- Email: [email protected]
Temperature dependence of the magnetic susceptibility of gadolinium clusters in faujasite type zeolites
Journal of Magnetism and Magnetic Materials 6 (1977) 220-222 0 North-Holland Publishing Company
TEMPERATURE DEPENDENCE OF THE MAGNETIC SUSCEPTIBILITY OF GADOLINIUM CLUSTERS IN FAUJASITE TYPE ZEOLITES F. SCHMIDT, W. GUNSSER and W. VOLLBERG Institute of Physical Chemistry,
University of Hamburg, Laufgraben 24, D-2000 Hamburg 13, W.-Germany
Received 5 April 1977
Electron microsdopy and magnetic measurements of the reduced gadolinium zeolites show formation of gadolinium particles with extremely narrow particle size distribution and diameters less than 1.3 nm. These particles are paramagnetic even at 5 K and 1 T.
1. Introduction
growth during investigation. After ion exchange the gadoliniumiions in dehydrated Gd3+-zeolite are reduced by alkali metal vapour. This should lead to small crystalline particles of gadolinium of definite size inside the zeolite holes. Fig. 1 shows part of the structure of a faujasite type zeolite. The sodium ions are partially replaced by Gd3+ ions. The size of the supercage is approximately 1.3 nm. This is the upper limit for the volume of the gadolinium particles. The purpose of this investigation is to prove whether the small metallic particles are really crystalline, to determine their size and size distribution function, and to study the magnetic properties of small metallic particles of gadolinium.
The low Curie temperature of gadolinium predestiminates the metal as a suitable material of which to investigate the magnetic properties of small metallic particles below, around and far above the magnetic phase transition of bulk material without any particle Si,Al
l
Na(%) Na(S2) Na(s,J Na(S3) Na(S,) Na(%)
2. Experimental
methods
The gadolinium zeolites were prepared by treating the sodium form of Linde Y zeolite with an aqueous solution of GdC13 [ 11. The composition of the Na-Y starting material was NaAIOz . 1.38 SiOZ on a dry weight basis. 80% of the Na+ ions were replaced by Gd3+ ions. Examination with X-rays showed the gadolinium zeolite samples to be crystalline before and after all experiments. The gadolinium samples were dehydrated at 700 K for two days and at a vacuum of 1.3 X 10e3 Pa. After dehydration, the samples were reduced with a calculated amount of sodium or potassium. The alkali metal was placed at the bottom of a
Fig. 1. Part of the Faujasite structure showing the location of the types of cation sites. 220
221
F. Schmidt et al. / Magnetic susceptibility of Gd clusters
glass tube. The dehydrated zeolite sample was supported above it in a small glass frit. The full-length heating mantle was brought to 500,600 and 700 K respectively. After a heating period of 5 hours the tube was allowed to cool. Then part of the zeolite was brought into the cell for magnetic measurements and another part was embedded in Spurr (Vinylcyclohexendioxide). All steps of preparation were made in a nitrogen or argon atmosphere. Electron micrographs as well as microelectrondiffraction and energydispersive analysis of ultramicrotom cut smaples were taken on a high-performance scanning transition electron microscope. Magnetic susceptibility data were obtained by a Foner magnetometer at temperatures between 5K and 500 K and variable fields up to 1 T.
