Ion channeling in the quasi-one dimensional blue bronzes A0.30MoO 3 (A = K, Rb)

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Ion channeling in the quasi-one dimensional blue bronzes A0.30MoO 3 (A = K, Rb)

B. Daudin, M. Dubus, J. Dumas, J. Marcus

To cite this version:

B. Daudin, M. Dubus, J. Dumas, J. Marcus. Ion channeling in the quasi-one dimensional blue bronzes A0.30MoO 3 (A = K, Rb). Journal de Physique, 1987, 48 (10), pp.1779-1786.

�10.1051/jphys:0198700480100177900�. �jpa-00210619�

Ion channeling ^{in the} quasi-one dimensional blue bronzes A0.30MoO3 (A = K, Rb)

B. Daudin, ^M. Dubus, ^{J. Dumas} (1) ^and J. Marcus (1)

Service des Basses Températures, C.E.N.G., ^{avenue des} Martyrs, ^{BP 85} X, 38041 Grenoble Cedex, ^France

(Reçu le 27 avril 1987, revis6 le 22 juin ^1987, accepté ^{le 25} juin 1987)

Résumé.

Nous avons mesuré les rendements de rétrodiffusion de protons ^{de 1 MeV} pour les bronzes bleus

quasi unidimensionnels K0,3MoO3 ^et Rb0,3MoO3 dans la gamme de température 50-350 K. Nous avons mis en

évidence d’importants phénomènes d’hystérésis ^et des effets dépendant ^du temps ^{sur de} longues périodes.

Nous discutons nos résultats en termes de domaines d’onde de densité de charge ^{et de} parois de domaines

couplées ^aux défauts du réseau.

Abstract.

We have measured backscattering yields ^{of 1 MeV} protons ^{in the} quasi-one dimensional blue bronzes Ka0.30MoO3 ^and Rb0.30MoO3 ^at temperatures between 50 and 350 K. Large hysteresis ^{and time} dependent ^effects involving long time scales are found. The measurements are discussed in terms of charge density ^wave domains and domain walls coupled ^to lattice defects.

Classification

Physics ^Abstracts

71.45

61.80

1. Introduction.

The molybdenum ^{blue bronzes} Ao.30MOO3 (A

^K, Rb) undergo ^a metal-to-semiconductor transition at

180 K [1]. Optical reflectivity measurements by Travaglini ^{and Wachter} [2] ^and resistivity ^measure-

ments by Perloff et al. [3] ^showed that these com-

pounds ^are quasi-one dimensional conductors at room temperature. X-ray ^diffuse scattering ^studies by Pouget et ^al. [4] ^have established that the transi- tion at 180 K is a Peierls transition towards an

incommensurate charge density ^{wave state which}

becomes quasi-commensurate ^{below 100 K. The} charge density ^wave (CDW) ^state results from a

nesting of the Fermi surface and from an electron-

phonon coupling ; ^{it consists of a} periodic ^modu-

lation of the electron density coupled ^{to a} periodic

lattice distortion. Sato et al. [5] ^and Pouget et ^al. [6]

have shown evidence of the formation of a Kohn

anomaly responsible ^{for the} softening ^{of a} phonon

mode at 180 K. In the semiconducting phase, ^Dumas

et al. [7] ^{have shown} that the blue bronze exhibits

non linear voltage-current characteristics beyond ^a

small threshold electric field. They ^{have attributed}

this non linearity ^{to the} sliding ^{of the} CDW. At low electric field, ^{the CDW is} pinned by ^its interactions with impurities ^{but above a threshold field the CDW}

is depinned ^and contributes to the conductivity [8].

Compelling evidence for the motion of the CDW has been given recently by Segransan et ^al. [9] by ^NMR studies of Rbo,3oMo03. A wealth of metastable

phenomena ^with long time scales both in the pinned

and current carrying ^{state have been} reported [10].

