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NEW SUPERCONDUCTIVE COMPOUNDS HAVING HIGH TRANSITION 
TEMPERATURE, AND METHODS FOR THEIR USE AND PREPARATION 



DESCRIPTION 

Technical Field 

Tliis invention relates to a new class of superconducting 
compositions having high superconducting transition 
temperatures and methods for using and preparing these 
compositions, and more particularly to superconducting 
compositions including copper and/or other transition 
metals, the compositions being characterized by a 
superconducting phase and a layer-like structure. 

Background Art 

Superconductivity is usually defined as the complete 
loss of electrical resistance of a material at a well- 
defined temperature. It is known to occur in many ma- 
terials, including about a quarter of the elements of 
the periodic table and over 1000 alloys and other 
multi-component systems. Generally, superconductivity 



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is considered to be a property of the metallic state of 
a material since all known superconductors are metallic 
under the conditions that cause them to be supercon- 
ducting. A few normally non-metallic materials, for 
example, become superconducting under very high pressure 
wherein the pressure converts them to metals before they 
exhibit superconducting behavior. 

Superconductors are known to be very attractive for the 
generation and energy-saving transport of electrical 
power over long distances, and as materials used to form 
the coils of very strong magnets. These magnets are used 
in, for example, plasma and nuclear physics, nuclear 
magnetic resonance medical diagnosis systems, and in 
connection with the magnetic levitation of fast trains. 
Other potential uses of superconducting materials occur 
in power generation systems using thermonuclear fusion 
where very large magnetic fields must be provided, 
superconducting magnets being the only possible means 
for providing such high fields. In addition to these 
applications, superconductors are known in high speed 
switching devices, such as Josephson type switches, and 
in high density packaging and circuit layouts. Super- 
conductors also are used in different types of elec- 



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tronic instrumentation, such as magnetic susceptometers 
and magnetometers. 

While the advantages of superconductors are quite obvi- 
ous to scientists and engineers, the common disadvantage 
of all presently known superconductive materials lies 
in their very low transition temperature. This temper- 
ature is often called the critical temperature and 
is the temperature above which superconductivity will 
not exist. Usually is on the order of a few degrees 
Kelvin- The element with the highest T^ is niobium whose 
T^ is 9.2^K. The composition having the highest previ- 
ously known T is Nb^Ge which exhibits a T of about 23^K 
c 3 c 

at ambient pressure. Transition metal alloy compounds 
of the A15(Nb^Sn) and Bl(NbN) structure have been shown 
to have high superconducting transition temperatures. 
Among the A 15 compounds is the aforementioned composi- 
tion Nb^Ge. Some of these compositions are described 
in J. Muller, Rep. Prog. Phys. 43, 663 (1980), and M, 
R. Beasley et al, Phys. Today, 37 (10), 60 (1984). 

It is known in the art that a small number of oxides will 
exhibit superconductivity. Reference is made to D.C. 
Johnston et al, Mat, Res. Bull. 8, 777 (1973), which 
describes high temperature superconductivity in the Li- 



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Ti-0 system with superconducting onsets as high as 
13.7^K. These materials have multiple crystal lographic 
phases including a spinel structure exhibiting the high 
T^. Other metallic oxides, such as the perovskite Ba- 
Pb-Bi-0 system can exhibit superconductivity due to high 
electron-phonon coupling in a mixed valent compound, as 
described by G. Binnig et al, Phys. Rev. Lett., 45, 1352 
(1980), and A.W. Sleight et al, Solid State Communi- 
cations, 17, 27 (1975). 

As is evident from the foregoing, superconductors pres- 
ently known require liquid helium for cooling and this, 
in turn, requires an elaborate technology and a consid- 
erable investment in cost and energy. Accordingly, it 
is a primary object of the present invention to provide 
new compositions which exhibit high and methods for 
using and producing the same. 

It is another object of the present invention to provide 
new superconducting compositions and methods for using 
and making them where cooling with liquid helium is not 
required in order to have superconductive properties in 
the compositions. 



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It is another object of tlio present invention to provide 
novel superconductive materials that are multi-valent 
oxides including transition metals, the compositions 
having a perovskite-like structure. 

