- Email: [email protected]
Charge-density-wave behaviour in intercalated single crystal Nb3Te4
.,__ .__ !bB
CwJ
SOLID
STATE IONICS
Solid State Ionics 100 (1997) 135-141
ELSFWIER
Charge-density-wave
behaviour
in intercalated
single crystal Nb,Te,
G.A. Scholz” Department
of Physics,
University
of Waterloo, Waterloo, Ontario,
NZL 3G1, Canada
Received 19 March 1997; accepted 26 April 1997
Abstract Judging by the influence of 12 guests on the CDW, they divide into two groups of which Ag and Tl appear representative. In both ternary compounds the CDW is suppressed when about 0.25 and 0.10 mole fraction respectively are intercalated. When the Ag content is increased a weak resistance peak near 46 K, also present in the pure host, grows in magnitude but retains a constant onset; the origin is unclear. Increasing the Tl concentration not only causes the CDW to reappear but the onset temperature is enhanced by up to 12.5 K over that in the pure host. For small mole fractions the CDW onset temperature appears to correlate with the conduction electron density. Although Tl appears to cause minimal disorder in the quasi 1-D Nb chains, initial results do not suggest non-ohmic CDW related behaviour up to fields of a few V cm-‘. No guest order/disorder transitions are apparent when cooling to 10 K. Keywords:
Charge-density-waves;
Intercalation;
Chalcogenides;
Resistivity
PACS: 71.45.L1
1. Introduction In numerous low dimensional transition metal chalcogenides (MCh) a Peierls transition [I] leading to a charge density wave (CDW) ground state is observed. Froehlich superconductivity [2], on the other hand, remains elusive. Quasi one dimensional NbSe, is particularly prevalent in the literature because it appears that the two CDW forming at 144 K and 59 K can be induced to slide through the host lattice by applying a relatively modest electric field
*Tel.: + 1 519 8851211/2213; [email protected].
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0167-2738/97/$17.00 0 1997 Elsevier PII SO167-2738(97)00344-5
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[3]. The related Nb,Ch, family of chalcogenides is also of interest. Structurally, Nb,Ch, is closely related to NbSe, in that both are build up of edge and face sharing NbCh, octahedra [4,5] but differ in the precise arrangement of these octahedra. The NbNb zigzag chains along the c-axis are common to both chalcogenide families and are responsible for their quasi 1-D character in that the intrachain NbNb distances are comparable to those in the pure Nb metal while the interchain distances are considerably larger. One very significant difference is that in Nb,Ch, the octahedra are organized so that the chalcogens form spacious hexagonal tunnel structures parallel to the Nb chains. Relatively large mole fractions (x) of guests can be intercalated into these tunnels without producing significant changes in the
136
G.A. Scholz I Solid State Ioaics 100 (1997) 135-141
host’s lattice parameters [6,8,9] and, perhaps most important, without introducing significant disorder along the Nb chains which would inhibit CDW formation. Since intercalation is a charge transfer process, by choosing the type and degree of guest species intercalated, one has the means of tuning the Fermi level over a large range without significantly altering the band structure or quenching the CDW. Of further interest may be the metallic guest atoms which can be expected to form additional quasi 1-D conducting chains if the degree of intercalation and therefore the accompanying orbital overlap is sufficient. Schoellhom and Schramm [6] were the first to intercalate alkali metals in Nb,Ch,, Boeller and Klepp [7] inserted thallium and more recently Huan and Greenblatt [8,9] went on to produce a large variety of these ternary compounds using intercalation and ion exchange techniques. Nb,Te, is a particularly interesting member of this family because, in part, the intra and interchain Nb-Nb distances show the greatest anisotropy and in that the telluride not only undergoes a Peierls transition [lO,ll] but it also appears to facilitate the coexistence of the CDW with superconductivity [lo121 usually two mutually exclusive phenomena. Ohtani et al. [13] have reported resistance features associated with a CDW transition in pressed polycrystalline pellets of indium intercalated Nb,Te, and they attribute changes in the CDW onset temperature to a charge transfer from In to the telluride. Single crystal electron diffraction results by Boswell and Bennett [ 141 confirm the diffraction results of Ohtani et al. [13] and show that when In or Tl is intercalated the CDW wave vector’s c* component increases by up to 10% and becomes incommensurate. The fact the CDW is incommensurate and exists for high guest concentrations suggests that pinning may not be severe and that even small fields of a few V cm-’ might induce non-ohmic behaviour. Single crystal resistivities of Nb,Te, intercalated with Tl, In, Ag, Cu, Sn, Bi, Ga, Cd and Pb are investigated. The diverging effects on the CDW consequent to intercalation is discussed with reference to charge transfer as suggested by Ohtani et al. [13] and to the electronic band structure calculation by Oshiyama [15,16] and more recently by Canadell and Whangbo
u71.
