Alkali halides (MX) are among the simplest binary systems to study under compression. The charge transfer from the alkali metal, M, to the halide, X, forming formally M + X -compounds, generates ions with closed-shell electronic configurations. Consequently, M + X -compounds are often described by rigid-sphere models, allowing for the derivation of intuitive trends based exclusively on the ionic size. However, this description might be over simplistic. In fact, already in the 1960s, J. B. Goodenough proposed that interactions between ions of the same kind, including cation-cation contacts, are important and should be taken into account. Alkali halides have often been used as model systems to investigate general trends at high pressure. The B1-B2 phase-transition is probably one of the most studied phenomenon of this class of materials, being measured multiple times in the past, sided by several theoretical analysis and transition mechanism studies. In particular, the B1-B2 transition in alkali fluorides is intriguing for many aspects: 1) The B1 phase exists for all the alkali metals at ambient pressure, including cesium (LiF, NaF, KF, RbF, CsF ); 2) the B2 phase transition occurs at the almost identical pressure for KF, RbF and CsF (between 1 -4 GPa), while 3) the B2 phase transition of NaF is triggered at much higher pressure (~ 28 GPa). Finally, 4) there is still no experimental evidence for a B2 phase-transition for LiF, which remains a matter of discussion among scientists. The typical highpressure "rules-of-thumb" would predict a linear lowering of the phase-transition's pressure moving from light to heavy elements, and not a discontinuity followed by a constant pressure value, as observed experimentally.