It is a further object of the present invention to pro- 
vide novel superconductive compositions that are oxides 
including rare earth and/or rare earth-like atoms, to- 
gether with copper or other transition metals that can 
exhibit mixed valent behavior - 

It is a still further object of the present invention 
to provide novel superconductive compositions exhibiting 
high T^, where the compositions are oxides including a 
phase having a layer- like structure and including cop- 
per. 

It is a still further object of the present invention 
to provide new superconductive compositions exhibiting 
high T^, where the superconductive compositions include 
layered structures including a rare earth and/or rare 
earth- like element and a transition metal. 

It is another object of this invention to provide a new 
class of superconducting compositions characterized by 



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a greater than 26 *^K, and methods for making and using 
these compos it ions . 

It is another object of this invention to provide now 
compositions and methods for using them, where the com- 
positions include a multi-valent oxide of copper and 
exhibit a greater than 26°K. 

The basis for our invention has been described by us in 
the following previously published article: J.G. 
Bednorz and K.A. Muller, Zeitschrift fur Physik B - 
Condensed Matter, 64, pp. 189-193. 

Another article of interest by us is J.G. Bednorz, K.A- 
Muller, M. Takashige, Europhysics Letters, 3(3), pp. 
379-385 (1987). 

Summary of the Invention 

This invention relates to novel compositions exhibiting 
superconductivity at temperatures higher than those ob- 
tained in prior known superconductive materials, and to 
methods for using and forming these compositions- These 
compositions can carry supercurrents (i,.e., electrical 



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currents in a substantially zero resistance state of the 
composition) at temperatures at greater than 26°K. In 
general, the compositions are characterized as mixed 
transition metal oxide systems where the transition 
metal oxide can exhibit multivalent behavior. These 
compositions have a layer-type crystalline structure, 
often perovskite-like, and can contain a rare earth or 
rare earth- like element, A rare earth- like element 
(sometimes termed a near rare earth element^ is one 
whose properties make it essentially a rare earth ele- 
ment. An example is a group IIIB element of the periodic 
table, such as La. Substitutions can be found in the 
rare earth (or rare earth-like) site or in the transi- 
tion metal sites of the compositions. For example, the 
rare earth site can also include alkaline earth elements 
selected from group IIA of the periodic table, or a 
combination of rare earth or rare earth- like elements 
and alkaline earth elements. Examples of suitable 
alkaline earths include Ca, Sr, and Ba. The transition 
metal site can include a transition metal exhibiting 
mixed valent behavior, and can include more than one 
transition metal. A particularly good example of a 
suitable transition metal is copper- As will be appar- 
ent later, Cu- oxide based systems provide unique and 
excellent properties as high T superconductors. 



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An example of a superconductive composition having high 

is the composition represented by the formula 
RE-TM-0, where RE is a rare earth or rare earth- like 
element, TM is a nonmagnetic transition metal, and 0 is 
oxygen. Examples of transition metal elements include 
Cu, Ni, Cr etc. In particular, transition metals that 
can exhibit multi-valent states are very suitable. The 
rare earth elements arc typically elements 58-71 of the 
periodic table, including Ce, Nd, etc. If an alkaline 
earth element (AE) were also present, the composition 
would be represented by the general formula RE-AE-TM-0. 

The ratio (AE,RE) : TM is generally approximately 1:1, 
but can vary from this as will be shown by examples where 
the ratio (AE,RE) : TM is 2:1. Of course, the amount 
of oxygen present in the final composition will adjust 
depending upon the processing conditions and will be 
such that the valence requirements of the system are 
satisfied. 

The methods by which these superconductive compositions 
can be made can use known principles of ceramic fabri- 
cation, including the mixing of powders containing the 
rare earth or rare earth-like, alkaline earth, and 



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transition metal elements, coprecipitation of these ma- 
terials, and heating steps in oxygen or air. 

