- Email: [email protected]
Structural and magnetic phase transitions in mixed-valence cobalt oxides REBaCo4O7 (RE=Lu, Yb, Tm)
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 300 (2006) 98–100 www.elsevier.com/locate/jmmm
Structural and magnetic phase transitions in mixed-valence cobalt oxides REBaCo4O7 (RE ¼ Lu, Yb, Tm) N. Nakayamaa,, T. Mizotaa, Y. Uedab, A.N. Sokolovc, A.N. Vasilievc a
Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube 755-8611, Japan Materials Design and Characterization Laboratory, Institute for Solid State Physics,University of Tokyo, Kashiwa 277-8581, Japan c Low Temperature Physics Department, Moscow State University, Moscow 119992, Russia
b
Available online 16 November 2005
Abstract New mixed valence Co oxides REBaCo4O7 (RE ¼ Lu, Yb, Tm) show structural first-order phase transitions from hexagonal (P63mc) to orthorhombic (Cmc21 or lower) system below 170, 180, and 230 K, respectively. The low-temperature phases order magnetically at 47, 75, and 105 K, respectively. r 2005 Elsevier B.V. All rights reserved. PACS: 75.20.Hr; 75.40.Cx Keywords: Mixed valence; Cobalt oxide; Structural phase transition; Magnetic ordering
The mixed valence transition metal oxides are of general interest, since they show a variety of physical phenomena, related to interplay of charge, spin and orbital degrees of freedom interacting with crystal lattice [1–4]. The recent discovery of superconductivity in cobalt oxyhydrate NaxCoO2 yH2O has stimulated the search of new cobalt oxide systems with possible fascinating features [5]. In the present paper, we report on synthesis and characterization of new mixed-valence cobalt oxides with general formula REBaCo4O7 (Re ¼ Lu, Yb, Tm). These compounds are isostructural to YBaCo4O7, where the Ba and O atoms form a close-packed structure with 4H (abac) stacking [6–7]. The Y and Co atoms occupy the octahedral and tetrahedral sites, respectively. The CoO4 tetrahedra form a three-dimensional network by the corner sharing in a non-centrosymmetric P63mc lattice. The structural features suggest the magnetic frustration in cobalt subsystem, whose formal valence is 2.25. The pellet samples of REBaCo4O7 (Re ¼ Lu, Yb, Tm) were prepared by heating the stoichiometric mixtures of BaCO3, RE2O3, and Co3O4 at 1050 1C in air for 2 days followed by the furnace cooling. More than twice calcinaCorresponding author.
E-mail address: [email protected] (N. Nakayama). 0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.10.041
tions at 1050 1C were performed. Samples were characterized by powder X-ray diffraction and transmission electron microscopy. Samples are almost single-phase materials of YBaCo4O7 type containing small amounts of RE2O3. Nearly stoichiometric oxygen composition, y ¼ 7:0 0:1 in REBaCo4Oy, was confirmed by the thermogravimetry measurements in the He-10%H2 mixed gas atmosphere. At room temperature, the X-ray patterns of REBaCo4O7 (Re ¼ Lu, Yb, Tm) were indexed in the hexagonal P63mc system. On lowering temperature, these compounds show structural phase transitions into the orthorhombic system below 165, 180, and 230 K, respectively. The powder X-ray diffraction patterns of the low-temperature phases can be indexed as the orthorhombic system with Cmc21 symmetry, which is a subgroup of P63mc and is reported for an oxygen deficient phase Ba2Er2Zn8O13 [8]. Cell parameters of hexagonal phase of REBaCo4O7 (Re ¼ Lu, Yb, Tm) are summarized in Table 1. The full crystallographic data on these compounds will be reported elsewhere. The characterization of physical properties of REBaCo4O7 (Re ¼ Lu, Yb, Tm) was done through the measurements of magnetization, specific heat and resistance in a range 5–300 K. The parent compound YBaCo4O7 shows a magnetic transition to a spin-glass like state at 60 K [6]. The replacement of Y with rare earth elements
ARTICLE IN PRESS N. Nakayama et al. / Journal of Magnetism and Magnetic Materials 300 (2006) 98–100 Table 1 Cell parameters a and c of hexagonal phase for REBaCo4O7 (Re ¼ Lu, Yb, Tm) and corresponding temperatures of structural (Ts) and magnetic (Tm) phase transitions Compound
a (A˚)
c (A˚)
Ts (K)
Tm (K)
LuBaCo4O7 YbBaCo4O7 TmBaCo4O7
6.263 6.267 6.276
10.225 10.233 10.240
161 178 224
47 75 105
90
M (H) TS
χ-1 (mol/emu)
80 70
0
60
2 Tm
4 6 8 H (kOe)
10 LuBaCo4O7
50
ZFC
40
FC
60
TS Tm
χ-1 (mol/emu)
50 40 30
YbBaCo4O7
20
ZFC FC
10 TS
χ-1 (mol/emu)
30 Tm
25 20 15
TmBaCo4O7
10
ZFC FC
5 0 0
50
100
150 200 T (K)
250
300
350
Fig. 1. Inverse magnetic susceptibility for REBaCo4O7 (RE ¼ Lu, Yb, and Tm) Magnetic phase transitions at Tm are indicated. Weak anomalies seen at TS are due to the structural phase transitions. The open and filled circles show the data obtained by cooling in the zero-field and 1000 Oe, respectively.
