- Email: [email protected]
Subsolidus phase equilibria and the Li5Nd4FeO10 phase in the Li2O–Nd2O3–Fe2O3 system
Materials Research Bulletin 40 (2005) 1856–1863 www.elsevier.com/locate/matresbu
Subsolidus phase equilibria and the Li5Nd4FeO10 phase in the Li2O–Nd2O3–Fe2O3 system I. Ban a,*, M. Drofenik a,b, D. Suvorov b, D. Makovec b a
Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia b ‘‘Jozef Stefan’’ Institute, Jamova 39, 1000 Ljubljana, Slovenia
Received 20 July 2004; received in revised form 16 February 2005; accepted 20 April 2005
Abstract A survey of the subsolidus phase equilibria in the system Li2O–Nd2O3–Fe2O3 was made at subsolidus temperatures in the range 1000–1050 8C. A ternary phase was identified. The phase is centered on Li5Nd4FeO10, ˚ . The compound melts incongruently at 1105 8C. The magnetic susceptibility was with a cubic lattice a = 11.9494 A measured in the temperature range 4–300 K. The compound is paramagnetic in the temperature range 150–300 K and follows the Curie–Weiss law. At about TN = 10 K, a long-range magnetic ordering is observed. # 2005 Elsevier Ltd. All rights reserved. Keywords: C. Differential scanning calorimetry (DSC); C. Thermogravimetric analysis (TGA); C. X-ray diffraction; D. Magnetic properties; D. Phase equilibrium
1. Introduction Several series of the lithium ferrites family have attracted considerable interest due to their potential for technological applications. These materials have a wide range of properties, making them suitable for use in microwave components (LiFe5O8). Low magnetic and dielectric losses are essential requirements for such an application [1,2]. In addition, LiFeO2 is important for cathodic materials and rechargeable lithium batteries. * Corresponding author. Tel.: +386 2 2294 417; fax: +386 2 2527 774. E-mail address: [email protected] (I. Ban). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.04.045
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
1857
This subsolidus phase equilibria implies possible solid-state reactions that can occur during the preparation of microwave ferrites based on Li ferrites doped with rare-earth oxides. Lithium ferrite is a low cost replacement for garnets. The remanence and squareness ratios of the lithium ferrite system are higher than those of other spinel ferries. The effect of various oxide additions on the magnetic and electric properties of ferrites is important and improves their performance. The aim of this work was to investigate the subsolidus phase equilibria in the Li2O–Nd2O3–Fe2O3 system [6,7]. Previous studies on the systems Li2O–La2O3(Nd2O3)–Fe2O3 were mainly concerned with the individual phases that belong to these systems. The structure of a ternary phase, La24Li22Fe6O56, identified, in these systems was determined using the P4/mbm space group and single crystal diffraction data [3–5,8]. However, no complete subsolidus phase equilibrium study for this system has been performed.
2. Experimental For the investigation of this system samples with selected compositions were prepared using Nd2O3 (Ventron 99.9%), Li2CO3 (Ventron 99.8%), Fe2O3 (Alfa product 99.8%) and Li2O (Alfa product 99.7%). Li2CO3 was used for the preparation and examination of the samples with various compositions, except in the Li2O-rich part of the diagram (LiFeO2–LiNdO2–Li2O), where Li2O was used. The samples were mixed in acetone, pressed into pellets and fired up to three times for 5 h at 1000 8C, with intermediate grinding. During firing the pellets were placed in an alumina crucible on a silver foil. The selected compositions within the Li2O–Nd2O3–Fe2O3 phase diagram are shown in Table 1. The compositions Table 1 Results of X-ray powder diffraction analyses of some composition in the Li2O–Nd2O3–Fe2O3 system fired in air at 1000 8C Samples code
Nominal composition
Phases identified
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
6Li2O + 3Fe2O3 + 1Nd2O3 2Li2CO3 + 6.5Fe2O3 + 1.5Nd2O3 6.6Li2O + 1.8Fe2O3 + 1.7Nd2O3 1Li2CO3 + 1Fe2O3 + 1Nd2O3 5Li2CO3 + 4Fe2O3 + 1Nd2O3 5Li2CO3 + 3Fe2O3 + 2Nd2O3 5Li2CO3 + 2Fe2O3 + 3Nd2O3 5Li2CO3 + 1Fe2O3 + 4Nd2O3 1Li2CO3 + 3Fe2O3 + 6Nd2O3 1Li2CO3 + 1Fe2O3 + 4Nd2O3 4Li2CO3 + 4Fe2O3 + 2Nd2O3 2Li2CO3 + 2Fe2O3 + 6Nd2O3 2.5Li2CO3 + 5Fe2O3 + 2.5Nd2O3 2.5Li2CO3 + 3Fe2O3 + 4.5Nd2O3 1Li2CO3 + 7Fe2O3 + 2Nd2O3 5Li2CO3 + 1Fe2O3 + 12Nd2O3 12Li2O + 1Fe2O3 + 5Nd2O3 4.5Li2CO3 + 0.5Fe2O3 + 5Nd2O3 0.7Li2CO3 + 8.3Fe2O3 + 1Nd2O3 5.5Li2O + 0.5Fe2O3 + 4Nd2O3
LNF, LiFeO2, Li2O LiFe5O8, LiFeO2, FeNdO2 LNF, LiFeO2, Li2O LNF, LiFeO2, FeNdO3 LNF, LiFeO2 LNF, LiFeO2 LNF, LiFeO2 LNF LNF, Nd2O3, FeNdO3 LNF, Nd2O3, FeNdO3 LNF, LiFeO2, FeNdO3 LNF, Nd2O3, FeNdO3 LiFeO2, FeNdO3 LNF, FeNdO3 LiFe5O8, FeNdO3 LNF, Nd2O3 LNF, Li2O LNF, LiNdO2, Nd2O3 LiFe5O8, Fe2O3, FeNdO2 LNF, LiNdO2, Li2O
Li5Nd4FeO10 LNF.
