Structural stability and doping in R2CuO4 (R = rare-earth)

PhysicaC 165 (1990) 357-363 North-Holland

STRUCTURAL

STABILITY AND DOPING IN R,Cu04 (R=RARE-EARTH)

Y.Y. XUE, P.H. HOR, R.L. MENG, Y.K. TAO, Y.Y. SUN, Z.J. HUANG, Texas Centerfor Superconductivity, University ofHouston, Houston, Texas, USA

L. GAO and C.W. CHU

Received 3 November 1989 Revised manuscript received 18 December 1989

Several new compounds have been synthesized. By analyzing the new compounds and reviewing data from existing compounds, we conclude that the atomic radius of R in R2Cu04 dictates the stability of and the type of doping permitted in the various homomorphies of the compounds. Our efforts have yet to induce a direct electron-to-hole HTS transition or to achieve 90 Kelectron HTS.

1. Introduction Three distinctly different and yet closely related structures have been reported in the 214 compound system R2Cu04 with R=rare-earth, depending on the specific R or dopant in the compound [ l-71. They are the so-called T-, T’-, and T*-phases and are shown in fig. 1. While the T- and T*-phases possess apical oxygen atoms, which are associated respectively with the CuO,-octahedra and the CuO,-squarepyramids, the T’-phase does not possess apical oxygen atoms and consists of only the square-planar CuO,-arrangement. Metallic behavior can be induced in these compounds by doping over a certain homogeneity range. Upon cooling, some become superconducting [ 8,9] with a transition temperature T

T’

T*

La

La

R

Fig. 1.The T-La$uO.,, T’-RzCu04 and T*-phases ( LaR’ )Cu04. 0921-4534/90/$03.50 0 Elsevier Science Publishers B.V. ( North-Holland )

T, up to 35 K. Hall measurements show that the carriers are holes in the T- and T*-phases [ lo,11 1, but electrons in the T’-phase [ 91. The difference has been attributed to the presence of the apical oxygen atoms near the CuOz-layers in the T-phase and their absence in the T’-phase [ 12,131. Many theoretical models have been advanced to account for the high temperature superconductivity HTS observed in the oxides. Some of them [ 141 are characterized by a charge symmetry whereas others [ 151 are not. Recently, a normal-insulator superconducting tunneling asymmetry has been proposed for the hole-pairing mechanism of superconductivity [ 16 1. This raises the possibility of a superconducting analogue with a semiconducting device. Therefore, the study of electron-HTS and the search for electron-HTS above 77 K will be of great interest both scientifically and technologically. To gain insight into the above problems, we chose to examine the structural stability and the dopability by electrons or holes in the T-, T’- and T*-phases. We succeeded in synthesizing many compounds with the T’- and T*-structures [ 171. By analyzing the new compounds and reviewing the data of the existing compounds, we found that the atomic radius R critically determines the stability and the type (i.e. hole or electron) of doping of a specific phase. The results also suggest that it is very difficult, if not impossible, to carry out a continuous transition from the holetype to the electron-type in the T*-phase by doping.

Y. Y. Xue et al. /Structural stability and doping in R2Cu0,

358

.4ttempts to achieve electron-HTS not yet been successful.

above 77 K have

2. Results and discussion 2.1. T-phase All three homomorphies of R2Cu04 can be pictured as a layered perovskite structure. A T-type R2Cu0, is very much like the cubic perovskite structure AB03, except that one slab of R2Cu04 is shifted by (a/2) ( 1, 1, 0) with respect to the one below. Consequently, similar to the cubic perovskite AB03, the structural stability of R2Cu0, depends on the proper matching of the CuOZ- and RO-layers as shown in fig. 2 for the T-phase of R2Cu04 [ 4,6]. For perfect matching, the ratio between the interatomic distances a=(A-O)/,/?(B-0) or (R-O)/ 1s one. A certain deviation from such a Jz(Cu-0) perfect matching has been found to be acceptable, i.e. 0.85 <(Y< 1.02 [ 18,191. The interatomic distances (R-O) and (Cu-0) depend on the atomic radii and the valences of R and Cu. In general, the (R-O) bond is more rigid than the (Cu-0) bond. The latter can be adjusted more easily through a Cuvalence change or a pseudo-Jahn-Teller distortion of the Cu06-octahedron. La2Cu04 crystalizes in the Tphase with an a~0.83. This shows that La&uO, is already at the critical point of structural stability. In fig. 3, we have plotted (Cu-0) / (Cu-O),, versus (R0) / (R-O),,,, for R = La, with Ba- or Sr-doping where the ( CU-O)~,, and (R-O),,, are the theoretical lengths of the respective bonds in their unstrained

