31 Charge-Ordered State in Single-Crystalline CaFeO 3 Thin Film Studied by X-ray Anomalous Diffraction Fig. 1. Crystal structure of CaFeO 3 . Two types of octahedra contain Fe(1) or Fe(2) alternatively. Spheres symbolize calcium ions. Charge ordering (CO) in perovskite-type oxides has been intensively investigated in recent years. CaFeO 3 (CFO) shows the CO transition at 290 K [1] with a change of the crystal space group from Pbnm in the normal state to P 2 1 / n in the CO state. T he transition accompanies the metal-insulator transition , as seen other mixed valence materials. On the other hand , S rFeO 3 ( S FO) , having the same nominal Fe valence of +4 and nearly the same structure , does not show CO even at 4 K. T he CFO crystal has a distorted perovskite-type structure ( G dFeO 3 structure) shown in Fig. 1 , where each FeO 6 octahedron is tilted. S ince the Fe 4+ -O bond has a strong d - p hybridi z ation , oxygen holes on the antibonding band are considered to be generated initially , and conse q uently , the system becomes metallic. T he Fe-O-Fe bond angle in CFO is 1 6 0 which is much reduced from the 1 8 0 in S FO. T his causes the d - p band in CFO to be narrower than that in S FO and , as a conse q uence , the system transforms to the CO state at a relatively high temperature. I n the CFO crystal , two ma j or steric configurations of the excess charge have been proposed for the CO state. One is that fully charged Fe 3 + (3 d 5 ) and Fe 5+ (3 d 3 ) are positioned separately with a rocksalt-type structure. T his concept was accepted at an early stage and supported by the experiment of the Mö ssbauer effect. T he other , based on XPS measurements , is that holes , as excess charge , are locali z ed around some Fe 3 + ions , such as 3 d 5 3 d 5 L 2 ( L denotes a ligand hole). T his suggests that the Fe ions of CFO in the CO state split into Fe 4+ δ and Fe 4 - δ (0 < δ ≤ 1) at the CO transition temperature. M ore recently , neutron and X -ray diffraction analyses of polycrystalline CFO have been performed [2]. T hese studies revealed that Fe-O bond lengths for each e q uivalent Fe site are distinct , e.g. , at 1 5 K for Fe(1) and Fe(2) sites , bond lengths are 0.1 87 2( 6 ) and 0.19 74 ( 6 ) nm , respectively [2]. E ven with these explicit studies , however , the physical picture appropriate for CO in CFO is still controversial , because of a lack of q uantitative information on each electronic state of Fe(1) and Fe(2). W e present explicit experimental evidence of the CO state reali z ed in CFO , using an X -ray anomalous diffraction ( XAD ) techni q ue , and a subse q uent electronic band-structure calculation is then compared with the experimental results to obtain the physical picture in terms of the q uantitative electronic state difference between Fe(1) and Fe(2) in the CO state of CFO [3]. CFO single-crystalline thin films with [001] orientation parallel to the growth direction were fabricated by pulsed laser deposition with epitaxial growth on the S r L a A lO 4 (001) substrate. A fter the deposition , the films were cooled by blowing partially o z oni z ed oxygen. T he si z e of the films obtained was 10 × 10 mm 2 in area and 5 0 nm in thickness. A n XAD experiment was carried out near the Fe- K absorption edge ( E = 7 112 e V ) with the κ -type six-circle diffractometer installed on beamline BL14B1 . T he X - rays were monochromati z ed by a S i(111) double- crystal system. T he XAD experiment is one of the powerful techni q ues for studying materials with CO. I n the present work , we have focused on a superlattice reflection (0. 5, 0. 5, 0. 5 ) p , where p denotes a primitive perovskite structure. S ince a structure factor for this reflection is obtained from the reported crystal structure [2] with two Fe atomic sites Fe(1) and Fe(2) , we can estimate the B ragg peak intensity with the anomalous scattering factors for the reflection. I f we observe the energy dispersion of such a superlattice reflection , there will be a characteristic structure on the dispersion curve , reflecting the difference between both environments of surrounding Fe(1) and Fe(2). T hen , in order to study the physical picture of CO in the CFO , we have carried out the XAD measurements of the superlattice reflection , (0. 5, 0. 5, 0. 5 ) p , for CFO thin film. I n our experimental geometry , the incident synchrotron beam was polari z ed perpendicular to the scattering plane ( σ σ ) and the diffracted beam was detected without regarding the polari z ation (i.e. , σ ’ + π ’ ). Figure 2 shows the energy dependence of the XAD intensity of the (0. 5, 0. 5, 0. 