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esearch papersActa Crystallographica Section BStructuralScienceISSN 0108-7681Crystal structure and charge distribution ofYbFeMnO 4Massimo Nespolo, a * MitsumasaIsobe, a ² Junji Iida b and NoboruKimizuka ca National Institute for Research in InorganicMaterials, Research Centre for Creating NewMaterials, 1-1 Namiki, Tsukuba, 305-0044Ibaraki, Japan, b Sumitomo Metal Mining Co.Ltd, Technology Division, 3-18-5 Nakakokubun,Ichikawashi, Chiba 272-0835, Japan,and c Universidad de Sonora, Departamento deInvestigaciones en Polimeros y Materiales,Hermosillo, Sonora, CP 83000 Mexico² Deceased 1994.Correspondence e-mail: nespolo@nirim.go.jpThe structure of synthetic YbFeMnO 4 has been re®ned bysingle-crystal X-ray diffraction. Space group R3Åm, a =3.4580 (1), c = 25.647 (3) A Ê , V = 265.59 (3) A Ê 3 , Z =3.Ybisin octahedral coordination, whereas Fe and Mn are disorderedon a single crystallographic type of trigonal bipyramid, inwhich the cation is off-centred from the basal plane. Assumingperfect stoichiometry, R 1 = 0.0195, but the charge distribution(CD) analysis suggests incomplete occupation of the Yb site.Re®nement of the occupancy lowers R 1 to 0.0175, resulting ins.o.f.(Yb) = 0.963 (3), with a signi®cant improvement of theFourier difference. The electroneutrality is likely preservedthrough incomplete occupancy of one of the two oxygen sites:the compound is thus non-stoichiometric, with the formulaYb 0.963 FeMnO 3.945 . Another mechanism for preserving theelectroneutrality is the oxidation of a small amount of Mn 2+ toMn 3+ , which is, however, less probable because of thereduction conditions in which the sample was synthesized.Both models give a satisfactorily CD result, but they cannot bede®nitively distinguished by X-ray data.Received 13 March 2000Accepted 7 April 2000# 2000 International Union of CrystallographyPrinted in Great Britain ± all rights reservedActa Cryst. (2000). B56, 805±810 Nespolo et al. YbFeMnO 4 8051. IntroductionCompounds with layer structure and high electric conductivityhave been the subject of intensive research over the last 30years. The binary R 2 O 3 ±MO and ternary R 2 O 3 ±A 2 O 3 ±MO orR 2 O 3 ±TiO 2 ±MO systems (R = In, Ga or a rare earth element;A = In, Ga, Fe, Al; M = Mg, Mn, Fe, Co, Cu, Zn, Cd) have beeninvestigated to clarify the synthesis conditions, phase relations,and stabilities and magnetic properties (Kimizuka et al., 1990,1993, 1995; Siratori, 1993; Iida et al., 1990; Nakamura et al.,1990, 1993a,b; Yamaguchi et al., 1991; Nakamura & Kimizuka,1993; Brown et al., 1999). Structural studies have consideredboth the local structure, through high-resolution transmissionelectron microscopy (HRTEM) and/or electron diffraction(Matsui et al., 1979; Matsui, 1980; Cannard & Tilley, 1988;Uchida et al., 1994; HoÈ rlin et al., 1998; Li et al., 1997, 1998,1999a,b; Li, Bando, Nakamura, Kurashima & Kimizuka, 1999;Wolf & Mader, 1999), and the global one, through powder(Cannard & Tilley, 1988; Nakamura & Kimizuka, 1993; Cava etal., 1998; Moriga et al., 1999) and single-crystal X-raydiffraction (Geller et al., 1975; Kato et al., 1975, 1976; Malamanet al., 1975, 1976; Isobe et al., 1990, 1991, 1994; Nespolo,Nakamura & Ohashi, 2000; Nespolo, Sato et al., 2000).The structural motif of this type of compound consists oflayers of octahedrally coordinated R 3+ cations alternated withlayers of M 2+ /M 3+ cations in trigonal bipyramidal coordination.Barbier (1989) has pointed out that the cation closepacking is much more regular than the oxygen packing. Thestacking of these layers along the c direction produces accreelectronicreprint
esearch papersTable 1Experimental details for YbFeMnO 4 .Crystal dataCell settingHexagonalSpace groupR3 ma (A Ê ) 3.4580 (1)c (A Ê ) 25.647 (3)V (A Ê 3 ) 265.59 (3)Z 3Radiation typeMo KWavelength (A Ê ) 0.71073No. of re¯ections for cell parameters 18 range ( ) 40.98±45.30Temperature (K) 293 (2)Crystal formSphereCrystal radius (mm) 0.055Crystal colourBlackData collectionDiffractometerEnraf±Nonius CAD-4Data collection method!± scansAbsorption correctionSphereT min 0.086T max 0.166No. of measured re¯ections 655No. of independent re¯ections 577No. of observed re¯ections 528Criterion for observed re¯ectionsI > 2(I)R int 0.0196 max ( ) 59.84Range of h, k, l 0 ! h ! 70 ! k ! 460 ! l ! 