- Email: [email protected]
C studied by EPR
Journal of Alloys and Compounds 470 (2009) 51–54
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Ageing effect in nanocrystalline TiCx /C studied by EPR N. Guskos a,b , J. Typek b,∗ , T. Bodziony b , G. Zolnierkiewicz b , M. Maryniak b , A. Biedunkiewicz c a
Solid State Physics, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece Institute of Physics, Szczecin University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland c Institute of Material Engineering, Szczecin University of Technology, Al. Piastow 17, 70-310 Szczecin, Poland b
a r t i c l e
i n f o
Article history: Received 18 January 2008 Received in revised form 1 March 2008 Accepted 4 March 2008 Available online 21 April 2008 Keywords: EPR of Ti3+ Titanium carbide
a b s t r a c t TiC/C nanocrystalline material: titanium carbide TiC dispersed in a carbon matrix has been prepared by a nonhydrolytic sol–gel process. Temperature dependence of the electron paramagnetic resonance (EPR) spectra of this material has been studied in the 3.5–120 K range. Two very different EPR lines have been recorded in fresh sample at temperatures below 120 K arising from the Ti(III) complex (broad and asymmetric line) and conduction electrons (very narrow line). In the same aged sample (1 year old) the magnetic anisotropy of Ti(III) line has increased while a narrow line attributed to conduction electrons has vanished. The existence of the paramagnetic centers connected with trivalent titanium ions could the result of disordering processes. The increase of anisotropy in Ti(III) line could be connected with the oxidation processes. The temperature dependence of the integrated intensity of the broad line revealed the presence of titanium antiferromagnetic dimers. The disappearance of a narrow EPR line suggests that the oxidation process (ageing effect) could influence also the electrical properties of titanium carbide. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Transition metal carbides posses numerous remarkable properties including high hardness and high melting points, making them useful materials for extreme applications. For example, their use as antiwear coating on tool and precision mechanical components is widespread [1–3]. These unique properties give continuously technological perspective in future production of new functional materials in several important engineering applications [4,5]. Titanium carbides (TiCx ) belong to strongly nonstoichiometric compounds and disordered states exist within an extremely broad homogeneity region extending from TiC0.47 to TiC0.98 with carbon atoms-structural vacancies forming a substitutial solid solution in the non-metallic sublattice [2,3]. Annealing titanium carbides with different carbon content could give rise to various types of ordered phases which could increase their microhardness [6]. The temperature dependence of electrical resistivity has shown a hysteresis connected with the disorder state inside titanium carbide through appearance of some trivalent titanium Ti(III) complexes, not existing in titanium nitride [7]. The electron paramagnetic resonance (EPR) study of titanium carbide and nitride has shown the presence of a very intense and narrow line arising from the conduction electrons [8,9]. Additionally, in titanium carbide the EPR spectrum
∗ Corresponding author. E-mail address: [email protected] (J. Typek). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.029
of titanium(III) ions was recorded that was not observed in titanium nitride [10]. Surface physical and chemical properties of titanium carbides play a very important role in understanding their interaction with the environment. They will dictate the performance of lubricants and boundary additives in mechanical contacts involving these materials and will provide insight into the potential use of the carbides as catalysts. In this context the role played by atmospheric oxygen should not be underestimated. In this report the ageing process in nanocrystalline titanium carbide dispersed in a carbon matrix (TiCx /C) synthesized by the nonhydrolytic sol–gel technique is studied by using temperature variable EPR method. The presence of titanium(III) paramagnetic centers could influence the reorganization processes connected with oxygen. This study could allow a better understanding of the oxidation phenomena in titanium carbide and it could provide explanation of anomalous behavior of conductivity, hardness, and resistance to corrosion of this compound. 2. Experimental The details of nonhydrolitic sol–gel method are described elsewhere [11]. The nanocomposite powder of the TiCx in a carbon matrix was prepared using organotitanium gel precursor. The process of the precursor preparation was described elsewhere [11]. Average crystallite size of titanium carbide was equal to 20 nm. The content of free carbon in samples is 50 wt%. Fig. 1 presents the TEM picture of the titanium carbide nanoparticles released from carbon. Carbon cages of sub-micron sizes are seen, containing TiC nanoparticles, with an average size in the 40–50 nm range. The content of free carbon in the sample is 3–5 wt%.
