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

Dielectric constants of BaO and melilites and

the oxide additivity rule



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


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


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



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 Ba1.92Nd0.04Ce0.04MgGe2O7. 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.