Polycrystalline material of the &#945;-phase of (BEDT-TTF)2I3 was compressed to small samples (4-mm x 1mm, thickness 0.3 mm typically) at a pressure of 10 kbar. Annealing at 70°C yields the superconducting &#945;t-phase. Microwaves (10,2 GHz) enable the measurements of the conductivity for stepwise annealing after every annealing step in always the same sample. For annealing times 10 min all conductivity versus temperature curves are intersecting in an isosbestlc point at 190 K. This behaviour can be described by a conductivity relation for a two component system, from which was determined the volume fraction of the new grown &#945;t-phase in dependence of the annealing time. Starting annealing (annealing times < 10 min) shows another unexpected phase transformation. After 2 min annealing the conductivity at 200 K increases by more than one order of magnitude, but then decreases of further annealing (5-10 min) down to the value for the unannealed sample

Helberg, Hans W.

Keller, Heimo J.

Müller, Gerhard

Schweitzer, Dieter

Online Publikationen der Universität Stuttgart

SynlhtliC Melah. 4/-43 (1991) 1999-2004 
MICROWAVE CONDUCTIVITY IN POLYCRYSTALLINE {BEDT-TTFl2IJ MATERIAL 
G. MULLER, H.W. HELBERG 
Drlttes Physikallsches Institut der UnIversitiit Gottingen (F.R.G.) 
D. SCHWEITZER 
Drlttes Physlkallsches Instltut der UnJversltiit Stuttgart (F.R.G.J 
HJ. KEllER 
InstJtut fUr AnorganJsche ChemJe der UnJversltat Heldelberg (F.R.G.J 
ABSTRACT 
1999 
Polycrystalline material of the tt-phase of <BEDT-1TF)213 was compressed to small 
samples (4- mm x t mm, thickness 0.3 mm typically) at a pressure of 10 kbat . Anneallng 
at 70°C Yields the superconducting elt. -phase. 
Microwaves (to.2 GHz) enable the measurements of the conductivity for stepwise 
annealing after every annealing step in always the same sample. For annealing times 
?:. 10 min all conductivity versus temperature curves are intersecting in an isosbestlc 
point at 190 K. This behaviour can be described by a conductivity relation for a two 
component system, from which was detennlned the volume fraction of the new grown 
at-phase In dependence of the annealIng time. 
Starting annealing (annealing times < 10 min) shows another unexpected phase 
transformation. After 2 min annealing the conductivity at 200 K increases by more 
than one order of magnitUde, but then decrea.ses of further annealing (5-10 min) down 
to the value for the unannealed sample. 
INTRODUCTION 
Crystals of a-<BEDT-TfF)213 are quasi two-dimensional organic metals [11. The 
crystals undergo a phase transition at 135 K. Cooling down. the dc conductivity increases 
sllghtly and then decreases sharply by about six orders of magnitude at 135 K. Cooling 
down, the microwave conductivity follows the de conductivity. But the drop at 135 K is 
only three orders of magnitude. Then, in contrast to the dc measurements between 
120 K and 40 K a plateau Is reached [21, obviously due to CDW. Tempering the crystals 
of the a-phase above 70°C for several days yields a structural phase transition. The 
new phase (at. -pha.se) is a quasi two-dimensional organic metal as well which has a 
stable superconducting state at 8 K and ambient pressure. This process can directly be 
observed in very small crystals by polarizing microscope techniques (31 The structure 
of the at. -phase is not known yet. Spectroscoplc methods lead to the speculation that 
0379·67791911$3.50 ~ Elsevier SeouoiaIPrinted in The Nethetl~n(1c; 
2000 
the structure of at is very similar to the one of crystals of a-(BEDT-TIF)zlJ [3-5) . A 
transfonnatlon by tempering like the transformation of the Cl-phase Into the at -phue 
Is also found In polycrlstalllne pressed samples [5-6] . An overview Is glvcn In (51. 
EXPERIMENTAL 
A grinding process of a-(BEDT-TIFl2.I, single crystals yields a powder of crystallites 
wIth typIcal diameters of O.S - 10 "rn. Applying pressure of 10 kbar to the powder 
