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Iron based oxypnictides: Structure and properties
Inorganica Chimica Acta 372 (2011) 2–7
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Review
Iron based oxypnictides: Structure and properties J. Prakash, A.K. Ganguli ⇑ Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India
a r t i c l e
i n f o
Article history: Available online 28 January 2011 On the occasion of the 70th birth anniversary of Prof. S.S. Krishnamurthy. Keywords: Oxypnictides Crystal structure Superconductivity
a b s t r a c t Metals, alloys and metal oxides have been investigated over several decades for their interesting electronic properties including superconductivity. However the pnictides (multinary compounds containing at least one pnictogen) have not been so popular for their superconducting properties. That has changed recently (2008) after the discovery of superconductivity at 26 K in fluoride-doped LaO0.9F0.1FeAs by Kamihara et al. [1] which has created a huge excitement in the field of superconductivity and pnictides. Since then a large number of pnictogen based superconductors were discovered. This article summarizes the structure and properties of the parent compounds, LnOFePn (Ln = La, Ce and Pr; Pn = P and As). Effect of Fe–Pn hybridization on structural distortion, electronic and magnetic properties of LnOFePn is examined. Ó 2011 Elsevier B.V. All rights reserved.
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystal structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. LnOFePn (Ln = La, Ce and Pr; Pn = P and As) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Structural transition in LnOFePn (Ln = La, Ce and Pr; Pn = P and As) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical and magnetic properties of LnOFePn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. LnOFeP (Ln = La, Ce and Pr) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. LnOFeAs (Ln = La, Ce and Pr). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Fe–As Hybridization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Pnictogens (P, As, Sb and Bi) form a large number of alloys and intermetallic compounds with most of the metals. The metal nitrides have significantly different properties as compared to the other pnictides. These pnictogen based compounds show very interesting and complex crystal structures. Most of the metal pnictides are either metallic or semiconducting in nature and are being used for variety of applications. For example gallium and indium pnictides (GaAs and InAs) are widely used as semiconductors while YbxCo4Sb12 and Bi2Te3 are very good thermoelectric materials [2,3]. In the past decade antimony and bismuth based compounds are mainly investigated in detail for their thermoelectric properties. ⇑ Corresponding author. Tel.: +91 11 26591511; fax: +91 11 26854715. E-mail address: [email protected] (A.K. Ganguli). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.01.055
2 3 3 4 5 5 5 6 6 6 6
SnAs and Sn4As3 are two pnictides known to have a superconducting property [4] till 2006 when surprisingly superconductivity in an oxyphosphide (LaOFeP) was discovered by Kamihara et al. [5] having a Tc of 4 K. However this report did not receive much attention due to the low superconducting transition temperature (Tc). After two years the same group reported superconductivity in fluoride-doped oxyarsenide (LaO1 xFxFeAs) with Tc 26 K which gave researchers a new hope to discover high temperature superconductors [1]. After this report, new rare earth based Ln(O/F)FeAs superconductors [6–9] were discovered and Tc was enhanced to 55 K [10] in SmO/FFeAs by substituting the smaller Sm ion in place of La. Superconductivity was also achieved by cobalt and thorium doping in LnOFe1 xCoxAs [11–13] and Ln1 xThxOFeAs [14,15], respectively. These LnOFeP and LnOFeAs pnictides were first synthesized by Jeitschko group in 1995 [16] and 2000 [17], respectively. These LnOFePn compounds crystallize in well known
J. Prakash, A.K. Ganguli / Inorganica Chimica Acta 372 (2011) 2–7
3
Fig. 2. FeAs4 tetrahedra in LnOFeAs (Ln = rare earth).
Fig. 1. Crystal structure of LaOFeAs (space group: P4/nmm).
tetragonal structure (space group: P4/nmm) [16,17]. Following this discovery other Fe-based superconducting compounds with general formula AFe2Pn2 (A = K, Na, Ca, Ba and Sr; Pn = P and As) [18–22], AFeAs (A = Na and Li) [23,24], FeS/Se/Te [25–27] were discovered. In this report we present a detailed review on structural, electrical and magnetic properties of undoped LnOFePn (Ln = La, Ce and Pr; Pn = P and As) compounds which show interesting structural and physical properties.
2. Crystal structure 2.1. LnOFePn (Ln = La, Ce and Pr; Pn = P and As) These compounds with the general formula LnOFePn were first discovered by Jeitschko’s group in 1995. Structure refinement on LnOFePn single crystals show that these compounds crystallize in the well known ZrCuSiAs type structure (Fig. 1) [28]. The ZrCuSiAs type structure is a filled variant of PbFCl structure [28]. The PbFCltype structure (sp. gp P4/nmm) could be understood as Pb atoms forming the cubic close packed type structure where Cl atoms occupy the octahedral and F atom occupy one-half of the tetrahedral voids in alternate layers. This arrangement of atoms gives ½ of the tetrahedral positions vacant. In this structure Pb and Cl atoms occupies 2c crystallographic site while F atoms occupies 2a crystallographic site [29]. ZrCuSiAs type structure is obtained by filling the vacant one half of the tetrahedral voids (2b crystallographic sites) in PbFCl structure [28]. The structure of LnOFePn (Fig. 1) can be regarded as alternative tetrahedral layers of Ln–O and Fe–Pn stacked in c-direction. Both rare-earth and pnictogen atoms occupy 2c crystallographic site while oxygen and iron atoms occupy the 2a and 2b crystallographic site, respectively [16,17]. The Ln ions form distorted square antiprism by coordinating with four pnictogen and four oxygen atoms forming. The iron and oxygen atoms are tetrahedrally connected to four pnictogen and four Ln atoms, respectively [16,17]. The tetrahedra FePn4 and OLn4 are distorted which results in two type of Pn–Fe–Pn and Ln–O–Ln bond angles (Fig. 2). The Pn– Fe–Pn bonds which are above and below the Fe plane with multiplicity of two designated as a while other Pn–Fe–Pn bonds with multiplicity of four are designated as b. The iron atoms are
Fig. 3. Square planar arrangement of Fe atom in LnOFePn (Ln = rare earth and Pn = pnictogen).
