Synthesis, characterisation and properties of a new three-dimensional Yttrium–Europium coordination polymer

Solid State Sciences 7 (2005) 1074–1082 www.elsevier.com/locate/ssscie

Synthesis, characterisation and properties of a new three-dimensional Yttrium–Europium coordination polymer Suzy Surblé a , Christian Serre a,∗ , Franck Millange a , Fabienne Pelle b , Gérard Férey a,c a Institut Lavoisier, UMR CNRS 8637, Université de Versailles St-Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France b Laboratoire de Chimie Appliquée de l’Etat Solide de l’ENSCP, Matériaux Inorganiques, CNRS UMR 7574, 11 rue Pierre et Marie Curie,

75231 Paris cedex 05, France c Institut Universitaire de France, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France

Received 6 March 2005; accepted 25 April 2005 Available online 23 June 2005

Abstract A new three-dimensional europium-doped Yttrium(III) tricarboxylate, MIL-92LT 1 or Y1−x Eux (H2 O)2 {C6 H3 –(CO2 )3 } (x ∼ 0.03) (LT: Low Temperature) has been synthesised hydrothermally. Its three-dimensional structure, determined using X-ray powder diffraction data, is built-up from isolated eight coordinated Ln(III) monocapped square antiprisms (Ln = Y, Eu) linked through carboxylate moieties. Its thermal behaviour, investigated using TGA and X-ray thermodiffractometry indicates that dehydration is irreversible giving the solid MIL-92HT or Y1−x Eux {C6 H3 –(CO2 )3 } (x ∼ 0.03) (HT: High Temperature). It induces a change in the connection mode of the carboxylate and a decrease in the coordination number of the rare-earth leading to an unusual octahedral environment for the rare-earth cation. After a structural analysis of the reconstructive phase transition, the optical properties of these solids have been investigated and show that dehydration leads to a strong increase in the optical output. Crystal data for MIL-92LT : monoclinic space group C2/c (n◦ 15) with a = 16.428(1) Å, b = 6.071(1) Å, c = 20.404(1) Å, β = 95.31(3)◦ and Z = 2. Crystal data for MIL-92HT : monoclinic space group C2/c (n◦ 15) with a = 17.390(1) Å, b = 5.521(1) Å, c = 19.487(1) Å, β = 105.35(2)◦ and Z = 2.  2005 Elsevier SAS. All rights reserved. Keywords: Hybrids; Carboxylates; Rare-earth; Phosphors

1. Introduction The synthesis of hybrid inorganic–organic solids is still a hot topic, because of their potential applications in catalysis, sorption, optical devices or magnetism [1–5]. The use of various organic acids with complexing functions (carboxylates, phosphonates. . .) and organic linkers, with a wide range of 3d transition or p-metals leads to a large variety of open-framework solids with different pore shapes and dimensions. Beside many works on these cations, we have fo* Corresponding author. Fax: (+33)1 39 25 43 58.

E-mail addresses: [email protected] (C. Serre), [email protected] (F. Millange), [email protected] (F. Pelle), [email protected] (G. Férey). 1 MIL for Material Institut Lavoisier. 1293-2558/$ – see front matter  2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.04.015

cused our attention since 1998 on rare-earth cations because their high coordination numbers may create unusual architectures. They may also give unique properties due to their f –f electronic transitions [6]. In the domain of rare-earth hybrid materials, most of the solids discovered up to now are coordination polymers in which the metallic moieties are either single polyhedra or isolated small clusters while the organics are carboxylates, sulfonates and bipyridine [7–15]. However, our studies in hydrothermal conditions on the rare-earth carboxylates led to several open-framework solids with a 1 or 2-D inorganic sub-networks [16–26]. Moreover, the use of Yttrium cations doped with optically active lanthanides (Eu, Tb) allow the introduction of optical properties in these solids [23–26]. This paper deals with the synthesis, the structure from powder data, the thermal behaviour of a new three-dimensional Yttrium–Europium tri-

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carboxylate MIL-92LT and its dehydrated form MIL-92HT : Y1−x Eux (H2 O)y {C6 H3 –(CO2 )3 } (x ∼ 0.03; y = 2, 0). The structural changes involved by dehydration are analysed as well as their consequences in term of optical properties.

