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Anticorrelation of atomic ordering with superconductivity in vanadium-based transition metal A15 alloys
Volume 46A, number 1
PHYSICS LETTERS
19 November 1973
ANTICORRELATION OF ATOMIC ORDERING WITH SUPERCONDUCTIVITY IN VANADIUM-BASED TRANSITION METAL A15 ALLOYS J.E. COX Naval Research Laboratory, Washington, D.C. 203 75, USA
R.M. WATERSTRAT National Bureau of Standards, Washington, D.C. 20234, USA Received 11 September 1973 Measurements of the superconducting transition temperature (T
0) of V-Rh A15 alloys reveal that long range order induced by low temperature annealing may cause T0 tp decrease rather than increase. The behavior of T0 with ordering is discussed for the systems V-Au, V-Pt, V-h, V-Os and V-Rh.
The Al 5 alloys have been in the forefront of superconducting materials interest for several years due to their unusually high transition temperatures (T0). Theories based on the suggestions of Weger [1] have been developed to explain the experimental effects observed with these materials. The structure of the Al 5 alloys is that of three mutually orthogonal linear chains formed by the A element atoms superposed pairwise on the faces of a body centered cube composed of the B element atoms. The theories usually relate increased T0s to the improved integrity of the linear chains, i.e. an increase in the degree of crystallographic long range order. This note is intended to emphasize that there are serious exceptions to previously accepted general trends. Ideal stoichiometric composition of the Al5 materials is frequently designated by the notation A3B. Deviations from stoichiometry and/or perfect order will often result from the melting procedure, causing some B atoms to occupy bcc sites. Previous studies [2—4] have indicated that T0 increases as B atoms on the A chains are replaced by A atoms. One can remove B atoms from the chains in either of two ways: by chemically altering the composition of the sample, or by low temperature (600—900C) thermal treatment. A good example of both of these effects is found in the V-Pt [2] system, wherein the Al 5 phase extends from approximately 67 to 81 at.%V. As the V content increases toward stoichiometry, the Pt atoms that had occupied chain sites are replaced by V atoms, and T0 increases. In addition to chemical alteration, a
low temperature anneal (800 C for one day) permits the Pt atoms that accidently were frozen on the Achain sites to interchange positions wit V atoms that were frozen into bcc sites. This increase in A-chain integrity also produces an increase in T0. Since a number of Al 5 alloy systems behave in this manner it was thought that this might be the universal behavior for the Al 5 alloys in general. However, we found that the V-Ir [5] system behaved in a considerably different manner. While well ordered stoichiometric V75 1r25 was not superconducting down to 15 mK, superconductivity sets in as more Ir is added to the alloy, and T0 is a monotonically increasing function of increasing Ir concentration up to a maximum T0 of 1.87K at the composition V61 1r39. That is, as the vandium chains are broken up by the addition of iridium, T0 increases. This behavior is clearly in contradiction with the Labbe’.Friedel [6,7] model, which postulated that T0 should be depressed as the chains are broken up. Although these same samples were subsequently subjected to a low temperature anneal (850 C—23 hr), similar to the anneals used in other systems, no change was observed in their respective transition temperatures. The A15 phase of the V-Os system exists in a narrow composition region near V450s55. Although the composition is far from stoichiometry, the sample had aT0 of 5.04K [3]. In the V-Os system, the A15 phase is unstable at the usual homogenization annealing temperatures, so this sample received only a 6 month, 620C anneal. The transition temperature is 21
Volume 46A, number I
Pl-IYSICS LETTERS
higher than any of the other vanadium-transition meta! alloys even though its composition deviates from stoichiornetry more than any other system.
We have recently investigated two samples in the V-Rh system to ascertain whether this system behaved analogously with the isoelectronic V-lr system. While the stoichiometric V75 Rh25 sample is non-superconducting down to 1 5 mK, the low temperature limit of the experimental apparatus, a V65 Rh35 sample was found to have a 7~of 1 .075 K. When this sample was subjected to an ordering anneal of 750C br 8 weeks, its 7~)decreased to I .036K~the first known decrease of T~)with thermal ordering when the B elenient is a transition metal. This behavior with ordering should not he too surprising, however, since thermal ordering is. in effect , tantamount to decreasing the percentage of Rh atoms on the V chain sites, just a soccurs when the chemical composition is changed to lower the Rh concentration in the alloy. Indeed, it was hoped that the low temperature annealing of the V-lr alloys would have produced the same result. Perhaps different thermal treatment would produce the desired effect. Thus, the simple correlation of chain integrity with transition temperature cannot be considered universally applicable to the Al 5 alloys since these data clearly emphasize the oversimplification of the Labbe’Friedel model. Rather, it now seems that when there is strong hybridization of the d-electron bands via alloying V with either Ir, Os or Rh, the V-lr. V-Os. or V-Rh bonds predominating over the V-V bonds found in other V3X alloys, where X is a transitional metal. As one proceeds in the periodic chart from Au to Pt, Ir and Os along the row of Sd transition metals (half filled 6s shells to a full 6s shell with appropriate changes in the Sd electrons), or vertically upward front Ir to Rh (5d-6s orbitals to 4d-5s orbitals). the effects of thermal treatment go from very pronounced
19 November 1973
and positive for V-Au [4] ,less pronounced for V-Pt. essentially nonexistent for V-lr, to negative for V-Rh. The chemical effects are roughly parallel in behavior. Since the Al 5 phase of V-Au is very narrow (— 3’3.)
