Octahedral niobium cluster-based solid state halides and oxyhalides: effects of the cluster condensation via an oxygen ligand on electronic and magnetic properties

The influences of an oxygen ligand on the structural, magnetic and electronic properties of octahedral niobium cluster-based oxides and oxychlorides are reported. The Nb-6 metal cluster is edge-bridged by twelve inner ligands and additionally bonded to six apical ligands to form (Nb6L12L6a)-L-i units (L = Cl, O) wherein oxygen and chlorine are perfectly ordered. Oxygen favours the interconnection of clusters via double Oi-a/Oa-i bridges in a similar way to the double Si-a/Sa-i bridges found in Chevrel phases based on face capped (Mo6L8L6a)-L-i units. Periodic density functional theory (DFT) calculations confirm that increasing the number of inner oxygen ligands at the expense of chlorine atoms favours the 14 metal-electron (ME) count per octahedral cluster unit. It is also shown that weak interactions occur between neighbouring clusters. Indeed, magnetic measurements performed on A(x)Nb(6)Cl(12)O(2) (A = Rb, x = 0.816(8); A = Cs, x = 1) series containing 15-ME species evidence antiferromagnetic interactions at low temperatures. Broken-symmetry DFT calculations of exchange parameters within spin dimer analysis confirm the experimental results.