1858
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
from the Nd2O3-rich part of the diagram were stored in petroleum oil after the last firing to prevent any reaction of the neodymium oxide with moisture. The fired samples were characterized by X-ray powder diffraction analyses with a Siemens D-5005 Xray diffractometer using Cu Ka radiation. The X-ray diffraction data were measured from 2u = 108 to 708 in steps of 0.028/s. A JEOL JXA-840A scanning electron microscope (SEM) equipped with an ISIS energy dispersive X-ray analyser was used for the overall microstructural and compositional analyses. Samples prepared for the SEM were mounted in epoxy in a cross-sectional orientation and then polished using standard metallographic techniques. Prior to analysis in the SEM, the samples were coated with carbon to provide electrical conductivity and avoid charging effects. The melting point of the ternary compound LFN was determined by monitoring the change in the dimension of the sample during heating it in air using a hot-stage microscope. The density of the powdered samples was measured using a helium picnometer. The magnetic susceptibility was measured using a SQUID magnetometer from room temperature down to liquid-helium temperature. The amount of Fe4+ in the samples was determined with the redox reaction: Fe4+ + Fe2+ = 2Fe3+. A weighed amount of samples was dissolved in a 0.05 M standardized Mohr salt solution, prepared in a 2 M H2SO4, and back titration of the remaining Fe2+ equivalents was performed with 0.01 M KMnO4 [9]. The composition of the ternary compound was determined with inductively coupled plasma atomic emission spectroscopy (ICP-AES) where a raw oxide mixture corresponding to the ternary phase composition was used as an internal standard.
3. Results and discussion 3.1. The subsolidus phase diagram The results of the heating experiments on 20 selected compositions are listed in Table 1. These data were used to construct the phase diagram shown in Fig. 2. One ternary phase, Li5Nd4FeO10, was
Fig. 1. TGA and DTA measurements of the oxide mixture of composition 8, Table 1 corresponding to the LNF compound: (a) during heating to 1000 8C and (b) during isothermal heating at 1000 8C in air.
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
1859
indicated as LFN. The studied subsolidus phase diagram relates to temperatures in the range 1000– 1050 8C. These temperatures are subsolidus temperatures; however, they are close to the melting points for most of the phase diagram. Seven compatibility triangles were established in the subsolidus region. The tie line LiFeO2–NdFeO3 divides the ternary system into two parts. The upper part, LiFeO2–NdFeO3–Fe2O3, is ‘‘empty’’, no ternary compound could be identified in this part of the system. In the lower part of the system, a ternary phase was identified with a nominal composition of Li5Nd4FeO10. The tie lines are between Li5Nd4FeO10 and LiFeO2, NdFeO3, LiNdO2 and Nd2O3. In addition, a tie line between LiFe5O8 and NdFeO3 was established. In the system Li2O–Nd2O3, the binary compound LiNdO2 was identified (PDF 30-755), while in the Fe2O3–Nd2O3 binary system the compound FeNdO3 was identified (PDF 25-1149). In the binary system Li2O–Fe2O3, two compounds were identified, LiFeO2 (PDF 17-0938) and LiFe5O8 (PDF 17-0114). All these compounds were identical to those found in the X-ray diffraction files [10]. Some attempts were made to estimate the solid solubility between LNF and LiFeO2. A preliminary result indicates that no solid solubility exists. The nominal compositions prior to the firing of the samples and the phases identified using X-ray diffraction analyses after the firing are given in Table 1. For the preparation and examination of the samples with various compositions Li2CO3 was used, except in the Li2O-rich part of the diagram where Li2O was used. The Li2CO3 melts at 723 8C [11]. The disintegration of Li2CO3 is associated with an evaporation temperature at 1310 8C. However, it decomposes much earlier when the Li2CO3 reacts with the constituent oxides. That the lithium carbonate (Li2CO3) decomposition is regulated by the ferrite formation and that the related amount of compatible phases is in turn determined by the composition of the starting mixture is well established. The mass spectroscopy-assisted TG/DTA of our samples containing a mixture including Li2CO3 is consistent with the results of other authors reporting that the lithium carbonate disintegrates at much lower temperatures when it reacts with iron oxide-forming compounds [12]. The formation of LNF occurs