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Preparation and structural properties of amorphous and polycrystalline Gd3CrxGa5−xO12 (x=0, 1, 2)
SOLID STATE
Solid State Ionics 63-65 (1993) 666-671 North-Holland
IONICS
Preparation and structural properties of amorphous and polycrystalline Gd3CrxGas_xO12 (x= 0, 1, 2) W. Gunsser, W.J. G i r n u s a n d K. M a t z e n Institute for Physical Chemistry, University of Hamburg, BundesstraJ3e45, D-2000 Hamburg, Germany
The preparation of amorphous mixed oxides with rare earth garnet stoichiometry by co-precipitation of the hydroxides is described. By means of DTA measurements, X-ray diffraction and absorption as well as by electron microscopy changes in structure and physical properties during the annealing process in a temperature range up to 1400°C are controlled.
I. Introduction The preparation of non-crystalline rare earth transition metal mixed oxides with perowskite and garnet stoichiometry has been studied in the last years by one of us as well as by other authors [ 1-4 ]. Several methods of characterizing the reaction products are used, predominantly X-ray diffraction. Some years ago [ 5 ] we used EPR technique for studying the garnet formation. Concerning the system G d / C r / G a / O the different preparation methods ("ceramic", thermal decomposition of nitrates and co-precipitation) were examined. By co-precipitation of rare earth and transition metal hydroxides and further annealing in a DTAfurnace X-ray amorphous precursors could be received. Atomic positions in an amorphous solid which determine, for example, magnetic properties of these compounds in a very sensible way, are far from being completely random in the sense of an ideal gas. Therefore the starting oxyhydroxide material as well as the final product and especially the transition from one to the other (the crystallization process) were studied with several methods. In order to investigate the short range order of our samples X-ray absorption technique was used. The annealing process results in a change of structural parameters as presented here. From X-ray diffraction patterns the development of the crystalline phase was controlled. From DTA and SEM investigations
we hoped to get more detailed information about the structural changes at the phase transition to G d / C r / Ga-garnet. Our DTA measurements regarding the crystallization process of thulium-iron garnets [7,8 ] show two exothermic peaks characterizing the transition to Tm3FesO]2. A structural interpretation of the exothermic peak at a temperature o f 50 K below the transition point failed. Magnetic investigations of amorphous rare earth garnets prepared with different methods were done by some of us [ 6 ] and several other authors [ 9 ]. In this study we only report our results from the crystalline products with different contents of chromium. The crystalline c o m p o u n d Gd3GasO]2 belongs to the space group Ia 3d (O~ °) [10]. Cation arrangement is determined by three coordination types with oxygen: tetrahedra, octahedra and dodecahedra. The magnetic structure is crucial, influenced by near neighbour and next near neighbour exchange interactions and therefore by the "degree of amorphicity" o f the noncrystalline substances. Magnetic properties are significantly determined by the local cation environments and therefore by the crystallization progress. Some of our results o f magnetic and M6ssbauer spectroscopy investigations on amorphous samples with garnet stoichiometry were presented at the Symposium [ 11 ].
0167-2738/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
667
W.. Gunsser et al. /Amorphous and polycrystalline Gd~CrxGas_~Ole
2. Experimental With the "ceramic method" and with the thermal decomposition of the nitrates we were not able to get X-ray amorphous materials. We found that in case of preparation of amorphous materials with Gd3CrxGas_xO~2 stoichiometry co-precipitation of the hydroxides is the only successful method. Because of desired purity and stoichiometry, organic precipitators had to be excluded. Precipitation in an ultra sonic field for preventing inhomogeneous nucleation, used for other systems, was not possible in this case either. Therefore co-precipitation of the oxyhydroxides with 0.1 M NaHCO3 together with the so-called glass ball method (see fig. 1 ) for suppressing nucleation was used. The mixed oxyhydroxides were washed carefully with water, dried for 10 h at 60 °C and grain-milled to give the precursors for all the samples examined. IR spectroscopy was used to ensure the absence of a residual solvent. XRD patterns are performed from samples annealed at different temperatures between 200 and 1400°C. Our EXAFS and XANES measurements were carried out at the EXAFS II beamline of the Hamburger Synchrotron Strahlungslabor HASYLAB. The powder samples (It. d ~ 2 ) have been milled together with granulated polyethylene. Ionization chambers were used to measure the beam intensity and a fit program to analyze the patterns. Measurements were carried out at 10 K in order to minimize the thermal Debye-Waller factor. Magnetic susceptibility and magnetization measurements were performed by a variable temperature
SQUID susceptometer (Biomagnetic Technology Inc. ) at temperatures between 2 and 400 K and magnetic fields up to 5 T, using the peak- to peak-mode. SEM and EDAX experiments were carried on with electron microscope SEM 515 (Philips) and DTA measurements with the equipment L 62 (Linseis).
