Effect of pressure gradient and new phases for 1,3,5-trinitrohexahydro-s-triazine (RDX) under high pressures

Abstract

Herein, pressure-induced phase transitions of RDX up to 50 GPa were systematically studied under different compression conditions. Precise phase transition points were obtained based on high-quality Raman spectra with small pressure intervals. This favors the correctness of the theoretical formula for detonation and the design of a precision weapon. The experimental results indicated that α-RDX immediately transformed to γ-RDX at 3.5 GPa due to hydrostatic conditions and possible interaction between the penetrating helium and RDX, with helium gas as the pressure-transmitting medium (PTM). Mapping of pressure distribution in samples demonstrates that the pressure gradient is generated in the chamber and independent of other PTMs. The gradient induced the first phase transition starts at 2.3 GPa and completed at 4.1 GPa. The larger pressure gradient promoted phase transition in advance under higher pressures. Experimental results supported that there existed two conformers of AAI and AAE for γ-RDX, as proposed by another group. δ-RDX was considered to only occur in a hydrostatic environment around 18 GPa using helium as the PTM. This study confirms that δ-RDX is independent of PTM and exists under non-hydrostatic conditions. Evidence for a new phase (ζ) was found at about 28 GPa. These 4 phases have also been verified via XRD under high pressures. In addition to this, another new phase (η) may exist above 38 GPa, and it needs to be further confirmed in the future. Moreover, all the phase transitions were reversible after the pressure was released, and original α-RDX was always obtained at ambient pressure.

The pressure effects and vibrational properties of energetic material: Hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (α‐RDX)

“…Under pressure, the calculated and experimental trends for NO 2 group and N―N bond modes, such as v 7 and v 21 , coincide well in Figure b,c. Among them, the high wavenumber NO 2 stretch has weak red shift at 3 GPa, which is in agreement with another experimental finds . Similarly, the modes related to C―H···O hydrogen bond (701 and 720 cm −1 ) are also observed weak red shift at 3 GPa, meaning that intramolecular and intermolecular hydrogen bond interactions are changed before phase transition (~4 GPa for experiments).…”

Predicted Melt Curve and Liquid Shear Viscosity of RDX up to 30 GPa

“…RDX solid exhibits a rich polymorphism with six phases confirmed through diffraction experiments. The α form [42] and β form [43] occur near normal conditions, although the β form is highly metastable and readily converts to α. Reversable transformations to high-pressure polymorphs have been identified near room temperature, including the γ form (P ~ 4 GPa) [44,45], the δ form (p ~ 18 GPa) [46], and the ζ form (P ~ 28 GPa) [46]. A highly reactive ε form has been isolated within a narrow range of elevated temperatures and pressures [47] that can be recovered at ambient pressure [48].…”

Pressure-Induced Phase Transition of β-RDX Single Crystals

“…The number of CH 2 stretching vibration modes increases from three to five at 0.4 GPa and further to six at 0.9 GPa. Asymmetric and symmetric modes from the same group are correlative to each other and can be approximately calculated according to the following equations. , where υ s is the frequency of the symmetry stretching mode; υ as is the frequency of the asymmetry stretching mode; , where μ i is the reciprocal of atomic mass m i ; φ is the bond angle; f r is the r bond stretching force constant; f rr is the force constant for the interaction between the bonds; and f r φ is the force constant for the interaction between the bond and bond angle. On the basis of eqs and , six CH 2 vibrational modes at 0.9 GPa are precisely assigned to three pairs of asymmetric and symmetric stretching modes [ν as , ν s ] as 1: [3097, 3012], 2: [3077, 3003], and 3: [3070, 2987], as shown in Figure c.…”

“…According to experimental and theoretical research, RDX exists in six different phases, α-, β-, γ-, δ-, ε-, and ζ-RDX, at different working conditions. − Only α-RDX and β-RDX exist at ambient conditions. The most stable α-RDX phase crystallizes in an orthorhombic structure (space group Pbca ) and possesses a C s symmetry and eight molecules (168 atoms) in the unit cell, as verified by single-crystal neutron diffraction. − It consists of three nitro groups, with two axial and one equatorial to the triazine ring, which is often referred to as the chair AAE conformer. − Thus far, the phase transitions of α-RDX have been studied extensively under high pressure and/or high temperature.…”

“…The most stable α-RDX phase crystallizes in an orthorhombic structure (space group Pbca ) and possesses a C s symmetry and eight molecules (168 atoms) in the unit cell, as verified by single-crystal neutron diffraction. − It consists of three nitro groups, with two axial and one equatorial to the triazine ring, which is often referred to as the chair AAE conformer. − Thus far, the phase transitions of α-RDX have been studied extensively under high pressure and/or high temperature. At about 4 GPa, α-RDX transforms into γ-RDX and further to δ-RDX, ζ-RDX, and η-RDX at pressures of 18.8, 28, and 38 GPa, respectively. − ε-RDX at high temperature and high pressure (489 K, 4.2 GPa) can only exist in a narrow range of pressures and temperatures. − …”