- Email: [email protected]
Crystal structure, magnetic and electrical properties of a new ternary arsenide U2Cu4As5
Journal ofthe Less-Common
255
Metals, 170( 1991) 255-262
Crystal structure, magnetic and electrical properties of a new ternary arsenide U,Cu,As, D. Kaczorowski W. Trzebiatowski Institute for Lo.w Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroctaw, P.O. Box 937(Poland)
H. No&l and R. TroC* Laboratoire de Chimie du Solide et Inotganique MoltGxlaire, U.R.A. No. 254 C.N.R.S., Rennes, Avenue du G&Pral Leclerc, 35042 Rennes (France)
Universiti de
(Received October 11,199O)
Abstract The crystal structure of the new ternary compound U,Cu,As, has been determined. It crystallizes with the body-centred tetragonal structure (space group I4/mmm) with Z = 2 and the lattice parameters: a = 3.990( 1) A and c = 24.299( 15) A. The crystal structure hitherto unknown was refined from singlecrystal X-ray data to a residual value R = 0.041 using 229 independent reflections. It appears that U,Cu,As, is an antiferromagnet with TN = 189 K. The electrical resistivity measured within the a-b plane behaves at low temperatures as T” (n = 2.1-2.5). At higher temperatures (above T,) the slope of p( T) is negative.
1. Introduction In recent years we have reported the crystal structures of several phases occurring in uranium-transition metal-pnictogen systems namely UCu,P, [ 11, UCuAs, [2], UCuP, [3], U,Cu,P, [4], UNi,,GAs, [5] and U,NiAs, [6]. Some new ternary p&tides belonging to this family of compounds have very recently been synthesized [7]. Among them we found a new arsenide with the chemical formula U,Cu,As, and a tetragonal crystal structure [8]. In this paper we report on the detailed crystallographic data of this compound as well as on its magnetic and electrical resistivity properties. 2. Experimental details 2.1. Sample preparation Polycrystalline U,Cu,As, was prepared from a mixture of the constituent elements enclosed in an evacuated quartz ampoule and heated at 750 “C for 2 weeks. The X-ray powder analysis performed on a DRON 1.5 diffractometer *On leave from the W. Trzebiatowski Institute for Low Temperature Polish Academy of Sciences, P.O. Box 937,50-950 Wroclaw, Poland. 0022-5088/91/$3.50
and Structure Research,
0 Elsevier Sequoia/Printed
in The Netherlands
256
showed a single-phase material with the lattice parameters: a= 3.99 A and c = 24.30 A, which are in fair agreement with our previously reported data [8]. Single crystals of U,Cu,As, were obtained by the chemical vapour transport method using iodine as a transporting agent in a two-zone furnace with the temperature gradient of 900 -+ 950 “C. The crystals had the form of thin plates with dimensions up to 3 mm X 2 mm X 0.1 mm. 2.2. Crystal structure determination A small single crystal with dimensions 0.1 mm X 0.1 mm X 0.1 mm, obtained by gently crushing a larger one, was selected for the structure determination. The tetragonal lattice parameters (a = 3.990( 1) A, c = 24.299( 15) A) were determined by least-squares refinements of the diffraction angles of 25 reflections collected using a four-circle diffractometer (Nonius CAD4, MO Ka radiation). The observed systematic extinctions (hkl: h + k + I# 2n) are consistent with a bodycentred lattice. The structure could be successfully refined in the highest symmetrical space group I4/mmm with Z= 2. A total number of 539 reflections were measured within the limits: 0 < 35”; 0 < h, k < 6; 0 < 1 < 39; of this number 417 intensities with Z > 3 a( I) were considered as observed. Averaging led to 229 independent reflections which were used for the structure determination. All calculations were performed using the S.D.P. system [9]. A spherical-type absorption correction was applied to the measured intensities, assuming the crystal to be a sphere, with pur= 3.6. It was deduced from the Patterson function that the uranium atom occupies a 4e position of the space group Z4/mmm. A difference Fourier map revealed the positions of the remaining atoms, confirming for this new compound the chemical formula U,Cu,As, determined from the synthesis experiments. Refinements of the atomic positions with isotropic thermal factors converged to z?=E[]F,]
- ]F,]]/~]F,]=O.O41
and
(weighting scheme based on counting statistics: o= l/c~(F)~). These values decreased slightly to R = 0.037 and R, = 0.05 1 when we performed a refining including the anisotropic thermal parameters. Looking at the B values collected in Table 1 we see that the thermal vibrations are only weakly anisotropic. The positional factors are given in Table 1 and the main interatomic distances are listed in Table 2. 2.3. Magnetic and electrical measurements Magnetic susceptibility measurements on the powder sample were performed in the temperature interval 4.2-300 K by using an R.H. Cahn electrobalance. Electrical resistivity was measured in the basal plane of the tetragonal unit cell between 4.2 and 300 K using a conventional four-point d.c. method.
