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Neutron powder diffraction studies of microtwinning in the high TC superconductor, YBa2Cu3O7
' ~ % S o l i d State Communications, Vol.66,No.5, pp.483-485, Printed in Great Britain.
0 0 3 8 - 1 0 9 8 / 8 8 $3.00 + .00 Pergamon Press plc
1988.
NEUTRON POWDER DIFFRACTION STUDIES OF MICROTWINNING IN THE HIGH T C SUPERCONDUCTOR, YBa2Cu 3° 7 W I F David Neutron Division Rutherford Appleton Laboratory Chllton, Dldcot, Oxon., O X l l 0QX, UK O Moze, F Licci and F Bolzonl Istituto MASPEC del CNR Via Chiavari 18/A Parma 43100, Italy (Received on November 11, 1987 by R. Fieschi) High resolution neutron powder diffraction undertaken at the spallatlon neutron source, ISIS, has been used to investigate the average mlcrostructure of the high temperature superconductor YBa=Cu3Ov. A Rletveld profile analysis incorporating an hkl-dependent line broadening formulation has been used to model parallel (110) microtwlnnlng. The average mlcrotwin separation is calculated to be 690(4) A, consistent with electron microscopy observations on the same material.
0
THE RECENT DISCOVERY of superconductivity at temperatures above 77 K in the material YBa=Cu307 has created unprecedented excitement. Detailed diffraction studies have shown that the average structure is based upon a tripled perovsklte cell wtth an ordered arrangement of large cations and oxygen vacancies. High resolution electron microscopy, however, has revealed a rich mlcrostructure that is dependent upon sarnple preparation [1-4]. Recent results have shown that, with careful preparation, the only detectable microstructure is twinning on (110) planes [5], a feature anticipated because of the pseudo-tetragonal nature of the orthorhombic lattice. In this paper, high resolution neutron powder diffraction results are presented thal, in addition to obtaining the average structure, confirm the existence of (110) parallel microtwins and indicate the mean separation between twin boundaries. The method is clearly less informative than high resolution electron microscopy yielding only average information but has the advantage thal the bulk of the sample is probed and that the sample environment is well-determined.
0.1
x '
0.2
75
I 80
, 85
90,
915
100 T (K)
Figure 1. Negative susceptibility in YBa2Cu307 indicating a maximum 22% Meissner effect. tained for 13 hours and cooled to room temperature under fluxing oxygen. The diffraction pattern was collected on the High Resolution Powder Diffractometer (HRPD) [11] at the ISIS Spallation Neutron Source. Profile refinement of the powder diffraction data, involving a standard time-of-flight lineshape consisting of the convolution of a Gaussian with two exponential functions, proceeded in a routine manner to yield an acceptable fit and structural parameters in agreement with previous observations 1610]. However, the unique resolution of HRPD allowed systematic differences to be observed between different reflections and a detailed examination of the difference between observed and calculated diffraction profiles indicated an hkl-dependent broadening of reflections, In parltcular, hhl reflections were observed to be narrower than calculated whereas hOI and 0kl reflections were broader (Fig. 2).. "1his corresponds to long range structural coherence within 110 planes and a shorter range within 100 and 010 planes. Seen in terms of an effective particle size this would imply a mor-
The material, corresponding to the chemical formula YBa2Cu3OT, was obtained by the high temperature reaction of stoiohiometrle amounts of Y2Oa, CuO and BaCO3, After grinding the reagents for 4 hours, the sample was slowly heated to 950°C, maintained at this temperature for 24 hours and cooled to room temperature: the whole process was carried out in an alumina crucible and under fluxing oxygen. X-ray powder diffraction Indicated that the resulting powder was indeed a single phase of orthorhombic YBa2Cu307 with lattice parameters coincident with that reported by olher authors (6-10]. When examined in a vibrating sample magnetometer in a weak magnetic field of 30 Oe, all samples exhibited a well-defined negative susceptibility between 77K and 95K (Fig. 1). In order to obtain sintered samples, the powders were pressed at 2500 Kgcm -2, slowly heated to 950°C, main-
483
484
SUPERCONDUCTOR YBa2Cu307
MICROTWINNING IN THE HIGH T
Vol. 66, Ne. 5
c
. . . . . . . . . . (I
i
(I
S)
(O 0
~)
6)
(I
I
4)
,o
w"
ss
to
is
,eo
los
Time-of-flight(microseconds)
Figure 2. The difference between observed and calculated powder diffraction profiles indicating a strong hkl-dependent peak broadening.
