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Thermoelectric properties of the intermediate valent cerium intermetallic Ce2Ni3Si5 doped with Pd, Co, and Cu
Journal of Alloys and Compounds 292 (1999) 124–128
L
Thermoelectric properties of the intermediate valent cerium intermetallic Ce 2 Ni 3 Si 5 doped with Pd, Co, and Cu K.J. Proctor, K.A. Regan, A. Littman, F.J. DiSalvo* Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA Received 6 July 1999; accepted 6 July 1999
Abstract The nickel site of Ce 2 Ni 3 Si 5 , which has the orthorhombic U 2 Co 3 Si 5 structure type, can be fully substituted with palladium and cobalt ˚ 3 to 704 A ˚ 3 upon substitution with palladium while and partially substituted with copper. The volume of the lattice expands from 635 A the volume contraction with cobalt and copper substitutions are much smaller. The thermopower of Ce 2 Ni 3 Si 5 is 32 mV/ K at room temperature and increases to 60 mV/ K at 40 K. This relatively high thermopower is decreased by substitution of the three metals studied here. The relatively temperature independent thermal conductivity of between 50 and 60 mW/ Kcm for Ce 2 Ni 3 Si 5 is decreased in magnitude by substitution of the heavier palladium, especially at temperatures below 150 K, and is changed to typical metallic behavior by cobalt substitution. Upon cooling from room temperature, the electrical resistivity of Ce 2 Ni 3 Si 5 displays a broad plateau of 300 mVcm until a precipitous drop below 120 K, indicative of coherence effects in the Kondo interactions between the cerium moments and conduction electrons. Copper and palladium substitutions result in a gradual reduction in the effects of cerium intermediate valence, whereas cobalt substitution drives the resistivity to metallic behavior but with a relatively large room temperature resistivity of 400 mVcm. 1999 Elsevier Science S.A. All rights reserved. Keywords: Intermediate valence; Cerium intermetallic; Electrical resistivity; Thermopower; Kondo behavior
1. Introduction The cerium intermetallic Ce 2 Ni 3 Si 5 with the orthorhombic U 2 Co 3 Si 5 structure type [1,2] was discovered in 1984 [3]. The properties of the material suggest that the cerium is intermediate valent (IV). The electrical resistivity of Ce 2 Ni 3 Si 5 increases slightly upon cooling and drops precipitously at low (|100 K) temperatures, typical of a coherent Kondo system, and the magnetic susceptibility does not follow the Curie-Weiss law [4–6]. In addition, the X-ray absorption spectrum of the Ce L III -edge can be fit to two oxidation states [7]. R 2 M 3 Si 5 forms with the U 2 Co 3 Si 5 structure type for R5Y, La–Er, and M5Co, Ni, Rh, Ir, Pd [3,8–12]. Some of these materials, including
*Corresponding author. Tel.: 11-607-255-7238; fax: 11-607-2554137. E-mail address: [email protected] (F.J. DiSalvo)
M5Ir, Rh, and R5Eu, also display intermediate valence of the rare earth element [8]. In addition, the only other known structure type for R 2 M 3 Si 5 materials (M5Mn, Fe, Re, Ru, Os) [13] is Sc 2 Fe 3 Si 5 [14,15] which is composed of U 2 Co 3 Si 5 - and CeMg 2 Si 2 -type building blocks [12]. This suggests that the U 2 Co 3 Si 5 structure of Ce 2 Ni 3 Si 5 is flexible and will allow full or partial substitutions of many other rare earths and transition metals for cerium and nickel, respectively. We have been searching for enhanced thermoelectric properties in cerium IV intermetallics in hopes of discovering materials for efficient thermoelectric cooling devices. IV materials often have large thermopowers (S¯100 mV/ K) and small electrical resistivities ( r ¯100 mVcm), a combination required for efficient devices. We hoped that the thermopower of Ce 2 Ni 3 Si 5 would be large and that its properties could be modified and improved by substitution of small amounts of other transition metals on the nickel site. This paper describes the synthesis of several substi-
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00483-1
K. J. Proctor et al. / Journal of Alloys and Compounds 292 (1999) 124 – 128
tuted versions of Ce 2 Ni 3 Si 5 and a comparison of their transport properties. Unfortunately, the properties of the substituted compounds indicate that they are less efficient thermoelectrics than the parent compound.
