Some new compounds with garnet-related structures have been examined by 57Fe Mössbauer spectroscopy. The results show that in compounds of the type YCa2SbFe4-xGaxO12 (x = 2, 3) the gallium is distributed over both the octahedral and tetrahedral sites. In these compounds, together with materials of composition Y3-2xCa2xSbxFe5-xO12 (x = 1.25, 1.5) and Y3-xCaxSnxFe5-xO12 (x = 1, 2), the results show evidence of more than one tetrahedral environment for the Fe3+ ions. The quadrupole splitting data for the Fe3+ ions in tetrahedral sites in some of these compounds are significantly larger than those previously reported for Fe3+ in tetrahedral sites in other garnets. The compounds YCa2SbFe4O12 and Y2CaSnFe4O12 magnetically order at similar temperatures and show comparable octahedral-and tetrahedral-hyperfine magnetic fields at 18 K. The substitution of Fe3+ by diamagnetic Sb5+ on the octahedral sites results in a considerable lowering of the magnetic ordering temperature. The compounds Y0.5Ca2.5Sb1.25Fe3.75O12 and Ca3Sb1.5Fe3.5O12 show magnetic ordering on both the octahedral and tetrahedral sublattices at 18 K. A similar effect is observed in the compound YCa2SbFe3GaO12 where 33% of the Fe3+ ions located at the tetrahedral sites are substituted by diamagnetic Ga3+ ions. The importance of a-d antiferromagnetic superexchange interactions is demonstrated in compounds of the type YCa2Sb Fe2Ga2O12 and YCa2SbFeGa3O12 where the Ga3+ ions substitute the Fe3+ ions on both the octahedral and tetrahedral sites. In these compounds the dilution of the magnetic ions at the tetrahedral sites results in the frustration of magnetic ordering on both the d-and a-sublattices. The compound YCa2Sn2Fe3O12 which does not contain Fe3+ on the octahedral sites shows magnetic ordering on the d-sublattice in the absence of magnetic ions on the a sublattice. The result demonstrates the importance of d-d antiferromagnetic superexchange interactions. © 1996 Academic Press, Inc.