llo
• tO :
The above equations indicate a distinction between hh0 and h-h0 broadening, these reflections In fact corresponding to the maximum (Ad = d2/T) and minimum (Ad = 0) broadening respectively. Thus, although the average structure is orthorhomblc, the broadening has a monoclinlc character. Accordingly the modified profile refinement code considered the overall microtwinned structure as monocfinic with respect to Miller Index broadening and orthorhombic with respect to the average structure. The above hkl-dependent broadening term, considered to be an extra, convoluted Gausslan term, when Included in the refinement gave an Improved structural fit with a X2 of 1.45 compared with 1.59 for the traditional refinement (see Fig. 3). R-factors (defined In [9]) were:
hkl.dependent-width refinement: R(Intensltles)= 3.89% : R(proflle)= 5.16% R(welghted profile)= 6.39% : R(exp)= 5.30%
isotropic broadening refinement. phology consisting of two-dimensional sheets with 110 or 1-10 plane normals. Noting the pseudo-tetragonal nature of the lattice !his may be simply explained in terms of parallel reflection microtwins where the reflection plane is 110 of one twin and 1-10 of the adjacent twin. These are strain-free microtwins. In principle, 90 ° twins are also permitted, but these are Inconsistent with our experimental hkl-dependent line-width observations, Although anticipated to occur in YBa2Cu3OT, their frequency of occurrence within the bulk of a well-prepared material is expected to be low: This Is because although an individual 90 ° twin boundary may be strain-free, a morphology of repeated 90 ° twins must eventually produce a strained lattice mismatch that is energetically unfavourable. 90 ° twins may, however, pin to surface effects such as grain boundaries or other crystal imperfections. Detailed calculations of the effects of parallel microtwinning indicate that the line-broadening is, to a good approximation, equivalent to that produced by isolated mtcrotwins. This is because the microtwin separation is not perfectly periodic and. thus, averaging over the bulk of the crystal there is no coherence between microtwins. Thus microtwins may be regarded as isolated two dimensional sheets with a 110 plane normal. Accordingly the broadening may be considered as a particle*size effect which from straightforward geometrical arguments has the form: P = T/sin {, where 9, the angle between the reciprocal lattice vector and the mlcrotwin plane normal, is, to a good approximation, given by cos ~ = (h + k) / /[2(h 2 + k2 + (la/c)2)] and T is the average microtwin separation. Since Ad/d = ~ . d / P for particle-size effects (e, the Scherrer constant is of the order of unity), the line-broadening is Ad = d 2 sin ~ / T
R(Intensitles)= 4.87% : R(proflle)= 5.28% R(weighted profile)= 6.66% : R(exp)= 5.28% The hkl-dependent refinement yielded a mean separation between twin boundaries of 690(4) A (Fig. 3). This has subsequently been corroborated by electron microscopy which gives a value of ~800A.
t ~¢oi¢
iwi. b~nd=r-/
=eporotlo~ -
690(4) A n i = ~ =
E 80 O
60
iIIllllllilllllll illllltlll ,i~[rf
l~illllllllllllllllllllllllllllllllll ~o
io~o Tlme-of-fllghl
iouo
lloo
(microseconds)
Figure 3. The 113 reflection. The hkl-dependent broadening leads to a significantly improved peak fit.
Given that the parallel 110 microtwins are a common feature in the YBa2Cu307 superconducting structure it is of interest to speculate on precisely which plane within the unit cell contains the reflection twin. There are two possibilities: (i) the plane containing the O atoms and (ii) the plane containing the cations. The attraction of the former option is that it preserves the integrity of the CuO, planar groups with the onedimensional chains (although their direction is altered by 90°). Bond-length calculations, however, indicate that this option involves Ba-Ba and Y-Y distances that are too short (~2.7,~). The second option may thus be regarded as an energetically more favourable configuration. This has indeed been suggested [12] and confirmed [13] by high resolution electron microscopy. Since this configuration requires a 'tetrahedral' Cu coordination in place of a square planar coordi-
.
lO
l
Vol. 66, No. 5
MICROTWINNING
nation it may be that this microtwinning places an upper limit on the critical current densities In this high Tc superconductor. Indeed, a recent survey on experimental and theoretical work on twinning plane superconductivity [14] has highlighted the Importance of the nature of twinning
IN THE HIGH T
c
SUPERCONDUCTOR
planes to properties.
