⁹³Nb NMR chemical shift scale for niobia systems

详细信息	查看全文 | 推荐本文 |

• 作者：Olga B. Lapina ; Dzhalil F. Khabibulin ; Konstantin V. Romanenko ; Zhehong Gan ; Mikhail G. Zuev ; Vladimir N. Krasil’ ; nikov and Vladimir E. Fedorov
• 关键词：⁹³Nb NMR ; Solid-state NMR ; Chemical shift scale ; Niobates ; High field MAS ; STMAS
• 刊名：Solid State Nuclear Magnetic Resonance
• 出版年：2005
• 期刊代码：83_09262040
• 类别：ce
• 出版时间：September 2005
• 卷：28
• 期：2-4
• 页码：204-224
• 文件大小：953 K

摘要

⁹³Nb solid-state NMR spectra of a series of inorganic niobates with Nb in different oxygen coordination environments were measured. For all studied compounds the chemical shielding and quadrupole tensor parameters were determined using conventional and ultrahigh field NMR facilities, ultrahigh speed MAS, DQ STMAS, solid-echo and computer modeling. It has been demonstrated that the ⁹³Nb isotropic chemical shift is sensitive to the coordination number of Nb sites. For the first time the ⁹³Nb NMR chemical shift scale for NbO_x polyhedra in solid materials has been proposed: for four-coordinated Nb sites, the isotropic shifts occur from −650 to −950 ppm; five-coordinated Nb sites have the isotropic shifts in the range of –900 to –980 ppm; for six-coordinated Nb sites the isotropic shifts vary from −900 to −1360 ppm; the shifts from −1200 to −1600 ppm are typical for seven-coordinated Nb sites; for eight-coordinated Nb sites the shifts are higher than −1400 ppm. The possible correlation between the value of the isotropic chemical shift and the ionic character of the NbO_x–MO_y polyhedra association has been suggested. The magnitude of the ⁹³Nb quadrupole coupling constant depends on the local symmetry of Nb sites and may vary from hundreds of kHz to hundreds of MHz.