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Some observations on the Gd-rich side of the Gd-C system
Journa| of
~L@YS A~qDC@~P©~DS ELSEVIER
Journal of Alloys and Compounds 269 ~1997) 107-I l0
Some observations on the Gd-fich side of the Gd-C system K.A. G s c h n e i d n e r Jr.*, A. P e c h a r s k y , K.W. D e n n i s Ames Laboratory and Department of Materials Science and Engineering, Iowa State Universi~, Ames, IA 50011-3020 USA
Received 21 February 1997; received in revised form 3 March 1997
Abstract
The phase relationships at the Gd-rich end of the Gd-C system were established. The addition of C expands the lattice parameters of pure Gd, indicating that C dissolves in Gd, but the solid solubility of C is less than 2 at.% below 600 °C. Below 600 °C the 'Gd2C' phase appears to exist as a line compound, but exhibits a narrow solid solution region above this temperature. This phase is sfightly C deficient, i.e., Gd_,C~_~, where x=0.015 [Gd2C,,~,.~]. 'Gd,C' has the rhombohedral C19, CdCl,-type structure with a=6.29 A, and a=33.70° The Gd-C alloys are quite reactive in moist air, forming gaseous hydrocarbons and a white powder which flakes off of the metallic ingots, and must be handled in glove boxes. Magnetic measurements indicated that 'Gd, C' orders ferromagnetically at 350 °C. This is the highest known magnetic ordering temperature fo: any binary lanthanide compound which doe~ not contain Fe or Co. © 1997 Elsevier Science S.A. Keywords: Gadolinium-carbon system; Crystal structure of Gd.,C; Hydrolysisof Gd-C alloys; Hardness of Gd-C alloys; Magnetic properties of Gd-C alloys; Curie temperature of Gd_~C
L Introduction
In the last few years there has been an interest in lanthanide based materials which have magnetic ordering temperatures at or above room temperature. In the late 1950s Spedding et al. [l} reported that Gd~C was ferromagnetic above room temperature, the first lanthanide material ever reported to order magnetically above 295K. Seven years later Lallement [2] reported that the Gd3C is ferromagnetic below 500K. Unfortunately, no other information or details concerning the magnetic properties of this phase were reported by either research group. Some crystallographic data were presented by Spedding et al. [1] for three intermediate phases in the Gd-C system: Gd3C, Gd2C 3 and GdC z. All of the known information on the phase relationships, crystal structures, transformation temperatures, and thermodynamic data on the Gd-C alloys have been reviewed by Gschneidner and Calderwood [3,4]. At the time we began a study of this system there was some uncertainty concerning the composition and the extent of the solid solution region (if it exists) around the Gd3C composition. Furthermore, two different crystal structures have been reported for the phase(s) near 25
*Correspondingauthor. Tel.: + 1 515 294 7931; fax: + l 515 294 9579; e-mail: [email protected] 0925-8388/97/$17.00 © 1997 Elsevier Science S.A. All fights reserved. P ! l S0925-8388(97)00146-1
at.%Cmthe fcc Fe4N-type (L I) and the rhombohedral CdCI z-type (C 19) [4]. Unfortunately, no information exists concerning the melting behaviours and high temperature phase relationships, especially in ~be 20 to 40 vt.%C region. As a first approximation, we assumed that the Gd-C system might be similar to the Y-C system: that is, the Y2.~C phase exhibits a solid solution region from 29 to 34 at.% at 700 °C, and from 25 to 44 at.% at 1400 °C, and a congruent melting point of 2000 °C at 30 at.% C [5]. On the basis of this information, we began a study of the magnetic properties, phase relationships and hydrolytic/ oxidation characteristics of the Gd-rich end of the Gd-C system. The results of this investigation are presented below.
