Thermochemistry of 3D and 2D Rare Earth Oxychlorides (REOCls) br

The thermodynamic stability of rare earth (RE)materials plays a key role in the design of separation and recyclingprocesses for RE elements. Thermodynamic stability is fundamentallyinfluenced by the lanthanide contraction, as observed in the systematicreduction of unit cell volumes with increasing atomic number. REmaterials are found in the form of solids having primary bonds in threedimensions (3D materials) as well as ones with primary bonds in twodimensions (2D materials) whose layers are held together by weak vander Waals (vdW) forces. While studies of synthesis, structure, andphysical properties of 2D RE materials are numerous, no systematicresearch has compared their thermodynamic stability to that of 3Dmaterials. In the present work, RE oxychlorides (REOCls), whichdisplay a structural transition from a 3D-polyhedral network (PbFCl-type) to a vdW-bonded layered one (SmSI-type) as the RE size decreases, were all synthesized by theflux method. High-temperatureoxide melt solution calorimetry was used to determine their formation enthalpies to enable Born-Haber cycles to calculate latticeenergies. Our results indicate that REOCl compounds are thermodynamically stable when compared to their binary oxides andchlorides. The lattice energies of 3D REOCls increase with decreasing RE size yet are insensitive to unit cell volumes for 2DREOCls. This is caused by interatomic interactions parallel and perpendicular to layers in the SmSI-type REOCls, causing a differentstructure response to the lanthanide contraction than 3D RE materials.