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Crystal structure and superconductivity in the Th-doped LaPtSi compounds
Physica C 470 (2010) S772–S773
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Crystal structure and superconductivity in the Th-doped LaPtSi compounds J.Y. Chen, H.H. Sung, K.J. Syu, W.H. Lee * Department of Physics, National Chung Cheng University, Ming-Hsiung, Chia-Yi, Taiwan, ROC
a r t i c l e
i n f o
Article history: Accepted 20 October 2009 Available online 24 October 2009 Keywords: (La1-xThx)PtSi Superconductivity
a b s t r a c t As observed with X-ray powder diffraction, the tetragonal structure of the parent compound LaPtSi, which crystallizes in the LaPtSi-type structure with space group I41md, is retained in (La1 xThx)PtSi up to the solubility limit near x = 0.5. By considering the size factor of Hume–Rothery theory of alloy phase formation, it is not marvelous that the extensive solid solutions cannot be fully completed in (La1 xThx)PtSi. We present the room temperature powder X-ray diffraction patterns, the room temperature lattice parameters and the dc magnetic susceptibility between 1.8 and 4.0 K for three single phase polycrystalline samples in (La1 xThx)PtSi with x = 0, 0.25 and 0.50. The refined lattice parameters show that both the a-axis and the volume of the unit cell v contract clearly, though the c-axis gives a less percentage expansion due to doping with thorium. It is found that the change in Tc with x is similar to the change in the lattice parameter a or v, which indicates that the stiffening of the lattice under pressure has a dominant effect on the decrease in Tc in this system. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Materials preparation
Previous reports showed that the suitable partial substitution of 4f rare-earth element in rare-earth element based carbide with the 5f element Th frequently results in an enhancement of superconducting critical temperature Tc. Some examples are: Y1 xThxC1.5 (Tc = 17 K with x 0.3) [1,2], (La1 xThx)NiC2 (Tc = 7.9 K with x 0.5) [3], (Y1 xThx)NiC2 (Tc = 8.3 K with x 0.5) [4]. Therefore, it is interesting to explore the consequence of Th substitutions at the La site in the ternary silicide LaPtSi, which crystallizes in a tetragonal structure with space group I41md and exhibits superconducting behaviour below 3.3 K [5,6]. Since Ni2+ ion has a smaller size compared with the Pt2+ ion and the Tc value decreases monotonically with the decreasing lattice parameters a, c and v in the pseudo-ternary series La(Pt1 xNix)Si [6], these effects may imply that the contraction of the unit cell or the chemical pressure effect of Ni is unfavorable, with respect to pure LaPtSi, to the superconducting transition temperature Tc. In this study, the system (La1 xThx)PtSi provides an extensive check on the role of chemical pressure influence on Tc in superconducting LaPtSi because of the smaller size of Th4+ compared to La3+. In addition, the factor of the different valence between La3+ and Th4+, which in general results in a change of density of states at Fermi surface (an important factor for a superconducting material), also will be considered.
According to Hume–Rothery theory of alloy phase formation [7], limited solid solubility will be expected if the size factor (= (rB – rA)/rA 100) is outside the range ±15%. Since (r(Th4+) – r(La3+))/r(La3+) = (0.95 – 1.15)/1.15 = –17.4%, it is not unforeseen that the extensive solid solutions cannot be fully completed in (La1 xThx)PtSi. In fact, ThPtSi crystallizes in a tetragonal structure with space group I41/amd [8], which is slightly different from that of LaPtSi (space group I41md). Our powder X-ray diffraction patterns indicated that the solubility limit in (La1 xThx)PtSi was close to x = 0.5. Fig. 1a–c displays the three powder X-ray diffraction patterns at room temperature for three samples LaPtSi, (La0.75Th0.25)PtSi and (La0.5Pt0.5)PtSi as prepared by arc-melting. The peaks of each observed pattern can be all indexed in a tetragonal structure with space group I41md. By using PowderCell program [9], the refined lattice parameters for the three compounds are: LaPtSi (a = 4.248 Å, c = 14.516 Å, v = 266.02 Å3), (La0.75Th0.25) PtSi (a = 4.219 Å, c = 14.574 Å, v = 259.45 Å3) and (La0.5Th0.5)PtSi (a = 4.191 Å, c = 14.595 Å, v = 256.43 Å3). It is found that the refined lattice parameters c and v decrease linearly with the Th concentration while the a and b lattice parameters increase linearly in less percentage with Th concentration.
