Cation coordination and oxygen vacancies in mixed oxide perovskites

SOLID STATE

Sohd State lomcs 53-56 (1992) 573-577 North-Holland

IOHICS Cation coordination and oxygen vacancies in mixed oxide perovskites J.T. Vaughey, E.F. H a s t y a n d K . R . P o e p p e l m e i e r Department ol Chemt~trv and the Sctence and Technology Center Jot Supe~condta ,vail', ,\o~lhwe~ter,7 Unt~er~tty. Evanston, IL 60208, USA

Single phase polycrystalhne LaSrCuGa~_,Zn~O5has been prepared with 15% substltutmn of gallium b.~ zinc The structure consists of neath planar [CuO_,] planes separated by tetrahedral [GaO4/e] chains Subst~tutmnwas found b) neutron diffraction to occur equally on both plane and chain sites despite their &fferent coordmatmn en*~ronments

1. Introduction The concept that a significant fraction of oxygen atoms may be removed by reduction from metallic oxide latttces and not have the vacancy e h m l n a t e d by crrystallographic shear was first proposed [1,2] debated [3] and studied over thirty years ago The perovsklte structure has been shown to persast over the composttlon range ABO3_,, 0 ~<.v~<0.5 for m a n y transition metal and some m a i n group metal cations. The first perovsklte c o m p o u n d s recognized [4,5] to have ordered oxide ion vacancies were CaeFe,O5 [6 ] and CaeAIFeOs [7]. Other superstructures based on the perovsklte StlUCture and oxide ion vacancies are known for manganese [8], iron [9]. cobalt [10], nickel [ 11 ] and copper [ 12 ]. Soon after the discovery of superconductivity in Y - B a - C u - O [13,14] system in 1987 the oxygen deficaent perovsklte YBaeCu~O~ _, with a structure [ 15,16 ] similar to that of La3Ba3Cu60]35+ ~ [17] was ldentafied to be the superconductor. In general the particular superstructure adopted depends on the electronic configuration (d ~) and corresponding coordination of the smaller B-site cation and the mntc radius of the larger, electroposmve A-cation [ 18,19 ]. In this paper we revaew the crystal structures and properties of the new phases LaSrCuAIOs [20]. LaSrCuGaO5 [21] and LnSr,To whom correspondence should be addressed

C u : G a O j [22]. all of which incorporate features c o m m o n l y seen in superconducting copper oxides, such as a close relatlonshap to the perovsklte structure and copper-oxygen planes In each example the gallium or a l u m i n u m cation has tetrahedral coordination by oxygen. Therefore zinc substltUtton of the tetrahedral site versus the copper plane sate might be expected. In contrast there is no aprtorl reason to expect chain site substitution over plane site substttutaon in YBaeCu307 and It has been shown that zinc substitutes primarily in the copper plane sites wtth the rapid loss of superconductivity [23], We have studied the site preference of zinc in LaSrCuGaOs by n e u t r o n diffraction.

2. Description of the structures 2 1 L a S , CuAIO~

The orthorhomblc structure of LaSrCuAIO5 (centrosymmetrlc space group P b c m ) is shown in fig 1, It should be vmwed as an oxygen deficient perovskate. The copper ions in the structure have very, dtstorted CuOo~ octahedra, so much so that four short (1.95 ( 2 × ) and 1.93 ( 2 × ) A) CuO bonds condense to CuO4 '2 planes. These planes are modulated along the b-axis The a l u m i n u m actions adopt tetrahedral coordination and the tetrahedral groups join corners to form chains that run throughout the struc-

0167-2738/92/$ 05 00 © 1992 Else~mrSoence Pubhshers B V All rights reserved

574

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~"

? 1

~l, ~ ~ : ]

;"~ ,

\,,~:-

A:(La/Sr)O // A:(La/Sr)O

',, 1

/

,

,

I'

-.<./)'

A~O ,,/'~'- / l A:(La/Sr)Ocuo2~-:.j~.3.-~

GaO

[

//

// \\_

Fig 2 The structure ofLaSrCuGaOs vmwcd down the ~-a,,ls F~g 1 The structure of LaSrCuA1Os wewed down the t-ax~s

ture parallel to the c-ax~s. The connectivity m the A102/2 layer ~s b r o k e n by the missing oxygen atoms that would, ~f present, give the n o r m a l perovsk~te structure and c o m p o s m o n ABO3. The La and Sr atoms are d~stnbuted unequally (69 31 ) over two e~ght and nine coordinate nonequivalent A-sttes, w~th the larger Sr ion preferring the more highly c o o r d i n a t e d s~te.

