Relationships between bridging oxygen O quadrupolar coupling parameters and structure in germanates

We have performed ab initio calculations on the model cluster (OH)3Ge±O±Ge(OH)3 in order to re®ne the relationships between O quadrupolar coupling parameters and the local structure around the bridging oxygen. From these calculations the trend in the bridging oxygen O quadrupolar coupling constant, Cq, and asymmetry parameter, gq, with changing Ge±O±Ge angle was found to be similar to previously established trends in silicate clusters with the overall Cq values being systematically increased in magnitude by approximately 3 MHz. Such a shift can be attributed to di€erences in cation±oxygen distances as well as coordinating cation electronegativity. In addition, we also investigated the e€ect of changing intratetrahedron bond angles over a range that has been suggested to exist in germanate systems. While such variations in intratetrahedron bond angles lead to a small variation in Cq, gq remains relatively una€ected. In such cases, gq can still be a reliable probe of Ge±O±Ge angle while Cq could serve as a probe of tetrahedron distortions. Ó 2000 Elsevier Science B.V. All rights reserved.