Materials Research Bulletin 48 (2013) 3262–3267

Phase transformation, thermal expansion and electrical conductivity oflanthanum chromite

Sapna Gupta, Manoj K. Mahapatra, Prabhakar Singh *

Center for Clean Energy Engineering, Department of Chemical, Materials, and Biomolecular Engineering, University of Connecticut, 44 Weaver Road, Storrs, CT 06269, United States

A R T I C L E I N F O

Article history:

Received 28 February 2013

Received in revised form 27 April 2013

Accepted 2 May 2013

Available online 18 May 2013

Keywords:

A. Oxides

C. Differential scanning calorimetry (DSC)

C. X-ray diffraction

D. Crystal structure

D. Defects

A B S T R A C T

This paper addresses discrepancies pertaining to structural, thermal and electrical properties of

lanthanum chromite. Experimental evidence is provided to support the hypothesis for poor densification

in air as well as reduction in electrical conductivity in reducing atmosphere. Sintering condition for the

synthesis of LaCrO3 was optimized to 1450 8C and 10 h. Thermo-analytical (differential scanning

calorimetry – DSC) and high temperature X-ray diffraction (HT-XRD) studies show that orthorhombic

lanthanum chromite transforms into rhombohedral structure at �260 8C and cubic structure above

1000 8C. Co-existence of the structural phases and the variation in each polymorph with temperature in

both air and 3%H2–Ar atmosphere is reported. Presence and absence of Cr-rich phase at inter-particle

neck are observed in oxidizing and reducing atmospheres respectively. The linear thermal expansion co-

efficient was calculated to be 10.8 � 0.2 � 10�6 8C�1 in the temperature range of RT–1400 8C. Electrical

conductivity of lanthanum chromite was found to be 0.11 S/cm in air. A decrease in electrical conductivity

(0.02 S/cm at 800 8C) of LaCrO3, as observed in reducing atmosphere (3%H2–Ar), corresponds to lattice

volume change as indicated by peak shift in HT-XRD results.

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Materials Research Bulletin

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1. Introduction

Lanthanum chromite (LaCrO3) based perovskite oxides havebeen proposed for use as electrode materials in magnetohydrody-namic (MHD) power generators [1], oxygen sensors [2], andheating element in high temperature furnaces [3]. They have alsobeen considered as electrodes and interconnect materials in solidoxide fuel cells [4], and as oxygen transport membrane for airseparation and oxy-fuel combustion with negligible greenhousegas emission [5]. The diverse application of LaCrO3 is attributed toits high melting point of �2510 8C [6], good electrical conductivityof �0.1 S/cm at 800 8C [7] and stability in both oxidizing(PO2

¼ 0:21 atm) and reducing atmospheres (PO2¼ 10�22 atm)

at 1000 8C [8]. Optimization of the required properties, for specificapplications, can be accomplished by doping of LaCrO3 by alkalineearth metals (Sr, Ca) at A-site and transition metals at B-site (Mn,Ni, Fe, Co) [9].

Discrepancies in the structure and properties of undopedlanthanum chromite have been reported in the literature. Koehleret al. [10] have reported the anti-ferromagnetic to paramagnetictransition temperature for lanthanum chromite to be 47 8C which isin disagreement with that of 14 8C [11] and 27 8C [12]. Variouscharacterization methods used in these studies may be the reason of

* Corresponding author. Tel.: +1 860 486 8379; fax: +1 860 486 8378.

E-mail address: singh@engr.uconn.edu (P. Singh).

