Study of a lacunary solid phase I—Thermodynamic and crystallographic characteristics of its formation

The dehydration of the barium acid oxalate H₂C₂O₄·BaC₂O₄·2H₂O, depending on the conditions in which it is performed, can lead to two different anhydrous salts. That designated αH₂C₂O₄·BaC₂O₄ has all the features of a stable lacunary phase. Its crystalline structure has been determined and the atomic movements during this structure's formation are described. The other variety βH₂C₂O₄·BaC₂O₄ can be prepared either directly from the dihydrate or by transformation of the α form at higher temperature. An infinite number of nonstoichiometric hydrates (designated as H₂C₂O₄·BaC₂O₄·?H₂O with 0 < ? < 2) can be prepared in particular conditions, which identifies the “hydrate-vapor” system studied as divariant. A structural interpretation of this phenomenon is proposed; it takes into account the particular role of water molecules in the coordination polyhedron of the Ba²⁺ cations.