Direct Observation of Charge Transfer at Jahn-Teller Transition in a Perovskite RbMn[Fe(CN) 6 ] Transition metal cyanides, A (I) M (II)[ N (III)(CN) 6 ] ( A = N a, K , Rb , C s ; M = M n, C o, C r ; N = F e, C r ) , h a v e b een attractin g t h e rene w ed interest o f materials scientists, b eca u se t h ey s h o w a no v el ph otoind u ced ma g neti z ation / dema g neti z ation in addition to t h e t h ermally ind u ced s p in - state transition . T h ey also h a v e t h e c h aracteristic str u ct u re : t h e transition metal ions are s u rro u nded b y si x cyanos (CN – ) and f orm a t h ree - dimensional — M -NC- N — net w or k analo g o u s to t h e do ub le -p ero v s k ite - ty p e transition metal o x ides . I n t h is sense, t h e st u dy o f t h e transition metal cyanides w ill contri bu te to a dee p er com p re h ension o f t h e stron g ly correlated electron system . M ost o f t h e transition metal cyanides, h o w e v er, contain considera b le nonstoic h iometric H 2 O molec u les, wh ic h ma k e t h e str u ct u ral analysis di ff ic u lt . A mon g t h e transition metal cyanides, RbM n [F e (CN ) 6 ] [1] does not contain e x tra H 2 O molec u les and is s u ita b le f or p recise str u ct u ral analysis on t h e c h ar g e density le v el . Ohk os h i et al. [1] o b ser v ed tem p erat u re - ind u ced c ub ic ( F 4-3 m : Z = 4) tetra g onal ( I 4- m 2: Z = 2) transition at aro u nd 220 K. T h e str u ct u ral transition is ascri b ed to t h e c h ar g e trans f er f rom t h e M n 2 + site to t h e F e 3 + site, b eca u se t h e res u ltant J a h n - Teller distortion o f M n 3 + ( d 4 ) can ca u se t h e tetra g onal str u ct u re [2]. W it h a fu rt h er decrease in tem p erat u re b elo w 12 K (= T C ) , t h e local s p ins at t h e M n sites are f erroma g netically ordered, as con f irmed b y ne u tron p o w der di ff raction e xp eriment [3]. To k oro et al. [4] o b ser v ed t h e s upp ression o f ma g neti z ation b y a v isi b le pu lse laser irradiation at 3 K. To o b tain X- ray p o w der data o f g ood co u ntin g statistics w it h h i gh an gu lar resol u tion, meas u rements w ere carried o u t at b eamline BL02B2 . T h e as - g ro w n sam p le p o w ders w ere sealed in a 0.3 mm φ L indemann ca p illary, wh ic h g a v e a h omo g eneo u s intensity distri bu tion in t h e D e b ye -S c h errer p o w der rin g. T h e w a v elen g t h o f t h e incident X- ray w as 0.82 Å ( sli gh tly lon g er t h an t h e K - ed g e o f Rb) , and t h e e xp os u re times w ere 26 min at 300 K and 67 min at 92 K. T h e electron density distri bu tions at b ot h tem p erat u res w ere v is u ali z ed b y t h e ma x im u m entro p y met h od (MEM) com b ined w it h t h e R iet v eld re f inement . T h e MEM analysis w as carried o u t w it h a p ro g ram ENIGMA u sin g 100 × 100 × × 100 p i x els (72 × 72 × 108 p i x els ) at 300 K ( at 92 K). I n F i g. 1 , t h e MEM c h ar g e density o f t h e Fig. 1. MEM electron density distribution of RbMn[Fe(CN) 6 ] for the (100) section at 300 K in the high-temperature cubic phase ( F 4-3 m : Z = 4). Contour lines are drawn from 0.0 to 4.0 e Å -3 at intervals of 0.2 e Å -3 . A schematic shows the crystal structure. (100) section containin g M n, F e, C , and N atoms is s h o w n at 300 K w it h sc h ematics o f crystal str u ct u re . T h e conto u r lines are dra w n only f or t h e lo w er density re g ion (4.0 e Å -3 ). T h e MEM c h ar g e density clearly e xh i b its t h e M n -N and t h e F e -C b ondin g f eat u res . T h e c h ar g e density distri bu tion is isotro p ic aro u nd M n 2 + b eca u se b ot h t h e e g or b itals, t h at is, dx 2 – y 2 and d 3 z 2 – r 2 , are occ up ied in t h e c ub ic ph ase . T h e minim u m o f t h e c h ar g e density (= 0.80 e Å -3 ) in t h e F e - C b ond is lar g er t h an t h at (= 0.60 e Å -3 ) in t h e M n -N b ond . T h is b ondin g electron distri bu tion is consistent w it h t h e p ict u re t h at RbM n [F e (CN) 6 ] consists o f F e (CN) 6 com p le x es and cations (Rb + and M n 2+ ). T h e c h ar g e density distri bu tion mar k edly c h an g es at t h e c ub ic - tetra g onal transition . F i gu re 2( a ) s h o w s t h e MEM c h ar g e density o f RbM n [F e (CN ) 6 ] f or t h e (110) section at 92 K. O ne may notice t h at t h e c h ar g e density distri bu tion aro u nd M n b ecomes anisotro p ic : t h e minim u m o f t h e c h ar g e density (= 0.65 e Å -3 ) in t h e M n -N xy b ond is h i gh er t h an t h at (= 0.28 e Å -3 ) in t h e M n -N z b ond . Obv io u sly, t h is anisotro p y is attri bu ted to t h e c h ar g e trans f er f rom t h e M n 2 + site to t h e F e 3 + site, wh ic h ca u ses a h ole on t h e e g or b ital o f t h e M n ion . 