The high-pressure phase stability of 2,4,6-trinitrotoluene (TNT)

Topological pressure-temperature state diagram of the crystalline dimorphism of 2,4,6-trinitrotoluene

“…This proves that the two polymorphs have an enantiotropic phase relationship and that, in accordance with the Le Chatelier principle, the orthorhombic form must be stable somewhere at low temperature. This assessment is in conflict with the conclusions drawn in previous papers [1][2][3]. However, the value of the enthalpy change associated with the transition is less clear, because the three observed values range from 2 to 4.9 J g -1 ( Table 1).…”

“…The second form crystallizes in a monoclinic system, space group P21/b, with V = 1828.92 Å 3 , Z = 8 and d = 1.650 g cm -3 . A final form was found to be orthorhombic with a volume of 3627 Å 3 (Table 1), however, the structure has not been solved. This latter form resembles the one initially found by Hultgren.…”

“…In 1996, Gallagher et al reported that the crystal structures of the monoclinic and orthorhombic phases had been solved previously by Duke, who could not publish the results, because they had been classified [9]. The authors provided the lattice parameters of the two structures communicated by Duke in 1981 [9]: monoclinic P21/c, with V = 1827.52 Å 3 and Z = 8 leading to d = 1.6511 g cm -3 and the orthorhombic form Pb21a with V = 1843.47 Å 3 and Z = 8 giving rise to d = 1.6368 g cm -3 . In the following years Gallagher et al produces more studies on the crystallization behavior of TNT and they come to the conclusion that even if the monoclinic form contains the lowest internal energy following lattice energy calculations, twinning of the monoclinic crystal tends to occur to a great extent and is only slightly more energetically unfavorable.…”

“…According to Vrcelj et al in 2003, the orthorhombic form is metastable and converts to the stable monoclinic form [1,2]. Bowden et al added to this in 2014: "the orthorhombic phase is metastable at ambient pressure/temperature and was observed at high pressure" [3], while referring to Stevens et al, who in fact had mentioned: "the high pressure-temperature(P-T) stability of TNT has not been investigated in detail" [4].…”

High-Pressure Study of Two Polymorphs of 2,4,6-Trinitrotoluene Using Neutron Powder Diffraction and Density Functional Theory Methods

“…PBE-D2 overestimates the bulk modulus of both the forms due to underestimating the volume at ambient pressure at 0 K. The neutron powder diffraction study failed to find any evidence for a phase transition at ∼2-4 GPa previously suspected from angle dispersive X-ray diffraction 22 and vibrational spectroscopy measurements. 23 Although we cannot discount the potential effects of sample deuteration, we believe that the discrepancy arises due to the nonhydrostatic compression conditions applied in the previous studies. The structural data presented herein are illustrative of a significant advance in the structural characterization of this molecular materials under extreme conditions.…”