Journal of Alloys and Compounds 620 (2015) 91–96

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Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Structural and electrical properties of strontium substituted Y2BaNiO5

http://dx.doi.org/10.1016/j.jallcom.2014.09.1270925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +351 234 370830; fax: +351 234 370953.E-mail address: duncan@ua.pt (D.P. Fagg).

Narendar Nasani a, Devaraj Ramasamy a, Isabel Antunes b, Budhendra Singh a, Duncan P. Fagg a,⇑a Nanotechnology Research Division, Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugalb Department of Materials and Ceramic Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal

a r t i c l e i n f o

Article history:Received 31 July 2014Received in revised form 12 September2014Accepted 15 September 2014Available online 22 September 2014

Keywords:Haldane energy gapBarium yttrium nickelateElectrical conductivitySolid solutionProtonic ceramics

a b s t r a c t

The Y2�xSrxBaNiO5 (x = 0, 0.1, 0.2 and 0.3) acceptor substituted system has been synthesized by solid statereaction. Structural and microstructural properties have been characterized by X-ray diffraction (XRD)and scanning electron microscopy (SEM), respectively. Lattice volume is shown to decrease linearly withincreasing Sr content until composition x = 0.2, highlighting the limit of the solid solution. The electricalresponse in the temperature range (700–100 �C) was assessed by A.C. impedance spectroscopy in wet anddry O2 and N2 atmospheres. Conductivity measurements as a function of oxygen partial pressure (pO2)were also performed. The data reveal that the conductivity Y2BaNiO5 can be increased by one and halforders magnitude by Sr-doping and is independent of both water vapour and oxygen partial pressures(pH2O and pO2). The low activation energy for electrical conduction (0.216–0.240 eV) suggests athermally activated electron hopping mechanism, while the observed pO2 and pH2O independence ofconductivity suggests that charge compensation for Sr is predominantly by formation of Ni3+ rather thanformation of oxygen vacancies.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The current article assesses the structural and electrical proper-ties of materials based on the base composition Y2BaNiO5; a mate-rial that is most commonly studied as Haldane energy gapmaterial. Since the discovery of the Haldane energy gap in onedimensional anti-ferromagnetic materials (1D AF), several materi-als types have been shown to offer this property [1,2]. Haldanematerials demonstrate an energy gap between singlet ground stateand the first excited triplet state when the spin quantum number isan integer. Such materials have generated interest as they canfacilitate the study of ground and excited states of quantum mod-els as well as offering the possibility of switchable devices. Rareearth doped R2BaNiO5 (R = Y, rare earth) compounds [3]. Tm2-

BaNiO5 and Yb2BaNiO5 were reported to be dimorphic with Ni ionsin pyramidal (Pnma) or octahedral (Immm) structural coordination[4,5]. The magnetic susceptibility [6,7] and spin thermal conductiv-ity [8] behaviour in these compounds have been extensivelyexplored, while electronic structures have been probed by usingvarious techniques viz. extended X-ray absorption fine structure(EXAFS), X-ray absorption near-edge structure (XANES) [9],electron energy loss spectroscopy (EELS) [10], nuclear magneticresonance spectroscopy (NMR) [11], X-ray and/or neutrondiffraction techniques [3] and photoemission spectroscopy [12].

Doped materials have also been studied, Novák et al. studied theelectronic structure of Y2�xMxBaNiO5 (M = Sr, Ca) by density func-tional theory (DFT) [13] while, Capsoni et al. studied the effect ofdifferent ion substitutions at the Ni-site (Y2BaNi1�xMxO5, M = Mg,Zn) using Raman spectroscopy [14]. In the case of Mg substitutions,structural symmetry (orthorhombic with Immm space group) withNiO6 octahedra was preserved. On the contrary, with the additionof Zn2+ ions, both Immm and Pnma symmetries were found to coex-ist in samples with x = 0.13 while, at higher concentrations of Zn2+

ions, single structural symmetry (Pnma) having NiO5 pyramidalcoordination was observed, with the assimilated Zn compositionshowing the existence of isolated pyramid structure. Structuralchanges and magnetic susceptibility of Y2BaNi1�xZnxO5 oxideswere also studied by Saez–Puche et al. noting similar structuraltransformations and shift to Curie–Weiss behaviour on inclusionof Zn [15].

