- Email: [email protected]
Physical and structural properties of Chevrel-phase selenides Mo3Se4 and RExMo6Se8: Crystal growth and mutual solubility
Journal of Magnetism and Magnetic Materials 140-144 (1995) 1171-1172
Jm
journal of magnetism and magnetic
J R materials
ELSEVIER
Physical and structural properties of Chevrel-phase selenides Mo3Se 4 and RExMo6Se 8" crystal growth and mutual solubility F. Le Berre a F. Maho a O. Pefia a,*, R. Horyfi b A. Wojakowski b a C.S.LM. URA 1495, Universitd de Rennes I, 35042 Rennes Cedex, France b Institute for Low Temperature and Structure Research, 50-950 Wroclaw, Poland
Abstract We present the synthesis and superconducting behaviours of the Chevrel-phase selenides series RExMo6Se 8 ( x > 0.5; RE = La, Ce, Ho, Yb). X-ray diffraction, lattice parameters and Tc variations clearly show that no solid solution exists between the binary and the appropriate ternary compounds despite of their isostructural character. Preliminary results obtained on single crystals of ternary compounds are also presented and discussed for comparison.
Chevrel-phase rare-earth molybdenum chalcogenides REMo6X 8 ( R E = r a r e earth; X = S , Se) have been well studied in the late 70's due to their remarkable properties [1]. Their structural arrangement can be described as two sub-lattices: one of them (Mo6X 8) is mainly responsible of the transport and superconducting properties, while in the other one, long-range magnetic interactions can take place between the RE ions. The selenides series REMo6Se s presents some unsolved problems since the isostructural superconductor Mo3Se 4 (Tc = 6.45 K) may hide some of their intrinsic properties. We have recently presented a new approach to grow single crystals of these materials [2] but the results still suggest the presence of small quantities of Mo3Se 4 as if they crystallize together with the ternary compound. A work was undertaken on the possible existence of a continuous solid solution RExMo6Se s (0 < x < 1.0). The choice of specific lanthanides (RE = La, Ce, Ho, Yb . . . . ) was based on the superconducting critical temperature and the lattice parameters [3], which could allow easy identification of the ternary phase with respect to Mo3Se 4. Starting components MoSe 2 and RESey (usually y = 3 / 2 ) were prepared from RE bars, Mo powder and Se drops. Synthesis was performed for 5 days at 1200°C under high vacuum using alumina holders to avoid any contact with the silica walls. Crystal growth of ternary compounds was done using highly Se-rich R E - M o - S e mixtures, sealed in Mo crucibles and melted at high temperatures (1650-1750°C), as previously described [2].
Crystallization of Mo3Se 4 was performed in a similar way by using a stoichiometric 3 : 4 mixture. We have investigated the compatibility line at 1200°C between Mo3Se 4 and REMo6Se 8 for cationic concentrations ranging from x = 0 up to x = 1.5. Three well-defined regions could be identified from the X-ray diffraction studies: (a) for x < 0.5, the characteristic lines of Mo3Se4 were mainly present, but very weak lines due to the ternary phase started to appear as early as x ~ 0.1; (b) for 0.5 < x < 1.0, lines of the RExMo6Se s phase predominate, while the Mo3Se 4 diffraction peaks weaken and disappear at x = 0.7-0.8; (c) for x > 1.0, extra lines due to the binary rare-earth selenides superpose to the ternary phase. We can then conclude that, for 0.1 < x < 0.75, samples contained both binary and ternary phases. The lattice parameters of the Mo3Se 4 phase (a H =9.555(1)A, c H = 11.166(1) A) remained constant as a function of x, while those of the ternary Chevrel phase mainly changed starting from x >
12"02f
f
11.91 f
9.465
°3 " ~ 9.460
9.45 0.5
Corresponding author. [email protected].
Fax: (33) 99.63.57.04;
email:
i ....
I.o
I
t J J
1.5 x ( L a )
Fig. 1. Lattice parameters of the hexagonal lattice for as a function of nominal concentration.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 0 9 9 1 - 0
....
