J O U R N A L O F C H E M I S T R Y Materials Structural investigations of new copper fluorides NaRECu2F8 (RE3+=Sm3+, Eu3+, Gd3+, Y3+, Er3+, Yb3+) C. De Nadaý�, A. Demourgues,* L. Lozano, P. Gravereau and J. Grannec ICMCB, Avenue du Dr A. Schweitzer, 33608 Pessac cedex, France. E-mail: demourg@icmcb.ubordeaux.fr Received 22nd April 1998, Accepted 30th June 1998 A series of copper(II ) and rare earth fluorides has been prepared.The crystal structure of NaGdCu2F8 has been solved from single crystal data and refined to conventional R=0.028 (wR=0.048) for 585 independent reflections. The symmetry is tetragonal (space group I422) with a=5.407(1) and c=10.382(1) A° (Z=2). A Rietveld analysis of NaYCu2F8 and refinements of the powder X-ray diVraction data for the other compounds of the series confirmed the I422 space group.The structure consists of layers of CuF4 square-planes perpendicular to the c-axis, interleaved alternatively by Na+ or RE3+ cations located in antiprisms. 4/mmm with systematic extinctions h+k+l=2n+1, consistent Introduction with space groups I422, I4mm, I492m, I49m2, I4/mmm. During recent years a wide range of ternary fluorides with Intensity data were collected on an Enraf Nonius CAD4 general formula AIMIIIF4 or AIIMIIF4 have been isolated: their four-circle automatic diVractometer using graphite-monochroinvestigation has led to the characterization of various struc- mated Mo-Ka radiation.Corrections were applied for Lorentz tural types given in ref. 1 and 2 (and references cited therein). and polarization eVects, followed by an empirical absorption When M is a d-transition element, the KBrF4-type3–5 is often correction using y-scan technique.For extinction corrections found in the presence of Jahn–Teller M2+ or M3+ cations. an empirical coeYcient e was used in the expression Fc(corr)= This is particularly true for CaCuF4 which exhibits a tetragonal Fc (1+10-3 eFc2 l3/sin2h).Intensity treatment and refinement symmetry6,7 (space group I4/mcm). This structure is related to calculations were performed using the SHELXL93 program.10 the fluorite-type by cationic ordering, yielding a doubling of Atomic scattering factors and anomalous dispersion paramthe c parameter value. The whole of the cations shows a fcc eters were taken from ref. 11. Crystal data and experimental stacking, but an important shift of the anionic sublattice leads conditions are listed in Table 1.The quality of the acquisition to define two types of environment for the cations: MF4 and refinement was based on the conventional reliability square-planes at z=0 and 1/2 which are isolated one from factors Rint and R1, wR2 respectively. another. Each layer of (MF4)2- polyhedra are shifted relative Full crystallographic details, excluding structure factors, to the other ones and the larger cations A are located between have been deposited at the Cambridge Crystallographic Data these layers at the center of AF8 antiprisms.Centre (CCDC). See Information for Authors, J. Mater. Some studies devoted to quaternary fluorides belonging to Chem., 1998, Issue 1.Any request to the CCDC for this the scheelite family8,9 have shown that it was possible to material should quote the full literature citation and the substitute the M2+–M4+ couple for trivalent ions in LiREF4. reference number 1145/108. In view of these results, it seemed worthwhile to investigate Complementary studies of NaGdCu2F8 and NaYCu2F8 appropriate substitutions in ternary fluorides belonging to the phases were carried out by the Rietveld method.12 Powder XKBrF4- type, which could induce a cationic