* Corresponding author. Tel.: +886 5 2720411; fax: +886 5 2720587. E-mail address: [email protected] (W.H. Lee). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.10.018
3. Results and discussion The ambient pressure superconducting transition temperatures, Tc’s, of the samples were determined from dc magnetic susceptibility measurements in a Quantum Design SQUID magnetometer with
S773
J.Y. Chen et al. / Physica C 470 (2010) S772–S773
112
FC 0.00
(a)
(b)
M (emu/g)
224 217,208 303 312 1110 305 219 0012
215 220
107 204 213
200 116
004
Intensity (arb. units)
101
103
105
-0.04
-0.08
ZFC -0.12
LaPtSi (La0.75Th0.25)PtSi (La0.5Th0.5)PtSi
-0.16 2
3
4
T (K)
(c)
20
30
40
50
60
70
80
2θ (deg) Fig. 1. Room-temperature powder X-ray diffraction patterns of (a) LaPtSi, (b) (La0.75Th0.25)PtSi, and (c) (La0.5Th0.5)PtSi using Cu Ka radiation.
a field of 10 Oe for the temperature range between 1.8 and 4.0 K. Fig. 2 depicts the zero-field-cooled (ZFC) and field-cooled (FC) magnetization for the three (La1 xThx)PtSi alloys with x = 0, 0.25 and 0.5. All measurements were carried out on bulk samples of about 0.2 g mass. For each sample, the ZFC curve shows sharp transition and reaches to saturation at the lower temperature. The large shielding signal for each sample indicates the perfect superconducting volume fraction. The 10–90% values of the transition are taken as the superconducting transition temperature. The data thus obtained are: LaPtSi (Tc = 3.18 3.37 K), (La0.75Th0.25)PtSi (Tc = 2.58 3.00 K) and (La0.5Th0.5)PtSi (Tc = 2.52 2.68 K). It is found that the Tc change rate dTc/dx = 1.35 ± 0.03 K and dTc/ dv = 0.070 ± 0.002 K/Å3 for the (La1 xThx)PtSi system. In a short concluding remark, above results demonstrate that the stiffening of the lattice under pressure has a more important
Fig. 2. Temperature dependence of ZFC and FC magnetization data for three compounds LaPtSi, (La0.75Th0.25)PtSi and (La0.5Th0.5)PtSi measured in a field of 10 Oe between 1.8 and 4.0 K.
effect on the decrease in Tc in this (La1 xThx)PtSi system. However, compared to the system La(Pt1 x Nix)Si (dTc/dv = 0.127 ± 0.043 K/ Å3) [6], the increase of charge carrier density by Th doping in LaPtSi indeed relax the Tc decreasing rate. Acknowledgment This work was supported by the National Science Council of the Republic of China under Contract No. NSC-96-2112-M-194-008MY3. References [1] M.C. Krupka, A.I. Giorgi, N.H. Krikorian, E.G. Szklarz, J. Less-Common Met. 19 (1969) 113. [2] A.I. Giorgi, E.G. Szklarz, M.C. Krupka, in: D.H. Douglas (Ed.), Superconductivity in d- and f-Band Metals, Am. Inst. Phys., New York, 1972, p. 147. [3] W.H. Lee, H.K. Zeng, Solid State Commun. 101 (1997) 323. [4] K.J. Syu, H.H. Sung, W.H. Lee, Solid State Commun. 141 (2007) 519. [5] J. Evers, G. Oehlinger, A. Weiss, C. Probst, Solid State Commun. 50 (1984) 61. [6] W.H. Lee, Solid State Commun. 94 (1995) 425. [7] L.H. Bennett (Ed.), Theory of Alloy Phases Formation, Proceedings of a Symposium at the 108th AIME Annual Meeting, New Orleans, LA, 1979. [8] W.X. Zhong, W.L. Ng, B. Chevalier, J. Etourneau, P. Hagenmuller, Mater. Res. Bull. 20 (1985) 1229. [9] G. Nolze, W. Kraus, PowderCell 2.3 Program, BAM, Berlin, 2000.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Crystal structure and superconductivity in the Th-doped LaPtSi compounds J.Y. Chen, H.H. Sung, K.J. Syu, W.H. Lee * Department of Physics, National Chung Cheng University, Ming-Hsiung, Chia-Yi, Taiwan, ROC
a r t i c l e
i n f o
Article history: Accepted 20 October 2009 Available online 24 October 2009 Keywords: (La1-xThx)PtSi Superconductivity
