Evidence for a Peierls transition in the blue bronzes K0.30MoO 3 and Rb0.30MoO3

HAL Id: jpa-00232150

https://hal.archives-ouvertes.fr/jpa-00232150

Submitted on 1 Jan 1983

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

Evidence for a Peierls transition in the blue bronzes

K0.30MoO 3 and Rb0.30MoO3

J.P. Pouget, S. Kagoshima, C. Schlenker, J. Marcus

To cite this version:

J.P. Pouget, S. Kagoshima, C. Schlenker, J. Marcus. Evidence for a Peierls transition in the blue

bronzes K0.30MoO 3 and Rb0.30MoO3. Journal de Physique Lettres, Edp sciences, 1983, 44 (3),

pp.113-120. �10.1051/jphyslet:01983004403011300�. �jpa-00232150�

Evidence for

a

Peierls transition

in the

blue

bronzes

K0.30MoO3

and

_Rb0.30MoO3

J. P.

_Pouget,

S.

_Kagoshima

Laboratoire de _Physique des Solides _(*), Université Paris-Sud, 91405 _Orsay, France

C. Schlenker and J. Marcus

Groupe des Transitions de _{Phases, C.N.R.S.,} B.P. 166, 38042 Grenoble Cedex, France (Re~u le 10 novembre 1982, accepte le 15 decembre 1982)

Résumé. 2014 Des études de diffusion de rayons X montrent _{que la transition métal-isolant} observée

à _Tc = 180 K _sur les bronzes bleus

K0,30MoO3

et

_Rb0,30MoO3,

_composés

_{quasi unidimensionnels,} est _accompagnée d’une distorsion

_périodique

de réseau. Une _analyse de la diffusion observée

au-dessus de

_Tc,

montre _que l’instabilité de réseau concerne _{principalement} les octaèdres

_MoO6.

L’ani-sotropie de la résistivité est

_compatible

avec une bande de conduction quasi unidimensionnelle construite sur les orbitales

_hybridées

4d des

_molybdènes

et _{p03C0 des} oxygènes. Les résultats des études

structurales

_joints

aux

_propriétés

de _{transport confirment que la transition de phase} des bronzes bleus est de _type Peierls.

Abstract 2014

X-ray diffuse

_scattering

studies of the _quasi one-dimensional blue bronzes

_K0.30MoO3

and

_Rb0.30MoO3

show that the metal insulator

_phase

transition observed at

_Tc

= 180 K is

accom-panied by

a

_periodic

lattice distortion.

_Analysis

of the diffuse

_scattering

observed well above _Tc

shows that the lattice _instability involves

_{mainly MoO6}

octahedra. The _anisotropy of the electrical

resistivity

is consistent with a _quasi one-dimensional conduction band built on

_hybridized

molyb-denum 4d and oxygen p03C0 orbitals.

_Together

with the electrical

_properties,

the structural results corroborate that the _phase transition of the blue bronzes can be viewed as a Peierls distortion.

Classification Physics Abstracts

61.10 - 61.60 - 64.70K - 72.20

1. Introduction. -

The so-called blue bronzes

_KO.30Mo03

and

_Rbo.30Mo03

_belong

to the class of the _ternary transition metal oxides

_{Mx TOm}

where T is a transition metal and M an alkaline metal

_[1].

In these

_compounds,

the alkaline metal

_usually

donates its outer electron to the

transi-tion metal and thus fills

_partially

the d states which are often

_unoccupied

in the oxide

_TO~.

The

molybdenum

bronzes are intermediate between the metallic _tungsten bronzes and the vanadium bronzes where d electrons are more localized. While the red bronze

_Ko,33Mo03

is a

semiconduc-tor at all _temperatures

_[2]

and the

_purple

bronze

_KO.9M06017

is a two-dimensional metal

exhibit-ing probably

a

charge density

wave

_(C.D.W.)

driven

_phase

transition at 120 K

_[3],

the blue bronze

(*) Laboratoire associe au C.N.R.S.

L-114 JOURNAL DE PHYSIQUE - LETTRES

Ko.3oMo03

exhibits at room

_temperature

_quasi

one-dimensional

_(ID)

metallic

_properties

and a metal-semiconductor

_phase

transition at 180 K

_{[4-6]. By}

_analogy

with the

_properties

of the

_{organic charge}

transfer salts such as

_TTF-TCNQ

and the

_Krogmann

salts such as

K2Pt(CN)4Bro.33H20

(KCP)

showing

similar electronic

_anisotropies

and

_undergoing

at low

temperature structural instabilities related to the formation of C.D.W.

