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Crystal structure of R3Ge4 compounds (R Er, Ho, Tm, Lu)
Journal of Alloys and Compounds, 210 (1994) 19-21 JALCOM 1115
19
Crystal structure of R3Ge4 compounds (R = Er, Ho, Tm, Lu) O.Ya. O l e k s y n a n d O.I. B o d a k Inorganic Chemistry Department, Lviv State University, Lomonosov St. 6, 290005 Lviv (Ukraine) (Received August 30, 1993; in final form September 30, 1993)
Abstract The crystal structure of binary germanides R3Ge4 (R-= Er, Ho, Tm, Lu) has been determined by means of powder X-ray diffraction. For Er3Ge4 a full-structure determination has been performed_using the Rietveld method (306 reflections, space group Cmcm, Z = 4, a = 4.00544(7), b = 10.5426(2), c = 14.1369(3) A, RI = 5.92, Rp = 10.69, R,,p = 9.54, Rcxp= 4.39%). The location of the atoms in Er3Ge4 is similar to that in the W3CoB 3 structure, with the Ge atoms substituting for Co and B. For the other R3Ge4 compounds (R-=Ho, Tm and Lu) the lattice parameters are given.
1. Introduction
Ingots with a mass of about 2 g of Ro.46Geo.64 composition were prepared by direct arc melting of the elemental components (germanium and rare earth metals were of purity 99.99 and 99.98 wt.% respectively) in an arc furnace under an argon atmosphere. Weight losses during synthesis were not more than 1 wt.%. The alloys were homogenized at 1070 K for 2 weeks in an evacuated quartz tube, followed by quenching in cold water.
application of the Ito method was successful, yielding an orthorhombic lattice with a = 4.0058(5), b = 10.544(1), c = 14.140(2) /~. The structure determination has been performed using the CSD programme package [3], According to the observed reflections, the selection of the space group is not unique. First we assumed the symmetry restrictions hkO: h + k = 2n, hOl: h + l = 2n, hO0: h = 2n, 0k0: k = 2n, 00/: l = 2n. On this basis, the structure has been solved in the P m n n group. However, a more precise inspection of the obtained model revealed that the symmetry might be higher, though the R values were low (RI=6.91, Rp=14.01, Rwp=10.90, R~xp= 4.45%). Refinement in the Crncm space group simplified the model, improved the temperature factors and made the fit better (RI=5.92, R=10.69, R m,=9.554, R~xo = 4.39%). This convinced us to choose the C m c m group. Figure 1 illustrates the correspondence between the observed and calculated patterns. The final atomic parameters are listed in Table 1. The sample was slightly oriented along the [100] axis; the texture parameter was 1.08(1). The projection of the EraGe4 unit cell along the shortest period and the coordination polyhedra (CPs)
3. Experimental details and results
TABLE 1. Atomic parameters of Er3Ge4
An investigation of the phase diagrams of R - G e systems ( R = H o , Er, Tm and Lu) performed by Eremenko and Obushenko [1] revealed the existence of binary phases R3Ge4. They form during the peritectoid reaction RGel.5 + RGe
~ R3Ge 4
and are isotypical. The crystal structure of these phases has not been determined until now, though numerous attempts have been made.
2. Sample preparation
Full-structure determination has been performed for the EraGe4 phase. X-ray powder data have been collected using an "HZG-4a" diffractometer (Cu Ka radiation) up to (sin 0)/A = 0.598 with a 0.05 ° step width and an average time of 80 s per step. For indexing the pattern, several programmes from the GUFI-3 package [2] have been used. However, only
Atom
Position
x/a
y/b
z/c
Bi~o
Erl Er2 Gel Ge2 Ge3
8(f) 4(c) 80') 4(c) 4(a)
0 0 0 0 0
0.3323(2) 0.0475(2) 0.3779(3) 0.7762(5) 0
0.0966(1) • 0.8912(2) ,~ 0
0.41(3) 0.25(5) 0.67(8) 0.66(9) 0.67(9)
0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSD1 0925-8388(94)01115-X
20
O.Ya. Oleksyn, 0.I. Bodak / Crystal structure of R3Ge~ compounds
Difference
18.00
50.00
4
90.00
102.00
114.00
128.00
Two Thet.e
Fig. 1. Observed and calculated profiles of Er3Ge4 and difference between them. TABLE 2. Interatomic distances (/~) in Er3Ge4
Z
()0 0 0 oO 0 O ( 000 0
OOE 0 X: 0
0 6~
Erl-lGel 2Gel 2Ge3 2Ge2 1Gel 1Er2 2Er2 1Ge3 2Erl 2Erl 1Erl
2.943(4) 2.991(3) 3.000(1) 3.010(2) 3.060(4) 3.702(3) 3.722(2) 3.759(2) 3.805(2) 4.0047(1) 4.335 (3)
Gel-lGe2 2Ge3 2Er2 1Erl 2Erl 1Erl
2.573(5) 2.834(2) 2.935(3) 2.943(4) 2.991(3) 3.060(4)
Ge2-2Gel 1Er2 4Erl 2Er2
2.573(5) 2.860(6) 3.010(2) 3.134(4)
Er2-1Ge2 4Gel 2Ge2 2Ge3 2Erl 4Erl 2Er2
2.860(6) 2.935(3) 3.134(4) 3.5690(4) 3.702(3) 3.722(2) 4.0047(1)
Ge3-4Gel 4Erl 2Er2 2Er2 2Erl
2.834(2) 3.000(1) 3.5690(4) 3.134(4) 3.759(2)
112
Fig. 2. Projection of EraGe4 unit cell on YZ plane and coordination polyhedra of Er (a, b) and Ge (e-e) atoms.