257
TABLE 1 Positional parameters, isotropic and anisotropic thermal factors for atoms in U,Cu,As, Atom
Position
x
y
2
B(A22)
B(Ll)
i3(2,2)
8(3,3)
U CU As(l) AS(~) AS(~)
4e 8g 2a 4d 4e
0 0 0 0 0
0 0.5 0 0.5 0
0.15801(3) 0.05686(6) 0 0.25 0.37714(7)
0.51(l) 1.17(3) 0.81(3) 0.74( 1) 0.68(2)
OSO( 1) 1.45(5) 0.70(6~ 0.82(3) 0.61(4)
B(1,1)
0.52(2) 1.04(4) 1.05(7) 0.59(4) 0.83( 5)
1.03(6) B(l>l) B(l>l) B(l>l)
The form of the anisotropic displacement parameter is exp[ - 1/4(h~~*~~(l,l)+k”b*~j3(2,2)+ l’c**@(3,3)+ 2hka*b*/3(1,2) + Zhla*c*B( 1,3) + 2klb*c*B(2,3))] where a*, b* and c* are reciprocal lattice constants. For all atoms: #I(1,2) = #?(1,3) = /3(2,3) = 0.
TABLE 2 Interatomic distances (A) in U,Cu,As, u-4u U-4& U-4As( 2) U-4As( 3)
3.990( 1) 3.165(2) 2.996( 1) 2.948( 2)
cu-2u cu- 1Cu Cu-2As(l) Cu-ZAs( 3)
3.165(2) 2.763(4) 2.427(Z) 2.560( 3)
As(l)-Ku As{ l )-4As( 1)
2.427( 1) 3.990( 1)
AS(~)-4U AS(~)-4As(2)
2.996(O) 2.821(O)
As( 3)-4Cu As( 3)-4U AS(~)-4As(2)
2.560(I) 2.948(2) 3.678(2)
3. Results and discussion
The crystal structure of U,Cu,As, is shown in Fig. 1. This structure reveals a typical layered character with a succession of sheets normal to the tetragonal c-axis which can be represented by the sequence: U-As( 2)-U-As( 3)-Cu-As( 1 )-CuAS(~)-U. The U-U interlayer separation within the U-AS(~)-U slab amounts to 4.470 A. It should be noted that this slab has its reference in both the UAs, and U&As, unit cells where the corresponding U-U interlayer spacing is only slightly different, i.e. 4.596 and 4.465 8, for the former and latter compound, respectively. The other slab U-As( 3)-Cu-As( 1)-Cu-As( 3)-U, also occurring in UZCu,As,, is a feature characteristic of this new structure only and hence there is no simple relationship between this structure and those presented in ref. 2. One notes that the ura~um atom sheets are separated here by as many as five non-ma~etic atom layfrs and their separation is extremely large being 7.679 A. As a result, the c-parameter strongly increases and the c/a ratio in this compound is as large as
258
As(l)
cu AS(~) U Ad21
cu As(l) cu As (31 U AsPI U A~(31 cu As(l)
l U @I cu 0
As
Fig. 1. The unit cell of U,Cu,As,.
