might be a reason for the relatively low pressure of nitrogen polymerization in CsN 3 predicted theoretically. 38…”

Effect of pressure on sodium azide studied by spectroscopic method

“…A series of pressure-induced phase transitions were observed in these substances and the interlayered shearing of unit cell and rotation of azide ions were revealed as the pressure below 30.0 GPa [12][13][14][15]17]. With increasing pressure, the azide ions were found to transform into chain like or ring structures then polymeric nitrogen nets [18][19][20][21]. Furthermore, the alkali metal cations were found to donate their electronic to change the electronic properties of nitrogen atom and compounds.…”

Structural phase transition and bonding properties of high-pressure polymeric CaN3

“…Recently, metal azides can be used as the starting materials to synthesize numerous types of potential high-energy-density materials because the synthesis pressure of metal azides is potentially lower than that of pure nitrogen gas. Over the years, various metal azides have been proposed in numerous experimental and theoretical studies, for example, LiN 3 , [13][14][15][16] LiN 5 , 13 NaN 3 , [17][18][19][20][21] KN 3 , 20,[22][23][24][25][26][27] CsN 3 , 23,28 and AlN 3 . 29 Moreover, N 2 H, 30,31 C-N system, 32,33 S-N system, 34 and the P-N system 35 are also studied as high-energy materials.…”