Raman measurements of YBa2Cu3Ox as a function of oxygen content

Solid State Communications,

Vol. 77, No. 5, pp. 367-371,

1991.

Printed in Great Britain.

0038-1098/91$3.00+.00 Pergamon Press pit

RAMAN MEASUREMENTS of YBasCusO, as a FUNCTION of OXYGEN CONTENT Gerald

Burns,

F. H. Dacoi, C. Feild, and F. Holtzberg

IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598-0218, USA (Received

29 Oct. 1990 by E. Burstein)

We report polarized Raman measurements on single crystals of YBa2Cu30,, Yl23-Ox, as a function of oxygen content. For x=7 the usual five, c-axis polarized modes are observed; mode #/5 occurs -500 cm-l and is due to the oxygen atoms in the four-coordinated Cu chain. At oxygen contents near the metal-insulator (M-i) chase transition the intensitv of mode ##5raoidtv decreases and disappeais, tiding replaced by a strongly *(zz) polarized mod& at x600 cm-‘. Our results are in sharp disagreement with published Raman results on ceramics. We believe the 600 cm--l mode is due to oxygen atom vibration in the two-fold coordinated Cu “sticks“. At the oxygen content near the M-l phase transition, other lower energy modes appear but they disappear for oxygen content xx6 where the appropriate number (five) of symmetry allowed modes are again observed. A “drastic difference” in the phonon density of states between x =6 and x=7 has been obtained from inelastic neutron measurements. We believe that much of the difference in phonon frequencies is not due to a decrease in the force constants in going from x=6, to x=7, but is due to phonon differences resulting from the coordination changes of the chain-Cu atoms.

Introduction Considerable effort has be undertaken to understand the properties of YBanCu30,, (Y123-0,) where 7 ZZX26. The accepted structure’ of the x=7 superconducting crystal is shown In Fig. 1. Although the structure, and many properties of the end members x=7 and 6 are clear, there are conflicting suggestions regarding the structure for intermediate x. Neutron diffraction experiments2s3 report a continuous variation of the structural parameters as a function of x, with a rapid variation in the Cu,-O,(b) distance (Fig. la) near the metaiInsulator (M-i) phase transition at ~~6.35. On the other hand, extra diffraction spots are observed in diffraction Y123 electron measurements on crystaliites having intermediate oxygen content-. These results have been interpreted in terms of ;;ioo,s ordered structures involving missing O,(b)

of these results9 is the continuous frequency variation for all of the modes, #l to #5, as the oxygen content varies from 7 to 6. Our results from measurements on single crystals of Y123-0, are quite different. Experimental - The Y123 single crystals were oreoared bv methods aireadv describedlO,“. The &III; oxygenated crystals (Tc=93 K) were annealed in flowing 02 first at 600°C, then for 10 days at 420°C, after which the samples were furnace cooled. Intermediate oxygen content crystals were obtained by starting with fully oxygenated crystals and annealing them in flowing He for 16 hours at various temperatures, depending on the desired oxygen content. The change of Tc, with oxygen removal, was measured with an ac susceptibility technique. The Raman measurements were carried out at 5145A on an Instruments SA S-3000 Micro-Raman Spectrometer with a diode-array position sensitive detector. The micro-Raman sample chamber allows measurements of x(zz)si scattering on the thin edges of the crystals, which yield Al, modes, as well as measurements from the larger, flat ab-plane which yield the Al, plus 61, modes7m8.

Die to its sensitivity to structural variations, Raman spectroscopy is an appropriate technique to examine the discrepancies between the neutron and electron diffraction results. For motion along the c axis there are 4A1, + Bl, Raman active modes using the tetragonal notation, or 5A, in the orthorhombic notation7s8. The motion can be approximately described as follows* (in ascending order of oscillation frequency). Ba - Ba motion. #l ##2 Cu, - Cu, motion. ##3 The 61 mode which only involves motion of the B #.4 0, - o,~:%?ls. #/5 OZ - 0, motion. There have been many reports of Raman measurements on compressed pellets of Y123-Or as a function of oxygen contents. One common aspect

Results - Figure 2a shows the x(zz)si Raman spectrum for x=7.0. These results are similar to those observed by many workersg. The 81, mode, observed with z(xx)f polarization, is dashed to indicate its position, and the modes are numbered #1 to #5 for convenience. As the oxygen content is decreased, ##5 decreases in energy and, the less intense, ##4 increases in energy making the latter difficult to resolve, as shown in Fig. 2b. The results shown in Figs. 2c, 2d, and 3 (and even the low-energy tiodes in Fig. 2b) are different 367