connected to each other in a square planar fashion (Fig. 3) and pnictogen atoms are on the top and bottom of the square plane. Fig. 4 shows the variation of lattice parameters (a and c) with rare earth metals for LnOFePn (Pn = P and As) [5,16,17]. Both the lattice parameters (a and c) decrease with the substitution of smaller rare-earth metal ions. Fig. 5 shows the variation of Fe–As bond length with different rare earth metal in LnOFeAs [17,30,31]. The Fe–As bond length decreases with the decrease in ionic size of rare earth metal ion in LnOFeAs compounds. The Fe–P bond length in LnOFeP (Table 1) [16,32] is smaller as compared to LnOFeAs which could be attributed to smaller ionic size of phosphorous ion as compared to arsenic ion. The substitution of smaller rare-earth ion at the Ln site in LnOFeAs results in shortening of c lattice parameter. As a result the Ln– O layer comes closer to Fe–As layer, thereby reducing the Fe–As bond length. The decrease in Fe–As bond length results in enhancement of overlapping of Fe(3d) and As(4p) orbitals which affects the extent of hybridization between Fe and As orbitals. Similarly the Fe–Fe bond distance decreases on substitution of smaller rareearth metal ions in these LnOFeAs compounds due to smaller a lattice parameter (Fig. 6) [17,30,31]. The variation of ‘a’ and ‘b’ As–Fe–As bond angle with different rare earth ions in LnOFeAs [17,30,31] is shown in Fig. 7. The value
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8.8
4.05
(b)
LnOFeP LnOFeAs
c lattice parameter (Å)
a lattice parameter (Å)
(a) 4.00
3.95
LnOFeP LnOFeAs
8.7
8.6
8.5
8.4
8.3
3.90
La
La
Pr
Ce Ln
Ce
Pr
Ln
Fig. 4. Variation of (a) a-lattice parameter and (b) c-lattice parameter with rare-earth in LnOFePn (Ln = La, Ce and Pr; Pn = P and As). The a and c-lattice parameters are taken from Ref. [17] for LnOFeAs (Ln = La, Ce and Pr), Ref. [5] for LaOFeP and Ref. [16] for LnOFeP (Ln = Ce and Pr).
angle is close to ideal tetrahedral angle (109.47°) [33]. Claderon et al. [34] have reported the dependence of ‘a’ Pn–Fe–Pn angle on the band structure within a Slater–Koster-based tight-binding model for iron containing pnictides (LnOFePn and AFe2As2) where they show that the density of states (DOS) and Fermi surface are strongly sensitive to changes in the ‘a’ Pn–Fe–Pn angle.
2.46
LnOFeAs 2.45
Fe-As (Å)
2.44 2.43 2.42
2.2. Structural transition in LnOFePn (Ln = La, Ce and Pr; Pn = P and As)
2.41 2.40
La
Ce
Pr
Ln Fig. 5. Variation of Fe–As bond length with rare earth in LnOFeAs (Ln = La, Ce and Pr). The Fe-As distances are computed using atomic positions given in Ref. [30] for LaOFeAs, Ref. [31] for CeOFeAs and Ref. [17] for PrOFeAs.
of ‘a’ As–Fe–As bond angle is higher than the ‘b’ As–Fe–As bond angle in all LnOFeAs compounds. The variation of ‘a’ As–Fe–As and ‘b’ As–Fe–As bond angle in different rare earth based LnOFeAs compound is different. The a As–Fe–As bond angle first increases with the substitution of Ce at the La site and then decreases on substituting smaller rare earth metal ion (Pr). On the other hand the ‘b’ As–Fe–As bond angle first decreases for CeOFeAs and then increases on substituting smaller rare earth ions at the Ln site. For LnOFeP, ‘a’ P–Fe–P angle contracts on substituting smaller rare earth ion while ‘b’ P–Fe–P angle increases. Substitution of smaller rare earth (Pr) ion at the La site in LaOFeP results in a synergistic increase of the Fe–P bond distances (Table 1) which pushes the P ions further away from the Fe square plane thereby decreasing the ‘a’ P–Fe–P angle. In Ln(O/F)FeAs superconductors enhancement in Tc was observed on substituting smaller rare earth which could be attribute to reduction of ‘a’ As–Fe–As angle. It has been observed that the maximum Tc was achieved when ‘a’ Pn–Fe–Pn
LnOFeP (Ln = La, Ce and Pr) compounds do not show any structural distortion from tetragonal structure on cooling [32,35]. However, the As-compounds, LnOFeAs (Ln = La, Ce and Pr) show structural distortion from tetragonal to monoclinic/orthorhombic symmetry. Variable temperature neutron diffraction studies on LaOFeAs have been reported by Cruz et al. [36]. For the undoped LaOFeAs, the (2, 2, 0) nuclear reflection showed a splitting at 155 K confirming the structural phase transition below this temperature. The structure changes from tetragonal (space group: P4/ nmm) to monoclinic (space group: P112/n). This structural distortion is suppressed on fluorine doping (electron doping) as indicated by the absence of splitting of (2, 2, 0) nuclear reflection for the doped LaO0.92F0.08FeAs superconductor [36]. Variable temperature neutron diffraction studies have also been carried out for CeOFeAs [31] and PrOFeAs [37]. Structural distortion from tetragonal to orthorhombic symmetry was confirmed by observation of splitting of (2, 2, 0) reflection of tetragonal phase into the (0, 4, 0) and (4, 0, 0) reflections in the orthorhombic phase below 158 K [31] and 153 K [37] for CeOFeAs and PrOFeAs, respectively. The magnetic moment (contribution by Fe) for LaOFeAs was calculated by normalizing the magnetic intensity to neutron scattering and was found to be 0.36 lB/Fe [36] which is much smaller than the predicted value of 2.3 lB per iron atom [38]. Similarly, Fe moments have been estimated for CeOFeAs and PrOFeAs. The effective moment of ‘Fe’ and TN for LnOFeAs (Ln = La, Ce and Pr) compounds [36,31,39] are given in Table 2. For CeOFeAs, Ce
Table 1 Important structural parameters of LaOFeP and PrOFeP. S.No.
Composition
Lattice parameter (Å)
Fe–P (Å)
Fe–Fe (Å)
Ln–O (Å)
a-P–Fe–P (°)
b-P–Fe–P (°)
References
1
LaOFeP
2.290
2.802
2.351
119.87
104.53
[32]
2
PrOFeP
a = 3.96307(4) c = 8.5087(1) a = 3.9113(6) c = 8.345(2)
2.327
2.766
2.285
114.34
107.09
[16]
J. Prakash, A.K. Ganguli / Inorganica Chimica Acta 372 (2011) 2–7
LnOFeAs
Fe-Fe (Å)
2.85
2.84
2.83
2.82
La
Ce
Pr
Ln Fig. 6. Variation of Fe–As bond length with rare earth in LnOFeAs (Ln = La, Ce and Pr). The Fe-As distances are computed using atomic positions given in Ref. [30] for LaOFeAs, Ref. [31] for CeOFeAs and Ref. [17] for PrOFeAs.