2. Experimental The Nb,Te, host material is first grown via iodine vapour transport from 1020 to 990°C. Intercalated materials are prepared by adapting the method outlined by Huan and Greenblatt [8,9,14] to single crystals. In 10 cm by 10 mm ampoules roughly 100 mg of the host telluride is mixed with the appropriate mole fraction of the guest metal weighed to within t5 pg. The mole fractions subsequently quoted are a reflection of the ampouled stoichiometry and a conservative estimate for the error margin is below ?3%. Ag and Cu require a 800°C anneal all others 500°C for 3 weeks and are cooled to room temperature by turning the furnace off. X-ray micro analysis confirms that no guest residue remains after an anneal. For example, ampouling with x > 1 results in residual guest material and a p(T) comparable to x = 1.0 samples. For comparison, some Ag intercalates are also prepared electrochemically using a 0.1 N AgNO, electrolyte and proved similar to the annealed samples. Four probe dc resistance measurements are made along the c-axis by attaching 36 AWG copper wire using GC Electronics silver print, a colloidal silver suspension. The copper wires are first epoxied to a glass slide and bent upward to accommodate the sample. Selectable sample current in the range 0.1 pA to 1 mA is supplied by a HP 3486A multimeter and the voltage is measured using a Keithley 197A microvoltmeter. Total power delivered to the sample is 5 0.1 p,W and sample voltages are on the order of mV The sample slide is mounted on a doubly shielded cold finger which can be cooled to about 10 K with a CTI-Cryogenics 21C cryocooler. Temperatures are measured via a Lakeshore Cryotronics DT-470-SD-13 silicon diode having an absolute accuracy of 2 1 K using a Keithley 617 multimeter. Sample and temperature voltages are interfaced to an IBM personal computer. The sample and temperature sensor are mounted separately on the cold finger and calibration p(T) measurements indicate that the maximum temperature offset is 0.8 K which is removed by averaging the warming and cooling curves. Pulsed Z-V measurements having a 50 pS duty cycle (11 mS repeat) are obtained using a
137
G.A. Scholz I Solid State Ionics 100 (1997) 135-141
sample and hold circuit based on that described Gupta et al. [18].
by
3. Results In agreement with previous studies [lo-12,19,20], as-grown Nb,Te, has a strong resistance peak beginning near 114 K (Fig. 1) which Sekine et al. [20] associate with the onset of a CDW having wave vectors 4 = +(1/3a* + 1/3b* + 3/7c*). There is 2.5 K hysteresis in the onset and quenching of the CDW between about 80 K and 120 K (Fig. 1, insert). The highest local resistance minima found in a sample occur at 114.OkO.3 K on cooling and 11620.3 K on heating and the lowest occur at 11020.5 K and 11220.5 K respectively. The CDW onset temperature, T,, will refer to the average of these heating/cooling minima. Also present is a very small peak beginning near T* = 45 k-2 K which Sekine et al. [20] suggest is caused by a second CDW whose superlattice satellites (SLS) are too weak to be observed. 1.0
r
The guests influence on the CDW fall into two groups of which Ag and Tl appear representative. The temperature dependent resistance of AgJb,Te, and Tl,Nb,Te, having intercalated mole fraction in the range 0 5 x 5 1 is summarized in Figs. l-4. The resistance is normalized to unity at either 240 K or 300 K to provide a clearer overview. The room temperature resistances vary between about 1 and 50 ohms but sample cross section and length differences can, within error, account for these variations. No attempt is made to judge the resistivity changes consequent to intercalation because the needle like sample cross-sections are on the order of micrometers and difficult to measure with sufficient accuracy. Fig. 5 provides for an overview of T,(x) variations in Tl,Nb,Te,. Error bars indicate the error inherent in a particular measurement and multiple To’s for a given x represent the scatter observed for a minimum of four samples. Preliminary pulsed p(T) measurements to fields of a few volts per cm have not provided any marked evidence for non-ohmic behaviour below To. Sample heating is evident above about 1 V cm-’ (To’s appear to decrease by up to 20 K) and adds to the difficulty in ascertaining small changes in the resistivity. A full investigation of CDW related non-
0.6 h al ‘;; 0 m 0.5 * CL!
2 0.4 ? 3 .I: 2
0.2
L
0.0 0
50
100
Temperature
150
200
250
(K)
Fig. 1. Resistivity of AgJb,Te, normalized to unity at 240 K. (a) pure material and x = 0.10, no difference: (b) x = 0.15; (c) x = 0.20; (d) x = 0.25 lowest two curves, differences due to slight sample to sample concentration variations; (e) x = 0.50, (f) n = 1.0. Insert is CDW hysteresis detail in pure host, warming curve at right.
20
40
60
Temperature
80
100
120
140
(K)
Fig. 2. Resistivity of Tl,Nb,Te, normalized to unity at 300 K. (a) pure material; (b) x = 0.15, offset by - 0.05 for clarity; (c) x = 0.15.
138
G.A. Scholz I Solid State Ionics 100 (1997) 135-141
T1,NbgTeq 0.7
g
aJ
Y-ii ;
200
0.6
d
::
2
0.5
h 2 3
0.4
I
I
I
.z
“::I’
0
40
60
120
Temperature
160
200
1.0
a, ” 0.8 m
L
0
I
50
I
100
I
150
Temperature
/
200
0.2
0.4
mole
(K)
Fig. 3. Resistivity of Tl,Nb,Te, normalized to unity at 300 K. (a) pure material; (b) n = 0.10, partial; (c) n = 0.15, partial; (d) x = 0.20 and x = 0.25; (e) x = 0.30; (f) x = 0.40.
0.2
0.0
I
250
300
(K)
Fig. 4. Resistivity of Tl,Nb,Te, normalized to unity at 300 K. (a) pure material; (b) x = 0.50 and x = 0.85, both have the break in the slope near 80 K; (c) n = 0.75; (d) x = 1.0.
ohmic behaviour is presently continuing using shorter current pulses. Intercalating with silver to x = 0.10 has no discernible effect on the resistivity. Of seven samples tried all had a To’s in the range 111 K to 117 K as
0.6
fraction
0.8
1.0
(x)
Fig. 5. Variation of the CDW onset temperature T, with the degree of intercalation (x). The points are joined by a dotted line as a guide to the eye only.
