SrFeO2 bears a square planar oxygen coordination around Fe2+. The rarity of this structure led us to study the substitution of strontium by calcium and we obtained the solid solution Sr1-xCaxFeO2 (0 =< x =< 1) that undergoes a structural phase transition around x = 0.9. In SrFeO2, strontium has a rectangular parallelepiped 8-fold oxygen coordination while in CaFeO2, calcium has an octahedral coordination. The investigation of the expansion the solid solution at low temperature led us to discover a negative thermal expansion behavior (not occuring in SrFeO2 and CaFeO2). The phenomenon being correlated to the solid solution containing disordered Sr2+ and Ca2+ sites requires to be studied using the GEM beamline which specialises in understanding the structure of disordered materials. The structure of two compounds (one with NTE and one without) will be investigated at various temperature.