2. Experimental Prior to syntheses, commercial cerium (Cerac, 99.9%) was separated from its oxide coating by mechanical abrasion and drip melting [16]. Cerium metal was always handled under an inert atmosphere. The other elements were commercially available (purity .99.9%) and used as delivered. The elements (total mass ¯1 g) were arc melted in stoichiometric ratios as previously described [3–6] then annealed at 10508C for 10 days in Ta foil and quartz tubes followed by an air quench. Mass losses for the entire synthesis were less than 1%. Powder X-ray diffraction patterns were collected from all samples on a Scintag XDS2000 diffractometer with Cu Ka radiation and the lattice parameters of the orthorhombic Ce 2 M 3 Si 5 phases were determined from a least squares refinement of at least 18 reflections. The multiphase powder diffraction patterns of the as-cast samples change to single phase patterns after annealing. An electronically equivalent formulation of Ce 2 Co 1.5 Cu 1.5 Si 5 and substitutions with silver were also processed in the same manner but the resulting products were always multiphase. Since the structure of Ce 2 Ni 3 Si 5 is orthorhombic, we also wanted to measure the anisotropy of the transport properties on single crystals. Unfortunately, Czochralski growth in a triarc furnace from a stoichiometric melt (mass ¯6 g) yielded crystals of CeNiSi 2 instead of the desired ternary. Perhaps single crystals could be grown from an off-stoichiometric melt or from a flux. However, the lower peritectoid decomposition of the compound between 850 and 10008C limits the range of growth temperatures and the material’s small thermopower did not justify further efforts. The transport properties of the samples were measured as previously described [16]. In general, the cast and annealed samples were cut into bars of approximate dimensions 333310 mm with a diamond saw (Beuhler, Series 15 LC diamond wafering blade). Thermopower and thermal conductivity were measured simultaneously by the steady state method and electrical resistivity was measured by an AC four-probe technique. Electrical contacts were made with ultrasonic In and the voltage contacts for the resistivity measurement were made with silver epoxy (Epoxy Technology, H20E). The current contacts were found to be ohmic with a negligible resistance (,1 V) while the resistance between the voltage contacts was generally less than 20 V.
125
3. Results and discussion
3.1. Lattice parameters The lattice parameters of the nickel and cobalt end members of Ce 2 M 3 Si 5 agree with previous reports to ˚ (see Table 1). As metals are substituted within 60.01 A for nickel, the lattice parameters of Ce 2 M 3 Si 5 change linearly following Vegard’s Law but not entirely as predicted based on the atomic radii [17] of the transition metals (see Fig. 1). The larger palladium expands the ˚ 3 which can be volume of the lattice from 635 to 704 A explained by the difference in covalent radii between ˚ and palladium (1.28 A). ˚ For Co however, nickel (1.15 A) ˚ 3 whereas the the volume contracts from 635 to 616 A radius of cobalt is actually larger than nickel (1.15 versus ˚ In the cobalt case, simple predictions based on the 1.16 A). size of the metals may not be accurate because there is an electronic as well as a size difference between nickel and cobalt. This electronic change may affect the IV state of cerium which will cause a visible change in the lattice parameters ˚ versus the as well; the ionic radius of Ce 13 is 1.11 A 14 ˚ radius for Ce of 0.95 A [17]. This is indirect evidence that, with cobalt substitution, cerium is driven towards the 14 oxidation state and is no longer IV in Ce 2 Co 3 Si 5 , a hypothesis which is supported by our resistivity and thermopower measurements.