during the reaction of Fe2O3, Nd2O3 and Li2CO3 below 1000 8C. Lithium carbonate starts to decompose far below its normal disintegration temperature and below its melting point. Fig. 1 shows characteristic TGA and DTA spectra for composition 8. During heating up to 1000 8C three well-developed endothermic peaks can be seen. Two of them are associated with weight loss. The endothermic peaks at 364.7 and 465.8 8C are accompanied by CO2 release, as confirmed by the mass spectroscopy analysis. The X-ray diffraction powder pattern of the samples heated to 600 8C shows the diffraction pattern of the starting oxides reflections in addition to the diffraction pattern of FeNdO2. The endothermic peak at 733 8C is associated with the melting of Li2CO3. Here, no weight loss could be detected. However, on further heating a significant weight loss is detected, associated with CO2 loss. During this final heating of the reaction mixture, the heterogeneous phase composition reacts to form the stable ternary phase LNF, associated with the reaction of the rest of the Li2CO3 present in the reaction mixture. The formation of the ternary compound LNF was confirmed by X-ray powder diffraction measurements. During the isothermal heating at 1000 8C, after half an hour of heating a plateau is reached, after that a slight increase of the weight can be noted (Fig. 1b). Since the lithium and neodymium possess very stable oxidation states, it is most probable that the weight increase observed is a consequence of Fe3+ iron oxidation. During heating of the starting raw oxide mixture and consequently the formation of the LNF phase in a strong alkaline media the oxidation of the Fe3+ occurs associated with an uptake of oxygen, which is relatively small and was partially masked due to the simultaneous CO2 release.
1860
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
Fig. 2. Proposed subsolidus ternary phase diagram of the Li2O–Nd2O3–Fe2O3 system, with the composition of the ternary phase marked (composition 8).
Table 2 X-ray powder diffraction data for Li4.23Nd3.86Fe0.83+Fe0.24+O9.50 h
k
l
2u(obs)
2u(calc)
0 3 2 1 0 3 2 0 1 4 3 0 1 6 4 3 0 0 1 3 8 8 2 3 2
1 0 2 2 0 0 2 1 2 0 0 0 1 0 1 1 1 4 2 0 1 0 2 1 5
1 1 2 3 4 3 4 5 5 4 5 6 6 2 5 6 7 6 7 7 1 2 8 8 7
10.5234 23.6005 25.8782 27.9845 29.9679 31.8131 36.8736 38.4603 41.4318 42.8252 44.2407 45.5963 46.8866 48.2120 49.4798 51.9283 54.3114 55.4720 56.6353 58.8927 63.2542 64.3052 66.4036 67.4505 69.4620
10.4613 23.5245 25.8068 27.9148 29.8855 31.7447 36.8189 38.3799 41.3518 42.7734 44.1579 45.5087 46.8289 48.1212 49.3879 51.8523 54.2361 55.4014 56.5505 58.8047 63.1621 64.2245 66.3216 67.3575 69.4065
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
1861
In the Li2O-rich part of the system, which covers the compositions within the subsystem Li2O–LNF– Nd2O3 and Li2O–LNF–Nd2O3 the third compatible phase should probably be Li2CO3. This is due to the fact that after the melting of the Li2CO3 and the subsequent reaction with other constituent oxides forming binary or ternary phases, some excess of Li2CO3 remains, which is stable at that temperature and can be identified by the X-ray analyses. Thus, when considering this part of the subsolidus equilibrium phase diagram Li2O was used instead of Li2CO3 (Fig. 2). 3.2. X-ray powder diffraction data of Li5Nd4FeO10 LNF ˚ . The microThe X-ray powder diffraction data can be indexed to a cubic unit cell a = 11.9494 A structure of pure LNF shows no other phase present. The chemical analysis of the ternary phase LNF gave the following composition: 3.86Nd2O34.23Li2OFe2O3 and/or Li4.23Nd3.86FeO9.40, which is fairly close to the nominal composition Li5Nd4FeO10. The analyses of the oxidation number of iron shows that the compound contains 20 mol% of Fe4+. The stoichiometric composition of the compound considered was therefore Li4.23Nd3.86Fe1 x3+Fex4+O7.90+1.5(1 x)+2x, where x = 0.2 (Li4.23Nd3.86Fe0.83+Fe0.24+O9.50). With seven formula units per unit cell the estimated density of the compound is 5.409 g/cm3. The measured density of LNF is 5.58 0.05 g/cm3 (Table 2). 3.3. Thermal stability of the LNF phase The ternary phase with a nominal composition of Li5Nd4FeO10 lies on the tie line LiFeO2–LiNdO2. This phase (the compound LNF) melts incongruently at 1105 8C. Fig. 3 shows the microstructure of the melted sample. The microstructure of the melted sample is composed of a light phase Li5Nd4FeO10, a gray phase Nd2O3 and a dark gray phase FeNdO3. The X-ray powder diffraction analysis of the melted sample in Fig. 4 supports the microstructural analyses presented in Fig. 3.
