- Email: [email protected]
Magnetic and crystallographic structure of Th6Mn23Dx
Journal
of the Less-Common
MAGNETIC
Metals,
96(1984)
201
201-211
AND CRYSTALLOGRAPHIC
STRUCTURE
OF
TW+b3Dx
KAY HARDMAN-RHYNE ~utional Bureau of Standards,
Wushington, DC 20234~~.S.~.~
H. KEVIN SMITH and W. E. WALLACE Department of Chemistry, University of Pittsburgh,
Pittsburgh,
PA 1526O(U.S.A.)
(Received March 21,1983)
Summary Th,Mn,,D,, and Th,Mn,,D,, were studied by neutron diffraction Rietveld refinement methods. At low temperatures (below 78 K) ThcMn,,D,, undergoes a crystallographic change from f.c.c. (Fm3m) to primitive tetragonal (P4/mmm). This compound has no long-range magnetic ordering down to temperatures of 4K. Deuterium is found only in the f, (tetrahedral) and i (trigonal) sites; the octahedral a site is empty. Th,Mn,,D,, retains f.c.c. symmetry even at 4 K but exhibits ferromagnetism with a Curie temperature of 329 K. All moments are coupled parallel except those on the b site, which has one manganese moment and is coupled antiparallel to the other 22 manganese moments in the d, f, and f2 sites. The manganese magnetic moments are much smaller than those in Y,Mn,,.
1. Introduction Th,Mn,, is the prototype structure of a large class of 6-23 compounds of rare earths and actinides with manganese and iron, many of which have very interesting magnetic properties. In addition, the compounds generally absorb hydrogen readily, accompanied by significant increases in unit cell volume and, in some cases, pronounced effects on the magnetic ordering. The Th,Mn,, structure has an f.c.c. unit cell (space group, Fm3m) with a lattice parameter of 12.523 A at room temperature which contains 116 atoms (four formula units) [l]. There is one distinct thorium site (e) and four manganese sites (b, d, f, and fJ which have a maximum occupancy of 6,1,6,8 and 8 atoms respectively per formula unit. The interatomic distances of the manganese atoms vary significantly among different sites which partly accounts for the wide variations in manganese magnetic moments observed in the magnetically ordered compounds of this class [2, 31. Neutron diffraction, with its ability to probe ordered magnetic moments on different site sublattices, has ((2 Elsevier Sequoia/Printed
in The Netherlands
202
proven to be a valuable tool in understanding the magnetism in these compounds. In this paper we shall discuss the changes occurring in both the crystallographic structure, including a pronounced tetragonal distortion at low temperature, and the magnetic ordering of Th,Mn,, upon deuteration.
2. Experimental
considerations
Polycrystalline ThsMnZ3 was prepared at the University of Pittsburgh by induction melting in a water-cooled copper boat under an argon atmosphere [4]. The bulk material readily absorbs hydrogen at room temperature. The hydrogen uptake is instantaneous. The amount of loading of deuterium is determined by the pressure drop in a fixed volume apparatus. In samples prepared for these experiments high purity deuterium was used in lieu of hydrogen because of the large incoherent scattering cross section of hydrogen atoms for neutrons. Figure 1 shows the deuterium absorption isotherm for Th,Mn,,, illustrating the relatively low pressures required to achieve deuteration even up to the full saturation value of approximately 30 atoms per formula unit [2]. A broad absorption plateau is found to exist from 17 to 24 deuteriuln atoms, in contrast with YsMnZ3D,, Gd,Mn,,D, and other 6-23 systems which do not exhibit any plateaux [l, 51. Two samples were prepared for this study, one with 16 deuterium atoms (just below the plateau), as determined both from the volume of deuterium absorbed and from the structure refinement of the occupancy parameters, and a second near full saturation of 30 atoms. Most of the experimental results discussed are on the Th,Mn,,D,,, which was found from X-ray and neutron diffraction to be a completely single-phase material with a lattice parameter a, of 12.922& i.e. a 3.2% increase with respect to the Th,Mn,, compound. The Th~Mn~~D~* sample showed some evidence of a coexisting second hydride or Laves phase. Neutron diffraction data on both the pure and deuterated Th,Mn,, samples were taken at the National Bureau of Standards reactor utilizing a high resolution five-detector powder diffractometer. Neutrons of wavelength 1.542 A were obtained from a (220) reflection copper monochromator with a graphite filter to eliminate second-order contamination. Solfer slit collimators of angular divergences lo', 20’ and 10 were placed before and after the monoehromator and in front of the detector respectively. The powder data from the five detectors, displaced in angle by 20” from each other, covered the 20 range from 10” to 115” in increments of 0.05” and were analyzed using a modified Rietveld profile refinement technique [6] which allowed background parameters to be refined in addition to the other structural and instrumental variables. The use of high resolution techniques combined with profile refinement has been found to be particularly useful for the 6-23 compounds owing to the high degree of peak overlap in the diffraction pattern in medium to high 28 regions.
203
3. Magnetic
ordering
Bulk magnetic measurements have been made on Th,Mn,, by Malik et aE. [2] using a Faraday balance in a constant applied field of6.3 kOe. Figure 2 shows the resulting temperature dependence of the bulk magnetization for both Th,Mn,, and the fully saturated hydride Th,Mn,,H,,. These results show that the pure material is a Pauli paramagnet with only a weak temperature dependence of the susceptibility. The addition of hydrogen produces a dramatic change to long-range ordered magnetism as shown by Fig. 1. The fully hydrided compound has a Curie temperature Tc of 329 K. Malik et al. showed that the paramagnetic~ferrimagnetic transition accompanying the hydrogenation was reversible by removing the hydrogen under vacuum at elevated temperatures. The neutron results showed that the Th,Mn,,D,, compound did not order magnetically down to 4 K and thus may be below the critical deuterium concentration necessary to induce order. Th,Mn,,D,, has a 4 K bulk magnetic moment of 18.4 pe (formula unit)- ’ corresponding to an average magnetization of 0.8 pLR (Mn atom) I. This apparently low value of the manganese moment is the result of the averaging by the bulk magnetization measurements over the ferrimagnetic manganese spin arrangement as elucidated by the neutron diffraction results for the individual site magnetizations. 4, High temperature
structure
The basic f.c.c. structure of Th,Mn,,
is unaltered by the addition of 16 and
102 F IO'
I
20
15
_-TM ‘;:
15 -
sl
__._ ‘.
e
IO
d H
5
\
\
\
\
0 20
24
28
32
X - (D/f.u.)