2. Results and discussion 2.1. Experimental section MIL-92LT was first prepared by mixing at room temperature Yttrium nitrate Y(NO3 )3 ·6H2 O (Aldrich, 99.9%), Europium nitrate Eu(NO3 )3 ·6H2 O (Aldrich, 99.9%), trimellitic acid C6 H3 –(CO2 H)3 (Aldrich, 99%), sodium hydroxide (Aldrich, 97%) and deionised water in the following ratios: 0.97 : 0.03 : 1 : 2 : 180. The resulting gel was aged at 220 ◦ C four days in a Teflon-lined PARR bomb and cooled down to room temperature. The pH remained acidic throughout the synthesis in both cases (∼ 2–3). The light white solid is filtered, washed and dried at room temperature. The anhydrous form MIL-92HT is obtained by calcination of MIL-92LT at 200 ◦ C in an oven under air atmosphere overnight. X-ray powder data patterns of MIL-92LT,HT were performed using a conventional high resolution (θ –2θ ) Siemens D5000 Diffractometer with λCu Kα (Fig. 1). TGA experiments (Fig. 2(a)) were performed under air atmosphere on MIL-92LT and MIL-92HT using a TAInstrument type 2050 analyser apparatus. In both cases the residue is Yttrium oxide Y2 O3 (with traces of europium oxide). MIL-92LT exhibits two weight losses of 10.4% and 54.3% at 150 ◦ C and 425 ◦ C approximately corresponding respectively (calc.: 11.4 and 54.2%) to the departure of the bound water and the organic moieties followed by their partial replacement by oxygen atoms to form dense oxide at higher temperatures. MIL-92HT exhibits only one weight loss of 61.4% at 425 ◦ C (calc.: 61.4%).

Fig. 1. X-ray diffraction patterns (λCu = 1.5406 Å) of MIL-92LT and MIL-92HT .

(a)

(b) Fig. 2. (a) Thermogravimetric analysis under O2 atmosphere of MIL-92LT and MIL-92HT ; (b): Infra-red spectra of MIL-92LT and MIL-92HT .

The density measurements, performed on MIL-92LT and MIL-92HT using a Micromeretics apparatus Accupyc 1330, gave 2.16 g·cm−3 and 2.09 g·cm−3 , respectively (calc.: 2.15 and 2.18 g·cm−3 ). The infra-red spectra (Fig. 2(b)) of MIL-92LT and MIL92HT clearly show the presence of the vibrational bands characteristic of the –(O–C–O)– groups around 1550 and 1430 cm−1 confirming the presence of the dicarboxylate within the solids. A large band around 3500 cm−1 also confirmed the presence of OH/H2 O groups in MIL-92LT ; this latter becomes very weak (very small band due to the KBr pellet) in the anhydrous form MIL-92HT . Yttrium and carbon contents were determined for MIL92LT and MIL-92HT at the CNRS Central Laboratory of Analysis of Vernaison (69, France); the Y, Eu, C contents are 24.4, 1.5, 32.4% and 26.9, 1.5, 36.1% for MIL-92LT and MIL-92HT , respectively, which is in agreement with the theoretical values (25.8, 1.4, 32.4% and 29.0, 1.5, 36.3%). The luminescence excitation was provided by a Coherent Innova 300 argon laser and the fluorescence was dispersed through a HR1000 Jobin–Yvon monochromator. The signal was detected by a RTC 56 TVP photomultiplier and amplified by a PARR 128a lock-in amplifier with data acquisition on a microcomputer.

