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Local model for charge delocalization in partly substituted cuprate superconductors
Pergamon
MUarir* Raorrcb Bullet& Vol. 30, No. 8.p~. 9874’93.1993 Cqy6ght 0 1995 Ehvior Scieaca Ltd RiIlIdiOtbeUSA.AllfightS0025~5408/95 s9.30 + .oo
00255408(95)000852
LOCAL MODEL FOR CHARGE DEIAKALIZATION IN PARTLY SUBS-D CUPRATE SUPERCONDUCTORS
EL Oerternicbtr
Department of Chemistry University of California, San Diego La Jolla. California 92093-03 17, U.S.A. (Received April 3, 1995; Communicated by A. Wold)
ABSTRACT Partial substitutions according to YB~(Cu,_&),O, with M = Ni etc. in connection with various redox sequencings allow for complex strategies in obtaining novel materials. We analyze redox related structural data on selected model compounds with special emphasis on a new local structural approach, derive aspects of their electronic behavior, and gain an expanded understanding of the charge balance tbnction of Cu( 1). We elaborate on the conditions for antitypes to conventional behavior such as n type superconductivity at low y and semiconductivity at high y. We show that a localization catastrophe can occur when the population of (1)4 contigurations (Cu(1) coordinated by 4 0) is low. In this case (1)3 and (1)2 coordinations dictate the spatial behavior of (1)4 which leads to charge localization. We show that this corresponds to a Mott type transition which has a general implication in degrading superconducting properties on further reoxygenation. MATEFUALS INDEX: nickel, cuprates
The question of charge localization vs. delocalization in cuprate superconductors hinges on subtle structural aspects.’ One opportunity to modi@ these aspects is through sophisticated substitutional and thermal strategies. An example is partial substitution1S4according to YBa, (Cu,.,~),O, with M = Ni, etc. Such systems respond strongly to the respective redox potentials and sequencing during preparation. Various states of M nanoclustering and different
987
988
H. OESTERREXHER
Vol. 30, No. 8
site distributions are a natural consequence of the changing redox potentials. These responses can be predicted* on a local thermodynamic model and be employed in the design of novel materials with special properties. As an example, a one dimensional nanocluster structure with M = Ni on Cu( 1) situated along (100) (due to Ni square planar preference) was predicted2 for more oxygenating preparations (OP) compared to say M = Fe. This should counteract the trends to tetragonality with Fe substitution and extend orthorhombicity to higher annealing temperatures and low y which was indeed observed.3*4Such materials (Kp) can show weak paramagnetism or diamagnetism and further oxidize to become volume expanded semiconducting paramagnets (Ko) at compositions (y - 6.9) where they were expected to be superconducting. In fact, long term annealing of Ko at relatively low T transforms the behavior back to conventional with Tc - 75K for x = 0.05. In this paper we further analyze data and generalize on aspects of the novel phenomenology. We will make plausible on bond valence arguments and a model based on local configurations that n type conductivity or superconductivity may be stable in an extended orthorhombic regime under special redox conditions, a situation which may be approached with Kp. We discuss examples of charge localizations in the low y regime (Kp). When Kp are further oxidized to compositions with potential p type (hole) superconductivity, memory effects appear to preclude the attainment of hole superconductivity (Ko paramagnet). Long term low temperature annealing is then needed to relax the material into conventional hole superconductors. This paper enlarges our previous discussion3*’ of phenomena in YBaJCu,,NiJ,O,.
Sample preparation generally involved getter annealing or quenching from elevated temperatures with long term air annealings. A symbol for an oxidative, slow cooled preparation, OP, subsequently annealed at 1123 K in air and quenched into liquid N, is [OP, 1123 A Q1. Reoxygenation in air was carried out at 670 K with slow cooling to room temperature [OP, 1123 A Q, 670 C]. More details have been given elsewhere)*’ including the subsequent characterization of structural, magnetic and resistive behavior.