0025-5408/$ – see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2013.05.032

the variation in magnetic transition temperature [10–13]. LaCrO3

has orthorhombic crystal structure at room temperature andrhombohedral at �260 8C [11,13,14]. On the other hand, Oikawaet al. [15] reports the coexistence of orthorhombic and rhombohe-dral phase at the same transition temperature. It is also reported thatthe LaCrO3 transforms to cubic structure at �1600 8C [16–18]. Incontrast, Momin et al. [19] found the cubic transition to be at1027 8C. The relative density of LaCrO3 was reported to be �50% at1600 8C [20,21] and �64% at 1700 8C when sintered in air [22]. It wasproposed that the formation of CrO3 volatile specie above 1000 8C[23] and its condensation as Cr2O3 at the interparticle necks [24]during sintering results in the lower density of LaCrO3. These studies,however, do not provide the evidence of Cr2O3 deposition in themicrostructure. The volume expansion coefficient was reported tobe 19.7 � 10�6 (25–267 8C), 20.2 � 10�6 (277–1007 8C), and82.4 � 10�6 8C�1 (1027–1327 8C), for the orthorhombic, hexagonal(rhombohedral) and cubic phases, respectively [19]. The same isobtained as 22.8 � 10�6 8C�1 (22–260 8C) and 28.4 � 10�6 8C�1

(260–740 8C) for the orthorhombic and rhombohedral phases,respectively [15]. The linear thermal expansion coefficient of LaCrO3

was measured to be 7.7 � 10�6 8C�1, and 7.3 � 10�6 8C�1, in thetemperature range of �173 to 247 8C and 257–867 8C respectively[25], in disagreement with the values of 4.6 � 10�6 8C�1 in 40–274 8C temperature range and 9.4 � 10�6 8C�1 in 290–1052 8C [18].Electrical conductivity of LaCrO3 is reported to be 0.1 S/cm at 800 8C[7]. Ong et al. [26] found the conductivity as high as 0.34 S/cmat same temperature. These discrepancies can be attributed to the

-0.30

-0.25

-0.20

-0.15

-0.10

-100 0 100 200 300 400

Heat

Flo

w (W

/g)

Temperature (oC)

Cycle 1Cycle 2

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

100 300 500 700 900 1100 1300 1500

Heat

Flo

w (W

/g)

Temperature (oC)

Cycle 1Cycle 2

-0.70

-0.69

-0.68

-0.67

-0.66

-0.65

240 250 260 270

a

b

Fig. 1. DSC profiles of LaCrO3: (a) in �80 8C to 350 8C range and (b) in 100–1500 8C.

S. Gupta et al. / Materials Research Bulletin 48 (2013) 3262–3267 3263

(a) sintering temperature and time as it can change the density ofLaCrO3, (b) La:Cr ratio due to evaporation and condensation ofchromium vapor species, and (c) the amount of porosity that canaffect the electrical conductivity. The different property measure-ment techniques can also result in discrepancy. Furthermore,experimental techniques involving the equilibration time forattaining uniform temperature and controlled environment canalso affect the various property values.

Considering the above reasons for the causes of variation in thephysical properties of LaCrO3, this paper further investigates thestructural changes and properties of LaCrO3 using in operando andin situ characterization techniques. The possible reasons for thediscrepancies in the reported literature are addressed by compar-ing the results of this study. Although the physical propertieschange with exposed atmosphere, the corresponding change instructure is not found in public domain as per author’s knowledge.Accordingly, the change in structure and its relation to theelectrical property has also been investigated with respect toexposed atmospheres.

2. Experimental procedure

2.1. LaCrO3 synthesis

LaCrO3 was synthesized using lanthanum oxide (La2O3, 99.95%,Alfa Aesar, USA) and chromium oxide (Cr2O3, 99.9%, Sigma–Aldrich, USA) powders by solid state reaction technique. La2O3 andCr2O3 powders were mixed using wet-ball milling in ethanol for24 h. After drying in air, the mixed powders were uniaxiallypressed into cylindrical pellets at 287 MPa. Several sinteringexperiments were conducted in 1300–1650 8C for period rangingfrom 5 to 24 h for the optimization of LaCrO3 stoichiometry. X-raydiffraction (BRUKER-D8 ADVANCE, Bruker AXS Inc., Madison, WI)and energy dispersive spectroscopy (EDS) attached to scanningelectron microscope (FEI – ESEM Quanta 250, Hillsboro, OH)techniques were used to identify the single phase LaCrO3 and La:Crratio of the sintered samples to optimize sintering condition. Anoptimized sintering condition of 1450 8C with 10 h dwell time wasselected. The overall stoichiometry (La:Cr ratio = 1:1) was alsoconfirmed by inductively coupled plasma (ICP) mass spectroscopy.

