electronic reprint Crystal structure and charge distribution of ... - CRM2

esearch papersratio for the octahedral cation signi®cantly different from 1,thus questioning the correctness of the structure. A rere®nementof LuFeO 3 (ZnO), in which Fe/Zn are in type IItrigonal bipyramids, has demonstrated that the Lu atom is notat the centre of symmetry, as previously assumed, but isactually split between two half-sites along z (Nespolo,Nakamura & Ohashi, 2000), con®rming a similar resultobtained by neutron powder diffraction (Cava et al., 1998).The same analysis also suggested that in the higher memberscontaining type II trigonal bipyramids, the M 3+ cation tends toavoid the bipyramid directly bound to the R 3+ octahedron. Onthe contrary, structures containing only type I trigonal bipyramidshave a q/Q ratio very close to 1 for all the cations andR 3+ occupies the centre of symmetry, without splitting.The above results call for further studies on this class ofcompound. The simplest (m = 1) member of the series is themost suitable candidate for investigating the geometry of thecoordination polyhedra and its structural effects. In fact, itcontains a single crystallographically independent trigonalbipyramid and thus the shape of this polyhedron directlyin¯uences the R 3+ cation. For the higher members its effect isdiluted among the other bipyramids, in which the cationdistribution is often hardly determined by X-ray diffraction.2. Materials and methodsSingle crystals of YbFeMnO 4 were grown at 1723±1773 Kfrom a mixture of the ratio Yb 2 O 3 :Fe 2 O 3 :MnO = 1:1:2 by the¯oating zone method, as described in Iida et al. (1990), but in acontrolled CO 2 atmosphere to prevent as much as possibleoxidation to Mn 3+ . Layered compounds RFe 2 O 4 andRFeMnO 4 (R = rare-earth elements or Y) are known to meltincongruently (Shindo et al., 1976; Iida et al., 1990), with a ®nalcomposition of the solid phase that differs from that of theliquid one. The composition of the crystal is therefore determinedhereafter by re®nement of the occupancy factors.One of the synthesized crystals was chosen for X-raydiffraction investigation and mounted on an Enraf±NoniusCAD-4 diffractometer. Intensity collection was performedusing variable-rate !±2 scans, with the scan range 1.275 +0.525tan (details are given in Table 1). The Lp correctionswere performed through the XCAD4 program (Harms, 1996)and the spherical absorption correction with the PLATONpackage (Spek, 1990). The structure re®nement wasperformed against F 2 with SHELX97 (Sheldrick, 1997). Initialpositional parameters and displacement factors were takenfrom InGaZnO 4 (Nespolo, Sato et al., 2000).Fe and Mn can hardly be distinguished by X-ray diffractionand the determination of their ratio, oxidation state andpossible ordering scheme requires different techniques. Owingto the premature disappearance of one of the authors, it wasnot possible to locate the original sample. However, the hightemperature at which it was synthesized is in favour of a modelin which Fe and Mn are disordered on the single bipyramid.Besides, the previous reports of analogous structures [Zn/Fe(Isobe et al., 1994; Cava et al., 1998); Co/Fe (Isobe et al., 1990);Zn/Ga (Nespolo, Sato et al., 2000); Cu/Al (GeÂrardin et al.,Table 2Atomic coordinates and equivalent isotropic displacement parameters(A Ê 2 ) for YbFeMnO 4 re®ned to s.o.f.(Yb) = 0964 (3).U eq is de®ned as one third of the trace of the orthogonalized U C tensor.Wyckoff x = y z U eqYb 3(a) 0 0 0.01184 (5)Fe/Mn 6(c) 0 0.215706 (14) 0.00728 (7)O(1) 6(c) 0 0.12826 (13) 0.0208 (5)O(2) 6(c) 0 0.29345 (8) 0.0098 (2)1980)] support a model with disordered distribution for theM 2+ /M 3+ cations in the trigonal bipyramids. Barbier (1989) hasshown that in In 1.2 Ga 0.8 MgO 4 , analogous to YbFeMnO 4 , theexcess In is accommodated in the trigonal bipyramids and0.2In + 0.8Ga + Mg are completely disordered. Finally, theMoÈ ssbauer investigation of YFe 2+ Fe 3+ O 4 revealed that Fe 2+and Fe 3+ are disordered at room temperature, and only at lowtemperature two different antiferromagnetic sublattices arisedue, however, to electron diffusion between Fe 2+ and Fe 3+ .Disorder of Fe/Mn is therefore assumed hereafter.3. ResultsResults of the structure re®nement are given in Tables 1±3 1and the structure is shown in Fig. 1. At ®rst, re®nement wasperformed assuming perfect stoichiometry, with a ®nal R 1 of0.0195. The CD analysis (Table 4) shows, however, that thecomputed charge for Yb is not satisfactory: Q is in fact only2.87, against an expected value (q) of 3.0, leading to a q/Qratio of 1.04 5 , i.e. more than 4% deviation from the ideal ratio.Moreover, the Fourier difference has a deep hole close to Yb(Table 1).In the case of LuFeZnO 4 the same type of anomaly led tore-re®ning the structure adopting the split atom model for theLu 3+ cation, which was found disordered at z around theinversion centre. In the present case, however, splitting of Ybeither brings Yb again on the inversion centre or produces anunstable re®nement. The low formal charge obtained by theCD analysis may have its cause in a lack of stoichiometry, inparticular in the incomplete occupation of the octahedral site,a hypothesis that is also supported by the incongruent melt ofthese compounds when synthesized by the ¯oating-zonemethod. The structure was thus further re®ned, including thes.o.f.(Yb) in the least-squares parameters: it re®ned to0.963 (3), with a ®nal R 1 of 0.0175 and a signi®cant reductionof both the highest peak and the deepest hole in the Fourierdifference (Table 1). To check whether the above improvementsare statistically meaningful or rather an apparent effectof the different number of parameters (14 versus 13), theHamilton test (Hamilton, 1965), employing the ratio of wR 2for the two models, was performed. Interpolating the tabulatedvalues for a signi®cance level of 0.005 results in a 4%vacancy of Yb being correct with a signi®cance level of 99.5%.1 Supplementary data for this paper are available from the IUCr electronicarchives (Reference: NA0104). Services for accessing these data are describedat the back of the journal.Acta Cryst. (2000). B56, 805±810 Nespolo et al. YbFeMnO 4 807electronic reprint