485
YBa2Cu307
macroscopic
superconducting
The authors wish to thank the SERC for the provision of neutron scattering facilities and Dr. C.C. Wilson for helpful contrlbutlons. REFERENCES
1. C.H.Chen, D.J.Werder, S.H.Liou, J.R.Kwo and M.Hong, Phys Rev. B35, 8767-8769 (1987) 2. G.van Tendeloo, H.W.Zandbergen and S.Amerlinckx, Solid State Comm. 63, 389-393 (1987) 3. M.Hervleu, B.Domenges, C.Mtchel and B.Raveau, Europhys. Lett. 4, 205-210 (1987) 4. 8.Domenges, M.HerWeu, C.MIchel and B.Raveau, Europhys. Lett. 4, 211-214 (1967) 5. E.A.Hewat, M.Dupuy, A.Bourret, J.J.Capponi and M.Marezio, Nature 327, 400-402 (1987) 6. M.A.Beno, L.Soderholm, D.W.Capone, II, D.G.Hinks, J.D.Jorgensen, J.D.Grace, I.K.Schuller, C.U.Segre and K.Zhang, Appl. Phys. Lett. 51, 57-62 (1987) 7. J.E.Greedan, A.O'Rellly and C.V.Stager, Phys. Rev. B35 8770-8772 (1987) 8. J.J.Capponl, C.Chaillout, A.W.Hewat, P,Lejay, M.Marezio, N.Nguyon, B.Raveau,
J.L.Soubeyroux, J.L.Tholence and R.Tournler, Europhys. Left. 3 1301-1305 (1987) 9. W.l.F.David, W.T.A.Harrlson, J.M.F.Gunn, O.Moze, A.K.Soper, P.Day, J.D.Jorgensen, M.A.Beno, D.W.Capone, II, D.G.Hinks, I.K.Schuller, L.Soderholm, C.U.Segre, K.Zhang and J.D.Grace, Nature 327 310-312 (1987) 10. F.Beech, S.Miraglla, A.Santoro and R.S.Roth, Phys. Rev. B35 8778-8781 (1987) 11. W.l.F.David and M.W.Johnson, Rutherford Appleton Laboratory Report, RAL-85-112 (1985) 12. A.Brokman, Solid State Comm., 64, 257 (1987) 13. E.A.Hewat, M.Dupuy, A.Bourret, J.J.Capponi and M.Marezio, Solid State Comm. 64, 517-520 (1987) 14. I.N.Khlyuatlkov and A.I.Buzdln, Advancea In Physics, 36, 271 (1987)
0 0 3 8 - 1 0 9 8 / 8 8 $3.00 + .00 Pergamon Press plc
1988.
NEUTRON POWDER DIFFRACTION STUDIES OF MICROTWINNING IN THE HIGH T C SUPERCONDUCTOR, YBa2Cu 3° 7 W I F David Neutron Division Rutherford Appleton Laboratory Chllton, Dldcot, Oxon., O X l l 0QX, UK O Moze, F Licci and F Bolzonl Istituto MASPEC del CNR Via Chiavari 18/A Parma 43100, Italy (Received on November 11, 1987 by R. Fieschi) High resolution neutron powder diffraction undertaken at the spallatlon neutron source, ISIS, has been used to investigate the average mlcrostructure of the high temperature superconductor YBa=Cu3Ov. A Rletveld profile analysis incorporating an hkl-dependent line broadening formulation has been used to model parallel (110) microtwlnnlng. The average mlcrotwin separation is calculated to be 690(4) A, consistent with electron microscopy observations on the same material.
0
THE RECENT DISCOVERY of superconductivity at temperatures above 77 K in the material YBa=Cu307 has created unprecedented excitement. Detailed diffraction studies have shown that the average structure is based upon a tripled perovsklte cell wtth an ordered arrangement of large cations and oxygen vacancies. High resolution electron microscopy, however, has revealed a rich mlcrostructure that is dependent upon sarnple preparation [1-4]. Recent results have shown that, with careful preparation, the only detectable microstructure is twinning on (110) planes [5], a feature anticipated because of the pseudo-tetragonal nature of the orthorhombic lattice. In this paper, high resolution neutron powder diffraction results are presented thal, in addition to obtaining the average structure, confirm the existence of (110) parallel microtwins and indicate the mean separation between twin boundaries. The method is clearly less informative than high resolution electron microscopy yielding only average information but has the advantage thal the bulk of the sample is probed and that the sample environment is well-determined.