LaxMoeSe8
1172
F. Le Berre et al. / Journal of Magnetism and Magnetic Materials 140-144 (1995) 1171-1172
0.75. Fig. 1 shows the case of the L a - M o - S e series. A similar behavior was encountered for RE = Ho and Ce while for RE = Yb, the c-parameter keeps increasing until x = 1.5 [4]. Fig. 2a shows the variation of the Tc as a function of the lanthanum concentration. The increase of T~ in the region 0 . 7 < x < 1.1, from 10.2 up to 11.8 K, correlates with the variation of the c-parameter. The highest constant value of Tc confirms that the lanthanum content can not go beyond 1 ion per formula unit, as expected from the crystal structure [5]. At low x, Tc corresponds to the lowest limit of solubility of the Chevrel-phase, that is, for x = 0.6-0.7. Below this value, a second step becomes visible at 6.45 K, which corresponds to the critical temperature of Mo3Se 4. Fig. 2b and 2c shows the Tc variation as a function of the nominal concentration for RE = Ho and Yb. A constant value of 6.45 _+ 0.05 K (Tc of Mo3Se 4) was observed below x = 0.6. At higher content of holmium, Tc stabilizes at 6.35 K. In the case of ytterbium, T~ decreases, first smoothly (0.5 < x < 1.0) and then, more rapidly ( x > 1) going below our temperature limits, suggesting a non-superconducting phase [6]. The variations of T~ and of the c-parameter of the ytterbium phase point toward a particular situation which will be further studied on single crystals in a forthcoming work. Preliminary results were obtained on the C e - M o - S e series. Cerium was found non-superconducting for x > 0.9. Surprisingly, a slight but visible increase of Tc, from 6.4 up to almost 7 K, was observed at low concentrations of cerium. Further work in single crystals is needed in order to confirm this interesting behaviour. In conclusion, no continuous solid solutions exist between the binary Mo3Se 4 and the ternary REMo6Se s
12
(a) La
~'11 v 21o o/ 6.5:
6.4 t~
_t . . . . .L-"
/
~-
•/ "
[ 0.4 ~t
.\..
Ioo,~
°--°~,~° N (C)
phases. Instead, there are restricted regions of homogeneity for the Chevrel phases, extending approximately from x = 0.75 up to 1. However, since the extension of the Mo3Se 4 and REMo6Se s structure-type phases within the ternary system is still unknown, the observed variations of the lattice parameters of the coexisting phases and of their Tc's may not constitute a direct proof for their mutual solubility along the line RExMo6Se s. The immiscibility found has an important effect on the crystallization of the ternary phases, resulting in a pattern of continuous alternate very-thin layers of REMo6Se s and Mo3Se 4 occurring in the same crystal (Fig. 3). Its direct consequence is an overall deficit of rare-earth atoms per formula unit, as calculated from the high temperature magnetic susceptibility. In addition, the Tc may reflect the values of the low rare-earth boundary limits of the homogeneity domains. Optimization of the crystal growth processes should allow an ideal distribution and maximum concentration of the rare-earth atoms in the structure.
References
_ ~ . k ( b ) Ho
6.3
~6
Fig. 3. Immiscibility of the binary and the ternary phases in a crystal of RExMo6Se s.
Y b ~
I
IK
"\ ,\
0.5 1.0 1.5 nominal content (RE)
Fig. 2. Tc°n~ct for some RExMo6Se 8 systems as a function of nominal concentration (transition widths usually less than 0.05 K). The vertical bars indicate different temperature scales.
[1] 0. Fischer and M.B. Maple, eds, Topics in Current Physics, vols. 32 and 34: Superconductivity in Ternary Compounds, I and II (Springer, Berlin, 1982). [2] R. Horyfi, O, Pefia, A. Wojakowski and M. Sergent, Supercond. Sci. Technol. 7 (1994) 146. [3] R.N. Shelton, R.W. McCallum and H. Adrian, Phys. Lett. A 56 (1976) 213. [4] F. Maho, F. Le Berre, O. Pefia, R. Horyfi and A. Wojakowski, J. Phys. III (Paris) (1995), in press. [5] O. Pefia and M. Sergent, Prog. Solid State Chem. 19 (1989) 165. [6] J.M. Tarascon, D.C. Johnson and M.J. Sienko, Inorg. Chem. 21 (1982) 1505.
