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Anisotropic transport properties of Y1Ba2Cu3O7 − x and Bi2Sr2CaCu2Oy single crystals
Journal of the Less-Common
Metals, 151 (1989) 159 - 164
ANISOTROPIC TRANSPORT AND Bi,Sr,CaCu,O, SINGLE
E. SCHWEIGER, Znstitut
G. LEISING,
fzir FestkGrperphysik,
PROPERTIES CRYSTALS*
K. ROTH, 0. LEITNER
Technische
159
OF Y,Ba,Cu,O,
_x
and H. KAHLERT
Universitiit Graz, Petersgasse
16, A-8010 Graz (Austria)
P. POLT Zentrum (Received
fiir Elektronenmikroskopie, November
Steyrergasse
17, A-8010 Graz (Austria)
8, 1988)
Summary Single crystals of YBa,Cu,O, . x and Bi,Sr,CaCu,O, were prepared by different preparation techniques to obtain a high yield of crystals as perfect and as large as possible to facilitate measurements of their anisotropic physical properties. Investigations of their Laue X-ray diffraction patterns and of the anisotropy of their conductivity are reported. Resistivity measurements were carried out in a closed cycle cryostat by means of a lock-in technique.
1. Introduction Since the discovery of high-temperature superconductivity (HTSC) at the IBM Zurich Research Laboratory [l] and especially since the discovery of the most thoroughly investigated HTSC material Y,Ba,Cu,O, (YBC) [2], it has been realized that it is easy to fabricate superconducting ceramic samples, but much more difficult to produce single crystals. The problems in crystal growth of YBC are based on its limited thermal and chemical stability and on its low solubility in appropriate flux media. Similar problems occur in the preparation of single crystals of high T, superconductors without rare earth elements, such as the Bi-Sr-CapCu oxides [3,4] (BSCC). At least two different phases were identified with critical temperatures at 85 and 110 K [5,6]. Because of the layered perovskite structure of most HTSC materials their physical properties are expected to be anisotropic.
*Paper presented at the Symposium on High Temperature Superconductors and Applications, at the E-MRS Fall Meeting, Strasbourg, November 8 - 10, 1988. 0022.5088/89/$3.50
c~ Elsevier
Sequoia/Printed
Preparation
in The Netherlands
160
2. Crystal
growth
of Y,Ba,Cu,O,
It is possible to grow single crystals from a calcined mixture of nominal composition Y,Ba,Cu,O, by heating it to 1020 “C!. After cooling, crystals can be found in cavities of the solidified melt, but it is difficult to separate them. YBC single crystals can also be produced by growth from the pseudoternary system YBC-BaCuO,-CuO [7]. YBC, BaCuO, and CuO were ground in a molar ratio of 1:7:13.33, pressed into pellet form and melted in gold crucibles. The conditions for crystal growth were: (1) heating to 970 “C within 360 min, (2) maintaining a constant temperature of 970 “C for 90 min, (3) cooling slowly to 400 “C within 2300 min and (4) finally, more rapid cooling to 200 “C. It is possible to separate the crystals by using the wetting and creeping properties of the liquid solution on metals such as gold and platinum, so that free crystals remain in the gaps between the metal sheets. Several experiments with different forms of crucible were made to determine an optimal shape for a high yield of single crystals. The best results were obtained using tubes with dimensions of about 5 x 1 x 0.1 cm3, which were placed into Al,O, crucibles in a non-horizontal position. YBC crystals up to 1.5 x 1.5 x 0.05 mm3 were found. Defects such as intergrowth, stacking faults and especially twinning were common in many crystals, as can be seen by polarization microscopy, but some crystals were of good quality, as judged from such microscopic inspection.
3. Preparation
of contacts
Gold wire (10 pm) contacts were attached to crystals (a x b x c z 1 x 1 x 0.05 mm3), using silver paint, under a binocular microscope. In a first step, fine points of silver were painted onto the crystals in Van der Pauw geometry. They were annealed at a temperature of 400 “C under oxygen flow for 1 -3 h, and gold wires were then attached to the silver dots. The contact resistivities achieved ranged between 0.5 and 3 R.
