Present paper is an overview of our efforts during the past few years to un
derstand complicated corelations of physical phenomena related to pressure
in Fe(II) solid state spin transition systems. Some principal results conce
rning p, T, lambda - experiments are extracted. In the context of correlati
on of the crystallographic phase transition with simultaneous HS --> LS rel
axation and LS --> HS photopopulation, we show the latest results: Brilloui
n and magnetic measurements on the crystal [Fe(ptz)(6)](BF4)(2).