A particularly suitable superconducting material in ac- 
cordance with this invention is one containing copper 

^ 2+ 

as the transition metal. Copper can exist in a Cu or 
3+ 

Cu mixed valence state. The state(s) assumed by cop- 
per in the overall composition will depend on the amount 
of oxygen present and on any substitutions in the crys- 
talline structure. Very high has been found in Cu- 
oxide systems exhibiting mixed valence states, as 
indicated by conductivity and other measurements. Cop- 
per oxide systems including a rare earth or rare earth- 
like element, and an alkaline earth element, are unique 
examples of this general class of superconducting lay- 
ered copper oxides which exhibit greater than 26**K. 

These and other objects, features, and advantages will 
be apparent from the following more particular de- 
scription of the preferred embodiments. 



Brief Description of the Drawings 



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FIG. 1 is a schematic illustration of a representative 
circuit used to measure dc conductivity in the high 
superconductors of this invent ion . 



FIG. 2 is a plot of the temperature dependence and 
resistivity in the composition Ba^La^_^Cu^O^ for 
samples with x(Ba)=l (upper two curves, left scale) and 
x(Ba)=0.75 (lower curve, right scale). The influence 
of current density through the composition is also 
shown . 



FIG. 3 is a plot of the low temperature dependence of 

resistivity in the composition ^^^^^S-x^^S^S (3-y) ^^^^ 
x(Ba))=l, for different annealing conditions (i.e., 
temperature and oxygen partial pressure. 



FIG. 4 is a plot of the low-temperature resistivity of 

the composition ^^^^^S-x^^S^S (3-y ) ^^^^^ x(Ba)= 0.75, 
recorded for different densities of electrical current 
through the composition. 



Description of the Preferred Embodiments 



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The superconductive compositions of this invention are 
transition metal oxides generally having a mixed valence 
and a layer-like crystalline structure, and exhibit T^'s 
higher than those of previously known superconducting 
materials. These compositions can also include a rare 
earth site in the layer-like structure where this site 
can be occupied by rare earth and rare earth-like atoms, 
and also by alkaline earth substitutions such as Ca, Sr, 
and Ba, The amount of oxygen present will be such that 
the valence requirements of the system are satisfied, 
the amount of oxygen being somewhat a function of the 
processing steps used to make the the superconductive 
compositions. Non-stoichiometric amounts of oxygen can 
be present in these compositions. The valence state of 
the elements in the oxide will be determined by the final 
composition in a manner well known to chemists. For 

example, the transition metal Cu may be present in some 

2+ 3+ 
compositions in both a Cu and a Cu state. 

An example of a superconductive compound having a 
layer-type structure in accordance with the present in- 
vention is an oxide of the general composition RE^TMO^, 
where RE stands for the rare earths (lanthanides) or 
rare earth-like elements and TM stands for a transition 
metal. In these compounds the RE portion can be par- 



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tially substituted by one or more members of the 
alkaline earth group of elements. In these particular 
compounds, the oxygen content is at a deficit. 

For example, one such compound that meets this general 
description is lanthanum copper oxide La^CuO^ in which 
the lanthanum - which belongs to the 1 1 IB group of 
elements - is in part substituted by one member of the 
neighboring IIA group of elements, viz. by one of the 
alkaline earth metals (or by a combination of the mem- 
bers of the IIA group), e.g., by barium. Also, the ox- 
ygen content of the compound can be incomplete such that 
the compound will have the general composition 

La^ Ba CuO, , wherein x < 0:3 and y < 0.5, 
2-x X 4-y' 

Another example of a compound meeting this general for- 
mula is lanthanum nickel oxide wherein the lanthanum is 
partially substituted by strontium, yielding the general 

formula La^ Sr NiO, . Still another example is cerium 
2-x X 4-y 

nickel oxide wherein the cerium is partially substituted 
by calcium, resulting in ^^2-x^^x^^^4-y * 

The following description will mainly refer to barium 
as a partial replacement for lanthanum in a La^CuO^ . 
compound because it is in the Ba-La-Cu-0 system that 



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many laboratory tests have been conducted. Some com- 
pounds of the general Ba-La-Cu-0 system have been de- 
scribed by C. Michel and B. Raveau in Rev. Chim. Min. 
21 (1984) 407, and by C. Michel, L. Er-Rakho and B, 
Raveau in Mat. Res. Bull., Vol, 20, (1985) 667-671. They 
did not, however, find or try to find superconductivity. 
These references and their teachings regarding 
perovskite-like layered oxides of mixed valent transi- 
tion metals, and their preparation, are herein incorpo- 
rated by reference. 