[7],

it has been

_suggested

that the metal insulator

_phase

transition of the blue bronze

_might

be of the Peierls _type.

Structural evidences of an electronic

instability

towards the formation of C.D.W. in the blue bronzes

_KO.30Mo03

and

_Rbo.30Mo03

are the purpose of this letter.

Ko.3oMo03 belongs

to the _{monoclinic space group}

_C2/m

_[8]

with

_twenty

formulae _per unit cell.

The

_{crystallographic}

structure is built with infinite sheets of

_Mo06

octahedra

_{separated by}

the K ions. The

_{Mo06 layers}

consist of clusters of octahedra linked

_by

corners in the

_[010]

and

_[102]

directions;

the cluster itself contains 10

_Mo06

octahedra

_{sharing edges. Packing}

of clusters is

steplike along

the

_[102]

direction and in column

_along

the

_[010]

direction. The structure can then be viewed as

containing

infinite chains of

Mo06

octahedra

sharing

corners

_along

the mono-clinic b direction. There are three different Mo sites _per

_cluster;

_only

two of them

_(Mo(2)

and

Mo(3)

in Ref.

_[8])

are involved in these infinite chains.

Chenevas,

Ghedira and Marezio have

established that 80

%

of the 4d electronic

_density

is found on the

_Mo(2)

and

_Mo(3)

sites

_[9].

Above 180

K,

Ko,3oMo0~

is a

_quasi

one-dimensional metal as shown

_by

the

anisotropy

of the electrical

_resistivity

and

_by

the observation of a metallic-like

_{plasma edge}

in the

_{optical reflectivity}

when the

_light

is

_{polarized parallel}

to the b-monoclinic axis and its absence when the

_light

is

polarized perpendicular

to b

_[5,

_6].

It has been

_{proposed previously}

that the conduction band is built on

_{hybridized pn-d}

orbitals

_involving

the

_Mo(2)06

and

_Mo(3)06

octahedra

only [6].

One should also

_point

out

that,

if the

_{stoichiometry corresponds exactly}

to the formula

Mo.3oMo03,

all

_{crystallographic}

sites are

occupied

and therefore no

crystallographic

disorder is

_expected.

The

_transport

and structural

_properties

of

_Rbo-30MOO3

are

_completely

similar to those of

KO.30Mo03 [5, 9].

2.

_Experimental

_techniques.

-

Single crystals

of the blue bronzes have been obtained

_by

the

electrolytic

reduction of a

_{M2Mo04_Mo03}

_(M

=

K,

Rb)

melt.

Crystals

_are

platelets

of

typical

size 5 x 2 x 1

mm3

_parallel

to the

(201)

cleavage plane,

with b as the

_long

direction. Structural refinements show that the chemical formulae are close to

_KO.30Mo03

and

_Rbo,3oMo03

_[9]

(and

not

_Rbo.23MOO3

as

_proposed

in Ret

_[10]).

The electrical

_conductivity

has been measured between 4.2 K and 300 K

_by

the four

_point

technique using

a

_gold

_paste

on

_freshly

cleaved

_crystals.

The

_X-ray

diffuse

_scattering

_study

was

_{performed using}

the « monochromatic Laue _»,

oscillat-ing crystal

and

_{Weissenberg photographic techniques.}

The

_CuKa

radiation

_(1.542

_A)

was obtained

by

reflection,

on a

_doubly

bent

_graphite

monochromator

(002

reflection),

of the

_X-ray

beam

pro-duced

_by

a 12 KW

_{Rigaku rotating}

anode

_generator.

The

_crystals

were cooled

by

a

regulated

nitrogen

flux and

_{temperatures,}

between 295 K and 110

_K,

were measured

_by

a

thermocouple

glued

on the

_sample.

X-ray

patterns

were

_{analysed using}

a

Joyce-Loeble

microdensitometer, and

the half-width at

half maximum of the diffuse

_scattering,

shown in

_figure

4,

was obtained from such

reading.

3. Results. -

_Figure

1 shows a

_typical

_temperature

_dependence

of the electrical

resistivity

of

Rbo.3oMo03

measured

_along

three

_orthogonal

_{crystallographic}

directions. Similar results have been obtained with

_Ko.3oMo03

[5].

