Magneto-structural coupling in [Formula: see text].
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Two interactions, the Jahn-Teller interaction and the spin-Peierls-like interaction are co-exist in [Formula: see text] system.From these measurements the crystal structures are determined.The full width at half maximum and integrated intensity give the fruitful information for magnetic elastic interactions.
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PubMed Central - PubMed
Affiliation: Shahjalal University of Science and Technology, Sylhet, 3114 Bangladesh.
ABSTRACT
[Formula: see text] compound is well Known to show the frustration of the spin structure. At 12 K, [Formula: see text] distorts to break symmetry of the degenerated frustrated spin states by the spin-Peierls-like phase transition, accompanying with the antiferromagnetic ordering. On the other hand, [Formula: see text] undergoes a Jahn-Teller phase transition at a temperature of 310 K, differing from the low temperature ferrimagnetic transition temperature [Formula: see text] of about 60 K. It is also reported that [Formula: see text] shows another magnetic phase transition at about 30 K. These two phase transitions accompanying with the lattice change can be understood by the magneto-elastic interactions. Two interactions, the Jahn-Teller interaction and the spin-Peierls-like interaction are co-exist in [Formula: see text] system. In this report the [Formula: see text] compounds with x = 0.8, 0.6 and 1 are investigated by the X-ray diffraction measurements. From these measurements the crystal structures are determined. The full width at half maximum and integrated intensity give the fruitful information for magnetic elastic interactions. No MeSH data available. Related in: MedlinePlus |
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Mentions: The lattice constants obtained by the Rietveld refinement which are listed in Tables 2, 3 and 4 are plotted against temperature in Fig. 5. As shown in the table or in the figure the lattice constants a and b take nearly same value at 30 K. This, however, should be accidental. Because at 15 K, a and b take different values again. The (440) reflection is measured at various temperatures between 15 and 300 K. The spectrum was fitted to a Pseudo-Voigit function and the temperature dependence of the lattice constants is obtained and shown in Figs. 6, 7 and 8 and also I.I. is shown in Fig. 9. The FWHM, of the spectrum, together with that of the (440) reflection and (440) reflection are shown in Fig. 10. Here for compound the measurement was carried out twice. The two results are almost same, though there is a small scattering. At 300 K, the FWHM of compound is much smaller than that of . This is because the structural phase transition of is just at higher temperature of 310 K. But this result suggests that the homogeneity of the compound is rather good. At 250 K, the FWHM of is almost same as that of . It also supports the good homogeneity of compound. With decreasing the temperature the FWHM of starts to increase and reach the sharp peak at 160 K, then decreases rather rapidly down to about 40 K and finally again starts to increase below about 30 K. The result of the Rietveld fitting shows that the low temperature structures can be orthorhombic. In the distorted phase the S value is much larger than 2, suggesting the worse fitting of Rietveld. We have to discuss the origin of this worse Rietveld fitting. Doping the ion in place of , results in the mixed compound of and . In the mixed compound, the local distortion of and separately occurs so as to minimize the crystal distortion energy. Then as M. Kataoka and J. Kanamori has already mentioned in their theoretical work on system Kataoka and Kanamori (1972), the elongated c-axis of will align along one of a-axes of , say a- or b-axis which are also elongated in . If the alignment of c-axis prefers to a particular axis, say a-axis, a and b axes are not equal, so the result is orthorhombic. When the distorted with and the distorted with are mixed in compound below the structural transition temperature, the FWHM can be expected to increase. In Fig. 8 vs. T is plotted. Here is the average values of lattice constants a and b obtained from the reflection (440) measurement. The figure shows that the structure transition seems to be at 160 K which corresponds to the peak temperature of FWHM. With decreasing temperature, increases and is goes to saturate at low temperatures, while the FWHM decreases quickly. This result can be understood as follows. At the transition temperatures, the c-axis of local structure aligns rather randomly and with lowering the temperature the alignment becomes more stable to one of a-axes. In present Rietveld fitting, the S value is rather large in the distorted phase. The FWHM in starts to increase rather sharply below about 30 K (Fig. 10). This behavior is similar to one in below about 60 K which corresponds to the ferrimagnetic ordering temperature. In , the ferrimagnetic transition accompany with the crystal distortion from the tetragonal symmetry to the orthorhombic. is however, orthorhombic structure above the ferrimagnetic transition. So only the lattice constants can be changed with spin order. As shown in Fig. 7, the lattice constants a or b do not show the drastic change at the ferrimagnetic order, but the c axis as shown in Fig. 6 shows the sudden drop at the ordering temperature. The I.I. of in Fig. 9 also shows the sudden increase at about 25 K. The Rietveld fitting S value at 15 K shows a large value of 2.15. But when we analyze the spectrum measured at 15 K by using the model of Fddd and symmetry mixing, S value is 1.81, not so small but much smaller than the S = 2.15 obtained assuming only structure model for the same spectrum at 15 K. In the distorted phase between 30 and 160 K, it is better to analyze the spectrum by using the model of Fddd and a slight mixing of . At about 16 K, the I.I. also shows the abrupt drop and the lattice constant c and a, b also drop at this temperature and the FWHM almost saturate below this temperature. From the temperature variation of I.I. of and that of , it is found that these two figures are very similar to each other. At about 150 K and 60 K , I.I. decrease very sharply and at 25 K and 20 K , I.I. abruptly increase. The phase transition of at 60 K is the ferrimagnetic order accompanied with the structural change from tetragonal to orthorhombic phase. So, the structural phase transition is due to the magneto-elastic interaction. The structural phase transition of at 160 K may also be due to the magneto-elastic coupling, that is, the collaboration of JT interaction due to the ion and spin-Peierls-like interaction due to the ion. The phase transition of at 25 K is the ferrimagnetic transition, but it also accompany with the structure change. But it already has the orthorhombic symmetry. So it also does not change the symmetry and just changes the lattice constants, similar to the phase transition which occurs at 30 K in . In Ni, the ferrimagnetic transition occurs at 60 K, but it accompany with the crystal symmetry change. In the antiferromagnetic transition without the crystal symmetry change occurs at 30 K. In another phase transition also occurs at about 16 K which should be antiferromagnetic transition similar to one at 30 K in . |
View Article: PubMed Central - PubMed
Affiliation: Shahjalal University of Science and Technology, Sylhet, 3114 Bangladesh.
No MeSH data available.