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Tunable Phase Transitions in Conductive
Cu(2,5-Dimethyl-Dicyanoquinonediimine)2 Radical Ion Salts
Dagmar Gómez, Jost Ulrich von Schütz, Hans Christoph Wolf, Siegfried Hünig
To cite this version:
Dagmar Gómez, Jost Ulrich von Schütz, Hans Christoph Wolf, Siegfried Hünig. Tunable Phase Transi- tions in Conductive Cu(2,5-Dimethyl-Dicyanoquinonediimine)2 Radical Ion Salts. Journal de Physique I, EDP Sciences, 1996, 6 (12), pp.1655-1671. �10.1051/jp1:1996181�. �jpa-00247272�
Tunable Phase lhansitions in Conductive
Cu(2,5-Dimethyl-Dicyanoquinonediimine)2
Radical Ion Salts
Dagmar Gômez (~), Jost Ulrich Von Schütz (~.*), Hans Curistoph Wolf (~) and Siegfried Hünig (~)
(~) 3. Physikalisches Institut, Universitàt Stuttgart, 70550 Stuttgart, Germany (~) Institut für Organische Chemie, Universitàt Würzburg, 97074 Würzburg, Germany
(Received 25 April 1996, accepted 3 JuJJe 1996)
PACS.71.30.+h Metal-insulator transitions and other electronic transitions PACS.76.30.-v Electron paramagnetic resonance and relaxation
Abstract. The conduction process of the copper salts of DCNQI and the origin of the phase transition inducable in dioEerent ways, is explained in this review by an admixture of the copper
d states to the DCNQI px band. This admixture depends
on structural processes which shift the charge transfer à from the copper moiety to the DCNQI stack to langer values with reduced
lattice dimensions. Reaching exactly à
= 4/3 (band filling degree p
= 2/3), the trimerization in
the copper stack leads to the disproportion of the mixed valence state of Cu by the localization
of the d states as Cu~+. A charge density wave is formed, opening a gap at the Fermi level EF.
These statements
are acquired by selective deuteration, selective alloying, applymg pressure
and by simultaneous electron spm resonance (ESR) and conductivity experiments in the ESR equipment under pressure as a function of temperature. For ail systems, when reaching critical values of the unit cell volume, phase transitions take place. We could establish a general phase diagram which is based on an eoEective pressure scale mcluding internai (chemical) and externat
apphed pressure.
1. Introduction
A large variety of radical ion salts from 2,5-N,N'-DCNQI (dicyanoquinonediimine) witu metallic counterions bas been syntuesized (Fig. 1). Tue crystal structures of tuese salts are similar, tuey
can ail be classified as I41la. Due to tue beuavior of tueir conductivity and tueir magnetic properties [1-3] tuese salts can be separated into two groups:
. Tue non copper salts M(2-Ri,5-R2-DCNQI)2 witu M
= Li+, Na±, K+, Rb+, Tl+, Ag+ or
even NH( as countenons
can ail be cuaractenzed a8 one-dimensional systems Iii. Tuis is
seen from tueir cuaracteristic temperature dependences of tue conductivity, tue frequency dependence of tue nudear magnetic resonance data, and of tue electron spin resonance
(ESR) fines wuicu are generally narrow Ii,4].
. Altuougu uaving a very similar crystal structure tue copper salts are outstanding. Ail copper salts show a uigu metalhc conductivity at room temperature and (almost) no ESR (*) Author for correspondence le-mail: vs3@physik.um-stuttgart.de)
Q Les Éditions de Physique 1996
/cN
R~ M: Cu,Li, Na, K,
~ Rb,~,Ag
R~, R~: Cl, Br,I, CH~, CD~,CH~O Nc/
2
Fig. 1. The complex M(2-Ri,5-R2-DCNQI)2 with M being the metallic counterion and Ri, R2 being the substituents.
signal in tue metallic temperature region. Tuese features are ascribed to an admixture of Cu(d) states to tue conduction band of tue anion stack. We tuerefore describe tue
copper salts as quasi turee-dimensional systems. In tuese systems, a phase transition to an insulating state can be induced [Si. In tuis insulating state an ESR signal can
be detected [3,GI. Tuus, we bave an anticoincidence of tue uigu conductivity and tue detection of a strong ESR signal.
In tuis work we exclusively deal witu tue copper salts Cu(2-Ri,5-R2-DCNQI)2.
