Intermediate-valent CeCoAl – a commensurate modulated structure with short Ce–Co distances

Abstract

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.

The modulated structure of intermediate-valent CeCoGa

“…For CeCoAl a phase transition temperature of 271 K could be determined for the transition from the subcell (HT-CeCoAl type structure) to the commensurate modulated structure (LT-CeCoAl), which was classified as phase transition of second order according to a λ -anomaly in the heat capacity [42]. Due to the almost identical behavior of CeCoGa, the same phase transition can also be expected for this compound at higher temperatures.…”

“…The starting atomic parameters were deduced using the charge-flipping algorithm of Superflip [46] and the CeCoGa structure was refined with anisotropic displacement parameters for all atoms based on F 2 with Jana2006 [47]. As already known for CeRuSn [26] and CeCoAl [42] an enhanced U 33 (approx. 5 × U 11 ) parameter of 1060 pm 2 for the Co site is observed.…”

“…So far, no temperature-dependent diffraction experiments and only preliminary physical property studies have been reported [37][38][39][40][41]. In extension of our detailed studies on CeRuSn we have recently investigated the CeCoAl structure [42]. The aluminide shows a secondorder phase transition at 271 K and the low-temperature structure can be described in the (3 + 1)D space group C2/m(α0γ)00 with q = (2/3, 0, 2/5).…”

Revisiting Ce3Pt4Ge6– crystal structure and physical properties

“…This is achieved by a transition of index a4 where a is introduced synonymously for approximant. 39 Here two transitions are involved, a doubling of the a axis along with a loss of the centering and an origin shift of (1/4, 1/4, 0). The a4 transition is paralleled by two k2 transitions in 3D space.…”

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.…”