3. Results and discussion
A 500 000 magnification electron bright-field micrograph of a Gd ° zeolite reduced at 700 K with potassium shows small particles with mean diameter less than 2 nm. The question is whether these small spots are due to gadolinium or to any residue of potassium. To clarify this, energy-dispersive analysis of a small spot of 10 nm around such a particle has been taken. Thus we could make quite sure about the presence of small gadolinium particles. To identify the gadolinium being in the metallic state and to get some information about the degree of crystallinity, convergent-beam microelectrondiffraction of the same small spot has been taken. The diffraction pattern shows the same d-values as the bulk material but the ratio of the intensities was not comparable With that of the X-ray diffractogram of the bulk gadolinium. Because of the size of the diffraction spots the lattice constant could only be determined with an error of 5%, so that a slightly greater or smaller lattice constant than the bulk value cannot be excluded. Very interesting information was obtained from a dark-field image in the light of a gadolinium reflex using the same magnification and the same spot as above. This image gave us not only the size and size distribution of gadolinium but also the pattern of the filled-up supercages of the zeolite framework. Now we must mention some of the problems. To get complete reduction and crystallization it is necessary to use high reduction temperatures, but at high tempera-
tures, gadolinium diffuses out of the zeolite cavities formhag greater particles outside of the zeolite framework. Preparing gadolinium particles mostly inside the zeolite holes, we were only successful using potassium as a reducing agent and a reduction temperature of 600 K. Because potassium is not completely desorbed from the zeolite at this temperature (as was tested for comparison) it is necessary to prevent any excess of potassium. From electron microscopy we can conclude that it is possible to prepare crystalline particles of gadolinium of approximately 1.3 nm (supercage size) in diamter, separated approximately 2 nm from each other. Bearing in mind the results of electron microscopy we can formulate the question of what is to be expected from the magnetic measurements. The magnetic-field dependence of the magnetization of collective paramagnetic particles should be described by means of a Langevin function, if anisotropy energy and demagnetization and particle interaction can be neglected. Fig. 2 shows the conditions for the values of the volume of gadolinium particles with given parameters T and H for paramagnetic behaviour and magnetic saturation, respectively. Curve A shows that gadolinium particles of 1.3 nm should be saturated at 5 K and fields of 1 T. Fig. 3 shows that the measured magnetization curves are straight lines even at 5 K and up to 1 T. This behaviour must be due either to a lowering of the Curie temperature below 5 K, or to a temperature function
©
%.
i00
/
~ =f(H)
0,5
1.0
2.0
3.0
i
5.0
10.0
V!~ rnm7
Fig. 2. Calculated values of the argument x = pH/kT = IspV/~/kT, of the Langevinfunction for different particle volumes of gadolinium, p = collective moment, Isp = spontaneous magnetization of bulk gadolinium, v = volume.
222
ul
F. Schmidt et al. /Magnetic susceptibility o f Gd clusters
~6
o8 /
~07
EK]
61
/
/
Y
~ 05
j"
g O~ E gO3
.-
~3. ///
/I
C12
1.
oi
IO0[K]
6~
d2
o3
/
/
m
-~ 06
/
o~-o!s
_ -----------~-TK] oTB 017 018 019 I0 ITIBET]
Fig. 3. Measured magnetization curves of gadolinium particles with a mean diameter of 1.3 nm.
of the spontaneous magnetization, very different from that of the bulk material because the gadolinium is crystalline as well as paramagnetic, with a latticle constant of bulk material. Recently Darby (2) reported on exchange interactions in very small particles. Using the RKKY model and assuming that the particles are perfectly spherical, the radii of the particles are in the order of magnitude of 1 nm, surface effects can be neglected, and that the ionic spins lie on a simple cubic lattice he found the paramagnetic Curie temperature to be zero. Our experimental results are in agreement with the calculations of Darby (fig. 4). Cubic particles of gadolinium prepared by a condensation method which have a mean diameter of 20 nm are paramagnetic even at 5 K and 0.5 T, as reported by Morozov et al. (3) However looking at their experimental results, we found two slight cracks in their 1Ix curves, which seem to indicate a phase transition near the Curie temperature of the bulk material of hcp gadolinium. Their supposition that
/
/ ~
60
1~
~0
2~
360 --a~rCK~
Fig. 4. Temperature dependence of the magnetic susceptibility of gadolinium particles with a mean diameter of 1.3 nm.
small particles of hcp metals are generally instable could not be confirmed by our investigations.
Acknowledgements We thank Prof. Mix and Mrs. Manshard from the Institute fiJr Allgemeine Botanik der Universit/it Hamburg for preparing the microtom samples, and Priv. Doz. Dr. Harsdorff and Mrs. Nack from the Institut for Angewandte Physik for taking the electron micrographs.
References [1 ] A. Nicula, Trif, E., Magn. Res. and Related Phenomena, V. Hovi (ed.), XVII Congress Ampere 1973, (1974) 543. [2] M.I. Darhy, Phil. Mag. 33 (1976) 49. [3] Yu. G. Morozov, A.N. Kostygov, V.I. Petinov, P.E. Chizhov, phys. stat. sol. (a) 32 (1975) K 119.