produces mechanically very stable samples (named Clp -phase). The sample si%e Is 4 mm x 
t mm with thicknesses 0 .2 - O.S mm typically. The contactless microwave measurements 
were carried out In a cylindrical cavity. working In the TMolO mode at to.2 GHz by 
usual cavity perturbation technique [7]. This technJque enables the measurements of 
the conductivity for stepwlse annealing after every annealing step In always the same 
sample. Annealing Is done In a preheated drying cupboard at 7rfJC . Its large volume 
(about 5 dm3) allows treatment of the samples also for short tlmu. The samples are 
left In the styrofoam sample holder so that the position ID the cavity Is always exactly 
the same for the different measurements. 
RESULTS AND DISCUSSION 
For three pressed samples without anneaUng the relative microwave conductivity Is 
plotted versus temperature In Fig. 1. The behaviour Is the same for all three samples. 
In contrast to the sharp decrease of the mIcrowave conductivity at 135 K found in 
single crystal .. (21. the transition Is broadened and shifted to higher temperatures . Also 
the decrease to the plateau Is only one order of magnitude In contnut to three orders 
of magnitude in the case of single crystals. Repeated running through the temperature 
cycle has no influence on the conductivity behaviour. 
,;-
<; 
~ 
-
, 
., 
~ '0' 
~ 
c 
• 
" 
10··'-__ .. .-~-",--",O o ? lOO 
ttmptr~turt (K) 
Fig. 1. Microwave conductivity of pressed samples of Cl -(BEDT -TfF) I without anneal-
I p ., ing re ated to the room temperature values 0R.T = 10-15 (Ocm)-I. Frequency 10.2 GHz. 
Samples A +, B 11. C O. 
2001 
,,' 
'e I '00 
" j la "-~ '0' 
'" 
:E . 
- I " , ~ < 0 
" 
;; 
" 5 006 
~ 1O' 
• 
.j 
• 0 
• ~ 
,,' 
• .. '" 
,,, 300 
hmpenture (K I temperature (K) 
Fig. 2. a) Mlcrowavc conductivity of the pressed sample B of a t-(8EDT-TfF}2'3 after 
annealing for 3.5 days at 7rfJC without Interruption. Frequency fo.2 GHz. b) Resistivity 
versus temperature corresponding to a) . 
Annealing was applled to sample B without interruption for 3.5 days at 7fiJC. The 
new grown phase Is named Clpt.' FIg. 2a shows the microwave conductivity In agreement 
with de measuremenu. In Ag. 2b the resistivity Is plotted versus temperature. In the 
range of 200-300 K the resistivity Is proportional to the temperature corresponding to 
the metallic character . The measurements shown in Fig. 2 are done during the cooling 
down process. Measurements of the microwave conductivity during cooling down and 
warmIng up show a hysteresis between 160-300 K with a maxJmum at 230 K. probably 
due to phase transitions In the material . 
To Investigate the transformation (rom the cxp-phase Into the «pt-phase stepwlse 
annealing was applied beginnIng with short duration and goIng to longer times. After 
every annealing step the microwave conductivity was measured In dependence of the 
temperature. Ag. 3 summarizes results for anneal1ng times from 10 mln to 42.7 h. All 
curves are crossing In an Isosbestic poInt at 190 K. Therefore the assumption is made 
that the 10 mln curve corresponds to the (lp-phase and the 42.7 h curve to the (lpt -phase. 
respectively . Then all curves between these two curves are due to the mixture of the 
two phues. That means that the conductivity can be described by a parallel arrangement 
of these two conductivities rated by the volume fraction x of the growing pt-phase: 
The two extremal CUTVU Clp(T) and 0ptCn are fitted stepwlse parabollcally. All other 
curves between these two curves are fitted by using the above equation and sa deter-
mining the volume frac tion x. Fig. 3 shows a good agreement between this equation and 
the data. 
2002 
• 
.. 
u 
C 
-
" > ;: 
g 101 
" C o 
U 
• > 
o 
• 
42.1h X • . 0 
x 5 .31"1 X-O.S8 
X 2 . 1t'> X.O. 27 
v 20 "'n 
X",O.O 
o 
L ~~;;!i~~~::~~~~d ~ • ·0.0 E 10 0 '100 200 300 
tellperature (I() 
Fig. 3. Microwave conductivity of the pressed sample C of a p ' Clp t.-(BEDT-TIA2IJ after 
