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Preparation and characterization of the nanocrystalline Ti0.5Cr0.5OxNy powder
January 2003
Materials Letters 57 (2003) 1062 – 1065 www.elsevier.com/locate/matlet
Preparation and characterization of the nanocrystalline Ti0.5Cr0.5OxNy powder Yaogang Li, Lian Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, People’s Republic of China Received 9 April 2002; accepted 12 April 2002
Abstract Nanocrystalline bimetallic metal oxynitride, Ti0.5Cr0.5OxNy, was synthesized by ammonolysis of the nanosized Cr2O3/TiO2 composite powder at 800 jC for 8 h. The precursor and the resulting oxynitride were characterized by Auger electron spectroscope (AES), X-ray diffraction analysis (XRD), electron probe microanalysis (EP), transmission electron microscopy (TEM), and BET surface area techniques. The result indicated that the as-synthesized oxynitride powder contains only Ti0.5Cr0.5OxNy with cubic structure and the average particle size is about 30 nm. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Chromium oxide; Titanium oxide; Ammonolysis; Nanocrystalline; Oxynitride
1. Introduction Since their preparation requires certain precautions to be taken, the studies concerning nitrides and oxynitrides have not been as systematic as those regarding other families, such as oxides [1]. During the last two decades, there has been a growing interest in binary and ternary nitrides and oxynitrides of p-, d- and fblock elements [2]. There are several synthetic approaches which have been used to obtain bimetallic nitrides and oxynitrides [3]. Examples include the direct reaction of a metal nitride with a metal or
*
Corresponding author. Tel.: +86-21-6251-2990; fax: +86-216251-3903. E-mail address: [email protected] (L. Gao).
another metal nitride, the reaction of a mechanical mixture of two metal powders with nitrogen or ammonia, the reaction between a metal amide with a metal nitride at low pressures (under flowing nitrogen or ammonia) or at high pressures (autoclave), and the ammonolysis of mixed metallic oxides [4]. The use of oxide precursors has been well documented as an experimentally simple and inexpensive method for the synthesis of a wide variety of nitride and oxynitride products [5 – 8]. Ammonolysis reactions of ternary oxides containing rare earth [9] or vanadium [10] yield distinct oxynitride products. The tribological characteristics of TiN and CrN have been known for a long time. Both materials are very useful for hard, wear-resistant applications. There are numerous investigations made to prepare (Ti1 xCrx)N films [11 – 14]. To our knowledge, few
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 9 3 1 - X
Y. Li, L. Gao / Materials Letters 57 (2003) 1062–1065
studies on preparation of nitrides or oxynitride powders containing chromium and titanium have been reported. In this paper, nanocrystalline bimetallic oxynitride, Ti0.5Cr0.5O xNy, has been synthesized by ammonolysis of a homogenous nanosized Cr2O3/ TiO2 composite powder.
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observed using a transmission electron microscopy (TEM, JEOL JEM-200CX, Japan). The Brunauer – Emmett – Teller (BET) surface area was determined using a Micromeritics ASAP 2010 nitrogen adsorption apparatus.
3. Results and discussion 2. Experimental 2.1. Oxynitride synthesis Chromium titanium oxynitride powder was prepared by the ammonolysis of a homogenous Cr2O3/ TiO2 composite powder prepared by co-precipitation method. The chemicals used were of analytical grade: chromium nitrate and titanium tetrabutyl orthotitanate (TBOT). Alcohol solutions were prepared in each case by dissolving the salt or TBOT in absolute alcohol. The two solutions were mixed under vigorous stirring, yielding a clear, green solution. The mixed solution containing Cr3 + and Ti4 + in a molar ratio of 1:1 was slowly dropped into a rapidly stirred ammonia solution while keeping a constant pH = 9 – 10 by adding extra ammonia solution. The precipitate was washed with distilled water and ethanol, and filtered, ovendried at 100 jC for 24 h, and heated at 450 jC for 2 h to obtain the precursor powder. The Cr2O3/TiO2 composite powder was put into a tube furnace and subjected to the nitridation in the flow of ammonia gas. The flow rate of NH3 gas was 0.5 l/min. The nitridation temperature was 800 jC, while the nitridation time was 8 h. The sample was taken from the furnace until it was cooled down to room temperature in a flow of NH3 gas.
