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Synthesis, crystal structure and magnetic properties of a new lithium cobalt metaphosphate, LiCo(PO3)3
Materials Research Bulletin 40 (2005) 1787–1795 www.elsevier.com/locate/matresbu
Synthesis, crystal structure and magnetic properties of a new lithium cobalt metaphosphate, LiCo(PO3)3 Seungdon Choi, Seung-Tae Hong * LG Chem Research Park, Daejeon 305-380, Republic of Korea Received 3 August 2004; received in revised form 26 April 2005; accepted 17 May 2005
Abstract A new lithium cobalt metaphosphate, LiCo(PO3)3, is reported for the first time, which was discovered during the exploratory synthesis in Li–Co–P–O system by solid state reaction. The structure has been refined by powder X-ray ˚ , b = 8.6326(2) A ˚ and c = 8.3520(2) A ˚ , Z = 4, Rp = 13.6%, Rietveld refinement method (P212121, a = 8.5398(2) A 2 Rwp = 19.4%, Rexp = 17.7%, S = 1.11, x = 1.23). It is isostructural with LiM(PO3)3 (M = Fe, Cu). It contains (PO3)1 chains with the Co atoms localized in the octahedral sites, bridging four neighboring chains. The magnetic susceptibility measurement showed a typical paramagnetic behavior of high spin of Co2+, following the Curie– Weiss law in the temperature range of 5–300 K. Unlike the olivine type lithium cobalt phosphate, LiCoPO4, cyclic voltammetry of LiCo(PO3)3 assembled in the coin-type cell showed no electrochemical activity in the voltage region of 1–5 V versus Li/Li+. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; C. X-ray diffraction; D. Magnetic properties; D. Crystal structure
1. Introduction The phosphates of several transition metals are known to have various structures, showing interesting properties. For example, the family of LiMPO4 (M = Mn, Fe and Co) [1–3] with the olivine structure shows good electrochemical properties as positive electrode materials for potential application in lithium-ion batteries. In the Li–Co–P–O quaternary system, three compounds have been * Corresponding author. Tel.: +82 428662498; fax: +82 428626073. E-mail address: [email protected] (S.-T. Hong). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.05.021
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reported so far: LiCoPO4, LiCo2(P3O10) and Li6Co5(P2O7)4 [4]. LiCoPO4 shows not only high voltage (5 V versus Li/Li+) lithium ion de-intercalation [3] but also piezo-magnetoelectric properties [5,6]. Exploratory synthesis in the search for new compounds, structures and properties is an important aspect of materials chemistry, leading to the production of a large variety of new phases, sometimes unexpected or unforeseen. Considering that so far only three compounds are known in the Li–Co–P–O system, we expected that there would be new phases with interesting physicochemical properties. A lithium cobalt metaphosphate, LiCo(PO3)3, was discovered during the exploratory synthesis in our laboratory. It is isostructural with LiM(PO3)3 (M = Fe, Cu) [7,8]. Its structural refinement and magnetic properties, as well as electrochemical results, are reported here for the first time.
