Triplet spin excitons, structure and conductivity of (TMN)3(AsF 6)2 and (TMN)3(ClO4)2 , the radical cation salts of tetramethoxynaphthalene

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Triplet spin excitons, structure and conductivity of (TMN)3(AsF 6)2 and (TMN)3(ClO4)2 , the radical

cation salts of tetramethoxynaphthalene

J. Krzystek, J.U. von Schütz, G. Ahlgren, J. Hellberg, S. Söderholm, G.

Olovsson

To cite this version:

J. Krzystek, J.U. von Schütz, G. Ahlgren, J. Hellberg, S. Söderholm, et al.. Triplet spin excitons, structure and conductivity of (TMN)3(AsF 6)2 and (TMN)3(ClO4)2 , the radical cation salts of tetramethoxynaphthalene. Journal de Physique, 1986, 47 (6), pp.1021-1027.

�10.1051/jphys:019860047060102100�. �jpa-00210278�

Triplet spin excitons,

^structure

^and conductivity ^of (TMN)3(AsF6)2

and (TMN)3(ClO4)2, ^the radical cation salts of tetramethoxynaphthalene

J.

Krzystek(*),

^{J. U. von} Schütz, ^G.

Ahlgren (+),

^J.

Hellberg (+),

^S.

Söderholm(**)

^{and G.}

Olovsson(+ +)

3. Physikalisches Institut, Universitat Stuttgart, Pfaffenwaldring ^{57, D-7000} Stuttgart 80, ^West Germany (+) Department ^of Organic Chemistry, ^The Royal ^{Institute of} Technology, ^S-10044 Stockholm, ^Sweden (**) Department ^of Physics III, ^The Royal ^{Institute of} Technology, ^S-10044 Stockholm, ^Sweden

(+ +) ^{Institute of} Chemistry, University ^of Uppsala, ^{Box 531, S-75121} Uppsala, ^Sweden (Reçu le 21 octobre 1985, accepté ^{le 28} janvier 1986)

Résumé. ²⁰¹⁴ La stoechiométrie 3:2 du sel (TMN)3(AsF6)2 ^{se manifeste dans une} configuration ^en pile ^altemée

avec des groupes de 3 (TMN) conduisant à un état fondamental diamagnétique. ^{Les excitons de} spins triplets

sont accessibles avec des énergies d’activation de 0,19 ^eV (AsF6) ^et 0,28 ^eV (ClO4). ^Les paramètres respectifs ^de

structure fine sont D ⁼ ± 0,00667 cm-1, ^E = ~ 0,00117 ^{cm-1 et D =} ± 0,00829 cm-1, ^{E =} ~ 0,00090 cm-1.

L’échange ^de spin ^{conduit à un} rétrécissement de raie à haute température. ^La conductivité est très faible

(03B4(300

K)) ~ 10-4 03A9-1 cm-1

(TMN)3(AsF6)2)

^et peut ^{résulter de} porteurs ^de charge ^activés thermiquement

à partir ^{d’états de défauts et à travers la bande} interdite, respectivement.

Abstract. ²⁰¹⁴ The 3 : 2 stoichiometry ^{of the} (TMN)3(AsF6)2 ^{salt is reflected in an} alternating ^stack configuration

with TMN-triads resulting ^{in a} diamagnetic ground ^state. Triplet spin ^{excitons are} accessible with activation

energies ^{of 0.19 eV} (AsF6) ^{and 0.28 eV} (ClO4). ^The respective ^{fine structure} parameters ^{are D =} ± 0.00667 cm-1,

E = ~ ^0.00117 cm-1 and D ⁼ ± 0.00829 cm-1, ^{E =} ~ ^0.00090 cm-1. Spin exchange ^{leads to line} narrowing

at higher temperatures. ^The conductivity ^is quite ^low

(03B4(300

K) ~ 10-403A9-1 cm-1

(TMN)3(AsF6)2)

^and may result from charge carriers, thermally activated from defect states and across the band _gap, respectively.