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N. Guskos et al. / Journal of Alloys and Compounds 470 (2009) 51–54
Fig. 1. Transmission electron micrograph (TEM) of the TiCx /C system. Fig. 3. Comparison of the EPR spectra of TiCx /C of (a) fresh sample and (b) aged sample (1 year old) at three different temperatures. EPR measurement were performed on powder samples placed in quartz tubes using Bruker E 500 X-band spectrometer ( = 9.46 GHz) with 100 kHz field modulation and an Oxford liquid helium flow cryostat for temperature dependent measurements in the 3.5–300 K range. The measurements on fresh sample have been done during the first weak after its preparation. The sample had a free access to fresh air all the time and after 1 year it has been designated as aged sample. The simulation of the EPR experimental spectra was done with the computer program Simfonia.
3. Experimental results and discussion Fig. 2 shows a selection of registered EPR spectra of the nanocrystalline TiCx /C sample at different temperatures after the ageing process lasting 1 year. The amplitude of the observed EPR signal decreases with temperature increase and the spectra are seen only at temperature below 120 K. Highly anisotropic EPR spectrum in the 335–375 mT range of magnetic field is observed that could be attributed to the Ti(III) ions. A very weak and narrow (B = 0.17 mT) line centered at geff = 2.0026(3) is apparently arising from the conduction electrons [8]. Fig. 3 presents the EPR spectra for fresh and aged samples at almost the same three temperatures. The effect of a strong time-dependent reorganization process is observed. The g-factor of the narrow line remains very close to the free electron g-value (ge = 2.0023) but its EPR signal amplitude has drastically diminished. Another visible temporal change is the increase of magnetic
Fig. 2. The temperature dependence of the EPR spectra of TiCx /C for aged sample; upper panel: low temperatures (T < 21 K), lower panel: high temperatures (T > 28 K).
anisotropy of the main broad line from Ti(III) ions. For fresh sample the asymmetrical lineshape indicates on the presence of two unresolved components (axial symmetry) while for aged sample lowering of Ti(III) coordination symmetry to rhombic (three unresolved components) is visible. To calculate the effect of increased magnetic anisotropy with time the simulation of the observed EPR spectra was carried out. As an example, in Fig. 4 the results of such simulations are presented for fresh and aged sample at 35 K. The simulated spectra differ slightly from the experimental ones what may indicate on the presence of another component. This aspect will be discussed later after presentation of the integrated intensity thermal dependence. For fresh sample the following values of the EPR parameters (g-factors and anisotropic linewidths) were obtained: gx = 1.947(1), gy = 1.948(1), gz = 1.972(1), Bx = 7.0(5) mT, By = 6.0(5) mT, Bz = 2.2(3) mT. These values indicate, within experimental errors, on axial symmetry of the crystal field (gx = gy = g⊥ ) acting on the Ti(III) ions in fresh sample. For aged sample the following values were calculated: gx = 1.952(1), gy = 1.879(1), gz = 1.974(1), Bx = 3.0(5) mT, By = 4.5(5) mT, Bz = 2.1(3) mT. As the temperature decreases there is only unimportant decrease of all three anisotropic g values and a small increase of anistropic linewidths for aged sample. Trivalent titanium ion has one unpaired electron d1 and 2 D ground state. In a cubic crystalline field this 5-fold orbital degenerate state splits into orbital doublet 2 E and triplet 2 T states. An additional trigonal or tetragonal distortion lifts further the degeneracy of the doublet and triplet levels and results in anisotropic g values. In a tetrahedral coordination, if the tetragonal distortion is positive (compression), the d3z2 −r 2 orbital lies lower in energy and the g values vary in the order g⊥ < g// ≈ ge , where ge = 2.0023. For a tetragonal elongation that order is reversed and g// < g⊥ < ge [12]. In an octahedral coordination, the ground state retains some orbital momentum even in the zero-order. For tetragonal compression in this geometry the ground state is the dxy orbital the g// depends on the energy separation between triplet and doublet levels (g// = ge − (8/), where is the spin-orbit constant, equal 154 cm−1 for Ti(III)), while g⊥ depends on the energy separation between dxy and dxz /dyz orbitals (g⊥ = ge − (2/ı)). Low-symmetry rhombic distortion (D2h symmetry) results in three different g vales [12]. Comparison of the obtained results for fresh sample (g⊥ < g// ) with these theoretical predictions indicates that the Ti(III) ion is placed in octahedral crystal field with tetragonal distortion (compression). Such order of g values is expected for a not so big extent of tetragonal distortion (ı/). The obtained g values for aged sam-
N. Guskos et al. / Journal of Alloys and Compounds 470 (2009) 51–54
53
Fig. 4. Experimental (black) and simulated (red) EPR spectra of fresh (left panel) and aged (right panel) sample at 35 K. The parameters of the simulated spectra are given in the text (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).