4. Resistivity
measurements
on YBC crystals
For resistivity measurements crystals with shiny black surfaces were chosen under the microscope. To measure resistivities in the a, b plane and in the c axis direction with a single contact arrangement, contacts were attached to the crystals as shown in the insert of Fig. 1. At first, the temperature dependence of resistivity in the a, b plane was determined with both the current and voltage terminals, one on each large face of the crystal. The resistivity showed the expected metallic behaviour, decreasing as the crystal was cooled. To determine the temperature dependence of the resistivity normal to the a, b plane, the current was applied through contacts positioned along the c axis direction. The resistance parallel to the c axis increased with
161
-I110-Z
5c
L~l~.~~r~~~,l,,~,l,,,,l,,,,J-
50
100 150 200 250 Temperature (K)
300 Temperature
(K)
Fig. 1. Temperature dependence of the resistance of an YBC single crystal in the a, b plane and in the c axis direction, with contacts as shown in the insert. Fig. 2. Temperature
dependence
of the resistivity
determined
by the Montgomery
method.
decreasing temperature. The curves are shown in Fig. 1. To analyse the anisotropic resistivity the Montgomery method was used, which considers the sample geometry [8,9] (Fig. 2). The absolute resistivity values obtained by this procedure should not be considered as very accurate because of the large area of the contacts compared with their distance apart, which results in an imprecise knowledge of the actual geometry. The different critical temperatures seem to be caused by local changes of the oxygen content, which is known to shift T, to lower values with increasing oxygen deficiency.
5.
Laue diffraction
patterns
of YBC crystals
To prove the quality of Y,Ba,Cu,O, platelike single crystals, they were investigated by X-ray diffraction in the Laue geometry. The crystals were chosen from the same preparation as the samples on which conductivity measurements were carried out, all of which exhibited a superconducting transition. Figure 3 shows Laue diffraction patterns of two different samples in transmission geometry, obtained with the incident beam parallel to the 001 direction. The conditions for the selection of particular crystals were the smoothness and cleanliness of their surface, judged by light microscopic inspection. The forms of the diffraction spots are quite different, but are directly correlated with the appearance of the samples in the microscope. One face of sample 1 reflected HeNe laser light more specularly than did the other face, which exhibited irregular steps on the surface. The more perfect face was adjusted perpendicular to the incident X-ray beam, facing the X-ray source. The X-ray diffraction spots, shown in Fig. 3a, are elongated towards the center of the pattern. They exhibit an inhomogeneous intensity distribution and are frayed at the edges. The sharpness of the
Fig. 3. Laue diffraction sample 2.
pattern
in transmission
geometry
of YBC crystals:
(a) sample
1, (b)
spots is a measure of the fluctuation of the orientation of the subdomains in the single crystal. The elongated spots result from a continuous rotation of the orientation of such subdomains around an axis by small angles [lo]. Sample 2 exhibited much better specular reflection of visible light from both surfaces of the platelet in comparison with sample 1. The Laue diffraction spots, shown in Fig. 3b, are slightly frayed at the edges and differ from the ideal elliptic shape, but they are not elongated. The Laue diagrams taken in the 001 direction have a four-fold axis of rotation. For a discussion of the location of spots belonging to the horizontal and vertical zone, the following lattice constants were used: a = 3.88 A, b = 3.82 A and c = 11.66 A [ll]. The difference Ax of a spot reflected by an (h01) plane and a (Ohl) plane depends on the difference between the lattice constants a and b and the distance x of the Laue spots from the center of the Laue pattern (X = cl tan 20), where d = 40 mm is the distance of the sample to the film and 8 is the angle between the incident beam and the reflecting plane. For the case of x z 30 mm, Ax is about 0.5 mm, increasing for larger x values. For adjustment, the sample can be rotated around three different axes on the goniometer. The largest inaccuracy is about 0.5” around the axis which determines the position of the spots in the horizontal zone. This results in Ax z 1 mm for x z 30 mm. Therefore it is hardly possible to discern whether a sample is orthorhombic or tetragonal. There exist some indications that at least sample 2 has a tetragonal unit cell. The surface of this sample shows uniform reflection of light in a polarization microscope. In the case of an orthorhombic twin structure, the surface would exhibit a checkered reflection pattern. In Raman spectroscopy, the Cu-0 vibrations in the orthorhombic YBa,Cu,O, are located at about 500 wavenumbers. This peak shifts to about 480 wavenumbers in this sample. This fact indicates oxygen deficiency and consequently a tetragonal structure. Unfortunately, conductivity measurements could not be performed on this sample and therefore we could not check whether it becomes superconducting.
