X-ray scattering evidence for dimerization (4 kF) and spin-Peierls distortion (2 kF) in silver salts of dicyanoquinodiimine (2,5 MR-DCNQI )2 Ag (R = CH3, Cl or Br, M = CH3)

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X-ray scattering evidence for dimerization (4 kF) and spin-Peierls distortion (2 kF) in silver salts of

dicyanoquinodiimine (2,5 MR-DCNQI )2 Ag (R = CH3, Cl or Br, M = CH3)

R. Moret, P. Erk, S. Hünig, J.U. von Schütz

To cite this version:

R. Moret, P. Erk, S. Hünig, J.U. von Schütz. X-ray scattering evidence for dimerization (4 kF) and spin-Peierls distortion (2 kF) in silver salts of dicyanoquinodiimine (2,5 MR-DCNQI )2 Ag (R = CH3, Cl or Br, M = CH3). Journal de Physique, 1988, 49 (11), pp.1925-1931.

�10.1051/jphys:0198800490110192500�. �jpa-00210872�

X-ray scattering evidence for dimerization (4 kF) ^and spin-Peierls

distortion (2 kF) in silver salts of dicyanoquinodiimine

(2,5 ^MR-DCNQI )2 Ag (R ⁼ CH3, ^{Cl or Br, M =} CH3)

R. Moret (1), ^{P. Erk} (2), ^S. Hünig (2) ^{and J.} U. Von Schütz (3)

(1) Laboratoire de Physique ^des Solides, UA2, Université Paris-Sud, ⁹¹⁴⁰⁵ Orsay, ^France

(2) Institut für Organische Chemie, Universität Würzburg ^Am Hubland, ^D-8700 Würzburg, ^F.R.G.

(3) Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, ^D-7000 Stuttgart, ^{80 F.R.G.}

(Reçu ^{le 9} juin 1988, accepté ^{le 5} juillet 1988)

Résumé.

Nous présentons ^{une étude,} par diffraction ^et diffusion des rayons X à basse température, ^des

instabilités structurales de (2,5 ^MR-DCNQI )2 Ag (R

CH3, ^{Cl ou Br, M}

CH3). ^Pour (DM-DCNQI)2Ag (DM

diméthyl) ^{on observe une} diffusion diffuse quasi-1D à ⁴ kF ^{dès la} température ambiante. Elle donne naissance à des satellites à q4kF

(0, 0, 1/2) ^en dessous de T4kF = 100 K. Une deuxième transition avec de faibles effets précurseurs quasi-1D intervient à T2kF = 83 K ^avec des réflexions satellites à q2 kF

(0, 0, 1/4). En liaison avec les propriétés électriques ^et magnétiques ^{de ce} composé, ^on suggère que la 1re transition localise les porteurs alors que la 2e conduit à ^un état fondamental de type spin-Peierls. ^Pour (MC1-DCNQI )2Ag ^et (MBr-DCNQI )2Ag les diffusions à 4 kF ^{et 2} kF ^{ne se} condensent pas ^et il n’y ^a pas de transition. Ceci est dû vraisemblablement au désordre inhérent aux substituents MC1 et MBr. Des effets

supplémentaires de diffusion diffuse sont aussi présentés.

Abstract. 2014 We have studied the structural instabilities of 2,5 (MR-DCNQI )2Ag (R

CH3, ^{Cl or Br,}

M

CH3) using low-temperature X-ray ^diffuse scattering and diffraction techniques. ^In (DM-DCNQI)2Ag (DM

dimethyl) ^{we observe} quasi-1D ⁴ kF ^diffuse scattering ^{at room} temperature which transforms into satellite reflections at

q4kF = (0, 0, 1/2) ^below T4 kF ~100 K. A second transition with limited quasi-1D

precursor scattering ^{occurs at} T2 kF ~ ^{83 K} with satellite reflections at q2 k F

(0, 0, 1/4). In relation with the transport ^and magnetic properties ^{it is} suggested that the upper transition localizes the charge carriers while the lower one leads to a spin-Peierls ground ^{state. In} (MCI-DCNQI )2Ag ^and (MBr-DCNQI )2Ag ^the quasi-1D

4 kF ^{and 2} kF ^diffuse scatterings ^{do not} condense and there is no transition, probably because of the inherent disorder caused by the MC1 and MBr substituents. Supplementary ^diffuse scattering features and irradiation effects are also reported.

