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Crystal structure of the R(NbPtO) phase
Journal of the Less-Common Metals, 71 (1980) P9 - P12 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
CRYSTAL
R. HORYfi
STRUCTURE
OF THE R(NbPt0)
PHASE
and R. ANDRUSZKIEWICZ
Institute for Low Temperature and Structure PI. Katedralny 1, 50-950 Wroctaw (Poland) (Received
P9
Research
of the Polish Academy
of Sciences,
June 28,1979)
Summary
The crystal structure of the R(NbPt0) phase was determined using Xray diffraction. The R phase of chemical composition Nb,Pt80 is not superconducting down to 4 K, in contrast with the H(NbIr0) phase (I!‘, = 11 K) which represents the same type of structure.
1. Introduction
We have recently reported [l] some results on phase equilibria in the Nb-Pt-0 system. Here we provide further data concerning the crystal structure of the R phase found in this system.
2. Experimental Because of considerable difficulties in preparing single crystals of the R phase we decided to determine its crystal structure by the powder diffraction method. An X-ray diffractometer with a scintillation counter and Cu K~s radiation were used for measuring the intensities. The values of the structure factors lFhkll were corrected for Lorentz and polarization factors. The absorption factor was omitted, since the technique applied was the symmetrical case of Bragg reflection for an infinitely thick polycrystalline platelet. The chemical composition of the sample used for the structure analysis was Nbo.8888Pto.848oOo.oss4. This sample contained some traces of secondary phases; however, of all the Nb-Pt-0 samples available its chemical composition was the closest to the stoichiometry of the R phase (see the figure given in ref. l), i.e. to Nb5Pt30. The lattice constants were calculated by the least-squares method. Crystal structure calculations were initiated using the atomic positions found
PlO TABLE 1 X-ray analysis of the R(NbPt0) Crystallographic formula: Symmetry : Lattice constants: Densities: Space group: Structure type:
phase (Cu Ko radiation)
NbBPtsO hexagonal a = 7.952 + 0.002 A, c = 5.045 * 0.004 A, c/a = 0.6345 d, = 12.89 g cme3, dx = 12.82 g cmm3, 2 = 2 P63lmcm TigGa4 (filled version of the D8a type)
Special positions:
2(b), 4(d), 6(g)I with x = i, G(g)11 with x = 0.59375
Discrepancy factor:
R = 0.136
hkl 100 110 200 111 210 002 102 2 1.1 300 .l 1 2 202 220 310 221 212 311 400 302 320 222 113 312 321 410 411 402 213 500 322 330 420 412 223 331 313 004 421 104 510
sin2ec 0.012511 0.037533 0.050044 0.060844 0.087577 0.093244 0.105755 0.110888 0.112599 0.130777 0.143288 0.150132 0.162643 0.173443 0.180821 0.185954 0.200175 0.205843 0.237709 0.243376 0.247332 0.255887’ 0.261020 0.262731
I, 0.012549 0.037522 0.049995 0.060874 0.087572 0.093075 0.105632 0.111580 0.130952 0.143259 0.150213 0.162625 0.173459 0.180909 0.186103 0.200130 0.205901 0.237885 0.243338 0.246991 0.255873 0.262353
0.293419 0.297376 0.312775 0.330953 _ 0.350308 0.355975 0.359931 0.361108 0.372442 0.372976 0.373619 0.387841
0.293350
0.297151 0.313071 0.331159 0.350377 0.356194 0.361044
151.66 61.47 67.35 156.33 123.51 162.72 4.23 710.85 277.85 411.47 126.46 8.20 59.05 43.64 6.72 300.69 5.37 17.33 9.04 42.83 25.50 7.44 24.72 16.30 3.91 31.37 157.79 63.50 58.00 0 54.32 27.93 15.14 73.37
0.372691 0.387567
40 34 40 116 123 145 4 1049 386 94 6 55 63 19 257 11 18 4 62 23 6 51 0 43 214 81 93 0 79 40 126 249
3.95 29.51
0 25
Pll TABLE l(continued) hkl
sin2tIc
sin20,
502 114 511 204 332 422 600 430 512 431 520
0.406019 0.410509 0.411152
0.406928
0.431041 0.443552 0.481085 0.486218 0.487929
_ 0.430855 0.443714 _ _ 0.486855
1, 148.03 5.60> 6.55 7.68 39.35 34.57 0.57 0 16.86 8.80> 83.82
10
201 0 45 33 0 0 143
for the H(NbIr0) phase [ 21; however, it was assumed that all corresponding positions in the R phase were occupied by pure niobium, platinum and oxygen atoms and that their ratios were given by the chemical formula Nb,PtsO. The atomic scattering factors for niobium, platinum and oxygen were taken from ref. 3.
