Multi-layer ceramic capacitors having, as the conductive elements therein, layers of perovskites containing oxygen and nitrogen

The present invention relates to new conductive perovskites containing oxygen and nitrogen, their preparation and their use particularly as electrode material. The conductive perovskites containing oxygen and nitrogen according to the invention correspond to the general formula I EQU AB(O,N).sub.3 (I) in the cationic lattice of which: PA0 A denotes a metal chosen from the metals of groups IA and IIA, yttrium and the lanthanides, PA0 B denotes a metal chosen from the transition metals of groups IVA to IB, on the express condition that at least one of the metals A and B is present in an oxidation state other than its normal maximum oxidation state.

The present invention relates to new perovskites containing oxygen and 
nitrogen and endowed with conductive properties permitting their use as 
electrode material, in particular as an electrode in multilayer ceramic 
capacitors. The invention also relates to the process for the preparation 
of these new perovskites. 
Patent Application EP-A-No. 0,184,951 describes perovskites containing 
oxygen and nitrogen of formula ABO.sub.3 -.sub.n N.sub.n, in the cationic 
lattice of which the metal cations A and B are present in their normal 
maximum oxidation state. Such perovskites exhibit dielectric properties. 
The present invention relates to perovskites containing oxygen and nitrogen 
corresponding to the general formula I: 
AB(O,N).sub.3 (I) 
in the cationic lattice of which: 
A denotes a metal chosen from the metals of groups IA and IIA, yttrium and 
the lanthanides, 
B denotes a metal chosen from the transition metals of groups IVA to IB, 
on the express condition that at least one of the metals A and B is present 
in an oxidation state other than its normal maximum oxidation state.

The invention is aimed more particularly at the perovskites of general 
formula I in which A is chosen from the group comprising Ca, Sr, Ba, La, 
Pr, Nd, Sm, Eu and Ce, and B is chosen from the group comprising W, Mo, V, 
Nb, Ta and Ti. 
In the cationic lattice of the perovskites according to the invention, the 
metal B will generally be present in an oxidation state lower than its 
normal maximum oxidation state, that is to say that the presence of, for 
example, Ti(III), V(IV) or V(III), Mo(V) and W(V) will be observed. 
However, in some particular cases, the metal B will be present in an 
oxidation state higher than its normal maximum oxidation state. Thus, the 
presence of Cu(III) or of Fe(IV) will be observed, for example. In the 
case of copper, there will therefore be a delocalization of the electrons 
or a mixed valency characterized by the simultaneous presence of Cu(II) 
and Cu(III). 
According to another feature of the invention, the perovskites containing 
oxygen and nitrogen are those wherein at least one of the metals A and B 
is present in a mixed valency state. 
The presence in the cationic site of certain elements such as copper, which 
has a preference for a lower coordination and tends towards the planar 
square coordination, gives rise, in the perovskites of the invention, to 
vacant anionic sites which may attain up to approximately 20% of the 
anionic elements. 
According to another feature of the invention, the perovskites may also 
have a cationic vacancy in the metal A. Thus, for example, in the case 
where the metal A is lanthanum, stoichiometry defects exist in the site A, 
and these lead to a mixed valency in the associated site B, for example 
the presence of vanadium with valencies III and IV. 
All these stoichiometric anomalies, vacant anionic or cationic sites are 
reflected in a mixed valency of one of the elements in the cationic 
lattice, that is to say by an electron delocalization which is responsible 
precisely for the conductive properties of the perovskites of the 
invention. These electrical properties, as a function of the precise 
nature of the perovskites according to the invention, correspond to a 
semiconductor state of n or p type, semimetallic or else metallic. In all 
the cases, these properties may be exploited when the conductive 
perovskites according to the invention are employed as electrode material, 
particularly as a capacitor electrode. The preferential application of 
these conductive perovskites of general formula I relates to capacitors 
with a multilayer ceramic structure which are obtained by cosintering 
conductive perovskites according to the invention with dielectric 
perovskites such as described in patent application No. EP-A-No. 0 184 
951. 