Classification

Physics ^Abstracts

76.30 ^- 76.30R

1. Introduction.

Radical cation salts

(rcs)

^of

naphthalene

^{have raised}

considerable interest

recently

^{in two} respects :

they

started the renascence of ^rcs of aromatic

hydro-

carbons

[1-3]

^and

they

^exhibited

high conductivity

and the narrowest ESR line

(2.5 mG)

^{found so far}

[4].

But _among further

interesting

^{features like}

phase

^tran-

sitions which _may lift a dimerization in a narrow tem-

perature range, there is ^one unwanted property : ^the (*) On leave from the Institute of Physics, ^{Polish Aca-} demy ^of Sciences, Warsaw, ^Poland.

crystals

^{are not stable !}

They

^{have to be grown at}

about - 40 OC and

decompose

within minutes at room temperature. ^{This has} limited the

exploration

^so

far

considerably.

After the successful

synthesis

^of

tetramethoxy- naphthalene,

^this compound ^{was used as} substitution of the

simple naphthalene

^{in the rcs.}

Beautiful

crystals

^were

obtained,

^{but - nature does}

not yet ^{allow us} the dictation of the

crystal

structure - trimers of TMN molecules were found

having instead

of metallic

properties

^{and doublet ESR}

signals, semiconducting

^{behaviour and}

triplet

^ESR spectra.

These results are

presented

^{in the}

following.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019860047060102100

1022

2.

Experimental.

2.1 SYNTHESIS. 2013 The precursor 2,

6-dibromo-3,

^7-

dimethoxynaphthalene 1

^was

synthesized by

^literature

methods

[5].

Treating

1 with sodium methoxide in N,N-dime-

thylformamide

in the presence of _copper

(I)

^iodide

gave 2, 3, 6,

7-tetramethoxynaphthalene 2 (TMN)

ⁱⁿ

71

% yield

^after

chromatography.

Mp ²⁴⁴ -C, ^Lit.

[6].

After further

purification by recrystallization

^and

gradient

sublimation,

2 was electrolyzed

^{in a 0.05 M}

solution of

(nBU)4NAsF6/(nBU)4CIO4

in dichloro- methane. The

electrolysis

^was

performed

^{at a constant}

current of 4-6

gA

^{and at 4-8 OC}

(refridgerator).

^The

black

shining crystals (approximate

^{size 2 x 0.5 x}

0.25

mm’)

^{were collected from the}

slightly

^blue

solution

by

filtration, washed with dichloromethane,

and dried. The

crystals

^{seem to have limited}

stability

in ambient

atmosphere;

^{a grey «} beard » is thus

developed.

^No

degradation

^is observed if stored under an argon

atmosphere. Elementary analysis

^gave

a 3 : 2

composition

^{with no chlorine} present

(

^0.3

%).

Cyclovoltammetry

^{of TMN revealed a}

quasirever-

sible

peak

^with

E1/2

^{= 1.25 V}

(vs SCE). Sample

concentration was 2 mM in 0.15 M

(nBu)4NBF4

ⁱⁿ

dichloromethane. Scan rate was

100 mV/s.

2.2 CRYSTAL STRUCTURE. ^-

Single crystal

X-ray ^dif- fraction

experiment (MoKa-radiation)

gave ^a triclinic

structure Pl, a ^{= 10.156}

(2),

b = 11.032

(1),

^{c =}

11.915 (1) A, cx. =

^77.84

(1), P

^{= 65.94}

(1),

y ⁼ 76.63

(1),

V ⁼

1176 A3,

^{Z =} 2, ^T = 295 K. A refinement of 404

independent

parameters

using

^{4 263}

unique

reflexions

(sin 9/2 =

^0.717

A -1)

gave ^an agreement- factor

R(F 2)

^{= 5.0}

%

^and

Q(C-C)

^{= 0.003}

A.

The

asymmetric

unit consists of

1/2

^TMN

(centro

symme-

tric),

^{1 TMN}

(general position)

^{and 1}

AsF6 (general position).