368

Vol. 77, No. 5

YBa2Cu30 x AS A FUNCTION OF OXYGEN CONTENT

De(a)

i

ZUc De(b) 3z

Dp(a) ~Up 3p(b) O

(a)

(b)

Fig. 1 Representations of the orthorhombic Y123 structure. (a) shows the labeling of the atom positions. A normally unoccupied position Oc(a) is indicated for discussion purposes. (b) shows the Cu coordination polyhedra.

from those reported for Raman results in oxygen deficient Y123 ceramics 9. As the oxygen content is further decreased to the insulator side of the M-I phase transition, the intensity of #5 rapidly diminishes while a sharp, intense mode appears at ~600 cm - t (Fig. 2c). Note also in Fig. 2c that there are again two low energy modes as for x = 7 . 0 , although the frequencies have shifted. Figure 2d shows the spectrum obtained when the oxygen content is further reduced. Mode #5 is absent and replaced by the 600 cm -1 line, while mode #1 and #2 remain. From Fig. 2d alone it appears that ~ also disappears for x = 6 , but this is not the case. Figure 3 shows results measured on the same crystal as in Fig. 2c but for a different Raman polarization. In Fig. 3, mode #4 at 458 cm - t is clearly visible (mode #5 is absent in all polarizations) but the 600 cm -1 mode is not visible, indicating a strong polarization dependence of m o d e #1 and the 600 c m - 1 mode 12. In fact z(xx)~ spectra in Fig. 3 does not vary as x is further decreased to x = 6 , in agreement with earlier results 13. By annealing the crystals in flowing oxygen all of the results shown here can be reversed. Also, the same effects have been observed in rare-earth substituted Y123 single crystals. Discussion - A number of observations can be made From the single crystal Raman results shown in Fig. 2. (1) Mode #5 decreases in frequency as the oxygen content is reduced from 07 to an oxygen content near the M-I phase transition. This frequency shift is similar to the results reported in ceramic samples 9. However, in sharp contrast with the ceramic results, we find that with further reduction in oxygen content this mode then rapidly decreases in intensity and disappears; it is replaced by a sharp, intense phonon at ~600 cm -1, which remains

until x = 6 . Thus, we do not see mode #5 below ~483 cm - t (Fig. 2c). We can shift mode #5 to lower energies by heating the sample as shown in Fig. 4 for a superconductor on the metallic side of the M-I phase transition. As the sample temperature is raised from 23°C to 350°C (Fig. 4a and 4b) mode #5 moves from ~486 to 470 cm -~. At the same time the ~600 cm -1 increases in intensity. This result is similar to that reported in our previous paper 14. Figure 4c shows a spectrum similar to Fig. 4b, but now the measured spot is laser heated resulting in a larger signal to noise ratio. Sample heating may cause some of the differences between our single crystal results and those of the ceramics s. (2) In the vicinity of the M-I phase transition, four lines can be detected with frequencies close to modes #1 and #2 and this can be seen in Figs. 2b and 4a. For many crystals with oxygen content near the M-I phase transition, we observe these four lines as well as both #5 and the 600 cm -1 mode. At lower oxygen contents, a two line spectrum is again observed (Fig. 2d). This two-line spectrum is also observed as the sample is heated and the 600 cm -1 mode increases in intensity (Figs. 4b and 4c). Thus, as the intensity shifts from mode #5 to the 600 cm - t mode, the low energy spectra again simplify to a two line spectrum. (3) In the vicinity of the M-I phase transition, a broad, weak feature in the 270 cm -1 region can be seen (Fig. 2c). This feature is easily observed by heating the sample 14 (Figs. 4b and 4c). However, we have not observed it in the x = 6 crystals. (4) For motion along the c axis, only five Raman modes are allowed in Y123-O 7 and they are all observed (Fig. 2a). Since the O¢(b), Cuc, and Y atoms (Fig. 1) are at centers of symmetry, they do not partake in Raman active modes 7,8. Hence, five Raman active modes are allowed in the other end

Vol. 77, No. 5

Y B a 2 C u 3 0 x AS A F U N C T I O N

12

l

i

~ # 1 149 (a, O7 1!7'[1!7[ (To = 93 K)

"-~ O

6-

E

5-

m

J

I

8

'

] J

O O

4

369

CONTENT i

YBa2Cu3OT-8= YI23-Ox #5 504 Ip x(zz)~

10

OF O X Y G E N

i

i

#2 141

YBa2Cu3OT.,~ z(xx)g

#3 342 ~

-

i

~ 06.3

(Not SC)

O3 rf-

g

2

I

I

I

I

.1487 I

02

i 0

I

I ' 200

I

I

'I'

400

I 600

Energy shift ( c m -1) P

6

(b)

126

~t [

~

06. 4

(Tc = 38 K)

I/ /

vii161145

Fill. 3 A z(xx)~ single crystal measurement of the same sample as in Fig. 2c. All mode positions are in cm-1. i

/ /

2

!