Lastly, the present invention relates to the process for the preparation of 
the conductive perovskites of general formula I. This process consists in 
subjecting a mixed oxide of metals A and B to nitriding under a stream of 
ammonia at a temperature of between 700.degree. C. and 900.degree. C. In 
particular, the oxide will be chosen from tungstates such as CaWO.sub.4, 
SrWO.sub.4, BaWO.sub.4 and Ln.sub.2 W.sub.2 O.sub.9, molybdates such as 
SrMoO.sub.4 and vanadates such as LaVO.sub.4 and LaVO.sub.3. 
Some particular examples of the abovementioned perovskites containing 
oxygen and nitrogen and corresponding to the general formula I will be 
mentioned below by way of illustration. All these perovskites exhibit a 
remarkable electrical conductivity. 
Other features and benefits of the present invention will become apparent 
from reading the detailed description which is given below with reference 
to various particular examples of preparation and of characterization of 
conductive perovskites, which are given by way of illustration. 
EXAMPLES 1 to 4 
Ca(Sr,Ba)-W and Sr-Mo pairs 
Double oxides with a scheelite structure of the CaWO.sub.4 type are used as 
starting materials. The alkaline-earth metal or lanthanide tungstates or 
molybdates are prepared, account being taken of the sublimation of 
tungsten or molybdenum oxides at high temperature. 
When the reaction between these oxides and the alkaline-earth metal oxide 
originating from the thermal decomposition of the carbonate takes place at 
a temperature such that the losses of tungsten or molybdenum oxide by 
sublimation can be neglected, the former may be carried out in a single 
stage. Thus, for example, the preparation of the scheelite phases 
Ca(Sr,Ba)WO.sub.4 is carried out by heating stoichiometric quantities of 
calcium carbonate and of the oxide WO.sub.3 in a muffle furnace, in an 
alumina crucible. The temperature is set at 750.degree. C. for a period of 
72 hours. 
On the other hand, preparation of compounds of formula Ln.sub.2 W.sub.2 
O.sub.9 or Ln.sub.2 WO.sub.6 requires considerably higher temperatures. 
The operation is then performed in two successive stages. 
The first stage consists in precombining the oxide WO.sub.3 into the mixed 
Ln.sub.2 O.sub.3-x WO.sub.3 (x=2 or 1) by heating for 14 hours in a muffle 
furnace at a temperature of 880.degree. C. After cooling, the product 
obtained is carefully ground up in an agate mortar and is then reheated to 
temperatures of between 1,100.degree. and 1,200.degree. C. depending on 
the lanthanide employed. The heating time is set at 72 hours. 
In all cases, the purity of the reaction products is checked by x-ray 
crystallographic analysis. 
The nitriding under a stream of ammonia is carried out at temperatures of 
between 700.degree. and 900.degree. C. for a period of approximately 14 
hours. 
The flow rate of ammonia is proportionately higher the higher the 
temperature, so as to prevent, as far as possible, the dissociation into 
nitrogen and hydrogen before contact with the product. The overall 
reaction can be written, schematically: 
EQU ABO.sub.x +NH.sub.3 .fwdarw.ABO.sub.y N.sub.z +H.sub.2 O.uparw. 
It is clear that the reaction is not total. Some of the ammonia 
dissociates, and the nitrogen and hydrogen produced by this dissociation 
entrain the water vapor originating from the reaction. 
During the cooling, in order to avoid any hydrolysis, the ammonia stream is 
substituted with a purge using U-grade nitrogen. 
The nitrogen-oxygen substitution of the preceding formation reaction is 
reflected in a loss in mass which is verified by weighing the materials 
before and after reaction. 
The various experiments carried out while the temperature, in particular, 
was varied, demonstrate that, after reaction, a pure phase composition 
which always remains substantially identical is obtained. 
Thus, nitrogen determination in the case of the Ca-W-O-N compounds varies 
between 7.5 and 7.7% for samples originating from 13 different 
preparations. The reproducibility of the determination has been verified 
for one and the same preparation. These experimental values are not 
significantly different to make it possible to conclude that a very 
extensive single-phase domain exists under our operating conditions. 