Rbo,3oMo03

shows a metal-semiconductor

phase

transition

at the same critical

_temperature,

Tc

=

180

K,

_as

KO.30Mo03

[5].

In both materials the

highest

Fig. 1. -

Electrical _{resistivity (logarithmic} scale) of

_Rbo.30Mo03

versus _{reciprocal temperature,} measured

along b, along [102] and

_{perpendicular}

to the

₍₂₀₁₎

_plane.

the

_[102]

direction and the smallest

_conductivity

in the direction

_{perpendicular}

to the infinite

layers

(201 )

of oxygen

octahedra,

as

expected

from the structural

anisotropy.

At room

_temperature

the ratio of resistivities measured in the

(201)

plane,

p[102]/p~,

is ~ 76 for

Rbo.30Mo03

and ~ 33

for

_{Ko 3oMo03}

and the ratio between the smallest and the

_highest

_{resistivities,}

_{pi Pb’}

is ~ 1450 in

_Rbo.30Mo03

and", 580 in

_Ko,30M0O3.

These

_anisotropies

are

_comparable

to those

found

in the

_quasi

one-dimensional conductor

_HMTSF-TCNQ

_[11].

X-ray

evidence of a C.D.W.

_{instability leading}

to a

_periodic

lattice distortion below

_7~

in

K.o 3oMoOg

and

_Rbo.30Mo03

are well summarized

by

the two

_X-ray

_patterns shown in

_figure

2. At room _temperature

_(R.T.),

the « monochromatic Laue »

_X-ray

_pattern

of

_figure

2a shows

clearly

thin diffuse _segments

_{perpendicular}

to the b direction and situated at

_{+ qb}

b* from

_layers

of

_Bragg

reflections with odd K

indices;

with for both

_compounds

the same incommensurate wave

_{vector qh}

= 0.28 ± 0.01 b*. Moreover

figure

2a shows that this

scattering

does _not

corres-pond

to diffuse sheets in the

_reciprocal

space. More

clearly

X-ray

Weissenberg

[h,

1

_{± qb,}

_l]

reciprocal planes

reveal broad diffuse streaks

_parallel

to the 2 a* - c*

_reciprocal

direction

(per-pendicular

to the octahedra

_layers),

as

_{schematically}

shown in

_figure

3a.

_Weissenberg

_X-ray

patterns

taken with

_KO.30Mo03

and

_Rbo.30Mo03

show a similar wave vector

_dependence

of the diffuse

_scattering.

L-116 JOURNAL DE PHYSIQUE - LETTRES

Fig. 2. 2013 « _{Monochromatic Laue » (a)} and _{oscillating crystal (0 - 100) (b) X-ray patterns} from

Rbo.30Mo03

at 295 K and

_KO.30Mo03

at 110 K

_{respectively.}

Arrows _point to the diffuse _{elongated platelets} in (a) and

to satellite reflections in _(b). On both _patterns the b* direction is vertical. On the X-ray pattern (a), note

the broad diffuse lines of weaker _intensity directed along b* and _containing the diffuse _platelets and the

main _Bragg reflections with even K indices. Similar X-ray patterns were obtained from

_KO.30Mo03

at

295 K and

_Rbo.30Mo03

at 110 K.

Fig. 3. - Schematic

representation

of the

_position

in _reciprocal space of diffuse

platelets

for T > _{T ~ (a)}

and satellites reflections for T _T~

_(b).

Solid circles _represent the main Bragg reflections and open circles

A further

_inspection

of the

_X-ray

_pattern of

_figure

2a shows that the diffuse segments

corres-pond

to a

_{strengthening}

at ±

( 1 -

qb)

b* = ± 0.72 b* of the

_intensity

of an

_anisotropic

thermal diffuse

_{scattering surrounding Bragg}

reflections with even H and K indices. This thermal

scatter-ing

also _appears under the form of streaks directed in the 2 a* - c* direction in the

_[h,

_0,

_l]

recipro-cal

_plane,

and shows the same

intensity dependence

with h and I as the diffuse streaks observed in the

_{[A, 1}

±

qb,1] planes.

For these reasons, one should more

_likely

consider the diffuse

_scattering

as satellite streaks situated at ±

( 1

-

qh)

b* from the above mentioned

_Bragg

reflections.

