Neutron structural characterization and transport properties of the oxidized and reduced LaCo0.5Ti0.5O3 perovskite oxide

Polycrystalline oxygen-stoichiometric LaCo0.5Ti0.5O3 perovskite oxide has been prepared by soft-chemistry procedures followed by annealing in air at 800°C. A new reduced LaCo0.5Ti0.5O3-δ specimen has been obtained by topotactical oxygen removal in an H2/N2 (5%/95%) flow at 600°C. The structural characterization has been conducted from neutron powder diffraction (NPD) data, very sensitive to the contrast between Co and Ti and the oxygen stoichiometry. Both perovskites (oxidized and reduced) crystallize in the orthorhombic Pbnm, space group. The partial reduction of Ti4+ to Ti3+ in the reduced phase is accompanied with the occurrence of oxygen vacancies, located at the axial octahedral sites, and it is expected to support the ionic conductivity, as usually observed in oxygen-defective perovskites. Thermogravimetric analysis (TGA) substantiates the oxygen stoichiometry and the stability range of the reduced sample. All the samples in study display a semiconductor-like behavior with values that not reach below to 0.5 Scm−1 for all the phases. Moreover, the measured thermal expansion coefficients perfectly match with the values usually displayed by SOFC electrolytes.


Introduction
Materials with perovskite-like structure are receiving great attention due to their vast range of possible applications. Perovskite-type oxides are widely used as functional materials in ferroelectric, thermoelectric, dielectric, magnetoelectronic devices and in solid oxide fuel cells (SOFC), among wide panoply of applications. Their unique properties derive from an unmatched chemical flexibility in spite of their relatively simple crystal structure [1]. The reduction of stoichiometric phases into novel oxygen hypo-stoichiometric oxides is a powerful tool for the development of new materials with novel magnetic or transport properties [2]. Moreover, the stabilization of transition-metal perovskites with an adequate concentration of oxygen vacancies under reducing atmosphere can also be of application in new mixed electronic-ionic conductors for energy-conversion devices such as solid oxide fuel cells, oxygen separation membranes or solid oxide electrolyzers.
Owing to many interesting properties such as rich structural characteristics, transport and magnetic properties [3][4][5][6], perovskites oxides of R(III)Ti(III)O 3 (R= rare earth) have been one of the hot subjects of recent studies. Among perovskite oxides that exhibit appealing properties, ceramic titanates are recognized to present high stability under reducing environments with high tolerance to sulphur poisoning. The formation of R(M,Ti)O 3 (R= rare earth; M= metal transition) is may be feasible either for trivalent M and Ti ions (having in mind that Ti 3+ ions are difficult to stabilize) or with a combination of divalent M and tetravalent Ti ionic states, under the appropriate atmosphere during the high-temperature synthesis [7]. When Co(III) ions are substituted by Ti(IV), as reported in the LaCo 1- x Ti x O 3 solid solution [8], a mixed valence system is obtained as the charge neutrality requires the partial reduction to divalent Co ions. LaCo 0.5 Ti 0.5 O 3 perovskite has been previously reported by several researchers, which defined the crystal structure by neutron powder diffraction in both orthorhombic and monoclinic unit-cell [9][10][11]; also x-ray diffraction, thermogravimetric analysis and magnetic susceptibilities were used to characterize this material [12]. Although there is a comprehensive study of this perovskite, the research of the reducibility has not been yet considered. In this work we have analyzed the reducibility of the LaCo 0.5 Ti 0.5 O 3 perovskite and the crystal structure evolution from the oxidized to the reduced phase. The analysis of NPD data for both oxidized and reduced specimens suggests the presence of Ti 3+ -Ti 4+ mixed valence in the reduced phase. The characterization has been completed with thermal expansion, electric conductivity measurements and thermal analysis under oxidizing and reducing atmosphere.