ordering in eight- ray diVraction (XRD) profiles were recorded on a Philips PW fold coordination.The present paper reports the structural determination of Table 1 Crystal data and experimental conditions for data collection several quaternary NaRECu2F8 fluorides.formula NaGdCu2F8 symmetry tetragonal Experimental space group I422 (no. 97) Preparation of the compounds unit-cell parameters/A° a=5.407(1) c=10.382(1) Polycrystalline samples were synthesized by solid state reac- volume/A° 3 303.5(1) tions from stoichiometric mixtures of the binary fluorides. The Z 2 starting materials were mixed under a dry argon atmosphere Dc/g cm-3 5.03 crystal size/mm 0.19×0.14×0.21 in a glove box due to oxygen and moisture sensitivity.The radiation Mo-Ka (l=0.71073 A° ), reactions were carried out in sealed platinum tubes for 15 h at graphite monochromator 550 °C. The reaction mixture was annealed several times in absorption coeYcient, m/mm-1 18.0 the same conditions with intermediate grindings in order to F(000) 410 obtain pure powders.All reactions were followed by a tempera- measuring range/degrees hmax=45 ture quenching. scan type v–2h index ranges -10h10, -10k10, 0l20 reflections collected 2578 X-Ray diVraction analysis independent reflections 636 [585 Fo2>2s(Fo2)] extinction coeYcient, e 0.06723 Single crystals were obtained by melting about 2 g of Rint 0.0826 NaGdCu2F8 at 700 °C followed by cooling to room tempera- final R indices [I>2s(I )] R1=0.0279; wR2=0.0484 ture at a rate of 3 °C h-1.goodness of fit, S 1.201 Weissenberg and precession photographs of a single crystal final Fourier residuals/eA° -3 -1.57, 1.33 showed a tetragonal symmetry belonging to the Laue class J. Mater. Chem., 1998, 8(11), 2487–2491 2487Table 2 Rietveld data and experimental conditions for data collection Table 3 Comparison of refined isotropic atomic displacements (A° 2) in NaGdCu2F8 formula NaGdCu2F8 NaYCu2F8 symmetry tetragonal tetragonal Atom Ueq (I4/mmm) Ueq (I422) twinned single crystal space group I422 (no. 97) I422 (no. 97) unit-cell parameters/A° a=5.408(1) a=5.370(1) Gd (2a) 0.0063(2) (2a) 0.0066(1) Na (2b) 0.0231(11) (2b) 0.0107(5) c=10.390(1) c=10.291(1) volume/A° 3 303.9(1) 296.8(1) Cu (4d) 0.0023(2) (4d) 0.0099(1) F (32o) 0.0163(7) (16k) 0.0161(4) Z 2 2 Dc/g cm-3 5.02 4.37 radiation Cu-Ka (l=1.5418 A° ), graphite monochromator peak shape function g=0.58 g=0.25 PV=gL+(1-g)G crystallizes in the I422 space group with unit-cell parameters FWHM function H2=Utan2h+Vtanh+W a=5.407 and c=10.382 A° .The final values of atomic coordimeasuring range/degrees 15<2h<100 16<2h<100 nates and anisotropic atomic displacements are listed in reflections collected 126 126 Table 4.Table 5 gives the main interatomic distances and parameters used 29 24 angles. The bond lengths obtained are close to the sum of the in refinement Shannon radii18 (for Na+ and Gd3+ with C.N.=8 and for cRp 0.190 0.152 cRwp 0.150 0.118 Cu2+ with C.N.=4; in each case F- is threefold coordinated RI 0.042 0.038 to cations as in CaCuF4).In addition, piezoelectric measurements were carried out in order to show the occurrence of a non-centrosymmetrical group. Confirmation of the postulated space group was not 1050 diVractometer in Bragg–Brentano geometry, using graphpossible using this technique.ite-monochromated Cu-Ka radiation. The sample was set into Finally, a powder diVraction study seemed necessary to an air-tight cell, filled in a dry atmosphere, by dusting the avoid the twinning eVect observed in the single-crystal investi- powder through a 20 mm sieve in order to minimize orientation gation. The