- Email: [email protected]
Crystal structure and magnetic behaviour of the ternary uranium and thorium-molybdenum chalcogenides UMo6S8, UMo6Se8 and ThMo6S8
ELSEVIER
Journal of Alloys and Compounds
232 (1996) 180-185
Crystal structure and magnetic behaviour of the ternary uranium and thorium-molybdenum chalcogenides UMo,S,, UMo,Se, and ThMo,S, A. Daoudi”3b, M. Potel”, H. NoEl” “Laboratoire de Chimie du Solide et Inorganique ‘Laboratoire
MoEculaire, U.R.A. C.N.R.S.1495, Universiti de Rennes, 1 Avenue du GCntral Leclerc, F-35042 Rennes, France de Chimie du Solide, Faculte’ des Sciences Ain Chock, Universiti Hassan II, Casablanca, Morocco
Received 17 July 1995
Abstract Single crystals of UMo,S,, UMo,Se, and ThMo,S, were obtained using the slow cooling method. Refinements of their crystal structure (R = 0.030, 0.032 and 0.029 respectively) revealed a non-integral occupancy of the actinide sites in the UMagnetic measurements showed weak selenide and the Th- sulfide, giving the formula U,,, Mo,Se, and Th,,,,Mo,S,. ferromagnetism behaviour below 25 K for U,,,, Mo,Se,, and a transition from weak paramagnetism to diamagnetism at 3 K for Th,,,Mo,S,, suggesting the onset of superconductivity. UMo,S, follows a Curie-Weiss type behaviour down to 2 K. Keywords:
Crystal structure; Magnetic behaviour; Uranium-molybdenum
1. Introduction The ternary molybdenum chalcogenides of formula M,Mo,X, (M = Ag, Cu, Pb, Sn, 3d-transition metals, rare earths, and X = S, Se, Te) exhibit a large variety of physical properties related to superconductivity such as lattice instabilities at low temperature, high critical fields, pressure sensitivity and the coexistence of superconductivity with magnetic ordering [ 1,2]. These properties have made them the subject of numerous studies since the initial report of the existence of the first series of compounds by Chevrel and coworkers [3]. The possibility of replacing the M element by a great variety of elements that differ in atomic number, size, electronegativity and concentration has been an attractive feature in the study of their physical properties. Up to now, only limited studies of phases with typically tetravalent M elements have been carried out. In particular, they were performed on powders for the compounds UMo,S,, UMo,Se, and ThMo,S, (4). In addition, only few selenides have been the subject of structural determinations [l]. In this paper, we report on the crystal structure refinement and single 0925-8388/96/$15.00 0 1996 SSDI 0925-8388(95)02000-4
Elsevier Science S.A. All rights reserved
chalcogenides; Thorium-molybdenum
chalcogenides
crystal magnetic measurements for UMo,S,, UMo,Se, and ThMo,S,. A comparison with the (RE)Mo,S, phases is made to shed more light on the role of the M element in the structure and on the physical properties.
2. Synthesis and characterization The ternary actinide-molybdenum chalcogenides UMo,S,, UMo,Se, and ThMo,S, have been prepared from stoichiometric mixtures of the starting materials: MX, (M = U, Th), MO powder and MoX, (X = S, Se). The mixtures, pressed into pellets, were heated at 1300°C for 12 h in sealed molybdenum crucibles. All materials were handled in a dry argon atmosphere glove box to avoid the formation of the actinide oxychalcogenides. The reaction products were analysed by the X-ray powder diffraction method. The X-ray diffraction patterns obtained are indexed in the hexagonal system with the parameters given in Table 1. The X-ray powder diffraction pattern of UMo,S, is given in Table 2. Single crystals of different sizes, up to about 1 mm
A. Daoudi et al. I Journal of Alloys and Compounds Table 1
UMo,S, ThMo,S, UMo,Se,
aH (A)
cki (A)
v, (iv)
9.008( 2) 9.027( 3) 9.369(2)
11.3.53(3) 11.424(5) 11.840(6)
797.7 806.2 899.9
232 (1996) 180-185
181
termination. X-ray diffraction intensities were collected on a Nonius-Cad4 four circle diffractometer with the experimental conditions given in Table 3. A numerical absorption correction was applied to the data, using the program Abscor [5]. All calculations were performed using the Molen’s program system
lI61.
