J. LECOMTE e t al. : Non-Stoichiometry and Electrical Conductivity (I) 743

phys. stat. sol. (a) 65, 743 (1981)

Subject classification: 10.2 and 14.4; 22.8.1

Laboratoire de Physique Electronique et Thermodynamique des Oxydes, Facultk des Sciences, Tours1) (a) and Laboratoire de Chimie Minerale Industrielle, Groupe de Cristallographie et Chimie du Solide, L.A. 251, Universitk de Caen2) ( b )

Non-Stoichiometry and Electrical Conductivity of Strontium Niobates with Perovskite Structure I. Defect Structure of Sr(SrllsNbals)Os

BY J. LECOMTE (a), J. P. LOUP (a), M. HERVIEU (b), and B. RAVEAU (b)

The conductivity of compounds with perovskite structure characterized by the simultaneous occupancy of the twelvefold sites and a great part of the sixfold sites by the large cation is studied in terms of oxygen partial pressure a t high temperatures (900 to 1450 "C) ; it concerns the family: S1(Sr(1/3)+~Nb(2/3)-~)03--(3/2)~ ( 0 5 xs 1/6). For the x= 0.00 compound and for PO,^ 5 10-9 atm, the lg conductivity exhibits a linear relationship with lg Poe , lg u = - f lg P o , + + C. For 10-9 < P O , < 1 atm, the conductivity is partly ionic due to a great amount of oxygen vacancies created by a predominant Schottky disorder. Contrary t o other perovskite compounds (BaTiO,, etc.) the importance of the Schottky disorder is enhanced by the fact that Sr2+ is located on both, A and B sites of the perovskite. For the x = 0.03 compound, the conductivity is oxygen partial pressure independent for a wide range and it is then quite entirely ionic. An order- disorder transition occurs for both the chemical compositions between 1050 and 1200 "C which affects the ionic conductivity. Below the transition temperature, it seems that the mobility of the oxygen vacancies decreases.

Die Leitfahigkeit von Verbindungen mit Perovskitstruktur, die durch gleichzeitige Besetzung der zwolffachen Platze und eines groden Teils der sechsfachen Pliitze mit den groden Kationen charak- terisiert sind, wird in Abhangigkeit vom Sauerstoffpartialdruck bei hohen Temperaturen (900 bis 1450°C) untersucht; es betrifft die Familie: S1(Sr(1/3)+~Nb(2/3)-~)03-((3/2)~ (0 5 x I= 1/6). Fur die Verbindungen mit x = 0,OO und fur Po, 5 Atm zeigt der Logarithmus der Leitfahigkeit einen linearen Zusammenhang mit lg Po2, Ig u = - lg P o , + C . Bur 1O-O < P O , < 1 Atm ist die Leitfahigkeit infolge eines groden Anteils von Sauerstoffleerstellen, die durch eine vor- herrschende Schottkyfehlordnung gebildet werden, teilweise ionisch. I m Gegensatz zu den anderen Perovskitverbindungen (BaTiO,, usw.) wird die Bedeutung der Schottkyfehlordnung dadurch erhoht, dad Sr2+ sowohl a n den A- als auch an den B-Platzen des Perovskits lokalisiert ist. Fur die Verbindung mit x = 0,03 ist die Leitfiihigkeit in einem weiten Bereich vom Sauerstoffpartial- druck unabhiingig und ist dann vollstandig ionisch. Ein Ordnungs-Fehlordnungsubergang tritt fur beide chemischen Zusammensetzungen zwischen 1050 und 1200 "C auf und beeinfluat die Ionenleitfkhigkeit. Unterhalb der Ubergangstemperatur scheint die Beweglichkeit der Sauerstoff- leerstellen abzunehmen.