^{The TMN in}

general position

^is

nonplanar

with a dihedral

angle

^{of 1.50 between the two least-}

squares

planes

ⁱⁿ the carbon

rings.

^{The TMN mole-}

cules are stacked like

poker-chips along

the a-axis with

AsF6

between the stacks in the c-direction

(Fig. 1).

The

crystal

^structure determination

of(TMN)3(CI04)2

is in progress.

Preliminary

^data

give

^{triclinic structure}

P1 or

P1, a

^{= 9.835}

_(6),

^{b = 10.963}

_(4),

^{c = 11.797}

₍₄₎ A,

^{ex = 63.92}

(3), P

^{= 68.12}

(4),

y ⁼ 80.43

(4)0,

^{V =}

1060 A3,

T = 295 K. The

similarity

of the cell parameters ^{to those of}

(TMN)3 (AsF6)2

^{also indicates}

a

similarity

ⁱⁿ structure. The unit cell volume of

(TMN)3(CI04)2

is somewhat smaller than that of

(TMN)3(AsF6)2.

^{This is}

expected

^{as the}

C104

^{ion is}

smaller than the

AsF6

^{ion. The detailed structure of}

(TMN)3(AsF6)2

^and

(TMN)3(C104)2

^{will be} pu- blished in Acta

Cryst. by

^{G. Olovsson.}

Fig. 1. ^- Crystal ^{structure of} (TMN)3(AsF6)2. ^{The TMN}

molecules and AsF6 ^{ions form} separate ^stacks along ^the

a-axis. Triads of TMNt-TMN-TMNt are formed. The dotted lines represent the intrastack contacts 3.0 A.

These are H... 0 contacts and are in the range 2.55-2.98 (3) A.

2. 3 ESR. - The ESR

experiments

^were

performed

with a Varian E-type ^X-band spectrometer

Century

line E 109

equipped

^{with an Oxford} Instruments variable temperature unit ESR 109

working

^{in the}

range of 3.8-300 K. The

crystals

^{were mounted on a}

goniometer,

the rotation axis was

always perpendi-

cular to the

magnetic

^field.

2.4 DC-CONDUCTIVITY. - The measurements of the

conductivity

^were

performed

^{with a} standard four-

probe technique.

^{Contacts were} made with silver paste. ^{The contact}

integrity

^{was checked}

by

^{the ratio}

between the unnested and the nested

voltage,

^as

defined

by

Schafer et al.

[7]. Samples

^{with a ratio less}

than 10

%

^were chosen. The

samples

^{were cooled}

by

an Air Products closed

cycle refridgerator

system

working

^{in the range 10-300 K.}

3. Results and evaluation.

3.1

(TMN)3(AsF6)2

CRYSTAL. - The ESR spectrum of the

(TMN)3(AsF 6)2 crystals

^is observable at tempe-

ratures

higher

^{than 100 K}

(Fig. 2).

^At T _ ^{140 K it} consists of two Lorentzian formed

signals, approxi- mately

250 mG broad and

symmetrically placed

around

the g

^{= 2}

point.

^{For most}

crystal

orientations

a

weak g

^{= 2}

signal

^{of the same linewidth is observed.}

The

signals

^{show no trace of}

hyperfine

^structure.

The

signals

^are

highly anisotropic.

^Their

angular dependence (Fig. 3)

^{forms a} pattern

typical

^{for a}

triplet (S

⁼

1)

^state.

Anticipating

^the discussion, ^{it was} therefore

possible

^to attribute them to

triplet spin

excitons and to

analyse

^their

angular dependence

using ^a standard hamiltonian for S ⁼ 1 :

Fig. ^2. - (TMN)3(AsF6)2 : ^ESR spectra ^{at different tem-} peratures. ^The position ^{of the} crystal ^{relative to the} magnetic

field was chosen in a way that it is possible ^{to follow the} exchange ^{effects in} both, ^slow exchange ^{and fast} exchange

limits (see discussion).