Y2BaNiO5 (BYN) is one of the typical Haldane gap oxide materi-als having 1D AF nature [16] with orthorhombic symmetry (Immmspace group) having isolated 1D chains of vertex-sharing NiO6

octahedra along the z-axis. The NiO6 octahedra are compressedalong the z direction which leads to the presence of two short api-cal Ni–O bond distances (1.88 Å). Nevertheless, the ground statehas insulating properties with a large optical gap of about 2.3 eV[16]. The most important structural feature in these compoundsis their chemical bonding which leads to 1D magnetic behaviour,due to the absence of Ni–O–Ni interaction between neighboringchains, allowing Y2BaNiO5 to be an ideal 1D anti-ferromagnet with

92 N. Nasani et al. / Journal of Alloys and Compounds 620 (2015) 91–96

S = 1. Lannuzel et al. successfully incorporated acceptor dopants(Sr2+ and Ca2+) for Y3+ into Y2BaNiO5 compounds and their studiesrevealed that the magnetic and transport properties, measured atlow temperatures, could be modified by such doping, with adecrease in resistivity of BYN of two orders magnitude measuredat 300 K and the generation of new magnetic states with spingreater than S = 1/2 inside the Haldane gap [10]. Recently, Chenet al. studied the dielectric properties of Y2BaNiO5 in the frequencyrange of 20 Hz to 1 MHz between 20 K and room temperature anddemonstrated high dielectric permittivity (e0 � 104) at room tem-perature below 100 kHz [17].

As highlighted, most previous studies of BYN have been devotedtowards the understanding of basic structural, magnetic andelectrical properties of Haldane gap systems measured at lowtemperatures. Nevertheless, Y2BaNiO5 has also been identified asan impurity phase in proton conducting ceramic electrolyte materi-als, particularly yttrium doped barium cerates and zirconates(BaCe(1�x�y)ZryYxO3�d), when NiO is used as sintering additive[18–21]. It has also been reported to be present in nickel cermetanodes Ni–BaCe(1�x�y)ZryYxO3�d prepared by typical synthesis tech-niques [22,23]. Tong et al. mentioned that the Y2BaNiO5 impurityphase can play vital role in the grain growth and densification ofBa(Zr,Y)O3�d materials when one uses NiO as sintering aid to reducethe sintering temperature below 1600 �C [18,19]. Thus, the presenceof BYN as an impurity phase in typical components of protonic cera-mic fuel cells (PCFCs) may contribute to their subsequent electro-chemical performance. It is, therefore, necessary to understand theelectrical properties of BYN also at temperatures that are relevantfor operation of PCFC devices (500–700 �C). This factor, therefore,led the authors to study the conductivity behaviour of Y2BaNiO5 incontrolled atmospheres as a function of temperature to assess itssubsequent properties when incorporated in protonic ceramic fuelcells or in further electrochemical applications that operate at ele-vated temperatures. To the best of author’s knowledge, there areno conductivity studies previously available on BYN under suchconditions. In the current article, we also investigate the effect ofstrontium doping on structural and electrical properties ofY2�xSrxBaNiO5 for different doping concentrations (x = 0, 0.1, 0.2and 0.3).

2. Experimental section

2.1. Synthesis of Y2�xSrxBaNiO5 (x = 0, 0.1, 0.2 and 0.3)

Y2�xSrxBaNiO5 (x = 0, 0.1, 0.2 and 0.3) was prepared by a combination ofmechanical activation and solid state synthesis [24] using Y2O3 (Hermann C.Starck), SrO2 (Sigma Aldrich), BaO2 (Sigma Aldrich) and NiO (99% Strem ChemicalsABCR GmbH&Co.). Y2O3 was dried overnight at 900 �C and kept at 250 �C prior toweighing. Stoichiometric quantities of all the reactant powders were mixed and ballmilled in zirconia vials under high energy at 650 rpm for 420 min using zirconiaballs to mechanically activate the precursor powders. For simplicity the Y2�xSrx-

BaNiO5 compositions are given the nomenclature, SBYN0, SBYN1, SBYN2 andSBYN3, for x = 0, 0.1, 0.2, and 0.3, respectively. The obtained powder was darkbrown in colour. The activated powder was uniaxially pressed into pellets with1 cm diameter and sintered at 1400 �C for 6 h with a heating and cooling rate of4 �C/min in ambient air. Then the sintered pellets were crushed into powder inagate mortar using pestle and analyzed for phase purity. For electrical measure-ments, bar shaped pellets (�1.3 � 0.3 � 0.4 cm3) were prepared by cold isostaticpressing of the activated powder at 300 MPa for 15 min followed by similar sinter-ing conditions as above. The geometric density of sintered samples was found to be�85–90% that of theoretical density, with the higher density samples being thosewith the higher Sr-content.