Fig. 1. Absorption isotherm for deuterium in Th,Mn,, 17 to 24 deuterium atoms should be noted.
\ \
X--L 16
,O$ x ‘0 5’ x
0
100
200
300
400
5000
T(K)
at 295 K. The broad plateau extending from
Fig. 2. Magnetization curves for Th,Mn,, (------) andTh~Mn~~D~~ (---) at 4.2 K for an applied field of 6.3 kOe [Z]. The deuteride exhibits fe~omagnetism while the pure material is only weakly paramagnetic with an almost temperature-independent susceptibility.
204
30 deuterium atoms [a], showing only an increase in the lattice parameter of 3.2% and 5.9% respectively. The diffraction pattern, with the accompanying calculated profile fit, is shown in Fig. 3 for Th,Mn,,D,,. The fitted profile refined to an R,,(weighted profile) of 8.29, an R&expected) of 5.53 and a reliability factor ratio x = R,,(weighted profile)/R,(expected) of 1.50 [7]. It is noted that prototype structural models generally refine to a precision of x < 2 [S]. The atomic parameters determined in the refinement are given in Table 1 and show that the eight deuterium atoms fill the tetrahedral f3 site completely [4] while the trigonal i site is two-thirds filled with eight atoms. The refinement allowed no deuterium atoms in the octahedral a site [4]. This occupancy is substantially 1
(4
ANGLE
50.0 (b)
55.0
60.0
65.0
70.0
75.0
2G
80.0
85.0 ANGLE
90.0
95.0
100.0
105.0
110.0
15.0
2 e
Fig. 3. Neutron diffraction data on Th,Mn,,D,, (Fm3m) at 295 K which is above the crystallographic phase transition: (a) data and residues from the first two of the five-detector spectrum; (b) data from the last three detectors multiplied by a factor of 2 (-, calculated profile fit to the data; I, calculated relative intensity and position of the nuclear reflections).
205
different from that reported by Commandre et a2. [S] for Y,Mn,,D,, in which they found full occupancy of one atom in the octahedral a site and a half-full j tetrahedral site. The i site found in the thorium compound is of higher symmetry (mm) than the j site (m) with the deuterium atom being surrounded by three manganese atoms rather than by three manganese atoms and one thorium atom as in the case of the j site. It is noted that after deuteration the manganese positions in the unit cell undergo negligible shifts and the thorium position undergoes a modest shift, as shown by the refined coordinates in Table 1. The principal effect is in the isotropic expansion of the lattice parameter on deuteration. TABLE
1
Atomic parametersfor Th,Mn,,D,
6 at 295 K and Th,Mn,,D,,
at 150 K in the (Fm3m) structure
Site
Parameter”
ThGMnz3” (a0 = 12.523 A)
ThfiMn23D16c (a0 = 12.921 A)
Th&fndbo (a0 = 13.203A.)
Th e
N(6) (x, 0, 0) B
6 z = 0.203
6 (0.002) 0.214 (0.0002) 0.85 (0.04)
6 0.213 (0.004) 0.81(0.09)
Mnb
N(1) (0.5,0.5,0.5) I3 N(6) (0,0.25,0.25) B N(8) (1, r, 2) B N(8) (2, -GXI B N(8) (r, 2, XI B
:0.5,0.5,0.5)
fO.5,0.5,0.5) 0.35 (0.05) 6 (0,0.25,0.25) 0.35 (0.05) 8 0.179 (0.0002) 0.35 (0.05) 8 0.368 (0.0002) 0.35 (0.5) 8 (0.07) 0.101(0.0001) 1.65 (0.08) S.l(O.08) 0.140 (0.~3) 4.91(0.15)
:0.5,0.5,0.5) 0.80 (0.12) 6 (0,0.25,0.25) 0.80 (0.12) 8 0.187 (0.0004, 0.80 (0.12) 8 0.366 (0.0004) 0.80 (0.12) 5.5 (0.09) 0.092 (0.0005) 2.07 (0.30) ll(O.05) 0.152 (0.~03} 2.14 (0.11) 12 (0.07) .x = 0.158 (0.0003) z = 0.046 (0.0005) l.ll(O.12)
Mnd
Mn f,
Mn f,
D
f,
D
i
N(12) (0.5,x,x) B
D
k
N(24) (x, r, a)
6 (0,0.25,0.25) 8 x = 0.178 8 x = 0.378
B
“N, refined occupancy (maximumoccupancy); (x, y, z), position; B (A’), Debye-Waller factor. b From ref. 1. ‘Statistical deviations are given in parentheses for unconstrained variables
5. Low temperature
temperature
neutron results
A previously unreported structural distortion, from the f.c.c. unit cell to a primitive tetragonal cell (P4/mmm), was found in Th,Mn,,D,,. The lattice
206
parameters of this cell are a, = 9.076 A and q, = 12.961 A, giving a ratio of c to 2ij2a of 1.010. If the cell had remained f.c.c. without distortion the corresponding tetragonal cell parameter would be a, = 9.11 A and c,, = 21120, = 12.88 A showing that significant distortion has occurred. Graphic evidence of the crystallographic distortion is seen from the 4 K diffraction pattern in Figs. 4 and 5. In particular, the latter figure shows an enlargement of a limited high angle range of the diffraction pattern at both 295 and 4 K with the f.c.c. profile fit applied to both. It is noted that a pronounced splitting of peaks has occurred (e.g. the fully resolved coincident peaks (100 0), (860) at 28 = 75”) and that the relative structure factors of the partially overlapping peaks (e.g. at 28 = 73”) have changed, reflecting the cell distortion and the atomic rearrangement. Of the possible space groups the one that provides the best fit to the nuclear structure of Th,Mn,,D,, is P4/mmm. As shown in Table 2, this structure splits the single thorium e site into four sites (g, h, j and k) and the manganese b site into a b and a c site. The manganese d site is split into an e, f and r1 site. The
+
10.0
15.0
20.0
25.0
30.0
x-
35.0 ANGLE
50.0
55.0
60.0
65.0
70.0
75.0
40.0
45.0
50.0
2 0
80.0
05.0 ANGLE
DO.0
05.0
100.0
105.0
, 10.0
116.0
29
Fig. 4. Neutron diffraction data taken on Th,Mn,,D,, at 4.2 K. The pronounced peak splitting indicates that a structural distortion has occurred which is best fit by the primitive tetragonal space group P4Jmmm.