Table 2 X-ray powder diffraction data of UMo,S, hkl
d,,,, (A)
d,,, (A)
Ill,
101 102 110 003 201 202 113 211 203 212 204 204 105 303 311 214 312 205 223 006 321 116 314 206 404 107 315 413 306 324
6.4518 4.5883 4.5031 3.7819 3.6865 3.2154 2.8975 2.8545 2.7199 2.6184 2.2947 2.2947 2.1796 2.1412 2.1246 2.0445 2.0212 1.9623 1.9350 1.8926 1.7680 1.7448 1.7214 1.7040 1.6075 1.5875 1.5660 1.5520 1.5299 1.5134
6.4294 4.5899 4.5038 3.7844 3.6888 3.2147 2.8973 2.8538 2.7161 2.6165 2.2950 2.2950 2.1801 2.1431 2.1253 2.0448 2.0217 1.9623 1.9352 1.8922 1.7678 1.7445 1.7207 1.7024 1.6073 1.5879 1.5664 1.5525 1.5300 1.5138
19 77 70 28 42 8 40 95 (3 100 21 21 73 20 89 85 40 5 98 10 26 6 15 30 7 8 <3 16 35
in the largest dimension, could be obtained using the slow-cooling method. The samples, pressed into pellets, were heated in sealed molybdenum crucibles up to their melting temperature and then cooled at experimentally determined rates found most appropriate for a given system. The sulfide UMo,S, forms peritecticaly but single crystals could be obtained from a uranium-rich composition U11,4M034.3S54.3 (i.e. UMo,S, + l/3 U,S,) at 1750°C with a cooling rate of 100°C h-‘. ThMo,S,and UMo,Se, form congruently at 1750°C and crystals were obtained with a cooling rate of 25°C h-‘.
Uranium and thorium, molybdenum and chalcogen atoms X(1) and X(2) were positioned as for the (RE)Mo,X, Chevrel phases in the la, 6f, 6f and 2c positions of the R-3 space group (No. 148 of International Tables of X-ray Crystallography). Refinements of the positional and isotropic thermal parameters led to R = 0.038,0.050 and 0.051 respectively for UMo,S,, UMo,Se, and ThMo,S,. Anormally high values of the thermal factors of the actinide element in both UMo,Se, and ThMo,S, suggested a partial occupation of the corresponding position. Refinements of the occupancy factor of the la site in UMo,S,, UMo,Se, and ThMo,S, were made and showed no significant deviation from full occupancy in the uranium sulfide while it converged to the values r = 0.80 and r = 0.81 respectively for UMo,Se, and ThMo,S, concomitantly with a significant decrease of the reliability factors to R = 0.037 and 0.038. Further refinements including anisotropic thermal parameters yielded the final values R = 0.030, 0.032 and 0.029 respectively for UMo,S,, UMo,Se, and ThMo,S,. These results show that, for two of the phases, about 18% of the actinide sites are empty in the investigated crystals and suggest the existence of a non-stoichiometry range. We tried to obtain single crystals in the uranium-rich side, using the nominal starting composition U11,4M034.4Se54,3 (UMo,Se, + 1/3U,Se,) and crystal growth was achieved using the same heating-cooling program. A platelet crystal was chosen for X-ray diffraction intensity collection with the same experimental conditions. The final refinement cycles with anisotropic thermal parameters gave the values of the reliability factors R = 0.036 and R, = 0.043 with a convergence of the occupancy factor of the uranium site to r = 0.82, which is not significantly different from the previous value. This result indicates the existence of an upper limit of -0.82 for the occupancy factor of the la site by the actinide element in UMo,Se,. A similar situation also very probably exists in ThMo,S,. Thus the formula of these compounds should be written U,,,,Mo,Se, and Th,,,,Mo$,. The positional and thermal parameters are given in Table 4 and the principal interatomic distances in Table 5.
3. Crystal structure determination
4. Crystal structure description and discussion
Single crystals with regular parallelepipedic forms were chosen to undertake the crystal structure de-
The crystal structure of the M,Mo,X, Chevrel phases has often been described in the literature. As
182
A. Daoudi et al. I Journal of Alloys and Compounds
Table 3 Crystallographic data for UMo,S,,
U,,,,Mo,Se,
Crystal dimensions (mm’) Linear absorption coefficient (cm-‘) Crystal* parameters (from CAD4): aR (A) % (“) OH CH Unit-cell volume V, (A’) Calculated density (g cm-‘) Radiation Scan mode Scan range hkl range
Setting Total observed reflections Independent reflections with I > 3m Reliability factors: R = c~[lF,l - IF,I]IPIF,I Rw = [Z,(IF,I - IFcl)2/~JFolz]“z Goodness of fit GOF
Table 4 Positional and thermal parameters of UMo,S,,
232 (19%)
180-185
and Th,,Mo,S, UMo,S,
U,, xzMo,Se,
Th, x,Mo&
0.06 x 0.10 x 0.02 225.6
0.02 x 0.02 x 0.02 430.5
0.06 x 0.04 x 0.03 222.7
6.4302( 4) 88.85(l) 9.002( 1) 11.359(2) 797.1 6.69 (MO Ko) O-28 tJ<50 -13shc13 Ochc13 OC1613 Rhombohedral 2070 1217
6.6773(7) 88.79( 1) 9.343( 1) 11.807(4) 892.6 8.07 A( MO Ka) 0 - 28 l3<40” -16
6.4611(3) 88.85(l) 9.045( 2) 11.414(2) 808.7 6.55 A(Mo KLY) w-20 l9<50 -13
0.030 0.032 0.989
0.032 0.044 1.029
0.029 0.037 1.059
Th,,,,Mo,S,
and U, RZMo6SeH
Atom
site
X
Y
Z
R,, (A?
