Electronic parameters of Sr2Nb2O7 and chemical bonding

X-ray photoelectron spectroscopy (XPS) measurements were carried out on a strontium pyroniobate (Sr2Nb2O7) powder sample, which was synthesized using standard solid-state method. The binding energy (BE) differences between the O 1s and cation core levels, Δ(O–Nb)=BE(O 1s)–BE(Nb 3d5/2) and Δ(O–Sr)=BE(O 1s)–BE(Sr 3d5/2), were used to characterize the valence electron transfer on the formation of the Nb–O and Sr–O bonds. The chemical bonding effects were considered on the basis of our XPS results for Sr2Nb2O7 and earlier published structural and XPS data for other Sr- or Nb-containing oxide compounds. The new data point for Sr2Nb2O7 is consistent with the previously derived relationship for a set of Nb5+-niobates that Δ(O–Nb) increases with increasing mean Nb–O bond distance, L(Nb–O). A new empirical relationship between Δ(O–Sr) and L(Sr–O) was also obtained. Interestingly, the correlation between Δ(O–Sr) and L(Sr–O) was found to differ from that between Δ(O–Nb) and L(Nb–O). Possible cause for the difference is discussed.