Structural Studies of NaPO3-MoO3 Glasses by Solid-State Nuclear Magnetic Resonance and Raman Spectroscopy
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- 作者：Silvia H. Santagneli ; Carla C. de Araujo ; Wenzel Strojek ; Hellmut Eckert ; Gaë ; l Poirier ; Sidney J. L. Ribeiro ; Younes Messaddeq
- 刊名：Journal of Physical Chemistry B
- 出版年：2007
- 出版时间：August 30, 2007
- 年：2007
- 卷：111
- 期：34
- 页码：10109 - 10117
- 全文大小：241K
- 年卷期：v.111,no.34(August 30, 2007)
- ISSN：1520-5207
文摘
Vitreous samples were prepared in the (100 - x)% NaPO3-x% MoO3 (0 
x 70) glass-forming system bya modified melt method that allowed good optical quality samples to be obtained. The structural evolution ofthe vitreous network was monitored as a function of composition by differential scanning calorimetry (DSC),Fourier transform infrared spectroscopy (FT-IR), Raman scattering, and solid-state nuclear magnetic resonance(NMR) for 31P, 23Na, and 95Mo nuclei. Addition of MoO3 to the NaPO3 glass melt leads to a pronouncedincrease in the glass transition temperatures up to x = 45, suggesting a significant increase in networkconnectivity. For this same composition range, vibrational spectra suggest that the Mo6+ ions are bonded tosome nonbridging oxygen atoms (Mo-O- or Mo=O bonded species). Mo-O-Mo bond formation occursonly at MoO3 contents exceeding x = 45. 31P magic-angle spinning (MAS) NMR spectra, supported by two-dimensional J-resolved spectroscopy, allow a clear distinction between species having two, one, and zeroP-O-P linkages. These sites are denoted as Q(2)2Mo, Q(2)1Mo, and Q(2)0Mo, respectively. For x < 0.45, thepopulations of these sites can be described along the lines of a binary model, according to which each unitof MoO3 converts two Q(2)nMo sites into two Q(2)(n+1)Mo sites (n = 0, 1). This structural model is consistentwith the presence of tetrahedral Mo(=O)2(O1/2)2 environments. Indeed, 95Mo NMR data suggest that themajority of the molybdenum species are four-coordinated. However, the presence of additional six-coordinatemolybdenum in the MAS NMR spectra indicates that the structure of these glasses may be more complicatedand may additionally involve sharing of network modifier oxide between the network formers phosphorusand molybdenum. This latter hypothesis is further supported by 23Na{31P} rotational echo double resonance(REDOR) data, which clearly reveal that the magnetic dipole-dipole interactions between 31P and 23Na areincreasingly diminished with increasing molybdenum content. The partial transfer of modifier from thephosphate to the molybdate network former implies a partial repolymerization of the phosphate species, resultingin the formation of Q(3)nMo species and accounting for the observed increase in the glass transition temperaturewith increasing MoO3 content that is observed in the composition range 0 x 45. Glasses with MoO3contents beyond x = 45 show decreased thermal and crystallization stability. Their structure is characterizedby isolated phosphate species [most likely of the P(OMo)4 type] and molybdenum oxide clusters with a largeextent of Mo-O-Mo connectivity.
x 70) glass-forming system bya modified melt method that allowed good optical quality samples to be obtained. The structural evolution ofthe vitreous network was monitored as a function of composition by differential scanning calorimetry (DSC),Fourier transform infrared spectroscopy (FT-IR), Raman scattering, and solid-state nuclear magnetic resonance(NMR) for 31P, 23Na, and 95Mo nuclei. Addition of MoO3 to the NaPO3 glass melt leads to a pronouncedincrease in the glass transition temperatures up to x = 45, suggesting a significant increase in networkconnectivity. For this same composition range, vibrational spectra suggest that the Mo6+ ions are bonded tosome nonbridging oxygen atoms (Mo-O- or Mo=O bonded species). Mo-O-Mo bond formation occursonly at MoO3 contents exceeding x = 45. 31P magic-angle spinning (MAS) NMR spectra, supported by two-dimensional J-resolved spectroscopy, allow a clear distinction between species having two, one, and zeroP-O-P linkages. These sites are denoted as Q(2)2Mo, Q(2)1Mo, and Q(2)0Mo, respectively. For x < 0.45, thepopulations of these sites can be described along the lines of a binary model, according to which each unitof MoO3 converts two Q(2)nMo sites into two Q(2)(n+1)Mo sites (n = 0, 1). This structural model is consistentwith the presence of tetrahedral Mo(=O)2(O1/2)2 environments. Indeed, 95Mo NMR data suggest that themajority of the molybdenum species are four-coordinated. However, the presence of additional six-coordinatemolybdenum in the MAS NMR spectra indicates that the structure of these glasses may be more complicatedand may additionally involve sharing of network modifier oxide between the network formers phosphorusand molybdenum. This latter hypothesis is further supported by 23Na{31P} rotational echo double resonance(REDOR) data, which clearly reveal that the magnetic dipole-dipole interactions between 31P and 23Na areincreasingly diminished with increasing molybdenum content. The partial transfer of modifier from thephosphate to the molybdate network former implies a partial repolymerization of the phosphate species, resultingin the formation of Q(3)nMo species and accounting for the observed increase in the glass transition temperaturewith increasing MoO3 content that is observed in the composition range 0 x 45. Glasses with MoO3contents beyond x = 45 show decreased thermal and crystallization stability. Their structure is characterizedby isolated phosphate species [most likely of the P(OMo)4 type] and molybdenum oxide clusters with a largeextent of Mo-O-Mo connectivity.