Jm
journal of magnetism and magnetic
J R materials
ELSEVIER
Physical and structural properties of Chevrel-phase selenides Mo3Se 4 and RExMo6Se 8" crystal growth and mutual solubility F. Le Berre a F. Maho a O. Pefia a,*, R. Horyfi b A. Wojakowski b a C.S.LM. URA 1495, Universitd de Rennes I, 35042 Rennes Cedex, France b Institute for Low Temperature and Structure Research, 50-950 Wroclaw, Poland
Abstract We present the synthesis and superconducting behaviours of the Chevrel-phase selenides series RExMo6Se 8 ( x > 0.5; RE = La, Ce, Ho, Yb). X-ray diffraction, lattice parameters and Tc variations clearly show that no solid solution exists between the binary and the appropriate ternary compounds despite of their isostructural character. Preliminary results obtained on single crystals of ternary compounds are also presented and discussed for comparison.
Chevrel-phase rare-earth molybdenum chalcogenides REMo6X 8 ( R E = r a r e earth; X = S , Se) have been well studied in the late 70's due to their remarkable properties [1]. Their structural arrangement can be described as two sub-lattices: one of them (Mo6X 8) is mainly responsible of the transport and superconducting properties, while in the other one, long-range magnetic interactions can take place between the RE ions. The selenides series REMo6Se s presents some unsolved problems since the isostructural superconductor Mo3Se 4 (Tc = 6.45 K) may hide some of their intrinsic properties. We have recently presented a new approach to grow single crystals of these materials [2] but the results still suggest the presence of small quantities of Mo3Se 4 as if they crystallize together with the ternary compound. A work was undertaken on the possible existence of a continuous solid solution RExMo6Se s (0 < x < 1.0). The choice of specific lanthanides (RE = La, Ce, Ho, Yb . . . . ) was based on the superconducting critical temperature and the lattice parameters [3], which could allow easy identification of the ternary phase with respect to Mo3Se 4. Starting components MoSe 2 and RESey (usually y = 3 / 2 ) were prepared from RE bars, Mo powder and Se drops. Synthesis was performed for 5 days at 1200°C under high vacuum using alumina holders to avoid any contact with the silica walls. Crystal growth of ternary compounds was done using highly Se-rich R E - M o - S e mixtures, sealed in Mo crucibles and melted at high temperatures (1650-1750°C), as previously described [2].
Crystallization of Mo3Se 4 was performed in a similar way by using a stoichiometric 3 : 4 mixture. We have investigated the compatibility line at 1200°C between Mo3Se 4 and REMo6Se 8 for cationic concentrations ranging from x = 0 up to x = 1.5. Three well-defined regions could be identified from the X-ray diffraction studies: (a) for x < 0.5, the characteristic lines of Mo3Se4 were mainly present, but very weak lines due to the ternary phase started to appear as early as x ~ 0.1; (b) for 0.5 < x < 1.0, lines of the RExMo6Se s phase predominate, while the Mo3Se 4 diffraction peaks weaken and disappear at x = 0.7-0.8; (c) for x > 1.0, extra lines due to the binary rare-earth selenides superpose to the ternary phase. We can then conclude that, for 0.1 < x < 0.75, samples contained both binary and ternary phases. The lattice parameters of the Mo3Se 4 phase (a H =9.555(1)A, c H = 11.166(1) A) remained constant as a function of x, while those of the ternary Chevrel phase mainly changed starting from x >
12"02f
f
11.91 f
9.465
°3 " ~ 9.460
9.45 0.5
Corresponding author. [email protected].
Fax: (33) 99.63.57.04;
email:
i ....
I.o
I
t J J
1.5 x ( L a )
Fig. 1. Lattice parameters of the hexagonal lattice for as a function of nominal concentration.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 0 9 9 1 - 0
....