2. Experimental LiCo(PO3)3 was first discovered as an unidentified phase with a pink color in the mixture of LiCo(PO3)3, Li3PO4 and Co3O4 during the attempts to look for new compounds in the system, Li2CO3:Co(CH3COO)24H2O:(NH4)H2PO4. The phase appeared in the reaction products from various compositions of Li:Co:P. Once it was identified as LiCo(PO3)3, the single phase was synthesized readily from the stoichiometric ratio (1:2:6) of Li2CO3, Co(CH3COO)24H2O and (NH4)H2PO4. These were mixed together, pressed into pellets and heated at 350 8C for 10 h as a calcination step, and at 600 8C for 12 h in the air with intermittent mixing and pressing. X-ray powder diffraction data were collected for LiCo(PO3)3 on a Bruker D4 Endeavor diffractometer ˚ ) and a diffracted beam graphite monochromator. Data were equipped with a Cu X-ray tube (l = 1.5418 A collected in the range of 128 2u 1008 with a step of 0.028 and time per step of 6 s. The structure of LiCo(PO3)3 was refined by the Rietveld method using the GSAS program [9]. The atomic parameters of LiFe(PO3)3 [7] were used as a starting model for the refinement of LiCo(PO3)3. A total of 73 parameters were refined including 50 structural (3 cell parameters, 42 atomic coordinates and 5 isotropic thermal parameters), 12 background parameters, 6 peak profile parameters, 2 absorption correction parameters, polarization factor, zero shift correction and scale factor. Magnetic susceptibilities were measured using a SQUID magnetometer in the temperature range of 5– 300 K. After zero cooling and stabilization of the temperature at 5 K, a magnetic field of 0.5 T was applied. The magnetic susceptibilities were measured with increasing temperature up to 300 K. Electrochemical properties of LiCo(PO3)3 were evaluated with coin-type cells, assembled with circular cathode of 1.49 cm2, metallic lithium anode and 1 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) solvent (1:2, v/v). The cathodes were fabricated by knife-coating the slurry mixture of active material, Super-P and PVdF (85:10:5, w/w/w) on aluminum foil.
3. Results and discussion 3.1. Crystal structure The powder X-ray Rietveld refinement confirmed that the LiCo(PO3)3 structure is isostructural with LiFe(PO3)3 [7] and LiCu(PO3)3 [8]. Crystallographic data are listed in Table 1. The refined atomic and
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Table 1 Crystallographic data for LiCo(PO3)3 at room temperature Formula Formula weight Crystal system Space group
LiCo(PO3)3 302.8 g/mol Orthorhombic P212121 (no. 19)
Unit-cell dimensions (at 298 K) and Z a b c Z
˚ 8.5398(2) A ˚ 8.6326(2) A ˚ 8.3520(2) A 4
Cell volume Density, calculated No. of variables Rp Rwp Rexp S (goodness of fit) x2
˚3 615.71(2) A 3.267 g/cm 3 73 13.6% 19.4% 17.7% 1.11 1.23
isotropic thermal parameters are given in Table 2. Fig. 1 compares the observed and calculated intensities. Selected bond distances and angles are in Table 3. The crystal structure of LiCo(PO3)3 projected onto the b–c plane is shown in Fig. 2a, where only PO4 tetrahedra are shown and Li and Co atoms are omitted for clarity. Fig. 2b, the circled part in Fig. 2a after 908 rotation along c-axis, clearly shows that each tetrahedron is corner-sharing with two neighboring tetrahedra, forming a one-dimensional (PO2O2/2)1 tetrahedral or (PO3)1 zigzag chain along a-axis. Fig. 2a also shows that such (PO3)1 chains are separated from each other. Table 2 Atomic coordinates and thermal parameters for LiCo(PO3)3 at room temperature with standard deviations in parentheses Atom Co P1 P2 P3 O1 O2 O3 O4 O5 O6 O7 O8 O9 Li a
x
y
Z
0.3561(5) 0.8434(5) 0.6278(6) 0.192(1) 0.5595(9) 0.2021(8) 0.4809(9) 0.6876(8) 0.3131(9) 0.6794(8) 0.6848(9) 0.0283(8) 0.140(2) 0.405(2) 0.156(2) 0.091(2) 0.655(2) 0.313(2) 0.229(2) 0.657(2) 0.042(2) 0.363(2) 0.548(2) 0.270(2) 0.385(2) 0.820(2) 0.382(2) 0.614(2) 0.635(2) 0.408(2) 0.536(2) 0.736(2) 0.134(2) 0.621(2) 0.586(2) 0.888(2) 0.807(2) 0.618(2) 0.130(2) 0.137(6) 0.879(4) 0.338(5) Thermal parameters for all oxygens and Li were constrained to be equal to each other.