Fig. 3. SEM image of melted sample with composition 8 (compound LNF). Light phase represents the compound LNF, gray phase is Nd2O3 and dark gray phase is FeNdO2.
1862
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
Fig. 4. X-ray powder diffraction patterns: (a) diffraction patterns of melted and crystallized LNF after cooling, where L = LNF, N = Nd2O3, FN = FeNdO3 and (b) diffraction patterns of the ternary compound LNF.
3.4. Magnetic measurements The magnetic susceptibility of Li4.23Nd3.86Fe0.83+Fe0.24+O9.50 followed the Curie–Weiss law in the temperature range 150–300 K. The corrected susceptibilities for the diamagnetic contribution data are shown in the plot of molecular susceptibility against temperature (Fig. 5). The diamagnetic contribution was taken as the sum of Pascal’s constants, amounting to 213 10 6 emu/g [13]. A least-squares fitting of the data in the temperature range 150–300 K gave a Curie constant, C = 10.3 emu K/mol, and a paramagnetic Curie temperature, u = 29 K, indicating an antiferromagnetic interaction between the iron atoms. A maximum in the magnetic susceptibility in the x(T) relation is observed at TN = 10 K, indicating the onset of a long-range magnetic ordering. The Curie constant agrees with the calculated value C = 10.7 emu K/mol assuming that spins of 3.87 neodymium Nd3+ and one iron, i.e. (Fe1 x3+Fex4+, x = 0.2) contribute to the susceptibility of one formula
Fig. 5. Susceptibility of LNF plotted against temperature.
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
1863
unit. The estimated effective magnetic moment of the compound is meff = 9.12 B.M. This value is close to the measured effective moment of 9.09 B.M.
4. Conclusions A ternary phase with the nominal composition Li5Nd4FeO10 was identified in the ternary system ˚ . The compound melts Li2O–Nd2O3–Fe2O3, and was indexed on the basis of a cubic cell a = 1.9494 A incongruently at 1105 8C. The compound is paramagnetic and follows the Curie–Weiss law in the temperature range 150–300 K. At about TN = 10 K, a long-range magnetic ordering is observed.
Acknowledgements The authors would like to thank Dr. Z. Jaglicˇicˇ for the SQUID measurements, Dr. B. Jancˇar for the SEM analyses and Dr. B. Budicˇ for the chemical analyses. We also gratefully acknowledge the support of the Ministry of Education, Science and Sport of the Republic of Slovenia for their support of this work.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Y. Sakurai, H. Arai, S. Okada, J. Yamaki, J. Power Sources 68 (1997) 711–715. N. Kumar, P. Kishan, Z.H. Zaid, J. Magn. Magn. Mater. 184 (1998) 335–357. D. Mazza, F. Abatista, M. Vallino, J. Less Common Metals 106 (1985) 277–285. F. Abbattista, D. Mazza, M. Vallino, M. Gazzano, J. Less Common Metals 142 (1988) 203–211. F. Abbattista, D. Mazza, M. Vallino, M. Gazzano, J. Less Common Metals 144 (1988) 311–319. E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase Diagrams for Ceramists, 1964 Supplement, American Ceramic Society, Columbus, OH, 1964, p. 54. L.P. Shklover, I.F. Zakharchenko, L.M. Shkolnokova, A.E. Obodobskaja, Russ. J. Inorg. Chem. 20 (7) (1975) 983–984. D. Mazza, F. Abatista, M. Vallino, Atti della Academia della Scienze di Torno 122 (3–4) (1988) 155–163. P. Porta, S. Cimino, S. De Rossi, M. Faticanti, G. Minelli, I. Pettiti, Mater. Chem. Phys. 71 (2001) 165–173. DIFFRACplus, Seach, ICDD database PDF, 1997. R.C. West, Handbook of Chemistry and Physics, CRC Press Inc., Cleveland, OH, 1973. V. Berbenni, A. Marini, G. Bruni, R. Riccardi, Thermochim. Acta 346 (2000) 115–132. K.H. Helluege (Ed.), Landolt–Bo¨rnstein Numerical Data and Functional Relationship in Science and Technology, New Series, Group 3, Crystal and Solid State Physics, vol. 2, Springer-Verlag, New York, 1966, pp. 1–13.
