High-Pressure Investigation of 2,4,6-Trinitro-3-bromoanisole (TNBA): Structural Determination and Piezochromism

Understanding
phase transitions in energetic materials is crucial
for developing predictive models of detonation. 2,4,6-Trinitro-3-bromoanisole
(TNBA), an energetic material, was studied in its single-crystal form
up to pressures of 45 GPa in a diamond anvil cell. The material was
characterized by using X-ray, Raman, and optical transmission measurements.
From single-crystal X-ray diffraction, the ambient structure of TNBA
was determined which crystallizes in the P21/c space group having four molecular units per unit
cell. The X-ray data up to 9.2 GPa were fitted to a third-order Birch–Murnaghan
equation of state by using the parameters K0 = 13.2(2.4) GPa and Kp = 5.1(1.4). Between
6.8 and 7.3 GPa, a phase transition was inferred in TNBA from concurrent
fading of X-ray diffraction, disappearance of Raman peaks, increase
in sample fluorescence, and discontinuous color change. The new phase
was consistent with an amorphous state of at least partially intact
molecules judging from the presence of higher-order Raman modes and
irreversibility of the Raman spectra upon release. Piezochromism was
observed with the translucent yellow TNBA gradually darkening and
becoming opaque black at ∼25 GPa. This correlated to the absorption
edge gradually shifting to the red in the visible spectrum. Signs
of two possible additional structural transitions were detected in
the 32.4–41.0 GPa range as suggested by a jump in the absorption
edge, the irreversible changes in the absorption spectrum upon release
to ambient pressure, and by the lack of Raman modes in recovered samples.
The crystal and electronic structures of TNBA were also investigated
up to 10 GPa by using DFT calculations and crystal structure prediction
(CSP) simulations. In agreement with the experimentally observed transition
at 7 GPa, the simulations at 10 GPa found a bevy of polymorphs lower
in enthalpy and higher in density than P21/c. The lowest calculated enthalpy structure was
determined to be P212121, being in a different space group than the ambient experimental
result.