˚ 3 100) a Uiso (A 1.5(6) 1.7(7) 1.5(7) 1.6(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6)
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Fig. 1. Observed (dots) and calculated (solid line) profiles for the Rietveld refinement of the LiCo(PO3)3 structure by powder Xray diffraction at room temperature. The difference between the observed and calculated profiles is plotted at the bottom. The vertical bars indicate the positions of Bragg reflections.
Fig. 3a shows the local environment of the Co atom. The Co is localized in the octahedral sites formed by six neighboring PO4 tetrahedra, three of them from a (PO3)1 chain (see also Fig. 2b) and the other three from three neighboring chains. Thus, the Co atoms play a role of interconnecting four separate (PO3)1 chains. Fig. 3b shows the local environment of P1 atom. The PO4 tetrahedron is corner-sharing with two neighboring PO4 tetrahedra, and two CoO6 octahedra. The local environments of P2 and P3 atoms are similar to that of P1. The Li is localized in the octahedron as shown in Fig. 3c, interleaving the space, corner-sharing with six PO4 tetrahedra and edge-sharing with three CoO6 octahedra. There are nine crystallographically distinct oxygen atoms. Six oxygens, O(1, 2, 5, 6, 8, 9), are coordinated to three cations, Co, Li and P(1, 2 or 3), whereas the other oxygens, O(3, 4, 7), are coordinated to two phosphorous cations. 3.2. Magnetic properties Fig. 4 shows the temperature dependence of molar magnetic susceptibilities for LiCo(PO3)3, which represents typical paramagnetic behavior. The inverse susceptibility plot showed the LiCo(PO3)3 follows a Curie–Weiss law, and the data were fitted with the equation: xm = Cm/(T u). The Curie constants and Curie–Weiss temperature were Cm = 3.89, u = 31.9 K, respectively, calculated in the temperature range of 100–300 K. The inverse plot shows only small deviations from linearity even at low temperatures, indicating almost no significant long-range ordering between Co2+ ion above 5 K. From the obtained Curie constant, the effective magnetic moment per Co2+ ion, meff = 5.57 B.M. is obtained, whose value is much larger than the spin-only value of high-spin Co2+ (3.87 mB; mso = [4S(S + 1)]1/2; S = 3/2) but closer to the value expected when the spin momentum and orbital momentum exist independently [5.20 mB; mls = [L(L + 1) + 4S(S + 1)]1/2; L = 3, S = 3/2]. This indicates a contribution of the orbital angular momentum of Co2+ (HS) ion.
S. Choi, S.-T. Hong / Materials Research Bulletin 40 (2005) 1787–1795 Table 3 ˚ ) and angles (8) for LiCo(PO3)3 Selected interatomic distances (A Co–O1 Co–O2 Co–O5 Co–O6 Co–O8 Co–O9
2.16(2) 2.07(1) 2.08(2) 2.09(1) 2.11(1) 2.09(1)
P1–O1 P1–O2 P1–O3 P1–O4
1.46(2) 1.50(2) 1.61(1) 1.57(2)
P2–O4 P2–O5 P2–O6 P2–O7
1.61(2) 1.52(2) 1.46(2) 1.62(2)
P3–O3 P3–O7 P3–O8 P3–O9
1.54(1) 1.57(2) 1.53(2) 1.49(1)
Li–O1 Li–O2 Li–O5 Li–O6 Li–O8 Li–O9
2.38(5) 1.99(4) 2.22(5) 2.14(4) 1.92(4) 2.14(4)
O2–Co–O5 O2–Co–O6 O2–Co–O8 O2–Co–O9 O5–Co–O6 O5–Co–O8 O5–Co–O9 O6–Co–O8 O6–Co–O9 O8–Co–O9
96.9(6) 172.0(7) 85.1(6) 87.8(6) 89.3(6) 91.3(6) 174.1(6) 89.8(6) 85.6(6) 85.6(6)
O1–P1–O2 O1–P1–O3 O1–P1–O4 O2–P1–O3 O2–P1–O4 O3–P1–O4