Fig. ^{3. -} (TMN)3(AsF 6)2 : angular dependence ^{of the}

ESR signals ^{at 125 K.} a) ^{Rotation about} the needle axis;

b) ^{Rotation about an axis} perpendicular ^{to it. The crosses} represent ^the experimental points, ^the curves were plotted using ^best fitted zfs parameters : ^{D = ±} 0.00667, ^{E =} ± 0.00117 cm-1 (Rf-frequency vrf ⁼ 8.976 GHz).

By fitting

^the

zero-field-splitting (zfs)

parameters ^D and E to the

experimental points, following

^values

were obtained at 125 K :

The ESR spectrum

changes considerably

with increas-

ing

temperature.

Starting

from about 140 K the lines

become wider and

asymmetric, approach

^{each other,}

merge into one

signal

^and

undergo subsequent narrowing,

^{so that at room} temperature

only

^one

signal

^{is observed}

(Fig. 2).

^Its linewidth varies between 250 and 500 mG

depending

^on orientation.

To determine the temperature

dependence

^of

spin

carrier concentration, ^{the most} convenient

procedure

is to follow the temperature

changes

^of

signal

^inten-

sity

^{for the} orientation where the two

low-temperature signals

^coincide

(this

orientation will be later called

«

crossing point »).

^The

single

^observed

signal

^remains

then

symmetrical

^and

Lorentzian-shaped,

^{and its}

intensity

^can be estimated

according

^{to the formula :}

with Y ⁼

signal amplitude and ABpp

⁼

peak-to-peak

linewidth.

The

integral signal intensity

increases with tempe-

rature

(Fig. 4)

^{and can be fitted to an}

exponential expression :

For

(TMN)3(AsF 6)2

^{the least mean} square deviation between the

experiment

and the fit is obtained for an

activation energy of 0.19 ± ^{0.01 eV.}

The room temperature

conductivity along

^the

long

axis of the

crystals,

^{i.e. the} a-axis, ^{is 5 x 10- 4}

(Qem) - 1

The temperature

dependence

^{of the}

conductivity

^is

characteristic for ^a semi-conductor with a

slight upward

^curvature

(Fig. 5).

^An attempt ^{to fit the}

high

temperature part, ^above 180 K, ^{to a constant acti-} vation _energy

gives

^{AE = 0.21} ± ^{0.02 eV. Conducti-}

vity

measurements in other directions could not be

performed

^{so far due to} the smallness of the

crystals.

Fig. ^{4. -}

(TMN)3(AsF6)2 :

temperature dependence ^{of the}

ESR signal intensity

multiplied

by ^the temperature. ^The

crosses represent experimental ^values while the straight

line corresponds ^{to an} activation energy of 0.192’eV.

1024

Fig. ^{5. -} Normalized DC-conductivity ^of (TMN)3(AsF6)z along ^{the needle axis.} Qrt ⁼ 5 x 10-4 Q-1 cm-1.

3.2

(TMN)3(C’04)2-

^{- The ESR} spectrum ^{of the}

(TMN)3(CI04)2 crystals

^is observable at temperatures above 170 K. It consists of two Lorentzian formed

signals,

^{about 700 mG broad and}

symmetrically placed around g

^{= 2.} Also, ^{there is}

usually

^{a weak}

g ⁼ 2

signal

present

(Fig. 6).

The dominant

signals

^are

typical

^for

triplet

^{excitons, and show}

angular depen-

dence similar to that of

(TMN)3(AsF 6)2 (Fig. 7).

^The

zfs parameters ^{at 230 K} obtained from the

experiments

are :

D ⁼ ± ^0.00829

(2)

^{and E = + 0.00090}

(2)

^cm-1.

Upon increasing

temperature ^the

integral signal intensity

increases with an activation energy of 0.28 ± ^{0.01 eV}

(Fig. 8).