The half-width at half maximum

_{(H. W. H. M.),}

d, of the diffuse streaks has been measured

_along

the

_[010]

and

_{[ 102]}

directions. Their _temperature

_{dependence, again}

identical for both

_compounds,

is

_{given together}

with the

_experimental

resolution in

_figure

4. This shows that above

_7~,

_{d [ 102]}

is several times

_larger

than

_{A [010],}

so that the diffuse streaks are in

_shape

of

_{elongated platelets}

_(or

flattened

_cigars).

When T

_approaches

_T~

from

above,

d [ 102]

reaches the

_experimental

resolution with a _square root-like

_law,

while A

_[010]

reaches

_linearly

the

_experimental

resolution. Somewhat similar _temperature

_dependence

of intrachain and interchain widths of the diffuse

scattering

have been observed in the 1 D

_organic

conductor

_{TMTSF-DMTCNQ [12].}

Fig. 4. -

Temperature dependence

of the half-width at half maximum, J, of the diffuse _{platelets along} the [010] and [102] directions for

_KO.30Mo03

(solid circles) and

_Rbo.30Mo03

_(open triangles). The

_experimental

resolutiori in these two directions is also indicated.

Below

_Tr

180

_K,

well defined

(i.e.

resolution

_limited)

reflections are observed

(Fig.

2b).

From their

_position

in

_reciprocal

_space

_(Fig.

_{3b) they}

can be considered as the « condensation of the

_high

_temperature

diffuse streaks into satellite reflections. Their narrowness shows that

they

correspond

to a

_long

_range

_periodic

lattice distortion.

The

_X-ray

_pattern

of

_figure

2b shows that in addition to

_strong

satellite reflections at

_{± q,,}

b* from

_layers

of

_Bragg

reflections with odd K

_indices,

weak satellite reflections can be observed at

± qb

b* from

_layers

of

_Bragg

reflections with even K indices. These last reflections are announced

by

streaks of _very weak

_intensity

below 200 K.

_{X-ray Weissenberg [h,}

_qb,

_{l], [A, 1}

_{± qb,}

_fl

_reciprocal

planes

locate satellite reflections at a wave vector _q =

[0,

±

(1 -

_~),

_1/2]

from the allowed main

L-118 JOURNAL DE _{PHYSIQUE -} LETTRES

temperature

precursor

scattering

and has the same wave vector

_dependence

for the two blue bronzes. At 110 K the incommensurate wave vector _{component q,,} = 0.26 ± 0.01 b* _appears to

have

_slightly

decreased from its value at R.T.

4. Discussion. - The main result of this

_study

is to show that the blue bronzes

_undergo

a

periodic

lattice distortion at the same

_temperature

₄

= 180

K,

where _a metal insulator

phase

transition has been

_{already reported}

on _transport

_[4,

_5]

and

_{optical [6] properties.}

This

_phase

transition can also be detected

_by

an

_anomaly

on

_specific

heat data

_{[ 13]}

and

_by

a decrease of the

magnetic susceptibility [5].

This _suggests that a

_coupling

between the lattice and electronic

degrees

of freedom are

_responsible

for the

_phase

transition. Further

_proofs

will be

_{brought by}

the

analysis

of the structural data

_given

below.

The existence of the same critical _temperatures for both

_Ko.3oMo03

and

_{Rbo 3oMoO~}

as well as the same

_temperature

_dependence

of precursor

_{scattering (Fig.}

_4),

show that the alkaline metals are not the

_driving

force of the

_phlse

transition. In

_addition,

the same wave vector

depen-dence of the structure factor of the satellite reflections and of the precursor

scattering,

in

_spite

of the

_large

difference of atomic form factor between K and

Rb,

also shows that the alkaline metals are not

_greatly

affected

_by

the

_phase

transition.

Furthermore,

it is

_possible

to

_find,

_by

_symmetry

considerations,

which

_Mo06

octahedra are

mainly

involved in the structural distortion. Let us first examine the data obtained at room

temperature.

The observation of diffuse streaks

_parallel

to the 2 a* - c*

_reciprocal

direction shows that there is no correlation in atomic

_{displacements}

between

_neighbouring

_{(201 )}

sheets

_(octahedra

layers),

as

expected by

the structural

_anisotropy

of the blue bronze. In these sheet

directions,

the observation of diffuse

_scattering

at ±

(I -

qb)

b* from the allowed main

_Bragg

reflections with even indices

_(Fig.