Fig. 1. X-ray diffraction patterns of Y2�xSrxBaNiO5 (X = 0, 0.1, 0.2 and 0.3) calcinedat 1400 �C for 6 h in ambient air.

2.2. Materials characterization

The powder X-ray diffraction (XRD) data and phase purity were analyzed byusing a Philips X’Pert diffractometer in the 2h = 20–80� angular range, DetectorX’Celerator, active length 2.5460�, step width 0.02� and counting time 30 s/step,operated at 45 kV and 40 mA with Cu Ka radiation. The obtained diffraction pattern

was analysed by structural and profile fitting model [25]. The microstructure of sin-tered pellets were observed using Scanning Electron Microscopy (SEM) (model Hit-achi SU-70).

Electrical measurements were performed on bar shaped samples (4 probe con-figuration) with an Autolab (Electrochemie) PGSTAT302N frequency response ana-lyzer using 50 mV amplitude in 1 MHz–0.01 Hz frequency range in the direction ofdecreasing temperature in the range 700–100 �C with 50 �C intervals. Prior to theelectrical measurements, contacts were painted on the dense bars with Pt pasteand fired at 900 �C for 30 min with a heating/cooling rate of 4 �C/min. All sampleswere measured under dry/wet N2 and O2 atmospheres with a flow rate of 50 ml/min. Humidification was achieved by bubbling gases through distilled water in asaturated KCl solution, producing approximately 86% relative humidity at ambientconditions. Drying was performed using Varian gas driers. Humidity levels weremeasured using a humidity meter (Jumo). The oxygen partial pressure (pO2) wascontrolled by mixing O2 with N2 with 2 h stabilization times at each measurementduring the pO2 experiment. Stability of the data was confirmed by performingrepeated impedance measurements at each condition after a further 30 min. Theobtained impedance data was analyzed by using ZView software (ScribnerAssociates).

3. Results and discussion

3.1. Phase formation and structural analysis

For the composition x = 0 (SBYN0) the desired BYN (JCPDS-081-0002, Y2BaNiO5) phase was identified in the XRD patterns, alongwith small traces of Y2O3 (JCPDS-155173) and Ba3Y4O9 (JCPDS-087118), Fig. 1. For SBYN1, SBYN2 and SBYN3, the BYN phasewas identified with minor traces of the Ba3Y4O9 impurity peak,but without any Y2O3 impurities. Capsoni et al. also found theimpure phases Y2O3 and BaY2O4 in Y2BaNi1�xMxO5 (M = Mg, Zn)polymorphs prepared by solid state reaction under synthesis con-ditions involving multiple firing, grinding steps with a final maxi-mum firing temperature of 1200 �C [14]. The impurity phasesobserved in the current work and in the work of Capsoni are inaccordance with the phase equilibrium study of Lopato et al. ofthe Ba–Y–O system, who showed that BaY2O4 is the stable phasefor temperatures <1400 �C while the Ba3Y4O9 phase is stableat >1400 �C. The phase diagram of the Y2O3–BaO–NiO system con-structed by Buttrey et al. in the 1000–1350 �C temperature range,suggests that BaY2NiO5 is a point composition [26]. Hence, anyslight deviation of stoichiometry would lead to the presence ofimpurity phases. From the phase diagram, the observed impurity,Ba3Y4O9, would suggest Ba-excess and entrance into theBaY2NiO5 + Ba3NiO4+x + Ba3Y4O9 region. Nonetheless, the composi-tion of each compound was confirmed by energy dispersive X-rayanalysis (SEM-EDAX) and was shown to be stoichiometricallyaccurate at the resolution of this technique. Attempts to remove

Table 1Rietveld refinement results for pure phase of Y2�xSrxBaNiO5 materials synthesized bymechano-synthesis.