The observation of electrical and thermal hys-

teresis [10] dielectric polarization [11], ^{various mem-}

ory effects [12] ^{as well as of time} dependent 87 Rb NMR [13] ^{and M05}

^EPR [14] lineshapes ^and

metastable loss of transverse order under application

of an electric field [15] indicate that many metastable

states are involved in the pinned ^{state of the CDW}

even at temperatures ^{as low as = 4.2 K.}

To obtain a more detailed understanding ^{of the}

Peierls distorted state we have investigated ^{the blue}

bronzes Ko.3oMo03 ^and Rbo,3oMo03 by proton ^chan- neling technique ^{in the} temperature range 50 K- 350 K. This technique is very sensitive to detect small atomic displacements, of the order of 0.1 A.

The channeled particles ^are dechanneled due to the effects of thermal vibrations, impurities, ^{lattice im-}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198700480100177900

1780

perfections. ^The study ^{of a CDW} transition by ^this technique ^{has been} reported for the first time by Haga ^{et al.} [16] ^then by Nunez-Regueiro ^{et al.} [17]

in the case of TiSe2 ^{and 1} T-TaS2. ^{The first} attempt

of ion channeling ⁱⁿ Kfl.3oMo03 ^{with He+ ions in the}

temperature range 100 K-300 K has been reported by ^{Abe et al.} [18]. ^{These authors have observed an}

enhancement of the minimum backscattering yield,

X min’ in the vicinity ^{of 180 K and have} attributed this

anomaly ^{to the} softening ^{of a} phonon ^mode.

We report ⁱⁿ this paper ^{a more} detailed study ^of

X min and show that this quantity depends strongly ^on

the thermal history ^{of the} sample ^{and is a function of}

time. Some preliminary results have been published

elsewhere [19]. Anomalies in the vicinity ^{of 180 K}

and 100 K are found on Ko.30MOO3 ^and Rbo.30Mo03.

Complementary resistivity measurements show that the resistance of the blue bronzes is also strongly hysteretic ^{and time} dependent. ^{This non} equilibrium

behaviour is discussed in relation with other metasta-

bility phenomena ^{in the blue} bronzes. We propose that the observed phenomena result from the pre-

sence of CDW domains and domain walls coupled ^to

lattice defects.

The blue bronze shows a layered type crystal

structure. The structure is monoclinic, built with clusters of ten distorted Mo06 octahedra. The clusters share corners along ^the monoclinic b-axis and the [102] direction and form infinite two-dimen- sional sheets. The alkali ions A+ lie between the sheets [20]. The structure, illustrated in figure 1,

contains three Mo sites. The 4d electrons are mainly

located on the Mo(2) ^and Mo (3 ) ^{sites which are}

Fig. ^1.

Crystal ^structure of the blue bronze showing ^the

infinite sheets of Mo06 ^{octahedra 11} (201), separated by

the alkaline ions (8). The sheets contain the infinite chains 11 ^b.

involved in infinite chains of Mo06 ^octahedra along

the b-axis [21]. ^{There are two} alkali sites with large

and anisotropic Debye-Waller ^factors.

The structure can also be viewed as made of infinite chains of clusters of ten Mo06 ^octahedra running along ^{the b-axis.} X-ray ^diffuse scattering

studies [4] have shown that precursor diffuse

platelets ^{above 180 K condensed} into weak satellite spots. ^The superstructure reflections are charac- terized by ^{the wave vector q}

10 a *, (1 - qb) b *, 0.5 c ^{with qb = 0.26 at 110 K.}

(1- qb) ^increases strongly ^with decreasing tempera- ture, ^is temperature independent ^{below 100 K and} extremely ^{close to the} commensurate value 0.75 [22].

2. Experimental techniques.

The single crystals used in these investigations ^were prepared ^at L.E.P.E.S. by electrolytic reduction of a

A2Mo04-Mo03 (A

^K, Rb) ^{melt at 550} °C. Details of the preparation ^are given in reference [23]. ^The crystals appear ^as platelets ^with typical ^{size 5 x}

2 x 1 mm3 parallel ^{to the} (201) cleavage plane ^with

the b-axis as the long ^{direction as} illustrated in

figure ^2a.