119(1) 108.6(9) 109(1) 109.7(9) 110.1(8) 98.7(9)
O4–P2–O5 O4–P2–O6 O4–P2–O7 O5–P2–O6 O5–P2–O7 O6–P2–O7
108(1) 112(1) 99.6(8) 117(1) 108(1) 110.5(9)
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Table 3 (Continued ) O3–P3–O7 O3–P3–O8 O3–P3–O9 O7–P3–O8 O7–P3–O9 O8–P3–O9
100.3(9) 107.0(8) 111(1) 109.2(9) 110.8(9) 117.2(9)
LiCoPO4 with the olivine structure shows anti-ferromagnetic behavior with TN 22 K [10–12]. On the other hand, LiCo(PO3)3 shows a paramagnetic behavior in the temperature range of 5–300 K with the same Co2+ (HS) ions. This may be explained by the difference in the CoO6 octahedral linkages in their structures. Fig. 5a shows that the CoO6 octahedra in LiCo(PO3)3 are separated from each other, interconnected by at least one P atom (or PO4 tetrahedron) with the shortest Co–Co distance of
Fig. 2. (a) View of the LiCo(PO3)3 structure projected onto the b–c plane, where only PO4 tetrahedra are shown and Li and Co atoms are omitted for clarity. (b) The circled part in (a), after 908 rotation along c-axis, showing a one-dimensional (PO3)1 zigzag chain along a-axis. The unit cell is outlined.
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Fig. 3. Local environments of (a) Co, (b) P1 and (c) Li in LiCo(PO3)3. The Co, Li and O atoms are shown as dark, hatched and open circles, respectively. The P atoms are in the center of the dark polyhedra.
˚ . On the other hand, in the LiCoPO4 structure (Fig. 5b), each CoO6 octahedron is corner-shared 5.04 A ˚ [13]. The difference in with four neighboring CoO6 octahedra. The shortest Co–Co distance is 3.821 A the CoO6 octahedra linkages between two compounds seems to be responsible for the difference in magnetic behaviors. 3.3. Electrochemical properties measurement Olivine type transition metal phosphate, LiMPO4 (M = Mn, Fe and Co), shows good lithium-ion deintercalation properties by utilizing 3D channel, and intensively investigated as potential candidate of
Fig. 4. Temperature dependence of magnetic and reciprocal susceptibilities measured at 0.5 T for LiCo(PO3)3 in the temperature range of 5–300 K.
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Fig. 5. Comparison of the CoO6 octahedral networks in the: (a) LiCo(PO3)3 and (b) LiCoPO4 structures. The unit cells are outlined.
cathode materials for popular lithium-ion battery [1–3]. Therefore, newly found LiCo(PO3)3 was tested to find out whether it has any electrochemical activities. Unlike olivine LiCoPO4, LiCo(PO3)3 did not show any evidence of lithium de-intercalation in the range of 1–5 V versus Li/Li+.
Acknowledgments We are grateful to Dr. Hyeong-Cheol Ri and Dr. Sang-Jun Oh at Korea Basic Science Institute for their kind help for the magnetic susceptibility measurement.
References [1] [2] [3] [4] [5] [6]
G. Li, H. Azuma, M. Tohda, Electrochem. Solid State Lett. 5 (2002) 135. A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. K. Amine, H. Yasuda, M. Yamachi, Electrochem. Solid State Lett. 3 (2000) 178. F. Sanz, C. Parada, C. Ruiz-Valero, Chem. Mater. 12 (2000) 671. J.P. Rivera, H. Schmid, Ferroelectrics 161 (1994) 91. I. Kornev, J.-P. Rivera, S. Gentil, A.G.M. Jansen, M. Bichurin, H. Schmid, P. Wyder, Physica B 271 (1999) 304.