The highest nitrogen concentration obtained (7.7.sub.4 %) corresponds to 
the formula: 
EQU CaWO.sub.1.5.sbsb.1 N.sub.1.4.sbsb.9 
very close to CaWO.sub.1.5 N.sub.1.5, for which calculation gives a 
nitrogen content of 7.81%. The absolute difference of 1% between this 
value and the experimental value is of the same order of magnitude as the 
accuracy of the determination. 
In this example, tungsten has a mean oxidation state very close to 5.5 and 
the phase may be formally written: 
EQU CaW(VI).sub.1-x W(V).sub.x O.sub.1+x N.sub.2-x 
with x.sub.exp =0.5.sub.1 in this case. 
Insofar as the pair Sr-W is concerned, nitrogen determination (% N=4.8%) 
leads to the formula SrWO.sub.1.9 N.sub.1.1, in which the oxidation state 
of the tungsten is very close to V. Using the same formulation as 
previously, the value of x is 0.9. 
In an analogous manner, the corresponding Ba-W perovskite phase has been 
identified by x-ray crystallographic analysis in the reaction mixture. 
In the case of the pair Sr-Mo, nitrogen determinations give contents of 
between 6.5 and 7.2% and lead to the formula: 
EQU SrMo(VI).sub.1-x Mo(V).sub.x O.sub.1+x N.sub.2-x 
with 0.8.sub.3 &lt;x&lt;0.9.sub.5 that is, an average formula SrMoO.sub.1.9 
N.sub.1.1. 
All these compounds have been characterized by x-ray crystallography 
analysis. 
In all the cases the lattice appears to be cubic and the analysis of the 
powder diagrams is given in Table I. 
Calculation of the data, which are refined using a least-squares methods, 
gives the following values: 
EQU CaWO.sub.1.5 N.sub.1.5 a=3.924 (1) .ANG. 
EQU SrWO.sub.1.9 N.sub.1.1 a=3.989 (3) .ANG. 
EQU BaWO.sub.1+x N.sub.2-x a=4.117 (3) .ANG. 
EQU SrMoO.sub.1.9 N.sub.1.1 a=4.005 (3) .ANG. 
In a powdered state, all these compounds exhibit a marked hygroscopic 
character. The rate of this degradation is the highest in the case of 
SrWO.sub.1.9 N.sub.1.1. Release of ammonia is observed and the initial 
oxide is obtained by an oxidation process. 
TABLE I 
__________________________________________________________________________ 
Powder diagrams for the oxynitrides A.sup.(III) W(O,N).sub.3 (A.sup.(II) 
= Ca, Sr, Ba) and 
SrMoO.sub.1.9 N.sub.1.1 
CaWO.sub.1.5 N.sub.1.5 
SrWO.sub.1.9 N.sub.1.1 
BaWO.sub.1+x N.sub.2-x 
SrMoO.sub.1.9 N.sub.1.1 
d.sub.hkl 
hkl I/I.sub.o 
d.sub.hkl 
hkl 
I/I.sub.o 
d.sub.hkl 
hkl I/I.sub.o 
d.sub.hkl 
hkl 
I/I.sub.o 
__________________________________________________________________________ 
3.926 
100 60 3.978 
100 
ms 4.114 
100 &lt;1 4.012 
100 
5 
2.776 
110 100 
2.818 
110 
VS 2.917 
110 100 11 
2.830 
110 
100 
2.265 
111 10 2.315 
111 
&lt;1 
1.961 
20 30 1.983 
200 
mS 2.058 
200 30 2.016 
200 
30 
1.756 
210 25 1.783 
210 
W 
1.603 
211 45 1.622 
211 
S 1.684 
211 40 1.634 
211 
30 
1.387 
220 15 1.401 
220 
mS 1.453 
220 50 1.415 
220 
15 
1.308 
300 10 1.332 
300 
W 1.370 
300 25 
1.241 
310 20 1.263 
310 
mS 1.244 
311 15 1.266 
310 
10 
__________________________________________________________________________ 
EXAMPLES 5 to 8 
Ln-W(Ln=La,Pr,Nd,Eu) pairs 
When considering the double cation substitution Ti(IV)-W(VI) and 
Ba(II)-Ln(III) in relation to barium titanate BaTiO.sub.3, it might be, a 
priori, expected that a completely nitrogenated perovskite of ABN.sub.3 
stoichiometry would be obtained. 