5 ) p reflection with two different a z imuthal angles observed at 1 5 0 K. T he anomalous diffraction intensity has a significant peak at E 7 12 4 e V . T he antiferromagnetic order ( T N = 11 5 K) does not appear at that temperature. T hus the structure of the XAD spectra is considered to derive Ca Fe (1) O 6 Fe (2) O 6 Fig. 2. Energy dependence of ( a) t he intensity of (0.5, 0.5, 0.5) p with the filled circles for the azimuthal angles of and at 150 K, and ( b) the absorption with the solid line at 300 K, near the Fe K edge. A typical error of the intensity is represented only for the point of 7030 eV. The solid lines in (a) show the calculated dispersion curve as described in the text. Inset in (b) shows a calculated azimuthal angle dependence of the XAD intensity at 7124 eV. References [ 1] M. Takano e t al. : Ma ter . Res . Bull . 12 ( 1 977) 923 . [ 2 ] P . M. W oo dw a rd et al. : Phys . Rev . B 62 ( 2000) 844 . [ 3 ] T. A k ao , Y . A zum a , M. U sud a , Y . Ni shih a t a , J . M izu k i, N . H a m a d a , N . H a y a shi, T . T e r a shim a a n d M. Takano : Phys . Rev . Lett . 91 ( 2003) 1 56405 . [ 4 ] I . S o l o vyev e t al. : Phy s . R ev . B 53 ( 1 996) 7 1 58 . [ 5 ] M . Taka h a shi a n d J . I g a r a shi: Phys . Rev . B 64 (200 1 ) 075 11 0 . T. A kao a , J . M izu k i b an d N . H a m a d a c a a ( a ) F a culty o f E n g i n eeri n g , To tt o ri U n iversity (b) SPri n g -8 / JAERI (c) F a culty o f Scie n ce an d T ech no l o g y, Scie n ce U n iversity o f Tok y o E-m a il: akao @m ech . t o tt o ri-u .a c . j p Absorption (a.u.) Energy (eV) Intensity (a.u.) (b) 7000 7050 7 100 7150 7200 7250 0 1 2 3 4 5 6 ( a) 0. 0 6 6 0. 5 1.0 1.5 2.0 2.5 fr o m the ch a rge o rder a t t he Fe i o n s . T he X-r a y a bs o rpti on s pe c trum (XAS) o f the C FO thi n film is sh o w n i n F ig . 2(b) a n d t he c a lcul a ted dispersi on cu rves f o r the tw o differe n t a zimuth a l an g les a re superp o sed on the o bse rved XAD curve i n Fig . 2( a ) . G e n er a lly, the s tructure f a ct o r o f reflecti o n s with c o n tributi o n s fr o m i o n s i n cludi n g e lectr o n s with a l o c a l an is o tr o py, such a s Fe i o n s i n CFO, sh o uld be tre a ted a s a t e n s o r a n d s h o w t h e a z i m u t h a l a n g l e depe n de n ce . I n t he prese n t c a se, h o wever, th e dispersi on cu rves a re a lm o st ide n tic a l . T his is re a s ona bly supp o rted by t he c a lcul a ti o n which expects th a t a zimuth a l depe n de n ce w o uld no t a ppe a r i n the reflecti on , a s sh o w n i n the i n set o f Fig . 2(b), due t o the rel a tively l a rge T h o ms o n sc a tteri n g term a n d o u r o p tic a l c on figur a ti on w here the diffr a cted be a m is c o llected with o ut p o l a riz a ti on. T he c a lcul a ted dispersi on o f the XAD i n te n sity seems well fitted t o the experime n t a lly o bt a i n ed cu rve . T he p r o cedure o f the c a lcul a ti on is briefly expl a i n ed a s f o ll o ws . T he first-pri n ciples electr o n ic structure c a lcul a ti o n is perf o rmed f o r the an tiferr o m a g n etic CFO i n the l o w- temper a ture p h a se with the rep o rted cryst a l structure d a t a [ 2 ] . T he f ull-p o te n ti a l li n e a rized a ugme n ted- pl an e-w a ve meth o d is empl o y ed with the LDA- U scheme [ 4 ] : U ef U U f = 2 eV f o r the Fe 3 d o rbit a l . T h e t he o retic a l cu rves i n the figure were o bt a i n ed by c a lcul a ti n g t he ( 1 s - 4 p ) p p dip o le-tr an siti on a mplitude usi n g the Fe 4 p st a tes . T he c a lcul a ti o n sc heme w a s est a blished by Taka h a shi a n d Ig a r a shi [ 5 ] . T he CO st a te is ch a r a cterized by t he hybridiz a ti on be twee n the Fe 3 d o rbit a l a n d t he O 2 p o rbit a l . We s h o w a rem a r ka ble fe a ture th a t is, the t o t a l n u mbers o f 3 d electr o n s a re a lm o st the s a me f o r Fe( 1 ) ~ Fe +5 a n d Fe(2) ~ Fe +3 , a s s h o w n i n Ta b le I, where Fe 3 d e lectr o n n u mbers i n the muffi n -ti n sp heres o f Fe( 1 ) an d Fe(2) a re t a bul a ted . T hus, we h a ve a picture f o r t he CO st a te simil a r t o the o xyge n h o le sce na ri o i n which the 3 d e lectr o n n u mber a t the Fe site is fixed a t 5, while the ch an g e o f the electr o n n u mber is a ttributed t o the o xyge n h o le . O ur picture, h o wever, a sserts th a t the Fe 3 d e lectr o n n u mber is l a rger th a n 5 an d th a t the mi no rity-spi n 3 d o rbit a l is a ls o p a rti a lly o ccupied; thus, the Fe m a g n etic m o me n t is sig n ific an tly reduced fr o m 5 μ B . T his reducti o n is a ls o o bserved by the n eu tr o n sc a tteri n g me a sureme n t: 2 . 5 μ B f o r Fe( 1 ) an d 3 . 5 μ B f o r Fe(2) [ 2 ] . T he m a g n etic m o me n ts a re a ls o c o n s iste n t with t he result o f the M össb a uer experime n t . Every O a t o m is a c o mm on n eighb o r on Fe( 1 ) an d Fe(2); the ch a rge a t the O s ite is, theref o re, u n ch an g ed by the CO tr a n siti o n. Tab le I . Numbers of 3 d e lectrons and magnetic moments for the two iron sites in CaFeO 3 . 2 2 2 2 2 2 2 2 2 2 2 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 32 32 32 32 32 32 32 32 32 32 32 32 32 32 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
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