Fig. ^2a.

Schematic view of the samples ^{in the two} configurations ^{used :} i) ^beam perpendicular ^{to the cleav-}

age plane ; ii) ^beam parallel ^{to the} cleavage plane.

Fig. ^2b.

Typical energy spectrum of backscattered

protons. ^{r :} random ; ^{c :} channeled. The oxygen front is

shown and the energy window used is visualized : (1).

The ion channeling measurements were carried out using ^{the 2 MeV Van de Graaf ion} accelerator of the SBT/L ^ACC.

As the blue bronzes are highly ^{sensitive to irradia-}

tion effects [24], considerable care was taken to minimize defect production by ^{the ion beam. In} particular, preliminary experiments ^{with He+ ions}

indicated noticeable permanent ^radiation damage ^of

the samples.

All the experiments reported ^{in this} paper ^were therefore performed using ^{1 MeV} protons ^{as it was} checked that they produced ^no significant damage.

On the other hand, ^the penetration depth ^{of 1 MeV}

protons in these material is as large ^{as 10.5 um}

whereas the energy window shown in figure ^2b corresponds ^{to an} analysed depth of about 0.8 _lim.

As a consequence, ^the number ,of ^defects produced by irradiation in the analysed region ^{of the} sample

was negligible. ^{The beam was} approximately ^{1 mm}

in diameter and the intensity ^{never exceeded} 3 x 10- 9 A. The samples ^{were cleaved} prior ^to experiments ^{and fixed to the} sample ^{holder with}

silver paint. ^A permanent helium flow was used to cool the sample ^{holder so that the} measurement

temperature range ^was 50 K-350 K. The heating ^and colling ^{rate were} approximately ^{1 K/minute.}

The backscattered particles ^{were counted} using ^a

multichannel analyser ^{and a} typical spectrum ^is shown in figure 2b where the energy window used for the experiments ^is visualized. The minimum

backscattering yield, X miD’ is defined ^as the ratio

NCIN, ^{where Nc and Nr are} the number of particle

backscattered in, respectively, ^{the channeled and}

random geometry.

As far as Ko,3Mo03 ^is concerned, ^two geometries

were used (see Fig. 2a) :

i) ^{In most} experiments, ^the sample ^{was oriented}

so that the beam direction was along ^{a channel} perpendicular ^{to the} cleavage plane ^{and to the b-}

axis.

ii) Additional experiments ^{were also} performed

with the beam axis parallel ^{to the} cleavage plane ^of

the platelets ^and perpendicular ^{to the b-axis. In this}

last _case, the samples ^{were about 1.5 mm thick and}

the beam diameter was reduced to 0.5 mm.

The experiments ^on Rbo.3Mo03 ^were performed using only ^the geometry ^{defined in} i).

The usual experimental procedure ^{was to orientate}

the sample ^{at room} temperature ^{and to measure}

X min ^{as a} function of temperature ^{down to 50 K.}

Then, ^the alignement ^was controlled in order to make sure that the sample ^{had not moved} during ^the cooling process. X min ^was subsequently ^measured

from 50 K to 350 K and a new alignement ^control

was done at high temperature.

Resistivity measurements were made on freshly

cleaved samples ^{with the} standard four probes

configuration. ^{Gold wires were} attached with silver

paint ^{on indium} evaporated ^areas.

3. Results.

3.1 THERMAL CYCLING EXPERIMENTS.

In a first set of experiments, X min ^was measured as a function of temperature ^{in the} 50 K-350 K range and several

cycles ^were completed. The results for two of them

are plotted ⁱⁿ figure 3a and 3b and correspond ^to

Fig. ^3a.

^Minimum backscattering yield ^{as a} function of temperature.

Fig. ^3b.