Tue metuyl substituted sait Cu(2,5-(CH3 )2-DCNQI)2 (u8 shows a uigu metallic conductivity
a = 10~ S cm~~ at
room temperature wuicu increases witu decreasing temperature (Figs. 2 and 3, /l). At 20 mK, a m 106 Scm~~ is reacued. No ESR signal is found for u8 (Fig. 3, /l).
Phase transitions to an insulating state can be forced by tue substitution of tue side groups Ri and for R2 in Cu(2-Ri,5-R2-DCNQI)2 by ualogens (Cl, Br, I) [1, 2]. Here a principal regularity
is found. Tue less space tue ualogen requires tue uiguer is tue phase transition temperature (~).
Simultaneously witu tue loss of tue conductivity at tue phase transition an ESR signal is found
(anticoincidence of conductivity and ESR activity). Its intensity increases (Curie like) witu
decreasing temperature [2].
A mucu smaller variation of tue "metallic" Cu(2,5-(CH3)2-DCNQI)2 (u8) is acuieved by tue substitution of tue uydrogen atoms by deuterium. A phase transition to tue insulating state is forced even by tue substitution of only turee uydrogens (one metuyl group) by deuterium, givmg Cu(2-CH3,5-CD3-DCNQI)2 (d3, Fig. 2, D). Tue phase transition occurs at a uiguer
temperature wuen replacing six uydrogens (two metuyl groups) by deuterium, giving Cu(2,5- (CD3 )2-DCNQI)2 (d6, Fig. 2, O). Tue phase transition temperature is even more mcreased by
tue supplementary substitution of tue uydrogens at tue ring by deutenum (ds, Fig. 2, i?).
If only tuese ring atoms are substituted by deutenum (d2) no phase transition occurs, but tue onset of a phase transition is observed (Fig. 2, O) là,7]. For tue substitution by deutenum
we tuerefore agam find a regularity. Tue more uydrogens are substituted by deuterium tue
uiguer is tue phase transition temperature. Tuis is m accordance witu tue regulanty found for tue ualogen substituents. Tue CD3 group needs less space tuan tue CH3 group [7] and so
less space again means a uiguer pua8e transition temperature (or tue inducement of a phase transition).
As described before, simultaneously witu tue loss of tue conductivity an ESR signal is de- tected in deuterium substituted systems, too (Fig. 3, O, d6 on beualf of ail deuterated systems
(1) This can aise mean that a phase transition is induced by substituents wuicu need less space.
à
~ '*~,~
= 4
fl 3 ,j
u~
~
2
,# l ~
~
.~ o A h~
q o ~
~ _i
g g . d~
~/ _2 ~~
~ ~ o d~
- ./~v v d~
à$
g
-5 °
0 50 100 150 200 250 300
temperature / K
Fig. 2. Temperature dependence of the conductivity of Cu(2,5-(CH3)2-DCNQI)2 (h8, A) and its deuterated pendants d~ with x being the number of deuterium atoms per DCNQI: d2 (O), d3 ID), d6 (O), and d8 Iv).
witu phase transition). Tue ESR intensity increases Curie like witu decreasing temperature and is lost at about 8 K due to an antiferromagnetic ordenng. Tue linewidtu is ABpp m 2 mT
in tue insulating state. It is broadened near tue phase transitions. Apart from tue antiferro- magnetic ordenng state we again observe tue known anticoincidence of tue uigu conductivity
and tue detection of an ESR signal altuougu a uigu paramagnetic susceptibility is present
(determined by SQUID measurements) [GI.
We present tue latest expenmental results obtained by varying tue internai (Sect. 2.1) and tue externat pressure (Sect. 2.2). (For experimental details see ref. [8].) From tuese data
we deduced a generalized phase diagram for ail systems examined (Sect. 3.i) and we found
a tueoretical model for tue explanation of tue phase diagram (Sect. 3.2). In Section 4 we
applicate tuis model to otuer systems.
2. Results
2. i. ALLOYS. To acuieve even minor changes in tue crystal structure (and tuerefore minute suifts of tue phase transition) alloyed crystals bave been syntuesized. Tuey consist of tue
undeuterated substance u8 (no phase transition, Fig. 3, /l) and tue deuterated substance d6 (phase transition, Fig. 3, O), giving Cu[(2,5-(CH3)2-DCNQI)~(2,5-(CD3)2-DCNQI)~_~]2u8id6
in diiferent ratios 0 < x < i.
Tue conductivity mea8urements of various alloys are suown in Figure 4 (cooling cycles).