Subsolidus phase equilibria and the Li5Nd4FeO10 phase in the Li2O–Nd2O3–Fe2O3 system I. Ban a,*, M. Drofenik a,b, D. Suvorov b, D. Makovec b a
Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia b ‘‘Jozef Stefan’’ Institute, Jamova 39, 1000 Ljubljana, Slovenia
Received 20 July 2004; received in revised form 16 February 2005; accepted 20 April 2005
Abstract A survey of the subsolidus phase equilibria in the system Li2O–Nd2O3–Fe2O3 was made at subsolidus temperatures in the range 1000–1050 8C. A ternary phase was identified. The phase is centered on Li5Nd4FeO10, ˚ . The compound melts incongruently at 1105 8C. The magnetic susceptibility was with a cubic lattice a = 11.9494 A measured in the temperature range 4–300 K. The compound is paramagnetic in the temperature range 150–300 K and follows the Curie–Weiss law. At about TN = 10 K, a long-range magnetic ordering is observed. # 2005 Elsevier Ltd. All rights reserved. Keywords: C. Differential scanning calorimetry (DSC); C. Thermogravimetric analysis (TGA); C. X-ray diffraction; D. Magnetic properties; D. Phase equilibrium
1. Introduction Several series of the lithium ferrites family have attracted considerable interest due to their potential for technological applications. These materials have a wide range of properties, making them suitable for use in microwave components (LiFe5O8). Low magnetic and dielectric losses are essential requirements for such an application [1,2]. In addition, LiFeO2 is important for cathodic materials and rechargeable lithium batteries. * Corresponding author. Tel.: +386 2 2294 417; fax: +386 2 2527 774. E-mail address: [email protected] (I. Ban). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.04.045
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
1857
This subsolidus phase equilibria implies possible solid-state reactions that can occur during the preparation of microwave ferrites based on Li ferrites doped with rare-earth oxides. Lithium ferrite is a low cost replacement for garnets. The remanence and squareness ratios of the lithium ferrite system are higher than those of other spinel ferries. The effect of various oxide additions on the magnetic and electric properties of ferrites is important and improves their performance. The aim of this work was to investigate the subsolidus phase equilibria in the Li2O–Nd2O3–Fe2O3 system [6,7]. Previous studies on the systems Li2O–La2O3(Nd2O3)–Fe2O3 were mainly concerned with the individual phases that belong to these systems. The structure of a ternary phase, La24Li22Fe6O56, identified, in these systems was determined using the P4/mbm space group and single crystal diffraction data [3–5,8]. However, no complete subsolidus phase equilibrium study for this system has been performed.
2. Experimental For the investigation of this system samples with selected compositions were prepared using Nd2O3 (Ventron 99.9%), Li2CO3 (Ventron 99.8%), Fe2O3 (Alfa product 99.8%) and Li2O (Alfa product 99.7%). Li2CO3 was used for the preparation and examination of the samples with various compositions, except in the Li2O-rich part of the diagram (LiFeO2–LiNdO2–Li2O), where Li2O was used. The samples were mixed in acetone, pressed into pellets and fired up to three times for 5 h at 1000 8C, with intermediate grinding. During firing the pellets were placed in an alumina crucible on a silver foil. The selected compositions within the Li2O–Nd2O3–Fe2O3 phase diagram are shown in Table 1. The compositions Table 1 Results of X-ray powder diffraction analyses of some composition in the Li2O–Nd2O3–Fe2O3 system fired in air at 1000 8C Samples code
Nominal composition
Phases identified
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
6Li2O + 3Fe2O3 + 1Nd2O3 2Li2CO3 + 6.5Fe2O3 + 1.5Nd2O3 6.6Li2O + 1.8Fe2O3 + 1.7Nd2O3 1Li2CO3 + 1Fe2O3 + 1Nd2O3 5Li2CO3 + 4Fe2O3 + 1Nd2O3 5Li2CO3 + 3Fe2O3 + 2Nd2O3 5Li2CO3 + 2Fe2O3 + 3Nd2O3 5Li2CO3 + 1Fe2O3 + 4Nd2O3 1Li2CO3 + 3Fe2O3 + 6Nd2O3 1Li2CO3 + 1Fe2O3 + 4Nd2O3 4Li2CO3 + 4Fe2O3 + 2Nd2O3 2Li2CO3 + 2Fe2O3 + 6Nd2O3 2.5Li2CO3 + 5Fe2O3 + 2.5Nd2O3 2.5Li2CO3 + 3Fe2O3 + 4.5Nd2O3 1Li2CO3 + 7Fe2O3 + 2Nd2O3 5Li2CO3 + 1Fe2O3 + 12Nd2O3 12Li2O + 1Fe2O3 + 5Nd2O3 4.5Li2CO3 + 0.5Fe2O3 + 5Nd2O3 0.7Li2CO3 + 8.3Fe2O3 + 1Nd2O3 5.5Li2O + 0.5Fe2O3 + 4Nd2O3
LNF, LiFeO2, Li2O LiFe5O8, LiFeO2, FeNdO2 LNF, LiFeO2, Li2O LNF, LiFeO2, FeNdO3 LNF, LiFeO2 LNF, LiFeO2 LNF, LiFeO2 LNF LNF, Nd2O3, FeNdO3 LNF, Nd2O3, FeNdO3 LNF, LiFeO2, FeNdO3 LNF, Nd2O3, FeNdO3 LiFeO2, FeNdO3 LNF, FeNdO3 LiFe5O8, FeNdO3 LNF, Nd2O3 LNF, Li2O LNF, LiNdO2, Nd2O3 LiFe5O8, Fe2O3, FeNdO2 LNF, LiNdO2, Li2O
Li5Nd4FeO10 LNF.