109.2
LnOFeAs
112.0
109.0 108.8
111.5
108.6
111.0
108.4 108.2
110.5
As-Fe-As (β) (in degree)
As-Fe-As(α) (in degree)
112.5
108.0 110.0
La
Ce
Pr
Ln Fig. 7. Variation of ‘a’ and ‘b’ As–Fe–As angle with rare earth in LnOFeAs (Ln = La, Ce and Pr). The As–Fe–As angles are computed using atomic positions given in Ref. [30] for LaOFeAs, Ref. [31] for CeOFeAs and Ref. [17] for PrOFeAs.
Table 2 Observed Fe moment and TN for LnOFeAs from neutron diffraction. S. No.
Compounds
Fe moment (lB)
TN (K)
References
1 2 3
LaOFeAs CeOFeAs PrOFeAs
0.36 0.8 0.35
137 140 136
[36] [31] [39]
moments order at 4 K [31] with moments lying in the a–b plane while Pr moments order below 14 K [37] with the Pr spins aligned along the c axis.
3. Electrical and magnetic properties of LnOFePn 3.1. LnOFeP (Ln = La, Ce and Pr) Resistivity measurements on polycrystalline LaOFeP shows metallic behavior in the temperature range between 5–300 K. A sudden drop in resistivity was observed near 4 K indicating onset of superconductivity [5]. LaOFeP is superconducting without additional doping of electrons or holes. Doping of electrons by substitution of fluoride ions at oxygen site in LaO1 xFxFeP results in enhancement of Tc to 8 K [5]. Negative sign of the Seebeck
5
coefficient indicates electrons as charge carrier in LaO1 xFxFeP superconductors [5]. McQueen et al. [32] reported the DC and AC magnetization, resistivity and specific-heat measurement of polycrystalline LaOFeP. They have synthesized LaOFeP using stoichiometric amounts of LaP, Fe and Fe3O4 by sealed tube method. They added 5% excess P to compensate for any P loss during heat treatment. Neutron diffraction studies of LaOFeP at various temperatures showed full occupancy of La, O, P and Fe ions. Metallic behavior was observed for this compound and no trace of superconductivity was detected above T = 0.35 K. McQueen et al. [32] suggested that the presence of superconductivity in LaOFeP reported by Kamihara et al. [5] is due to oxygen deficiency. Hamlin et al. [40] reported superconductivity in LaOFeP single crystals with the superconducting transition (Tc) of 6.6 K which could be enhanced to 7.8 K on reducing the compound in flowing Ar at 700 °C suggesting that decrease in oxygen content may lead to enhancement in Tc [40]. Pressure dependent resistivity studies [40] of LaOFeP single crystal reveals enhancement in Tc to 14 K from 7 K under 110 kbar pressure. Further increase in pressure results in suppression of Tc in LaOFeP superconductor. However, specific heat measurement of LaOFeP single crystal does not show a discontinuity at Tc, suggesting that stoichiometric LaOFeP does not exhibit bulk superconductivity, though some reports do suggest possibility of bulk superconductivity at low temperature [41]. The resistivity varies linearly with temperature in CeOFeP [42] between 300 K and 30 K. PrOFeP show metallic behavior up to 3.2 K below which superconductivity was observed [43]. It could be seen that the Tc decreases on substitution of smaller rare earth in LnOFeP superconductors. Thus, the variation of Tc with rare earth in LnOFeP is opposite to Ln(O/F)FeAs where substitution of smaller rare earth results in the enhancement of Tc. A strong sensitivity of the valence-band (VB) structure to the lattice parameters and to interaction with localized f states is responsible for variation of properties of LnOFePn (Pn = P and As) [44–46]. The electronic structure of LnOFePn near the Fermi level (EF) is dominated by five energy bands which have mainly Fe 3d character [44,45]. Small variations of the lattice parameters mainly affect the bands formed from dxy and d3z2 r2 orbitals. The dxy and d3z2 r2 derived bands shift towards lower and higher binding energies (B.E.), respectively on increasing the distance between Pn ions and Fe plane. Holder el al. [46] has studied the electronic structure of CeOFeP by angle-resolved photoemission (ARPES) spectroscopy to get insights for the different behavior of CeOFeP as compared to the La analog. They found marked change in the position and the dispersion of the valence bands near EF for CeOFeP as compared to LaOFeP due to interactions with Ce 4f states. Also the hybridization between Fe 3d-derived valence-bands and the Ce 4f states leads to a reconstruction of the Fermi surface which may be responsible for absence of superconductivity in CeOFeP. 3.2. LnOFeAs (Ln = La, Ce and Pr) Fig. 8 shows the variation of resistivity with temperature for LnOFeAs (Ln = La, Ce and Pr) which reveals semimetallic behavior of all the three arsenides. LaOFeAs exhibits a sudden decrease of resistivity at 150 K due to structural transition from tetragonal to monoclinic symmetry. The resistivity continues to decrease below 150 K and shows a minimum at 70 K. For CeOFeAs, the resistivity first increased up to 160 K and then a sudden fall was observed which can be attributed to the structural distortion from tetragonal to orthorhombic symmetry [31]. Further cooling leads to decrease in resistivity up to the lowest temperature measured. PrOFeAs also shows an anomaly characterized by a drop in resistivity at 155 K. This anomalous resistivity drop is associated with a
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smaller in LaOFeAs as compared to LaOFeP which accounts for the better metallic character (low value of resistivity) of latter as compared to the former. The magnetic studies of LnOFeAs (Ln = La, Ce and Pr) show linear temperature dependence of magnetic susceptibility in the high temperature paramagnetic state. This type of behavior is neither Pauli nor Curie-Wess like and is similar to the behavior of metallic Cr which also exhibits a spin density wave (SDW) state. All the undoped LnOFeAs compounds show long range antiferromagnetic order. The TN for LnOFeAs compounds are given in Table 2.
Resistivity (mohm-cm)
8 CeOFeAs PrOFeAs LaOFeAs
6
4
2
3.3. Fe–As Hybridization 0
25
50
75
100
125 150
175
200
Temperature (K) Fig. 8. The temperature dependence of resistivity of LnOFeAs (Ln = La, Ce and Pr).