3a)

means that the modulation involves entities with

b/2

and

a/2

+ c

periodi-cities.

_Considering,

for the moment, the ideal Mo bronze

_K3Mo10030

_(see

the

_crystal

structures

described in

_Figs.

6c and 6d in Ref.

_[8]),

it is _easy to

_reproduce

this

_{periodicity by ignoring}

the K

atoms and

_Mo(l)06

octahedra. The

_pseudo-unit

cell then includes 2

_Mo(2)06

and 2

_Mo(3)06

octahedra,

which forms the infinite chains directed

_along

b. The

_{pseudo-symmetry}

of the diffuse

scattering

can then be

_explained

if the lattice modulation involves

_mainly

these octahedra. At room

_temperature

when the atomic

_{displacements}

are

_small,

deviations from the ideal structure

lead to

_secondary

effects.

Moreover,

when the

_amplitude

of the atomic

_{displacements}

_increases,

when

₄

is

_approached

from

above,

deviations from the ideal structure, as well as

_possible

dis-tortion of

_Mo(l)0~

octahedra _may become relevant and break the

_high

_temperature

pseudo-symmetry.

But below 200 K the

_large

difference between the intensities of the satellite reflections around the main

_Bragg

reflections with even K indices and those with odd K

_indices,

still

_keep

the

memory of this

pseudo-symmetry.

As the

_Mo(2)-06

and

_Mo(3)-06

octahedra are involved in the

_quasi

one-dimensional

conduc-tion

band,

it is natural to _propose,

_{by analogy}

with other ID

conductors,

that the

_phase

transition is Peierls _type, as it was

already suggested

in reference

[6].

In 1 D

conductors,

coupling

between the 2

_kF

electronic

_instability

and some

_special

low

_{frequency phonon}

modes leads to the forma-tion of C.D.W. which fluctuate

_incoherently

from chain to

_chain,

at a

_phonon

_frequency

above

7~,

and thus

_gives

rise to

_X-ray

diffuse sheets

_[7].

Below

_7~

these C.D.W. become static and

_couple

three-dimensionally, leading

to new

_{superlattice periodicities.}

The structural data obtained with the blue bronze present some difference from those observed on more conventional ID conductors like

_TTF-TCNQ

or KCP

_{[7]. They}

are

_{critically analysed}

below.

The _strongest difference is that _precursor effects observed above

_Tc

are not

_exactly

in

_shape

of diffuse sheets often observed in the

_region

of 1D C.D.W. fluctuations.

_Figure

4 shows that between R.T. and

_Tc

the diffuse

_scattering

appears as

_{elongated platelets}

with a much

_larger

width in the

_[102]

direction than in the

[010]

direction. For

_example

at

_R.T.,

the H.W.H.M. of the streak

(8

pseudo-unit

or octahedra

_spacing)

and

_{~o2] ~}

8

A (less

than 1

_{pseudo-unit spacing).}

This shows that at R.T. the C.D.W. fluctuations are well

_established

in the b direction and exhibit weak correlations

_along

_[102]

and no correlation between

(201)

sheets. In this _respect structural corre-lation

_lengths

follow the

_anisotropy

of the electrical

_{resistivity (Fig.}

_1).

Diffuse streaks

_might

be viewed either as a Kohn

_anomaly

associated to a 2D Fermi surface or to the

_coupling

between

ID C. D. W. Observation of a different

_temperature

dependence

between

~2] ~ ~/TT 2013

_{T ~}

_(mean

field

_behaviour)

and

_{ç[oto] ""}

_{(T -}

_{Tc) (quasi}

one ID fluctuation

_regime)

seems to accredite the latter

_possibility.

Similar

_temperature

_dependence

has been

_reported

in the 1 D conductor

TMTSF-DMTCNQ

[12]

where lateral

_coupling

between C.D.W. fluctuations is observed in a

_large

tem-perature range above

7~,

and is

_{predicted by}

a theoretical treatment of ID

_coupled

fluctua-tions

_[14].

The Peierls transition is related to the

_opening

of a _gap at the Fermi wave vectors _±

_kF

of a ID

band structure. This leads to a

_{semiconducting}

state. The structural data obtained at room tem-perature, with _{1 - qb} = 0.72 b* _are consistent either with

2 kF

= 0.72 b*

(=

0.36 x 2 b* if one considers the

_{pseudo-periodicity}

_b/2

discussed

_previously)

or with

2 kF -

_{1.28 b* ( =}

0.64 x 2

_b*).

