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Crystal structure of UNi1.6As2
Journul of the Less-Common
Metals, 159 ( 1990) 12 1-
125
121
CRYSTAL STRUCTURE OF UNi,,,As, R. TROC and D. KACZOROWSKI W. Trzebiutowski Institute for Low Temperature and Structure Research, Polish Academy of Sciences. SO-950 Wroclaw, P.O. Box 937(Poland)
H. NOEL and R. GUERIN Laboratoire de Chirnie Minerale B, U.R.A. No. 254 C.N.R.S., Leclerc, 35042 Rennes (France)
Universite de Rennes, Avenue du General
(Received June 14,1989)
Summary A new nickel-defective ternary compound UNi,,6As, was prepared in single crystalline form. It crystallizes with the primitive tetragonal (space group P4/nmm) CaBe,Ge,-type structure, with the lattice parameters a=3.994( 1) A and c = 9.28 l( 11) A. The crystal structure was refined on single-crystal X-ray data to a residual value R = 0.058 using 3 15 independent reflections.
1. Introduction In recent years, we have reported the refined crystal structures of new ternary compounds occurring in the U-Cu-(P, As) systems [l-4]. Similar efforts undertaken for the compounds existing in the U-Ni-(P, As) systems have failed because of the problems in obtaining single crystals of nickel-containing phases by chemical transport. In this paper we present the successful synthesis and the structural results for one of the compounds representing the above system, i.e. UNi,,6As,, crystallizing in a defective tetragonal system. Previously, the existence of UN&As, with an ideal composition and with the CaBe,Ge,-type structure has been reported by Jeitschko et al. [5].
2. Experimental details A polycrystalline sample with nominal composition UN&As, was prepared by heating a stoichiometric mixture of the constituent elements in an evacuated sealed quartz tube at 900°C for several days. The product was multiphased. We used this product as a starting material for preparing single crystals. Small single 0022-5088/90/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
122
crystals were obtained by melting the above multiphased product at 1500 “C in a sealed molybdenum crucible placed in a graphite resistor furnace. Heating and slow cooling cycles were controlled by a programmer. 3, Structure
re~nem~nt
The crystals obtained were characterized using a four-circle diffractometer (Enraf-Nonius CAD-4) equipped with graphite monochromated MO Ka radiation, giving least-squares refined tetragonal lattice parameters of a = 3.994( 1) A and e systematic extinctions of h + k# 2n for h/c0 reflections were c=9.281(11)A.Th consistent with the space group P4/n, P4/nmm. A total number of 9 11 reflections were measured within the limits 8 < 50”, 0 < h, k < 8 and 0 < I< 20, of which 542 intensities with I> 3afI) were considered as observed. Averaging led to 3 15 independent reflections which were used for the structure determination. All calculations were performed using the SDP system 161; absorption corrections were made using the program DZFABS. The uranium position was determined from the Patterson function and the positions of the remaining atoms were obtained from subsequent Fourier synthesis. The crystal data are given in Table 1. As Table 1 indicates, the Ni( 2) and As{ 1) atoms exhibit very large thermal factors suggestive of a lower electron density and/ or atomic disorder on these crystallographic sites. The occupancy factors were refined, together with the positional and anisotropic thermal parameters; the occupancy factor of Ni(2) converged to d=0.592, while that of As(l) did not deviate significantly from t = 1. Thus, the chemical formula obtained from the above refinement is close to UNi,,AsZ. The final residuals were R=2(IF,l-lF,I)/CIF,I=0.058 and R,=(~w(lF;I-IF,I)*/~olF01*1”*=0.065 TABLE 1 Positional and thermal parameters in UNi,,,Asza
U Ni(1) Ni(2) As(l) As(Z)
2c 2b 2c 2a 2c
1 1 0.592( 5) I 1
0.25 0.75 0.25 0.75 0.25
0.25 0.25 0.25 0.25 0.25
0.2509( 1) 0.5 0.8909(9) 0 0.6423(4)
0.32( 1) 0.56(3) 2.41(9) 4.58(8) 0.48( 2)
0.29( 1) 0.67( 5) 3.5(2) 5.3(Z) 0.56(4)
0.39(Z) 0.33(9) 0.2( 2) 3.2(Z) 0.33(8)
“The form of the anisotropic displacement parameter is exp( - 1/4(hZa**#?,, + kZb*2/3,,+ Izc2/3,, + 2hka*b*#l,, + 2hla*c*/3,, + Zklb*c*&)), where a*, b* and c* are reciprocal lattice constants. For all posltions & = & = j3zj = 0.