Table 3 Superconducting resistivity(q(RT)).
transition
temperature
(Tc)
and
room
temperature
S. No.
Compounds
q (RT) (m O cm)
Tc (K)
References
1 3 4 5 6 7 7 8
LaOFeP LaOFeP CeOFeP PrOFeP LaOFeAs LaOFeAs CeOFeAs PrOFeAs
1.51 2.1 5.2 0.3 5 4.8 5.4 3
Non 4 Non 3.2 Non Non Non Non
[32] [5] [42] [43] [1] [47] [48] [49]
SC SC SC SC SC SC
Jishi et al. [51] have reported the effect of hybridization on structural and electronic properties of LnOFeAs compounds. Their theoretical calculations suggest strong hybridization between the Fe 3d orbitals and As 4p orbitals. The occupation scheme of Fe 3d levels in tetragonal and low temperature orthorhombic structure in LnOFeAs is illustrated in Fig. 9. Structural distortion from tetragonal to orthorhombic symmetry in parent LnOFeAs results in change in electronic energy which removes degeneracy of the Fe 3dxz and 3dyz levels [50]. Their studies also indicate that electron doping by substitution of fluoride ion at the oxygen site induces lowering of symmetry which suppresses structural distortion and Fe magnetic moments. Hybridization between the Fe 3d and the As 4p states leads to spin fluctuations on the Fe sites which may play a role in suppressing the magnetic order and the emergence of superconductivity in these arsenides [51]. 4. Conclusions
structural distortion from tetragonal to orthorhombic symmetry. Below the occurrence of structural transition and SDW, the resistivity drops steeply and then becomes almost independent of temperature (below 84 K). A down turn was observed below 10 K which is attributed to the decrease in scattering due to the ordering of the Pr moments. Unlike LaOFeP, where differing reports (superconducting by some and non superconducting (metallic) by other groups) exist, the arsenide analog, LnOFeAs have been found to show semimetallic behavior by all groups [47–49]. Yan et al. [50] have studied the temperature dependence of in-plane electrical resistivity on single crystals of LaOFeAs. Similar to polycrystalline LaOFeAs, semimetallic behavior was observed with anomaly at 153 K in resistivity measurement for LaOFeAs single crystals [50]. The room temperature resistivity of LnOFeAs [1,5,32,42,43,47–49] is given in Table 3. It may be note that the As compounds show high room temperature resistivity as compared to the phosphides (LnOFeP). Vildosola et al. [44] have studied the band structure of LaOFeAs and LaOFeP using first-principles full-potential electronic structure calculations. Their calculations showed the band width of Fe near the Fermi surface is
Fig. 9. The occupation scheme of Fe(II) 3d levels in tetragonal and low temperature orthorhombic structure in LnOFeAs (adopted from Ref. [51]).
Both LnOFeP and LnOFeAs compounds crystallize in tetragonal ZrCuSiAs type structure at room temperature. LnOFeAs exhibit structural distortion on cooling while LnOFeP retain tetragonal structure at low temperature. Dramatic change in electrical and magnetic properties of LnOFeP is observed on replacing P with As. LnOFeP are superconducting while LnOFeAs compounds are antiferromagnetic semimetals. These variation in structure (at low temperature), electrical and magnetic properties of LnOFePn (Ln = rare earth; Pn = P and As) may be attributed to extensive hybridization between iron and pnictogen. Acknowledgments A.K.G. thanks DST, Govt. of India for financial support. J.P. thanks CSIR Govt. of India, for fellowship. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130 (2008) 3296. [2] N.R. Dilley, E.D. Bauer, M.B. Maple, Phys. Rev. B 61 (2000) 4608. [3] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’Quinn, Nature 13 (2001) 597. [4] S. Geller, G.W. Hull, Phys. Rev. Lett. 13 (1964) 127. [5] Y. Kamihara, H. Hiramatsu, M. Hirano, R. Kawamura, H. Yanagi, T. Kamiya, H. Hosono, J. Am. Chem. Soc. 128 (2006) 10012. [6] J. Prakash, S.J. Singh, S. Patnaik, A.K. Ganguli, Physica C 469 (2009) 82. [7] J. Prakash, S.J. Singh, A. Banerjee, S. Patnaik, A.K. Ganguli, App. Phys. Lett. 95 (2009) 262507. [8] J. Yang, Z.C. Li, W. Lu, W. Yi, X.L. Shen, Z.A. Ren, G.C. Che, X.L. Dong, L.L. Sun, F. Zhou, Z.-X. Zhao, Supercond. Sci. Technol. 21 (2008) 082001. [9] A. Ren, J. Yang, W. Lu, W. Yi, G.C. Che, X.L. Dong, L.L. Sun, Z.X. Zhao, Mater. Res. Innovations 12 (2008) 105. [10] Z.A. Ren, L. Wei, Y. Jie, Y. Wei, S.X. Li, L.Z. Cai, C.G. Can, D.X. Li, S.L. Ling, Z. Fang, Z.Z. Xian, Chin. Phys. Lett. 25 (2008) 2215. [11] A.S. Sefat, A. Huq, M.A. McGuire, R. Jin, B.C. Sales, D. Mandrus, L.M.D. Cranswick, P.W. Stephens, K.H. Stone, Phys. Rev. B 78 (2008) 104505. [12] J. Prakash, S.J. Singh, S. Patnaik, A.K. Ganguli, Solid State Commun.149 (2009) 181.
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Mr. Jai Prakash obtained his B.Sc. (Hons) in Chemistry from Atma Ram Sanatan Dharma College, University of Delhi in 2004. Later he obtained his M.Sc. in chemistry from I.I.T Roorkee with specialization in Analytical Chemistry in 2006. Currently he is working as a Ph.D. student in the Department of Chemistry, I.I.T Delhi under the supervision of Prof. A.K. Ganguli on New Iron Containing Oxypnictide Superconductors.
Prof. Ashok K. Ganguli obtained his M.Sc. degree in Chemistry from the University of Delhi (1984) and Ph.D. from Indian Institute of Science Bangalore, India (1990). He then worked at the Central Research and Development Department of DuPont Company at Wilmington Delaware, USA (1990–91). and Ames Laboratory, USDOE at Iowa state University. In 1995 he joined the department of Chemistry at the Indian Institute of Technology, Delhi. Dr. Ganguli has contributed extensively to the field of Solid State and Materials Chemistry especially oxides (dielectric and superconducting, polar intermetallics (Zintl phases).and nanocrystalline materials. He has published around 150 papers in international journals and contributed 6 chapters in books. Prof. Ganguli is the recipient of the Materials Research Society of India Medal for 2006, the Chemical Research Society of India Medal for 2007 and is a Fellow of the Indian Academy of Sciences. He is an Associate editor of Bulletin of Materials Sciences and an editorial Board member of Indian J. Chem. A.