163
6. Growth of Bi,Sr,Ca,Cu,O,
crystals
Crystals of Bi,Sr,Ca,Cu,O, have been grown from a melt of Bi,Sr,Ca,C&O, in self-flux method. The ingot was prepared from high-purity BiO,, SrCO,, CaO and CuO, which were ground and then calcined in an Al,O,, crucible at 840 “C for 12 h, then reground. The crystal growth conditions in air were: (1) heating to 890 “C within 4 h, (2) maintaining a constant temperature of 890 C for 3 h, (3) cooling to 400 “C! in 6 h, (4) and, finally, furnace-cooling to room temperature. The crystals were rod shaped. The quality of the crystals appeared to be not very satisfactory. Energy-dispersive X-ray (EDX) analysis at different points on one sample showed different compositions. There were regions with bismuth deficiency and strontium, calcium, and copper enrichment. Other parts of the crystals appeared to contain less calcium and copper and were rich in strontium and bismuth. The concentrations deduced by EDX analysis should be considered as qualitative rather than as quantitative information, because it is difficult to treat the matrix effects exactly.
7. Resistivity
measurements
on BSCC crystals
Contacts were made in Van der Pauw geometry, using platinum paint and gold wires. Because of the irregular shape of the crystals it was not possible to determine the crystal directions for proper arrangement of the contacts. Therefore the resistance measurements shown in Fig. 4 give the temperature dependence of the resistance with the electric field E in the a, c plane and with E in the b, c plane respectively, verified by polarized microRaman spectroscopy. Because of the layered morphology of the crystals, seen by scanning electron microscopy, the measured anisotropy might be the result not only of the anisotropy of the material, but also of these layers.
Fig. 4. Temperature dependence E in the b, c plane. Fig. 5. Laue diffraction
of the resistance
pattern in transmission
of a BSCC crystal with E in the a, c plane and
geometry
of a BSCC crystal agglomerate.
164
8. Laue diffraction
patterns
of BSCC samples
The Laue pattern also reflects the polycrystalline stratified structure. The spots are blurred and can be associated only partly with Laue zones which should appear as ellipses, hyperbolas or parabolas. This fact indicates the coexistence of arbitrarily oriented single crystals in the sample. Together with the results of the EDX analysis described above, Fig. 5 can be interpreted as a superimposition of the Laue patterns of a number of different single crystals.
9. Conclusion Whereas the available methods allow us to grow sufficiently high-quality single crystals of YBC, which enabled us to determine the anisotropy of the normal state resistivity, the “crystals” of BSCC that we obtained appear to be agglomerates of individual single crystals with varying composition, and care should be taken in the interpretation of anisotropic behaviour of such crystals, which does not necessarily reflect the true intrinsic anisotropic behaviour.
References 1 J. G. Bednorz and K. B. Miiller, 2. Phys., B, 64 (1986) 189. 2 M. K. Wu, J. R. Ashburn, C. J. Torney, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Q. Wang and C. W. Chu, Phys. Rev. Lett., 58 (1987) 908. 3 C. Michel, M. Hervieu et al., 2. Phys., B, 68 (1987) 421. 4 H. Medea, Y. Tanaka, M. Fukutomi and T. Asano, Jpn. J. AppZ. Phys. Lett., 27(2) (1988). 5 Y. F. Yan, C. Z. Li, J. H. Wang, Y. C. Chang, Q. S. Yang, D. S. Hou, X. Chu, Z. H. Mai, H. Y. Zhang, D. N. Zheng, Y. M. Ni, S. L. Jia, D. H. Shen and Z. X. Zhao, Mod. Phys. Mt., B, 2 (1988) 571. 6 P. Bordet, J. J. Capponi, C. Chaillout, J. Chenavas, A. W. Hewat, J. L. Hodeau, M. Marezio, J. L. Tholence and D. Tranqui, Physica, C, 153- 155 (1988) 623. 7 D. L. Kaiser, F. Holzberg, B. A. Scott and T. R. McGuire, A@. Phys. Lett., 51 (1987) 1040. 8 H. C. Montgomery, J. A&. Phys., 42 (1971) 2971. 9 B. F. Logan, S. 0. Rice and R. F. Wick, J. Appl. Phys., 42 (1971) 2975. 10 K. H. Jost, RSntgenbeugung an Kristallen, Akademieverlag, Berlin, 1975. 11 Y. Ishirawa, 0. Fukunaga, H. Nozaki and T. Tanaka, Jpn. J. AppZ. Phys. Lett., 26 (1987) L676.