1. Introduction

The point defect studies made on oxides with a perovskite structure up to the present time have been focused principally on the compounds with composition ABO, and &4B03-,. These compounds are characterized by a great size difference between the A

37200 Tours, France. 2, 14032 Caen Cedex, France.

48'

744 J. LECOMTE, J. P. LOUP, M. HERVIEU, and B. RAVEAU

and B cations which are located on twelvefold and sixfold sites, respectively: the deviations from stoichiometry observed for these oxides are thus very small (x 5 lo-,). The principal results obtained in this domain concern the relations between the semi- conductive properties of these oxides and the nature of the point defects in the struc- ture [ l to 51 a t high temperature. The recent results in crystal chemistry have shown the ability of perovskites to accomodate large deviations from stoichiometry on the oxygen sites. This is indeed the case in Ba(Ba0.~Ta~.5)02.,~ [B], Sr(Sr0.5Nb0.5)02.,5 "71, or Ba(Be0.5Mo0.5)02.75 [81.

These materials which are characterized by relatively great amounts of anionic defects are thus susceptible to present an important ionic contribution to the conduc- tivity [9]. On the other hand, the great amount of A cations (A/B < 1) involves complex distributions of these cations, and thus order-disorder phenomena which could influence the conduction properties of these oxides.

The present work deals with the perovskites

Sr(Sr(l/3)+2Nb(2/3)-2)03-(3/2)~ ( O f 5 $) *

2. Experimental Procedure

2.1 Basic materials and synthesis

Strontium niobates were prepared from "specpure" Johnson Mattey SrCO, and Nb,O,. A batch analysis of these basic materials is reported by the manufacturer in Table 1.

Table 1 Basic material impurities (ppm)

batch No. S. 86 282 Ba 10 A1 1 Ca 10 Si less than 1 Na 1 Ta not detected

less than 50 other not detected E} each less

Mg than 1

other not detected

batch No. S. 82 413

Ag

The components were weighed in proper proportions and mixed in an agate mortar according to the equation

and heated in Pt crucibles. All the preparations were carried out by stirring the product after short periods of heating a t 1000 "C and grounding again till the mass loss equals the theoretical departure of CO,. The powders were then heated a t 1450 "C for 20 h. After this treatment the total mass loss never exceeded 0.3% of the theoret- ical mass of CO,.

The products were studied by X-ray powder diffraction methods using a CGR diffractometer and a Guinier-De Wolf camera. A single-phase perovskite was always observed.

Non-Stoichiometry and Electrical Conductivity of Strontium Niobates (I) 745

2.2 Electrical conductivity measurements

The powders were hydrostatically pressed into rods under 3000 kp cm-, and heated a t 1450 "C during 15 h in air and then slowly cooled. The specimens for conductivity measurements were cut from these rods in order to have a cylindrical sample: 0 = = 0.4 cm, 1 = 2 cm. The electrical contacts for the four-probe conductivity measure- xnents [lo] were made by wrapping the sample with a Pt wire 0.2 mm in diameter and sealing by Pt paint.

Specimen densities were calculated from weight and geometric measurements. The apparent densities of all samples ranged from 85 to 95% of the theoretical density which is computed from the X-ray measured cell dimensions.

The electrical conductivity measurements were carried out in an electric furnace a t temperatures up to 1450 "C; the atmosphere of the furnace was regulated by a flow of gas mixtures of 0,-Ar or CO-CO, which were given by mixing pumps (Wosthoff SA l8/2F and SA 27/2F). The purity of each gas was 99.95% and the flow rate in the furnace was about 16 cm min-l.

The 0,-Ar mixtures covered a range of oxygen partial pressure (Po,) between 1 and 10-4 atm. Lower partial pressures were achieved with CO-CO, mixtures with nominal CO content ranging from 0.1 to 99.0%. The free-energy value [ll] for the CO/CO, reaction was used to calculate the 0, partial pressure,

100 -% CO /o CO

lg Po, = -29502.8/T + 9.0694 + 2 lg 7

So, some intermediate PO, could not be achieved by these mixtures [12] especially a t low temperatures.

During the experiments the oxygen partial pressure of the atmosphere of the furnace was controlled by a stabilized ZrO, solid electrolytic cell [13].

The conductivity was determined by measuring the voltage across the inner probes and across a standard resistor in series with the specimen. Electric power was supplied by a constant voltage source (Lambda LP410A) and the voltages were measured with an electronic voltmeter (Keithley 177 microvolt DDM). The dc power supply was adjusted to measure voltages in the range 100 to 400 mV in both forward and reverse directions. Conductivity was determined as a function of the gas mixture a t constant temperature for both increasing and decreasing Po,.