0.1
x '
0.2
75
I 80
, 85
90,
915
100 T (K)
Figure 1. Negative susceptibility in YBa2Cu307 indicating a maximum 22% Meissner effect. tained for 13 hours and cooled to room temperature under fluxing oxygen. The diffraction pattern was collected on the High Resolution Powder Diffractometer (HRPD) [11] at the ISIS Spallation Neutron Source. Profile refinement of the powder diffraction data, involving a standard time-of-flight lineshape consisting of the convolution of a Gaussian with two exponential functions, proceeded in a routine manner to yield an acceptable fit and structural parameters in agreement with previous observations 1610]. However, the unique resolution of HRPD allowed systematic differences to be observed between different reflections and a detailed examination of the difference between observed and calculated diffraction profiles indicated an hkl-dependent broadening of reflections, In parltcular, hhl reflections were observed to be narrower than calculated whereas hOI and 0kl reflections were broader (Fig. 2).. "1his corresponds to long range structural coherence within 110 planes and a shorter range within 100 and 010 planes. Seen in terms of an effective particle size this would imply a mor-
The material, corresponding to the chemical formula YBa2Cu3OT, was obtained by the high temperature reaction of stoiohiometrle amounts of Y2Oa, CuO and BaCO3, After grinding the reagents for 4 hours, the sample was slowly heated to 950°C, maintained at this temperature for 24 hours and cooled to room temperature: the whole process was carried out in an alumina crucible and under fluxing oxygen. X-ray powder diffraction Indicated that the resulting powder was indeed a single phase of orthorhombic YBa2Cu307 with lattice parameters coincident with that reported by olher authors (6-10]. When examined in a vibrating sample magnetometer in a weak magnetic field of 30 Oe, all samples exhibited a well-defined negative susceptibility between 77K and 95K (Fig. 1). In order to obtain sintered samples, the powders were pressed at 2500 Kgcm -2, slowly heated to 950°C, main-
483
484
SUPERCONDUCTOR YBa2Cu307
MICROTWINNING IN THE HIGH T
Vol. 66, Ne. 5
c
. . . . . . . . . . (I
i
(I
S)
(O 0
~)
6)
(I
I
4)
,o
w"
ss
to
is
,eo
los
Time-of-flight(microseconds)
Figure 2. The difference between observed and calculated powder diffraction profiles indicating a strong hkl-dependent peak broadening.
llo
• tO :
The above equations indicate a distinction between hh0 and h-h0 broadening, these reflections In fact corresponding to the maximum (Ad = d2/T) and minimum (Ad = 0) broadening respectively. Thus, although the average structure is orthorhomblc, the broadening has a monoclinlc character. Accordingly the modified profile refinement code considered the overall microtwinned structure as monocfinic with respect to Miller Index broadening and orthorhombic with respect to the average structure. The above hkl-dependent broadening term, considered to be an extra, convoluted Gausslan term, when Included in the refinement gave an Improved structural fit with a X2 of 1.45 compared with 1.59 for the traditional refinement (see Fig. 3). R-factors (defined In [9]) were:
hkl.dependent-width refinement: R(Intensltles)= 3.89% : R(proflle)= 5.16% R(welghted profile)= 6.39% : R(exp)= 5.30%
isotropic broadening refinement. phology consisting of two-dimensional sheets with 110 or 1-10 plane normals. Noting the pseudo-tetragonal nature of the lattice !his may be simply explained in terms of parallel reflection microtwins where the reflection plane is 110 of one twin and 1-10 of the adjacent twin. These are strain-free microtwins. In principle, 90 ° twins are also permitted, but these are Inconsistent with our experimental hkl-dependent line-width observations, Although anticipated to occur in YBa2Cu3OT, their frequency of occurrence within the bulk of a well-prepared material is expected to be low: This Is because although an individual 90 ° twin boundary may be strain-free, a morphology of repeated 90 ° twins must eventually produce a strained lattice mismatch that is energetically unfavourable. 90 ° twins may, however, pin to surface effects such as grain boundaries or other crystal imperfections. Detailed calculations of the effects of parallel microtwinning indicate that the line-broadening is, to a good approximation, equivalent to that produced by isolated mtcrotwins. This is because the microtwin separation is not perfectly periodic and. thus, averaging over the bulk of the crystal there is no coherence between microtwins. Thus microtwins may be regarded as isolated two dimensional sheets with a 110 plane normal. Accordingly the broadening may be considered as a particle*size effect which from straightforward geometrical arguments has the form: P = T/sin {, where 9, the angle between the reciprocal lattice vector and the mlcrotwin plane normal, is, to a good approximation, given by cos ~ = (h + k) / /[2(h 2 + k2 + (la/c)2)] and T is the average microtwin separation. Since Ad/d = ~ . d / P for particle-size effects (e, the Scherrer constant is of the order of unity), the line-broadening is Ad = d 2 sin ~ / T
R(Intensitles)= 4.87% : R(proflle)= 5.28% R(weighted profile)= 6.66% : R(exp)= 5.28% The hkl-dependent refinement yielded a mean separation between twin boundaries of 690(4) A (Fig. 3). This has subsequently been corroborated by electron microscopy which gives a value of ~800A.