3. Results and discussion 3.1. X R D
X-ray diffraction as a function of annealing temperature shows that all samples are X-ray amorphous up to about 700°C. In the case of G d / G a / O system no intermediate compound could be found. Chromium-containing samples in contrast, showed t~d rel. units
600°C solution of metal nitrates
\
I /
precipitant 400°C
~ .
" ~
reactionvessel with glass balls ~
L~~/
]~/{
membranefilter (pore width: 0.24tm)
Fig. 1. Equipment for hydroxide preparation.
5.9
~
, 6.0 eoergy [keV]
611
6.2
Fig. 2. Temperature development of Cr-edge XANES spectra for 3 Gd/Cr/GaJxO.
W. Gunsser et al. /Amorphous and polycrystalline Gd3CrxGas_x012
668
in the higher temperature region, some additional lines which could be associated by ASTM. The chromium-free compound shows an inset of crystallization at about 800°C, whereas the G d / C r / G a / O systems show garnet reflections together with an orthorhombic phase in this temperature region. The intermediate compound could be identified as Gd3GaO6 besides an only chromium containing phase (CrO3). This result is confirmed by our X-ray distribution images. 3.2. X A N E S
The temperature development of Cr-K-edge XANES spectra for the chromium compounds between 200 and 1400°C is shown in figs. 2 and 3.
Gd3CrGa40~2 stoichiometry sample shows a pre-peak up to an annealing temperature of 1200 ° C, while the compound ending at polycrystalline Gd3Cr2Ga30~2 has a Cr pre-peak still at 1300°C. From our Cr-XANES spectra we derive a change from a symmetric (octahedral) surrounding of the chromium ion to an asymmetric (tetrahedral) and further to a symmetric octahedral polyheder. A detailed analysis could show that this transition starts already at 200°C and is not completed at the higher temperatures. In fig. 4 the percentage fraction of chromium tetrahedra in dependence on annealing temperature derived from XANES intensity measurements is demonstrated. 3.3. DTA
DTA diagrams for continuously annealed 3 G d / Cr/4 G a / x O and 3 G d / 2 Cr/3 Ga/xO-samples are shown in figs. 5 and 6. In the lower temperature region the calcination process is determined by an endothermic decomposition of the hydroxides which is
rel. units
II/ICll~,
,
[rcrtmit~]
:
3oo
1200°C
t
t
...............
t
,
T
20o
1100°C
J lOO
IO00*C 200
400
rio0
900°C
800
I000
1200
1400
I¢111p¢l;ihH {" [ " c ]
Fig. 4. Relative intensity of the pre-edge peak of Cr-K edge of 3 Gd/2 Cr/3 Ga/xO.
B
A
5:9
6:1
6.o
6.2
erllergy [keV] Fig. 3. Temperaturedevelopment of Cr-edge XANES spectra for 3 Gd/2 Cr/3 Ga/xO.
.
.
.
.
.
.
.
.
200
r
400
. . . .
i
600
. . . .
i
800
. . . .
i
,
,
,
1000 temperature r°c]
Fig. 5. DTA diagram for the 3 G d / C r / 4 G a / x O system.
IV. Gunsseret aL /Amorphous and polycrystalline Gd3CrxGas_x012
i
imarph
_
669
I
A
,_J .
.
.
.
i
200
.
.
.
.
I
.
.
400
.
.
r
•
•
•
600
i
800
i
.
.
1000
temperature [°c] Fig. 6. DTA diagram for the 3 Gd/2 Cr/3 Ga/xO system.