results in a systematic shift of magnetic transition temperature in clear correspondence to RE ions size. The temperature dependences of inverse magnetic susceptibility w1 in REBaCo4O7 (RE: Lu, Yb, and Tm) measured by a SQUID ‘‘Quantum Design’’ magnetometer at 0.1 T are shown in Fig. 1. The structural phase transitions are seen in w1 vs. T dependences as small discontinuities at TS. The determination of Co valence
99
states from the slope of w1 vs. T curves is strongly affected by the magnetic frustration inherent to the structure of these compounds and by the presence of appreciable temperature-independent term in magnetic susceptibility w0. Moreover, in cases of YbBaCo4O7 and TmBaCo4O7 compounds the magnetic susceptibility contains a fairly large paramagnetic contribution from Yb3+ (S ¼ 1=2) or Tm3+ (S ¼ 1). This term is absent in LuBaCo4O7 because Lu3+ is non-magnetic (S ¼ 0). Fairly below the structural phase transition temperature, the magnetic susceptibility shows anomaly indicating the magnetic phase transition. The w1 vs. T curve of LuBaCo4O7 shows a sharp peak at T m ¼ 47 K suggesting the antiferromagnetic ordering. The measurements of magnetization vs. field (inset on Fig. 1) confirm this suggestion showing no hysteresis of magnetization. However, a weak thermo-remanent magnetization is observed at w1 vs. T curves on field cooling. The inverse magnetic susceptibility of YbBaCo4O7 and TmBaCo4O7 show the deviation from the Curie–Weiss law below 75 and 105 K, respectively, particularly when samples are cooled in the magnetic field. The field-induced thermo-remanence is dependent on the applied field and is enhanced for the small field. The magnetic behaviors of Yb and Tm phases are similar to that reported for YBaCo4O7. In case of LuBaCo4O7 we can estimate the effective moment for high-temperature phase. It is close to the spin only value of the S ¼ 1=2 state (peff ¼ 1:73 mB ). Assuming the formal valence of 2.25, the one-forth of the Co atoms is trivalent and the three-forth of Co atoms is divalent. The high spin state, S ¼ 3=2 for Co(II), S ¼ 2 for Co(III) and peff ¼ 4:15 mB , does not agree with the observations. Also the low spin state in the regular tetrahedral crystal field, S ¼ 3=2 for Co(II), S ¼ 1 for Co(III), and peff ¼ 3:64 mB , does not agree. The low-spin configurations in the distorted C3v tetrahedral crystal field [9], S ¼ 1=2 for Co(II), S ¼ 0 for Co(III), and peff ¼ 1:50 mB , almost agree with the observation. Otherwise, the contribution of unquenched orbital moment should be taken into the consideration. Accompanied with the structural phase transition, the electrical resistivity of the sintered body shows an anomaly as shown in Fig. 2. REBaCo4O7 phases are semiconducting at room temperature (r ¼ 1210 O cm) and the activation energies derived from the temperature dependence of r are in the range of 0.10–0.15 eV. Below the structural phase transition temperatures, the electrical resistivity deviates from the temperature dependence of normal activationtype one and gradually increases. The deviations start at 162, 175, and 215 K for the Lu, Yb, and Tm compounds, respectively. Almost 50 K below the phase transition temperatures, the thermal dependence of the electrical resistivity recovers, however, to the activation type one, where the activation energies are slightly larger than those of the high- temperature hexagonal phases. Such behavior of electrical resistivity at the temperature of structural transition may be due to redistribution of mixed valence Co-ions in tetrahedra.
