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MPt2Si2 compounds of the ThCr2Si2 type
Journal of the Less-Common Metals, 55 (1977) 171 - 176 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
MPtsSis
COMPOUNDS
OF THE ThCrsSis
171
TYPE
I. MAYER and P. D. YETOR* Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem (Israel) (Received
February 4, 1977)
Summary MPtsSia-type compounds of the rare earth metals and Sr crystallize in the ThCrs Sis-type body-centered tetragonal structure. Structure factor calculations reveal that Pt and Si atoms are statistically distributed among their crystallographic positions. Interatomic distances were calculated and show relatively strong Si-Si bonds similar to the Cu-, Pd- and Au-containing ThCrsSis-type compounds.
Introduction The investigation of ternary metallic compounds with the general formula AMsX2 started with the actinide compounds where A = Th or U, M is one of the 3d transition metals and X = Si or Ge [l].In the last two decades the study of this type of compound has been extended to compounds in which A is one of the alkaline earth or lanthanide metals and M is one of the 3d or 4d transition metals [2 - 51. The crystal structure of all of these compounds is body-centred tetragonal of the ThCrsSis type with the space group I4/mmm. The A atoms in this structureare located at the center of a polyhedron composed of the transition metal and Si or Ge atoms. The polyhedron is somewhat distorted because the Si or Ge atoms present in it cause two different A-Si(Ge) distances. The equivalent positions of all the atoms are fixed, except the z parameter of the Si(Ge) atoms. The shortest Si-Si distances are affected by the value of this parameter. ThCrsSis-type compounds having the EuMsSis composition are of special interest because of the intermediate valency of Eu in EuCusSis and the presence of di- and trivalent Eu in the compounds with M = Fe, Co and Ni [6,7]. Unusual magnetic behavior was found in the case of the LnFesXs compounds, the iron sublattice being partly ferromagnetic [8].
*Present address: Universidad
Complutense
de Madrid, Madrid, Spain.
172
Among the structural properties of the ThCrz& -type compounds the distribution of the transition metal (M) and Si or Ge atoms between the 4(d) and 4(e) sites of the I4/mmm space group is of considerable interest. The possibility was raised that these atoms interchange their positions or alternatively that they are statisti~~y dist~bu~d among their crystallographic sites. The ~~on~n~g compounds reported in the present paper are the first representatives of the ThCrsSi, structure type in which the Pt and Si atoms were found to be randomly distributed between the 4(d) and 4(e) positions. Experimental The compounds were prepared by melting together the rare earth, strontium, platinum and silicon metals (each of 99.9% purity). The platinum was in foil form and the other metals were folded in this foil. The samples were placed in alumina crucibles and induction heated under the protective atmosphere of helium. Powdered samples were X-ray analyzed by a Philipps diffractometer. using monochromatiz~ Cu & radiation. X-ray intensity measurement were carried out by scanning the samples at a rate of l/4” (20) per minute. The integrated intensity of the reflections was determined by measuring the area of the peaks. The structure factor calculations were made by a least squares program [9] .
Results The powder patterns of the samples could be indexed on the basis of a body-centered tetragonal cell. The unit cell constants of the compounds are listed in Table 1. The tetragonal phase of most of the compounds was accompanied by a few lines of another phase, which was probably one of the silicides of Pt. The variation of the unit cell constants uersus the atomic number of the rare earth metals is illustrated in Fig. 1. Estimation of the relative intensities of the reflections of the different LnPtsSia compounds, based on their peak heights, has shown that these intensities cannot correspond to the “normal” distribution of the Pt and Si atoms, namely that Pt and Si occupy the 4(d) and 4(e) sites of the 14lmmm space group, respectively. If this were the case, the relative intensities of the 103,112 and 200 reflections should be around 15%, 100% and 40%; in fact they were found to be 85%, 100% and 75%, respectively. It was necessary therefore to carry out structure factor calculations in order to determine the exact location of the Pt and Si atoms in the structure. These calculations were performed for ErPtsSis. Each unit cell contains two ErPtsSiz; the available equivalent positions of the I4lmmm space group for the above 10 atoms are: 2(a) 000; 4(d) 0 1,1/4, l/20 j/4;4(c) OOz,OO27
173 TABLE 1 Lattice constants of the L&&Ii*
LaPt$iz NdPt$& EuPtzSiz GdPt#i, DyPt$$ ErPt$Q TmPtzSiz LuPtzSiz SrPtzSi2
4.01 a 1 ’ IA Ce Pr Nd
4.277 4.218 4.215 4.175 4.149 4.115 4.113 4.075 4.270
0
and SrPtzSiz compounds
9.822
2.296
9.790
2.321 2.295 2.343 2.360 2.387 2.392 2.442 2.317
9.810 9.783 9.790 9.822 9.838 9.952
9.895
’ ’ I 0 ’ 0 ’ ’ ’ Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 1. Variation of the lattice constants a and c with atomic number for the LnPt2Siz compounds.