6.09. Although a considerably higher value of 9.16 for the c/a ratio was previously found for the phosphide U,Cu,P, [4], the largest U-U interlayer separation in this compound is only 5.324 A. The coordination polyhedra in U2Cu4As5 are similar to those observed in the UCuAs,-type structure. The uranium atom is located in a site of C,, point symmetry where it is coordinated by 12 nearest-neighbours (4As( 3) + 4As( 2) + 4Cu). The AS(~) and AS(~) atoms form a square antiprism with the base sides equal to a and a/2l/*, respectively. The Cu atoms are placed above this antiprism in a square arrangement with the side a/2l’*. The coordination sphere of the copper atom contains as many as 11 atoms (2As( l)-2As( 3) + 1Cu + 4Cu + 2U). The near-neighbour Cu atoms form a square pyramid with the base side a, being slightly flattened along the c-axis, while the As( 1) and AS(~) atoms together form a distorted tetrahedron. The Cu coordination polyhedron is completed by two U atoms placed in a linear arrangement below the tetrahedron.
259
The coordination number for all arsenic atoms is eight, but the locations of their near neighbours are different. The As( 1) atom is surrounded by eight copper atoms forming a regular square parallelepiped with a base side equal to ~7/2’/~.The AS(~) atom has four of its nearest neighbours, which are the AS(~) atoms, in a square arrangement with a base side a, and four uranium atoms which form a tetrahedron. The coordination sphere of AS(~) is like that for the U atoms, i.e. it contains four Cu and four U atoms which form a square antiprism with the base sides being equal to a/2l/’ and a, respectively. Considering the shortest interatomic distances in U,Cu,As, (Table 2), one can say that they are essentially at the same distances as those observed in the UCuAs,-type structure. The latter, in turn, is regarded as a main block arrangement reproduced in other-type structures [4]. The U-As spacings are only slightly lower than the sum of the ionic radii of the U4+ and As’- ions. This indicates that the U-As chemical bonding is mostly ionic. An important feature of this structure is the short distances between the copper and arsenic atoms (2.427 and 2.560 A) which reflect a strongly covalent character of the Cu-As bonding. This interaction leads to the formation of pyramidal layers built of both transition-metal and pnictogen atoms. Such an arrangement reminds one of the series of the uranium and rare-earth ternaries crystallizing in some derivatives of the BaAl,-type structure [lo]. Moreover, it is clear that due to the covalency of the Cu-As bonds the effective oxidation number for arsenic is less than - 3 at least for the As( 1) atoms (the Cu-As( 1) bond length is the shortest). A lower oxidation number may also be ascribed to the AS(~) atoms for which some covalent interaction between the arsenic atoms themselves can be deduced from the somewhat short As( 2)-As( 2) separation-2.821 A. In view of the remark previously given, it seems reasonable to represent a formal valence scheme for U,Cu,As, by the formula: U;+Cui+As( 1)2-As(2)z-As( 3)$-. On the other hand, the metallic conductivity of this compound (to be discussed later) indicates that the charge balance is also made here by the conduction electrons and possibly by the valence-band holes. Thus, the compensation scheme previously proposed cannot be taken too literally. Finally, it is worthwhile considering the shortest U-U interatomic distances characteristic of the U,Cu,As, structure. Two such distances can be distinguished within the a-b plane of the tetragonal unit cell of this compound. The shortest U-U separation of 3.990 A is along the basal axis, and the other along the diagonal of the plane, which is 5.643 A. Comparing these values with corresponding U-U interatomic spacings in other uranium compounds, it is reasonable to expect a strong ferromagnetic coupling of uranium magnetic moments within the (001) plane as is the case for example for all semimetallic uranium pnictides with a tetragonal symmetry [ 111. The distance between the uranium atoms, taken along the c-axis, i.e. within the U-AS(~)-U slab (5.286 A) and across the U-AS(~)-CuAs( 1)-Cu-AS(~)-U slab (7.679 A) excludes the presence of any appreciable U-U exchange interactions along the c-axis. The latter distance can only be compared with that found recently in the oxysulphide of uranium and erbium (UO),ErS, which have a similar tetragonal structure with c/a = 5.50 and in which the U-U distance in the slab U-S-(Er,S)-S-U is as large as 7.82 A [ 121.
260
3.2. Physical properties The temperature variation of the magnetic susceptibility of U,Cu,As, measured on the powder specimen, x,( T ), is presented in Fig. 2. As deduced from the pronounced maximum in x,(T), this arsenide orders antiferromagnetically below 189 K. The thermal variatron of x p 1 is slightly curvilinear and above 195 K this curve can be approximated to the modified Curie-Weiss law with the param-
120 IO0 1
30 24 -
I
- 80 z 18 -
- 60 ; E
12 -
40 s
6-
20
I_-_-1 50
150
100
00 300
250
200
T(K) Fig. 2. Molar susceptibility (left-hand scale) and inverse molar susceptibility U,Cu,As, as a function of temperature measured on a powder sample.