In fact, perovskites are obtained as the lanthanide goes from lanthanum to 
neodymium, but these are partially oxygenated because of the fact that the 
tungsten does not maintain its maximum oxidation state. 
In the case of neodymium, the formation of a scheelite phase of formula 
NdW(VI)O.sub.3 N is also observed. 
Europium forms a special case because of the stability of the oxidation 
state II, and a perovskite phase is obtained with this element instead of 
the scheelite which is normally expected. 
The preparation of these compounds is carried out by nitriding, in a stream 
of ammonia, the tungstates Ln.sub.2 W.sub.2 O.sub.9 which are prepared as 
indicated earlier. The reaction temperatures are between 700.degree. and 
900.degree. C. 
The pure perovskite phase is obtained when Ln=La or Pr. 
In the case of neodymium, if the operation is carried out close to the 
thermal threshold of the reaction (approximately 700.degree. C.), the 
scheelite phase is obtained predominantly. The proportion of this phase 
decreases when the heating is prolonged or when the operation is carried 
out at a higher temperature. 
The composition of these phases has been determined by chemical 
determination of nitrogen. In the case of the La-W pair, the experimental 
content is between 8.4 and 9.2%. In the case of the formula: 
EQU LaW(VI).sub.1-x W(V).sub.x O.sub.x N.sub.3-x 
this gives a value of x of between 0.8 and 0.6. 
The same applies to the case of neodymium, in whose case the nitrogen 
content is between 8.2 and 9.2% (0.8&gt;x&gt;0.6). 
Lastly, the results of analysis for a praseodymium compound give a value of 
x=0.75 (experimental nitrogen content: 8.5%), and hence the formula 
EQU PrWO.sub.0.7.sbsb.5 N.sub.2.2.sbsb.5. 
For a given lanthanide, no shift in the diffraction lines is observed by 
x-ray crystallographic analysis for these various compositions. 
While the kinetics of formation of the perovskite phase become 
progressively slower as the radius of the lanthanide decreases, this phase 
is easily obtained with europium, and this, in itself, proves the presence 
of divalent europium of a greater ionic radius than for the valency III in 
the compound. 
The experimental results are as follows: 
______________________________________ 
Experimental 
Temperature of 
nitrogen content 
preparation (% by weight) Formula 
______________________________________ 
740.degree. C. 
6.4.sub.0 EuWO.sub.1.25 N.sub.1.75 
860.degree. C. 
7.3.sub.5 EuWO.sub.1.0 N.sub.2.0 
______________________________________ 
The compounds involved in this case exhibit elements with mixed valency in 
the two sites A and B of the perovskite. 
The interreticular distances corresponding to the h k 1 planes for the 
various compounds are collated in Table II. 
These patterns fit a cubic lattice, the data for which, refined by a 
least-squares method, are given below: 
EQU LaWO.sub.0.6 N.sub.2.4 a=3.994 (1) .ANG. 
EQU PrWO.sub.0.7.sbsb.5 N.sub.2.2.sbsb.5 a=3.967 (1) .ANG. 
EQU NdWO.sub.0.7 N.sub.2.3 a=3.964 (1) .ANG. 
EQU EuWO.sub.1 N.sub.2 a=3.974 (1) .ANG. 
In contrast to the preceding perovskites which involved an alkaline-earth 
element, the equivalent phases with a lanthanide do not exhibit any 
hygroscopic behavior. 
The action of oxygen has been investigated by thermogravimetric analysis in 
the case of the lanthanum compound. The reaction begins at 300.degree. C. 
and leads to the tungstate La.sub.2 W.sub.2 O.sub.9. 
