The modulated structure of intermediate-valent CeCoGa

CeCoGa was synthesized by melting of the elements in an arc-melting furnace as well as in a sealed niobium tube in an induction furnace. A further annealing step improves the purity and crystallinity of the samples significantly. Its structure was refined on the basis of single-crystal X-ray diffractometer data at different temperatures. Already at room temperature CeCoGa crystallizes in a superstructure of the HT-CeCoAl type. This superstructure can be described in the (3+1)D superspace group C2/m(α0γ)00; α=2/3, γ=1/3 with a temperature independent q-vector (Z=4). For the 300 K data (also for 90 K) the commensurate modulated structure could be refined with 1336 F² values, 56 variables and residuals of wR=0.0348 for the main and wR=0.0605 for the satellites of 1^st order [a=1101.7(1), b=436.0(1) and c=482.4(1) pm, β=103.2(1) ]. Furthermore a description in a transformed 3D supercell with the space group C2/m and Z=12 is possible. For the 90 K data this 3D supercell was refined with 1289 F² values, 56 variables and a residual of wR=0.0409 [a=1618.8(1), b=435.3(1) and c=1094.1(1) pm, β=119.3(1) ]. The relation of the HT-CeCoAl type structure, the (3+1)D modulated and the 3D supercell structure are discussed on the basis of a group-subgroup relation. By thermal analysis and magnetic measurements the phase transition temperature to the HT-CeCoAl type structure is stated slightly above 475 K. Furthermore intermediate cerium valence was identified by the magnetic susceptibility.