ARTICLE IN PRESS N. Nakayama et al. / Journal of Magnetism and Magnetic Materials 300 (2006) 98–100
100
16
600
TS
14
LuBaCo4O7 500
12
TmBaCo4O7 400 C (J/mol K)
ln (r)
10 8 6 LuBaCo4O7
4
3RN = 324.1 J/mol K
300 200
2 0
100
16 14
TS
0
12 ln (r)
YbBaCo4O7
0
10 8
100
150 T (K)
200
250
300
Fig. 3. Heat capacity curves of REBaCo4O7 (RE ¼ Lu, Yb, and Tm). Sharp peaks at 160, 178, and 224 K correspond to the structural phase transition and the anomaly in the electrical resistivity. Despite the magnetic anomalies in w1 vs. T curves below 100 K, no anomaly is found in the heat capacity.
6 YbBaCo4O7
4
50
2 0 14
tures. This fact probably signifies that the magnetic heat capacity is spread over wide temperature range. In summary, new mixed valence compounds REBaCo4O7 (RE ¼ Lu, Yb, and Tm) show the first-order structural phase transitions at high temperatures and the magnetic phase transition at lower temperatures. The origin for the structural phase transitions may be the charge ordering in the Co subsystem.
12 ln (r)
10
TS
8 6 4
TmBaCo4O7
2 0 0.002
0.004
0.006 0.008 T-1 (K-1)
0.010
0.012
Fig. 2. Electrical resistivity curves of sintered samples for REBaCo4O7 (RE ¼ Lu, Yb, and Tm). The inflections at 162, 175, and 215 K, respectively, correspond to the structural phase transition from hexagonal to orthorhombic system.
This work is supported by Grants-in-Aid for Scientific Research No. 14038234 from the Ministry of Education, Culture, Sports, Science and Technology, Japan and by Russian Foundation for Basic Research RFBR Grant 0302-16108. References
The temperature dependences of specific heat of REBaCo4O7 (RE ¼ Lu, Yb, and Tm) are shown in Fig. 3. Each of these dependences reveals a peak-like anomaly at 160, 178 and 224 K, respectively. These peaks indicate the first-order phase transitions and their temperatures are in good agreement with the results of other measurements. The peak at phase transition in YbBaCo4O7 is more pronounced than in TmBaCo4O7 and LuBaCo4O7, which probably is due to the higher homogeneity of this sample. At high temperatures, heat capacities of all these compounds reach the theoretical limit 3RN ¼ 324.1 J/ mol K (dotted line in Fig. 3). No anomalies are seen in specific heat measurements at magnetic ordering tempera-
[1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 36 (1997) R12685. [2] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, Phys. Rev. B 62 (2000) 166. [3] F. Faut, E. Suard, V. Caignaert, B. Domeng‘es, I. Mirebeau, L. Keller, Eur. Phys. J. B 21 (2001) 163. [4] T. Vogt, P. Woodward, P. Karen, B. Hunter, P. Henning, A. Moodenbaugh, Phys. Rev. Lett. 84 (2000) 2969. [5] D. Akahoshi, Y. Ueda, J. Solid State Chem. 156 (2001) 355. [6] M. Valldor, M. Andersson, Solid State Sciences 4 (2002) 923. [7] M. Valldor, Solid State Science 6 (2004) 251. [8] Ch. Rabbow, Hk. Mu¨ller-Buschbaum, Z. Anorg. Allg. Chem. 620 (1994) 527. [9] D.M. Jenkins, A.J. Di Billo, M.J. Allen, T.A. Betley, J.C. Peters, J. Am. Chem. Soc. 124 (2002) 15336.