structure of NaGdCu2F8 was refined by the eVects. Owing to the Bragg–Brentano geometry of the equip- Rietveld method.12 However, during the synthesis it was ment, the mylar windows of the cylindrical cell are crossed by impossible to avoid traces of hexagonal NaGdF4;19 this phase the X-ray beam over a quasi-constant thickness.Data were has been taken into account in the refinement on the basis of collected over 152h100°, in 0.02° steps, with integration the structural hypothesis of NaNdF4.20 One should note that times of 20 s.the RI factor relating to this impurity remains high (ca. 30%) The refinements were performed with the FULLPROF owing to its rather small quantity. Moreover the peaks of program package.13 The background level was optimized with NaGdF4 are not well separated from the main diVraction lines a polynomial function and the peak shape s fitted by a of the new compound.The I422 and I4/mmm space groups pseudo-Voigt function. The change in the peak full-width at were tested, following exactly the same procedure. The com- half-maximum (FWHM) across the diVraction pattern was parison of the reliability factors and of the isotropic thermal defined by the function determined by Caglioti et al.14 The displacements showed that the best results were obtained with reliability factors were the usual ones in Rietveld method (RI, I422 space group (Table 6).In addition, the negative value of Rp and Rwp).15 The powder data and experimental conditions Biso found for the sodium led us to assume a small exchange are given in Table 2. between Gd3+ and Na+ ions in the 2a and 2b sites respectively.As is shown in Table 6, the best refinement corresponds to an Structural determination exchange rate equal to 3.5%. This is consistent with the similarity between both these sites. Nevertheless such a result NaGdCu2F8 was not observed in the single crystal study. It could be explained considering the mode of cooling in each case : the First, the structure was solved in the I4/mmm space group, by conventional Patterson method for the gadolinium and copper single crystal was subjected to a very slow cooling which favours a complete ordering, whereas the powder was cooled atoms positions; sodium and fluorine atoms were located from a diVerence-Fourier synthesis.The results were in good agree- by a temperature quenching which could generate an exchange between two similar sites.However this exchange rate of 3.5% ment with a previous structural hypothesis concerning NaEuCu2F8.16 The refinement led to R1=0.0341 and wR2= has been tested for the single crystal, and the results of refinement are quite acceptable. Thus, the structural param- 0.0786 with anisotropic atomic displacements, but required to take into account a random distribution of fluorine atoms eters of powder XRD are in good agreement with the crystal study, and the reduced coordinates correspond as near as one around copper, the site (32o) being 50% occupied.Secondly, calculations were carried out in the other space groups; the and the half standard deviation. Moreover the best argument in favour of I422 space group best residual R values were obtained for the I422 space group if considering a twinned single crystal (R1=0.0279, wR2= was the determination of the structure factors for the (211) and (213) reticular planes.Indeed the intensity of the XRD 0.0484 with anisotropic thermal factors). The value of Flack’s index17 equal to 0.5 for the refinement in the I422 space group lines according to the Debye law [F(hkl )=A+iB]21 with hk0 and l0 diVers strongly depending on the space could