3.2. Thermopower and thermal conductivity This is the first report of the thermopower and thermal conductivity of Ce 2 Ni 3 Si 5 , Ce 2 Co 3 Si 5 , and Ce 2 Pd 3 Si 5 . Unlike a normal metal whose thermopower is generally less than 10 mV/ K and decreases with decreasing temperature, the thermopower of Ce 2 Ni 3 Si 5 (see Fig. 2) is 32 mV/ K at room temperature and increases to 60 mV/ K at 40 K. Unfortunately, all of the transition metal substitutions performed here result in lower thermopowers at all temperatures between 40 and 300 K. A comparison of the substituted materials, Ce 2 Ni 32x M x Si 5 where x50.5, suggests that copper lowers the thermopower more gradually than cobalt or palladium. The values of the thermopower
Table 1 Comparison of the orthorhombic lattice parameters and cell volume of Ce 2 Ni 3 Si 5 and Ce 2 Co 3 Si 5 with previous reports Material
Source
˚ a (A)
˚ b (A)
˚ c (A)
˚ 3) Volume (A
Ce 2 Ni 3 Si 5
Presented here [6] [4] [3]
9.644 9.65 9.639 9.649
11.427 11.42 11.402 11.421
5.764 5.76 5.754 5.763
635 635 632 635
Ce 2 Co 3 Si 5
Presented here [3]
9.624 9.623
11.306 11.306
5.660 5.652
616 615
126
K. J. Proctor et al. / Journal of Alloys and Compounds 292 (1999) 124 – 128
Fig. 2. The thermopower of Ce 2 Ni 32x M x Si 5 for M5Co, Cu, and Pd. Each data set is labeled with the corresponding M and / or x. The dotted lines are guides to the eye.
Fig. 1. The orthorhombic lattice constants and cell volume for Ce 2 Ni 32x M x Si 5 where M5Pd, Cu, and Co with the U 2 Co 3 Si 5 structure type. Note that x50.5 Cu and Co data overlap.
no gain in thermopower associated with this temperature as in CePd 3 .
3.3. Electrical resistivity for Ce 2 Co 3 Si 5 and Ce 2 Pd 3 Si 5 are more typical of normal metals. Ce 2 Co 3 Si 5 has a broad maximum of about 11 mV/ K between 100 K and .300 K while Ce 2 Pd 3 Si 5 has a more pronounced maximum at about 150 K but only of 7 mV/ K. The thermal conductivity of Ce 2 Ni 3 Si 5 (see Fig. 3) is almost temperature independent and not as sensitive to substitutions on the metal site as the thermopower. For palladium substitution, it seems clear that the heavier metal is decreasing the conductivity at all temperatures, although most dramatically at low temperatures. Small copper substitutions produce a decrease in the thermal conductivity with little change in the temperature dependence. In contrast, cobalt substitution seems to make the conductivity more metallic; Ce 2 Co 3 Si 5 displays an increasing thermal conductivity with decreasing temperature. In some intermediate valent compounds such as CePd 3 , the thermal conductivity displays a minimum at temperatures below 300 K [16,18]. A weak minimum is observed for small substitutions (x50.5) of cobalt but. unfortunately, there is
The electrical resistivity of Ce 2 Ni 3 Si 5 is shown in Fig. 4. Both the room temperature magnitude of about 300 mVcm and the unusual temperature behavior match a previous report [4] which has been duplicated in the inset plot. This unusual temperature dependence is a characteristic of IV systems and suggests the presence of strong Kondo interactions between the cerium magnetic moments and conduction electrons. The drop in resistivity at low temperature is usually ascribed to the onset of coherence in the Kondo lattice. Substitutions for nickel cause changes in both the magnitude and temperature dependence of the resistivity (see Fig. 5). The changes are most dramatic for cobalt substitutions where the Kondo behavior changes to metallic behavior with a strictly decreasing resistivity with decreasing temperature. However, the magnitude of the resistivity is rather large for a simple metal suggesting there remains some scattering by the cerium magnetic moments. With palladium substitution, the drop in resistivity at
K. J. Proctor et al. / Journal of Alloys and Compounds 292 (1999) 124 – 128
Fig. 3. The thermal conductivity of Ce 2 Ni 32x M x Si 5 for M5Co, Pd, and Cu. Each data set retains the symbol from Fig. 2 and is labeled with the corresponding M and / or x. The dotted lines are guides to the eye.