2 2 LaSrCuGaO: The o r t h o r h o m b l c structure of LaSrCuGaO~ ( n o n c e n t r o s y m m e t n c space group I m a 2 ) ~s shown m fig 2 The structure of LaSrCuGaO5 is closel~ related to that of Ca2FeA105 or Ca2Fe20~. C o p p e r adopts octahedral c o o r d m a t m n w~th corner shared oxygen atoms to form two-dimensional [ C u O , ] planes The axml c o p p e r - o x y g e n b o n d s are 2 48 /k ( × 2), whde the equatorml oxygen distances are 1 89 2X ( X 2) and 1 96 ,~. ( X 2). C o p p e r is u n s y m m e t n calIy b o u n d to the four near neighbor oxygen atoms w~thm the copper-oxygen plane w~th alternating 1 96 ,~ and 1.89 A C u - O ( l ) b o n d s throughout the CuO~,~ "'square" net The two d i m e n s i o n a l [ C u b 2 ] planes are separated by tetrahedra o f g a l h u m and oxygen that repeat their orientation in every other layer In contrast to the ordered covalent B O 4 / : framework, the large electroposit~ve l a n t h a n u m and stron-

tlUm A-cations are staUst~cally d l s m b u t e d over one s~te The A-cation site has a coordination n u m b e r of e~ght and ~t ~s displaced from the vacant oxygen site m the g a l h u m - o x y g e n layer. The l a n t h a n u m / s t r o n t r a m - o x y g e n b o n d lengths vary. from 2,48 A - 2 92/k C a n o n s of similar size and oxidation state have been observed statistically d~stnbuted over one s~le m a large n u m b e r o f perovsk~te compounds.

2 3 LnSr~Cu:GaO-, (Ln=La-I'b, Y) The o r t h o r h o m b l c structure of L n S r 2 C u 2 G a O 7 ( n o n c e n t r o s y m m e t r l c space group I m a 2 ) is similar to YBa2Cu30, and is sho,~n m fig 3 The square planar copper chains m YBa2CuO7 are replaced by chains of corner-shared GaO4/~ tetrahedra, This replacement creates a large supercell of the ideal, cubic perovskite structure (see table 1) The large lanthanldes and Sr are &strxbuted over the A and A ' cat~on sites but the small lanthamdes and y t t r m m occup~ onl~ the A' site between the copper oxide planes within the double C u b e layer The copper coo r d i n a t m n for the L n = L a ~+ and Ho ~+ c o m p o u n d s was found to be square p y r a m i d a l w~th four short in plane distances averaging 1 96 and 1 94 ,~ and one long apical b o n d of 2 34 and 2 37 A respectively C o m p a r e d to LaSrCuMO5 [ M = A I ~+, Ga ~+ ] with one C u b : plane and an AI, Gab4/2 chain, the two layer structure has amon vacancies m both the G a b 4 : : chain and the Y-layer

J T Vaughey et al/Oxygen vacanctes m mlaed o:~lde perovskttes 2~ c,

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GaO r~

A:

SrO CuO 2

A:

,~I

L 12/

c, ~j

~.,-~_

Ln/Y CuO 2

0

0

O

Fig 3 The structure of LnSr2Cu2GaO7 viewed down the b-axis

3. Experimental A polycrystalhne sample of LaSrCuGa~_,Zn~Os ( x = 0 . 1 5 ) was prepared by heating stoichxometrtc amounts of La203 (Aldrich, 99.99%), SrCO3 (Aidrich, 99.99%), Ga203 (Aldrich, 99.99%), ZnO (Aldrich, 99.99%), and CuO (Aldrich, 99.999%). The sample was heated at 980 °C for two months with intermittent grindmgs. A time-of-fhght data set was collected at room temperature and ambient pressure at the Intense Pulse Neutron Source (IPNS) facihty at Argonne National Laboratory. Approximately 8 g of the sample was contained m a thin walled vanadium can. Data were collected for six hours. The data

