We report on a Ni L$_{2,3}$ edges x-ray absorption spectroscopy (XAS) study
in $R$NiO$_3$ perovskites. These compounds exhibit a metal to insulator ($MI$)
transition as temperature decreases. The L$_{3}$ edge presents a clear
splitting in the insulating state, associated to a less hybridized ground
state. Using charge transfer multiplet calculations, we establish the
importance of the crystal field and 3d spin-orbit coupling to create a
mixed-spin ground state. We explain the $MI$ transition in $R$NiO$_3$
perovskites in terms of modifications in the Ni$^{3+}$ crystal field splitting
that induces a spin transition from an essentially low-spin (LS) to a
mixed-spin state.Comment: 4 pages, 4 figures, accepted as PRB - Rapid Comm. Dez. 200

A. Y. Ramos

C. Piamonteze

F. M. F. de Groot

H. C. N. Tolentino

J. A. Alonso

J. B. Torrance

M. J. Martínez-Lope

N. E. Massa

English

arXiv

We report on a Ni L-2,L-3 edge x-ray absorption spectroscopy study in RNiO3 perovskites. These compounds exhibit a metal-to-insulator (MI) transition as temperature decreases. The L-3 edge presents a clear splitting in the insulating state, associated to a less hybridized ground state. Using charge transfer multiplet calculations, we establish the importance of the crystal field and 3d spin-orbit coupling to create a mixed-spin ground state. We explain the MI transition in RNiO3 perovskites in terms of modifications in the Ni3+ crystal field splitting that induces a spin transition from an essentially low-spin to a mixed-spin state.71

Piamonteze, C

de Groot, FMF

Tolentino, HCN

Ramos, AY

Massa, NE

Alonso, JA

Martinez-Lope, MJ

Repositorio da Producao Cientifica e Intelectual da Unicamp

Spin-orbit-induced mixed-spin ground state in RNiO3 perovskites probed by x-ray absorption spectroscopy: Insight into the metal-to-insulator transition

We report on a Ni L-2,L-3 edge x-ray absorption spectroscopy study in RNiO3 perovskites. These compounds exhibit a metal-to-insulator (MI) transition as temperature decreases. The L-3 edge presents a clear splitting in the insulating state, associated to a less hybridized ground state. Using charge transfer multiplet calculations, we establish the importance of the crystal field and 3d spin-orbit coupling to create a mixed-spin ground state. We explain the MI transition in RNiO3 perovskites in terms of modifications in the Ni3+ crystal field splitting that induces a spin transition from an essentially low-spin to a mixed-spin state

Utrecht University Repository

We report on a Ni L2,3 edge x-ray absorption spectroscopy study in RNiO₃ perovskites. These compounds exhibit a metal-to-insulator (MI) transition as temperature decreases. The L₃ edge presents a clear splitting in the insulating state, associated to a less hybridized ground state. Using charge transfer multiplet calculations, we establish the importance of the crystal field and 3d spin-orbit coupling to create a mixed-spin ground state. We explain the MI transition in RNiO₃ perovskites in terms of modifications in the Ni³⁺ crystal field splitting that induces a spin transition from an essentially low-spin to a mixed-spin state.Facultad de Ciencias ExactasCentro de Química Inorgánic

Piamonteze, Cinthia

Groot, F. M. F. de

Tolentino, Hélio C. N.

Ramos, A. Y.

Massa, Néstor Emilio

Alonso, José Antonio

Martínez Lope, María Jesús

LAReferencia - Red Federada de Repositorios Institucionales de Publicaciones Científicas Latinoamericanas

