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Coercive force and stability of SmCo5 and GdCo5
Journal of the Less-Common Metals Elsevier Sequoia %A., Lausanne - Printed
COERCIVE
65 in The Netherlands
FORCE AND STABILITY
F. J. A. DEN BROEDER
OF SmCo, AND GdCo,
AND K. H. J. BUSCHOW
Philips Research Laboratories, Eindhoven (The Netherlands) (Received
February
21qt 1972)
SUMMARY
At low temperatures the compounds GdCo, and SmCo, decompose into the corresponding phases R&o7 and R&o, ,. This transformation is connected with the dependence of the coercive force of powder compacts on sintering temperature.
INTRODUCTION
High coercive forces can be introduced into SmCo5 by grinding’,2, while the coercivity of SmCo, powder compacts can be raised by sintering3. Westendorp4 observed that the coercivity is very sensitive to the sintering temperature. From a value of 11 kOe in the ground- and as-pressed state, the coercive force first decreases with the heating temperature to a value of 4 kOe near 750°C. It increases again to a maximum value of about 36 kOe at a sintering temperature T,,, = 108O”C,followed by a steep decrease at higher temperatures. Westendorp also reported that a heat treatment at 750°C after a first sintering treatment at T,,, again results ‘in a drastic drop of the coercive force. This led this author to suppose that at high temperature SmCo, is in a single phase state (low ,H,), that near T,,, precipitation occurs (high &), while at lower temperatures the precipitate coarsens (low ,H,). According to the tentative form of the homogeneity regions of SmCo, and GdCos as proposed by Buschow and van der Goat’, which shows an extension of this region at higher temperatures to a higher cobalt content, the adjacent compound, R2Co, 7 (R= Sm or Gd), can precipitate at low temperature from a non-stoichiometric RCo, +x-single phase. Moreover, Strnat has obtained indications that Sm,Co,, has a much lower magnetocrystalline anisotropy than SmCo,6. This is in accordance with our experience that in pure Sm2Co 1, no appreciable coercive force can be introduced either by milling or by sintering. In the course of our investigations it appeared, however, that sintered samples corresponding to compositions within the two-phase region Sm,Co,-SmCo, also exhibited a decrease of the coercive force on annealing at a low temperature after first being sintered at T,,,. This result cannot be explained by precipitation of Sm,Co,, solely, due to the smaller cobalt-solubility in SmCo, at lower temperatures. It is, however, known that the RCo,-type compounds, in which R is a heavier rare-earth metal such as Dy, Ho and Er, are only stable at higher temperatures and that they decompose at lower temperatures into R,Co,, and R2C0,7. Therefore it .J.Less-Common
Metals, 29 (1972)
66
F. J.A. DEN BROEDER, K. H. J. BUSCHOW
was decided to investigate whether a similar transformation also occurs in the Sm-Co system and whether this might be correlated with the behaviour of the coercive force. The compound GdCo, was included in this investigation since it also shows a high magnetocrystalline anisotropy and its stability presumably lies between that of SmCo5 and the other RCo~-compounds mentioned. EXPERIMENTAL
PROCEDURE
The alloys were prepared by melting weighed amounts of 99.9 wt.% pure components in an argon-arc furnace as well as in an induction furnace under purified argon. Annealing of the as-cast material was carried out in sintered A1,03 crucibles enclosed in sealed silica tubes filled with purified argon, as well as in iron capsules which were closed by cold pressing at 30 kbar. The different preparation and annealing methods gave identical results. The powder compacts were prepared in a way described by Westendorp4. All handling of the powder was carried out in a protective atmosphere, The sintering treatment of the powder compacts was carried out in the iron capsules mentioned above. In spite of all precautions, minor oxidation of the samples took place leading to a small shift in the composition (about 1 at.% Co) of the alloy. Therefore the experiments on powdered materials were performed with alloys having a slight excess of Sm or Gd with respect to the composition of RCo, . The metallographic investigation was carried out by standard techniques. A nital etch (1%) was used. The coercive forces were measured at room temperature with a vibrating sample magnetometer after premagnetizing in a pulsed field of about 100 kOe. RESULTS AND DISCUSSION
Figure 1 shows the measured intrinsic coercive force (&) of SmCo,.,
and
SmCo,, . GdCo,,
0
LO
IkOel t
Tempera!ure