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Review
Iron based oxypnictides: Structure and properties J. Prakash, A.K. Ganguli ⇑ Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India
a r t i c l e
i n f o
Article history: Available online 28 January 2011 On the occasion of the 70th birth anniversary of Prof. S.S. Krishnamurthy. Keywords: Oxypnictides Crystal structure Superconductivity
a b s t r a c t Metals, alloys and metal oxides have been investigated over several decades for their interesting electronic properties including superconductivity. However the pnictides (multinary compounds containing at least one pnictogen) have not been so popular for their superconducting properties. That has changed recently (2008) after the discovery of superconductivity at 26 K in fluoride-doped LaO0.9F0.1FeAs by Kamihara et al. [1] which has created a huge excitement in the field of superconductivity and pnictides. Since then a large number of pnictogen based superconductors were discovered. This article summarizes the structure and properties of the parent compounds, LnOFePn (Ln = La, Ce and Pr; Pn = P and As). Effect of Fe–Pn hybridization on structural distortion, electronic and magnetic properties of LnOFePn is examined. Ó 2011 Elsevier B.V. All rights reserved.
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystal structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. LnOFePn (Ln = La, Ce and Pr; Pn = P and As) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Structural transition in LnOFePn (Ln = La, Ce and Pr; Pn = P and As) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical and magnetic properties of LnOFePn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. LnOFeP (Ln = La, Ce and Pr) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. LnOFeAs (Ln = La, Ce and Pr). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Fe–As Hybridization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Pnictogens (P, As, Sb and Bi) form a large number of alloys and intermetallic compounds with most of the metals. The metal nitrides have significantly different properties as compared to the other pnictides. These pnictogen based compounds show very interesting and complex crystal structures. Most of the metal pnictides are either metallic or semiconducting in nature and are being used for variety of applications. For example gallium and indium pnictides (GaAs and InAs) are widely used as semiconductors while YbxCo4Sb12 and Bi2Te3 are very good thermoelectric materials [2,3]. In the past decade antimony and bismuth based compounds are mainly investigated in detail for their thermoelectric properties. ⇑ Corresponding author. Tel.: +91 11 26591511; fax: +91 11 26854715. E-mail address: [email protected] (A.K. Ganguli). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.01.055
2 3 3 4 5 5 5 6 6 6 6
SnAs and Sn4As3 are two pnictides known to have a superconducting property [4] till 2006 when surprisingly superconductivity in an oxyphosphide (LaOFeP) was discovered by Kamihara et al. [5] having a Tc of 4 K. However this report did not receive much attention due to the low superconducting transition temperature (Tc). After two years the same group reported superconductivity in fluoride-doped oxyarsenide (LaO1 xFxFeAs) with Tc 26 K which gave researchers a new hope to discover high temperature superconductors [1]. After this report, new rare earth based Ln(O/F)FeAs superconductors [6–9] were discovered and Tc was enhanced to 55 K [10] in SmO/FFeAs by substituting the smaller Sm ion in place of La. Superconductivity was also achieved by cobalt and thorium doping in LnOFe1 xCoxAs [11–13] and Ln1 xThxOFeAs [14,15], respectively. These LnOFeP and LnOFeAs pnictides were first synthesized by Jeitschko group in 1995 [16] and 2000 [17], respectively. These LnOFePn compounds crystallize in well known
J. Prakash, A.K. Ganguli / Inorganica Chimica Acta 372 (2011) 2–7
3
Fig. 2. FeAs4 tetrahedra in LnOFeAs (Ln = rare earth).
Fig. 1. Crystal structure of LaOFeAs (space group: P4/nmm).
tetragonal structure (space group: P4/nmm) [16,17]. Following this discovery other Fe-based superconducting compounds with general formula AFe2Pn2 (A = K, Na, Ca, Ba and Sr; Pn = P and As) [18–22], AFeAs (A = Na and Li) [23,24], FeS/Se/Te [25–27] were discovered. In this report we present a detailed review on structural, electrical and magnetic properties of undoped LnOFePn (Ln = La, Ce and Pr; Pn = P and As) compounds which show interesting structural and physical properties.
2. Crystal structure 2.1. LnOFePn (Ln = La, Ce and Pr; Pn = P and As) These compounds with the general formula LnOFePn were first discovered by Jeitschko’s group in 1995. Structure refinement on LnOFePn single crystals show that these compounds crystallize in the well known ZrCuSiAs type structure (Fig. 1) [28]. The ZrCuSiAs type structure is a filled variant of PbFCl structure [28]. The PbFCltype structure (sp. gp P4/nmm) could be understood as Pb atoms forming the cubic close packed type structure where Cl atoms occupy the octahedral and F atom occupy one-half of the tetrahedral voids in alternate layers. This arrangement of atoms gives ½ of the tetrahedral positions vacant. In this structure Pb and Cl atoms occupies 2c crystallographic site while F atoms occupies 2a crystallographic site [29]. ZrCuSiAs type structure is obtained by filling the vacant one half of the tetrahedral voids (2b crystallographic sites) in PbFCl structure [28]. The structure of LnOFePn (Fig. 1) can be regarded as alternative tetrahedral layers of Ln–O and Fe–Pn stacked in c-direction. Both rare-earth and pnictogen atoms occupy 2c crystallographic site while oxygen and iron atoms occupy the 2a and 2b crystallographic site, respectively [16,17]. The Ln ions form distorted square antiprism by coordinating with four pnictogen and four oxygen atoms forming. The iron and oxygen atoms are tetrahedrally connected to four pnictogen and four Ln atoms, respectively [16,17]. The tetrahedra FePn4 and OLn4 are distorted which results in two type of Pn–Fe–Pn and Ln–O–Ln bond angles (Fig. 2). The Pn– Fe–Pn bonds which are above and below the Fe plane with multiplicity of two designated as a while other Pn–Fe–Pn bonds with multiplicity of four are designated as b. The iron atoms are
Fig. 3. Square planar arrangement of Fe atom in LnOFePn (Ln = rare earth and Pn = pnictogen).