ple, gy < gx < gz < ge , point out on larger energy separation of the dxy and dxz orbitals than dxy and dyz orbitals (ıx > ıy ). The electronic structure of transition metal carbides has been described in a variety of publications and extensively reviewed by Johansson [13]. The materials crystallize in the rocksalt structure and have been termed as interstitial compounds to explain the octahedral bonding environment of the C atoms. Strong metal–carbon covalent bonding exists, resulting in the increased hardness and melting points. There is also fairly close proximity of the neighbor metal atoms in the lattice, resulting in some metal–metal bonds. Finally, some charge transfer from metal to the carbon is also apparent from XPS core level studies, indicating the presence of ionic interactions. The understanding the interplay of all these forces is a big challenge. The titanium carbide system is usually in a non-stechiometric state and the disorder processes play a very important role what was evidenced by an extraordinary behavior of the thermal dependence of conductivity [7]. It is suggested that the above behavior could be explained by disorder processes connected with the existence the trivalent states of titanium ions. Temporal decrease of the EPR signal from conducting electrons and an increase of magnetic anisotropy of Ti(III) centers could be connected with the reorganization processes in disordered system that also changes the electric conductivity of the material. The oxidation studies of transition metal carbides in general, and in particular TiC, have been undertaken by many authors [14–18]. Because these materials have been traditionally used under extreme conditions where environmental oxygen is the main corrosive agent the stability against oxygen exposure is an important topic. Usually oxidation of the (1 0 0) surface, which is the most stable surface, has been studied. Oxygen atoms preferentially adsorb on the top layer carbon sites of TiC(1 0 0). For that layer the most stable final configuration corresponds to a situation in which two O atoms sit on two neighbor 3-fold coordinated sites. To explain our EPR results it will be assumed that oxygen reacts with the carbide phase according to the reaction of point defects TiC + y(1/2O2 ) = TiC1−y Oy + yC, where the oxygen goes to the site of carbon, producing an oxygen vacancy, and carbon is dis-
placed in interstitial position. This would lower the crystal field symmetry acting on Ti(III) ion that has been observed in our EPR study. Fig. 4 shows the temperature dependence of the EPR integrated intensity I(T) for aged sample. The integrate intensity was calculated by numerical integration of the EPR absorption spectrum (not the usual first derivative spectrum registered by the spectrometer). This quantity obtained from EPR measurements is proportional to the static (dc) spin susceptibility. The I(T) curve has not the expected shape of the Curie–Weiss law, ICW (T) = const/(T − CW ), in the whole investigated temperature range. To reveal the presence of different magnetic interactions in the investigated system the temperature dependence of the inverse of integrated intensity, I−1 , and the product of the integrated intensity and temperature, I(T)·T, is calculated and presented in Fig. 5. The later quantity is proportional
Fig. 5. The temperature dependences of the integrated intensity of the EPR spectrum of aged sample (squares). The lines are fittings with the Curie–Weiss law (dashed line), antiferromagnetic dimer (dotted line) and the sum of both contributions (solid line).
