8 Mössbauer Spectroscopy in External Magnetic Fields
409
Na, K, Tl most with Pn = Sb, but also some phosphorous based compounds was performed. Figure 8.28 shows the low temperature spectrum of two phosphorous based
skutterutides PrFe 4 P 12 and NdFe 4 P 12 in zero field. The spectra can be fitted by only
one doublet without any sign of line broadening. This is as expected, because in
the structure only one crystallographic Fe site is present and X-ray diffraction have
confirmed full occupation of the RE sublattice. In contrast for the antimony based
RE compounds low temperature zero-field spectra are slightly asymmetric. This
asymmetry is more pronounced in the spectra taken in external fields (Figs. 8.29 and
8.30). Because of the structure type, texture as reason for the asymmetry can be ruled
out. Two subspectra are necessary to fit the spectra. Only the Yb sample needs only
one subspectrum to fit the data satisfactorily well within the measuring accuracy, in
agreement with the fact that Yb sublattice is fully occupied [95]. The intensities of
the two subspectra is given in Table 8.2. Intensity of the majority subspectrum is
within measuring accuracy in good agreement with the occupation number of the
RE-atoms. Looking on the local surrounding of the Fe atoms there are 6 pnictogenic atoms forming the octahedron in the first shell and 12 Sb atoms in the next
nearest shell together with two electropositive filler atoms (Fig. 8.31). The fact that
especially in the RE sublattice the filling factor is smaller than 1, the Fe atom may
have 0, 1 or 2 filler atoms in the second shell. The other Fe atoms are in a larger
distance and may thus influence the central Fe atom only by a small amount. According to the filling factor x, probabilities concerning the frequency of the respective
surrounding can be calculated by binomial distribution. E.g. for x = 0.9 probabilities
of 0.81, 0.18, and 0.01 are obtained for the case to have 2, 1, or 0 RE atoms in the
next nearest neighbour shell. Since the probability to have no filler atom in the next
nearest neighbour shell is rather small, it can be added to the case to have 1 filler
atom in this shell, thus giving an expected intensity ratio for the two subspectra of 81
to 19. The thus obtained values are in good agreement to the ones obtained from the
Mössbauer fits (Table 8.2). A similar scenario was suggested for the Co based skutterudites Tl 0,8 Co 3 FeSb 12 and Tl 0,5 Co 0,35 Fe 0,5 Sb 12 by Long et al. [96], but called into
question, because of deviations of the observed area ratio of the subspectra from the
ones calculated by statistical distribution. Above 4 T the spectra are fully polarized—
visible by the vanishing of the m = 0 transitions (Figs. 8.28, 8.29 and 8.30). For
the La and Yb compounds which show no magnetic order [76, 95] the values of the
measured hyperfine fields B h f for the subspectrum allocated to the component with
the high intensity either coincide with the one of the applied field B a , or were slightly
larger. Significant deviations from the value of B a were only obtained for Fe atoms
allocated to the spectra with the small area. Similar behaviour is obtained for the
Pr and Nd compounds, although according to bulk magnetic measurements they are
magnetically ordered (ordering 5 K and 13 K for Pr [84] and Nd [76] (Fig. 8.27)). For
the Eu compound an ordering temperature of 84 K is present [76, 77]. Calculated
induced hyperfine fields B ind = B h f − B a are shown in Fig. 8.32. For the La, Pr,
and Nd compounds B ind exhibits some tendency towards saturation at high applied
fields for the Fe site with the low intensity subspectrum, whereas for the Fe atoms
allocated to the high intensity subspectrum B ind scatters around zero (Fig. 8.32). For
Yb the induced hyperfine field is within measuring accuracy also zero. In case of Eu
409
Na, K, Tl most with Pn = Sb, but also some phosphorous based compounds was performed. Figure 8.28 shows the low temperature spectrum of two phosphorous based
skutterutides PrFe 4 P 12 and NdFe 4 P 12 in zero field. The spectra can be fitted by only
one doublet without any sign of line broadening. This is as expected, because in
the structure only one crystallographic Fe site is present and X-ray diffraction have
confirmed full occupation of the RE sublattice. In contrast for the antimony based
RE compounds low temperature zero-field spectra are slightly asymmetric. This
asymmetry is more pronounced in the spectra taken in external fields (Figs. 8.29 and
8.30). Because of the structure type, texture as reason for the asymmetry can be ruled
out. Two subspectra are necessary to fit the spectra. Only the Yb sample needs only
one subspectrum to fit the data satisfactorily well within the measuring accuracy, in
agreement with the fact that Yb sublattice is fully occupied [95]. The intensities of
the two subspectra is given in Table 8.2. Intensity of the majority subspectrum is
within measuring accuracy in good agreement with the occupation number of the
RE-atoms. Looking on the local surrounding of the Fe atoms there are 6 pnictogenic atoms forming the octahedron in the first shell and 12 Sb atoms in the next
nearest shell together with two electropositive filler atoms (Fig. 8.31). The fact that
especially in the RE sublattice the filling factor is smaller than 1, the Fe atom may
have 0, 1 or 2 filler atoms in the second shell. The other Fe atoms are in a larger
distance and may thus influence the central Fe atom only by a small amount. According to the filling factor x, probabilities concerning the frequency of the respective
surrounding can be calculated by binomial distribution. E.g. for x = 0.9 probabilities
of 0.81, 0.18, and 0.01 are obtained for the case to have 2, 1, or 0 RE atoms in the
next nearest neighbour shell. Since the probability to have no filler atom in the next
nearest neighbour shell is rather small, it can be added to the case to have 1 filler
atom in this shell, thus giving an expected intensity ratio for the two subspectra of 81
to 19. The thus obtained values are in good agreement to the ones obtained from the
Mössbauer fits (Table 8.2). A similar scenario was suggested for the Co based skutterudites Tl 0,8 Co 3 FeSb 12 and Tl 0,5 Co 0,35 Fe 0,5 Sb 12 by Long et al. [96], but called into
question, because of deviations of the observed area ratio of the subspectra from the
ones calculated by statistical distribution. Above 4 T the spectra are fully polarized—
visible by the vanishing of the m = 0 transitions (Figs. 8.28, 8.29 and 8.30). For
the La and Yb compounds which show no magnetic order [76, 95] the values of the
measured hyperfine fields B h f for the subspectrum allocated to the component with
the high intensity either coincide with the one of the applied field B a , or were slightly
larger. Significant deviations from the value of B a were only obtained for Fe atoms
allocated to the spectra with the small area. Similar behaviour is obtained for the
Pr and Nd compounds, although according to bulk magnetic measurements they are
magnetically ordered (ordering 5 K and 13 K for Pr [84] and Nd [76] (Fig. 8.27)). For
the Eu compound an ordering temperature of 84 K is present [76, 77]. Calculated
induced hyperfine fields B ind = B h f − B a are shown in Fig. 8.32. For the La, Pr,
and Nd compounds B ind exhibits some tendency towards saturation at high applied
fields for the Fe site with the low intensity subspectrum, whereas for the Fe atoms
allocated to the high intensity subspectrum B ind scatters around zero (Fig. 8.32). For
Yb the induced hyperfine field is within measuring accuracy also zero. In case of Eu