Fig. 1 shows the AES spectra of the precursor powder. The peaks for Cr, Ti, and O can be clearly seen in the spectra. The spectra taken at six different areas exhibit nearly identical intensity, indicating the composition homogeneity of the precursor and thus, its suitability for preparing homogenous oxynitride. Fig. 2 shows the XRD patterns of the Cr2O3/TiO2 precursors calcined at 450 jC (a) and 800 jC (b) for 2 h, respectively. For the precursor calcined at 450 jC for 2 h, rhombohedral Cr2O3 crystalline diffraction peaks can be detected, while the diffraction peaks attributed to TiO2 crystalline is not distinct. The diffraction peaks corresponding to rhombohedral Cr2O3 and rutile TiO2 crystalline phases can be observed for the powder calcined at 800 jC for 2 h. This result indicates that the precursor was the mixture of Cr2O3 and TiO2, and no solid solution was formed by them. Table 1 shows the results of EP analysis of the precursor and synthesized oxynitride. The Ti/Cr ratios of the precursor and oxynitride are 1.07 and 1.01, respectively. The contents of Ti, Cr, O, and N of the
2.2. Sample characterization The homogeneity of the precursor was detected by Auger electron spectroscope (AES, Microlab-310F model). The powder structure was determined by Xray diffraction (XRD, D/max-2550 V) using Cu Ka radiation. To obtain more accurate peak positions, a low-speed step scanning XRD method with a speed of 0.05j/min was also used. Chemical composition measurements were performed by electron probe microanalysis (EP, EPMA-8705QH2, Shimadazu). The morphology and size of the resulting oxynitride were
Fig. 1. AES spectra of the precursor calcined at 450 jC for 2 h ((a) – (f) corresponding to six different areas).
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Y. Li, L. Gao / Materials Letters 57 (2003) 1062–1065
Fig. 2. XRD patterns of the precursor calcined at (a) 450 jC (b) 800 jC for 2 h.
oxynitride lead to a composition of Ti0.487Cr0.483O0.197 N0.833, therefore, we conclude that the compound is nearly stoichiometric Ti0.5Cr0.5O0.2N0.83. Besides the incorporation of oxygen atoms in the lattice, surfaceabsorbed oxygen affects the detected oxygen content. This is to say that the suggested composition of Ti0.5 Cr0.5O0.2N0.83 is somewhat approximate. It is more reasonable to name the oxynitride as Ti0.5Cr0.5OxNy . Fig. 3 shows the XRD patterns of the Ti0.5Cr0.5Ox Ny powder. (111) (200) and (220) diffraction peaks ˚ were corresponding to cubic phase with a = 4.1763 A detected. According to the phase diagram, there is a broad phase zone of the type (Ti,Cr)N + Cr2N, in the vicinity of the (Ti,Cr)N phase zone. This region is characterized by phases having understoichiometric nitrogen content [11]. Vetter et al. [12] found the Cr2N phase when the Cr content (x) in the (Ti,Cr)N coating was >0.1. However in our study, hexagonal Cr2N diffraction peaks could not be detected in the Ti0.5 Cr0.5OxNy powder. This is consistent with the result reported by Lee et al. [11] for (Ti1 xCrx)N coatings prepared by ion-plating method. No Cr2O3 and TiO2
Fig. 3. XRD pattern of the oxynitride Ti0.5Cr0.5OxNy .
diffraction peaks were detected, which indicated the oxygen atoms located in the lattice of oxynitride. Fig. 4 shows the results from low-speed step scanning XRD of the TiN, CrN and Ti0.5Cr0.5OxNy powders obtained in this study from 2h = 41– 45j. Only one (200) diffraction peak corresponding to the Ti0.5Cr0.5OxNy phase could be detected for the as-synthesized powder. The peak shifts toward higher diffraction angle for Ti0.5Cr0.5OxNy powder (2h = 43.281j) compared to TiN (2h = 42.880j) and toward lower diffraction angle compared to CrN (2h = 43.620j), which is the result of the formation of solid solution. Hones et al. [13] also observed the B1 NaCl phase exclusively in (Ti,Cr)N coatings produced by reactive magnetron sputtering. However, Nainaparampil et al. [14] regarded the (Ti,Cr)N deposited by cathodic arc evaporation as a mixture
Table 1 Chemical composition of the precursor and the as-prepared oxynitride Oxygen Nitrogen Chromium Titanium Proposed stoichiometry (atom %) Precursor 63.76 Oxynitride 9.85
41.64
17.53 24.15
18.73 24.36
Ti0.5Cr0.5 O0.2N0.83
Fig. 4. Step scanning XRD patterns of TiN, CrN, and Ti0.5Cr0.5 OxNy.