123
(weighting scheme based on counting statistics: w = 1/a( F )*). The interatomic distances in UNi,,6Asz are listed in Table 2. An “ortep” representation of the structure together with the inner coordinations of the central atoms of uranium, Ni( 1) and Ni( 2) are given in Fig. 1.
TABLE 2 Interatomic distances in UNi, eA~Z Atoms
Distance (A)
Atoms
Distunce
u-4u U-4As( 2) U-4As( 1) U-4Ni( 1) U-4Ni( 2) U-lNi(2)
3.994( 1) 2.993( 1) 3.067( 1) 3.055( 1) 3.115(3) 3.342(8)
As( l)-4Ni(2) As( l )-4As( 1) As( l)-4U As( l)-4As(2)
2.239(4) 2.824(O) 3.067( 1) 3.874( 3)
Ni( l )-4As( 2) Ni( l)-4Ni( 1) Ni( l )-4U Ni(2)-4As( 1) Ni(2)-lAs(2) Ni( 2)-4U Ni(2)-1U Ni( 2)-4Ni( 2)
2.394(2) 2.824(O) 3.055( 1) 2.239(4) 2.307(8) 3.115(3) 3.342(8) 3.476( 8)
As(2)-lNi(2) As( 2)-4Ni( 1) As( 2)-4U
2.307(8) 2.394(2) 2.993( 1)
(A)
Ni 121 U As 121
Ni 11) As 121 U Nil21 As(l)
Fig. 1. Three-dimensional “ortep” representation of the UNi, hA~Zunit cell. The Ni(2) site is partially filled (population factor t-0.592). The inner coordination of the three different sites around the central atoms: (a) uranium (square antiprismatic), (b) Ni( 1) (tetrahedral) and (c) Ni(2) (pyramidal).
124
4. Discussion The crystal structure of UNi,,Asz is of the defective CaBe?Ge, type, but with the transition metal nickel occupying the germanium sites, as found previously in the case of the rare earth ternary arsenides RNi,As, [S, 7) with R E La-Sm. It is interesting to note that the latter arsenides, for which the CaBe,Ge,-type structure is the high temperature (ht) modification, were found to be stoichiometric, whereas those with the low temperature (It) modification (ThCr,Si,-type) occur with a slightly lower nickel content [5, 71. Contrastingly, all the rare earth nickel antimonides and bismuthides which crystallize with the CaBe,Be,-type structure at room temperature (except for the europium-based compound which adopts the ThCr,Si,-type) are strongly deficient in their nickel content [8]. For example, the single-crystal structure refinements performed for both the lanthanum- and the europium-contai~g ~timonides have yielded the compositions LaNi I.srSb, and EuNi,.,,Sb, [S]. These results indicate that the formation of nickel vacancies is invariant of crystal structure type. For the former compound this deficiency involves the nickel atoms in both positions 2a (90% Ni( 1)) and 2c (61%Ni(2)). Nevertheless, the defective RNi, _ .&Sb, Bi), compounds, as well as the other stoichiometric compounds presently known with the CaBe,Ge,-type structure, e.g. LaIr,Si, (ht) [93, RIr,Ge* (R= La-Nd) [lo] and RR~,(P,As)~ (RE La-Nd) [ll], have the proper beryllium site occupied by the transition metal. It is remarkable that the defects at the nickel position have recently also been observed for UNi,.54Pz by neutron diffraction measurements [ 121. Since this compound crystallizes with the ThCr,Si,-type structure [ 13, 141, it becomes clear that the occurrence of nickel voids in the crystal lattice is characteristic of both types of structures, in a similar way as observed for the rare earth antimonides. The difference