1858
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
from the Nd2O3-rich part of the diagram were stored in petroleum oil after the last firing to prevent any reaction of the neodymium oxide with moisture. The fired samples were characterized by X-ray powder diffraction analyses with a Siemens D-5005 Xray diffractometer using Cu Ka radiation. The X-ray diffraction data were measured from 2u = 108 to 708 in steps of 0.028/s. A JEOL JXA-840A scanning electron microscope (SEM) equipped with an ISIS energy dispersive X-ray analyser was used for the overall microstructural and compositional analyses. Samples prepared for the SEM were mounted in epoxy in a cross-sectional orientation and then polished using standard metallographic techniques. Prior to analysis in the SEM, the samples were coated with carbon to provide electrical conductivity and avoid charging effects. The melting point of the ternary compound LFN was determined by monitoring the change in the dimension of the sample during heating it in air using a hot-stage microscope. The density of the powdered samples was measured using a helium picnometer. The magnetic susceptibility was measured using a SQUID magnetometer from room temperature down to liquid-helium temperature. The amount of Fe4+ in the samples was determined with the redox reaction: Fe4+ + Fe2+ = 2Fe3+. A weighed amount of samples was dissolved in a 0.05 M standardized Mohr salt solution, prepared in a 2 M H2SO4, and back titration of the remaining Fe2+ equivalents was performed with 0.01 M KMnO4 [9]. The composition of the ternary compound was determined with inductively coupled plasma atomic emission spectroscopy (ICP-AES) where a raw oxide mixture corresponding to the ternary phase composition was used as an internal standard.
3. Results and discussion 3.1. The subsolidus phase diagram The results of the heating experiments on 20 selected compositions are listed in Table 1. These data were used to construct the phase diagram shown in Fig. 2. One ternary phase, Li5Nd4FeO10, was
Fig. 1. TGA and DTA measurements of the oxide mixture of composition 8, Table 1 corresponding to the LNF compound: (a) during heating to 1000 8C and (b) during isothermal heating at 1000 8C in air.
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
1859
indicated as LFN. The studied subsolidus phase diagram relates to temperatures in the range 1000– 1050 8C. These temperatures are subsolidus temperatures; however, they are close to the melting points for most of the phase diagram. Seven compatibility triangles were established in the subsolidus region. The tie line LiFeO2–NdFeO3 divides the ternary system into two parts. The upper part, LiFeO2–NdFeO3–Fe2O3, is ‘‘empty’’, no ternary compound could be identified in this part of the system. In the lower part of the system, a ternary phase was identified with a nominal composition of Li5Nd4FeO10. The tie lines are between Li5Nd4FeO10 and LiFeO2, NdFeO3, LiNdO2 and Nd2O3. In addition, a tie line between LiFe5O8 and NdFeO3 was established. In the system Li2O–Nd2O3, the binary compound LiNdO2 was identified (PDF 30-755), while in the Fe2O3–Nd2O3 binary system the compound FeNdO3 was identified (PDF 25-1149). In the binary system Li2O–Fe2O3, two compounds were identified, LiFeO2 (PDF 17-0938) and LiFe5O8 (PDF 17-0114). All these compounds were identical to those found in the X-ray diffraction files [10]. Some attempts were made to estimate the solid solubility between LNF and LiFeO2. A preliminary result indicates that no solid solubility exists. The nominal compositions prior to the firing of the samples and the phases identified using X-ray diffraction analyses after the firing are given in Table 1. For the preparation and examination of the samples with various compositions Li2CO3 was used, except in the Li2O-rich part of the diagram where Li2O was used. The Li2CO3 melts at 723 8C [11]. The disintegration of Li2CO3 is associated with an evaporation temperature at 1310 8C. However, it decomposes much earlier when the Li2CO3 reacts with the constituent oxides. That the lithium carbonate (Li2CO3) decomposition is regulated by the ferrite formation and that the related amount of compatible phases is in turn determined by the composition of the starting mixture is well established. The mass spectroscopy-assisted TG/DTA of our samples containing a mixture including Li2CO3 is consistent with the results of other authors reporting that the lithium carbonate disintegrates at much lower temperatures when it reacts with iron oxide-forming compounds [12]. The formation of LNF occurs