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N. Guskos et al. / Journal of Alloys and Compounds 470 (2009) 51–54
for hamiltonian H = −2JSa Sb , where J is the antiferromagnetic intra-dimer coupling. The fit of the observed I(T) by the sum of paramagnetic (Curie–Weiss) and dimer (Bleaney–Bowers) contributions allowed to calculate the value of inter-dimer coupling J/k = −65(10) K. The fit is not entirely satisfactory, especially in the high-temperature range, what indicates on the possibility of existence of yet another subsystem of interacting ions (clusters). 4. Conclusion Magnetic effects of aging of TiC/C sample have been registered by the EPR method. A narrow line attributed to conduction electrons has vanished and a broad line due to Ti(III) centers has increased its anisotropy by reduction of an axial symmetry to rhombic symmetry. As titanium carbide is well covered by carbon the internal oxidation processes could be very complicated. Probably the distortion phenomena, especially acting during a long time could transform drastically the physical properties of this material. This transformation is very important from the application point of view because various properties of nanocrystalline titanium carbide such as hardness and corrosion resistance could be modified during this process. Acknowledgments
Fig. 6. Temperature dependence of the inverse integrated intensity (upper panel) and the product of integrated intensity and temperature (lower panel) for aged sample. The straight line in the upper panel is the Curie–Weiss law with the Curie–Weiss constant CW = 2.4 K.
to the square of the effective magnetic moment. As could be seen in Fig. 5, the Curie–Weiss law holds in the low temperature range, for T < 15 K, and the value of the Curie–Weiss constant in that range is CW = 2.4 K indicating on ferromagnetic character of magnetic interactions. In general, as the magnetic interactions are concerned there are three temperatures ranges (see Fig. 5b). Above 45 K the magnetic moment increases with temperature decrease and this is a signature of prevailing ferromagetic interactions. Between 45 and 15 K the trend of magnetic moment variations reverses and antiferromagnetic interactions dominate. Below 15 K once again the ferromagnetic interactions take over (Fig. 6, lower panel). The obtained temperature dependence of integrated intensity I(T) indicates on the presence of additional, apart from paramagnetic Ti(III) ions, contribution from another center that displays maximum of susceptibility at ∼45 K and diminish at low temperatures (Fig. 4). An obvious candidate for such a defect center will be the appearance of titanium dimers coupled by antiferromagnetic exchange interaction. This leads to a singlet ground state and a thermally populated triplet excited state with the temperature dependence of integrated intensity (susceptibility) modelled by the Bleaney–Bowers expression [19] I(T ) =
C 1 1 + exp T 3
2J −1 kT
This paper and the work it concerns were partially generated in the context of the MULTIPROTECT project, funded by the European Community as contract No. NMP3-CT-2005-011783 under the 6th Framework Programme for Research and Technological Development. References [1] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. [2] A.A. Rembel, Usp. Fiz. Nauk (Russia) 166 (1996) 33. [3] A.I. Gusev, S.Z. Nazarov, Usp. Fiz. Nauk (Russia) 175 (2005) 682. [4] F.G. Wei, K. Tsuzaki, Metall. Mater. Tran. A- Phys. Metall. Mater. Sci. 37A (2006) 331. [5] W.H. Kao, Y.L. Su, S.H. Yao, Vacuum 80 (2006) 604. [6] V.N. Lipatnikov, A.A. Rempel, A.I. Gusev, Int. J. Recfractory Met. Hard Mater. 15 (1997) 61. [7] N. Guskos, A. Biedunkiewicz, J. Typek, S. Patapis, M. Maryniak, K.A. Karkas, Rev. Adv. Mater. Sci. 8 (2004) 49. [8] N. Guskos, T. Bodziony, A. Biedunkiewicz, A. Adinis, Acta Phys. Pol. 108 (2005) 311. [9] T. Bodziony, N. Guskos, A. Biedunkiewicz, J. Typek, R. Wrobel, M. Maryniak, Mater. Sci. Poland 23 (2005) 899. [10] N. Guskos, T. Bodziony, M. Maryniak, J. Typek, A. Biedunkiewicz, J. Alloy Compd. 