TABLE II 
__________________________________________________________________________ 
Powder diagrams of the oxynitrides LnW(O,N).sub.3 - Ln = La, Pr, Nd, Eu 
LaWO.sub.0.7 N.sub.2.3 
PrWO.sub.0.75 N.sub.2.25 
NdWO.sub.0.7 N.sub.2.3 
EuWON.sub.2 
d.sub.hkl 
hkl I/I.sub.o 
d.sub.hkl 
hkl 
I/I.sub.o 
d.sub.hkl 
hkl I/I.sub.o 
d.sub.hkl 
hkl 
I/I.sub.o 
__________________________________________________________________________ 
3.995 
100 5 3.969 
100 
5 3.960 
100 5 3.978 
100 
5 
2.821 
110 100 
2.805 
110 
100 
2.805 
110 100 
2.814 
110 
100 
1.998 
200 25 1.981 
200 
25 1.981 
200 25 1.987 
200 
25 
1.787 
210 &lt;1 1.774 
210 
2 1.774 
210 3 1.778 
210 
3 
1.631 
211 35 1.618 
211 
40 1.618 
211 40 1.623 
211 
35 
1.412 
220 15 1.402 
220 
15 1.400 
220 15 1.404 
220 
15 
1.262 
310 15 1.255 
310 
15 1.254 
310 15 1.256 
310 
15 
__________________________________________________________________________ 
EXAMPLE 9 
La-V pair 
In this system, in contrast to those preceding, there exists an oxygen 
compound of formula LaVO.sub.3 of perovskite structure. This is not an 
ideal structure: it exhibits an orthorhombic deformation identical with 
that observed in GdFeO.sub.3. In the present case, the lattice appears to 
be square. 
This compound introduces defects in stoichiometry in the A site. These 
result in a single-phase composition domain formulated as La.sub.2/3+y 
.quadrature..sub.1/3-y VO.sub.3, in which the vanadium has the valencies 
III and IV. Thus the overall formula La.sub.0.9 VO.sub.3 corresponds to 
the value of y equal to 0.23. The developed electronic formula may be 
written: 
EQU La.sub.0.9 .quadrature..sub.0.1 V.sub.0.7.sup.(III) V.sub.0.3.sup.(IV) 
O.sub.3 
The substitution of some of the oxygen by nitrogen is another way of 
increasing the formal charge on the vanadium while maintaining the La/V 
ratio equal to 1. 
The preparation of LaVO.sub.3 may be carried out by reduction of the 
vanadate LaVO.sub.4 in a stream of hydrogen, in the temperature range 
700.degree.-800.degree. C. 
The use of ammonia makes it possible to have an atmosphere which is both 
nitriding and reducing. The overall reaction can then be written: 
##EQU1## 
Various experiments have been carried out at temperatures of between 
650.degree. and 800.degree. C. In all cases, the reaction kinetics are 
slow and the final composition undergoes a change, even after heating 
which may exceed several days. 
There exists a wide range of composition which extends from LaV(III)O.sub.3 
to LaV(III).sub.0.1 V(IV)0.9O.sub.2.1 N.sub.0.9. This latter phase has 
been obtained after two heating periods of a week with a grinding 
operation in between. Manipulations for shorter periods result in 
intermediate compositions. Long-term tests make it possible to obtain the 
nitrogen-richest composition in the range, that is to say LaV(IV)O.sub.2 
N, which has excellent electroconductive properties. 
In all the preceding cases, the composition is established by chemical 
determination of nitrogen. This determination requires a digestion of the 
product using molten potassium hydroxide. It has been observed that this 
digestion was more difficult in the case of the phases of the La-V pair 
than with the other perovskites containing nitrogen. 
Oxidation has been investigated by thermogravimetry in oxygen. It begins at 
250.degree. C. and leads to LaVO.sub.4. 
X-ray crystallographic analysis of the various phases in the domain of 
composition shows a change in the crystal lattice. 