during the reaction of Fe2O3, Nd2O3 and Li2CO3 below 1000 8C. Lithium carbonate starts to decompose far below its normal disintegration temperature and below its melting point. Fig. 1 shows characteristic TGA and DTA spectra for composition 8. During heating up to 1000 8C three well-developed endothermic peaks can be seen. Two of them are associated with weight loss. The endothermic peaks at 364.7 and 465.8 8C are accompanied by CO2 release, as confirmed by the mass spectroscopy analysis. The X-ray diffraction powder pattern of the samples heated to 600 8C shows the diffraction pattern of the starting oxides reflections in addition to the diffraction pattern of FeNdO2. The endothermic peak at 733 8C is associated with the melting of Li2CO3. Here, no weight loss could be detected. However, on further heating a significant weight loss is detected, associated with CO2 loss. During this final heating of the reaction mixture, the heterogeneous phase composition reacts to form the stable ternary phase LNF, associated with the reaction of the rest of the Li2CO3 present in the reaction mixture. The formation of the ternary compound LNF was confirmed by X-ray powder diffraction measurements. During the isothermal heating at 1000 8C, after half an hour of heating a plateau is reached, after that a slight increase of the weight can be noted (Fig. 1b). Since the lithium and neodymium possess very stable oxidation states, it is most probable that the weight increase observed is a consequence of Fe3+ iron oxidation. During heating of the starting raw oxide mixture and consequently the formation of the LNF phase in a strong alkaline media the oxidation of the Fe3+ occurs associated with an uptake of oxygen, which is relatively small and was partially masked due to the simultaneous CO2 release.
1860
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
Fig. 2. Proposed subsolidus ternary phase diagram of the Li2O–Nd2O3–Fe2O3 system, with the composition of the ternary phase marked (composition 8).
Table 2 X-ray powder diffraction data for Li4.23Nd3.86Fe0.83+Fe0.24+O9.50 h
k
l
2u(obs)
2u(calc)
0 3 2 1 0 3 2 0 1 4 3 0 1 6 4 3 0 0 1 3 8 8 2 3 2
1 0 2 2 0 0 2 1 2 0 0 0 1 0 1 1 1 4 2 0 1 0 2 1 5
1 1 2 3 4 3 4 5 5 4 5 6 6 2 5 6 7 6 7 7 1 2 8 8 7
10.5234 23.6005 25.8782 27.9845 29.9679 31.8131 36.8736 38.4603 41.4318 42.8252 44.2407 45.5963 46.8866 48.2120 49.4798 51.9283 54.3114 55.4720 56.6353 58.8927 63.2542 64.3052 66.4036 67.4505 69.4620
10.4613 23.5245 25.8068 27.9148 29.8855 31.7447 36.8189 38.3799 41.3518 42.7734 44.1579 45.5087 46.8289 48.1212 49.3879 51.8523 54.2361 55.4014 56.5505 58.8047 63.1621 64.2245 66.3216 67.3575 69.4065
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
1861
In the Li2O-rich part of the system, which covers the compositions within the subsystem Li2O–LNF– Nd2O3 and Li2O–LNF–Nd2O3 the third compatible phase should probably be Li2CO3. This is due to the fact that after the melting of the Li2CO3 and the subsequent reaction with other constituent oxides forming binary or ternary phases, some excess of Li2CO3 remains, which is stable at that temperature and can be identified by the X-ray analyses. Thus, when considering this part of the subsolidus equilibrium phase diagram Li2O was used instead of Li2CO3 (Fig. 2). 3.2. X-ray powder diffraction data of Li5Nd4FeO10 LNF ˚ . The microThe X-ray powder diffraction data can be indexed to a cubic unit cell a = 11.9494 A structure of pure LNF shows no other phase present. The chemical analysis of the ternary phase LNF gave the following composition: 3.86Nd2O34.23Li2OFe2O3 and/or Li4.23Nd3.86FeO9.40, which is fairly close to the nominal composition Li5Nd4FeO10. The analyses of the oxidation number of iron shows that the compound contains 20 mol% of Fe4+. The stoichiometric composition of the compound considered was therefore Li4.23Nd3.86Fe1 x3+Fex4+O7.90+1.5(1 x)+2x, where x = 0.2 (Li4.23Nd3.86Fe0.83+Fe0.24+O9.50). With seven formula units per unit cell the estimated density of the compound is 5.409 g/cm3. The measured density of LNF is 5.58 0.05 g/cm3 (Table 2). 3.3. Thermal stability of the LNF phase The ternary phase with a nominal composition of Li5Nd4FeO10 lies on the tie line LiFeO2–LiNdO2. This phase (the compound LNF) melts incongruently at 1105 8C. Fig. 3 shows the microstructure of the melted sample. The microstructure of the melted sample is composed of a light phase Li5Nd4FeO10, a gray phase Nd2O3 and a dark gray phase FeNdO3. The X-ray powder diffraction analysis of the melted sample in Fig. 4 supports the microstructural analyses presented in Fig. 3.