TEMPERATURE DEPENDENCE OF THE MAGNETIC SUSCEPTIBILITY OF GADOLINIUM CLUSTERS IN FAUJASITE TYPE ZEOLITES F. SCHMIDT, W. GUNSSER and W. VOLLBERG Institute of Physical Chemistry,
University of Hamburg, Laufgraben 24, D-2000 Hamburg 13, W.-Germany
Received 5 April 1977
Electron microsdopy and magnetic measurements of the reduced gadolinium zeolites show formation of gadolinium particles with extremely narrow particle size distribution and diameters less than 1.3 nm. These particles are paramagnetic even at 5 K and 1 T.
1. Introduction
growth during investigation. After ion exchange the gadoliniumiions in dehydrated Gd3+-zeolite are reduced by alkali metal vapour. This should lead to small crystalline particles of gadolinium of definite size inside the zeolite holes. Fig. 1 shows part of the structure of a faujasite type zeolite. The sodium ions are partially replaced by Gd3+ ions. The size of the supercage is approximately 1.3 nm. This is the upper limit for the volume of the gadolinium particles. The purpose of this investigation is to prove whether the small metallic particles are really crystalline, to determine their size and size distribution function, and to study the magnetic properties of small metallic particles of gadolinium.
The low Curie temperature of gadolinium predestiminates the metal as a suitable material of which to investigate the magnetic properties of small metallic particles below, around and far above the magnetic phase transition of bulk material without any particle Si,Al
l
Na(%) Na(S2) Na(s,J Na(S3) Na(S,) Na(%)
2. Experimental
methods
The gadolinium zeolites were prepared by treating the sodium form of Linde Y zeolite with an aqueous solution of GdC13 [ 11. The composition of the Na-Y starting material was NaAIOz . 1.38 SiOZ on a dry weight basis. 80% of the Na+ ions were replaced by Gd3+ ions. Examination with X-rays showed the gadolinium zeolite samples to be crystalline before and after all experiments. The gadolinium samples were dehydrated at 700 K for two days and at a vacuum of 1.3 X 10e3 Pa. After dehydration, the samples were reduced with a calculated amount of sodium or potassium. The alkali metal was placed at the bottom of a
Fig. 1. Part of the Faujasite structure showing the location of the types of cation sites. 220
221
F. Schmidt et al. / Magnetic susceptibility of Gd clusters
glass tube. The dehydrated zeolite sample was supported above it in a small glass frit. The full-length heating mantle was brought to 500,600 and 700 K respectively. After a heating period of 5 hours the tube was allowed to cool. Then part of the zeolite was brought into the cell for magnetic measurements and another part was embedded in Spurr (Vinylcyclohexendioxide). All steps of preparation were made in a nitrogen or argon atmosphere. Electron micrographs as well as microelectrondiffraction and energydispersive analysis of ultramicrotom cut smaples were taken on a high-performance scanning transition electron microscope. Magnetic susceptibility data were obtained by a Foner magnetometer at temperatures between 5K and 500 K and variable fields up to 1 T.