connected to each other in a square planar fashion (Fig. 3) and pnictogen atoms are on the top and bottom of the square plane. Fig. 4 shows the variation of lattice parameters (a and c) with rare earth metals for LnOFePn (Pn = P and As) [5,16,17]. Both the lattice parameters (a and c) decrease with the substitution of smaller rare-earth metal ions. Fig. 5 shows the variation of Fe–As bond length with different rare earth metal in LnOFeAs [17,30,31]. The Fe–As bond length decreases with the decrease in ionic size of rare earth metal ion in LnOFeAs compounds. The Fe–P bond length in LnOFeP (Table 1) [16,32] is smaller as compared to LnOFeAs which could be attributed to smaller ionic size of phosphorous ion as compared to arsenic ion. The substitution of smaller rare-earth ion at the Ln site in LnOFeAs results in shortening of c lattice parameter. As a result the Ln– O layer comes closer to Fe–As layer, thereby reducing the Fe–As bond length. The decrease in Fe–As bond length results in enhancement of overlapping of Fe(3d) and As(4p) orbitals which affects the extent of hybridization between Fe and As orbitals. Similarly the Fe–Fe bond distance decreases on substitution of smaller rareearth metal ions in these LnOFeAs compounds due to smaller a lattice parameter (Fig. 6) [17,30,31]. The variation of ‘a’ and ‘b’ As–Fe–As bond angle with different rare earth ions in LnOFeAs [17,30,31] is shown in Fig. 7. The value
4
J. Prakash, A.K. Ganguli / Inorganica Chimica Acta 372 (2011) 2–7
8.8
4.05
(b)
LnOFeP LnOFeAs
c lattice parameter (Å)
a lattice parameter (Å)
(a) 4.00
3.95
LnOFeP LnOFeAs
8.7
8.6
8.5
8.4
8.3
3.90
La
La
Pr
Ce Ln
Ce
Pr
Ln
Fig. 4. Variation of (a) a-lattice parameter and (b) c-lattice parameter with rare-earth in LnOFePn (Ln = La, Ce and Pr; Pn = P and As). The a and c-lattice parameters are taken from Ref. [17] for LnOFeAs (Ln = La, Ce and Pr), Ref. [5] for LaOFeP and Ref. [16] for LnOFeP (Ln = Ce and Pr).
angle is close to ideal tetrahedral angle (109.47°) [33]. Claderon et al. [34] have reported the dependence of ‘a’ Pn–Fe–Pn angle on the band structure within a Slater–Koster-based tight-binding model for iron containing pnictides (LnOFePn and AFe2As2) where they show that the density of states (DOS) and Fermi surface are strongly sensitive to changes in the ‘a’ Pn–Fe–Pn angle.
2.46
LnOFeAs 2.45
Fe-As (Å)
2.44 2.43 2.42
2.2. Structural transition in LnOFePn (Ln = La, Ce and Pr; Pn = P and As)
2.41 2.40
La
Ce
Pr
Ln Fig. 5. Variation of Fe–As bond length with rare earth in LnOFeAs (Ln = La, Ce and Pr). The Fe-As distances are computed using atomic positions given in Ref. [30] for LaOFeAs, Ref. [31] for CeOFeAs and Ref. [17] for PrOFeAs.
of ‘a’ As–Fe–As bond angle is higher than the ‘b’ As–Fe–As bond angle in all LnOFeAs compounds. The variation of ‘a’ As–Fe–As and ‘b’ As–Fe–As bond angle in different rare earth based LnOFeAs compound is different. The a As–Fe–As bond angle first increases with the substitution of Ce at the La site and then decreases on substituting smaller rare earth metal ion (Pr). On the other hand the ‘b’ As–Fe–As bond angle first decreases for CeOFeAs and then increases on substituting smaller rare earth ions at the Ln site. For LnOFeP, ‘a’ P–Fe–P angle contracts on substituting smaller rare earth ion while ‘b’ P–Fe–P angle increases. Substitution of smaller rare earth (Pr) ion at the La site in LaOFeP results in a synergistic increase of the Fe–P bond distances (Table 1) which pushes the P ions further away from the Fe square plane thereby decreasing the ‘a’ P–Fe–P angle. In Ln(O/F)FeAs superconductors enhancement in Tc was observed on substituting smaller rare earth which could be attribute to reduction of ‘a’ As–Fe–As angle. It has been observed that the maximum Tc was achieved when ‘a’ Pn–Fe–Pn
LnOFeP (Ln = La, Ce and Pr) compounds do not show any structural distortion from tetragonal structure on cooling [32,35]. However, the As-compounds, LnOFeAs (Ln = La, Ce and Pr) show structural distortion from tetragonal to monoclinic/orthorhombic symmetry. Variable temperature neutron diffraction studies on LaOFeAs have been reported by Cruz et al. [36]. For the undoped LaOFeAs, the (2, 2, 0) nuclear reflection showed a splitting at 155 K confirming the structural phase transition below this temperature. The structure changes from tetragonal (space group: P4/ nmm) to monoclinic (space group: P112/n). This structural distortion is suppressed on fluorine doping (electron doping) as indicated by the absence of splitting of (2, 2, 0) nuclear reflection for the doped LaO0.92F0.08FeAs superconductor [36]. Variable temperature neutron diffraction studies have also been carried out for CeOFeAs [31] and PrOFeAs [37]. Structural distortion from tetragonal to orthorhombic symmetry was confirmed by observation of splitting of (2, 2, 0) reflection of tetragonal phase into the (0, 4, 0) and (4, 0, 0) reflections in the orthorhombic phase below 158 K [31] and 153 K [37] for CeOFeAs and PrOFeAs, respectively. The magnetic moment (contribution by Fe) for LaOFeAs was calculated by normalizing the magnetic intensity to neutron scattering and was found to be 0.36 lB/Fe [36] which is much smaller than the predicted value of 2.3 lB per iron atom [38]. Similarly, Fe moments have been estimated for CeOFeAs and PrOFeAs. The effective moment of ‘Fe’ and TN for LnOFeAs (Ln = La, Ce and Pr) compounds [36,31,39] are given in Table 2. For CeOFeAs, Ce
Table 1 Important structural parameters of LaOFeP and PrOFeP. S.No.
Composition
Lattice parameter (Å)
Fe–P (Å)
Fe–Fe (Å)
Ln–O (Å)
a-P–Fe–P (°)
b-P–Fe–P (°)
References
1
LaOFeP
2.290
2.802
2.351
119.87
104.53
[32]
2
PrOFeP
a = 3.96307(4) c = 8.5087(1) a = 3.9113(6) c = 8.345(2)
2.327
2.766
2.285
114.34
107.09
[16]
J. Prakash, A.K. Ganguli / Inorganica Chimica Acta 372 (2011) 2–7
LnOFeAs
Fe-Fe (Å)
2.85
2.84
2.83
2.82
La
Ce
Pr
Ln Fig. 6. Variation of Fe–As bond length with rare earth in LnOFeAs (Ln = La, Ce and Pr). The Fe-As distances are computed using atomic positions given in Ref. [30] for LaOFeAs, Ref. [31] for CeOFeAs and Ref. [17] for PrOFeAs.