A compilation of relevant data for selected model compounds exemplifying a broad range of behavior is given in table 1. In order to discuss these data we will introduce a local perspec-
tive. For this purpose we shall make distinctions of the various 0 in the structure. We denote p0 for the ones in the CuO, planes, a0 for the apical 0, CO for the 0 in the chains (in the plane of Cu(1) atoms). We depict Cu( 1) with its various 0 coordinations by either (1)2, (1)3 or (1)4 and Cu(2) by (2)s. For YBa&O, it was shown’ that practically the only critical feature for the redox mechanism is the relative location of a0. We present in fig. 1 relevant neutron information from work on getter annealed unsubstituted materials concerning the two important distances of a0 to Cu( 1) designated d 1 and of aOCu(2) designated d 2. The most important parameter for the oxidation state of the Cu(2) planes is d 2. We note that in getter annealed materials this parameter increases from y = 7 to y = 6 with one small step near y = 6.8 and one major step near y = 6.4. The major step in d 2 near y = 6.4 is also reflected5 in a strong increase in c or the ratio c/p where p represents the prism basis taken as (a+b)3/2. Specifically, we note the extrapolative behavior of d 2 vs. y rather than the plateaus as emphasized earlier.s The be-
Vol. 30, No. 8
CHARGE DEIBCALIZATION MODEL
TABLE
989
1
Variety of Redox Response in YBa#u,,Ni),O, S’,
C”
1 1 2 3 3 4 5 6 7 7 8 8 9 10 12
2 op 01 01 Tl Op : 2 01 TP Tl Tl
Y 7.0 7.0 6.83 6.9 6.8 6.7 6.7 6.7 6.45 (6.35) 6.4 6.4 (6.5) 6.35 6.0
92 75 90 65 35 60 45 60 T,,=SOO
a(A)
b(A)
3.815 3.810 3.82 3.824 3.835 3.87 3.82 3.840 3.830 3.840 3.860 3.855 3.865 3.86 3.86
3.885 3.877 3.88 3.893 3.893 3.88 3.896 3.875 3.882 3.872 3.880 -
I.Deeigaationinfigl.S~acllexrmpl~wgivoninKwe~ 2. Cham&r. 0 MKIT rtrndfa ceihohombiomd totrag&
a(A)
v(A’)
3.885 3.876 3.89 3.895 3.893 3.87 3.89 3.910 3.903 3.910 3.939 3.939 3.890 3.93 3.94
172.7 171.8 172.9 174.0 174.6 174.0 173.1 175.4 173.8 174.9 176.6 176.8 174.4 175.7 176.1
1for locked,
c/P p++b)3/2 1.009 1.008 1.010 1.009 1.008 1.00 1.011 1.011 1.013 1.013 1.018 1.018 1.006 1.018 1.021
TlWbIUllP G,Lx+ OP (%x=0 Ko GA 1053K. x=0 wx+ Qsx+ ollffo VA75OK,x=O C+A713K,rO VA7OOK.x-O 673 K wx-0 CiA, x==O
Ref. 5 3.4 5 3.4 3.4 7 5 P 5 P 5 8.P 9 5 5
p for positivechargecurier. Symbolsin bruk&
YS, tertfa detail If not othewhe shted x=0.05. GA utd VA hmd for gotieruul wcuum umotig Phadsfapfowntwofk. DtiinKdesignateumealhgkmpuxhw.
md Q fa quench@.
havior of d 1 in the orthorhombic range is more complex. A&r an initial increase (not plotted earlier) it can be roughly taken as linear although a strong shortening of d 1 can be extrapolated towards the range of orthorhombicity at low y as indicated by dashed lines. d 1 stays constant in the tetragonal range. From such data band valence sums (v) have been determined5 for the Cu(2) and the Cu(1) site. They indicate that v for Cu(2) is practically identical in shape with d 2 (y) and also decreases in two steps with decrease of y which is in turn similar to T&J). We indicate in fig 1 accordingly recalibrated values of vCu(2) = 2.3+ for y = 7 and vCu(2) = 2+ for y = 6. We have previously outlined6 that if Cu( 1) plays the role of a charge balance then vCu( 1) has to show a reciprocal behavior to vCu(2). that is when vCu(2) increases strongly then vCu(1) should decrease. However, so far only a charge “reservoir” function was considered for Cu( 1) in line with a linear increase in v with y. We present a local analysis which brings the data into better conformity with a charge balance i&a as it would be natural to expect lost charge on Cu(2) to show up on Cu( 1). We assume that measured values of d 1 are averages between individual local values for configurations l(2) and l(4) for the orthorhombic range. d1for1(2)isamallerthanfor1(4)fortheboundaqphases~=6and7respectively),andwe assume this also to hold for some intermediate range. One can now calculate weighted averages ford l( 1)2 and d l(l)4 such that their bond valence sums reciprocate with v&t(Z). We will similarly consider vCu(2) to be composed of weighted local averages. A schematic represuttation of this situation is given in fig 2. This means that d 2 for Cu(2) adjacent to (1)4 designatedd2(1)4shouldstayconstantnear2.3Aandd2(1)2shouldremainnear2.5A.This should result in the connecting line in fig 1 for the overall values of d 2 which is nearly observedforthe expekental points of the orthorhombic getter annealed materials save for the exceptional materials 2 and 8 outlined below.
H. OES’IERREICHER
990
Vol. 30, No. 8
7 FIG. 1. a) Values of d I(A), ‘OCu(1) versus y in YBa#hl_&&Oy. Solid points are Finn neutron diEaction (5). Open circles are assumed values for or&rhombic, open squares for tetrm materials. The crossed symbols are speculative. Numbers ColTespondto samples in table 1. b) Values of d 2, ‘OCu(2) versus y. Also plotted are approximate renormalized band valence values (v) for Cu(2).
