Oxygen nonstoichiometry and electrical conductivity of the solid solutions La2−xSrxNiOy (0≤x≤0.5)

Solid State Ionics 110 (1998) 245–253

Oxygen nonstoichiometry and electrical conductivity of the solid solutions La 22x Sr x NiO y (0 # x # 0.5) a, a a a a V.V. Vashook *, S.P. Tolochko , I.I. Yushkevich , L.V. Makhnach , I.F. Kononyuk , H. Altenburg b , J. Hauck c , H. Ullmann d a

Institute of General and Inorganic Chemistry, National Academy of Sciences of Belarus, Minsk, Belarus b Fachhochschule Munster, Steinfurt, Germany c Institut fur Festkorperforschung, Forschungszentrum Julich GmbH, Julich, Germany d Technische Universitaet Dresden, Dresden, Germany Received 22 September 1997; accepted 6 April 1998

Abstract Oxygen nonstoichiometry and electrical resistance of a series La 22x Srx NiO y solid solutions, where x 5 0.0, 0.2 and 0.5 in argon flows at oxygen partial pressures 1.5, 10.2, 49.2, 100 and 286 Pa within the temperature range of 20–10508C were studied. Nickelate oxygen desorption / sorption spectra when heating–cooling at constant rate demonstrated strong dependence of cation composition of the samples. Unlike La 1.5 Sr 0.5 NiOy compounds those of La 2 NiOy and La 1.8 Sr 0.2 NiOy have weakly bonded oxygen, capable to exchange reversibly with the gas phase at the temperatures higher than 2508C. The equilibrium values of oxygen nonstoichiometry and specific resistance for the these nickelates were determined at 300–10508C and pO 2 51.5–286 Pa as a functions of temperature versus oxygen partial pressure. All nickelate studied appear to be p-type conductors with metal electric conductivity at equilibrium states.  1998 Elsevier Science B.V. All rights reserved. Keywords: Lanthanum–strontium nickelates; Phase transformation; Oxygen stoichiometry; Electrical conductivity

1. Introduction A series of La 22x Sr x NiO y solid solutions based on lanthanum strontium nickelate La 2 NiO y with the perovskite-like structure of K 2 NiF 4 have been synthesised and investigated by a number of authors [1–8] Ishikawa et al. [4] have studied nonstoichiometry and electrical conductivity in La 2 NiO y and LaSrNiO y within the temperature range of 200–9008C and at *Corresponding author.

oxygen partial pressures from 98–58 000 Pa. Both compounds had the K 2 NiF 4 structure at 9008C in this oxygen pressure range. It was found that LaSrNiO y had a higher concentration of oxygen vacancies than La 2 NiO y at 9008C and under the same oxygen partial pressures. Takeda et al. [5] have shown that the tetragonal distortion of NiO 6 -octahedra in La 22x Sr x NiO y decreases monotonously with x increasing for 0#x#1.4 and the La(Sr)–O(2) bond length increases abnormally in the range of 0,x, 0.6. The electrical conductivity type of the solutions changes with temperature. The transition from a

0167-2738 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00134-9

246

V.V. Vashook et al. / Solid State Ionics 110 (1998) 245 – 253

semiconductor-type of conductivity to a metal one lowers at the temperature from about 4008C for x50 to 22538C for x51.2. Cava et al. [6] have investigated magnetic and electrical properties of solid solutions near LaSrNiO 4 composition in the low temperatures range and did not find any superconductivity up to 30 mK. Makhnach et al. [7] have studied oxygen nonstoichiometry and electrical conductivity of La 12x Sr 11x NiO y solid solutions at 0# x#1 depending on temperature in the air and oxygen flows at the temperatures 2196–11008C. Anomalies in the resistance curves versus temperature within the temperature range 847–9478C were interpreted in terms of the changes in nickelates of oxygen stoichiometry and structure. Nickelate dissolution in HCl was found to be accompanied with both oxygen and chlorine which reveals the existence of two bond types in the oxygen of the crystal lattice. The investigations carried out recently show these compounds to be promising materials applied as electrodes in various oxygen-ion–solid electrolyte devices and as selective oxygen permeable ceramic membranes. At the same time we know very little about the oxygen non-stoichiometry and electrical conductivity of these compounds. Thus, this paper presents the investigation of oxygen non-stoichiometry and electrical conductivity of the three La 22x Sr x NiO y solid solutions where x5 0, 0.2 and 0.5 with satisfactory ceramic properties [2,3] as a functions of temperature and oxygen partial pressure.