Experiments conducted in connection with the present 

invention have revealed that high-T^ superconductivity 

is present in compounds where the rare earth or rare 

earth-like element is partially replaced by any one or 

more of the members of the IIA group of elements, i.e., 

the alkaline earth metals. Actually, the T^ of 

2+ 

La„CuO, with the substitution Sr is higher and its 
2 4-y 

superconductivity- induced diamagnetism larger than that 

2+ 2+ 
found with the substitutions Ba and Ca 



The Ba-La-Cu-0 system can exhibit a number of 

crystal lographic phases, namely with mixed-valent copper 

constituents which have itinerant electronic states be- 

3+ 2+ 
tween non-Jahn-Tel ler Cu and Jahn-Teller Cu ions. 

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This applies likewise to systems where nickel is used 

3+ 

in place of copper, with Ni being the Jahn-Teller 

2+ 

constituent, and Ni being the non-Jahn -Teller con- 
stituent. The existence of Jahn-Teller polarons in 
conducting crystals was postulated theoretically by K.H. 
Hoeck, H- Nickisch and H. Thomas in Helv. Phys. Acta 56 
(1983) 237. Polarons have large electron-phonon inter- 
actions and, therefore, are favorable to the occurrence 
of superconductivity at higher critical temperatures. 

Samples in the Ba-La-Cu-0 system, when subjected to X- 
ray analysis, revealed three individual crystal lographic 
phases , viz. 

• a first layer-type perovskite-like phase, related 
to the 

K^NiF^ structure, with the general composition 

La^ Ba CuO, , with 
2-x X 4-y' 

X « 1 and y > 0; 

• a second, non-conducting CuO phase; and 

• a third, nearly cubic perovskite phase of the 
general composition La^ ^Ba^CuO^ ^ which appears 
to be independent of the exact starting composi- 
tion. 



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Of these three phases the first one appeared to be re- 
sponsible for the observed high-T^ superconductivity, 
the critical temperature showing a dependence on the 

2+ 

barium concentration in that phase. Obviously, the Ba 

2+ 

substitution caused a mixed-valent state of Cu and 
Cu^^ to preserve charge neutrality. It is assumed that 
the oxygen deficiency, y, is the same in the doped and 
undoped crystallites. 



In this application, the terms transition metal oxide, 
copper oxide, Cu-oxide, etc. are meant to broadly in- 
clude the oxides which exhibit superconductivity at 
temperatures greater than 26**K. Thus, the term copper 
oxide can mean, among other things, an oxide such as 

CuO, in the mixed oxide composition La ^Ba CuO,^ . 
A— V z—x X ^ y 



Both La^CuO, and LaCuO„ are metallic conductors at high 
temperatures in the absence of barium. Actually, both 
are metals like LaNiO^. Despite their metallic charac- 
ter, the Ba-La-Cu-0 type materials are essentially ce- 
ramics, as are the other compounds of the RE^TMO^, type, 
and their manufacture generally follows the known prin- 
ciples of ceramic fabrication. The preparation of a 
superconductive Ba-La-Cu-0 compound, for example, in 



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accordance with the present invention typically involves 
the following manufacturing steps.: 

• Preparing aqueous solutions of the respective 
nitrates of barium, lanthanum and copper and 
coprecipitatiori thereof in tlieir appropriate ra- 
tios , 

• adding the coprecipitate to oxalic acid and 
forming an intimate mixture of the respective 
oxalates . 

• decomposing the precipitate and causing a solid- 
state reaction by heating the precipitate to a 
temperature between 500 and 1200^C for one to eight 
hours . 

• pressing the resulting product at a pressure of 
about 4 kbar to form pellets. 

• re-heating the pellets to a temperature between 
500 and 900*^C for one half hour to three hours for 
sintering. 