Classification

Physics ^Abstracts

61.50K

64.70

61.80K

1. Introduction.

There is a growing interest for a new class of highly conducting organic ^{salts based on} N,N’

dicyanoquinodiimine (DCNQI), ^{which is a} strong acceptor, and cations such as Cu+ and Ag+ [1].

They ^{are formed of stacks of the} organic ^molecules

connected by the metal ions in a distorted tetrahedral coordination [1, 2]. ^The overlapping of the p 7T orbitals from the quasi-planar ^{DCNQI molecules} along ^the stacking axis leads to quasi-one-dimen-

sional energy bands filled by the electrons donated

by the cations. If the oxydation ^{state of the cations is}

+ 1 these conduction bands are quarter-filled ^be-

cause of the stoichiometry [3]. Most of these com-

pounds undergo low-temperature metal-semicon- ductor transitions [3-5] ^{related to the strong one-}

dimensional character. In contrast (2,5 ^DM- DCNQI) 2Cu exhibits metallic conductivity ^{down to}

0.45 K [6]. However, ^{under a} modest pressure of about 100 bar the metallic state is suppressed [7, 8]

and the temperature-pressure phase diagram displays

an unusual reentrant behaviour at low temperature

[9]. ^A peculiar feature of the copper derivatives is the intermediate oxydation ^{state of} the copper ion estimated to be about 1.3 from XPS data [2]. ^This

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198800490110192500

1926

agrees with X-ray scattering observations of diffrac- tion satellites due to a lattice modulation of reduced

wave vector q =1 c * below the metal-insulator 3

transitions in (2,5 MCI-DCNQI)2Cu [2] ^and (2,5 MBr-DCNQI)2Cu [10]. ^These transitions have been attributed to cooperative Jahn-Teller and

charge-density ^wave (CDW) instabilities [10].

In the Ag salts which are the subject ^{of the} present study ^the oxydation ^{state of} Ag ^{is + 1. The room} temperature conductivity of the three derivatives is moderate (o- - ^{50-100 0-1} cm-1 ) ^and weakly ^tem- perature dependent ^{down to about 120 K} [5]. ^The resistivity ^increases continuously below about 100 K- 120 K although the rise is a little steeper ^{for the} dimethyl derivative (noted ^DM below) [5]. ^More precisely ^{in DM one observes a} resistivity ^minimum

near 170 K, ^{a slow increase down to about 80-100 K}

and a faster one below, ^{with a} nearly ^constant

d ln p _[11]. While there is no anomaly in d ln p ^at

dT y ⁱⁿ

dT ^at ambient pressure ^a metal-insulator transition can be detected above 4 kbar [11].

The spin susceptibility (ESR) ^is high ^{at room} temperature (,y :--. ^{5 x 10-4} emu/mole), ^decreases slowly ^{at T is reduced, with a Curie} tail below about 30 K [4, 5]. ^{However, in the case of DM one}

observes a marked drop of X ^{at about 80 K. The}

behaviour of these three Ag salts has been inter-

preted ^{in terms of} charge localization in a 1D system together with orientational disorder effects in the

case of (MCl-DCNQI) 2Ag (noted MCI) ^and (MBr- DCNQI)2Ag (noted MBr)) [4, ^5, 7]. ^To investigate possible structural instabilities and transitions as-

sociated with the transport ^and magnetic properties

of these materials we have performed ^{a low-tem-} perature X-ray ^diffuse scattering study ^on single crystals grown in Wfrzburg.

2. Experimental procedure and results.

Diffuse scattering patterns ^{were obtained} using ^the

so-called monochromatic Laue technique (CuKa

radiation focussed by ^a doubly-bent graphite ^mono- chromator, stationnary crystal, cylindrical photo- graph). The needle crystals ^were placed ^{into a}

helium cryogenerator ^{and the} temperature ^{was vari-} ed in the 295 K-20 K range. Photographs ^{were taken}

every 10 K near phase transitions with exposure times of 3 to 12 hours (sealed X-ray ^tube operated ^at

40 kV, ²⁰ mA).