Tue conductivities of us Ill) and d6 (O) are suown for comparison. In tue cooling cycle, tue
alloyed substance u8/d6 50 : 50 (Fig. 4, o) shows a Sharp phase transition to tue insulating
state at a temperature Tci (upper phase transition temperature, see Fig. 5) wuicu is smaller
tuan tue phase transition temperature Tci of d6. On furtuer coohng, a second suarp phase
~
'~AA
t
~
~4S4tdM~M
E ~
£l ,
~ o
s _~ f
m _~ ~f
_~
'
d 100
d
~
80
#
~ 20
u~
Ul 0
6
£
ce ~ *
u ,
~
~
4 j
É 3 #
e
~ 2 O44#
q~ 0
0 50 100 150 200 250 300
temperatwre / K
Fig. 3. Temperature dependence of the conductivity of h8 IA and d6 (O) and of the ESR intensity and ESR linewidth of d6 (O). In the substance h8 no ESR signal can be detected.
transition back to tue metallic state occurs at tue phase transition temperature Tc2 (see Fig. 5).
Tuis transition is called re-entry [GI. Qualitatively tue same temperature dependence of tue
conductivity is found in tue ueating cycle. It shows a large hysteresis at botu phase transitions [GI and tuerefore is omitted for clarity.
Tue alloy u8/d6 70 30 (Fig. 4, D) shows two phase transitions, too. In comparison witu
u8/d6 50 50, tue upper phase transition at Tci is suifted to a lower temperature, wuereas tue lower phase transition at Tc2 is suifted to a uiguer temperature [GI. Again, large uysteresis at botu pua8e transitions are found.
Tue alloy h8/d6 90 :10 (Fig. 4, i7) does not show any phase transition, it stays metallic until trie lowest temperature reached.
For ail alloys, trie ESR intensity again is in anticoincidence to trie high conductivity [GI (data not shown). This means, for h8/d6 90 :10 no ESR signal is detected, and we are able to detect a high ESR intensity for h8/d6 50 50 and h8/d6 70 30 between Tci and Tc2. Near trie phase transition temperatures and in trie re-entry state we found a small ESR intensity in trie temperature region of high conductivity. To prove this coemstence of conducting (ESR
à 5
~£ 4
À 3
~ ~ ,
jp
,# o
_g ,
~
-l
q ~ é $
~ / : ~ h8
§ -3 '
/
v hg/d~ 90:10~ ~ ~
. hg/d~ 70:30
~ _~ ~* 3 h/d~ 50:50
~ __, , d~
20 40 60 80 100
temperatwre / K
Fig. 4. Temperature dependence of the conductivity of h8 (A) and d6 (O) and the alloys h8/d6 90 10 iv), h8/d6 70:30 (D), and h8/d6 50:50 (o).
~@
fÉ
j 'é ~cÎ
E
~ l _,
~ l~Î~tÉlÎ
~ /Î $ ~
~ Î/~"_ Î,
.x&[., 4u .-Ii,"~.~ ~
j / .~ '/ ~ c2
pressure p -
Fig. 5. Schematic phase diagram as determmed for undeuterated Cu(2,5-(CH3 )2-DCNQI)2 (h8 (9].
Depending on the operation point, selected by pressure, there
are: none (zone0), two (zone2) or one (zone 1) pha8e transitions.
silent) and insulating (ESR active) areas we performed similltaneoils conductivity and ESR measurements (see below).
In summary this means:
. Alloying h8 with trie "smaller" d6 induces a phase transition in accordance with trie
halogen substituents and trie deuterated systems.
. Alloys h8/d6 with a higher ratio of trie "smaller" d6 show a higher phase transition temperature Tci This is again in accordance with trie regulanty found for trie halogen
substituents and for trie deuterated systems.
. Alloys with ratios ofd6 between 30% and 50% show a second phase transition at Tc2 Trie
"window of insulation" between Tci and Tc2 is trie narrower in temperature trie smaller trie ratio of d6 is.
. In u8, a phase transition to tue insulating state can be induced by pressilre [9]. We there- fore identify tue substances witu ~'less space substituents" as substances witu an internai
pressilre. As reference witu an internai pressure po
" 0 bar we take tue undeuterated
substance u8.
Witu tuat reference, all above introduced Cu(2-Ri,5-R2-DCNQI)2 systems and alloys bave an internai pressure po > 0 bar. Of course, Cu(2-Ri,5-R2-DCNQI)2 systems witu po < 0 bar
are possible, e-g- Cu(2,5-(CH30)2-DCNQI)2 [10]. In tuis case, tue side groups are very large, expanding tue lattice.
