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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.45 n.2 Concepción jun. 2000

http://dx.doi.org/10.4067/S0366-16442000000200022 

COMUNICACION PREVIA

SYNTHESIS, CRYSTAL STRUCTURE, MAGNETIC PROPERTIES
AND THERMAL STABILITY OF GD(REO4)3(H2O)3

*Carlos Mujica1, Jaime Llanos1, Karl Peters2, Eva-Maria Peters2 and Hans Georg von Schnering2

1Departamento de Química, Universidad Católica del Norte, Casilla 1280 Antofagasta, Chile
2Max-Planck-Institut FKF, Heisenbergstr. 1 D-70506 Stuttgart, Germany
(Received: January 13,2000 - Accepted: March 13, 2000)

In memoriam of Doctor Guido S. Canessa C.

ABSTRACT

Trisaquatris(tetraoxorhenate(VII)) gadolinium(III), Gd(ReO4)3(H2O) 3, was prepared by the reaction of Gd2O3 with HReO4 at room temperature. The colorless compound crystallizes in the orthorhombic space group Pna21 (No. 33) with four formula units per unit cell (a = 1470.9 pm , b = 843.6 pm, c = 1146.1 pm ). The main feature of the crystal structure is the formation of a non-centric 3D network. This arrangement shows distorted tricapped trigonal prisms of Gd(H2O)3(ReO4) 6/2 interconected via the ReO4- ligands. The compound is paramagnetic and follows the Curie-Weiss law with a magnetic moment of 7.6 BM. It decomposes above 1170 K with evolution of volatile rhenium oxides and formation of Gd3ReO8.

KeyWords: Rhenium, gadolinium, oxide, crystal structure, magnetic properties.

RESUMEN

Se preparó Trisaquatris(tetraoxorhenate(VII)) gadolinium(III), Gd(ReO4)3(H2O) 3 a temperatura ambiente por reacción de Gd2O3 con HReO4. El compuesto incoloro cristaliza en el grupo espacial Pna21 (No. 33) con cuatro unidades de fórmula por celda (a = 1470.9 pm , b = 843.6 pm , c = 1146.1 pm ). La principal característica de la estructura es la formación de una red 3D acentrosimétrica. Este ordenamiento presenta los prismas trigonales tricofiados distorsionados, Gd(H2O)3(ReO4) 6/2, interconectados a través de los ligantes ReO4-. El compuesto es paramagnético y sigue la ley de Curie-Weiss con un momento magnético de 7.6 MB. Descompone sobre 1170 K con evolución de óxidos de renio volatiles y formación de Gd3ReO8.

PALABRAS CLAVES: Renio, gadolinio, óxido, estructura cristalina, propiedades magnéticas.

INTRODUCTION

In previous papers we have reported the syntheses and crystal structures of tetraoxorhenates(VII) of some divalent cations of first row transition metals as well as of lanthanum and europium [1-7]. In all these structures the coordination sphere of the central ion depends mainly on the size of the cation, being the larger La(III) and Eu(III) ions ninefold coordinated by three H2O molecules and six ReO4- ligands. As a part of the studies in the search of precursors to complex rare earth-rhenium oxides, we report here the synthesis, the crystal structure, magnetic properties and thermal stability of the title compound.

Experimental:

Synthesis: Gd2O3 was added stepwise to a 30wt% solution of HReO4, until a small portion remained undissolved. Unreacted Gd2O3 was filtered and washed with water. Colorless crystals of Gd(ReO4)3(H2O) 3 were grown by slow concentration of the mother solution. A well-shaped colorless prismatic crystal was chosen for X-ray single crystal investigation. The rest of the solution was concentrated until almost dryness. The crystal bulk was filtered, and dried under vacuum.

Structure determination: A crystal of Gd(ReO4)3(H2O) 3 was fixed into a glass capillary and mounted on a Siemens P4 four circle diffractometer. The unit cell was obtained by least-squares analysis of the setting of 25 reflections automatically centered in the range 10.6 < 2 q < 22.6. Intensity data were collected by the w-scan technique. The structure was solved by direct methods in the space group Pna21 and subsequently refined by least-squares methods using the SHELXTL PLUS program system [8]. Some crystallographic details are given in Table I. The final parameters are listed in Table II. Selected bond lengths are listed in Table 3.

Table I Crystallographic data and details of the structure analysis.


  Formula; molar mass Gd(ReO4)3(H2O) 3; 961.91 amu
  Crystal Transparent colorless prism;
    (0.1x0.2x0.25 mm)
  Space group; formula units Pna21 (No. 33); Z = 4
  Unit cell (294 K) a = 1470.9(3) pm
    b = 843.6 (2) pm
    c = 1146.1(2) pm
  Volume; density 1422.1 (4) Å3; 4.448 g cm-3
  Data collection Siemens-P4 four-circle diffractometer
    MoKa radiation (l = 0,71073 Å); graphite
    monochromator, w-scan mode, 2q < 60º
  Structure refinement SHELXTL PLUS [8], direct methods,
    empirical absorption correction (y-scan method),
    m = 30.50 mm-1
  Measured/unique reflections 2485 / 1869
  Observed reflections (Fo>3.0 s(Fo)) 1793
  Weighting s-2
  R; Rw 0.061 ; 0.074

Table II. Positional and equivalent displacement parameters Ueq (in pm2). All atoms are at the Wyckoff position 4a. The oxygen atoms were refined isotropically. Standard deviations are given in parentheses.