2. Experimental procedures Fifteen alloys were prepared (2, 2.5, 3, 3.5, 4, 4.5, 5, 23, 25, 30, 33, 33.3, 35, 40 and 45 at.%C) by arc-melting weighed amounts of 99.8 at.% pure Gd from the Ames Laboratory's Materials Preparation Center and 99.98 at.% pure spectrographic graphite. The alloys weighed about 5g each and were melted six times, turning over the sample after each melt. The alloys were stored in z dry box because the alloys readily react with normally moist dr. The samples were examined in the as-cast condition and also after heat treatment at 400, 600, 800, 1000 and
108
K.A. Gschneidner Jr. et al. I Journal of Alloys and Compounds 260 (1997) 107-110
11t30 °C for one week at each temperature followed by a quench to room temperature. The X-ray powder diffraction data were recorded using a Scintag automated diffractometer and monochromatic Cu Kot radiation. All crystallographic calculations were carried out using the CSD program system [6] and a 486/87-based personal computer. Optical metallography was used to supplement the X-ray diffraction measurements. After mechanical polishing with AI20 3 using ethyl alcohol as a lubricant, the samples were etched with a 2% nitric acid-methyl alcohol solution for 2 to 5 s. Since the Gd-C alloys undergo hydrolysis in moist air, we measured the weight change at various time intervals using an analytical balance to try to obtain some quantitative data concerning this reaction as a function of the carbon content. Three different methods were used to make the magnetic measurements. From 4.2 to 325K a Lake Shore ac sasceptometer/dc magnetometer was used to measure the dc magnetization, M, as a function of magnetic field, H, (up to 5.0 T) and temperature. The high temperature measurements (295-725K) were made using either a Quantum Design SQUID magnetometer in fields up to 5 T, or a vibrating sample magnetometer (VSM) in fields up to 2 T. Arrott plots ( M 3 v s . H ) [7] were used to determine the ferromagnetic Curie temperature from the SQUID measurements. The paramagnetic Curie temperature was calculated from the VSM data using the Curie-Weiss law (1/~ vs. T). 3. Phase relafionsMps X-ray and metallographic examinations indicate that the solid solubility of C in Gd is less than 2 at.% from room temperature to 1000 °C. The increase in the lattice parameters tbr the 5 at.% C alloys (a=3.650-±8 and c=5.87___ IA) relative to those of pure Gd (a=3.619_2 and c = 5.774~3A) indicates that some C is soluble in Gd. Since the 2 at.%C as-cast arc-melted alloy is single phase, this suggests that the solubility of C increases to a value greater than 2 at.%C above 1000 0(2. Thermal analysis of the 5 at.%C alloy using a PerkinElmer DTA (Differential Thermal Analysis) apparatus showed a weak thermal arrest at 1295 °C, a strong peak at 1320 °C and a small anomaly at 1383 °C. We have assumed the 1295°C arrest corresponds to the et-Gd--->[3-Gd+ 'Gd2C' peritectoid horizontal, the 1320°C peak corresponds to the [3-Gd-->L+'Gd2C' peritectic horizontal, and the 1383 °C anomaly to the liquidus on the basis of the similarity of the Gd-C system to the Y-C system [5]. Fhis interpretation of the DTA data is consistent with the 1235 °C ~-{B transformation and the 1313 °C melting point of pure Gd. Examination of the annet, led alloys between 23 and 45 at.%C indicates that below 1000°C the Gd-rich carbide
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Composition (~, o L) A - sin~e-phase sample (Gd) D - singie-ph~_m sampl," ("Gd2C")
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phase exists over a solid solution region from at least 30.0 to 33.0 at.%, indicating that this phase is C deficient Gd2C~_ ~ with x ranging from 0.015 to ->0.143 (see Fig, 1). Since the composition is so close to the Gd~C stoichiometry, we will refer to this phase as Gd:C for the remainder of this paper. Gd2C has the rhombohedral C19, CdCl:-type structure, a=6.29+2/~ and a = 3 3 . 7 0 _ 4 ° (the equivalent hexagoonal lattice parameters are a=3.648_+9 and c = 17.79_7A). Within experimental error, no variation was observed in the lattice parameters of the Gd2C phase for the two phase alloys on either side of this composition, confirming the metallographic data. Furthermore, no evidence was found for the defect NaCI fee structure reported earlier [1,2]. Alloys containing less than 30.0 at.% carbon below 1000°C consisted of Gd with some dissolved C solid solution plus Gd2C, while ',hose containing more than 33.0 at.%C consisted of Gd2C+Gd.,C 3. At 1100°C the solid solution region expands on the C-rich side of the compound to at least 33.3 at.%, indicating that at high temperature the Gd2C solid solution range expands just as in the Y-C system. Metallographic and X-ray examination of the as-cast arc-melted alloys showed that all compositions consisted of two phases [(Gd~_~C~+Gd2C ) or (Gd2C+Gd2C3) ] except the 2, 33.0 and 33.3 at.% alloys which were single phase materials. This suggests that at temperatures > I I00 0(2 Gd dissolves between 2 at.%C and 2.5 at.%C, and the Gd2C solid solution changes from substoichiometric with respeet to the C content to superstoichiome~c.