of the atoms are shown in Fig. 2. The coordination number for Er atoms is 17: the CPs are described by the general formula [Erl(2)GegErs] (Figs. 2a and 2b). Around Ge one finds archimedean antiprisms
[GelGe3Er6] (Fig. 2c), trigonal prisms with three additional atoms [Ge2Ge3Er6] (Fig. 2d), and icosahedra [Ge3Ge4Ers] (Fig. 2e). The interatomic distances in Er3Ge4 are typical for intermetallic compounds (Table 2). The shortest distances are: Er-Ge, 2.860 /~; Ge-Ge, 2.573 ~; these are nearly 8.9% and 7.5% less than the sums of the
O.Ya. Oleksyn, 0.I. Bodak / Crystal structure of R3Ge 4 compounds TABLE 3. Lattice parameters and unit cell volume of R3Ge4 phases R
Ho Er Tm Lu
a
b
c
V
(A)
(A)
(A)
(A3)
4.012(2) 3.998(2) 3.980(1) 3.968(1)
10.562(3) 10.523(3) 10.491(2) 10.438(3)
14.120(6) 14.082(6) 14.049(5) 14.040(8)
598.3(7) 592.4(7) 586.6(5) 581.7(6)
corresponding metallic radii (Er, 1.75 /~; Ge, 1.39 [41). For the phases with Ho, Tm and Lu the lattice constants have been refined using data gathered with a DRON-3.0 powder diffractometer (Table 3).
4. Discussion
The peritectoid nature of R3Ge4 phase formation caused problems in single-crystal synthesis and necessitated the application of powder X-ray diffraction. A similar structure was solved previously by Jedlicka et al. [5] for the ternary compound W3CoB3, where the B and Co atoms occupy 4(c), 4(a) and 8(f) sites, i.e. substituting for the Ge atoms in the Er3Ge4 structure. The relation between Er3Ge 4 and W3CoB 3 is identical with the relations between BaA14 and CeGa2AI2 and
21
between CaCu5 and CeCo3B z structure types. Therefore the structure of W3CoB 3 is a superstructure of substitution to the Er3Ge4 structure. According to Table 3, the unit cell volume of the binary germanides R3Ge 4 ( R ~ - H o , Er, Tm, Lu) vs. the atomic number of the rare earth component decreases simultaneously with the R atom radius. This indicates the normal valency state of the R atoms in the phases discussed.
Acknowledgment
One of the authors (O.O) wishes to thank Dr L.G. Akselrud for discussions.
References 1 V.N. Eremenko and I.M. Obushenko, Izv. Vuzov. Zvet. Met., 3 (1981) 59. 2 R.E. Dinnebier, W. Eysel and E.J. Sonneveld, Proc. lind Eur. Powder Diffraction Conf., Enschede, July-August 1992, p. 47. 3 L.G. Akselrud, Yu.N. Grin, P.Yu. Zavalij, V.K. Pecharsky and V.S. Fundamensky, Proc. Xllth Eur. Crystallography Meeting, Moscow, August 1989, Viniti, Moscow, 1989, p. 155. 4 G.B. Bokij, Vvedenije v Kristallochimiju, Isdatelstvo MGU, Moscow, 1954, p. 171. 5 H. Jedlicka, F. Benesovsky and H. Novotny, Monatsh. Chem., 100 (1969) 844.