observed for the pure host. However, for x = 0.15 the CDW may already be quenched since beginning near 50 K (Fig. lb) only a plateau remains in p(T) and there is no significant change for x = 0.20 (Fig. lc). Compare this with the nominally x = 0.10 lead sample (Fig. 6a) where both a reduced T,, and a strong peak at T* are visible. Two nominally x = 0.25 (Fig. Id) samples proved interesting because of small differences in x. At the lower concentration limit all traces of a resistance anomaly are absent but at slightly higher concentrations a low temperature feature at 46.020.5 K begins to emerge and develops into a strong peak as the Ag content increases to x = 0.50 and 1.0 (Fig. le, f). Notable is that independent of x the onset temperature remains at 46.02 1 K, strikingly similar to the T* feature in the pure host. In contrast to the CDW peak hysteresis is not observed to an uncertainty of better than 20.5 K, nor are there any associated SLS [21]. The resistivity behaviour when intercalating Cu, Bi, Ga, Cd, Sn and Pb to x = 1.0 is very similar to Ag (examples in Fig. 6) but the individual T* do vary slightly and are respectively 45.6 K, 48.6 K, 46.2 K, 48.0 K, 48.0 K and 48.5 K all to.5 K. The CDW behaviour contrasts sharply with the above if Tl (or In) are guests. For Tl the resistance
G.A. Scholz I Solid State Ionics 100 (1997) 135-141
139
beginning near 46 K is only observed for the x = 0.15 samples but not for any other Tl mole fractions. Otherwise only the very similar, judging by p(T), x = 0.50 and 0.85 samples show a break in the slope near 80 K (Fig. 4b). For In the CDW peak is considerably smaller but the T,, again vary with x (differing in detail) and is 1572 1 K for x = 1.0, somewhat higher than the 140 K observed by Ohtani et al. [13].
0.6 -
4. Discussion
0.0 0
I 20
/ 40
, 60
Temperature
I 60
I 100
1 120
140
(K)
Fig. 6. Resistivity of AJVb,Te, normalized to unity at 240 K. (a) A, = Pb,,; (b) A, = Pb,.,; (c) Ax = Sn,,,; (d) A, = Cu,.,.
peak magnitude and onset again decrease for small x. For x = 0.10 (Fig. 2b) the CDW is already nearly suppressed and may well be completely so for values very close to and somewhat below 0.10 mole fraction. However, in stark contrast to the Ag inter&ate, when the Tl content is further increased the Peierls transition to a CDW ground state not only reappears but increases in temperature and magnitude. For x = 0.15 the CDW onset is at 97 K (Fig. 2c) and appears to peak somewhat for 0.20 5 x 5 0.25 (Fig. 3d) in the range 112 K to 116 K because for x = 0.30 (Fig. 3e) the onset again decreases to 10720.5 K. This small peaking of T,(x) (Fig. 5) is judged to be real in that r, for all x = 0.30 samples is consistently smaller than those for x = 0.20 even though both ternary compounds are from the same host ampoule and anneal batch. Above x = 0.30 to x = 0.50 (Fig. 4b) the CDW onset increases steeply and then more gradually to x = 0.75 (Fig. 4c) where in one sample it reaches 235 K. Whereas in the pure host and for small x the CDW onset is characterized by a fairly sharp cusp in the resistance, for large mole fractions the local resistivity minimum becomes progressively broader. The 2.5 K wide hysteresis observed in the host remains about the same in the intercalated material (only for x = 0.15 is it just over 5 K) and no systematic trends are observed. A resistance peak
The CDW is a collective interaction between the phonon system and the conduction electrons and is sensitive to the density of states (DOS) at the Fermi level (Er) and the degree to which the Fermi surface can nest. Because the tunnels into which the guest migrates provide ample room [8,9], a reasonable starting point for understanding the present results is a charge transfer [ 131 from the guests to a rigid host lattice. Nesting conditions in ideal 1-D materials are perfect and simply correspond to Fermi surface ‘sheets’ separated by -+k,. Considering that the host is a quasi 1-D material, a further reasonable simplification for small mole fractions is that nesting conditions will not change greatly. The LCAO band structure calculations for Nb,Ch, by Oshiyama [15,16] and the tight binding approach for Nb,S, by Canadell and Whangbo [17] come to similar conclusions; significant is that EF is sheet like and that near EF the DOS is small and decreasing but then increases via three minor peaks to a major peak at considerably higher energy. Therefore, following intercalation, the conduction electron density and CDW wave vector should increase only after first decreasing. Accordingly, if To is primarily sensitive to the conduction electron density it should also first decrease, possibly disappear if the electronic energy gain associated with the Peierls transition is insufficient, and then increase as more guests are intercalated. In contrast to other low-D CDW systems [22,23] where guest introduced disorder rapidly quenches the CDW, in the Tl (In) ternary tellurides disorder along the conducting Nb chains does not appear to be as significant considering the CDW persist to x = 1.0 [13,14]. The surprisingly similar resistance ratios also suggest that the guests do not
140
G.A. S&ok
I Solid State Ionics 100 (1997) 135-141