3. Results and discussion
A 500 000 magnification electron bright-field micrograph of a Gd ° zeolite reduced at 700 K with potassium shows small particles with mean diameter less than 2 nm. The question is whether these small spots are due to gadolinium or to any residue of potassium. To clarify this, energy-dispersive analysis of a small spot of 10 nm around such a particle has been taken. Thus we could make quite sure about the presence of small gadolinium particles. To identify the gadolinium being in the metallic state and to get some information about the degree of crystallinity, convergent-beam microelectrondiffraction of the same small spot has been taken. The diffraction pattern shows the same d-values as the bulk material but the ratio of the intensities was not comparable With that of the X-ray diffractogram of the bulk gadolinium. Because of the size of the diffraction spots the lattice constant could only be determined with an error of 5%, so that a slightly greater or smaller lattice constant than the bulk value cannot be excluded. Very interesting information was obtained from a dark-field image in the light of a gadolinium reflex using the same magnification and the same spot as above. This image gave us not only the size and size distribution of gadolinium but also the pattern of the filled-up supercages of the zeolite framework. Now we must mention some of the problems. To get complete reduction and crystallization it is necessary to use high reduction temperatures, but at high tempera-
tures, gadolinium diffuses out of the zeolite cavities formhag greater particles outside of the zeolite framework. Preparing gadolinium particles mostly inside the zeolite holes, we were only successful using potassium as a reducing agent and a reduction temperature of 600 K. Because potassium is not completely desorbed from the zeolite at this temperature (as was tested for comparison) it is necessary to prevent any excess of potassium. From electron microscopy we can conclude that it is possible to prepare crystalline particles of gadolinium of approximately 1.3 nm (supercage size) in diamter, separated approximately 2 nm from each other. Bearing in mind the results of electron microscopy we can formulate the question of what is to be expected from the magnetic measurements. The magnetic-field dependence of the magnetization of collective paramagnetic particles should be described by means of a Langevin function, if anisotropy energy and demagnetization and particle interaction can be neglected. Fig. 2 shows the conditions for the values of the volume of gadolinium particles with given parameters T and H for paramagnetic behaviour and magnetic saturation, respectively. Curve A shows that gadolinium particles of 1.3 nm should be saturated at 5 K and fields of 1 T. Fig. 3 shows that the measured magnetization curves are straight lines even at 5 K and up to 1 T. This behaviour must be due either to a lowering of the Curie temperature below 5 K, or to a temperature function
©
%.
i00
/
~ =f(H)
0,5
1.0
2.0
3.0
i
5.0
10.0
V!~ rnm7
Fig. 2. Calculated values of the argument x = pH/kT = IspV/~/kT, of the Langevinfunction for different particle volumes of gadolinium, p = collective moment, Isp = spontaneous magnetization of bulk gadolinium, v = volume.
222
ul
F. Schmidt et al. /Magnetic susceptibility o f Gd clusters
~6
o8 /
~07
EK]
61
/
/
Y
~ 05
j"
g O~ E gO3
.-
~3. ///
/I
C12
1.
oi
IO0[K]
6~
d2
o3
/
/
m
-~ 06
/
o~-o!s
_ -----------~-TK] oTB 017 018 019 I0 ITIBET]
Fig. 3. Measured magnetization curves of gadolinium particles with a mean diameter of 1.3 nm.
of the spontaneous magnetization, very different from that of the bulk material because the gadolinium is crystalline as well as paramagnetic, with a latticle constant of bulk material. Recently Darby (2) reported on exchange interactions in very small particles. Using the RKKY model and assuming that the particles are perfectly spherical, the radii of the particles are in the order of magnitude of 1 nm, surface effects can be neglected, and that the ionic spins lie on a simple cubic lattice he found the paramagnetic Curie temperature to be zero. Our experimental results are in agreement with the calculations of Darby (fig. 4). Cubic particles of gadolinium prepared by a condensation method which have a mean diameter of 20 nm are paramagnetic even at 5 K and 0.5 T, as reported by Morozov et al. (3) However looking at their experimental results, we found two slight cracks in their 1Ix curves, which seem to indicate a phase transition near the Curie temperature of the bulk material of hcp gadolinium. Their supposition that
/
/ ~
60
1~
~0
2~
360 --a~rCK~
Fig. 4. Temperature dependence of the magnetic susceptibility of gadolinium particles with a mean diameter of 1.3 nm.
small particles of hcp metals are generally instable could not be confirmed by our investigations.
Acknowledgements We thank Prof. Mix and Mrs. Manshard from the Institute fiJr Allgemeine Botanik der Universit/it Hamburg for preparing the microtom samples, and Priv. Doz. Dr. Harsdorff and Mrs. Nack from the Institut for Angewandte Physik for taking the electron micrographs.
References [1 ] A. Nicula, Trif, E., Magn. Res. and Related Phenomena, V. Hovi (ed.), XVII Congress Ampere 1973, (1974) 543. [2] M.I. Darhy, Phil. Mag. 33 (1976) 49. [3] Yu. G. Morozov, A.N. Kostygov, V.I. Petinov, P.E. Chizhov, phys. stat. sol. (a) 32 (1975) K 119.