2020 Volume 61 Issue 8 Pages 1480-1482
This study presents results on crystal structure and phase transition in La1−yCeyFe11.44Si1.56 (y = 0.1 and 0.3) compounds. All the samples were prepared by using arc-melting method in argon atmosphere. The effect of temperature was studied by X-ray diffraction method. As shown, the compounds possess clear NaZn13-type cubic phase with ferromagnetic spin arrangement of Fe atoms. There reveals a sudden change of lattice constants at 195 K (y = 0.1) and 175 K (y = 0.3) which corresponds to type 1 phase transition.
Fig. 2 Variation of peak (422) maxima according to T for (a) y = 0.1 and (b) y = 0.3.
The magnetocaloric effect (MCE) created by interaction between lattice and spin dynamics of a ferromagnetic system putting in varying applied magnetic field couples with large change of temperature of system and heat transfer with environment in many LaFe13 type alloys. Although the relatively large changes of magnetic entropy (ΔSm) are also detected in many types of magnetic materials such as perovskites ABO3, Heusler alloys, rare earth-transition metal alloys,1–7) the LaFe13 compounds with NaZn13 type cubic structure are still considered as ones of best candidates for application in modern gas free refrigeration technology. Recently, the partial replacements of La by other rare earth elements (R) are reported to show improvements on MCE of doped compounds La1−yRyFe13−xSix.8–11) The achievement of larger MCE in lower magnetic field is very important for application in order to improve both cooling performance and mechanical endurance of materials, together with effective controls of working temperature (i.e. magnetic phase transition temperature TC), while lowering the costs. Therefore, in this study we report the effect of temperature on the crystal structure of La1−yCeyFe11.44Si1.56 (y = 0.1 and 0.3) compounds to shed lights on structural phase transition within the temperature range of MCE regime.
The La1−yCeyFe11.44Si1.56 (y = 0.1 and 0.3) compounds were prepared from the precursor materials consisting of purified metallic element (La, Ce 99.9%; Fe 99.99%; Si 99.999%) by using arc-melting method in argon atmosphere with pressure P = 10−5 Torr. Because the rare earth elements La and Ce could easily evaporate during the melting process, they were supplied with 5% excess in mass to compensate the possible loss. After the precursors were melted together, the samples were inserted into a sealed quartz tube vacuumed at 10−5 Torr and incubated at 1100°C for 7 days. After that, the samples were fast cooled by immediately putting in ice water. The crystal structure of powder compounds were studied by X-ray diffraction method in varying temperatures from 100 to 295 K, with Cu-Kα radiation of wavelength λ = 1.54056 Å.
The X-ray diffraction diagrams of La1−yCeyFe11.44Si1.56 (y = 0.1 and 0.3) measured at different temperatures are shown in Fig. 1. As seen, the structures do not change a lot from the original NaZn13 phase for both doped cases, but shifts of positions signifying the lattice parameter variation can be recognized. Looking closer at the peak (422) (Fig. 2) for a sample with y = 0.1 we can reveal that the position of this peak does not change very much in the temperature range from 100 to 190 K but a sudden shift Δ(2θ) ≈ 0.15° to the right can easily be recognized at 195 K. This means that the lattice constant reduces ≈0.07 Å correspondingly as temperature increases from 190 (a = 11.544 Å) to 195 K (a = 11.474 Å). Above 195 K the lattice undergoes a regular thermal expansion. Under normal thermodynamic conditions the increase of temperature always leads to the increase of lattice constant, referring to as the thermal expansion of lattice. However, what we express at 190–195 K is an inverse process of lattice collapse under increase of temperature, which naturally argues that an expansion below 190 K is certainly due to the Coulomb repulsion caused by ferromagnetic spin arrangement below Curie temperature TC (193 K10)).
X-ray diffraction patterns of the samples (a) y = 0.1 and (b) y = 0.3 at different temperatures.
Variation of peak (422) maxima according to T for (a) y = 0.1 and (b) y = 0.3.
A similar picture is also seen for a sample with y = 0.3 (Fig. 2). Here a collapse of lattice appears earlier at T = 170 K (corresponds to a bit lower TC of this sample, 175 K10)) and is ≈0.031 Å, i.e. from a = 11.497 to 11.466 Å. From above 180 K this sample undergoes a clear thermal expansion and at 290 K the lattice constant relaxes to a = 11.471 Å.
The evolution of lattice constants for both doped cases is summarized in Fig. 3 where a temperature of structural phase change Tp is shown with respect to the Curie temperature TC for individual case. The results show that despite the change of lattice constant, the symmetry remains unchanged and the compounds are still having the NaZn13 original symmetry of the undoped compound.
Dependence of lattice constant a on the temperature for La1−yCeyFe11.44Si1.56 (y = 0.1 and 0.3).
Therefore it is clear that around TC both compounds undergo the first order phase transition associated with the volume expansion caused by ferromagnetic spin arrangement below TC. For La0.9Ce0.1Fe11.44Si1.56 the lattice constant expands about 0.61% at TC in comparison with that of 0.27% for La0.7Ce0.3Fe11.44Si1.56. A larger is Ce substituted content a smaller is volume expansion at TC. The larger structural phase transition is usually connected to a more efficient MCE.
We have successfully fabricated the compounds of composition La1−yCeyFe11.44Si1.56 (y = 0.1 and 0.3) and investigated their structures according to temperature change from 100 to 295 K. The results clearly show the first order phase transition around their Curie temperature TC and that the volume expansion below TC should be accompanied by the ferromagnetic arrangement of spin. The study reveals an important role of Ce doping in LaFe13 compounds, as the larger Ce content led to a smaller volume expansion below TC.
This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2017.326.