In addition to the characteristic lines of the perovskite, the LaVO.sub.3 
pattern shows low intensity lines due to the symmetry of the pseudosquare 
lattice which is derived from the cubic lattice according to: 
##EQU2## 
As the nitrogen content increases, these low intensity lines are seen to 
disappear gradually and the lattice seems to appear cubic. However, a 
marked dissymmetry in the profile of certain lines must be noted, 
indicating that the actual symmetry is not cubic, but the accuracy of the 
x-ray diffraction patterns does not permit the lattice symmetry to be 
suggested unambiguously. 
The lines seen in the diffraction patterns of LaVO.sub.3 and LaVO.sub.2.1 
N.sub.0.9 are collated in Table III. 
TABLE III 
______________________________________ 
Powder pattern of the oxynitride LaVO.sub.2.1 N.sub.0.9 
compared with that of the oxide LaVO.sub.3. 
LaVO.sub.3 LaVO.sub.2.1 N.sub.0.9 
.sup.d hkl 
hkl I/I.sub.o 
.sup.d hkl 
hkl I/I.sub.o 
______________________________________ 
3.296 110 20 3.909 100 20 
3.496 111 5 
2.772 200 100 2.759 110 100 
2.614 201 5 
2.362 211 5 
2.265 202 20 2.257 111 20 
2.171 113 5 
1.961 220 30 1.951 200 30 
1.903 221 5 
1.754 310 10 1.743 210 10 
1.713 311 5 
1.601 312 45 1.593 211 40 
1.457 313 5 
1.388 400 15 1.376 220 15 
1.365 401 5 
1.308 330 10 1.302 300 5 
______________________________________ 
The lattice data for phases exhibiting various compositions have been 
refined from experimental interreticular distances. The results are shown 
in Table IV. 
TABLE IV 
______________________________________ 
Composition 
a.sub.cub. (.ANG.) 
a.sub.sq. (.ANG.) 
c.sub.sq. (.ANG.) 
V (.ANG..sup.3) 
______________________________________ 
LaVO.sub.3 5.547 (1) 7.847 (6) 
241.44 
LaVO.sub.2.8 N.sub.0.2 
5.542 (1) 7.825 (6) 
240.33 
LaVO.sub.2.3 N.sub.0.7 
3.907 (1) 238.59 
LaVO.sub.2.1 N.sub.0.9 
3.899 (2) 237.15 
______________________________________ 
STRUCTURAL STUDY OF LaWO.sub.x N.sub.3-x (x=0.6) 
The neutron diffraction study of the oxynitride LaWO.sub.0.6 N.sub.2.4 
shows that the crystal lattice is not cubic in symmetry, but square, with 
##EQU3## 
The space group is I4. Calculation shows that the oxygen and nitrogen atoms 
are not ordered. FIG. 1 is a projection of the structure along c, in which 
a deformation in the chain arrangement of the tungsten coordination 
octahedra can be seen. 
ELECTRICAL PROPERTIES 
The compounds whose synthesis has been described above were pressed into 
the form of 0.1.times.0.3.times.1.2 cm parallelepipedal bars, at a 
pressure of 5.times.10.sup.3 kg cm.sup.-2. In order to avoid a partial 
decomposition of the samples after pressing, the bars were not subjected 
to any heat treatment. Under these conditions the compactness of the bars 
is close to 50%. 
The conductivity was measured using the method of four aligned points, 
described by J. Laplume "L'Onde Electrique", 335 (1955), 113. It has been 
calculated by measuring the current intensity/voltage ratio between the 
points in both directions of current flow to minimize the dissymmetry 
effects, which are always possible, between the contacts. 
The thermoelectric power was measured in the following manner. The bar was 
held between two platinum heads. An oven permitted a temperature 
difference .DELTA.T=10 K. to be maintained between the two heads, and 
hence between the two ends of the bar. The e.m.f. generated at the sample 
boundaries was measured with the aid of a Keithley model 616 electrometer. 
Under these conditions, the values of the thermoelectric power were 
determined with an error of the order of .+-.1 .mu.V K.sup.-1. 
