Long-Range Order and Short-Range Order inPd3V: Breakdown of the Mean-Field Theory

Diffuse scattering experiments of electrons, neutrons, and x rays have been performed on ${\mathrm{Pd}}_3$V. They all give rise to intensity maxima at positions (100), which indicates a local order different from that of the low-temperature $D{0}_{22}$ ordered phase, characterized by (1 ½ 0) diffraction spots. This contradicts the predictions of the usual mean-field theory. By use of the cluster variation method (octahedron-tetrahedron approximation) it is shown that this surprising effect can be explained in a model with first-and second-neighbor interactions $V_1$, $V_2$, of the same sign, with $\frac{V_2}{V_1}\ll 1$.