2. Experimental La 2 NiO y , La 1.8 Sr 0.2 NiO y and La 1.5 Sr 0.5 NiO y powders were obtained by the citrate process [7]. Quantitative analysis of nickelate samples was carried out means of atomic-absorption spectroscopy (AAS-3 spectrometer, Carl Zeiss, Jena). The La:Sr:Ni ratio was found to be equal to the required atomic index ratio in each compound with an accuracy of 60.01 mol. X-ray phase analysis was carried out within the temperature range 20–10008C in air using a DRON3 (USSR) diffractometer and Cu Ka radiation. The sample temperature was maintained with constant accuracy 618C.

The brick-shaped samples (103333 mm 3 ) were pressed under the pressure of 500 MPa for simultaneous measurements of oxygen non-stoichiometry and electrical resistance. These samples had four platinum electrodes each, which were introduced into the sample body during pressing. The samples were sintered at 12708C in air for 8 h. Experimental density of the samples after sintering was about 80–90% of the theoretical value. The initial content of oxygen in the samples was determined by thermogravimetry in hydrogen flow at the temperature up to 9508C. This method allowed us to determine the initial oxygen stoichiometry of nickelates with an accuracy of 60.01 of the oxygen atomic index. The coulometric measuring system ‘OXYLYT’ (SensoTech Magdeburg) [9] was employed for creation and step change of oxygen partial pressure over the samples as well as for monitoring of the oxygen exchange kinetic. The oxygen stoichiometry changes were calculated on the basis of the coulometric titration in the second cell of this measuring complex. The variation factor when determining oxygen index change did not exceed 1% in the 98% confidence interval (CI). The experiments on the investigations of oxygen non-stoichiometry were performed on powder samples and also on ceramics with simultaneous registration of oxygen exchange kinetics and electrical resistance. Electrical resistance has been measured by the DC four-point method in the range from room temperature up to 1050618C. The systematic error did not exceed 1% for a range of specific resistance of 10 23 –10 Ohmpcm. Argon with pO 2 50.5 Pa content was used as carrier-gas in all experiments. To increase the oxygen partial pressure the gas flow was sometimes enriched with oxygen by penetrating through the silicon rubber tube wall from the air or pure oxygen flows (1 atm). The tube was kept in a thermostat, where definite differential temperature was maintained. Two types of experiments were carried out. The aim of the first one was to define a qualitative picture of the oxygen desorption / sorption spectra during a heating up and cooling down of the powder samples with the constant rate of temperature change. The other type of experiment was employed for the determination of oxygen stoichiometry and elec-

V.V. Vashook et al. / Solid State Ionics 110 (1998) 245 – 253

247

trical conductivity under equilibrium conditions. The measurements were performed at constant pO 2 and stepwise temperature changes (heating up and subsequent cooling down) and waiting for equilibrium at each fixed temperature. The establishment of equilibrium required from 30 min up to a several dozens of hours depending on the sample type, temperature and oxygen partial pressure. This has been detected by reaching the value of current equal to that measured before the temperature change in the second cell of OXYLYT and also by the invariability of sample resistance versus time. All powder samples were pretreated in air at 10508C up to equilibrium state and after that they were cooled to room temperature with a rate of 78C / min.

