Interplay of two magnetic sublattices in related compounds Sm 2 Mn 1- x Ga 6- y Ge y ( x = 0.1-0.3, y = 0.6-1.0) and Sm 4 MnGa 12- y Ge y ( y = 3.0-3.5) with different ordering of empty and filled (Ga,Ge) 6 octahedra.

Single crystals of two new intermetallic phases Sm 2 Mn 1- x Ga 6- y Ge y ( x = 0.1-0.3, y = 0.6-1.0) and Sm 4 MnGa 12- y Ge y ( y = 3.0-3.5) were grown using a self-flux technique. According to single crystal X-ray diffraction data, Sm 4 MnGa 12- y Ge y is characterised by the Y 4 PdGa 12 structure type ( a ∼ 8.65 Å; Im 3̄ m ), while Sm 2 Mn 1- x Ga 6- y Ge y formally adopts the K 2 PtCl 6 structure type ( a ∼ 8.71 Å; Fm 3̄ m ). The general features of both compounds with rather similar crystal structures are represented by the alternation of empty and Mn-filled p-element octahedra, the order of which is determined by the Mn concentration. The diffraction data for Sm 2 Mn 1- x Ga 6- y Ge y reveal a large concentration of Mn vacancies ( x ∼ 0.3), which affects adjacent Ga/Ge atoms leading to their shift towards the vacancy. Both compounds demonstrate two ferromagnetic-like transitions and the presence of two interacting Mn and Sm magnetic sublattices. The Mn sublattice orders at T C1 of 143 K and 318 K, while the Sm one orders at lower temperatures at T C2 of 50 K and 280 K for Sm 4 MnGa 8.6 Ge 3.4 and Sm 2 Mn 0.74 Ga 5.1 Ge 0.9 , respectively. The increase in Mn content not only increases the ordering temperatures, but also dramatically decreases the coercivity μ 0 H C from 230 mT to just 6.5 mT at 2 K. Despite the presence of two magnetically active sublattices in Sm 2 Mn 0.74 Ga 5.1 Ge 0.9 , the magnetic entropy change is quite low and only reaches 0.3 J kg -1 K -1 at T = 300 K and μ 0 H = 5 T, while the estimated relative cooling power (RCP) is about 36 J kg -1 at 5 T.