about the V
785 Au21 .s coniposition, One cannot readily make a series of alloys. In the V-Pt system, becuase of its wide phase field, compositional ef6.~cts have been studied and 7~>increases with increasing V composition up to the stoichiometric composition, after which it tails off slowly over the remainder of the phase field. The V-lr systeni forms the AIS phase between 61 aiid 75~ V. and 7~)decreases as V increases. The V-Rh system whose Al 5 phase field cxtends [81 from approximat ely 63 to 75~V. behaves in the same manner as the V-Ir alloys. It would appear then that for sonic V-transition metal Al 5 alloys, the electron bonding orbitals are more fundamental in determining superconductivity than is the “integrity” of the vanadium chains. However, one cannot exclude completely the influence of phonons on the superconductovoty-producing mechanism, nor can one at this point determine how the magnetic properties of the B element may effect the T0’s of these alloys.
References 11] M. Weger, Rev. Mod. Phys. 36 (1964) 175. [21 J.E. Cox. R.A. Hem, R.M. Waterstrat, Proc. LT 12 (1970) 131 RD. Blaugher et at., J.
Low Temp. Phys. 1 (1969) 539 . . [4[ R.A. 11cm U at.. PhysiLa 55 (1971) 523. [51 J.E. Cox, 3. Bostock, R.M Watcrstrat. Proc. LTI 3 (to be published). 161 J. Labbe’ and J. Friedel, J. Physique 27 (1966) 153, 393.
708.
171 J.Labbe’ and P.C. van Reuth, Phys. Rev. Lett. 24 (1970) [81 R.M. Watersirat, to be published
PHYSICS LETTERS
19 November 1973
ANTICORRELATION OF ATOMIC ORDERING WITH SUPERCONDUCTIVITY IN VANADIUM-BASED TRANSITION METAL A15 ALLOYS J.E. COX Naval Research Laboratory, Washington, D.C. 203 75, USA
R.M. WATERSTRAT National Bureau of Standards, Washington, D.C. 20234, USA Received 11 September 1973 Measurements of the superconducting transition temperature (T
0) of V-Rh A15 alloys reveal that long range order induced by low temperature annealing may cause T0 tp decrease rather than increase. The behavior of T0 with ordering is discussed for the systems V-Au, V-Pt, V-h, V-Os and V-Rh.
The Al 5 alloys have been in the forefront of superconducting materials interest for several years due to their unusually high transition temperatures (T0). Theories based on the suggestions of Weger [1] have been developed to explain the experimental effects observed with these materials. The structure of the Al 5 alloys is that of three mutually orthogonal linear chains formed by the A element atoms superposed pairwise on the faces of a body centered cube composed of the B element atoms. The theories usually relate increased T0s to the improved integrity of the linear chains, i.e. an increase in the degree of crystallographic long range order. This note is intended to emphasize that there are serious exceptions to previously accepted general trends. Ideal stoichiometric composition of the Al5 materials is frequently designated by the notation A3B. Deviations from stoichiometry and/or perfect order will often result from the melting procedure, causing some B atoms to occupy bcc sites. Previous studies [2—4] have indicated that T0 increases as B atoms on the A chains are replaced by A atoms. One can remove B atoms from the chains in either of two ways: by chemically altering the composition of the sample, or by low temperature (600—900C) thermal treatment. A good example of both of these effects is found in the V-Pt [2] system, wherein the Al 5 phase extends from approximately 67 to 81 at.%V. As the V content increases toward stoichiometry, the Pt atoms that had occupied chain sites are replaced by V atoms, and T0 increases. In addition to chemical alteration, a
low temperature anneal (800 C for one day) permits the Pt atoms that accidently were frozen on the Achain sites to interchange positions wit V atoms that were frozen into bcc sites. This increase in A-chain integrity also produces an increase in T0. Since a number of Al 5 alloy systems behave in this manner it was thought that this might be the universal behavior for the Al 5 alloys in general. However, we found that the V-Ir [5] system behaved in a considerably different manner. While well ordered stoichiometric V75 1r25 was not superconducting down to 15 mK, superconductivity sets in as more Ir is added to the alloy, and T0 is a monotonically increasing function of increasing Ir concentration up to a maximum T0 of 1.87K at the composition V61 1r39. That is, as the vandium chains are broken up by the addition of iridium, T0 increases. This behavior is clearly in contradiction with the Labbe’.Friedel [6,7] model, which postulated that T0 should be depressed as the chains are broken up. Although these same samples were subsequently subjected to a low temperature anneal (850 C—23 hr), similar to the anneals used in other systems, no change was observed in their respective transition temperatures. The A15 phase of the V-Os system exists in a narrow composition region near V450s55. Although the composition is far from stoichiometry, the sample had aT0 of 5.04K [3]. In the V-Os system, the A15 phase is unstable at the usual homogenization annealing temperatures, so this sample received only a 6 month, 620C anneal. The transition temperature is 21
Volume 46A, number I
Pl-IYSICS LETTERS
higher than any of the other vanadium-transition meta! alloys even though its composition deviates from stoichiornetry more than any other system.