2.2. Characterization

The phase transition temperatures of LaCrO3 were determinedusing differential scanning calorimeters (DSC; DSCQ100-TA for�80 to 350 8C temperature range and STDQ600-TA for 100–1500 8C range; TA Instruments, OH, USA) with 20 8C/min heatingrate. The same amount of LaCrO3 powder (�10 mg) was used forboth the DSC studies. The heat of transformation required for eachphase transition was calculated using TA Instruments UniversalAnalysis 2000 software.

High temperature XRD (HT-XRD) studies of LaCrO3 powderwere performed in RT–1200 8C temperature range both in air andreducing gas atmosphere (3%H2–Ar) to identify the change incrystal structure with temperature and exposed atmospheres. Theheating and cooling rate of 5 8C/min was used during theexperiment with the holding time of 48 h at 1200 8C and 1 h atremaining temperatures. The scan step was 0.028 using Cu Ka

radiation (l = 1.5406 A). The amount of different polymorphs ofLaCrO3 was calculated semi-quantitatively using ratio of intensitysum of the corresponding phase to that of all the phases.

A cylindrical bar (0.56 cm � 0.33 cm � 0.17 cm) was cut fromthe sintered pellet to evaluate the linear thermal expansioncoefficient (TEC) of the LaCrO3 using dilatometer (NETZSCH – DIL402 PC, NETZSCH Instruments, and Burlington, USA). The TECmeasurement was carried out in the temperature range of

RT–1400 8C in air with a heating rate of 3 8C/min. Prior to theactual measurement; the dilatometer was calibrated usingalumina (Al2O3) standard.

A thin cylindrical bar (0.56 cm � 0.33 cm � 0.17 cm) was cutfrom the sintered pellet to determine the electrical conductivity ofLaCrO3 using standard four-probe DC technique in 100–1000 8C inair and reducing gas atmosphere (3%H2–Ar). The heating andcooling rates were 3 8C/min. At least 30 min was elapsed tostabilize the temperature before collecting the data. Also,minimum of 4 days was provided at 950 8C for the equilibrationof 3%H2–Ar gas atmosphere. The reliability of the experiment wasinitially confirmed by measuring the conductivity of well-studiedYSZ.

Microstructure was investigated for sintered LaCrO3 byscanning electron microscope. The sample used for electricalconductivity measurement was also studied to identify the effectof reducing atmosphere on the microstructure.

3. Results and discussion

3.1. Phase transition and crystal structure

The DSC profiles obtained for sintered LaCrO3 to identify thephase transition temperatures are shown in Fig. 1. Fig. 1a showstwo endotherms at �15 8C and at �260 8C while Fig. 1b showsendotherms at �260 8C and �1300 8C corresponding to structuralphase transition. The endotherm at �15 8C shown first time usingDSC corresponds to a magnetic order–disorder transition of LaCrO3

[11] because of changes in the spin orientation of Cr3+ ions [27]leading from anti-ferromagnetic to paramagnetic state. Theendotherms at �260 8C and �1300 8C correspond to the changein crystal structure and will be discussed in the subsequentparagraph. The measured heat of transformation required for themagnetic transition and the polymorphic transformations are0.635 J/g at�15 8C, 1.768 J/g at �260 8C, and 728.2 J/g at �1300 8C.

20 30 40 50 60 70 80 90 100 110 120 130

Rela

tive

Inte

nsity

2θ(°)

RT260°C1000°C1200°C

* * *** *** *

32 32.5 33

Rel

ativ

e In

tens

ity

2θ(°)

RT260°C1000°C1200°C

a

b

Fig. 2. (a) In situ XRD patterns for undoped LaCrO3 at RT, 260 8C, 1000 8C and

1200 8C in air. (b) Magnified image of the HT-XRD for LaCrO3 at RT, 260 8C, 1000 8Cand 1200 8C for 2u = 32–338.

