Skip to main content

Full text of "USPTO Patents Application 08479810"

See other formats


JOURNAL OF SOLID STATE CHEMISTRY 39, 120-127 (1981) 



Oxygen Defect K 2 NiF 4 -Type Oxides: The Compounds 

La 2 -jSr x Cu04- x / 2 +6 



NINH NGUYEN, JACQUES CHOISNET, 1 MAR YVONNE HERVIEU, 
and BERNARD RAVEAU 

Laboratoire de Cristallographie et Chimie du Solide, L.A. 251 , ISMRA, 
Universite de Caen, 14032 Caen Cedex, France 

Received December 29, 1980; in final form February 18, 1981 



Oxygen defect K 2 NiF 4 -type oxides La^S^di 0 4 - xn + 6 have been synthesized for a wide composition 
range :0 < x < 1.34. From the X-ray and electron diffraction study three domains have been 
characterized: orthorhombic compounds with La 2 CuO< structure for 0 ^ jr < 0. 10, tetragonal oxides 
similar to LaSrCu0 4 for 0. 10 ^ x < I and several superstructures derived from the tetragonal cell {a = 
rt-flLasrcuo, w * tn " = 3» 4, 4.5, 5, 6) for 1 ^ x ^ 1.34. The compounds corresponding to 0 < x < 1 differ 
from the other oxides in that they are characterized by the presence of copper with two oxidation 
states: + 2 and + 3. A model structure for Lao.8Sri. 2 CuA0 3 . 4 , in which copper has only the + 2 oxidation 
state, and for which the actual cell is tetragonal — a = 18.80< A and c = 12.94 A — has been established. 
The particular structural evolution of these compounds is discussed in terms of a competition between 
the capability of Cu(LT) to be oxidized to Cu(III) and the ordering of oxygen vacancies. 



Introduction 

A lot of oxides, with the A 2 M0 4 formula, 
characterized by the intergrowth of 
perovskite- and sodium chloride-type 
layers are known at the present time. Con- 
trary to the perovskite oxides, no oxygen 
defect has been observed for this structural 
series to our knowledge. Copper, due to its 
ability to take different coordinations 
smaller than six, is a potential candidate 
which could form such anion defect com- 
pounds. However the only isostructural 
copper compounds which have been syn- 
thesized, La2Cu0 4 (/, 2) and SrLaCu0 4 (J) 
are stoichiometric. Nevertheless, the re- 
cent results concerning the oxides 
Laa-^Aj+xOe-^ (A = Ca, Sr) (4), whose 



1 Author to whom reprint requests should be ad- 
dressed. 



structure is strongly related to that of 
Sr 3 Ti 2 0 7 (5) suggest the possibility of oxy- 
gen defect for A 2 Cu0 4 compounds. Thus, 
the present work deals with the oxides 
La 2 _xSr J Cu0 4 _x/2+6 ) for which the replace- 
ment of lanthanum by strontium leads to 
the formation of oxygen vacancies, involv- 
ing order phenomena. 

Experimental 

For the synthesis of the compounds of 
the system La 2 Cu0 4 -Sr 2 Cu0 3 , SrC0 3 , CuO 
and La20 3 were mixed according to the 
following ratios: (2 - x)/2La 2 0 3 /x SrC0 3 /l 
CuO. All these reactions were made in a 
platinum crucible in air. The synthesis of 
the compounds with high purity strongly 
depends on the temperature for a fixed 
pressure. The mixtures were thus first 
heated for 5 hr at 9(XT C, and then at tem- 



0022-4596/81/100I20-08$02.00/0 

Copyright © 1981 by Academic Press, Inc. 
All rights of reproduction in any form reserved. 



120 



peratures ranging : 
12 hr. 

The oxidation s 
oxygen defect, was 
the compounds by 
reactions were foll< 
try using a Setaran 

The crystallogn 
lished by two com; 
ray diffractometry 
with a Philips goni« 
fraction using an 
scope. 

