  The 4d-transition-metals carbides (ZrC, NbC) and nitrides (ZrN, NbN) in the
rocksalt structure, as well as their ternary alloys, have been recently studied
by means of a first-principles full potential linearized augmented plane waves
method within the local density approximation. These materials are important
because of their interesting mechanical and physical properties, which make
them suitable for many technological applications. Here, by using a simple
theoretical model, we estimate the bulk moduli of their ternary alloys
Zr$_x$Nb$_{1-x}$C and Zr$_x$Nb$_{1-x}$N in terms of the bulk moduli of the end
members alone. The results are comparable to those deduced from the
first-principles calculations.
