Article
Reference
Piezoelectric and dielectric relaxation in nickel iodine boracite at low temperature
RIVERA, Jean-Pierre, SCHMID, Hans
Abstract
With a view to elucidating the anomaly of various phys. properties found near 120 K, the complex dielec. const. was measured vs. temp. (4-300 K) and frequency (10 kHz-1 MHz) and is compared with previous piezoelec. studies. Substantial shifts in temp. of the ε''-peak and 1/Qm piezo-peak with frequency indicate a Debye-type relaxation mechanism due to defects and exclude new phase transitions as previously claimed in the literature. A similar, but weaker anomaly occurs near 15 K. Relaxation times and activation energies are given.
RIVERA, Jean-Pierre, SCHMID, Hans. Piezoelectric and dielectric relaxation in nickel iodine boracite at low temperature. Ferroelectrics , 1984, vol. 54, no. 1, p. 103-106
DOI : 10.1080/00150198408215826
Available at:
http://archive-ouverte.unige.ch/unige:32132
Disclaimer: layout of this document may differ from the published version.
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PIEZOELECTRIC A~lD DIELECTRIC RELAX.ATION IN NICKEL IODI~E
BORACITF. AT LOiv TE~IPERATURE
JEA~-PIF.RRE RIVERA and HANS SCHmD
tabo~ato~y of Applied Chemist~y, Unive~sity of Geneva, CH-1211 Geneva 4, Switze~land
Abst~act With a view to elucidating the anomaly of va~ious
physical prooe~ties found near 120K, the complex dielectric constant has been measured versus tempe~atu~e (4K ~ 300K) and frequency (10kHz ~ 1~1Hz) and is compared to p~evious piezoelect~ic studies. Substantial shifts in temperature of the £"-peak and 1/C/rn piezo-peak with frequency indicate a Debye type relaxation mechanism due to defects and exclude new phase transitions as previously claimed in literature.
A similar, but weaker anomaly occurs near lSK. Relaxation times and activation energies are given.
I~TRODt:CTIO~
~ickel iodine boracite (Ni3B,013I =Nil) has been the object of controversies concerning the number of existing phase transitions, the symmetry of phases and~the simultaneity of the onset of
spontaneous polarization, Ps, and spontaneous magnetization, ~s·
!1o1•ever, this simultaneous onset of Ps and ~s is nO\v rather well est.Jblished forT = 61.5K 1 , though first evid~nc·~ for it was already obtained in 1966 2• During those first experiments 2 a strong anomaly of dielectric constant ~· and magnetic suscepti- bility had been noted at approximately 120K. Although anomalies of the cubic lattice parameter have been reported for about 120K 3,~, no deviation from the cubic structure (F43c) has been detected, neither by X-rays 5, nor by neutron diffraction 6. An
isomorphous phase transition was therefore proposed to occur at about 120K 7• Finally, also Rarr..:m effect 8 and piezoelectric experi- ments 9,1~ clearly showed anomalies in that temperature range. Our piezoelectric measurements at the fun~.J::Jental frequenc:' and third overtone (of a length extensional mode) showed 10, however, that the anomaly shifted in temperature for a change of measurin~
frequency, thus suggesting an explanation in terms of defects rather than by an isomorphous transition. That is why more syste- matic measurements of dielectric constant (::* = e:' - ie:") ver.;us temperature and frequency have been undertaken in this work. So far only a study~ £* versus frequency at 20°C, and of e:' and .:-" vs.
temperature at a fixed frequency has been reported for Nil 11
(44311103
1().11~1 J.·P. RIVERA A:-10 H SCH~1!0
EXPERWE~TS
For both the piezoelectric 10 and dielectric experiments the same sample, cut out from a (lOO)c growth pyramid of a vapour transport grown 12 crystal, was used and had the following dimensions : R. = 1.86mm (// [ 110]c), w = 0.66mm (// [ llO]c) and t = 0.1 18mm
( .L [001lc). Gold wires (D. 40l.Jm) were attached to the centre of the
electrodcd (semitransparent Au on Cr) (100)c - facPs.
An Impedance ~leter (HP 4815A), linked via a synthesizer and two digital voltmeters to a desktop computer, was used for the piezo- electric measurements by the techniques of admittance circle and motional capacitance 10 An LCR Meter (HP 4275A) directly connected to a computer served for dielectric measurements.
RESULTS
Figure 1a shows the piezoelectric series resonance frequencies of the fundamental mode and of the third overtone mode of the same crystal, (note that the sound velocity is V = 2.l .f ). In Figure lb, the intern3l mechanical losses (1/Qm) are
g~ven
10,5also for thefundamental frequency and the third overtone. The absorption peak is clearly displaced in temperature. Figures lc and ld show the dielectric constant s' and s", vs.temperature for some frequencies.