low temperatures is lost as the coherence of the lattice is disturbed by alloying. As more palladium is substituted, the maximum in resistivity moves to lower temperatures and the room temperature resistivity drops suggesting a weakening of the Kondo interactions. The bump at around 6 K in electrical resistivity of Ce 2 Pd 3 Si 5 may be evidence of magnetic ordering. We note that the room temperature magnitude does not appear to monotonically decrease as
127
Fig. 5. The electrical resistivity of Ce 2 Ni 32x M x Si 5 for M5Co, Pd, and Cu. Each curve is labeled with the corresponding M and / or x. Fully substituted samples (x50 or 3) are thick lines and partially substituted samples are thin lines.
the palladium concentration increases. However, such small deviations in the magnitude may be caused by cracks in the polycrystalline samples or by the error associated with measuring the area factors of the sample bars which we estimate at less than 10%. Interpreting the effect of copper substitution is difficult with only one sample, but it is similar to adding palladium. For the x50.5 copper sample, the room temperature resistivity and the temperature at which the resistivity drops are lower. Thus it appears that the strength of the Kondo interactions have been reduced in the x50.5 copper sample a similar amount as in the x51 palladium sample.
4. Conclusions
Fig. 4. The measured electrical resistivity of Ce 2 Ni 3 Si 5 with the previously reported [4] resistivity inset for comparison.
As expected, Ce 2 Ni 3 Si 5 was found to have a larger thermopower than typical metals and a smaller resistivity than current thermoelectric materials which are typically highly doped semiconductors. However, substitutions of other transition metals did not improve the transport properties so these materials are not suited for thermoelectric cooling applications.
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K. J. Proctor et al. / Journal of Alloys and Compounds 292 (1999) 124 – 128
Acknowledgements This research was funded by a grant from the Office of Naval Research. K.A.R. would like to acknowledge support from the REU (Research Education for Undergraduates) program through the CCMR (Cornell Center for Materials Research) under award DMR-9632275. The cutting operations of the sample preparation made use of the Technical Operations Laboratory of the CCMR, also supported by award DMR-9632275.
References [1] L.G. Aksel’rud, Ya.P. Yarmolyuk,, E.I. Gladyshevskii, Kristallografiya 22 (1977) 861–863. [2] L.G. Aksel’rud, Ya.P. Yarmolyuk, E.I. Gladyshevskii, Sov. Phys. Crystallogr. 22–4 (1977) 492–493. ´ J. Less-Common Met. 97 (1984) 285–290. [3] B. Chabot, E. Parthe, [4] C. Mazumdar, R. Nagarajan, S.K. Dhar, L.C. Gupta, R. Vijayaraghavan, B.D. Padalia, Phys. Rev. B 46 (14) (1992) 9009– 9012. [5] C. Mazumdar, R. Nagarajan, C. Godart, L.C. Gupta, B.D. Padalia, R. Vijayaraghavan, J. Appl. Phys. 79–8 (1996) 6347–6348.
