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Phase behavior of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is investigated by X-ray powder diffraction (XRD). The XRD patterns at elevated temperature show that there is a co-existing temperature range of b-and d-phase during the phase transition process. Additionally, mechanical forces can catalyze the conversion from d-back to b-phase. Based on the diffraction patterns of b-and d-phase at different temperatures, we calculate the coefficients of thermal expansion by Rietveld refinement. For b-HMX, the linear coefficients of thermal expansion of a-axis and b-axis are about 1.37 Â 10 À5 and 1.25 Â 10 À4 8C À1 . A slight decrease in c-axis with temperature is also observed, and the value is about À 0.63 Â 10 À5 8C À1 . The volume coefficient of thermal expansion is about 1.60 Â 10 À4 8C À1 , with a 2.2% change from 30 to 170 8C. For d-HMX, the linear coefficients of thermal expansion of a-axis and c-axis are found to be 5.39 Â 10 À5 and 2.38 Â 10 À5 8C À1 , respectively. The volume coefficient of thermal expansion is about 1.33 Â 10 À4 8C À1 , with a 2.6% change from 30 to 230 8C. The results indicate that b-HMX has a similar volume coefficient of thermal expansion compared with d-HMX, and there is about 10.5% expansion from b-HMX at 30 8C to d-HMX at 230 8C, of which about 7% may be attributed to the reconstructive transition.

Si-Jia(刘思佳) a) , Peng Tong-Jiang(彭同江) a) , Song Zhen(宋 振) b) , Bian Liu(边 柳) b) , Song Gong-Bao(宋功保) a) , and Liu Quan-Lin(刘泉林) b) †

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