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It will be evident to those skilled in the art that if 
the partial substitution of lanthanum by another 
alkaline earth element, such as strontium or calcium, 
is desired, the particular nitrate thereof will have to 
be used in place of the barium nitrate of the example 
process described above. Also, if the copper of this 
example is to be replaced by another transition metal, 
the nitrate thereof will obviously have to be employed. 
Other precursors of metal oxides, such as carbonates or 
hydroxides, can be chosen in accordance with known 
principles . 



Experiments have shown that the partial contents of the 

individual compounds in the starting composition play 

an important role in the formation of the phases present 

in the final product. While, as mentioned above, the 

final Ba-La-Cu-0 system obtained generally contains the 

said three phases, with the second phase being present 

only in a very small amount, the partial substitution 

of lanthanum by strontium or calcium (and perhaps 

beryllium) will result in only one phase existing in the 

final La^ Sr CuO. or La^ Ca CuO, , respectively, 
2-x X 4-y 2-x x A-y • 

provided x < 0.3. 



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With a ratio of 1:1 for the respective (Ba, La) and Cu 
contents, it is expected that the three phases will oc- 
cur in the final product. Setting aside the second 
phase, i.e. the CuO phase whose amount is negligible, 
the relative volume amounts of the other two phases are 

dependent on the barium content in the La_ Ba CuO, 
^ 2-x X 4-y 

complex. At the 1:1 ratio and with an x = 0.02, the 
onset of a localization transition is observed, i.e., 
the resistivity increases with decreasing temperature, 
and there is no superconductivity. 



With X = 0.1 at the same 1:1 starting ratio, there is a 
resistivity drop at the very high critical temperature 
of 35^K. 



With a (Ba, La) versus Cu ratio of 2:1 in the starting 

composition, the composition of the La2CuO^:Ba phase, 

which appears to be responsible for the 

superconductivity, is imitated, with the result that now 

only two phases are present, the CuO phase not existing. 

With a barium content of x = 0.15, the resistivity drop 

occurs at T = 26^K. 
c 

The method for preparing these Ba-La-Cu-0 sample com- 
plexes used two heat treatments for the precipitate at 



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an elevated temperature for several hours. In tlie ex- 
periments carried out in connection with the present 
invention it was found that best results were obtained 
at 90d*^C for a decomposition and reaction period of 5 
hours, and again at 900^C for a sintering period of one 
hour. These values apply to a 1:1 ratio composition as 
well as to a 2:1 ratio composition. 

For the 2:1 ratio composition, a somewhat higher tem- 
perature is permissible owing to the higher melting 
point of the composition in the absence of excess copper 
oxide. However, a one-phase compound was not achieved 
by a high temperature treatment . 



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Conductivity Measurements (FIGS. 1-4) 

The dc conductivity of representative Ba-La-Cu-0 com- 
positions was measured to determine their low temper- 
ature behavior and to observe their high T - These 

^ c 

5 measurements were performed using the well known four- 

point probe technique, which is schematically illus- 
trated in FIG. 1. Rectangular shaped samples 10 of 

Ba >La^ >Cu^O^,^ ^ were cut from sintered pellets, and 
X 5-x 5 5(3-y) 

provided with gold sputtered electrodes 12A and 12B, 
10 about 0.5 microns thick. Indium wires 14A and 14B con- 

tact electrodes 12A and 12B, respectively. The sample 
was contained in a continuous flow cryostat 16 
(Leybold-Hereaus) and measurements were made over a 
temperature range 300-412^K. 

15 Electrodes 12A and 12B are connected in a circuit in- 

cluding a current source 18 and a variable resistor 20, 
Indium leads 22A and 22B are pressed into contact with 
sample 10 and fixed with silver paint 24. Leads 22A, 
22B are connected to a voltage reading instrument 26. 

20 Since the current and voltage are accurately determined, 

the resistivity of the sample 10 is then known. In the 
configuration used for these measurements, a computer 
was used to provide a computer-controlled fully- 



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automatic system for temperature variation, data acqui- 
sition and processing. 