-
Fig. 1. Coercive force of powder compacts of SmCo4,8 and GdCo,., as a function of the sintering temperature. J. Less-Common
Metals, 29 (1972)
COERCIVE
FORCE
AND STABILITY
OF SmCo,
67
AND GdCo,
GdCo,,, powder compacts as a function of the sintering temperature. The curve shown for the Sm compound has a shape similar to that reported by Westendorp4, although the maximum is found at a somewhat lower temperature. We observed, in addition, that the coercive force for the Sm compound first sintered at 975°C drops to the much lower value of 5 kOe after a second anneal at 700°C and then rises again to the original value if the sample is reheated at 975°C. This change in coercive force does not occur in a completely reversible way at intermediate temperature intervals but shows a temperature hysteresis’. For both SmCo5 and GdCo, the changes in coercive force can be related to a change of the microstructure. The samples sintered at 975°C have a microstructure which is similar to those of the as-cast samples, i.e., some islands of the phase R&o, occurring in an otherwise completely homogeneous RCo, matrix. The samples which have been heated for rather long periods (24 h to several months) at about 700°C show a quite different microstructure (Figs. 2 and 3). It can be seen that, mainly along the grain boundaries and partly in the interior of the grains, the phase RZCol, (appearing white) has precipitated, while the grains themselves show a large amount of straight lines. These lines are all oriented parallel to each other within one grain. This peculiar microstructure disappears completely on heating at higher temperatures. It reappears again on heating a second time at about 700°C. So, apart from a possible temperature hysteresis, the change in microstructure has the same reversible character as observed for the coercive force. By milling a sample of a microstructure such as that shown in Figs. 2 and 3, and mounting the powder in araldite while aligning the particles in a magnetic field
Fig. 2. Microstructure J. Less-Common
of as-cast
GdCo,
Metals, 29 (1972)
after an annealing
treatment
of 3 months
at 700°C. ( x 1600)
68
F. J. A. DEN BROEDER, K. H. J. BUSCHOW
Fig. 3. Microstructure of as-cast SmCo,,, after an annealing treatment of 7 weeks at 700°C. ( x 1600)
it was observed that these straight lines are all oriented perpendicularly to the direction of the applied field. Since in SmCo, and GdCo, the crystallographic c-axis is the easy magnetization direction, this means that these lines are all oriented parallel to the hexagonal basal planes of the matrix. Electron microscopy of indirect replicas revealed that these lines probably represent intersections of platelets which are about 0.1 pm thick. If a SmCo,,, powder compact is sintered at llOO”C, cooled very slowly to 730°C and kept for 24 h at the latter temperature, the platelets will appear to have grown thicker and give the impression of a precipitate (Fig. 4). The appearance of this phase has much in common with the striated form of some R2Ni, compounds observed earlier by Buschow and van der Goat’. This suggests that the occurrence of a microstructure such as that shown in Figs. 2-4 is the result of a eutectoid reaction in which the compound RCo, decomposes on annealing at temperatures well below 900°C into the adjacent compounds R,Co, and R,Co, ,. Such a decomposition of the CaCu,-type phase has already been observed in several other rare earth-cobalt systems’. The occurrence of the typical microstructure accompanying the phase transition can be explained by the following considerations. The difference in unit cell dimension in the hexagonal u-direction between RCo, and R&o, is appreciably less than between RCo, and R,Co,, (taking account of the additional factor J3 in the case of R&o,,)~. From this fact one may assume that the platelike phase represents R&o, since it is likely that only this phase can easily precipitate coherently on the basal plane of the RCo, matrix. This assumption is supported by a study of the deJ. Less-Common Metals, 29 (1972)
COERCIVE FORCE AND STABILITY OF SmCo, AND GdCo,
Fig. 4. Microstructure of a powder compact of SmCo,,, after sintering 1 h at 1IOO’Cfollowed by annealing 24 h at 730°C. The large dark circular areas represent pores. ( x 1600)
Fig. 5. Microstructure of HoCo 5.s annealed 60 h at 1100°C. (x 250)
composition process of the CaCu, type phase in the Ho-Co system where this phase is only stable above 1150°C7. Figure 5 is a micrograph of HoCo,,, which was heattreated for 60 h at 11OO’C. The decomposition into HZCol, ~appearing white) and J.
