A measurement has been made on a crystal showing multiple resonîmces due to the présence ..... [9] J . F . Nye, Physical Properties of Crystals (Oxford Science.
Journal of the Korean Physical Society, Vol. 32, February 1998, pp. S1261~S1264
Piezoelectric EfFect and Ferroelectric Properties in MnsBrOisl Boracite O. CROTTAZ, J . -P. RIVERA and H . SCHMID Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, CH-1211 Geneva 4, Switzerland Crystcils of M n a B r O i a l boracite with transparent gold électrodes have been prepared and, after an electrical poling, the single domain state of the sample verified by polarized light microscopy. Ferroelectric hystérésis loops have been measured at room température. Spontaneous polarization ajid pyroelectric coefficient of the orthorhombic phase are reported as a function of température. Piezoelectric measurements have been made using an admittance circle and motionai capacitance method. The piezoelectric coefficients are reported as a function of température for the cubic and orthorhombic phases. The efFect of the magnetic phase transition on the séries résonance frequency is also shown. A measurement has been made on a crystal showing multiple resonîmces due to the présence of tiny ferroelectric/ferroelastic domains to illustrate the necessity of opticaJ control during measurements on ferroelcistic compounds.
I. I N T R O D U C T I O N
Manganèse iodine boracite MnsByOïsI is a member of the boracite family with the gênerai formula A/aByOïaX (hereafter M — X), where Mis usually a divalent transition métal ion and X a halogen ion. Most of the boracites undergo a séries of phase transitions from a high température cubic phase with 43ml' symmetry to phases with lower symmetry (42ml', mm21', 3ml', ml',...- see Réf. [1]). At low température some of thèse compounds become simultaneously ferroelectric and ferromagnetic. The séquence of phases in Mn-I is from a cubic 43ml' phase (with a= 12.3404(3) Â at 421 K) [2] to an orthorhombic mm21' one (with a= 8.7643(4), b= 8.6980(5) and c= 12.351(1) Â at room température [3]) and to a weakly ferromagnetic m'm2' phase. The transition températures are 407 K [4] and 26 K [5], respectively. Ali three phases, being non-centrosymmetric, allow the piezoelectric effect. In addition the polar orthorhombic phases permit ferroelectricity and pyroelectricity. Piezoelectric measurements have previously been made on différent boracites using either a résonance - antiresonance method (see Réf. [6] and références therein) or the admittance circle and motionai capax:itance method [7]. In this work we measured the ferroelectric hystérésis cycles and the pyroelectric effect of Mn-I on a single domain crystal under optical control. The piezoelectric effect has been investigated using the admittance circle and motionai capacitance method. II.
facets. After removing the gold on the sides of the rods a gold wire is attached to the centre of the electroded faces. In order to obtain a single domain sample poling with an electric field of 40 kV/cm was necessary. During this procédure the crystal is put under a polarised light microscope in order to see if a single domain state is obtained. The size of the crystals for the piezoelectric measurements were 1.7 x 0.325 x 0.075 mm^ in the orientation 31 (crystal #1) and 2.96 x 0.44 x 0.075 mm^ in the orientation 32 (crystal #2), permitting a length extensionnal mode vibration along the long side of the crystal. The ferroelectric/pyroelectric measurements were also made on crystal #1. The piezoelectric measurements on a twinned sample were made on another crystal (#3) with dimensions 2.14 x 0.36 x 0.075 mm^. The form of the tensors for the piezoelectric coefficients dij and for the elastic compliances sij in the différent point groups are given in Réf. [9]. On the Mn-I samples
o r t h o r h o m b i c : dj,, s„ c u b i c : d,4, s ' „
EXPERIMENTAL
The Mn-I single crystals used in thèse measurements are prepared by a chemical vapor transport method [8]. Elongated rods parallel to the cubic direction were eut from a (OOl)cubic platelet using a wire saw. Transparent gold électrodes were deposited on the (OOl)cubic
o r t h o r h o m b i c : dgj, c u b i c : d,4, s',,
Fig. 1 . The différent piezoelectric and elastic coefficients obtainable on Mn-I boracite piezoelectric oscillators dépend -ing on the température and the sign of the electric field applied. The crystallographic directions a,b,c and the axes of the optical indicatrix n » , n^, n-, (abbreviated a , / 3 , 7 on the drawing) refer to the orthorhombic pheise.