Tuese results can be summarized in a scuematic phase diagram, Figure 5, wuicu shows tue
phase transition temperature Tc as a function of tue (internal) pressure p. For clarity, tue
uysteresis is not involved. Tue broken lines show tue course of tue temperature dependent
measurements.
Witu no or small pressure, we do not bave any pua8e transition (zone 0). Tue system stays metallic. In a narrow pressure region, we can find two phase transitions (zone 2): metal-
insulator-metal. Witu uiguer pressure, we bave only one phase transition left (zone i) and
no re-entry any more. In tuis diagram (Fig. 5), we can see tue increase of tue upper phase
transition at Tci witu increasing pressure and in zone 2 supplementary tue decrease of tue lower phase transition at Tc2 witu increasing pressure. Tuerefore tue "window of insulation"
becomes broader witu increasing pressure.
2.2. EXTERNAL PRESSURE. Until Dow, we Orly talked about trie inducement or trie shirt of phase transition in tuese substances by internai pressilre. For lue examination of tue coex-
istence of insulating and conducting areas and for pressure dependent measurements we bave
performed similltaneoils condilctiuity and ESR ezpenments ilnder ezternal pressilre.
We began witu pressure dependent conductivity measurements on d6 (Fig. 6). Tuis sub-
stance refers to zone i of Figure 5 by its uigu internal pressure. Wituout external pressure
(Fig. 3, /l), d6 is uiguly conductive between 300 K and 60 K. At Tci
" 60 K, in tue cooling
cycle (open triangles), it undergoes a suarp phase transitiop to tue insulating state and uere shows a typically semiconductive beuavior. Tue ueating curve (full triangles) is similar, witu
a uysteresis ATci-~ 4 K. Witu an externat pressure p
= 200 bar, tue phase transition is suifted to a uiguer temperature (circles). Tuis means, we suift tue broken hne in Figure 5, zone i, to tue rigut. Notewortuy in Figure 6 is tue coincidence of all coohng (open symbols) and ueating (full symbols) curves witu (circles) and wituout (triangles) pressure above and below Tc.
Figure î shows tue similltaneoils pressure dependent conductivity a and ESR measurements of d6 at diiferent temperatures just above TciIi bar)
= 60 K (pressure up cycles). Tue uiguer
tue given temperature is tue more pressure is needed to induce tue phase transition to tue insulating state. Tuese simultaneous measurements clearly show tue coemstence of conducting (ESR silent) and insulating (ESR active) areas just below tue "critical" pressure at wuicu tue conductivity breaks down. Tuere is even a quantitative evaluation possible. Tue phase
transition m a occurs wuen 20§lo of tue maximum ESR intensity are reached. Tuis means that 20§lo of tue total crystal volume consists of insulating (ESR active) areas and 80% consists of
conducting (ESR silent) areas. Tue conductivity breaks down abruptly, 1.e., tue percolation of tue conducting areas is cut off by tue msulating areas. Tue ESR intensity is not percolation
limited and so it is a measure for tue integral volume of insulating areas. Because of tuis percolation bruit ii-e-, phase transition m a if 20% ESR intensity are reacued), we can obtain
3 2
=
'E
à
~ 0 1~
O
_#
.k -2 '
G
é
( _~
~' -4
m -5 -6
0 20 40 60 80 100
temperatwre / K
Fig. 6. Temperature dependence of the conductivity of d6 under ambient pressure (A) and highest
pressure available (200 bar) (O). The empty and fuit symbols refer to the cooling and the heating
cycle, respectively.
tue cntical pressure for T
= 65 K and T
= 66 K. At tuese temperatures tue conductivity could not be measured because of flaked off contacts.
In a furtuer step, we performed simultaneous conductivity and ESR experiments under pressure on a re-entry system, 1. e., we took crystals wuicu under ambient pressure refer to zone 2 in Figure 5. As an example, m Figure 8 tue temperature dependence of a, ESR intensity and
ESR lmewidtu of u8/d6 70 30 are given for diiferent extemal pressure values
m tue cooling cycle. Tue initial window of insulation between TciIl bar) and Tc2Ii bar) becomes larger witu
mcreasing pressure. Above 95 bar we only bave one phase transition, leaving zone 2 in Figure 5 to tue rigut uand side. Concomitantly witu tue phase transition in a, tuere appears a strong ESR signal wuicu is ratuer constant in linewidtu if we exdude tue temperature ranges close to tue phase transitions. By careful analysis we could show again tuat at a conversion degree
of 20% (1.e. 80% of tue total volume are still m tue metallic phase) tue conductivity breaks down abruptly. Tuese simultaneous measurements prove tue coexistence of conducting and
msulating areas near tue pua8e transitions and in tue wuole temperature range of tue re-entry
state. Tuis uolds for all systems. We like to note tuat tue decrease in tue ESR intensity below 10 K is due to tue above mentioned antiferromagnetic ordering and not due to a re-entry in
a. Tuis uolds for all systems, too. Tue ueating curves (data not suown) are quahtatively very similar, tue respective phase transition temperatures are suifted to uiguer temperatures only.
