Resolving Composition and Structure of RE–Sb–O–C Natural Superlattice Phases (RE = La, Ho)

A family of rare earth antimonide oxycarbides have been prepared and structurally characterized. These superlattice phases are constructed from NaCl-type RESb slabs sandwiched between RE–O–C layers. Depending on the carbon content and synthetic conditions, three different RE–Sb–O–C structures can be obtained. At lower temperatures,RE9–δSb5(O,C)5 phases are obtained for RE = La, Ho. These phases adopt a stuffed Sc2Sb-type structure with P4/nmm symmetry. An O/C mixture, in which the O/C ratio is larger than 4:1, is randomly distributed within the RE–O–C layers. The RE atoms are highly disordered within the oxide layer. At temperatures above the melting point of the samples, RE9Sb5O4C phases with P4/n symmetry are produced. The RE–O–C layers in RE9Sb5O4 are fully ordered; the RE sites are well defined, and the O and C atoms occupy the tetrahedral and square-pyramidal voids, respectively. At high temperatures, a new ordered La14Sb8O7C structure with P4bm symmetry was discovered. The La14Sb8O7C phase is structurally similar to RE9Sb5O4C and features orderedarrangements of La and O/C atoms in the La–O–C layer. The RE9–δSb5(O,C)5, RE9Sb5O4C and La14Sb8O7C phases appear to be charge-balanced, and their compositions and structures are controlled by the O/C ratio. Parallel preparative experiments revealed the importance of carbon in the formation of these layered phases. In addition, it has been established that the purity of the rare earth metals influences the compositions and structures of the products.