cause significant disorder. The initial decrease both in T,, and the resistance peak for both (all) ternary tellurides can therefore be understood as a direct consequence of a decreasing DOS at EF and a shrinking Fermi surface caused by the initial guest to host charge transfer. It is interesting to note that whereas in Ag,Nb,Te, the CDW disappears near x = 0.25, in Tl,Nb,Te, it is strongly suppressed and likely disappears a little below x = 0.10. In both cases EF should be near the DOS minimum. The nominal oxidation of Tl is + 3 and a three times greater charge transfer is ideally expected compared to Ag, in agreement with the roughly factor of three difference in the mole fractions required to suppress the CDW. With further increasing Ag or Tl content the behaviour of the CDW ground state diverges strongly. Whereas in Tl,Nb,Te, the CDW reemerges above x - 0.10 and moves to higher temperatures, there is no sign of a CDW ground state in Ag@,Te, above x - 0.25. The resistance ratio for the Ag intercalates do improve significantly and may be related to the intercalated silver but the formation of very high quality crystals during the anneal can not be ruled out. Surprising is that the CDW reappears only when Tl (In) is intercalated. Since there is no reason for, say, Ag to produce a greater amount of disorder along the Nb chains compared to Tl (resistance ratios suggest just the opposite), the reason for the failure of the CDW to reemerge for higher Ag concentrations must be found elsewhere. In any case, a strong association strictly between the DOS at EF and the CDW onset temperature is clearly an oversimplification at higher guest concentrations. For small guest concentrations the To’s do likely mirror the host DOS and the small peak (Fig. 5) in T,, near x = 0.25 may well be an example. For large Tl concentrations the local resistivity minimum broadens (Figs. 3 and 4) and the sample to sample scatter in the To’s increases. Presumably the CDW develops locally over a range of temperatures depending on local conditions until coherence in the bulk of the crystal is established. It is not possible to say whether slight intercalate inhomogeneities or disorder along the Nb chains is the primary cause for the broadening. The decreasing To’s for x Z 0.75 samples are likely caused by disorder finally appearing along the Nb chains at these high guest con-
centrations. The possibility that the peaking of To near x = 0.75 simply mirrors EF passing through the main DOS peak is problemsome. It has already been argued that for large x a strong correlation between To and the DOS at EF is unlikely. Furthermore it also ignores the strong possibility that because of Coulomb effects the nominal Tl valence will decrease at higher intercalate concentrations [24] which in combination with an estimated transfer of 3 to 4 electrons/mole fraction [17] makes it unlikely that EF will ever reach this DOS peak. Although a marked non-ohmic behaviour is not observed, it was expected in view of the CDW being incommensurate and in that its presence to large x does suggest that disorder related pinning might not be a significant problem in this chalcogenide. The resistivity peak near 46 K for AgxNb,Te, (x > 0.25) and Tl,,,,Nb,Te, (Figs. 2 and 3c) is at the same temperature as the low temperature peak T* in the pure host and may therefore be related. When the CDW is suppressed in either ternary compound (Fig. Id and Fig. 2b) so is this peak and although this does appear to support an electronic origin, a proposed [20] low temperature CDW is unlikely. Otherwise one would then need to explain why, when the peak’s magnitude is increasing following intercalation, an expected hysteresis in p(T) is not observed, why the onset temperature is not affected although the DOS and Fermi surface surely are and why SLS still do not appear. The absence of SLS also rules out intercalate ordering as a cause for this peak because, as in InNb,Te, [14], SLS would be expected for at least some concentrations. There may be a degree of complementarity between the 46 K peak and the CDW in that either develops fully only when the other is absent. Both are simultaneously present only ‘transition’ concentrations such as in during Pb,,,,Nb,Te, (Fig. 6a) or TJ&b,Te, (Fig. 2~). The resistivity gives no indication of intercalate ordering to 10 K for any of the ternary compounds. This is surprising considering that the extremely high temperature factors in X-ray work [7] and the ease of electrochemical intercalation [6,8,9 and presently for Ag] do suggest rather mobile guests. Since SLS point to ordering in In,Nb,Te, up to room temperature [14], the absence of any order/disorder transition in p(T) may therefore indicate that guests are already ordered near room temperature. Therefore,
G.A. Scholz I Solid State Ionics 100 (1997) 135-141
when guests are electrochemically intercalated, instead of diffusing uniformly along the tunnels they are likely to cluster and move rigidly in groups.
5. Conclusion The CDW onset temperature when Tl is intercalated increases over a significant range and only above x- 0.75 does it appear that crystal disorder reverses this trend. Marked non-ohmic behaviour is not observed even though two primary causes for pinning, crystal disorder and a commensurate CDW, are addressed. For small x the CDW behaviour can be understood in terms of existing band structure calculations and a rigid band model. Consequently the CDW To’s may well mirror the host DOS and the small peak in T,(x) near x = 0.25 may be an example of this. Judging by the mole fractions required to suppress the CDW in the Ag and Tl compounds, their respective ionicity differs roughly by a factor of three in agreement with their nominal valence. For larger x the CDW reappearance and growth both in strength and onset temperature for the Tl but not the Ag (and similar) samples is surprising and suggests that for larger guest concentrations a simple charge transfer picture is incomplete. The resistance peak near 46 K in the pure host and when Ag is intercalated appear to be related, but the cause for the peak is not clear. Pointing to an electronic origin is its disappearance, along with the CDW, for small x. However, a second low temperature CDW seems unlikely considering that for larger x the peak’s onset temperature remains constant and no hysteresis in p(T) nor SLS are observed.
Acknowledgments I am grateful to F.W. Boswell and P. Stillwell samples, and to NSERC for support.