These values should be

compared

with

_possible

values of 2

_kF

deduced from the chemical formula and the band structure.

_{Unfortunately,}

the band structure

_{of Ko.3oMo03}

is not

_accurately

known and

_only

_speculations

can be made at this

_point.

Let us assume in a first

_{approximation}

that all electrons

_given

_by

the alkaline metals

_participate

to the conduction

band,

so that there is 1.5 con-duction electron _per

_pseudo-unit

cell

_{(pseudo-cluster)}

of 2

_Mo(2)06

+ 2

_Mo(3)06

octahedra. A value of 2

_kF

= 0.75 b*

(=

0.375 x 2

_b*)

would then

_correspond

to a ID conduction band with a

degeneracy

of 4

(orbital

and

spin degeneracy

of

2).

It had been

_previously

_proposed

that the band

structure of the Mo bronzes should be similar to that of

_Re03

_[6].

But in the blue

_bronzes,

the orbital

_degeneracy

of the d states has been lifted

_by

the low

_crystal

symmetry and the distortion of the

_Mo06

octahedron,

so that the

corresponding

orbital

degeneracy

of the

hybridized d-p

states should be I for a

_Mo03

molecule.

_Furthermore,

in the

_{pseudo-cluster}

of 4

_Mo06

_octahedra,

one

_expects

the formation of molecular orbitals and a further removal of

degeneracy, possibly

leading

to 2 molecular states of lower _energy

_(instead

of 4 orbital

_states)

for the

_pseudo-unit

_cell,

which

_might

be

_{partially occupied}

_by

electrons

_{given by}

the alkaline metals. This would be the case if the

_splitting

of the

_subbands,

due both to the distortion of the relevant

_Mo06

octahedra and to the existence of the

clusters,

is

_{large enough}

to _prevent subbands

_overlap

at the Fermi level. The

_experimental

value

_{of 2 kF}

= 0.72 b* could be consistent with this

picture.

One should also

notice that the deviation of 2

_{k’ F}

from the theoretical value of 0.75 b* can be accounted for

_by

several mechanisms.

First,

the

_{stoichiometry}

of the blue bronze may be

slightly

different from

KO.30Mo03. Secondly,

the number of electrons _per

_{pseudo-cluster}

should be

_slightly

smaller than 1.5 as some d-electron

_density,

found on the

_Mo(l)

sites

_{[9], might belong}

to localized states not

_contributing

to the

_conduction

band.

Let us now describe the

_coupling

between C.D.W. fluctuations. At room _temperature, the observation of diffuse streaks distant of ±

( 1

-

qb)

b* from the main

_Bragg

reflections

_{(Fig. 3a)}

means that there is

_{in-phase ordering}

of the modulation of the

_pseudo-unit

cells

_along

the

_[102]

direction. This

_periodicity

is difficult to understand if there is one C.D.W. _per

_{pseudo-cluster.}

In ID conductors built with one

_type

of chain

_only,

like

_KCP,

_nesting

effects on the Fermi sur-face

_[15]

as well as Coulomb interactions between C.D.W.

_[16]

lead to an

_{antiphase ordering.}

A suitable

explanation

for the blue bronzes

_might

be that there is

_{antiphase ordering}

between 2 C.D.W. _per

_{pseudo-cluster.}

Each C.D.W.

_{might belong}

to a chain built with entities of two

octahedra :

_Mo(2)06

+

_Mo(3)06.

This

_{might provide}

a

_physical

basis for the two-fold orbital

degeneracy

per

pseudo-cluster, suggested

above. In addition the

_strong

_coupling

between these chains

_might

also

_explain

the short range order observed in the

[102]

direction.

At

_T~,

diffuse streaks « _{condense » into satellite reflections}

_midway

_{between the main}

_Bragg

reflections as shown in

_figure

3b. This

_{ordering corresponds}

to a

_doubling

of the lattice

periodicity

L-120 JOURNAL DE _{PHYSIQUE -} LETTRES

in the c direction and no

_change

of

_periodicity

_along

a. As the

layers

of octahedra are

_parallel

to the

[ 102]

direction,

an

_{antiphase ordering}

of the sheets of octahedra

_explains

also the lateral

periodi-cities observed below

_7~.