We will now discuss, on a local model, the variety in redox behavior observed or postulated as a function of d&rent thermal sequencing and partial substitution. In the previous section we have intmduced the new notion that local configurations (e.g. (1)2 and (1)4) maintain more or less their characteristic interatomic distances of the boundary phases over some intermediary range in y and add in a weighted mannertoproducetheoveralldi&mcesandbond valencies. A second new notion pertains to the collapse of the distances of the minority local components into the majority components at low minurity compositions. Examples are the collapse of d 2(1)2 into d 2( 1)4 distances at high (1)4 concentrations (y - 6.8) and a oollapse of d 2(1)4 into d 2(1)2 distances at high (1)2 concentrations (y - 6.4) as shown in fig 2. The collapse of d 2( 1)2 iuto d 2( 1)4 can be seen in the decrease to constant d 2 from y - 6.8 to 7 and
Vol. 30, No. 8
CHAROEDELOCALIZATION MODEL
Sl
s2
s5
S3,Ko 2.6+2+1+
991
SqOl
S12
2.6+1+
l-t-
2.3+ 2.3+.8+ 22+1+
d 1=1.83A
cum 1.78 a0
1 2.48
d2=2.3aF 2.3+
2.3+2.3+2.2+21+ 2-t-2+2+ 2t2+
2+
cut21
FIG. 2. schematic representation of d 1 and d 2 for samples in fig 1. A full diagonal line represents (1)4, ahalfline(1)3andnodiagonal(1)2.Estimatedvaregiven.S5maintainsd1andd2closetothe ones of boundary phases, while in S 2 and S 8 majorities dominate minorities. Reoxygenation of S8toS3keeps(1)4underthedominsnceof(1)2and(1)3,leadingtoabsenceofsuperconducting and large V (thick family).
the rise in d 1 followed by charge compensating decreasing d 1. The collapse of d 2( 1)4 into d 2(1)2 on going from 57 to S8 is seen in the increasing d 2 and decreasing d 1 near y = 6.4. Generally (1)4 acts as an oxidator configuration responsible for the charge delocalization between (1)4 and (2)5. We suppose accordingly that (1)2 is not involved in the common bands and stays in various degrees of localization. This situation stays stable from y = 6.8 to about y = 6.4 where d 2 follows roughly an extrapolative line between y = 7 and 6. This indicates tbat the number of oxygenator configurations (1)4 is linearly decreasing but that their d 2( I)4 are stable explaining a Tc plateau. A dramatic change occurs neary = 6.4 where an expansion of d 2 can be surmised from the c axis and T, trends. This appears to correspond to a jump of d 2( 1)4 into values characteristic of d 2( 1)2. This results in a loss of oxygenator function of (1)4. Towards the stoichiometry where the (1)4 row is surrounded by two (1)2 rows, (I)4 cannot hold its characteristic distances and collapses into a common a0 location. In fact, this step appears to occur at relatively diminished orthorhombicity which means that the population of (1)4 has been even further reduced beyond the one straightforwardly extrapolated. The actual composition then includes (1)2, (I)3 in addition to smaller amounts of (1)4 so that (1)4 intluence can be overwhelmed. The main result is a charge localization connected with a dramatic increase of bond valence to roughly 2.6+ on Cu( 1)4 while Cu(2) drops to 2+. The overall charge localization is then responsible for both the loss of Tc in or&rhombic materials and the V expansion. We shall take this localization catastrophe near y = 6.4 as a pivotal occurrence which is involved in a variety of phenomena as outlined below. We suggest that whenever a sample has enough time to equilibrate in this localized regime (near 1020 K in air) further oxidation to an ordered material with strong or&rhombic splitting can be hindered due to memory effects of this localization. This is taken to be the origin of Ko paramagnetism in Ni substituted materials, the genesis of the T phase, the di&ulty’ in achieving Tc = 90 K in LaE&Cu,O, as well as the general dete-
992
H. OESTERRFJCBER
Vol. 30, No. 8
rioration of Tc in as quenched or in partly substituted materials when annealing encompasses sufficient time at the relevant temperatures. The delocalized phase (eg SS) is stabilized by the common valence of (1)4 and (2)5 and destabilized by the separate a0 location between (1)4 and (1)2. Conversely, the localized situation (S8) is stabilized by common a0 distances but destabilized by the strong differences in local charges. Near y = 6.4 energies of both states cross and lead to an internal redox rearrangement. The step between samples 8 and 7 is therefore identified as an insulator metal transition of a Mott type. Such transitions should also be possible as a result of external pressure near y - 6.4. The localization catastrophe appears to need long term annealing to develop and can be avoided by fast reoxidation kinetics from low y. When (1)4 in the localized regime is spatially dictated by (1)3 and (1)2 then further oxidation of (1)2 is not required to go to solely (1)4 but is spatially permitted to go to (1)3 with similar d 1. This creates a nonideal orthorhombic material (Ko) in which (1)4 stays spatially enslaved by the growing amount of (1)3. In order to avoid Cu(2) oxidation, the basal plane expands beyond the one characteristic for ideal materials. This results in relatively low c/p although the material is not superconducting. A similar situation applies to the T phase, which has relatively large a axis and unusually low c. This counterbalance keeps Cu(2) near 2+ and obviates in this case the utility of c/p in predicting oxidation state. We note that there are therefore two families of compounds which achieve a given oxidation state by either high or low c/p respectively. Comparisons of their oxidation state through c/p can be made only within one family. The conventional elongated c/p family (thin family) is exemplified by samples 12, 10,s with CUE’ and c/p - 1.02 or samples 1,s with CU(~)~.