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Phase equilibria in the niobium-rich corner of the niobium-platinum-oxygen system at 1373 K
Journal of the Less-Common Metals, 69 (1980) 321- 330 0 Elsevier Sequoia S.A., Lausaune - Printed in the Netherlands
327
PHASE EQUILIBRIA IN THE NIOBIUM-RICH CORNER OF THE NIOBIUM-PLATINUM-OXYGEN SYSTEM AT 1373 K
R. HORYfi and R. ANDRUSZKIEWICZ Institute forLow Temperature and Structure PI. Katedralny 1, 50-950 Wroctaw (Poland)
Research
of the Polish Academy
of Sciences,
(Received April 26,1979)
Summary
The phase equilibria of the Nb-Pt-0 system were studied by the X-ray powder method and by chemical analysis. The results obtained are shown in a phase diagram. A new ternary phase described as R(NbPt0) was identified. Its density and hexagonal unit cell parameters are as follows: d, = 12.89 g cmp3, a = 7.935 f 0.004 A, c = 5.066 f 0.003 A, c/a = 0.638. Superconductivity of the R(NbPt0) phase was not observed above 4 K.
1. Introduction
This work is a continuation of a programme of exploration for new superconducting materials. Phase equilibria investigations in the Nb-Ir-0 ternary system [l] have revealed that the H(NbIr0) phase with a D&J structure type [2] is responsible for the superconductivity of Nb-Ir binary alloys reported recently by several investigators [ 3 - 71. We have also pointed out [ 11 that other such phases may exist in similar oxygen-transition metal ternary systems, of which Nb-Pt-0 seems to be the most promising.
2. Experimental Samples were prepared from niobium (Starck, Berlin), 99.9% platinum (Polish Mint) and 99.9% niobium pentoxide (Fluka, Buchs). The finely powdered components required for a given composition were mixed in an agate mortar, were compressed into pellets 6 mm in diameter weighing 0.5 g and were subjected to vacuum heating. Grinding and pelleting were repeated before the sintering and annealing processes were performed. Degassed samples were heated overnight at 1173 K in a quartz tube under a reduced pressure of argon. After this pre-sintering procedure speci-
mens were reacted in an r.f. heating furnace under an argon atmosphere for 8 h at 1673 K. Annealing to attain a phase equilibrium was subsequently carried out in a resistance furnace for 15 d at 1373 K. Phases were identified from X-ray diffraction powder patterns obtained with Cu Ka radiation in a Guinier-type camera using NaCl as an internal standard. The compositions of annealed samples were determined by chemical analysis to correct for contamination by air. A simple gravimetric method was used. First, the pulverized sample was placed in a platinum boat and was fully oxidized in air at 1173 K, and then the weight increase was determined gravime trically . Assuming that the metal components A and B (for the general case of the ternary system A-B-O) oxidize to A,O, and BpOp oxides and that the A/B weight ratio is constant throughout the history of the sample, we can determine the total amount of each component from the equations [A] = _?(Y+0/K [B] =-!!? p+Ko
PI = mo- [Al - PI where a =
M AxOy XMA
m. is the initial weight of the sample, m the weight of the oxidized sample, K = A/B and M is the molecular weight. In the case of the Nb-Pt-0 system /3 = 1. The density was measured pycnometrically by weighing the wellground degassed sample in CCll at 298 + 0.1 K. The R(NbPt0) phase was tested for superconductivity by the selfinductance method. No transition to a superconducting state was found above 4 K.
3. Results and discussion The construction of the phase diagram was started by using the available data on the homogeneity range of NbO [8] and Nb-0 solid solution [ 91 (Fig. 1). The homogeneity range of NbOs is very narrow [lo] and was neglected in this work. The phase NbPt seems to be stoichiometric [ 111. In the present work, however, the homogeneity ranges for NbsPt (19.8 - 27
329
Fig. 1. The niobium-rich portion of the Nb-Pt-0 phase diagram at 1373 K: R, new hexagonal phase; 1, Nb,, + NbaPt; 2, Nb,, + NbO + NbaPt; 3, (NbaPt),, + NbO; 4, Nb3Pt + NbO + R; 5, Nb3Pt + R + Nb-Pt; 6, (NbePtlss + R; 7, Nb&‘t + R + NH%; 8, R + NbO + NbPt; 9, NbO + NbOz + NbPt; 10, R + NbO; 11, (Nb3Pt),,; 12, (Nb+Pt)s,.
at.% Pt) and Nb,aPt (32.5 - 38 at.% Pt) are broadened in comparison with published data [ 12, 131. Moehlecke et al. [ 141 have observed an even wider range of homogeneity for Nb,Pt. Because of difficulty in reaching the complete equilibrium state for the R(NbPt0) phase, its probable position is circled by a broken line in the phase diagram. A sample of composition Nb0.558sPt,-,34so00.0ssr lying in this circle was used for X-ray analysis and pycnometric measurements, in spite of the presence of some traces of foreign phases. The powder pattern of the R(NbPt0) phase was indexed on the basis of a hexagonal unit cell with lattice constants a = 7.935 A, c = 5.066 A and c/a = 0.638. Its density was determined experimentally as 12.89 g cme3. Assuming 18 average atoms per unit cell the calculated X-ray density is 13.08 g cme3. The systematic extinctions for h01 with I = 2n + 1 lead to possible space groups PG,/mcm, P&2, P6,cm and P&l.