Fig. 2. The projections of R$uO, on the a-b plane: (a) T’-phase; (b) T*-phase: ( ) Cu and ( o ) oxygen in the CuOz layer; (0 ) R and (0 ) oxygen in the RO layer. l

-O),, vs. (R-O)/ (R-O),, for the T-phase: Fig. 3. (Cu-O)/(CU’~ ( A ) LazCu04. ( 0 ) (La, Sr)#ZuOI and ( 0 ) (La, Ba )#ZuO,.

state with appropriate coordination numbers (the (Cu-O),, and (R-O),, have been corrected for the Cu valence-change and the R size-change due to Srand Ba-doping) [ 201. Bond-lengths were obtained based on the neutron and X-ray data with an estimated uncertainty between 1OP3 and 1O-‘; however, the relative uncertainty is much smaller [ 2 11. When either of these normalized quantities is greater than one, the corresponding bond is under tension; when either of them is smaller than one, the corresponding bond is under compression. Therefore, it is evident that the (La-O) bond in La,CuO, is under tension while the (Cu-0) bond is under great compression. The absence of any T-phase in R2Cu04 with any other R’s can thus be understood; their smaller-thanLa size makes the T-structure no longer stable. To improve the matching in La$ZuO_,, i.e. to enhance (Y, one may deform the CuO,-octahedron to an elongated form known as the pseudo-John-Teller distortion, lengthen the (La-O) bond. and/or shorten the (Cu-0) bond. The bond-length of (La0) can be lengthened through proper doping and that of (Cu-0) can be shortened by increasing the Cuvalence and/or the (Cu-0)-covalency, or by tilting the CuOb-octahedra. In fact, a pseudo-John-Teller distortion and a freezing of the tilt-mode of the CuOboctahedra have already occurred naturally in La*CuO, to make it more stable. Any attempt to induce metallic behavior in La,CuO, through doping can be achieved only without degrading the critical structural stability. In other words, one cannot reduce the (La-O) bond-length

359

Y. Y. Xue et al. /Structural stabilityand doping in R2Cu04

and/or increase the (Cu-0) bond-length upon doping. The substitution of La by any other trivalent rareearths which are all smaller than La will invariably lead to a reduction in the (R-O) bond-length and thus to an eventual collapse of the T-phase, which is consistent with the experimental observation. On the other hand, the partial replacement of La by a divalent alkaline earth element D= Ba, Sr, or Ca results in an increase in the (La-O) due to the greater radius of D (even in the case of covalent Ca [ 22 ] ) and, more importantly, a decrease in (Cu-0) due to the increase of the Cu-valence. Such a partial La-replacement by D, corresponding to a hole-doping, is thus allowed. The accompanying structural stability improvement is evident from fig. 3 [ 221. It should be noted that a local shrinkage of the chemical bond between Cu and the apical 0 near the dopant site (e.g. Sr) has been detected in (La, _Sr,) &u04 [ 23 1, suggesting a possible charge inhomogeneity in these superconductors. It is also interesting to point out that there appears to be a drastic drop in the Cu-0 (apical) bond-length with x between 0.08 and 0.1, as shown in fig. 4, near the beginning of the disappearance of superconductivity [ 241. This may signal a doping-induced charge transfer, which appears to be different from that previously reported in other HTS systems [25,26,27], near the metal-insulator phase boundary.

since La2Cu04 is already at the critical point of structural instability due to the large tension experienced by the (La-O) bond. One may rotate these bonds by 45” with respect to the (Cu-0) bond, as shown schematically in fig. 2, to improve the match of (La-O) with (Cu-0) and achieve the atomic arrangement of the T’-phase. As a result of this rotation, the (R-O) bond in the T’-phase is under compression in contrast to the T-phase for La2Cu04. La becomes too large to form a T’-LazCu04. Smaller R’s are thus more favorable for the T’-R2Cu04, consistent with the experimental observations of T’phase with R=Pr, Nd, Sm, Eu, and Gd. For these smaller R’s, the oxygen atoms tend to move toward the space between the R-layers due to the stronger attraction associated with R and due to the conservation of charge. As shown in fig. 5a, within the ex-

P,(o

1.01

Nd

t

2.2. T’ -phase It was pointed out above that the T-phase cannot be stabilized in pure R2Cu04 if R is smaller than La,

2.4 c-z

.