Application of the phase rule indicates that four degrees of freedom must be fixed to establish thermodynamic equilibrium. Temperature and 0, partial pressure were controlled experimentally and the total pressure (atmospheric pressure) was supposed to be constant. The vapour pressure of Sr and Nb being very low, the fourth constant must be the Sr/Nb ratio [l, 21.

3. Results 3.1 Crystallographb data

I n the system Sr-Nb-0 a new solid solution with the perovskite structure, Sr(Sr(l/3)+zNb(2/3)-2)03--(3/2)s, has been isolated for 0 5 x 5 +. The compounds of the series were obtained in the form of a well-crystallized powder, the colour of which varies gradually from white (x = 0) to violet (x = +). Their X-ray patterns were indexed on the basis of a cubic cell, with a parameter close to that of Sr(Sr1/3Nb2/3)03 [14] (Table 2). Due to the great amount of strontium, a part of the strontium atoms are located with the niobium atoms on the octahedral sites B. The great size difference between strontium and niobium ions involves an ordered distribution of these ions over two sorts of octahedral sites, B' and B"; an order noted 1.1 type corresponding

746 J. LECOMTE, J. P. LOUP, M. HERVIEU, and B. R,AVEAU

0.00 8.268 0.01 8.272 0.02 8.274 0.03 8.277 0.05 8.283 0.08 8.290 0.16 8.312

to the stacking along the (111) direction of the B' planes and B" planes alternatively can be considered; this type of order has been first observed for (NH,),FeF, [15] and for oxides A [(M1/2)B, (M;/2)~*,]0~. It has indeed also been proposed for some complex perovskites with general formula A(M1/3Mk/3)03 [ 161. It is the case of Sr(Sr1/3Nb2/3)03 for which the B' planes are mixed and contain strontium and niobium simultaneously, while the B" planes are only occupied by the niobium atoms. An analogous distri- bution of the strontium and niobium atoms over the B' and B" sites can thus be proposed for the solid solution, leading to the formula (Sr(l/3)+2Nb(l/~)-m)~, (Nb1/2)B"03-(3/2)2. This shows that the substitution of the niobium atoms for strontium atoms in the B' planes does not involve a drastic modification of the structure. A preliminary study by electron diffraction of the limit Sr(Srl/2Nb1/2)02,75 shows, however, that the actual structure is more complex. Several types of crystals have in fact been observed, they are characterized by superstructure reflections whose intensity changes from one crystal to the other. Numerous crystals have a tetragonal cell with a + 2a, fz and c = 413,. I n some cases satellite reflections have been observed. These observations suggest the possibility of partial order especially in the B' planes. A syst,ematic study

T=14509 - 1300

-2 1200

Fig. 1. Isothermal Ig cr vs. Ig Po, for specimen a) x = 0.00 and b) x = 0.03 at several temperatures

Eon-Stoichiometry and Electrical Conductivity of Strontium Niobates (I) 747

rig. 2. Temperature dependence of the conductivity of the ipecimen x = 0.00 for different Po,: x 1, o lo-*, 0 B 10-15atm

will be necessary for different conditions of syn- ihesis quenching and annealing in order to elucidate ;hese phenomena.

3 2 Electrical conductimity

[n this Part I, the results about

sr(sr(1/3) +zNb(2/3) - % I 0 3 -( 3/2)z m4- T / K 3 - -

with x=O.OO and x=O.O3 o d y are submitted. A next paper will deal with other x + 0.00 compounds.

The oxygen partial pressure dependence of the isothermal electrical conductivity is illustrated in Pig. l a and b. For both specimens, there are two behaviours de- pending principally on temperature :

(i) Above 1200 "C, for x = 0.03, the oxygen partial pressure has no effect upon the electrical conductivity for a wide range, for x = 0.00, the presence of regions of n-type and p-type behaviour is in agreement with measurements on other perovskites like BaTiO, [l to 51, KTaO, [17], SrTiO, [18]. But, contrary to the latter compounds the total conductivity exhibits rather weak variations for PO, ranging from to 1 atm. Nevertheless as the conductivity strongly increases as the oxygen partial pressure decreases, we shall assume that the conductivity is essentially electronic for Po, below atm.