Results 

Study of the Systen 
The Compounds Lc 

According to the 
scribed, K 2 NiF 4 -t) 
sponding to the 
La 2 _ x SrxCu»0 4 _ x/2 
large composition r 
microthermogravim 
ides under hydros 
that a part of Cu(I 



( 



Range 


X 




I 


0 


0 




0.08 


o.o: 


II 


0.25 


0.0( 




0.33 3 


o.i: 




0.50 


0.K 




0.66 6 


0.05 




0.88o 


0.0* 


III 


1.00° 


0.0 




1.28° 


0.0 




1.34° 


0.0 




1.20 


0.0 



° The "a" parameters 
composition are given in 



o 

o 

a 

fe 

0> 



OXYGEN DEFECT K 2 NiF<-TYPE OXIDES 



121 



peratures ranging from 1000 to 1200°C for 
12 hr. 

The oxidation state of copper, i.e., the 
oxygen defect, was determined by reducing 
the compounds by hydrogen: the reduction 
reactions were followed by thermogravime- 
try using a Setaram microbalance. 

The crystallographic data were estab- 
lished by two complementary methods: X- 
ray diffractometry using CuKa radiation 
with a Philips goniometer and electron dif- 
fraction using an EM 200 Philips micro- 
scope. 

Results 

Study of the System La 2 Cu0 4 -Sr 2 Cu0 3 : 
The Compounds La 2 - x Sr x CuO A _ xn + h 

According to the methods previously de- 
scribed, K 2 NiF 4 -type compounds corre- 
sponding to the nominal composition 
La 2 _ x Sr x Cu II 0 4 _ J/ 2 were synthesized in a 
large composition range: 0 ^ x ^ 1 .34. The 
microthermogravimetric study of these ox- 
ides under hydrogen showed, however, 
that a part of Cu(II) had been oxidized to 



Cu(III), leading to the formula 
La 2 _ x Sr x Cu0 4 - x/ 2+6 with 0 < 5 < 0. 12. For 
jc > 1.34 a mixture of the K 2 NiF 4 -type 
phase and Sr 2 Cu0 3 (6) was observed. 

The crystallographic data of different 
compositions are summarized in Table I. 
The study of the X-ray patterns showed a 
continuous evolution of the structure and 
allowed to characterize three composition 
ranges which were studied by electron dif- 
fraction. 

(I) 0 < x < 0.10. The X-ray patterns 
very similar to that of La2Cu0 4 (/) were 
indexed in an orthorhombic cell with: 

a, = 2a p sin fi/2 = a^cuo** 
b x = 2a p cos p/2 = 6 La2 cuo 4 > 

C l — C La 2 Cu0 4 » 

where a p is the parameter of the perovskite 
cubic cell, and /J defines the monoclinic 
distortion of the cell. 

From the conditions limiting possible 
reflections— AW :h + k 9 l + h, k + I = In— 
three space groups are possible: Fmmm, 
Fmm2, and F222. 



TABLE I 

Crystallographic Data of La 2 _ x Sr J .Cu0 4 „ x /2 + « Compounds 



Heating 



Range 


X 


5 


Composition 


a 

(A) 


b 

(A) 


c 

(A) 


temperature 

rc) 


I 


0 


0 


La,Cu0 4 


5.366(2) 


5.402(2) 


13.149(4) 


uoo 




0.08 


0.030(1) 


LaiaSrooeCuOaw 


5.351(1) 


5.368(2) 


13.200(5) 


1000 


II 


0.25 


0.060(4) 


La! 75Sro.15CuOj.935 


3.775(2) 




13.247(5) 


1000 




0.33 3 


0.119(4) 


La 1 . 66 S S I"o .33 jCU 0 3 . 95 


3.776(1) 




13.250(2) 


uoo 




0.50 


0.100(4) 


L^ t . 50 . 50 Cll Oj . fig 


3.773(1) 




13.204(3) 


1160 




0.66 6 


0.092(4) 


Lai.33 s Sro.« 6 Cu03.75 B 


3.775(1) 




13.150(4) 


1170 




0.88o 


0.088(4) 


La 1 . l2 Sr 0 .88CuO 3 . e4l 


3.773(1) 




13.073(5) 


1170 


III 


1.00° 


0.0 


LaSrCuO 3 .50 


3.767(1) 




13.002(3) 


1200 




1.28° 


0.0 


La 0 . 72 Sr 1 . M CuO 3 . 36 


3.761(2) 




12.922(9) 


1200 




1.34° 


0.0 


Lao.GeS^^CuOs^ 


3.759(3) 




12.907(9) 


1200 




1.20 


0.0 


Lao.goSri^oCuOj^o 


18.803(7) 




12.941(7) 


1200 



° The "a" parameters of these compounds (range III) are those of the tetragonal subcell; n values for every 
composition are given in Table II. 



a 
o 

o 

O 



< 

CO 



122 



NGUYEN ET AL. 