(right-hand
scale) of
1.5
1.0
a5
I
0
1
so
I
ml
6
I
150
200
2%
300
T(K) Fig. 3. Electrical resistivity of a U,Cu,As, single crystal measured within the a-b plane. The inset shows the temperature dependence of dp/d T around the NeCl point.
261
70 -
?,..:~~id35 ,./' :. : ;.' :'
60U,Cu,AsS 50 f&o e 2 30 I ' 20 1
1
:
:' .Y' ,..'
,. '.
..,,,,.
_‘_I
0
5000
15m
10000 T"[K
--I
2ooClo
1”
Fig. 4. Electrical resistivity of U,Cu,As, as a function of T”. Curve 1, n =2.4.55, and curve 2, n = 2.135. were found in the temperature range 4.2-50 and 4.2-100 K, respectively.
eters: x0=926 x 10-h e.m.u. mall’, C=O.473 K e.m.u. mol-’ (,M~,,=1.95 ,u,/U atom)and 13,=168K. In Fig. 3 we show the temperature dependence of the electrical resistivity of U&&As, measured within the a-b plane. As seen, the resistivity of U,Cu,As, has a distinct knee at TN and a negative slope above TN. The inset of this figure gives evidence of the rapid fall of the derivative dp/d T at TN, the value of which is also close to 189 K. However, the behaviour of p( T) at low temperatures is atypical. In Fig. 4 one sees that p(T) can be approximated as T “, where y1is equal either to 2.14 or 2.46, depending on the temperature range considered. These values are found from the regions 4.2-100 and 4.2-50 K, respectively. The latter IZvalue is close to 5/2. So far such a value has been found in two cases-USb, [13] and UNiGe [ 14]-but the reason of its occurrence is not yet clear. A further attempt to fit the low temperature resistivity data to the exponential function p( T) = T 2 x exp( -A/T ) gave no satisfactory result even in the lowest range of temperatures studied. References I Z. Zofnierek, H. Noel and D. Kaczorowski, J. Less-Common Met., 128 (1987) 265. D. Kaczorowski and R. Trod, J. Less-Common Met., 132 (1987) 15. 3 H. Noel, 2. Zobierek, D. Kaczorowski and R. Trod, J. Less-Common Met., 132 (1987) 327. 4 H. Noel, Z. Zofnierek, D. Kaczorowski, R. Trod and J. Stepien-Damm, J. Less-Common Met., I35 2 J. Stepieri-Damm,
(1987)61. 5 R. Trot, D. Kaczorowski,
H. Noel and R. Guerin, J. Less-Common 6 R. Trot, D. Kaczorowski, H. Noel and R. Guerin, .I. Less-Common
Met., 1.59( 1990) 12 1. Met., 157( 1990) Li.
262 7 D. Kaczorowski, Abstracts of “19’“‘” Journtes des Actinides”, Madonna di Campiglio, Italy, 29-31 March, 1989, p. 17. 8 D. Kaczorowski and R. Trod, Abstracts of “17”“” Journe’es des Actinides”, Signal des Chexbres, Switzerland, 26-28 March, 1987, p. 11.
9 B. A. Frenz, Enraf Nonius CAD4 SDP, in H. Schenk, R. Olthof-Hazekamp, H. Von Koningsveld and G. C. Bassy (eds.), Computing in Crystallography, Delft University Press, Delft, 1978. 10 G. Cordier, B. Eisenmann and H. Schafer, Z. Anorg. Allg. Chem., 426 (1976) 205. 11 R. TroC, Inorg. Chimica Acta, 140 (1987) 67. 12 S. Jaulmes, M. Julien-Pouzol, M. Guittard, T. Vovan, P. Laruelle and J. Flahaut, Acta Crystullogr. C,42(1986) 1109. 13 Z. Henkie and Z. Kletowski, Acta Phys. Polon. A, 42 (1972) 405. 14 V. H. Tran, R. Trot and D. Badurski, J. Magn. Magn. Mater., 87( 1990) 291.