for
141
References [II R.E. Peierls, Quantum Theory of Solids, Clarendon Press, Oxford, 1955. 121H. Froehlich, Proc. Royal Sot. A 223 (1954) 296. J. [31 P. Monceau, N.P. Ong, A.M. Portis, A. Meerschaut, Rouxel, Phys. Rev. Lett. 37 (1976) 602. [41 K. Selte, A. Kjekshus, Acta Crystallogr. 17 (1964) 1568. [51 A.F. Ruysink, A. Kaduk, A.J. Wagner, F. Jellinek, Acta Crystallogr. B 24 (1968) 1614. WI R. Schoellhom, W. Schramm, 2. Naturforsch. B 34 (1979) 697. [71 H. Boeller, K. Klepp, Mat. Res. Bull. 18 (1983) 437. [81 G. Huan, M. Greenblatt, Mater. Res. Bull. 22 (1987) 943. t91 G. Huan, M. Greenblatt, Mater, Res. Bull. 22 (1987) 505. UOI Y. Ishihara, I. Nakada, Solid State Commun. 45 (1983) 129. Cl11 K. Suzuki, M. Ichihara, I. Nakada, Y. Ishihara, Solid State Commun. 52 (1984) 743. WI E. Amberger, K. Pobom, P. Grimm, M. Dietrick, B. Obst, Solid State Commun. 26 (1978) 943. u31 T. Ohtani, Y. Sano, Y. Yokota, J. Solid State Chem. 103 (1993) 504. u41 F.W. Boswell, J.C. Bennett, Mat. Res. Bull. 31 (1996) 1083. 1151 A. Oshiyama, Solid State Commun. 43 (1982) 607. U61 A. Oshiyama, J. Phys. Sot. Japan 52 (1983) 587. u71 E. Canadell, M.H. Whangbo, Inorg. Chem. 25 (1986) 1488. ml SK. Gupta, S.C. Gadkari, D.K. Aswal, M.K. Gupta, Rev. Sci. Instrum. 65 (1994) 2065. [191 K. Suzuki, Y.M. Ichihara, I. Nakada, Y. Ishihara, Solid State Commun. 52 (1984) 743. K. Uchinokura, R. WI T. Sekine, Y. Kiuchi, E. Matsuura, Yoshizaki, Phys. Rev. B 36 (1987) 3153. WI F.W. Boswell (private communication). [W G.A. Scholz, 0. Singh, R.F. Frindt, A.E. Curzon, Solid State Commun. 44 (1982) 1455. t231 R.H. Friend, A.D. Yoffe, Advances in Physics 36 (1987) 1. Mat. Res. Bull. 18 (1983) r241 A. Schramm, R. Schoellhom, 1283.
CwJ
SOLID
STATE IONICS
Solid State Ionics 100 (1997) 135-141
ELSFWIER
Charge-density-wave
behaviour
in intercalated
single crystal Nb,Te,
G.A. Scholz” Department
of Physics,
University
of Waterloo, Waterloo, Ontario,
NZL 3G1, Canada
Received 19 March 1997; accepted 26 April 1997
Abstract Judging by the influence of 12 guests on the CDW, they divide into two groups of which Ag and Tl appear representative. In both ternary compounds the CDW is suppressed when about 0.25 and 0.10 mole fraction respectively are intercalated. When the Ag content is increased a weak resistance peak near 46 K, also present in the pure host, grows in magnitude but retains a constant onset; the origin is unclear. Increasing the Tl concentration not only causes the CDW to reappear but the onset temperature is enhanced by up to 12.5 K over that in the pure host. For small mole fractions the CDW onset temperature appears to correlate with the conduction electron density. Although Tl appears to cause minimal disorder in the quasi 1-D Nb chains, initial results do not suggest non-ohmic CDW related behaviour up to fields of a few V cm-‘. No guest order/disorder transitions are apparent when cooling to 10 K. Keywords:
Charge-density-waves;
Intercalation;
Chalcogenides;
Resistivity
PACS: 71.45.L1
1. Introduction In numerous low dimensional transition metal chalcogenides (MCh) a Peierls transition [I] leading to a charge density wave (CDW) ground state is observed. Froehlich superconductivity [2], on the other hand, remains elusive. Quasi one dimensional NbSe, is particularly prevalent in the literature because it appears that the two CDW forming at 144 K and 59 K can be induced to slide through the host lattice by applying a relatively modest electric field
*Tel.: + 1 519 8851211/2213; [email protected].
fax:
0167-2738/97/$17.00 0 1997 Elsevier PII SO167-2738(97)00344-5
+ 1 519 7468115;
e-mail:
Science BY. All rights reserved.
[3]. The related Nb,Ch, family of chalcogenides is also of interest. Structurally, Nb,Ch, is closely related to NbSe, in that both are build up of edge and face sharing NbCh, octahedra [4,5] but differ in the precise arrangement of these octahedra. The NbNb zigzag chains along the c-axis are common to both chalcogenide families and are responsible for their quasi 1-D character in that the intrachain NbNb distances are comparable to those in the pure Nb metal while the interchain distances are considerably larger. One very significant difference is that in Nb,Ch, the octahedra are organized so that the chalcogens form spacious hexagonal tunnel structures parallel to the Nb chains. Relatively large mole fractions (x) of guests can be intercalated into these tunnels without producing significant changes in the
136
G.A. Scholz I Solid State Ioaics 100 (1997) 135-141
host’s lattice parameters [6,8,9] and, perhaps most important, without introducing significant disorder along the Nb chains which would inhibit CDW formation. Since intercalation is a charge transfer process, by choosing the type and degree of guest species intercalated, one has the means of tuning the Fermi level over a large range without significantly altering the band structure or quenching the CDW. Of further interest may be the metallic guest atoms which can be expected to form additional quasi 1-D conducting chains if the degree of intercalation and therefore the accompanying orbital overlap is sufficient. Schoellhom and Schramm [6] were the first to intercalate alkali metals in Nb,Ch,, Boeller and Klepp [7] inserted thallium and more recently Huan and Greenblatt [8,9] went on to produce a large variety of these ternary compounds using intercalation and ion exchange techniques. Nb,Te, is a particularly interesting member of this family because, in part, the intra and interchain Nb-Nb distances show the greatest anisotropy and in that the telluride not only undergoes a Peierls transition [lO,ll] but it also appears to facilitate the coexistence of the CDW with superconductivity [lo121 usually two mutually exclusive phenomena. Ohtani et al. [13] have reported resistance features associated with a CDW transition in pressed polycrystalline pellets of indium intercalated Nb,Te, and they attribute changes in the CDW onset temperature to a charge transfer from In to the telluride. Single crystal electron diffraction results by Boswell and Bennett [ 141 confirm the diffraction results of Ohtani et al. [13] and show that when In or Tl is intercalated the CDW wave vector’s c* component increases by up to 10% and becomes incommensurate. The fact the CDW is incommensurate and exists for high guest concentrations suggests that pinning may not be severe and that even small fields of a few V cm-’ might induce non-ohmic behaviour. Single crystal resistivities of Nb,Te, intercalated with Tl, In, Ag, Cu, Sn, Bi, Ga, Cd and Pb are investigated. The diverging effects on the CDW consequent to intercalation is discussed with reference to charge transfer as suggested by Ohtani et al. [13] and to the electronic band structure calculation by Oshiyama [15,16] and more recently by Canadell and Whangbo
u71.