(//) 0. 10 < .r < I . The symmetry is tetra- 
gonal like that of LaSrCu0 4 (3); the cell 
parameters are related to the latter and to I 
in the following manner: 

«II =* 0,/2 1/2 =* fl p = aLaSrCuCV 
C H ~~ C, ~ CLaSrCuQ^j. 

The reflection conditions are those of 
LaSrCu0 4 — hkl: h + k + 1= In— involving 
the space groups: I4/mmm t I4/m, 1411 and 
1 41m. 

(Ill) I < x < 1.34. The X-ray diffracto- 
grams are characterized by the existence of 
a system of strong peaks, which was al- 
ready observed for the compounds (II), 
involving at least the existence of a tetra- 
gonal subcell of the same type. However, 
for all these patterns, weak peaks were 
always observed which could not be in- 
dexed in this cell. An electron diffraction 
study was thus undertaken: about 50 crys- 
tals were examined for each value of x 
given in Table II. Several types of crystals 
were isolated: 

— Small number of crystals, about 10%, 
were characterized by a tetragonal cell sim- 
ilar to that of LaSrCu0 4 : 

a m ~ All ~~ O p - flLaSrCu0 4 » 
C \U — c ll — C x ~ CLaSrCuo^. 

— Most of the crystals, i.e., about 90%, 
presented, in addition to the fundamental 
reflections previously described, super- 
structure reflections with a variable inten- 



sity. The electron diffraction patterns al- 
lowed us to find the following relations for 
the actual tetragonal cell for a composition 

x: 



a in - na m - na lu 
c \l\ 



TABLE II 

n Values Observed by Electron Diffraction 
for Compounds of Range III 



Composition 


X 




n 


LaSrCu0 3 . 5 


1.00 


1; 


4.5 


LaossSr, uCuOa^ 


1.12 


I; 


4.5; 5 


Lao.80SiY20CuO.M0 


1.20 


5 




Lao.72SiY2aCuO3.36 


1.28 


1; 


4.6; 5; 5.3; 5.4 


Lao.eeSr^CuO^ 


1.34 


i; 


4; 5; 5.6; 6 



For a same composition jc, several sorts of 
superstructures were generally observed, 
characterized either by integral n values (n 
= 4, 5, or 6) or nonintegral values of n (n 
ranging from 4.5 to 5.6), as shown for 
several compositions in Table II. Figure 1 
shows, as an example, the electron diffrac- 
tion patterns of the (001) planes for 
La^Sr^CuC},.*- From Table II it can be 
seen that a pure term, characterized by a 
superstructure with an integral value of n (n 
= 5), is only obtained for x = 1.20. It has 
thus been attempted to elaborate a struc- 
tural model for this phase. 

A Structural Model for La 0S Sr u2 CuO 34 

The actual cell of this compound is tetra- 
gonal: a = I8.80 4 A and c = 12.94 A (Z = 
50). The conditions limiting possible 
reflections are the same as those of the 
subcell (a = 3.760, c = 12.94 A; Z = 2), 
leading to the space groups I4/mmm t 
I4/m, 1411, and /42m. The intensity calcu- 
lations were first made in the K 2 NiF 4 type 
cell, with the most symmetric space group 
lA/mmm. For these calculations, 
reflections corresponding only to the 
subcell were used. Copper atoms were 
placed on 1(a) , lanthanum and strontium 
atoms were statistically distributed on 4e, 
and oxygen atoms and anionic vacancies 
were statistically distributed over two sorts 
of sites 4e (d) and 4c (O n ). After 
refinement of the atomic parameters the 
discrepancy factor could not be lowered 
below R = 0. 104. The possibility of an 
order of the oxygen atoms and vacancies 
over the O, and 0„ sites was thus consid- 
ered. The occupancy factors of both sites 



were refined successi 
neously and the bes 
(Table IV) was obtaii 

TAB 

Lao.8oSr 120 0 3 ,4o* Atom P< 





Sites 


-V 


y 


La] 








