Oxalate coprecipitation synthesis and transport properties of polycrystalline Sr1−xLaxPbO3−δ solid solutions

Journal of Alloys and Compounds 367 (2004) 246–250

Oxalate coprecipitation synthesis and transport properties of polycrystalline Sr1−x Lax PbO3−δ solid solutions Vadim A. Drozd a,∗ , Alexander M. Gabovich b , Marek P˛ekała c , Sergey A. Nedilko a , Piotr Gierłowski d a

Department of Chemistry, Kiev Taras Shevchenko National University, Volodymyrs’ka Street 64, Kiev 01033, Ukraine b Crystal Physics Department, Institute of Physics, Prospekt Nauki 46, Kiev 03028, Ukraine c Department of Chemistry, University of Warsaw, Al. Zwirki ˙ i Wigury 101, Warsaw PL-02-089, Poland d Institute of Physics of PAS, Al. Lotników 32/46, Warsaw PL-02-668, Poland

Abstract Solid solutions Sr1−x Lax PbO3−δ with x = 0, 0.05, 0.1, 0.15, 0.2 and 0.25 were synthesized by oxalate precursor coprecipitation methods. The solid solutions obtained were analyzed by the XRD technique and iodometric titration. Resistivity ρ and thermoelectric power S were measured for temperatures 20 K < T < 300 K. The parent compound SrPbO3−δ shows a shallow minimum of ρ(T) at about 120 K, whereas ρ(T) for the La-doped samples are rather smooth with weakly manifested inflection points for all investigated compositions. The thermoelectric power is negative. The transport data demonstrate that the ceramic samples Sr1−x Lax PbO3−δ are unstable towards electron spectrum dielectrization or localization of current carriers. From the analysis of the measured properties it may be inferred that the effects of disorder and granularity and the inter-grain tunneling, in particular, strongly influence transport phenomena. © 2003 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Precipitation; Crystal structure and symmetry; Electronic transport

1. Introduction In the past various semiconducting and semimetallic compounds that acquire metallic properties when doped were often considered as candidates for high-Tc superconductors [1]. Solid solutions BaPb1−x Bix O3−δ (BPB) appeared to be the first oxide system to live up to such expectations [2]. On the other hand, different kinds of Coulomb- and crystal lattice-driven instabilities may distort the primordial electron spectrum in such compounds, so that the ground state becomes insulating, partially dielectrized or magnetic [3]. Then possible superconductivity runs into danger due to the lack of the available Fermi surface. Weak or strong current carrier localization is also detrimental to the development of the Cooper-paired state [4]. Nevertheless, Müller and Bednorz discovered high-Tc superconductivity in 1986 and it turned out that the theoretically suggested upper bounds on Tc (see a recent survey of relevant discussions in [5]) for pairing triggered by the conventional electron-phonon mediation could be ∗

Corresponding author. Tel.: +38-44-239-33-06. E-mail address: [email protected] (V.A. Drozd).

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overcome. (Here, Tc means the critical temperature of the normal metal—superconductor phase transition). Of course, in the cuprate case some kind of non-phonon mechanism may be the main cause of superconductivity, which seems highly probable in view of the antiferromagnetic insulating nature of the ground state for undoped Cu-based oxides. Therefore, syntheses and studies of various new classes of unequivocally nonmagnetic oxides are necessary not only because of their practical value but also in order to elucidate the ability of the electron-phonon interaction to induce high-Tc superconductivity. The existence of oxygen octahedra similar to those in cuprates and concomitant lattice vibrations would ensure a fruitful comparison of the superconducting properties (if any) for new substances and the layered cuprates. Finally, one would expect the solution of the so-called pseudogap puzzle, because pseudogapping manifests itself not only in high-Tc cuprates but in the BPB solid solutions as well [3,6,7]. There have been a lot of investigations of promising oxides and the highest unambiguously achieved Tc is about 30 K in Ba1−x Kx BiO3−δ [8]. Evidence also exists concerning possible room-Tc superconductivity in Cu24 Pb2 Sr2 Ag2 Ox [9] and Agx Pb6 CO9±δ [10]. Resistive

