Abstract:
Properties of the lanthanide–oxygen bonds in lanthanide niobates LnNb$_7$O$_{12}$ (Ln = La, Ce, Pr, Nd, Pm, Sm, Eu) have been studied using the ab initio cluster X$_\alpha$ discrete variational method with two different basis sets of molecular orbitals. It has been found that the structural instability of lanthanides LnNb$_7$O$_{12}$ (Ln = Nd, Sm) observed in experiments is due to the combined effect of decrease in the stability of [LnO$_8$]$^{13-}$ cluster and increase in quantum-mechanical Ln–O repulsion when the minimum near neodymium (in the first model of the basis set of molecular orbitals) or praseodymium (in the second model of the basis set of molecular orbitals) atoms is passed.