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2.2. Structure determination The patterns of MIL-92LT and MIL-92HT were indexed using the Dicvol program [27], both in the space group C2/c (n◦ 15) (see cell parameters in Table 1). The pattern matching was performed with Fullprof2k using the WinPLOTR package [28,29]. Then, a direct method was realised in both cases using the Expo software to localise the heavy atoms (Y, Eu) and most of the oxygen and carbon atoms. A Fourier difference was performed, using the Shelxtl97 program [30], to locate the missing atoms. A polynomial function was used to adjust the background with a Pseudo-Voigt function to determine the peak profile. Two asymmetry parameters, one overall thermal parameter and a preferred orientation correction parameter were also applied during refinements. Distance and angle constraints were used during the refinement especially to refine the trimellitate anion as a rigid body. Finally, both structures were refined using Fullprof and its Winplotr package [28, 29]. Details of the structures determinations are reported in Table 1. The formula deduced from the structure determinations for MIL-92LT and MIL-92HT are LnIII (H2 O)2 {C6 H3 – (CO2 )3 } and LnIII {C6 H3 –(CO2 )3 } (Ln = Y, Eu). The final agreement factors [31] are satisfactory (see Table 1) even if their values are still relatively high for MIL-92LT due to a strong preferred orientation. The final Rietveld plots, atomic coordinates and the principal interatomic distances can be find in Appendix A.

2.2.1. Results and discussion TGA results (Fig. 2(a)) indicate that MIL-92LT contains two water molecules per Yttrium (O(7) and O(8) in Table 2a). The X-ray powder diffraction pattern of a sample of MIL-92LT calcined under air atmosphere overnight indicates that dehydration induces a structural change (Fig. 1) leading to the anhydrous form MIL-92HT . Moreover, the X-ray pattern of the latter remained unchanged after one month at room temperature and therefore demonstrates the irreversibility of the dehydration, confirmed also by the absence of the band at 3500 cm−1 in IR experiments on MIL-92HT (see Fig. 2(b)). This water departure is followed by an important contraction of the cell (MIL-92LT : V = 3 3 2026.3(4) Å ; MIL-92HT : V = 1804.2(1) Å ) but with no significant decrease of the crystallinity. Such a cell contraction was already observed for other lanthanide carboxylates upon dehydration [22,26]. At first glance, the three-dimensional structures of MIL92LT and MIL-92HT are very closely related (Fig. 3). They are built up from isolated Yttrium polyhedra, linked by the benzene tricarboxylate groups which ensure the 3D character of the two solids. Their topologies, seeming very similar, could suggest a displacive structural phase transition. It is not the case as it will be explained further in the paper. Interatomic distances are usual: Y–O within the 2.24– 2.64 Å range, as well as C–O (1.25–1.38 Å) and C–C distances (1.36–1.55 Å). Besides, the steric hindrance of the carboxylate leaves no real porosity in both cases. In MIL92LT , Yttrium (and Europium) atoms are eightfold coordi-

Table 1 Crystal data and structure refinement parameters for MIL-92LT and MIL-92HT Formula Molar mass (g·mol−1 ) System Calculated density (g·cm−3 ) Observed density (g·cm−3 ) Space group Cell parameters (a modifier)

MIL-92LT

MIL-92HT

Y0.97 Eu0.03 (H2 O)2 [(C6 H3 )–(CO2 )3 ] 334.5 monoclinic 2.15 2.16(1) C2/c (n◦ 15) a = 16.428(1) Å b = 6.071(1) Å c = 20.404(1) Å β = 95.31(3)◦

Y0.97 Eu0.03 [(C6 H3 )–(CO2 )3 ] 298.5 monoclinic 2.18 2.09(9) C2/c (n◦ 15) a = 17.390(2) Å b = 5.521(1) Å c = 19.487(2) Å β = 105.35(1)◦

3

V = 2026.3(4) Å Figures of merit M20 = 23, F20 = 48 (0.0047, 89) 1.5406, 1.5444 Radiation (λCu (Å): Kα1 , Kα2 ) Data collection (◦ ) 7–70 Number of independent reflections 617 Number of intensity-dependent parameters 55 Number of profile parameters 12 RP 16.1 22.0 RWP RB 13.3 RF 8.9



2 2 2 R1 = (|Fo | − |Fc |)/ |Fo | and wR2 = [ w(Fo − Fc ) / w(Fo2 )2 ]1/2 .