109.2
LnOFeAs
112.0
109.0 108.8
111.5
108.6
111.0
108.4 108.2
110.5
As-Fe-As (β) (in degree)
As-Fe-As(α) (in degree)
112.5
108.0 110.0
La
Ce
Pr
Ln Fig. 7. Variation of ‘a’ and ‘b’ As–Fe–As angle with rare earth in LnOFeAs (Ln = La, Ce and Pr). The As–Fe–As angles are computed using atomic positions given in Ref. [30] for LaOFeAs, Ref. [31] for CeOFeAs and Ref. [17] for PrOFeAs.
Table 2 Observed Fe moment and TN for LnOFeAs from neutron diffraction. S. No.
Compounds
Fe moment (lB)
TN (K)
References
1 2 3
LaOFeAs CeOFeAs PrOFeAs
0.36 0.8 0.35
137 140 136
[36] [31] [39]
moments order at 4 K [31] with moments lying in the a–b plane while Pr moments order below 14 K [37] with the Pr spins aligned along the c axis.
3. Electrical and magnetic properties of LnOFePn 3.1. LnOFeP (Ln = La, Ce and Pr) Resistivity measurements on polycrystalline LaOFeP shows metallic behavior in the temperature range between 5–300 K. A sudden drop in resistivity was observed near 4 K indicating onset of superconductivity [5]. LaOFeP is superconducting without additional doping of electrons or holes. Doping of electrons by substitution of fluoride ions at oxygen site in LaO1 xFxFeP results in enhancement of Tc to 8 K [5]. Negative sign of the Seebeck
5
coefficient indicates electrons as charge carrier in LaO1 xFxFeP superconductors [5]. McQueen et al. [32] reported the DC and AC magnetization, resistivity and specific-heat measurement of polycrystalline LaOFeP. They have synthesized LaOFeP using stoichiometric amounts of LaP, Fe and Fe3O4 by sealed tube method. They added 5% excess P to compensate for any P loss during heat treatment. Neutron diffraction studies of LaOFeP at various temperatures showed full occupancy of La, O, P and Fe ions. Metallic behavior was observed for this compound and no trace of superconductivity was detected above T = 0.35 K. McQueen et al. [32] suggested that the presence of superconductivity in LaOFeP reported by Kamihara et al. [5] is due to oxygen deficiency. Hamlin et al. [40] reported superconductivity in LaOFeP single crystals with the superconducting transition (Tc) of 6.6 K which could be enhanced to 7.8 K on reducing the compound in flowing Ar at 700 °C suggesting that decrease in oxygen content may lead to enhancement in Tc [40]. Pressure dependent resistivity studies [40] of LaOFeP single crystal reveals enhancement in Tc to 14 K from 7 K under 110 kbar pressure. Further increase in pressure results in suppression of Tc in LaOFeP superconductor. However, specific heat measurement of LaOFeP single crystal does not show a discontinuity at Tc, suggesting that stoichiometric LaOFeP does not exhibit bulk superconductivity, though some reports do suggest possibility of bulk superconductivity at low temperature [41]. The resistivity varies linearly with temperature in CeOFeP [42] between 300 K and 30 K. PrOFeP show metallic behavior up to 3.2 K below which superconductivity was observed [43]. It could be seen that the Tc decreases on substitution of smaller rare earth in LnOFeP superconductors. Thus, the variation of Tc with rare earth in LnOFeP is opposite to Ln(O/F)FeAs where substitution of smaller rare earth results in the enhancement of Tc. A strong sensitivity of the valence-band (VB) structure to the lattice parameters and to interaction with localized f states is responsible for variation of properties of LnOFePn (Pn = P and As) [44–46]. The electronic structure of LnOFePn near the Fermi level (EF) is dominated by five energy bands which have mainly Fe 3d character [44,45]. Small variations of the lattice parameters mainly affect the bands formed from dxy and d3z2 r2 orbitals. The dxy and d3z2 r2 derived bands shift towards lower and higher binding energies (B.E.), respectively on increasing the distance between Pn ions and Fe plane. Holder el al. [46] has studied the electronic structure of CeOFeP by angle-resolved photoemission (ARPES) spectroscopy to get insights for the different behavior of CeOFeP as compared to the La analog. They found marked change in the position and the dispersion of the valence bands near EF for CeOFeP as compared to LaOFeP due to interactions with Ce 4f states. Also the hybridization between Fe 3d-derived valence-bands and the Ce 4f states leads to a reconstruction of the Fermi surface which may be responsible for absence of superconductivity in CeOFeP. 3.2. LnOFeAs (Ln = La, Ce and Pr) Fig. 8 shows the variation of resistivity with temperature for LnOFeAs (Ln = La, Ce and Pr) which reveals semimetallic behavior of all the three arsenides. LaOFeAs exhibits a sudden decrease of resistivity at 150 K due to structural transition from tetragonal to monoclinic symmetry. The resistivity continues to decrease below 150 K and shows a minimum at 70 K. For CeOFeAs, the resistivity first increased up to 160 K and then a sudden fall was observed which can be attributed to the structural distortion from tetragonal to orthorhombic symmetry [31]. Further cooling leads to decrease in resistivity up to the lowest temperature measured. PrOFeAs also shows an anomaly characterized by a drop in resistivity at 155 K. This anomalous resistivity drop is associated with a
6
J. Prakash, A.K. Ganguli / Inorganica Chimica Acta 372 (2011) 2–7
smaller in LaOFeAs as compared to LaOFeP which accounts for the better metallic character (low value of resistivity) of latter as compared to the former. The magnetic studies of LnOFeAs (Ln = La, Ce and Pr) show linear temperature dependence of magnetic susceptibility in the high temperature paramagnetic state. This type of behavior is neither Pauli nor Curie-Wess like and is similar to the behavior of metallic Cr which also exhibits a spin density wave (SDW) state. All the undoped LnOFeAs compounds show long range antiferromagnetic order. The TN for LnOFeAs compounds are given in Table 2.
Resistivity (mohm-cm)
8 CeOFeAs PrOFeAs LaOFeAs
6
4
2
3.3. Fe–As Hybridization 0
25
50
75
100
125 150
175
200
Temperature (K) Fig. 8. The temperature dependence of resistivity of LnOFeAs (Ln = La, Ce and Pr).
