Substitution of TMTSeF with TMTTF in (TMTSeF)2ClO4 : high pressure studies

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Substitution of TMTSeF with TMTTF in (TMTSeF)2ClO4 : high pressure studies

S.S.P. Parkin, C. Coulon, D. Jérome, J.M. Fabre, L. Giral

To cite this version:

S.S.P. Parkin, C. Coulon, D. Jérome, J.M. Fabre, L. Giral. Substitution of TMTSeF with TMTTF in (TMTSeF)2ClO4 : high pressure studies. Journal de Physique, 1983, 44 (5), pp.603-607.

�10.1051/jphys:01983004405060300�. �jpa-00209637�

Substitution of TMTSeF with TMTTF in (TMTSeF)2ClO4 :

high ^{pressure studies}

S. S. P. Parkin (**), C. Coulon (*), ^{D. Jérome}

Laboratoire de Physique ^{des Solides,} Université Paris-Sud, ⁹¹⁴⁰⁵ Orsay, ^France

J. M. Fabre and L. Giral

Laboratoire de Chimie Structurale Organique (U.S.T.L.), ³⁴⁰⁶⁰ Montpellier, ^France (Reçu le 22 novembre 1982, accepté ^{le 21} janvier 1983 )

Resumé. 2014 On étudie la résistivité électrique ^des alliages [(TMTSeF)1-x(TMTTF)x]2ClO4 ^à pression ^élevée Gusqu’à ¹⁸ kbar). ^En substituant TMTSeF par TMTTF, ^on supprime ^l’état supraconducteur ^de (TMTSeF)2ClO4

et on observe, ^à pression ambiante, ^une transition métal-isolant (M-I) ^{à basse} température. ^{Celle-ci est} supprimée

au-delà d’une pression critique Pc ^{et un état} métallique ^{est stabilisé} jusqu’aux plus ^basses températures ^atteintes

lors de notre étude. Les diagrammes ^de phase ^des différents alliages ^sont similaires; cependant Pc augmente ^avec la concentration en TMTTF. Enfin, ^nous n’avons pas observé l’apparition ^{d’un état} supraconducteur ^{dans le} régime métallique (P > Pc).

Abstract.

[(TMTSeF)1-x(TMTTF)x]2ClO4 alloys ^{have been studied} under pressure for pressures up ^to 18 kbar

through resistivity measurements. On substitution of TMTSeF in the superconducting compound (TMTSeF)2ClO4

with TMTTF a low temperature metal-insulator (M-I) transition is found at ambient pressure; above ^some cri- tical pressure, Pc, the M-I transition is suppressed giving ^{rise to a metallic state to} the lowest temperatures ^considered in this study. ^The phase diagrams ^of the various alloys ^{are of the same form} although Pc ^{increases with} increasing

TMTTF concentration. Superconductivity ^{was not} observed in any of these materials in the metallic high pressure

regime.

Classification Physics ^Abstracts

72.15 - 71. 30

1. Introduction. - The discovery ^of superconduc- tivity [ 1-3] ^and antiferromagnetism [4-6] ^{in the} (TMTSeF)2X ^compounds caused extensive studies to be made ^on these materials. The isostructural sulphur compounds (TMTTF)2X ^were already known, although poorly characterized [7]. Improved ^under- standing ^{of their} properties ^{has been} gained ^{in subse-} quent ^{work at} ambient pressure [8-9] ^and through

measurements of their pressure-temperature phase diagrams [10-12]. These latter studies have shown that at sufficiently high pressures the (TMTTF)2X compounds behave like the (TMTSeF)2X ^materials

even though their ambient pressure electrical _pro-

perties ^are somewhat different and that the pressure-

temperature phase diagrams ^{of these families are of the}

same form [11]. ^In particular ^a highly conducting

metallic phase is stabilized in (TMTTF)2Br ^{at 25 kbar}

with the possibility ^{of a} superconducting ^transition

near 3.5 K [10]. Recently it has been shown that at ambient _pressure (TMTTF)2Br [13-14] ^and (TMTTF)2 SCN [15] ^have antiferromagnetic ground

states; ^{in contrast a} structural distortion has been observed in the PF6 ^salt [9].