Less-Common Metais, 29 (1972)
70
F. J. A. DEN BROEDER,
K. H. J. BUSCHOW
Ho&o, (dark gray) proceeds mainly in a cellular way. One sees, however, that within the grains, the phase Ho&o, precipitates independently in the form of large, rather thin discs, which also appear to be oriented parallel to the basal plane of the HoCo5,s matrix. Finally we should like to mention that in the SmCo, case we observed by microscopic and electron microprobe investigations that the phase Sm,Co,, also precipitates very often at the interfaces between SmCo, and Sm,Co, islands (the latter being already present in an as-cast sample of SmCo,.,). From all these facts and consideratlbns one might conclude that SmCo, and GdCo, are, indeed, not stable below about 8OO”C,but decompose into the respective R,Co, and R&o,, compounds. It must be noted, however, that X-ray diffraction analysis gave only weak evidence of this transformation. After annealing for several months, new reflections belonging to the R&o 17 phase were very distinct. The intensity of the R&o, reflections, on the other hand, increased only slightly as a result of the heat treatment. Several possibilities have, however, to be considered. (1) The decomposition process proceeds very sluggishly. The etching treatment needed to reveal the microstructure leads to a great over estimation of the amount of the R&o, phase. (2)Theplatelets ofthe precipitate are so thin that the number ofrepeat distances of R&o, unit cells along the c-axis is too low to result in reflections of significant peak intensity. (3) It is known that the R,Co, compounds can occur in a hexagonal (c ~24 A) and in a rhombohedral (c x 36 A) moditication5,7. The different types of stacking of both modifications are shown in Fig. 6. For Sm&o, and Gd&!o, these forms are of almost equal stability. It is possible, therefore, that both types of stacking occur simultaneously and more or less at random. In that case one will observe an X-ray diffraction pattern which is practically indistinguishable from that of pure RCo,. (4) The R&o, precipitate has the proper stoichiometry, but is still disordered and occurs without any supercell formation at all along the c-axis. In the last three cases the only repeat distance which is reasonably well developed is that of the RCo, unit cell. The R&o, phase will, therefore, give rise to a
mm-
69
drh
-
-
-
mm_
s-e
-w-m
___
-_-
-e-m
---
B
Fig. 6. Stacking orders of the two R,Co, hedral form : a z 5 A, c z 36 A. J. Less-Common
Metals, 29 (1972)
modifications
: A, hexagonal
form:
aN 5 & c z 24 A ; B, rhombo-
COERCIVE FORCE AND STABILITY OF SmCo, AND GdCo5
71
somewhat broadened pattern of the RCo, structure. En view of the fact that the microstructure always shows only a partial conversion of RCo, into R,Co,, and R&o,, these broadened lines will be obscured by the strong, norma RCo, X-ray pattern. CONCLUSIONS
The behaviour of the coercive force of sintered powder compacts of SmCoS and GdCo, when subjected to different annealing temperatures between 700°C and 1000°C can be refated to the phase transformation of RCo, into R,Co, and R,Co,, and vice versa. While it is known that RCo, compounds, in which R is a heavier rareearth metal such as Dy, Ho, Er, are only stable at higher temperatures’, it appears now that the Sm and Gd compounds form an extension of this RCo, series in which the eutectoid transformation temperature rises with the R atom number, ACKNOWLEDGEMENTS
The authors wish to thank G. D. Westerhout and C. F. P. Naninck for the assistance given in sample preparation and in the magnetic measurements. REFERENCES 1 2 3 4 5 6
7 8
9
K. Sttnat, G. Hoffer, J. Olson, W. Ostertag and J. J. Becker, J. Appl. Phys., 38 (1967) 1001. K. H. J. Buschow, P. A. Naastepad and F. F. Westendorp, f. Appl. Phys., 40 (1969) 4029. D. K. Das, IEEE Truns. Magn., MAG-5 (1969) 214. F. F. Westendorp, Solid State Comm., 8 (1970) 139. K. H. J. Buschow and A. S. van der Goot, J. Less-Common Metals, 14 (1968) 323 ; 17 (1969) 249. K. J. Strnat, Proc. 7th Rare Earth Res. Conf:, Coronado. 1968, p. IS. K. H. J. Buschow, Philips Res. Rep&, 26 (1971) 49. F. .I. A. den Broeder and G. D. Westerhout, to be published. K. H. J. Buschow and A. S. van der Coot, J. Less-Common Metals, 22 (1970) 419.