Journal of Magnetism and Magnetic Materials 300 (2006) 98–100 www.elsevier.com/locate/jmmm
Structural and magnetic phase transitions in mixed-valence cobalt oxides REBaCo4O7 (RE ¼ Lu, Yb, Tm) N. Nakayamaa,, T. Mizotaa, Y. Uedab, A.N. Sokolovc, A.N. Vasilievc a
Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube 755-8611, Japan Materials Design and Characterization Laboratory, Institute for Solid State Physics,University of Tokyo, Kashiwa 277-8581, Japan c Low Temperature Physics Department, Moscow State University, Moscow 119992, Russia
b
Available online 16 November 2005
Abstract New mixed valence Co oxides REBaCo4O7 (RE ¼ Lu, Yb, Tm) show structural first-order phase transitions from hexagonal (P63mc) to orthorhombic (Cmc21 or lower) system below 170, 180, and 230 K, respectively. The low-temperature phases order magnetically at 47, 75, and 105 K, respectively. r 2005 Elsevier B.V. All rights reserved. PACS: 75.20.Hr; 75.40.Cx Keywords: Mixed valence; Cobalt oxide; Structural phase transition; Magnetic ordering
The mixed valence transition metal oxides are of general interest, since they show a variety of physical phenomena, related to interplay of charge, spin and orbital degrees of freedom interacting with crystal lattice [1–4]. The recent discovery of superconductivity in cobalt oxyhydrate NaxCoO2 yH2O has stimulated the search of new cobalt oxide systems with possible fascinating features [5]. In the present paper, we report on synthesis and characterization of new mixed-valence cobalt oxides with general formula REBaCo4O7 (Re ¼ Lu, Yb, Tm). These compounds are isostructural to YBaCo4O7, where the Ba and O atoms form a close-packed structure with 4H (abac) stacking [6–7]. The Y and Co atoms occupy the octahedral and tetrahedral sites, respectively. The CoO4 tetrahedra form a three-dimensional network by the corner sharing in a non-centrosymmetric P63mc lattice. The structural features suggest the magnetic frustration in cobalt subsystem, whose formal valence is 2.25. The pellet samples of REBaCo4O7 (Re ¼ Lu, Yb, Tm) were prepared by heating the stoichiometric mixtures of BaCO3, RE2O3, and Co3O4 at 1050 1C in air for 2 days followed by the furnace cooling. More than twice calcinaCorresponding author.
E-mail address: [email protected] (N. Nakayama). 0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.10.041
tions at 1050 1C were performed. Samples were characterized by powder X-ray diffraction and transmission electron microscopy. Samples are almost single-phase materials of YBaCo4O7 type containing small amounts of RE2O3. Nearly stoichiometric oxygen composition, y ¼ 7:0 0:1 in REBaCo4Oy, was confirmed by the thermogravimetry measurements in the He-10%H2 mixed gas atmosphere. At room temperature, the X-ray patterns of REBaCo4O7 (Re ¼ Lu, Yb, Tm) were indexed in the hexagonal P63mc system. On lowering temperature, these compounds show structural phase transitions into the orthorhombic system below 165, 180, and 230 K, respectively. The powder X-ray diffraction patterns of the low-temperature phases can be indexed as the orthorhombic system with Cmc21 symmetry, which is a subgroup of P63mc and is reported for an oxygen deficient phase Ba2Er2Zn8O13 [8]. Cell parameters of hexagonal phase of REBaCo4O7 (Re ¼ Lu, Yb, Tm) are summarized in Table 1. The full crystallographic data on these compounds will be reported elsewhere. The characterization of physical properties of REBaCo4O7 (Re ¼ Lu, Yb, Tm) was done through the measurements of magnetization, specific heat and resistance in a range 5–300 K. The parent compound YBaCo4O7 shows a magnetic transition to a spin-glass like state at 60 K [6]. The replacement of Y with rare earth elements
ARTICLE IN PRESS N. Nakayama et al. / Journal of Magnetism and Magnetic Materials 300 (2006) 98–100 Table 1 Cell parameters a and c of hexagonal phase for REBaCo4O7 (Re ¼ Lu, Yb, Tm) and corresponding temperatures of structural (Ts) and magnetic (Tm) phase transitions Compound
a (A˚)
c (A˚)
Ts (K)
Tm (K)
LuBaCo4O7 YbBaCo4O7 TmBaCo4O7
6.263 6.267 6.276
10.225 10.233 10.240
161 178 224
47 75 105
90
M (H) TS
χ-1 (mol/emu)
80 70
0
60
2 Tm
4 6 8 H (kOe)
10 LuBaCo4O7
50
ZFC
40
FC
60
TS Tm
χ-1 (mol/emu)
50 40 30
YbBaCo4O7
20
ZFC FC
10 TS
χ-1 (mol/emu)
30 Tm
25 20 15
TmBaCo4O7
10
ZFC FC
5 0 0
50
100
150 200 T (K)
250
300
350
Fig. 1. Inverse magnetic susceptibility for REBaCo4O7 (RE ¼ Lu, Yb, and Tm) Magnetic phase transitions at Tm are indicated. Weak anomalies seen at TS are due to the structural phase transitions. The open and filled circles show the data obtained by cooling in the zero-field and 1000 Oe, respectively.