The interplay ^between magnetism ^and superconduc- tivity ^{in the} (TMTSeF)2X salts has long ^{been an} important question particularly ^at pressures close ^to the critical pressure, Pc above which these materials become superconducting. Early measurements showed for pressures ^near P, resistance upturns ^with decreasing temperature just ^{above the} superconducting ^transi-

tion temperature, T,, in several salts [2,16] perhaps

of magnetic character. The most extensive studies have been made on the CI04 salt for which P, ^{is close}

to 1 bar [2, 3]. ^A magnetic ^{state has} clearly ^been

observed in this compound ^above T, ^{with the obser-}

vation of antiferromagnetic resonances and resur-

rection of the ESR signal ^at high ^microwave power [17],

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01983004405060300

604

although ^{more recent} studies show the _presence of this state to be dependent ^{on the rate of} cooling ^{at low} temperatures [18]. A useful method for further exa-

mining ^the competition ^between superconductivity magnetism ^{and other lattice} instabilities is

through substitution of TMTSeF with TMTTF in

(TMTSeF)2C’04 ^{to form} alloys of chemical for- mula [(TMTSeF)1-x(TMTTF)x]2CI04. ^{Note that} (TMTTF)2C’04 undergoes ^{an anion} ordering ^transi-

tion near 70 K at 1 bar [8, 9] ^{which is} supressed ^under

moderate _pressure (~ 10 kbar) giving way ^{to a} low temperature ^M-I transition (below - ²⁰ K) presumed

to be associated with some stack instability [ 10,12].

..

The ambient _pressure behaviour of the alloys ^consi-

dered here has been described in a previous paper

(paper I) [19]. These results are briefly summarized in the following section. In section 3 we present ^resistance

measurements on the same alloys ^{as a} function of pressure, for pressures up ^to 18 kbar. Some discussion of these results is given ^at the end of the _paper.

2. Summary of ambient pressure results.

Charac- terization of the alloys used in this study is described in I. The chemical composition ^{of the} alloys ^{is of the}

form [(TMTSeF), -.,,(TMTTF),,12C’04 ^consistent

with the 2 : 1 stoichiometry ^{of the} pure TMTSeF and TMTTF salts suggesting ^the alloys follow the same

crystal structure, although ^{no detailed} X-ray ^measure-

ments have been made. Preliminary X-ray ^{data show}

that the lattice parameters ^{of the} alloys ^{are interme-}

diate between those of the undoped ^{TMTSeF and}

TMTTF salts, suggesting ^{that the} doped samples ^are

true homogeneous alloys ^without significant ^domain

structure (D. Chasseau, C. Hauw and J. Gaultier, private communication). ^The composition ^{of the}

various alloys ^is given ^{in table I} together ^{with a}

summary of their behaviour at 1 bar.

For the alloys weakly doped ^{with TMTTF, with}

x ⁼ 0.2 % ^{and 0.5} %, broad ill-defined drops ⁱⁿ

resistance of approximately ⁶⁰ % ^{and 15} % ^{are observ-}

ed near 800 mK and 200 mK respectively, suggestive

of partial superconducting phase transitions, perhaps

related to inhomogeneities ^{in these} systems. ^The

Table I.

Summary of ambient pressure data on various

[(TMTSeF) , - x(TMT-fF)xl 2C’04 alloys.

M

magnetic phase transition; ^N

Non-magnetic (see text);

RT = room temperature; TM -,

metal-insulator transition tem-

perature ; g(RT)

corresponds ^{to room} temperature g ^{value for} applied magnetic ^field along stacking axis. Data from paper I [19].

needle axis electrical conductivity ^{of the} alloys strongly

decreases on doping ^{which was} attributed in I to electron localization. Similar behaviour has been observed for alloys ^{of the same} type but for the PF6

anion [20].

Extensive EPR studies on the [(TMTSeF),-., (TMT’TF),12C’04 alloys ^are presented ^{in I. Both}

EPR and conductivity measurements show that a

M-I transition, although ^smeared out, ^{is exhibited} by

the alloys for x > 0.02 % ^{at low} temperatures (below

~

20 K). ^Different temperature dependences ^{of the}

EPR linewidth are observed as x is varied. For x - XC2 - 5 % ^{the EPR} line broadens below Tm-1 indicating a spin density ^wave (SDW) ^{state and for}

x > X,,,2 the intensity of the ESR line simply ^decreases suggesting ^a non-magnetic transition. Values of g,

for the static magnetic ^field applied along ^the stacking axis, change abruptly ^{for x} = XxI ~ 3 % ^{from those}

characteristic of TMTSeF to those corresponding ^to TMTTF, although the linewidth of the ESR signal

varies more smoothly and is much broader in the TMTTF rich alloys ^{than is} found in the undoped

TMTTF salts. (Other components ^of the g ^{tensor vary}

less markedly ^with x). Further discussion of these results is given ^{in I.}

3. Experimental.

^The experimental arrange- ments are the same as those described in [2]. ^Needle

axis resistivity measurements were made with gold paint ^contacts using the standard 4-probe ^method.

