Itinerant and local magnetism, superconductivity and mixed valency phenomena in RM₂Si₂, (R = rare earth, M = Rh, Ru)°

X-Ray, magnetization and Mossbauer (¹⁵¹Eu, ¹⁵⁵Gd, ¹⁶¹Dy and dilute ⁵⁷Fe) studies of RM₂Si₂ reveal that when R is a magnetic ion the compounds order antiferromagnetically. For M = Rh a second antiferromagnetic phase transition is observed, corresponding to Rh itinerant electron magnetic ordering. In EuRh₂Si₂ the Eu ion is predominantly divalent with a mixed valent component. In EuRu₂Si₂ the Eu is predominantly trivalent. LaRu₂Si₂ and LuRu₂Si₂ display enhanced electron paramagnetism and become superconducting at 3.5 K and 2.4 K respectively. LaRh₂Si₂, YRh₂Si₂ and LuRh₂Si₂ display an itinerant electron magnetic phase transition, T_M (LaRh₂Si₂) = 7 K, and at lower temperatures a superconducting phase transition, T_c(LaRh₂Si₂) = 3.8 K. There is evidence that in the superconducting phase the itinerant magnetic order survives.