between UNi,,6Asz and LaNi,.,Sb, is the presence of nickel vacancies at the 2a positions for the latter arsenide. Another example of a defective uranium ternary pnictide is URh1.62A~1,85,As Zemmi et al. [ 151 have shown by X-ray single-crystal refinement of this compound, a superstructure is formed owing to the specific arrangement of rhodium and arsenic voids in the lattice. Here, the rhodium vacancies were also found only at the 2c sites. Nevertheless, the structures of nickel- and rhodium-based arsenides are almost the same, but there is less disorder in the nickel than in the rhodium compound. Considering the thermal vibration parameters of Ni(2) and As( 1) atoms (Table l), it can be seen that they are large, but weaker than those of the corrcsponding atoms in URh,,,As,,, [ 151. Furthermore, we did not observe any supplementary weak reflections in our X-ray data, suggesting a superstructure in the nickel-containing arsenide. In general, the structural framework of UNi,,6As, is the same as that of UCuAs, [2]. Uranium and arsenic atoms are localized on the same 2c positions in both structures, Ni( 1) occupies the copper site and there is a partial filling of the 2c position by Ni( 2). This aspect of similarity between these structures has been considered in detail in ref. 16. From the point of view of bonding considerations, it was previously assumed
12.5
[17] for the copper-defective ternary phosphides CaCu,,,,P, and SrCu,,,,P, that the deviation from the full occupancy of the copper atom positions in the structure allows the Cu-Cu antibonding states to remain empty and consequently the compound is not being destabilized. According to another crude model, taking into account charge equilibrium [18], the formal valence of a transition metal is related to the nature of the sites occupied by this metal (i.e. oxidation number + 1 for the tetrahedral (tet) and + 2 for a pyramidal (py) site), the full charge compensation for the U-Ni-As compound is reached along the scheme: U ‘“Ni~Jtet)Ni~~(py)As~‘. This simple model applied to the uranium nickel phosphide UNi,,,,P2 needs to also take into account a P-P bonding interaction. Qualitatively, this problem is similar to that considered, for example, for the isotypic compound LaNi,P, [19], which has a full stoichiometry of 1 : 2 : 2. We now consider the interatomic distances between the nickel and arsenic atoms (Table 2). One can see that the separations of four AS(~) neighbours from the central atom Ni( 1) at a tetrahedron (2.4 A) and of four As( 1) and one AS(~) atoms from Ni(2) at a pyramid (about 2.3 A) (see also Fig. 1) are comparable with the corresponding Ni-As distances in the rare earth nickel arsenides [5, 71 as well as to the value of 2.42 A for the sum of the metallic radius of nickel for coordination number 12 and the tetrahedral radius of arsenic [20]. References 1 Z. Zolnierek, H. Noel and D. Kaczorowski, J. Less-Common Met., 126:(1987) 265. 2 .I. Stepieti-Damm, D. Kaczorowski and R. Trot, J. Less-Common Met., 132 (1987) 15. 3 H. Noel, Z. Zolnierek, D. Kaczorowski and R. Trod, J. Less-Common Met., 132 (1987) 327. 