Univ Sao Paulo, Inst Fis Sao Carlos, CEP 369, BR-13560590 Sao Carlos - Brazil
Número total de Afiliações: 2
Tipo de documento:
Journal of Physical Chemistry C;
FEB 18 2016.
Citações Web of Science:
Network former-network modifier interactions in three sodium borophosphate glasses have been probed by B-11[Na-23] and P-31[Na-23] rotational echo double resonance (REDOR) experiments. For the compositions chosen essentially all of the boron atoms in these glasses are four-coordinated and, thus, of formally anionic character, whereas the majority of the phosphorus species are of the (nominally charge-neutral) P-(3) type. Despite this fact, the dipolar second moments M-2(B-11[Na-23]) and M-2(P-31[Na-23]) extracted from these REDOR data indicate that the sodium cations interact significantly more strongly with the phosphate than with the borate species. As a matter of fact, the distance sums Sigma r(P-Na)(-6) calculated from these second moments are close to those measured in sodium ultraphosphate glasses with similar sodium concentrations, whereas the corresponding Sigma r(B-Na)(-6) values are significantly smaller than those measured in sodium borate glasses with comparable sodium concentrations. These results are corroborated by complementary Na-23[P-31] and Na-23[B-11] REDOR experiments. As supported by bond valence arguments, the formation of P-(3)-O-B-(4) linkages in borophosphate glasses results in a charge displacement away from the four-coordinated boron atoms toward the nonbridging oxygen atoms on the P-(3) units. These charge dispersal effects result in shallower Coulomb traps, thereby facilitating ionic mobility. (AU)