A system wuicu under ambient pressure refers to zone 0 in Figure 5 is u8/d6 75: 25. Otuer systems wuicu refer to zone 0 are tue "basic" undeuterated substance u8 and tue deuterated d2 Tue phase transition temperatures of botu salts are listed in Table I for some pressure values.
Toi is tue upper phase transition temperature, Tc2 tue lower one (if re-entry is acuieved). Heat- ing (Tci,h, Tc2,h) and cooling (Tci,c, Tc2,c) Phase transition temperatures diifer considerably,
exuibiting uuge uysteresis eifects.
pressure / bar
0 50 100 150 200
= m~ u au u
£ 4 à~
~ o * A~ T=62K
u'
~
A T=63K
zy D T=64K
~ ,
.£ _2 v T=65K
( D Ô T=66K
~ 3 ~
g j
Î ~4 . ÎÉ~ÎW~©
Ç~ D@ Do
~
100 e d
i~
~ ~Ù il
ÉÎ ~
( j
à ~~
~'
iÎ 20
-j
~~
ill o
oe ~ o(~
~ ~ o ~/ i~
1 5
~ ~,~A~#
,j oeoj
Î
4
. ~Î~#
.% f*
e ~
- V
~ É12 1(
q
p~(62K) (p~(MK) p~(66K)
Pc(63K) p~(65K)
Fig. 7. Pressure dependence of the conductivity, the ESR intensity and the ESR linewidth of d6
measured simultaneously at various temperatures T (o : T
= 62 K, A : T
= 63 K, D ; T
= 64 K
,
V T
= 65 K, O
: T
= 66 K). Only the plots for "pressure up" are shown. Being qmte sensitive to
pressure cycles, the electrical contacts just survive 2-3 cycles. Therefore only the ESR data could be
given for T
= 65 K and T
= 66 K. The fines are guides for the eye.
On beualf of tuese '~zone 0" systems, m Figure 9 we show exclusively the ESR data of
u8/d6 î5 : 25 as a function of temperature for diiferent pressure values (cooling cycle). Al- tuougu tuere exists a metallic conductivity in tue wuole temperature range, a weak ESR signal
is detectable belo~. 60 K. Referring to tue comments above, we conclude a coexistence of in-
sulating areas (mmonty) and conducting ranges. Witu a minor pressure of 20 bar we exceed
a volume conversion of more tuan 20% into an msulating phase and tuerefore a phase transi- tion at Tci and a re-entry at Tc2. At pressure values above 100 bar, tuere is only one phase
4 AA~ ~
_ 2
À~~~
~é o
À
~
~
é~ ~
l bar
bo -6 A 25 bar
'~
~ . 95 bar
.170 bar
j 100
d
~
80
#
3 .1
c~2
~ 6
&
j 5 o
~ 4 A o*
~ f~~§,
~éÀÎ
~ 2 0@0##*
1
~Î o Coolillg
0 10 20 30 40 50 60
temperatwe / K
Fig. 8. Temperature dependence of the conductivity, the ESR intensity and the ESR linewidth of
h8/d6 70:30 for dioEerent externat pre88ure8 in the coohng cycle.
Table I. Phase transition temperatilres ofh8 and d2 at dijferent pressilre ua1iles.
substance pressure cooling ueating uysteresis phase
p Tci,c Tc2,c Tc2,h Tci,h ATci ATc2 transition(s)
150 bar
none
u8 180 bar 46 K 18 K 33 K Si K 5 K là K "re-entry"
215 bar 52 K là K 36 K 53 K i K 21 K "re-entry"
80 bar none
d2 1là bar 40 K 17 K 32 K 50 K 10 K là K "re-entry"
150 bar 43 K 10 K 30 K 53 K 10 K 20 K "re-entry"
210 bar 52 K 60 K 8 K one
transition left. Tuis means, we suift tue broken line m zone 0, Figure 5, to tue rigut, we pass
zone 2 and we arrive m zone i at pressure values above 100 bar. Notewortuy in Figure 9 is
tuat tue linewidtu is not broadened by tue re-entry phase transition.