Metals, 151 (1989) 159 - 164
ANISOTROPIC TRANSPORT AND Bi,Sr,CaCu,O, SINGLE
E. SCHWEIGER, Znstitut
G. LEISING,
fzir FestkGrperphysik,
PROPERTIES CRYSTALS*
K. ROTH, 0. LEITNER
Technische
159
OF Y,Ba,Cu,O,
_x
and H. KAHLERT
Universitiit Graz, Petersgasse
16, A-8010 Graz (Austria)
P. POLT Zentrum (Received
fiir Elektronenmikroskopie, November
Steyrergasse
17, A-8010 Graz (Austria)
8, 1988)
Summary Single crystals of YBa,Cu,O, . x and Bi,Sr,CaCu,O, were prepared by different preparation techniques to obtain a high yield of crystals as perfect and as large as possible to facilitate measurements of their anisotropic physical properties. Investigations of their Laue X-ray diffraction patterns and of the anisotropy of their conductivity are reported. Resistivity measurements were carried out in a closed cycle cryostat by means of a lock-in technique.
1. Introduction Since the discovery of high-temperature superconductivity (HTSC) at the IBM Zurich Research Laboratory [l] and especially since the discovery of the most thoroughly investigated HTSC material Y,Ba,Cu,O, (YBC) [2], it has been realized that it is easy to fabricate superconducting ceramic samples, but much more difficult to produce single crystals. The problems in crystal growth of YBC are based on its limited thermal and chemical stability and on its low solubility in appropriate flux media. Similar problems occur in the preparation of single crystals of high T, superconductors without rare earth elements, such as the Bi-Sr-CapCu oxides [3,4] (BSCC). At least two different phases were identified with critical temperatures at 85 and 110 K [5,6]. Because of the layered perovskite structure of most HTSC materials their physical properties are expected to be anisotropic.
*Paper presented at the Symposium on High Temperature Superconductors and Applications, at the E-MRS Fall Meeting, Strasbourg, November 8 - 10, 1988. 0022.5088/89/$3.50
c~ Elsevier
Sequoia/Printed
Preparation
in The Netherlands
160
2. Crystal
growth
of Y,Ba,Cu,O,
It is possible to grow single crystals from a calcined mixture of nominal composition Y,Ba,Cu,O, by heating it to 1020 “C!. After cooling, crystals can be found in cavities of the solidified melt, but it is difficult to separate them. YBC single crystals can also be produced by growth from the pseudoternary system YBC-BaCuO,-CuO [7]. YBC, BaCuO, and CuO were ground in a molar ratio of 1:7:13.33, pressed into pellet form and melted in gold crucibles. The conditions for crystal growth were: (1) heating to 970 “C within 360 min, (2) maintaining a constant temperature of 970 “C for 90 min, (3) cooling slowly to 400 “C within 2300 min and (4) finally, more rapid cooling to 200 “C. It is possible to separate the crystals by using the wetting and creeping properties of the liquid solution on metals such as gold and platinum, so that free crystals remain in the gaps between the metal sheets. Several experiments with different forms of crucible were made to determine an optimal shape for a high yield of single crystals. The best results were obtained using tubes with dimensions of about 5 x 1 x 0.1 cm3, which were placed into Al,O, crucibles in a non-horizontal position. YBC crystals up to 1.5 x 1.5 x 0.05 mm3 were found. Defects such as intergrowth, stacking faults and especially twinning were common in many crystals, as can be seen by polarization microscopy, but some crystals were of good quality, as judged from such microscopic inspection.
3. Preparation
of contacts
Gold wire (10 pm) contacts were attached to crystals (a x b x c z 1 x 1 x 0.05 mm3), using silver paint, under a binocular microscope. In a first step, fine points of silver were painted onto the crystals in Van der Pauw geometry. They were annealed at a temperature of 400 “C under oxygen flow for 1 -3 h, and gold wires were then attached to the silver dots. The contact resistivities achieved ranged between 0.5 and 3 R.
