perovskites_lect24

perovskites_lect24 - Transition Metal Oxide Perovskites:...

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Unformatted text preview: Transition Metal Oxide Perovskites: Band Structure, Electrical and Magnetic Properties Chemistry 754 Solid State Chemistry Lecture #24 May 27, 2003 Chemistry 754 - Solid State Chemistry Transition Metal Oxides To illustrate the relationship between crystal structure, bonding, band structure, electrical and structure, bonding, structure, magnetic properties we are going to consider transition metal oxides of three structure types. Perovskite (AMO3)/ReO3 Rock Salt (MO) Rutile (MO2) For all three structures M-O interactions will Mdictate the properties. In the latter two structure types we also need to consider M-M bonding. MChemistry 754 - Solid State Chemistry 1 Perovskites and Band Structure Octahedral Molecular Orbital Diagram *(t2g) and *(eg) Bands *(e Orbital Overlap and Bandwidth (ReO3 vs. MnO32-) Structural Distortions (Octahedral Tilting) Exchange Splitting (Spin Pairing Energy) The d-electron count (SrTiO3 to SrFeO3) dInstabilities and the d4 electron count SrFeO3 LaMnO3 CaFeO3 Chemistry 754 - Solid State Chemistry Perovskite Crystal Structure M O A Chemistry 754 - Solid State Chemistry 2 Generic Octahedral MO Diagram t1u ( + ) a1g ( ) (n+1)p (n+1)s nd eg (dx2-y2, dz2) (n+1)d t2g (dxy, dxz, dyz) t1g & t2u Oxygen ) eg ( ) t2g ( O 2p (6) - t2g, t1u O 2p NB (6)-t1g, t2u t2g () O 2p (6) a1g, t1u, eg Transition Metal eg () t1u ( + ) a1g () Chemistry 754 - Solid State Chemistry Simplified Band Structure (n+1)p [4] Bands of interest (n+1)s nd eg (dx2-y2, dz2) (n+1)d t2g (dxy, dxz, dyz) M-O [2] M-O [3] Oxygen O 2p (12) O 2p NB Transition Metal M-O M-O O 2p (6) a1g, t1u, eg Chemistry 754 - Solid State Chemistry 3 Orbital Overlap * and * Bands * Overlap (M d t2g O 2p ) Band Runs Uphill point (kx=ky=0) non-bonding point (kx=ky=/a) antibonding Greater Spatial Overlap W(*) > W(*) * Overlap (M d eg O 2p ) Band Runs Uphill Chemistry 754 - Solid State Chemistry Overlap in 3D So far we have been working mostly in 1D and 2D. In 3D keep the following overlap considerations in mind: X Point (kx=/a, ky=kz=0) (k dxy, dxz 1/2 antibonding dyz nonbonding 2 degenerate bands M Point (kx=ky=/a, kz=0) (k dxy, antibonding dyz, dxz 1/2 antibonding 2 degenerate bands R Point (kx=ky=kz= /a) (k dxy, dyz, dxz antibonding 3 degenerate bands y x X point Chemistry 754 - Solid State Chemistry 4 Band Structure ReO3 and MnO3230 30 O 2p M t2g M eg EF 25 *(eg) *(e W~7 eV 25 Energy (vs. O 2s) 20 *(t2g) W~5 eV 20 *(eg) *(e W~4 eV *(t2g) W~2 eV 15 15 10 10 X R M R0 3 X R M R0 6 ReO3 k DOS 3 Chemistry 754 - Solid State Chemistry MnO k DOS 2- Structural Distortions: CaMnO3 Cubic (Pm3m) Linear Mn-O-Mn MnOrthorhombic (Pnma) (Pnma) Bent Mn-O-Mn Mn- Mn O Mn Mn O Mn Chemistry 754 - Solid State Chemistry 5 Octahedral Tilting & Band Structure Cubic (Pm3m) Linear Mn-O-Mn Mn*(eg) *(e W~4 eV Orthorhombic (Pnma) (Pnma) Bent Mn-O-Mn Mn- 20 20 Energy (vs. O 2s) *(t2g) W~2 eV *(eg) *(e W~2.5 eV *(t2g) W~1.5 eV 15 15 10 10 X R M R k Chemistry 754 - Solid State Chemistry k Z T Y X U R Spin Polarized Band Structure low spin spin polarized 20 20 eg(*) t2g(*) EF eg(*) t2g(*) DOS Energy (vs. O 2s) 15 15 10 10 X R M R O 2p Mn 3d t2g Mn 3d eg X R s = +1/2 s = -1/2 M R k k CaMnO3 is a Mott-Hubbard Insulator, rather than a metal! Chemistry 754 - Solid State Chemistry 6 3d TM Oxide Perovskites Compound SrTiO3 (d0) SrVO3 (d ) SrCrO3 (d ) CaMnO3 (d ) SrFeO3 (d4) 3 2 1 Electron Config. *0*0 *1 *0 Electrical Properties Semiconductor Metallic Metallic Magnetic Properties Diamagnetic Pauli Paramagnetic Pauli Paramagnetic *2*0 t2g3*0 t2g3*1 Semiconductor Antiferromagnetic TN = 110 K Spiral AFM Metallic TN ~ 130 K *, * implies delocalized electrons t2g, eg implies localized electrons Chemistry 754 - Solid State Chemistry SrFeO3-The Edge of Instability eg t2g eg t2g EF eg(*) t2g(*) eg(*) t2g(*) DOS Fe4+ Fe4+ Cubic Structure No Jahn-Teller Distortion JahnAll Fe atoms equivalent Localized t2g electrons Delocalized eg electrons Metallic to at least 4 K Chemistry 754 - Solid State Chemistry 7 Cubic Band Structure Calculations CaMnO3 (a = 3.81 A) SrFeO3 (a = 3.86 A) LaMnO3 (a = 4.03 A) 20 20 20 Energy (vs. O 2s) 15 15 15 10 10 10 X R M R X R M k k Chemistry 754 - Solid State Chemistry k R X R M R LaMnO3-Cooperative Jahn Teller Dist. Fe(Mn)-O Distances Fe(Mn) LaMnO3 2 1.907(1) 2 2.178(1) 2 1.968(1) SrFeO3 6 1.92 Fe(Mn)-O-Fe(Mn) Angles Fe(Mn) Fe(Mn) CaFeO3 Octahedral tilting and decreased covalency both narrow the * (eg) band. (e This leads to electron localization and a cooperative Jahn-Teller Distortion Jahn155.48(5) 155.11(5) SrFeO3 180 Chemistry 754 - Solid State Chemistry 8 LaMnO3-Cooperative Jahn Teller Dist. eg t2g Mn3+ Symmetric MnO6 Mn3+ dx2-y2 dz2 t2g EF dx2-y2 (*) dz2(*) t2g(*) dx2-y2 (*) dz2(*) t2g(*) Jahn-Teller JahnDistortion DOS Orthorhombic Structure Pronounced Jahn-Teller JahnDistortion All Mn atoms equivalent Localized t2g & eg electrons Semiconductor Chemistry 754 - Solid State Chemistry CaFeO3-Charge Disproportionation Fe-O Distances FeCaFeO3 2 1.919(2) 2 1.927(2) Ca 2 1.919(1) SrFeO3 6 1.92 Fe-O-Fe Angles FeCaFeO3 158.1(1) 158.4(2) Octahedral tilting narrows * (eg) (e band, leads to electron localization! SrFeO3 180 Chemistry 754 - Solid State Chemistry 9 Soft Mode Condensation (290 K) eg t2g eg t2g Fe3+ eg eg t2g Fe5+ t2g Oxygen shift alters crystal field splitting Localizes the eg electrons Drives Metal to Semiconductor Transition Chemistry 754 - Solid State Chemistry 10 ...
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This note was uploaded on 06/11/2011 for the course CHEM 101 taught by Professor Stegemiller during the Spring '07 term at Ohio State.

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