ON INTERRELATIONSHIPS OF PHYSICAL PROPERTIES OF LANTHANIDE COMPOUNDS - MELTING POINT HEAT OF FORMATION AND LATTICE PARAMETER(S)

The relative heats of formation and the reduced melting temperatures (the melting point of the compound divided by the melting point of the metal) of the lanthanide compounds are correlated with the magnitude of the lanthanide contraction in these compounds relative to the lanthanide contraction in the pure metals. If the lanthanide contraction (as determined from the lattice constants) in the compounds is greater than that observed for the metals, the heat of formation is expected to increase as one proceeds from lanthanum to lutetium. The opposite is expected if the lanthanide contraction is less in the compounds than in the metals. This has been substantiated for the five series for which sufficient thermodynamic data exist, specifically, RCd, RCl3, RH2, RMg and R2O3. For the melting points, 15 of 17 lanthanide compound series show agreement between the observed reduced melting points and that expected from the relative lanthanide contraction. Discrepancies are more likely to occur when correlating the melting points, since only part of the various bonds, RM, MM and RR, are broken upon melting, than for the heats of formation since this quantity is a measure of forming all of the bonds in the crystal. © 1969.