455 (2008) 52. [11] A. Biedunkiewicz, Mater. Sci. Poland 21 (2003) 445. [12] R. Bal, K. Chaudhari, D. Srinivas, S. Sivasanker, P. Ratnasamy, J. Mol. Catal. A: Chem. 162 (2000) 199. [13] L.I. Johansson, Surf. Sci. Rep. 21 (1995) 177. [14] W.W. Webb, J.T. Norton, C. Wagner, J. Electrochem. Soc. 103 (1956) 112. [15] E.K. Storms, The Refractory Carbides, Academic Press, New York, 1967. [16] S. Shimada, F. Yunazar, S. Otani, J. Am. Ceram. Soc. 83 (2000) 721. [17] A. Bellucci, D. Gozzi, M. Nardone, A. Sodo, Chem. Mater. 15 (2003) 1217. [18] Y.F. Zhang, F. Vines, Y.J. Xu, Y. Li, J.Q. Li, F. Illas, J. Phys. Chem. B 110 (2006) 15454. [19] B. Bleaney, K.D. Bowers, Proc. R. Soc. Lond. A 214 (1952) 451.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Ageing effect in nanocrystalline TiCx /C studied by EPR N. Guskos a,b , J. Typek b,∗ , T. Bodziony b , G. Zolnierkiewicz b , M. Maryniak b , A. Biedunkiewicz c a
Solid State Physics, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece Institute of Physics, Szczecin University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland c Institute of Material Engineering, Szczecin University of Technology, Al. Piastow 17, 70-310 Szczecin, Poland b
a r t i c l e
i n f o
Article history: Received 18 January 2008 Received in revised form 1 March 2008 Accepted 4 March 2008 Available online 21 April 2008 Keywords: EPR of Ti3+ Titanium carbide
a b s t r a c t TiC/C nanocrystalline material: titanium carbide TiC dispersed in a carbon matrix has been prepared by a nonhydrolytic sol–gel process. Temperature dependence of the electron paramagnetic resonance (EPR) spectra of this material has been studied in the 3.5–120 K range. Two very different EPR lines have been recorded in fresh sample at temperatures below 120 K arising from the Ti(III) complex (broad and asymmetric line) and conduction electrons (very narrow line). In the same aged sample (1 year old) the magnetic anisotropy of Ti(III) line has increased while a narrow line attributed to conduction electrons has vanished. The existence of the paramagnetic centers connected with trivalent titanium ions could the result of disordering processes. The increase of anisotropy in Ti(III) line could be connected with the oxidation processes. The temperature dependence of the integrated intensity of the broad line revealed the presence of titanium antiferromagnetic dimers. The disappearance of a narrow EPR line suggests that the oxidation process (ageing effect) could influence also the electrical properties of titanium carbide. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Transition metal carbides posses numerous remarkable properties including high hardness and high melting points, making them useful materials for extreme applications. For example, their use as antiwear coating on tool and precision mechanical components is widespread [1–3]. These unique properties give continuously technological perspective in future production of new functional materials in several important engineering applications [4,5]. Titanium carbides (TiCx ) belong to strongly nonstoichiometric compounds and disordered states exist within an extremely broad homogeneity region extending from TiC0.47 to TiC0.98 with carbon atoms-structural vacancies forming a substitutial solid solution in the non-metallic sublattice [2,3]. Annealing titanium carbides with different carbon content could give rise to various types of ordered phases which could increase their microhardness [6]. The temperature dependence of electrical resistivity has shown a hysteresis connected with the disorder state inside titanium carbide through appearance of some trivalent titanium Ti(III) complexes, not existing in titanium nitride [7]. The electron paramagnetic resonance (EPR) study of titanium carbide and nitride has shown the presence of a very intense and narrow line arising from the conduction electrons [8,9]. Additionally, in titanium carbide the EPR spectrum
