Chapter 18
Influence of Chemical Composition on Heavy-Electron Behavior 1
H. R. Ott and Z. Fisk
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The Challenge of d and f Electrons Downloaded from pubs.acs.org by TUFTS UNIV on 07/13/18. For personal use only.
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Laboratorium für Festkörperphysik, ETH-Hönggerberg, 8093 Zürich, Switzerland Los Alamos National Laboratory, Los Alamos, N M 87545 2
The drastic influence of small changes in chemical composition on the low-temperature properties of heavy-electron materials is demonstrated. Prevention of the heavy-electron state itself or suppression of phase transitions out of this state are among the most interesting experimental observations.
The o c c u r r e n c e o f a h e a v y - e l e c t r o n s t a t e i n m e t a l s i s most d i s t i n c t l y observed i n compounds where one o f the c h e m i c a l c o n s t i t u e n t s i s an element o f the r a r e - e a r t h ( 4 f ) o r a c t i n i d e ( 5 f ) s e r i e s . W i t h i n t h e s e s e r i e s , i t i s t h e elements at the b e g i n n i n g o r the end o f the r e s 8 e c t i v e row o f the p e r i o d i c system t h a t are most l i k e l y i n v o l v e d i n t h i s effect (Ce, Yb, U , Np). The h e a v y - e l e c t r o n s t a t e m a n i f e s t s i t s e l f at low temperatures by a l a r g e and temperature-dependent r a t i o C J 5 V T , where c | * i s t h e s p e c i f i c heat due t o c o n d u c t i o n - e l e c t r o n e x c i t a t i o n s . T h i s r a t i o i n c r e a s e s w i t h d e c r e a s i n g temperature and, f o r Τ + 0 K , reaches v a l u e s t h a t are one t o t h r e e o r d e r s o f magnitude l a r g e r than observed i n u s u a l m e t a l s . These l a r g e r a t i o s i m p l y a very h i g h e f f e c t i v e den s i t y o f e l e c t r o n i c s t a t e s at t h e Fermi energy N(Ep) due t o a l a r g e enhancement o f the e f f e c t i v e mass m* o f the i t i n e r a n t e l e c t r o n s . T h i s i s c o n s i s t e n t w i t h the l a r g e magnetic s u s c e p t i b i l i t i e s t h a t are o b s e r v e d i n these m a t e r i a l s at low t e m p e r a t u r e s . Another d i s t i n c t f e a t u r e o f these m a t e r i a l s i s the temperature dependence o f the e l e c t r i c a l r e s i s t i v i t y p(T) and a t y p i c a l example ( C e A l a ) i s shown i n f i g . 1. C h a r a c t e r i s t i c i s a r e l a t i v e l y l a r g e r e s i s t i v i t y at room temperature which s t a y s almost c o n s t a n t or even i n c r e a s e s w i t h d e c r e a s i n g t e m p e r a t u r e , f o l l o w e d by a d i s t i n c t drop o f ρ i n a f a i r l y narrow temperature range and low v a l u e s o f ρ at v e r y low temperatures where a Τ v a r i a t i o n i s observed [see i n s e t o f f i g . 1 ] . T h i s k i n d o f b e h a v i o u r must be t r a c e d back t o s t r o n g i n t e r a c t i o n s i n the e l e c t r o n i c subsystem and hence these m a t e r i a l s a r e i d e a l l y s u i t e d t o study many-body e f f e c t s among e l e c t r o n s which form some k i n d o f Fermi l i q u i d . There are not many g e n e r a l r u l e s t o p r e 0097-6156/89/0394-O260$06.00/0 ο 1989 American Chemical Society
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OTT & FISK
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diet the observation of a heavy-electron state. A favourable condi tion i s certainly a d i s t i n c t separation of the f-electron carrying atoms within the c r y s t a l l a t t i c e , both i n distance and with respect to the coordination of the ligands. It should be stressed, however, that this requirement only appears to be a necessary but by no means a s u f f i c i e n t condition. This paper i s intended to show that changes i n chemical composition may affect the formation of the heavy-elec tron state i t s e l f or, i f i t i s preserved, influence i t s ground-state properties d r a s t i c a l l y (J_). Chemical Composition, Crystal Structure and Heavy-Electron
State
In order to elucidate some of the points raised i n the introduc tion, we f i r s t mention a few interesting facts related to the che mical composition and the c r y s t a l structure of compounds that show heavy-electron behaviour. Certain c r y s t a l structures seem to favour the formation of a heavy-electron state. We note that compounds adopting the cubic CU3AU structure have no strong tendency i n t h i s respect, with the exception of U(In]_ Sn )3 where, with varying x, the low-temperature properties change quite d r a s t i c a l l y 02) and NpSn3 (3). URh3 and U I r 3 , for example, show e s s e n t i a l l y non-magnetic behaviour because of a strong hybridization of the 5f wave functions with wave functions of other symmetries (4), but no features that are of interest here. I f the anion i s chosen from the neighbouring column of the periodic table, UPt3 c r y s t a l l i z e s i n the hexagonal stacking of the same structure, i . e . , the Ni3Sn structure, and i s an outstanding example of heavy-electron materials 05), just as i s the aforemention ed CeAl3(6) which adopts the same c r y s t a l structure. As a side remark we mention that UPd3 c r y s t a l l i z e s i n a combination of cubic and hexa gonal c r y s t a l structure, the double-hexagonal closed-packed struc ture, and undergoes s t r u c t u r a l phase t r a n s i t i o n s at low temperatures without any trace of heavy-electron behaviour (7). Another structure which seems favourable for the formation of the heavy-electron state i s the cubic AuBes structure which i s shown in f i g . 