t ~¢oi¢
iwi. b~nd=r-/
=eporotlo~ -
690(4) A n i = ~ =
E 80 O
60
iIIllllllilllllll illllltlll ,i~[rf
l~illllllllllllllllllllllllllllllllll ~o
io~o Tlme-of-fllghl
iouo
lloo
(microseconds)
Figure 3. The 113 reflection. The hkl-dependent broadening leads to a significantly improved peak fit.
Given that the parallel 110 microtwins are a common feature in the YBa2Cu307 superconducting structure it is of interest to speculate on precisely which plane within the unit cell contains the reflection twin. There are two possibilities: (i) the plane containing the O atoms and (ii) the plane containing the cations. The attraction of the former option is that it preserves the integrity of the CuO, planar groups with the onedimensional chains (although their direction is altered by 90°). Bond-length calculations, however, indicate that this option involves Ba-Ba and Y-Y distances that are too short (~2.7,~). The second option may thus be regarded as an energetically more favourable configuration. This has indeed been suggested [12] and confirmed [13] by high resolution electron microscopy. Since this configuration requires a 'tetrahedral' Cu coordination in place of a square planar coordi-
.
lO
l
Vol. 66, No. 5
MICROTWINNING
nation it may be that this microtwinning places an upper limit on the critical current densities In this high Tc superconductor. Indeed, a recent survey on experimental and theoretical work on twinning plane superconductivity [14] has highlighted the Importance of the nature of twinning
IN THE HIGH T
c
SUPERCONDUCTOR
planes to properties.
485
YBa2Cu307
macroscopic
superconducting
The authors wish to thank the SERC for the provision of neutron scattering facilities and Dr. C.C. Wilson for helpful contrlbutlons. REFERENCES
1. C.H.Chen, D.J.Werder, S.H.Liou, J.R.Kwo and M.Hong, Phys Rev. B35, 8767-8769 (1987) 2. G.van Tendeloo, H.W.Zandbergen and S.Amerlinckx, Solid State Comm. 63, 389-393 (1987) 3. M.Hervleu, B.Domenges, C.Mtchel and B.Raveau, Europhys. Lett. 4, 205-210 (1987) 4. 8.Domenges, M.HerWeu, C.MIchel and B.Raveau, Europhys. Lett. 4, 211-214 (1967) 5. E.A.Hewat, M.Dupuy, A.Bourret, J.J.Capponi and M.Marezio, Nature 327, 400-402 (1987) 6. M.A.Beno, L.Soderholm, D.W.Capone, II, D.G.Hinks, J.D.Jorgensen, J.D.Grace, I.K.Schuller, C.U.Segre and K.Zhang, Appl. Phys. Lett. 51, 57-62 (1987) 7. J.E.Greedan, A.O'Rellly and C.V.Stager, Phys. Rev. B35 8770-8772 (1987) 8. J.J.Capponl, C.Chaillout, A.W.Hewat, P,Lejay, M.Marezio, N.Nguyon, B.Raveau,
J.L.Soubeyroux, J.L.Tholence and R.Tournler, Europhys. Left. 3 1301-1305 (1987) 9. W.l.F.David, W.T.A.Harrlson, J.M.F.Gunn, O.Moze, A.K.Soper, P.Day, J.D.Jorgensen, M.A.Beno, D.W.Capone, II, D.G.Hinks, I.K.Schuller, L.Soderholm, C.U.Segre, K.Zhang and J.D.Grace, Nature 327 310-312 (1987) 10. F.Beech, S.Miraglla, A.Santoro and R.S.Roth, Phys. Rev. B35 8778-8781 (1987) 11. W.l.F.David and M.W.Johnson, Rutherford Appleton Laboratory Report, RAL-85-112 (1985) 12. A.Brokman, Solid State Comm., 64, 257 (1987) 13. E.A.Hewat, M.Dupuy, A.Bourret, J.J.Capponi and M.Marezio, Solid State Comm. 64, 517-520 (1987) 14. I.N.Khlyuatlkov and A.I.Buzdln, Advancea In Physics, 36, 271 (1987)