Spin-orbit-induced mixed-spin ground state in RNiO3 perovskites probed by x-ray absorptionspectroscopy: Insight into the metal-to-insulator transitionC. Piamonteze,1,2 F. M. F. de Groot,3 H. C. N. Tolentino,1 A. Y. Ramos,1,4 N. E. Massa,5J. A. Alonso,6 and M. J. Martínez-Lope61Laboratório Nacional de Luz Síncrotron, Caixa Postal 6192, 13084-971, Campinas, São Paulo, Brazil2IFGW/UNICAMP, 13083-970, Campinas, São Paulo, Brazil3Department of Inorganic Chemistry and Catalysis, Utrecht University, Sorbonnelaan 16, 3584 CA, Utrecht, The Netherlands4LMCP-CNRS, Université de Paris 6, Paris, France5Laboratorio Nacional de Investigación y Servicios en Espectroscopía Optica, Centro CEQUINOR, UNLP, 1900 La Plata, Argentina6Instituto de Ciencia de Materiales de Madrid, C.S.I.C, Cantoblanco, E-28049 Madrid, SpainsReceived 17 June 2004; revised manuscript received 5 November 2004; published 26 January 2005dWe report on a Ni L2,3 edge x-ray absorption spectroscopy study in RNiO3 perovskites. These compoundsexhibit a metal-to-insulator sMId transition as temperature decreases. The L3 edge presents a clear splitting inthe insulating state, associated to a less hybridized ground state. Using charge transfer multiplet calculations,we establish the importance of the crystal field and 3d spin-orbit coupling to create a mixed-spin ground state.We explain the MI transition in RNiO3 perovskites in terms of modifications in the Ni3+ crystal field splittingthat induces a spin transition from an essentially low-spin to a mixed-spin state.DOI: 10.1103/PhysRevB.71.020406 PACS numberssd: 75.10.Dg, 75.25.1z, 78.70.Dm, 71.30.1hRare-earth nickel perovskites sRNiO3, R=rare earthdpresent a sharp well-defined metal-to-insulator sMId transi-tion as temperature decreases.1 The transition temperature,TMI, increases with reducing the R ion size, which deter-mines the degree of distortion of the structure.2 It wasproposed that the gap opening would be due to a smallerNi-O-Ni superexchange angle leading to a reduction of thebandwidth.3 However, non-negligible electron-phononinteractions4 and a shift in TMI with oxygen isotopesubstitution5 evidenced the importance of modifications inNi-O interatomic distances, suggesting a phonon-assistedmechanism for conduction. As temperature decreases, thesenickelates undergo a magnetic transition to an unusual anti-ferromagnetic order.6–8 The magnetic arrangement for thelighter R compounds sR=Pr, Nd, Sm, Eud was refined with asingle Ni moment s0.9 mBd and required nonequivalent cou-plings among Ni ions to stabilize the structure.6,7 This is aquite unusual situation in an orthorhombic crystallographicstructure whose Ni sites are all equivalent.2 For the heavier Rcompounds, Alonso et al.8,9 established a monoclinic distor-tion in the crystallographic structure, leading to two differentNi sites with longer and shorter NiuO distances alternatingalong the three axes. The antiferromagnetic structure wasexplained by a charge ordering defined among the differentNi sites, each one with different magnetic moments s1.4 and0.7 mB for YNiO3d.8 More recently, some evidences that thelow-temperature distortion is shared by all members of theRNiO3 family were reported.10 Concerning the Ni localstructure, our recent extended x-ray absorption fine structuresEXAFSd results demonstrated the existence of two differentNi sites in all RNiO3 compounds, regardless of their long-range crystallographic structure.11,12From spectroscopic data together with configuration inter-action calculations, Mizokawa et al.13 established for PrNiO3at room temperature a metallic ground state composed of34% 3d7+56% 3d8LI +10% 3d9LI 2, where LI stands for aligand hole. Owing to the high degree of hybridization in theground state, with hole transfer from Ni 3d to O 2p orbitals,these compounds have been classified as self-doped Mottinsulators.14 Such a mixed metallic ground state, mostly−3d8LI , is compatible with a Ni3+ low-spin configuration,because the amount of charge transfer for parallel spin al-most equals that for antiparallel spin.13 However, the spectralshape in RNiO3 compounds is very sensitive to the transitionfrom metallic to insulating states15 and a complete descrip-tion of the spin degree of freedom remains to be given. As inrecent outcomes on Co3+ oxides,16,17 where unconventionalspin states exist due to the competition between crystal fieldsplitting and effective 3d exchange interaction, assignmentsmade so far about Ni3+ in RNiO3 compounds have to bereexamined.We report here Ni L edge absorption measurements,which probes directly the available Ni 3d states, togetherwith charge transfer multiplet calculations. We establish theimportance of the crystal field and 3d spin-orbit sl ·sd cou-pling to create a mixed-spin ground state. We explain the MItransition in RNiO3 perovskites in terms of modifications inthe Ni3+ crystal field splitting that localizes the electronicstates and induces a spin transition from an essentially low-spin sLSd to a mixed-spin state, with a quite large contribu-tion from a high-spin sHSd state.Soft x-ray absorption measurements at the Ni L2,3 edgeswere carried out at the spherical grating monochromatorsSGMd beamline of Laboratório Nacional de Luz SíncrotronsLNLSd, Brazil, using a spherical grating monochromatorwith an energy resolution of 0.8 eV at 845 eV. Data werecollected at 300 and 96 K using a liquid-nitrogen cold finger.The bulk polycrystalline RNiO3 sR=Pr, Nd, Eu, Y, and Ludsamples were obtained by a wet-chemistry technique, as de-scribed elsewhere.18 The experimental spectra measured at300 K for LuNiO3, YNiO3, EuNiO3, NdNiO3, and PrNiO3are shown in Fig. 1. The MI transition for these compoundsPHYSICAL REVIEW B 71, 020406sRd s2005dRAPID COMMUNICATIONS1098-0121/2005/71s2d/020406s4d/$23.00 ©2005 The American Physical Society020406-1takes place at 599, 582, 460, 200, and 130 K, respectively.9At 300 K, NdNiO3 and PrNiO3 are at the metallic state,whereas the other compounds are at the insulating state. Forthe insulators, a clear splitting is observed at the L3 edge s,854 eVd, while for the metals this splitting is absent. Dif-ferences among spectra measured in the different electronicstates are also noticeable at the L2 edges,872 eVd. PrNiO3 and NdNiO3 compounds spectra, mea-sured at 96 K, when both are insulators, are analogous tothose of R=Eu, Y, and Lu at 300 K, as shown by the splittingat the L3 edge for the PrNiO3 compound sinset Fig. 1d. Suchmodifications in the experimental spectra, unambiguously as-sociated to the MI transition, demonstrate that importantchanges take place at the Ni local electronic structure. As thespectral shape depends mostly if the compound is metal orinsulator, we conclude that the short-range scale electronicstructure of all compounds shares a common basis.To identify the electronic interactions accounting for theexperimental data, charge transfer sconfiguration interactiondmultiplet19,20 sCTMd calculations were carried out. The cal-culations take into account interactions between three con-figurations, 3d7, 3d8LI , and 3d9LI 2, in D4h symmetry. A seriesof calculations varying the crystal field splitting parameter10Dq, keeping Ds=0.1 eV, Dt=0.2 eV fixed, is presented inFig. 2. The configuration interaction parameters are D=0.5 eV, Udd−Upd=−1 eV, and the transfer integrals TsB1d=TsA1d=2TsB2d=2TsEd=2 eV. The calculations matchingthe best with the experimental spectra for the metallic com-pounds are those with 10Dq=2.2 eV. The Ni3+ metallicground state turns out to be composed of 49% 3d7+47% 3d8LI +4% 3d9LI 2. The relative proportions for thethree configurations are in reasonable agreement with thosefound by Mizokawa et al. for metallic PrNiO3.13 The slightdifference can be partially ascribed to the way the ligandhole is treated in the codes.21 For the insulating