19
Crystal structure of R3Ge4 compounds (R = Er, Ho, Tm, Lu) O.Ya. O l e k s y n a n d O.I. B o d a k Inorganic Chemistry Department, Lviv State University, Lomonosov St. 6, 290005 Lviv (Ukraine) (Received August 30, 1993; in final form September 30, 1993)
Abstract The crystal structure of binary germanides R3Ge4 (R-= Er, Ho, Tm, Lu) has been determined by means of powder X-ray diffraction. For Er3Ge4 a full-structure determination has been performed_using the Rietveld method (306 reflections, space group Cmcm, Z = 4, a = 4.00544(7), b = 10.5426(2), c = 14.1369(3) A, RI = 5.92, Rp = 10.69, R,,p = 9.54, Rcxp= 4.39%). The location of the atoms in Er3Ge4 is similar to that in the W3CoB 3 structure, with the Ge atoms substituting for Co and B. For the other R3Ge4 compounds (R-=Ho, Tm and Lu) the lattice parameters are given.
1. Introduction
Ingots with a mass of about 2 g of Ro.46Geo.64 composition were prepared by direct arc melting of the elemental components (germanium and rare earth metals were of purity 99.99 and 99.98 wt.% respectively) in an arc furnace under an argon atmosphere. Weight losses during synthesis were not more than 1 wt.%. The alloys were homogenized at 1070 K for 2 weeks in an evacuated quartz tube, followed by quenching in cold water.
application of the Ito method was successful, yielding an orthorhombic lattice with a = 4.0058(5), b = 10.544(1), c = 14.140(2) /~. The structure determination has been performed using the CSD programme package [3], According to the observed reflections, the selection of the space group is not unique. First we assumed the symmetry restrictions hkO: h + k = 2n, hOl: h + l = 2n, hO0: h = 2n, 0k0: k = 2n, 00/: l = 2n. On this basis, the structure has been solved in the P m n n group. However, a more precise inspection of the obtained model revealed that the symmetry might be higher, though the R values were low (RI=6.91, Rp=14.01, Rwp=10.90, R~xp= 4.45%). Refinement in the Crncm space group simplified the model, improved the temperature factors and made the fit better (RI=5.92, R=10.69, R m,=9.554, R~xo = 4.39%). This convinced us to choose the C m c m group. Figure 1 illustrates the correspondence between the observed and calculated patterns. The final atomic parameters are listed in Table 1. The sample was slightly oriented along the [100] axis; the texture parameter was 1.08(1). The projection of the EraGe4 unit cell along the shortest period and the coordination polyhedra (CPs)
3. Experimental details and results
TABLE 1. Atomic parameters of Er3Ge4
An investigation of the phase diagrams of R - G e systems ( R = H o , Er, Tm and Lu) performed by Eremenko and Obushenko [1] revealed the existence of binary phases R3Ge4. They form during the peritectoid reaction RGel.5 + RGe
~ R3Ge 4
and are isotypical. The crystal structure of these phases has not been determined until now, though numerous attempts have been made.
2. Sample preparation
Full-structure determination has been performed for the EraGe4 phase. X-ray powder data have been collected using an "HZG-4a" diffractometer (Cu Ka radiation) up to (sin 0)/A = 0.598 with a 0.05 ° step width and an average time of 80 s per step. For indexing the pattern, several programmes from the GUFI-3 package [2] have been used. However, only
Atom
Position
x/a
y/b
z/c
Bi~o
Erl Er2 Gel Ge2 Ge3
8(f) 4(c) 80') 4(c) 4(a)
0 0 0 0 0
0.3323(2) 0.0475(2) 0.3779(3) 0.7762(5) 0
0.0966(1) • 0.8912(2) ,~ 0
0.41(3) 0.25(5) 0.67(8) 0.66(9) 0.67(9)
0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSD1 0925-8388(94)01115-X
20
O.Ya. Oleksyn, 0.I. Bodak / Crystal structure of R3Ge~ compounds
Difference
18.00
50.00
4
90.00
102.00
114.00
128.00
Two Thet.e
Fig. 1. Observed and calculated profiles of Er3Ge4 and difference between them. TABLE 2. Interatomic distances (/~) in Er3Ge4
Z
()0 0 0 oO 0 O ( 000 0
OOE 0 X: 0
0 6~
Erl-lGel 2Gel 2Ge3 2Ge2 1Gel 1Er2 2Er2 1Ge3 2Erl 2Erl 1Erl
2.943(4) 2.991(3) 3.000(1) 3.010(2) 3.060(4) 3.702(3) 3.722(2) 3.759(2) 3.805(2) 4.0047(1) 4.335 (3)
Gel-lGe2 2Ge3 2Er2 1Erl 2Erl 1Erl
2.573(5) 2.834(2) 2.935(3) 2.943(4) 2.991(3) 3.060(4)
Ge2-2Gel 1Er2 4Erl 2Er2
2.573(5) 2.860(6) 3.010(2) 3.134(4)
Er2-1Ge2 4Gel 2Ge2 2Ge3 2Erl 4Erl 2Er2
2.860(6) 2.935(3) 3.134(4) 3.5690(4) 3.702(3) 3.722(2) 4.0047(1)
Ge3-4Gel 4Erl 2Er2 2Er2 2Erl
2.834(2) 3.000(1) 3.5690(4) 3.134(4) 3.759(2)
112
Fig. 2. Projection of EraGe4 unit cell on YZ plane and coordination polyhedra of Er (a, b) and Ge (e-e) atoms.