P,,
P22
P 33
P 12
P,,
UMo,S, U MO S(1) S(2)
la 6f 6f 2c
0 0.22253( 5) 0.3677( 2) 0.2388(2)
0 0.42125(5) 0.1263( 1) 0.2388(2)
0 0.56222(5) 0.7580(2) 0.2388(2)
0.597( 1) 0.310(3) 046(l) 0.50( 1)
0.601(4) 0.305(7) 0.51(2) 0.51(2)
0.303(7) 0.37(2)
PII
PI,
0.315(7) 0.49(2)
0.046( 6) 0.12(2)
%52(6) 0.09(2)
P,1
P,,
-0.104 0.028( 6) O.OO(2) -0.03(2)
Pi,
P,,
0 0.22421(4) 0.3720( 1) 0.2406( 1)
0 0.42016(4) 0.1256(l) 0.2406( 1)
0 0.56248(4) 0.7541( 1) 0.2406(2)
0.612( 1) 0.342( 3) 0.537(9) 0.612( 1)
0.616(4) 0.335(6) 0.40( 2) 0.53( 1)
0.325(6) 0.60(2)
0.359(6) 0.61(2)
0.037(5) 0.05(2)
0.044( 5) 0.09(2)
P,,
P,,
-0.107(4) 0.041(5) O.Ol(2) -0.07( 1)
P,,
P,*
0 0.2310( 1) 0.3722( 1) 0.2368( 1)
0 0.4228( 1) 0.1214(l) 0.2368( 1)
0 0.5601( 1) 0.7593( 1) 0.2368( 1)
0.778( 4) 0.521(9) 0.70( 1) 0.707(5)
0.78(4) OSO(2) 0.73(2) 0.71(2)
0.50(2) 0.57( 2)
0.55(2) 0.79(2)
0.03(2) 0.09(2)
%3(2) 0.07(2)
Pi,
Pi,
-0.16( 1) 0.63(2) -0.01(2) -0.05(2)
P,*
Pi,
Tho.,,Mo& Th MO S(1) S(2)
la 6f 6f 2c
U, ,,Mo,%
U MO Se(l) Se(2)
la 6f 6f 2c
can be seen in Fig. 1, it consists of a stacking of Mo,X, pseudo-cubes with chalcogen atoms in the comers and molybdenum atoms slightly outside the middle of the faces. The six molybdenum atoms form a distorted octahedron involving short and unequal MO-MO distances: (MO-MO), intracluster distances between two molybdenum atoms belonging to the same triangular plane perpendicular to the ternary axis, and MO,-MO, intracluster distances between two molybdenum atoms belonging to two different triangular planes. The MO, clusters linked together with intercluster distances larger than the intracluster ones. This particular arrangement of the Mo,X, pseudo-cubes leaves two
41
PI,
PI,
PI,
P,*
P,,
PI,
P*,
PI,
types of cavity in the chalcogen atom network. These cavities are empty in the binaries Mo,X, and are filled by the M atoms in the ternary phases. One of these cavities, the largest one, is situated at the origin of the rhombohedral unit cell with point symmetry - 3. The actinide atom resides in this cavity with a pseudo-cubic environment (2 + 6). Unlike thorium, which is typically a tetravalent element, uranium can exhibit two valence states: +3 and +4 (i.e. 5f3 and 5f2 electronic configurations) in its chalcogenides. It has been shown that the valency of the uranium atoms with sulfur or selenium environment can be postulated by considering the mean
A. Daoudi
et al. I Journal of Alloys
Table 5 Interatomic distances (A) LJMo,S,
Th,, ,,Mo,Se,
U,, R2M06SeH
6 X(1) 6 MO
2.713( 1) 2.924( 1) 4.123(l)
2.746( 1) 2.972( 1) 4.140( 1)
2.797( 1) 3.054(l) 4.327( 1)
Mo-
X(2) X(1) X(1) X(1) X(1)
2.408( 2.436( 2.448( 2.551( 2.571(
2.401( 1) 2.438( 1) 2.452( 1) 2.539( 1) 2.569( 1)
2.515( 1) 2.559( 1) 2.570( 1) 2.655( 1) 2.701(l)
Mo-
2 MO” 2 Mob MO MO
2.660( 1) 2.704( 1) 3.131( 1) 3.794( 1) 3.862( 1) 4.123( 1)
2.661( 2.708( 3.170( 3.797( 3.891( 4.140(
1) 1) 1) 1) 1) 1)
2.682( 1) 2.734( 1) 3.348( 1) 3.830( 1) 4.063( 1) 4.327( 1)
2.436( 1) 2.448( 1) 2.551( 1) 2.571( 1) 2.924( 1) 3.276( 1) 3.359( 1) 3.417( 1) 3.429( 1)
2.438( 2.452( 2.539( 2.569( 2.972( 3.334( 3.403( 3.410( 3.431(
1) 1) 1) 1) 1) 1) 1) 1) 1)
2.559( 1) 2.570( 1) 2.655( 1) 2.701( 1) 3.054( 1) 3.403( 1) 3.507( 1) 3.513( 1) 3.585(l)
2.408( 2.713( 3.276( 3.417(
2.401( 1) 2.746( 1) 3.334( 1) 3.410(l)
2.515( 1) 2.797( 1) 3.403( 1) 3.513(l)
M-
2 X(2)
X(2) M MO MO MO MO M
X(1)
X(2) 2 X(1) X(2) 2 X(1) 3 MO M
X(2)
3 X(l) 3 X(l) ’ (MO-MO),
1) 1) 1) 1) 1)
1) 1) 1) 1)
and b MO&-MO, intracluster distances.
