1 / 82

Experimental Tests of Charge/Valence and Orbital Ordering

Experimental Tests of Charge/Valence and Orbital Ordering. J.Rodríguez-Carvajal 1,2 and M.A.Daoud-Aladine 1,3 1 Laboratoire Léon Brillouin, CEA/SACLAY , France 2 Service de Physique Statistique Magn é tisme et Supraconductivit é , CEA, Grenoble, France

adelinet
Download Presentation

Experimental Tests of Charge/Valence and Orbital Ordering

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Experimental Tests of Charge/Valence and Orbital Ordering J.Rodríguez-Carvajal1,2 and M.A.Daoud-Aladine1,3 1Laboratoire Léon Brillouin, CEA/SACLAY, France 2Service de Physique Statistique Magnétisme et Supraconductivité, CEA, Grenoble, France 3Paul Scherrer Institut, Villigen, Switzerland SCOOTMO Workshop

  2. 2001 SCOOTMO Workshop

  3. Nature 416, March 2002 SCOOTMO Workshop

  4. Nature 416, April 2002 SCOOTMO Workshop

  5. Charge ordering concept at the heart of phase transitions in localised/itinerant strongly correlated systemsregain of interest: superconductivitycolossal magnetoresistance SCOOTMO Workshop

  6. Occupation matrix{n}(LSDA + U): Orbital Ordering Local charge re-distribution around an atom propagating to long range in an spatially ordered way. The total ground state electronic density ((r)=G(r)2), contains the most relevant contribution from the “valence electrons” around the transition metal sites. This component of the wave function contains the information about orbital ordering. The angular distribution of the d-electron-spin density around a particular site is: G: Green-function matrix, nl: principal and orbital quantum numbers (eg. 3d), m: index of a particular d-orbital, : spin projection index SCOOTMO Workshop

  7. Charge Ordering Term used to describe a situation in which “ions” in a crystal pass from a homogeneous intermediate valence state to an ordered mixed valence state. Special case of a charge density wave (CDW). • R: lattice vector, ri position of ions inside the basic unit cell • The high temperature disordered state is “metallic” or • “semiconductor” with a relatively high carrier mobility • Carriers: electron or holes eventually accompanied by a • distortion of the surrounding ions (polarons). • Coulomb repulsion between carriers is, in principle, the driving • force of the charge localisation and ordering. 2Mv(t2gx*1) Mv-(t2gxeg1+) + Mv+(t2gxeg1-) SCOOTMO Workshop

  8. Charge Disproportionation: High Temperature Homogeneous Formal Valence RNiO3 (Ni3+), CaFeO3 (Fe4+), BaBiO3 (Bi4+) High Temperature State  Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Low Temperature State  Mv+ Mv- Mv+ Mv- Mv+ Mv- Mv+ Mv- Mv+ Mv- Mv+ Mv- Mv+ Mv- Mv+ Mv- Mv+ Mv- SCOOTMO Workshop

  9. Charge Ordering: Intermediate/Mixed Valence (Mn3+/Mn4+ in manganites) Solid solutions: R1-xDxMnO3 (R=La,Pr,…D=Sr,Ca…) Self-doped: LiMn2O4, NaV2O5, … Intermediate homogeneous valence  metal Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Mv Localised Charge (Disordered)  semiconductor M MM M M M M M M MM M M M M MM M M M M MM M M M Charge Ordered  insulator or semiconductor M M M M M M M M M M M M M M M M M M M M M M M M M M M M SCOOTMO Workshop

  10. Colossal Magnetoresistance  Regain of interest in electronic properties of (semi)conducting magnetic oxides Questions!  Charge ordering: In the charge ordered state, is the charge localization at atomic level? What does the conventional ionic states Mn3+/Mn4+ mean? … CDW, charge disproportionation, polarons, …  Orbital ordering: Is that concept really relevant for explaining the electronic/structural transitions in manganites? Is there any difference with respect to a MO6 distorting structural transition? SCOOTMO Workshop

  11.  Electronic transitions and phase separation: Role of the chemistry and the disorder. Homogeneous/inhomogeneous states: what is a phase? Role of thedefects and microstrains …  Magnetic exchange interactions: Are the Goodenough-Kanamori-Anderson rules still applicable? How to explain the canted magnetic structures? Are there other magnetic structures compatible with the observed magnetic powder diffraction patterns? Is the CE structure really collinear? SCOOTMO Workshop

  12. Diffraction techniques: Synchrotron and neutron powder diffraction: problem of weak effects, asymmetric peak shapes, pseudo symmetry  wrong results? Single crystals: twinning, homogeneity, …difficult synthesis Spectroscopy: XANES, EXAFS, NMR Combined: RIXS, RXS, DAFS Disorder/order in A-sites: Local distortions versus average structure PDF analysis SCOOTMO Workshop