4 H. Noel, Z. Zolnierek, D. Kaczorowski, R. Trot and J. Stepien-Damm, J. Less-Common Met., 135, (1987)61. 5 W. Jeitschko, W. K. Hofmann and L. J. Terbiichte, J. Less-Common Met., 137( 1988) 133. 6 B. A. Frenz, Enraf-Nonius CAD 4 SDP, in H. Schenk, R. Olthof-Hazekamp, H. Von Konigsveld, G. C. Bassy (eds), Computing in Crystallography Delft University Press, Delft, 1978. 7 E. H. El Ghadraoui, J. Y. Pivan, R. Guerin, 0. Pena and J. Padiou, Muter. Res. Bull., 23 (1988) 1345. 8 W. K. Hofmann and W. Jeitschko, J. Less-Common Met., 138 (1988) 3 13. 9 H. F. Braun, N. Engel and E. Parthe, Phys. Rev. B, 28 ( 1983) 1389. 10 M. Francois, G. Venturini, J. F. Mar&he, B. Malaman and B. Roques, J. Less-Common Met., 113 (1985) 231. 11 R. Madar, P. Chaudouet and J. P. Senateur, J. Less-Common Met., 133 (1987) 303. 12 Fischer, A. Murasik, D. Kaczorowski and R. Trod, Physica B, 156-/57( 1989) 829. 13 W. K. Hofmann and W. Jeitschko, J. Solid State Chem., 51(1984) 152. 14 Z. Zolnierek, D. Kaczorowski, R. Trot and H. Noel, J. Less-Common Met., 121(1986) 193. 15 S. Zemmi, J. Vicat, B. Lambert, R. Madar, P. Chaudouet and J. Senateur, J. Less-Common Met., 143(1988) 113. 16 E. H. El Ghadraoui, J. Y. Pivan and R. Guerin, J. Less-Common Met., 136( 1988) 303. 17 W. H. Hofmann and W. Jeitschko, Monatsch. Chem., 116 (1985) 569. 18 R. Madar, V. Ghetta, E. Dhahri, P. Chaudouet and J. P. Senateur, .I. Solid Stare Chem., 66 (1987)
667. 19 W. Jeitschko and M. Reehuis, J. Phys. Chem. Solids, 45 (1987) 667. 20 L. Pauling, The Nature of the Chemical Bond, Cornell University 1960.
Press, Ithaca, NY, 3rd edn.,
Metals, 159 ( 1990) 12 1-
125
121
CRYSTAL STRUCTURE OF UNi,,,As, R. TROC and D. KACZOROWSKI W. Trzebiutowski Institute for Low Temperature and Structure Research, Polish Academy of Sciences. SO-950 Wroclaw, P.O. Box 937(Poland)
H. NOEL and R. GUERIN Laboratoire de Chirnie Minerale B, U.R.A. No. 254 C.N.R.S., Leclerc, 35042 Rennes (France)
Universite de Rennes, Avenue du General
(Received June 14,1989)
Summary A new nickel-defective ternary compound UNi,,6As, was prepared in single crystalline form. It crystallizes with the primitive tetragonal (space group P4/nmm) CaBe,Ge,-type structure, with the lattice parameters a=3.994( 1) A and c = 9.28 l( 11) A. The crystal structure was refined on single-crystal X-ray data to a residual value R = 0.058 using 3 15 independent reflections.
1. Introduction In recent years, we have reported the refined crystal structures of new ternary compounds occurring in the U-Cu-(P, As) systems [l-4]. Similar efforts undertaken for the compounds existing in the U-Ni-(P, As) systems have failed because of the problems in obtaining single crystals of nickel-containing phases by chemical transport. In this paper we present the successful synthesis and the structural results for one of the compounds representing the above system, i.e. UNi,,6As,, crystallizing in a defective tetragonal system. Previously, the existence of UN&As, with an ideal composition and with the CaBe,Ge,-type structure has been reported by Jeitschko et al. [5].