LaxMoeSe8
1172
F. Le Berre et al. / Journal of Magnetism and Magnetic Materials 140-144 (1995) 1171-1172
0.75. Fig. 1 shows the case of the L a - M o - S e series. A similar behavior was encountered for RE = Ho and Ce while for RE = Yb, the c-parameter keeps increasing until x = 1.5 [4]. Fig. 2a shows the variation of the Tc as a function of the lanthanum concentration. The increase of T~ in the region 0 . 7 < x < 1.1, from 10.2 up to 11.8 K, correlates with the variation of the c-parameter. The highest constant value of Tc confirms that the lanthanum content can not go beyond 1 ion per formula unit, as expected from the crystal structure [5]. At low x, Tc corresponds to the lowest limit of solubility of the Chevrel-phase, that is, for x = 0.6-0.7. Below this value, a second step becomes visible at 6.45 K, which corresponds to the critical temperature of Mo3Se 4. Fig. 2b and 2c shows the Tc variation as a function of the nominal concentration for RE = Ho and Yb. A constant value of 6.45 _+ 0.05 K (Tc of Mo3Se 4) was observed below x = 0.6. At higher content of holmium, Tc stabilizes at 6.35 K. In the case of ytterbium, T~ decreases, first smoothly (0.5 < x < 1.0) and then, more rapidly ( x > 1) going below our temperature limits, suggesting a non-superconducting phase [6]. The variations of T~ and of the c-parameter of the ytterbium phase point toward a particular situation which will be further studied on single crystals in a forthcoming work. Preliminary results were obtained on the C e - M o - S e series. Cerium was found non-superconducting for x > 0.9. Surprisingly, a slight but visible increase of Tc, from 6.4 up to almost 7 K, was observed at low concentrations of cerium. Further work in single crystals is needed in order to confirm this interesting behaviour. In conclusion, no continuous solid solutions exist between the binary Mo3Se 4 and the ternary REMo6Se s
12
(a) La
~'11 v 21o o/ 6.5:
6.4 t~
_t . . . . .L-"
/
~-
•/ "
[ 0.4 ~t
.\..
Ioo,~
°--°~,~° N (C)
phases. Instead, there are restricted regions of homogeneity for the Chevrel phases, extending approximately from x = 0.75 up to 1. However, since the extension of the Mo3Se 4 and REMo6Se s structure-type phases within the ternary system is still unknown, the observed variations of the lattice parameters of the coexisting phases and of their Tc's may not constitute a direct proof for their mutual solubility along the line RExMo6Se s. The immiscibility found has an important effect on the crystallization of the ternary phases, resulting in a pattern of continuous alternate very-thin layers of REMo6Se s and Mo3Se 4 occurring in the same crystal (Fig. 3). Its direct consequence is an overall deficit of rare-earth atoms per formula unit, as calculated from the high temperature magnetic susceptibility. In addition, the Tc may reflect the values of the low rare-earth boundary limits of the homogeneity domains. Optimization of the crystal growth processes should allow an ideal distribution and maximum concentration of the rare-earth atoms in the structure.
References
_ ~ . k ( b ) Ho
6.3
~6
Fig. 3. Immiscibility of the binary and the ternary phases in a crystal of RExMo6Se s.
Y b ~
I
IK
"\ ,\
0.5 1.0 1.5 nominal content (RE)
Fig. 2. Tc°n~ct for some RExMo6Se 8 systems as a function of nominal concentration (transition widths usually less than 0.05 K). The vertical bars indicate different temperature scales.
[1] 0. Fischer and M.B. Maple, eds, Topics in Current Physics, vols. 32 and 34: Superconductivity in Ternary Compounds, I and II (Springer, Berlin, 1982). [2] R. Horyfi, O, Pefia, A. Wojakowski and M. Sergent, Supercond. Sci. Technol. 7 (1994) 146. [3] R.N. Shelton, R.W. McCallum and H. Adrian, Phys. Lett. A 56 (1976) 213. [4] F. Maho, F. Le Berre, O. Pefia, R. Horyfi and A. Wojakowski, J. Phys. III (Paris) (1995), in press. [5] O. Pefia and M. Sergent, Prog. Solid State Chem. 19 (1989) 165. [6] J.M. Tarascon, D.C. Johnson and M.J. Sienko, Inorg. Chem. 21 (1982) 1505.