330
Determination of the crystal structure of the R(NbPt0) progress and the results will be published in a separate paper.
phase is in
Acknowledgments The authors wish to thank Professor W. Trzebiatowski for his interest in this work. Thanks are also due to Dr. J. Sosnowski for testing the samples for superconductivity.
References 1 R. Horyli, L. Folcik-Kokot and N. Biev, J. Less-Common Met., 57 (1978) P69. 2 R. Horyli and L. Folcik-Kokot, J. Less-Common Met., 57 (1978) P75. 3 V. B. Compton, E. Corenzwit, J. P. Maita, B. T. Matthias and F. J. Morin, P!rys. Rev., 123 (1961) 1567. 4 E. Bucher, F. Heiniger and J. Muller, Helu. Phys. Acta.. 34 (1961) 843. 5 R. D. Blaugher and J. K. Hulm, J. Phys. Chem. Solids, 19 (1961) 124. 6 C. C. Koch and J. 0. Scarbrough, Phys. Rev., Sect. B, 3 (1971) 742. 7 W. L. Johnson and S. J. Poon, J. Less-Common Met., 42 (1975) 355. 8 G. Brauer, 2. Anorg. Chem., 248 (1941) 1. 9 A. U. Seybolt, pans. Metall. Sot. AZME, 200 (1954) 774. 10 A. Magneli, G. Andersson and G. Sundkvist, Acta Chem. &and., 9 (1955) 1402. 11 V. Sadagopan and H. C. Gatos, Phys. Status Solidi, 13 (1966) 423. 12 J. Miiller, R. Fliikiger, A. Junod, F. Heiniger and C. Susz, J. Low. Temp. Phys., 13, (3) (1975) 446. 13 R. P. Elliott, Constitution of Binary Alloys, 1st Suppl., McGraw-Hill, New York, 1965, p. 267. 14 S. Moehlecke, D. E. Coxand and A. R. Sweedler, Solid State Commun., 23 (1977) 703.
327
PHASE EQUILIBRIA IN THE NIOBIUM-RICH CORNER OF THE NIOBIUM-PLATINUM-OXYGEN SYSTEM AT 1373 K
R. HORYfi and R. ANDRUSZKIEWICZ Institute forLow Temperature and Structure PI. Katedralny 1, 50-950 Wroctaw (Poland)
Research
of the Polish Academy
of Sciences,
(Received April 26,1979)
Summary
The phase equilibria of the Nb-Pt-0 system were studied by the X-ray powder method and by chemical analysis. The results obtained are shown in a phase diagram. A new ternary phase described as R(NbPt0) was identified. Its density and hexagonal unit cell parameters are as follows: d, = 12.89 g cmp3, a = 7.935 f 0.004 A, c = 5.066 f 0.003 A, c/a = 0.638. Superconductivity of the R(NbPt0) phase was not observed above 4 K.
1. Introduction
This work is a continuation of a programme of exploration for new superconducting materials. Phase equilibria investigations in the Nb-Ir-0 ternary system [l] have revealed that the H(NbIr0) phase with a D&J structure type [2] is responsible for the superconductivity of Nb-Ir binary alloys reported recently by several investigators [ 3 - 71. We have also pointed out [ 11 that other such phases may exist in similar oxygen-transition metal ternary systems, of which Nb-Pt-0 seems to be the most promising.
2. Experimental Samples were prepared from niobium (Starck, Berlin), 99.9% platinum (Polish Mint) and 99.9% niobium pentoxide (Fluka, Buchs). The finely powdered components required for a given composition were mixed in an agate mortar, were compressed into pellets 6 mm in diameter weighing 0.5 g and were subjected to vacuum heating. Grinding and pelleting were repeated before the sintering and annealing processes were performed. Degassed samples were heated overnight at 1173 K in a quartz tube under a reduced pressure of argon. After this pre-sintering procedure speci-
mens were reacted in an r.f. heating furnace under an argon atmosphere for 8 h at 1673 K. Annealing to attain a phase equilibrium was subsequently carried out in a resistance furnace for 15 d at 1373 K. Phases were identified from X-ray diffraction powder patterns obtained with Cu Ka radiation in a Guinier-type camera using NaCl as an internal standard. The compositions of annealed samples were determined by chemical analysis to correct for contamination by air. A simple gravimetric method was used. First, the pulverized sample was placed in a platinum boat and was fully oxidized in air at 1173 K, and then the weight increase was determined gravime trically . Assuming that the metal components A and B (for the general case of the ternary system A-B-O) oxidize to A,O, and BpOp oxides and that the A/B weight ratio is constant throughout the history of the sample, we can determine the total amount of each component from the equations [A] = _?(Y+0/K [B] =-!!? p+Ko
PI = mo- [Al - PI where a =
M AxOy XMA
m. is the initial weight of the sample, m the weight of the oxidized sample, K = A/B and M is the molecular weight. In the case of the Nb-Pt-0 system /3 = 1. The density was measured pycnometrically by weighing the wellground degassed sample in CCll at 298 + 0.1 K. The R(NbPt0) phase was tested for superconductivity by the selfinductance method. No transition to a superconducting state was found above 4 K.