K.A. Gschneidner :~r. et a L I Journal of Alloys and Compounds 260 (1997} 107-110
109
4. Hydrolysis behavmur The rare-earth carbides are known to r~ ~ct quite readily with moist air to form hydrocarbons: the R3C-R~C phase yields CH~+ H a and are called metha;~ides, while the R~C 3 and RCa phases yield C2H~+ H~+ high hydrocarbons (including C z H 4 and C2H6) l~d are called acetylides [1]. The other reaction product is a white powder which readily flakes off and is probably a mixture of all or some of the following comp::,unds: RO(OH), ~(OH)3, RzO3, RzOs'xH20. Becau~,e t~e Gd-C alloys react so readily with the moist air, evea w~zhin one day the white powder falls off of the metallic ingest. So in order to determine the weight change (a loss), ~h,e white powder ls brushed off and then the sample is placeci in acetone and ultrasonically cleaned before it is weighed. Thus, we actually measured the amount of sample n:maJning after a given time of exposure to moist air in ~he laboratory,. In general, the as-cast alloys are m-~;e reactive than those annealed at 800 and 1000 °C for on," week, and those annealed at 1000 °C were the least reacti~ e. As might be expected, the 5 at.%C all% was by far the most stable alloy, exhibiting hardly any weight change over 225 days. The weight loss over a 2Y-day period vs. time for the alloys containing 25 to 45 at.'~C is shown in Fig. 2. The Gd~C (33.3 at.%) alloy was t?r-. most stable of these alloys. For the I000 °C annealed all~ys, the 25 at.% alloy was the least reactive ('after the Gd2C phase), followed by the 40 and 45 at.% alloys, whi;ie the 30 and 35 at.% alloys were the most reactive (Fig. 3). This observa-
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tion is quite interesting because the two two-phase alloys having the largest fraction of Gd2C are generally the most reactive, while the single-phase Gd2C alloy, relatively speaking, is quite stable. It appears that the presence of a second phase catalyses the reaction of H20 with Gd2C.
5. Magnetic properties 3,5
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The magnetic properties are summarized in Table 1 for the Gd-t alloys. Above the Curie temperature the susceptibility follows Curie-Weiss behaviour. The effective magnetic moments for the 30 to 45 at.%C a'loys are generally significantly lower than the theoretical value for a Gd 3÷ ion (7.941~a), except for the 40 at.% alloy. The ferromagnetic Curie temperature for Gd2C is found to be 350K and it should be the same for all alloys between 25 and 45 at.%C, which it is for the 25 at.% alloy, since these alloys are two-phase alloys (Gd+Gd2C below 33.3 at.%C and Gd2C+Gd2C 3 above 33.3 at.%C. The paramagnedc Curie temperature (0p) varies around the 350K ferromagnetic Curie temperature by a few percent (the maximum deviation is - 7 % for the 45 at.%C alloy and +5% for the 33.3 at.% alloy). This difference between the ferromagnetic and paramagnetic Curie temperatures is not unusual. The 350K Curie temperature for Gd2C is the highest ferromagnetic ordering temperature known for a binary Gd compound which does not contain Fe or Co. This value was also recently confirmed by Shiet al. [8]. Our results suggest that a previously reported upper limit for the ferromagnetic ordering temperature (500K)
i 10
K.A. Gschneidner Jr. et al. / Journal of Alloys and Compounds 260 (1097) 107- ! 10
Table i Magnetic properties of some Gd-C alloys Composition (at.%)
Curie temp? (K)
X = CM1(1"-0o)
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CM (emuK moleGd-')
P~,, (P-B)
0p (K)
Temp. range (K)
8.26
8.13
318
6. ! 8 6.20 5.71 7.86 5.33
7.03 7.05 6.76 7.93 6.53
344 368 340 357 326
300-380 -300-600 300-400 300-450 300-450 300-650
294 350 350
~From SQUID measurements using Arrott plots.
for GdsC [21 is much too high. But since no experimental details were presented, it is difficult to determine how this upper limit was established.