reveal the occurrence of a twin in our crystal; a and baxes inversion (50%) brought the value of Flack’s index close group: the centrosymmetric space group I4/mmm leads to B= 0 whereas in the I422 space group B diVers from 0.The to zero and could explain the results obtained in the I4/mmm space group.Furthermore the refined atomic displacements U comparison of X-ray diVraction patterns for both hypotheses in Fig. 1 and the values of structure factors in Table 7 showed considering the I4/mmm space group or the hypothesis of a twinned single crystal with the I422 space group have been clearly these diVerences and a best fit in I422 space group. Finally the powder diVraction study confirms the single compared for each element (see Table 3).As far as the I4/mmm hypothesis is concerned, owing to the results found for Gd crystal structure determination and one can conclude that NaGdCu2F8 crystallizes in the I422 space group. In order to and F atoms, the Ueq value of Na atoms appeared too large whereas that of Cu atoms was too small.The refinement in verify if all the compounds of the NaRECu2F8 series exhibit exactly the same structure, another phase involving a smaller the I422 space group gives more suitable Ueq values. According to the crystallographic data, NaGdCu2F8 rare earth ion such as Y3+ was studied by powder XRD. 2488 J. Mater. Chem., 1998, 8(11), 2487–2491Table 4 Atomic coordinates and displacements (A° 2) in NaGdCu2F8 [estimated standard deviations (esds) in parentheses] Atom Site x y z U11 U22 U33 U23 U13 U12 Ueq Gd 2a 0 0 0 0.0065(1) 0.0065(1) 0.0069(2) 0 0 0 0.0066(1) Na 2b 0 0 0.5 0.0102(7) 0.0102(7) 0.0116(11) 0 0 0 0.0107(5) Cu 4d 0 0.5 0.25 0.0099(2) 0.0099(2) 0.0097(2) 0 0 0 0.0099(1) F 16k 0.1872(5) 0.3303(5) 0.3763(2) 0.0158(8) 0.0132(8) 0.0191(10) 0.0025(6) -0.0083(7) -0.0018(9) 0.0160(4) Table 5 Interatomic distances (A° ) and angles (°) in NaGdCu2F8 (esds in parentheses) (4×) 1.893(2) CuMF (4× ) 2.785(2) GdMF (8× ) 2.314(2) NaMF (8× ) 2.422(2) FMCuMF 87.81(13) FMGdMF 72.05(4) FMNaMF 70.42(8) 92.34(13) 74.24(9) 73.66(4) 175.95(13) 86.02(12) 84.93(11) 112.56(9) 115.94(8) 133.29(13) 130.61(12) 152.61(13) 153.85(13) Table 6 Isotropic thermal displacements for diVerent structural hypotheses and corresponding R factors for NaGdCu2F8 (esds multiplied by Berar’s factor in parentheses) Biso/A° 2 Space group 2a 2b 4d 32o/16k RI cRp cRwp I4/mmm 0.07(16) 0.9(9) 0.9(3) 2.4(7) 0.070 0.214 0.176 I422 without exchange 0.50(13) -1.6(5) 0.6(2) 1.2(3) 0.044 0.195 0.154 I422 with exchange 96.5%/3.5% 0.31(12) 0.5(6) 0.9(2) 1.4(3) 0.042 0.190 0.150 Table 7 Comparison of the observed and calculated structural factors in arbitrary units for the two reticular planes (211) and (213) in the case of the single crystal study F(hkl )=A+iB I422 with twinned I422 single crystal I4/mmm hkl Fo Fc Fo Fc Fo Fc 211 8691 8690 8663 8684 8046 6466 213 9127 9230 9104 9254 8581 7244 NaYCu2F8 The structure of NaYCu2F8 was refined by the Rietveld method.The previous structural results of NaGdCu2F8 were used as a starting model. The I422 and I4/mmm space groups were considered as structural hypothesis; we did not succeed in obtaining good results with the latter (cRp=0.200, cRwp=0.184, RI=0.0875). On the contrary, the refinement in the I422 space group rapidly converged to cRp=0.162, cRwp=0.127 and RI=0.050. A negative thermal displacement on the sodium site and a large one on the yttrium site led us to consider an exchange rate of 8% between Y3+ and Na+ ions in the 2a and 2b sites respectively (cRp=0.152, cRwp=0.118, RI=0.038).The refined atomic coordinates