(ii) Below 1100 "C the semiconducting behaviour increases for both x = 0.00 and x = 0.03 specimens; this effect is particularly important a t low oxygen partial pres- sure (CO/CO, range) for x = 0.03 where the conductivity a t 900 to 1000 "C is almost the same as for x = 0.00.

For the latter, in the intermediate temperature range (1100 to 1200 "C) the temper- ature dependence of the conductivity exhibits an important discontinuity close to the minimum conductivity but no discontinuity occurs a t lower oxygen partial pressure. A typical behaviour of the conductivity as a function of the reciprocal temper- ature is illustrated in Fig. 2 for several oxygen partial pressures.

At low Poa, lg cr and lg PO, exhibit a reasonably linear relationship,

lg 0 = - ( 1 / 4 1g Po, + c 9 ( 1)

Table 3 hasure dependence of isothermal conductivity (values of m from lg u = -(l/m) lg PO, + const)

748 J. LECOMTE, J. P. LOUP, M. HERVIEU, and B. RAVEAU

where C is a constant and (T the total conductivity. Experimental values of rn are given in Table 3 ; they slightly increase with temperature. This effect certainly reflects the influence of the proximity of the conductivity minima, because the oxygen partial pressure which can be achieved by COjCO, mixture is not low enough.

4. Defect Models for Electrical Conductivity and Discussion

4.1 Sr /Nb = 2.00 (X = 0.00)

4.1.1 n-type conductivity

We assume that apparent variations of rn with temperature are not significant with a change of defect reaction and that m, a t any temperature, is theoretically equal to four for x = 0.00 and oxygen partial pressure lower than 10-9 atm. The electrical conductivity being predominantly n-type, the expression for the conductivity is

(T = nepn, ( 2 ) where n is the free-electron concentration, e the elementary charge, and pn the electron mobility.

We consider, like in other perovskite-type compounds that, a t low oxygen partial pressure, oxygen vacancies will be present in the material [l, 5, 17, 181. According to the defect equilibria and using Kroger's notation [19]

the law of mass action gives

KO = [VG] PZs2 = No exp ( - E , / ~ T ) , (6)

K, = [Vb] n/[V;] = iVl exp ( -E, /kT) . K, = [VV;;] n / [Vb] = N2 exp ( -E, /kT) ,

where square brackets indicate that the structure elements are expressed in terms of concentrations; KO, K,, and K, are the mass action constants for the formation of a neutral oxygen vacancy, the ionization of the first electron, and the ionization of the second electron, respectively; E,, El, and E, are the corresponding activation energies.

Now, many models of defect reaction can account for m = 4. First, this behaviour is explained on the assumption that oxygen vacancies are singly ionized ; the neutrality condition will then be noted as

n = [Vb]

(T = ep,(KoKl)1/2 P0,1/4 . and equations (2) to (a), (6), (7), (9) lead to

Although the slopes of the conductivity curves can be explained in the way referred to above, we prefer a different interpretation. Two reasons support our point of view. First, Vg and not TO is the predominant defect in SrTiOs [18, 201 and Smyth [23] has shown that oxygen vacancies in other perovskite-type compounds are entirely doubly ionized a t temperatures of 800 "C and above. Second, the rather weak variation of the conductivity over a wide range of Po, ( loe9 to 100 atm) is in reasonable agreement for a substantial contribution of the ionic conductivity.

Kon-Stoichiometry and Electrical Conductivity of Strontium Niobates (I) 749

The second model is derived from the last remark: [VS] > [Vb] > [V;] and the neutrality equation can be reduced to an equality involving [V,] and the concen- tration of negatively charged point defects.

I n the perovskite structure cation vacancies are found in preference to interstitial defects in a close packed structure. So, i t could be considered that the main defect for our material is derived from the Schottky disorder.

