Article
Reference
Dielectric properties of orthorhombic Mn
3B
7O
13Cl,Mn
3B
7O
13Br, Mn
3B
7O
13I boracites
CASTELLANOS-GUZMAN, Guillermo A., et al.
Abstract
The dielectric, pyroelectric, optical, and thermal properties of Mn-Cl, Mn-Br and Mn-I boracites were investigated in the ferroelectric orthorhombic phase (space group Pca21). The peculiar and very different behavior of the dielectric constant, e33, in each compd. is consistent with the predictions of current phenomenologic theories on improper phase transitions.
CASTELLANOS-GUZMAN, Guillermo A., et al. Dielectric properties of orthorhombic Mn3B7O13 Cl,Mn3B7O13Br, Mn3B7O13I boracites. Ferroelectrics, 1981, vol. 36, no. 1, p. 411-414
DOI : 10.1080/00150198108218141
Available at:
http://archive-ouverte.unige.ch/unige:33207
Disclaimer: layout of this document may differ from the published version.
1 / 1
Ferroelectrics, 1981, Vol. 36, pp. 411-414 0015-0193/81/3601-0411/$06.50/0
© 1981 Gordon and Breach, Science Publishers, Inc.
Printed in the United States of America
DIELECTRIC PROPERTIES OF ORTHORHOMBIC Mn 3B
7
o
13cl, Mn 3B
7
o
13Br, AND Mn
3B 7
o
13I BORACITES
a a b AND P.TISSOTb
A.G.CASTELLANOS-GUZMlN , J.C.BURFOOT , H.SCHMID ,
a Physics Department, Queen Mar.y College, University of London b Department of Mineral, Ana~tical and Applied Chemistr.y,
University of Geneva
Abstract. Dielectric, pyroelectric, optical and thermal properties of Mn-Gl, Mn-Br and Mn-I boracites have been investigated in the ferroelectric
orthorhombic phase (space-group Pca2
1). These are the first such
measurements made on a triad of closely related boracites. The peculiar and ver,y different behaviour of dielectric constant, E , in each compound seems to be consistent with the predictions of current 33phenomenological theories on improper phase transitions.
INTRODUCTION
Halogen boracites are compounds. with the general formula Me
3B7
o
13x,
-wh&re-.Me is a divalent metal and X stands for Cl, Br or I. A boracite isconventionally referred to by its metal and halogen only, with a hyphen, e.g. Mn-Gl is written for Mn
3B 7
o
13
c1.
Most haloge!l boraci tes have a_high-temperaturenon-polar phase (space-group F43c; point-group 43m) which transforms, on
~~ooling through the transition temperature T , to a pyroelectric improper
~erroelectric orthorhombic phase (space-gtou~ Pca2 ; point-group mm2). Interest in these materials arose because their unusual di~lectric, ferromagnetoelectric and optical properties make thea potentially attractive for device applications, and also for testing phenomenological theories on improper phase transitions.
This paper presents measurements of the temperature dependence of dielectric constant, f , pyroelectric coefficient p, spontaneous polarisation P , and
spontaneous33birefringence bn perpendicular to P , in the
orthorhom~ic
phase, as well as a DTA stu~, near T ,9of a triad of borac!tes which haveMn
as the metal, i.e. Mn-Gl, Mn-Br, and Mn£I.EXPERIMENTAL
We1grew Mn-X single cr,ystals by vapour transport reactions in closed quartz am- poules • Plates were ground parallel to cubic {100} natural faces and polished with one-micron diamond paste. Semitransparent gold on chromium electrodes were
exaporated onto the principal2faces. Sample2thickness ranged usually from 60flm to 80pm, and area from 0.008 cm to 0.025 cm • During dielectric and pyroelectric
411
412 A.G.CASTELLANOS-GUZM.AN, J .C.BURFOOT, H.SCHMID, P.TISSor
measurements below T the single-domain s~ate of the samples was controlled by continuous observatiog of the birefringence under a polarising microscope. Paling
-1 -1
was typical~ by application of 10 kVcm to 60 kVcm just below T for ten hours.