~+ and c/p - 1.O1. A second contracted (thick) family is exemplified by samples 4 with Cu(2)” although c/p - 1.O. Other examples are super or nonsuperconducting YSr2(CuFe)30,,10 (YCa),Ba,Cu, CO,O,” and LaBa$hO,‘. Changing the location of a0 is therefore not the only redox mechanism but size of basal plane can also play a reciprocal role, and oxidation of Cu(2) should be achievable by increasing or decreasing c/p. Another unusual material is sample 9. It shows a small c/p in a tetragonal superconducting material. It is however not clear whether this represents an inherent tetragonal material or orthorhombic micro domains, too small to be resolved by x-ray diffraction. At face value the sample should have low d 2 and high d 1 and could be considered a tetragonal hole superconductor. Superconductivity may be caused by a disproportionation according to (2)S2’ and ( 1)3’2+or by the presence of some ( 1)4’2+and 2(S)‘2+. Charge disproportionations can involve the temperature dependence of the tolerance facto? t for perovskite stacking. The charge disproportionation between (2)s and (1)4 in samples 8 has generally to do with the readiness of (1)4 to oxidize nea? AH* = 2 10 kJ to > 2+ while (2)5, at a relatively low T, needs slightly lower AH* (200 kJ) or higher y for this transition. At higher T more polarizability obtains as t is closer to 1 and a common charge-delocalized situation is favored explaining the observation (table 1) of extension of the delocalized phase to lower y at higher T. Generally the V expansion near y = 6.4 could also portend even further expansion potential beyond the one corresponding to d 2(1)2 and result in a situation corresponding to fictitious sample 11. In this case Cu(2) would decrease in oxidation number below 2+ (e.g. 1.S+) and Cu( 1) could shorten in a reciprocal way so that its oxidation on (1)2 would also be v - 1.8+. The general trends in the orthorhombic regimes are not out of line with this assumption. Sample 11 could therefore represent a n type superconductor with the additional possibility of hole contributions from a small amount of (1)4. It is possible that Kp materials3*’ partly represent this situation and that the unusual situation could lead to especially high Tc. Such internal
Vol. 30, No. 8
CHARCE DRLOCALIZATIONMODEL
993
redox equilibria may be the origin of filamentary type high Tc o&n reported. Generally such materials need high basal plane dimensions or cell V in order to allow electron doping into antibonding bands. Representatives of the thick family may be promising candidates. The prediction of superconductivity in electron doped systems involving a0 can include other systems. While it largely excludes Ce4+ doping due to the stability of the CeBaO, structure (Ba or Sr are needed to create aO), doping according to Y(LaBa),Cu,O,. or YBa, (Cu,,MJ,O, with M’+ are possibilities which should allow orthorhombicity (high 0 filling) at relatively low Cu valence. The case of M = Se4+ is particularly attractive as it has diameter comparable to Cu2+ and has a predicted redox response opposite to M = Fe pref~entially occupying Cu(2) at low T. This will not strongly influence orthorhombicity, making OP preparation suitable in this respect.
In disuGng novel aspects of the phenomenology of partly substituted materials we introduce several new concepts. We consider local intercalation con@urations to exist either with spatial chara&ristics of their pure form in the boundary phase over some intermediary range or with a collapse into values characteristic of the majority component when they become a minority. This leads to a new t&z&am@ of charge balancing and of a localization catastrophe near y = 6.4 which is capable of explaining a wide variety of detrimental effects in specially processed superconductors.
1. Raveau, B. et al., Crystal Chemistry of High Tc Copper Oxides, Springer (1991). Ocstcrrcicher, H., Mat. Res. Bull. 29,489 (1994).
2. 3. 4. 5. 6. 7.
8. 9. 10. 11.
Ko, D. and Cksterreicher, H., Physica, 23 1,252 (1994). Ko, D. and Oesterreicher H., Mat. Rae. Bull. 29,102s (1994). Cava, R. J. et al., Physica C165,419 (1990). Oesterreicher, H., J. Solid State Chemistry 80,142 (1989). A. Manthiram et al. Nature, 329,701 (1987), H. Oesterreicher et al., Mat. Res. Bull., 23,1327 (1988). hi. 0. Smith et al., Mat. Res. Bull. 23,563 (1988). D. Ko et al. (unpublished). J. O’Brien et al. (unpublished). B. Domcnges et al., J. Solid State Chem., 108,219 (1994).