The general defect reaction is

n i l ~ V A + a v B , + ~ v B , , + 3 V o . (11)

As V, is more realistic than VBj and VB’r in any perovskite structures [22] and if cation M lies in both A and B‘ sites and cation N lies in both B‘ and B“ sites, the following reactions can be written:

Then, the Schottky disorder will lead to

MA + $NB, 2VA + MB, + +-NB” + 3 v, . ( 14) I n our compound M = Sr2+ and N = Nb5+; the charge and geometric difference

[28] between the cations should not lead to a random distribution over the B’ sites. We assume that in the B‘ planes an octahedral NbO, has two adjacent octahedral SrO,. This considerations does not necessarily involve a long-range order over the B’ sites.

Thus, we shall write the cation in the B’ site as a triplet: (Sr, Sr, Nb)g and the Schottky disorder reads

6 Sri + 3 (Sr, Sr, Nb)& * 12 V i + 4(Sr, Sr, Sr):, + 3 Nb& + 18 Vg (15)

The Schottky constant Kh is defined as the mass action constant of the equilibrium if all defects are assumed fully ionized.

(1% Kb = [VXI2 [(Sr, Sr, Sr)3’I2/3 [&I3 . (16)

When the compound is equilibrated with the oxygen partial pressure P t 2 of minimum conductivity, we can write

[V;;] = + [VO] , (17) [(SrSrSr)3’] = $ [V@ (18)

== 1.38KL3/1’. (19)

(20)

and then

The electroneutrality condition is

Z [ V ~ I = 2 [VXI + 3 [(s~s~sP):~]. Another mechanism involving Va and V i is reported by Smyth [23, 241 for BaTiO,

when excess of TiO, is incorporated in the perovskite phase. If we develop this model for our compound when synthesis is carried out with an excess of SrO or Nb,O,, the electroneutrality conditions would be

2 [ V S ~ = 2 [VJ + 3 [ ( ~ r ~ r ~ r ) ; , ]

2 [Vg] + 3 [(SrNbNb);,] = 2 [VX] . and

‘750 J. LEGOMTE, J. P. LOUP, M. HERVIEU, and B. RAVEATJ

In this case the oxygen vacancy concentration does not depend on temperature and is fixed by the Sr/Nb ratio in the material.

So, close to the minimum conductivity, the concentration of oxygen vacancies is fixed by Schottky disorder or cation non-stoichiometry. When this sample is equili- brated with lower oxygen pressure, addition of oxygen vacancies and free electrons will be formed according to the reactions (3) to (5).

As long as the number of oxygen vacancies added by equilibrium with the ambient gas is small compared with those already present, then the electronic conductivity varies as

4.1.2 Minimum conductivity

For an electronic conductor the conductivity minimum is

ami, = 2a,

according to an = ap if i t could be assumed that the hole mobility pp equals the electron mobility p,. This is experimentally verified for BaTiO, [21j.

For our compound the electronic conductivity a, a t minimum conductivity amin is determined by extrapolation of the linear relationship (1): i t is found that amio is greater than 2an, Accordingly, ionic conductivity seems to take an important part of the total conductivity

amin = 20, + aim - (24) Direct measurements of the ionic contribution were performed using solid state

e.m.f. techniques and are given in Par t I1 of this paper. In the intermediate range of temperatures 1100 to 1200 “C, the discontinuity around

the minimum conductivity as shown in Fig. 2 is proposed to be due to an order- disorder transition. This ordering can be either a crystallographic order (electronic microscopy patterns of annealed materials below 1150 “C show superstructure reflec- tions) or associations between Vg and other charged species such as

v; + v;; 2 2 (v;;vy)x , 4(SrSrSr)3’ + 6V; + 6(Sr’, V& Sr’)* .

According to Kroger’s notation the commas in (Sr’, VG, Sr’) mean that the two Sr and the V; are not necessarily on adjacent sites [19]. Under the assumption that order-disorder transition occurs, the discontinuity of a close to the conductivity minima results either from a decrease of oe1 or from a decrease of cion as the tem- perature decreases.

As this discontinuity does not occur a t low Po, when the conductivity is essentially n-type, this implies that only aion decreases in the intermediate range of temperatures.