2. In t h i s respect i t i s i n s t r u c t i v e to discuss the proper t i e s of some compounds of the form UM5, where M = Ni, Cu, Pd and Pt. With the exception of UPds, a l l members of this group of compounds c r y s t a l l i z e i n the AuBes structure. The c r y s t a l structure of UPds i s not c l e a r l y established because i t i s d i f f i c u l t to prepare this com pound i n single phase form, confirming the view that Pd i s an element that often plays an extraordinary r o l e . With respect to the AuBes structure i t i s important to r e a l i z e that there are two inequivalent anion s i t e s with a r a t i o of 1 : 4 and forming large and small t e t r a hedra respectively. One of the anion s i t e s i s c r y s t a l l o g r a p h i c a l l y equivalent to the cation s i t e . It i s interesting to note that the occupation of a single element on these equivalent s i t e s results in the cubic Laves-phase structure. UAI2, which c r y s t a l l i z e s i n this l a t t e r structure, i s one of the e a r l i e s t examples for which features of heavy-electron behaviour was discovered (9). A common feature of the compounds that we are discussing here i s the large distance between the U atoms which ranges from 4.79 Â for UN15 to 5.29 Â for U P t s . The physical properties of UNis are similar to those of the compounds with the CU3AU structure that were mention ed above and i n t h i s sense not of interest here (10). UCU5, however, orders antiferromagnetically at T^ = 15 Κ and enters a heavy-elecx
x
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THE CHALLENGE OF d AND f ELECTRONS
Figure 1. Temperature dependence of the electrical resistivity p(T) of CeAl below room temperature. (Reproduced with permission from r é f . 1. Copyright 1987 Elsevier.) 3
Figure 2. The AuBe structure. The large open circles denote Au sites; the dotted large circles and the small open circles denote Be sites (see also ref. 8) . (Reproduced with permission from ref. 13. Copyright 1987 American Physical Society.) 4
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tron state at temperatures far below Τ Ν ( 1 2 ) . With a further decrease i n temperature, this state again becomes unstable and a further phase t r a n s i t i o n of s t i l l not well established nature occurs around 1 K. For our purposes i t i s now interesting to mention how these features described for UCU5 are varying by manipulating the chemical composition. At f i r s t we note that small amounts of Ni replacing Cu are devastating for both the magnetic ordering and the formation of the heavy-electron state ( 1 , 1 3 ) . This i s shown i n f i g . 3 where i t may be seen that the anomaly of the s p e c i f i c heat at TJ\J i s quenched very e f f e c t i v e l y by replacing a few atomic % of the Cu atoms with Ni. The c h a r a c t e r i s t i c upturn of C § V T below 4 Κ that i s observed i n UCU5 i s also wiped out with only 1% of Ni for Cu (see f i g . 4 ) . With a further increase i n Ni content, a rather abrupt drop of the low-temperature c/j^T ratio i s noted i f the Ni content exceeds 20% (10). This a l l occurs without any drastic changes i n the l a t t i c e constants, not to mention the structure. While the drastic effects at low Ni concentra tions are probably related with disorder effects on the Ni sublatt i c e , the loss i n electronic s p e c i f i c heat at larger Ni concentra tions might be due to band-structure e f f e c t s . Quite different i s the influence of substituting Cu with Ag. i f one Cu atom per formula unit i s replaced by Ag, Τ Ν i s raised to 16 Κ and the formation of the heavy-electron state below 5 Κ per s i s t s (13). What i s different now i s that the low-temperature phase t r a n s i t i o n i s quenched as i s demonstrated i n the C p ( T ) plot of f i g . 5. The s p e c i f i c heat varies l i n e a r l y with Τ down to the lowest temperatures investigated and γ = CJJVT i s roughly one hundred times larger than that observed i n the equivalent amount of Cu metal. This c l e a r l y demonstrates that Ni or Ag substitutions for Cu i n UCU5 i n duce quite different changes in the low-temperature behaviour but the reasons for i t are not yet clear. In U P t s , the shortest distance between U atoms was mentioned above and, at more than 5 Â, must be regarded as very large. Never theless, the expected magnetic ordering among localized 5f-electron moments i s not observed above 1.5 K. Indeed the temperature depen dence of the magnetic s u s c e p t i b i l i t y χ gives no evidence for well de fined moments on the U ions. This compound again behaves more l i k e a metal whose properties are dominated by electron energy bands of average width with a s l i g h t tendency to heavy-electron behaviour at the lowest temperatures (14). This tendency may now be grearjly en hanced by replacing Pt with Au. This may be seen i n f i g . 6 Where Cp/T versus Τ i s shown for UPts and UAuPti*. The C p / T ratio of UAuPtit increases dramatically at very low temperatures and reaches a value of more than 700 mJ/mole K as Τ 0 Κ (see inset of f i g . 6) (13). Concomittantly, the low-temperature magnetic s u s c e p t i b i l i t y χ of UAuPtit i s d i s t i n c t l y larger than that of U P t s . phase t r a n s i t i o n i s observed above 0.1 K. A further enhancement of the Au content re sults i n a suppression of the specific-heat enhancement and the γvalue of UAu2Pt3 i s considerably lower than that of UPts (13). On the other hand, χ s t i l l increases with increasing Au concentration but no magnetic ordering i s observed for UAu2Pt3 above 1.5 K. Again i t i s not obvious what exactly causes these d i s t i n c t changes i n the low temperature behaviour. From band-structure calculations i t may be concluded that no drastic changes are induced i n the electronic exci tation spectrum (16) and therefore again, many-body effects that are 2
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THE CHALLENGE OF d AND f ELECTRONS
r
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• UCu " UCu Ni * UCu |Ni 5
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