(a) Basic structural detailsStructure Space group

Orthorhombic Immm

(b) lattice parameters (in Å) and angle (in �)Sample a b c a b c Vol. (Å3)

SBYN0 3.75579 5.76000 11.32939 90 90 90 245.093SBYN1 3.75444 5.75655 11.32874 90 90 90 244.844SBYN2 3.75108 5.75465 11.32654 90 90 90 244.496SBYN3 3.75068 5.75604 11.33216 90 90 90 244.651

(c) Goodness of fita

Sample Rb Rf v2 c/a S (goodness of fit) = Rwp/Rexp

SBYN0 6.07 4.94 4.63 3.0165 0.8333SBYN1 8.75 7.66 5.05 3.0174 0.8165SBYN2 4.18 3.78 3.46 3.0195 0.8041SBYN3 6.84 5.15 7.28 3.0214 0.9001

a Rb: Bragg R-factor, Rf: Crystallographic Rf – factor and v2: Goodness of fit.

N. Nasani et al. / Journal of Alloys and Compounds 620 (2015) 91–96 93

the minor impurity phase of Ba3Y4O9 by further mechanical activa-tion were unsuccessful. Nonetheless, Rietveld refinement high-lighted that the content of impurity phases in all compositionswas �2 wt%, Fig. 2; a very minor level (�1 mol%) that can be con-sidered negligible. Moreover, the absence of any strontium relatedimpurity phases suggests the incorporation of the Sr2+ dopant inthe base composition BYN, in agreement with the article aim.

All the Y2�xSrxBaNiO5 (x = 0, 0.1, 0.2 and 0.3) compositions exhi-bit an orthorhombic system with space group Immm. The latticeparameters and the atomic positions were refined using the Riet-veld method with profile and structural model using Fullprof soft-ware, Table 1. Satisfactory global agreements for Rietveldrefinement were obtained. The unit cell parameters are shown todecrease with increasing strontium content up to the compositionx = 0.2, Table 1, and a corresponding decrease in unit cell volume isalso noted, Fig. 3. As the ionic size of Sr is greater than that of the Yhost, Sr2+ (1.13 Å) cf. Y3+ (0.93 Å) [10,27], the observed lattice vol-ume contraction can only be explained by consideration of thepotential charge compensation mechanisms for the acceptor dop-ant. For instance, due to a decreasing unit cell size related to theformation of oxygen vacancies or due to the oxidation of Ni2+ toNi3+ with an associated reduction in ionic size. The most likely ofthese charge compensation mechanisms will be discussed in thelater text based on the electrochemical results. The inversion ofthe lattice parameters and associated lattice volume at the compo-sition x = 0.3, suggests that this composition exceeds the solid sol-ubility of Sr in the BYN structure. The solid solution limit of x = 0.2corresponds well with that previously noted by Lannuzel et al.[10], while being smaller than that reported for the Ca-substitutedsystem, Y2�xCaxBaNiO5, where solubilities up to x = 0.5 have beendocumented [28]. The lower solubility in the current case is likely

SBYN0

SBYN2

Fig. 2. Observed, calculated, and difference X-ray powder diffraction profile fromthe Rietveld refinement for SBYN0 and SBYN2 calcined at 1400 �C for 6 h.

Fig. 3. Variation of lattice volume as a function of the strontium content forY2�xSrxBaNiO5.

to be due to the larger ionic size mismatch between Sr and the Yhost than that for Ca. Lattice volume decreases with increasingacceptor substitution have also been documented in the BaY2�xCax-

NiO5 system, in agreement with the current work [28].

3.2. Microstructure

The microstructure of Y2BaNiO5 (BYN) was observed by scan-ning electron microscopy (SEM). Fig. 4 represents the fracturedcross sectional SEM images of series of compositional pellets sin-tered at 1400 �C for 6 h. The SBYN0 pellet can be observed to havenot attained full densification at 1400 �C and to contain pores, cor-responding to its lower measured density of 85%. In contrast, the Srsubstituted BYN analogues are all observed to be dense, Fig. 4, andto exhibit larger grain sizes. An example Energy Dispersive X-rayAnalysis image (EDAX) for the sample x = 0.1 is included inFig. 4e and highlights that the composition is homogenousthroughout the material. Note also that no impurities can beobserved by this technique, due to their aforementioned low con-centration (<1 mol%).

3.3. Electrical conductivity of Y2�xSrxBaNiO5

The conductivity of pure and strontium modified BYN sampleswere evaluated as a function of temperature under a range of

(a) SBYN0 (b) SBYN1

(c) SBYN2 (d) SBYN3

(e) SBYN1-EDS

Fig. 4. Cross sectional SEM image of (a) undoped and Sr doped BYN (b), (c), (d) and EDAX elemental mapping of Sr doped BYN (e) pellet sintered at 1400 �C for 6 h in ambientair.