and Compounds
232 (1996)
180-18-5
183
be in an intermediate valence state (between U”+ and u4+). The mean U-Se distance in U,,,Mo,Se, is d = 2.990 A, a value which is closer to the suggested one (2.97 A for a U4+ ion in an eight-fold selenium coordination). A comparison of these M,Mo,X, (M = U, Th) with the (RE)Mo,X, (RE = rare earths) phases shows that (MO-MO), intracluster distances are very similar in both series, while the MO,-MO, intracluster as well as the the MO-MO intercluster distances decrease slightly from the Actinides to the RE compounds. Thus the MO, cluster contracts more in the uranium phases and tends to be more regular. The MO-MO intracluster distances vary mainly as a function of the valence state of the M element in the M,Mo,X, phases. These distances decrease when the valence of the M element increases and the MO, cluster becomes more regular. It has been shown that the large electronegativity differences between the two metals M and MO and the chalcogen in M,Mo,X, (0
Fig. 1. View of the UMo,S, crystal structure (white circles = U, black circles = MO, grey circles = S).
5. Magnetic properties uranium Thus in 2.871 A U4’-8s
to chalcogen distances in the structure [7]. the UMo,S, phase, the mean U-S distance of is significantly larger than that suggested for a distance (2.82 A), which suggests uranium to
Magnetic measurements were performed using an S.H.E. SQUID magnetometer. The thermal variation of the reverse susceptibility of
A. Daoudi et al. I Journal of Alloys and Compounds
184
232 (1996) 180-185
H = 2 Kgauss
I
.__
___
TEMPERATURE Fig. 2. Thermal
variation
$ y 6 El.0
I
of UMo,S,.
k/U which cannot be directly related to any valence state of uranium, on account of large crystal field effects. The susceptibility of U,.,, Mo,Se, follows a modified Curie-Weiss law above 60 K with ,uefr = 3.70 h/U
I
I
I
I
I
“\
pKiz&q
1 “\
-
'0 I
I
10
20
'0 -0-o-q I
30
8
.0*O ,0*O CO ,O'Q,000 ,o 0*0 ,O?? .0*O .OdO
1“\0
3 0
susceptibility
0.
i3 go.50.0
I
I
O-OdO,
(K)
of the reverse
UMo,S, is presented in Fig. 2. UMo,S, shows paramagnetic behaviour down to 2 K and a curve fitting using a modified Curie-Weiss law x = x0 + CI(T - e) leads to a paramagnetic temperature $., = -13.6 K and a rather low effective magnetic moment of ,u = 2.39 2.0 -
^^ so0
ZW
100
0
40
TEMPERATURE (K)
,O’O 000
.O’O
#O
3.5
O.O' O.Os ocO'
0
50
100
*c)O
LW
ZW
150
TEMPERATURE (K) Fig. 3. Thermal variation variation of magnetization
of the reverse susceptibility (upper inset).
of U,,,, Mo,Se,.
Magnetization
as a function
of magnetic
field (lower
inset).
Thermal
A. Daoudi et al. I Journal of Alloys and Compounds
and eP = -8.4. K. An onset of weak ferromagnetism is observed below 25 K (Fig. 3). The value of the ordered remanent magnetic moment at 5 K is 0.25 ,+,/U, which could not be revealed in a preliminary neutron powder diffraction experiment. Magnetic susceptibility measurements under 20 gauss were performed dow to 2.3 K on a pressed pellet of Th,.,,Mo,S, powder. A transition from weak paramagnetism to diamagnetism was observed at 3 K, independently of the sample holder. This behaviour is considered to be intrinsic and is reminiscent of a superconducting transition. The lowering of T, compared to that of the RE counterparts is related to the MO d-band fillings. The ternary neptunium selenide NpMo,Se, was also reported to exhibit superconductivity below 6 K [9], but Np is expected to be in a pure trivalent state in this compound.
232 (1996) 180-185
185
References [l] K. Yvon, in E. Kaldis (ed.), Current Topics in Materials Science, Vol. 3, Elsevier, Amsterdam, 1979, p. 53. [2] 0. Fischer and M.B. Maple, Topics of Current Physics, Vol. 32, Superconductivity in Ternary Compounds I, Springer-Verlag, Berlin, 1982. [3] R. Chevrel, M.Sergent and J. Prigent, J. Solid State Chem., 3 (1971) 515-519. Actinides 1981, Abstracts [4] H. Noel, R. Chevrel and M.Sergent, ht. Conf Pacific Grove, California, Lawrence Berkeley Lab., University of California, LBL-12441. [5] N. Walker and D. Stuart, Acta Crystallogr., A39 (1983) 158-166. [6] C.K. Fair, in Molen Users Manual. An Interactive Intelligent 1989. System For Crystal Structure Analysis, Delft, Netherlands. [7] H. Noel, J. Solid State Chem., 52 (1984), 303. [8] K. Yvon, A. Paoli, R. Fhigiger and R. Chevrel, Acta Crystallogr., B33 (1977) 3066-3072. [9] D. Damien, C.H. De Novion and J. Gall, Solid State Commun.. 38 (1981) 437.