207
(b)
72
73
74
75 ANGLE
76
77
78
28
Fig. 5. Expanded high angle data for Th,Mn,,D,, at (a) 295 K and(b) 4 K showing the peak splitting accompanying the tetragonal distortion below the crystal structure transition temperature: --, the profile fit for the 295 K f.c.c. data.
manganese f sites and the deuterium f site are split into s and t sites. The trigonal deuterium i site is split into four sites which are m, n and two r sites in the P4/mmm structure. Table 2 also gives the refined atomic positions for the distorted structure and the equivalent f.c.c. cell coordinates indexed on the tetragonal basis. In most cases the fractional atomic coordinates have changed relatively little, but the lattice has undergone a stretching in the c axis direction and a compression along the basal plane relative to the f.c.c. cell. The profile fit was to approximately 2120 data points, excluding those omitted for the aluminum container that was used to hold the sample. The model included 500 reflections and 54 parameters were refined. The statistical standard deviations of the refined lattice positions listed in Table 2 ranged from 0.0008 for the thorium j and k sites to 0.0022 for the deuterium r3 site. The deviations of the temperature factor B were low for thorium (0.065) and manganese (0.10) somewhat higher for deuterium s3 (0.18) and deuterium t, (0.23), and much higher for deuterium n (0.45), deuterium r2 (0.37) and deuterium r3 (0.48). The deviations in the population parameters for deuterium s1 and deuterium s2 were about 0.05 atom and about 0.25 atom for deuterium n, deuterium r2 and deuterium r3. The lattice parameters showed very small deviations (0.0001 A). The site preference and occupancy of the f.c.c. structure are perhaps fortuitously in agreement with theoretical predictions for preferred hydrogen sites given by Westlake’s model for A,B,,H, compounds [9] except that no deuterium atoms were found in the a site. The results for Th,Mn,,D,, are not in
m (x,0,+) r2 (x, X, 2) r3 (x, r, 2)
D n(x,f,O)
C C C C C C 0.350 (0.360) 0.140 (0.140)
0.223 (0.236) 0.203 (0.202) 0.296 (0.298) 0.248 (0.220) - (0.280) 0.350 (0.360) 0.140 (0.140)
8 (8)
8 (8) 4 (4) 0 (4) 8 (8) 4 (8)
8 (8) 8 (8)
0.248 (0.264)
0.244 (0.25)
0.244 (0.25)
C C 0.135 (0.140) 0.352 (0.360)
0.099 (0.101) 0.402 (0.399)
2.8 0.22
0.03
0.75 1.87
0.00 0.00
0.00 0.00
0.184 (0.179) 0.317 (0.321) 0.356 (0.368) 0.133 (0.132)
0.00 0.00 0.00
0.00 0.00
0.66 0.66 0.66 0.66
C C 0.748 (0.75)
C C
0.214 (0.214) 0.285 (0.286) C C
*
B
The first two columns indicate the equivalency of sites in the f.c.c. and tetragonal cells. The x, y and z values were refined. C indicates positions that were held constant. The equivalent undistorted f.c.c. lattice indexed to the tetragonal cell is given in parentheses.
D i (O,Y,Z)
ss (x,O,y) t, (%tY)
D
D
f3 (x, r, X)
Mn s2 (r, 0,~) t, (r,tY)
8 (8)
t, (r. kY)
C C
C C
C C
C C
C C 0.218 (0.214) 0.289 (0.286)
Y
C C
8 (8)
Mn s1 (~,O,Y)
coordinates
C C 0.218 (0.214) 0.289 (0.286)
x
Refined
structure
0.354 (0.358) 0.137 (0.142)
2 (2) 2 (2) 8 (8)
10) l(1)
2 (2) 2 (2) 4 (4) 4 (4)
N (mar)
tetragonal
f (k 0,O) rl (x, 2, z)
Mn e (O,$f)
c ($9f, 0)
Mn f2 (x, X, X)
Mn f, (x,x, 4
Mn d (0,0.25,0.25)
Mn b
Mn h (0.5,0.5,0.5)
(0,0,f)
Th g (0,0, z) h (t f, z) j (x, z, 0) k (rr r, b)
Th e(x,O,O)
sites
Tetragonal
at 4 K in the P4/mmm
F.c.c. sites
Atomic parameters of ThsMn2sD,,
TABLE 2
209
accord with Westlake’s model (see Section 4). The tetragonal deformation results from the atomic ordering of the deuterium atoms at low temperature among the four possible sites derived from the i site in the f.c.c. structure. Them site is empty and the r3 site is only half-full (Table 2). It should be noted that the low temperature diffraction scans show the presence of three peaks that are not fitted accurately in the non-magnetic primitive tetragonal (P4/mmm) structure. Since the room temperature data fit exceptionally well, non-magnetic impurities are not likely to be the cause of the discrepancy. Additional magnetic contributions were considered but the lowest angle peaks which would have the greatest contribution from magnetism show no additional intensity and fit a strictly nuclear model quite well.