[6] R.J. Cava, A.P. Ramirez, H. Takagi, J.J. Krajewski, W.F. Peck Jr., J. Magn. Magn. Mater. 128 (1993) 124–128. [7] C. Mazumdar, R. Nagarajan, C. Godart, L.C. Gupta, B.D. Padalia, R. Vijayaraghavan, J. Appl. Phys. 79 (8) (1996) 6347–6348. [8] C. Godart, L.C. Gupta, C.V. Tomy et al., Physica B 163 (1990) 163–166. [9] B.P. Chevalier, P. Lejay, M. Vlasse, J. Etourneau, P. Hagenmuller, in: Solid State Chem. Conf., Veldhoven, The Netherlands, Paper FII, 1982, p. 19. [10] B. Chevalier, P. Lejay, J. Etourneau, M. Vlasse, P. Hagenmuller, in: VIIth Intern. Conf. on Solid Compounds of Transition Elements, Grenoble, France, Abstract II B, 1982, p. 16. [11] B. Chevalier, P. Lejay, J. Etourneau, M. Vlasse, P. Hagenmuller, Mater. Res. Bull. 17 (1982) 1211–1220. ´ B. Chabot, in: K.A. GschneiderJr., L. Eyring (Eds.), [12] E. Parthe, Handbook on the Physics and Chemistry of Rare Earths, Vol. 6, North-Holland, Amsterdam, 1984, pp. 222–224. [13] H.F. Braun, Phys. Let. A 75A (1980) 386–388. [14] O.I. Bodak, B.Y. Kotur, Ya.I. Yarovets, E.I. Gladyshevskii, Kristallografiya 22 (1977) 385–388. [15] O.I. Bodak, B.Y. Kotur, Ya.I. Yarovets, E.I. Gladyshevskii, Sov. Phys. Crystallogr. 22 (1977) 217–219. [16] K.J. Proctor, C.D.W. Jones, F.J. DiSalvo, J. Phys. Chem. Solids 60 (1999) 663–671. [17] R.D. Shannon, Acta Crystallographica A32 (1976) 751. [18] H. Schneider, D. Wohlleben, Z. Phys. B 44 (1981) 193.
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Thermoelectric properties of the intermediate valent cerium intermetallic Ce 2 Ni 3 Si 5 doped with Pd, Co, and Cu K.J. Proctor, K.A. Regan, A. Littman, F.J. DiSalvo* Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA Received 6 July 1999; accepted 6 July 1999
Abstract The nickel site of Ce 2 Ni 3 Si 5 , which has the orthorhombic U 2 Co 3 Si 5 structure type, can be fully substituted with palladium and cobalt ˚ 3 to 704 A ˚ 3 upon substitution with palladium while and partially substituted with copper. The volume of the lattice expands from 635 A the volume contraction with cobalt and copper substitutions are much smaller. The thermopower of Ce 2 Ni 3 Si 5 is 32 mV/ K at room temperature and increases to 60 mV/ K at 40 K. This relatively high thermopower is decreased by substitution of the three metals studied here. The relatively temperature independent thermal conductivity of between 50 and 60 mW/ Kcm for Ce 2 Ni 3 Si 5 is decreased in magnitude by substitution of the heavier palladium, especially at temperatures below 150 K, and is changed to typical metallic behavior by cobalt substitution. Upon cooling from room temperature, the electrical resistivity of Ce 2 Ni 3 Si 5 displays a broad plateau of 300 mVcm until a precipitous drop below 120 K, indicative of coherence effects in the Kondo interactions between the cerium moments and conduction electrons. Copper and palladium substitutions result in a gradual reduction in the effects of cerium intermediate valence, whereas cobalt substitution drives the resistivity to metallic behavior but with a relatively large room temperature resistivity of 400 mVcm. 1999 Elsevier Science S.A. All rights reserved. Keywords: Intermediate valence; Cerium intermetallic; Electrical resistivity; Thermopower; Kondo behavior
1. Introduction The cerium intermetallic Ce 2 Ni 3 Si 5 with the orthorhombic U 2 Co 3 Si 5 structure type [1,2] was discovered in 1984 [3]. The properties of the material suggest that the cerium is intermediate valent (IV). The electrical resistivity of Ce 2 Ni 3 Si 5 increases slightly upon cooling and drops precipitously at low (|100 K) temperatures, typical of a coherent Kondo system, and the magnetic susceptibility does not follow the Curie-Weiss law [4–6]. In addition, the X-ray absorption spectrum of the Ce L III -edge can be fit to two oxidation states [7]. R 2 M 3 Si 5 forms with the U 2 Co 3 Si 5 structure type for R5Y, La–Er, and M5Co, Ni, Rh, Ir, Pd [3,8–12]. Some of these materials, including