I

I

I

I

I

sl 6

602 II I/

(c) ~ 06.3 (Not SC)

p

I

/i

24 ~ ~ 141 ~ , ~ 1 2458483 7 554 II

g.. a

I

I

I

I

(d) ~ 06 (Not SO)

c

s •~

I

"'

t 6oo II

Jill

.2

IIII

c2 g 2

E

o ¢v"

I

0 0

I 200

I

I 400

I

I 600

Energy shift (cm -1) Fig, 2 (a) to (d) are x(zz)~ single crystal Raman spectra measured at room temperature for Y123 with various oxygen contents as noted. All mode positions are in cm-L For the crystals that are not superconductors, we write "Not SC', and "P" indicates a plasma line from the laser.

m e m b e r Y123-O s and five are o b s e r v e d (Figs. 2d and 3). It is only for i n t e r m e d i a t e c o m p o s i t i o n s that m o r e c o m p l e x i t y arises (Figs. 2b and 2c).

Conclusions - (1) The a p p e a r a n c e of the 600 cm -1 line and the d i s a p p e a r a n c e of m o d e #5 with decreasing oxygen content can be u n d e r s t o o d simply. The Oz-Oz v i b r a t i o n is very different when these oxygen a t o m s are part of f o u r - c o o r d i n a t e d Cu

chains (Fig. 1) and when they are part of twoc o o r d i n a t e d Cu sticks (Oz-CUc-Oz). In fact, it is difficult to imagine how this v i b r a t i o n a l frequency variation could be continuous through a change of coordination. Thus, we propose that the 600 cm -1 m o d e is associated with the Oz-Oz v i b r a t i o n in the t w o - c o o r d i n a t e d Cu sticks, while the ~500 cm -1 m o d e #5 is associated with Oz-Oz v i b r a t i o n in the f o u r - c o o r d i n a t e d Cu chains. As evidence for Oz-Oz vibration of the t w o - c o o r d i n a t e d Cu sticks being higher than 500 cm - I , we cite the results in the unusual s u p e r c o n d u c t o r Pb2Sr2YCu308. The structure of this material 15.1 has s i m i l a r O-Cu-O sticks to those found in Y123-O 6 and an intense Raman m o d e is found is at 577 cm -1, which is near the 600 cm -1 line that we observe. A n o t h e r piece of evidence is found in IR measurements. There is a ,-075 cm -1 difference between the 648 cm -1 IR mode assigned to the Cuc-Oz stretch in Y123-O s single crystal m e a s u r e m e n t s 17 and the c o m p a r a b l e e i g e n v e c t o r m o d e at 572 cm -1 m e a s u r e d in Y123-O 7 c e r a m i c s 18. (2) We p r o p o s e that the four l o w - e n e r g y lines (Fig. 2b), o b s e r v e d in the vicinity of the M-I phase transition, with frequencies near m o d e s #1 and #2, are due to Ba-Ba and CUp-CUp, each in t w o different environments. These atoms can be in a stick-only o r in a f o u r - c o o r d i n a t e d Cu-chain-only e n v i r o n m e n t . That is, Ba atoms that are in a stick-only environi'nent have one vibration frequency w h i l e those in a f o u r - c o o r d i n a t e d Cu-only e n v i r o n m e n t have a slightly different vibrational frequency. From this proposal and that in conclusion (1), it w o u l d seem that the chain Cu atoms have pred o m i n a n t l y either two- o r four-fold c o o r d i n a t i o n . We see no m o d e s that are associated with three-fold c o o r d i n a t e d Cu atoms. Further, since the low energy lines (near #1 and #2) are relatively sharp, we conclude that there are p r e d o m i n a t e l y long rows of either two- o r four-coordinated Cu a t o m s and our low energy Raman signals are coming f r o m regions that have only two- o r four-coordinated Cu atoms. This is consistent with a proposal of Reyers et al s. .(3) The origin of the broad features at ~270 cm -1is" uncertain. However, near the M-I phase change there must be considerable d i s o r d e r due to some. regions with rows of t w o - c o o r d i n a t e d and some with rows of four-coordinated Cu a t o m s in the chains. The features in this region are possibly diso r d e r induced spectra o r possibly v i b r a t i o n s from the cross-over regions. Also, we o b s e r v e these features with m o r e intensity at higher t e m p e r a t u r e s

370

YBa2Cu30x AS A FUNCTION OF OXYGEN CONTENT 8

l

,

l

,

i

Y 123-O6.5 6

x(zz)~

II I/

4

(a)

1 16.4

T=23°C

/ I

/ 144.6

o

I

J

I

3

I

(b) T = 3 5 0

1

I

•m

E;

t 548 . . .