6. Structural
and magnetic properties of Th,Mn,,D,,
The second sample studied was of nominal composition Th,Mn,,D,, which is the fully saturated deuteride which had been found to have magnetic order in bulk magnetization studies [2]. Unfortunately, the Th,Mn,,D,, sample had Laves phase impurities present which limited the accuracy of the neutron results. This impurity contamination is reflected in the higher R factor in Table 3. The diffraction scans andrefinement showed that Th,Mn,,D,, is f.c.c. down to 4 K, in contrast with Th,Mn,,D 16. It also exhibits magnetic contributions to the Bragg peaks which are best represented by a magnetic model of 22 ferromagnetic manganese magnetic moments ordered antiparallel to the twenty third moment. TABLE 3 Reliability
factors for Th,Mn,,D,
compounds
ThcMn23DlG(295
RP R WP R erP x = Rw,,‘R,x, R”“, R ma*
6.68 8.86 5.61 1.58 6.60
K)
TbMn23D16 8.75 11.53 6.46 1.78 9.09
(4 K)
Th6Mn23D30 (150 K) 6.32 8.24 3.85 2.14 13.45 34.19
The nuclear structure refinement parameters are given in Table 1. The f site has “lost” deuterium atoms which allows occupation of the nearby k site [S]. The occupation of the k site by 12 atoms is greater than predicted by the Westlake model with simultaneous f site occupation by 5.5 atoms. The individual site magnetization refinement shows the manganese magnetic moments on the two f sites to be coupled in the opposite direction to the moment on the b site. The manganese atoms in the d site have very small, if any, magnetic moments. On the whole the manganese magnetic moments are much smaller than those found in Y,Mn,, [lo]. The b site prefers a large magnetic moment and will refine to values up to 8.0~~. This moment value was
210
constrained to 3.5 pz (typical for manganese compounds) and is coupled opposite the other manganese moments to form an almost ferromagnetic structure which produced the lowest R factor. This b site is completely surrounded by manganese atoms in the f2 site. The magnetic moments of the d, f, and f2 sites were found to be 0.80 uz, 1.84 ~(zand 1.50 pLgrespectively with a standard deviation of 0.1 pz (Mn atom))‘.
7. Conclusions These studies show that the compound Th,MnZ3D i6 exhibits no long-range magnetic order; however, the addition of deuterium (or hydrogen) atoms to produce Th,Mn,,D,, transforms the compound to one with long-range magnetic ordering. The moments found for the four transition metal sites are smaller in magnitude than in the prototype Y,Mn,, compound and show an almost ferromagnetic structure (the single-atom b site is coupled antiparallel to the other three sites). In Th,Mn,,D,, the crystal structure changes on cooling from room temperature to 4 K which is suggested to be the result of the atomic ordering of the deuterium atoms. The addition of hydrogen (or deuterium) atoms to the Th,Mn2, compound has two effects that are presumably related to the development of ordered magnetism. First, it can be assumed that the manganese d bands in the pure compound are essentially filled as evidenced by the non-magnetic order [a]. The addition of hydrogen induces a transfer of electron charge either to or from the manganese and thorium, in essence perturbing the manganese d bands and allowing the magnetic exchange coupling to proceed. Second, the introduction of the hydrogen atom produces a significant expansion of the lattice parameter and also promotes the structural change to tetragonal at low temperature, both of which affect the electronic band structure. The differences in magnetic ordering between the isostructural Y,Mn,,D, and Th,Mn,,D, compounds and the dependence of magnetic order on deuterium concentration suggest that the Mn-Mn atomic distances do not play a dominant role in determining magnetic order in these compounds.
Acknowledgments The authors wish to thank S. K. Malik and J. J. Rhyne for continual interest and helpful discussions concerning the magnetization data. They also thank E. B. Boltich for preparing the Th,Mn,, compound. We also wish to thank E. Prince for many discussions on crystal symmetry and possible crystal structures. Work at the University of Pittsburgh was supported by the National Science Foundation Grant CHE-8208048.
211
References 1 2 3 4 5 6 7 8 9 10
J. V. Florio, R. E. Rundle and A. I. Snow, Acta Crystallogr., 5(1952) 449. S. K. Malik, T. Takeshita and W. E. Wallace, Solid State Commun., 23(1977) 599. M. Commandre, D. Fruchart, A. Rouault, D. Sauvage, C. B. Shoemaker and D. P. Shoemaker, J. Phys. (Paris), 40 (1979) L639. K. Hardman, J. J. Rhyne, K. Smith and W. E. Wallace, J. Less-Common Met., 74(1980) 97. F. Pourarian, E. B. Boltich, W. E. Wallace and S. K. Malik, J. ~~ss-cornrno~ Met., 74 (1980) 153. H. M. Rietveld, J. Appl. Crystallogr., 2(1969) 65. R. A. Young, E. Prince and R. A. Sparks, J. Appl. Crystallogr., 15 (1982) 351. C. Greaves, A. J. Jacobson, B. C. Tofield and B. E. F. Fender, Acta Crystallogr., Sect. B, 31(1975) 641. 13. G. Westlake, Argonne National Laboratory, personal communication, 1982. A. Delapalme, J. Deportes, R. Lemaire, K. Hardman and W. J. James, J. Appt. Phys., 50 (1979) 1987.
of the Less-Common
MAGNETIC
Metals,
96(1984)
201
201-211
AND CRYSTALLOGRAPHIC
STRUCTURE
OF
TW+b3Dx
KAY HARDMAN-RHYNE ~utional Bureau of Standards,
Wushington, DC 20234~~.S.~.~
H. KEVIN SMITH and W. E. WALLACE Department of Chemistry, University of Pittsburgh,
Pittsburgh,
PA 1526O(U.S.A.)