61No. of standard re¯ections 3Frequency of standard re¯ectionsEvery 240 minIntensity decay (%) 0.5Re®nementRe®nement on F 2s.o.f. (Yb) = 1 s.o.f. (Yb) = 0.963Chemical formula weight 347.83 341.49D x (Mg m 3 ) 6.524 6.405 (mm 1 ) 33.593 32.637R‰F 2 >2…F 2 †Š 0.0195 0.0175wR…F 2 † 0.0524 0.0437S 1.153 1.184No. of re¯ections used 577 577in re®nementNo. of parameters used 13 14…=† max 0.000 0.001 max (e A Ê 3 ) 3.39 at 2/3,1/3,0.1059(0.30 A Ê from Fe/Mn)2.55 at 0,0,0.1109(0.44 A Ê from O1) min (e A Ê 3 ) 6.20 at 0,0,0.0214(0.55 A Ê from Yb)4.88 at 0,0,0.0217(0.56 A Ê from Yb)Extinction method SHELXL (Sheldrick,1997)SHELXL (Sheldrick,1997)Extinction coef®cient 0.0237 0.0290 (11)tional homologous series, i.e. series in which the type(s) andthe general shapes of building blocks, as well as the principlesde®ning their mutual relationships, are preserved, but the sizeof these blocks increases with the number of coordinationpolyhedra in them (Makovicky, 1997).Ideal trigonal bipyramids, with two identical apical bonds,have been found only in compounds where the bipyramidshost a single type of cation, such as InGaO 3 (II) (Shannon &Prewitt, 1968). When different cations enter the bipyramids,these no longer have their two apical bonds identical and havebeen classi®ed into two types. Type I has the three basal MÐObonds shorter than the two apical ones: the cation is stillrelatively close to the basal plane, and the bipyramid is notmuch distorted. Type II has the cation signi®cantly off-centredfrom the basal plane, towards one of the corners: one of theapical bonds is shorter and the other is longer than the threebasal ones (Nespolo, Sato et al., 2000).The above compounds can be classi®ed into two accretionalhomologous series: (R 3+ M 3+ O 3 ) n M 2+ O, analogous to the(YbFeO 3 ) n FeO series (Kato et al., 1975, 1976; Malaman et al.,1975, 1976; Kimizuka et al., 1976), and R 3+ M 3+ O 3 (M 2+ O) m ,analogous to the LuFeO 3 (ZnO) m series (Isobe et al., 1994).The latter has recently drawn attention from differentresearch groups, because of the unusual features of highermembers. In particular, for R = In or Ga, M 3+ = Fe and M 2+ =Zn, extra re¯ections in the selected-area electron diffractionpattern were reported (Li et al., 1997, 1998, 1999a; Li, Bando,Nakamura, Kurashima & Kimizuka, 1999) and interpreted interms of superspace group analysis (Li et al., 1998). TEMobservations have revealed the existence of a modulationwave involving the octahedral cation (Li et al., 1997, 1998,1999a; Li, Bando, Nakamura, Kurashima & Kimizuka, 1999;HoÈ rlin et al., 1998; Wolf & Mader, 1999), whose origin is atpresent unclear. No modulated structure has been reported sofar in members with m < 6. The structure of this series hasbeen recently rationalized in terms ofm ˆ 2K ‡ L;…1†where K 0 and L =0or1(K and L are both integers). Theideal symmetry is P6 3 /mmc and R3Åm for L = 0 and L =1,respectively. In higher members with a modulated structurethe symmetry is actually reduced, at least locally, to orthorhombicand monoclinic, respectively (Li et al., 1998).Important insights into the structural details of thesecompounds can be obtained by the charge distribution (CD)analysis (Hoppe et al., 1989; Nespolo et al., 1999). The CDmethod is the most recent development of the classical theoryof bond strength (Pauling, 1929) and differs from the bondvalence (BV) approach (Brown, 1978) in exploiting the truebond distances in a kind of self-consistent computation ratherthan employing empirical curves. Differently from the BVmethod, in the CD approach the computed `charges' (Q) ofthe cations and of the anions convey different information. Inshort, by labelling q the formal oxidation number, the q/Qratio for the cations indicates the correctness of the structuredetermination, whereas the same ratio for the anionsmeasures the degree of over- or under-bonding. The analysisof the structural details on the basis of the strength of eachbond (called `bond valence' in the BVand `bond weight' in theCD) is meaningful only when the structure is correctly re®nedand the empirical method itself is applicable. The BV methoddoes not contain any internal criterion for such an evaluation,while the CD method has in the q/Q ratio for the cationsprecisely this kind of criterion (for details refer to Nespolo etal., 1999).The CD analysis of several compounds belonging to the twoaccretional homologous series described above has revealedthat those containing type II trigonal bipyramids have a q/Q806 Nespolo et al. YbFeMnO 4 Acta Cryst. (2000). B56, 805±810electronic reprint
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