I

0 N

g

+5

1.00 -

"7 4 P 2.3

2 : 2.

P 1 z.

Q--

_

x increase

2.2 I

Fig. 5. (Cu-O)/ Fig. 4. [Cu-O(apical)] and Sr.

bondvs.

xin (Lal_xDx)2Cu04for

D=Ba

(Cu-O),,,

vs. (R-O)/ ( R-O),h for the T’-phase. and (0) R1.85-Ce0.15Cu04 (Nd2_xCe,)Cu0,,0~x~0.3.

(A) (a) (0) R2Cu04, (LaR,,.85Ce0.15)CuO~. (b) (0)

360

Y. Y. XUP et al. / Siruciural

perimental resolution. the (R-O) bond is mainly under compression and the (Cu-0) bond is either under tension (R= Nd and Pr) or slight compression (R=Eu and Gd) in these compounds. The stretched (Cu-0) bond corresponds to a lower Cuvalence. Hole-doping of the T’-R,Cu04 by partial replacement of R with the divalent Ba or Sr tends to enhance the compression in the (R-O) bond and to cause a structural instability which has been observed by us and others [ 28 1. For instance, T-phase appears simultaneously with the T’-phase in a Srdoped T’-NdzCu04. On the other hand, electrondoping of T’-R&uO, with Ce or Th, reduces the stress in the (R-O) bond without destabilizing the T’-phase, accompanied by an increase in the (Cu0) bond-length. This is demonstrated in fig. 5b for (Nd,~..Ce,),CuO,. The doping of the Ce in (Nd,_.Ce,),CuO, reduces (Nd-0) due to the smaller atomic radius of Ce+4 and increases ( Cu-0), thus relaxing the stress of the (Nd-0) slab. In fact it even goes into a tensile state with large x (fig. 5b), but only slightly, particularly in comparison with the T-La2Cu04. The (Cu-0) bond becomes longer with doping, demonstrating a reduction of the Cu-valence. Similar effects are also observed in samples with R=Pr, Sm, Eu, and Gd, also shown in fig. 5a. It should be noted that the (Cu-0) bond-length in the T’-R2Cu04 is very close to the ideal (Cu+‘0)bond. This may be responsible for the contradictory nature (i.e. electrons or holes) of the Hall carriers reported in Ce-doped NdzCu04. Without the apical oxygen to facilitate the pseudo-John-Teller distortion, the CuO,-planar configuration associated with the T’-phase is less compressible than the CuO,octahedra in the T-phase. The small dT,/dp [27,29,35], the limited homogeneity range for doping, and a very similar lattice constant a can then be understood. Judging from the (Cu-0) / ( CU+“-O),,, value, the (Cu-0) bond in Gd&uO, is under compression and farthest away from the stable point among all known T’-R&uO,. Since no T’-phase exists beyond R = Gd, there may therefore exist a limit beyond which small R can no longer stabilize the T’-phase. In other words, the smaller (R-O) bond-length and bigger (Cu-0) bond-length can no longer match each other. To test this hypothesis, we decided to increase the

stabiliry

and doping

rn R2C’u0,

range of R for the T’-phase by enlarging the effective (R-O) bonds through doping R with La, which has a larger ionic radius [ 17.13 1. Indeed, ( LaR) CuO, single T’-phase has been successfully synthesized with R extending beyond Gd to Dy [ 17 1. As displayed in fig. 5a, the (Cu-0) bonds in (LaR)CuO, are further (although slightly) stretched, implying a tendency towards electron doping. When R varies from the bigger Pr, Nd to the smaller Eu, Gd, the (Cu-0) bond changes from being stretched to compressed. As a result, one would expect: i. the Ce solubility in (RLa)CuO, will be larger than that in R&uO,, especially for smaller R; ii. the La solubility will be larger in the Ce-doped R&uO, compared to the undoped R2Cu04; iii. the Ce-concentration for maximum 7, (with a constant lattice parameter a) will be smaller in (LaR),CuO, than in R2Cu04. All of these predictions have been confirmed by us [17] and others [28]. Our (LaR,,,Ce,,,,)CuO, (R= Pr. Nd. Sm. Eu) becomes superconducting after reduction in a N,-atmosphere. while ( LaGdo.8sCeo.is)Cu04 still remains semiconducting.