Y. Li, L. Gao / Materials Letters 57 (2003) 1062–1065
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solution phase with cubic structure, and no impurities such as CrN, Cr2N and TiN phases were detected by XRD analysis. The result of EP analysis indicated that the oxynitride was nearly stoichiometric Ti0.5Cr0.5O0.2 N0.83. The as-prepared oxynitride powder had a spherical morphology and an average particle size of about 30 nm.
Acknowledgements
Fig. 5. TEM micrographs of the as-synthesized oxynitride with selected-area diffraction pattern.
of TiN and CrN. In our study, only Ti0.5Cr0.5OxNy solid solution phase was obtained. The use of homogenous mixed oxide precursors offers the advantage of molecular-level mixing of the metals, which decreases the diffusion distances of the cations and may lower the temperature and shorten the time necessary for ammonolysis reaction. TEM micrograph of the Ti0.5Cr0.5OxNy powder is given in Fig. 5. It can be seen that the particle size of the oxynitride is about 30 nm, the shape is spherical, and little agglomerate exits. The particle size calculated from specific surface area of Ti0.5Cr0.5OxNy powder is 28.8 nm, which is close to that observed under TEM. The electron diffraction rings that were observed are typical for cubic-phase. This is consistent with the result of XRD analysis.
4. Conclusion The nanosized Cr2O3/TiO2 mixed powder was prepared by co-precipitation method. The detection of AES indicated that the powder was fairly homogenous, which is beneficial to forming bimetallic oxynitride. Nanocrystalline bimetallic oxynitride, Ti0.5 Cr0.5OxNy, was synthesized by the ammonolysis of the Cr2O3/TiO2 precursor at 800 jC for 8 h. The assynthesized powder contains only Ti0.5Cr0.5OxNy solid
This work was funded by the National Key Fundamental Research Project (Grant No. G1999064506). The authors are grateful to Ms. Ling Yu, Ms. Meiling Ruan, and Mr. Wei Shi for the measurements of Auger electron spectra and transmission electron microscopy.
References [1] R. Marchand, Y. Laurent, J. Guyader, P. L’Haridon, P. Verdier, J. Eur. Ceram. Soc. 8 (1991) 197. [2] J. Grins, P.-O. Ka¨ll, G. Svensson, J. Mater. Chem. 5 (4) (1995) 571. [3] S. Alconchel, F. Sapin˜a, D. Beltra´n, A. Beltra´n, J. Mater. Chem. 9 (1999) 749. [4] H.-C. Zur Loye, J.D. Houmes, D.S. Bem, The Chemistry of Transition Metal Carbids and Nitrides, in: S.T. Oyama (Ed.), Blackie Academic and Professional, Champan and Hall, London, 1996. [5] D.S. Bem, H.P. Olsen, H.-C. Zur Loye, Chem. Mater 7 (1995) 1824. [6] S.H. Elder, L.H. Doerrer, F.J. Disalvo, J.B. Parise, D. Guyomard, J.M. Tarascon, Chem. Mater 4 (1992) 928. [7] M. Lerch, J. Lerch, R. Hock, J. Wrba, J. Solid State Chem. 128 (1997) 282. [8] S.H. Elder, F.J. Disalvo, J.B. Parise, J.A. Hrilijac, J.W. Richardson, J. Solid State Chem. 108 (1994) 73. [9] R. Marchand, P. Antoine, Y. Laurent, J. Solid State Chem. 107 (1993) 34. [10] C.C. Yu, S.T. Oyama, J. Solid State Chem. 116 (1995) 205. [11] K.H. Lee, C.H. Park, Y.S. Yoon, J.J. Lee, Thin Solid Films 385 (2001) 167. [12] J. Vetter, H.J. Scholl, O. Knotek, Surf. Coat. Technol. 74/75 (1995) 286. [13] P. Hones, R. Sanjine´s, F. Le´vy, Thin Solid Films 332 (1998) 240. [14] J.J. Nainaparampil, J.S. Zabinski, A. Korenyi-Both, Thin Solid Films 333 (1998) 88.