V.A. Drozd et al. / Journal of Alloys and Compounds 367 (2004) 246–250

superconducting-like anomalies for the temperature range 190 K < T < 270 K in Cu24 Pb2 Sr2 Ag2 Ox were subsequently observed by another group [11]. Although there are no unambiguous confirmations of high-temperature superconductivity occurring in the above-mentioned silver–copper–lead and silver–lead oxides, the search is being continued [12,13]. Other oxide families related to the parent compound SrPbO3−δ also merit notice in connection with possible superconductivity. Relevant transport investigations were made for La-doped [14–16] and K-doped [17] ceramics SrPbO3−δ . In the rigid-band picture the first family would be expected to possess an electron type of conductivity, whereas the second one has to manifest a hole-like conductivity. However, this turned out not to be the case, so inter-electron correlations are significant for these solid solutions, making simple predictions about resistivity ρ and other transport properties highly unreliable. Moreover, as we have shown in the previous work for Sr1−x Kx PbO3−δ [17], in polycrystalline samples the granular structure itself may determine the temperature dependence of ρ, so that possible tendencies towards a metal-insulator transition may have an extrinsic rather than an intrinsic origin. In this study, we extended to 0.25 the degree of La-doping for SrPbO3−δ in comparison to earlier studies of Sr1−x Lax PbO3−δ [15], where x did not exceed 0.02, and carried out measurements of both ρ(T) and the thermoelectric power S(T). The results are compared with those for the nominally hole-doped solid solutions Sr1−x Kx PbO3−δ [17].

247

Table 1 Results of trilonometric titration of SrPbO3−δ on different stages of thermal treatment Analyzed sample

Molar ratio Sr:Pb

Initial mixture of aqueous solutions of Sr(NO3 )2 and Pb(NO3 )2 Coprecipitated oxalates of Sr and Pb(II) Product of oxalate thermal decomposition at 600 ◦ C for 5 h Product of oxalate thermal decomposition at 750 ◦ C for 24 h Product of oxalate thermal decomposition at 750 ◦ C for 48 h

1:1 1:0.998 1:0.993 1:0.990 1:0.990

It is known that lead oxides have high volatility that might cause uncontrolled changes in the chemical composition of the lead-containing oxide materials during their synthesis. The Sr1−x Lax PbO3−δ solid solutions were chemically analyzed (trilonometric titration) to check the stoichiometry of the final product after the thermal synthesis. The results of trilonometric titration for the sample with a starting molar ratio Sr:La:Pb=1:0:1 are presented in Table 1. According to these data the loss of lead during the heat treatment does not exceed 1 at.%. Oxygen indices in solid solutions were determined by a modified method of the iodometric titration [17–19]. Fig. 1 displays the total oxygen content for Sr1−x Lax PbO3−δ and an average lead valence calculated from the iodometric titration data. The X-ray investigations were carried out by the powder method using a diffractometer DRON-3 (Cu K␣ radiation).

4.00 3.96 3.92 3.88 3.84 3.02

Total oxygen content (y)

Solid solutions Sr1−x Lax PbO3−δ with 0 ≤ x ≤ 0.25 were prepared by oxalate precursor coprecipitation method. Aqueous solutions (with concentrations of about 0.2 M) of Sr(NO3 )2 , La(NO3 )3 and Pb(NO3 )2 were chosen as starting reagents. Sesquialteral excess (in the molar ratio to the sum of the moles of cations in the solution) of the oxalic acid (∼0.8 M solution) was added to the appropriately mixed solutions of the Sr, La and Pb(II) nitrates. In order to reduce the oxalate solubility, 2-propanol was added (in the volume ratio 1:1) to the resulting solution of coprecipitated oxalates. Ammonium hydroxide was used to adjust the final pH to a range of 6–7. The precipitates were held for 2 h in the mother solution, then filtrated and rinsed sequentially by a mixture of 2-propanol and distilled water, distilled water and finally, acetone. The powders were dried in air at 80 ◦ C. Oxalate coprecipitates were converted to Sr1−x Lax PbO3−δ solid solutions by slowly heating up to 600 ◦ C in air atmosphere with subsequent annealing at this temperature for several hours. Then the powders were ground, pressed into pellets and annealed in an oxygen flow at 750–850 ◦ C for 24 h. The final products varied in color from deep brown (x = 0) to brown (x = 0.25).

Average Pb valence

2. Synthesis and sample characterization

3.00 2.98 2.96 2.94 2.92 0.00

0.05

0.10

0.15

0.20

0.25

Substitution degree (x) Fig. 1. Results of the iodometric titration of the Sr1−x Lax PbO3−δ solid solutions.

