170
5 Vibrational Up-Pumping in Polymorphic Materials
Table 5.6 Band gap values
for the three polymorphs of
FOX-7. Band gaps are
labelled as direct (D) or
indirect (I) in each case, and
values are reported in eV
Material
B3PW91
PBE
HSE06
α-FOX-7
3.9833 (I)
2.4483 (I)
3.6719 (I)
β-FOX-7
4.2252 (I)
2.6643 (I)
3.9169 (I)
γ -FOX-7
4.2462 (I)
2.5866 (D)
3.9430 (I)
5.4.2.2 Electronic Structure
Despite the failings of the ‘band gap’ criterion across the series of molecular energetic
compounds in Sect. 4.5.1, it was nevertheless worthwhile considering this effect for
the polymorphic forms of FOX-7. The electronic structure of the three polymorphs
of FOX-7 were therefore calculated using the same three functionals as in Sect. 4.5.2
and 5.4.1.1 and are summarised in Table 5.6. There does appear to be a very slight
increase in the band gap as the material becomes increasingly layered.
5.4.2.3 Vibrational Up-Pumping in FOX-7 Polymorphs
Note that the phonon dispersion curve for γ -FOX-7 was calculated by Ms S. Piggott.
(Master’s student, EaStCHEM School of Chemistry, University of Edinburgh)
It is hence worth considering the sensitivity of the polymorphs within the uppumping models of impact sensitivity that have been built in Chaps. 3 and 4. The
phonon dispersion curves and associated density of states for all three polymorphs
of FOX-7 are given in Fig. 5.7. Note the imaginary frequency associated with the
Fig. 5.7 Phonon dispersion curves for three polymorphs of FOX-7. The gap between the phonon
bath and doorway modes is highlighted with a green box
5 Vibrational Up-Pumping in Polymorphic Materials
Table 5.6 Band gap values
for the three polymorphs of
FOX-7. Band gaps are
labelled as direct (D) or
indirect (I) in each case, and
values are reported in eV
Material
B3PW91
PBE
HSE06
α-FOX-7
3.9833 (I)
2.4483 (I)
3.6719 (I)
β-FOX-7
4.2252 (I)
2.6643 (I)
3.9169 (I)
γ -FOX-7
4.2462 (I)
2.5866 (D)
3.9430 (I)
5.4.2.2 Electronic Structure
Despite the failings of the ‘band gap’ criterion across the series of molecular energetic
compounds in Sect. 4.5.1, it was nevertheless worthwhile considering this effect for
the polymorphic forms of FOX-7. The electronic structure of the three polymorphs
of FOX-7 were therefore calculated using the same three functionals as in Sect. 4.5.2
and 5.4.1.1 and are summarised in Table 5.6. There does appear to be a very slight
increase in the band gap as the material becomes increasingly layered.
5.4.2.3 Vibrational Up-Pumping in FOX-7 Polymorphs
Note that the phonon dispersion curve for γ -FOX-7 was calculated by Ms S. Piggott.
(Master’s student, EaStCHEM School of Chemistry, University of Edinburgh)
It is hence worth considering the sensitivity of the polymorphs within the uppumping models of impact sensitivity that have been built in Chaps. 3 and 4. The
phonon dispersion curves and associated density of states for all three polymorphs
of FOX-7 are given in Fig. 5.7. Note the imaginary frequency associated with the
Fig. 5.7 Phonon dispersion curves for three polymorphs of FOX-7. The gap between the phonon
bath and doorway modes is highlighted with a green box