2. Experimental The Nb,Te, host material is first grown via iodine vapour transport from 1020 to 990°C. Intercalated materials are prepared by adapting the method outlined by Huan and Greenblatt [8,9,14] to single crystals. In 10 cm by 10 mm ampoules roughly 100 mg of the host telluride is mixed with the appropriate mole fraction of the guest metal weighed to within t5 pg. The mole fractions subsequently quoted are a reflection of the ampouled stoichiometry and a conservative estimate for the error margin is below ?3%. Ag and Cu require a 800°C anneal all others 500°C for 3 weeks and are cooled to room temperature by turning the furnace off. X-ray micro analysis confirms that no guest residue remains after an anneal. For example, ampouling with x > 1 results in residual guest material and a p(T) comparable to x = 1.0 samples. For comparison, some Ag intercalates are also prepared electrochemically using a 0.1 N AgNO, electrolyte and proved similar to the annealed samples. Four probe dc resistance measurements are made along the c-axis by attaching 36 AWG copper wire using GC Electronics silver print, a colloidal silver suspension. The copper wires are first epoxied to a glass slide and bent upward to accommodate the sample. Selectable sample current in the range 0.1 pA to 1 mA is supplied by a HP 3486A multimeter and the voltage is measured using a Keithley 197A microvoltmeter. Total power delivered to the sample is 5 0.1 p,W and sample voltages are on the order of mV The sample slide is mounted on a doubly shielded cold finger which can be cooled to about 10 K with a CTI-Cryogenics 21C cryocooler. Temperatures are measured via a Lakeshore Cryotronics DT-470-SD-13 silicon diode having an absolute accuracy of 2 1 K using a Keithley 617 multimeter. Sample and temperature voltages are interfaced to an IBM personal computer. The sample and temperature sensor are mounted separately on the cold finger and calibration p(T) measurements indicate that the maximum temperature offset is 0.8 K which is removed by averaging the warming and cooling curves. Pulsed Z-V measurements having a 50 pS duty cycle (11 mS repeat) are obtained using a
137
G.A. Scholz I Solid State Ionics 100 (1997) 135-141
sample and hold circuit based on that described Gupta et al. [18].
by
3. Results In agreement with previous studies [lo-12,19,20], as-grown Nb,Te, has a strong resistance peak beginning near 114 K (Fig. 1) which Sekine et al. [20] associate with the onset of a CDW having wave vectors 4 = +(1/3a* + 1/3b* + 3/7c*). There is 2.5 K hysteresis in the onset and quenching of the CDW between about 80 K and 120 K (Fig. 1, insert). The highest local resistance minima found in a sample occur at 114.OkO.3 K on cooling and 11620.3 K on heating and the lowest occur at 11020.5 K and 11220.5 K respectively. The CDW onset temperature, T,, will refer to the average of these heating/cooling minima. Also present is a very small peak beginning near T* = 45 k-2 K which Sekine et al. [20] suggest is caused by a second CDW whose superlattice satellites (SLS) are too weak to be observed. 1.0
r
The guests influence on the CDW fall into two groups of which Ag and Tl appear representative. The temperature dependent resistance of AgJb,Te, and Tl,Nb,Te, having intercalated mole fraction in the range 0 5 x 5 1 is summarized in Figs. l-4. The resistance is normalized to unity at either 240 K or 300 K to provide a clearer overview. The room temperature resistances vary between about 1 and 50 ohms but sample cross section and length differences can, within error, account for these variations. No attempt is made to judge the resistivity changes consequent to intercalation because the needle like sample cross-sections are on the order of micrometers and difficult to measure with sufficient accuracy. Fig. 5 provides for an overview of T,(x) variations in Tl,Nb,Te,. Error bars indicate the error inherent in a particular measurement and multiple To’s for a given x represent the scatter observed for a minimum of four samples. Preliminary pulsed p(T) measurements to fields of a few volts per cm have not provided any marked evidence for non-ohmic behaviour below To. Sample heating is evident above about 1 V cm-’ (To’s appear to decrease by up to 20 K) and adds to the difficulty in ascertaining small changes in the resistivity. A full investigation of CDW related non-
0.6 h al ‘;; 0 m 0.5 * CL!
2 0.4 ? 3 .I: 2
0.2
L
0.0 0
50
100
Temperature
150
200
250
(K)
Fig. 1. Resistivity of AgJb,Te, normalized to unity at 240 K. (a) pure material and x = 0.10, no difference: (b) x = 0.15; (c) x = 0.20; (d) x = 0.25 lowest two curves, differences due to slight sample to sample concentration variations; (e) x = 0.50, (f) n = 1.0. Insert is CDW hysteresis detail in pure host, warming curve at right.
20
40
60
Temperature
80
100
120
140
(K)
Fig. 2. Resistivity of Tl,Nb,Te, normalized to unity at 300 K. (a) pure material; (b) x = 0.15, offset by - 0.05 for clarity; (c) x = 0.15.
138
G.A. Scholz I Solid State Ionics 100 (1997) 135-141
T1,NbgTeq 0.7
g
aJ
Y-ii ;
200
0.6
d
::
2
0.5
h 2 3
0.4
I
I
I
.z
“::I’
0
40
60
120
Temperature
160
200
1.0
a, ” 0.8 m
L
0
I
50
I
100
I
150
Temperature
/
200
0.2
0.4
mole
(K)
Fig. 3. Resistivity of Tl,Nb,Te, normalized to unity at 300 K. (a) pure material; (b) n = 0.10, partial; (c) n = 0.15, partial; (d) x = 0.20 and x = 0.25; (e) x = 0.30; (f) x = 0.40.
