Measurements of chain length distributions in calcium phosphate glasses using 2D 31P double quantum NMR

P-31 double quantum (DQ) NMR is used for improved studies of the structure of phosphate glasses. The common Q(n) ts with n bridging oxygen atoms is extended to Q(n,jkl) indicating the Q type of the bonded adjacent units by the additional superscripts j,k and l. It will be shown that: (i) Q(1) units have different isotropic chemical shifts depending on whether they form diphosphate anions in the amorphous state (Q(1,1)) or end groups of chains (Q(1,2)) and (ii) that even Q(2) units possess different isotropic chemical shifts such that in the DQ dimension Q(2,11) can be distinguished from Q(2,12) and middle groups of longer chains or rings (Q(2,22)). Based on this result the opportunity for measuring chain length distributions in the amorphous state is discussed and first results for binary calcium phosphate glasses are presented. The advantages and also the limitations of this novel approach will be discussed. (C) 1998 Elsevier Science B.V. All rights reserved.