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[7] E.A. Genkina, B.A. Maksimov, Yu.K. Kabalov, O.K. Mel’nikov, Doklady Akademii Nauk SSSR 270 (1983) 1113 (ICSD #20610, Inorganic Crystal Structure Database, v.1.3.1, Fachinformationszentrum Karlsruhe, Germany, December 2003). [8] M. Laugt, I. Tordjman, J.C. Guitel, M. Roudaut, Acta Crystallogr. B 28 (1972) 2352. [9] A.C. Larson, R.B. Von Dreele, General Structure Analysis System, Report no. LA-UR-86-748, Los Alamos National Laboratory, Los Alamos, NM, 1987. [10] R.P. Santoro, R.E. Newnham, Acta Crystallogr. 22 (1967) 344. [11] J.G. Creer, G.J. Troup, Phys. Lett. 32A (1970) 439. [12] K. Rissouli, K. Benkhouja, M. Bettach, A. Sadel, M. Zahir, A. Derrory, M. Drillon, Ann. Chim. (Paris) 23 (1998) 85. [13] F. Kubel, Zeit. Krist. 209 (1994) 755.
Synthesis, crystal structure and magnetic properties of a new lithium cobalt metaphosphate, LiCo(PO3)3 Seungdon Choi, Seung-Tae Hong * LG Chem Research Park, Daejeon 305-380, Republic of Korea Received 3 August 2004; received in revised form 26 April 2005; accepted 17 May 2005
Abstract A new lithium cobalt metaphosphate, LiCo(PO3)3, is reported for the first time, which was discovered during the exploratory synthesis in Li–Co–P–O system by solid state reaction. The structure has been refined by powder X-ray ˚ , b = 8.6326(2) A ˚ and c = 8.3520(2) A ˚ , Z = 4, Rp = 13.6%, Rietveld refinement method (P212121, a = 8.5398(2) A 2 Rwp = 19.4%, Rexp = 17.7%, S = 1.11, x = 1.23). It is isostructural with LiM(PO3)3 (M = Fe, Cu). It contains (PO3)1 chains with the Co atoms localized in the octahedral sites, bridging four neighboring chains. The magnetic susceptibility measurement showed a typical paramagnetic behavior of high spin of Co2+, following the Curie– Weiss law in the temperature range of 5–300 K. Unlike the olivine type lithium cobalt phosphate, LiCoPO4, cyclic voltammetry of LiCo(PO3)3 assembled in the coin-type cell showed no electrochemical activity in the voltage region of 1–5 V versus Li/Li+. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; C. X-ray diffraction; D. Magnetic properties; D. Crystal structure
1. Introduction The phosphates of several transition metals are known to have various structures, showing interesting properties. For example, the family of LiMPO4 (M = Mn, Fe and Co) [1–3] with the olivine structure shows good electrochemical properties as positive electrode materials for potential application in lithium-ion batteries. In the Li–Co–P–O quaternary system, three compounds have been * Corresponding author. Tel.: +82 428662498; fax: +82 428626073. E-mail address: [email protected] (S.-T. Hong). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.05.021
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reported so far: LiCoPO4, LiCo2(P3O10) and Li6Co5(P2O7)4 [4]. LiCoPO4 shows not only high voltage (5 V versus Li/Li+) lithium ion de-intercalation [3] but also piezo-magnetoelectric properties [5,6]. Exploratory synthesis in the search for new compounds, structures and properties is an important aspect of materials chemistry, leading to the production of a large variety of new phases, sometimes unexpected or unforeseen. Considering that so far only three compounds are known in the Li–Co–P–O system, we expected that there would be new phases with interesting physicochemical properties. A lithium cobalt metaphosphate, LiCo(PO3)3, was discovered during the exploratory synthesis in our laboratory. It is isostructural with LiM(PO3)3 (M = Fe, Cu) [7,8]. Its structural refinement and magnetic properties, as well as electrochemical results, are reported here for the first time.