94 N. Nasani et al. / Journal of Alloys and Compounds 620 (2015) 91–96

controlled atmospheres. Fig. 5a depicts the conductivity of SBYN0,SBYN1 and SBYN2 compounds as a function of temperature in N2

gas atmospheres. The conductivities of all samples exhibit positivetemperature dependences, while the conductivities in dry and wetN2 atmospheres are observed to be effectively equal. With theincorporation of Sr2+ in the BYN lattice, a significant increase inconductivity can be observed, yielding an increase of approxi-mately one and half orders of magnitude over that of the base com-position [17,28]. However, the measured conductivities forsamples SBYN1 and SBYN2 are shown to be similar across mostof the temperature range, only revealing slight improvements withincreased Sr substitution at the highest temperatures. To clarifythis feature, Fig. 5b plots this data on a linear conductivity scale.The observed non-linear increase in conductivity with increasingacceptor dopant content is in agreement with that previouslyobserved in the system Er2�xCaxBaNiO5, where compositionsx = 0.34 and x = 0.19 showed similar levels of conduction, whichwere an order of magnitude higher than the base composition [28].

Fig. 6 shows the variation of the conductivity for SBYN0, SBYN1and SBYN2 compositions as a function of temperature measured inwet and dry O2 atmospheres. The resultant trends are shown tomirror those measured under N2, in all aspects.

Previously, Lannuzel et al. proposed that Sr/Ca doping canstrongly affect the magnetic and transport properties of BYN byalterations in its electronic structure [10], a suggestion supportedby DFT calculations by Novak et al. [13]. For the current BYN

system, the conductivities measured in Figs. 5 and 6 are shownto be thermally activated, with activation energies, determinedusing the Arrhenius relation in the temperature range 500–100 �C, outlined in Table 2. Activation energy values lie in the range0.216–0.240 eV for all samples and can be noted to be effectivelyindependent of atmosphere. A significant feature of the presentwork is that the activation energy 0.216–0.240 eV of BYN is signif-icantly lower than the expected energy gap�2.3 eV of the undopedBYN Haldane system, in agreement with that observed by DiTusa[29] for single crystal samples measured in a sub-ambient temper-ature range. Recently Chen et al. documented dielectric and resis-tivity behaviours of polycrystalline samples of the undoped BYNsystem and also reported activation energies of Ea � 0.12 eV attemperatures below 25 �C [17]. Thus, the current activation ener-gies of Table 2, measured at much higher temperatures, concurwith those at lower temperature, which have previously been sug-gested to be due to a small polaronic electron hopping conductionmechanism where electrons are localized due to the presence oflocal distortions in the crystal lattice sites [28]. Such values of acti-vation energy are significantly lower than those which could berelated oxide-ion conduction. This concurs with the absence ofdirect O–O bond linkages between NiO6 chains in the BYN crystal-lographic structure; a factor which would be expected to hinderpotential oxide-ion conduction. Protonic conductivity can also benegated due to the water vapour partial pressure independenceof the conductivities demonstrated in Figs. 5 and 6.

Fig. 5. Conductivity of Y2�xSrxBaNiO5 (x = 0, 0.1 and 0.2) measured in N2 gasatmosphere.

Fig. 6. Conductivity of Y2�xSrxBaNiO5 (x = 0, 0.1 and 0.2) measured in O2 gasatmosphere.

Table 2Activation energy of Y2�xSrxBaNiO5 (X = 0, 0.1 and 0.2) system in O2 and N2

atmospheres.

X Activation Energy, Ea (eV) (500–100 �C)

Oxygen Nitrogen

Dry Wet Dry Wet

0 0.216 0.216 0.223 0.2240.1 0.221 0.227 0.223 0.2290.2 0.233 0.232 0.234 0.240

N. Nasani et al. / Journal of Alloys and Compounds 620 (2015) 91–96 95

3.3.1. Conductivity dependence on oxygen partial pressure (pO2)Fig. 7 plots the oxygen partial pressure (pO2) dependence of

conductivity of compositions within the solid solution, at temper-atures in the range 600–700 �C. No significant variations in con-ductivity can be observed with changing pO2 under theseconditions, for any composition. The conductivities of SBYN1 andSBYN2 are shown to be higher than SBYN0, as expected from theresults of Figs. 5 and 6. If charge compensation in the generic mate-rial, Y2�xSrxBaNiO5, would exclusively determined by ionic defects,the substitution of Y for Sr would be described in Kröger-Vinknotation as