The crystals ^were fragile ^with poor morphology. ^No

absolute measurements of resistivity ^were attempted,

so only normalized resistivities are given.

Fig. ^1.

Resistivity ^versus temperature ^{curves for various} pressures (the figures ^{on the curves} correspond ^{to the} applied pressure in kbar) ^for (TMTSeF),CIO, doped ^with

2 % ^{of TMTTF.}

Figure ¹ gives resistivity ^versus temperature ^curves

at different _pressures for the x ⁼ 2 % alloy (sample 3).

With increasing pressure the M-I transition seen

clearly ^{at 4 kbar} (and ^{at 1} bar, ^{as shown in} I) ^is gra-

dually weakened and finally suppressed above 10 kbar.

In figure 2a the low temperature portion ^{of these}

curves is shown on a ln(p) ^versus 1/T plot. ^The

inflexion points for the various curves give ^{an estimate}

of the M-I transition temperatures, ^{so that the} phase diagram ^{can be drawn as shown in} figure ^2b. Pc ^is

Fig. ^2.

(a) ^The same curves shown in figure ¹ plotted

on a logarithmic ^{scale versus inverse} temperature (nor-

malized to 1 at 50 K), from which the phase diagram ^shown

in (b) has been derived.

Fig. ^3.

Resistivity ^{curves for a} (TMTSeF),CIO, sample doped ^{with 30} % TMTTF, normalized to 1 at 300 K.

thus about 10 kbar. Note that above 10 kbar the resis- tance saturates at low temperatures : ^{at 10 kbar satu-} ration of the resistance was found to 200 mK with no

sign ^of superconductivity (M. Ribault, private ^commu- nication).

Similar phase diagrams ^were found for the alloys

with x - 6.5 % ^{and 30} %. ^{Results for} the latter are

shown in figure ^{3. The curve at} 12 kbar is similar to that at 1 bar with a broad minimum in _p at high tempe-

rature. Metallic behaviour is found at 18 kbar indi-

cating ^a Pc of about 17 kbar. Figure ^{4 shows} P, ^for

several alloys plotted ^{as a} function of x. Also included in the figure ^is P, ^for pure (TMTTF)2CI04 ^obtained

in previous measurements by Parkin et al. [10, 12].

4. Discussion.

Stack instabilities (SDW ^or CDW), (which ^{must be} distinguished from anion order- disorder transitions which occur at much higher temperatures) ^{are found at similar} temperatures (- ²⁰ K) in both the (TMTSeF)2X ^and (TMTT’F)2X

salts, ^even though the electrical conductivities of these materials ^are _very different [21, 8]. ^{In the same}

way the [(TMTSeF)1-x(TMTIF)x]2CI04 alloys

which display ^{a wide} range of conductivities exhibit M-I transitions at 1 bar at very similar temperatures (see ^table I; Tm - I - ¹⁰ K). ^{In I this} pbint ^{is discussed}

in more detail, ^{where it is} suggested ^{that the} disper-

sion in Q values and the weak metallic behaviour found in the more heavily doped ^TMTTF alloys

may be associated with electron localization through

the growth ^{of a} 4 kF ^{CDW on the organic stacks as}

described in a recent theory ^of Emery et ^al. [22].

As mentioned above in addition to stack insta-

bilities, for salts of TMTTF and TMTSeF containing non-centrosymmetric anions there is the possibility

of anion ordering transitions which can give ^rise

to either metal-insulator transitions or weak resistivity

anomalies. There is an anion order-disorder tran- sition in (TMTTF)2C’04 ^{near 70 K} [8, 9] ^where

the CI04 anions order on a 2 x 2 x 2 _array. This transition is suppressed with pressure [12] although

the possibility ^{of an anion} ordering ^{with a different}

structure under _pressure cannot be ruled out, in view of recent studies on (TMTSeF)2C’04’ ^{Careful mea-}

surements on this compound have revealed a weak knee in the resistance versus temperature ^{curve near} 24 K [23] associated with an anion ordering [24] ^for

which there is no change in the a-axis periodicity.

NMR studies on (TMTSeF)2C104 suggest ^{the anion} ordering is absent under _pressure [18]. ^The possibility

of anion ordering ^{in the} alloys studied here cannot be ruled out by ^these resistivity measurements, although

no indications of anion ordering through ^anomalies

in resistivity ^{were found.}

The nature of the low temperature insulating phases

in the [(TMTSeF),-.,(TMTTF).,]*C’04 alloys ^has

been examined in I through ^EPR studies; it appears

that the phase ^has magnetic character in the alloys

lightly doped with TMTTF but is non-magnetic

606

in the alloys rich in TMTTF. It is not unlikely ^that

the non-magnetic ^M-I transitions in the TMTTF rich

alloys ^{at 1 bar} develop ^some magnetic ^character

under _pressure as has been postulated for the pure

(TMT’TF)2X compounds ^{which are} non-magnetic

at ambient _pressure [11, 12]. ^The pressure ^measure- ments presented ^{above cannot} address this question,

since both SDW and CDW transitions affect simi-

larly ^the resistivity of these materials.