c DIELECTRIC MEASUREMENTS
The stress-free dielectric constant f parallel to P was measured at 1 MHz, as a function of temperature, for the thre~3 compositions,swith a test signal of 15 mV. The results are shown in figure 1. The dielectric anoma~ in Mn-Cl is similar to that reported for some other boracites3,4,5, i.e. E
33 jumps downward on cooling through the transition. Mn-Hr shows almost no anomaly. A prominent feature in this boracite is that €
33 is independent of the temperature above Tc' thus fulfilling the
7predictions of the theories of Dvorak and Petzelt6
and of Gufan and Sakhnenk.o on improper phase transitions. In Mn-I £
3;_ jumps upward
8at Tc .-.
on cooling. Such behaviour had onzy- previous~ been observed in ""tru-61 boraci te • Our results for Mn-I are in agreement with an independent measurement made at 10 kHz9. These different anomalies can be explained by the theor.y10• However another theoretical model exists11'12 which can explain not
on~
such behaviour but also the variation ofE
33 with temperature above Tc as observed in Mn-Cl and Mn-I. As determined optical~, T was 411±2°C in Mn-Cl, 276±2°C in Mn-Br, and
134±2°C in Mn-I. c
PYROELECTRIC MEASUREMENTS
We used the charge-integration technique of Glass 13 which measures the total pyroelectric coefficient, primar.y plus secondar.y. The specimen mounts were designed to be as near~ as possible stress-free. We measured p from specimens selected to be visual~ free of defects and domain walls, and we measured from room
temperature to a few degrees below T • Our resmlts are shown in figure 2. Values c
of P at 25°C were 0.761±0.014x10-2Cm-2 in Mn-Cl, -0.262t0.024x10-2Cm-2 in Mn-Br, s
and 0.924+0.015x10-2
em-
2 in Mn-I. In Mn-Br, p reverses its sign at about 140°C.This behaviour was observed in all samples studied. The origin of this effect is as yet ~~wn though several possible explanations have been given in the
literature • We believe
14that the region of negative values is not due to a metastable phase transition for we observed no dependence of p on dT/dt.
BIREFRINGENCE MEASUREMENTS
The behaviour ofDn versus temperature is well documented for most boracites2
so we concentrated oursattention on~n (perpendicular toP) in Mn-I. Birefringence,
s s
dn , was measured for Mn-I samples, ranging in thickness from 75 to 240pm, by means ofs a Leitz tilting compensator type M and compensating the path difference visual~.
Our measurements in white light gave indications of a high~ dispersive crystal
(figure 3). As a function of temperature, ~n increases as temperature is raised, and starts to decrease at about 110°C. In the cas~s of Mn-cl and Mn-Br our measurements of 2 6n as a function of temperature are in agreement with those reported in the literature •
S
DIELECTRIC PROPERTIES OF ORTHORHOMBIC Mn 3B
7
o
13
x
BORACITES 413-32
r1
""
w
f-
z 24
~ (f) 0 z
u u 16
0:::
f-u
Mn-CI
~
w 8298t~ __ L _ _ L I 398 _ _ L__!L__L~IL_~~~L-~
498 598 698
Figure l .
...
07
~s lC
TEMPERATURE (K)
Temperature Dependence of the Dielectric Constant in MN-X Boracites
0
D
D D
0: 1600 u
~ w
I
u 1200 u:::
u-
w-o._l (f).::.::: 800 o:::-
:5~ 0 0
~E
'400
u~ - : : l O:::o
<t:-,
co-o
(f)
400 500 600 700 ... 370
WAVELENGTH ( nm)
'.::.:::
N 'E
'f'o260
~ t:
~
f--~ 180
u
fE
w8
100398 498 598 698
TEMPERATURE (K)
Figure 2. Temperature Dependence of the Pyroelectric Coefficient in MN- X Boraci tes
Mn-I Mn-Br
410 V 520 560 TEMPERATURE
/630 (K)
Mn- Cl
670 710 I'
Figure 3. Birefringence as Function of Wavelength in MN-I Boracite
(Sample Thickness 240 ~m) Figure 4. Isobaric Molar Specific Heat, Cpt of MN-X Boracites in the Vicinity of the Cubic-Orthorhombic Phase- Transition
414 A.G.CASTELLANOS-GUZMAN, J .C.BURFOOT, H.SCHMID, P.TISSOT ORDER OF TRANSITION
Gufan and Sakhnenko's theory7 shows that the transition should be first order, but we have not been able to observe any displacement of T larger than
t
degree byc
applied fields up to 40 kVcm- 1; nor have we seen any ~steresis larger than 2°. A differential thermal ana~sis (DTA) near T was made in a Mettler,type TA2000. Samples of Mn-Gl, Mn-Br, and Mn-I were as-grownc single crystals weighing 7.32, 16.07, and 43.87 mg respective~. The ref~rence substance used was Al
2
o
3 powder heat-treated at 1000°C and we used the 429.75°! transition in indium metal for calibration.