(Received March 21,1983)
Summary Th,Mn,,D,, and Th,Mn,,D,, were studied by neutron diffraction Rietveld refinement methods. At low temperatures (below 78 K) ThcMn,,D,, undergoes a crystallographic change from f.c.c. (Fm3m) to primitive tetragonal (P4/mmm). This compound has no long-range magnetic ordering down to temperatures of 4K. Deuterium is found only in the f, (tetrahedral) and i (trigonal) sites; the octahedral a site is empty. Th,Mn,,D,, retains f.c.c. symmetry even at 4 K but exhibits ferromagnetism with a Curie temperature of 329 K. All moments are coupled parallel except those on the b site, which has one manganese moment and is coupled antiparallel to the other 22 manganese moments in the d, f, and f2 sites. The manganese magnetic moments are much smaller than those in Y,Mn,,.
1. Introduction Th,Mn,, is the prototype structure of a large class of 6-23 compounds of rare earths and actinides with manganese and iron, many of which have very interesting magnetic properties. In addition, the compounds generally absorb hydrogen readily, accompanied by significant increases in unit cell volume and, in some cases, pronounced effects on the magnetic ordering. The Th,Mn,, structure has an f.c.c. unit cell (space group, Fm3m) with a lattice parameter of 12.523 A at room temperature which contains 116 atoms (four formula units) [l]. There is one distinct thorium site (e) and four manganese sites (b, d, f, and fJ which have a maximum occupancy of 6,1,6,8 and 8 atoms respectively per formula unit. The interatomic distances of the manganese atoms vary significantly among different sites which partly accounts for the wide variations in manganese magnetic moments observed in the magnetically ordered compounds of this class [2, 31. Neutron diffraction, with its ability to probe ordered magnetic moments on different site sublattices, has ((2 Elsevier Sequoia/Printed
in The Netherlands
202
proven to be a valuable tool in understanding the magnetism in these compounds. In this paper we shall discuss the changes occurring in both the crystallographic structure, including a pronounced tetragonal distortion at low temperature, and the magnetic ordering of Th,Mn,, upon deuteration.
2. Experimental
considerations
Polycrystalline ThsMnZ3 was prepared at the University of Pittsburgh by induction melting in a water-cooled copper boat under an argon atmosphere [4]. The bulk material readily absorbs hydrogen at room temperature. The hydrogen uptake is instantaneous. The amount of loading of deuterium is determined by the pressure drop in a fixed volume apparatus. In samples prepared for these experiments high purity deuterium was used in lieu of hydrogen because of the large incoherent scattering cross section of hydrogen atoms for neutrons. Figure 1 shows the deuterium absorption isotherm for Th,Mn,,, illustrating the relatively low pressures required to achieve deuteration even up to the full saturation value of approximately 30 atoms per formula unit [2]. A broad absorption plateau is found to exist from 17 to 24 deuteriuln atoms, in contrast with YsMnZ3D,, Gd,Mn,,D, and other 6-23 systems which do not exhibit any plateaux [l, 51. Two samples were prepared for this study, one with 16 deuterium atoms (just below the plateau), as determined both from the volume of deuterium absorbed and from the structure refinement of the occupancy parameters, and a second near full saturation of 30 atoms. Most of the experimental results discussed are on the Th,Mn,,D,,, which was found from X-ray and neutron diffraction to be a completely single-phase material with a lattice parameter a, of 12.922& i.e. a 3.2% increase with respect to the Th,Mn,, compound. The Th~Mn~~D~* sample showed some evidence of a coexisting second hydride or Laves phase. Neutron diffraction data on both the pure and deuterated Th,Mn,, samples were taken at the National Bureau of Standards reactor utilizing a high resolution five-detector powder diffractometer. Neutrons of wavelength 1.542 A were obtained from a (220) reflection copper monochromator with a graphite filter to eliminate second-order contamination. Solfer slit collimators of angular divergences lo', 20’ and 10 were placed before and after the monoehromator and in front of the detector respectively. The powder data from the five detectors, displaced in angle by 20” from each other, covered the 20 range from 10” to 115” in increments of 0.05” and were analyzed using a modified Rietveld profile refinement technique [6] which allowed background parameters to be refined in addition to the other structural and instrumental variables. The use of high resolution techniques combined with profile refinement has been found to be particularly useful for the 6-23 compounds owing to the high degree of peak overlap in the diffraction pattern in medium to high 28 regions.
203
3. Magnetic
ordering
Bulk magnetic measurements have been made on Th,Mn,, by Malik et aE. [2] using a Faraday balance in a constant applied field of6.3 kOe. Figure 2 shows the resulting temperature dependence of the bulk magnetization for both Th,Mn,, and the fully saturated hydride Th,Mn,,H,,. These results show that the pure material is a Pauli paramagnet with only a weak temperature dependence of the susceptibility. The addition of hydrogen produces a dramatic change to long-range ordered magnetism as shown by Fig. 1. The fully hydrided compound has a Curie temperature Tc of 329 K. Malik et al. showed that the paramagnetic~ferrimagnetic transition accompanying the hydrogenation was reversible by removing the hydrogen under vacuum at elevated temperatures. The neutron results showed that the Th,Mn,,D,, compound did not order magnetically down to 4 K and thus may be below the critical deuterium concentration necessary to induce order. Th,Mn,,D,, has a 4 K bulk magnetic moment of 18.4 pe (formula unit)- ’ corresponding to an average magnetization of 0.8 pLR (Mn atom) I. This apparently low value of the manganese moment is the result of the averaging by the bulk magnetization measurements over the ferrimagnetic manganese spin arrangement as elucidated by the neutron diffraction results for the individual site magnetizations. 4, High temperature
structure
The basic f.c.c. structure of Th,Mn,,
is unaltered by the addition of 16 and
102 F IO'
I
20
15
_-TM ‘;:
15 -
sl
__._ ‘.
e
IO
d H
5
\
\
\
\
0 20
24
28
32
X - (D/f.u.)