Sr] 


4(f) 


0 


0 


Cu 


2(a) 


0 


0 


o, 


Me) 


0 


0 




4(c) 


0 


0,5 


a a 


« 3.760 A; 


c = 


12.9-= 



OXYGEN DEFECT K 2 NiF 4 -TYPE OXIDES 



123 



erns al- 
ions for 
position 



sorts of 
served, 
ilues (n 
of n (n 
wn for 
? igure 1 
diffrac- 
tes for 
can be 
id by a 
; of n (/i 
. It has 
i struc- 



s tetra- 

Mz = 

ossible 
of the 

: = 2), 

Immm, 
calcu- 
F 4 type 
: group 
ations, 
:o the 
; were 
Dntium 
on 4e, 
■ancies 
b sorts 
After 
:rs the 
>wered 
of an 
:ancies 
:onsid- 
h sites 



4 




Fig. 1. Electron diflFraction patterns of the (001) planes for La 2/3 Sr 4/3 Cu0 3 . 33 : (a) n = I; (b) 4; (c) 5.6; 
(d) 6. 



were refined successively and then simulta- 
neously and the best value of R = 0.081 
(Table IV) was obtained for a total occupa- 

TABLE III 

I^o.wSrj.joOa^o: Atom Positions in the Subcell q 



B 





Sites 


X 


y 




2 




(A 2 ) 


La] 


Me) 














Sr] 


0 


0 


0.357 




0.001 


0.88 


Cu 


2(a) 


0 


0 




u 




0.85 


0, 


4(e) 


0 


0 


0.168 




0.002 


1.68 


o„ 


4(c) 


0 


0,5 




0 




4.25 


Q a 


= 3.760 A; 


c = 


12.94 A. 











tion of the 0! sites, while vacancies and 
oxygen atoms were distributed over the O n 
sites. The location of the vacancies prefer- 
entially on the 0„ sites, at the same level as 
the copper atoms, can be considered as 
significant, on account of the relatively 
weak scattering factor of oxygen. This is 
confirmed by the high R value (R = 0. 153) 
obtained for a total occupation of the O n 
sites, vacancies and oxygen atoms being 
distributed on the Oi sites. The first results 
which are summarized in Table III show 
the atoms are located in positions very 
close to those usually observed in K 2 NiF 4 
type structures. The main difference with 



124 



NGUYEN ET AL. 



TABLE IV 



Lao.8Sr,. 2 Cu03.4: Observed and 
Calculated Intensities for Atomic 
Positions of Table III 0 



n k I 




'calc 


0 0 2 


4.0 


4.0 


1 0 1 


13.0 


15.1 


0 0 4 


17.0 


16.9 


1 0 3 


164.0 


156.1 


1 1 0 


114.0 


115.1 


1 1 2 


0.1 


1.7 


0 0 6 


29.0 


23.6 


1 0 5 


27.0 


23.5 


1 1 4 


35.0 


34.6 


2 0 0 


44.0 


49.8 


2 0 2 


0.1 


0.4 


1 1 61 


26.0 


25.2 


2 1 |J 


3.9 


3.8 


1 0 7 


12.0 


11.1 


2 0 4] 


10.3 


8.2 


0 0 8j 


6.6 


5.3 


2 1 3 


48.0 


48.1 


2 0 6] 


15.8 


18.1 


2 I 5J 


8.1 


9.4 


1 1 8 


9.0 


7.5 


1 0 9 


0.1 


1.7 


22 0 


9.0 


12.4 


2 2 2 


0.1 


0.1 


0 0 10 


0.1 


0.8 


3 0 1 


0.1 


0.7 


2 1 7 


6.0 


7.0 


2 2 4] 


3.3 


3.0 


2 0 8] 


7.6 


6.9 


30 3 


7.0 


8.8 



a Subcell, space group lAjmmm \ R = 
0.081. 



the ideal structure concerns the existence 
of vacancies located in the same plane as 
the copper atoms (Fig. 2). Moreover, the 
high B value for oxygen of O, sites (4.2 A 2 ) 
suggests that in this plane oxygen and va- 
cancies were ordered. 