In all the calculations Er atoms were put in the 2(a) positions. For Pt and Si the following three cases were examined: Pt in 4(d) and Si in 4(e); Pt in 4(e) and Si in 4(d); Pt and Si occupy the 4(d) and 4(e) positions randomly. The free parameter of the 4(e) site was determined to be 0.370. In Table 2 the observed and calculated intensities are compared for the above three cases. Three significantly different R values were found for the three types of calculations. The lowest value of R was obtained for the case when Pt and Si are statistically distributed between the 4(d) and 4(e) positions.
174 TABLE 2 Relative integrated intensities of ErPt&& hkl
002 101 110 103 112 004 200 202 114 23.1 105 006 213 204 220 116 222 301 215 107 310 206 303 312
Gb*.a
Pt(H, 0, j/l) Si(0, 0,0.370)
Pt(0, 0,0.370)
2.0 n.0. 21.6 82.4 100.0 2.0 78.2 n.0. 20.9 n.0. 29.9 n.0. 30.8 3.0 24.8 28.0 n.0. n.0. 24.6 n.0. n.0. n.0. 15.3 35.0
21.27 15.37 5.94 15.68 100.0 19.0 35.03 6.90 10.65 4.19 5.20 1.09 1.49 20.26 11.66 18.06 2.57 0.82 4.17 0.77 1.09 1.90 1.72 23.60
7.64 11.63 89.95 100.0 13.29 3.36 42.03 2.62 11.02 3.04 24.62 1.38 49.46 3.74 14.01 5.61 0.97 0.60 19.76 1.49 16.70 2.45 11.23 3.06
R
0.590
Mixed
Si(?h, 0, f/4)
0.731
4.77 0.54 15.34 87.15 100.0 4.93 75.34 1.39 22.62 0.16 22.88 0.01 42.63 5.09 25.07 23.44 0.52 0.03 18.36 0.03 2.64 0.01 9.70 23.39 0.156
%_o., not observed.
Discussion The variation of the unit cell constant a versus the atomic number of the rare earths can be attributed to the lanthanide contraction of the trivalent rare earths. An exception is EuPt,Si,; in this case the value of a is much higher than that found for the neighboring lanthanides. This seems to be a clear indication that Eu is divalent in this compound, as in many Eu compounds of the ThCraSis type. The changes observed for the unit cell constant c have a different character; c remains constant in the La - Dy compounds and increases from then on. A similar behavior of c was also found in other LnMzSiz compounds with the same structure in which the transition metal is Cu, Pd or Au. The C/Q values of all these compounds have been found to be relatively low. The shortest Si-Si bond is located along the c axis; relatively low and constant values of c therefore mean short and rigid Si-Si bonds. Another consequence of the relatively short c values is that the
175
Ln-8Pt and Ln-8Si distances are very similar and not very different from the Ln-2Si distance. This means that the polyhedron around the lanthanide atoms is less distorted in these compounds. In Table 3 the interatomic distances for the LnPt& compounds are listed. This table shows the bond distances in the polyhedron mentioned above and it also shows that the Si-Si distance remains almost constant in all the compounds. The M-M and M-Si bonds are relatively long in all the compounds with low c/u values. This suggests that in compounds in which the Si-Si bonds are strong the interaction between the transition metals and Si becomes weaker. TABLE
3
Interatomic
distance
(8)
of SrPtpSil
and LnPtzSiz
compounds
Sr
La
Nd
EU
Gd
Dy
Er
Tm
Lu
Ltv-8Pt Ln--8Si Ln--2Si
3.285 3.267 3.661
3.256 3.264 3.634
3.231 3.224 3.622
3.253 3.262 3.630
3.215 3.195 3.620
3.208 3.179 3.622
3.204 3.158 3.634
3.206 3.163 3.640
3.216 3.139 3.682
Pt,- 4Pt Pt-4Si Si-Si
3.020 2.473 2.596
3.025 2.466 2.554
2.983 2.438 2.545
3.023 2.464 2.551
2.953 2.419 2.544
2.934 2.409 2.545
2.910 2.396 2.554
2.909 2.396 2.558
2.882 2.388 2.588
The most significant difference found between the Pt-containing compounds studied in this work and all the other ThCrpSi2-type compounds is the distribution of the Pt and Si atoms in the structure. The X-ray diffraction intensity calculations definitely prove that Pt and Si are statistically distributed between the 4(d) and 4(e) sites. The lowest R value (0.156) was obtained in this case and was much lower than the values obtained when Pt is in 4(d) and Si in 4(e) (0.590) or the inverse case (0.731). This leaves no doubt that the best agreement between observed and calculated X-ray intensities is obtained when Pt and Si are randomly distributed between the 4(d) and 4(e) positions and that this is the real structure of the Pt-containing ThCr2Si2-type silicides. As was pointed out above, the possibility that the transition metal and Si atoms are randomly distributed has been raised in many studies. So far the Pt-containing ThCr2Si,-type silicides are the only examples in which the existence of this arrangement has been confirmed. It is hard to tell at this stage what the role of the Pt atoms is in the formation of such a structure. Pt is known to form chains in nonmetals in many coordination compounds. It may well be that this property of Pt is responsible for the statistical distribution of the Pt and Si atoms, thus forming Pt-Si-Pt-Si chains in the structure. Pt is known to appear in the tetravalent form, in contrast to other transition metals occurring in ThCr,Si,-type compounds. This property might also be responsible for the tendency of Pt to distribute statistically with the Si atoms, which are as a rule tetravalent like the Pt atoms.