Fig. 1. Absorption isotherm for deuterium in Th,Mn,, 17 to 24 deuterium atoms should be noted.
\ \
X--L 16
,O$ x ‘0 5’ x
0
100
200
300
400
5000
T(K)
at 295 K. The broad plateau extending from
Fig. 2. Magnetization curves for Th,Mn,, (------) andTh~Mn~~D~~ (---) at 4.2 K for an applied field of 6.3 kOe [Z]. The deuteride exhibits fe~omagnetism while the pure material is only weakly paramagnetic with an almost temperature-independent susceptibility.
204
30 deuterium atoms [a], showing only an increase in the lattice parameter of 3.2% and 5.9% respectively. The diffraction pattern, with the accompanying calculated profile fit, is shown in Fig. 3 for Th,Mn,,D,,. The fitted profile refined to an R,,(weighted profile) of 8.29, an R&expected) of 5.53 and a reliability factor ratio x = R,,(weighted profile)/R,(expected) of 1.50 [7]. It is noted that prototype structural models generally refine to a precision of x < 2 [S]. The atomic parameters determined in the refinement are given in Table 1 and show that the eight deuterium atoms fill the tetrahedral f3 site completely [4] while the trigonal i site is two-thirds filled with eight atoms. The refinement allowed no deuterium atoms in the octahedral a site [4]. This occupancy is substantially 1
(4
ANGLE
50.0 (b)
55.0
60.0
65.0
70.0
75.0
2G
80.0
85.0 ANGLE
90.0
95.0
100.0
105.0
110.0
15.0
2 e
Fig. 3. Neutron diffraction data on Th,Mn,,D,, (Fm3m) at 295 K which is above the crystallographic phase transition: (a) data and residues from the first two of the five-detector spectrum; (b) data from the last three detectors multiplied by a factor of 2 (-, calculated profile fit to the data; I, calculated relative intensity and position of the nuclear reflections).
205
different from that reported by Commandre et a2. [S] for Y,Mn,,D,, in which they found full occupancy of one atom in the octahedral a site and a half-full j tetrahedral site. The i site found in the thorium compound is of higher symmetry (mm) than the j site (m) with the deuterium atom being surrounded by three manganese atoms rather than by three manganese atoms and one thorium atom as in the case of the j site. It is noted that after deuteration the manganese positions in the unit cell undergo negligible shifts and the thorium position undergoes a modest shift, as shown by the refined coordinates in Table 1. The principal effect is in the isotropic expansion of the lattice parameter on deuteration. TABLE
1
Atomic parametersfor Th,Mn,,D,
6 at 295 K and Th,Mn,,D,,
at 150 K in the (Fm3m) structure
Site
Parameter”
ThGMnz3” (a0 = 12.523 A)
ThfiMn23D16c (a0 = 12.921 A)
Th&fndbo (a0 = 13.203A.)
Th e
N(6) (x, 0, 0) B
6 z = 0.203
6 (0.002) 0.214 (0.0002) 0.85 (0.04)
6 0.213 (0.004) 0.81(0.09)
Mnb
N(1) (0.5,0.5,0.5) I3 N(6) (0,0.25,0.25) B N(8) (1, r, 2) B N(8) (2, -GXI B N(8) (r, 2, XI B
:0.5,0.5,0.5)
fO.5,0.5,0.5) 0.35 (0.05) 6 (0,0.25,0.25) 0.35 (0.05) 8 0.179 (0.0002) 0.35 (0.05) 8 0.368 (0.0002) 0.35 (0.5) 8 (0.07) 0.101(0.0001) 1.65 (0.08) S.l(O.08) 0.140 (0.~3) 4.91(0.15)
:0.5,0.5,0.5) 0.80 (0.12) 6 (0,0.25,0.25) 0.80 (0.12) 8 0.187 (0.0004, 0.80 (0.12) 8 0.366 (0.0004) 0.80 (0.12) 5.5 (0.09) 0.092 (0.0005) 2.07 (0.30) ll(O.05) 0.152 (0.~03} 2.14 (0.11) 12 (0.07) .x = 0.158 (0.0003) z = 0.046 (0.0005) l.ll(O.12)
Mnd
Mn f,
Mn f,
D
f,
D
i
N(12) (0.5,x,x) B
D
k
N(24) (x, r, a)
6 (0,0.25,0.25) 8 x = 0.178 8 x = 0.378
B
“N, refined occupancy (maximumoccupancy); (x, y, z), position; B (A’), Debye-Waller factor. b From ref. 1. ‘Statistical deviations are given in parentheses for unconstrained variables
5. Low temperature
temperature
neutron results
A previously unreported structural distortion, from the f.c.c. unit cell to a primitive tetragonal cell (P4/mmm), was found in Th,Mn,,D,,. The lattice
206
parameters of this cell are a, = 9.076 A and q, = 12.961 A, giving a ratio of c to 2ij2a of 1.010. If the cell had remained f.c.c. without distortion the corresponding tetragonal cell parameter would be a, = 9.11 A and c,, = 21120, = 12.88 A showing that significant distortion has occurred. Graphic evidence of the crystallographic distortion is seen from the 4 K diffraction pattern in Figs. 4 and 5. In particular, the latter figure shows an enlargement of a limited high angle range of the diffraction pattern at both 295 and 4 K with the f.c.c. profile fit applied to both. It is noted that a pronounced splitting of peaks has occurred (e.g. the fully resolved coincident peaks (100 0), (860) at 28 = 75”) and that the relative structure factors of the partially overlapping peaks (e.g. at 28 = 73”) have changed, reflecting the cell distortion and the atomic rearrangement. Of the possible space groups the one that provides the best fit to the nuclear structure of Th,Mn,,D,, is P4/mmm. As shown in Table 2, this structure splits the single thorium e site into four sites (g, h, j and k) and the manganese b site into a b and a c site. The manganese d site is split into an e, f and r1 site. The
+
10.0
15.0
20.0
25.0
30.0
x-
35.0 ANGLE
50.0
55.0
60.0
65.0
70.0
75.0
40.0
45.0
50.0
2 0
80.0
05.0 ANGLE
DO.0
05.0
100.0
105.0
, 10.0
116.0
29
Fig. 4. Neutron diffraction data taken on Th,Mn,,D,, at 4.2 K. The pronounced peak splitting indicates that a structural distortion has occurred which is best fit by the primitive tetragonal space group P4Jmmm.