30 40 50 60 70 80 90 100 110 120 130

Rel

ativ

e in

tens

ity

RT - air RT - 3% H -Ar1200⁰C - air 1200⁰C - 3% H -Ar

2

2

32 32.5 33

Rel

ativ

e In

tens

ity

2θ(°)

2θ(°)

RT260°C1000°C1200°C

a

b

Fig. 3. (a) Comparison of HT-XRD patterns at RT and 1200 8C in air and in reducing

atmosphere (3%H2–Ar) showing intensity variation. (b) Magnified image of the HT-

XRD for LaCrO3 at RT, 260 8C, 1000 8C and 1200 8C for 2u = 32–338 in reducing

atmosphere (3%H2–Ar).

57 57.5 58 58.5 59

Rel

ativ

e in

tens

ity

2θ (⁰)

RT - airRT 3% H - Ar2

Fig. 4. Comparison of HT-XRD patterns showing peak shifting in reducing

atmosphere (3%H2–Ar) at RT.

S. Gupta et al. / Materials Research Bulletin 48 (2013) 3262–32673264

The above transformations were observed at the same temperatureduring the subsequent cycles. This suggests the reproducibility andreversibility of the magnetic and structural transition. The magnetictransformation temperature is in agreement with those determinedusing thermo-analytical techniques [11,13] but in disagreementwith 47 8C [10] and 27 8C [12] determined using static magneticsusceptibility and neutron diffraction technique respectively.

HT-XRD patterns of the LaCrO3 at different temperatures (RT–1200 8C) in air are shown in Fig. 2a. The change in the crystalstructure at 260 8C and 1000 8C is observed (corresponding XRDpatterns are shown for brevity). From the XRD pattern, LaCrO3 isdetermined to be orthorhombic (JCPDS-01-070-2694) at roomtemperature. The orthorhombic (O) structure changes to rhombo-hedral (JCPDS-00-033-0702) at 260 8C. This transition is marked bythe disappearances of certain peaks (as indicated by asterisk inFig. 2a) as well as peak splitting indicative of appearance of newpolymorph in Fig. 2b. For example, the peak corresponding to2u ! 32.58 splits at 260 8C as shown in Fig. 2b, corresponding torhombohedral (R) polymorph. It is also found that the LaCrO3

coexists in both orthorhombic (�51%) and rhombohedral structure(�49%) at 260 8C. With increase in temperature the splitted peaksstart to combine at �1000 8C as shown in Fig. 2b. Further mergingof peaks at 1200 8C (selected for brevity) indicates that the LaCrO3

partially transforms into cubic (C) polymorph (JCPDS-01-074-1961, Fig. 2b). The co-existence of orthorhombic (C) (�23%),rhombohedral (R) (�58%) and cubic (C) phase (�19%) was found at1200 8C. With the temperature increase, the higher amount ofcubic and rhombohedral polymorphs compare to orthorhombicstructure suggest that the orthorhombic polymorph is transform-ing into rhombohedral and cubic structure.

Similar XRD patterns features are observed in reducing gasatmosphere (3%H2–Ar) as shown in Fig. 3a and b. The peakintensity, however, decreases in comparison with those in air asshown in Fig. 3b. Also, a slight peak shifting to the left is observedin reducing atmosphere. For instance, the peak corresponding to(0 4 2) plane at 58.138 appears at 58.098 in reducing atmosphere asshown in Fig. 4 with the dotted lines. This suggests the increase inunit cell volume for the sample treated in 3%H2–Ar and the valuesare given in Table 1. The change in lattice volume has not been

Table 1Lattice parameters and unit cell volume of LaCrO3 at room temperature in air and

reducing atmosphere.

Composition

(LaCrO3)

a (A) b (A) c (A) Volume (A3)

Air 5.489 � 0.002 7.762 � 0.004 5.515 � 0.004 234.993 � 0.152

3%H2–Ar 5.489 � 0.002 7.7676 � 0.003 5.5169 � 0.003 235.222 � 0.120

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0 200 400 600 800 1000 1200 1400

Ther

mal

Exp

ansi

on (d

L/L o

)

Temperature (oC)

Cycle 1Cycle 2

Fig. 6. Thermal expansion of LaCrO3 from RT–1400 8C in air.