176
References 1 2 3 4 6 6
Z. Ban and M. Sikirica, Acta Crystallogr., 18 (1965) 594. W. Rieger and E. Parthe, Monatsh. Chem., 100 (1969) 444. I. Mayer, J. Cohen and I. Felner, J. Less-Common Met., 30 (1973) 181. R. Balestracci, C. R. Acad. Sci. Paris Ser. B, 282 (1976) 291. W. Dorrscheidt, N. Niess and H. Schafer, Z. Naturforsch., Teil B, 31 (1976) 890. E. R. Bauminger, D. Froindlich, I. Nowik, S. Ofer, I. Felner and I. Mayer, Phys. Rev. Lett., 30 (1973) 1053. 7 I. Mayer and I. Felner, J. Phys. Chem. Solids, in the press. 8 I. Felner, I. Mayer, A. Grill and M. Schieher, Solid State Commun., 16 (1975) 1005. 9 M. H. Muller, L. Heaton and K. I. Miller, Acta Crystallogr., 13 (1960) 828.
MPtsSis
COMPOUNDS
OF THE ThCrsSis
171
TYPE
I. MAYER and P. D. YETOR* Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem (Israel) (Received
February 4, 1977)
Summary MPtsSia-type compounds of the rare earth metals and Sr crystallize in the ThCrs Sis-type body-centered tetragonal structure. Structure factor calculations reveal that Pt and Si atoms are statistically distributed among their crystallographic positions. Interatomic distances were calculated and show relatively strong Si-Si bonds similar to the Cu-, Pd- and Au-containing ThCrsSis-type compounds.
Introduction The investigation of ternary metallic compounds with the general formula AMsX2 started with the actinide compounds where A = Th or U, M is one of the 3d transition metals and X = Si or Ge [l].In the last two decades the study of this type of compound has been extended to compounds in which A is one of the alkaline earth or lanthanide metals and M is one of the 3d or 4d transition metals [2 - 51. The crystal structure of all of these compounds is body-centred tetragonal of the ThCrsSis type with the space group I4/mmm. The A atoms in this structureare located at the center of a polyhedron composed of the transition metal and Si or Ge atoms. The polyhedron is somewhat distorted because the Si or Ge atoms present in it cause two different A-Si(Ge) distances. The equivalent positions of all the atoms are fixed, except the z parameter of the Si(Ge) atoms. The shortest Si-Si distances are affected by the value of this parameter. ThCrsSis-type compounds having the EuMsSis composition are of special interest because of the intermediate valency of Eu in EuCusSis and the presence of di- and trivalent Eu in the compounds with M = Fe, Co and Ni [6,7]. Unusual magnetic behavior was found in the case of the LnFesXs compounds, the iron sublattice being partly ferromagnetic [8].
*Present address: Universidad
Complutense
de Madrid, Madrid, Spain.