It is interesting ^to compare the effect of irradiation

damage ^{on the} properties ^{of the} (TMTSeF)2X ^salts

with that of TMTTF doping. Bouffard et al. [25]

have shown ^that the M-I transition in (TMTSeF)zPF6 8is smeared out and the temperature depressed ^on

irradiation. (T M- falls by ^{about 40} % ^{for a molecular}

defect concentration of about 6 x 10- 4 at 4 and 6.5 kbar). ^{Similar results have} been obtained by

Choi et al. [26]. ^{In contrast} doping with TMTTF

causes an increase in Tm-1 of ^{about 40} % ^{for a 3} % dopant concentration with smaller increases in TM - ^I

observed for dopant concentrations as low as

0.5 % [20]. Mortensen et al. [20] discuss in ^some detail possible explanations for the increase in TM _ ⁱ

and suggest ^that they arise from a sharp ^decrease

in bandwidth. The contrasting ^{behaviour observed}

on irradiation and alloying clearly suggests ^{that the} disorder introduced by ^TMTTF doping ^{is of a} very different character from that associated with radiation

damage which introduces breaks in the organic

chains giving ^{rise to} magnetic ^defects [25]. ^{The absence}

of a Curie tail in EPR measurements on the

[(TMTSeF)1-x(TMTIF)x]2CI04 alloys ^considered

Fig. 4.

^{Variation of} the critical pressure, P,, (defined ^{in the} text) with TMTTF concentration (x)

for [(TMTSeF)1 _x(TMTTF)x2C104 alloys.

here indicates no spin ^{defects are} introduced by

TMTTF alloying. ^A similar result is found for

[(TMTSeF),-,,(TMTTF).,12PF6 alloys [20]. ^The

different behaviours of irradiated an doped (TMTSeF)2X ^salts may therefore be attributed, ^at

least in part, ^{to the} magnetic ^and non-magnetic ^cha-

racter of the defects respectively.

Consistent with the above discussion is the mono-

tonic variation of P, ^{with x} (see Fig. 4), indicating

that the alloys ^become increasingly ^{like the} undoped (TMTTF)2C’04 compound ^{as the TMTTF concen-}

tration is increased. This is also seen, ^as previously

mentioned through ^the gradual reduction in Q as x increases to a value corresponding ^{to that of the} undoped ^{TMTTF salt and} through changes in g

factors. Thus various measurements indicate that the

properties ^of doped (TMTSeF)2C’04 ^are intermediate between those of the pure TMTSeF and TMTTF salts suggesting ^{these materials are true} alloys.

The data shown in figure ⁴ suggest ^that P, ^increases

more rapidly ^{at low values of x with} perhaps ^{a cross-}

over to a slower increase in P, for x > 2 %, although

the data is limited One might ^describe this behaviour

as resulting ^{from the} competition ^{between two} types ^of instabilities, ^affected differently by dopant. ^One possi- bility ^{is the} competition ^between superconductivity

at low x to other stack instabilities at higher values

of x.

5. Conclusion.

The pressure temperature phase diagrams of various [(TMTSeF) , - x(TMTTF)xl 2 C’04 alloys have been determined through resistivity

measurements. Pressure suppresses insulating ^states

observed at 1 bar at low temperatures restoring

metallic states above some critical _pressure, Pc

which increases monotonically ^with increasing

TMTTF concentration, ^to that previously ^{found in} undoped (TMTTF)2C’04 [12]. ^The behaviour of various properties ^{and the} very ^{different effects of} radiation damage ^{on other} (TMTSeF)2X ^{salts sug-}

gests ^{that these materials are true} alloys ^{with few} spin defects. The absence of superconductivity ⁱⁿ

these compounds may therefore reflect the intrinsic

behaviour of the TMTTF salt, ^or may simply ^be

associated with the limited number of defects. A recent theory suggests ^{that defects} by limiting ^the possible ^coherence lengths may ^more easily ^suppress long range order in the superconducting ^{state than}

for other electronic instabilities [27].

Acknowledgments.

^{We are} grateful ^{to M. Ribault}

for help ^{with the} experiments below 2 K. We thank

C. Andrezjewski and A. Andrieux for expert ^technical

help.

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