Materials Letters 57 (2003) 1062 – 1065 www.elsevier.com/locate/matlet
Preparation and characterization of the nanocrystalline Ti0.5Cr0.5OxNy powder Yaogang Li, Lian Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, People’s Republic of China Received 9 April 2002; accepted 12 April 2002
Abstract Nanocrystalline bimetallic metal oxynitride, Ti0.5Cr0.5OxNy, was synthesized by ammonolysis of the nanosized Cr2O3/TiO2 composite powder at 800 jC for 8 h. The precursor and the resulting oxynitride were characterized by Auger electron spectroscope (AES), X-ray diffraction analysis (XRD), electron probe microanalysis (EP), transmission electron microscopy (TEM), and BET surface area techniques. The result indicated that the as-synthesized oxynitride powder contains only Ti0.5Cr0.5OxNy with cubic structure and the average particle size is about 30 nm. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Chromium oxide; Titanium oxide; Ammonolysis; Nanocrystalline; Oxynitride
1. Introduction Since their preparation requires certain precautions to be taken, the studies concerning nitrides and oxynitrides have not been as systematic as those regarding other families, such as oxides [1]. During the last two decades, there has been a growing interest in binary and ternary nitrides and oxynitrides of p-, d- and fblock elements [2]. There are several synthetic approaches which have been used to obtain bimetallic nitrides and oxynitrides [3]. Examples include the direct reaction of a metal nitride with a metal or
*
Corresponding author. Tel.: +86-21-6251-2990; fax: +86-216251-3903. E-mail address: [email protected] (L. Gao).
another metal nitride, the reaction of a mechanical mixture of two metal powders with nitrogen or ammonia, the reaction between a metal amide with a metal nitride at low pressures (under flowing nitrogen or ammonia) or at high pressures (autoclave), and the ammonolysis of mixed metallic oxides [4]. The use of oxide precursors has been well documented as an experimentally simple and inexpensive method for the synthesis of a wide variety of nitride and oxynitride products [5 – 8]. Ammonolysis reactions of ternary oxides containing rare earth [9] or vanadium [10] yield distinct oxynitride products. The tribological characteristics of TiN and CrN have been known for a long time. Both materials are very useful for hard, wear-resistant applications. There are numerous investigations made to prepare (Ti1 xCrx)N films [11 – 14]. To our knowledge, few
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 9 3 1 - X
Y. Li, L. Gao / Materials Letters 57 (2003) 1062–1065
studies on preparation of nitrides or oxynitride powders containing chromium and titanium have been reported. In this paper, nanocrystalline bimetallic oxynitride, Ti0.5Cr0.5O xNy, has been synthesized by ammonolysis of a homogenous nanosized Cr2O3/ TiO2 composite powder.
1063
observed using a transmission electron microscopy (TEM, JEOL JEM-200CX, Japan). The Brunauer – Emmett – Teller (BET) surface area was determined using a Micromeritics ASAP 2010 nitrogen adsorption apparatus.
3. Results and discussion 2. Experimental 2.1. Oxynitride synthesis Chromium titanium oxynitride powder was prepared by the ammonolysis of a homogenous Cr2O3/ TiO2 composite powder prepared by co-precipitation method. The chemicals used were of analytical grade: chromium nitrate and titanium tetrabutyl orthotitanate (TBOT). Alcohol solutions were prepared in each case by dissolving the salt or TBOT in absolute alcohol. The two solutions were mixed under vigorous stirring, yielding a clear, green solution. The mixed solution containing Cr3 + and Ti4 + in a molar ratio of 1:1 was slowly dropped into a rapidly stirred ammonia solution while keeping a constant pH = 9 – 10 by adding extra ammonia solution. The precipitate was washed with distilled water and ethanol, and filtered, ovendried at 100 jC for 24 h, and heated at 450 jC for 2 h to obtain the precursor powder. The Cr2O3/TiO2 composite powder was put into a tube furnace and subjected to the nitridation in the flow of ammonia gas. The flow rate of NH3 gas was 0.5 l/min. The nitridation temperature was 800 jC, while the nitridation time was 8 h. The sample was taken from the furnace until it was cooled down to room temperature in a flow of NH3 gas.
