The crystal and magnetic structures of LaCa ₂Fe _3-xM _xO ₈ (M=Al, Ga, In)

LaCa ₂Fe ₃O ₈ (A ₃B ₃O ₈) is an example of a layered structure in that it consists of pairs of octahedral, perovskite-like layers alternating with a single tetrahedral layer. This work explores the doping of non-magnetic group 13 elements, M=Al, Ga and In, onto the B-site of LaCa ₂Fe _3-xM _xO ₈ as a function of x. The structural and magnetic effects are examined using a combination of neutron and X-ray diffraction. Solubility limits are established. It is found that for M=Ga the solubility limit occurs between x=1.0 and x=1.25, for the synthesis conditions used, while there is evidence for low (x<0.25) but non-zero substitution of Al. Structural refinements at x=1 suggest that Ga prefers neither the tetrahedral nor octahedral sites. The magnetic structure of LaCa ₂Fe ₂GaO ₈ has been examined using neutron diffraction at 3.2 K and room temperature. At low temperature the staggered moment per Fe ³ is 3.8(1) ^μB in LaCa ₂Fe ₃O ₈ and 4.8(1) ^μB in LaCa ₂Fe ₂GaO ₈. The magnetic space group (P _2b2 ₁′ma′) and moment direction (along c) does not appear to change with Ga substitution.