4. Resistivity
measurements
on YBC crystals
For resistivity measurements crystals with shiny black surfaces were chosen under the microscope. To measure resistivities in the a, b plane and in the c axis direction with a single contact arrangement, contacts were attached to the crystals as shown in the insert of Fig. 1. At first, the temperature dependence of resistivity in the a, b plane was determined with both the current and voltage terminals, one on each large face of the crystal. The resistivity showed the expected metallic behaviour, decreasing as the crystal was cooled. To determine the temperature dependence of the resistivity normal to the a, b plane, the current was applied through contacts positioned along the c axis direction. The resistance parallel to the c axis increased with
161
-I110-Z
5c
L~l~.~~r~~~,l,,~,l,,,,l,,,,J-
50
100 150 200 250 Temperature (K)
300 Temperature
(K)
Fig. 1. Temperature dependence of the resistance of an YBC single crystal in the a, b plane and in the c axis direction, with contacts as shown in the insert. Fig. 2. Temperature
dependence
of the resistivity
determined
by the Montgomery
method.
decreasing temperature. The curves are shown in Fig. 1. To analyse the anisotropic resistivity the Montgomery method was used, which considers the sample geometry [8,9] (Fig. 2). The absolute resistivity values obtained by this procedure should not be considered as very accurate because of the large area of the contacts compared with their distance apart, which results in an imprecise knowledge of the actual geometry. The different critical temperatures seem to be caused by local changes of the oxygen content, which is known to shift T, to lower values with increasing oxygen deficiency.
5.
Laue diffraction
patterns
of YBC crystals
To prove the quality of Y,Ba,Cu,O, platelike single crystals, they were investigated by X-ray diffraction in the Laue geometry. The crystals were chosen from the same preparation as the samples on which conductivity measurements were carried out, all of which exhibited a superconducting transition. Figure 3 shows Laue diffraction patterns of two different samples in transmission geometry, obtained with the incident beam parallel to the 001 direction. The conditions for the selection of particular crystals were the smoothness and cleanliness of their surface, judged by light microscopic inspection. The forms of the diffraction spots are quite different, but are directly correlated with the appearance of the samples in the microscope. One face of sample 1 reflected HeNe laser light more specularly than did the other face, which exhibited irregular steps on the surface. The more perfect face was adjusted perpendicular to the incident X-ray beam, facing the X-ray source. The X-ray diffraction spots, shown in Fig. 3a, are elongated towards the center of the pattern. They exhibit an inhomogeneous intensity distribution and are frayed at the edges. The sharpness of the
Fig. 3. Laue diffraction sample 2.
pattern
in transmission
geometry
of YBC crystals:
(a) sample
1, (b)
spots is a measure of the fluctuation of the orientation of the subdomains in the single crystal. The elongated spots result from a continuous rotation of the orientation of such subdomains around an axis by small angles [lo]. Sample 2 exhibited much better specular reflection of visible light from both surfaces of the platelet in comparison with sample 1. The Laue diffraction spots, shown in Fig. 3b, are slightly frayed at the edges and differ from the ideal elliptic shape, but they are not elongated. The Laue diagrams taken in the 001 direction have a four-fold axis of rotation. For a discussion of the location of spots belonging to the horizontal and vertical zone, the following lattice constants were used: a = 3.88 A, b = 3.82 A and c = 11.66 A [ll]. The difference Ax of a spot reflected by an (h01) plane and a (Ohl) plane depends on the difference between the lattice constants a and b and the distance x of the Laue spots from the center of the Laue pattern (X = cl tan 20), where d = 40 mm is the distance of the sample to the film and 8 is the angle between the incident beam and the reflecting plane. For the case of x z 30 mm, Ax is about 0.5 mm, increasing for larger x values. For adjustment, the sample can be rotated around three different axes on the goniometer. The largest inaccuracy is about 0.5” around the axis which determines the position of the spots in the horizontal zone. This results in Ax z 1 mm for x z 30 mm. Therefore it is hardly possible to discern whether a sample is orthorhombic or tetragonal. There exist some indications that at least sample 2 has a tetragonal unit cell. The surface of this sample shows uniform reflection of light in a polarization microscope. In the case of an orthorhombic twin structure, the surface would exhibit a checkered reflection pattern. In Raman spectroscopy, the Cu-0 vibrations in the orthorhombic YBa,Cu,O, are located at about 500 wavenumbers. This peak shifts to about 480 wavenumbers in this sample. This fact indicates oxygen deficiency and consequently a tetragonal structure. Unfortunately, conductivity measurements could not be performed on this sample and therefore we could not check whether it becomes superconducting.