575

from 0 568 A to 2 8931 A were used to refine the structure The unit cell was determined using a nonhnear least squares Marquet method after Bevmgton [24,25] to fit the peaks to an exponential rise and fall convoluted into a gausslan shape which is charactenstlc of the spallat~on neutron source [26]. The sample was essentially single phase, with a small unidentified ( < 1%) impurity. A umt cell was determined from the peak positrons using the program THEOR [27 ] and refined with the program POLSQ [28] to a 16.517(3) A × 5 . 5 1 8 ( 2 ) A × 5 350(6) A orthorhomblc cell. The lndexatlon revealed that body centering was one of the reflection condlttons The structure of LaSrCuGao ssZno j505 was solved by using a umt cell and the non-centrosymmetnc space group lma2 (#46) similar to LaSrCuGaO5 [21 ] as a starting model. The structure was refined by the Rletveld method [29]. The scattering lengths used were 8,24, 7.02, 7.718, 7.288, 5.680, and 5 804 fm, for the lanthanum, strontmm, copper, gallium, zmc and oxygen atoms, respectively. Thirty-nine parameters were refined. In the final cycle all parameters were allowed to refine undamped, including the scale factor, six-peak shape parameters, five background parameters, the umt cell parameters, posmonaL and ~sotropic thermal factors, the zero point s h i f t , the diffractometer constant, as well as the absorption and e x t i n c t i o n parameters. The a t o m i c p o s i t i o n s a r e shown in table 2. The final R-factor was 4 6% (7.4 Rwtd). The observed and calculated diffraction patterns and difference plot from 0.55 A ~
Table 1 Comparison of lattice parameters

LaSrCuGaO5 LaSrCuAIO5 LaSrzCu2GaO7

a

b

~

V a)

(A)

(A)

cA)

IA3)

16.383 (4ap) 7 922 C a p ) 23 143 (6ao)

5 533 (vF2-ga~) 11 020 (2x/2a,)~" 5.566 (x/2ao)r-

5 330 (x/-2ap) 5.424 ( ~ 2 a p ) 5 465 (\/-2a o)

60 4 59 2 58 7

~ Volume per one poovsk~te formula unit

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Table 2 Atomic posmons for LaSrCuGao s~Zno ~Os ~ktom

Wyckofl" s~mmetry

x

l

z

B

O¢c

La/Sr ('u Zn ( 1 ) Ga Zn(2) O(1 ) O12) O(3)

8c 4a 4a 4b 4b 8c 8c 4b

0 6074( 1 05 05 075 075 0 4945( 1 0 6484( 1 075

00123(3) 00 00 0 5666(4) 05666(4) 02492(3) 00491(3) 0617316)

00378(2) 0 5402(8 ) 0 540218) 00 00 0791819) 05617(7) 0639717)

05112) 0 28(4) 0 28(4)

I0 0 92( I ) 0 08( I ) 093(1) 007(1) 10 I () I1)

Space group lma2 (~46 w l t h a = 1 6 5 2 6 ( l ) A , h = 5

071(7)

07117) 05413) I 1915) 09316)

5177(5) -~,~=5 3523(5)

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Fig 4 0 b s e r ~ ed ( + ), calculated (sohd hne ). and difference ( bottom ) neutron dlflracUon pattei n for LaSrCuGaL at room temperature The nc marks indicate the posmons of the Bragg tefelctmns

,Zn,O~ ( ~ = 0 15 )

4. Results and discussion

t a h e d r a l p l a n e site. T h e z i n c is e q u a l l y d i s t r i b u t e d

The doped compound LaSrCuGaL_,Zn,O> (a = 0.15) was found to be ldenUcal to LaSrCuGaO~ [21 ] e x c e p t f o r t h e a v e r a g e l e n g t h e n i n g o f t h e t e t r a -

o ~ e r b o t h s i t e s W e p r e s u m e t h a t c o p p e r is d i s p l a c e d f r o m t h e o c t a h e d r a l site t o t h e c h a i n s R e t o o , b u t b e cause copper and galhum have similar scattering lengths we did not attempt to add this to our model

hedral (Ga/Zn)O4/2 b o n d s f r o m 1 . 8 3 1 1 ) /k t o 1 8 8 ( 1 ) A. a n d a s m a l l i n c r e a s e m t h e u m t cell exp e c t e d f o r t h e s u b s t i t u t i o n o f Z n 2+ ( 0 . 6 0 fk) f o r G a 3+ (0.47 ~). Surprisingly there was no preference for t h e t e t r a h e d r a l c h a i n s~te v e r s u s t h e ( d i s t o r t e d ) oc-

5. Conclusion W e h a v e f o u n d t h a t t h e z i n c t o n ( Z n ~+ ) d o e s n o t

J T Vaughey et al/Oxygen vacanctes m m t x e d oxtde perovskttes

substitute preferentmlly on the tetrahedral cham stte m LaSrCuGaO~ despite the very different coordlnatron environments of the two B-cattons sttes in thts structure. Therefore doping with zinc to oxldtze or hole dope the copper oxygen planes does not appear to be substantmlly different than m the other cuprates where tt is well known that small amounts of transmon metal substltUt~on result m loss of superconductivity.