Fig. ^{6. -} (TMN)3(AsF6)2 : ^ESR spectrum ^{at 300 K for} different orientations of the crystal ^{relative to the} magnetic

field. a) ^An arbitrary orientation where 2 signals ^{can be} observed; b) Orientation corresponding ^{to the} overlapping

of the two signals. ^The origin ^{of the} weak g’ a ² signal ⁱⁿ figure ^{6a is discussed in the text.} Vrf ⁼ 8.977 GHz.

Fig. ^{7. -} (TMN)3(C104)2 : ^the angular dependence ^of

the ESR signals ^{at 230 K.} a) Rotation about the needle

axis; b) ^{Rotation about an axis} perpendicular ^{to it. The}

crosses represent ^the experimental points ^{while the curves}

were plotted using best-fitted zfs-parameters : ^{D = ±} 0.00829, ^{E =} :+ ^0.00090 cm-1 (v,f ^{= 8.977} GHz).

Fig. ^{8. -} (TMN)3(C104)2 : temperature dependence ^of

the ESR signal intensity multiplied by ^the temperature.

The crosses represent ^the experimental values while the

straight ^line corresponds ^{to an} activation _energy of 0.277 eV.

Starting

from about 230 K to

higher

temperatures, the

signals

broaden, ^become

asymmetrically shaped

and

approach

each other.

Contrary

^{to the}

(TMN)3(AsF 6)2 crystal,

however,

they

^{do not} merge, except for ^a limited _range of

crystal

orientations close to the

crossing point.

^{At 300 K the}

signals

^{are about}

12 G broad

(Fig. 6),

^{except for the}

crossing point region,

^{where the}

single

^observed

signal gets

^narrowed

to 250 mG.

So far, ^{there are no}

conductivity

^{data available on}

(TMN)3(C104)2.

4. Discussion.

The presence of cation radical stacks is characteristic for all

crystal

structures of rcs. The internal structure of the stacks, however, determines

predominantly

the electric and

magnetic properties

of the solids.

It is found that dimerization in

quasi

one-dimensional chains of systems ^{with 1:1}

stoichiometry

^{or double}

dimerization in systems ^{of 2 : 1}

stoichiometry

^{leads to}

the

opening

^{of a} gap in the middle of the band and therefore to a metal to insulator transition. Concomi-

tantly spin pairing

^{causes the}

disappearance

^{of the}

Pauli-like

susceptibility.

In our systems ^{we have a 3 : 2 ratio} between the number of cations and anions, ^{which means that two}

charges (and spins)

^{have to} be attributed to three TMN-molecules :

(TMN)2 -(AsF )2-.

Furthermore, ^{the structure}

clearly

^{shows a}

perio- dicity comprising

cation triads of three TMN-mole- cules

(alternating stacks) resulting

^therefore in full and

empty

bands,

respectively. Isolating

^or

semiconducting

electric

properties

^and

diamagnetism (for

^{a usual}

negative exchange

interaction _energy J between the

two

spins)

^{are therefore}

expected.

Considering

^the

stoichiometry,

^the symmetry ^{of the}

two TMN molecules and the

interplanar spacings

ⁱⁿ

the stack, ^{there is} strong ^{evidence that the stack} contains triads of TMNt-TMN-TMNt

(Fig. 1).

This is

analogous

^{to the} distribution of

charges

and

spins suggested

ⁱⁿ tetraselenatetracene

(TSeT)3(Hg2Br6) [8],

^a

compound

^{which is}

nearly

isostructural with

(TMN)3(AsF6)2.

^The

average

^dis-

tances between the

least-squares planes

^{within the}

triads are both

3.27 A (the

^dihedral

angle

^between

TMN

(1)

^and

(2’)

^is

0.60),

^{while the} distance between the triads in

3.362 A (Fig. 1).

^Molecules

TMNt(2)

and

TMNt(2’)

^{are related}

by

^{a centre of} symmetry and are thus

mutually parallel,

^{whereas TMN}

(1)

^is

located around a centre of symmetry and turned

by

350 around the

stacking

direction with respect ^to TMNt

(2)

^{and TMNt}

(2).