3

V = 1804.2(1) Å M20 = 13, F20 = 17 (0.007, 156) 1.5406, 1.5444 7–70 440 55 12 11 15.3 7.4 4.5

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coordination) is observed in a lanthanide or yttrium hybrid solid upon dehydration while keeping close structural relationships between the frameworks. 2.3. Structural relationships

(a)

(b) Fig. 3. View of the structure of MIL-92LT (a) and MIL-92HT (b) along the b axis. Lanthanide polyhedra, oxygen atoms and carbon atoms from the benzene ring are in light gray, gray and black. For a better understanding, the carbon atoms from the three carboxylate functions are in three different gray colours.

nated by six oxygen atoms from the carboxylates groups and two terminal water molecules. After dehydration, the coordination of Yttrium becomes a distorted octahedron with only oxygen atoms from the 1,2,4-benzene tricarboxylate moieties. One can notice, to our knowledge, this is the first time that an Yttrium (or lanthanide) carboxylate is reported with an only octahedral coordination, even if an Ytterbium carboxylate solid exhibiting several coordination modes (6, 7) was characterised before [32]. This is also the first time that such a decrease in coordination (from eight to six-

Indeed, an apparent close analogy between the two structures exists. From it, one could believe that the only difference is a slight rotation of the benzenic rings during the dehydration, which would also induce the change of the β angle of the monoclinic unit cell and therefore, just a displacive phase transition. The reality, more subtle, and can be analysed when considering, at the local level, the benzenic ring as the invariant of the structure. For an easier comparison, the same atoms will have the same labels in both structures (except O(7) and O(8) which represent the two water molecules of MIL-92LT ). In these conditions, the six carbon of the benzene ring are labelled C(1)–C(6) and the three carbons of the carboxylates functions C(7), C(8) and C(9), respectively. In both structures, the O–C(7)–O function is in the same plane as the benzene ring, whereas the other O–C(9)–O, in para-position, makes an angle of 60(3)◦ with the latter. Only the positions of the two oxygens of the O–C(8)–O moiety is affected by dehydration, far beyond the inaccuracies due to the solution obtained from powder diffraction. Therefore, for the comparison of the two structures, the local structural invariance applies not only to the benzene ring, but also to the grafted carbons of the carboxylate functions. When looking both MIL-92LT and MIL-92HT in a direction perpendicular to the benzene rings with the same orientation for C(7) and C(9) (Fig. 4), the first striking difference concerns the number of Yttrium polyhedra linked to the 1,2,4-BTC. The organic moiety is shared between five Yttrium eightfold-coordinated polyhedra in MIL-92LT and six Yttrium octahedra in MIL-92HT . The change in the connection mode of the O–C(9)–O function is responsible of this difference, O–C(8)–O and O–C(7)–O having the same bridging function in both structures. Indeed, the chelating role that O–C(9)–O had in MIL-92LT is transformed into a bridging mode in MIL-92HT . This means that, in the dehydrated form, each carboxylate bridged two isolated Y (Eu) octahedral, which explains the presence of six polyhedra around one 1,2,4-BTC. As far as the 1,2,4-BTC is considered as the local invariant, the above analysis first shows that this change of role for the O–C(9)–O function implies, during or just after the departure of the water molecules, a cooperative motion along the [010] of the rows of isolated Y polyhedra. Moreover, the comparison between Figs. 4(a) and (b) clearly evidences a spatial organization of the polyhedra around the 1,2,4-BTC. These two facts undoubtly prove that, despite the expectation of a displacive phase transition, it is a reconstructive one which really occurs during the dehydration. This topological change also appears on Fig. 5, in which the center of the benzene ring is evidenced by a pseudo-atom

