Dielectric properties of boracites and evidence for ferroelectricity. ASCHER, Edgar, SCHMID, Hans, TAR, D. Abstract

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1 Article Dielectric properties of boracites and evidence for ferroelectricity ASCHER, Edgar, SCHMID, Hans, TAR, D. Abstract The dielectric constants of Ni-Cl, Ni-Br, and Co-I boracites have each a maximum at a temperature hich coincides ith that of a polar to a nonpolar phase transformation hich as observed optically. The motion of ferroelectric domain alls under an applied electric field has been sought in Ni-Cl boracite and as found to occur at temperatures as far as 8 belo that of the phase transition. Reference ASCHER, Edgar, SCHMID, Hans, TAR, D. Dielectric properties of boracites and evidence for ferroelectricity. Solid State Communications, 1964, vol. 2, no. 2, p DOI : 1.116/38-198(64)9571-X Available at: Disclaimer: layout of this document may differ from the published version.

2 Solid State Communications Vol. 2, pp , Pergamon Press, Inc. Printed in the United States. AND EVIDENCE FOR FERROELECTRICITY E. Ascher, H. Schmid and D. Tar Battelle Memorial Institute, International Division Geneva, Sitzerland (Received 17 January 1964 by G. Busch) The dielectric constants of Ni-Cl, Ni-Br, Co-Br and Co-l boracites have each a maximum at a temperature hich coincides ith that of a phase transformation e have observed optically. Motion of ferroelectric domain alls under an applied electric field has been sought for in Ni-Cl boracite and found to occur at temperatures as far as soo belo that of the phase transition. Introduction BORACITES are compounds ith the composi - tion M3B713X here M stands for one of the divalent ions Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, and X denotes one of the halogens Cl, Br, I. Solid solutions of these compounds exist. We have also indications that boracites may be formed ith other divalent ions than those mentioned above and that halogens may be replaced by OH. Ito and collaboratorsl have determined the crystal structure of the mineral called boracite: Mg3B713Cl; there exists a piezoelectric, optically isotropic high temperature phase ith space-group Tg (F43c) and a pyreelectric, optically anisotropico-temperature phase ith space group C2v (-Pea). Boracites have been investigated already for more than one century; recently Le Corre, Jona, and Sonine and Zheludev2 have investigated hether boracites are ferro- electric. With a vie to verifying a orking hypothesis on the existence of electric polarization in paramagnetic crystals, e have prepared single crystals of boracites and measured their electric, magnetic and optical properties. Here our principal aim is 45 to report our first findings concerning the dielectric properties of boracites. Other qu,estions ill be taken up in subsequent papers. Dielectric constant FIG. 1 shos the dielectric constants of the Ni-Cl, Ni-Br, Co-Br, Co-l boracites as a function of temperature. In all these cases e find a maximum of the dielectric constant. The temperature at hich the maximum occurs for each of these compounds is indicated in Table 1. In to cases (Ni-Br, Co-l) e observe a thermal hysteresis. Ni-l boracite as also investigated; the dielectric constant shos no maximum beteen soo and 5 K. Phase transition WE have investigated hether there is a phase transition corresponding to the maximum of the dielectric constant. The phase transition can be observed optically, because the lo temperature phase is optically anisotropic, hereas the high temperature phase is not. The temperatures of the phase transition are reported in Table 1 together ith data obtained by Heide3 and Jona2. From this table it may be seen that the maximum of the dielectric constant occurs at

3 46 Vol. 2, No z < 1-8 en z 6 ii 1-4..J c.o 2.2 a:: t;.1 en (I)..J N1 3 B 7 13 Cl 48MHz V (loo) V...,.. V V V < I CI) 1 ii 9 I- ()..J 8 Co 3 B 7 13 Br (Ill) 1kHz /. J r---t TEMPERATURE ( C) TEMPERATURE ( C) r-... t t 1_.. Co I lookhz -1 -eo -6o -4o -2 TEMPERATURE ( C) (loo) z <en 1-!::: (I) z 6 >- ::4 oct -a::!:: 2..Jet 25 Nl 3 B 7 13 Br (Ill) 5 khz, recorded...-:::-,-;:;-!, TEMPERATURE ( C) 175 FIG. 1 Dielectric constants of some boracites

4 Vol. 2, No a) E = kv 1 cm b) E=O c) kv E=-35- cm d) E=O e) kv E=-53-, cm f) E=O FIG. 2 Motion of ferroelectric domain alls in Ni3B713Cl

5 ' 48 Vol. 2, No. 2 TABLE 1 I' Cl Br I '... phase maximum phase transition of e transition OK OK OK Mg 538 ) Cr <1 * Mn G *; Fe 67 k Co G23 * 458 c Ni 61, Cu 365 * Zn 723 * Cd X maximum phase maximum of e transition of OK OK OK < <SO < the transition beteen a polar and a non-polar phase. Domain all motion WE have looked for domain all motion in Ni Cl boracite under an applied electric field and found this phenomenon in several cases, of hich an example is shon in Fig. 2. The specimen as prepared as a (111) plate ith a surface of about 4 mm2 and a thickness of. 22 mm. The all movement as observed at 52 7 ± 1 o K, i. e. ell belo the temperature of phase transition (61 K). In the photographs one sees essentially three domains that are separated by to (loo) planes hich appear as bands of interference fringes. The angle hich these planes make ith the free surface (as calculated from the idth of the bands and the thickness of the crystal) is, ithin experimental error, equal to the theoretical value of 5444'. The direction of polarization is that of one of the six [1] diretions; thus the polarization is either perpendicular or parallel to the domain all and makes an angle of 1516' ith the surface. The photographs 2a to 2f are given in chronological order. The voltage is applied in the plane of the plate, to its upper and loer edges. The average field strength is indicated, a plus sign meaning that the positive voltage is applied to the loer edge. Conclusion WE have shon that Ni-Cl boracite is ferroelectric. Experiments in progress lead us to believe that this is also the case for most of the other boracites. Acknoledgement We ish to thank Prof. A. Janner and Prof. B. Elschner for their decisive help in the initial stages of our research on boracites.

6 Vol. 2, No References 1.!TOT., MORIM:OTO N. and SADANAGA R., Acta Cryst. 34 (1951). 2. LE CORRE Y., J. Phys. Radium 18, 629 (1957); JONAF., J. Phys. Chem. 63, 175 (1959); SONINE A. S. and ZHELUDEV I. K', Kristallograiiya ' 183 (1963). 3. HEIDE F., WALTER G., and URLAU R., Naturissenschaften 48, 97 (1961). 4. See e. g. SMOLENSKI G. A., Izvestija Akademii Nauk SSR, serija fizicheskaja 2, 163 (1956). Les constantes dielectriques des boracites de Ni-Cl, de Ni-Br et de Co-I passent par un maximum a une temperature qui coi"cide avec celle d'un changement de phase observe par voie optique. Dans la boracite de Ni-Cl, les parois de domaine se deplacent sous l'effet d'un champ electrique applique, jusqu'a des temperatures inferieures de 8 a celle du changement de phase.