a b s t r a c t As observed with X-ray powder diffraction, the tetragonal structure of the parent compound LaPtSi, which crystallizes in the LaPtSi-type structure with space group I41md, is retained in (La1 xThx)PtSi up to the solubility limit near x = 0.5. By considering the size factor of Hume–Rothery theory of alloy phase formation, it is not marvelous that the extensive solid solutions cannot be fully completed in (La1 xThx)PtSi. We present the room temperature powder X-ray diffraction patterns, the room temperature lattice parameters and the dc magnetic susceptibility between 1.8 and 4.0 K for three single phase polycrystalline samples in (La1 xThx)PtSi with x = 0, 0.25 and 0.50. The refined lattice parameters show that both the a-axis and the volume of the unit cell v contract clearly, though the c-axis gives a less percentage expansion due to doping with thorium. It is found that the change in Tc with x is similar to the change in the lattice parameter a or v, which indicates that the stiffening of the lattice under pressure has a dominant effect on the decrease in Tc in this system. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Materials preparation
Previous reports showed that the suitable partial substitution of 4f rare-earth element in rare-earth element based carbide with the 5f element Th frequently results in an enhancement of superconducting critical temperature Tc. Some examples are: Y1 xThxC1.5 (Tc = 17 K with x 0.3) [1,2], (La1 xThx)NiC2 (Tc = 7.9 K with x 0.5) [3], (Y1 xThx)NiC2 (Tc = 8.3 K with x 0.5) [4]. Therefore, it is interesting to explore the consequence of Th substitutions at the La site in the ternary silicide LaPtSi, which crystallizes in a tetragonal structure with space group I41md and exhibits superconducting behaviour below 3.3 K [5,6]. Since Ni2+ ion has a smaller size compared with the Pt2+ ion and the Tc value decreases monotonically with the decreasing lattice parameters a, c and v in the pseudo-ternary series La(Pt1 xNix)Si [6], these effects may imply that the contraction of the unit cell or the chemical pressure effect of Ni is unfavorable, with respect to pure LaPtSi, to the superconducting transition temperature Tc. In this study, the system (La1 xThx)PtSi provides an extensive check on the role of chemical pressure influence on Tc in superconducting LaPtSi because of the smaller size of Th4+ compared to La3+. In addition, the factor of the different valence between La3+ and Th4+, which in general results in a change of density of states at Fermi surface (an important factor for a superconducting material), also will be considered.
According to Hume–Rothery theory of alloy phase formation [7], limited solid solubility will be expected if the size factor (= (rB – rA)/rA 100) is outside the range ±15%. Since (r(Th4+) – r(La3+))/r(La3+) = (0.95 – 1.15)/1.15 = –17.4%, it is not unforeseen that the extensive solid solutions cannot be fully completed in (La1 xThx)PtSi. In fact, ThPtSi crystallizes in a tetragonal structure with space group I41/amd [8], which is slightly different from that of LaPtSi (space group I41md). Our powder X-ray diffraction patterns indicated that the solubility limit in (La1 xThx)PtSi was close to x = 0.5. Fig. 1a–c displays the three powder X-ray diffraction patterns at room temperature for three samples LaPtSi, (La0.75Th0.25)PtSi and (La0.5Pt0.5)PtSi as prepared by arc-melting. The peaks of each observed pattern can be all indexed in a tetragonal structure with space group I41md. By using PowderCell program [9], the refined lattice parameters for the three compounds are: LaPtSi (a = 4.248 Å, c = 14.516 Å, v = 266.02 Å3), (La0.75Th0.25) PtSi (a = 4.219 Å, c = 14.574 Å, v = 259.45 Å3) and (La0.5Th0.5)PtSi (a = 4.191 Å, c = 14.595 Å, v = 256.43 Å3). It is found that the refined lattice parameters c and v decrease linearly with the Th concentration while the a and b lattice parameters increase linearly in less percentage with Th concentration.