^Minimum backscattering yield ^{as a} function of temperature,. The dotted line is only ^an eye guide (see

text). The transition temperature, Tc, is indicated.

two different impact point ^{on the same} sample. ^The

characteristic features exhibited are :

a) ^A smooth variation of _{X min} for decreasing

temperature ^{and no} sharp anomaly ^around Tc = 180 ^K.

b) ^A large effect for increasing temperature

which corresponds ^{to an increase of X min around}

100 K and a slow recovering ^{achieved for} tempera-

ture between 300 and 350 K, well above the transi- tion temperature. It is worth noting ^{that the} highest

value of _{X min} obtained during cycle ^b (see Fig. 3b) ^is

close to the random value and therefore indicates that a very strong disorder is present ^{in the} sample.

For the experiments reported ⁱⁿ figure ^{3a, the}

sample ^was aligned ^{at 50 K and a new} impact point

1782

was chosen before increasing ^the temperature. ^{As a} consequence, ^the large increase in _{X min} which was

observed could definitely ^{not be} attributed to poss- ible irradiation defects.

c) ^{For the} decreasing temperature part ^{of the} cycle ^{shown in} figure 3b, ^the experimental points ^are

fitted by ^a straight line between 200 and 300 K but a

noticeable deviation from this line is observed for T 220 K, ^{in a} range where premonitory ^{effects of}

the transition are known to take place [4].

This deviation which was not always present ^was nevertheless observed during ^various experiments,

for decreasing temperature, ^{and we} attribute it ot the establishment of long-range ^{order in the} sample.

3.2 HYSTERESIS EFFECTS.

The results plotted ⁱⁿ figure ⁴ correspond ^to various thermal cycles ^below

and above Tc. ^{The most} striking ^{result is that the} change ^of slope ^{observed on the} heating ^curve

around 100 K when the minimum temperature

reached was 50 K (see Fig. ^{3a and} 3b) ^{can be shifted}

to higher temperature. ^{It is} actually ^{clear in} figure ⁴

Fig. ^4.

Hysteresis in the minimum backscattering yield

versus temperature ^{curves for} Ko.3Mo03 after thermal

cycling. ^Closed symbols correspond ^to increasing tempera-

ture (1, 3, 5). Open symbols correspond ^to decreasing temperature (2, 4).

that the temperature associated to this change ^of slope ^is strongly correlated to the maximum tempera-

ture reached during ^the cycle just ^before and that the

system ^{has a} memory of the previous stages. ^This

memory effect has ^some analogy ^{with that found} by

Mutka et al. (12) ^{in their} resistivity measurements.

It is also clear that this effect is present ^for temperatures larger ^than Tc ^{and the evolution of}

X min plotted ⁱⁿ figure ^{5 as a} function of time is an

evidence for metastability.

Fig. ^5.

Time-dependance of the minimum backscatter-

ing yield ^{at T}

^{200 K} (open lozenges), ^{taken on a} heating cycle. The closed triangle is the value measured at room

temperature ^{on the} virgin sample (i.e. before any thermal

cycle).

3.3 IRREVERSIBILITY EFFECTS.

In figure ^{6 the}

result for thermal cycling between 350 K and 190 K is shown. An hysteresis ^is present but, ^{in contrast}

with figure 5, ^the stabilization which was done at 190 K before heating ^the sample again ^{was uneffec-} tive, ^{as shown in the inset of} figure ^6.

Fig. ^6.

Hysteresis in the minimum backscattering yield

versus temperature ^{curve above} Tc. ^{Inset :} Time-depend-

ance of X min at ¹⁹⁰ K, ^between cycle ^{1 and 2.}

In a following step, ^{the time} dependance ^of

X min ^was investigated ^below Tc (see Fig. 7). ^As

shown in the inset of figure 7, X min ^was measured as a function of time at T

164 K. It was found that the time scale was very large ^and the stabilization could not be achieved. The sample ^was then cooled

again prior ^{to a further} heating up ^to 350 K. It appears in figure ^{7 that the maximum of} X min around T

220 K was therefore larger ^{than in most ex-} periments, indicating ^{that the} memory of ^the stabili- zation at 164 K was kept by ^the sample. ^In addition,

a strong annealing of X min is observed between 250 and 350 K which is consistent with the increase of the disorder observed during the stabilization process at 164 K.

Fig. ^7.