We have recently investigated two samples in the V-Rh system to ascertain whether this system behaved analogously with the isoelectronic V-lr system. While the stoichiometric V75 Rh25 sample is non-superconducting down to 1 5 mK, the low temperature limit of the experimental apparatus, a V65 Rh35 sample was found to have a 7~of 1 .075 K. When this sample was subjected to an ordering anneal of 750C br 8 weeks, its 7~)decreased to I .036K~the first known decrease of T~)with thermal ordering when the B elenient is a transition metal. This behavior with ordering should not he too surprising, however, since thermal ordering is. in effect , tantamount to decreasing the percentage of Rh atoms on the V chain sites, just a soccurs when the chemical composition is changed to lower the Rh concentration in the alloy. Indeed, it was hoped that the low temperature annealing of the V-lr alloys would have produced the same result. Perhaps different thermal treatment would produce the desired effect. Thus, the simple correlation of chain integrity with transition temperature cannot be considered universally applicable to the Al 5 alloys since these data clearly emphasize the oversimplification of the Labbe’Friedel model. Rather, it now seems that when there is strong hybridization of the d-electron bands via alloying V with either Ir, Os or Rh, the V-lr. V-Os. or V-Rh bonds predominating over the V-V bonds found in other V3X alloys, where X is a transitional metal. As one proceeds in the periodic chart from Au to Pt, Ir and Os along the row of Sd transition metals (half filled 6s shells to a full 6s shell with appropriate changes in the Sd electrons), or vertically upward front Ir to Rh (5d-6s orbitals to 4d-5s orbitals). the effects of thermal treatment go from very pronounced
19 November 1973
and positive for V-Au [4] ,less pronounced for V-Pt. essentially nonexistent for V-lr, to negative for V-Rh. The chemical effects are roughly parallel in behavior. Since the Al 5 phase of V-Au is very narrow (— 3’3.)
about the V
785 Au21 .s coniposition, One cannot readily make a series of alloys. In the V-Pt system, becuase of its wide phase field, compositional ef6.~cts have been studied and 7~>increases with increasing V composition up to the stoichiometric composition, after which it tails off slowly over the remainder of the phase field. The V-lr systeni forms the AIS phase between 61 aiid 75~ V. and 7~)decreases as V increases. The V-Rh system whose Al 5 phase field cxtends [81 from approximat ely 63 to 75~V. behaves in the same manner as the V-Ir alloys. It would appear then that for sonic V-transition metal Al 5 alloys, the electron bonding orbitals are more fundamental in determining superconductivity than is the “integrity” of the vanadium chains. However, one cannot exclude completely the influence of phonons on the superconductovoty-producing mechanism, nor can one at this point determine how the magnetic properties of the B element may effect the T0’s of these alloys.
References 11] M. Weger, Rev. Mod. Phys. 36 (1964) 175. [21 J.E. Cox. R.A. Hem, R.M. Waterstrat, Proc. LT 12 (1970) 131 RD. Blaugher et at., J.
Low Temp. Phys. 1 (1969) 539 . . [4[ R.A. 11cm U at.. PhysiLa 55 (1971) 523. [51 J.E. Cox, 3. Bostock, R.M Watcrstrat. Proc. LTI 3 (to be published). 161 J. Labbe’ and J. Friedel, J. Physique 27 (1966) 153, 393.
708.
171 J.Labbe’ and P.C. van Reuth, Phys. Rev. Lett. 24 (1970) [81 R.M. Watersirat, to be published