*Corresponding author. Tel.: 11-607-255-7238; fax: 11-607-2554137. E-mail address: [email protected] (F.J. DiSalvo)
M5Ir, Rh, and R5Eu, also display intermediate valence of the rare earth element [8]. In addition, the only other known structure type for R 2 M 3 Si 5 materials (M5Mn, Fe, Re, Ru, Os) [13] is Sc 2 Fe 3 Si 5 [14,15] which is composed of U 2 Co 3 Si 5 - and CeMg 2 Si 2 -type building blocks [12]. This suggests that the U 2 Co 3 Si 5 structure of Ce 2 Ni 3 Si 5 is flexible and will allow full or partial substitutions of many other rare earths and transition metals for cerium and nickel, respectively. We have been searching for enhanced thermoelectric properties in cerium IV intermetallics in hopes of discovering materials for efficient thermoelectric cooling devices. IV materials often have large thermopowers (S¯100 mV/ K) and small electrical resistivities ( r ¯100 mVcm), a combination required for efficient devices. We hoped that the thermopower of Ce 2 Ni 3 Si 5 would be large and that its properties could be modified and improved by substitution of small amounts of other transition metals on the nickel site. This paper describes the synthesis of several substi-
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00483-1
K. J. Proctor et al. / Journal of Alloys and Compounds 292 (1999) 124 – 128
tuted versions of Ce 2 Ni 3 Si 5 and a comparison of their transport properties. Unfortunately, the properties of the substituted compounds indicate that they are less efficient thermoelectrics than the parent compound.
2. Experimental Prior to syntheses, commercial cerium (Cerac, 99.9%) was separated from its oxide coating by mechanical abrasion and drip melting [16]. Cerium metal was always handled under an inert atmosphere. The other elements were commercially available (purity .99.9%) and used as delivered. The elements (total mass ¯1 g) were arc melted in stoichiometric ratios as previously described [3–6] then annealed at 10508C for 10 days in Ta foil and quartz tubes followed by an air quench. Mass losses for the entire synthesis were less than 1%. Powder X-ray diffraction patterns were collected from all samples on a Scintag XDS2000 diffractometer with Cu Ka radiation and the lattice parameters of the orthorhombic Ce 2 M 3 Si 5 phases were determined from a least squares refinement of at least 18 reflections. The multiphase powder diffraction patterns of the as-cast samples change to single phase patterns after annealing. An electronically equivalent formulation of Ce 2 Co 1.5 Cu 1.5 Si 5 and substitutions with silver were also processed in the same manner but the resulting products were always multiphase. Since the structure of Ce 2 Ni 3 Si 5 is orthorhombic, we also wanted to measure the anisotropy of the transport properties on single crystals. Unfortunately, Czochralski growth in a triarc furnace from a stoichiometric melt (mass ¯6 g) yielded crystals of CeNiSi 2 instead of the desired ternary. Perhaps single crystals could be grown from an off-stoichiometric melt or from a flux. However, the lower peritectoid decomposition of the compound between 850 and 10008C limits the range of growth temperatures and the material’s small thermopower did not justify further efforts. The transport properties of the samples were measured as previously described [16]. In general, the cast and annealed samples were cut into bars of approximate dimensions 333310 mm with a diamond saw (Beuhler, Series 15 LC diamond wafering blade). Thermopower and thermal conductivity were measured simultaneously by the steady state method and electrical resistivity was measured by an AC four-probe technique. Electrical contacts were made with ultrasonic In and the voltage contacts for the resistivity measurement were made with silver epoxy (Epoxy Technology, H20E). The current contacts were found to be ohmic with a negligible resistance (,1 V) while the resistance between the voltage contacts was generally less than 20 V.