The obvious strong

overlap

^{between TMN+}

(2)

^and

TMN+.

(2)

favourizes the

pairing

^{of the}

spins leading

to S ⁼ 0. Further hints for this association of the electrons are

given by

^the

nonplanarity

^{of TMNt}

(2, 2)

ⁱⁿ

conjunction

^with

planarity

^{of TMN}

(1)

^{and the}

absolute value of the fine structure which, ^as

originat- ing

^from

magnetic dipolar

interaction,

depends

^{on the}

separation

^{of the two}

spins.

If the

exchange

interaction J in the 2, 2’ dimer is in the order of kT,

triplet

^{states can be activated with a}

concentration of

This

expression

coincides with

equation (3)

^as

long

^as

k T

EA

⁼ J,

being

^{the case in both}

compounds

^with

and

These

triplet

^{states become mobile}

(without

^a

charge transportation) by exchange

interaction J’ with other dimers in the

alternating

^{stack or the}

adjacent

^chains

[9]. Depending

^{on the}

mobility,

^{the ESR-line}

shape

^of

triplet

^{excitons can exhibit residual} unresolved

hyper-

fine interaction

(hfs)

^or

motionally

narrowed lines

as it is here. the case even at the lowest temperature ^of detectable excitation

(Fig. 2).

Temperature-influenced changes

^{of ESR}

signal intensity

^{and width are} well known in the literature

[10-14]

^{and can} be attributed to

spin exchange

^between

mobile

triplet

excitons. This is favourized at

higher

temperatures ^{due to a}

higher

concentration of excitons and reflects therefore their activation. The

exchange frequency

^{of this} biexcitonic _process can be deter- mined in two limits

[ 15] :

- slow

exchange :

^{the two}

low-temperature

^ESR

signals

broaden and

approach

^{each other. For}

(TMN)3(AsF 6)2

^{this limit is valid for 140 TZ 200} K;

- fast

exchange :

^the

single averaged

^ESR

signal

becomes narrowed. This

happens

ⁱⁿ

(TMN)3(AsF6)2

for T > 200 K.

The

exchange frequency

^values calculated in both limits from linewidth measurements are shown in

figure

^{9. For the}

AsF6 compound

^{it can be seen that for}

both limits the same activation

energy

^{of 0.19 ± 0.01 eV}

is obtained, when it is fitted to the calculated values

Fig. ^{9. -} (TMN)3(AsF6)2 : temperature dependence ^{of the} triplet ^exciton exchange frequency. ^{The crosses} represent the values calculated from the ESR linewidth ^measure- ments while the straight ^line corresponds ^{to an activation}

energy of o.189 eV. The values below 200 K were calculated in the slow-exchange limit while those above 200 K in the fast-exchange ^limit.

1026

according

^{to the}

expression :

The

exchange frequency

ⁱⁿ

(TMN)3(AsF 6)2

^{at 273 K}

is about 520 MHz, vo about 1012 Hz.

For

(TMN)3(CI04)2’

^{the effects of}

triplet

^exciton

exchange

^on

signal position

^{are shown in}

figure

¹⁰

for the

vicinity

^{of the}

crossing point.

^{At 230 K there is}

practically

^no

exchange.

^{At 300 K the}

signals

^are

shifted towards each other

(slow exchange limit),

^and

there is a

region

^{of 100 where}

they

merge into ^one

line, ^{as the fast}

exchange

limit is reached.

The

triplet

^exciton

exchange frequency

^{was calcu-}

lated for the

(TMN)3(C’04)2

^{in the same way as for}

(TMN)3(AsF 6)2’

^but

^only

^{in the slow}

^exchange

^limit,

as the fast

exchange

^limit

applies only

^{to a} very limited range of

crystal

orientations. The activation energy of this process ^was found to be 0.27 ± ^{0.01 eV}

(Fig. 11).

The

exchange frequency

^{at 273 K is about 12} MHz,

vo about 1012 Hz

again,

^{as found in the}

AsF6

^com-

pound.