J. Less-Common
Metals, 29 (1972)
COERCIVE
65 in The Netherlands
FORCE AND STABILITY
F. J. A. DEN BROEDER
OF SmCo, AND GdCo,
AND K. H. J. BUSCHOW
Philips Research Laboratories, Eindhoven (The Netherlands) (Received
February
21qt 1972)
SUMMARY
At low temperatures the compounds GdCo, and SmCo, decompose into the corresponding phases R&o7 and R&o, ,. This transformation is connected with the dependence of the coercive force of powder compacts on sintering temperature.
INTRODUCTION
High coercive forces can be introduced into SmCo5 by grinding’,2, while the coercivity of SmCo, powder compacts can be raised by sintering3. Westendorp4 observed that the coercivity is very sensitive to the sintering temperature. From a value of 11 kOe in the ground- and as-pressed state, the coercive force first decreases with the heating temperature to a value of 4 kOe near 750°C. It increases again to a maximum value of about 36 kOe at a sintering temperature T,,, = 108O”C,followed by a steep decrease at higher temperatures. Westendorp also reported that a heat treatment at 750°C after a first sintering treatment at T,,, again results ‘in a drastic drop of the coercive force. This led this author to suppose that at high temperature SmCo, is in a single phase state (low ,H,), that near T,,, precipitation occurs (high &), while at lower temperatures the precipitate coarsens (low ,H,). According to the tentative form of the homogeneity regions of SmCo, and GdCos as proposed by Buschow and van der Goat’, which shows an extension of this region at higher temperatures to a higher cobalt content, the adjacent compound, R2Co, 7 (R= Sm or Gd), can precipitate at low temperature from a non-stoichiometric RCo, +x-single phase. Moreover, Strnat has obtained indications that Sm,Co,, has a much lower magnetocrystalline anisotropy than SmCo,6. This is in accordance with our experience that in pure Sm2Co 1, no appreciable coercive force can be introduced either by milling or by sintering. In the course of our investigations it appeared, however, that sintered samples corresponding to compositions within the two-phase region Sm,Co,-SmCo, also exhibited a decrease of the coercive force on annealing at a low temperature after first being sintered at T,,,. This result cannot be explained by precipitation of Sm,Co,, solely, due to the smaller cobalt-solubility in SmCo, at lower temperatures. It is, however, known that the RCo,-type compounds, in which R is a heavier rare-earth metal such as Dy, Ho and Er, are only stable at higher temperatures and that they decompose at lower temperatures into R,Co,, and R2C0,7. Therefore it .J.Less-Common
Metals, 29 (1972)
66
F. J.A. DEN BROEDER, K. H. J. BUSCHOW
was decided to investigate whether a similar transformation also occurs in the Sm-Co system and whether this might be correlated with the behaviour of the coercive force. The compound GdCo, was included in this investigation since it also shows a high magnetocrystalline anisotropy and its stability presumably lies between that of SmCo5 and the other RCo~-compounds mentioned. EXPERIMENTAL
PROCEDURE
The alloys were prepared by melting weighed amounts of 99.9 wt.% pure components in an argon-arc furnace as well as in an induction furnace under purified argon. Annealing of the as-cast material was carried out in sintered A1,03 crucibles enclosed in sealed silica tubes filled with purified argon, as well as in iron capsules which were closed by cold pressing at 30 kbar. The different preparation and annealing methods gave identical results. The powder compacts were prepared in a way described by Westendorp4. All handling of the powder was carried out in a protective atmosphere, The sintering treatment of the powder compacts was carried out in the iron capsules mentioned above. In spite of all precautions, minor oxidation of the samples took place leading to a small shift in the composition (about 1 at.% Co) of the alloy. Therefore the experiments on powdered materials were performed with alloys having a slight excess of Sm or Gd with respect to the composition of RCo, . The metallographic investigation was carried out by standard techniques. A nital etch (1%) was used. The coercive forces were measured at room temperature with a vibrating sample magnetometer after premagnetizing in a pulsed field of about 100 kOe. RESULTS AND DISCUSSION
Figure 1 shows the measured intrinsic coercive force (&) of SmCo,.,
and
SmCo,, . GdCo,,
0
LO
IkOel t
Tempera!ure
-
Fig. 1. Coercive force of powder compacts of SmCo4,8 and GdCo,., as a function of the sintering temperature. J. Less-Common