Experimental
LaCo 0.5 Ti 0.5 O 3 perovskite was prepared as a polycrystalline powder from citrate precursors obtained by soft-chemistry procedures. Stoichiometric amounts La 2 O 3 (pre-dried at 900ºC), Co(NO 3 ) 2 ·6H 2 O and TiC 10 H 14 O 5 were solved in citric acid and some drops of nitric acid. The solution was then slowly evaporated, in order to favor the dissolution of the rare-earth oxide, leading to organic resins that contain a homogeneous distribution of the involved cations. The formed resins were dried at 120 ºC and decomposed at 600 ºC for 12 h, heating with a 50 ºC·h -1 ramp, in air. All the organic materials and nitrates were eliminated in a subsequent treatment at 800°C in air, for 2 hours, which gave rise to the pure perovskite oxide phase. The reduced LaCo 0.5 Ti 0.5 O 3-δ perovskites were prepared by treating the oxidized phase under an 5%H 2 /95%N 2 flow (60 mL min -1 ) at 600 ºC for 4 h in alumina boats. The initial characterization of the product was carried out by XRD with a Bruker-axs D8 Advanced diffractometer (40 kV, 30 mA), controlled by a DIFFRACT PLUS software, in Bragg-Brentano reflection geometry with Cu K α radiation (λ = 1.5418 Å) and a PSD (Position Sensitive Detector). A filter of nickel allows the complete removal of Cu K β radiation. For the structural refinement NPD patterns were collected at the D1A diffractometer of the ILL, Grenoble, with a wavelength λ= 1.910 Å at room temperature. About 2 g of the sample were contained in a vanadium can and placed in the isothermal zone of a furnace with a vanadium resistor operating under vacuum (P O2 ≈ 10 -6 Torr), and the counting time was 2 h per pattern in the high-intensity mode. The NPD data were analyzed by the Rietveld method [13] with the FULLPROF program [14]. A pseudo-Voigt function was chosen to generate the line shape of the diffraction peaks. The following parameters were refined in the final run: scale factor, background coefficients, zero-point error, pseudo-Voigt corrected for asymmetry parameters, positional coordinates and isotropic thermal factors for all the atoms. The coherent scattering lengths for La, Co, Ti and O were 8.24, 2.49, -3.438 and 5.803 fm, respectively [14]. Thermal analysis was carried out in a Mettler TA3000 system equipped with a TC10 processor unit. Thermogravimetric (TG) curves were obtained in a TG50 unit, working at a heating rate of 10 °C min -1 , in a reducing H 2 (5%)/N 2 (95%) flow of 0.3 L min -1 . The heating rate was 10 °C min −1 , using about 50 mg of sample in each experiment. Measurements of the thermal expansion coefficient and electrical conductivity required the use of sintered samples. The obtained density is around 90-95%. Thermal expansion of the sintered samples was performed in a dilatometer Linseis L75HX1000, between 300 and 800 ºC in air and The conductivity was measured between 25 and 850 ºC in the requested atmosphere, by the four-point method in bar-shaped pellets under DC currents between 0.05 and 0.10 A. The currents were applied and collected with a Potenciostat-Galvanostat AUTOLAB PGSTAT 302 from ECO CHEMIE.

Crystal structure
The oxidized and reduced LaCo 0.5 Ti 0.5 O 3 perovskites were obtained as well-crystallized powders.
Single-orthorhombic perovskite phases were identified from laboratory XRD ( Fig. 1  To carry out a more accurate structural study of the LaM 0.5 Ti 0.5 O 3 (M= Co and Ni) oxides, we performed a NPD investigation at room temperature for all the perovskites; neutrons are especially sensitive to the nature of these atoms since they show very contrasting (positive for Co and negative for Ti) scattering lengths. The crystal structure of the oxidized and reduced LaCo 0.5 Ti 0.5 O 3 is defined in the orthorhombic Pbnm space group (No. 62), Z=4, as was previously reported by Clairns et al. [11]. La and O1 atoms are located at 4c (x,y,1/4) positions, M and Ti distributed at random at 4b (1/2,0,0), and oxygen atoms O2 at 8d (x,y,z). Therefore, for both oxidized and reduced phases the Co and Ti atoms are randomly distributed and no crystallographic long-range order was observed. The refinement of the occupancy factors of the oxygen atoms for the oxidized phase led to a full stoichiometry, while the reduced phase shows oxygen vacancies concomitant with the presence of Ti 3+ . The vacancies are concentrated at O1 sites (axial oxygen atoms); O2 showed occupancies slightly higher than 1 and was then fixed to unity. The refined occupancy factors of oxygen atoms for the reduced phase lead to the LaCo 0.5 Ti 0.5 O 2.91(1) stoichiometry. Fig. 2 illustrates the good agreement between the observed and calculated NPD patterns for the oxidized and reduced LaCo 0.5 Ti 0.5 O 3 at room temperature.  An alternative refinement of the crystal structure of the oxidized and reduced samples was carried out in the monoclinic P2 1 /n space group; in this model the Rietveld refinements display the existence of a certain level of anti-site disorder between Co and Ti cations. However, the results of the refinement at P2 1 /n are similar to those obtained with the Pbnm; the R Bragg discrepancy factor is practically the same (10%) and the displacement B factors for Zn and Mn become unrealistically high (around 5 Å 2 ); therefore this model was discarded. This is in disagreement with the results by Rodriguez [10], who described an monoclinic P2 1 /n symmetry for the La(Co,Ti)O 3 from NPD data. Previous reports of the LaCo 0.5 Ti 0.5 O 3 perovskite have achieved similar R Bragg discrepancy factor for the disordered model in Pnma and the ordered model in P2 1 /n [10,11]. Refinements in the ordered model revealed small amounts of intermixing (5%) of Co and Ti on the B and B′ sites. The refinements show that the higher-temperature synthesis (1300 °C) method gives more complete B site ordering than was found for the lower-temperature synthesis is temperature (900 °C) method previously reported [10].