S. Gupta et al. / Materials Research Bulletin 48 (2013) 3262–3267 3265

calculated for the high temperatures due to limited accuracy incalculated phase quantity. The existence of Cr4+ ions in undopedLaCrO3 is considered due to the inherent structural defects [28].The change in lattice parameter is considered to be due to thechange in valence state of Cr4+ to Cr3+ in reducing atmospherewhich has bigger ionic radii (r

Cr3þ > rCr4þ ). The calculated phase

fraction of polymorphs (orthorhombic, rhombohedral and cubic)phase at different temperature in air and reducing atmosphere isshown in Fig. 5. Similar to air, the orthorhombic polymorphtransforms into rhombohedral and cubic structure in reducingatmosphere.

The polymorphic transition temperatures obtained from HT-XRD are consistent with those found in DSC profiles (Fig. 1). Thechange in crystal structure corresponding to the endotherm at1300 8C was not determined due to the experimental limitation.The HT-XRD result confirms that the cubic transition initiates at�1000 8C and continues to 1200 8C in agreement with Momin et al.[19] and Malghe et al. [29] but in disagreement with thetransformation temperature �1600 8C [16–18]. The DSC-endo-therm at 1300 8C is suggested to correspond to polymorphictransition to the cubic structure. The reversibility of the phasetransition has also been confirmed by superimposing the XRDpatterns during heating and cooling cycles (The patterns are notshown for brevity).

3.2. Thermal expansion

The linear thermal expansion of LaCrO3, obtained fromdilatometer, is shown in Fig. 6. A discontinuity at �260 8C isobserved in the thermal expansion profile because of transfor-mation from orthorhombic to rhombohedral structure, consistentwith the DSC and HT-XRD results and in agreement with Sakaiet al. [11] and Hayashi et al. [13]. Deviation from continuity in thethermal expansion curve, observed at �1350 8C, is consistent withDSC endotherm suggesting further change in crystal structure. Itwas found that the linear thermal expansion coefficients are8.2 � 0.1 8C�1, 11.4 � 0.2 8C�1, and 7.3 � 1.4 � 10�6 8C�1 in thetemperature range of RT–300 8C, 300–1200 8C and 1200–1400 8C,

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400

Phas

e Fr

actio

n (%

)

Temperature (ºC)

Orthorhombic - air Rhombohedral - airCubic - air Orthorhombic - 3% H - Ar Rhombohedral - 3% H - Ar Cubic - 3% H - Ar

2

22

Fig. 5. Phase fraction (orthorhombic, rhombohedral and cubic) in the temperature

range of RT–1200 8C.

respectively. The linear thermal expansion coefficient value is inclose agreement with 7.7 � 10�6 8C�1 in the temperature range of�173 to 247 8C [25]. A small variation can be attributed to thedifferent temperature ranges considered in calculation. The resultsare in disagreement with 4.6 � 10�6 8C�1 in 40–274 8C temperaturerange.

3.3. Electrical conductivity

The measured electrical conductivity is 0.11 S/cm at 800 8C inagreement with 0.1 S/cm [7]. However, it decreases to 0.02 S/cmin 3%H2–Ar atmosphere. This is considered to be due to the changein valence state of Cr4+ to Cr3+ during exposure to reducing gasatmosphere leading to reduction in the charge carrier concentra-tion. This is also supported by an increase in lattice parameter inreducing atmosphere (Table 1). Fig. 7 shows the linear relationshipbetween log sT and 1/T for electrical conductivity of LaCrO3 in thetemperature range of 100–1000 8C in air and reducing gas (Ar–3%H2) atmosphere. Reproducible results were obtained duringboth heating and cooling cycles. With increase in temperature, theconductivity increases due to increase in the lower thermalactivated barrier. The above observation is consistent with theconductivity reported by Ruangvittayanon et al. [7] but indisagreement with Ong et al. [26]. The inset in the conductivityplot (Fig. 7) shows previously undocumented information relatedto the deviation from continuity at �260 8C corresponding to theorthorhombic to rhombohedral transition of LaCrO3. The activationenergy was calculated to be 0.12 eV and 0.34 eV in air and reducingatmosphere (3%H2–Ar).