3. Results and discussion The supposition that the R phase represents the filled D8,-type ( Ti5Ga4-type) structure is fully confirmed. A comparison of the experimental
data with the proposed model is given in Table 1. The discrepancy factor R (= Z (I F,I -- IF, 1)/Z IF, I) obtained for all reflections presented in Table 1 is 0.136. Special positions occupied in the space group P6s/mcm are as follows: 2(b), oxygen atoms; 4(d), niobium atoms; 6(g)I, niobium atoms; G(g)II, platinum atoms. The results presented in Table 1 show a satisfactory agreement of the experimental data with the proposed model, although we realise that they require further confirmation by a single-crystal analysis. The most interesting feature of the R phase, when compared with the H phase [ 21, is that it does not exhibit any transition to a superconducting state down to 4 K. This is thought to be a consequence of considerable differences in distribution of the atoms within the unit cells of these two phases. The 4(d) and 6(g)II atomic positions in the H(NbIr0) phase are randomly filled with Nb-0 and/or b-0 atoms, whilst in the R phase these positions are purely metallic. Such a situation can create a specific electronic structure of the material which may not be favourable for superconductivity. This work and that on phase equilibria in the Nb-Pt-0 system [l] evidently support the idea outlined previously [2] that it is necessary to search for new Nowotny-type phases in regions of concentration of a given system where they have not so far been expected.
P12
References 1 R. Horyd and R. Andruszkiewicz, J. Less-Common Met., 69 (1980) 327. 2 R. Horyd and L. Folcik-Kokot, J. Less-Common Met., 57 (1978) P75. 3 International Tables for X-ray Crystallography, Vol. III, Kynoch Press, Birmingham, 1962.
CRYSTAL
R. HORYfi
STRUCTURE
OF THE R(NbPt0)
PHASE
and R. ANDRUSZKIEWICZ
Institute for Low Temperature and Structure PI. Katedralny 1, 50-950 Wroctaw (Poland) (Received
P9
Research
of the Polish Academy
of Sciences,
June 28,1979)
Summary
The crystal structure of the R(NbPt0) phase was determined using Xray diffraction. The R phase of chemical composition Nb,Pt80 is not superconducting down to 4 K, in contrast with the H(NbIr0) phase (I!‘, = 11 K) which represents the same type of structure.
1. Introduction
We have recently reported [l] some results on phase equilibria in the Nb-Pt-0 system. Here we provide further data concerning the crystal structure of the R phase found in this system.
2. Experimental Because of considerable difficulties in preparing single crystals of the R phase we decided to determine its crystal structure by the powder diffraction method. An X-ray diffractometer with a scintillation counter and Cu K~s radiation were used for measuring the intensities. The values of the structure factors lFhkll were corrected for Lorentz and polarization factors. The absorption factor was omitted, since the technique applied was the symmetrical case of Bragg reflection for an infinitely thick polycrystalline platelet. The chemical composition of the sample used for the structure analysis was Nbo.8888Pto.848oOo.oss4. This sample contained some traces of secondary phases; however, of all the Nb-Pt-0 samples available its chemical composition was the closest to the stoichiometry of the R phase (see the figure given in ref. l), i.e. to Nb5Pt30. The lattice constants were calculated by the least-squares method. Crystal structure calculations were initiated using the atomic positions found
PlO TABLE 1 X-ray analysis of the R(NbPt0) Crystallographic formula: Symmetry : Lattice constants: Densities: Space group: Structure type:
phase (Cu Ko radiation)