Table 3 Superconducting resistivity(q(RT)).
transition
temperature
(Tc)
and
room
temperature
S. No.
Compounds
q (RT) (m O cm)
Tc (K)
References
1 3 4 5 6 7 7 8
LaOFeP LaOFeP CeOFeP PrOFeP LaOFeAs LaOFeAs CeOFeAs PrOFeAs
1.51 2.1 5.2 0.3 5 4.8 5.4 3
Non 4 Non 3.2 Non Non Non Non
[32] [5] [42] [43] [1] [47] [48] [49]
SC SC SC SC SC SC
Jishi et al. [51] have reported the effect of hybridization on structural and electronic properties of LnOFeAs compounds. Their theoretical calculations suggest strong hybridization between the Fe 3d orbitals and As 4p orbitals. The occupation scheme of Fe 3d levels in tetragonal and low temperature orthorhombic structure in LnOFeAs is illustrated in Fig. 9. Structural distortion from tetragonal to orthorhombic symmetry in parent LnOFeAs results in change in electronic energy which removes degeneracy of the Fe 3dxz and 3dyz levels [50]. Their studies also indicate that electron doping by substitution of fluoride ion at the oxygen site induces lowering of symmetry which suppresses structural distortion and Fe magnetic moments. Hybridization between the Fe 3d and the As 4p states leads to spin fluctuations on the Fe sites which may play a role in suppressing the magnetic order and the emergence of superconductivity in these arsenides [51]. 4. Conclusions
structural distortion from tetragonal to orthorhombic symmetry. Below the occurrence of structural transition and SDW, the resistivity drops steeply and then becomes almost independent of temperature (below 84 K). A down turn was observed below 10 K which is attributed to the decrease in scattering due to the ordering of the Pr moments. Unlike LaOFeP, where differing reports (superconducting by some and non superconducting (metallic) by other groups) exist, the arsenide analog, LnOFeAs have been found to show semimetallic behavior by all groups [47–49]. Yan et al. [50] have studied the temperature dependence of in-plane electrical resistivity on single crystals of LaOFeAs. Similar to polycrystalline LaOFeAs, semimetallic behavior was observed with anomaly at 153 K in resistivity measurement for LaOFeAs single crystals [50]. The room temperature resistivity of LnOFeAs [1,5,32,42,43,47–49] is given in Table 3. It may be note that the As compounds show high room temperature resistivity as compared to the phosphides (LnOFeP). Vildosola et al. [44] have studied the band structure of LaOFeAs and LaOFeP using first-principles full-potential electronic structure calculations. Their calculations showed the band width of Fe near the Fermi surface is
Fig. 9. The occupation scheme of Fe(II) 3d levels in tetragonal and low temperature orthorhombic structure in LnOFeAs (adopted from Ref. [51]).
Both LnOFeP and LnOFeAs compounds crystallize in tetragonal ZrCuSiAs type structure at room temperature. LnOFeAs exhibit structural distortion on cooling while LnOFeP retain tetragonal structure at low temperature. Dramatic change in electrical and magnetic properties of LnOFeP is observed on replacing P with As. LnOFeP are superconducting while LnOFeAs compounds are antiferromagnetic semimetals. These variation in structure (at low temperature), electrical and magnetic properties of LnOFePn (Ln = rare earth; Pn = P and As) may be attributed to extensive hybridization between iron and pnictogen. Acknowledgments A.K.G. thanks DST, Govt. of India for financial support. J.P. thanks CSIR Govt. of India, for fellowship. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130 (2008) 3296. [2] N.R. Dilley, E.D. Bauer, M.B. Maple, Phys. Rev. B 61 (2000) 4608. [3] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’Quinn, Nature 13 (2001) 597. [4] S. Geller, G.W. Hull, Phys. Rev. Lett. 13 (1964) 127. [5] Y. Kamihara, H. Hiramatsu, M. Hirano, R. Kawamura, H. Yanagi, T. Kamiya, H. Hosono, J. Am. Chem. Soc. 128 (2006) 10012. [6] J. Prakash, S.J. Singh, S. Patnaik, A.K. Ganguli, Physica C 469 (2009) 82. [7] J. Prakash, S.J. Singh, A. Banerjee, S. Patnaik, A.K. Ganguli, App. Phys. Lett. 95 (2009) 262507. [8] J. Yang, Z.C. Li, W. Lu, W. Yi, X.L. Shen, Z.A. Ren, G.C. Che, X.L. Dong, L.L. Sun, F. Zhou, Z.-X. Zhao, Supercond. Sci. Technol. 21 (2008) 082001. [9] A. Ren, J. Yang, W. Lu, W. Yi, G.C. Che, X.L. Dong, L.L. Sun, Z.X. Zhao, Mater. Res. Innovations 12 (2008) 105. [10] Z.A. Ren, L. Wei, Y. Jie, Y. Wei, S.X. Li, L.Z. Cai, C.G. Can, D.X. Li, S.L. Ling, Z. Fang, Z.Z. Xian, Chin. Phys. Lett. 25 (2008) 2215. [11] A.S. Sefat, A. Huq, M.A. McGuire, R. Jin, B.C. Sales, D. Mandrus, L.M.D. Cranswick, P.W. Stephens, K.H. Stone, Phys. Rev. B 78 (2008) 104505. [12] J. Prakash, S.J. Singh, S. Patnaik, A.K. Ganguli, Solid State Commun.149 (2009) 181.
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Mr. Jai Prakash obtained his B.Sc. (Hons) in Chemistry from Atma Ram Sanatan Dharma College, University of Delhi in 2004. Later he obtained his M.Sc. in chemistry from I.I.T Roorkee with specialization in Analytical Chemistry in 2006. Currently he is working as a Ph.D. student in the Department of Chemistry, I.I.T Delhi under the supervision of Prof. A.K. Ganguli on New Iron Containing Oxypnictide Superconductors.
Prof. Ashok K. Ganguli obtained his M.Sc. degree in Chemistry from the University of Delhi (1984) and Ph.D. from Indian Institute of Science Bangalore, India (1990). He then worked at the Central Research and Development Department of DuPont Company at Wilmington Delaware, USA (1990–91). and Ames Laboratory, USDOE at Iowa state University. In 1995 he joined the department of Chemistry at the Indian Institute of Technology, Delhi. Dr. Ganguli has contributed extensively to the field of Solid State and Materials Chemistry especially oxides (dielectric and superconducting, polar intermetallics (Zintl phases).and nanocrystalline materials. He has published around 150 papers in international journals and contributed 6 chapters in books. Prof. Ganguli is the recipient of the Materials Research Society of India Medal for 2006, the Chemical Research Society of India Medal for 2007 and is a Fellow of the Indian Academy of Sciences. He is an Associate editor of Bulletin of Materials Sciences and an editorial Board member of Indian J. Chem. A.