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V.A. Drozd et al. / Journal of Alloys and Compounds 367 (2004) 246–250

Intensity (a.u.)

112

220

024

200

110

204 132 020 222

012 022 013 200 113

111

20

30

40

023

131

50

2Θ/degree CuK α

Fig. 2. X-ray diffraction pattern for Sr0.9 La0.1 PbO3−δ .

Crystal lattice parameters were calculated according to the least squares method using sets of 15–20 reflections. Crystalline Si was used as an internal standard. According to the X-ray diffraction analysis, a single-phase region for the Sr1−x Lax PbO3−δ solid solutions exists for 0 ≤ x ≤ 0.15. Samples with x ≥ 0.20 contain small amounts of impurity phases (most likely La2 O3 and Pb2 O3 ) with the strongest diffraction lines corresponding to d = 3.207, 3.147 and 2.843 Å. All XRD patterns of Sr1−x Lax PbO3−δ solid solutions were indexed assuming an orthorhombically distorted perovskite-type structure. No splitting of the diffraction lines indicating symmetry changes or superstructure formation was observed. An X-ray powder diffraction pattern of Sr0.9 La0.1 PbO3−δ is shown in Fig. 2. Crystal lattice parameters are plotted in Fig. 3. In the undoped SrPbO3−δ , oxygen vacancies are compensated according to the following scheme: Sr 2+ Pb1−δ 4+ Pbδ 2+ O3−δ 2− , i.e. the enhancement of the index ␦ is accompanied by an increase of the Pb(II) cation concentration. The substitution of La3+ for Sr2+ in Sr1−x Lax PbO3−δ solid solutions is heterovalent, so that there are two possibilities to maintain the charge neutrality of the compound: Sr 1−x 2+ Lax 3+ Pb1−δ−(x/2) 4+ Pbδ+(x/2) 2+ O3−δ 2− and Sr 1−x 2+ Lax 3+ Pb1−δ 4+ Pbδ 2+ O3−δ+(x/2) 2− . The former one implies a compensation of the extra charge introduced into the lattice by La3+ by the appearance of Pb2+ ions instead of Pb4+ . According to the latter scheme the charge neutrality is preserved by the disappearance of oxygen vacancies inherent to the parent SrPbO3−δ compound. It can be seen (Fig. 1) that all samples have deviations from the ideal oxygen stoichiometry. This deviation, ␦, decreases with the La concentration. This means that the second charge redistribution scheme plays an essential role for the 0 ≤ x ≤ 0.15 substitution range. On the other hand, the increase of the crystal lattice parameters with x (Fig. 3) for 0 < x < 0.10 can be explained assuming that the Sr2+ substitution for La3+ results in a reduction of Pb4+ to Pb2+ . The latter ion is larger than Pb4+ and will expand the crystal lattice. The decrease of the

Fig. 3. Crystal lattice parameters of Sr1−x Lax PbO3−δ solid solutions.

crystal lattice parameters for x > 0.10 can be explained by a size effect (difference in ionic radii of Sr2+ and La3+ ) as well as heterogenization of the solid solutions with x ≥ 0.15. This explanation of the oxygen content variation cannot be directly applied to solid solutions with x ≥ 0.20 because of their multi-phase nature. The oxygen index y found from iodometric titration (Fig. 1) reflects only the total oxygen content in the analyzed system, but not its distribution among phases that constitute the mixture.

3. Electrical measurements The samples under investigation were rectangular bars with approximate dimensions 2 mm × 2 mm × 6 mm, which were cut from the initially produced pellets. Transport measurements were performed between 20 and 300 K in a closed cycle refrigerator. The temperature was stabilized with an accuracy of 0.02 K. A four-probe technique was used for electrical resistivity ρ(T) measurements. For the differential thermoelectric power S(T) measurements a small temperature gradient of about 1 K along the sample was created by a milliwatt heater. A HP 3457A voltmeter was used for precise voltage measurements.