MUarir* Raorrcb Bullet& Vol. 30, No. 8.p~. 9874’93.1993 Cqy6ght 0 1995 Ehvior Scieaca Ltd RiIlIdiOtbeUSA.AllfightS0025~5408/95 s9.30 + .oo
00255408(95)000852
LOCAL MODEL FOR CHARGE DEIAKALIZATION IN PARTLY SUBS-D CUPRATE SUPERCONDUCTORS
EL Oerternicbtr
Department of Chemistry University of California, San Diego La Jolla. California 92093-03 17, U.S.A. (Received April 3, 1995; Communicated by A. Wold)
ABSTRACT Partial substitutions according to YB~(Cu,_&),O, with M = Ni etc. in connection with various redox sequencings allow for complex strategies in obtaining novel materials. We analyze redox related structural data on selected model compounds with special emphasis on a new local structural approach, derive aspects of their electronic behavior, and gain an expanded understanding of the charge balance tbnction of Cu( 1). We elaborate on the conditions for antitypes to conventional behavior such as n type superconductivity at low y and semiconductivity at high y. We show that a localization catastrophe can occur when the population of (1)4 contigurations (Cu(1) coordinated by 4 0) is low. In this case (1)3 and (1)2 coordinations dictate the spatial behavior of (1)4 which leads to charge localization. We show that this corresponds to a Mott type transition which has a general implication in degrading superconducting properties on further reoxygenation. MATEFUALS INDEX: nickel, cuprates
The question of charge localization vs. delocalization in cuprate superconductors hinges on subtle structural aspects.’ One opportunity to modi@ these aspects is through sophisticated substitutional and thermal strategies. An example is partial substitution1S4according to YBa, (Cu,.,~),O, with M = Ni, etc. Such systems respond strongly to the respective redox potentials and sequencing during preparation. Various states of M nanoclustering and different
987
988
H. OESTERREXHER
Vol. 30, No. 8
site distributions are a natural consequence of the changing redox potentials. These responses can be predicted* on a local thermodynamic model and be employed in the design of novel materials with special properties. As an example, a one dimensional nanocluster structure with M = Ni on Cu( 1) situated along (100) (due to Ni square planar preference) was predicted2 for more oxygenating preparations (OP) compared to say M = Fe. This should counteract the trends to tetragonality with Fe substitution and extend orthorhombicity to higher annealing temperatures and low y which was indeed observed.3*4Such materials (Kp) can show weak paramagnetism or diamagnetism and further oxidize to become volume expanded semiconducting paramagnets (Ko) at compositions (y - 6.9) where they were expected to be superconducting. In fact, long term annealing of Ko at relatively low T transforms the behavior back to conventional with Tc - 75K for x = 0.05. In this paper we further analyze data and generalize on aspects of the novel phenomenology. We will make plausible on bond valence arguments and a model based on local configurations that n type conductivity or superconductivity may be stable in an extended orthorhombic regime under special redox conditions, a situation which may be approached with Kp. We discuss examples of charge localizations in the low y regime (Kp). When Kp are further oxidized to compositions with potential p type (hole) superconductivity, memory effects appear to preclude the attainment of hole superconductivity (Ko paramagnet). Long term low temperature annealing is then needed to relax the material into conventional hole superconductors. This paper enlarges our previous discussion3*’ of phenomena in YBaJCu,,NiJ,O,.
Sample preparation generally involved getter annealing or quenching from elevated temperatures with long term air annealings. A symbol for an oxidative, slow cooled preparation, OP, subsequently annealed at 1123 K in air and quenched into liquid N, is [OP, 1123 A Q1. Reoxygenation in air was carried out at 670 K with slow cooling to room temperature [OP, 1123 A Q, 670 C]. More details have been given elsewhere)*’ including the subsequent characterization of structural, magnetic and resistive behavior.