Irreversibility in the minimum backscattering yield ^versus temperature ^{curve. Inset :} Time-dependance

of _{X min} at 164 K corresponding ^{to the} part ^{2 of the curve.}

During ^the heating (curve 4) steps of Xmin ^{seem to}

occur. This effect was observed on other samples ^but

the results are not reported ^here.

In order to clarify ^{the time} dependant effects,

another tefnperature cycle ^{was done} using ^different

Fig. ^8.

^Minimum backscattering yield ^{as a} function of temperature. Cooling ^and heating ^{rate were 20 K/hour.}

experimental conditions : the temperature ^{was var-} ied step by step ^{as usual but a 20 min} stabilization

was achieved for each experimental point. ^The

results are plotted ⁱⁿ figure ^{8 and an increase of}

X min is clearly ^shown upon heating. ^{This effect is not}

so large ^{as in} figure 3 but is nevertheless more

important ^{than for most} experiments (not reported here) ; ^{This is} consistent with the time dependant

effects displayed ⁱⁿ figure ^7.

3.4 BEAM ALIGNED PARALLEL TO THE CLEAVAGE PLANE.

An additional experiment ^{was done} using

the geometry ^described in section 2.ii) (see Fig. 2a)

as it was assumed that the channeling ^{would be}

a priori better in this direction due to the large interplanar spacing. Actually, Xmin ^was found to be

comparable ^to previous ^{values and no} significant improvement ^was found. The results are plotted ⁱⁿ figure ^{9 and a} significant ^{increase of} Xmin is observed but, surprisingly, for decreasing temperature. ^Fur- thermore, ^{this effect was} present only for the first

cycle ^{and the} following cycles have reavealed (see Fig. 9) ^a behaviour similar to most experiments performed ^{whith the} previous geometry.

Fig. ^9.

^Minimum backscattering yield ^{as a} function of temperature. ^The channeling ^{direction was} parallel ^{to the} cleavage plane (see text). ^{Curve 1} (closed dots) ^corres- ponds ^to the first cooling ^{of the} sample. Curve 2 and 3

were measured next.

3.5 SUMMARY OF EXPERIMENTAL DATA. - As far

as the channeling yield ^is concerned, the exper- imental features can be summarized in the following

way :

-

strong ^time dependant ^{effects were observed}

above and below Tc ;

-

hysteresis ^{effects were} observed above and below T, ;

-

all these effects were reproducible ^but strongly

sample-dependant. One should note that it is also

the case for transport properties [10] ;

1784

-

an evolution was observed towards a state

where, after several cyclings, ^no strong ^anomalies

were present. ^{For most}

virgin

samples, ^no large

anomalies were present ^{and we stress} again ^{that the} sample quality ^is very important ^{as far as these} experiments ^are concerned. (The « virgin » ^state

refers to a sample prior ^to any thermal cycling, ^with

no consideration, a priori, of the defects ^concen-

tration. )

-

similar experiments performed ^of Rbo.3MoO3

revealed no significant differences with respect ^to

Ko.3Mo03.

4. Resistivity measurements.

It has been previously reported ^{that the} transport

properties ^of Ko.3oMo03 ^and Rbo,3Mo03 ^are entirely

similar [10].

Figure ^{10 shows} the thermal hysteresis ^{of the}

Ohmic resistance of a Rbo.3oMo03 sample ^{in the} temperature range 90 K-110 K. Hysteresis always

appears upon thermal cycling whatever the explored

temperature interval. These effects are more striking

in electron irradiated samples, ^as demonstrated by

Mutka et al. [12].

Fig. ^10.

Hysteresis ^{in the} resistance vs. temperature

curves for a Rbo.30MoO 3 sample ^{after thermal} cyclings.

Cooling ^or heating ^{rate :} dT/dt

^{2 K/min.}

Figure 11 shows the variation of the resistance after a thermal cycling and relaxation at two different temperatures. ^The resistance is always larger ^on heating ^{than on} cooling. ^{On the} cooling curve, the resistance increases as a function of time at a given temperature ^{while on the} heating curve, it decreases with time. Similar results have been observed by

several authors in the blue bronzes and in TaS3 [25].