2. Experimental details A polycrystalline sample with nominal composition UN&As, was prepared by heating a stoichiometric mixture of the constituent elements in an evacuated sealed quartz tube at 900°C for several days. The product was multiphased. We used this product as a starting material for preparing single crystals. Small single 0022-5088/90/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
122
crystals were obtained by melting the above multiphased product at 1500 “C in a sealed molybdenum crucible placed in a graphite resistor furnace. Heating and slow cooling cycles were controlled by a programmer. 3, Structure
re~nem~nt
The crystals obtained were characterized using a four-circle diffractometer (Enraf-Nonius CAD-4) equipped with graphite monochromated MO Ka radiation, giving least-squares refined tetragonal lattice parameters of a = 3.994( 1) A and e systematic extinctions of h + k# 2n for h/c0 reflections were c=9.281(11)A.Th consistent with the space group P4/n, P4/nmm. A total number of 9 11 reflections were measured within the limits 8 < 50”, 0 < h, k < 8 and 0 < I< 20, of which 542 intensities with I> 3afI) were considered as observed. Averaging led to 3 15 independent reflections which were used for the structure determination. All calculations were performed using the SDP system 161; absorption corrections were made using the program DZFABS. The uranium position was determined from the Patterson function and the positions of the remaining atoms were obtained from subsequent Fourier synthesis. The crystal data are given in Table 1. As Table 1 indicates, the Ni( 2) and As{ 1) atoms exhibit very large thermal factors suggestive of a lower electron density and/ or atomic disorder on these crystallographic sites. The occupancy factors were refined, together with the positional and anisotropic thermal parameters; the occupancy factor of Ni(2) converged to d=0.592, while that of As(l) did not deviate significantly from t = 1. Thus, the chemical formula obtained from the above refinement is close to UNi,,AsZ. The final residuals were R=2(IF,l-lF,I)/CIF,I=0.058 and R,=(~w(lF;I-IF,I)*/~olF01*1”*=0.065 TABLE 1 Positional and thermal parameters in UNi,,,Asza
U Ni(1) Ni(2) As(l) As(Z)
2c 2b 2c 2a 2c
1 1 0.592( 5) I 1
0.25 0.75 0.25 0.75 0.25
0.25 0.25 0.25 0.25 0.25
0.2509( 1) 0.5 0.8909(9) 0 0.6423(4)
0.32( 1) 0.56(3) 2.41(9) 4.58(8) 0.48( 2)
0.29( 1) 0.67( 5) 3.5(2) 5.3(Z) 0.56(4)
0.39(Z) 0.33(9) 0.2( 2) 3.2(Z) 0.33(8)
“The form of the anisotropic displacement parameter is exp( - 1/4(hZa**#?,, + kZb*2/3,,+ Izc2/3,, + 2hka*b*#l,, + 2hla*c*/3,, + Zklb*c*&)), where a*, b* and c* are reciprocal lattice constants. For all posltions & = & = j3zj = 0.
123
(weighting scheme based on counting statistics: w = 1/a( F )*). The interatomic distances in UNi,,6Asz are listed in Table 2. An “ortep” representation of the structure together with the inner coordinations of the central atoms of uranium, Ni( 1) and Ni( 2) are given in Fig. 1.
TABLE 2 Interatomic distances in UNi, eA~Z Atoms
Distance (A)
Atoms
Distunce
u-4u U-4As( 2) U-4As( 1) U-4Ni( 1) U-4Ni( 2) U-lNi(2)
3.994( 1) 2.993( 1) 3.067( 1) 3.055( 1) 3.115(3) 3.342(8)
As( l)-4Ni(2) As( l )-4As( 1) As( l)-4U As( l)-4As(2)
2.239(4) 2.824(O) 3.067( 1) 3.874( 3)
Ni( l )-4As( 2) Ni( l)-4Ni( 1) Ni( l )-4U Ni(2)-4As( 1) Ni(2)-lAs(2) Ni( 2)-4U Ni(2)-1U Ni( 2)-4Ni( 2)
2.394(2) 2.824(O) 3.055( 1) 2.239(4) 2.307(8) 3.115(3) 3.342(8) 3.476( 8)
As(2)-lNi(2) As( 2)-4Ni( 1) As( 2)-4U
2.307(8) 2.394(2) 2.993( 1)
(A)
Ni 121 U As 121
Ni 11) As 121 U Nil21 As(l)
Fig. 1. Three-dimensional “ortep” representation of the UNi, hA~Zunit cell. The Ni(2) site is partially filled (population factor t-0.592). The inner coordination of the three different sites around the central atoms: (a) uranium (square antiprismatic), (b) Ni( 1) (tetrahedral) and (c) Ni(2) (pyramidal).