Journal of Alloys and Compounds
232 (1996) 180-185
Crystal structure and magnetic behaviour of the ternary uranium and thorium-molybdenum chalcogenides UMo,S,, UMo,Se, and ThMo,S, A. Daoudi”3b, M. Potel”, H. NoEl” “Laboratoire de Chimie du Solide et Inorganique ‘Laboratoire
MoEculaire, U.R.A. C.N.R.S.1495, Universiti de Rennes, 1 Avenue du GCntral Leclerc, F-35042 Rennes, France de Chimie du Solide, Faculte’ des Sciences Ain Chock, Universiti Hassan II, Casablanca, Morocco
Received 17 July 1995
Abstract Single crystals of UMo,S,, UMo,Se, and ThMo,S, were obtained using the slow cooling method. Refinements of their crystal structure (R = 0.030, 0.032 and 0.029 respectively) revealed a non-integral occupancy of the actinide sites in the UMagnetic measurements showed weak selenide and the Th- sulfide, giving the formula U,,, Mo,Se, and Th,,,,Mo,S,. ferromagnetism behaviour below 25 K for U,,,, Mo,Se,, and a transition from weak paramagnetism to diamagnetism at 3 K for Th,,,Mo,S,, suggesting the onset of superconductivity. UMo,S, follows a Curie-Weiss type behaviour down to 2 K. Keywords:
Crystal structure; Magnetic behaviour; Uranium-molybdenum
1. Introduction The ternary molybdenum chalcogenides of formula M,Mo,X, (M = Ag, Cu, Pb, Sn, 3d-transition metals, rare earths, and X = S, Se, Te) exhibit a large variety of physical properties related to superconductivity such as lattice instabilities at low temperature, high critical fields, pressure sensitivity and the coexistence of superconductivity with magnetic ordering [ 1,2]. These properties have made them the subject of numerous studies since the initial report of the existence of the first series of compounds by Chevrel and coworkers [3]. The possibility of replacing the M element by a great variety of elements that differ in atomic number, size, electronegativity and concentration has been an attractive feature in the study of their physical properties. Up to now, only limited studies of phases with typically tetravalent M elements have been carried out. In particular, they were performed on powders for the compounds UMo,S,, UMo,Se, and ThMo,S, (4). In addition, only few selenides have been the subject of structural determinations [l]. In this paper, we report on the crystal structure refinement and single 0925-8388/96/$15.00 0 1996 SSDI 0925-8388(95)02000-4
Elsevier Science S.A. All rights reserved
chalcogenides; Thorium-molybdenum
chalcogenides
crystal magnetic measurements for UMo,S,, UMo,Se, and ThMo,S,. A comparison with the (RE)Mo,S, phases is made to shed more light on the role of the M element in the structure and on the physical properties.
2. Synthesis and characterization The ternary actinide-molybdenum chalcogenides UMo,S,, UMo,Se, and ThMo,S, have been prepared from stoichiometric mixtures of the starting materials: MX, (M = U, Th), MO powder and MoX, (X = S, Se). The mixtures, pressed into pellets, were heated at 1300°C for 12 h in sealed molybdenum crucibles. All materials were handled in a dry argon atmosphere glove box to avoid the formation of the actinide oxychalcogenides. The reaction products were analysed by the X-ray powder diffraction method. The X-ray diffraction patterns obtained are indexed in the hexagonal system with the parameters given in Table 1. The X-ray powder diffraction pattern of UMo,S, is given in Table 2. Single crystals of different sizes, up to about 1 mm
A. Daoudi et al. I Journal of Alloys and Compounds Table 1
UMo,S, ThMo,S, UMo,Se,
aH (A)
cki (A)
v, (iv)
9.008( 2) 9.027( 3) 9.369(2)
11.3.53(3) 11.424(5) 11.840(6)
797.7 806.2 899.9
232 (1996) 180-185
181
termination. X-ray diffraction intensities were collected on a Nonius-Cad4 four circle diffractometer with the experimental conditions given in Table 3. A numerical absorption correction was applied to the data, using the program Abscor [5]. All calculations were performed using the Molen’s program system
lI61.
Table 2 X-ray powder diffraction data of UMo,S, hkl
d,,,, (A)
d,,, (A)
Ill,
101 102 110 003 201 202 113 211 203 212 204 204 105 303 311 214 312 205 223 006 321 116 314 206 404 107 315 413 306 324
6.4518 4.5883 4.5031 3.7819 3.6865 3.2154 2.8975 2.8545 2.7199 2.6184 2.2947 2.2947 2.1796 2.1412 2.1246 2.0445 2.0212 1.9623 1.9350 1.8926 1.7680 1.7448 1.7214 1.7040 1.6075 1.5875 1.5660 1.5520 1.5299 1.5134
6.4294 4.5899 4.5038 3.7844 3.6888 3.2147 2.8973 2.8538 2.7161 2.6165 2.2950 2.2950 2.1801 2.1431 2.1253 2.0448 2.0217 1.9623 1.9352 1.8922 1.7678 1.7445 1.7207 1.7024 1.6073 1.5879 1.5664 1.5525 1.5300 1.5138
19 77 70 28 42 8 40 95 (3 100 21 21 73 20 89 85 40 5 98 10 26 6 15 30 7 8 <3 16 35
in the largest dimension, could be obtained using the slow-cooling method. The samples, pressed into pellets, were heated in sealed molybdenum crucibles up to their melting temperature and then cooled at experimentally determined rates found most appropriate for a given system. The sulfide UMo,S, forms peritecticaly but single crystals could be obtained from a uranium-rich composition U11,4M034.3S54.3 (i.e. UMo,S, + l/3 U,S,) at 1750°C with a cooling rate of 100°C h-‘. ThMo,S,and UMo,Se, form congruently at 1750°C and crystals were obtained with a cooling rate of 25°C h-‘.
