Preface to Volume 18............................................................................................v Contents........................................................................................................ix Contents of Volumes 1–17..................................................................................xi Contributors....................................................................................................xv 1. Magnetic Properties of Filled Skutterudites H. Sato, H. Sugawara, Y. Aoki and H. Harima.....................1 2. Spin Dynamics in Nanometric Magnetic Systems David Schmool..................................................111 3. Magnetic Sensors: Principles and Applications Pavel Ripka and Karel Záveta.............................347 Author Index....................................................................................................421 Subject Index...................................................................................................453 Materials Index.................................................................................................455
H. Sato, H. Sugawara, Y. Aoki and H. Harima
Contents
1. Introduction 2 2. Synthesis of Filled Skutterudites 3 2.1. Synthesis under ambient pressure 4 2.2. Synthesis under high pressures 6 3. Background that Realizes Unique Behaviors in the Filled Skutterudites 8 3.1. Characteristics of the band structure and strong c-f hybridization 8 3.2. The interaction among multipoles via a main conduction band 11 3.3. Crystalline electric field effect 13 4. Filled Skutterudite with Rare Earth or Actinide Elements 15 4.1. La-based filled skutterudites 15 4.2. Ce-based filled skutterudites 19 4.3. Pr-based filled skutterudites 26 4.4. Nd-based filled skutterudites 50 4.5. Sm-based filled skutterudites 54 4.6. Eu-based filled skutterudites 64 4.7. Gd-based filled skutterudites 65 4.8. Tb-, Dy-, Ho-, Er-, and Tm-based filled skutterudites 66 4.9. Yb-based filled skutterudites 68 4.10.Filled skutterudites with actinoid ions 69 5. Other Filled Skutterudites 72 5.1. AT4Sb12 (A = alkaline and alkaline earth; T = Fe, Ru, Os) 72 5.2. Ge cage-forming filled skutterudite APt4Ge12 (A = Sr, Ba, La, Ce, Pr, Nd, Eu, U, Th) 75 5.3. YT4P12 (T = Fe, Ru, Os) and I0.9 Rh4Sb12 78 6. Summary 78 Acknowledgments 79 References 80 Appendix A 92
1. Introduction
The word "skutterudites" was first used by Haidinger (1845) as the name of a new mineral with chemical formula (Co, Ni, Fe)As3 found in a cobalt mining at "Skutterud", Modum, Norway. Nowadays, "skutterudite" is used as a name for a series of cubic compounds with chemical formula TX3 (T = Co, Ni, Rh, etc.; X = pnictogen). The crystal structure shown in Fig. 1.1a was first determined by Oftedal (1927). The main subject of this section are the filled skutterudites AT4X12 which were synthesized by Jeitschko and Braun (1977) for the first time in the course to search for new ternary metal-P compounds. In the conventional cubic unit cell made of eight cubes shown in Fig. 1.1a, there are two large empty spaces around dotted circles surrounded by 12 pnictogens called hereafter as a cage, and the filled skutterudite is the filled up version of the cage by an element A (dotted circles in Fig. 1.1a) that could be rare earth, actinide, alkaline, or alkaline earth elements. This system has been intensively investigated from two viewpoints: as scientific target materials to investigate their novel attractive features and as a potential candidate for thermoelectric material of the next generation. For the latter viewpoint, a nice review article by Sales (2003) has already been reported. Therefore, in this article, we are going to focus on the former subject, putting emphasis on the electronic structures and magnetic properties related with the 4f- and d-electrons. For the detailed explanation of the thermoelectric characteristics, the crystal chemistry, and the more intense discussion on the crystal stability, please refer to the review work of Sales.
The crystal structure of the filled skutterudite AT4X12 is shown in Fig. 1.1b. It belongs to the space group Im]bar.3] (T5h, #204) and the atomic positions of A, transition metal T, and pnictogen X ions are (0, 0, 0), (1/4, 1/4, 1/4), and (0, u, v), respectively. The values of the parameters u and v depend on the combination of A, T, and X, at around u = 0.335–20.360 and v = 0.142–0.16 (Kaiser and Jeitschko, 1999; Sales, 2003; Uher, 2001). The distorted icosahedron cage is formed by 12 pnictogen atoms, and a rare earth ion is located at the center of the cage. The transition metal ions T are located between the cages, forming a simple cubic sublattice. The size of the icosahedron cage formed by the pnictogen ions increases with increasing ionic radius of pnictogen as P -> As -> Sb. The lattice constant shown in Fig. 1.2 reflects this feature. The similarities between the filled skutterudite structure and the perovskite structures (for example, SrTiO3) have been pointed out by Jeitschko and Braun (1977) (refer to Fig. 1.2 in Sales, 2003). From such a viewpoint, the characteristic feature for the filled skutterudite structure is the tilting of the octahedra, resulting in the formation of the enlarged voids, in which rare earth ions are accommodated.
2. Synthesis of Filled Skutterudites
The preparation of filled skutterudite samples is not so easy a task, especially when one needs high-quality single crystals with a large enough size for particular experiments. That is because this family of compounds itself is usually incongruent, and constituent elements include both high melting temperature materials and high vapor pressure ones. In addition, some elements are toxic. The most widely used way to grow single crystals is the flux method under ambient pressure (Braun and Jeitschko, 1980a, 1980b; Jeitschko and Braun 1977). The method has been successfully applied to P- and Sb-based filled skutterudites containing light rare earth elements, although there is a limit on the size of grown samples. However, for the filled skutterudites containing As and/or heavy rare earth elements, the synthesis under high pressures was practically proven to be useful (Shirotani et al., 1996). In the following, the basic part of crystal growth methods is explained, under ambient pressure and under high pressures in separate subsections.