In FIG. 2, the low temperature dependence of resistivity 

(p, measured in ohm-cms) in the composition 

Ba La^ Cu^O^.^ ^ is plotted for two different values 
X 5-x 5 5(3-y) 

of X. For the upper two curves, the value of x(Ba) is 
1 and the left side vertical scale is used. For the 
lower curve, the value of x is 0.75, and the resistivity 
scale on the right hand side of the figure is used. The 

data is taken for different values of current density: 

2 2 
0.25 A/cm for the top curve and 0,50 A/cm for the 

middle and bottom curves . 

For barium-doped samples with x(Ba) < 1.0, for example 

2 

with X < 0.3, at current densities of 0.5A/cm , a high- 
temperature metallic behavior with an increase in 
resistivity at low temperatures was found as depicted 
in FIG, 2. At still lower temperatures, a sharp drop 
in resistivity (> 90%) occurred which for higher current 
densities became partially suppressed (FIG. 1 upper 
curves, left scale). This characteristic drop was 
studied as a function of the annealing conditions, i.e. 
temperature and oxygen partial pressure as shown in FIG, 
2. For samples annealed in air, the transition from 



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itinerant to localized behavior , as indicated by the 
minimum in resistivity in the 80°K range, was not found 
to be very pronounced. Annealing in a slightly reducing 
atmosphere, however, led to an increase in resistivity 
and a more pronounced localization effect. At the same 
time, the onset of the resistivity drop was shifted to- 
wards the 30°K region. Curves 4 and 5 (FIG. 3), recorded 
for samples treated at 900**C, show the occurrence of a 
shoulder at still lower temperatures, more pronounced 
in curve 6. At annealing temperatures of 1040*^C, the 
highly conducting phase has almost vanished. Long 
annealing times and/or high temperatures will generally 
destroy the superconductivity. 

The mixed-valent state of copper is of importance for 
elect ron-phonon coupling. Therefore, the concentration 
of electrons was varied by the Ba/La ratio, A typical 
curve for a sample with a lower Ba concentration of 0.75 
is shown in FIG. 2(right scale). Its resistivity de- 
creases by at least three orders of magnitude, giving 
evidence for the bulk being superconducting below 13°K 
with an onset around 35°K, as shown in FIG. 4 on an ex- 
panded temperature scale. FIG. 4 also shows the influ- 
ence of the current density, typical for granular 



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compounds. Current densities of 7.5, 2.5, and 0.5 A/cm 
were passed through the superconducting composition. 

When cooling the samples from room temperature, the 
resistivity data first show a mctal-like decrease. At 
low temperatures, a change to an increase occurs in the 
case of Ca substituted compounds and for the Ba- 
substituted samples. This increase is followed by a 
resistivity drop, showing the onset of superconductivity 
at 22 ± 2^K and 33 ± 2° K for the Ca and Ba compounds, 
respectively- In the Sr compound, the resistivity re- 
mains metallic down to the resistivity drop at 40 ± 1*^K. 
The presence of localization effects, however, depends 
strongly on alkaline-earth ion concentration and sample 
preparation, that is to say, on annealing conditions and 
also on the density, which have to be optimized. All 
samples with low concentrations of Ca, Sr, and Ba show 
a strong tendency to localization before the resistivity 
drops occur. 

Apparently, the onset of the superconductivity, i.e. the 
value of the critical temperature T^, is dependent on, 
among other parameters, the oxygen content of the final 
compound. It seems that for certain materials, an oxy- 
gen deficiency is necessary for the material to have a 



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high-T^ behavior. In accordance with the present in- 
vention^ the method described above for making the 
La2CuO^:Ba complex is complemented by an annealing step 
during which tlie oxygen content of the final product can 
be adjusted. Of course, what was said in connection with 
the formation of the La^CuO^rBa compound likewise ap- 
plies to other compounds of the general formula RE^TMO^ 
: AE (where AE is an alkaline earth element), such as, 
e.g. Nd^NiO^rSr, 

In the cases where a heat treatment for decomposition 
and reaction and/or for sintering was performed at a 
relatively low temperature, i.e., at no more than 950^C, 
the final product is subjected to an annealing step at 
about 900*^0 for about one hour in a reducing atmosphere. 
It is assumed that the net effect of this annealing step 
is a removal of oxygen atoms from certain locations in 
the matrix of the RE^TMO^ complex, thus creating a dis- 
tortion in its crystalline structure. The 0^ partial 
pressure for annealing in this case may be between 10 
and 10 ^ bar. 