4. Results In Fig. 4, the T-dependence of the resistivity ρ is shown for a number of solid solutions Sr1−x Lax PbO3−δ . For the parent compound (x = 0) the shallow minimum [15,17] in

V.A. Drozd et al. / Journal of Alloys and Compounds 367 (2004) 246–250

down to 20 K. It would be desirable to test these samples for superconductivity both resistively and magnetically at lower T, bearing in mind the 12 and 11 K superconductivity onsets in the related perovskite families Sr1−x Kx BiO3−δ [20] and Ba1−x Lax PbO3−δ [21]. The qualitative difference between the low-T thermal resistivity coefficients dρ/dT for Sr1−x Lax PbO3−δ and Sr1−x Kx PbO3−δ [17] may be due either to the bulk intrinsic asymmetry of electrons and holes in the parent substance SrPbO3−δ , or to the detrimental influence of the impurity phases on the conductivity in the inter-grain gap for Sr1−x Kx PbO3−δ . Notwithstanding the metallic behavior of the polycrystalline Sr1−x Lax PbO3−δ , the quantitative analysis of its resistivity must be performed taking into account the granular structure of the samples. The intrinsic ρ(T) may be inferred only from single crystal investigations. Fig. 5 demonstrates the temperature dependence of the thermoelectric power (Seebeck coefficient) S for the same samples of Sr1−x Lax PbO3−δ . For all x, S(T) < 0 and the amplitudes |S(T)| grow with increasing T. This means that electrons comprise the current carrier majority. The results for zero La-content are quite similar to previous ones [15]. The overall tendency of |S(T)| reduction with increasing x also agrees well with the data of Terasaki and Nonaka [15]. Our measurements, however, are made for much larger doping, so that we can clearly see not only the decrease of the absolute values of S(T), but also a saturation of the slopes for x > 0.2. In the free-electron picture for the degenerate electron gas it can be treated as a growth with a subsequent saturation of the current carrier concentration, since in this approximation for low T [22]

ρ · 102 Ohm·cm

7 6 x =0.00

5 4 x =0.05

3 2 28

4

ρ · 10 Ohm·cm

26

x =0.15

24 x=0.20

22 20 18

x =0.25

16 14 0

50

100

150

200

250

249

300

Temperature, K Fig. 4. Temperature dependence of the electrical resistivity of the investigated compounds.

the range 100 K < T < 150 K is reproduced. It is clear from the plots that La-doping, in contrast to K-doping [17], leads to a more metallic behavior, totally suppressing the semiconducting-like upturn for lower T. The results qualitatively agree with the trend observed for the data of Terasaki and Nonaka [15], obtained, however, for much lower doping levels. Nevertheless, superconductivity was not observed

S=

π2 kB2 T . 3eEF

(1)

0 -30 -10

S, µ V/K

S, µV/K

-60 -90

x=0.25 x=0.20

-20 x=0.15

-120 x=0.05

-30 0

-150

50

100

150

200

250

300

T, K

-180

x = 0.00 0

50

100

150

200

250

300

T, K Fig. 5. Temperature variations of the Seebeck coefficients S(T) for Sr1−x Lax PbO3−δ solid solutions. The insert shows S(T) dependences for 0.05 ≤ x ≤ 0.25 samples on an enlarged scale.

250

V.A. Drozd et al. / Journal of Alloys and Compounds 367 (2004) 246–250

Here, e is the current carrier charge (negative for electron-like quasiparticles), kB the Boltzmann constant, EF is the Fermi energy. However, the linearity of S(T) required by the basic theory is not observed in Sr1−x Lax PbO3−δ even at the lowest T attained in our experiment. It should be mentioned that such nonlinearities are often seen for compounds and elementary metals [22]. Moreover, the whole simplified picture may be misleading, since for the related oxide family Sr1−x Kx PbO3−δ the Seebeck coefficient is also negative despite the hole-like doping.

[2] [3] [4] [5] [6] [7]

[8]

5. Conclusions To summarize, we synthesized the solid solutions Sr1−x Lax PbO3−δ with x ≤ 0.25 and measured their resistivity ρ and thermoelectric power S. It was shown that the parent compound SrPbO3−δ , being on the verge of a metal– insulator transition, transforms into metallic state with doping. Superconductivity was not detected down to 20 K, although its existence for lower T is not improbable. Further investigations with other dopants seem very promising.

[9] [10] [11] [12]

[13] [14]

Acknowledgements

[15]

Two of the authors (V.A.D. and A.M.G.) are grateful to the J. Mianowski Foundation and Foundation for Support of the Polish Science (Fundacja na Rzecz Nauki Polskiej) for the support of this investigation during their stay at Warsaw, P.G. and M.P. acknowledge partial support of Polish Committee for Scientific Research, grant nos. PBZ-KBN-013/T08/19 and 7T08 02820 correspondingly. This work was also partially supported by NATO under grant no PST.CLG 979446.

[16]

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