255
Metals, 170( 1991) 255-262
Crystal structure, magnetic and electrical properties of a new ternary arsenide U,Cu,As, D. Kaczorowski W. Trzebiatowski Institute for Lo.w Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroctaw, P.O. Box 937(Poland)
H. No&l and R. TroC* Laboratoire de Chimie du Solide et Inotganique MoltGxlaire, U.R.A. No. 254 C.N.R.S., Rennes, Avenue du G&Pral Leclerc, 35042 Rennes (France)
Universiti de
(Received October 11,199O)
Abstract The crystal structure of the new ternary compound U,Cu,As, has been determined. It crystallizes with the body-centred tetragonal structure (space group I4/mmm) with Z = 2 and the lattice parameters: a = 3.990( 1) A and c = 24.299( 15) A. The crystal structure hitherto unknown was refined from singlecrystal X-ray data to a residual value R = 0.041 using 229 independent reflections. It appears that U,Cu,As, is an antiferromagnet with TN = 189 K. The electrical resistivity measured within the a-b plane behaves at low temperatures as T” (n = 2.1-2.5). At higher temperatures (above T,) the slope of p( T) is negative.
1. Introduction In recent years we have reported the crystal structures of several phases occurring in uranium-transition metal-pnictogen systems namely UCu,P, [ 11, UCuAs, [2], UCuP, [3], U,Cu,P, [4], UNi,,GAs, [5] and U,NiAs, [6]. Some new ternary p&tides belonging to this family of compounds have very recently been synthesized [7]. Among them we found a new arsenide with the chemical formula U,Cu,As, and a tetragonal crystal structure [8]. In this paper we report on the detailed crystallographic data of this compound as well as on its magnetic and electrical resistivity properties. 2. Experimental details 2.1. Sample preparation Polycrystalline U,Cu,As, was prepared from a mixture of the constituent elements enclosed in an evacuated quartz ampoule and heated at 750 “C for 2 weeks. The X-ray powder analysis performed on a DRON 1.5 diffractometer *On leave from the W. Trzebiatowski Institute for Low Temperature Polish Academy of Sciences, P.O. Box 937,50-950 Wroclaw, Poland. 0022-5088/91/$3.50
and Structure Research,
0 Elsevier Sequoia/Printed
in The Netherlands
256
showed a single-phase material with the lattice parameters: a= 3.99 A and c = 24.30 A, which are in fair agreement with our previously reported data [8]. Single crystals of U,Cu,As, were obtained by the chemical vapour transport method using iodine as a transporting agent in a two-zone furnace with the temperature gradient of 900 -+ 950 “C. The crystals had the form of thin plates with dimensions up to 3 mm X 2 mm X 0.1 mm. 2.2. Crystal structure determination A small single crystal with dimensions 0.1 mm X 0.1 mm X 0.1 mm, obtained by gently crushing a larger one, was selected for the structure determination. The tetragonal lattice parameters (a = 3.990( 1) A, c = 24.299( 15) A) were determined by least-squares refinements of the diffraction angles of 25 reflections collected using a four-circle diffractometer (Nonius CAD4, MO Ka radiation). The observed systematic extinctions (hkl: h + k + I# 2n) are consistent with a bodycentred lattice. The structure could be successfully refined in the highest symmetrical space group I4/mmm with Z= 2. A total number of 539 reflections were measured within the limits: 0 < 35”; 0 < h, k < 6; 0 < 1 < 39; of this number 417 intensities with Z > 3 a( I) were considered as observed. Averaging led to 229 independent reflections which were used for the structure determination. All calculations were performed using the S.D.P. system [9]. A spherical-type absorption correction was applied to the measured intensities, assuming the crystal to be a sphere, with pur= 3.6. It was deduced from the Patterson function that the uranium atom occupies a 4e position of the space group Z4/mmm. A difference Fourier map revealed the positions of the remaining atoms, confirming for this new compound the chemical formula U,Cu,As, determined from the synthesis experiments. Refinements of the atomic positions with isotropic thermal factors converged to z?=E[]F,]
- ]F,]]/~]F,]=O.O41
and
(weighting scheme based on counting statistics: o= l/c~(F)~). These values decreased slightly to R = 0.037 and R, = 0.05 1 when we performed a refining including the anisotropic thermal parameters. Looking at the B values collected in Table 1 we see that the thermal vibrations are only weakly anisotropic. The positional factors are given in Table 1 and the main interatomic distances are listed in Table 2. 2.3. Magnetic and electrical measurements Magnetic susceptibility measurements on the powder sample were performed in the temperature interval 4.2-300 K by using an R.H. Cahn electrobalance. Electrical resistivity was measured in the basal plane of the tetragonal unit cell between 4.2 and 300 K using a conventional four-point d.c. method.