Measurements were made under open-circuit conditions. In all cases the transition was an endothermic process. Visual observation of th~
1
tramsition at the same scanning speed as in the DTA measurements (2 deg min ) showed it to occur over a small temperature range (4 K for Mn-Gl and Mn-Br, and 10 K for Mn-I). The specific heat, C , for the three compositions, near T , is shown in figure 4 •• The1enthalpy, ~
~Ht' p and entropy, 6St,of transition werec found to be 5527t404 Jmole- , 8.09±0.60 J mole-1
K-1
for Mn-Cl; 3196±141 J mole-1,
5.84±0.25
J mole-1K-1 for Mn-Br, and 1869±103 J mole-1, 4.63t0.25 J mole-1K-1 for Mn-I. We observed that many samples showed multiple peaks in the measurements near T similar to those reportei for other boracites15. It had been suggested that the mul~iple peaks might be caused by
internal stresses in the crystals. In our case, however, annealing procedures tried for Mn-X boracites were ineffective; we discarded samples, showing multiple peaks, for DTA evaluation. Possibly this effect is due to different sector growths having
slight~ different transition temperature.
It has been suggested that the first-order character of this transition
decreases with decreasing mass and size of the halogen 16 , so that Mn-I would be more
strong~ first order. However our transition enthalpy values imply that the transition in Mn-Gl and Mn-Br is more strongly first-order whilst that of Mn-I is
close to being second order; this is further supported by our pyroelectric m~ements.
Support by Consejo Nacional de Ciencia y Tecnologia de Mexico for one of us
(A.G.C-G) and by the Fonds National Suisse de la Recherche Scientifique is gratefully acknowledged.
REFERENCES
(1) H.Scbmid, J.Pgys.Chem.Sol. 26 973 (1965)
(2) H.Schmid and H.Tippmann, Ferroelectrics 20 21 (1978)
(3) F.Ascher, H.Schmid and D.Tar, Sol.State Commun. 2 45 (1964) (4) F.Smutny and C.Konak, Pgys.Stat.Sol.(a)
l1
151 (1975)(5) B.G.Bochkov,H.D.Gavrilova,V.K.Novik,V.A.Kopsik, Sov.Pgys.C;ystal.20 404 (1975) (6) V.Dvorak and J.Petzelt, Czech.J.Pgys. B21 1141 (1971)
(7) Yu M.Gufan and V.P.Sakhnenko, Sov.P s:SOl.State
1!
1660 (1973) (8) H.Schmid and L.A.P~termann, Pgys.Stat.Sol. a11
K147 (1977) (9) A.Shaulov, W.A.Smith and H.Schmid, Berroelectrics (in the press) {10) V.Dvorak, Ferroelectrics 1 1 (1974)(11) A.P.Lev~ and D.G.Sannikov, Sov.P~s.Sol.State 17 327 (1975) (12) D.G.Sannikov, JETP Letts., lt(6) 313 1980) --
(13) A.M.Glass, J.Appl.Phys., ~ 4699 (1969) (14) S.B.Lang, Pgys.Rev.!
i
3603 (1971)(15) M.Delfino, G.M.Loicono, W.A.Smith, P.S.Gentile,J.Sol.State Chem.33 107 (1980) (16) P.Felix, M.Lambert, R.Comes and H.Schmid, Ferroelectrics