Atom X Y Z Ueq/Uiso

Re1 0.6798(1) 0.1017(1) 0.6759(2) 220(4)
Re2 0.6726(1) 0.6018(1) 0.5862(1) 223(4)
Re3 0.4156(1) 0.1943(1) 0.4872(1) 211(3)
Eu 0.6665(1) 0.2698(1) 0.3451(2) 138(3)
O1 0.695(2) 0.134(3) 0.538(3) 438(65)
O2 0.706(2) 0.461(3) 0.499(3) 376(53)
O3 0.523(2) 0.268(3) 0.444(2) 337(52)
O4 0.571(2) 0.247(3) 0.182(3) 363(58)
O5 0.578(2) 0.517(3) 0.328(2) 283(49)
O6 0.626(2) 0.990(3) 0.344(3) 555(77)
O11 0.572(3) 0.052(5) 0.723(3) 650(95)
O12 0.758(1) 0.958(2) 0.722(2) 218(40)
O13 0.702(2) 0.276(3) 0.753(2) 325(50)
O21 0.579(2) 0.540(5) 0.662(4) 594(87)
O22 0.637(2) 0.772(5) 0.540(4) 763(114)
O23 0.755(2) 0.634(3) 0.700(2) 422(63)
O31 0.411(2) 0.989(4) 0.504(4) 627(91)
O32 0.389(3) 0.282(4) 0.631(3) 555(80)
O33 0.335(2) 0.248(3) 0.394(3) 320(57)

Table III. Bond lengths (in pm). The distance from Re to the ligator oxygen atoms are marked (*). Standard deviations are given in parentheses.


Bond lengths              
                 
Re(1) - O(1)* 162 (3) Re(2) - O(2)* 163 (3) Re(3) - O(3)* 177 (3)
  - O(11) 172 (4)   - O(21) 172 (4)   - O(31)* 175 (4)
  - O(12)* 177 (2)   - O(22) 162 (4)   - O(32) 184 (4)
  - O(13) 175 (3)   - O(23)* 180 (3)   - O(33)* 165 (3)
                 

Gd

- O(1) 253 (3) Gd - O(4) 235 (3) Gd - O(12) 237 (2)
  - O(2) 247 (3)   - O(5) 246 (2)   - O(23) 232 (3)
  - O(3) 240 (3)   - O(6) 243 (3)   - O(33) 255 (3)

Results and Discussion

Crystal structure. Gd(ReO4)3(H2O) 3 is isotypic with Eu(ReO4)3(H2O) 3 [ 7 ]. The compound crystallizes non-centric in the space group Pna21, with four formula units in the unit cell. The gadolinium ions are ninefold coordinated in a distorted tricapped trigonal prism of global formula Gd(H2O)3(ReO4) 6/2. The three crystallographic independent ReO4 tetrahedra of the new compound act as bidentate ligands. The ReO4- anions of Re1 and Re2 connect the gadolinium ions along [011] and [011] and the ReO4- tetrahedra of Re3 connect them along [001] (Fig. 1).

Fig.1 Stereo view along [100] of the polyhedral network of Gd(ReO4)3(H2O)3.

The ReO4- tetrahedra are only slightly distorted with the mean bond length d(Re-O) = 172 pm. The Re-O distances to the ligator oxygen atoms do not exhibit any elongation indicating ionic interactions between the ReO4- ligand and the central Gd cation. The bond lengths Gd-OReO3 range from 240(3) pm to 253(3) pm with the mean value d(Gd-O) = 247 pm.

Magnetic behavior of Gd(ReO4)3(H2O) 3. Magnetic measurements were carried out in the temperature range of 5 - 300 K. The data were corrected for the diamagnetic contribution as calculated from Selwood data [9]. The temperature dependence of the reciprocal magnetic susceptibility is shown in Fig 2.

Fig.2 Reciprocal magnetic
susceptibility vs. temperature.

The effective magnetic moment of the Gd3+ ion and the Curie-Weiss temperature were determined from the linear c-1(T) relation: meff = 7.6 MB , q = 0.33 K. The meff lies very close to the calculated value of about 7.94 MB, for the free Gd3+ ion. This fact as well as the small value of the Weiss temperature confirms the lack of any antiferromagnetic interaction between the gadolinium ions, and is consistent with the mainly ionic interaction between gadolinium ions and ReO4- ligands.

Thermal decomposition and synthesis of Gd3ReO8: Like the europium compound, the new Gd(ReO4)3(H2O) 3 loses the coordination water below 450 K and decomposes above 1170 K in volatile rhenium oxides and Gd3ReO8. The latter was obtained as about 0.3 mm long orange needles, and the corresponding structure was determined by means of single crystal X-ray diffraction [10].

ACKNOWLEDGEMENTS

This work was partially supported by Fondecyt (Contract # 1980882)

REFERENCES

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