3. Results and discussion The primary oxygen content ( y) in the initial powders determined by reduction in H 2 flow, was found to be 4.1460.01 for La 2 NiO y , 4.1160.01 for La 1.8 Sr 0.2 NiO y and 4.0560.01 for La 1.5 Sr 0.5 NiO y . The value of y obtained by the authors for La 2 NiO y is in agreement with the data of [8] and is a little higher than that found in [4]. We believe that the reported value of y54.04 for La 2 NiO y quenched in liquid nitrogen from 13008C is in good conformity with our data, too.

3.1. Investigation of powders in polythermic conditions Fig. 1a–c show the time (t ) dependencies of titration current (I) on the second cell of the complex OXYLYT, temperature (t) and oxygen index ( y) for the powder samples of La 2 NiO y , La 1.8 Sr 0.2 NiO y and La 1.5 Sr 0.5 NiO y during their heating, isothermal exposition and subsequent cooling at constant rate in gas flow at pO 2 510.2 Pa as an example. Similar experiments were carried out in gas flows at 1.5, 49.2 100 and 286 Pa. The investigations showed, that the character of oxygen desorption and sorption spectra in the same gaseous atmospheres differ greatly for powders, with different cation composition and they are similar to the samples with identical cation composition in gas flows at different pO 2 .

Fig. 1. Titration current (I), and oxygen content ( y) of pretreated in air powder samples of nickelates during heating and cooling in argon flow at 10.2 Pa in the temperature region 20–10508C: (a) La 2 NiO y , (b) La 1.8 Sr 0.2 NiO y , (c) La 1.5 Sr 0.5 NiO y .

In Fig. 1a the fact that two oxygen desorption maxima appeared during heating of La 2 NiO y powder at 10.2 Pa in the temperature range 20–10608C is shown. Oxygen desorption began at about 2508C and continued up to 10508C. The first peak was observed at ¯4508C and the second one only at 10508C. It is possible that the displacement of the second peak was connected only with stopping of temperature. Relatively high, almost constant oxygen desorption was observed during heating of the sample in the temperature range of 600–9008C between these two peaks. The oxygen sorption by the powder while cooling is accompanied with the formation of two diffused maxima. The amount of oxygen sorbed

248

V.V. Vashook et al. / Solid State Ionics 110 (1998) 245 – 253

while cooling is much less, than the amount of oxygen, desorbed while heating. If the initial oxygen stoichiometry came to 4.15, the final one was only 4.06. The sorption of oxygen was observed practically up to room temperature while cooling the powder. Two oxygen desorption maxima appeared during heating La 1.8 Sr 0.2 NiO y powder (Fig. 1b), also. Both peaks were observed within the same temperature range as in the case of La 2 NiO y . Oxygen sorption while cooling was also accompanied by the formation of two diffused maxima. The amount of oxygen sorbed while cooling is less in comparison with the amount of oxygen desorbed with heating. The initial oxygen stoichiometry was equal to 4.07 and the its final value was equal to 4.02. As well as in the case of La 2 NiO y the chosen modes of cooling, apparently, did not provide the possibility of the necessary amount of oxygen sorption to achieve the initial stoichiometry because of the low contents of oxygen in a gas phase. The oxygen sorption by that powder while cooling was completed practically at 3008C. Only a single oxygen desorption maximum was observed during heating of La 1.5 Sr 0.5 NiO y powder at 10.2 Pa (Fig. 1c). Oxygen desorption began at the temperatures higher 7008C, that is about 4008C higher than that for both La 2 NiO y and La 1.8 Sr 0.2 NiO y . Also the single oxygen sorption maximum accompanied cooling of that powder. The amount of oxygen sorbed by powder cooling is practically equal to the amount of oxygen desorbed at heating. The repeated heating–cooling of La 1.5 Sr 0.5 NiO y powder in a gas flow at pO 2 510.2 Pa gives the same spectrum of oxygen exchange as in the case of this powder pretreated in air. On the other hand the repeated oxygen exchange spectra of La 2 NiO y and La 1.8 Sr 0.2 NiO y in gas flow at pO 2 510.2 Pa were quite different in comparison with Fig. 1a and b. The principal difference was the decrease in areas of the first peaks of oxygen desorption in the case of repeated heating as the previous cooling of samples in a gas flow with lower oxygen content did not allow the attainment of the initial oxygen stoichiometry (Fig. 1a, b). It is seen from the comparison of Fig. 1a–c that the increase in the amount of strontium in the nickelates results in a decrease in the amount of desorbed oxygen during heating of powder samples in a gas flow at pO 2 510.2 Pa. The similar situation

was also marked for investigations at other pO 2 values.