Table 1 Exothermic peak positions from the DTA diagrams. Temperature ( ° C)
Peak A Peak BI Peak Bn
x=0
x=l
x=2
200-300 688 -
150-250 668 698
100-300 679 709
a c c o m p a n i e d by a weight decrease. F o r the chrom i u m free c o m p o u n d from D T A m e a s u r e m e n t s a crystallization t e m p e r a t u r e o f 690 ° C is derived, but a pure phase is to be found at reaction time used not before 1100 ° C. C h r o m i u m - c o n t a i n i n g samples show exothermic peaks at 668°C (for x = l ) and 679°C (for x = 2 ) with a shoulder which is situated at a t e m p e r a t u r e o f about 30 K higher. The peak positions o f the 2 or 3 D T A peaks for the three c o m p o u n d s are listed in table 1. After an annealing time o f 24 h in case o f the G d / C r / G a / O system only at t e m p e r a t u r e s higher than 1400°C pure polycrystalline garnets could be identified. 3.4. S E M
The existence o f i n t e r m e d i a t e phases in case o f the c h r o m i u m - c o n t a i n i n g c o m p o u n d s could be confirmed by Scanning Electron Microscopy. Fig. 7 shows, as an example, SEM images for sample 3 G d / 2 C r / 3 G a / x O annealed at 700, 800 a n d 1400°C. By
Fig. 7. SEM images as an example for the temperature development: (a) 700°C, (b) 800°C, (c) 1400°C for 3 Gd/2 Cr/3 Ga/ xO system
X-ray contribution images we were able to ensure that a homogeneous distribution o f Gd, G a a n d Cr can be taken into account. Gd3GaO6 as an i n t e r m e d i a t e product is forming needles with a length o f approximately 15 p.m, shown in fig. 7b. Besides, there exist spheres with a d i a m e t e r o f about 0.1 mm. We assume that these are formed from c h r o m i u m ( V I )
IV.. Gunsser et aL /Amorphous and polycrystalline Gd3CrxGas_x012
670
Table 2 Magnetic data for the polyerystallinecompounds. Calculated x=0
Experimental
C = 2 . 9 7 2 × 10 -4
1 0 - 4 m 3 Kmol -~ ncxp= 22.35/za O=0.2 K_+ 10 K C=2.887× 10 - 4 m 3 K molnexp=27.12 ,uB 0=0.6 K+ 10 K C= 3.079 X 10-4 m3 K tool- i
m3K mol-~
C=2.615×
near=23.82/zB x=l
C= 3.207 X 10-4 m3K mol- i neff=27.69 #e
x=2
C= 3.427 × 10-4 m 3 K mo1-4 neff=31.56gB
n~xp= 31.30/.tB
O=0.4 K_+ 10 K
xTE10-4m3K/moll ~[
"
"1
. . . .
i
. . . .
i
. . . .
i
. . . .
~
. . . .
3.5- /
i
. . . .
i
. . . .
Gd3CrxGa~Ot2
2.5 30
iii2-%
5o
lOO
12o
2oo
220
300
320
T [K~]
Fig. 8. Magnetic susceptibilityX'temperature T versus T.
oxide which is melted during the annealing and solidified during the cooling processes. 3.5. M a g n e t i s m
Concerning the magnetic properties of the three c o m p o u n d s investigated, this paper deals only with their reaction products. Calculated a n d experimental data are compared in table 2. At temperatures lower than approximately 50 K, there begins an ordering process. With the c h r o m i u m - c o n t a i n i n g samples the ferrimagnetic ordering can be concluded. The Curie temperature for the x = 1 sample exists at about 20 K whereas for the x = 2 sample a transition temperature of about 12 K is to be concluded. At temperatures lower than I 0 K the magnetic behaviour is more complicated (fig. 8). This will be studied in a further investigation.