207
(b)
72
73
74
75 ANGLE
76
77
78
28
Fig. 5. Expanded high angle data for Th,Mn,,D,, at (a) 295 K and(b) 4 K showing the peak splitting accompanying the tetragonal distortion below the crystal structure transition temperature: --, the profile fit for the 295 K f.c.c. data.
manganese f sites and the deuterium f site are split into s and t sites. The trigonal deuterium i site is split into four sites which are m, n and two r sites in the P4/mmm structure. Table 2 also gives the refined atomic positions for the distorted structure and the equivalent f.c.c. cell coordinates indexed on the tetragonal basis. In most cases the fractional atomic coordinates have changed relatively little, but the lattice has undergone a stretching in the c axis direction and a compression along the basal plane relative to the f.c.c. cell. The profile fit was to approximately 2120 data points, excluding those omitted for the aluminum container that was used to hold the sample. The model included 500 reflections and 54 parameters were refined. The statistical standard deviations of the refined lattice positions listed in Table 2 ranged from 0.0008 for the thorium j and k sites to 0.0022 for the deuterium r3 site. The deviations of the temperature factor B were low for thorium (0.065) and manganese (0.10) somewhat higher for deuterium s3 (0.18) and deuterium t, (0.23), and much higher for deuterium n (0.45), deuterium r2 (0.37) and deuterium r3 (0.48). The deviations in the population parameters for deuterium s1 and deuterium s2 were about 0.05 atom and about 0.25 atom for deuterium n, deuterium r2 and deuterium r3. The lattice parameters showed very small deviations (0.0001 A). The site preference and occupancy of the f.c.c. structure are perhaps fortuitously in agreement with theoretical predictions for preferred hydrogen sites given by Westlake’s model for A,B,,H, compounds [9] except that no deuterium atoms were found in the a site. The results for Th,Mn,,D,, are not in
m (x,0,+) r2 (x, X, 2) r3 (x, r, 2)
D n(x,f,O)
C C C C C C 0.350 (0.360) 0.140 (0.140)
0.223 (0.236) 0.203 (0.202) 0.296 (0.298) 0.248 (0.220) - (0.280) 0.350 (0.360) 0.140 (0.140)
8 (8)
8 (8) 4 (4) 0 (4) 8 (8) 4 (8)
8 (8) 8 (8)
0.248 (0.264)
0.244 (0.25)
0.244 (0.25)
C C 0.135 (0.140) 0.352 (0.360)
0.099 (0.101) 0.402 (0.399)
2.8 0.22
0.03
0.75 1.87
0.00 0.00
0.00 0.00
0.184 (0.179) 0.317 (0.321) 0.356 (0.368) 0.133 (0.132)
0.00 0.00 0.00
0.00 0.00
0.66 0.66 0.66 0.66
C C 0.748 (0.75)
C C
0.214 (0.214) 0.285 (0.286) C C
*
B
The first two columns indicate the equivalency of sites in the f.c.c. and tetragonal cells. The x, y and z values were refined. C indicates positions that were held constant. The equivalent undistorted f.c.c. lattice indexed to the tetragonal cell is given in parentheses.
D i (O,Y,Z)
ss (x,O,y) t, (%tY)
D
D
f3 (x, r, X)
Mn s2 (r, 0,~) t, (r,tY)
8 (8)
t, (r. kY)
C C
C C
C C
C C
C C 0.218 (0.214) 0.289 (0.286)
Y
C C
8 (8)
Mn s1 (~,O,Y)
coordinates
C C 0.218 (0.214) 0.289 (0.286)
x
Refined
structure
0.354 (0.358) 0.137 (0.142)
2 (2) 2 (2) 8 (8)
10) l(1)
2 (2) 2 (2) 4 (4) 4 (4)
N (mar)
tetragonal
f (k 0,O) rl (x, 2, z)
Mn e (O,$f)
c ($9f, 0)
Mn f2 (x, X, X)
Mn f, (x,x, 4
Mn d (0,0.25,0.25)
Mn b
Mn h (0.5,0.5,0.5)
(0,0,f)
Th g (0,0, z) h (t f, z) j (x, z, 0) k (rr r, b)
Th e(x,O,O)
sites
Tetragonal
at 4 K in the P4/mmm
F.c.c. sites
Atomic parameters of ThsMn2sD,,
TABLE 2
209
accord with Westlake’s model (see Section 4). The tetragonal deformation results from the atomic ordering of the deuterium atoms at low temperature among the four possible sites derived from the i site in the f.c.c. structure. Them site is empty and the r3 site is only half-full (Table 2). It should be noted that the low temperature diffraction scans show the presence of three peaks that are not fitted accurately in the non-magnetic primitive tetragonal (P4/mmm) structure. Since the room temperature data fit exceptionally well, non-magnetic impurities are not likely to be the cause of the discrepancy. Additional magnetic contributions were considered but the lowest angle peaks which would have the greatest contribution from magnetism show no additional intensity and fit a strictly nuclear model quite well.