Uranium and thorium, molybdenum and chalcogen atoms X(1) and X(2) were positioned as for the (RE)Mo,X, Chevrel phases in the la, 6f, 6f and 2c positions of the R-3 space group (No. 148 of International Tables of X-ray Crystallography). Refinements of the positional and isotropic thermal parameters led to R = 0.038,0.050 and 0.051 respectively for UMo,S,, UMo,Se, and ThMo,S,. Anormally high values of the thermal factors of the actinide element in both UMo,Se, and ThMo,S, suggested a partial occupation of the corresponding position. Refinements of the occupancy factor of the la site in UMo,S,, UMo,Se, and ThMo,S, were made and showed no significant deviation from full occupancy in the uranium sulfide while it converged to the values r = 0.80 and r = 0.81 respectively for UMo,Se, and ThMo,S, concomitantly with a significant decrease of the reliability factors to R = 0.037 and 0.038. Further refinements including anisotropic thermal parameters yielded the final values R = 0.030, 0.032 and 0.029 respectively for UMo,S,, UMo,Se, and ThMo,S,. These results show that, for two of the phases, about 18% of the actinide sites are empty in the investigated crystals and suggest the existence of a non-stoichiometry range. We tried to obtain single crystals in the uranium-rich side, using the nominal starting composition U11,4M034.4Se54,3 (UMo,Se, + 1/3U,Se,) and crystal growth was achieved using the same heating-cooling program. A platelet crystal was chosen for X-ray diffraction intensity collection with the same experimental conditions. The final refinement cycles with anisotropic thermal parameters gave the values of the reliability factors R = 0.036 and R, = 0.043 with a convergence of the occupancy factor of the uranium site to r = 0.82, which is not significantly different from the previous value. This result indicates the existence of an upper limit of -0.82 for the occupancy factor of the la site by the actinide element in UMo,Se,. A similar situation also very probably exists in ThMo,S,. Thus the formula of these compounds should be written U,,,,Mo,Se, and Th,,,,Mo$,. The positional and thermal parameters are given in Table 4 and the principal interatomic distances in Table 5.
3. Crystal structure determination
4. Crystal structure description and discussion
Single crystals with regular parallelepipedic forms were chosen to undertake the crystal structure de-
The crystal structure of the M,Mo,X, Chevrel phases has often been described in the literature. As
182
A. Daoudi et al. I Journal of Alloys and Compounds
Table 3 Crystallographic data for UMo,S,,
U,,,,Mo,Se,
Crystal dimensions (mm’) Linear absorption coefficient (cm-‘) Crystal* parameters (from CAD4): aR (A) % (“) OH CH Unit-cell volume V, (A’) Calculated density (g cm-‘) Radiation Scan mode Scan range hkl range
Setting Total observed reflections Independent reflections with I > 3m Reliability factors: R = c~[lF,l - IF,I]IPIF,I Rw = [Z,(IF,I - IFcl)2/~JFolz]“z Goodness of fit GOF
Table 4 Positional and thermal parameters of UMo,S,,
232 (19%)
180-185
and Th,,Mo,S, UMo,S,
U,, xzMo,Se,
Th, x,Mo&
0.06 x 0.10 x 0.02 225.6
0.02 x 0.02 x 0.02 430.5
0.06 x 0.04 x 0.03 222.7
6.4302( 4) 88.85(l) 9.002( 1) 11.359(2) 797.1 6.69 (MO Ko) O-28 tJ<50 -13shc13 Ochc13 OC1613 Rhombohedral 2070 1217
6.6773(7) 88.79( 1) 9.343( 1) 11.807(4) 892.6 8.07 A( MO Ka) 0 - 28 l3<40” -16
6.4611(3) 88.85(l) 9.045( 2) 11.414(2) 808.7 6.55 A(Mo KLY) w-20 l9<50 -13
0.030 0.032 0.989
0.032 0.044 1.029
0.029 0.037 1.059
Th,,,,Mo,S,
and U, RZMo6SeH
Atom
site
X
Y
Z
R,, (A?