3. Results and discussion The construction of the phase diagram was started by using the available data on the homogeneity range of NbO [8] and Nb-0 solid solution [ 91 (Fig. 1). The homogeneity range of NbOs is very narrow [lo] and was neglected in this work. The phase NbPt seems to be stoichiometric [ 111. In the present work, however, the homogeneity ranges for NbsPt (19.8 - 27
329
Fig. 1. The niobium-rich portion of the Nb-Pt-0 phase diagram at 1373 K: R, new hexagonal phase; 1, Nb,, + NbaPt; 2, Nb,, + NbO + NbaPt; 3, (NbaPt),, + NbO; 4, Nb3Pt + NbO + R; 5, Nb3Pt + R + Nb-Pt; 6, (NbePtlss + R; 7, Nb&‘t + R + NH%; 8, R + NbO + NbPt; 9, NbO + NbOz + NbPt; 10, R + NbO; 11, (Nb3Pt),,; 12, (Nb+Pt)s,.
at.% Pt) and Nb,aPt (32.5 - 38 at.% Pt) are broadened in comparison with published data [ 12, 131. Moehlecke et al. [ 141 have observed an even wider range of homogeneity for Nb,Pt. Because of difficulty in reaching the complete equilibrium state for the R(NbPt0) phase, its probable position is circled by a broken line in the phase diagram. A sample of composition Nb0.558sPt,-,34so00.0ssr lying in this circle was used for X-ray analysis and pycnometric measurements, in spite of the presence of some traces of foreign phases. The powder pattern of the R(NbPt0) phase was indexed on the basis of a hexagonal unit cell with lattice constants a = 7.935 A, c = 5.066 A and c/a = 0.638. Its density was determined experimentally as 12.89 g cme3. Assuming 18 average atoms per unit cell the calculated X-ray density is 13.08 g cme3. The systematic extinctions for h01 with I = 2n + 1 lead to possible space groups PG,/mcm, P&2, P6,cm and P&l.
330
Determination of the crystal structure of the R(NbPt0) progress and the results will be published in a separate paper.
phase is in
Acknowledgments The authors wish to thank Professor W. Trzebiatowski for his interest in this work. Thanks are also due to Dr. J. Sosnowski for testing the samples for superconductivity.
References 1 R. Horyli, L. Folcik-Kokot and N. Biev, J. Less-Common Met., 57 (1978) P69. 2 R. Horyli and L. Folcik-Kokot, J. Less-Common Met., 57 (1978) P75. 3 V. B. Compton, E. Corenzwit, J. P. Maita, B. T. Matthias and F. J. Morin, P!rys. Rev., 123 (1961) 1567. 4 E. Bucher, F. Heiniger and J. Muller, Helu. Phys. Acta.. 34 (1961) 843. 5 R. D. Blaugher and J. K. Hulm, J. Phys. Chem. Solids, 19 (1961) 124. 6 C. C. Koch and J. 0. Scarbrough, Phys. Rev., Sect. B, 3 (1971) 742. 7 W. L. Johnson and S. J. Poon, J. Less-Common Met., 42 (1975) 355. 8 G. Brauer, 2. Anorg. Chem., 248 (1941) 1. 9 A. U. Seybolt, pans. Metall. Sot. AZME, 200 (1954) 774. 10 A. Magneli, G. Andersson and G. Sundkvist, Acta Chem. &and., 9 (1955) 1402. 11 V. Sadagopan and H. C. Gatos, Phys. Status Solidi, 13 (1966) 423. 12 J. Miiller, R. Fliikiger, A. Junod, F. Heiniger and C. Susz, J. Low. Temp. Phys., 13, (3) (1975) 446. 13 R. P. Elliott, Constitution of Binary Alloys, 1st Suppl., McGraw-Hill, New York, 1965, p. 267. 14 S. Moehlecke, D. E. Coxand and A. R. Sweedler, Solid State Commun., 23 (1977) 703.