0.2
0.0
I
250
300
(K)
Fig. 4. Resistivity of Tl,Nb,Te, normalized to unity at 300 K. (a) pure material; (b) x = 0.50 and x = 0.85, both have the break in the slope near 80 K; (c) n = 0.75; (d) x = 1.0.
ohmic behaviour is presently continuing using shorter current pulses. Intercalating with silver to x = 0.10 has no discernible effect on the resistivity. Of seven samples tried all had a To’s in the range 111 K to 117 K as
0.6
fraction
0.8
1.0
(x)
Fig. 5. Variation of the CDW onset temperature T, with the degree of intercalation (x). The points are joined by a dotted line as a guide to the eye only.
observed for the pure host. However, for x = 0.15 the CDW may already be quenched since beginning near 50 K (Fig. lb) only a plateau remains in p(T) and there is no significant change for x = 0.20 (Fig. lc). Compare this with the nominally x = 0.10 lead sample (Fig. 6a) where both a reduced T,, and a strong peak at T* are visible. Two nominally x = 0.25 (Fig. Id) samples proved interesting because of small differences in x. At the lower concentration limit all traces of a resistance anomaly are absent but at slightly higher concentrations a low temperature feature at 46.020.5 K begins to emerge and develops into a strong peak as the Ag content increases to x = 0.50 and 1.0 (Fig. le, f). Notable is that independent of x the onset temperature remains at 46.02 1 K, strikingly similar to the T* feature in the pure host. In contrast to the CDW peak hysteresis is not observed to an uncertainty of better than 20.5 K, nor are there any associated SLS [21]. The resistivity behaviour when intercalating Cu, Bi, Ga, Cd, Sn and Pb to x = 1.0 is very similar to Ag (examples in Fig. 6) but the individual T* do vary slightly and are respectively 45.6 K, 48.6 K, 46.2 K, 48.0 K, 48.0 K and 48.5 K all to.5 K. The CDW behaviour contrasts sharply with the above if Tl (or In) are guests. For Tl the resistance
G.A. Scholz I Solid State Ionics 100 (1997) 135-141
139
beginning near 46 K is only observed for the x = 0.15 samples but not for any other Tl mole fractions. Otherwise only the very similar, judging by p(T), x = 0.50 and 0.85 samples show a break in the slope near 80 K (Fig. 4b). For In the CDW peak is considerably smaller but the T,, again vary with x (differing in detail) and is 1572 1 K for x = 1.0, somewhat higher than the 140 K observed by Ohtani et al. [13].
0.6 -
4. Discussion
0.0 0
I 20
/ 40
, 60
Temperature
I 60
I 100
1 120
140
(K)
Fig. 6. Resistivity of AJVb,Te, normalized to unity at 240 K. (a) A, = Pb,,; (b) A, = Pb,.,; (c) Ax = Sn,,,; (d) A, = Cu,.,.
peak magnitude and onset again decrease for small x. For x = 0.10 (Fig. 2b) the CDW is already nearly suppressed and may well be completely so for values very close to and somewhat below 0.10 mole fraction. However, in stark contrast to the Ag inter&ate, when the Tl content is further increased the Peierls transition to a CDW ground state not only reappears but increases in temperature and magnitude. For x = 0.15 the CDW onset is at 97 K (Fig. 2c) and appears to peak somewhat for 0.20 5 x 5 0.25 (Fig. 3d) in the range 112 K to 116 K because for x = 0.30 (Fig. 3e) the onset again decreases to 10720.5 K. This small peaking of T,(x) (Fig. 5) is judged to be real in that r, for all x = 0.30 samples is consistently smaller than those for x = 0.20 even though both ternary compounds are from the same host ampoule and anneal batch. Above x = 0.30 to x = 0.50 (Fig. 4b) the CDW onset increases steeply and then more gradually to x = 0.75 (Fig. 4c) where in one sample it reaches 235 K. Whereas in the pure host and for small x the CDW onset is characterized by a fairly sharp cusp in the resistance, for large mole fractions the local resistivity minimum becomes progressively broader. The 2.5 K wide hysteresis observed in the host remains about the same in the intercalated material (only for x = 0.15 is it just over 5 K) and no systematic trends are observed. A resistance peak
The CDW is a collective interaction between the phonon system and the conduction electrons and is sensitive to the density of states (DOS) at the Fermi level (Er) and the degree to which the Fermi surface can nest. Because the tunnels into which the guest migrates provide ample room [8,9], a reasonable starting point for understanding the present results is a charge transfer [ 131 from the guests to a rigid host lattice. Nesting conditions in ideal 1-D materials are perfect and simply correspond to Fermi surface ‘sheets’ separated by -+k,. Considering that the host is a quasi 1-D material, a further reasonable simplification for small mole fractions is that nesting conditions will not change greatly. The LCAO band structure calculations for Nb,Ch, by Oshiyama [15,16] and the tight binding approach for Nb,S, by Canadell and Whangbo [17] come to similar conclusions; significant is that EF is sheet like and that near EF the DOS is small and decreasing but then increases via three minor peaks to a major peak at considerably higher energy. Therefore, following intercalation, the conduction electron density and CDW wave vector should increase only after first decreasing. Accordingly, if To is primarily sensitive to the conduction electron density it should also first decrease, possibly disappear if the electronic energy gain associated with the Peierls transition is insufficient, and then increase as more guests are intercalated. In contrast to other low-D CDW systems [22,23] where guest introduced disorder rapidly quenches the CDW, in the Tl (In) ternary tellurides disorder along the conducting Nb chains does not appear to be as significant considering the CDW persist to x = 1.0 [13,14]. The surprisingly similar resistance ratios also suggest that the guests do not
140
G.A. S&ok
I Solid State Ionics 100 (1997) 135-141