Thermal analysis (TGA)
The thermal evolution of the sample was studied by recording TGA curves. Heating LaCo 0.5 Ti 0.5 O 3 in 5%H 2 /95%N 2 atmosphere leads to the reduction of the sample to give LaCo 0.5 Ti 0.5 O 3-δ with the same crystal structure. The left panel of Fig 4 depicts the stability of the oxidized sample; in the right panel the auxotherm run is followed by an isotherm treatment at 600ºC required to stabilize the stoichiometry of the reduced sample, displaying the loss of 0.12 oxygen atoms at this temperature. The calculated value is in good agreement with the NPD data (Table I), leading to a LaCo 0.5 Ti 0.5 O 2.88 composition. The thermal analysis confirmed the existence of a mixed valence in the reduced phase. A thermal treatment of the resulting reduced phase in oxidizing (air) atmosphere restores the perovskite phase, thus confirming the required reversibility upon cycling in oxidizing-reducing atmospheres.

Thermal expansion measurements.
Aiming to determine the mechanical compatibility of our material with the other cell components, thermal expansion measurement of the dense ceramic was carried out in different atmospheres. The thermal expansion of the perovskite phases were measured in sintered pellets, initially heated in air at 900 ºC for 12 h; the reduced phase was finally treated in a 5% H 2 flow at 800 ºC for 4h. A dilatometric analysis was performed between 35 and 800 ºC for several cycles; the data where only recorded during the heating runs. Fig. 5 shows no abrupt changes in the thermal expansion of oxidized and reduced LaCo 0.5 Ti 0.5 O 3 in all the temperature range under measurement. The TEC measured in air atmosphere between 100 and 800 ºC is 10.43x10 -6 K -1 for the oxidized phase. The thermal expansion of the reduced phase shows a value of 12.63x10 -6 K -1 when heating the sample in H 2 (5%)/N 2 (95%), very similar to that obtained for the oxidized perovskite. The determined TEC values match with those of other cell components and are certainly lower than those reported for some cobaltites, as expected by the partial substitution of Co by Ti.   Fig. 6 shows the thermal variation of the electrical conductivity of LaCo 0.5 Ti 0.5 O 3 measured twice, in order to be sure, in sintered bars in 5%H 2 /95%N 2 atmosphere by the dc four-probe method. The reduced phase shows a semiconductor-like behavior under reducing conditions with a maximum value of 0.27 S⋅cm -1 at 850 ºC. Although the conductivity values obtained are lower than 0.5 Scm -1 , it is not necessary such high values for the anode materials in single oxide fuel cells. Although the reduced LaCo 0.5 Ti 0.5 O 3-δ perovskite was prepared under a 5%H 2 /95%N 2 flow at 600 ºC, this phase is stable up to 750 ºC, as checked by TG measurements. Beyond this temperature there is a slight weight loss which suggests a partial reduction of the oxygen stoichiometry; this would correspond only to the last two conductivity points, and, indeed, both conductivity and thermal expansion curves show a monotonic behavior in all the temperature range Fig. 6 also illustrates the electrical conductivity of the oxidized LaCo 0.5 Ti 0.5 O 3 perovskite, measured in an air atmosphere. The phase display also a semiconductor-like behavior, with conductivity values much lower than for the reduced sample with a maximum value of 0.

Conclusions
In this work, we have prepared oxygen-stoichiometric LaCo 0.5 Ti 0.5 O 3 perovskite, containing Co 2+ and Ti 4+ , by soft chemistry procedures followed by thermal treatments in air. A topotactic reduction of the stoichiometric perovskites, in a reduced atmosphere, leads to oxygen-deficient phase with LaCo 0.5 Ti 0.5 O 2.91 compositions, where Ti 4+ is partially reduced to Ti 3+ , as shown by both neutron diffraction and thermogravimetric analysis. The expansion of the unit-cell volume ions in the reduced sample, the increase of the <Co,Ti-O> bond lengths and the localization of oxygen vacancies at the axial positions of the perovskite are sizeable proofs of the presence of Ti 3+ cations in the specimen. The crystal structure of both the oxidized and reduced LaCo 0.5 Ti 0.5 O 3 have been refined at RT in the orthorhombic Pbnm space group; the Co and Ti atoms are randomly distributed and no crystallographic long-range order was observed. The electrical characterization evidences a semiconductor behavior in all the samples displaying a maximum value of 0.27 Scm -1 in the reduced specimen. The thermal expansion coefficients for the oxidized and reduced phases perfectly match with the standard values of SOFCs electrolytes. The reversibility of the reduction-oxidation of the LaCo 0.5 Ti 0.5 O 3 makes it possible the required cyclability of the cells.