Calculations in the actual cell in space 
group I4/mm> were undertaken with 136 
possible reflections, including superstruc- 
ture reflections. Using the position and 
distributions determined from the subcell, 
the R factor increased to 0. 104, showing, of 
course, a weak contribution of the super- 
structure reflections to the R value. The 




Fig. 2. Ideal drawing of the tetragonal K 2 NiF 4 -type 
structure showing the localization of oxygen vacancies 
for Lao. 8 Sr 12 Cu0 3 . 4 . 

atomic parameters were then refined and 
the R value was lowered to 0.07 for the final 
atomic parameters given in Table V. From 
this table it can be seen that copper atoms 
are not significantly displaced from their 
ideal positions, while the bigger cations La, 
Sr, and the oxygen atoms are only slightly 
displaced from their ideal positions, but 
enough to produce the superstructure 
reflections. These small displacements are 
certainly induced by an order of the oxygen 
vacancies, whose contribution to intensi- 
ties is too small to be detected here. Thus, 
on account of the numerous possibilities of 
order between vacancies, and oxygen 
atoms, and of the weak scattering power of 
these atoms, we did not try any hypothesis 
of distribution. Nevertheless, the very 
likely ordering of vacancies in the 4 'copper 
plane," should also involve an ordering of 
lanthanum and strontium over the different 
sites. Refining the occupancy factors of La 
and Sr, led to an R value of 0.064 which is 
not very significant due to the weak contri- 
bution of La and Sr to the superstructure 
reflexions; a preferential occupation of the 
different sites is, however, likely: A l9 A 4 , 
and A h would only be occupied by stron- 
tium, while lanthanum would occupy 90% 
of A 6 sites, the remaining strontium and 
lanthanum atoms being located statistically 
over the A 2 and A 3 sites. 



T. 

Lao^Sr^SuOa.,: At 
Act 



Sites 




A Me) 


o 


A,(16/i) 


0. 194 


A«(l6n) 


0.403 


A A 16m) 




AA\6m) 


0.410 


A«02O) 


0 IRQ 


AJ7n) 


n 




u.zuu 


AJ&i) 




A..(8A) 


0.200 




0.405 


A. .(16/1 




A l3 (4e) 


o 


A„(I6*) 


0.216 


A I5 (I6*) 


0.382 


A 16 (I6m) 


0.182 


A 17 (l6m) 


0.400 


A IB (320) 


0.400 


A l9 m 


0.100 


A zo m 


0.300 


A»(4c) 


0 


A n (\6l) 


0.214 


^ n (l6/) 


0.430 


A M (I6/) 


0.300 


A t5 (16/) 


0.390 


A„(8/) 


0.200 


A„m 


0.400 


a a = 18.804 


A; c = 


IJmmm). 





Discussion 

The stabilization, 
Cu(III) by only heati 
worthy of note. Bu 
characteristic of this 
existence of a Cu(III 
< v < I) which lie; 
regions (x = 0 and ; 
strongly related one i 
be explained by two ( 
are competitive: the 
stoichiometric K 2 Nr 
La2Cu0 4 and LaSrO 
form a related defect 




OXYGEN DEFECT K 2 NiF,-TYPE OXIDES 



F 4 -type 
cancies 



:d and 
le final 
From 
atoms 
i their 
>ns La, 
lightly 
is, but 
ucture 
tits are 
oxygen 
ntensi- 
. Thus, 
ities of 
sxygen 
>wer of 
othesis 
i very 
copper 
:ring of 
iflferent 
s of La 
/hich is 
contri- 
ructure 
i of the 
A i , A 4 , 
/ stron- 
py 90% 
urn and 
istically 



TABLE V 

LaoaSrj jSuOa^: Atomic Parameters of the 
Actual Cell" 