∗ Corresponding author. E-mail address: [email protected] (J. Typek). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.029
of titanium(III) ions was recorded that was not observed in titanium nitride [10]. Surface physical and chemical properties of titanium carbides play a very important role in understanding their interaction with the environment. They will dictate the performance of lubricants and boundary additives in mechanical contacts involving these materials and will provide insight into the potential use of the carbides as catalysts. In this context the role played by atmospheric oxygen should not be underestimated. In this report the ageing process in nanocrystalline titanium carbide dispersed in a carbon matrix (TiCx /C) synthesized by the nonhydrolytic sol–gel technique is studied by using temperature variable EPR method. The presence of titanium(III) paramagnetic centers could influence the reorganization processes connected with oxygen. This study could allow a better understanding of the oxidation phenomena in titanium carbide and it could provide explanation of anomalous behavior of conductivity, hardness, and resistance to corrosion of this compound. 2. Experimental The details of nonhydrolitic sol–gel method are described elsewhere [11]. The nanocomposite powder of the TiCx in a carbon matrix was prepared using organotitanium gel precursor. The process of the precursor preparation was described elsewhere [11]. Average crystallite size of titanium carbide was equal to 20 nm. The content of free carbon in samples is 50 wt%. Fig. 1 presents the TEM picture of the titanium carbide nanoparticles released from carbon. Carbon cages of sub-micron sizes are seen, containing TiC nanoparticles, with an average size in the 40–50 nm range. The content of free carbon in the sample is 3–5 wt%.
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N. Guskos et al. / Journal of Alloys and Compounds 470 (2009) 51–54
Fig. 1. Transmission electron micrograph (TEM) of the TiCx /C system. Fig. 3. Comparison of the EPR spectra of TiCx /C of (a) fresh sample and (b) aged sample (1 year old) at three different temperatures. EPR measurement were performed on powder samples placed in quartz tubes using Bruker E 500 X-band spectrometer ( = 9.46 GHz) with 100 kHz field modulation and an Oxford liquid helium flow cryostat for temperature dependent measurements in the 3.5–300 K range. The measurements on fresh sample have been done during the first weak after its preparation. The sample had a free access to fresh air all the time and after 1 year it has been designated as aged sample. The simulation of the EPR experimental spectra was done with the computer program Simfonia.
3. Experimental results and discussion Fig. 2 shows a selection of registered EPR spectra of the nanocrystalline TiCx /C sample at different temperatures after the ageing process lasting 1 year. The amplitude of the observed EPR signal decreases with temperature increase and the spectra are seen only at temperature below 120 K. Highly anisotropic EPR spectrum in the 335–375 mT range of magnetic field is observed that could be attributed to the Ti(III) ions. A very weak and narrow (B = 0.17 mT) line centered at geff = 2.0026(3) is apparently arising from the conduction electrons [8]. Fig. 3 presents the EPR spectra for fresh and aged samples at almost the same three temperatures. The effect of a strong time-dependent reorganization process is observed. The g-factor of the narrow line remains very close to the free electron g-value (ge = 2.0023) but its EPR signal amplitude has drastically diminished. Another visible temporal change is the increase of magnetic
Fig. 2. The temperature dependence of the EPR spectra of TiCx /C for aged sample; upper panel: low temperatures (T < 21 K), lower panel: high temperatures (T > 28 K).