29 Charge Density ( e Å -3 ) at 300 K at 92 K [001] at 92 K [110] Mn 0.5 1 0 0.5 1 NC Fe Distance (arb. units) Fig. 3. Charge density of RbMn[Fe(CN) 6 ] along the Mn-NC-Fe bond at 300 and 92 K, determined by MEM/Rietveld analysis. Kenichi Kato a and Yutaka Moritomo b (a) JASRI / SPring-8 (b) Department of Applied Physics, Nagoya University E-mail: katok@spring8.or.jp Fig. 2. ( a ) MEM electron density distribution of RbMn[Fe(CN) 6 ] for the (110) section in the low-temperature tetragonal phase ( I 4- m 2: Z = 2) at 92 K. ( b ) Electron density distribution of RbMn[Fe(CN) 6 ] for the (110) section determined by the LDA first-principles calculation based on the actual atomic coordinates. Contour lines are drawn from 0.0 to 4.0 e Å -3 at intervals of 0.2 e Å -3 . References [1] S. Ohkoshi et al. : J. Phys. C hem. B 106 ( 2002 ) 2423 . [ 2 ] Y. Moritomo et al. : J. Phys. Soc. Jpn. 71 ( 2002 ) 207 8. [ 3 ] Y. Moritomo et al. : J. Phys. Soc. Jpn. 72 ( 2003 ) 456 . [ 4 ] H . T okoro et al. : Appl. Phys. L ett. 82 ( 2003 ) 1 245 . [ 5 ] K. Kato, Y. Moritomo, M. T akata, M. Sakata, M. Umeka w a, N. H amada, S. Ohkoshi, H . T okoro, K. H ashimoto: Phys. Rev. L ett. 91 ( 2003 ) 255502 . It is interesting to investigate the variation of the charge densities around the Mn ion and the F e ion at the cubic-tetragonal transition. F igure 3 sho w s the charge density of RbMn[ F e( C N) 6 ] along the Mn-N C - F e bond at 300 and 92 K. T he charge density around the Mn ion significantly decreases in the tetragonal phase. W e estimated the total charges around the Mn ion by spherical integration up to the midpoint of the Mn-N bond, i.e., 1. 09 Å for the cubic phase and 1. 06 Å for the tetragonal phase. T he total charge decreases from 23 . 0 ( 2 ) e to 22 . 2 ( 2 ) e at the cubic-tetragonal transition, indicating the valence change from Mn 2+ to Mn 3+ . W e have calculated the charge density distribution w ith the full-potential lineari z ed augmented plane- w ave method w ithin the local density appro x imation ( L DA) scheme. T he actual tetragonal lattice parameters at 92 K determined by Rietveld analysis w ere used. T he calculated spin moment μ Mn at the Mn site ( = 3 . 2 μ B ) is nearly consistent w ith the e x perimental results [ μ Mn = 3 . 2 ( 7 ) μ B [ 3 ]]. F igure 2 (b) sho w s the calculated charge density of RbMn[ F e( C N) 6 ] for the (11 0 ) section. H eights and interval of the contour lines are the same for both the e x perimental [ F ig. 2 (a)] and calculated [ F ig. 2 (b)] images. T he L DA calculation q uantitatively reproduces the MEM charge density, including the anisotropic charge density around Mn 3+ . Actually, the calculated minima of the charge densities are 0 . 59 e Å and 0 . 32 e Å - 3 - 3 in the Mn-N xy and Mn-N z bonds, r e s p e c t i v e l y , w h i c h a r e c l o s e t o t h e e x perimentally obtained values ( 0 . 65 e Å - 3 and 0 . 2 8 e Å - 3 in the Mn-N xy and Mn-N z bonds, respectively). In summary, w e have investigated the variation of charge density at the Jahn- T eller transition of RbMn[ F e( C N ) 6 ] and directly observed the charge transfer from the Mn site to the F e site at the cubic- tetragonal transition [ 5 ]. W e further interpreted the anisotropic charge density distribution around Mn 3 + in terms of the bonding electrons. T hus, the MEM/ Rietveld charge density analysis proved to be effective for the deeper comprehension of the transition metal compounds. (a) (b) 30
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