Fig. 7. Electrical conductivity of LaCrO3 100–1000 8C in air and reducing

atmosphere (3%H2–Ar).

Fig. 8. SEM images of LaCrO3: (a) lower magnification in air, (b and c) higher magnifications in air, (d) lower magnification in 3%H2–Ar, and (e) higher magnification in

3%H2–Ar.

S. Gupta et al. / Materials Research Bulletin 48 (2013) 3262–32673266

3.4. Density and microstructure

The relative density of sintered LaCrO3 pellet is �41%. SEMimages of the sintered LaCrO3 show the porous microstructure(Fig. 8a). The grain size varies from 1.5 to 4.5 mm with an average of1.7 mm.

The observed lower density is attributed to chromiumevaporation (CrO3) as it is known to occur above 1000 8C [23].The vapor species of chromium evaporate from the bulk grainsand condenses onto irregular contact (between grain boundaries)due to the surface energetics. As a result, the vapor species depositas Cr2O3 at the particle necks and the neck growth continueswithout the mass-transport of the material from the bulk of the

grains [30–32]. The Cr2O3 evaporation and condensation reac-tions are shown below [23]:

Cr2O3þ ð3=2ÞO2 ! 2CrO3 " ðevaporationÞ (1)

2CrO3 " ! Cr2O3þ ð3=2ÞO2ðcondensationÞ (2)

The chromia (Cr2O3) layer at the inter-particle neck inhibitsfurther densification [30–32]. Based on EDS analysis, the deposi-tion of chromium oxide is evident (La:Cr = 1 near the bulk surface,and La:Cr < 1 near the neck area) from the chromium enrichedphase in the microstructures shown in Fig. 8b and c. After theelectrical conductivity measurement in reducing gas atmosphere(Ar–3%H2), microstructural analysis of the treated sample are

S. Gupta et al. / Materials Research Bulletin 48 (2013) 3262–3267 3267

shown in Fig. 8d and e. The extensive surface rearrangement(compared to air) may attribute to the intensity decreases in thecorresponding XRD pattern (Fig. 4) [33]. The samples also did notshow the formation of chromium enriched phases.

This study provides experimental evidence to support thehypothesis for reduced densification in air and decreased electricalconductivity in reducing atmosphere for undoped LaCrO3.Presence of chromium oxide in the microstructure (Fig. 8b andc) validates chromium oxide evaporation-condensation attributingto poor densification of LaCrO3. Peak shift in the XRD pattern ofLaCrO3 in 3%H2–Ar atmosphere confirms the change in the valencestate of Cr4+ to Cr3+ resulting in lower electrical conductivity. HT-XRD patterns confirm the orthorhombic transformation torhombohedral polymorph at �260 8C and to cubic polymorph at�1000 8C. Consistency of phase transformation temperatures hasbeen confirmed by different complimentary techniques (DSC,Dilatometer, HT-XRD, and electrical conductivity).

4. Conclusions

LaCrO3 has been synthesized and its structure and propertieshave been investigated in oxidizing (air) and reducing (3%H2–Ar)atmospheres. Transition from orthorhombic to rhombohedralstructure at 260 8C has been confirmed by HT-XRD, DSC,dilatometer, and 4-probe DC conductivity studies. HT-XRD showsthe coexistence of orthorhombic and rhombohedral phases at260 8C along with transformation to cubic phase above 1000 8C.This study suggests that the disagreement in magnetic andpolymorphic transition temperatures, as reported in literature,arises due to the use of different characterization techniques.Decrease in electrical conductivity in reducing atmosphere iscorroborated to reduction of chromium ions in lanthanumchromite as supported by increase in lattice volume.

Acknowledgements

Authors sincerely thank the Center for Clean Energy Engineer-ing and the University of Connecticut for financial support. We

sincerely acknowledge Ms. Laura Pinatti at Institute of MaterialScience, University of Connecticut for assistance in conducting theDSC experiment.

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