* Corresponding author. Tel.: +886 5 2720411; fax: +886 5 2720587. E-mail address: [email protected] (W.H. Lee). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.10.018
3. Results and discussion The ambient pressure superconducting transition temperatures, Tc’s, of the samples were determined from dc magnetic susceptibility measurements in a Quantum Design SQUID magnetometer with
S773
J.Y. Chen et al. / Physica C 470 (2010) S772–S773
112
FC 0.00
(a)
(b)
M (emu/g)
224 217,208 303 312 1110 305 219 0012
215 220
107 204 213
200 116
004
Intensity (arb. units)
101
103
105
-0.04
-0.08
ZFC -0.12
LaPtSi (La0.75Th0.25)PtSi (La0.5Th0.5)PtSi
-0.16 2
3
4
T (K)
(c)
20
30
40
50
60
70
80
2θ (deg) Fig. 1. Room-temperature powder X-ray diffraction patterns of (a) LaPtSi, (b) (La0.75Th0.25)PtSi, and (c) (La0.5Th0.5)PtSi using Cu Ka radiation.
a field of 10 Oe for the temperature range between 1.8 and 4.0 K. Fig. 2 depicts the zero-field-cooled (ZFC) and field-cooled (FC) magnetization for the three (La1 xThx)PtSi alloys with x = 0, 0.25 and 0.5. All measurements were carried out on bulk samples of about 0.2 g mass. For each sample, the ZFC curve shows sharp transition and reaches to saturation at the lower temperature. The large shielding signal for each sample indicates the perfect superconducting volume fraction. The 10–90% values of the transition are taken as the superconducting transition temperature. The data thus obtained are: LaPtSi (Tc = 3.18 3.37 K), (La0.75Th0.25)PtSi (Tc = 2.58 3.00 K) and (La0.5Th0.5)PtSi (Tc = 2.52 2.68 K). It is found that the Tc change rate dTc/dx = 1.35 ± 0.03 K and dTc/ dv = 0.070 ± 0.002 K/Å3 for the (La1 xThx)PtSi system. In a short concluding remark, above results demonstrate that the stiffening of the lattice under pressure has a more important
Fig. 2. Temperature dependence of ZFC and FC magnetization data for three compounds LaPtSi, (La0.75Th0.25)PtSi and (La0.5Th0.5)PtSi measured in a field of 10 Oe between 1.8 and 4.0 K.
effect on the decrease in Tc in this (La1 xThx)PtSi system. However, compared to the system La(Pt1 x Nix)Si (dTc/dv = 0.127 ± 0.043 K/ Å3) [6], the increase of charge carrier density by Th doping in LaPtSi indeed relax the Tc decreasing rate. Acknowledgment This work was supported by the National Science Council of the Republic of China under Contract No. NSC-96-2112-M-194-008MY3. References [1] M.C. Krupka, A.I. Giorgi, N.H. Krikorian, E.G. Szklarz, J. Less-Common Met. 19 (1969) 113. [2] A.I. Giorgi, E.G. Szklarz, M.C. Krupka, in: D.H. Douglas (Ed.), Superconductivity in d- and f-Band Metals, Am. Inst. Phys., New York, 1972, p. 147. [3] W.H. Lee, H.K. Zeng, Solid State Commun. 101 (1997) 323. [4] K.J. Syu, H.H. Sung, W.H. Lee, Solid State Commun. 141 (2007) 519. [5] J. Evers, G. Oehlinger, A. Weiss, C. Probst, Solid State Commun. 50 (1984) 61. [6] W.H. Lee, Solid State Commun. 94 (1995) 425. [7] L.H. Bennett (Ed.), Theory of Alloy Phases Formation, Proceedings of a Symposium at the 108th AIME Annual Meeting, New Orleans, LA, 1979. [8] W.X. Zhong, W.L. Ng, B. Chevalier, J. Etourneau, P. Hagenmuller, Mater. Res. Bull. 20 (1985) 1229. [9] G. Nolze, W. Kraus, PowderCell 2.3 Program, BAM, Berlin, 2000.