Sites 


X 


y 


z 


B 

(A 2 ) 


A x m 


0 


0 


0.347 


0.35 


A,(l6n) 


0.194 


0 


0.359 


1.00 


A 3 (I6*) 


0.403 


0 


0.356 


0.39 


A 4 (16m) 


0.200 


0.200 


0.357 


0.32 


A 5 (l6m) 


0.410 


0.410 


0.358 


1. 00 


A 6 (320) 


0.389 


0.192 


0.357 


0.86 


A 7 (2a) 


0 


0 


0 


0.80 


A 8 (8i) 


0.200 


0 


0 


0.51 


A,(8/) 


0.400 


0 


0 


0.43 


A 10 (8/i) 


0.200 


0.200 


0 


0.31 


A u (Sh) 


0.405 


0.405 


0 


1.00 


A U (I6/) 


0.403 


0.205 


0 


0.37 


A l3 (4e) 


0 


0 


0.168 


1.00 


A 14 (l6/i) 


0.216 


0 


0.168 


1.00 


A 15 (16/i) 


0.382 


0 


0.168 


1.00 


A 16 (l6m) 


0.182 


0.182 


0.172 


1.00 


A i7 (l6m) 


0.400 


0.400 


0.168 


1.00 


A l8 (320) 


0.400 


0.202 


0.163 


1.00 


A 1B (8/) 


0.100 


0 


0 


1.00 


A 20 (8» 


0.300 


0 


0 


1.00 


A n (4c) 


0 


0.500 


0 


1.00 


A«(I6/) 


0.214 


0.100 


0 


1.00 


A B (I6() 


0.430 


0.100 


0 


1.00 


A«(I6/) 


0.300 


0.200 


0 


1.00 


A„(160 


0.390 


0.310 


0 


1.00 


A„(8j) 


0.200 


0.500 


0 


1.00 


A^j) 


0.400 


0.500 


0 


1.00 


° « = 18.804 
IJmmm). 


A; c 


- 12.941 


A (space 


group 



Discussion 

The stabilization, in this system, of 
Cu(III) by only heating the oxides in air is 
worthy of note. But the most important 
characteristic of this system concerns the 
existence of a Cu(III) composition range (0 
< x < 1) which lies between two Cu(II) 
regions (x = 0 and x > 1), for structures 
strongly related one to the other. This can 
be explained by two opposite effects which 
are competitive: the trend to preserve a 
stoichiometric K 2 NiF 4 structure as for 
La2Cu0 4 and LaSrCu0 4 and the trend to 
form a related defect structure but with an 



ordering of the oxygen vacancies. Thus, 
rather close to the stoichiometric com- 
pound La2Cu0 4 (x < I), the trend to stoi- 
chiometry is favored and the vacancies 
formed from the nominal compositions in- 
volving only Cu(II) are partly balanced by 
the oxidation of Cu(II) to Cu III. For x > 1 , 
i.e., rather far from stoichiometry, the 
La 2 Cu0 4 or "LaSrCuCV' stoichiometric 
compounds cannot be stabilized any more 
and orderings of the oxygen vacancies ap- 
pear leading to different microphases as 
observed from the electron diffraction 
study, favoring Cu(II) with smaller coordi- 
nations (2, 5). 

Structure is not, of course, the only fac- 
tor governing the relative stability of Cu(II) 
and Cu(III) in these oxides. Kinetics play 
an important part for determining the ratio 
Cu(III)/Cu(II) in the richer Cu(III) oxides. 
For 0 < x < 1 , we have indeed noticed that 
the pure compounds could only be synthe- 
sized by heating at least 12 hr at the forma- 
tion temperature (Table I) in order to en- 
sure a good crystallization. Annealing the 
same samples at the same temperature, 
during longer periods (24 hr) allows us to 
prepare pure phases with the same struc- 
ture, but with greater amount of Cu(III). 
The oxygen pressure will also influence the 
Cu(III)/Cu(II) ratios. Heating, for exam- 
ple, some Cu(III) samples at low tempera- 
ture under vacuum, involves a decrease of 
Cu(III) amount without destroying the 
structure. In the same way, a reaction 
under oxygen allows us to increase the 
Cu(III) amount. 