  13. SCOOTMO Workshop

  14. The Bond Valence approach... The information contained in data bases suggest a strong correlation between average M-O distances and formal valence of M ions What can we get from precise crystallographic structures? SCOOTMO Workshop

  15. Measure of the Jahn-Teller effect. Distortion: Valence state associated to a Mn site : Directly related to the average Mn-O distance <d>=<dMn-O> o e- The importance of precise structural refinements  Nature of the octahedral distortions ? Mn4+(Non JT) ions in CaMnO3, T=RT <dMn-O >= 1.90 Å, D = 0,03.10-4 Mn3+(JT) ions in LaMnO3, T=RT< TJT  <dMn-O >= 2,02 Å l= 2.18, m=1.97, s=1.91 Å, D = 33,1.10-4 SCOOTMO Workshop

  16. The Bond-Valence analysis Correlation between formal charges on ions and inter-atomic distances. sij : bond-strength or bond-valence (valence units) B=0.37, Roij depends on the chemical species “i” and “j”. • Rule 1 (usually verified) • The Bond-Valence Sum Rule states that Vi deviates from • the formal valence less than a few percent in inorganic structures. • (information about CHARGE ordering) • Rule 2 (usually violated) • The individual sij tends to s=V/N in non-frustrated inorganic • structures. N is the co-ordination number. • (information about ORBITAL ordering) • Predicted average distance: dP= Ro- B ln (V/N) • Example: • Predicted d(Mn3+-O = 2.0165 Å  2.02 Å • Predicted d(Mn4+-O = 1.9030 Å  1.90 Å SCOOTMO Workshop

  17. Structure at HT  Disordered Charges Orthorhombic Perovskites Site B of spinels Fe3+[Fe2.5+]2O4 Li[Mn3.5+]2O4 Pr1/2Ca1/2[Mn3.5+]O3 Structure at LT- M3+/M4+ or M2+/M3+ Ordered Charges T<290K T<TV=120K T<TCO=240K (a,2b,c) P1121/m ? Monoclinic (2a, 2a,2a) Cc ? (3a,3a,a) Fddd Intensity of superstructures / fundamental peaks Strong >4-5% <2% <3% “Known” examples of Charge Ordering Transitions Characteristics of the transition: (1) Jump in resistivity (eventually a metal-insulator transition), (2) Observation of a structuralphase transition, (3) Antiferromagnetic properties, (4) Opening of a gap in the optical conductivity. SCOOTMO Workshop

  18. Electronic structure changes: The valence state of Mn XANES at the Mn K edge : (LURE) Y1/2Ca1/2MnO3 XANES at different temperatures Comparison with Mn2O3 (Mn3+) and MnO2(Mn4+) 1.2  The edge position is related to the valence state of the Mn Mn2O3 1 MnO2 0.8 ) 1 /I 0 0.6 Log(I 0.4 0.2 0 6530 6550 6570 E(eV) SCOOTMO Workshop

  19. Electronic structure changes: The valence state of Mn XANES at the Mn K edge : (LURE) Y1/2Ca1/2MnO3 XANES at different temperatures Comparison with Mn2O3 (Mn3+) and MnO2(Mn4+) 1.2  The edge position is related to the valence state of the Mn Mn2O3 1 MnO2 10K 0.8 ) 1 250K /I 0 0.6 350K TCO=300K Log(I 0.4 TN=115K 0.2 0 6530 6550 6570 E(eV) SCOOTMO Workshop

  20. 10K 250K 350K Electronic structure changes: The valence state of Mn XANES at the Mn K edge : (LURE) Y1/2Ca1/2MnO3 XANES at different temperatures 1.2  The edge position is related to the valence state of the Mn 1 0.8 ) 1 /I  Its evolution with temperature do not display any changes norat the structural neither at the magnetic phase transitions. 0 0.6 TCO=300K Log(I 0.4 TN=115K 0.2 0 6530 6550 6570 E(eV) SCOOTMO Workshop

  21. 10K 250K 350K * If we had a Mn3+/ Mn4+ charge ordering one would expect a broader shape of the edge structure corresponding to the mixing of different Mn3+ and Mn4+ electronic configurations Electronic structure changes: The valence state of Mn XANES at the Mn K edge : (LURE) Y1/2Ca1/2MnO3 XANES at different temperatures 1.2 * for a sharpedge, Mn are in an intermediate valence electronic state 1 0.8 ) 1 /I 0 0.6 TCO=300K Log(I 0.4 TN=115K 0.2 0 6530 6550 6570 E(eV) SCOOTMO Workshop