results in a systematic shift of magnetic transition temperature in clear correspondence to RE ions size. The temperature dependences of inverse magnetic susceptibility w1 in REBaCo4O7 (RE: Lu, Yb, and Tm) measured by a SQUID ‘‘Quantum Design’’ magnetometer at 0.1 T are shown in Fig. 1. The structural phase transitions are seen in w1 vs. T dependences as small discontinuities at TS. The determination of Co valence
99
states from the slope of w1 vs. T curves is strongly affected by the magnetic frustration inherent to the structure of these compounds and by the presence of appreciable temperature-independent term in magnetic susceptibility w0. Moreover, in cases of YbBaCo4O7 and TmBaCo4O7 compounds the magnetic susceptibility contains a fairly large paramagnetic contribution from Yb3+ (S ¼ 1=2) or Tm3+ (S ¼ 1). This term is absent in LuBaCo4O7 because Lu3+ is non-magnetic (S ¼ 0). Fairly below the structural phase transition temperature, the magnetic susceptibility shows anomaly indicating the magnetic phase transition. The w1 vs. T curve of LuBaCo4O7 shows a sharp peak at T m ¼ 47 K suggesting the antiferromagnetic ordering. The measurements of magnetization vs. field (inset on Fig. 1) confirm this suggestion showing no hysteresis of magnetization. However, a weak thermo-remanent magnetization is observed at w1 vs. T curves on field cooling. The inverse magnetic susceptibility of YbBaCo4O7 and TmBaCo4O7 show the deviation from the Curie–Weiss law below 75 and 105 K, respectively, particularly when samples are cooled in the magnetic field. The field-induced thermo-remanence is dependent on the applied field and is enhanced for the small field. The magnetic behaviors of Yb and Tm phases are similar to that reported for YBaCo4O7. In case of LuBaCo4O7 we can estimate the effective moment for high-temperature phase. It is close to the spin only value of the S ¼ 1=2 state (peff ¼ 1:73 mB ). Assuming the formal valence of 2.25, the one-forth of the Co atoms is trivalent and the three-forth of Co atoms is divalent. The high spin state, S ¼ 3=2 for Co(II), S ¼ 2 for Co(III) and peff ¼ 4:15 mB , does not agree with the observations. Also the low spin state in the regular tetrahedral crystal field, S ¼ 3=2 for Co(II), S ¼ 1 for Co(III), and peff ¼ 3:64 mB , does not agree. The low-spin configurations in the distorted C3v tetrahedral crystal field [9], S ¼ 1=2 for Co(II), S ¼ 0 for Co(III), and peff ¼ 1:50 mB , almost agree with the observation. Otherwise, the contribution of unquenched orbital moment should be taken into the consideration. Accompanied with the structural phase transition, the electrical resistivity of the sintered body shows an anomaly as shown in Fig. 2. REBaCo4O7 phases are semiconducting at room temperature (r ¼ 1210 O cm) and the activation energies derived from the temperature dependence of r are in the range of 0.10–0.15 eV. Below the structural phase transition temperatures, the electrical resistivity deviates from the temperature dependence of normal activationtype one and gradually increases. The deviations start at 162, 175, and 215 K for the Lu, Yb, and Tm compounds, respectively. Almost 50 K below the phase transition temperatures, the thermal dependence of the electrical resistivity recovers, however, to the activation type one, where the activation energies are slightly larger than those of the high- temperature hexagonal phases. Such behavior of electrical resistivity at the temperature of structural transition may be due to redistribution of mixed valence Co-ions in tetrahedra.