257
TABLE 1 Positional parameters, isotropic and anisotropic thermal factors for atoms in U,Cu,As, Atom
Position
x
y
2
B(A22)
B(Ll)
i3(2,2)
8(3,3)
U CU As(l) AS(~) AS(~)
4e 8g 2a 4d 4e
0 0 0 0 0
0 0.5 0 0.5 0
0.15801(3) 0.05686(6) 0 0.25 0.37714(7)
0.51(l) 1.17(3) 0.81(3) 0.74( 1) 0.68(2)
OSO( 1) 1.45(5) 0.70(6~ 0.82(3) 0.61(4)
B(1,1)
0.52(2) 1.04(4) 1.05(7) 0.59(4) 0.83( 5)
1.03(6) B(l>l) B(l>l) B(l>l)
The form of the anisotropic displacement parameter is exp[ - 1/4(h~~*~~(l,l)+k”b*~j3(2,2)+ l’c**@(3,3)+ 2hka*b*/3(1,2) + Zhla*c*B( 1,3) + 2klb*c*B(2,3))] where a*, b* and c* are reciprocal lattice constants. For all atoms: #I(1,2) = #?(1,3) = /3(2,3) = 0.
TABLE 2 Interatomic distances (A) in U,Cu,As, u-4u U-4& U-4As( 2) U-4As( 3)
3.990( 1) 3.165(2) 2.996( 1) 2.948( 2)
cu-2u cu- 1Cu Cu-2As(l) Cu-ZAs( 3)
3.165(2) 2.763(4) 2.427(Z) 2.560( 3)
As(l)-Ku As{ l )-4As( 1)
2.427( 1) 3.990( 1)
AS(~)-4U AS(~)-4As(2)
2.996(O) 2.821(O)
As( 3)-4Cu As( 3)-4U AS(~)-4As(2)
2.560(I) 2.948(2) 3.678(2)
3. Results and discussion
The crystal structure of U,Cu,As, is shown in Fig. 1. This structure reveals a typical layered character with a succession of sheets normal to the tetragonal c-axis which can be represented by the sequence: U-As( 2)-U-As( 3)-Cu-As( 1 )-CuAS(~)-U. The U-U interlayer separation within the U-AS(~)-U slab amounts to 4.470 A. It should be noted that this slab has its reference in both the UAs, and U&As, unit cells where the corresponding U-U interlayer spacing is only slightly different, i.e. 4.596 and 4.465 8, for the former and latter compound, respectively. The other slab U-As( 3)-Cu-As( 1)-Cu-As( 3)-U, also occurring in UZCu,As,, is a feature characteristic of this new structure only and hence there is no simple relationship between this structure and those presented in ref. 2. One notes that the ura~um atom sheets are separated here by as many as five non-ma~etic atom layfrs and their separation is extremely large being 7.679 A. As a result, the c-parameter strongly increases and the c/a ratio in this compound is as large as
258
As(l)
cu AS(~) U Ad21
cu As(l) cu As (31 U AsPI U A~(31 cu As(l)
l U @I cu 0
As
Fig. 1. The unit cell of U,Cu,As,.