3.2. Nature of oxygen desorption peaks The non-monotonous change of oxygen desorption during heating of the powders at a constant rate can be caused by several factors. In our opinion one of them can be connected with the running of phase transformations in a material, and another one with unequal bonding energy of oxygen atoms that are at various crystallographic positions in a crystal lattice of the compounds. In the first case due to the changes in the structure of the material the abrupt change in bonding energy of oxygen with the lattice takes place and as a consequence it can cause an anomaly in the oxygen desorption rate. In the second case at the increase of temperature the separate consecutive desorption of oxygen from the different position can proceed in the existence area of the compound in accordance with the increase in bonding energy of oxygen in the crystal lattice. To choose one of the assumptions the XRD analysis of all nickelates in air at temperatures 20, 200, 400, 600, 800 and 10008C was carried out. As a result of these studies we have not found any significant differences in the X-ray diffraction spectra for all nickelates at the above-mentioned temperatures. The diffraction peaks registered for all samples in the interval of 2Q 510–908 are known well for the structure of K 2 NiF 4 type. Some data on phase transition investigations in lanthanum nickelate La 2 NiO y are known from the literature. Kajitani and al. [10] showed the existence of tetragonal and orthorhombic structures of La 2 NiO y by high resolution XRD analysis and neutron diffraction methods. They found that La 2 NiO y at room temperature has the pseudotetragonal (orthorhombic) structure, that turns into real tetragonal structure at heating above 1578C. The transition temperature was connected with oxygen stoichiometry. Gopalan et al. [11] showed that single crystals of La 2 NiO 42d (d 50...0.067) transform from high temperature tetragonal modification into low temperature orthorhombic phase at 4078C and then turn into low temperature tetragonal modification at about 22038C at further cooling.

V.V. Vashook et al. / Solid State Ionics 110 (1998) 245 – 253

In view of our own XRD analysis and the data [10,11] we can not relate the two oxygen desorption peaks due to the heating of nickelate powders presented in Fig. 1 to any phase transformations and we like to relate them to removal of oxygen from different crystallographic positions of nickelates in that area of the compounds. According to [12] overstoichiometric oxygen in La 2 NiO 41d structure is located in interstitial positions with the coordinates 1 / 4 1 / 4 1 / 4, close to layers formed by NiO 6 octahedra in the perovskite like K 2 NiF 4 -type lattice. These sites are preferable for oxygen ions due to the convenience of their disposition between lanthanum cations. These interstitial oxygen ions, apparently, are the most weakly bonded in the nickelate lattice. As follows from the data on nickelate reduction in hydrogen, the amount

249

of interstitial ions of oxygen in the investigated nickelates decrease with the increase in strontium content from 0.15 oxygen atomic index in La 2 NiO 4.15 up to 0.11 in La 1.8 Sr 0.2 NiO 4.11 and 0.05 in La 1.5 Sr 0.5 NiO 4.05 . It is possible that during the heating of nickelates interstitial oxygen ions leave the compound lattice at lower temperatures and cause the occurrence of the first oxygen desorption peaks in the range 300–7008C in the case of La 2 NiO y and La 1.8 Sr 0.2 NiO y compounds. After reaching some lower oxygen overstoichiometry the further allocation of oxygen apparently occurs accounting for the removing of oxygen from its regular positions. Oxygen allocated in this case causes the appearance of oxygen desorption peaks in the temperature range 700–10508C shown in Fig. 1a–c. The overstoichiometric contents of oxygen in La 2 NiO y at 7008C at the occurrence of the second peak of oxygen desorption comes at about 0.08 (Fig. 1a), and in the case of La 1.8 Sr 0.2 NiO y , at 0.07 (Fig. 1b). The overstoichiometric contents of oxygen in the initial La 1.5 Sr 0.5 NiO y makes up 0.05 of the oxygen atomic index and it is possibly so strongly kept by the nickelate lattice that it does not result in desorption of oxygen at heating the powders up to temperatures 700–8008C (Fig. 1c). The appearance of the two oxygen desorption peaks at heating of La 2 NiO y and La 1.8 Sr 0.2 NiO y confirms the assumption [8] concerning the presence of two types of oxygen in given nickelates, resulted in simultaneous allocation of oxygen and chlorine when dissolving in hydrochloric acid.

