The Tc-SSR correlation in A3B(A15-type) compounds

Solid State Communications, Vol. 23, pp. 803-808, 1977.

Pergamon Press.

Printed in Great Britain

THE To-SSR CORRELATION IN A 3B(A 15-TYPE) COMPOUNDS F.E. Wang

Research and Technology Department, Naval Surface Weapons Center, Silver Spring, MD 20910, U.S.A.

(Received 29 April 1977 by A.G. Chynoweth) The 28A3B (AI 5-type) compounds, for which both the phase equilibrium diagram and the superconducting critical temperature, To, are known, can be divided into two groups. The first group consists of compounds whose SSR (solid solution range) is either zero or extends only to the A-rich side of the stoichiometric composition. The second group consists of the compounds whose SSR extends to both sides or only to the B-rich side. The first group of compounds generally has high Tc's and follow the Tc-SSR correlation proposed earlier. Both the grouping and the Tc-SSR correlation are directly relatable to "A-chain integrity" and thus lend further support to the thesis that ",4-chain integrity" is of fundamental importance in the superconductivity of these compounds. IN OUR continuing efforts [1-3] to better understand the nature of superconductivity, particularly that associated with A aB (A15-type) compounds, we observed [4] earlier some interesting empirical correlations between the superconducting characteristics and the phase equilibrium diagrams of these compounds. The correlations were principally based on whether the compound was of the congruent-melting or incongruent-melting type in that: (a) Congruent-melting compounds are nonsuperconducting (or are poor superconductors, i.e. low To) and have SSR's (solid solution ranges) extending to both sides of the stoichiometric composition. In contrast, incongruent-melting compounds are superconductors (with high To's) and have their SSR's extending only to the A-rich side of the stoichiometric composition. (b) The Te of an incongruent-melting (and superconducting) compound is directly proportional to its SSR (the Tc-SSR correlation). These correlations were interpreted [4] in terms of "A.chain integrity" [5] - a proposed principal operating mechanism of superconductivity in A15 compounds. This was in turn related to the electronic structure and the thermodynamics of the phase diagrams. However, as was indicated in our original paper [4], there were a few eases in which the correlations were not obeyed rigorously (e.g. V3Si, MoaOs). It was pointed out that these deviations may be attributable to the following causes: (1) error in the reported phase diagrams - particularly in terms of the exact SSR's, (2) inaccuracy in the reported Tc's (or the Te's may be accurate but the material under investigation may be poorly characterized thus leading to an erroneous To). Therefore, the final judgement 803

(acceptance, abandonment or modification) of the correlations required more (in number) and better (more accurate) phase diagrams and Tc data. A significant number of new or revised phase diagrams and Tc's have appeared in the literature since our original paper [4], the Cr-Ir [7], V - A u [8], V - G a [9], V - R h [ 1 0 ] , V - S i [ l l ] , V - P t [12], Nb-Au [13], Nb-Ga [14], Nb-Os [15], and Nb-Pt [13] systems. The purpose of this paper is to evaluate the validity of our proposed correlations [4] in light of these newly reported systems. Dramatic support of the Tc-SSR correlation is found in the revised phase diagrams of V-Si and Nb-Ga (Fig. 1) in which the SSR's are shown to be 5 and 3.5 at.% respectively (these were originally reported to have no SSR) in agreement with their high Te's. The V-Au, V-Ga, Nb-Au and Nb-Pt systems, although less dramatic, also conform to the correlation, ha sharp contrast, there are a number of systems, e.g. the Cr-Ir, V-Pt, V - R h and Nb-Os, which appear to directly contradict the correlation. For example, VaRh and NbaOs are both incongruent-type and have SSR's of 12 and 4 at.% respectively (Fig. 2); yet, VaRh is not superconducting down to 0.015 K [16] and NbaOs has a Te of only 0.94 K [16]. Contradictions of this magnitude cannot be attributed to possible errors in the investigations. A close inspection of these phase diagrams, however, indicates (Fig. 2) that the SSR's of both systems extend only to the B-rich side of the stoichiometric composition. The implication of this observation is clear - in these systems, the B-element can readily substitute for the A-element, but the A-element does not substitute for the B-element. Therefore, the interaction between B (element) atoms is rather strong. Conversely, the

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interaction between A (element)-atoms, the basis for "'A-chain integrity", is practically non-existent (or extremely weak). Consequently, these compounds are either not superconducting or very poor superconductors. These two systems, therefore, not only do not contradict the correlation (based on the "A-chain integrity" concept) but in fact lend further support to it. With this logic in mind, the SSR's for the 28 A15 compounds (whose phase diagrams as well as Tc's are known) are listed in Fig. 3. The A15 compounds in this list (Fig. 3) can be divided into two groups. The first group consists of compounds whose SSR's are either zero or extend only to the A-rich side of the stoiehiometric composition. The second group consists of compounds whose SSR's extend to both sides or only to the B-rich side. As discussed above, the distinction between the first group and the second group can be related to the stability of the "A-chain integrity". Thus, if "A-chain integrity" is indeed an important factor in the superconductivity of AI 5 compounds, the two groups should show different superconductivity characteristics. This anticipated differenee is demonstrated in Fig. 4 in which the Te is plotted