Fig. 1 shows the AES spectra of the precursor powder. The peaks for Cr, Ti, and O can be clearly seen in the spectra. The spectra taken at six different areas exhibit nearly identical intensity, indicating the composition homogeneity of the precursor and thus, its suitability for preparing homogenous oxynitride. Fig. 2 shows the XRD patterns of the Cr2O3/TiO2 precursors calcined at 450 jC (a) and 800 jC (b) for 2 h, respectively. For the precursor calcined at 450 jC for 2 h, rhombohedral Cr2O3 crystalline diffraction peaks can be detected, while the diffraction peaks attributed to TiO2 crystalline is not distinct. The diffraction peaks corresponding to rhombohedral Cr2O3 and rutile TiO2 crystalline phases can be observed for the powder calcined at 800 jC for 2 h. This result indicates that the precursor was the mixture of Cr2O3 and TiO2, and no solid solution was formed by them. Table 1 shows the results of EP analysis of the precursor and synthesized oxynitride. The Ti/Cr ratios of the precursor and oxynitride are 1.07 and 1.01, respectively. The contents of Ti, Cr, O, and N of the
2.2. Sample characterization The homogeneity of the precursor was detected by Auger electron spectroscope (AES, Microlab-310F model). The powder structure was determined by Xray diffraction (XRD, D/max-2550 V) using Cu Ka radiation. To obtain more accurate peak positions, a low-speed step scanning XRD method with a speed of 0.05j/min was also used. Chemical composition measurements were performed by electron probe microanalysis (EP, EPMA-8705QH2, Shimadazu). The morphology and size of the resulting oxynitride were
Fig. 1. AES spectra of the precursor calcined at 450 jC for 2 h ((a) – (f) corresponding to six different areas).
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Y. Li, L. Gao / Materials Letters 57 (2003) 1062–1065
Fig. 2. XRD patterns of the precursor calcined at (a) 450 jC (b) 800 jC for 2 h.
oxynitride lead to a composition of Ti0.487Cr0.483O0.197 N0.833, therefore, we conclude that the compound is nearly stoichiometric Ti0.5Cr0.5O0.2N0.83. Besides the incorporation of oxygen atoms in the lattice, surfaceabsorbed oxygen affects the detected oxygen content. This is to say that the suggested composition of Ti0.5 Cr0.5O0.2N0.83 is somewhat approximate. It is more reasonable to name the oxynitride as Ti0.5Cr0.5OxNy . Fig. 3 shows the XRD patterns of the Ti0.5Cr0.5Ox Ny powder. (111) (200) and (220) diffraction peaks ˚ were corresponding to cubic phase with a = 4.1763 A detected. According to the phase diagram, there is a broad phase zone of the type (Ti,Cr)N + Cr2N, in the vicinity of the (Ti,Cr)N phase zone. This region is characterized by phases having understoichiometric nitrogen content [11]. Vetter et al. [12] found the Cr2N phase when the Cr content (x) in the (Ti,Cr)N coating was >0.1. However in our study, hexagonal Cr2N diffraction peaks could not be detected in the Ti0.5 Cr0.5OxNy powder. This is consistent with the result reported by Lee et al. [11] for (Ti1 xCrx)N coatings prepared by ion-plating method. No Cr2O3 and TiO2
Fig. 3. XRD pattern of the oxynitride Ti0.5Cr0.5OxNy .
diffraction peaks were detected, which indicated the oxygen atoms located in the lattice of oxynitride. Fig. 4 shows the results from low-speed step scanning XRD of the TiN, CrN and Ti0.5Cr0.5OxNy powders obtained in this study from 2h = 41– 45j. Only one (200) diffraction peak corresponding to the Ti0.5Cr0.5OxNy phase could be detected for the as-synthesized powder. The peak shifts toward higher diffraction angle for Ti0.5Cr0.5OxNy powder (2h = 43.281j) compared to TiN (2h = 42.880j) and toward lower diffraction angle compared to CrN (2h = 43.620j), which is the result of the formation of solid solution. Hones et al. [13] also observed the B1 NaCl phase exclusively in (Ti,Cr)N coatings produced by reactive magnetron sputtering. However, Nainaparampil et al. [14] regarded the (Ti,Cr)N deposited by cathodic arc evaporation as a mixture
Table 1 Chemical composition of the precursor and the as-prepared oxynitride Oxygen Nitrogen Chromium Titanium Proposed stoichiometry (atom %) Precursor 63.76 Oxynitride 9.85
41.64
17.53 24.15
18.73 24.36
Ti0.5Cr0.5 O0.2N0.83
Fig. 4. Step scanning XRD patterns of TiN, CrN, and Ti0.5Cr0.5 OxNy.