In those cases where a relatively high temperature 
(i.e., above 950^C) is employed for the heat treatment, 
it might be advantageous to perform the annealing step 



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in a slightly oxidizing atmosphere. This would make up 
for an assumed exaggerated removal of oxygen atoms from 
the system owing to the high temperature and resulting 
in a too severe distortion of the system's crystalline 
structure. 



Resistivity and susceptibility measurements as a func- 

-y 



2+ 2+ 

tion of temperature of Sr and Ca^ -doped La2CuO^_ 



ceramics show the same general tendency as the 

9+ 

Ba" -doped samples: a drop in resistivity p (T) , and a 

crossover to diamagnetism at a slightly lower temper- 

2+ 

ature. The samples containing Sr actually yielded a 

2+ 2+ 

higher onset than those containing Ba and Ca*" . Fur- 
thermore, the diaraagnetic susceptibility is about three 

times as large as for the Ba samples. As the ionic ra- 

2+ 3+ 
dius of Sr nearly matches that of La , it seems that 

the size effect does not cause the occurrence of 

superconductivity. On the contrary, it is ratlier ad- 

2+ 2+ 

verse, as the data on Ba and Ca indicate. 



The highest T^ for each of the dopant ions investigated 
occurred for those concentrations where, at room tem- 
perature, the RE^ TM 0. structure is close to the 
^ * 2-x X 4-y 

orthorhombic-tetragonal structural phase transition, 
which may be related to the substantial elect ron-phonon 



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interaction enhanced by the substitution. The 

alkaline-earth substitution of the rare earth metal is 

clearly important, and quite likely creates TM ions with 

no e Jahn-Tellcr orbitals. Therefore, the absence of 
S 

these Jahn-Teller orbitals, that is, Jahn-Teller holes 
near the Fermi energy, probably plays an important role 

in the T enhancement. 

.c 

While examples have been given using different transi- 
tion metal elements in the superconducting compositions, 
copper oxide compositions having mixed valence appear 
to be unique and of particular importance, having 
superconducting properties at temperatures in excess of 
26*'K. These mixed valent copper compositions can in- 
clude a rare earth element and/or a rare earth-like el- 
ement which can be substituted for by an alkaline earth 
element. The amount of oxygen in these compositions 
will vary depending upon the mode of preparation and 
will be such as to meet the valence requirements of the 
composition. These copper-based compositions have a 
layer-like structure, often of a perovskite type. For 
a more detailed description of some of the types of 
crystallographic structures that may result, reference 
is made to the aforementioned publication by Michel and 



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Raveau in Rev. Chim. Min. 21, 407 (1984), and to C. 
Michel et al. Mat. Res. Bull., Vol. 20, 667-671 (1985). 

While the invention has been described with respect to 
particular embodiments thereof, it will be apparent to 
those of skill in the art that variations can be made 
therein without departing from the spirit and scope of 
the present invention- For example, while the range of 
compositions includes rare earth elements and transition 
metal elements, the ratios of these elements can be 
varied because the crystalline structure can accommodate 
vacancies of these elements and still retain a layer- 
like structural phase exhibiting superconductivy . 

Further, the stoichiometry or degree of non- 
stoichiometry of oxygen content (i.e., oxygen deficit 
or surplus) of these compositions can be varied by using 
reducing or oxidizing atmospheres during formation of 
the compounds and by using different doping amounts in 
the rare earth and transition metal sites of the crystal 
structure. This type of distortion of the crystal 
structure and the many forms that it can encompass are 
readily apparent from reference to the aforementioned 
Michel and Raveau publications. Thus, the invention 
broadly relates to mixed (doped) transition metal oxides 



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having a layer- like structure that exhibit supercon- 
ducting behavior at temperatures in excess of 26°K. Of 
these materials, a mixed copper oxide having multi- 
valent states provides high and favorable supercon- 
ducting properties. 



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