172
Among the structural properties of the ThCrz& -type compounds the distribution of the transition metal (M) and Si or Ge atoms between the 4(d) and 4(e) sites of the I4/mmm space group is of considerable interest. The possibility was raised that these atoms interchange their positions or alternatively that they are statisti~~y dist~bu~d among their crystallographic sites. The ~~on~n~g compounds reported in the present paper are the first representatives of the ThCrsSi, structure type in which the Pt and Si atoms were found to be randomly distributed between the 4(d) and 4(e) positions. Experimental The compounds were prepared by melting together the rare earth, strontium, platinum and silicon metals (each of 99.9% purity). The platinum was in foil form and the other metals were folded in this foil. The samples were placed in alumina crucibles and induction heated under the protective atmosphere of helium. Powdered samples were X-ray analyzed by a Philipps diffractometer. using monochromatiz~ Cu & radiation. X-ray intensity measurement were carried out by scanning the samples at a rate of l/4” (20) per minute. The integrated intensity of the reflections was determined by measuring the area of the peaks. The structure factor calculations were made by a least squares program [9] .
Results The powder patterns of the samples could be indexed on the basis of a body-centered tetragonal cell. The unit cell constants of the compounds are listed in Table 1. The tetragonal phase of most of the compounds was accompanied by a few lines of another phase, which was probably one of the silicides of Pt. The variation of the unit cell constants uersus the atomic number of the rare earth metals is illustrated in Fig. 1. Estimation of the relative intensities of the reflections of the different LnPtsSia compounds, based on their peak heights, has shown that these intensities cannot correspond to the “normal” distribution of the Pt and Si atoms, namely that Pt and Si occupy the 4(d) and 4(e) sites of the 14lmmm space group, respectively. If this were the case, the relative intensities of the 103,112 and 200 reflections should be around 15%, 100% and 40%; in fact they were found to be 85%, 100% and 75%, respectively. It was necessary therefore to carry out structure factor calculations in order to determine the exact location of the Pt and Si atoms in the structure. These calculations were performed for ErPtsSis. Each unit cell contains two ErPtsSiz; the available equivalent positions of the I4lmmm space group for the above 10 atoms are: 2(a) 000; 4(d) 0 1,1/4, l/20 j/4;4(c) OOz,OO27
173 TABLE 1 Lattice constants of the L&&Ii*
LaPt$iz NdPt$& EuPtzSiz GdPt#i, DyPt$$ ErPt$Q TmPtzSiz LuPtzSiz SrPtzSi2
4.01 a 1 ’ IA Ce Pr Nd
4.277 4.218 4.215 4.175 4.149 4.115 4.113 4.075 4.270
0
and SrPtzSiz compounds
9.822
2.296
9.790
2.321 2.295 2.343 2.360 2.387 2.392 2.442 2.317
9.810 9.783 9.790 9.822 9.838 9.952
9.895
’ ’ I 0 ’ 0 ’ ’ ’ Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 1. Variation of the lattice constants a and c with atomic number for the LnPt2Siz compounds.
In all the calculations Er atoms were put in the 2(a) positions. For Pt and Si the following three cases were examined: Pt in 4(d) and Si in 4(e); Pt in 4(e) and Si in 4(d); Pt and Si occupy the 4(d) and 4(e) positions randomly. The free parameter of the 4(e) site was determined to be 0.370. In Table 2 the observed and calculated intensities are compared for the above three cases. Three significantly different R values were found for the three types of calculations. The lowest value of R was obtained for the case when Pt and Si are statistically distributed between the 4(d) and 4(e) positions.
174 TABLE 2 Relative integrated intensities of ErPt&& hkl
002 101 110 103 112 004 200 202 114 23.1 105 006 213 204 220 116 222 301 215 107 310 206 303 312
Gb*.a
Pt(H, 0, j/l) Si(0, 0,0.370)
Pt(0, 0,0.370)
2.0 n.0. 21.6 82.4 100.0 2.0 78.2 n.0. 20.9 n.0. 29.9 n.0. 30.8 3.0 24.8 28.0 n.0. n.0. 24.6 n.0. n.0. n.0. 15.3 35.0
21.27 15.37 5.94 15.68 100.0 19.0 35.03 6.90 10.65 4.19 5.20 1.09 1.49 20.26 11.66 18.06 2.57 0.82 4.17 0.77 1.09 1.90 1.72 23.60
7.64 11.63 89.95 100.0 13.29 3.36 42.03 2.62 11.02 3.04 24.62 1.38 49.46 3.74 14.01 5.61 0.97 0.60 19.76 1.49 16.70 2.45 11.23 3.06
R
0.590
Mixed
Si(?h, 0, f/4)
0.731
4.77 0.54 15.34 87.15 100.0 4.93 75.34 1.39 22.62 0.16 22.88 0.01 42.63 5.09 25.07 23.44 0.52 0.03 18.36 0.03 2.64 0.01 9.70 23.39 0.156
%_o., not observed.