Moreover, as i t will be discussed in Par t 11, the ionic conductivity is due to oxygen migration by a vacancy mechanism as in calcia or yttria-stabilized zirconia 1125 to 271. If aion decreases either [VGJ or the oxygen ion mobility decreases. The assumption that [V;] decreases implies that an should be affected a t low PO,, a, being related to

As this is not the case, the discontinuity around the minimum conductivity seems to be due to a decrease of the ionic conductivity because the oxygen ion mobility diminishes.

IVEI by (22).

Non-Stoichiometry and Electrical Conductivity of Strontium Niobates (I) 751

4.2 S r / N b = 2(2 + 9 z) (z = 0.03)

Material with Sr/Nb > 2 can formally be viewed as having been prepared by equili- brating the x = 0.00 compound with an external source of SrO.

The conservation of mass (atomic species) , charge, crystal structure, and electronic states has been maintained. Then, the preparation should be considered as the in- corporation of an acceptor, because, in fact, Nb5+ are being substituted by Sr2+ on B’ sites. I n consequence, oxygen vacancies are created.

This increase of the concentration of oxygen vacancies affects the equilibrium (15), which split towards the decrease of site A vacancies. Thus, for a substantially large x, the concentration of site A vacancies is so weak that the chemical formula may be written

Sr6[(Sr2Nb)l-6a’ (sr3)6zIBp Nb3~*,018-9z - The total concentration of oxygen vacancies is then proportional to x defined by

Sr/Nb = 2(1 + + x) (27) and the electroneutrality equation is reduced to

3[(SrSrSr)3’] = 2[VS] . (28) At temperatures ranging above 1200 “C, the electrical conductivity is almost in-

dependent of oxygen partial pressure and should be entirely ionic. The oxygen vacancy concentration being related to x, the ionic conductivity is

proportional to x. An extrapolation of this latter remark to the x = 0.00 compound shows us that a great oxygen vacancy concentration is present in the stoichiometric compound.

If this large oxygen vacancy concentration is due to an excess of one of the basic components, this implies an error 20 times as high as the accuracy of the weighing. So, the great amount of oxygen vacancies in SrS~1/3Nb2/~0, leading to a partly ionic conduction a t high temperature seems to be in agreement with a predominant Schottky disorder rather than a cation non-stoichiometry.

At high temperature (>1200 “C) the increase of the oxygen vacancy concentration with x affects both ionic and electronic conductivity:

At Po, 5 10-l2 atm and a t 1300 “C i t is shown on Pig. l a and 2b that the total conductivity of the x = 0.03 compound is lower than those of the x = 0.00 compound which in these conditions is an n-type conductor. This is in agreement with (22): if [Vb;] arises then n decreases, so

0, (X = 0.00) > 0, (X = 0.03) . (29) A slipping of the point where n = towards the lowest PO. is consistent with the

presence of the dominant acceptor species (SrSrSr)$, which can also be written (Sr&,;b)B,. On Pig. l a and b it can be seen that the minimum of conductivity occurs for Po, x 10-4 atm for x = 0.00 and for PO, = atm for x = 0.03.

5. Conclusions

The behaviour of the strontium niobates Sr(Sr(1,3)+zNb(2/3)--2)03-(3/2)s is quite different from that of other perovskites BaTiO,, KTaO, which were studied by identical methods. Due to the large size of strontium, which is distributed over both A and B sites, a “pseudo Schottky disorder” seems to be the predominant defect in this mate- rial. Consequently, this ionic defect leads to a partly ionic conductivity a t high tem- peratures in a large range of oxygen pressure close to the minimum conductivity.

752 J. LECOMTE e t al. : Non-Stoichiometry and Electrical Conductivity (I)

At lower temperature, crystallographic order on B’ sites occurs. The oxygen vacancy mobility is affected when the ionic conductivity is lowered.

The importance of the “Schottky disorder” and the decrease of the ionic mobility can explain why this perovskite structure can allow a so great non-stoichiometry.

This work shows that electric transport phenomena are not only closely dependent on crystallographic structure but also on the chemical species of the cations which are located on certain sites.

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A, Ed. N. M. TALLAN, M. Dekker, Inc., New York L974.

edition, Pergamon Press, London 1967 (p. 421).

American Elsevier, 2974.

(Received April 2, 1981)