compounds,a very good agreement with experiments is achieved by de-creasing 10Dq down to 2.0 or 1.9eV. Such a decrease in10Dq leads to a less hybridized ground state composed of61% 3d7+37% 3d8LI +2% 3d9LI 2.Figure 3sad shows the simulated spectrum for 10Dq=1.9 eV taking into account the l ·s coupling, as in Fig. 2sed,while Fig. 3sbd shows the calculation without the l ·s cou-pling. The effect on the spectral shape is remarkable. Theground state without the l ·s coupling is a HS state, schemati-cally represented in Fig. 3sdd. Figure 3scd shows a calcula-tion without the l ·s coupling, but where the initial state ofthe transition is the first excited state, which is a LS state,schematically represented in Fig. 3sed. The energy differencebetween these two states is found to be ,0.1 eV. Since thel ·s coupling is of the same order, the insulating ground stateturns out to be a mixing of LS and HS states. In addition, thecalculation of the metallic state fFig. 2sbdg is quite similar tothat shown in Fig. 3scd, indicating that the LS state gives themain contribution to the metallic ground state. We concludeFIG. 1. Ni L2,3 edge spectra at 300 K for PrNiO3, NdNiO3,EuNiO3, YNiO3, and LuNiO3, whose TMI are 130, 200, 462, 582and 599 K, respectively. Inset: L3 edge spectra for PrNiO3 at 300 Kssolid lined and at 96 K sopen circlesd.FIG. 2. CTM calculations of the Ni L XAS spectra in D4h sym-metry. The value of 10Dq is varied between 2.3 eV sad and 1.8 eVsfd, in steps of 0.1 eV. The other parameters used are described inthe text.FIG. 3. CTM calculations for 10Dq=1.9 eV sad with l ·s cou-pling, sbd without l ·s coupling, and scd without l ·s coupling butwith the initial state being the first excited state; sdd scheme of HS4E and sed LS 2A1 states.PIAMONTEZE et al. PHYSICAL REVIEW B 71, 020406sRd s2005dRAPID COMMUNICATIONS020406-2that the ground state composition is very sensitive to thespin-orbit sl ·sd coupling and that modifications in the crystalfield splitting s10Dqd change the relative contribution of bothstates to the mixed ground state.The ground state sGSd can be decomposed into differentsymmetries that are mixed up by the l ·s coupling,CGS = oi;k+l+m+n=3aiua1kb1l b2menl+ oj;k8+l8+m8+n8=2a jua1k8b1l8b2m8en8LI l+ op;k9+l9+m9+n9=1apua1k9b1l9b2m9en9LI 2l . s1dIn this expression k+ l+m+n, k8+ l8+m8+n8, and k9+ l8+m9+n9 are the number of d holes in 3d7, 3d8LI , and 3d9LI 2configurations, respectively. The ai,a j,ap values can be ob-tained as described by Wasinger et al.22 The contributions ofthese different symmetries to the ground state and to spinsSzd and orbital angular sLzd moments as functions of 10Dqare shown in Fig. 4. The labels in Fig. 4sbd refer to thedistribution of the d holes among B1, A1, B2, and E orbitals.At the metallic state, with 10Dq=2.2 eV, the ground state iscomposed of ,38% B1B1A1I sLSd and a small amounts,8% d of B1A1EI sHSd, the rest being charge transfer con-figurations. This leads to a net spin moment Sz,0.5, in ac-cordance with previous results that found a LS state for themetallic PrNiO3.13 Despite this strongly hybridized groundstate, the expected value for Sz is close to the Ni3+ LS ionicvalue. The reason is essentially due to the charge transferconfigurations. The amount of charge transfer with spin upsSz=1 from B1A1LI d is compensated by that of spin downsSz=0 from B1B1LI d. The ground-state transition takes placeat 10Dq=2.1 eV, where we obtain the same contributions forHS and LS states, that is, the ground state is made of ,25%B1A1EI and of ,25% B1B1A1I . The spin moment assumes anintermediate value Sz,0.7. At the insulating state, with10Dq=2.0 eV, the ground state is composed of ,46%B1A1EI sHSd and a small amount s,9% d of B1B1A1I sLSd, thecomplement coming from charge transfer configurations.This leads to a net spin moment Sz,1.0. The mixed-spinstate arises because these compounds are between the strongand weak field limit