2. Experimental LiCo(PO3)3 was first discovered as an unidentified phase with a pink color in the mixture of LiCo(PO3)3, Li3PO4 and Co3O4 during the attempts to look for new compounds in the system, Li2CO3:Co(CH3COO)24H2O:(NH4)H2PO4. The phase appeared in the reaction products from various compositions of Li:Co:P. Once it was identified as LiCo(PO3)3, the single phase was synthesized readily from the stoichiometric ratio (1:2:6) of Li2CO3, Co(CH3COO)24H2O and (NH4)H2PO4. These were mixed together, pressed into pellets and heated at 350 8C for 10 h as a calcination step, and at 600 8C for 12 h in the air with intermittent mixing and pressing. X-ray powder diffraction data were collected for LiCo(PO3)3 on a Bruker D4 Endeavor diffractometer ˚ ) and a diffracted beam graphite monochromator. Data were equipped with a Cu X-ray tube (l = 1.5418 A collected in the range of 128 2u 1008 with a step of 0.028 and time per step of 6 s. The structure of LiCo(PO3)3 was refined by the Rietveld method using the GSAS program [9]. The atomic parameters of LiFe(PO3)3 [7] were used as a starting model for the refinement of LiCo(PO3)3. A total of 73 parameters were refined including 50 structural (3 cell parameters, 42 atomic coordinates and 5 isotropic thermal parameters), 12 background parameters, 6 peak profile parameters, 2 absorption correction parameters, polarization factor, zero shift correction and scale factor. Magnetic susceptibilities were measured using a SQUID magnetometer in the temperature range of 5– 300 K. After zero cooling and stabilization of the temperature at 5 K, a magnetic field of 0.5 T was applied. The magnetic susceptibilities were measured with increasing temperature up to 300 K. Electrochemical properties of LiCo(PO3)3 were evaluated with coin-type cells, assembled with circular cathode of 1.49 cm2, metallic lithium anode and 1 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) solvent (1:2, v/v). The cathodes were fabricated by knife-coating the slurry mixture of active material, Super-P and PVdF (85:10:5, w/w/w) on aluminum foil.
3. Results and discussion 3.1. Crystal structure The powder X-ray Rietveld refinement confirmed that the LiCo(PO3)3 structure is isostructural with LiFe(PO3)3 [7] and LiCu(PO3)3 [8]. Crystallographic data are listed in Table 1. The refined atomic and
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Table 1 Crystallographic data for LiCo(PO3)3 at room temperature Formula Formula weight Crystal system Space group
LiCo(PO3)3 302.8 g/mol Orthorhombic P212121 (no. 19)
Unit-cell dimensions (at 298 K) and Z a b c Z
˚ 8.5398(2) A ˚ 8.6326(2) A ˚ 8.3520(2) A 4
Cell volume Density, calculated No. of variables Rp Rwp Rexp S (goodness of fit) x2
˚3 615.71(2) A 3.267 g/cm 3 73 13.6% 19.4% 17.7% 1.11 1.23
isotropic thermal parameters are given in Table 2. Fig. 1 compares the observed and calculated intensities. Selected bond distances and angles are in Table 3. The crystal structure of LiCo(PO3)3 projected onto the b–c plane is shown in Fig. 2a, where only PO4 tetrahedra are shown and Li and Co atoms are omitted for clarity. Fig. 2b, the circled part in Fig. 2a after 908 rotation along c-axis, clearly shows that each tetrahedron is corner-sharing with two neighboring tetrahedra, forming a one-dimensional (PO2O2/2)1 tetrahedral or (PO3)1 zigzag chain along a-axis. Fig. 2a also shows that such (PO3)1 chains are separated from each other. Table 2 Atomic coordinates and thermal parameters for LiCo(PO3)3 at room temperature with standard deviations in parentheses Atom Co P1 P2 P3 O1 O2 O3 O4 O5 O6 O7 O8 O9 Li a
x
y
Z
0.3561(5) 0.8434(5) 0.6278(6) 0.192(1) 0.5595(9) 0.2021(8) 0.4809(9) 0.6876(8) 0.3131(9) 0.6794(8) 0.6848(9) 0.0283(8) 0.140(2) 0.405(2) 0.156(2) 0.091(2) 0.655(2) 0.313(2) 0.229(2) 0.657(2) 0.042(2) 0.363(2) 0.548(2) 0.270(2) 0.385(2) 0.820(2) 0.382(2) 0.614(2) 0.635(2) 0.408(2) 0.536(2) 0.736(2) 0.134(2) 0.621(2) 0.586(2) 0.888(2) 0.807(2) 0.618(2) 0.130(2) 0.137(6) 0.879(4) 0.338(5) Thermal parameters for all oxygens and Li were constrained to be equal to each other.