A compilation of relevant data for selected model compounds exemplifying a broad range of behavior is given in table 1. In order to discuss these data we will introduce a local perspec-
tive. For this purpose we shall make distinctions of the various 0 in the structure. We denote p0 for the ones in the CuO, planes, a0 for the apical 0, CO for the 0 in the chains (in the plane of Cu(1) atoms). We depict Cu( 1) with its various 0 coordinations by either (1)2, (1)3 or (1)4 and Cu(2) by (2)s. For YBa&O, it was shown’ that practically the only critical feature for the redox mechanism is the relative location of a0. We present in fig. 1 relevant neutron information from work on getter annealed unsubstituted materials concerning the two important distances of a0 to Cu( 1) designated d 1 and of aOCu(2) designated d 2. The most important parameter for the oxidation state of the Cu(2) planes is d 2. We note that in getter annealed materials this parameter increases from y = 7 to y = 6 with one small step near y = 6.8 and one major step near y = 6.4. The major step in d 2 near y = 6.4 is also reflected5 in a strong increase in c or the ratio c/p where p represents the prism basis taken as (a+b)3/2. Specifically, we note the extrapolative behavior of d 2 vs. y rather than the plateaus as emphasized earlier.s The be-
Vol. 30, No. 8
CHARGE DEIBCALIZATION MODEL
TABLE
989
1
Variety of Redox Response in YBa#u,,Ni),O, S’,
C”
1 1 2 3 3 4 5 6 7 7 8 8 9 10 12
2 op 01 01 Tl Op : 2 01 TP Tl Tl
Y 7.0 7.0 6.83 6.9 6.8 6.7 6.7 6.7 6.45 (6.35) 6.4 6.4 (6.5) 6.35 6.0
92 75 90 65 35 60 45 60 T,,=SOO
a(A)
b(A)
3.815 3.810 3.82 3.824 3.835 3.87 3.82 3.840 3.830 3.840 3.860 3.855 3.865 3.86 3.86
3.885 3.877 3.88 3.893 3.893 3.88 3.896 3.875 3.882 3.872 3.880 -
I.Deeigaationinfigl.S~acllexrmpl~wgivoninKwe~ 2. Cham&r. 0 MKIT rtrndfa ceihohombiomd totrag&
a(A)
v(A’)
3.885 3.876 3.89 3.895 3.893 3.87 3.89 3.910 3.903 3.910 3.939 3.939 3.890 3.93 3.94
172.7 171.8 172.9 174.0 174.6 174.0 173.1 175.4 173.8 174.9 176.6 176.8 174.4 175.7 176.1
1for locked,
c/P p++b)3/2 1.009 1.008 1.010 1.009 1.008 1.00 1.011 1.011 1.013 1.013 1.018 1.018 1.006 1.018 1.021
TlWbIUllP G,Lx+ OP (%x=0 Ko GA 1053K. x=0 wx+ Qsx+ ollffo VA75OK,x=O C+A713K,rO VA7OOK.x-O 673 K wx-0 CiA, x==O
Ref. 5 3.4 5 3.4 3.4 7 5 P 5 P 5 8.P 9 5 5
p for positivechargecurier. Symbolsin bruk&
YS, tertfa detail If not othewhe shted x=0.05. GA utd VA hmd for gotieruul wcuum umotig Phadsfapfowntwofk. DtiinKdesignateumealhgkmpuxhw.
md Q fa quench@.
havior of d 1 in the orthorhombic range is more complex. A&r an initial increase (not plotted earlier) it can be roughly taken as linear although a strong shortening of d 1 can be extrapolated towards the range of orthorhombicity at low y as indicated by dashed lines. d 1 stays constant in the tetragonal range. From such data band valence sums (v) have been determined5 for the Cu(2) and the Cu(1) site. They indicate that v for Cu(2) is practically identical in shape with d 2 (y) and also decreases in two steps with decrease of y which is in turn similar to T&J). We indicate in fig 1 accordingly recalibrated values of vCu(2) = 2.3+ for y = 7 and vCu(2) = 2+ for y = 6. We have previously outlined6 that if Cu( 1) plays the role of a charge balance then vCu( 1) has to show a reciprocal behavior to vCu(2). that is when vCu(2) increases strongly then vCu(1) should decrease. However, so far only a charge “reservoir” function was considered for Cu( 1) in line with a linear increase in v with y. We present a local analysis which brings the data into better conformity with a charge balance i&a as it would be natural to expect lost charge on Cu(2) to show up on Cu( 1). We assume that measured values of d 1 are averages between individual local values for configurations l(2) and l(4) for the orthorhombic range. d1for1(2)isamallerthanfor1(4)fortheboundaqphases~=6and7respectively),andwe assume this also to hold for some intermediate range. One can now calculate weighted averages ford l( 1)2 and d l(l)4 such that their bond valence sums reciprocate with v&t(Z). We will similarly consider vCu(2) to be composed of weighted local averages. A schematic represuttation of this situation is given in fig 2. This means that d 2 for Cu(2) adjacent to (1)4 designatedd2(1)4shouldstayconstantnear2.3Aandd2(1)2shouldremainnear2.5A.This should result in the connecting line in fig 1 for the overall values of d 2 which is nearly observedforthe expekental points of the orthorhombic getter annealed materials save for the exceptional materials 2 and 8 outlined below.
H. OES’IERREICHER
990
Vol. 30, No. 8
7 FIG. 1. a) Values of d I(A), ‘OCu(1) versus y in YBa#hl_&&Oy. Solid points are Finn neutron diEaction (5). Open circles are assumed values for or&rhombic, open squares for tetrm materials. The crossed symbols are speculative. Numbers ColTespondto samples in table 1. b) Values of d 2, ‘OCu(2) versus y. Also plotted are approximate renormalized band valence values (v) for Cu(2).