For the DM derivative a more quantitative study

was performed using ^a low-temperature diffractome-

ter (MoKa radiation, rotating ^anode generator, Displex cryogenerator, lifting-detector geometry).

This enabled us to estimate the transition tempera-

tures more accurately ^{and to determine the trans-}

verse wave vector components, ^{which was difficult} from the photographic ^{data. We now} describe and

compare the various structural instabilities and the diffuse scattering features observed in the three derivatives.

2.1 4 kF INSTABILITY.

At room temperature ^one observes diffuse scattering planes (visible ^{as continu-}

ous lines on the photographs) ^located halfway ^be-

tween the successive layers ^of Bragg reflections in the c * direction (Figs. la, ^c, e). ^{The c * wave vector}

component q = - q = ^{c *} corresponds ^{to 4} k k , ^Fermi

2 ^{F F}

wave vector) ^{for a} quarter-filled ^{band in a 1D}

electron scheme. The intensity ^is weakly ^modulated

in the diffuse planes indicating short-range ^corre-

lations. This scattering ^{is a} signature ^{of a} quasi-lD instability.

The diffuse scattering ^{is weaker in the case of MCI} (Fig. 1c) ^{and MBr} (Fig. 1e) than for DM and its

dependence upon temperature is different. For DM the intensity modulations get stronger ^{and the diffuse} planes sharpen ^as T is reduced. A continuous transition (second order) ^{occurs at} T4kF ^{=100 ±}

5 K with the development ^of superstructure ^reflec- tions defined by q4 kF

(0, ^0, 1/2). This is shown both from the photographic ^data (Fig. Ib) ^{and from}

diffractometer scans of reflections like (2, ^6, 1.5)

whose peak intensity ^{versus T curve is} given ⁱⁿ figure 2. ^The superstructure reflections, though sharp, ^are still broader than the main Bragg ^ones, especially along ^{c *. For} instance half widths at half maximums HWHM’s are 0.020 A- 1, ^{0.033 Å - 1 and}

0.056 A-1 for (2, 6, 1.5) ^as compared ^{to 0.017} A-1,

0.026 A-1 and 0.024 A-1 ¹ for (1, ^5, 2) along ^the a *, ^{b * and c *} directions, respectively. ^{We have also} plotted the inverse susceptibility X4 kF

^{T// de-}

duced from microdensitometry ^{of the diffuse scat-} tering plane ^above T4 kF. ^{Despite the limited data}

and the lack of reliable data points ^near T4 kF (because ^{of some} uncertainty ^{in the} temperature determination from the photographic measurements)

a linear extrapolation ^of X4 kF (Curie-Weiss law)

agrees with the above value of T 4 kF.

In the case of MCI and MBr there is a slow increase of the diffuse scattering ^{and a weak im-} provement of the correlations (the ^diffuse planes sharpen ^{and the} modulations increase) ^{when the} samples ^{are cooled from room} temperature ^{to 20 K}

(Figs. 1d, f). ^{However no} satellites appear and there is no transition towards long-range ^order.

2.2 2 kF INSTABILITY.

A second type ^{of diffuse} scattering ^{becomes visible below about} T4 kF ^{in the}

form of very weak quasi-lD ^diffuse planes. ^{Its wave}

vector component along ^{c *} is q

^{± 1/4 c *, that is}

± 2 kF. ^{In DM a} transition occurs towards 3D long-

range order ^at T2 kF

^{83 K ± 3 K with the formation}

of satellite reflections (Fig. 1 b ). ^{Their transverse}

Fig. ^1. - X-ray monochromatic (CuK«) ^Laue photographs ^of (a, b) (DM-DCNQI)2Ag, (c, d) (MCI-DCNQI)2Ag ^and (e, f) (MBr-DCNQI )2Ag ^{at room} temperature (left ^{side a, c,} e) ^{and 20 K} (right ^side b, d, f). ^The c axes are horizontal, typical exposure times 3-6 hours). ^{Some 2} kF ^{and 4} kF ^{lines are noted and some diffuse} scattering features described in the text are shown by ^arrows.

wave vector components ^{have been} determined with the diffractometer leading ^to _{q2 kF}

(0, ^{0, ±} 1/4).