and isotropic thermal displacements are listed in Table 8. The main interatomic distances and angles are given Table 8 Atomic coordinates and isotropic thermal displacements (A° 2) in NaYCu2F8 (esds multiplied by Berar’s factor in parentheses) Occupancy Atom Site x y z Biso (%) Y 2a 0 0 0 0.6(2) 92 Na 2a 0 0 0 0.6(2) 8 Y 2b 0 0 0.5 1.1(7) 8 Fig. 1 X-Ray diVraction patterns of NaGdCu2F8: (a) observed (,), Na 2b 0 0 0.5 1.1(7) 92 calculated (——) and (b) diVerence; the tick marks labelled (c) repre- Cu 4d 0 0.5 0.25 1.0(2) 100 sent the position of the diVraction lines for NaGdF4 (top) and for F 16k 0.1897(14) 0.3332(14) 0.3775(12) 1.2(3) 100 NaGdCu2F8 (bottom).J. Mater. Chem., 1998, 8(11), 2487–2491 2489Table 9 Interatomic distances (A° ) and angles (°) in NaYCu2F8 (esds multiplied by Berar’s factor in parentheses) (4×) 1.887(10) CuMF (4× ) 2.775(9) YMF (8× ) 2.274(9) NaMF (8× ) 2.414(9) FMCuMF 88.3(8) FMYMF 72.1(4) FMNaMF 69.4(5) 91.9(6) 74.3(6) 74.2(4) 174.7(9) 85.7(6) 84.3(6) 112.6(5) 117.0(5) 133.6(7) 130.1(6) 152.2(7) 153.9(7) Table 10 Unit-cell parameters of the NaRECu2F8 series Compounds a/A° c/A° V/A° 3 NaSmCu2F8 5.426(1) 10.430(3) 307.1(2) NaEuCu2F8 5.412(2) 10.398(2) 304.5(3) NaGdCu2F8 5.407(1) 10.382(1) 303.5(1) NaErCu2F8 5.360(1) 10.270(2) 295.0(1) NaYbCu2F8 5.354(2) 10.225(2) 293.1(3) NaYCu2F8 5.370(1) 10.290(1) 296.8(3) in Table 9.The NaMF and CuMF bond lengths are of the same order of magnitude as those obtained in NaGdCu2F8. The YMF distance is shorter than GdMF for the same site 2a, in good agreement with the smaller ionic radius of Y3+.18 The NaRECu2F8 series This series has been studied for several rare earths.As far as the largest rare earth ion is concerned, only the phase containing Sm3+ has been isolated when SmF3 was stabilized in the variety which crystallizes in orthorhombic symmetry. The hexagonal form of SmF3 did not react with the other binary fluorides to give the quaternary compounds. Therefore the series exists only with rare earth fluorides which adopt the YF3-type structure (space group Pnma).22 The Lu3+ ion is expected to be the lower limit of the series.The phases containing the cations Sm3+, Eu3 +, Gd3 +, Er3+, Yb3+ and Y3+ have been studied. For each compound, the unit-cell parameters have been determined from slow powder diVraction pattern (angle step: 0.02° and counting time: 10 s) using a silicon standard, followed by least-squares refinement.All parameters are listed in Table 10. The unit-cell volume decreases gradually as a function of the rare earth size. Fig. 3 (001)-projection of NaGdCu2F8 structure: (a) Gd environment, (b) Na environment (surrounded fluorine atoms are located below the Na or RE atom plane). The respective distances are indicated Fig. 4 (001)-projection of CaCuF4 structure (surrounded fluorine Fig. 2 Perspective view of NaGdCu2F8 structure. atoms are located below the Ca atom plane). 2490 J. Mater. Chem., 1998, 8(11), 2487–2491square-planes is smaller compared to NaGdCu2F8 compound Description of the structure and discussion for instance. Thus the raising of ion exchange rate could be The structure of NaRECu2F8 can be related to that of CaCuF4 associated with the decreasing of the [CuF4]2- square-plane (space group I4/mcm)6,7 which crystallizes in the KBrF4-type.5 distortion, and also are related to the electronic configuration A perspective view is represented in Fig. 2.Projections of of the rare earth. NaGdCu2F8 and CaCuF4 on the (001) plane are shown in Fig. 3 and 4. References The CaMF bond length in CaCuF4 is 2.349 A° .7 Owing to the cationic ordering in NaGdCu2F8, two distinct GdMF and 1 D. Babel, Struct.Bonding (Berlin), 1967, 3, 32. NaMF bond distances are found at 2.314 and 2.422 A° , 2 D. Babel and A. Tressaud, in Inorganic Solid Fluorides, Chemistry and Physics, Academic Press Inc., 1985, ch. 3, Crystal Chemistry respectively. of Fluorides. In the fluorite-type structure, the Ca2+ ions are eight-fold 3 S.Siegel, Acta Crystallogr., 1956, 9, 493. coordinated to fluorine and occupy a site of cubic symmetry. 