We will now discuss, on a local model, the variety in redox behavior observed or postulated as a function of d&rent thermal sequencing and partial substitution. In the previous section we have intmduced the new notion that local configurations (e.g. (1)2 and (1)4) maintain more or less their characteristic interatomic distances of the boundary phases over some intermediary range in y and add in a weighted mannertoproducetheoveralldi&mcesandbond valencies. A second new notion pertains to the collapse of the distances of the minority local components into the majority components at low minurity compositions. Examples are the collapse of d 2(1)2 into d 2( 1)4 distances at high (1)4 concentrations (y - 6.8) and a oollapse of d 2(1)4 into d 2(1)2 distances at high (1)2 concentrations (y - 6.4) as shown in fig 2. The collapse of d 2( 1)2 iuto d 2( 1)4 can be seen in the decrease to constant d 2 from y - 6.8 to 7 and
Vol. 30, No. 8
CHAROEDELOCALIZATION MODEL
Sl
s2
s5
S3,Ko 2.6+2+1+
991
SqOl
S12
2.6+1+
l-t-
2.3+ 2.3+.8+ 22+1+
d 1=1.83A
cum 1.78 a0
1 2.48
d2=2.3aF 2.3+
2.3+2.3+2.2+21+ 2-t-2+2+ 2t2+
2+
cut21
FIG. 2. schematic representation of d 1 and d 2 for samples in fig 1. A full diagonal line represents (1)4, ahalfline(1)3andnodiagonal(1)2.Estimatedvaregiven.S5maintainsd1andd2closetothe ones of boundary phases, while in S 2 and S 8 majorities dominate minorities. Reoxygenation of S8toS3keeps(1)4underthedominsnceof(1)2and(1)3,leadingtoabsenceofsuperconducting and large V (thick family).
the rise in d 1 followed by charge compensating decreasing d 1. The collapse of d 2( 1)4 into d 2(1)2 on going from 57 to S8 is seen in the increasing d 2 and decreasing d 1 near y = 6.4. Generally (1)4 acts as an oxidator configuration responsible for the charge delocalization between (1)4 and (2)5. We suppose accordingly that (1)2 is not involved in the common bands and stays in various degrees of localization. This situation stays stable from y = 6.8 to about y = 6.4 where d 2 follows roughly an extrapolative line between y = 7 and 6. This indicates tbat the number of oxygenator configurations (1)4 is linearly decreasing but that their d 2( I)4 are stable explaining a Tc plateau. A dramatic change occurs neary = 6.4 where an expansion of d 2 can be surmised from the c axis and T, trends. This appears to correspond to a jump of d 2( 1)4 into values characteristic of d 2( 1)2. This results in a loss of oxygenator function of (1)4. Towards the stoichiometry where the (1)4 row is surrounded by two (1)2 rows, (I)4 cannot hold its characteristic distances and collapses into a common a0 location. In fact, this step appears to occur at relatively diminished orthorhombicity which means that the population of (1)4 has been even further reduced beyond the one straightforwardly extrapolated. The actual composition then includes (1)2, (I)3 in addition to smaller amounts of (1)4 so that (1)4 intluence can be overwhelmed. The main result is a charge localization connected with a dramatic increase of bond valence to roughly 2.6+ on Cu( 1)4 while Cu(2) drops to 2+. The overall charge localization is then responsible for both the loss of Tc in or&rhombic materials and the V expansion. We shall take this localization catastrophe near y = 6.4 as a pivotal occurrence which is involved in a variety of phenomena as outlined below. We suggest that whenever a sample has enough time to equilibrate in this localized regime (near 1020 K in air) further oxidation to an ordered material with strong or&rhombic splitting can be hindered due to memory effects of this localization. This is taken to be the origin of Ko paramagnetism in Ni substituted materials, the genesis of the T phase, the di&ulty’ in achieving Tc = 90 K in LaE&Cu,O, as well as the general dete-
992
H. OESTERRFJCBER
Vol. 30, No. 8
rioration of Tc in as quenched or in partly substituted materials when annealing encompasses sufficient time at the relevant temperatures. The delocalized phase (eg SS) is stabilized by the common valence of (1)4 and (2)5 and destabilized by the separate a0 location between (1)4 and (1)2. Conversely, the localized situation (S8) is stabilized by common a0 distances but destabilized by the strong differences in local charges. Near y = 6.4 energies of both states cross and lead to an internal redox rearrangement. The step between samples 8 and 7 is therefore identified as an insulator metal transition of a Mott type. Such transitions should also be possible as a result of external pressure near y - 6.4. The localization catastrophe appears to need long term annealing to develop and can be avoided by fast reoxidation kinetics from low y. When (1)4 in the localized regime is spatially dictated by (1)3 and (1)2 then further oxidation of (1)2 is not required to go to solely (1)4 but is spatially permitted to go to (1)3 with similar d 1. This creates a nonideal orthorhombic material (Ko) in which (1)4 stays spatially enslaved by the growing amount of (1)3. In order to avoid Cu(2) oxidation, the basal plane expands beyond the one characteristic for ideal materials. This results in relatively low c/p although the material is not superconducting. A similar situation applies to the T phase, which has relatively large a axis and unusually low c. This counterbalance keeps Cu(2) near 2+ and obviates in this case the utility of c/p in predicting oxidation state. We note that there are therefore two families of compounds which achieve a given oxidation state by either high or low c/p respectively. Comparisons of their oxidation state through c/p can be made only within one family. The conventional elongated c/p family (thin family) is exemplified by samples 12, 10,s with CUE’ and c/p - 1.02 or samples 1,s with CU(~)~.