Discussion The variation of the unit cell constant a versus the atomic number of the rare earths can be attributed to the lanthanide contraction of the trivalent rare earths. An exception is EuPt,Si,; in this case the value of a is much higher than that found for the neighboring lanthanides. This seems to be a clear indication that Eu is divalent in this compound, as in many Eu compounds of the ThCraSis type. The changes observed for the unit cell constant c have a different character; c remains constant in the La - Dy compounds and increases from then on. A similar behavior of c was also found in other LnMzSiz compounds with the same structure in which the transition metal is Cu, Pd or Au. The C/Q values of all these compounds have been found to be relatively low. The shortest Si-Si bond is located along the c axis; relatively low and constant values of c therefore mean short and rigid Si-Si bonds. Another consequence of the relatively short c values is that the
175
Ln-8Pt and Ln-8Si distances are very similar and not very different from the Ln-2Si distance. This means that the polyhedron around the lanthanide atoms is less distorted in these compounds. In Table 3 the interatomic distances for the LnPt& compounds are listed. This table shows the bond distances in the polyhedron mentioned above and it also shows that the Si-Si distance remains almost constant in all the compounds. The M-M and M-Si bonds are relatively long in all the compounds with low c/u values. This suggests that in compounds in which the Si-Si bonds are strong the interaction between the transition metals and Si becomes weaker. TABLE
3
Interatomic
distance
(8)
of SrPtpSil
and LnPtzSiz
compounds
Sr
La
Nd
EU
Gd
Dy
Er
Tm
Lu
Ltv-8Pt Ln--8Si Ln--2Si
3.285 3.267 3.661
3.256 3.264 3.634
3.231 3.224 3.622
3.253 3.262 3.630
3.215 3.195 3.620
3.208 3.179 3.622
3.204 3.158 3.634
3.206 3.163 3.640
3.216 3.139 3.682
Pt,- 4Pt Pt-4Si Si-Si
3.020 2.473 2.596
3.025 2.466 2.554
2.983 2.438 2.545
3.023 2.464 2.551
2.953 2.419 2.544
2.934 2.409 2.545
2.910 2.396 2.554
2.909 2.396 2.558
2.882 2.388 2.588
The most significant difference found between the Pt-containing compounds studied in this work and all the other ThCrpSi2-type compounds is the distribution of the Pt and Si atoms in the structure. The X-ray diffraction intensity calculations definitely prove that Pt and Si are statistically distributed between the 4(d) and 4(e) sites. The lowest R value (0.156) was obtained in this case and was much lower than the values obtained when Pt is in 4(d) and Si in 4(e) (0.590) or the inverse case (0.731). This leaves no doubt that the best agreement between observed and calculated X-ray intensities is obtained when Pt and Si are randomly distributed between the 4(d) and 4(e) positions and that this is the real structure of the Pt-containing ThCr2Si2-type silicides. As was pointed out above, the possibility that the transition metal and Si atoms are randomly distributed has been raised in many studies. So far the Pt-containing ThCr2Si,-type silicides are the only examples in which the existence of this arrangement has been confirmed. It is hard to tell at this stage what the role of the Pt atoms is in the formation of such a structure. Pt is known to form chains in nonmetals in many coordination compounds. It may well be that this property of Pt is responsible for the statistical distribution of the Pt and Si atoms, thus forming Pt-Si-Pt-Si chains in the structure. Pt is known to appear in the tetravalent form, in contrast to other transition metals occurring in ThCr,Si,-type compounds. This property might also be responsible for the tendency of Pt to distribute statistically with the Si atoms, which are as a rule tetravalent like the Pt atoms.
176
References 1 2 3 4 6 6
Z. Ban and M. Sikirica, Acta Crystallogr., 18 (1965) 594. W. Rieger and E. Parthe, Monatsh. Chem., 100 (1969) 444. I. Mayer, J. Cohen and I. Felner, J. Less-Common Met., 30 (1973) 181. R. Balestracci, C. R. Acad. Sci. Paris Ser. B, 282 (1976) 291. W. Dorrscheidt, N. Niess and H. Schafer, Z. Naturforsch., Teil B, 31 (1976) 890. E. R. Bauminger, D. Froindlich, I. Nowik, S. Ofer, I. Felner and I. Mayer, Phys. Rev. Lett., 30 (1973) 1053. 7 I. Mayer and I. Felner, J. Phys. Chem. Solids, in the press. 8 I. Felner, I. Mayer, A. Grill and M. Schieher, Solid State Commun., 16 (1975) 1005. 9 M. H. Muller, L. Heaton and K. I. Miller, Acta Crystallogr., 13 (1960) 828.