6. Hardness The Vickers hardness values ,br pure Gd and most of the Gd-C alloys were measured on the as-cast and the heat treated at 800 °C alloys, see Fig. 4. It is seen that small C additions increase the hardness of pure Gd metal from -62 to 76. The hardness of .the two-phase alloys Gd+GdEC increases with increasing C content, and then rises rapidiy as the concentration approaches the Gd2C composition. On the GdEC+Gd2C 3 side of GdEC, the hardness drops off rapidly and seems to reach a minimum of 40 to 45 at.%C; presumably it rises again as the composition approaches the Gd,C~ stoichiometry. These results also indicate that Gd,C is about thr re times harder than pure Gd metal.
7. Conclusions Our studies indicate that below 1100 °C the first Gd-rich Gd-C compound is a phase which exhibits a narrow solid 220 200
Acknow|edgemen~ The authors wish to thank Dr. V.K. Pecharsky, H.E. Saiisbury and J.O. Moorman for their helpful discussions and/or assistance in carrying out some of the experiments. This work was carried out under a Laboratory Directed Research and Development research grant. The Ames Laboratory is supported by the U. S. Department of Energy, Office of R ~ i , . q,-'.. . . . . . . . a~.. r, ...... " W-7405ENG-82.
References @ cast
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solution region from - 3 0 to 33.0 at.%C, i.e., substoichiometric Gd:C with respect to C. This behaviour is significantly different from the Y-C system which exhibits an extensive solid solution region from -28 to - 3 4 at.%C at 700 °C and even wider at higher temperatures. The ferromagnetic Curie temperature was established to be 350K, the h,~ :"~'""" ..... magnetic ordering temperature of a lanthanide binary compound which does not contain Fe or Co. Unfortunately the Gd-C alloys containing more than 5 at.%C are very reactive with moist air and must be carefully handled while carrying out experimental studies. However, the stoichiometric GdaC phase is significantly less reactive than the other alloys containing more than 20 at.%C.
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\
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20
30
40
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as-cast and heat treated Gd-C alloys. The microhardness values were determined using a 500 mgf load.
Ill F.H. Spedding, K. Gschneidner Jr., A.H. Daane, J. Am. Chem. 5oc. 80 (1958) 4499. [2] R. Lallement, Rare Earth Research-Ill. in: L. Eyring (Ed.), Gordon and Breach, New York, 1965, p. 55. [3] K.A. Gschneidner Jr., F.W. Calderwood, Bull. Alloy Phase Diagrams 7 (t986) 421. [4] K.A. Gschneidner Jr., F.W. Calderwood, Bull. Alloy Phase Diagrams 7 (1986b 443. [5] K.A. Gschneidner Jr., F.W. Calder N, Bull. Alloy Phase Diagrams 7 (1996) 564. [6] L.G. Akselmd, Yu N. Grin, P. Yu. Zavalij, V.K. Pecharsky, V.S. Fundamensky, Xllth European Crystailogr. Meet., Coll. Abstr., Moscow, 3 (1989) 155. [7] A. Arrott, Phys. Rev. 108 (1957) 1394. [8] J. $hi, H. Izumi, K Machida, G. Adachk J. Alloys Comp. 240 (1996) 156.