125
3. Results and discussion
3.1. Lattice parameters The lattice parameters of the nickel and cobalt end members of Ce 2 M 3 Si 5 agree with previous reports to ˚ (see Table 1). As metals are substituted within 60.01 A for nickel, the lattice parameters of Ce 2 M 3 Si 5 change linearly following Vegard’s Law but not entirely as predicted based on the atomic radii [17] of the transition metals (see Fig. 1). The larger palladium expands the ˚ 3 which can be volume of the lattice from 635 to 704 A explained by the difference in covalent radii between ˚ and palladium (1.28 A). ˚ For Co however, nickel (1.15 A) ˚ 3 whereas the the volume contracts from 635 to 616 A radius of cobalt is actually larger than nickel (1.15 versus ˚ In the cobalt case, simple predictions based on the 1.16 A). size of the metals may not be accurate because there is an electronic as well as a size difference between nickel and cobalt. This electronic change may affect the IV state of cerium which will cause a visible change in the lattice parameters ˚ versus the as well; the ionic radius of Ce 13 is 1.11 A 14 ˚ radius for Ce of 0.95 A [17]. This is indirect evidence that, with cobalt substitution, cerium is driven towards the 14 oxidation state and is no longer IV in Ce 2 Co 3 Si 5 , a hypothesis which is supported by our resistivity and thermopower measurements.
3.2. Thermopower and thermal conductivity This is the first report of the thermopower and thermal conductivity of Ce 2 Ni 3 Si 5 , Ce 2 Co 3 Si 5 , and Ce 2 Pd 3 Si 5 . Unlike a normal metal whose thermopower is generally less than 10 mV/ K and decreases with decreasing temperature, the thermopower of Ce 2 Ni 3 Si 5 (see Fig. 2) is 32 mV/ K at room temperature and increases to 60 mV/ K at 40 K. Unfortunately, all of the transition metal substitutions performed here result in lower thermopowers at all temperatures between 40 and 300 K. A comparison of the substituted materials, Ce 2 Ni 32x M x Si 5 where x50.5, suggests that copper lowers the thermopower more gradually than cobalt or palladium. The values of the thermopower
Table 1 Comparison of the orthorhombic lattice parameters and cell volume of Ce 2 Ni 3 Si 5 and Ce 2 Co 3 Si 5 with previous reports Material
Source
˚ a (A)
˚ b (A)
˚ c (A)
˚ 3) Volume (A
Ce 2 Ni 3 Si 5
Presented here [6] [4] [3]
9.644 9.65 9.639 9.649
11.427 11.42 11.402 11.421
5.764 5.76 5.754 5.763
635 635 632 635
Ce 2 Co 3 Si 5
Presented here [3]
9.624 9.623
11.306 11.306
5.660 5.652
616 615
126
K. J. Proctor et al. / Journal of Alloys and Compounds 292 (1999) 124 – 128
Fig. 2. The thermopower of Ce 2 Ni 32x M x Si 5 for M5Co, Cu, and Pd. Each data set is labeled with the corresponding M and / or x. The dotted lines are guides to the eye.
Fig. 1. The orthorhombic lattice constants and cell volume for Ce 2 Ni 32x M x Si 5 where M5Pd, Cu, and Co with the U 2 Co 3 Si 5 structure type. Note that x50.5 Cu and Co data overlap.
no gain in thermopower associated with this temperature as in CePd 3 .
3.3. Electrical resistivity for Ce 2 Co 3 Si 5 and Ce 2 Pd 3 Si 5 are more typical of normal metals. Ce 2 Co 3 Si 5 has a broad maximum of about 11 mV/ K between 100 K and .300 K while Ce 2 Pd 3 Si 5 has a more pronounced maximum at about 150 K but only of 7 mV/ K. The thermal conductivity of Ce 2 Ni 3 Si 5 (see Fig. 3) is almost temperature independent and not as sensitive to substitutions on the metal site as the thermopower. For palladium substitution, it seems clear that the heavier metal is decreasing the conductivity at all temperatures, although most dramatically at low temperatures. Small copper substitutions produce a decrease in the thermal conductivity with little change in the temperature dependence. In contrast, cobalt substitution seems to make the conductivity more metallic; Ce 2 Co 3 Si 5 displays an increasing thermal conductivity with decreasing temperature. In some intermediate valent compounds such as CePd 3 , the thermal conductivity displays a minimum at temperatures below 300 K [16,18]. A weak minimum is observed for small substitutions (x50.5) of cobalt but. unfortunately, there is
The electrical resistivity of Ce 2 Ni 3 Si 5 is shown in Fig. 4. Both the room temperature magnitude of about 300 mVcm and the unusual temperature behavior match a previous report [4] which has been duplicated in the inset plot. This unusual temperature dependence is a characteristic of IV systems and suggests the presence of strong Kondo interactions between the cerium magnetic moments and conduction electrons. The drop in resistivity at low temperature is usually ascribed to the onset of coherence in the Kondo lattice. Substitutions for nickel cause changes in both the magnitude and temperature dependence of the resistivity (see Fig. 5). The changes are most dramatic for cobalt substitutions where the Kondo behavior changes to metallic behavior with a strictly decreasing resistivity with decreasing temperature. However, the magnitude of the resistivity is rather large for a simple metal suggesting there remains some scattering by the cerium magnetic moments. With palladium substitution, the drop in resistivity at
K. J. Proctor et al. / Journal of Alloys and Compounds 292 (1999) 124 – 128
Fig. 3. The thermal conductivity of Ce 2 Ni 32x M x Si 5 for M5Co, Pd, and Cu. Each data set retains the symbol from Fig. 2 and is labeled with the corresponding M and / or x. The dotted lines are guides to the eye.
