Santagneli Silvia H, de Araujo Carla C, Strojek Wenzel, Eckert Hellmut, Poirier Gaël, Ribeiro Sidney J L, Messaddeq Younes
Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany.
J Phys Chem B. 2007 Aug 30;111(34):10109-17. doi: 10.1021/jp072883n. Epub 2007 Aug 7.
Vitreous samples were prepared in the (100 - x)% NaPO(3)-x% MoO(3) (0 <or= x <or= 70) glass-forming system by a modified melt method that allowed good optical quality samples to be obtained. The structural evolution of the 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 (31)P, (23)Na, and (95)Mo nuclei. Addition of MoO(3) to the NaPO(3) glass melt leads to a pronounced increase in the glass transition temperatures up to x = 45, suggesting a significant increase in network connectivity. For this same composition range, vibrational spectra suggest that the Mo(6+) ions are bonded to some nonbridging oxygen atoms (Mo-O- or Mo=O bonded species). Mo-O-Mo bond formation occurs only at MoO(3) contents exceeding x = 45. (31)P magic-angle spinning (MAS) NMR spectra, supported by two-dimensional J-resolved spectroscopy, allow a clear distinction between species having two, one, and zero P-O-P linkages. These sites are denoted as Q(2)(2Mo), Q(2)(1Mo), and Q(2)(0Mo), respectively. For x < 0.45, the populations of these sites can be described along the lines of a binary model, according to which each unit of MoO(3) converts two Q(2)(nMo) sites into two Q(2)((n+1)Mo) sites (n = 0, 1). This structural model is consistent with the presence of tetrahedral Mo(=O)2(O(1/2))2 environments. Indeed, (95)Mo NMR data suggest that the majority of the molybdenum species are four-coordinated. However, the presence of additional six-coordinate molybdenum in the MAS NMR spectra indicates that the structure of these glasses may be more complicated and may additionally involve sharing of network modifier oxide between the network formers phosphorus and molybdenum. This latter hypothesis is further supported by (23)Na{(31)P} rotational echo double resonance (REDOR) data, which clearly reveal that the magnetic dipole-dipole interactions between (31)P and (23)Na are increasingly diminished with increasing molybdenum content. The partial transfer of modifier from the phosphate to the molybdate network former implies a partial repolymerization of the phosphate species, resulting in the formation of Q(3)(nMo) species and accounting for the observed increase in the glass transition temperature with increasing MoO(3) content that is observed in the composition range 0 <or= x <or= 45. Glasses with MoO(3) contents beyond x = 45 show decreased thermal and crystallization stability. Their structure is characterized by isolated phosphate species [most likely of the P(OMo)4 type] and molybdenum oxide clusters with a large extent of Mo-O-Mo connectivity.
通过一种改进的熔融法在（100 - x）% NaPO₃ - x% MoO₃（0 ≤ x ≤ 70）玻璃形成体系中制备玻璃样品，该方法能够获得具有良好光学质量的样品。通过差示扫描量热法（DSC）、傅里叶变换红外光谱（FT - IR）、拉曼散射以及针对³¹P、²³Na和⁹⁵Mo核的固态核磁共振（NMR），监测玻璃网络的结构演变作为组成的函数。向NaPO₃玻璃熔体中添加MoO₃会导致玻璃化转变温度显著升高，直至x = 45，这表明网络连接性显著增加。对于相同的组成范围，振动光谱表明Mo⁶⁺离子与一些非桥氧原子键合（Mo - O - 或Mo = O键合物种）。仅在MoO₃含量超过x = 45时才会形成Mo - O - Mo键。二维J - 分辨光谱支持的³¹P魔角旋转（MAS）NMR光谱能够清晰区分具有两个、一个和零个P - O - P键的物种。这些位点分别表示为Q₂(2Mo)、Q₂(1Mo)和Q₂(0Mo)。对于x < 0.45，这些位点的分布可以按照二元模型来描述，根据该模型，每个MoO₃单元将两个Q₂(nMo)位点转化为两个Q₂((n + 1)Mo)位点（n = 0, 1）。这个结构模型与四面体Mo(=O)₂(O(1/2))₂环境的存在一致。实际上，⁹⁵Mo NMR数据表明大多数钼物种是四配位的。然而，MAS NMR光谱中额外六配位钼的存在表明这些玻璃的结构可能更复杂，并且可能还涉及网络形成剂磷和钼之间网络改性剂氧化物的共享。²³Na{³¹P}旋转回波双共振（REDOR）数据进一步支持了后一种假设，该数据清楚地表明随着钼含量的增加，³¹P和²³Na之间的磁偶极 - 偶极相互作用逐渐减弱。改性剂从磷酸盐向钼酸盐网络形成剂的部分转移意味着磷酸盐物种的部分再聚合，导致形成Q₃(nMo)物种，并解释了在0 ≤ x ≤ 45组成范围内观察到的随着MoO₃含量增加玻璃化转变温度升高的现象。MoO₃含量超过x = 45的玻璃显示出热稳定性和结晶稳定性降低。它们的结构特征是孤立的磷酸盐物种[最可能是P(OMo)₄类型]和具有大量Mo - O - Mo连接性的氧化钼簇。