~L@YS A~qDC@~P©~DS ELSEVIER
Journal of Alloys and Compounds 269 ~1997) 107-I l0
Some observations on the Gd-fich side of the Gd-C system K.A. G s c h n e i d n e r Jr.*, A. P e c h a r s k y , K.W. D e n n i s Ames Laboratory and Department of Materials Science and Engineering, Iowa State Universi~, Ames, IA 50011-3020 USA
Received 21 February 1997; received in revised form 3 March 1997
Abstract
The phase relationships at the Gd-rich end of the Gd-C system were established. The addition of C expands the lattice parameters of pure Gd, indicating that C dissolves in Gd, but the solid solubility of C is less than 2 at.% below 600 °C. Below 600 °C the 'Gd2C' phase appears to exist as a line compound, but exhibits a narrow solid solution region above this temperature. This phase is sfightly C deficient, i.e., Gd_,C~_~, where x=0.015 [Gd2C,,~,.~]. 'Gd,C' has the rhombohedral C19, CdCl,-type structure with a=6.29 A, and a=33.70° The Gd-C alloys are quite reactive in moist air, forming gaseous hydrocarbons and a white powder which flakes off of the metallic ingots, and must be handled in glove boxes. Magnetic measurements indicated that 'Gd, C' orders ferromagnetically at 350 °C. This is the highest known magnetic ordering temperature fo: any binary lanthanide compound which doe~ not contain Fe or Co. © 1997 Elsevier Science S.A. Keywords: Gadolinium-carbon system; Crystal structure of Gd.,C; Hydrolysisof Gd-C alloys; Hardness of Gd-C alloys; Magnetic properties of Gd-C alloys; Curie temperature of Gd_~C
L Introduction
In the last few years there has been an interest in lanthanide based materials which have magnetic ordering temperatures at or above room temperature. In the late 1950s Spedding et al. [l} reported that Gd~C was ferromagnetic above room temperature, the first lanthanide material ever reported to order magnetically above 295K. Seven years later Lallement [2] reported that the Gd3C is ferromagnetic below 500K. Unfortunately, no other information or details concerning the magnetic properties of this phase were reported by either research group. Some crystallographic data were presented by Spedding et al. [1] for three intermediate phases in the Gd-C system: Gd3C, Gd2C 3 and GdC z. All of the known information on the phase relationships, crystal structures, transformation temperatures, and thermodynamic data on the Gd-C alloys have been reviewed by Gschneidner and Calderwood [3,4]. At the time we began a study of this system there was some uncertainty concerning the composition and the extent of the solid solution region (if it exists) around the Gd3C composition. Furthermore, two different crystal structures have been reported for the phase(s) near 25
*Correspondingauthor. Tel.: + 1 515 294 7931; fax: + l 515 294 9579; e-mail: [email protected] 0925-8388/97/$17.00 © 1997 Elsevier Science S.A. All fights reserved. P ! l S0925-8388(97)00146-1
at.%Cmthe fcc Fe4N-type (L I) and the rhombohedral CdCI z-type (C 19) [4]. Unfortunately, no information exists concerning the melting behaviours and high temperature phase relationships, especially in ~be 20 to 40 vt.%C region. As a first approximation, we assumed that the Gd-C system might be similar to the Y-C system: that is, the Y2.~C phase exhibits a solid solution region from 29 to 34 at.% at 700 °C, and from 25 to 44 at.% at 1400 °C, and a congruent melting point of 2000 °C at 30 at.% C [5]. On the basis of this information, we began a study of the magnetic properties, phase relationships and hydrolytic/ oxidation characteristics of the Gd-rich end of the Gd-C system. The results of this investigation are presented below.