low temperatures is lost as the coherence of the lattice is disturbed by alloying. As more palladium is substituted, the maximum in resistivity moves to lower temperatures and the room temperature resistivity drops suggesting a weakening of the Kondo interactions. The bump at around 6 K in electrical resistivity of Ce 2 Pd 3 Si 5 may be evidence of magnetic ordering. We note that the room temperature magnitude does not appear to monotonically decrease as
127
Fig. 5. The electrical resistivity of Ce 2 Ni 32x M x Si 5 for M5Co, Pd, and Cu. Each curve is labeled with the corresponding M and / or x. Fully substituted samples (x50 or 3) are thick lines and partially substituted samples are thin lines.
the palladium concentration increases. However, such small deviations in the magnitude may be caused by cracks in the polycrystalline samples or by the error associated with measuring the area factors of the sample bars which we estimate at less than 10%. Interpreting the effect of copper substitution is difficult with only one sample, but it is similar to adding palladium. For the x50.5 copper sample, the room temperature resistivity and the temperature at which the resistivity drops are lower. Thus it appears that the strength of the Kondo interactions have been reduced in the x50.5 copper sample a similar amount as in the x51 palladium sample.
4. Conclusions
Fig. 4. The measured electrical resistivity of Ce 2 Ni 3 Si 5 with the previously reported [4] resistivity inset for comparison.
As expected, Ce 2 Ni 3 Si 5 was found to have a larger thermopower than typical metals and a smaller resistivity than current thermoelectric materials which are typically highly doped semiconductors. However, substitutions of other transition metals did not improve the transport properties so these materials are not suited for thermoelectric cooling applications.
128
K. J. Proctor et al. / Journal of Alloys and Compounds 292 (1999) 124 – 128
Acknowledgements This research was funded by a grant from the Office of Naval Research. K.A.R. would like to acknowledge support from the REU (Research Education for Undergraduates) program through the CCMR (Cornell Center for Materials Research) under award DMR-9632275. The cutting operations of the sample preparation made use of the Technical Operations Laboratory of the CCMR, also supported by award DMR-9632275.
References [1] L.G. Aksel’rud, Ya.P. Yarmolyuk,, E.I. Gladyshevskii, Kristallografiya 22 (1977) 861–863. [2] L.G. Aksel’rud, Ya.P. Yarmolyuk, E.I. Gladyshevskii, Sov. Phys. Crystallogr. 22–4 (1977) 492–493. ´ J. Less-Common Met. 97 (1984) 285–290. [3] B. Chabot, E. Parthe, [4] C. Mazumdar, R. Nagarajan, S.K. Dhar, L.C. Gupta, R. Vijayaraghavan, B.D. Padalia, Phys. Rev. B 46 (14) (1992) 9009– 9012. [5] C. Mazumdar, R. Nagarajan, C. Godart, L.C. Gupta, B.D. Padalia, R. Vijayaraghavan, J. Appl. Phys. 79–8 (1996) 6347–6348.
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