  22. LaMnO3 LaMnO3 + CaMnO3 CaMnO3 Mn4+ Mn3+ La1/2Ca1/2MnO3 Absence of “change of valence” from Mn-K XANES J.García et al. J.Phys. Cond. Matt. 13, 3243 (2001) Comparison of observed spectrum for La1/2Ca1/2MnO3 and the weighted mixture of LaMnO3 and CaMnO3 spectra XANES spectra of LaMnO3 and CaMnO3 SCOOTMO Workshop

  23. SCOOTMO Workshop

  24. Interpretation of physical properties of RNiO3 compounds.Example of charge disproportionation Magnetic structure: Orbital ordering without structural distortions? …. Metal-Insulator transition: Charge disproportionation ... SCOOTMO Workshop

  25. Phase diagram of RNiO3 compounds SCOOTMO Workshop

  26. Magnetic structure of RNiO3 Propagation vector with respect to the Pbnm group: k=(1/2, 0, 1/2), implying arrangements of types: + + - - + + - - + + - - + + - - + + - - + + - - …. +0-0+0-0+0-0+0-+0-+0-+0…. along the pseudo-cubic axes. k referred to pseudo-cubic perovskite cell: kc=(1/4, 1/4, 1/4) SCOOTMO Workshop

  27. Along the line k=(k, k, k) the Fourier transform of the exchange interactions for a simple cubic lattice up to fourth nearest-neighbours can be written as: k=1/4 may be the k-position for a maximum of J(k) only if: Conditions to get kc=(1/4,1/4,1/4) as the propagation vector of the magnetic ground state in the homogeneous case (no orbital ordering) SCOOTMO Workshop

  28. 2 limiting options: a) Spin/charge transition : 2Ni(III)  Ni2+(LS) + Ni2+(HS) + 2 holes in Oxygen b) Charge disproportionation ( commensurate CDW) : 2Ni(III)  Ni2+ Ni4+ In both cases there is a diamagnetic ion, Ni2+(LS) and Ni4+, so in case of a NaCl-like ordering J1=0, considering AF exchange to second neighbours (J2< 0) the propagation vector k=(1/4,1/4,1/4) is the optimum SCOOTMO Workshop

  29. dx2-y2 dz2 dz2 dx2-y2 dx2-y2 dz2 Two types of orbital orderings may be proposed to explain the magnetic structure of RNiO3 compounds SCOOTMO Workshop

  30. Average Ni-O distance for HS Ni2+: 2.06 Å Average Ni-O distance for LS Ni3+: 1.94 Å Average Ni-O distance for Ni4+: 1.88 Å (?) d(Ni-O)= 1.958 Å Metal Insulator d(Ni1-O)= 1.994 Å d(Ni2-O)= 1.923 Å J.A. Alonso et al., PRL 82, 3871 (1999) SCOOTMO Workshop

  31. Structural changes at the Metal-Insulator Transition in LuNiO3 SCOOTMO Workshop

  32. Nature of the M-I transition in RNiO3 The single Ni site in the metallic phase breaks up into two sublattices in the insulating low temperature phase Alternating expanded and contracted NiO6 octahedra can be taken as an evidence for the stabilisation of a partial charge disproportionation, 2Ni3+ Ni3+d+Ni3-d Unequal magnetic moments for the two Ni sites corroborates the charge disproportionation. But , what about exchange interactions explaining the observed magnetic structure? SCOOTMO Workshop

  33. Electronic crystallization in a Li battery material: columnar ordering of electron and holes in the spinel LiMn2O4 LiTd [Mn2]Oct.O4 : Mn3.5+ High temperature: mixed valence state J. Rodríguez-Carvajal, G. Rousse, Ch. Masquelier and M. Hervieu Physical Review Letters, 81, 4660 (1998) SCOOTMO Workshop

  34. LiMn2O4 : Electron Diffraction 230 K 320 K 400 12 0 0 040 0 12 0 Orthorhombic Fddd a = 24.7435(5) Å b = 24.8402(5) Å c = 8.1989(1) Å x 9 Cubic Fd3m a = 8,248 (1) Å J. Rodríguez-Carvajal et al, PRL, 81, 4660 (1998) SCOOTMO Workshop

  35. 350K 230K 30 40 50 60 70 80 Orthorhombic Distortion Superstructure reflections LiMn2O4 : Neutron Diffraction, 3T2 (LLB) Charge ordered state 2 theta (°) SCOOTMO Workshop

  36. <Mn-O> = 1,996(4) Å Mn(2) = 3.27+ D = 19.4 <Mn-O> = 2,020(5) Å Mn(3) = 3.12+ D = 36.6 <Mn-O> = 2,003(2) Å Mn(1) = 3.20+ D = 20.6 c b a <Mn-O> = 1,915(4) Å Mn(5) = 3.90+ D = 6.1 <Mn-O> = 1,903(4) Å Mn(4) = 4.02+ D = 4.6 LiMn2O4 : Partial Charge Ordering 64 « Mn4+ » 80 « Mn3+ -like» 8 delocalised holes SCOOTMO Workshop