Figure 3 shows the temperature dependence ^{of the} peak intensity ^{for the} (3, 5, 1.25) reflection together

with its scan along ^c* (inset). ^The transition is second order as indicated by the continuous growth

of the intensity. It is noted that the intensity ^{of these}

2 kF reflections are 2 or 3 times as large ^{as that of the}

4 kF ^{ones, on} the average.

In contrast the MCI and MBr derivatives do not

undergo ^{such a} transition. The quasi-lD ^diffuse

planes ^{are weaker in} intensity ^and although they

become stronger ^and sharper ^{as the} temperature ^is lowered there is no condensation into satellite reflec- tion (Figs.1d, f).

2.3 OTHER TYPES OF DIFFUSE SCATTERING.

The above 4 kF ^{and 2} kF ^diffuse scattering ^features (and

the associated phase transitions in the DM com-

pound) ^{are common to the 3} derivatives of

(DCNQI )2Ag. ^{In addition, one} observes other types

of diffuse scattering ^{which are} present ^{in one or two}

of the derivatives only.

1928

Fig. ^2.

⁴ kF transition in (DM-DCNQI )2Ag. Tempera-

ture dependence of the inverse susceptibility X4 kF ^{and of}

the peak intensity ^{of a} superstructure reflection (2, 6, 1.5).

Both curves indicate T4 kF

^{100 K ± 5.}

-

In DM and MCI one sees very broad diffuse

scattering ^{sheets located close to the} Bragg ^reflection layers ^{and shown} by ^{arrows in} figures la, ^{c. This} scattering ^{is the} strongest ^near the f = 1 Bragg layer

and it is remarquable ^{that it seems to be} present ^on

one side of the layer. ^It is visible from room

temperature ^{in both cases} (DM ^and MCI) although

much stronger in DM. However its temperature

dependence is different. In MCI it stays roughly ^the

same down to 20 K while in DM one observes first a

slow decrease of its intensity ^{down to about 50 K}

where it condenses partly ^{into a} complex array of broad spots. ^These spots ^{can be divided} up into ^two sets with wave vector components q, == 0.12 and q2 = 0.17 along ^{c *. Froni the} photographs it appears that their transverse components along ^{a * and}

b

are equal ^{to 0 but no} confirmation was attempted

in the diffractometer study. ^{This diffuse} scattering ^is

absent in MBr.

-

In MCI and MBr there exists a strong ^diffuse spot ^near the f

2 main Bragg layer (shown by

arrows in Figs. Ic, ^{d, e,} f). ^{As T} is lowered its

intensity grows in MBr while it stays nearly ^constant

in MCI. We note that, surprisingly, ^its position ^and

therefore its Bragg angle change very little with temperature.

- In the case of MCI and MBr there are well defined diffuse scattering planes superimposed ^on

the main Bragg reflection layers (Figs. 1c, e). ^This scattering ^is essentially temperature independent ⁱⁿ

both cases (Figs. 1d, f). ^{In the DM} compound ^there

is practically ^{no such diffuse} scattering.

- Finally ^{in the case of MBr} only, ^{one observes}

very broad spots ^{of diffuse} scattering ^{located at or}

near q = 1 ^{c * (halfway} between the Bragg ^reflection

2

planes). ^This scattering ^is strong ^and temperature independent ^too (see Figs. 1e, f).

2.4 IRRADIATION EFFECTS.

In the course of the

experiments ^{we have} discovered that photographs

taken in the same conditions but at various stages ^of irradiation by ^the X-ray beam showed differences.

This is illustrated in figure ^{4 taken at 20 K and where}

the 3 photographs correspond ^{to about 12} h, ^{30 h}

and 48 h of irradiation. There is little effect on the 4 kF ^{and 2} kF satellite reflections although ^{a close}

examination reveals a weak transverse broadening (in ^the (a*, b*) plane). ^{However a more dramatic}

effect occurs for the diffuse scattering described in 2.3. In figure ^{3a the two sets} of broad reflections defined by ql and q2 ^are clearly ^{visible but as the}

irradiation time increases these reflections get broader and merge into ^a broad diffuse _{sheet very} much like what is observed at higher temperatures.

Fig. ^3.

² kF transition in (DM-DCNQI )2Ag. Tempera-

ture dependence ^{of the} peak intensity ^{for the} (3, ^5, 1.25) superstructure reflection. The inset shows it profile along

c*.