BaOþ ð2� xÞYO1:5 þ xSrOþ NiO

! BaxBa þ xSr=Y þ ð2� xÞYx

Y þ NixNi þ 0:5xV��O þ ð5� 0:5xÞOx

O ð1Þ

However, R. Castañer et al., using X-ray absorption spectros-copy, highlighted that acceptor doping can also influence thevalence state of nickel cation in BYN, leading to a mixed Ni2+ toNi3+ valence state [28]. This alterative charge compensation mech-anism can be expressed by the equilibrium of oxygen vacanciesand Ni valence with the surrounding atmosphere, described bythe equation

V��O þ12

O2ðgÞ þ 2NixNi ()

KoxOx

O þ 2Ni�Ni ð2Þ

With mass action constant:

Kox ¼½Ox

O�½Ni�Ni�2

½NixNi�

2½V��O �p1=2O2

ð3Þ

Under oxidising conditions the associated electroneutrality con-dition can be simplified to:

bNi�Nic þ 2bV��O c � bSr=Yc ð4Þ

leading to the mass action relation

Fig. 7. Conductivity of Y2�xSrxBaNiO5 (X = 0, 0.1 and 0.2) measured as a function ofoxygen partial pressure (pO2) from 700 to 600 �C temperature range.

96 N. Nasani et al. / Journal of Alloys and Compounds 620 (2015) 91–96

Kox ¼½Ox

O�½Ni�Ni�2

½NixNi�

2 12 ð½Sr=Y� � ½Ni�Ni�Þp

1=2O2

ð5Þ

If all charge compensation were by the formation of oxygenvacancies, ½Sr=Y � � ½V

��O � � constant, and Eq. (5) would suggest an

oxygen partial pressure dependence of ½Ni�Ni� of pO21/4. On the other

hand, if charge compensation were predominately by the forma-tion of Ni3+, then the electroneutrality equation (Eq. (4)) wouldbe reduced to ½Sr=Y � � ½Ni�Ni�, signifying an effective independenceof the concentration of the charge carrier ½Ni�Ni� on changing pO2.The observed pO2 independence of conductivity in Fig. 7, therefore,best relates to the latter hypothesis, of a predominant charge com-pensation mechanism of formation of Ni3+ where the ½Ni�Ni� concen-tration would be fixed by the level of acceptor doping, ½Sr=Y �, ratherthan the alternative charge compensation mechanism of formationof oxygen vacancies. This conclusion is in agreement with that sug-gested by Alonso [28] and Fagot–Revurat [12] by neutron diffrac-tion, infrared absorption spectra and photo emission experimentsperformed at lower temperatures. The current results support thecontinued presence of Ni3+ at higher temperatures that lead tothe enhanced conductivity noted in the Sr-substituted composi-tions in Figs. 5–7 due to thermally induced small polaronic hop-ping between Ni2+ and Ni3+.

4. Conclusions

The Y2�xSrxBaNiO5 (x = 0, 0.1, 0.2 and 0.3) series was synthe-sized by a combination of mechanical activation and solid statesynthesis. The unit cell parameters and lattice volume are shownto decrease with increasing of strontium content up to compositionx = 0.2. The expansion of the lattice at composition x = 0.3 suggeststhis composition to be beyond the solubility of Sr in the BYN lat-tice. The electrical conductivity was measured in wet and dry O2

and N2 containing atmospheres in the temperature range700–100 �C. The results demonstrate that an increased conductiv-ity of one and half orders magnitude can be obtained by Sr2+ sub-stitutions in Y2BaNiO5 beyond that of undoped Y2BaNiO5. The lowactivation energy 0.216–0.240 eV for conduction, suggests a ther-mally activated small polaronic hopping mechanism, while negat-ing the presence of oxide-ion conductivity. The conductivity wasshown to have no appreciable dependence on atmosphere, eitherwith water vapour or oxygen partial pressure. This result con-cludes the absence of protonic conductivity, while the absence ofa pO2 dependence of conductivity suggests that the principalcharge compensation mechanism for Sr substitution is that of theformation of Ni3+.

Acknowledgements

The authors acknowledge kind support from the FCT, FEDERand COMPETE, PTDC/CTM/100412/2008, PTDC/CTM/105424/2008,and Narendar Nasani is grateful to FCT for doctoral research GrantSFRH/BD/80949/2011. Isabel Antunes is thankful to the FCT fordoctoral research grant SFRH/BD/76738/2011.

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