6.09. Although a considerably higher value of 9.16 for the c/a ratio was previously found for the phosphide U,Cu,P, [4], the largest U-U interlayer separation in this compound is only 5.324 A. The coordination polyhedra in U2Cu4As5 are similar to those observed in the UCuAs,-type structure. The uranium atom is located in a site of C,, point symmetry where it is coordinated by 12 nearest-neighbours (4As( 3) + 4As( 2) + 4Cu). The AS(~) and AS(~) atoms form a square antiprism with the base sides equal to a and a/2l/*, respectively. The Cu atoms are placed above this antiprism in a square arrangement with the side a/2l’*. The coordination sphere of the copper atom contains as many as 11 atoms (2As( l)-2As( 3) + 1Cu + 4Cu + 2U). The near-neighbour Cu atoms form a square pyramid with the base side a, being slightly flattened along the c-axis, while the As( 1) and AS(~) atoms together form a distorted tetrahedron. The Cu coordination polyhedron is completed by two U atoms placed in a linear arrangement below the tetrahedron.
259
The coordination number for all arsenic atoms is eight, but the locations of their near neighbours are different. The As( 1) atom is surrounded by eight copper atoms forming a regular square parallelepiped with a base side equal to ~7/2’/~.The AS(~) atom has four of its nearest neighbours, which are the AS(~) atoms, in a square arrangement with a base side a, and four uranium atoms which form a tetrahedron. The coordination sphere of AS(~) is like that for the U atoms, i.e. it contains four Cu and four U atoms which form a square antiprism with the base sides being equal to a/2l/’ and a, respectively. Considering the shortest interatomic distances in U,Cu,As, (Table 2), one can say that they are essentially at the same distances as those observed in the UCuAs,-type structure. The latter, in turn, is regarded as a main block arrangement reproduced in other-type structures [4]. The U-As spacings are only slightly lower than the sum of the ionic radii of the U4+ and As’- ions. This indicates that the U-As chemical bonding is mostly ionic. An important feature of this structure is the short distances between the copper and arsenic atoms (2.427 and 2.560 A) which reflect a strongly covalent character of the Cu-As bonding. This interaction leads to the formation of pyramidal layers built of both transition-metal and pnictogen atoms. Such an arrangement reminds one of the series of the uranium and rare-earth ternaries crystallizing in some derivatives of the BaAl,-type structure [lo]. Moreover, it is clear that due to the covalency of the Cu-As bonds the effective oxidation number for arsenic is less than - 3 at least for the As( 1) atoms (the Cu-As( 1) bond length is the shortest). A lower oxidation number may also be ascribed to the AS(~) atoms for which some covalent interaction between the arsenic atoms themselves can be deduced from the somewhat short As( 2)-As( 2) separation-2.821 A. In view of the remark previously given, it seems reasonable to represent a formal valence scheme for U,Cu,As, by the formula: U;+Cui+As( 1)2-As(2)z-As( 3)$-. On the other hand, the metallic conductivity of this compound (to be discussed later) indicates that the charge balance is also made here by the conduction electrons and possibly by the valence-band holes. Thus, the compensation scheme previously proposed cannot be taken too literally. Finally, it is worthwhile considering the shortest U-U interatomic distances characteristic of the U,Cu,As, structure. Two such distances can be distinguished within the a-b plane of the tetragonal unit cell of this compound. The shortest U-U separation of 3.990 A is along the basal axis, and the other along the diagonal of the plane, which is 5.643 A. Comparing these values with corresponding U-U interatomic spacings in other uranium compounds, it is reasonable to expect a strong ferromagnetic coupling of uranium magnetic moments within the (001) plane as is the case for example for all semimetallic uranium pnictides with a tetragonal symmetry [ 111. The distance between the uranium atoms, taken along the c-axis, i.e. within the U-AS(~)-U slab (5.286 A) and across the U-AS(~)-CuAs( 1)-Cu-AS(~)-U slab (7.679 A) excludes the presence of any appreciable U-U exchange interactions along the c-axis. The latter distance can only be compared with that found recently in the oxysulphide of uranium and erbium (UO),ErS, which have a similar tetragonal structure with c/a = 5.50 and in which the U-U distance in the slab U-S-(Er,S)-S-U is as large as 7.82 A [ 121.
260
3.2. Physical properties The temperature variation of the magnetic susceptibility of U,Cu,As, measured on the powder specimen, x,( T ), is presented in Fig. 2. As deduced from the pronounced maximum in x,(T), this arsenide orders antiferromagnetically below 189 K. The thermal variatron of x p 1 is slightly curvilinear and above 195 K this curve can be approximated to the modified Curie-Weiss law with the param-
120 IO0 1
30 24 -
I
- 80 z 18 -
- 60 ; E
12 -
40 s
6-
20
I_-_-1 50
150
100
00 300
250
200
T(K) Fig. 2. Molar susceptibility (left-hand scale) and inverse molar susceptibility U,Cu,As, as a function of temperature measured on a powder sample.