2. Experimental procedures Fifteen alloys were prepared (2, 2.5, 3, 3.5, 4, 4.5, 5, 23, 25, 30, 33, 33.3, 35, 40 and 45 at.%C) by arc-melting weighed amounts of 99.8 at.% pure Gd from the Ames Laboratory's Materials Preparation Center and 99.98 at.% pure spectrographic graphite. The alloys weighed about 5g each and were melted six times, turning over the sample after each melt. The alloys were stored in z dry box because the alloys readily react with normally moist dr. The samples were examined in the as-cast condition and also after heat treatment at 400, 600, 800, 1000 and
108
K.A. Gschneidner Jr. et al. I Journal of Alloys and Compounds 260 (1997) 107-110
11t30 °C for one week at each temperature followed by a quench to room temperature. The X-ray powder diffraction data were recorded using a Scintag automated diffractometer and monochromatic Cu Kot radiation. All crystallographic calculations were carried out using the CSD program system [6] and a 486/87-based personal computer. Optical metallography was used to supplement the X-ray diffraction measurements. After mechanical polishing with AI20 3 using ethyl alcohol as a lubricant, the samples were etched with a 2% nitric acid-methyl alcohol solution for 2 to 5 s. Since the Gd-C alloys undergo hydrolysis in moist air, we measured the weight change at various time intervals using an analytical balance to try to obtain some quantitative data concerning this reaction as a function of the carbon content. Three different methods were used to make the magnetic measurements. From 4.2 to 325K a Lake Shore ac sasceptometer/dc magnetometer was used to measure the dc magnetization, M, as a function of magnetic field, H, (up to 5.0 T) and temperature. The high temperature measurements (295-725K) were made using either a Quantum Design SQUID magnetometer in fields up to 5 T, or a vibrating sample magnetometer (VSM) in fields up to 2 T. Arrott plots ( M 3 v s . H ) [7] were used to determine the ferromagnetic Curie temperature from the SQUID measurements. The paramagnetic Curie temperature was calculated from the VSM data using the Curie-Weiss law (1/~ vs. T). 3. Phase relafionsMps X-ray and metallographic examinations indicate that the solid solubility of C in Gd is less than 2 at.% from room temperature to 1000 °C. The increase in the lattice parameters tbr the 5 at.% C alloys (a=3.650-±8 and c=5.87___ IA) relative to those of pure Gd (a=3.619_2 and c = 5.774~3A) indicates that some C is soluble in Gd. Since the 2 at.%C as-cast arc-melted alloy is single phase, this suggests that the solubility of C increases to a value greater than 2 at.%C above 1000 0(2. Thermal analysis of the 5 at.%C alloy using a PerkinElmer DTA (Differential Thermal Analysis) apparatus showed a weak thermal arrest at 1295 °C, a strong peak at 1320 °C and a small anomaly at 1383 °C. We have assumed the 1295°C arrest corresponds to the et-Gd--->[3-Gd+ 'Gd2C' peritectoid horizontal, the 1320°C peak corresponds to the [3-Gd-->L+'Gd2C' peritectic horizontal, and the 1383 °C anomaly to the liquidus on the basis of the similarity of the Gd-C system to the Y-C system [5]. Fhis interpretation of the DTA data is consistent with the 1235 °C ~-{B transformation and the 1313 °C melting point of pure Gd. Examination of the annet, led alloys between 23 and 45 at.%C indicates that below 1000°C the Gd-rich carbide
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phase exists over a solid solution region from at least 30.0 to 33.0 at.%, indicating that this phase is C deficient Gd2C~_ ~ with x ranging from 0.015 to ->0.143 (see Fig, 1). Since the composition is so close to the Gd~C stoichiometry, we will refer to this phase as Gd:C for the remainder of this paper. Gd2C has the rhombohedral C19, CdCl:-type structure, a=6.29+2/~ and a = 3 3 . 7 0 _ 4 ° (the equivalent hexagoonal lattice parameters are a=3.648_+9 and c = 17.79_7A). Within experimental error, no variation was observed in the lattice parameters of the Gd2C phase for the two phase alloys on either side of this composition, confirming the metallographic data. Furthermore, no evidence was found for the defect NaCI fee structure reported earlier [1,2]. Alloys containing less than 30.0 at.% carbon below 1000°C consisted of Gd with some dissolved C solid solution plus Gd2C, while ',hose containing more than 33.0 at.%C consisted of Gd2C+Gd.,C 3. At 1100°C the solid solution region expands on the C-rich side of the compound to at least 33.3 at.%, indicating that at high temperature the Gd2C solid solution range expands just as in the Y-C system. Metallographic and X-ray examination of the as-cast arc-melted alloys showed that all compositions consisted of two phases [(Gd~_~C~+Gd2C ) or (Gd2C+Gd2C3) ] except the 2, 33.0 and 33.3 at.% alloys which were single phase materials. This suggests that at temperatures > I I00 0(2 Gd dissolves between 2 at.%C and 2.5 at.%C, and the Gd2C solid solution changes from substoichiometric with respeet to the C content to superstoichiome~c.
