based perovskites Alan Nicholls, University of Illinois at Chicago, IL Miladin Radovic, Oak Ridge National Laboratory, TN

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1 Phase Click Transitions to edit Master in LaCoO title 3 and style LaGaO 3 based perovskites Alan Nicholls, University of Illinois at Chicago, IL Miladin Radovic, Oak Ridge National Laboratory, TN Edgar Lara-Curcio, Oak Ridge National Laboratory, TN Andrew Third Payzant, level Oak Ridge National Laboratory, TN Mike Reece, Queen Mary University of London, UK Students: John Fifth Lloyd, level Laura Fegely, David Steimentz, Siddhartha Pathak This research was supported by NSF, Division of Materials Research and DoE s Faculty and Student Teams program. This research was also supported in part by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR Vehicle Technologies and NATO Linkage grant.

2 Characterize the defect structure of LaCoO 3 based rhombohedral perovskites Demonstrate a ferroelastic to paraelastic phase transition Second level in La.6 Ca.4 CoO 3 by in-situ high temperature TEM Objectives Investigate in situ microstructural changes during heating, annealing and cooling in the microscope column Fifth level Study the structural stability of LaCoO 3 in reducing environment by XRD Demonstrate the temperature induced orthorhombic to rhombohedral LaGaO 3 phase transition

3 JEOL JEM-31 TEM Experimental Gatan Model 652 double tilt heating stage was used to heat Second perovskite level foils up to 85 C at a controlled heating rate of 1 C or 2 C per minute. Oxygen partial pressure in the microscope column is about Fourth 5 1 level -8 Pa Scintag PADV diffractometer (CuK radiation) Thermal Mechanical Analyzer (TMA) Optical microscope

4 LaCoO 3 La.8 Ca.2 CoO 3 La.6 Ca.4 CoO 3 La.45 Ca.55 CoO 3 T C R = 16ºC; T C R = 95 º C; T C R = 7 º C cubic Grain size m Polycrystalline ceramics prepared by Praxair Surface Technologies Lanthanum based cobaltites High temperature Cubic structure with the space group Upon cooling, LaCoO 3 exhibits a displacive phase transition at T c Room temperature Rhombohedral structure with the space group T c is decreases with a divalent cation doping Pm3m Rc 3

5 a, Å CTE a, 1-6 K -1 c, Å CTE c, 1-6 K -1 Intensity,a.u. Department of Mechanical, Materials and Aerospace Engineering Stress, MPa o C o C Theta, o LaCoO ~ 1% porosity Strain

6 Intensity,a.u. a, Å c, Å Intensity, a.u. Department of Mechanical, Materials and Aerospace Engineering Stress,MPa 9 o C Theta, 4 3Click to edit Master o text styles 2 1Second level Theta, o o C La.8 Ca.2 CoO ~ 5% porosity CTE a, 1-6 K CTE c, 1-6 K Strain

7 a, Å Intensity, a.u. c, Å Intensity, a.u. Department of Mechanical, Materials and Aerospace Engineering Stress, MPa 7 6 La.6 Ca.4 CoO o C o C Second level Theta, o Theta, o ~ 5% porosity CTE a, 1-6 K CTE c, 1-6 K Strain

8 Intensity, a.u. a, Å c, Å Intensity,a.u. Department of Mechanical, Materials and Aerospace Engineering Stress,MPa 7 6 La.45 Ca.55 CoO 3 9 o C CTE a, 1-6 K Theta, 3 Click to edit Master o text styles Theta, o CTE c, 1-6 K o C ~ 5% porosity Strain

9 ά ά = 6 Ferroelastic Phase Transition High Temperature High symmetry prototypic phase a = b = c; = b = g = 9 Cubic Oxygen T C Lanthanum Cobalt T C R = LaCoO 3 at 16ºC; La.8 Ca.2 CoO 3 at 95 º C; La.6 Ca.4 CoO 3 at 7 º C Low Temperature Low symmetry phase a = b = c; = b = g 9 ά = 6.78 Rhombohedral Stress relaxation by twinning