anisotropy of the main broad line from Ti(III) ions. For fresh sample the asymmetrical lineshape indicates on the presence of two unresolved components (axial symmetry) while for aged sample lowering of Ti(III) coordination symmetry to rhombic (three unresolved components) is visible. To calculate the effect of increased magnetic anisotropy with time the simulation of the observed EPR spectra was carried out. As an example, in Fig. 4 the results of such simulations are presented for fresh and aged sample at 35 K. The simulated spectra differ slightly from the experimental ones what may indicate on the presence of another component. This aspect will be discussed later after presentation of the integrated intensity thermal dependence. For fresh sample the following values of the EPR parameters (g-factors and anisotropic linewidths) were obtained: gx = 1.947(1), gy = 1.948(1), gz = 1.972(1), Bx = 7.0(5) mT, By = 6.0(5) mT, Bz = 2.2(3) mT. These values indicate, within experimental errors, on axial symmetry of the crystal field (gx = gy = g⊥ ) acting on the Ti(III) ions in fresh sample. For aged sample the following values were calculated: gx = 1.952(1), gy = 1.879(1), gz = 1.974(1), Bx = 3.0(5) mT, By = 4.5(5) mT, Bz = 2.1(3) mT. As the temperature decreases there is only unimportant decrease of all three anisotropic g values and a small increase of anistropic linewidths for aged sample. Trivalent titanium ion has one unpaired electron d1 and 2 D ground state. In a cubic crystalline field this 5-fold orbital degenerate state splits into orbital doublet 2 E and triplet 2 T states. An additional trigonal or tetragonal distortion lifts further the degeneracy of the doublet and triplet levels and results in anisotropic g values. In a tetrahedral coordination, if the tetragonal distortion is positive (compression), the d3z2 −r 2 orbital lies lower in energy and the g values vary in the order g⊥ < g// ≈ ge , where ge = 2.0023. For a tetragonal elongation that order is reversed and g// < g⊥ < ge [12]. In an octahedral coordination, the ground state retains some orbital momentum even in the zero-order. For tetragonal compression in this geometry the ground state is the dxy orbital the g// depends on the energy separation between triplet and doublet levels (g// = ge − (8/), where is the spin-orbit constant, equal 154 cm−1 for Ti(III)), while g⊥ depends on the energy separation between dxy and dxz /dyz orbitals (g⊥ = ge − (2/ı)). Low-symmetry rhombic distortion (D2h symmetry) results in three different g vales [12]. Comparison of the obtained results for fresh sample (g⊥ < g// ) with these theoretical predictions indicates that the Ti(III) ion is placed in octahedral crystal field with tetragonal distortion (compression). Such order of g values is expected for a not so big extent of tetragonal distortion (ı/). The obtained g values for aged sam-
N. Guskos et al. / Journal of Alloys and Compounds 470 (2009) 51–54
53
Fig. 4. Experimental (black) and simulated (red) EPR spectra of fresh (left panel) and aged (right panel) sample at 35 K. The parameters of the simulated spectra are given in the text (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).
ple, gy < gx < gz < ge , point out on larger energy separation of the dxy and dxz orbitals than dxy and dyz orbitals (ıx > ıy ). The electronic structure of transition metal carbides has been described in a variety of publications and extensively reviewed by Johansson [13]. The materials crystallize in the rocksalt structure and have been termed as interstitial compounds to explain the octahedral bonding environment of the C atoms. Strong metal–carbon covalent bonding exists, resulting in the increased hardness and melting points. There is also fairly close proximity of the neighbor metal atoms in the lattice, resulting in some metal–metal bonds. Finally, some charge transfer from metal to the carbon is also apparent from XPS core level studies, indicating the presence of ionic interactions. The understanding the interplay of all these forces is a big challenge. The titanium carbide system is usually in a non-stechiometric state and the disorder processes play a very important role what was evidenced by an extraordinary behavior of the thermal dependence of conductivity [7]. It is suggested that the above behavior could be explained by disorder processes connected with the existence the trivalent states of titanium ions. Temporal decrease of the EPR signal from conducting electrons and an increase of magnetic anisotropy of Ti(III) centers could be connected with the reorganization processes in disordered system that also changes the electric conductivity of the material. The oxidation studies of transition metal carbides in general, and in particular TiC, have been undertaken by many authors [14–18]. Because these materials have been traditionally used under extreme conditions where environmental oxygen is the main corrosive agent the stability against oxygen exposure is an important topic. Usually oxidation of the (1 0 0) surface, which is the most stable surface, has been studied. Oxygen atoms preferentially adsorb on the top layer carbon sites of TiC(1 0 0). For that layer the most stable final configuration corresponds to a situation in which two O atoms sit on two neighbor 3-fold coordinated sites. To explain our EPR results it will be assumed that oxygen reacts with the carbide phase according to the reaction of point defects TiC + y(1/2O2 ) = TiC1−y Oy + yC, where the oxygen goes to the site of carbon, producing an oxygen vacancy, and carbon is dis-