P,,
P22
P 33
P 12
P,,
UMo,S, U MO S(1) S(2)
la 6f 6f 2c
0 0.22253( 5) 0.3677( 2) 0.2388(2)
0 0.42125(5) 0.1263( 1) 0.2388(2)
0 0.56222(5) 0.7580(2) 0.2388(2)
0.597( 1) 0.310(3) 046(l) 0.50( 1)
0.601(4) 0.305(7) 0.51(2) 0.51(2)
0.303(7) 0.37(2)
PII
PI,
0.315(7) 0.49(2)
0.046( 6) 0.12(2)
%52(6) 0.09(2)
P,1
P,,
-0.104 0.028( 6) O.OO(2) -0.03(2)
Pi,
P,,
0 0.22421(4) 0.3720( 1) 0.2406( 1)
0 0.42016(4) 0.1256(l) 0.2406( 1)
0 0.56248(4) 0.7541( 1) 0.2406(2)
0.612( 1) 0.342( 3) 0.537(9) 0.612( 1)
0.616(4) 0.335(6) 0.40( 2) 0.53( 1)
0.325(6) 0.60(2)
0.359(6) 0.61(2)
0.037(5) 0.05(2)
0.044( 5) 0.09(2)
P,,
P,,
-0.107(4) 0.041(5) O.Ol(2) -0.07( 1)
P,,
P,*
0 0.2310( 1) 0.3722( 1) 0.2368( 1)
0 0.4228( 1) 0.1214(l) 0.2368( 1)
0 0.5601( 1) 0.7593( 1) 0.2368( 1)
0.778( 4) 0.521(9) 0.70( 1) 0.707(5)
0.78(4) OSO(2) 0.73(2) 0.71(2)
0.50(2) 0.57( 2)
0.55(2) 0.79(2)
0.03(2) 0.09(2)
%3(2) 0.07(2)
Pi,
Pi,
-0.16( 1) 0.63(2) -0.01(2) -0.05(2)
P,*
Pi,
Tho.,,Mo& Th MO S(1) S(2)
la 6f 6f 2c
U, ,,Mo,%
U MO Se(l) Se(2)
la 6f 6f 2c
can be seen in Fig. 1, it consists of a stacking of Mo,X, pseudo-cubes with chalcogen atoms in the comers and molybdenum atoms slightly outside the middle of the faces. The six molybdenum atoms form a distorted octahedron involving short and unequal MO-MO distances: (MO-MO), intracluster distances between two molybdenum atoms belonging to the same triangular plane perpendicular to the ternary axis, and MO,-MO, intracluster distances between two molybdenum atoms belonging to two different triangular planes. The MO, clusters linked together with intercluster distances larger than the intracluster ones. This particular arrangement of the Mo,X, pseudo-cubes leaves two
41
PI,
PI,
PI,
P,*
P,,
PI,
P*,
PI,
types of cavity in the chalcogen atom network. These cavities are empty in the binaries Mo,X, and are filled by the M atoms in the ternary phases. One of these cavities, the largest one, is situated at the origin of the rhombohedral unit cell with point symmetry - 3. The actinide atom resides in this cavity with a pseudo-cubic environment (2 + 6). Unlike thorium, which is typically a tetravalent element, uranium can exhibit two valence states: +3 and +4 (i.e. 5f3 and 5f2 electronic configurations) in its chalcogenides. It has been shown that the valency of the uranium atoms with sulfur or selenium environment can be postulated by considering the mean
A. Daoudi
et al. I Journal of Alloys
Table 5 Interatomic distances (A) LJMo,S,
Th,, ,,Mo,Se,
U,, R2M06SeH
6 X(1) 6 MO
2.713( 1) 2.924( 1) 4.123(l)
2.746( 1) 2.972( 1) 4.140( 1)
2.797( 1) 3.054(l) 4.327( 1)
Mo-
X(2) X(1) X(1) X(1) X(1)
2.408( 2.436( 2.448( 2.551( 2.571(
2.401( 1) 2.438( 1) 2.452( 1) 2.539( 1) 2.569( 1)
2.515( 1) 2.559( 1) 2.570( 1) 2.655( 1) 2.701(l)
Mo-
2 MO” 2 Mob MO MO
2.660( 1) 2.704( 1) 3.131( 1) 3.794( 1) 3.862( 1) 4.123( 1)
2.661( 2.708( 3.170( 3.797( 3.891( 4.140(
1) 1) 1) 1) 1) 1)
2.682( 1) 2.734( 1) 3.348( 1) 3.830( 1) 4.063( 1) 4.327( 1)
2.436( 1) 2.448( 1) 2.551( 1) 2.571( 1) 2.924( 1) 3.276( 1) 3.359( 1) 3.417( 1) 3.429( 1)
2.438( 2.452( 2.539( 2.569( 2.972( 3.334( 3.403( 3.410( 3.431(
1) 1) 1) 1) 1) 1) 1) 1) 1)
2.559( 1) 2.570( 1) 2.655( 1) 2.701( 1) 3.054( 1) 3.403( 1) 3.507( 1) 3.513( 1) 3.585(l)
2.408( 2.713( 3.276( 3.417(
2.401( 1) 2.746( 1) 3.334( 1) 3.410(l)
2.515( 1) 2.797( 1) 3.403( 1) 3.513(l)
M-
2 X(2)
X(2) M MO MO MO MO M
X(1)
X(2) 2 X(1) X(2) 2 X(1) 3 MO M
X(2)
3 X(l) 3 X(l) ’ (MO-MO),
1) 1) 1) 1) 1)
1) 1) 1) 1)
and b MO&-MO, intracluster distances.