NbBPtsO hexagonal a = 7.952 + 0.002 A, c = 5.045 * 0.004 A, c/a = 0.6345 d, = 12.89 g cme3, dx = 12.82 g cmm3, 2 = 2 P63lmcm TigGa4 (filled version of the D8a type)
Special positions:
2(b), 4(d), 6(g)I with x = i, G(g)11 with x = 0.59375
Discrepancy factor:
R = 0.136
hkl 100 110 200 111 210 002 102 2 1.1 300 .l 1 2 202 220 310 221 212 311 400 302 320 222 113 312 321 410 411 402 213 500 322 330 420 412 223 331 313 004 421 104 510
sin2ec 0.012511 0.037533 0.050044 0.060844 0.087577 0.093244 0.105755 0.110888 0.112599 0.130777 0.143288 0.150132 0.162643 0.173443 0.180821 0.185954 0.200175 0.205843 0.237709 0.243376 0.247332 0.255887’ 0.261020 0.262731
I, 0.012549 0.037522 0.049995 0.060874 0.087572 0.093075 0.105632 0.111580 0.130952 0.143259 0.150213 0.162625 0.173459 0.180909 0.186103 0.200130 0.205901 0.237885 0.243338 0.246991 0.255873 0.262353
0.293419 0.297376 0.312775 0.330953 _ 0.350308 0.355975 0.359931 0.361108 0.372442 0.372976 0.373619 0.387841
0.293350
0.297151 0.313071 0.331159 0.350377 0.356194 0.361044
151.66 61.47 67.35 156.33 123.51 162.72 4.23 710.85 277.85 411.47 126.46 8.20 59.05 43.64 6.72 300.69 5.37 17.33 9.04 42.83 25.50 7.44 24.72 16.30 3.91 31.37 157.79 63.50 58.00 0 54.32 27.93 15.14 73.37
0.372691 0.387567
40 34 40 116 123 145 4 1049 386 94 6 55 63 19 257 11 18 4 62 23 6 51 0 43 214 81 93 0 79 40 126 249
3.95 29.51
0 25
Pll TABLE l(continued) hkl
sin2tIc
sin20,
502 114 511 204 332 422 600 430 512 431 520
0.406019 0.410509 0.411152
0.406928
0.431041 0.443552 0.481085 0.486218 0.487929
_ 0.430855 0.443714 _ _ 0.486855
1, 148.03 5.60> 6.55 7.68 39.35 34.57 0.57 0 16.86 8.80> 83.82
10
201 0 45 33 0 0 143
for the H(NbIr0) phase [ 21; however, it was assumed that all corresponding positions in the R phase were occupied by pure niobium, platinum and oxygen atoms and that their ratios were given by the chemical formula Nb,PtsO. The atomic scattering factors for niobium, platinum and oxygen were taken from ref. 3.
3. Results and discussion The supposition that the R phase represents the filled D8,-type ( Ti5Ga4-type) structure is fully confirmed. A comparison of the experimental
data with the proposed model is given in Table 1. The discrepancy factor R (= Z (I F,I -- IF, 1)/Z IF, I) obtained for all reflections presented in Table 1 is 0.136. Special positions occupied in the space group P6s/mcm are as follows: 2(b), oxygen atoms; 4(d), niobium atoms; 6(g)I, niobium atoms; G(g)II, platinum atoms. The results presented in Table 1 show a satisfactory agreement of the experimental data with the proposed model, although we realise that they require further confirmation by a single-crystal analysis. The most interesting feature of the R phase, when compared with the H phase [ 21, is that it does not exhibit any transition to a superconducting state down to 4 K. This is thought to be a consequence of considerable differences in distribution of the atoms within the unit cells of these two phases. The 4(d) and 6(g)II atomic positions in the H(NbIr0) phase are randomly filled with Nb-0 and/or b-0 atoms, whilst in the R phase these positions are purely metallic. Such a situation can create a specific electronic structure of the material which may not be favourable for superconductivity. This work and that on phase equilibria in the Nb-Pt-0 system [l] evidently support the idea outlined previously [2] that it is necessary to search for new Nowotny-type phases in regions of concentration of a given system where they have not so far been expected.
P12
References 1 R. Horyd and R. Andruszkiewicz, J. Less-Common Met., 69 (1980) 327. 2 R. Horyd and L. Folcik-Kokot, J. Less-Common Met., 57 (1978) P75. 3 International Tables for X-ray Crystallography, Vol. III, Kynoch Press, Birmingham, 1962.