Also here, the absorption peak is clearly displaced in tempera- ture. Smaller absorption peaks (1/Qm and t:") are also observed between 10K and 20K.
DISCL'SSION
By plotting log versus 1/Tp, where w
=
Z1f (f =measuring frequency) and T [K] the temperature of the peak of 1/Qm or£" (t:" a: tg 6,
s~nce
E' -;; const.), a straight line is obtained (Fig. 2). In first approximation such a relationship can be explained by a Debye type relaxation mechanism, both for 1/Qm 13 and£" 14• The:n it follows that lnW = - lnTo - E/(kTP)' where Tois a relaxation time, E an activation energy and k Boltzmann's constant. Whereas the peak centred at 95K for f
=
10kHz, shifting to 152k for f = 6~lliz, can be attributed to a relaxation mechanism withE = 0. 14 e\"(1130 cm-1) and To= 7.8 x 10-13s, the peak moving from 10K to 20K is characterized by E=
0.0092 eV (74 cm-1) and To = 1.9 x 10-1~s. Figure 3 shows a Cole-Cole plot for 128K.Figure 4 raveals , also for 128K, that the width of the peak is larger than1.144 decade at half height 13• Both figures indicate a mechanism different from a tru~ Debye-type relaxation with a single
relaxation time. There is no doubt that the origin of these relaxations are defects (Or - vacancies, impurities, . . . ), and not phase transition~ near lOK and 120K as previously claimed in the literature. No anomaly of Ps• birefringence and
Faraday rotation is observed near 10K; the only ?hase transition
REL.-\\A TIO:" 1:" :"ICKEL IODt;>;E BORA CITE
"¥ , , /1 ~--•\ ~J· 0 - IOIW
~zoo
/
'-'
18-" I/ 6.10
i
,I"'
"
~ 198 I
' '
16.,
...
i 605, .
"
,!,
J .
~i
z
14/'~
~
•
"'
}..,.600 B 10K<T- <20K
"
E = O.Q091 ~V ~
12 r,= 19 -10_,. s
"
~10 6 8 10 12
• - 60 eo 100 120
UXJ/ TEMPERATURE {l/K]
FIGl!RE 2 log .J vs. 1/Tp of the absorption peaks of 1/Qm and £".
1- 1.0
z
e"
<{
1- I/) z
c. 0
u
u 10
a:
1-22 20
~
o . o
-' 18 19 20
w
Ci DIELECTRIC CONSTANT
e'
,_ z
FIGt:~E J Cole-Cole plot at 128K.
~ 18
"'
z
8
16u
e·
~ 14L---~--~---L--~--~----L-__J
A true Debye type relaxation would be a circle on the L'-axis.
1.0 r - - - -- - - ,
~ ~
u 0~··- ffi~ -
iOO~ ~:::.-...&...-·
200 ____ l:;....,.:::aL-.,J·300 Ii ii 0.::
4
TEMPER.l.TURE [K] ,_
. • ~ 0.2
FIC.CRf. Ptezoelectrtc resonance ~
frequencies of the funda~ental and 0 00~---~----~~----~~----~
third overtone of a bar vs. T (a), 10' v:f 10' 10' 10'
intern.:ll :::echa:1ical loss (b), :::' FREQUENCY (t"z]
(c) and ::" (d) fvr some frequen- :rct:RE 4 __ , vs.log f at 128K. cies. In (b) and (d) the shift of
the absorption peaks for different frequencies are clearly evidenced.
A Mechanis~ with a single relaxa- tion ti~e ~ould have a width, at half maxi:::um, of 1.144 decade.
l.·P RIVERA ASD H SCH~IID
of Nil is that found at 61.5K l , u, It is note~o.•orthv that :-liBr shot•s no an0maly of c wh3tsoever do~o.rn to 4K 15• Thi~ differen~.:e ---in behaviour betwe~n NiBr aQd NiL_seems tg be related to the
frequent observation that iodine boracites tend to show strong
"parasitic" birefringence and deep coloration in cert.1in growth sectors 16 - indicating a strong defect concentration - whereas in Br-and Cl-boracites such anomalies are usually weaker or hardly perceptible.
ACKNQI,'LEDGC~IE;\TS
Thanks go to R. Boutellier, E. Burkhardt and R. Cros for assistance, to ~rs 0. Hirth for typing and to the Fonds National Suisse de la Recherche Scientifique for support.
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