4. C o n c l u s i o n The formation process of Gd3CrxGas_xO12 is completed after an annealing process of hydroxides (24 h) for the chromium-free c o m p o u n d at about 1200°C, for the x = l and x = 2 systems at about 1400°C. In the system m e n t i o n e d first (3 G d / 5 G a / x O ) no intermediate c o m p o u n d exists. But in the case of Gd3CrGa4012 and Gd3CrEGa3012 stoichiometry at 7 0 0 - 8 0 0 ° C Gd3GaO6 (orthorhombic structure, needles) and c h r o m i u m (VI)-oxide could be identified (table 3). We assume that the formation of Gd3GaO6 is caused by a segregative (precipitation) of chromium. Because of the loss of Cr the forming of garnets is being developed not from a composition-like garnet stoichiometry but from a deficiency of chromium. Magnetic properties of mixed polycrystalline oxides in the temperature region lower than 50 K are
W. Gunsser et al. /Amorphous and polycrystalline Gd3CrxGas_x012
671
Table 3 Reaction scheme of garnet forming in G d / C r / G a / O system. ~680°C
NaHCO3
Ga 3+ Gd 3+
, Ga(OH)3 NaHCO3
homogeneous phase
' Gd(OH)3
NaHCO3
Cr s+
, Cr(OH)3 NaHCO3
Ga 3÷
, Ga(OH)3 NaHCO3
Gd 3+
, Gd(OH)3
(Ga 3+/Gd 3+ ) amorphous
Gd3GasOl2
crystalline Cr (VI)-oxide Gd3CrGa4Oj2 Gd3GaO6 homogeneous phase --, rsp. Ga3+/Gd3+/Cr3+/Cr6+ ) Gd3CrGa4Ot2 rsp. GdaCr2Ga3Ot2 GdaCr2Ga3Ol2
d o m i n a t e d b y a n t i f e r r o m a g n e t i c c o u p l i n g effects. T h e b e h a v i o u r o f c h r o m i u m c o n t a i n i n g s u b s t a n c e s at T < 20 K is v e r y i n t e r e s t i n g b e c a u s e o f t h e c o n c u r rence of different exchange interactions.
Acknowledgement This work was supported by the Bundesministerium rdr Forschung und Technologie (BMFT), Proj e c t no. 0 3 - G U 3 - H A M .
References [ 1 ] V.P. Chalyi, E.N. Lukachina and L.M. Simonovich,Izv. Akad. Nauk. SSSR Neorg. Mater. 12 (1976) 607. [ 2 ] W. Girnus, H. Beuthien, R. Priess and W. Gunsser, J. Magn. Magn. Mater. 54-57 (1986) 225.
[ 3 ] H. Beuthien, A. Torkler, W. Gunsser and W. Niemann, Ber. Bunsenges. Physik. Chem. 91 (1987) 1296. [ 4 ] A. Torkler, H. Niemann, W. Gunsser and W. Niemann, Solid State Ionics 32/33 (1989) 278. [ 5 ] W. Gunsser and U. Wolfmeier, Proc. 8th Intern. Symp. Reactivity of Solids (Elsevier, Amsterdam, 1976 ) p. 587. [ 6 ] W. Girnus, H. Beuthien, R. Priess and W. Gunsser, J. Magn. Magn. Mater. 54-57 (1986) 225. [ 7 ] H. Beuthien, A. Torkler, W. Gunsser and W. Niemann, Ber. Bunsenges. Physik. Chem. 91 (1987) 1296. [8] H. Beuthien, D. v. Ahlften, W. Girnus, A. Torkler, W. Gunsser and W. Niemann, J. Phys. (Paris) C8, 47 (1986) 733. [9] N. Schultes, H. Schieder, F.J. Litterst and G.W. Kalvius,J. Mag. Magn. Mater. 91 ( 1983 ) 1507. [ 10] E.L. Dukhovskaya, Y.G. Saksonov and A.G. Titova, Inorg. Mat. USSR 9 (1973) 724, 1074. [ 11 ] W.J.J. Girnus and W. Gunsser, paper presented at the Conf. on Reactivity of Solids, Madrid, 1992.
Solid State Ionics 63-65 (1993) 666-671 North-Holland
IONICS
Preparation and structural properties of amorphous and polycrystalline Gd3CrxGas_xO12 (x= 0, 1, 2) W. Gunsser, W.J. G i r n u s a n d K. M a t z e n Institute for Physical Chemistry, University of Hamburg, BundesstraJ3e45, D-2000 Hamburg, Germany
The preparation of amorphous mixed oxides with rare earth garnet stoichiometry by co-precipitation of the hydroxides is described. By means of DTA measurements, X-ray diffraction and absorption as well as by electron microscopy changes in structure and physical properties during the annealing process in a temperature range up to 1400°C are controlled.