K.A. Gschneidner :~r. et a L I Journal of Alloys and Compounds 260 (1997} 107-110
109
4. Hydrolysis behavmur The rare-earth carbides are known to r~ ~ct quite readily with moist air to form hydrocarbons: the R3C-R~C phase yields CH~+ H a and are called metha;~ides, while the R~C 3 and RCa phases yield C2H~+ H~+ high hydrocarbons (including C z H 4 and C2H6) l~d are called acetylides [1]. The other reaction product is a white powder which readily flakes off and is probably a mixture of all or some of the following comp::,unds: RO(OH), ~(OH)3, RzO3, RzOs'xH20. Becau~,e t~e Gd-C alloys react so readily with the moist air, evea w~zhin one day the white powder falls off of the metallic ingest. So in order to determine the weight change (a loss), ~h,e white powder ls brushed off and then the sample is placeci in acetone and ultrasonically cleaned before it is weighed. Thus, we actually measured the amount of sample n:maJning after a given time of exposure to moist air in ~he laboratory,. In general, the as-cast alloys are m-~;e reactive than those annealed at 800 and 1000 °C for on," week, and those annealed at 1000 °C were the least reacti~ e. As might be expected, the 5 at.%C all% was by far the most stable alloy, exhibiting hardly any weight change over 225 days. The weight loss over a 2Y-day period vs. time for the alloys containing 25 to 45 at.'~C is shown in Fig. 2. The Gd~C (33.3 at.%) alloy was t?r-. most stable of these alloys. For the I000 °C annealed all~ys, the 25 at.% alloy was the least reactive ('after the Gd2C phase), followed by the 40 and 45 at.% alloys, whi;ie the 30 and 35 at.% alloys were the most reactive (Fig. 3). This observa-
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45
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At%C Fig. 3. The weight loss (see text) after 25 days ~xposure to ai~ for a series o f annealed (at 1000 °C for one week) G d - C alloys, except for the 33.3 at.%C alloy which was in the as-cast condition.
tion is quite interesting because the two two-phase alloys having the largest fraction of Gd2C are generally the most reactive, while the single-phase Gd2C alloy, relatively speaking, is quite stable. It appears that the presence of a second phase catalyses the reaction of H20 with Gd2C.
5. Magnetic properties 3,5
....
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30
25
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GdTsCa5 (1000°C1 GdroC3o(100O*C) Gd.C3s (10600C) Gd6oC4o{1000°C) GdssC4s[I0000C) Gd~eTC33~ (c&st)
A , A
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5
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10
15
20
VVV
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25
30
Days Fig. 2. The weigh! !oss (see ~ext) vs. ~me for a nurnl~r of Gd-C alloys w h i c h had been heat treated for 1OO0°C for 1 wee~, e~cept for the 33.3 at.%C alloy which was in the as-cast condition.
The magnetic properties are summarized in Table 1 for the Gd-t alloys. Above the Curie temperature the susceptibility follows Curie-Weiss behaviour. The effective magnetic moments for the 30 to 45 at.%C a'loys are generally significantly lower than the theoretical value for a Gd 3÷ ion (7.941~a), except for the 40 at.% alloy. The ferromagnetic Curie temperature for Gd2C is found to be 350K and it should be the same for all alloys between 25 and 45 at.%C, which it is for the 25 at.% alloy, since these alloys are two-phase alloys (Gd+Gd2C below 33.3 at.%C and Gd2C+Gd2C 3 above 33.3 at.%C. The paramagnedc Curie temperature (0p) varies around the 350K ferromagnetic Curie temperature by a few percent (the maximum deviation is - 7 % for the 45 at.%C alloy and +5% for the 33.3 at.% alloy). This difference between the ferromagnetic and paramagnetic Curie temperatures is not unusual. The 350K Curie temperature for Gd2C is the highest ferromagnetic ordering temperature known for a binary Gd compound which does not contain Fe or Co. This value was also recently confirmed by Shiet al. [8]. Our results suggest that a previously reported upper limit for the ferromagnetic ordering temperature (500K)
i 10
K.A. Gschneidner Jr. et al. / Journal of Alloys and Compounds 260 (1097) 107- ! 10
Table i Magnetic properties of some Gd-C alloys Composition (at.%)
Curie temp? (K)
X = CM1(1"-0o)
5 25 30 33.3 35 40 45
CM (emuK moleGd-')
P~,, (P-B)
0p (K)
Temp. range (K)
8.26
8.13
318
6. ! 8 6.20 5.71 7.86 5.33
7.03 7.05 6.76 7.93 6.53
344 368 340 357 326
300-380 -300-600 300-400 300-450 300-450 300-650
294 350 350
~From SQUID measurements using Arrott plots.