˚ 3 100) a Uiso (A 1.5(6) 1.7(7) 1.5(7) 1.6(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6) 1.1(6)
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Fig. 1. Observed (dots) and calculated (solid line) profiles for the Rietveld refinement of the LiCo(PO3)3 structure by powder Xray diffraction at room temperature. The difference between the observed and calculated profiles is plotted at the bottom. The vertical bars indicate the positions of Bragg reflections.
Fig. 3a shows the local environment of the Co atom. The Co is localized in the octahedral sites formed by six neighboring PO4 tetrahedra, three of them from a (PO3)1 chain (see also Fig. 2b) and the other three from three neighboring chains. Thus, the Co atoms play a role of interconnecting four separate (PO3)1 chains. Fig. 3b shows the local environment of P1 atom. The PO4 tetrahedron is corner-sharing with two neighboring PO4 tetrahedra, and two CoO6 octahedra. The local environments of P2 and P3 atoms are similar to that of P1. The Li is localized in the octahedron as shown in Fig. 3c, interleaving the space, corner-sharing with six PO4 tetrahedra and edge-sharing with three CoO6 octahedra. There are nine crystallographically distinct oxygen atoms. Six oxygens, O(1, 2, 5, 6, 8, 9), are coordinated to three cations, Co, Li and P(1, 2 or 3), whereas the other oxygens, O(3, 4, 7), are coordinated to two phosphorous cations. 3.2. Magnetic properties Fig. 4 shows the temperature dependence of molar magnetic susceptibilities for LiCo(PO3)3, which represents typical paramagnetic behavior. The inverse susceptibility plot showed the LiCo(PO3)3 follows a Curie–Weiss law, and the data were fitted with the equation: xm = Cm/(T u). The Curie constants and Curie–Weiss temperature were Cm = 3.89, u = 31.9 K, respectively, calculated in the temperature range of 100–300 K. The inverse plot shows only small deviations from linearity even at low temperatures, indicating almost no significant long-range ordering between Co2+ ion above 5 K. From the obtained Curie constant, the effective magnetic moment per Co2+ ion, meff = 5.57 B.M. is obtained, whose value is much larger than the spin-only value of high-spin Co2+ (3.87 mB; mso = [4S(S + 1)]1/2; S = 3/2) but closer to the value expected when the spin momentum and orbital momentum exist independently [5.20 mB; mls = [L(L + 1) + 4S(S + 1)]1/2; L = 3, S = 3/2]. This indicates a contribution of the orbital angular momentum of Co2+ (HS) ion.
