Indium segregation in Gd5(Si, Ge)4 magnetocaloric materials

•Indium chemical substitution is promising for tuning and for unveiling magnetocaloric atomic scale behaviour.•Independently of the In substitution on Gd5Si1.2Ge2.8 (by Si, Ge, and Gd), In is insoluble in the magnetocaloric 5:4 phase.•Indium promotes and segregates into the 5:3 phase leading to a change of Si/Ge ratio on the 5:4 phase.•In substitution promotes: atomic disorder, unit cell expansion and a reduction of Gd5Si1.2Ge2.8 magnetocaloric effect.