Charge Compensation in Sodium Borophosphate Glasses Studied by ¹¹B{²³Na} and ³¹P{²³Na} Rotational Echo Double Resonance Spectroscopy

Abstract

Network former–network modifier interactions in three sodium borophosphate glasses have been probed by 11B{23Na} and 31P{23Na} 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 M2(11B{23Na}) and M2(31P{23Na}) 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 ∑rP–Na–6 calculated from these second moments are close to those measured in sodium ultraphosphate glasses with similar sodium concentrations, whereas the corresponding ∑rB–Na–6 values are significantly smaller than those measured in sodium borate glasses with comparable sodium concentrations. These results are corroborated by complementary 23Na{31P} and 23Na{11B} 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.