The influence of the Cu(III) amount can 
also be detected by considering the struc- 
tural evolution, especially the c parameter, 
of these compounds as a function of com- 
position (Fig. 3). This evolution is rather 
complex and quite different from that usu- 
ally observed for single solid solutions. The 
substitution of strontium for lanthanum, 
should not affect this evolution, due to the 
similar sizes of these cations. It seems 
interesting to take the Cu(II) compounds as 



126 



NGUYEN ET AL. 



The influence of the o: 
formation of these str 
tigated.The relations 1 
properties and the stn 
will be studied. 



112 - 



'La 2 Cu 0 A 



13.1. 



References 

/. J. M. LONGO AND P. M 

Chem. 6, 526 (1973). 



13.0- 



[ia Sr Cu0 35 



, , ! | 1 1 1 1 1 1 1 

02 0< 06 0.8 io 1.2 

Fig. 3. Evolution of the "c" parameter as a function of x. 



a reference (dotted lines). Although we 
have only our compositions for comparison 
it can be seen that from La2Cu n 0 4 to 
La^Sr^Cu'^s, a continuous decrease of 
a and c parameters could be foreseen for all 
Cu(II) compounds, as x increases, in agree- 
ment with the increase of oxygen vacan- 
cies. This evolution is not linear, probably 
due to ordering of the vacancies observed 
for different compositions. What is worthy 
of note is the large deviation from this law 
observed for the only compounds contain- 
ing Cu(III) (continuous line): the c parame- 
ter is greater than that obtained from the 
"reference line" corresponding to the pres- 
ence of Cu(II) only, while the a parameter 
is smaller. Moreover, the largest devia- 
tions are observed for x = 0.33 which 
corresponds to the maximum value of 5 (5 
= 0.119), i.e., for the greatest amount of 
Cu(IlI). It can thus be observed that the c/a 



ratio increases with the Cu(III)/Cu(II) ratio 
in agreement with the observations previ- 
ously made by Goodenough et al. (3). At- 
tempts to modify the a and c parameters for 
x = 0. 16 and 0.5, were successful: heating 
these compounds under vacuum at 500°C 
led to a decrease of c and a slight increase 
of a , while a decrease of the Cu(III)/Cu(II) 
ratio was confirmed. 

Conclusion 

The stabilization of a great number of 
oxygen vacancies in the K 2 NiF 4 -type struc- 
ture has been shown. It is easily explained 
by the ability of copper to show square and 
square-pyramidal coordinations. During 
the synthesis in air, two phenomena are 
competitive: the substitution of Cu 3+ for 
Cu 2+ , and ordering of oxygen and vacancies 
involving the existence of microphases. 



O 
O 

JD 
2 



CO 

CD 
CO 



OXYGEN DEFECT K 2 NiF 4 -TYPE OXIDES 



127 



The influence of the oxygen pressure on the 
formation of these structures will be inves- 
tigated. The relations between the electrical 
properties and the structure of these oxides 
will be studied. 



References 

/. J. M. LONGO AND P. 

Chem. 6, 526(1973). 



4. 



5. 



M. Raccah, J. Solid State 6. 



H. K. MULLER-BUSCHBAUM, AND M. 

Z. Anorg. Allg. Chem. 428, 120 



B. Grande 
Schweizer 
(1977). 

J. B. Goodenough, G. Demazeau, M. Pou- 
chard, and P. Hagenmuller, J. Solid State 
Chem. 8, 325 (1973). 

N. Nguyen, L. Er-Rakho, C. Michel, J. 
Choisnet, and B. Raveau. Mater. Res. Bull. 15, 
891 (1980). 

S. N. Ruddlesden and P. Popper, Acta Crystal- 
logr. 11, 54 (1958). 

C. H. R. L. Teske and H. K. Muller-Busch- 
baum, Z. Anorg. Allg. Chem. 371, 325 (1969). 



[I) ratio 
s previ- 
J). At- 
ters for 
heating 
t 500°C 
ncrease 
./Cu(II) 



aber of 
e struc- 
plained 
are and 
During 
ena are 
:u 3+ for 
cancies | 
phases. $ 

1