Variations in lattice parameters with annealing temperature for L-Pd₅Ce

The lattice parameters of L-Pd₅Ce have been determined as a function of annealing temperature. L-Pd₅Ce has a one-dimensional long-period superstructure derived from L1₂, and the long periodic direction is along [110] of the L1₂ lattice. At 873 K, the distance between adjacent anti-phase boundaries, or the domain size, is almost exactly M≡3 in units of the lattice spacing of (110) planes of L1₂ (≡=_co), and the crystal structure is Pmma with the lattice parameters a=0.5700 mn, b=0.4062 nm and c=3_co=0.8462 nm. The axes correspond to [110], [001] and [110] respectively of L1₂. When M is even, the space group of the structure is regarded as Bmmm for the above setting of the axes. As the annealing temperature is elevated to 973 K, the quantities a/2^1/2, b and 2^1/2c_o, which correspond to the lattice dimensions of the pseudocubic lattice of L1₂, then have an identical value to one another. The variation in lattice parameters is explained in terms of a rapid increase in M associated with atomic disordering. The observed lattice parameters indicate that the averaged atomic volume varies less than 0.2%. The small variation cannot be a cause of the distinct dependence of electrical resistivity upon the annealing temperature.