Metals, 29 (1972)
COERCIVE
FORCE
AND STABILITY
OF SmCo,
67
AND GdCo,
GdCo,,, powder compacts as a function of the sintering temperature. The curve shown for the Sm compound has a shape similar to that reported by Westendorp4, although the maximum is found at a somewhat lower temperature. We observed, in addition, that the coercive force for the Sm compound first sintered at 975°C drops to the much lower value of 5 kOe after a second anneal at 700°C and then rises again to the original value if the sample is reheated at 975°C. This change in coercive force does not occur in a completely reversible way at intermediate temperature intervals but shows a temperature hysteresis’. For both SmCo5 and GdCo, the changes in coercive force can be related to a change of the microstructure. The samples sintered at 975°C have a microstructure which is similar to those of the as-cast samples, i.e., some islands of the phase R&o, occurring in an otherwise completely homogeneous RCo, matrix. The samples which have been heated for rather long periods (24 h to several months) at about 700°C show a quite different microstructure (Figs. 2 and 3). It can be seen that, mainly along the grain boundaries and partly in the interior of the grains, the phase RZCol, (appearing white) has precipitated, while the grains themselves show a large amount of straight lines. These lines are all oriented parallel to each other within one grain. This peculiar microstructure disappears completely on heating at higher temperatures. It reappears again on heating a second time at about 700°C. So, apart from a possible temperature hysteresis, the change in microstructure has the same reversible character as observed for the coercive force. By milling a sample of a microstructure such as that shown in Figs. 2 and 3, and mounting the powder in araldite while aligning the particles in a magnetic field
Fig. 2. Microstructure J. Less-Common
of as-cast
GdCo,
Metals, 29 (1972)
after an annealing
treatment
of 3 months
at 700°C. ( x 1600)
68
F. J. A. DEN BROEDER, K. H. J. BUSCHOW
Fig. 3. Microstructure of as-cast SmCo,,, after an annealing treatment of 7 weeks at 700°C. ( x 1600)
it was observed that these straight lines are all oriented perpendicularly to the direction of the applied field. Since in SmCo, and GdCo, the crystallographic c-axis is the easy magnetization direction, this means that these lines are all oriented parallel to the hexagonal basal planes of the matrix. Electron microscopy of indirect replicas revealed that these lines probably represent intersections of platelets which are about 0.1 pm thick. If a SmCo,,, powder compact is sintered at llOO”C, cooled very slowly to 730°C and kept for 24 h at the latter temperature, the platelets will appear to have grown thicker and give the impression of a precipitate (Fig. 4). The appearance of this phase has much in common with the striated form of some R2Ni, compounds observed earlier by Buschow and van der Goat’. This suggests that the occurrence of a microstructure such as that shown in Figs. 2-4 is the result of a eutectoid reaction in which the compound RCo, decomposes on annealing at temperatures well below 900°C into the adjacent compounds R,Co, and R,Co, ,. Such a decomposition of the CaCu,-type phase has already been observed in several other rare earth-cobalt systems’. The occurrence of the typical microstructure accompanying the phase transition can be explained by the following considerations. The difference in unit cell dimension in the hexagonal u-direction between RCo, and R&o, is appreciably less than between RCo, and R,Co,, (taking account of the additional factor J3 in the case of R&o,,)~. From this fact one may assume that the platelike phase represents R&o, since it is likely that only this phase can easily precipitate coherently on the basal plane of the RCo, matrix. This assumption is supported by a study of the deJ. Less-Common Metals, 29 (1972)
COERCIVE FORCE AND STABILITY OF SmCo, AND GdCo,
Fig. 4. Microstructure of a powder compact of SmCo,,, after sintering 1 h at 1IOO’Cfollowed by annealing 24 h at 730°C. The large dark circular areas represent pores. ( x 1600)
Fig. 5. Microstructure of HoCo 5.s annealed 60 h at 1100°C. (x 250)
composition process of the CaCu, type phase in the Ho-Co system where this phase is only stable above 1150°C7. Figure 5 is a micrograph of HoCo,,, which was heattreated for 60 h at 11OO’C. The decomposition into HZCol, ~appearing white) and J.