As was pointed out early, the (R-O) bond is under tension in the T-phase, whereas it is under compression in the T’-phase, with the less rigid (Cu0) bond adjusted to match the more rigid (R-O) bond. It has also been shown that hole-doping through a partial substitution of Sr for R in T-RZCu04 reduces the tension in the (R-O) bond and electrondoping through a partial replacement of R by Ce (or Th) in T’-R,Cu04 diminishes the compression in the (R-O) bond. Therefore. it appears possible to create a structure with a stable configuration by fusing the T- and T’-phases together. In other words. one may be able to construct a T*-structure (LaR)CuO, where R is a rare-earth element through proper doping. The unit cell of the T*-phase will then be a hybrid of the T- and T’-half-cells. The following compounds have been successfully synthesized by us with a pure T*-phase:

Y. Y. Xue et al. /Structural stability and doping in R,CuO,

(Lao.-1~Sr~.2~Nd~.85Ceo.ls)C~Oy ( Lao.7s%.2sSm)CuO, and (Lao.75Sr0.25SmO.&eo. l ) CuO,

( Lao.75 Sro.2s Eu )CuO, and ( hTs

sro.,, Euo.ssCeo.ls KuO,

( Lao.7s Sro.2s Gd PO,

and

(Lao.7s sro.2s Gb9 Ceo.I )CuO, (h.~Dyo.9)Cu0, (G4.2Ceo.3sSro.4s KuO, and (Prl.2Ceo.~sSr0.45)C~Oy . It

should

be

pointed

out

that

(Nd1.&e0.&-0.41 )CuO, and (Lao.&o.IsSm)CuO, have been previously stabilized [ 3,3 11. All of them show net hole-conductivity based on our Hall measurements. In section 2.2, it has been shown that ( LaR)Cu04 tends to favor the T’-structure. The severe mismatch between bonds of (La-O), (R-O) and (Cu-0) in the T*-configuration shown in fig. 1 makes it extremely difficult to stabilize the pure T*-( LaR)Cu04 without line-tuning the various bond-lengths through proper doping. This is clearly demonstrated by the above results and work by others [ 3,30-32 1. One of our motivations to synthesize the T*-phase is to search for a system where a continuous hole-toelectron doping can be carried out in order to gain insight into the problem of charge symmetry of the HTS models. This is because, intuitively, the T*phase consists of two parts in a unit cell: one similar to the T-phase and another to the T’-phase, as depicted in fig. 1, in which hole-and-electron dopings are expected to be made, respectively. Indeed, in our compounds ( LaR)Cu04 with R=Nd, Sm, Eu, and Gd, shown above, we have performed hole-doping by partially replacing La with Sr and then electrondoping by partially substituting Ce for R. Unfortunately, all of these doped T*-compounds [ (La, _,Sr,) ( R1 _$ez) ]CuO, have hole-carriers as determined by Hall measurements. When z exceeds the values given above, second phases appear. Again, this can be understood in terms of the relationship described earlier in this paper between atomic radius and structural stability. In order to form the T*-phase,

361

we have to bring all the relevant bonds from two extremes to match, i.e. the stretched (La-O) in the Tstructure versus the compressed (R-O) in the T’structure in the presence of the CuO,-square-pyramids. Since both the (La-O) bond and the (R-O) bond are rather rigid, to make a T*-phase one must shorten the (R-O) bond and lengthen the (La-O) bond with proper doping. Therefore, the (Cu-0) bond in the T*-phase has to lie somewhere between those in the T- and T’-phases. On the other hand, the coordinate numbers of Cu are 4, 5, and 6 in the T’, T*- and T-phases, respectively. The (Cu-0) bondlength should be the shortest in the T’-phase, and the longest in the T-phase, even if the Cu-valences are the same. Since the (Cu-0) bonds are only slightly stretched in the T’-phase, but highly compressed, as observed in the T-phase, the Cu-valence in the T*phase would be lower than 2, corresponding to a holedoping. Such a picture would also predict a metal atom ordering, i.e. the bigger Sr ion can only be inserted into the (La-O) slab, with the smaller Ce ion for the (R-O) slab, as is evident [ 321 in the (RL.2Ce0.35Sr0.45)Cu02 with R=Pr, Nd and Gd. Since the (Cu-0) bond is closely related to the lattice parameter u and a correlation between II and T, has been proposed [33,36] for the T-phase, we have extended the correlation to the T’-phases. The two superconducting phases clearly fall into two separate regions as shown in fig. 6. The lattice parameter a for the T*-phase lies between 3.84 and 3.88 A. Until now, all of our efforts have failed in inducing “electron’‘-superconductivity in the T*-phase com-