I

I

°C

,20.4

Z"

(Fig. 4) w h e n the e x t r a o x y g e n d o e s not h a v e t i m e to l e a v e the s a m p l e 14. Thus, t h e s e f e a t u r e s m a y be a s s o c i a t e d with a d i s o r d e r - i n d u c e d e n v i r o n m e n t of the p o s i t i o n s of t h e s e e x t r a o x y g e n a t o m s 14. (4) The end m e m b e r Y123-O 6 has o n l y 4 A l g + Big R a m a n a l l o w e d m o d e s 7,8 for v i b r a t i o n s with m o t i o n a l o n g the c axis; the actual m o t i o n s a r e des c r i b e d in the I n t r o d u c t i o n . In a g r e e m e n t w i t h the group theory for x=6, we observe only five modes (Figs. 2d a n d 3). A d d i t i o n a l l y , w e find that t h e B a - B a a n d 600 cm -1 m o d e s a r e s t r o n g l y (zz) p o l a r i z e d .

,

~ 486.0

Te ~ 50 K

470

1 596

We conclude, f r o m o u r s i n g l e c r y s t a l results, that b o t h end m e m b e r s Y123-O 7 a n d Y123-O 8 s h o w the c o r r e c t n u m b e r of c a x i s p o l a r i z e d R a m a n lines (five). H o w e v e r , the h i g h e s t f r e q u e n c y l i n e s a r e f r o m v i b r a t i o n s of Oz a t o m s in r a t h e r d i f f e r e n t e n v i ronments. F u r t h e r , f o r i n t e r m e d i a t e o x y g e n concentrations, particularly near the metal-insulator p h a s e t r a n s i t i o n , the s i t u a t i o n is m o r e c o m p l i c a t e d . We p r o p o s e that t h e o b s e r v e d l o w e n e r g y m o d e s , n e a r #1 a n d #2, a r e a s s o c i a t e d w i t h a t o m s v i b r a t i n g in e i t h e r a t w o - c o o r d i n a t e d c h a i n - C u e n v i r o n m e n t or a four-coordinated chain-Cu environment. Inelastic n e u t r o n d i f f r a c t i o n has b e e n used to m e a s u r e the p h o n o n d e n s i t y of states, F((~), of h i g h Tc m a t e r i a l s . " D r a s t i c d i f f e r e n c e s " a r e f o u n d 19,2° b e t w e e n F(~) in Y123-O 6 a n d Y123-O 7 w h i c h a r e exp l a i n e d in t e r m s of d e c r e a s e s in t h e f o r c e c o n s t a n t s in g o i n g f r o m x = 6 t o x = 7 . T h e s e c h a n g e s in F(~) a r e m u c h l a r g e r t h a n o b s e r v e d in, f o r e x a m p l e , t h e d o u b l e l a y e r e d B i - m a t e r i a l , B i ( n = 2 ) 19,2°. O u r results i n d i c a t e that the h i g h e s t f r e q u e n c y m o d e s in x=6 a r e d u e to o x y g e n a t o m s in t h e t w o c o o r d i n a t e d c o p p e r c h a i n s (600 a n d 648 cm -1 in R a m a n a n d IR ~7, r e s p e c t i v e l y ) . F o r x = 7 crystals, t h e c o r r e s p o n d i n g m o d e s a r e f r o m o x y g e n a t o m s in t h e f o u r - c o o r d i n a t e d c o p p e r c h a i n s ( ~ 500 a n d 572 cm -1, r e s p e c t i v e l y ) . It a p p e a r s to us that t h e d r a s t i c d i f f e r e n c e s f o u n d by i n e l a s t i c n e u t r o n s c a t t e r i n g in Y123 a r e p r e d o m i n a t e l y d u e to this c h a n g e of o x y gen c o o r d i n a t i o n of t h e h i g h e s t e n e r g y m o d e s a n d not to a g e n e r a l s o f t e n i n g of the f o r c e c o n s t a n t s in g o i n g f r o m x = 6 to x = 7 .