3.3. Oxygen stoichiometry and conductivity near equilibrium

Fig. 2. Oxygen content y (a) and specific electrical resistance r (b) of La 2 NiO y ceramic sample during step heating and waiting up to constant values of these parameters at various oxygen partial pressures.

The agreement of both of r and y values for La 2 NiO y at the same temperatures in temperature region 400–10008C during step heating and subsequent step cooling of a sample by 100 or 508C every time in argon flow with oxygen partial pressure 100 Pa allowed us to attribute such values to equilibrium ones. Similar results were obtained for all three nickelates studied at oxygen partial pressures 1.5, 10, 49, 100 and 286 Pa. The determined values of specific electrical resistance and oxygen stoichiometry of investigated nickelates are presented in Figs. 2–4. These values were

250

V.V. Vashook et al. / Solid State Ionics 110 (1998) 245 – 253

Fig. 3. Oxygen content y (a) and specific electrical resistance r (b) of La 1.8 Sr 0.2 NiO y ceramic sample during step heating and waiting up to constant values of these parameters at various oxygen partial pressures.

reproduced in the temperature range 300–10508C at all oxygen partial pressures for all nickelates. With decreasing pO 2 values the duration of the equilibrium establishment increased. Sometimes it required more than 20 h to establish the equilibrium states. It should be noticed, that for La 1.5 Sr 0.5 NiO y within the experience error limits in the temperature range 300– 7008C and at pO 2 51.5–286 Pa no changes in oxygen stoichiometry and conductivity were observed. These oxygen stoichiometry and conductivity values cannot be considered as «frozen» values since at the approach to any chosen temperatures within 300– 7008C with various rates of cooling from higher temperatures r and y values were always reproduced. The equilibrium state was not attained for

Fig. 4. Oxygen content y (a) and specific electrical resistance r (b) of La 1.5 Sr 0.5 NiO y ceramic sample during step heating and waiting up to constant values of these parameters at various oxygen partial pressures.

La 2 NiO y in gas flow at pO 2 51.5 Pa and temperature 10508C. The sample continues losing its oxygen and increasing its resistance with constant rate for 25 h. It is possible that a slow decomposition of the material took place under these experimental conditions. In Figs. 2 and 3a relating to La 2 NiO y and La 1.8 Sr 0.2 NiO y it is possible to allocate two sites, distinguished by the character of oxygen index ( y) change versus temperature (t). The low temperature range is characterized by positive value of the second derivative of oxygen stoichiometry versus tempera≠ 2y ture s ] . 0d. And on the contrary, their high-tem≠t 2 perature ranges are characterized by the negative one ≠ 2y , 0d. The inflection points in both cases lay in s] ≠t 2 the temperature range of 600–8008C. Each of these two regions can be brought into conformity with one