124
4. Discussion The crystal structure of UNi,,Asz is of the defective CaBe?Ge, type, but with the transition metal nickel occupying the germanium sites, as found previously in the case of the rare earth ternary arsenides RNi,As, [S, 7) with R E La-Sm. It is interesting to note that the latter arsenides, for which the CaBe,Ge,-type structure is the high temperature (ht) modification, were found to be stoichiometric, whereas those with the low temperature (It) modification (ThCr,Si,-type) occur with a slightly lower nickel content [5, 71. Contrastingly, all the rare earth nickel antimonides and bismuthides which crystallize with the CaBe,Be,-type structure at room temperature (except for the europium-based compound which adopts the ThCr,Si,-type) are strongly deficient in their nickel content [8]. For example, the single-crystal structure refinements performed for both the lanthanum- and the europium-contai~g ~timonides have yielded the compositions LaNi I.srSb, and EuNi,.,,Sb, [S]. These results indicate that the formation of nickel vacancies is invariant of crystal structure type. For the former compound this deficiency involves the nickel atoms in both positions 2a (90% Ni( 1)) and 2c (61%Ni(2)). Nevertheless, the defective RNi, _ .&Sb, Bi), compounds, as well as the other stoichiometric compounds presently known with the CaBe,Ge,-type structure, e.g. LaIr,Si, (ht) [93, RIr,Ge* (R= La-Nd) [lo] and RR~,(P,As)~ (RE La-Nd) [ll], have the proper beryllium site occupied by the transition metal. It is remarkable that the defects at the nickel position have recently also been observed for UNi,.54Pz by neutron diffraction measurements [ 121. Since this compound crystallizes with the ThCr,Si,-type structure [ 13, 141, it becomes clear that the occurrence of nickel voids in the crystal lattice is characteristic of both types of structures, in a similar way as observed for the rare earth antimonides. The difference between UNi,,6Asz and LaNi,.,Sb, is the presence of nickel vacancies at the 2a positions for the latter arsenide. Another example of a defective uranium ternary pnictide is URh1.62A~1,85,As Zemmi et al. [ 151 have shown by X-ray single-crystal refinement of this compound, a superstructure is formed owing to the specific arrangement of rhodium and arsenic voids in the lattice. Here, the rhodium vacancies were also found only at the 2c sites. Nevertheless, the structures of nickel- and rhodium-based arsenides are almost the same, but there is less disorder in the nickel than in the rhodium compound. Considering the thermal vibration parameters of Ni(2) and As( 1) atoms (Table l), it can be seen that they are large, but weaker than those of the corrcsponding atoms in URh,,,As,,, [ 151. Furthermore, we did not observe any supplementary weak reflections in our X-ray data, suggesting a superstructure in the nickel-containing arsenide. In general, the structural framework of UNi,,6As, is the same as that of UCuAs, [2]. Uranium and arsenic atoms are localized on the same 2c positions in both structures, Ni( 1) occupies the copper site and there is a partial filling of the 2c position by Ni( 2). This aspect of similarity between these structures has been considered in detail in ref. 16. From the point of view of bonding considerations, it was previously assumed
12.5
