Long-Period SuperlatticePd3Mn II and Its Large Tetragonal Distortion

The origin of the stabilization of the long-period structure in a transition-metal alloy and of the large tetragonal distortion which is a function of the composition in such an alloy series has been investigated. For this purpose characteristics of the long-period superlattice ${\mathrm{Pd}}_3$Mn II were investigated over a wide composition range in relation to its neighboring phases in the Pd-Mn system. It was found that a ${\mathrm{Cu}}_3$Au-type super-lattice exists at Mn compositions below 25 at.%, while for Mn contents above the ${\mathrm{Pd}}_3$Mn II range, a CuAu-type superlattice is formed. The concentration range with the CuAu-type ordered structure had previously been reported as an independent ${\beta }_1$ phase with a tetragonally distorted CsCl-type structure. Moreover, as the Mn content increases, the cubic $A_{3}B$-type superlattice was found to transform continuously to the $\mathrm{AB}$-type superlattice through the long-period structure without the formation of a two-phase region. ${\mathrm{Pd}}_3$Mn II has a fixed domain size of $M=2\mathrm{}$, irrespective of the composition in its range of stability. This structure was found to be stabilized by a tetragonal distortion which keeps the Brillouin-zone boundaries at the Fermi surface as the electron-atom ratio varies, instead of by changing the period of the structure as observed in other long-period superlattices. The large distortion which occurs above the stoichiometric composition is sustained by a preferential distribution of the excess Mn atoms. From these observations it was concluded that the origin of the stability in ${\mathrm{Pd}}_3$Mn II is similar to that in the noble-metal alloys with long-period structures, and hence the number of conduction electrons of the Mn and Pd atoms could be deduced to be 3 and 0.6, respectively.