and also due to the strong hybridizationthat mixes the Ni 3d and O 2p orbitals. In case there wouldbe no charge transfer at all, the high- to low-spin transitionwould take place over a crystal field range of less than0.1 eV. It is due to the strong covalence that there is agradual change from the high- to low-spin ground state.For the B1B1A1I sLSd state, the electron coming from the O2p ligand band occupies the B1 orbital, even though it has ahigher energy than A1, owing to the gain in exchange energy.So, B1A1LI is the most important charge transfer configura-tion at the metallic state. On the other hand, for the B1A1EIsHSd state, the ligand electron must occupy an orbital alreadyfilled and there is no gain in exchange energy. In this case,the ligand electron occupies the A1 orbital, which is lower inenergy than B1 and has a transfer integral twice larger thanfor the E orbital. Moreover, the B1B1A1I sLSd state has all itsthree holes in orbitals that have strong mixing with the O 2pligand band sA1 and B1 come from the eg orbital in Oh sym-metryd, whereas the B1A1EI sHSd state has a more localizedhole in an E orbital that has less efficient overlap with the O2p ligand band. So, the increase of the HS component in theground state and the decrease of the hybridization for smaller10Dq are related to each other.The physical reason for a smaller 10Dq at the insulatingstate originates in the coexistence of two nonequivalent Nisites and overall increase of the average Ni-O bonding in allRNiO3 compounds.2,8,9 Nonequivalent Ni sites coexist evenat the metallic state11,12 and are compatible with a segrega-tion into two phases, where a more localized electron phaseis embedded in a conducting background.23 The MI transi-tion takes place with the increase of the less hybridized phaserelated to the other. Alonso et al.8 found for the heavierRNiO3 compounds, by refining neutron diffraction data inthe monoclinic symmetry, an antiferromagnetic structurecompatible with two nonequivalent moments. The smallerand larger Ni sites display magnetic moments of 0.7 and1.4 mB, respectively. The values that we predict here for Szare larger because the spin moments of the 3d elements re-tain most of their isolated ion properties in x-ray absorptionspectroscopy sXASd. Owing to the short time scale in XAS,essentially all electrons appear localized. Delocalized elec-trons should reduce these values in the solid. The same ar-gument holds for the angular moment Lz, which is normallyquenched in these compounds. Even if somewhat overesti-mated, our predictions show that an important parameterFIG. 4. sad Variation of Sz, Lz and sbd ground-state compositionas a function of 10Dq. The configuration B1B1A1I corresponds to theLS 2A1 state in Fig. 3sed and the configuration B1A1EI correspondsto the HS 4E state in Fig. 3sdd. The others correspond to the chargetransfer configurations.SPIN-ORBIT-INDUCED MIXED-SPIN GROUND STATE… PHYSICAL REVIEW B 71, 020406sRd s2005dRAPID COMMUNICATIONS020406-3controlling the properties of these compounds is the smallsplitting in distances, which leads to a different crystal fieldin each site. For the lighter RNiO3 powder compounds, thelong-range structure has always been refined using a singleNi site and for that reason no such different spin values havebeen found. Recently, a monoclinic distortion was observedin thin films of NdNiO3,24 questioning the orthorhombicsymmetry obtained previously by neutrons. Our results showthat the lighter and heavier RNiO3 compounds are very simi-lar from the point of view of the local electronic structure.Experimental spectra look very similar and depend only ifthe compound is insulator or metal. So, we believe that thesame physics observed in the heavier RNiO3 compoundsmay be involved in the lighter ones.In conclusion, we presented experimental evidences ofsignificant changes in the local electronic structure aroundNi3+ in