Relativistic effects in general lead to a radial contraction of the s and p orbitals and to an expansion of the d this is particularly true in the case of Hf because of the increase of relativistic effects with Z2.[351 The result is a volume contraction of the Hf phosphide because of its smaller, structure-determining sand p orbitals. However, since the radii used for calculating the bond orders are based on experimental results, and the experiments are necessarily relativistic, relativistic effects are included by comparisons based on radii. Therefore, the significant higher bond orders of Hf,P4 indicate that relativistic effects alone are not responsible for the observed cell contraction.
All viewpoints complement one another and lead to the conclusion that compared with Zr, Hf will be more strongly bonded to itself in metal-rich compounds and more covalently bonded to the nonmetal. In general, these conclusions should also be true for the structural differences of metal-rich compounds of Nb and Ta. In summary, the general trend in the different bonding behavior of Zr and Hf, observed in many examples with very different crystal structures, can also be studied based on isostructural compounds like Zr7P4 and Hf7P4.