3. Discussion.

It appears ^{that the} Ag ^{salts of DCNQI} exhibit very rich structural features which are important ^{for the} understanding of their low temperature ^behaviour.

In the following ^{we shall} try ^to explain ^{most of the}

results and to rely them with the transport ^and

magnetic properties.

Beginning ^with (DM-DCNQI )2Ag ^{the first} important result is the observation of quasi-lD scattering ^at 4 kF ^{at room} temperature ^{and at} 2 kF below about 100 K. This is a signature ^{of 1D}

character and it confirms previous structural consid- erations and _energy band calculations. The organic

stacks are bridged by ^{the metal atoms} (Ag ^{in this} case) ^{which are} tetrahedrally coordinated to the terminal N atoms of the DCNQI molecules. Mixing

of the p’7T orbitals of DCNQI forming ^{1D conduction}

bands with the 3d orbitals of the metal (M) ^is

conditionned mainly by ^{the M-N} distance, ^{the distor-}

tion of the tetrahedron and the valency ^{of M} [2, 3].

As compared ^{to the case of Cu the Ag-N distance is} larger (2.308 ^{A versus 1.976} A) ^{and the} tetrahedron is squeezed along ^c resulting in smaller intermolecu- lar distances along ^{the chains} [1, 2]. Moreover the

oxydation ^{state of} Ag ^is undoubtedly ^{fixed to + 1}

while that of Cu can be intermediate as confirmed by

XPS [2] ^and X-ray scattering [10] ^{results. Therefore}

little mixing between the p7T ^{and d} orbitals should

occur which is expected ^to result in a strong ^1D character. This is demonstrated by ^the present observations and the absence of _{any 1D} scattering ⁱⁿ

(DM-DCNQI )2Cu [12].

The 4 kF transition at T4 kF ’- ^{100 K} corresponds ^to

a doubling ^{of the c} axis and thus to a dimerization of the DM-DCNQI molecules along the chains. Actu-

ally the existence of c components for the distortion is clearly visible from the gradual increase of the

intensity of the satellite reflections for increasing ^f

values. The observation of quasi-lD precursor ^scat-

tering ^{over a wide} temperature range (more ^than

200 K) ^{indicates a} strong ⁴ kF electron-phonon coupl- ing.

The lower 2 kF transition (T2kF ’- ^{83 K) involves a}

further doubling ^{of the c axis and a} tetramerization of the chains which is also associated with an

increase of the satellite reflection intensities for

increasing ^{f. In contrast the} quasi-lD precursor

scattering ^{is much more} limited in temperature

(about ^20-30 K).

These transitions shed light ^{on the} transport ^and magnetic properties ^of (DM-DCNQI )2Ag. ^At T4 kF

there is no anomaly ^{in the} resistivity ^{or the} magnetic susceptibility [4, 6, 11]. ^{However, below a} resistivity

minimum at Tp ^{= 170 K there is a} continuous locali- zation with a faster rate of increase of _p below 80- 100 K [6, 11]. ^The origin ^of Tp ^{may come from both}

4 kF electron-phonon ^and electron-electron

Umklapp scattering processes in the wide regime ^of

4 kF fluctuations that we have observed. The occur- rence of the 4 kF dimerization at T 4 kF ^{(- 100 K ) in a}

1D system where the intrasite electron-electron interactions are moderately large (relatively large U,

where U is the Hubbard gap) should open ^a small dimerization gap and freeze the charge carriers, ^as

observed. While the opening of this gap does ^not

give ^{rise to a clear} metal-insulator (M-I) transition in the resistivity ^data [4, ^5, 11] ^a drop in the ther- mopower is initiated near 100 K [11]. Actually ^as

noted in [11] ^the thermopower ^{is more sensitive to}

the gap than the resistivity. ^{Moreover a M-I tran-}

sition is induced by ^the application ^of pressure ^above 4 kbar [11]. ^In the copper salts the effect of pressure is believed to be similar to reducing ^the temperature, which increases the one-dimensionality through ^a

marked shortening ^{of the c} parameter ^{and a distor-}

tion of the Cu tetrahedron [3, 8]. ^{If this holds for the}

silver salts, ^{then a better 1D} character would presum-

ably ^{involve a} larger dimerization gap and explain

that a true M-I transition is observed under pressure

[11].