10 TEM micrographs of La.6 Ca.4 CoO 3 perovskite grains showing a complex configuration of twins

11 TEM micrograph showing a configuration of twins in LaCoO 3 Electron diffraction pattern exhibits splitting of the spots. The magnitude of the splitting increases with the distance from the center of the pattern

12 Domain wall in LaCoO Å Domain wall [12] [12] = º [112] [112] No splitting of diffraction spots in the SAD pattern has been observed

13 A B C RT 4 C 7 C The micrographs showing detwinning upon heating in La.6 Ca.4 CoO 3 A B C 4 C 59 C RT The micrographs showing reappearance of twins during cooling

14 4 C 6 C Twins in La.6 Ca.4 CoO 3 perovskite at 4 C. A dislocation is observed near the twins tips. The dislocation serves as a strong pinning point delaying disappearance of the twins during heating.

15 RT 3 C RT 6 C Stacking faults in La.6 Ca.4 CoO 3 perovskite

16 4 C 7 C Detwinning process during heating of La.6 Ca.4 CoO 3

17 Click LaCoO to edit Master title style 3 perovskite undergoes topotactic LaCoO 2.5 structure reduction to vacancy-ordered phase at high temperatures in reducing atmosphere 2 LaCoO 3 (s) La 2 Co 2 O 5 (s) + ½ O 2 (g) Orthorhombic brownmillerite structure Perovskite type structure Alternate layers of Co in octahedral and tetrahedral sites Every second row of oxygen atoms is removed from alternate planes Stemmer, S., Sane, A., Browning, N.D., Mazanec, T. Solid State Ionics, 13, 71-8, 2

18 7 C 7 C, A Click to edit Master 25 minutes title of style annealing B 15 min 19 min TEM micrographs of La.6 Ca.4 CoO 3 perovskite at 7 C. A) Micrograph taken immediately after heating. A single domain grain can be clearly seen. B) Micrograph taken after 25 min of annealing at 7 C

19 Twins in La.8 Ca.2 CoO 3 8 C Wedge shaped twins 5 nm Wedge shaped twins along with short parallel to each other domains are stable up to 85 C in La.8 Ca.2 CoO 3 perovskite

20 Co oxide surface precipitations during annealing of La.8 Ca.2 CoO 3 for 1.5 hours at 8 C. Precipitations are favored along the grain boundaries. 2 Å EDS x-ray analysis taken from the surface precipitation. Co-rich composition is detected

21 HV, GPa LaCoO 3 in oxidizing and reducing environment In air CTE Loss modulus Linear expansion Storage modulus In 4%H 2 /96%Ar CTE 1 o C Linear expansion Vickers Hardness Temperature, C LaCoO 3 is not stable in reducing atmosphere with the most significant changes starting to occur at 8-9 o C

22 Intensity, a.u. LaCoO 3 reduction steps x 3 #1 6LaCoO 3 2La 3 Co 3 O 8 +O 2 4La 3 Co 3 O 8 6La 2 Co 2 O 5 +O 2 Fourth #2level 8LaCoO 3 2La 4 Co 3 O 1 +2CoO+O 2 4LaCoO Fifth 3 2La level 2 CoO 4 +2CoO+3O 2 La 2 Co2O 5 La 2 CoO 4 +CoO #3 La 4 Co 3 O 1 2La 2 O 3 +Co 3 O 4 La 2 CoO 4 La 2 O 3 +CoO #4 2Co 3 O 4 6CoO+3O 2 2CoO 2Co+O 2 Point # min in 4%H 2 /96% Ar Point # min in 4%H 2 /96% Ar Point # min in 4%H 2 /96% Ar Point # 3 88 min in 4%H 2 /96% Ar Point # 1 in air x x x Co- hexagonal xcoo x x x x CoO Theta, o xcoo La 2 O 3 x Co 3 O 4 x La 4 Co 3 O 1 x La 2 CoO 4 La 3 Co 3 O 8 + La 2 Co 2 O 5 La 2 CoO 4 La(OH) 3 Co- cubic La 2 O 3 La 2 CoO 4 x x x LaCoO 3 LaCoO 3 LaCoO 3