Non-Debye character of the relaxation was observed

by L. Mihaly ^{et al.} [11].

After a thermal cycling the resistance measured at

190 K is still weakly ^time dependent. ^{The relative} change OR/R ^{is less than one} per ^{cent over a} period

of one hour. More pronounced ^{effects were observed}

on different samples [26]. For T > 190 K we find no

time dependence ^{of the} resistance.

Fig. ^11.

Hysteresis ⁱⁿ the resistance vs. temperature

curve for a Ko.30M003 sample after relaxation at 109 K (a)

for 30 min then at 117 K (b) ^{for 30} min ; dT/dt

2 K/min.

5. Discussion.

It is well accepted ^{that the} metastability phenomena

in incommensurate CDW systems ^{are related to the} random distribution of the pinning strengths ^{of the}

CDW by impurities ^{or other} point lattice defects.

The disorder due to random pinning ^{leads to}

glass

like » characteristics of the CDW state which are

particularly striking in pure, W-doped ^{or electron}

irradiated blue bronzes [10, 12]. ^{The CDW distorted}

state can be understood only ⁱⁿ taking ^{into account}

the large ^{number of internal} degrees ^{of freedom of}

the CDW condensate. Only deformable models can account for hysteresis ^{and various time} dependent

and memory effects. In this context, ^the existence of CDW domains and CDW domain walls has been

proposed by ^{several authors} [27]. ^{Due to the} pinning

forces the CDW condensate is always ^{in a stressed}

state. It can be viewed as an

electronic solid » [28]

which possesses its ^own defects [29].

We discuss our results in terms of CDW defects

interacting with lattice defects, first the results in the

temperature range 180 K-350 K, then those in the

temperature range 50 K-180 K.

5.1 TEMPERATURE ^INTERVAL 180 K T 350 K.

-

In the explored temperature range and using

different cooling ^{rates most} surprising is the absence of marked anomaly ⁱⁿ X min during cooling ^{from a} virgin ^{state. The} observation of an anomaly ^{in the} Young’s ^modulus [30] ^{in the vicinity} of the Peierls transition temperature ^{in common} with other CDW materials as well as anomalies in the lattice parame- ters [21] ^would suggest ^a possible change ⁱⁿ Xmin-

However, ^{when the} sample is heated from 50 K,

X min becomes strongly temperature dependent ^and

its values are much larger than those observed on

cooling especially ^{in the} vicinity of 200 K. An increase of _xm;n always corresponds ^{to an increase of}

disorder. The hysteresis disappears only ^{at ~ 350 K}

and the sample returns to its virgin ^{state. The time} dependence ^{of X min is found} only ^{on the} heating

curve. For T ~ 200 K _{X min} decreases slowly ^with

time. This clearly indicates that the system ^{has been} driven far from equilibrium. ^If X min ^was only ^{due to}

the softening ^{of a} phonon ^{mode one would not}

expect hysteresis ^{and time} dependent effects. On the other hand the temperature dependence ^of X m;n does

not follow that of the order parameter [22] ^{unlike in} TiSe2 [17]. ^{Therefore we} rather believe that Xmin is dominated by the presence of mobile defects. Since the anomaly ^of x min is rather large in view of the small ion displacements associated with the Peierls

transition, the defects involved might ^{be incommen-}

surate CDW domains slowly rearranging ^{and inside}

which the wavevector of the modulation is pinned ^to

out of equilibrium values. This hypothesis is suppor- ted by the fact that reasonable point ^{defects concen-}

trations cannot account for such a large increase of X min’

No firm conclusion on the possible shape ^{of CDW}

domains can be given, ^{even with the two} configura-

tions employed. ^In particular, ^the possible ^existence

of domains elongated parallel ^{or normal to be b-axis}

cannot be deduced from the above data, although ^it

is desirable to elucidate this point ^{for a better} understanding ^{of CDW} transport.