V.V. Vashook et al. / Solid State Ionics 110 (1998) 245 – 253

of two maxima of oxygen desorption observed in the titration current curve change during polythermal experiments on powders (Fig. 1a and b). Unlike La 2 NiO y and La 1.8 Sr 0.2 NiO y the dependencies of equilibrium values of oxygen stoichiometry for La 1.5 Sr 0.5 NiO y are characterized by negative value of the second derivative of oxygen index ≠ 2y versus temperature s ] , 0d within the whole tem≠t 2 perature range 300–10508C (Fig. 5a). Contrary to oxygen index dependencies versus temperature, the temperature dependencies of specific electrical resistance r (t) for all nickelates are characterized by positive value of the second deriva≠ 2r tive at all oxygen partial pressures s ] . 0d. In other ≠t 2 words, all nickelates in the investigated interval of temperatures and oxygen partial pressures reveal a metallic character of conductivity. The decrease in electrical conductivity with the decrease in oxygen partial pressure indicates the fact, that electron carriers of hole type are predominant for all nickelates. The very weak (slight) dependence of r ( pO 2 ) for La 1.5 Sr 0.5 NiO y in the temperature range 300–7008C confirms the data [5] concerning the close to zero Seebek coefficient, which is determined for the given compound. Apparently, in the given compound at temperatures 300–7008C the contribution of electronic and hole type charge carriers to total conductivity is quite comparable. Comparison of y(t) (Fig. 2a–4a) and r (t) (Fig. 2b–4b) dependencies show, that the change in the oxygen stoichiometry index of La 2 NiO y and La 1.8 Sr 0.2 NiO y nickelates in the temperature range

Fig. 5. Equilibrium values of oxygen content ( y) in nickelates as functions of temperature at different oxygen partial pressures.

251

300–8008C does not have the essential influence on the character of temperature dependencies of electrical resistance of the samples. The obtained data correspond with the above assumption (Section 3.2) that the oxygen stoichiometry change of nickelates in the mentioned-above temperature region occurs because of the exchange of weak-bonded oxygen atoms (ions), which do not influence essentially the state of chemical bonds, involved in the electrical charge carrying. The absence of oxygen stoichiometry anomalies in the given temperature range for La 1.5 Sr 0.5 NiO y apparently can serve as confirmation of the absence of such weak-bonded oxygen in this compound. A monotonous conductivity change in all nickelates at 300–10508C allows us to assume the presence of the only conductivity mechanism. It is most probably, that the carrying of charge takes place through –Ni–O–Ni–O– chains, located in the equatorial planes of NiO 6 octahedra, forming perovskite layers of nickelates. It is possible to see in Figs. 5 and 6, showing only equilibrium values of oxygen stoichiometry and specific electrical resistance that the intervals of equilibrium values of y and r change in investigated nickelates under experimental conditions decrease with the increase in their strontium content. Established for the all compounds investigated in this work, nickelate p-type conductivity assuming the presence of interstitial oxygen ions in this structure

Fig. 6. Equilibrium values of specific electrical resistance ( r ) of nickelates as functions of temperature at different oxygen partial pressures.

252

V.V. Vashook et al. / Solid State Ionics 110 (1998) 245 – 253

of nickelate allows us to offer the following scheme of disordering of nickelates as the most suitable one: 1 / 2O 2 ⇔O i9 1 h? ⇔O i99 1 2h? , If we are to accept that in nickelates structure the oxidation states of lanthanum, strontium and oxygen ions are constant and are equal to 31, 21 and 22 respectively it is necessary to accept, that the holes are located on nickel ions, which have variable valence, and taking into account the oxidation states of individual elements the nickelate formulas run as 21 31 31 21 follows: La 31 La 1.8 Sr 21 2 Ni 922y Ni 2y28 O y , 0.2 Ni 8.822y31 21 21 31 Ni 31 2y27.8 O y and La 1.5 Sr 0.5 Ni 8.522y Ni 2y27.5 O y . Thus, the investigations carried out have shown that lanthanum–strontium nickelates studied have relatively high total conductivity (around 10 2 Ohm 21 pcm 21 ) and they are stable in the pO 2 interval of 1.5–286 Pa at the temperatures from 3008C up to 10508C, excepting La 2 NiO y , which apparently is decomposed or undergoes polymorphic transformation at the temperatures higher than 900–10008C depending on the value of oxygen partial pressure. The width of oxygen homogeneity area of nickelates decreases with the substitution of lanthanum by strontium. Preliminary research showed high oxygen diffusivity in nickelates at the same conditions. Chemical diffusion coefficients of oxygen in La 1.8 Sr 0.2 NiO y , for example, varied in the interval of 26 27 2 21 10 –10 cm sec [13] at the same temperatures and oxygen partial pressures considered in the presented work. The obtained results confirm the utility of these nickelates for the production of electrode materials for yttrium stabilised zirconia-based high-temperature solid-electrolyte devices and possibly for making selective ceramic membranes for separation of oxygen from gaseous mixtures.