placed in interstitial position. This would lower the crystal field symmetry acting on Ti(III) ion that has been observed in our EPR study. Fig. 4 shows the temperature dependence of the EPR integrated intensity I(T) for aged sample. The integrate intensity was calculated by numerical integration of the EPR absorption spectrum (not the usual first derivative spectrum registered by the spectrometer). This quantity obtained from EPR measurements is proportional to the static (dc) spin susceptibility. The I(T) curve has not the expected shape of the Curie–Weiss law, ICW (T) = const/(T − CW ), in the whole investigated temperature range. To reveal the presence of different magnetic interactions in the investigated system the temperature dependence of the inverse of integrated intensity, I−1 , and the product of the integrated intensity and temperature, I(T)·T, is calculated and presented in Fig. 5. The later quantity is proportional
Fig. 5. The temperature dependences of the integrated intensity of the EPR spectrum of aged sample (squares). The lines are fittings with the Curie–Weiss law (dashed line), antiferromagnetic dimer (dotted line) and the sum of both contributions (solid line).
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N. Guskos et al. / Journal of Alloys and Compounds 470 (2009) 51–54
for hamiltonian H = −2JSa Sb , where J is the antiferromagnetic intra-dimer coupling. The fit of the observed I(T) by the sum of paramagnetic (Curie–Weiss) and dimer (Bleaney–Bowers) contributions allowed to calculate the value of inter-dimer coupling J/k = −65(10) K. The fit is not entirely satisfactory, especially in the high-temperature range, what indicates on the possibility of existence of yet another subsystem of interacting ions (clusters). 4. Conclusion Magnetic effects of aging of TiC/C sample have been registered by the EPR method. A narrow line attributed to conduction electrons has vanished and a broad line due to Ti(III) centers has increased its anisotropy by reduction of an axial symmetry to rhombic symmetry. As titanium carbide is well covered by carbon the internal oxidation processes could be very complicated. Probably the distortion phenomena, especially acting during a long time could transform drastically the physical properties of this material. This transformation is very important from the application point of view because various properties of nanocrystalline titanium carbide such as hardness and corrosion resistance could be modified during this process. Acknowledgments
Fig. 6. Temperature dependence of the inverse integrated intensity (upper panel) and the product of integrated intensity and temperature (lower panel) for aged sample. The straight line in the upper panel is the Curie–Weiss law with the Curie–Weiss constant CW = 2.4 K.
to the square of the effective magnetic moment. As could be seen in Fig. 5, the Curie–Weiss law holds in the low temperature range, for T < 15 K, and the value of the Curie–Weiss constant in that range is CW = 2.4 K indicating on ferromagnetic character of magnetic interactions. In general, as the magnetic interactions are concerned there are three temperatures ranges (see Fig. 5b). Above 45 K the magnetic moment increases with temperature decrease and this is a signature of prevailing ferromagetic interactions. Between 45 and 15 K the trend of magnetic moment variations reverses and antiferromagnetic interactions dominate. Below 15 K once again the ferromagnetic interactions take over (Fig. 6, lower panel). The obtained temperature dependence of integrated intensity I(T) indicates on the presence of additional, apart from paramagnetic Ti(III) ions, contribution from another center that displays maximum of susceptibility at ∼45 K and diminish at low temperatures (Fig. 4). An obvious candidate for such a defect center will be the appearance of titanium dimers coupled by antiferromagnetic exchange interaction. This leads to a singlet ground state and a thermally populated triplet excited state with the temperature dependence of integrated intensity (susceptibility) modelled by the Bleaney–Bowers expression [19] I(T ) =
C 1 1 + exp T 3
2J −1 kT
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