S. Choi, S.-T. Hong / Materials Research Bulletin 40 (2005) 1787–1795 Table 3 ˚ ) and angles (8) for LiCo(PO3)3 Selected interatomic distances (A Co–O1 Co–O2 Co–O5 Co–O6 Co–O8 Co–O9
2.16(2) 2.07(1) 2.08(2) 2.09(1) 2.11(1) 2.09(1)
P1–O1 P1–O2 P1–O3 P1–O4
1.46(2) 1.50(2) 1.61(1) 1.57(2)
P2–O4 P2–O5 P2–O6 P2–O7
1.61(2) 1.52(2) 1.46(2) 1.62(2)
P3–O3 P3–O7 P3–O8 P3–O9
1.54(1) 1.57(2) 1.53(2) 1.49(1)
Li–O1 Li–O2 Li–O5 Li–O6 Li–O8 Li–O9
2.38(5) 1.99(4) 2.22(5) 2.14(4) 1.92(4) 2.14(4)
O2–Co–O5 O2–Co–O6 O2–Co–O8 O2–Co–O9 O5–Co–O6 O5–Co–O8 O5–Co–O9 O6–Co–O8 O6–Co–O9 O8–Co–O9
96.9(6) 172.0(7) 85.1(6) 87.8(6) 89.3(6) 91.3(6) 174.1(6) 89.8(6) 85.6(6) 85.6(6)
O1–P1–O2 O1–P1–O3 O1–P1–O4 O2–P1–O3 O2–P1–O4 O3–P1–O4
119(1) 108.6(9) 109(1) 109.7(9) 110.1(8) 98.7(9)
O4–P2–O5 O4–P2–O6 O4–P2–O7 O5–P2–O6 O5–P2–O7 O6–P2–O7
108(1) 112(1) 99.6(8) 117(1) 108(1) 110.5(9)
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Table 3 (Continued ) O3–P3–O7 O3–P3–O8 O3–P3–O9 O7–P3–O8 O7–P3–O9 O8–P3–O9
100.3(9) 107.0(8) 111(1) 109.2(9) 110.8(9) 117.2(9)
LiCoPO4 with the olivine structure shows anti-ferromagnetic behavior with TN 22 K [10–12]. On the other hand, LiCo(PO3)3 shows a paramagnetic behavior in the temperature range of 5–300 K with the same Co2+ (HS) ions. This may be explained by the difference in the CoO6 octahedral linkages in their structures. Fig. 5a shows that the CoO6 octahedra in LiCo(PO3)3 are separated from each other, interconnected by at least one P atom (or PO4 tetrahedron) with the shortest Co–Co distance of
Fig. 2. (a) View of the LiCo(PO3)3 structure projected onto the b–c plane, where only PO4 tetrahedra are shown and Li and Co atoms are omitted for clarity. (b) The circled part in (a), after 908 rotation along c-axis, showing a one-dimensional (PO3)1 zigzag chain along a-axis. The unit cell is outlined.
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Fig. 3. Local environments of (a) Co, (b) P1 and (c) Li in LiCo(PO3)3. The Co, Li and O atoms are shown as dark, hatched and open circles, respectively. The P atoms are in the center of the dark polyhedra.
˚ . On the other hand, in the LiCoPO4 structure (Fig. 5b), each CoO6 octahedron is corner-shared 5.04 A ˚ [13]. The difference in with four neighboring CoO6 octahedra. The shortest Co–Co distance is 3.821 A the CoO6 octahedra linkages between two compounds seems to be responsible for the difference in magnetic behaviors. 3.3. Electrochemical properties measurement Olivine type transition metal phosphate, LiMPO4 (M = Mn, Fe and Co), shows good lithium-ion deintercalation properties by utilizing 3D channel, and intensively investigated as potential candidate of
Fig. 4. Temperature dependence of magnetic and reciprocal susceptibilities measured at 0.5 T for LiCo(PO3)3 in the temperature range of 5–300 K.
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Fig. 5. Comparison of the CoO6 octahedral networks in the: (a) LiCo(PO3)3 and (b) LiCoPO4 structures. The unit cells are outlined.
cathode materials for popular lithium-ion battery [1–3]. Therefore, newly found LiCo(PO3)3 was tested to find out whether it has any electrochemical activities. Unlike olivine LiCoPO4, LiCo(PO3)3 did not show any evidence of lithium de-intercalation in the range of 1–5 V versus Li/Li+.
Acknowledgments We are grateful to Dr. Hyeong-Cheol Ri and Dr. Sang-Jun Oh at Korea Basic Science Institute for their kind help for the magnetic susceptibility measurement.
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