stepwise annealing at 70°C. Parameter: added up annealing duration and volume frac-
tion x of the growing apt.-phase. x is determined by fitting the curves from open and 
0pt.(T) using 0(T) = U-x}Op(TI + x·Opt.cn . 
>.0 
• 
0 .8 
, 
• ~ 
" 
0 .' 
• ~ A 
• E o • 2 
0 
• 
• 
0 .2 
• 
0 .0 • 
' 0 ' ' 0' '0 ' '0' 
annealing time (mln ) 
Fig. 4. Volume fraction of the growing apt. -phase in the pressed sample C of (lp, 
IIpt. -(BEDT - TJ"F)2I3 taken from Fig , 3 (0) versus annealing duration . Sample A is annealed 
for 1 h without interruption (6). See Fig. S. 
In Fig. 4 the values of the volume fraction x of the pt-phue taken from Fig. 3 are 
plotted versus annealing duration . The volume fraction Increases with annealing duration 
and saturates above two days. Half the materIal Is transformed after about 5 h for the 
interrupted annealing shown In Fig. 3. The anneaUng of another sample (A) Is Inter-
rupted not before t h. These results plotted In Fig. S In comparison to the results of 
sample C show that half of the material Is traMformed after 1 b if the annealing Is 
applied without intelTUpUons. Obvlousty the IntelTUpted annealing causes baniers to 
the transformation process. 
,., 
~ 
'> ;; 
u , 
" •o 
u 
• > 10· 
o 
• o 
L 
U 
• 
• > ;; 
o 
A +1h L:> 2 h 
o 1.3 h 
C 0 2.7 h 
X 5.3 h 
~ ~ 10" !:-~~....,;!;,.~~...,;!;;-~~-';.;!, 
teMp.,.oture HO 
2003 
Ag. S. Mlcrowav~ conductivity of the pressed sample A of Cl: • et t.-(BEDT-1TF)213 after 
t h (+) and 2 h (11.) annealing at 7(/JC. Results of sample C ~e plotted for comparison 
(from Fig. 3), 
-
, 
e 
u 
c:: 
-
>. 10' 
• 
. . ~--:---;,-~ 
1 0 -' LO-_ _ ~,*""---·_-,*,··_·'--'--'--'-d 
1'1 100 200 300 
temperature (K) 
Ag. 6. Microwave conductivity of the pressed sample 0 of apt G't p t.-<BEDT-TTFl2I3 after 
stepwise annealing at 70°C. Parameter: added up annealing duration. Inserts: conduct-
ivIty at 40 K and 200 K, respectively, versus annealing duration. 
In the beginning of annealing there is another unexpected phase transformation. The 
values measured for a few minutes of annealing dW"ation therefore are omitted In Ag. 
3 and Inveatlgated in more detaJl on sample D. Fig. 6 shows the results. The conduct-
ivity increases with increasing annealing time and decreases after passing a maximum. 
Inserts in Fig. 6 describe this behaviour at 40 K and 200 K. respectively . In the high 
temperature range (200 K) the increase of the conductivity is more than one order of 
2004 
magnitude. The origin for this behaviour is not known yet. Probably the high pres8ure 
procedure yields metastable pbases dIsappearing In the first mInutes of annealing. Due 
to the high heat conductivity and the small thicknesses of the samples annealing 
works In the whole sample volume. particularly In the case of short annealing tlmes, too. 
ruling out surface effects . 
REFERENCES 
1 K. Bender. l. Hennlg. D. Schweitzer. K. Dletz. H. Endres and HJ. KeJler, MoL Cryst. 
Uq. Cry_t., 108 ((984) 359. 
2 K. Bender, K. Dletz., H. Endres, H.W. Helberg. J. Hennlg, HJ. Keller. H.W. Schafer 
and D. Schweltzer. Mol. Cryst. Uq. Cryst., 107 (1984) 4.5 . 
3 H.W. Hetherg. D. Schweitzer and HJ. Keller, S,oth. Met . • 27 (1988) A347. 
4 O. Schweitzer. P. Bele. H. Brunner. E. Gogu, U. Haeherlen. t. Hennlc, I. Kluu. R. 
SwletUk and HJ. Keller, Z. Phys. B Condensed Matter , 67 (1987) 489. 
5 D. Schweitzer, S. KahUch, S. Giirtner, E. Gogu, H. Grlmm, I. HeineD, T. Kutz and R. 
Zamboni. in RM. Metzger, P. Day and G. Papavas:lilIJou (eds.). Lower Dimensional 
Systems and Molecular ElectronlC3, Plenum Press, 1989. 
6 D. Schweitzer, S. Giirtner, H. Grlmm. E. Gogu, HJ. Keller, Solid State Comm .• 69 
(1988) 843. 
7 H. W. Helberg. B. Wartenberg, Z. angew. Phys .• 20 (1966) SOS. 


Microwave conductivity in polycrystalline (BEDT-TTF)2I3 material

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Microwave conductivity in polycrystalline (BEDT-TTF)2I3 material

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