  37. b 1 1 1 1 1 x x x x x x 2 2 2 2 3 3 3 3 c x x x x x x 2 2 3 2 2 3 3 3 x x x x x x 1 1 1 1 1 x x x x x x Mn3+ 3 2 3 2 3 2 3 2 x x x x x x Mn4+ 3 2 3 2 2 3 2 3 x x x x x x 1 1 1 1 1 Li+ X/ x x x x x x 2 2 2 2 3 3 3 3 x x x x x x 3 2 2 2 3 3 2 3 x x x x x x 1 1 1 1 1 x x x x x x 3 3 2 3 2 2 3 2 x x x x x x 3 2 3 2 3 2 3 2 x x x x x x 1 1 1 1 1 a LiMn2O4 : Partial Charge Ordering Mn(2) Mn(3) SCOOTMO Workshop

  38. Nature of the phase transition in LiMn2O4  The structural distortions and average distances around the Mn ions support the atomic scale charge ordered state below the transition temperature.  The structural transition is driven by a charge ordering process. The Coulomb repulsion is however of secondary importance compared to electron-lattice coupling via the Jahn-Teller effect  The distorted pyrochlore lattice of Mn-ions is then half doped with a partial charge ordering at low temperature. The magnetic ground state is incommensurate and very complex. SCOOTMO Workshop

  39. Co+3 Co+2 SCOOTMO Workshop

  40. E. Suard, F. Fauth, V. Caignaert, I. Mirebeau, and G. Baldinozzi, Phys. Rev. B. 61, R11871 (2000). SCOOTMO Workshop

  41. Co+3 Co+2 YBaCo2O5 T. Vogt, P.M. Woodward, P. Karen, B.A. Hunter, P. Henning, and A.R. Moodenbaugh Phys. Rev. Lett. 84, 2969 (2000). SCOOTMO Workshop

  42. SCOOTMO Workshop

  43. Charge ordering in R1/2D1/2MnO3 (TN<TCO) Zener Polaron Ordering Half-DopedManganites A.Daoud-Aladine, J.Rodríguez-Carvajal*, L. Pinsard-Gaudart, M.T. Fernández-Díaz and A. Revcolevschi Physical Review Letters89, 097205 (2002) SCOOTMO Workshop

  44. SCOOTMO Workshop

  45.  Phase diagram of half doped manganites (<RA>,T) Ferromagnetic Metal TC=350 T(K) La0.5Sr0.5MnO3 TC=255 La0.5Ca0.5MnO3 T(K) TN=TCO=135 Paramagnetic Insulator TCO=240 TN=170 Pr0.5Ca0.5MnO3 T(K) AF Insulator(CO) TCO320 TN=115 Y0.5Ca0.5MnO3 T(K) Paramagnetic Charge- Ordered state Decreasing <RA> Stability of the CO-phase (x = 1/2) SCOOTMO Workshop

  46. t a cos(j/2) e- e- j Ferromagnetism-delocalization correlation: The double exchange (DE) in metallic phases Large <RA> C. Zener Phys. Rev. 82,440 (1951) P. G. de Gennes Phys. Rev. 118,141 (1960) JH Optimization of the kinetic energy give rise to ferromagnetic correlations SCOOTMO Workshop

  47. e- e- e- e- Ferromagnetism-delocalization correlation: The double exchange (DE) in metallic phases Large <RA> C. Zener Phys. Rev. 82,440 (1951) P. G. de Gennes Phys. Rev. 118,141 (1960) JH Ferromagnetic Metal SCOOTMO Workshop

  48. TN=TCO=135 Paramagnetic Insulator TCO=240 TN=170 Pr0.5Ca0.5MnO3 T(K) AF Insulator(CO) TCO320 TN=115 Y0.5Ca0.5MnO3 T(K) Paramagnetic Charge- Ordered state Decreasing <RA> Stability of the CO-phase (x = 1/2), Low <RA>  Phase diagram of half doped manganites (<RA>,T) SCOOTMO Workshop

  49. AF F  Goodenough-Kanamori-Anderson Super-exchange rules for Super Exchange Magnetism of insulating phases: Orbital ordering and Super-Exchange (SE) Super-exchange? Orbital Ordering c A phenomenological approach : J. Goodenough Phys. Rev. 100, 564 (1955) b a Mn4+ Mn3+ A systematic phenomenological justification : J. Kanamori J. Phys. Chem. Solids 10, 87 (1959) Mn4+ Mn3+ Theoretical basis: P.W Anderson Phys. Rev. 118,141 (1960) Mn3+ Mn4+ SCOOTMO Workshop

  50. c c b a b a AF F Magnetism of insulating phases: The AF-CE order of charge ordered compounds Is there an Orbital Order explaining it? SCOOTMO Workshop

More Related