Eur. J. Mineral. 1992, 4, 1241-1249

Dielectric constants of BaO and melilites and

the oxide additivity rule

Robert D. SHANNON

E.I. Du Pont de Nemours and Co., Inc., Central Research and Development, Experimental Station
Wilmington, DE 19880-0356, USA

Kazuaki LISHI

Department of Mineralogical Sciences and Geology, Yamaguchi University
Yoshida, Yamaguchi 753, Japan

Toomas H. ALLIK

Science Applications Intemational Corporation, 1710 Goodridge Dr.
P.O. Box 1303, McLean, VA 22102, USA

George R. ROSSMAN

Division of Geological and Planetary Sciences, California Institute of Technology
Pasadena, CA, 91125, USA

Josef LIEBERTZ

Institut fur Kristallographie der Universität zu Köln, Ziilpicher Strasse 49
D-5000 Köln 1, Germany

Abstract

The constant-stress "free" dielectric constants and dissipation factors of BaO and a series of X₂YZ₂O₇ melilites were measured at 1 MHz using a two-terminal method and empirically determnined edge corrections. The results are:

BaO K = 31.1
Ca2MgSi2O₇ KTa = 11.83 KTc = 7.97
Ca2CoSi₂O₇ KTa = 12.44 KTc = 9.00
Ca2ZnSi₂O₇ KTa = 12.03 KTc = 8.21
Sr2CoSi₂O₇ KTa = 8.93 KTc = 6.87
Ba2MgGe2O₇:Nd,Ce KTa = 8.75 KTc = 7.51

The discrepancies between measured dielectric polarizabilities as determined from the Clausius-Mosotti equation and those calculated fromn the sum of oxide polarizabilities according to (aD(X₂YZ₂0₇) = 2 (aD(XO) + aD(YO) + 2 (aD(Z0₂) are + 13.6 to + 16.0% for Ca-containing melilites, + 6.1% for Sr₂CoSi₂O₇ and + 1.0% for Ba_1.92Nd_0.04Ce_0.04MgGe₂O₇. The deviations from additivity in the Ca- and Sr-mnelilites are believed to result from structural misfit between the alkaline earth X ion and the YZ₂O₇ layers which leads to rattling X ions.