Acknowledgement Support of this research mcludmg use of the Intense Pulsed Neutron Source at Argonne Nattonal Laboratory was from the Sctence and Technology Center for Superconduct1vity (NSF DMR-8809854) and the Summer Program in Solid State Chemtstry for Undergraduate Students and College Faculty, NSF DMR-8905605 (E.F.H.). References [ I ] G H Jonker, Physica 20 (1954) 1118 [2] M Kestlglan, J G Dlckenson and R. Ward, J. Am Chem, Soc 79 (1960) 5598 [3] S Anderson and A D Wadslev, Nature 187 (1960) 499 [4] H Watanabe, M Suglmoto, M. Fukase and T Hlrone, J Appl Phys 36 (1965)988 [5] S Geller, R W Grant, U. Gonser, H Wlederslch and G P Espmosa, Phys. Len 20 (1966) 115 [ 6 ] E F Bertaut, P Blum and A Sagnleres, Acta Crystallogr. 12 (1959) 149 [7]WC, H a n s e n a n d L T Brownmfller, Am J Scl 15 (1928) 224 [8] K R Poeppelmeler, M E Leonowlcz, J C Scanlon, J M Longo and W B Yelon, J, Sohd State Chem 45 (1982) 71 [9] J C Gremer, M Pouchard and P Hagenmuller, Struct Bonding47 (1981) 1 [ 10] K Vldyasagar, J Gopalaknshnan and C N R Rao, Inorg Chem 23 (1984) 1206.

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[ 11 ] K Vldyasagar, A Relier, J Gopalakrlshnan and C N R Rao, J Chem Soc, Chem Commun 7 (1985) [12]C Mlcheland B Raveau, Rev Chlm Miner 21 (1984) 407. [13]MK, Wu, J R Ashburn, C J Torng, P H Hur, R L Meng, L. Gao, Z J Huang, Y Q. Wang and C W Chu, Phys Rev Lett. 58 (1987) 908 [ 1 4 ] P H Hur, L Gao, R L Meng, Z J Huang, K Forster, J Vasslhous and C W Chu, Phys Rev Len 58 (1987) 911 [ 15 ] M A Beno, L Soderholm, D W Capone, J D Jorgensen, I,K Schuller, C U Segre, K Zhang and J.D Grace, Appl. Phys Lett 51 (1987)57 [ 1 6 ] R J Cava, B Batlogg, RB van Dover, D W Murphy, S Sunshine, T Slegnst, J P Remelka, E A ReUman, S Zahurak and G P Esplnosa, Phys Re'¢ Lett 58 (1987) 1676 [ 17] L Er-Rakho, C Michel, J Pro'~ost and B Raveau, J Solid State Chem 37 (1981) 151 [18] J C Gremer, J Parnet and M Pouchard, Mat Res Bull 11 (1976) 1219 [ 1 9 ] K R Poeppelmeler ME l_eonowlcz and J M Longo, J Solid State Chem 44 (1982) 89 [20] J B Wiley, M Sabat. S-J Hwu, K R Poeppelmeler, A Relier and T Williams, J Solid State Chem 87 (1990) 250 [21 ] J T Vaughey, J B Wiley and K R Poeppelme~er, Z Anorg Allg Chem 599/600 (1991) 343 [ 2 2 ] J T Vaughey J P Thlel, E F Hasty, DA, Groenke, C L Stern, K R Poeppelmeler, B Dabrowskl, D G. Hmks and A W Mitchell, Chem Mater 3 (1991) 935 [23] R Jones, P P Edwards. M R Harrison, T Thanyasm and E Smn, J A m , Chem Soc 110(1988)6716 [24] P.R Bevmgton, in Data Reduction and Error Analysis for the Ph~ slcal Sciences ( McGraw-Hill, New York, 1963 ) [25] J P Thlel and K.R Poeppelmeler, Neutron Fit Program Dept of Chemistry Northwestern University, Evanston, IL, 1990) [26] F Rotella, Users Manual for Rletveld Analysis at IPNS [27] P E Werner, TREOR (Umverslty of Stockholm, Sweden, 1984) [28]D Kezler and J Ibers, Modified POLSQ (Dept of Chemistry, Northwestern Umverslty, Evanston, IL, 1983 ) [29] R B Von Dreele, J D Jorgensen and C G Windsor, J Appl Crwst 15 (1982) 58l