The electrical properties of the samples were measured as a function of 
temperature in the range 80.degree.-400.degree. K. Temperatures above 
400.degree. K. were rejected to avoid the risk of changing the composition 
of the bars. In the case of low temperatures (T&lt;290.degree. K.), the 
measurements were carried out under helium at a pressure of 200 mbar, and 
under the same pressure of argon in the case of temperatures above ambient 
temperature. 
1. Compounds LnW(O,N).sub.3 (Ln=La, Nd) 
Three samples were studied: LaWO.sub.0.6 N.sub.2.4, LaW.sub.0.7 N.sub.2.3 
and NdW.sub.0.8 N.sub.2.2. The change in conductivity as a function of 
temperature for these three samples is shown in FIGS. 2 and 3. Two 
approximately linear sections can be made out in each of these curves, one 
at low temperature (T&lt;170.degree. K.) and one at higher temperature 
(T&gt;.degree. 200 K.). In each of these sections the conductivity may be 
expressed in the conventional form for the semiconductors 
.sigma.=.sigma..sub.o exp(-.DELTA.E/KT). The activation energies which can 
be calculated from the experimental results are very low and practically 
independent of the composition (cf. following table). The transition 
between the two temperature regions takes place gently. 
______________________________________ 
.DELTA.E (eV) 
Composition T &lt; 170 K T &gt; 200 K 
______________________________________ 
LaWO.sub.0.6 N.sub.2.4 
0.002 0.012 
LaWO.sub.0.7 N.sub.2.3 
0.003 0.013 
NdWO.sub.0.8 N.sub.2.2 
0.003 0.015 
______________________________________ 
The change in the thermoelectric power as a function of temperature is 
shown for the two lanthanum-containing phases in FIG. 4. The negative 
value of .alpha. shows that the predominant charge carriers are electrons. 
.alpha. is very low, since at normal temperature 
.vertline..alpha..vertline.&lt;5 .mu.V K.sup.-1, and is practically 
independent of temperature, within experimental error. 
The low value of .alpha. shows that the density of carriers is very high, 
which agrees with the number of potential carriers deduced from the mean 
oxidation state of the tungsten, determined from the results of analysis, 
which is between 5.2 and 5.4 for all the phases investigated. The apparent 
temperature-independence of .alpha. shows that the number of carriers 
remains practically unchanged throughout the temperature region 
investigated. From this it follows that in the transport mechanism of 
these phases it is the mobility of the carriers which is activated. In 
effect, the conductivity of a sample is expressed as: 
EQU .sigma.=ne.mu. 
where 
n denotes the number of carriers per unit volume, 
e their charge, and 
.mu. the mobility. 
2. Compounds CaW(O,N).sub.3, SrMo(O,N).sub.3 and LaV(O,N).sub.3 
In a first stage, exploratory measurements were carried out on the 
following compounds: 
2.1 CaWO.sub.1.5 N.sub.1.5 
The conductivity was measured as a function of temperature. FIG. 5 shows 
the curve log .sigma.=f(1/T). 
The increase in conductivity with temperature is indicative of a 
semiconductive character. The value of .sigma. is lower than in the case 
of the Ln-W pairs investigated earlier. 
Two values of the activation energies .DELTA.E can be calculated, depending 
on temperature ranges considered. 
EQU 85&lt;T&lt;125.degree. K. .DELTA.E=0.011 eV. 
EQU 200&lt;T&lt;300.degree. K. .DELTA.E=0.033 eV. 
These results can be linked with the value of the formal oxidation state of 
tungsten which is in this case equal to 5.5, a value which is higher than 
the mean value of 5.3 found for the compounds LnW(O,N).sub.3. 
2.2 SrMoO.sub.1.9 N.sub.1.1 
The curve in FIG. 6, representing log .sigma.=f(1/T) shows that this 
compound is semiconductive. 
The conductivity and activation energy values are of the same order of 
magnitude as for the Ln-W systems: 
EQU 85&lt;T&lt;125.degree. K. .DELTA.E=0.004 eV 
EQU 200&lt;T&lt;300.degree. K. .DELTA.E=0.016 eV 
In this compound, the formal oxidation state of molybdenum is equal to 5.1. 
