Intermediate-valent CeCoAl - a commensurate modulated structure with short Ce-Co distances
Résumé
CeCoAl was synthesized by melting of the
elements
in a sealed niobium tube in an induction
furnace.
Annealing of the sample gave access to a single
phase sample. Its structure was refined on the basis of single-
crystal X-ray diffractometer data at different temperatures.
Above 271 K CeCoAl crystallizes in its own structure
type in the space group C2/m [a = 1107.4(2), b = 440.6(1)
and c = 479.6(1) pm, β = 104.6(1)°]. Data obtained at 300 K
lead to 511 F 2 values with 20 variables and a residual of
[I ≥ 3σ(I)] wR = 0.0539. Below 271 K satellites give rise to
the superspace group C2/m(α0γ)00; α = 2/3, γ = 2/5 with a
temperature independent q-vector. For the 90 K data (also
for 180 and 220 K) the commensurate modulated structure
could be refined with 4817 F 2 values, 129 variables and
residuals of wR = 0.0347 (main), wR = 0.1927 (satellites 1st
order), wR = 0.1541 (satellites 2nd order) and wR = 0.1768
(satellites 3rd order) [a = 1107.5(1), b = 440.3(1) and
c = 479.0(1) pm, β = 104.7(1)°]. For the three temperatures
only minor variations of the modulation amplitudes are
observed. The relation of the low temperature (3+1)D 3a ×
5c approximant and the room temperature 3D structure
is discussed on the basis of a group–subgroup relation.
By investigation of the heat capacity, the phase transition
could be identified as a second order one with a transition
temperature of 271 K. Magnetic measurements clearly
prove the intermediate cerium valence which is in line
with the short Ce–Co distances.
Domaines
Matériaux
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