Because of the _strong

_anisotropy

of the electron _gas, this

_periodicity

can be more

_likely

understood as

_being

that

_minimizing

the Coulomb interaction between the C.D.W.

_belonging

to

_neighbouring

sheets.

A

_specific

feature of the diffuse

_scattering

of the blue

_bronzes,

_compared

to that

_{of TTF-TCNQ}

and KCP

_[7],

is that the diffuse streaks _appear in Brillouin zones where a thermal diffuse

_scattering

in broad sheets

_{perpendicular}

to the

_[102]

direction is observed

_(see

_Fig.

_2a).

This

_might

indicate that the Kohn

_anomaly

takes

_place

in a

_valley

of softer

_phonons.

(The

thermal

_scattering

observed around

_strong

_Bragg

reflections is

_probably

the

_signature

of softer

_phonons

with acoustic-like

displacements).

It is thus

_tempting

to

_suggest

that,

in the blue

_bronzes,

the Kohn

_anomaly

is forced to

_develop

in a

_{pre-existing valley}

of low

_{frequency phonons,}

where there is a _strong

elec-tron

_{phonon coupling}

as a _consequence of

larger

atomic

displacements.

Inelastic neutron

scatter-ing

experiments

are now _necessary to corroborate the

_picture.

Acknowledgments. -

The authors wish to thank E. Bervas for the electrical

_resistivity

measu-rements, J. Dumas and C.

Filippini

for

_extremely

_helpful

discussions and M. Marezio for

stimulat-ing

comments.

References

[1] HAGENMULLER, P., Prog. Solid State Chem. 5 ₍₁₉₇₂₎ 71.

[2] BOUCHARD Jr., G. H., PERLSTEIN, J. and SIENKO, M. _{J., Inorg.} Chem. 6 ₍₁₉₆₇₎ 1682.

[3] BUDER, R., DEVENYI, J., DUMAS, J., MARCUS, J., MERCIER, J. and _SCHLENKER, C., J. _Physique Lett. 43 (1982) L-59.

[4] FOGLE, W. and PERLSTEIN, J. M., Phys. Rev. B 6 (1972) 1402.

[5] BRUSETTI, R., CHAKRAVERTY, B. K., DEVENYI, J., DUMAS, J., MARCUS, J. and SCHLENKER, C., Recent Developments in Cond. Matter _Phys., Ed. J. T. Devreese et al. _(Plenum) Vol. _{2, 1982,} p. 181.

BUDER, R., DUMAS, J., MARCUS, J., MERCIER, J. and _{SCHLENKER, C., Congrès} de la Soc. Française de Phys., Bulletin des Résumés, Clermont-Ferrand 1981, p. 204.

[6]

TRAVAGLINI, G., WACHTER, P., MARCUS, J. and SCHLENKER, C., Solid State Commun. 37 ₍₁₉₈₁₎ 599. [7] See for

_{example Highly Conducting}

One Dimensional Solids, edited _by J. T. Devreese et al. _(Plenum)

1979.

[8] GRAHAM, J. and WADSLEY, A. D., Acta

_Crystallogr.

20 (1966) 93.

[9]

CHENAVAS, J., GHEDIRA, M. and MAREZIO, M., Private comm. and to be published. See also Bull. Amer. Phys. Soc. 26 (1981) 449.

[10]

STROBEL, P. and GREENBLATT, M., J. Solid State Chem. 36 ₍₁₉₈₁₎ 331.

[11]

COOPER, J. R., JEROME, D., ETEMAD, S. and _ENGLER, E. M., Solid State Commun. 22 (1977) 257. [12] POUGET, J. P., Chemica _Scripta 17 ₍₁₉₈₁₎ 85.

FORRO, L., ZUPPIROLI, L., POUGET, J. P. and BECHGAARD, K., Phys. Rev. B _{(in press).}

[13] DUMAS, J., FILIPPINI, C., MARCUS, J. and SCHLENKER, C. (to be

_published).

[14]

See for

_example

DIETERICH, W., Adv.

_Phys.

25 ₍₁₉₇₆₎ 615.

[15] HOROWITZ, B., GUTFREUND, M. and WEGER, M.,

Phys.

Rev. B 12 (1975) 3174.

[16]

SAUB, K., BARISIC, S. and _{FRIEDEL, J., Phys.} Lett. 56A ₍₁₉₇₆₎ 302.