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(a) (a)

(b) Fig. 4. View of the connection mode of the 1,2,4-Benzene tricarboxylate moiety of MIL-92LT (a) and MIL-92HT (b). Lanthanide polyhedra, oxygen atoms and carbon atoms from the benzene ring are in light gray, gray and black. For a better understanding, the carbon atoms from the three carboxylate functions are in three different gray colours.

for a better evaluation of the distances. This time, the modification concerns not only the local changes around one 1,2,4-BTC but also the whole structure. In MIL-92LT , the position and the volume of the water molecules (Fig. 5(a)) make the stacking of the benzene rings along [100] roughly regular (distances in the 4.8–5.1 Å and in the (100) plane, a distance of 6.07 Å). The departure of the water molecules induces a decrease in the distance between two consecutive rings in each (100) plane (5.52 Å instead of 6.07 Å) but also, a quasi dimerisation of two consecutive (100) planes with an alternation of short (4.38 Å) and long (6.20 Å) along [100]. The short distances are in a range of weak π –π interactions, and the appearance of such bonds in MIL-92HT might participate to the strong (> 10%) cell volume decrease associated with the departure of water. Such π –π interactions

(b) Fig. 5. View of the stacking of the 1,2,4-Benzene tricarboxylate group of MIL-92LT (a) and MIL-92HT (b). A pseudo-atom (in green) is represented in the center of the benzene ring for a better evaluation of distances. Lanthanide polyhedra, oxygen atoms and carbon atoms from the benzene ring are in yellow, red and black. For a better understanding, the carbon atoms from the three carboxylate functions are in cyan, blue and purple, respectively.

could also be at the origin of the irreversibility of the dehydration. 2.4. Effect of size of the rare-earth The same synthesis conditions applied to Praseodymium cations, previously led to the solids MIL-81LT,HT or Pr(H2 O)x {1,2,4-BTC} (x = 1, 0). They both exhibit threedimensional structures built up from chains of edge-sharing rare-earth polyhedra and trimellitate anions [26]. The complete study of the lanthanide/1,2,4-BTC system under hydrothermal conditions shows that the largest rare-earth

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Fig. 6. Fluorescence spectra of MIL-92LT and MIL-92HT at room temperature (excitation at 467 nm).

cations (Ce, Pr, . . . , Gd) adopt the MIL-81 structural type while the use of smaller lanthanide cations (Tb, Dy, Er) lead to solids isostructural with MIL-92. 2.4.1. Optical properties The characteristic luminescence of Eu3+ ions is observed at room temperature in MIL-92LT and MIL-92HT under blue light. The luminescence spectra recorded with an excitation at 465.8 nm provided by an Argon Laser are represented on Fig. 6. The transitions observed between 580 and 740 nm are ascribed to the 5 D0 → 7 FJ (J = 0, 1, 2, 3, 4) transitions. One emission line is observed in both cases for the non-degenerated 5 D0 → 7 F0 transition which confirms the presence of one site of symmetry (Cs, Cn or Cnv) for the Eu3+ ions within these compounds. The intensity ratio of the 5 D0 → 7 F2 to 5 D0 → 7 F1 transitions is correlated to the degree of symmetry of the europium site within the solid. The calcined form exhibits a higher 5 D0 → 7 F2 to 5 D0 → 7 F1 intensity ratio (∼ 5.8) compared with the as-synthesised form (∼ 3.9). This is in agreement with a decrease in symmetry upon calcination since a distorted octahedral environment is present in MIL-92HT . To determine if a transfer effect occurs within these two solids, an excitation spectrum has been performed at room temperature using a Xenon Lamp. The intensity of the prin-

