Polarization-optical studies of the polymorphism of the ferroelectric phases of Zn-Cl-, Co-Co, Fe-Cl- and Fe-Br-boracites

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Polarization-optical studies of the polymorphism of the ferroelectric phases of Zn-Cl-, Co-Co, Fe-Cl- and Fe-Br-boracites

SCHMID, Hans, KLIEGL, G., KOBAYASHI, Jinzo

SCHMID, Hans, KLIEGL, G., KOBAYASHI, Jinzo. Polarization-optical studies of the polymorphism of the ferroelectric phases of Zn-Cl-, Co-Co, Fe-Cl- and Fe-Br-boracites.

Helvetica Physica Acta , 1969, vol. 42, no. 4, p. 599

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~l!j.lJT<tlUill

H E LV E TICA. PHYS lC A A CTA

\'olumcu 4-2, Fascic-ulus <Jlmrtus (19(>'))

I

Polarisation-optical Studies of the Polymo!·phism of the Ferroelectric Phases of zn-01-, Co-co··, Fe-el- and F!!-nr- "Cor~t-:::itc!)

by H. ScH~tw anti l;. 1\LII>Gr.

(Battelle Institute, Gencnt Research Centre, Switzerl:tnd)

and j. J<OOAYASH.I

(Wascda University, Tokyo, Japan)

The birefringence in the indicatrix' principal sections of the polymorphic ferro- electric ph;1 es of Zn-CJ-, Co-Cl-, Fe-CI- ami Fe-Br- boracites has been measured between room temperature and the cubic-orthorhombic transition temperature. Con- tributions of the spontaneous linear and quadratic electro-optic effect to the spon- taneous birefringen\"e are discussed .. ·\ new phasc was found to occur in t!te Co-CI-, Fe-Cl- and Zn-CJ- boraciles, intermediate between the miil2 and 3m phase; the obscn·ed relevant twinning law (composition plane perpendicular to the original mm2 polar direction, i.e. loss of the twofold axis) is in favour of the monoclinic point group m. Combinjng the data of the present study <mel of previous work [1) [2] [3], the aul11ors summarize, for those boracitcs that are characterized by a 31Jl. plta~e. the following phase seq ucnces and transition temperatures:

-

Fe-Cl-B 43m. -<- - -). ·mm2 <- - -> m ^{-<- ·- _,.} 3m ^<- - -)> 3m'

-

Fe-Br-B 43m <- - -)> 111'1112 -<

,..

-

Fe-1-B 43m -< - -+ ^11/,11/2 <-···· ... ... ,.. _3m -<- - -)> 31n'

-

Co-Cl-B 43m -<- - -)· mm2 -<

,..

Zn-Cl-B 43m

__ ^,..

<- - - mm2 ^---· _-< ^{· - -}_-

....

____ ^,.

< - - 3m

786°1\ 5G7°K ·1::!0°K

If not otherwise indic-ated, thermal liystnesis ranged between 0.1 and 1 deg-ree.

It is noteworthy that the ferromagnetic phase has most probably the ferromag- neto-electric point group 3m' L+].

Hrfcreucrs

[1] H. ScHMID and J. l\L TRoosmR, Solid State Comm. 5, 31 (1967).

[2) ]. 1\0BAYASIII, H. SCJH11l) and E .. \scHEll, rby~. St;"~t. Svl. ::6. 277 (1968j.

[3) G. Qul~ZEL ct H. SCHMID, Solid State Comm. 6, '14 7 (l9b~).

[4) H. Sctn: !D, to appear.

') A full account will be published elsewhere.