163
6. Growth of Bi,Sr,Ca,Cu,O,
crystals
Crystals of Bi,Sr,Ca,Cu,O, have been grown from a melt of Bi,Sr,Ca,C&O, in self-flux method. The ingot was prepared from high-purity BiO,, SrCO,, CaO and CuO, which were ground and then calcined in an Al,O,, crucible at 840 “C for 12 h, then reground. The crystal growth conditions in air were: (1) heating to 890 “C within 4 h, (2) maintaining a constant temperature of 890 C for 3 h, (3) cooling to 400 “C! in 6 h, (4) and, finally, furnace-cooling to room temperature. The crystals were rod shaped. The quality of the crystals appeared to be not very satisfactory. Energy-dispersive X-ray (EDX) analysis at different points on one sample showed different compositions. There were regions with bismuth deficiency and strontium, calcium, and copper enrichment. Other parts of the crystals appeared to contain less calcium and copper and were rich in strontium and bismuth. The concentrations deduced by EDX analysis should be considered as qualitative rather than as quantitative information, because it is difficult to treat the matrix effects exactly.
7. Resistivity
measurements
on BSCC crystals
Contacts were made in Van der Pauw geometry, using platinum paint and gold wires. Because of the irregular shape of the crystals it was not possible to determine the crystal directions for proper arrangement of the contacts. Therefore the resistance measurements shown in Fig. 4 give the temperature dependence of the resistance with the electric field E in the a, c plane and with E in the b, c plane respectively, verified by polarized microRaman spectroscopy. Because of the layered morphology of the crystals, seen by scanning electron microscopy, the measured anisotropy might be the result not only of the anisotropy of the material, but also of these layers.
Fig. 4. Temperature dependence E in the b, c plane. Fig. 5. Laue diffraction
of the resistance
pattern in transmission
of a BSCC crystal with E in the a, c plane and
geometry
of a BSCC crystal agglomerate.
164
8. Laue diffraction
patterns
of BSCC samples
The Laue pattern also reflects the polycrystalline stratified structure. The spots are blurred and can be associated only partly with Laue zones which should appear as ellipses, hyperbolas or parabolas. This fact indicates the coexistence of arbitrarily oriented single crystals in the sample. Together with the results of the EDX analysis described above, Fig. 5 can be interpreted as a superimposition of the Laue patterns of a number of different single crystals.
9. Conclusion Whereas the available methods allow us to grow sufficiently high-quality single crystals of YBC, which enabled us to determine the anisotropy of the normal state resistivity, the “crystals” of BSCC that we obtained appear to be agglomerates of individual single crystals with varying composition, and care should be taken in the interpretation of anisotropic behaviour of such crystals, which does not necessarily reflect the true intrinsic anisotropic behaviour.
References 1 J. G. Bednorz and K. B. Miiller, 2. Phys., B, 64 (1986) 189. 2 M. K. Wu, J. R. Ashburn, C. J. Torney, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Q. Wang and C. W. Chu, Phys. Rev. Lett., 58 (1987) 908. 3 C. Michel, M. Hervieu et al., 2. Phys., B, 68 (1987) 421. 4 H. Medea, Y. Tanaka, M. Fukutomi and T. Asano, Jpn. J. AppZ. Phys. Lett., 27(2) (1988). 5 Y. F. Yan, C. Z. Li, J. H. Wang, Y. C. Chang, Q. S. Yang, D. S. Hou, X. Chu, Z. H. Mai, H. Y. Zhang, D. N. Zheng, Y. M. Ni, S. L. Jia, D. H. Shen and Z. X. Zhao, Mod. Phys. Mt., B, 2 (1988) 571. 6 P. Bordet, J. J. Capponi, C. Chaillout, J. Chenavas, A. W. Hewat, J. L. Hodeau, M. Marezio, J. L. Tholence and D. Tranqui, Physica, C, 153- 155 (1988) 623. 7 D. L. Kaiser, F. Holzberg, B. A. Scott and T. R. McGuire, A@. Phys. Lett., 51 (1987) 1040. 8 H. C. Montgomery, J. A&. Phys., 42 (1971) 2971. 9 B. F. Logan, S. 0. Rice and R. F. Wick, J. Appl. Phys., 42 (1971) 2975. 10 K. H. Jost, RSntgenbeugung an Kristallen, Akademieverlag, Berlin, 1975. 11 Y. Ishirawa, 0. Fukunaga, H. Nozaki and T. Tanaka, Jpn. J. AppZ. Phys. Lett., 26 (1987) L676.