This domain structure acquired ^{at low} tempera- ture, ^would persist above 180 K. The hysteresis

above - 180 K may have the ^same origin ^{as the}

remanent extra 87 Rb NMR line [13]. This latter effect has been discussed in the framework of the defect density ^wave concept (DDM) developed by

Lederer et al. [31]. ^{In this} model, ^{a small fraction of}

the lattice defects order periodically ^{and are} coupled

to the CDW modulation.

A weak electrical hysteresis in the Ohmic resist-

ance also persist ^{above 180} K, AR/R ^decreases by

-

1 % over one hour at 190 K. These results seem to be also in agreement with the DDW model. In

addition, ^this picture ^seems consistent with the observations in 1 T-TaS2 ^{and in} (TaSe4)2I ^{near the}

Peierls transition of an acoustic noise emission [32]

which corresponds ^{to a release of} elastic energy.

5.2 TEMPERATURE INTERVAL 50 K T 180 K.

-

Below ~ 100 K, ^the wavevector of the modulation is temperature independent ^and extremely ^{close to}

the commensurate value 0.75 [22]. ^An incommensu- rate CDW can be described in terms of commensu- rate domains separated by ^narrow regions, ^{the so-}

called discommensurations (DC) [33], ^{where the} phase of the CDW necessary ^{to account} for the deviation of the Fermi wavevector q

2 kf ^{from the}

commensurate value changes rapidly. ^{These DC’s}

are delimited by phase dislocation loops [34]. They

can be pinned by lattice defects and can give ^{rise to} long equilibrium times if the barrier energy for

pinning ^is larger ^{than kT} [35]. The crucial role of defects on a CDW has been investigated by ^Mutka

et al. in electron irradiated blue bronzes 12 and 1 T-TaS2 [36]. ^{However in the case of blue bronzes,}

it seems that the possibility ^{of DC’s has to be ruled}

out from recent NMR work [13]. Domain walls

parallel ^{to the b-axis with an} arbitrary ^thickness

would be likely [37].

The hysteresis in X min is barely measurable be- tween 50 K and 100 K and becomes more pro- nounced only above 100 K. A possible origin ^{of the}

time dependance ^of X min would be the rearranging ^of

CDW domains or CDW phase ^defects coupled ^to

mobile lattice defects. In this context the modifi- cation of the Ohmic resistance as a function of time would arise from rearrangements ^{of CDW} phase

defects or CDW domains. The Ohmic resistance is extrinsic at low temperatures ^{and has to be attributed}

to non-stoichiometry ^and/or impurity levels in the Peierls gap. ^{If the} lattice defects are coupled ^to

CDW defects the population ^{of the} corresponding

levels in the gap would be metastable. This mechan- ism can explain qualitatively ^{the time} dependence ^of

the resistance and also the existence of precursor

voltage pulses ^or steps in V-I characteristics at low

temperature ^{observed in some} samples [7, 25]. ^In

this context, ^{it is not} surprising ^{that the} resistivity

and X min measurement ^are sample dependent.

The results show no clear cut evidence for a lock- in transition at 100 K.

6. Conclusion.

The ion channeling experiments ^and resistivity ^meas-

urements reported ^{in the} present ^{work have} provided

evidence for the presence of defects in the Peierls distorted state of the blue bronzes. The observed remarkable hysteresis ^{and time} dependence ^{of the} backscattering yield and of the Ohmic resistance have been attributed to CDW structural defects

(possibly ^walls parallel ^{to the} b-axis) coupled ^to

mobile point lattice defects (impurities, ^{alkali vacan-} cies...). Further work would involve a comparative study ^of backscattering yield ^on samples doped ^with impurities giving ^{rise to} strong ^{or weak} pinning ^of

the CDW in the sense of the Lee and Rice theory. ^In

relation with CDW defects, ^more direct methods of

study ^{of the CDW state} would be desirable.

7. Acknowledgments.

The authors wish to thank B. K. Chakraverty,

D. Feinberg, ^C. Filippini, ^G. Mihaly ^and

C. Schlenker for fruitful discussions.

1786

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