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Fig. 2. The phase equilibrium diagrams of Nb-Os and V - R h (references [ 10] and [ 15 ]) showing: (1) incongruentmelting nature, and (2) wide SSR's of the A15 compounds in these two systems. should be abandoned. The correlation should be revised therefore to read: A15 compounds, whose SSR's are zero or extend only to the A -rich side of the stoichiometric composition are generally good superconductors and conform to the Tc-SSR correlation; whereas A15 compounds, whose SSR's extend only to the B-rich side or to both sides of the stoichiometric composition, are poor superconductot's and to not follow the Tc-SSR correlation. The correlation thus described is aesthetically satisfying on two accounts: (I) The inferred importance of "A-chain integrity" to the superconductivity of these compounds is supported by a number of other experiments and observations; (e.g. a dramatic difference in the degradation of Te depending on whether the isotropic substitution is for the A or B element [17], Matthias' rule is followed only when the e/a (electron to atom) ratio is reached by B element substitution (,4 element substitution invariably degrades Te) [18], and more recently direct evidence of Te degradation due to "A-chain disruption by thermal neutron bombardment [19].

(2) Since a phase equilibrium diagram is a manifestation of all the thermodynamic properties, it must contain information concerning electronic arrangements as well as atomic arrangements. The manifestations of the electronic arrangement (such as in SSR or congruent vs incongruent-melting) in phase diagrams, can therefore be related to a phenomenon that is supersensitive to electronic properties - superconductivity. The strong B - B interactions (so strong that the A-element does not substitute for the B-element) demonstrated by the SSR's of the V - R h and Nb-Os systems are rather surprising. Since every compound in the second group (with the exception of CrsSi) is formed with a B-element that contains a 4d or 5d orbital (i.e. a transition element), it may be argued that d-bonding between B elements accounts for the strong B - B interactions. However, the B - B interatomic distances, 4.143 A (Rh-Rh) [20] and 4.443 A (Os-Os) [21] in VsRh and NbsOs as calculated from their crystal structures are far greater than the 2.70 A and 2.66 ,a, found in their respective metallic states [22]. Interatomic distances of about 1.5 times those found in their metallic state are simply not conducive for forming direct

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B-B interactions - be it d-bonding or sp-bonding. It is likely that the B-B interactions in these systems are indirect, i.e. via the A-elements. As a result, it can be reasoned, that the A - A interaction is perturbed to the extent that it is practically non-existent. Within this context, it is reasonable to anticipate the phonon specta of the A15 compounds from the first and the second group (as differentiated in Figs. 3 and 4) to be dramatically different. Whether these arguments are correct can be determined when data from further investigations or reinvestigations of phase diagrams which contain A15 compounds become available. In view of the observations described above, a conclusion that the strong A - A interaction in terms of "A-chain integrity" is indeed an important factor in the superconductivity of A15 compounds is inevitable. The fact that the A-atom (in AI 5 compounds) can come only from 4A (Ti, Zr, Hf), 5A (V, Nb, Ta) and 6,4 (Cr, Mo, W) transition elements [1 ] tends to suggest chemical bondings involving special atomic orbitals to be operative within the A-chain. With this in mind, a possibility of A-chain being a conjugated molecular chain having "double"-"single" alternate bonding along the chain is considered. We regard this possibility as not unrealistic in that: (1) the fact that A15 compounds tend to be metastable (by

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having T,., a manenistic transition [23]), is attributable to the instability of a conjugated chain (saturated bonding involving only "single" bonds are more stable), (2) according to Axe and Shirane [24], at Tm (typified in Nb3Sn) two of the three mutually perpendicular (to one another) A-chains assume " l o n g " - " s h o r t " alternating interatomic distances along the chain. This is reminiscent of "double"-"single" bond alternation in a conjugated chain, and (3) it has been shown in our recent theoretical treatments of conjugated molecular chains [25, 26], invoking vibronic interactions, that a high electron conduction along the chain involving pairwise charge transfer is possible in such a chain. For these reasons, a possible formation of "double" -"single" bond alternating molecular orbitals along the A-chain in AI 5 compounds of the first group and a possible B - A - B indirect interactions in A15 compounds of the second group are under study. Acknowledgement - I wish to thank Dr. R.M. Waterstrat of the National Bureau of Standards for calling my attention to a number of new systems in the literature and allowing me to use his pre-publication data on the V - R h system.

808

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Vol. 23, No. 11

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