T

T’

40 % 30 -

B 0

;

20 -

Nd 0 Pr Sm O

0

0 0

Euo

10 -

Gd

0

0 3.74

3.82 Lattice

3.90 constants a

1Du(

uuw. -3.M

Fig. 6. T, vs. IIfor the T- and T’-phase: (0 ) La,_,(Sr, Ba)$uO,, (A ) NdI.3zSro.41Ceo.27Cu04 and (0 ) R1.8SCe0.1SCu04.

362

Y. Y. Xue er al. /Srructural

pounds by doping. The aforementioned results seem to suggest that it is very difficult to induce a hole-toelectron HTS transition continuously in the T*-phase [131. 2.4.Search ,fbr electron-HTS

above 77 K

The T’-(R,_,Ce,)2Cu0,, has been considered to belong to the same group of HTS cuprate oxides previously discovered because of its CuO*-layers and high-jr,. However, it may be rather different from the latter material. This is particularly true in view of its very small pressure effect on T, [ 351, the possibly much longer coherence length, and the complicated band structure, in contrast to its T-phase homologue. It would be interesting to determine if an electron-HTS exists above 77 K, so that a direct comparison can be made with the hole-HTS with T,> 90 K. It has been proposed that the dopability of the Tand T’-R2Cu04 with holes or electrons depends critically on the presence or absence of the apical oxygen atoms near the CuO*-layers [ 12 1. For instance, the negatively charged apical oxygen atoms near the CuO,-layers in the T-phase will make these layers more negatively charged, and hence, will favor positively charged hole-doping, while the opposite is true in the absence of the apical oxygen atoms from the T’-phase. The middle CuOz-layers in the 3-layered BizCazSrzCu~OY and Tl,CazBazCu30, with T, > 110 K do not possess any apical oxygen atoms. Therefore, we have attempted to stabilize the insulating parent of these 3-layered compounds, in which electron-doping might be carried out to achieve electron-HTS above 77 K. Following the steps to synthesize the insulating parent of the 2-layered Bi2CaSr2Cu20,,, we tried to replace the Ca by a rareearth element in BizCazSrzCu,O,,. Unfortunately, the 3-layered structure collapsed into the 2-layered structure when more than 15% of the Ca was replaced by Cd. Recently, there have been claims of the discovery of several electron-HTS compounds above 80 K, solely based on their starting stoichiometry [ 37 1. Our careful investigations of structural identification and Hall measurements have shown that all of the above display only an overall hole-carrier behavior [ 13 1.

stabrlity

and doping

in R,C‘uO,

3. Conclusion We have succeeded in synthesizing a series of T’and T*-compounds. By analyzing the data of these and other existing compounds, we found that the atomic radius of R dictates the stability of the three homomorphies of R,Cu04 as well as the type of doping of these compounds. For instance, the T-phase exists only for the largest R, i.e. La$ZuO,, the T’phase for smaller effective R, i.e. R,Cu04 where R=Pr, ... Cd, or (LaR)Cu04 where R=Pr, . Dy; and the T*-phase for inetermediate effective R with proper doping, i.e. (La, _.,Sr,R, _.Ce,)CuO, or even ( La,. ,Dy, 9 ) CuO,. For better matching between the different (R-O) and (Cu-0) bonds, the T-phase favors only hole-doping, the T’-phase slight electrondoping, and the T*-phase hole-doping. Consequently, no continuous electron-to-hole doping has been successfully induced in the T*-phase. Finally, careful examination reveals that the Hall carriers of the previously reported “electron”-HTS’s with T,> 80 K are holes.

4. Acknowledgements This work is supported in part by the NSF Low Temperature Physics Program Grant No. DMR 86126539, DARPA Grant No. MDA 972-88-G-002, NASA Grant No. NAGW-977, Texas Center for Superconductivity at the University of Houston, and the T.L.L. Temple Foundation at Houston. The authors wish to thank M. Bonvalot, Q. Xiong and J.W. Chu for preparing the sample and related work.

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