Family of (NxH)-polytypes with La2WO6-related stoichiometry

Abstract

Structure and chemical composition of non-stoichiometric $\mathrm{(N \times H)}$-polytypes, $\mathrm{N} \in \{3,\,4,\,5,\,6,\,7\}$, belonging to the family of La2WO6-related tungstates, are presented and the basic rules behind their construction are formulated. The polytypes are validated from the point of view of the internal energy per atom against all the competing La-O-W compounds by means of the ternary convex hull computed by DFT. According to DFT, the ground state of the La2WO6 family surprisingly turned out to be the as-yet hypothetical tungstate La2WO6.oP36, isostructural with the known compound Gd2WO6, closely related to the basic building H-block of the polytypes. One more candidate for an experimentally accessible low-energy structure, La2WO6.mC72, derived from the already observed Sm2MoO6, has been identified to lie only $ΔE \approx 5\:\mathrm{meV/at.} \cong 60\:\mathrm{K}$ above the convex envelope.