The modulated structure of intermediate-valent CeCoGa

Abstract

International audienceCeCoGa 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 1st 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

A ZrNiAl related high-pressure modification of CeRuSn

“…The transformation from (3 + 1)D to the √3a × √3a × 3c approximant structure is classified as an a3. 54,55 Unfortunately, no refinement of HP-CeRuSn is possible in this 3D approximant structure, which is most likely due to the lack of a sufficient amount of satellite reflections. In this case the data analyses with the (3 + 1)D superspace approach is superior to the 3D superstructure refinement.…”

“…The indices for the translationengleiche (t ) and klassengleiche (k) symmetry reductions, the unit cell transformations, as well as the evolution of the atomic parameters are given. The index a54,55 is introduced for the formation of the approximant of the modulated structure.…”