I. Introduction The preparation of non-crystalline rare earth transition metal mixed oxides with perowskite and garnet stoichiometry has been studied in the last years by one of us as well as by other authors [ 1-4 ]. Several methods of characterizing the reaction products are used, predominantly X-ray diffraction. Some years ago [ 5 ] we used EPR technique for studying the garnet formation. Concerning the system G d / C r / G a / O the different preparation methods ("ceramic", thermal decomposition of nitrates and co-precipitation) were examined. By co-precipitation of rare earth and transition metal hydroxides and further annealing in a DTAfurnace X-ray amorphous precursors could be received. Atomic positions in an amorphous solid which determine, for example, magnetic properties of these compounds in a very sensible way, are far from being completely random in the sense of an ideal gas. Therefore the starting oxyhydroxide material as well as the final product and especially the transition from one to the other (the crystallization process) were studied with several methods. In order to investigate the short range order of our samples X-ray absorption technique was used. The annealing process results in a change of structural parameters as presented here. From X-ray diffraction patterns the development of the crystalline phase was controlled. From DTA and SEM investigations
we hoped to get more detailed information about the structural changes at the phase transition to G d / C r / Ga-garnet. Our DTA measurements regarding the crystallization process of thulium-iron garnets [7,8 ] show two exothermic peaks characterizing the transition to Tm3FesO]2. A structural interpretation of the exothermic peak at a temperature o f 50 K below the transition point failed. Magnetic investigations of amorphous rare earth garnets prepared with different methods were done by some of us [ 6 ] and several other authors [ 9 ]. In this study we only report our results from the crystalline products with different contents of chromium. The crystalline c o m p o u n d Gd3GasO]2 belongs to the space group Ia 3d (O~ °) [10]. Cation arrangement is determined by three coordination types with oxygen: tetrahedra, octahedra and dodecahedra. The magnetic structure is crucial, influenced by near neighbour and next near neighbour exchange interactions and therefore by the "degree of amorphicity" o f the noncrystalline substances. Magnetic properties are significantly determined by the local cation environments and therefore by the crystallization progress. Some of our results o f magnetic and M6ssbauer spectroscopy investigations on amorphous samples with garnet stoichiometry were presented at the Symposium [ 11 ].
0167-2738/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
667
W.. Gunsser et al. /Amorphous and polycrystalline Gd~CrxGas_~Ole
2. Experimental With the "ceramic method" and with the thermal decomposition of the nitrates we were not able to get X-ray amorphous materials. We found that in case of preparation of amorphous materials with Gd3CrxGas_xO~2 stoichiometry co-precipitation of the hydroxides is the only successful method. Because of desired purity and stoichiometry, organic precipitators had to be excluded. Precipitation in an ultra sonic field for preventing inhomogeneous nucleation, used for other systems, was not possible in this case either. Therefore co-precipitation of the oxyhydroxides with 0.1 M NaHCO3 together with the so-called glass ball method (see fig. 1 ) for suppressing nucleation was used. The mixed oxyhydroxides were washed carefully with water, dried for 10 h at 60 °C and grain-milled to give the precursors for all the samples examined. IR spectroscopy was used to ensure the absence of a residual solvent. XRD patterns are performed from samples annealed at different temperatures between 200 and 1400°C. Our EXAFS and XANES measurements were carried out at the EXAFS II beamline of the Hamburger Synchrotron Strahlungslabor HASYLAB. The powder samples (It. d ~ 2 ) have been milled together with granulated polyethylene. Ionization chambers were used to measure the beam intensity and a fit program to analyze the patterns. Measurements were carried out at 10 K in order to minimize the thermal Debye-Waller factor. Magnetic susceptibility and magnetization measurements were performed by a variable temperature
SQUID susceptometer (Biomagnetic Technology Inc. ) at temperatures between 2 and 400 K and magnetic fields up to 5 T, using the peak- to peak-mode. SEM and EDAX experiments were carried on with electron microscope SEM 515 (Philips) and DTA measurements with the equipment L 62 (Linseis).
3. Results and discussion 3.1. X R D
X-ray diffraction as a function of annealing temperature shows that all samples are X-ray amorphous up to about 700°C. In the case of G d / G a / O system no intermediate compound could be found. Chromium-containing samples in contrast, showed t~d rel. units
600°C solution of metal nitrates
\
I /
precipitant 400°C
~ .
" ~
reactionvessel with glass balls ~
L~~/
]~/{
membranefilter (pore width: 0.24tm)
Fig. 1. Equipment for hydroxide preparation.
5.9
~
, 6.0 eoergy [keV]
611
6.2
Fig. 2. Temperature development of Cr-edge XANES spectra for 3 Gd/Cr/GaJxO.