Y. Li, L. Gao / Materials Letters 57 (2003) 1062–1065
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solution phase with cubic structure, and no impurities such as CrN, Cr2N and TiN phases were detected by XRD analysis. The result of EP analysis indicated that the oxynitride was nearly stoichiometric Ti0.5Cr0.5O0.2 N0.83. The as-prepared oxynitride powder had a spherical morphology and an average particle size of about 30 nm.
Acknowledgements
Fig. 5. TEM micrographs of the as-synthesized oxynitride with selected-area diffraction pattern.
of TiN and CrN. In our study, only Ti0.5Cr0.5OxNy solid solution phase was obtained. The use of homogenous mixed oxide precursors offers the advantage of molecular-level mixing of the metals, which decreases the diffusion distances of the cations and may lower the temperature and shorten the time necessary for ammonolysis reaction. TEM micrograph of the Ti0.5Cr0.5OxNy powder is given in Fig. 5. It can be seen that the particle size of the oxynitride is about 30 nm, the shape is spherical, and little agglomerate exits. The particle size calculated from specific surface area of Ti0.5Cr0.5OxNy powder is 28.8 nm, which is close to that observed under TEM. The electron diffraction rings that were observed are typical for cubic-phase. This is consistent with the result of XRD analysis.
4. Conclusion The nanosized Cr2O3/TiO2 mixed powder was prepared by co-precipitation method. The detection of AES indicated that the powder was fairly homogenous, which is beneficial to forming bimetallic oxynitride. Nanocrystalline bimetallic oxynitride, Ti0.5 Cr0.5OxNy, was synthesized by the ammonolysis of the Cr2O3/TiO2 precursor at 800 jC for 8 h. The assynthesized powder contains only Ti0.5Cr0.5OxNy solid
This work was funded by the National Key Fundamental Research Project (Grant No. G1999064506). The authors are grateful to Ms. Ling Yu, Ms. Meiling Ruan, and Mr. Wei Shi for the measurements of Auger electron spectra and transmission electron microscopy.
References [1] R. Marchand, Y. Laurent, J. Guyader, P. L’Haridon, P. Verdier, J. Eur. Ceram. Soc. 8 (1991) 197. [2] J. Grins, P.-O. Ka¨ll, G. Svensson, J. Mater. Chem. 5 (4) (1995) 571. [3] S. Alconchel, F. Sapin˜a, D. Beltra´n, A. Beltra´n, J. Mater. Chem. 9 (1999) 749. [4] H.-C. Zur Loye, J.D. Houmes, D.S. Bem, The Chemistry of Transition Metal Carbids and Nitrides, in: S.T. Oyama (Ed.), Blackie Academic and Professional, Champan and Hall, London, 1996. [5] D.S. Bem, H.P. Olsen, H.-C. Zur Loye, Chem. Mater 7 (1995) 1824. [6] S.H. Elder, L.H. Doerrer, F.J. Disalvo, J.B. Parise, D. Guyomard, J.M. Tarascon, Chem. Mater 4 (1992) 928. [7] M. Lerch, J. Lerch, R. Hock, J. Wrba, J. Solid State Chem. 128 (1997) 282. [8] S.H. Elder, F.J. Disalvo, J.B. Parise, J.A. Hrilijac, J.W. Richardson, J. Solid State Chem. 108 (1994) 73. [9] R. Marchand, P. Antoine, Y. Laurent, J. Solid State Chem. 107 (1993) 34. [10] C.C. Yu, S.T. Oyama, J. Solid State Chem. 116 (1995) 205. [11] K.H. Lee, C.H. Park, Y.S. Yoon, J.J. Lee, Thin Solid Films 385 (2001) 167. [12] J. Vetter, H.J. Scholl, O. Knotek, Surf. Coat. Technol. 74/75 (1995) 286. [13] P. Hones, R. Sanjine´s, F. Le´vy, Thin Solid Films 332 (1998) 240. [14] J.J. Nainaparampil, J.S. Zabinski, A. Korenyi-Both, Thin Solid Films 333 (1998) 88.