Single-crystal X-ray diffraction and neutron powder diffraction analyses were employed to study the reversible intercalation of oxygen in the phase (\mathrm{LaSr}{3} \mathrm{Fe}{3} \mathrm{O}{10-\delta}) and in the mixed (\mathrm{Fe} / \mathrm{Al}) analogue. A single-crystal X-ray diffraction study was done on (\mathrm{LaSr}{3}\left(\mathrm{Fe}{0.8} \mathrm{Al}{0.2}\right){3} \mathrm{O}{8.95}, M_{\mathrm{r}}=695.18), tetragonal, (I 4 / \mathrm{mmm}, a=3.8665(8) \AA, c=28.369(4) \AA, V=424.1(2) \AA^{3}, Z=2, D_{x}=5.44 \mathrm{~g} \mathrm{~cm}^{-3}, M o K \alpha=) (0.71069 \AA, \mu=272.1 \mathrm{~cm}^{-1}, F(000)=625.6, R=0.025) for 227 reflections with (F>5 \sigma(F)). The crystal structure consists of a triple layer of octahedra separated by (\mathrm{La} / \mathrm{Sr}-\mathrm{O}) layers. The oxygen stoichiometry is variable and Rietveld refinement of neutron powder diffraction data in addition to the X-ray study was used to determine the vacancy sites. The vacancies occur primarily in the equatorial layer of the central octahedron. The value of the (c)-axis parameter is a linear function of the oxygen stoichiometry and increases with decreasing oxygen content. It varies from (28.04 \AA) for the phase (\mathrm{LaSr}{3} \mathrm{Fe}{3} \mathrm{O}{9.9}) to (28.52 \AA) for the phase containing 9.2 oxygen atoms. The range of the (c)-axis variation is narrower for the Al-containing phase than for the pure (\mathrm{Fe}) compounds. It ranges from about (28.1 \AA) for (\mathrm{LaSr}{3}) (\mathrm{Fe}{2.4} \mathrm{Al}{0.6} \mathrm{O}{9.3}) to (28.4 \AA) for the phase containing about 8.9 oxygen atoms. Intercalation of oxygen occurs reversibly as a function of temperature. A given oxygen stoichiometry can be obtained by quenching the material from an elevated temperature into liquid nitrogen. The thermal displacement parameter of the cation occupying the central octahedron increases in the (001) plane with increasing oxygen deficiency in the equatorial layer of this octahedron. This displacement is interpreted as motion of the cation into a tetrahedral coordination environment. The range of oxygen stoichiometry for (\mathrm{LaSr}{3} \mathrm{Fe}{3} \mathrm{O}{10-\delta}) is (0.1 \leq \delta \leq 0.8); for the Al-containing phase it is (0.65 \leq \delta \leq 1.1). Single-phase material can be synthesized with (\mathrm{La}, \mathrm{Nd}, \mathrm{Pr}), and Gd but not with Er. No substitution of Sr by Ba or Ca was possible. The solid solution range (\mathrm{Sr}{4-x} \mathrm{La}{5}) is very narrow, (0.96 \leq x \leq 1.04). Ferromagnetic and antiferromagnetic exchange interactions are present in (\mathrm{LaSr}{3} \mathrm{Fe}{3} \mathrm{O}_{10-\delta}); the former predominates at small values of (\delta) and the latter when (\delta) becomes large. 1993 Academic Press. Inc.