cipal peak of the 5 D0 → 7 F2 emission line is thus plot as a function of the excitation wavelength (Fig. 7). It appears in both cases that a large band maximum of intensity is present around 300 nm. This corresponds to a maximum of UV absorption for a carboxylic acid possessing a single aromatic ring [33], and thus validates the hypothesis of a transfer effect between the carboxylate moieties and the europium polyhedra. The intensity of the red-emission of europium has also been estimated under UV-radiation (at ∼ 310 nm) on the two solids. It appears that the calcined phase exhibits a much higher optical activity compared with the as-synthesised solid (Fig. 8). This was expected since MIL-92LT is hydrated which usually quenches the luminescence quantum efficiency.

3. Conclusion Finally, we report the synthesis and structure determination from X-ray powder of a new three-dimensional Yttrium–Europium carboxylate, built up from isolated eightcoordinated rare-earth polyhedra and unsaturated organic tricarboxylates. The structure of its dehydrated form has been determined showing an irreversible change in the coor-

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Table 2a Atomic coordinates (Å2 ) for MIL-92LT Atom

Site

x/a

y/b

z/c

Y O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9)

8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f

0.119(1) 0.035(2) 0.136(1) 0.285(1) 0.388(2) 0.103(1) 0.125(2) 0.185(2) 0.036(2) −0.128(2) 0.119(2) 0.122(3) 0.134(2) 0.148(3) 0.138(3) 0.118(4) 0.129(1) 0.125(1)

0.025(1) 0.186(4) 0.307(3) 0.139(5) 0.372(5) −0.001(5) 0.338(3) −0.331(4) 0.719(4) 0.431(3) 0.335(3) 0.205(5) 0.299(3) 0.532(3) 0.662(4) 0.251(3) 0.312(5) 0.200(5)

0.188(1) 0.328(1) 0.279(1) 0.186(1) 0.210(1) 0.074(1) 0.115(1) 0.167(1) 0.177(1) 0.111(1) −0.116(1) −0.060(1) 0.003(1) 0.009(1) −0.045(1) 0.338(1) −0.186(1) 0.067(1)

Table 2b Atomic coordinates (Å2 ) for MIL-92HT Fig. 7. Excitation spectra of MIL-92LT and MIL-92HT at room temperature using a Xenon lamp (emission at 6106 and 6129 Å, respectively).

dination mode of the rare-earth and in the connection mode of the carboxylate. This implies a reconstructive process for the phase transition. In the dehydrated form, the lanthanide cations adopt an unusual octahedral environment. The optical activity of their yttrium analogue doped with europium has been investigated and shows that the dehydrated form is much more optically active than the as-synthesised solid. Other similar compounds are currently under study to enlarge the field of luminescent hybrid phases based on rare-earth metals [34]. Appendix A. Structure determination of MIL-92LT and MIL-92HT from powder data The powder diffraction patterns of MIL-92LT, HT were collected on a D5000 (θ –2θ mode) Siemens diffractometer

Atom

Site

x/a

y/b

z/c

Y O(1) O(2) O(3) O(4) O(5) O(6) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9)

8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f 8f

0.111(1) 0.143(2) 0.250(2) 0.123(2) 0.025(2) 0.089(1) 0.115(2) 0.164(5) 0.129(3) 0.112(5) 0.128(4) 0.172(4) 0.193(4) 0.190(4) 0.101(4) 0.115(4)

0.009(2) 0.356(6) 0.582(8) 0.846(7) 0.909(6) 0.717(6) 0.335(4) 0.467(2) 0.665(8) 0.675(9) 0.474(1) 0.280(1) 0.280(2) 0.461(3) 0.845(2) 0.502(1)

0.183(1) 0.254(1) 0.309(1) 0.288(2) 0.339(1) 0.587(1) 0.621(2) 0.376(3) 0.395(3) 0.463(4) 0.509(3) 0.493(3) 0.493(3) 0.312(3) 0.345(5) 0.577(3)

with λCu (Kα1 , Kα2 ) = 1.54059, 1.54439 Å. Thier patterns were indexed with the Dicvolgv program. Monoclinic solutions with adequate figures of merit were found. Systematic absences were consistent with the space group C2/c

Fig. 8. MIL-92LT and MIL-92HT observed at room temperature under a UV light (λ ∼ 310 nm).