and Compounds
232 (1996)
180-18-5
183
be in an intermediate valence state (between U”+ and u4+). The mean U-Se distance in U,,,Mo,Se, is d = 2.990 A, a value which is closer to the suggested one (2.97 A for a U4+ ion in an eight-fold selenium coordination). A comparison of these M,Mo,X, (M = U, Th) with the (RE)Mo,X, (RE = rare earths) phases shows that (MO-MO), intracluster distances are very similar in both series, while the MO,-MO, intracluster as well as the the MO-MO intercluster distances decrease slightly from the Actinides to the RE compounds. Thus the MO, cluster contracts more in the uranium phases and tends to be more regular. The MO-MO intracluster distances vary mainly as a function of the valence state of the M element in the M,Mo,X, phases. These distances decrease when the valence of the M element increases and the MO, cluster becomes more regular. It has been shown that the large electronegativity differences between the two metals M and MO and the chalcogen in M,Mo,X, (0
Fig. 1. View of the UMo,S, crystal structure (white circles = U, black circles = MO, grey circles = S).
5. Magnetic properties uranium Thus in 2.871 A U4’-8s
to chalcogen distances in the structure [7]. the UMo,S, phase, the mean U-S distance of is significantly larger than that suggested for a distance (2.82 A), which suggests uranium to
Magnetic measurements were performed using an S.H.E. SQUID magnetometer. The thermal variation of the reverse susceptibility of
A. Daoudi et al. I Journal of Alloys and Compounds
184
232 (1996) 180-185
H = 2 Kgauss
I
.__
___
TEMPERATURE Fig. 2. Thermal
variation
$ y 6 El.0
I
of UMo,S,.
k/U which cannot be directly related to any valence state of uranium, on account of large crystal field effects. The susceptibility of U,.,, Mo,Se, follows a modified Curie-Weiss law above 60 K with ,uefr = 3.70 h/U
I
I
I
I
I
“\
pKiz&q
1 “\
-
'0 I
I
10
20
'0 -0-o-q I
30
8
.0*O ,0*O CO ,O'Q,000 ,o 0*0 ,O?? .0*O .OdO
1“\0
3 0
susceptibility
0.
i3 go.50.0
I
I
O-OdO,
(K)
of the reverse
UMo,S, is presented in Fig. 2. UMo,S, shows paramagnetic behaviour down to 2 K and a curve fitting using a modified Curie-Weiss law x = x0 + CI(T - e) leads to a paramagnetic temperature $., = -13.6 K and a rather low effective magnetic moment of ,u = 2.39 2.0 -
^^ so0
ZW
100
0
40
TEMPERATURE (K)
,O’O 000
.O’O
#O
3.5
O.O' O.Os ocO'
0
50
100
*c)O
LW
ZW
150
TEMPERATURE (K) Fig. 3. Thermal variation variation of magnetization
of the reverse susceptibility (upper inset).
of U,,,, Mo,Se,.
Magnetization
as a function
of magnetic
field (lower
inset).
Thermal
A. Daoudi et al. I Journal of Alloys and Compounds
and eP = -8.4. K. An onset of weak ferromagnetism is observed below 25 K (Fig. 3). The value of the ordered remanent magnetic moment at 5 K is 0.25 ,+,/U, which could not be revealed in a preliminary neutron powder diffraction experiment. Magnetic susceptibility measurements under 20 gauss were performed dow to 2.3 K on a pressed pellet of Th,.,,Mo,S, powder. A transition from weak paramagnetism to diamagnetism was observed at 3 K, independently of the sample holder. This behaviour is considered to be intrinsic and is reminiscent of a superconducting transition. The lowering of T, compared to that of the RE counterparts is related to the MO d-band fillings. The ternary neptunium selenide NpMo,Se, was also reported to exhibit superconductivity below 6 K [9], but Np is expected to be in a pure trivalent state in this compound.
232 (1996) 180-185
185
References [l] K. Yvon, in E. Kaldis (ed.), Current Topics in Materials Science, Vol. 3, Elsevier, Amsterdam, 1979, p. 53. [2] 0. Fischer and M.B. Maple, Topics of Current Physics, Vol. 32, Superconductivity in Ternary Compounds I, Springer-Verlag, Berlin, 1982. [3] R. Chevrel, M.Sergent and J. Prigent, J. Solid State Chem., 3 (1971) 515-519. Actinides 1981, Abstracts [4] H. Noel, R. Chevrel and M.Sergent, ht. Conf Pacific Grove, California, Lawrence Berkeley Lab., University of California, LBL-12441. [5] N. Walker and D. Stuart, Acta Crystallogr., A39 (1983) 158-166. [6] C.K. Fair, in Molen Users Manual. An Interactive Intelligent 1989. System For Crystal Structure Analysis, Delft, Netherlands. [7] H. Noel, J. Solid State Chem., 52 (1984), 303. [8] K. Yvon, A. Paoli, R. Fhigiger and R. Chevrel, Acta Crystallogr., B33 (1977) 3066-3072. [9] D. Damien, C.H. De Novion and J. Gall, Solid State Commun.. 38 (1981) 437.