ARTICLE IN PRESS N. Nakayama et al. / Journal of Magnetism and Magnetic Materials 300 (2006) 98–100
100
16
600
TS
14
LuBaCo4O7 500
12
TmBaCo4O7 400 C (J/mol K)
ln (r)
10 8 6 LuBaCo4O7
4
3RN = 324.1 J/mol K
300 200
2 0
100
16 14
TS
0
12 ln (r)
YbBaCo4O7
0
10 8
100
150 T (K)
200
250
300
Fig. 3. Heat capacity curves of REBaCo4O7 (RE ¼ Lu, Yb, and Tm). Sharp peaks at 160, 178, and 224 K correspond to the structural phase transition and the anomaly in the electrical resistivity. Despite the magnetic anomalies in w1 vs. T curves below 100 K, no anomaly is found in the heat capacity.
6 YbBaCo4O7
4
50
2 0 14
tures. This fact probably signifies that the magnetic heat capacity is spread over wide temperature range. In summary, new mixed valence compounds REBaCo4O7 (RE ¼ Lu, Yb, and Tm) show the first-order structural phase transitions at high temperatures and the magnetic phase transition at lower temperatures. The origin for the structural phase transitions may be the charge ordering in the Co subsystem.
12 ln (r)
10
TS
8 6 4
TmBaCo4O7
2 0 0.002
0.004
0.006 0.008 T-1 (K-1)
0.010
0.012
Fig. 2. Electrical resistivity curves of sintered samples for REBaCo4O7 (RE ¼ Lu, Yb, and Tm). The inflections at 162, 175, and 215 K, respectively, correspond to the structural phase transition from hexagonal to orthorhombic system.
This work is supported by Grants-in-Aid for Scientific Research No. 14038234 from the Ministry of Education, Culture, Sports, Science and Technology, Japan and by Russian Foundation for Basic Research RFBR Grant 0302-16108. References
The temperature dependences of specific heat of REBaCo4O7 (RE ¼ Lu, Yb, and Tm) are shown in Fig. 3. Each of these dependences reveals a peak-like anomaly at 160, 178 and 224 K, respectively. These peaks indicate the first-order phase transitions and their temperatures are in good agreement with the results of other measurements. The peak at phase transition in YbBaCo4O7 is more pronounced than in TmBaCo4O7 and LuBaCo4O7, which probably is due to the higher homogeneity of this sample. At high temperatures, heat capacities of all these compounds reach the theoretical limit 3RN ¼ 324.1 J/ mol K (dotted line in Fig. 3). No anomalies are seen in specific heat measurements at magnetic ordering tempera-
[1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 36 (1997) R12685. [2] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, Phys. Rev. B 62 (2000) 166. [3] F. Faut, E. Suard, V. Caignaert, B. Domeng‘es, I. Mirebeau, L. Keller, Eur. Phys. J. B 21 (2001) 163. [4] T. Vogt, P. Woodward, P. Karen, B. Hunter, P. Henning, A. Moodenbaugh, Phys. Rev. Lett. 84 (2000) 2969. [5] D. Akahoshi, Y. Ueda, J. Solid State Chem. 156 (2001) 355. [6] M. Valldor, M. Andersson, Solid State Sciences 4 (2002) 923. [7] M. Valldor, Solid State Science 6 (2004) 251. [8] Ch. Rabbow, Hk. Mu¨ller-Buschbaum, Z. Anorg. Allg. Chem. 620 (1994) 527. [9] D.M. Jenkins, A.J. Di Billo, M.J. Allen, T.A. Betley, J.C. Peters, J. Am. Chem. Soc. 124 (2002) 15336.