[17] for the copper-defective ternary phosphides CaCu,,,,P, and SrCu,,,,P, that the deviation from the full occupancy of the copper atom positions in the structure allows the Cu-Cu antibonding states to remain empty and consequently the compound is not being destabilized. According to another crude model, taking into account charge equilibrium [18], the formal valence of a transition metal is related to the nature of the sites occupied by this metal (i.e. oxidation number + 1 for the tetrahedral (tet) and + 2 for a pyramidal (py) site), the full charge compensation for the U-Ni-As compound is reached along the scheme: U ‘“Ni~Jtet)Ni~~(py)As~‘. This simple model applied to the uranium nickel phosphide UNi,,,,P2 needs to also take into account a P-P bonding interaction. Qualitatively, this problem is similar to that considered, for example, for the isotypic compound LaNi,P, [19], which has a full stoichiometry of 1 : 2 : 2. We now consider the interatomic distances between the nickel and arsenic atoms (Table 2). One can see that the separations of four AS(~) neighbours from the central atom Ni( 1) at a tetrahedron (2.4 A) and of four As( 1) and one AS(~) atoms from Ni(2) at a pyramid (about 2.3 A) (see also Fig. 1) are comparable with the corresponding Ni-As distances in the rare earth nickel arsenides [5, 71 as well as to the value of 2.42 A for the sum of the metallic radius of nickel for coordination number 12 and the tetrahedral radius of arsenic [20]. References 1 Z. Zolnierek, H. Noel and D. Kaczorowski, J. Less-Common Met., 126:(1987) 265. 2 .I. Stepieti-Damm, D. Kaczorowski and R. Trot, J. Less-Common Met., 132 (1987) 15. 3 H. Noel, Z. Zolnierek, D. Kaczorowski and R. Trod, J. Less-Common Met., 132 (1987) 327. 4 H. Noel, Z. Zolnierek, D. Kaczorowski, R. Trot and J. Stepien-Damm, J. Less-Common Met., 135, (1987)61. 5 W. Jeitschko, W. K. Hofmann and L. J. Terbiichte, J. Less-Common Met., 137( 1988) 133. 6 B. A. Frenz, Enraf-Nonius CAD 4 SDP, in H. Schenk, R. Olthof-Hazekamp, H. Von Konigsveld, G. C. Bassy (eds), Computing in Crystallography Delft University Press, Delft, 1978. 7 E. H. El Ghadraoui, J. Y. Pivan, R. Guerin, 0. Pena and J. Padiou, Muter. Res. Bull., 23 (1988) 1345. 8 W. K. Hofmann and W. Jeitschko, J. Less-Common Met., 138 (1988) 3 13. 9 H. F. Braun, N. Engel and E. Parthe, Phys. Rev. B, 28 ( 1983) 1389. 10 M. Francois, G. Venturini, J. F. Mar&he, B. Malaman and B. Roques, J. Less-Common Met., 113 (1985) 231. 11 R. Madar, P. Chaudouet and J. P. Senateur, J. Less-Common Met., 133 (1987) 303. 12 Fischer, A. Murasik, D. Kaczorowski and R. Trod, Physica B, 156-/57( 1989) 829. 13 W. K. Hofmann and W. Jeitschko, J. Solid State Chem., 51(1984) 152. 14 Z. Zolnierek, D. Kaczorowski, R. Trot and H. Noel, J. Less-Common Met., 121(1986) 193. 15 S. Zemmi, J. Vicat, B. Lambert, R. Madar, P. Chaudouet and J. Senateur, J. Less-Common Met., 143(1988) 113. 16 E. H. El Ghadraoui, J. Y. Pivan and R. Guerin, J. Less-Common Met., 136( 1988) 303. 17 W. H. Hofmann and W. Jeitschko, Monatsch. Chem., 116 (1985) 569. 18 R. Madar, V. Ghetta, E. Dhahri, P. Chaudouet and J. P. Senateur, .I. Solid Stare Chem., 66 (1987)
667. 19 W. Jeitschko and M. Reehuis, J. Phys. Chem. Solids, 45 (1987) 667. 20 L. Pauling, The Nature of the Chemical Bond, Cornell University 1960.
Press, Ithaca, NY, 3rd edn.,