4 W. G. Sly and R. E. Marsh, Acta Crystallogr., 1957, 10, 378. In NaRECu2F8, the occurrence of Cu2+ cations in square 5 A. J. Edwards and G. R. Jones, J. Chem. Soc. A, 1969, 1936. planar environment constrains the larger cations (Na+ and 6 D. Dumora, J. Ravez and P. Hagenmuller, Bull.Soc. Chim. Fr., RE3+) to be in more or less distorted tetragonal antiprisms 1970, 4, 1301. 7 H. G. von Schnering, B. Kolloch and A. Kolodziejczyk, Angew. depending on their size and their charge. Chem., 1971, 83, 440. The four short CuMF distances (1.893 A° ) are of the same 8 A. Vedrine, L. Baraduc and J-C. Cousseins, Mater. Res. Bull., order of magnitude, but slightly longer, than those encountered 1973, 8, 581.in CaCuF4 (1.880 A° ) and SrCuF4 (1.858 A° ).7 All these struc- 9 A. Vedrine, D. Trottier, J-C. Cousseins and R. Chevalier, Mater. tures are constructed from layers of isolated [CuF4]2- units Res. Bull., 1979, 14, 583. perpendicular to the c-axis and shifted with respect to each 10 G. M. Sheldrick, SHELXL93, A Program for Refinement of Crystal Structure, University of Go� ttingen, Germany, 1993.other. In each layer a [CuF4]2- square-plane is oriented at 90° 11 International Tables for X-ray Crystallography, Kynoch Press, from each adjacent one. The Na+ and RE3+ ions are located Birmingham, 1974, vol. 4 (present distributor: Kluwer Academic between these layers. Publishers, Dordrecht). In this structure, compared to the I4/mcm space group of 12 H.M. Rietveld, J. Appl. Crystallogr., 1969, 2, 65. CaCuF4, the disappearance of the sliding plane c leads to a 13 J. Rodriguez-Carvajal, FULLPROF, ver. 3.2, jan. 1997, LLBCEA, Saclay, France. small tilting of the [CuF4]2- square-planes from the bisecting 14 G. Caglioti, A. Paoletti and F. P. Ricci, Nucl. Instrum. Methods, plane [(a+b)/2, c] in contrast with CaCuF4, inducing a 1958, 3, 223. FMCuMF angle slightly diVerent from 90° (Table 5, Fig. 3). 15 R. J. Hill and R. X. Fisher, J. Appl. Crystallogr., 1990, 23, 462. Such a distortion of the square-planes has already been 16 N. Ruchaud, Ph. D. Thesis, University of Bordeaux I, France, observed in KAuF4 5 where it could be due to the strong 1991. Jahn–Teller eVect of AuIII in low-spin state. In the case of 17 H. D. Flack, Acta Crystallogr., Sect. A, 1983, 39, 876. 18 R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751. NaRECu2F8, such a tilting must be related to the cationic 19 R. E. Thoma, H. Insley and G. M. Hebert, Inorg. Chem., 1966, ordering which takes into account the diVerent size and charge 5, 1222. of Na+ and RE3+. 20 J. H. Burns, Inorg. Chem., 1965, 4, 881. However the higher ion exchange rate equal to 8% has been 21 M. VanMeersche and J. Feneau-Dupont, Introduction a` la cristalfound surprisingly for NaYCu2F8 where the diVerence of ionic lographie et a` la Chimie Structurale, Vander Ed., 1973. radii between Na+ and Y3+ is the largest. Nevertheless one 22 O. Greis and T. Petzel, Z. Anorg. Allg. Chem., 1974, 403, 1. should notice that in this latter phase the FMCuMF angle is closer to 90° and consequently the distortion of [CuF4]2- Paper 8/03015D J. Mater. Chem., 1998, 8(11), 2487–2491 24