The magnetic susceptibility drops sharply ^near

80 K as found by ^ESR [4, 5]. ^This corresponds ^{to the} 2 kF transition at T2 kF ’- 83 K ^{described above and}

strongly supports ^an interpretation ^{in terms of a} spin-Peierls transition leading ^{to a} tetramerized non-

magnetic ground ^{state. It is} remarkable that the condensation of the spin degrees ^{of freedom occurs}

at a relatively high temperature ^{and close to the} condensation of the charges. ^This is unusual when

compared ^to well known spin-Peierls ^{materials like} MEM(TCNQ)2 [13] ^or (TN=)2PF6 [14] ^where

the spin-Peierls transition is in the 10-20 K range and there is on the order of 200 or 300 K difference between the 4 kF charge localization and the 2 kF spin condensation. However we recall that in

MEM (TCNQ )2 pressure lowers the _upper metal- insulator transition temperature very rapidly ^{while it}

raises the spin-Peierls transition temperature ^{so that}

the two transitions join ^and disappear eventually

above 4 kbar [15].

The low-temperature superstructure reflections induced by ^the dimerization and the tetramerization

are defined by (0, ^0, 1/2) ^and (0, ^{0, ±} 1/4) ^wave

vectors, respectively. ^{Thus there is no} change ^{of the}

transverse periodicity. It also appears that there is

no supplementary reflection at low temperature ^on the basic reciprocal lattice, meaning ^{that the room} temperature body-centred ^structure (space group ^I

4,la) ^is preserved, ^on average, through ^{the two}

transitions. However a preliminary examination re-

veals peculiar conditions limiting ^{the observed} superstructure reflections. For the (0, ^0, 1/2) ^reflec-

tions one has either h

2 n or k

2 n and this cannot be explained ^{with a} unique ^{lattice. It} probably

indicates that the symmetry is lowered to orthorhom- bic and that equivalent ^{domains related} by ^{a rotation}

of i- ^{are produced at T4 kF’ Quite} similar effects are

2

found for the (0, ^{0, ±} 1/4) reflections with either h = 2 n + 1 or k= 2n+l, ^which agrees with the above model. The complex arrangement ^{of the DM-} DCNQI chains in the structure makes any

hypothesis about the nature of the 4 kF ^{and 2} kF

distortions and their phasing very difficult and

premature ^{at this} point. Structural determinations in the dimerized and tetramerized phases ^{are much}

awaited. As noted before stronger intensities have been found for the 2 kF reflections than the 4 kF ^ones suggesting ^{that the} tetramerization involves larger displacements ^{than the} dimerization. The behavior of the MCI and MBr derivatives can be now com-

pared ^{to that} of DM. In both cases we have observed

weaker quasi-lD ^diffuse scattering than for DM and

1930

no transition to long-range order. A distinctive feature of MCI and MBr is the existence of disorder caused by ^the methyl and chlorine (bromine) ^sub-

stituents whose random distribution is inferred from the location of the molecules on inversion centers

[3]. This disorder may be responsible ^{for the tem-} perature independent ^diffuse scattering planes pas-

sing through ^{the main} Bragg layers ^{and described in}

3c. This sharp scattering ^{indicates that} along ^a given

chain the methyl and chlorine (bromine) groups ^are

reasonably ^{well ordered but no} order is achieved between different chains. In the case of MBr a

different arrangement ^{seems to} coexist with the

previous ^{one as} suggested by ^{the broad diffuse}

reflections at q = 1 c * ^{described in 3d.} _They ^corre-

2

spond presumably ^{to a} short-range alternation of

methyl ^{and bromine} groups ^{on the same chain. The}

Fig. ^4. - X-ray irradiation effects (CuKa) ^{on the low}

temperature ordering ⁱⁿ (DM-DCNQI)2Ag. ^{Some weak} broadening is observed for the 2 kF ^{and 4} kF ^satellite

reflections. More drastic effects occur for the scattering

which condenses at + ql and + q2 (arrows).

decay ^{of the} intensity ^{of these} spots ^{from the} origin

of the reciprocal space confirms that this disorder is substitutional. In MBr the two types of disorder described above may be present ^{in different} regions

of the crystal.