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Table 3a Principal interatomic distances (in Angströms) for MIL-92LT Y–O(8) Y–O(2) Y–O(4) Y–O(1)

2.26(3) 2.27(2) 2.39(3) 2.46(2)

Y–O(3) Y–O(7) Y–O(6) Y–O(5)

2.47(2) 2.48(3) 2.51(2) 2.64(2)

C(7)–O(1) C(8)–O(6) C(9)–O(4)

1.38(2) 1.27(3) 1.31(2)

C(7)–O(2) C(8)–O(5) C(9)–O(3)

1.40(3) 1.33(3) 1.38(3)

C(1)–C(2) C(2)–C(3) C(3)–C(4) C(4)–C(5) C(5)–C(6)

1.48(3) 1.40(3) 1.48(2) 1.47(2) 1.49(3)

C(6)–C(1) C(7)–C(1) C(8)–C(5) C(9)–C(4)

1.52(3) 1.55(3) 1.40(3) 1.53(3)

Table 3b Principal interatomic distances (in Angströms) for MIL-92HT

(a)

Y(1)–O(1) Y(1)–O(3) Y(1)–O(5)

2.37(4) 2.24(5) 2.24(5)

Y(1)–O(2) Y(1)–O(4) Y(1)–O(6)

2.44(4) 2.27(5) 2.35(5)

O(1)–C(7) O(3)–C(8) O(5)–C(9)

1.34(4) 1.28(4) 1.28(4)

O(2)–C(7) O(4)–C(8) O(6)–C(9)

1.25(4) 1.30(4) 1.31(4)

C(1)–C(2) C(1)–C(7) C(3)–C(4) C(4)–C(9) C(6)–C(8)

1.43(6) 1.42(9) 1.45(5) 1.42(9) 1.40(5)

C(1)–C(6) C(2)–C(3) C(4)–C(5) C(5)–C(6)

1.36(4) 1.41(4) 1.47(5) 1.42(4)

or Cc (n◦ 15). To minimise the preferred orientation, the powders were mounted in a top-loaded Mac Murdie type sample-holder. The pattern matchings were performed with Fullprof2k using the WinPLOTR package. Structure determinations were performed first using the EXPO package, which combines a full pattern decomposition program EXTRA and a direct method program SIR97 optimised for powder diffraction data. Yttrium atoms and most of the oxygen atoms were found during the direct method. The missing atoms were found using successive Fourier differences using the Shelxtl97 program. Both structures were finally refined using also Fullprof. Five parameters polynomial functions were used in both cases to adjust the background with a Pseudo-Voigt function to determine the peak profile. Two asymmetry parameters, one overall thermal parameter and a preferred orientation correction parameter were also applied during refinements. The preferred orientation vectors were chosen as the direction parallel to the inorganic chains. Distance and angle constraints were used during the refinement especially to refine the mellitate anions as rigid bodies. The formula deduced from the structure determination for MIL-92LT, HT are: Y0.97 Eu0.03 (H2 O)x [(C6 H3 )–(CO2 )3 ] (x = 2, 0). The final agreement factors (see Table 1) are on the whole satisfactory even if their values are still high for MIL-92LT due probably to a strong preferred orientation. Atomic coordinates, principal interatomic distances are reported in Tables 2 and 3. Final Rietveld plots of MIL-92LT and MIL-92HT are shown in Fig. 9.

(b) Fig. 9. Final Rietveld plots of MIL-92LT (a) and MIL-92HT (b).

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