Less-Common Metais, 29 (1972)
70
F. J. A. DEN BROEDER,
K. H. J. BUSCHOW
Ho&o, (dark gray) proceeds mainly in a cellular way. One sees, however, that within the grains, the phase Ho&o, precipitates independently in the form of large, rather thin discs, which also appear to be oriented parallel to the basal plane of the HoCo5,s matrix. Finally we should like to mention that in the SmCo, case we observed by microscopic and electron microprobe investigations that the phase Sm,Co,, also precipitates very often at the interfaces between SmCo, and Sm,Co, islands (the latter being already present in an as-cast sample of SmCo,.,). From all these facts and consideratlbns one might conclude that SmCo, and GdCo, are, indeed, not stable below about 8OO”C,but decompose into the respective R,Co, and R&o,, compounds. It must be noted, however, that X-ray diffraction analysis gave only weak evidence of this transformation. After annealing for several months, new reflections belonging to the R&o 17 phase were very distinct. The intensity of the R&o, reflections, on the other hand, increased only slightly as a result of the heat treatment. Several possibilities have, however, to be considered. (1) The decomposition process proceeds very sluggishly. The etching treatment needed to reveal the microstructure leads to a great over estimation of the amount of the R&o, phase. (2)Theplatelets ofthe precipitate are so thin that the number ofrepeat distances of R&o, unit cells along the c-axis is too low to result in reflections of significant peak intensity. (3) It is known that the R,Co, compounds can occur in a hexagonal (c ~24 A) and in a rhombohedral (c x 36 A) moditication5,7. The different types of stacking of both modifications are shown in Fig. 6. For Sm&o, and Gd&!o, these forms are of almost equal stability. It is possible, therefore, that both types of stacking occur simultaneously and more or less at random. In that case one will observe an X-ray diffraction pattern which is practically indistinguishable from that of pure RCo,. (4) The R&o, precipitate has the proper stoichiometry, but is still disordered and occurs without any supercell formation at all along the c-axis. In the last three cases the only repeat distance which is reasonably well developed is that of the RCo, unit cell. The R&o, phase will, therefore, give rise to a
mm-
69
drh
-
-
-
mm_
s-e
-w-m
___
-_-
-e-m
---
B
Fig. 6. Stacking orders of the two R,Co, hedral form : a z 5 A, c z 36 A. J. Less-Common
Metals, 29 (1972)
modifications
: A, hexagonal
form:
aN 5 & c z 24 A ; B, rhombo-
COERCIVE FORCE AND STABILITY OF SmCo, AND GdCo5
71
somewhat broadened pattern of the RCo, structure. En view of the fact that the microstructure always shows only a partial conversion of RCo, into R,Co,, and R&o,, these broadened lines will be obscured by the strong, norma RCo, X-ray pattern. CONCLUSIONS
The behaviour of the coercive force of sintered powder compacts of SmCoS and GdCo, when subjected to different annealing temperatures between 700°C and 1000°C can be refated to the phase transformation of RCo, into R,Co, and R,Co,, and vice versa. While it is known that RCo, compounds, in which R is a heavier rareearth metal such as Dy, Ho, Er, are only stable at higher temperatures’, it appears now that the Sm and Gd compounds form an extension of this RCo, series in which the eutectoid transformation temperature rises with the R atom number, ACKNOWLEDGEMENTS
The authors wish to thank G. D. Westerhout and C. F. P. Naninck for the assistance given in sample preparation and in the magnetic measurements. REFERENCES 1 2 3 4 5 6
7 8
9
K. Sttnat, G. Hoffer, J. Olson, W. Ostertag and J. J. Becker, J. Appl. Phys., 38 (1967) 1001. K. H. J. Buschow, P. A. Naastepad and F. F. Westendorp, f. Appl. Phys., 40 (1969) 4029. D. K. Das, IEEE Truns. Magn., MAG-5 (1969) 214. F. F. Westendorp, Solid State Comm., 8 (1970) 139. K. H. J. Buschow and A. S. van der Goot, J. Less-Common Metals, 14 (1968) 323 ; 17 (1969) 249. K. J. Strnat, Proc. 7th Rare Earth Res. Conf:, Coronado. 1968, p. IS. K. H. J. Buschow, Philips Res. Rep&, 26 (1971) 49. F. .I. A. den Broeder and G. D. Westerhout, to be published. K. H. J. Buschow and A. S. van der Coot, J. Less-Common Metals, 22 (1970) 419.
J. Less-Common
Metals, 29 (1972)