It is well known that disorder affects the 2 kF ^and

4 kF instabilities in organic charge transfer salts with two main results i) ^an increase of the diffuse scat-

tering intensity ^{due to the} formation of Friedel oscillations and ii) ^{a loss of} the correlations at low temperature because of phase pinning ^effects [16].

In the present ^{case we do not} observe the first type of effect as the quasi-lD scattering intensity appears to be weaker in MCI and MBr than in DM. On the other hand the absence of 4 kF ^{and 2} kF transitions in MCI and MBr can be attributed to the disorder of the methyl ^{and chlorine} (or bromine) groups. This is in agreement ^{with the} transport ^and magnetic ^data

that show a slower increase of the resistivity ^below

100-120 K and no drop ^{of the} susceptibility ^{in MCI}

and MBr [5].

Another source of disorder is the production ^of

defects caused by X-ray irradiation and its conse-

quences ^are similar to those of other types ^of disorder (substitutional ^or orientational) [16]. ^Our preliminary observations show a slight decrease of the correlation lengths ^{in DM} under 30 and 48 hours of irradiation (paragraph ^2.4, Fig. 4). ^It corresponds

to a low concentration of defects that we can

estimate by comparison ^{with a} previous study ^of TMTSF-DMTCNQ performed ^{on the same} set-up

[17] and where 0.02 mol % molecular defects were

created _per day of exposure. For 48 hours ^as in

figure ^4c this would give 0.04 mol % defects although

the actual rate should depend ^{on the nature of the} organic molecule. In any ^{case we are} certainly dealing ^{here with a} low defect concentration and one

could expect that further irradiation would eventu-

ally suppress ^{the 3D} long-range order and the transitions and result in a behaviour similar to that of MCI and MBr. This would be interesting ^{to confirm.}

We turn now to much less understood diffuse

scattering features. First the broad diffuse sheets described in paragraph ^{2.3 and} present ^{in DM and} MCI (although weaker). ^{In DM a} partial ^conden-

sation into broad incommensurate satellites at ql

0.12 c * and q2

^{0.17 c *} supports ^{the idea of a} softening ^of phonon ^{modes at the} corresponding

wave vectors. It may be related ^to Fermi surface instabilities associated with partial nesting ^within

closed Fermi surface pockets ^{as found in} tight- binding calculations for (MCl-DCNQI )2Cu [3]. ^A possible explanation ^{for the} asymmetry ^of the diffuse

scattering only ^{visible on one} side of the main Bragg

layers is that it may correspond ^{to a} maximum in the

structure factor of the DCNQI molecules. These

considerations are however very tentative and

further characterization is awaited.

Even more puzzling is the observation of a strong diffuse reflection in MCI and MBr as described in

paragraph 2.3. It appears ^to be the only ^{one of its}

kind on the stationnary photographs (Figs. ^{1c, d, e,} f). However it is not due to an artefact because it is observed in different crystals ^{and on} precession photographs. ^Its origin is unknown.

In conclusion we have obtained X-ray ^diffuse scattering and diffraction data which show the exist-

ence of 4 kF ^{and 2} kF structural instabilities in the

DM, MCI and MBr derivatives of (DCNQI )2Ag. ^In (DM-DCNQI)2Ag ^these instabilities give ^{rise to}

second-order structural transitions at 100 K and 83 K, respectively, ^{while in MCI} and MBr sub- stitutional disorder prevents ^the condensation of 3D

long-range order. The lower 2 kF transition in

(DM-DCNQI )2Ag ^can be attributed to the stabili- zation of a spin-Peierls ground ^state. The behaviour of these silver salts is mainly determined by ^a strong 1D character in contrast with the copper salts.

Further structural studies of the low temperature dimerized and tetramerized phases and of the influ-

ence of pressure ^are planned.

Acknowledgements.

We are very grateful ^to S. Tomic and D. J6rome who convinced us that we should undertake this

study. We would like to thank R. T. Henriques, ^W.

Kang, S. Tomic and D. J6rome for communicating

their results prior ^to publication. Very fruitful dis- cussions with them and with J. P. Pouget ^{are also} acknowledged.

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