23 a La c b a = 5.52Å b = 5.49Å c = 7.77Å Orthorhombic Room temperature LaGaO 3 Crystal Structure 145o C Rhombohedral The first-order transition orthorhombic ~145 o C leads to twinning of LaGaO 3 crystals change of volume (2) (1) I.K.Bidikin, I.M.S.k., A.M. Balbashov, A.V. Kazansky, Twinning of LaGaO3 Single Crystals. J. Appl. Cryst., : p (2) A.N.Morozov, O.Y.M., N.M.Ponomarev, Real Structure of single crystals of LaGaO3 grown by the Czochralski method. 1. X-ray diffraction and X-ray topographic methods. Kristallografiya, : p Ga O a c b a = 3.889Å α = 89.5 o

24 <1> LaGaO 3 single crystal in the [1] plane 9.7 o LaGaO 3 single crystals preparation LaGaO 3 single crystal in the [1] plane 45.1 o (1) (1) Twins Crystal growth direction <1> Porosity Twin lamellae 54 o 52 (1) Single-crystals of LaGaO 3 were grown by Czochralski technique LaGaO 3 (1) subjected to detwinning C. Klementz, University of Central Florida produced the crystals Single crystals of LaGaO 3 were grown at University of Central Florida by Prof. C.Klemenz

25 CTE 1^-16 K^-1 CTE 1^-16 K^-1 Δ L / L % Δ L / L % LaGaO 3 : Volume Change during Phase transition Click to edit Master text styles.2 -.2Second level LaGaO Fourth 3 (1) level Fifth level LaGaO 3 (1) Shrinkage on heating Temperature, o C Expansion on heating Temperature, o C Expansion on cooling Shrinkage on cooling Overall volume change in the phase transition <.1% Temperature Temperature

26 Intensity, a.u. Department of Mechanical, Materials and Aerospace Engineering Intensity, a.u. Optical microscopy: Heating of (1) LaGaO Heating o C x Raman shift,cm x 2 633nm Raman spectroscopy allows distinguishing the two phases. There are 24 allowed Raman modes in orthorhombic LaGaO3 and 5 Raman modes in rhombohedral LaGaO Cooling ~144 o C nm Raman shift, cm-1

27 Intensity, a.u. Intensity, a.u. Department of Mechanical, Materials and Aerospace Engineering Peak 2 Intensity (counts) Intensity (a.u.) Raman spectra of (1) LaGaO 3 Intensity map of 361cm -1 Raman band LaGaO Click to _1 Outside Twin 3, 1, outside twin, RT Outside twin edit Master text styles Third level Raman Shift, cm -1 LaGaO LaGaO 3 _1 inside Indentation 3, 1, inside Vickers impression, RT Raman Shift, cm Outside twin Twin width Twin width Peak 1 (337) Peak 2 (361) Raman Shift cm -1 Outside twin Distance (Micrometers)

28 Conclusions Twins, Click domain to walls, edit stacking Master faults and dislocations title style were observed in LaCoO 3 based perovskites. The mobility of domains was recorded in-situ by high temperature TEM in the La Click to.6 Ca edit.4 CoO Master 3 perovskite. The detwinning process during text the heating styles and reappearance of domains Second during cooling levelwere observed as a result of a ferroelastic phase transition. Annealing Third level of the LaCoO 3 based specimens at high temperatures in vacuum leads to progressive transitions accompanied by nucleation and movement Fourth of level new stacking faults. This can be explained by the formation of oxygen deficient brownmillerite-type LaCoO 2.5 structure. LaCoO 3 is not stable and reducing environment and can be easily reduced up to metallic cobalt. The orthorhombic to rhombohedral phase transition in LaGaO 3 occurs at ~145 o C. The significant thermal expansion/contraction occurs in the material during heating/cooling cycles.