of the atoms are shown in Fig. 2. The coordination number for Er atoms is 17: the CPs are described by the general formula [Erl(2)GegErs] (Figs. 2a and 2b). Around Ge one finds archimedean antiprisms
[GelGe3Er6] (Fig. 2c), trigonal prisms with three additional atoms [Ge2Ge3Er6] (Fig. 2d), and icosahedra [Ge3Ge4Ers] (Fig. 2e). The interatomic distances in Er3Ge4 are typical for intermetallic compounds (Table 2). The shortest distances are: Er-Ge, 2.860 /~; Ge-Ge, 2.573 ~; these are nearly 8.9% and 7.5% less than the sums of the
O.Ya. Oleksyn, 0.I. Bodak / Crystal structure of R3Ge 4 compounds TABLE 3. Lattice parameters and unit cell volume of R3Ge4 phases R
Ho Er Tm Lu
a
b
c
V
(A)
(A)
(A)
(A3)
4.012(2) 3.998(2) 3.980(1) 3.968(1)
10.562(3) 10.523(3) 10.491(2) 10.438(3)
14.120(6) 14.082(6) 14.049(5) 14.040(8)
598.3(7) 592.4(7) 586.6(5) 581.7(6)
corresponding metallic radii (Er, 1.75 /~; Ge, 1.39 [41). For the phases with Ho, Tm and Lu the lattice constants have been refined using data gathered with a DRON-3.0 powder diffractometer (Table 3).
4. Discussion
The peritectoid nature of R3Ge4 phase formation caused problems in single-crystal synthesis and necessitated the application of powder X-ray diffraction. A similar structure was solved previously by Jedlicka et al. [5] for the ternary compound W3CoB3, where the B and Co atoms occupy 4(c), 4(a) and 8(f) sites, i.e. substituting for the Ge atoms in the Er3Ge4 structure. The relation between Er3Ge 4 and W3CoB 3 is identical with the relations between BaA14 and CeGa2AI2 and
21
between CaCu5 and CeCo3B z structure types. Therefore the structure of W3CoB 3 is a superstructure of substitution to the Er3Ge4 structure. According to Table 3, the unit cell volume of the binary germanides R3Ge 4 ( R ~ - H o , Er, Tm, Lu) vs. the atomic number of the rare earth component decreases simultaneously with the R atom radius. This indicates the normal valency state of the R atoms in the phases discussed.
Acknowledgment
One of the authors (O.O) wishes to thank Dr L.G. Akselrud for discussions.
References 1 V.N. Eremenko and I.M. Obushenko, Izv. Vuzov. Zvet. Met., 3 (1981) 59. 2 R.E. Dinnebier, W. Eysel and E.J. Sonneveld, Proc. lind Eur. Powder Diffraction Conf., Enschede, July-August 1992, p. 47. 3 L.G. Akselrud, Yu.N. Grin, P.Yu. Zavalij, V.K. Pecharsky and V.S. Fundamensky, Proc. Xllth Eur. Crystallography Meeting, Moscow, August 1989, Viniti, Moscow, 1989, p. 155. 4 G.B. Bokij, Vvedenije v Kristallochimiju, Isdatelstvo MGU, Moscow, 1954, p. 171. 5 H. Jedlicka, F. Benesovsky and H. Novotny, Monatsh. Chem., 100 (1969) 844.