2.3 LaV(O,N).sub.3 
As indicated earlier, evidence has been found for a wide range of 
composition included between LaVO.sub.3 and LaVO.sub.2 N, for which the 
oxidation state of vanadium varies, therefore, between III and IV. 
Measurements have been carried out on a perovskite having the composition 
LaVO.sub.2.7 N.sub.0.3. 
The curve in FIG. 7, which shows the variation .sigma. as a function of 
1/T, shows a semiconductive behavior. There is a change in gradient at 
about 150.degree. K.; this phenomenon has been reported for LaVO.sub.3 
(137.degree. K.) and is attributed to an antiferromagnetic transition. 
The corresponding activation energies are: 
EQU .DELTA.E=0.06 for T&lt;150.degree. K. 
EQU .DELTA.E=0.08 for T&gt;150.degree. K. 
The conductivity .sigma. is equal to 0.1 .OMEGA..sup.-1 cm.sup.-1 at 
300.degree. K. All these values are comparable to those which have been 
determined for LaVO.sub.3. 
The change in the thermoelectric power .alpha. as a function of temperature 
has been investigated. In contrast to what has been observed earlier in 
the case of the perovskites LnW(O,N).sub.3, a positive value of .alpha. is 
observed in this case, indicating that a p-type semiconductivity is 
involved. 
The curve .alpha.=f(T) in FIG. 8 shows the decrease in the Seebeck 
coefficient with rising temperature, which expresses a concurrent increase 
in the number of charge carriers. It can be seen, furthermore, that the 
change is linear. 
The values of .alpha. at 250.degree. and 500.degree. K are as follows: 
______________________________________ 
T (K) .alpha. (.mu.V K.sup.-1) 
______________________________________ 
250 270 
500 75 
______________________________________ 
Because of their electroconductive properties, the perovskites according to 
the invention find application as electrode material, especially for 
capacitors. The perovskites of the invention can thus be advantageously 
cosintered with another dielectric material to manufacture ceramic 
capacitors. The associated dielectric may itself consist of a perovskite 
containing oxygen and nitrogen of the type of those described in Patent 
Application No. EP-A-0,184,951. Such cosintering is carried out in a 
nonoxidizing atmosphere, advantageously in a nitrogen atmosphere, and 
optionally in the presence of sintering additives such as lithium-based 
compounds. Multilayer ceramic capacitors are thus obtained under excellent 
conditions, chiefly because of the very high compatibility of the two 
types of perovskites which are brought together. In fact, on the one hand 
the conductive oxynitrides are stable under the same atmospheric 
conditions as the dielectric perovskites and, on the other hand, the 
crystallographic lattices are in both cases very similar in size. 
The electrical properties of the perovskites according to the invention 
also make it possible to envisage their use as a selective sensor. The use 
of semiconductive oxides as redox reaction catalysts has long been known. 
Their activity is generally linked with the presence of metallic elements 
with two oxidation states in the materials. This property has already been 
employed for the production of reductive gas sensors based on 
nonstoichiometric binary oxides such as TiO.sub.2, Fe.sub.2 O.sub.3 or 
SnO.sub.2. This work has subsequently been extended to ternary oxides of 
perovskite or derived structures such as LaMnO.sub.3 or LaCoO.sub.3, whose 
catalytic activity is also well known. 
Given that, in principle, the sensitivity of the materials is linked with 
the properties of adsorption of the gas onto the oxide, thick layers 
produced by silkscreen printing are generally employed and the variation 
in the electrical resistance of the material as a function of partial gas 
pressure is followed, the sensors needing, furthermore, to exhibit a gas 
detection response which is as fast as possible. 
In the case of the perovskites containing oxygen and nitrogen with mixed 
valency according to the invention, of formula AB(O,N).sub.3, the 
oxidation state of the element B, or even of the combination A+B (for 
example EuW(O,N).sub.3) is linked directly to the proportion of nitrogen 
to oxygen in the compound. 
In the case of this application, it has been shown that the sensitivity of 
the sensor is higher if the conductivity is not very high. Phases which 
are less conductive will therefore be preferably chosen in this case.