RNiO3 compounds. With the support of charge trans-fer multiplet calculations, we showed that, concomitantlywith the metal to insulator transition, a decrease of the crys-tal field splitting leads to a ground state transition from anessentially LS to a mixed-spin ground state. The mixed-spinground state is possible because the energy separation be-tween HS and LS states is of the same order of the spin-orbitcoupling. The smaller hybridization, intrinsic to the HS state,gives an explanation to the more localized character at theinsulating regime. The existence of a mixed-spin state, in-volving both LS and HS states and also charge transfer con-figurations, shed new light in the understanding of the un-usual antiferromagnetic order observed below TN, which isstill an open question.This work is partially supported by LNLS/ABTLuS. C.P.thanks FAPESP for support. A.Y.R. thanks CNPq for finan-cial support as a visiting scientist.1 P. Lacorre, J. B. Torrance, J. Pannetier, A. I. Nazzal, P. W. Wang,and T. C. Huang, J. Solid State Chem. 91, 225 s1991d.2 J. L. García-Muñoz, J. Rodríguez-Carvajal, P. Lacorre, and J. B.Torrance, Phys. Rev. B 46, 4414 s1992d.3 J. B. Torrance, P. Lacorre, A. I. Nazzal, E. J. Ansaldo, and C.Niedermayer, Phys. Rev. B 45, R8209 s1992d.4 N. E. Massa, J. A. Alonso, M. J. Martínez-Lope, and I. Rasines,Phys. Rev. B 56, 986 s1997d.5 M. Medarde, P. Lacorre, K. Conder, F. Fauth, and A. Furrer,Phys. Rev. Lett. 80, 2397 s1998d.6 J. L. García-Muñoz, J. Rodríguez-Carvajal, and P. Lacorre, Phys.Rev. B 50, 978 s1994d.7 J. Rodríguez-Carvajal, S. Rosenkranz, M. Medarde, P. Lacorre,M. T. Fernandez-Díaz, F. Fauth, and V. Trounov, Phys. Rev. B57, 456 s1998d.8 J. A. Alonso, J. L. García-Muñoz, M. T. Fernández-Díaz, M. A.G. Aranda, M. J. Martínez-Lope, and M. T. Casais, Phys. Rev.Lett. 82, 3871 s1999d.9 J. A. Alonso, M. J. Martínez-Lope, M. T. Casais, J. L. García-Muñoz, and M. T. Fernández-Díaz, Phys. Rev. B 61, 1756s2000d.10 F. de la Cruz, C. Piamonteze, N. E. Massa, H. Salva, J. A. Alonso,M. J. Martínez-Lope, and M. T. Casais, Phys. Rev. B 66,153104 s2002d.11 C. Piamonteze, H. C. Tolentino, A. Y. Ramos, N. E. Massa, J. A.Alonso, M. J. Martinez-Lope, and M. T. Casais, Phys. Scr. sto bepublishedd.12 C. Piamonteze, H. C. Tolentino, A. Y. Ramos, N. E. Massa, J. A.Alonso, M. J. Martinez-Lope, and M. T. Casais, Phys. Rev. B.sto be publishedd.13 T. Mizokawa, A. Fujimori, T. Arima, Y. Tokura, N. Mōri, and J.Akimitsu, Phys. Rev. B 52, 13865 s1995d.14 T. Mizokawa, D. I. Khomskii, and G. A. Sawatzky, Phys. Rev. B61, 11263 s2000d.15 C. Piamonteze, H. C. Tolentino, F. C. Vicentin, A. Y. Ramos, N.E. Massa, J. A. Alonso, M. J. Martinez-Lope, and M. T. Casais,Surf. Rev. Lett. 9, 1121 s2002d.16 Z. Hu et al., Phys. Rev. Lett. 92, 207402 s2004d.17 I. A. Nekrasov, S. V. Streltsov, M. A. Korotia, and V. I. Anisi-mov, Phys. Rev. B 68, 235113 s2003d.18 J. A. Alonso, M. J. Martínez-Lope, and I. Rasines, J. Solid StateChem. 120, 170 s1995d.19 F. M. F. de Groot, J. Electron Spectrosc. Relat. Phenom. 67, 529s1994d.20 F. M. F. de Groot, Coord. Chem. Rev. 249, 31 s2005d.21 The multiplet program used considers the ligand hole as having dsymmetry, whereas cluster programs take into account the realp-ligand states.22 E. C. Wasinger, F. M. F. de Groot, B. Hedman, K. O. Hodgson,and E. I. Solomon, J. Am. Chem. Soc. 125, 12894 s2003d.23 J.-S. Zhou, J. B. Goodenough, B. Dabrowski, P. W. Klamut, andZ. Bukowski, Phys. Rev. B 61, 4401 s2000d.24 U. Staub, G. I. Meijer, F. Fauth, R. Allenspach, J. G. Bednorz, J.Karpinski, S. M. Kazadov, L. Paolasini, and F. d’Acapito, Phys.Rev. Lett. 88, 126402 s2002d.PIAMONTEZE et al. PHYSICAL REVIEW B 71, 020406sRd s2005dRAPID COMMUNICATIONS020406-4

Spin-orbit-induced mixed-spin ground state in RNiO₃ perovskites probed by x-ray absorption spectroscopy: Insight into the metal-to-insulator transition

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