Fig. 3. SEM image of melted sample with composition 8 (compound LNF). Light phase represents the compound LNF, gray phase is Nd2O3 and dark gray phase is FeNdO2.
1862
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
Fig. 4. X-ray powder diffraction patterns: (a) diffraction patterns of melted and crystallized LNF after cooling, where L = LNF, N = Nd2O3, FN = FeNdO3 and (b) diffraction patterns of the ternary compound LNF.
3.4. Magnetic measurements The magnetic susceptibility of Li4.23Nd3.86Fe0.83+Fe0.24+O9.50 followed the Curie–Weiss law in the temperature range 150–300 K. The corrected susceptibilities for the diamagnetic contribution data are shown in the plot of molecular susceptibility against temperature (Fig. 5). The diamagnetic contribution was taken as the sum of Pascal’s constants, amounting to 213 10 6 emu/g [13]. A least-squares fitting of the data in the temperature range 150–300 K gave a Curie constant, C = 10.3 emu K/mol, and a paramagnetic Curie temperature, u = 29 K, indicating an antiferromagnetic interaction between the iron atoms. A maximum in the magnetic susceptibility in the x(T) relation is observed at TN = 10 K, indicating the onset of a long-range magnetic ordering. The Curie constant agrees with the calculated value C = 10.7 emu K/mol assuming that spins of 3.87 neodymium Nd3+ and one iron, i.e. (Fe1 x3+Fex4+, x = 0.2) contribute to the susceptibility of one formula
Fig. 5. Susceptibility of LNF plotted against temperature.
I. Ban et al. / Materials Research Bulletin 40 (2005) 1856–1863
1863
unit. The estimated effective magnetic moment of the compound is meff = 9.12 B.M. This value is close to the measured effective moment of 9.09 B.M.
4. Conclusions A ternary phase with the nominal composition Li5Nd4FeO10 was identified in the ternary system ˚ . The compound melts Li2O–Nd2O3–Fe2O3, and was indexed on the basis of a cubic cell a = 1.9494 A incongruently at 1105 8C. The compound is paramagnetic and follows the Curie–Weiss law in the temperature range 150–300 K. At about TN = 10 K, a long-range magnetic ordering is observed.
Acknowledgements The authors would like to thank Dr. Z. Jaglicˇicˇ for the SQUID measurements, Dr. B. Jancˇar for the SEM analyses and Dr. B. Budicˇ for the chemical analyses. We also gratefully acknowledge the support of the Ministry of Education, Science and Sport of the Republic of Slovenia for their support of this work.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Y. Sakurai, H. Arai, S. Okada, J. Yamaki, J. Power Sources 68 (1997) 711–715. N. Kumar, P. Kishan, Z.H. Zaid, J. Magn. Magn. Mater. 184 (1998) 335–357. D. Mazza, F. Abatista, M. Vallino, J. Less Common Metals 106 (1985) 277–285. F. Abbattista, D. Mazza, M. Vallino, M. Gazzano, J. Less Common Metals 142 (1988) 203–211. F. Abbattista, D. Mazza, M. Vallino, M. Gazzano, J. Less Common Metals 144 (1988) 311–319. E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase Diagrams for Ceramists, 1964 Supplement, American Ceramic Society, Columbus, OH, 1964, p. 54. L.P. Shklover, I.F. Zakharchenko, L.M. Shkolnokova, A.E. Obodobskaja, Russ. J. Inorg. Chem. 20 (7) (1975) 983–984. D. Mazza, F. Abatista, M. Vallino, Atti della Academia della Scienze di Torno 122 (3–4) (1988) 155–163. P. Porta, S. Cimino, S. De Rossi, M. Faticanti, G. Minelli, I. Pettiti, Mater. Chem. Phys. 71 (2001) 165–173. DIFFRACplus, Seach, ICDD database PDF, 1997. R.C. West, Handbook of Chemistry and Physics, CRC Press Inc., Cleveland, OH, 1973. V. Berbenni, A. Marini, G. Bruni, R. Riccardi, Thermochim. Acta 346 (2000) 115–132. K.H. Helluege (Ed.), Landolt–Bo¨rnstein Numerical Data and Functional Relationship in Science and Technology, New Series, Group 3, Crystal and Solid State Physics, vol. 2, Springer-Verlag, New York, 1966, pp. 1–13.