A

t\ ) 1

I

I

t

I

P

596

16

o

(C) Te,t ~ 3750C

~-~ 12

124.2

II

PIA 4 0 . 2

'~

O n~

|

0

I

47n

I

I

/I

I

200 400 Energy shift (cm -1)

Vol. 77, No. 5

I

600

Fig. 4 Single crystal x(zz)~ Raman spectra measured on the crystal edge. The measurements are at various temperatures as indicated. In (c), the heating is obtained by the laser beam itself. See reference 14 for the determination of Test. All mode positions are in cm -1.

References 1.

2. 3. 4. 5.

G. Burns and A. M. Glazer, "Space Groups for Solid State Scientists" (Academic Press, 1990). R. Beyers and T. M. Shaw, "Solid State Physics" (Academic Press, 1989) Vol. 42, page 135. K. Yzon and M. Frances, Z. Phys. B 76, 413 (1989), R. M. Hazen, "Physical Properties of High-Tc Superconductors" Ed. D. M. Ginsberg (World Scientific, 1990), Vol. 2, page 121. J.D. Jorgensen, B. W. Veal, A. P. Paulikas, L. J. Nowicki, G.W. Crabtree, H. Claus, and W. K. Kwok, Phys. Rev. B 41, 1863 (1990). R. J. Cava, A. W. Hewat, E. A. Hewat, B. Batlogg, M. Marezio, K. M. Rabe, J. J. Krajewski, W. F. Peck Jr. and L. W. Rupp Jr, Physica C 165, 419 (1990). C. Chaillout, M. A. Alario-Franco, J. J. Capponi, J. Chenavas, J. L. Hodeau, and M. Marezio, Phys. Rev. B 36, 7118 (1987). C. Chaillout, M. A. Alario-Franco, J. J. Capponi, J. Chenavas, P. Strobel, and M. Marezio, Solid State Commun. 66, 283 (1988).

6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

R. Beyers, B. T. Ahn, G. Gorman, V. Y Lee, S. S. P. Parkin, M. L Ramirez, K. P. Roche, J. E. Vazquez, T. M. G0r, and R. A. Huggins, Nature 340, 619 (1989). G. Burns, F. H. Dacol, P. Freitas, T. S. Plaskett, a,nd W. KOnig, Solid State Commun. 64, 471 (1987). R. Feile, Physica C 159, 1 (1989). For example, see the six references in the caption of Fig. 4 in the review paper, reference 8. F. Holtzberg and C. Feild, Eur. J. Solid State Inorg. Chem. 27, 107 (1990). F. Holtzberg and C. Feild, J. of Crystal Growth, 99, 915 (1990). G. Burns, F. H. Dacol, C. Feild, and F. Holtzberg, Physica C, submitted. G. Burns, F. H. Dacol, F. Holtzberg, and D. L. Kaiser, Solid State Commun. 66, 217 (1988). G. Burns, F. H. Dacol, C. Feild, and F. Holtzberg, Solid State Commun. 78, 893 (1990). R. J. Cava, B. Batlo.qg, J. J. Krajewski, L. W. Rupp, L. F.

Vol. 77, N o . 5

Yl,;12Cu3(): AS A FIINCTION OF ()XY(:I:.N CON'L'I'~NT

Schn9emeyer, T. Siegrist, R. B. vanDover, P. Marsh, W. F. Peck, Jr., P. K. Gallagher, S. H. Glarum, J. H. Marshall, R. C. Farrow, J. V. Waszczak, R. Hull and P. Trevor, Nature 136, 211 (1988). 16. C. Thomsen, M. Cardona, R. Liu, Hj. Mattausch, W. Konig, F. Garcia-Alvarado, B. Suarez, E. Moran and M. AlarioFranco, Solid State Commun. 69, 857 (1989). 17. M. K. Crawford, G. Burns, and F. Holtzberg, Solid State Commun. 70, 557 (1989).

371

18. M K. Crawford, W. E. Farneth, E. M. McCarron, III, and R. Bordia, Phys. Rev. B 38, 11382 (1988). 19. B. Renker, F. Gompf. E. Gering, G. Roth, W. Reichardt, D. Ewert, H. Rietschel, and H. Mutka, Z. Phys. B 71, 437 (1988). Ibid, 73, 309 (1988). Ibid, 77, 65 (1989). 20. L. Pinlschovius, in "Phonons 89", Eds. S. Hunklinger, W. Ludwig, and G. Weiss (World Scientific, 1990), p. 217.