W. Gunsser et al. /Amorphous and polycrystalline Gd3CrxGas_x012
668
in the higher temperature region, some additional lines which could be associated by ASTM. The chromium-free compound shows an inset of crystallization at about 800°C, whereas the G d / C r / G a / O systems show garnet reflections together with an orthorhombic phase in this temperature region. The intermediate compound could be identified as Gd3GaO6 besides an only chromium containing phase (CrO3). This result is confirmed by our X-ray distribution images. 3.2. X A N E S
The temperature development of Cr-K-edge XANES spectra for the chromium compounds between 200 and 1400°C is shown in figs. 2 and 3.
Gd3CrGa40~2 stoichiometry sample shows a pre-peak up to an annealing temperature of 1200 ° C, while the compound ending at polycrystalline Gd3Cr2Ga30~2 has a Cr pre-peak still at 1300°C. From our Cr-XANES spectra we derive a change from a symmetric (octahedral) surrounding of the chromium ion to an asymmetric (tetrahedral) and further to a symmetric octahedral polyheder. A detailed analysis could show that this transition starts already at 200°C and is not completed at the higher temperatures. In fig. 4 the percentage fraction of chromium tetrahedra in dependence on annealing temperature derived from XANES intensity measurements is demonstrated. 3.3. DTA
DTA diagrams for continuously annealed 3 G d / Cr/4 G a / x O and 3 G d / 2 Cr/3 Ga/xO-samples are shown in figs. 5 and 6. In the lower temperature region the calcination process is determined by an endothermic decomposition of the hydroxides which is
rel. units
II/ICll~,
,
[rcrtmit~]
:
3oo
1200°C
t
t
...............
t
,
T
20o
1100°C
J lOO
IO00*C 200
400
rio0
900°C
800
I000
1200
1400
I¢111p¢l;ihH {" [ " c ]
Fig. 4. Relative intensity of the pre-edge peak of Cr-K edge of 3 Gd/2 Cr/3 Ga/xO.
B
A
5:9
6:1
6.o
6.2
erllergy [keV] Fig. 3. Temperaturedevelopment of Cr-edge XANES spectra for 3 Gd/2 Cr/3 Ga/xO.
.
.
.
.
.
.
.
.
200
r
400
. . . .
i
600
. . . .
i
800
. . . .
i
,
,
,
1000 temperature r°c]
Fig. 5. DTA diagram for the 3 G d / C r / 4 G a / x O system.
IV. Gunsseret aL /Amorphous and polycrystalline Gd3CrxGas_x012
i
imarph
_
669
I
A
,_J .
.
.
.
i
200
.
.
.
.
I
.
.
400
.
.
r
•
•
•
600
i
800
i
.
.
1000
temperature [°c] Fig. 6. DTA diagram for the 3 Gd/2 Cr/3 Ga/xO system.
Table 1 Exothermic peak positions from the DTA diagrams. Temperature ( ° C)
Peak A Peak BI Peak Bn
x=0
x=l
x=2
200-300 688 -
150-250 668 698
100-300 679 709
a c c o m p a n i e d by a weight decrease. F o r the chrom i u m free c o m p o u n d from D T A m e a s u r e m e n t s a crystallization t e m p e r a t u r e o f 690 ° C is derived, but a pure phase is to be found at reaction time used not before 1100 ° C. C h r o m i u m - c o n t a i n i n g samples show exothermic peaks at 668°C (for x = l ) and 679°C (for x = 2 ) with a shoulder which is situated at a t e m p e r a t u r e o f about 30 K higher. The peak positions o f the 2 or 3 D T A peaks for the three c o m p o u n d s are listed in table 1. After an annealing time o f 24 h in case o f the G d / C r / G a / O system only at t e m p e r a t u r e s higher than 1400°C pure polycrystalline garnets could be identified. 3.4. S E M
The existence o f i n t e r m e d i a t e phases in case o f the c h r o m i u m - c o n t a i n i n g c o m p o u n d s could be confirmed by Scanning Electron Microscopy. Fig. 7 shows, as an example, SEM images for sample 3 G d / 2 C r / 3 G a / x O annealed at 700, 800 a n d 1400°C. By
Fig. 7. SEM images as an example for the temperature development: (a) 700°C, (b) 800°C, (c) 1400°C for 3 Gd/2 Cr/3 Ga/ xO system
X-ray contribution images we were able to ensure that a homogeneous distribution o f Gd, G a a n d Cr can be taken into account. Gd3GaO6 as an i n t e r m e d i a t e product is forming needles with a length o f approximately 15 p.m, shown in fig. 7b. Besides, there exist spheres with a d i a m e t e r o f about 0.1 mm. We assume that these are formed from c h r o m i u m ( V I )
IV.. Gunsser et aL /Amorphous and polycrystalline Gd3CrxGas_x012
670
Table 2 Magnetic data for the polyerystallinecompounds. Calculated x=0
Experimental
C = 2 . 9 7 2 × 10 -4
1 0 - 4 m 3 Kmol -~ ncxp= 22.35/za O=0.2 K_+ 10 K C=2.887× 10 - 4 m 3 K molnexp=27.12 ,uB 0=0.6 K+ 10 K C= 3.079 X 10-4 m3 K tool- i
m3K mol-~
C=2.615×
near=23.82/zB x=l
C= 3.207 X 10-4 m3K mol- i neff=27.69 #e
x=2
C= 3.427 × 10-4 m 3 K mo1-4 neff=31.56gB
n~xp= 31.30/.tB
O=0.4 K_+ 10 K
xTE10-4m3K/moll ~[
"
"1
. . . .