Therefore, the difference in the

exchange

^{rate at}

273 K between the two

compounds

^{is due}

solely

^{to the}

different

triplet

concentration.

It is worth to notice the _very

good

agreement between the activation _energy values obtained from ESR

signal intensity

^{and those obtained from line-}

width measurements.

Consequently,

^{it is not} necessary to account for the

discrepancy

^{in those} values like it

was done in

[ 13,14] by suggesting

^an additional _energy needed to activate a ^«

self-trapped » triplet

^exciton.

The

assignment

^{of the two}

charges

^to molecules 2 and 2’

(Fig. 1)

is,

although

^there are reasons for it,

quite arbitrary. Therefore,

it would be

interesting

^to

learn where the two

unpaired spins forming

^a

triplet

exciton reside. For this, however,.a ^detailed

knowledge

of the electron

density

^is necessary, ^as well as the

exact orientation of the zfs tensor relative to the

crystallographic

and molecular axes. Such examina- tions are in

progress.

For

(TMN)3(C104)2

^we

predict

^{a smaller distance}

or better electronic

overlap

between the TMN mole- cules

constituting

^the

triplet

ⁱⁿ

comparison

^{to the}

(AsF6)-salt.

^{This statement is based on the}

larger

activation

energy

^{and the}

bigger

^{fine structure}

splitting

constant D.

There is a

question

^whether

only

^{S = 1 states are}

observed in the ESR spectra ^{of the TMN salts concern-} ed In

particular,

^the

origin

^{of the}

weak g

^{= 2}

signals,

visible in almost _every spectrum ^{has to be} considered, Our

explanation

^{of those}

signals

^{is an} insufficient

crystal quality. Specifically,

^the

crystals

^{examined were}

not ideal

single crystals,

^{but had also some small} parts misoriented in relation to the bulk of the

crystal.

^As

those parts ^are very small,

they hardly

^{attribute to the}

ESR spectrum except ^{in the}

angular region

very close to their

crossing point.

^{At this}

point

^{the fast}

exchange

causes a

significant narrowing

of the lines

resulting

ⁱⁿ

an increase of their

amplitudes.

^This

explanation

^is

Fig. ^{10. -} (TMN)3(C’01)2 : angular dependence ^{of ESR} signals ^{at 230 K} (crosses) ^{and 300 K} (circles). Only ^{a part}

of the rotation about an axis perpendicular ^{to the needle}

axis is shown (v,f ^{= 8.977} GHz).

Fig. 11. - (TMN)3(CI04)2 : temperature dependence ^of triplet ^exciton exchange frequency. ^{The crosses} represent values calculated from the linewidth in the slow-exchange

limit. The straight ^line corresponds ^{to an} activation _energy of 0.274 eV.

based ^on two facts :

- the linewidth of

the g

^{= 2}

signals

^is

always

250 mG for both

crystals,

^that is characteristic for the

averaged signals originating

^{form the bulk at the}

crossing point;

- the

amplitude

^{of those}

signals

^is

extremely pnisotropic

^{- the}

signals

appear and

disappear

^{in a}

very ^narrow

angular

range.

4.1 CONDUCTIVITY. -

Systems

^with

diamagnetic ground

^{state have}

generally

^low

conductivity [16].

This is true in our case, too.

Although

^{the activation}

energy of the

conductivity

ⁱⁿ

(TMN)3(AsF 6)2 nearly

coincides with that of the

triplet spins

concentration this

might

^be accidental.

It should be mentioned however, that with the

given

activation, ^a

charge

^carrier

density

^{of about}

n ⁼

1017/cm3

^is calculated for 300 K. With a rea-

sonable

mobility

^{value of 10-2}

cm2/s

^{we can reach}

the

experimentally

determined

conductivity.

Acknowledgments.

This work was

supported by

^the

Stiftung

^Volkswa-

genwerk.

^{One of us}

(J. K.)

^is grateful ^{to the Max-} Planck-Gesellschaft and Grimmke

Stiftung

^{for finan-}

cial support.

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