for GdsC [21 is much too high. But since no experimental details were presented, it is difficult to determine how this upper limit was established.
6. Hardness The Vickers hardness values ,br pure Gd and most of the Gd-C alloys were measured on the as-cast and the heat treated at 800 °C alloys, see Fig. 4. It is seen that small C additions increase the hardness of pure Gd metal from -62 to 76. The hardness of .the two-phase alloys Gd+GdEC increases with increasing C content, and then rises rapidiy as the concentration approaches the Gd2C composition. On the GdEC+Gd2C 3 side of GdEC, the hardness drops off rapidly and seems to reach a minimum of 40 to 45 at.%C; presumably it rises again as the composition approaches the Gd,C~ stoichiometry. These results also indicate that Gd,C is about thr re times harder than pure Gd metal.
7. Conclusions Our studies indicate that below 1100 °C the first Gd-rich Gd-C compound is a phase which exhibits a narrow solid 220 200
Acknow|edgemen~ The authors wish to thank Dr. V.K. Pecharsky, H.E. Saiisbury and J.O. Moorman for their helpful discussions and/or assistance in carrying out some of the experiments. This work was carried out under a Laboratory Directed Research and Development research grant. The Ames Laboratory is supported by the U. S. Department of Energy, Office of R ~ i , . q,-'.. . . . . . . . a~.. r, ...... " W-7405ENG-82.
References @ cast
~
solution region from - 3 0 to 33.0 at.%C, i.e., substoichiometric Gd:C with respect to C. This behaviour is significantly different from the Y-C system which exhibits an extensive solid solution region from -28 to - 3 4 at.%C at 700 °C and even wider at higher temperatures. The ferromagnetic Curie temperature was established to be 350K, the h,~ :"~'""" ..... magnetic ordering temperature of a lanthanide binary compound which does not contain Fe or Co. Unfortunately the Gd-C alloys containing more than 5 at.%C are very reactive with moist air and must be carefully handled while carrying out experimental studies. However, the stoichiometric GdaC phase is significantly less reactive than the other alloys containing more than 20 at.%C.
'140
~ 120
~Ioo
0
\
10
20
30
40
50
AL%C Fig. 4. The Vickers hardness as a function of C concentratian for some
as-cast and heat treated Gd-C alloys. The microhardness values were determined using a 500 mgf load.
Ill F.H. Spedding, K. Gschneidner Jr., A.H. Daane, J. Am. Chem. 5oc. 80 (1958) 4499. [2] R. Lallement, Rare Earth Research-Ill. in: L. Eyring (Ed.), Gordon and Breach, New York, 1965, p. 55. [3] K.A. Gschneidner Jr., F.W. Calderwood, Bull. Alloy Phase Diagrams 7 (t986) 421. [4] K.A. Gschneidner Jr., F.W. Calderwood, Bull. Alloy Phase Diagrams 7 (1986b 443. [5] K.A. Gschneidner Jr., F.W. Calder N, Bull. Alloy Phase Diagrams 7 (1996) 564. [6] L.G. Akselmd, Yu N. Grin, P. Yu. Zavalij, V.K. Pecharsky, V.S. Fundamensky, Xllth European Crystailogr. Meet., Coll. Abstr., Moscow, 3 (1989) 155. [7] A. Arrott, Phys. Rev. 108 (1957) 1394. [8] J. $hi, H. Izumi, K Machida, G. Adachk J. Alloys Comp. 240 (1996) 156.