i
. . . .
i
. . . .
i
. . . .
~
. . . .
3.5- /
i
. . . .
i
. . . .
Gd3CrxGa~Ot2
2.5 30
iii2-%
5o
lOO
12o
2oo
220
300
320
T [K~]
Fig. 8. Magnetic susceptibilityX'temperature T versus T.
oxide which is melted during the annealing and solidified during the cooling processes. 3.5. M a g n e t i s m
Concerning the magnetic properties of the three c o m p o u n d s investigated, this paper deals only with their reaction products. Calculated a n d experimental data are compared in table 2. At temperatures lower than approximately 50 K, there begins an ordering process. With the c h r o m i u m - c o n t a i n i n g samples the ferrimagnetic ordering can be concluded. The Curie temperature for the x = 1 sample exists at about 20 K whereas for the x = 2 sample a transition temperature of about 12 K is to be concluded. At temperatures lower than I 0 K the magnetic behaviour is more complicated (fig. 8). This will be studied in a further investigation.
4. C o n c l u s i o n The formation process of Gd3CrxGas_xO12 is completed after an annealing process of hydroxides (24 h) for the chromium-free c o m p o u n d at about 1200°C, for the x = l and x = 2 systems at about 1400°C. In the system m e n t i o n e d first (3 G d / 5 G a / x O ) no intermediate c o m p o u n d exists. But in the case of Gd3CrGa4012 and Gd3CrEGa3012 stoichiometry at 7 0 0 - 8 0 0 ° C Gd3GaO6 (orthorhombic structure, needles) and c h r o m i u m (VI)-oxide could be identified (table 3). We assume that the formation of Gd3GaO6 is caused by a segregative (precipitation) of chromium. Because of the loss of Cr the forming of garnets is being developed not from a composition-like garnet stoichiometry but from a deficiency of chromium. Magnetic properties of mixed polycrystalline oxides in the temperature region lower than 50 K are
W. Gunsser et al. /Amorphous and polycrystalline Gd3CrxGas_x012
671
Table 3 Reaction scheme of garnet forming in G d / C r / G a / O system. ~680°C
NaHCO3
Ga 3+ Gd 3+
, Ga(OH)3 NaHCO3
homogeneous phase
' Gd(OH)3
NaHCO3
Cr s+
, Cr(OH)3 NaHCO3
Ga 3÷
, Ga(OH)3 NaHCO3
Gd 3+
, Gd(OH)3
(Ga 3+/Gd 3+ ) amorphous
Gd3GasOl2
crystalline Cr (VI)-oxide Gd3CrGa4Oj2 Gd3GaO6 homogeneous phase --, rsp. Ga3+/Gd3+/Cr3+/Cr6+ ) Gd3CrGa4Ot2 rsp. GdaCr2Ga3Ot2 GdaCr2Ga3Ol2
d o m i n a t e d b y a n t i f e r r o m a g n e t i c c o u p l i n g effects. T h e b e h a v i o u r o f c h r o m i u m c o n t a i n i n g s u b s t a n c e s at T < 20 K is v e r y i n t e r e s t i n g b e c a u s e o f t h e c o n c u r rence of different exchange interactions.
Acknowledgement This work was supported by the Bundesministerium rdr Forschung und Technologie (BMFT), Proj e c t no. 0 3 - G U 3 - H A M .
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