~+ and c/p - 1.O1. A second contracted (thick) family is exemplified by samples 4 with Cu(2)” although c/p - 1.O. Other examples are super or nonsuperconducting YSr2(CuFe)30,,10 (YCa),Ba,Cu, CO,O,” and LaBa$hO,‘. Changing the location of a0 is therefore not the only redox mechanism but size of basal plane can also play a reciprocal role, and oxidation of Cu(2) should be achievable by increasing or decreasing c/p. Another unusual material is sample 9. It shows a small c/p in a tetragonal superconducting material. It is however not clear whether this represents an inherent tetragonal material or orthorhombic micro domains, too small to be resolved by x-ray diffraction. At face value the sample should have low d 2 and high d 1 and could be considered a tetragonal hole superconductor. Superconductivity may be caused by a disproportionation according to (2)S2’ and ( 1)3’2+or by the presence of some ( 1)4’2+and 2(S)‘2+. Charge disproportionations can involve the temperature dependence of the tolerance facto? t for perovskite stacking. The charge disproportionation between (2)s and (1)4 in samples 8 has generally to do with the readiness of (1)4 to oxidize nea? AH* = 2 10 kJ to > 2+ while (2)5, at a relatively low T, needs slightly lower AH* (200 kJ) or higher y for this transition. At higher T more polarizability obtains as t is closer to 1 and a common charge-delocalized situation is favored explaining the observation (table 1) of extension of the delocalized phase to lower y at higher T. Generally the V expansion near y = 6.4 could also portend even further expansion potential beyond the one corresponding to d 2(1)2 and result in a situation corresponding to fictitious sample 11. In this case Cu(2) would decrease in oxidation number below 2+ (e.g. 1.S+) and Cu( 1) could shorten in a reciprocal way so that its oxidation on (1)2 would also be v - 1.8+. The general trends in the orthorhombic regimes are not out of line with this assumption. Sample 11 could therefore represent a n type superconductor with the additional possibility of hole contributions from a small amount of (1)4. It is possible that Kp materials3*’ partly represent this situation and that the unusual situation could lead to especially high Tc. Such internal
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CHARCE DRLOCALIZATIONMODEL
993
redox equilibria may be the origin of filamentary type high Tc o&n reported. Generally such materials need high basal plane dimensions or cell V in order to allow electron doping into antibonding bands. Representatives of the thick family may be promising candidates. The prediction of superconductivity in electron doped systems involving a0 can include other systems. While it largely excludes Ce4+ doping due to the stability of the CeBaO, structure (Ba or Sr are needed to create aO), doping according to Y(LaBa),Cu,O,. or YBa, (Cu,,MJ,O, with M’+ are possibilities which should allow orthorhombicity (high 0 filling) at relatively low Cu valence. The case of M = Se4+ is particularly attractive as it has diameter comparable to Cu2+ and has a predicted redox response opposite to M = Fe pref~entially occupying Cu(2) at low T. This will not strongly influence orthorhombicity, making OP preparation suitable in this respect.
In disuGng novel aspects of the phenomenology of partly substituted materials we introduce several new concepts. We consider local intercalation con@urations to exist either with spatial chara&ristics of their pure form in the boundary phase over some intermediary range or with a collapse into values characteristic of the majority component when they become a minority. This leads to a new t&z&am@ of charge balancing and of a localization catastrophe near y = 6.4 which is capable of explaining a wide variety of detrimental effects in specially processed superconductors.
1. Raveau, B. et al., Crystal Chemistry of High Tc Copper Oxides, Springer (1991). Ocstcrrcicher, H., Mat. Res. Bull. 29,489 (1994).
2. 3. 4. 5. 6. 7.
8. 9. 10. 11.
Ko, D. and Cksterreicher, H., Physica, 23 1,252 (1994). Ko, D. and Oesterreicher H., Mat. Rae. Bull. 29,102s (1994). Cava, R. J. et al., Physica C165,419 (1990). Oesterreicher, H., J. Solid State Chemistry 80,142 (1989). A. Manthiram et al. Nature, 329,701 (1987), H. Oesterreicher et al., Mat. Res. Bull., 23,1327 (1988). hi. 0. Smith et al., Mat. Res. Bull. 23,563 (1988). D. Ko et al. (unpublished). J. O’Brien et al. (unpublished). B. Domcnges et al., J. Solid State Chem., 108,219 (1994).