6. Structural
and magnetic properties of Th,Mn,,D,,
The second sample studied was of nominal composition Th,Mn,,D,, which is the fully saturated deuteride which had been found to have magnetic order in bulk magnetization studies [2]. Unfortunately, the Th,Mn,,D,, sample had Laves phase impurities present which limited the accuracy of the neutron results. This impurity contamination is reflected in the higher R factor in Table 3. The diffraction scans andrefinement showed that Th,Mn,,D,, is f.c.c. down to 4 K, in contrast with Th,Mn,,D 16. It also exhibits magnetic contributions to the Bragg peaks which are best represented by a magnetic model of 22 ferromagnetic manganese magnetic moments ordered antiparallel to the twenty third moment. TABLE 3 Reliability
factors for Th,Mn,,D,
compounds
ThcMn23DlG(295
RP R WP R erP x = Rw,,‘R,x, R”“, R ma*
6.68 8.86 5.61 1.58 6.60
K)
TbMn23D16 8.75 11.53 6.46 1.78 9.09
(4 K)
Th6Mn23D30 (150 K) 6.32 8.24 3.85 2.14 13.45 34.19
The nuclear structure refinement parameters are given in Table 1. The f site has “lost” deuterium atoms which allows occupation of the nearby k site [S]. The occupation of the k site by 12 atoms is greater than predicted by the Westlake model with simultaneous f site occupation by 5.5 atoms. The individual site magnetization refinement shows the manganese magnetic moments on the two f sites to be coupled in the opposite direction to the moment on the b site. The manganese atoms in the d site have very small, if any, magnetic moments. On the whole the manganese magnetic moments are much smaller than those found in Y,Mn,, [lo]. The b site prefers a large magnetic moment and will refine to values up to 8.0~~. This moment value was
210
constrained to 3.5 pz (typical for manganese compounds) and is coupled opposite the other manganese moments to form an almost ferromagnetic structure which produced the lowest R factor. This b site is completely surrounded by manganese atoms in the f2 site. The magnetic moments of the d, f, and f2 sites were found to be 0.80 uz, 1.84 ~(zand 1.50 pLgrespectively with a standard deviation of 0.1 pz (Mn atom))‘.
7. Conclusions These studies show that the compound Th,MnZ3D i6 exhibits no long-range magnetic order; however, the addition of deuterium (or hydrogen) atoms to produce Th,Mn,,D,, transforms the compound to one with long-range magnetic ordering. The moments found for the four transition metal sites are smaller in magnitude than in the prototype Y,Mn,, compound and show an almost ferromagnetic structure (the single-atom b site is coupled antiparallel to the other three sites). In Th,Mn,,D,, the crystal structure changes on cooling from room temperature to 4 K which is suggested to be the result of the atomic ordering of the deuterium atoms. The addition of hydrogen (or deuterium) atoms to the Th,Mn2, compound has two effects that are presumably related to the development of ordered magnetism. First, it can be assumed that the manganese d bands in the pure compound are essentially filled as evidenced by the non-magnetic order [a]. The addition of hydrogen induces a transfer of electron charge either to or from the manganese and thorium, in essence perturbing the manganese d bands and allowing the magnetic exchange coupling to proceed. Second, the introduction of the hydrogen atom produces a significant expansion of the lattice parameter and also promotes the structural change to tetragonal at low temperature, both of which affect the electronic band structure. The differences in magnetic ordering between the isostructural Y,Mn,,D, and Th,Mn,,D, compounds and the dependence of magnetic order on deuterium concentration suggest that the Mn-Mn atomic distances do not play a dominant role in determining magnetic order in these compounds.
Acknowledgments The authors wish to thank S. K. Malik and J. J. Rhyne for continual interest and helpful discussions concerning the magnetization data. They also thank E. B. Boltich for preparing the Th,Mn,, compound. We also wish to thank E. Prince for many discussions on crystal symmetry and possible crystal structures. Work at the University of Pittsburgh was supported by the National Science Foundation Grant CHE-8208048.
211
References 1 2 3 4 5 6 7 8 9 10
J. V. Florio, R. E. Rundle and A. I. Snow, Acta Crystallogr., 5(1952) 449. S. K. Malik, T. Takeshita and W. E. Wallace, Solid State Commun., 23(1977) 599. M. Commandre, D. Fruchart, A. Rouault, D. Sauvage, C. B. Shoemaker and D. P. Shoemaker, J. Phys. (Paris), 40 (1979) L639. K. Hardman, J. J. Rhyne, K. Smith and W. E. Wallace, J. Less-Common Met., 74(1980) 97. F. Pourarian, E. B. Boltich, W. E. Wallace and S. K. Malik, J. ~~ss-cornrno~ Met., 74 (1980) 153. H. M. Rietveld, J. Appl. Crystallogr., 2(1969) 65. R. A. Young, E. Prince and R. A. Sparks, J. Appl. Crystallogr., 15 (1982) 351. C. Greaves, A. J. Jacobson, B. C. Tofield and B. E. F. Fender, Acta Crystallogr., Sect. B, 31(1975) 641. 13. G. Westlake, Argonne National Laboratory, personal communication, 1982. A. Delapalme, J. Deportes, R. Lemaire, K. Hardman and W. J. James, J. Appt. Phys., 50 (1979) 1987.