(right-hand
scale) of
1.5
1.0
a5
I
0
1
so
I
ml
6
I
150
200
2%
300
T(K) Fig. 3. Electrical resistivity of a U,Cu,As, single crystal measured within the a-b plane. The inset shows the temperature dependence of dp/d T around the NeCl point.
261
70 -
?,..:~~id35 ,./' :. : ;.' :'
60U,Cu,AsS 50 f&o e 2 30 I ' 20 1
1
:
:' .Y' ,..'
,. '.
..,,,,.
_‘_I
0
5000
15m
10000 T"[K
--I
2ooClo
1”
Fig. 4. Electrical resistivity of U,Cu,As, as a function of T”. Curve 1, n =2.4.55, and curve 2, n = 2.135. were found in the temperature range 4.2-50 and 4.2-100 K, respectively.
eters: x0=926 x 10-h e.m.u. mall’, C=O.473 K e.m.u. mol-’ (,M~,,=1.95 ,u,/U atom)and 13,=168K. In Fig. 3 we show the temperature dependence of the electrical resistivity of U&&As, measured within the a-b plane. As seen, the resistivity of U,Cu,As, has a distinct knee at TN and a negative slope above TN. The inset of this figure gives evidence of the rapid fall of the derivative dp/d T at TN, the value of which is also close to 189 K. However, the behaviour of p( T) at low temperatures is atypical. In Fig. 4 one sees that p(T) can be approximated as T “, where y1is equal either to 2.14 or 2.46, depending on the temperature range considered. These values are found from the regions 4.2-100 and 4.2-50 K, respectively. The latter IZvalue is close to 5/2. So far such a value has been found in two cases-USb, [13] and UNiGe [ 14]-but the reason of its occurrence is not yet clear. A further attempt to fit the low temperature resistivity data to the exponential function p( T) = T 2 x exp( -A/T ) gave no satisfactory result even in the lowest range of temperatures studied. References I Z. Zofnierek, H. Noel and D. Kaczorowski, J. Less-Common Met., 128 (1987) 265. D. Kaczorowski and R. Trod, J. Less-Common Met., 132 (1987) 15. 3 H. Noel, 2. Zobierek, D. Kaczorowski and R. Trod, J. Less-Common Met., 132 (1987) 327. 4 H. Noel, Z. Zofnierek, D. Kaczorowski, R. Trod and J. Stepien-Damm, J. Less-Common Met., I35 2 J. Stepieri-Damm,
(1987)61. 5 R. Trot, D. Kaczorowski,
H. Noel and R. Guerin, J. Less-Common 6 R. Trot, D. Kaczorowski, H. Noel and R. Guerin, .I. Less-Common
Met., 1.59( 1990) 12 1. Met., 157( 1990) Li.
262 7 D. Kaczorowski, Abstracts of “19’“‘” Journtes des Actinides”, Madonna di Campiglio, Italy, 29-31 March, 1989, p. 17. 8 D. Kaczorowski and R. Trod, Abstracts of “17”“” Journe’es des Actinides”, Signal des Chexbres, Switzerland, 26-28 March, 1987, p. 11.
9 B. A. Frenz, Enraf Nonius CAD4 SDP, in H. Schenk, R. Olthof-Hazekamp, H. Von Koningsveld and G. C. Bassy (eds.), Computing in Crystallography, Delft University Press, Delft, 1978. 10 G. Cordier, B. Eisenmann and H. Schafer, Z. Anorg. Allg. Chem., 426 (1976) 205. 11 R. TroC, Inorg. Chimica Acta, 140 (1987) 67. 12 S. Jaulmes, M. Julien-Pouzol, M. Guittard, T. Vovan, P. Laruelle and J. Flahaut, Acta Crystullogr. C,42(1986) 1109. 13 Z. Henkie and Z. Kletowski, Acta Phys. Polon. A, 42 (1972) 405. 14 V. H. Tran, R. Trot and D. Badurski, J. Magn. Magn. Mater., 87( 1990) 291.