4. Conclusions The oxygen desorption maxima observed during the heating of La 2 NiO y and La 1.8 Sr 0.2 NiO y powders at temperatures 300–7008C in the interval of pO 2 5 1–286 Pa are apparently due to the allocation of weak-bonded overstoichiometric oxygen located in the interstitial sites of a crystal lattice of nickelate.

Absence of a similar maximum in the case of La 1,5 Sr 0.5 NiO y indicates the absence of such weakbonded oxygen in the compound lattice. The observed similar oxygen desorption maxima at temperatures above 7008C for all investigated compounds are apparently connected with allocation of oxygen from NiO 6 octahedra building perovskite layers of nickelates. The removal of oxygen from perovskite layers occurs when the lattice maintains some amount of overstoichiometric oxygen (0.05– 0.08 mol), located in interstitial sites. The equilibrium diagrams of «oxygen partial pressure–temperature–oxygen stoichiometry» and «oxygen partial pressure–temperature–specific electrical resistance» types for all investigated nickelates in the temperature region 300–10508C and oxygen partial pressure of 1.5–286 Pa are created. It was marked, that in accordance with the increase of strontium content the exchange interval of oxygen stoichiometry and electrical conductivity became smaller under the conditions which were studied. It was established, that all three nickelates in the investigated ranges of temperature and oxygen partial pressure are p-type conductors with metallic character of conductivity.

Acknowledgements This work was supported by the grant INTAS-94¨ Bildung und 1399 and by Bundesministerium fur Forschung, Bonn, in the framework of bilateral cooperation Germany–Belarus.

References [1] P. Ganguly, C.N.R. Rao, Mat. Res. Bull. 8 (1973) 405. [2] I.F. Kononyuk, N.G. Soormach, Neorganich. Mat. 18 (1982) 1222. [3] I.F. Kononyuk, N.G. Soormach, Neorganich. Mat. 18 (1982) 1029. [4] K. Ishikawa, S. Kondo, H. Okano, et al., Bull. Chem. Soc. (Japan) 60 (1987) 1295–1298. [5] Y. Takeda, R. Kanno, M. Sakano, et al., Mat. Res. Bull. 25 (1990) 293–306. [6] R.J. Cava, B. Batlogg, T.T. Palsta, et al., Phys. Rev. B. 43(N1) (1991) 1229–1232. [7] L.V. Makhnach, S.P. Tolochko, I.F. Kononyuk, et al., Neorganich. Mat. 29(12) (1993) 1678–1682.

V.V. Vashook et al. / Solid State Ionics 110 (1998) 245 – 253 [8] S.P. Tolochko, L.V. Makhnach, I.F. Kononyuk, V.V. Vashook, Zh. Neorganich. Khim. 37(7) (1994) 1092–1095. [9] M. Bode, K. Teske, H. Ullmann, Fachzeitschtift Lab. 38 (1994) 495–500. [10] T. Kajitani, Y. Kitagaki, S. Wada, K. Hiraga, S. Hosoya, Physics C, 185–189 (1991) 579–580.

253

[11] P. Gopalan, M.W. Mcelfresh, Z. Kakol, J. Spatek, Phys. Rev. B. 45(1) (1992) 249–253. [12] Z. Hiroi, M. Takano, Y. Bando, Supercond. Sci. Technol. 4(15) (1991) 139–141. [13] V.V. Vashook, M.V. Zinkevich, L.V. Makhnach, S.P. Tolochko, I.F. Kononyuk. Neorganich. Mat., 1998, in press.