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Journal of the Korean Physical Society, Vol. 32, February 1998
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shown in Fig. 1 it can be seen that, depending on the sign of the electric field applied during poling, two différent sets of orthorhombic coefficient can be measured. In order to simplify the notation the samples with the crystallographic direction a peurallel to the long side of the rod (allowing measurement of the coefficient dsi) are referred as "orientation 31 " and when the direction 6 is parallel to the long side of the rod (allowing meeisurement of the coefficient ^32) as "orientation 32'\e that in the cubic phase it is possible to détermine only a linear combination of the Sij cubic elastic compliances with s'il = l / 4 ( 2 s i i + 2 s i 2 + S44). The electric équivalent circuit of a piezoelectric oscillator consists in a séries RiLiCi circuit in parallel with capacitance CQ. The admittance circle and motionai capacitance technique that was used for the piezoelectric measurements has been extensively described elsewhere [7]. The main advantage of this technique is that, by measuring the whole admittance circle at the piezoelectric résonance, it allows the direct calculation of the entire set of parameters of the équivalent electric circuit. The experimentaJ détails as well as the équations used for the dérivation of thèse parameters are given in Réf. [7] and are not repeated here. A new program has been written in HP Instrument Basic language for a PC. m. FERROELECTRICITY AND PYROELECTRICITY
The Aizu species 43ml'Fmm21' being fuUy ferroelectric-fuUy ferroelastic [13], it is possible, in the orthorhombic mm21' phase, to switch the spontaneous polarization £ind the coupled corresponding ferroelastic deformation by applying a sufficiently strong electric field. The ferroelectric hystérésis loop observed by applying an a.c. electricfieldis shown in Fig. 2. The spontaneous polarization is found to be 2.5 fj.C/cra? and the rémanent
= 2.5 ^C/cm^
p, = 2.0 nC î/cm^--
\
J ^
/ -20
/ i
-10
0
10
20
ELECTRIC FIELD E [kV/cm]
Fig. 2. Hystérésis loop of Mn-I boracite at room température on a 1.7 X 0.325 x 0.075 mm^ crystal (synthesizer H P 3325A, operational amplifier K E P C O BOPlOOOM, oscilloscope HP 141B with a 1 p F capacitor and Sawyer and Tower electric circuit).
>
i
-
/
/
-
p = - d FV / d T
\ 150 O
0) 100 ' ô
_
50
t3
0)
0
20
40
60
80
100
120
140
Température [°C]
Fig. 3. Spontaneous polarization and pyroelectric coeffi -cient of orthorhombic Mn-I boracite vs. température (electrometer Keithley 617, A T / t a 3 "C/min). The Curie température of the 43ml'-mm2l' transition is 134 " C (407 K ) .
polarization 2.0 //C/cm^ (at 300 K). The coercive field is 9 kV/cm (measurement using the Sawyer and Tower method, [10] at 50 Hz). The rémanent polarization as a function of température is shown in Fig. 3. The sUght différence between its room température value and that reported in Fig, 2 is due to expérimental inaccuracies. Contrary to what has been reported elsewhere [11] the phase transition is clearly of first order when measuring spontaneous polarization. This discrepancy is due to the fact that in Réf. [11] the pyroelectric coefficient was measured and the spontaneous polarization obtained by intégration, but without taking account of the intégration constant. This makes the phase transition to appear of second order and gives an erroneous value of the spontetneous polarization (which is also indicated in [12], where the values for Mn-Br and Mn-Cl are probably also incorrect for the same reason). The pyroelectric coefficient, obtained as the derivative of the spontstneous polarization with respect to température, is also shown in Fig. 3. The values obtained are close to those determined in [12]. IV.
= 9 kV/