2.1. Synthesis under ambient pressure
From the viewpoint of sample quality for various physical property measurements, the flux method is the best way to grow single crystals. For example, single crystals of LaFe4P12 with residual resistivity ratio, RRR ( = ρRT/ρ0, where ρRT is the room temperature resistivity and ρ0 the residual resistivity), greater than 1000 and those of CeRu4Sb12 with the size larger than 5 mm x 5 mm x 5 mm have been obtained (Sugawara et al., 2000, 2002a), in which de Haas–van Alphen effect has been observed. The flux method under ambient pressure is the most convenient way without any special apparatus. Basically, only a high-vacuum pump and a resistance heater furnace are necessary, and that is a laboratory-friendly method for the high-quality single crystal growth. In this section, the method of crystal growth under ambient pressure is briefly described. For a complete review on the general principle and the growth techniques by the flux method, the following review papers are recommended: Canfield and Fisk (1992) and Fisk and Remeika (1989).
The method we applied is basically the same as was reported by Jeitschko and Braun (1977). However, we have modified various conditions, in the process to search for the best condition to grow larger and higher quality samples, which are described below. The raw materials, better than 3N (99.9% pure)-rare earth (ingot), 4N-Fe, Ru, Os (powder), 6N-P (chunk), and 5N-Sn (shot), were encapsulated in a quartz tube with a molar ratio R:T:P:Sn = 1:4:20:40. Sometimes, the reaction between the constituent materials and quartz is a serious problem, for example, Os attacks quartz at high temperatures above 950°C. For such a case, Al2O3 can be used as a crucible to avoid direct contact between the raw materials and quartz. The carbon coating on inside wall of quartz tube is another possible solution to avoid the reaction (Bauer et al., 2001c).
The ampoule is heated up to 1050°C in a furnace, kept at this temperature for 100 hours, cooled down to 650°C at the rate of 1°C/hour, and then furnace is cooled down to room temperature. The estimation of minimum Sn ratio needs special care in order to avoid explosion of the ampoule during the heating process. Actually, when we tried crystal growth in the ratio R:T:P:Sn = 1:4:20:20, the ampoule exploded at around 850°C. The excess Sn-flux was dissolved in concentrated hydrochloric acid (HCl), since HCl does not practically attack filled skutterudite.
For the RT4Sb12 system, one can use Sb itself as flux. In the Sb self-flux method, the growth in the molar ratio R:T:Sb = 1:4:20 is usually successful. The crystal growth process is basically the same as in the previous report (Braun and Jeitschko, 1980b). The crystal growth of RRu4Sb12 and ROs4Sb12 is easier compared to that of RFe4Sb12. After growing crystals, the excess Sb-flux was dissolved in aqua regia. In this process, the soak interval in aqua regia requires special care, since aqua regia also attacks the sample. Typically, a few hours are enough for removing Sb-flux. In the case of RFe4Sb12, single crystal growth is promoted by the troublesome solid-state reaction, since this compound is not stable above ~700°C (Morelli and Meisner, 1995), which is far below the melting temperature. Moreover, this compound is easily attacked by aqua regia, which makes it difficult to extract grown single crystals from Sb-flux. Single crystals might be mechanically isolated from Sb-flux (Mori et al., 2007). In addition, the quality of RFe4Sb12 crystals is generally worse compared to other Sb-based skutterudites, partly due to the incomplete filling of R sites, typically 70–90% (Butch et al., 2005; Ikeno et al., 2008). Better crystals with almost fully occupied R sites can be obtained using the high-pressure synthesis described in Section 2.2.
Recently, Ge-based filled skutterudites MPt4Ge12 have been synthesized by Bauer et al. (2007) and Gumeniuk et al. (2008a) in polyscrystalline form, almost simultaneously. The synthesizing process is basically the same as that used for polycrystalline RFe4Sb12 (Danebrock et al., 1996; Morelli and Meisner, 1995; Toda et al., 2008). Fortunately in this case, we can use an arc furnace in the first step to mix the constituent materials because of relatively low vapor pressure of the raw materials. For M = Sr or Ba, excess amount of M in the ratio of M:Pt:Ge = 1.2:4:12 is prepared to compensate the loss of M due to evaporation (Sugawara et al., unpublished data).
2.2. Synthesis under high pressures
Single crystals of both P- and Sb-based skutterudites could be grown by the flux method at ambient pressure, at least for light rare earth components. However, the method does not work for the systems containing heavy rare earth elements except for the divalent or intermediate valence Eu and Yb with a larger ionic radius. Therefore, samples for such systems are hardly obtainable even in polycrystalline form. To overcome the difficulty, Shirotani et al. (2003b, 2005a) applied the high-pressure technique to obtain polycrystalline samples of heavy rare earth-filled skutterudites. They have succeeded in synthesizing RFe4P12 for all the rare earth elements by wedge-type cubic anvil pressure cell up to ~4 GPa.
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