cause significant disorder. The initial decrease both in T,, and the resistance peak for both (all) ternary tellurides can therefore be understood as a direct consequence of a decreasing DOS at EF and a shrinking Fermi surface caused by the initial guest to host charge transfer. It is interesting to note that whereas in Ag,Nb,Te, the CDW disappears near x = 0.25, in Tl,Nb,Te, it is strongly suppressed and likely disappears a little below x = 0.10. In both cases EF should be near the DOS minimum. The nominal oxidation of Tl is + 3 and a three times greater charge transfer is ideally expected compared to Ag, in agreement with the roughly factor of three difference in the mole fractions required to suppress the CDW. With further increasing Ag or Tl content the behaviour of the CDW ground state diverges strongly. Whereas in Tl,Nb,Te, the CDW reemerges above x - 0.10 and moves to higher temperatures, there is no sign of a CDW ground state in Ag@,Te, above x - 0.25. The resistance ratio for the Ag intercalates do improve significantly and may be related to the intercalated silver but the formation of very high quality crystals during the anneal can not be ruled out. Surprising is that the CDW reappears only when Tl (In) is intercalated. Since there is no reason for, say, Ag to produce a greater amount of disorder along the Nb chains compared to Tl (resistance ratios suggest just the opposite), the reason for the failure of the CDW to reemerge for higher Ag concentrations must be found elsewhere. In any case, a strong association strictly between the DOS at EF and the CDW onset temperature is clearly an oversimplification at higher guest concentrations. For small guest concentrations the To’s do likely mirror the host DOS and the small peak (Fig. 5) in T,, near x = 0.25 may well be an example. For large Tl concentrations the local resistivity minimum broadens (Figs. 3 and 4) and the sample to sample scatter in the To’s increases. Presumably the CDW develops locally over a range of temperatures depending on local conditions until coherence in the bulk of the crystal is established. It is not possible to say whether slight intercalate inhomogeneities or disorder along the Nb chains is the primary cause for the broadening. The decreasing To’s for x Z 0.75 samples are likely caused by disorder finally appearing along the Nb chains at these high guest con-
centrations. The possibility that the peaking of To near x = 0.75 simply mirrors EF passing through the main DOS peak is problemsome. It has already been argued that for large x a strong correlation between To and the DOS at EF is unlikely. Furthermore it also ignores the strong possibility that because of Coulomb effects the nominal Tl valence will decrease at higher intercalate concentrations [24] which in combination with an estimated transfer of 3 to 4 electrons/mole fraction [17] makes it unlikely that EF will ever reach this DOS peak. Although a marked non-ohmic behaviour is not observed, it was expected in view of the CDW being incommensurate and in that its presence to large x does suggest that disorder related pinning might not be a significant problem in this chalcogenide. The resistivity peak near 46 K for AgxNb,Te, (x > 0.25) and Tl,,,,Nb,Te, (Figs. 2 and 3c) is at the same temperature as the low temperature peak T* in the pure host and may therefore be related. When the CDW is suppressed in either ternary compound (Fig. Id and Fig. 2b) so is this peak and although this does appear to support an electronic origin, a proposed [20] low temperature CDW is unlikely. Otherwise one would then need to explain why, when the peak’s magnitude is increasing following intercalation, an expected hysteresis in p(T) is not observed, why the onset temperature is not affected although the DOS and Fermi surface surely are and why SLS still do not appear. The absence of SLS also rules out intercalate ordering as a cause for this peak because, as in InNb,Te, [14], SLS would be expected for at least some concentrations. There may be a degree of complementarity between the 46 K peak and the CDW in that either develops fully only when the other is absent. Both are simultaneously present only ‘transition’ concentrations such as in during Pb,,,,Nb,Te, (Fig. 6a) or TJ&b,Te, (Fig. 2~). The resistivity gives no indication of intercalate ordering to 10 K for any of the ternary compounds. This is surprising considering that the extremely high temperature factors in X-ray work [7] and the ease of electrochemical intercalation [6,8,9 and presently for Ag] do suggest rather mobile guests. Since SLS point to ordering in In,Nb,Te, up to room temperature [14], the absence of any order/disorder transition in p(T) may therefore indicate that guests are already ordered near room temperature. Therefore,
G.A. Scholz I Solid State Ionics 100 (1997) 135-141
when guests are electrochemically intercalated, instead of diffusing uniformly along the tunnels they are likely to cluster and move rigidly in groups.
5. Conclusion The CDW onset temperature when Tl is intercalated increases over a significant range and only above x- 0.75 does it appear that crystal disorder reverses this trend. Marked non-ohmic behaviour is not observed even though two primary causes for pinning, crystal disorder and a commensurate CDW, are addressed. For small x the CDW behaviour can be understood in terms of existing band structure calculations and a rigid band model. Consequently the CDW To’s may well mirror the host DOS and the small peak in T,(x) near x = 0.25 may be an example of this. Judging by the mole fractions required to suppress the CDW in the Ag and Tl compounds, their respective ionicity differs roughly by a factor of three in agreement with their nominal valence. For larger x the CDW reappearance and growth both in strength and onset temperature for the Tl but not the Ag (and similar) samples is surprising and suggests that for larger guest concentrations a simple charge transfer picture is incomplete. The resistance peak near 46 K in the pure host and when Ag is intercalated appear to be related, but the cause for the peak is not clear. Pointing to an electronic origin is its disappearance, along with the CDW, for small x. However, a second low temperature CDW seems unlikely considering that for larger x the peak’s onset temperature remains constant and no hysteresis in p(T) nor SLS are observed.
Acknowledgments I am grateful to F.W. Boswell and P. Stillwell samples, and to NSERC for support.
for
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