406
M. Reissner
Fig. 8.26 Crystal structure
of filled skutterudites
Table 8.2 Intensities of the two subspectra necessary to fit the Mössbauer spectra of RE x Fe 4 Sb 12
compounds and effective magnetic moments from susceptibility measurements
RE-atom
x
Int1/%
Int2/%
μ eff /μ B
La
0.80
78
22
3.0
Pr
0.73
64
36
4.2
Nd
0.72
69
31
4.5
Eu
0.88
84
16
8.4
Yb
1.00
100
0
4.5
These ternary skutterudites are one example of so-called cage compounds. The
first ternary skutterudite LaFe 4 P 12 was synthesized by Jeitschko et al. in 1977 [65].
Since then a large amount of such compounds were found. As filler atoms (A) trivalent
light RE (La, Ce, Pr, Nd, Sm, Eu) elements and Yb, as well as Th and U were
successfully build into the structure. But also divalent ions as earth alkali (Ca, Sr,
Ba) and also monovalent alkali ions (Na, K) and also Tl could be incorporated into
the structure. Heavy RE skutterudites (Ho, Er, Tm) could be synthezised under high
pressure. In contrast to binary skutterudites, in the ternary compounds also Fe, Os,
and Ru are possible on the M site. Skutterudites are of special interest, because of
their potential for thermoelectric applications [66–70]. Thermoelectric materials can
convert heat in electricity. They are used as power generators in satellites, to cool
computer chips and to convert heat of exhaust fumes of trucks.
To be a good thermoelectric material the figure of merit ZT = S
2 (λ/σ ) with S
the Seebeck coefficient, λ the thermal conductivity and σ the electrical conductivity
should be at least 1. Such high value is possible, if good electrical conductivity is,
against Wiedemann-Franz law, accompanied by low thermal conductivity, which is
possible, if there are low lying Einstein modes in the phonon spectrum, which can
be realized by the filler ions which are only weakly bonded in the oversized cages
and therefore tend to rattle, thus hindering phonon propagation. In that sense filled
skutterudites are archetypes of so-called phonon glass electron crystals, a concept
introduced by Slack in 1995 [71].
M. Reissner
Fig. 8.26 Crystal structure
of filled skutterudites
Table 8.2 Intensities of the two subspectra necessary to fit the Mössbauer spectra of RE x Fe 4 Sb 12
compounds and effective magnetic moments from susceptibility measurements
RE-atom
x
Int1/%
Int2/%
μ eff /μ B
La
0.80
78
22
3.0
Pr
0.73
64
36
4.2
Nd
0.72
69
31
4.5
Eu
0.88
84
16
8.4
Yb
1.00
100
0
4.5
These ternary skutterudites are one example of so-called cage compounds. The
first ternary skutterudite LaFe 4 P 12 was synthesized by Jeitschko et al. in 1977 [65].
Since then a large amount of such compounds were found. As filler atoms (A) trivalent
light RE (La, Ce, Pr, Nd, Sm, Eu) elements and Yb, as well as Th and U were
successfully build into the structure. But also divalent ions as earth alkali (Ca, Sr,
Ba) and also monovalent alkali ions (Na, K) and also Tl could be incorporated into
the structure. Heavy RE skutterudites (Ho, Er, Tm) could be synthezised under high
pressure. In contrast to binary skutterudites, in the ternary compounds also Fe, Os,
and Ru are possible on the M site. Skutterudites are of special interest, because of
their potential for thermoelectric applications [66–70]. Thermoelectric materials can
convert heat in electricity. They are used as power generators in satellites, to cool
computer chips and to convert heat of exhaust fumes of trucks.
To be a good thermoelectric material the figure of merit ZT = S
2 (λ/σ ) with S
the Seebeck coefficient, λ the thermal conductivity and σ the electrical conductivity
should be at least 1. Such high value is possible, if good electrical conductivity is,
against Wiedemann-Franz law, accompanied by low thermal conductivity, which is
possible, if there are low lying Einstein modes in the phonon spectrum, which can
be realized by the filler ions which are only weakly bonded in the oversized cages
and therefore tend to rattle, thus hindering phonon propagation. In that sense filled
skutterudites are archetypes of so-called phonon glass electron crystals, a concept
introduced by Slack in 1995 [71].