Anisotropic Thermal Expansion of SiO₂ and AlPO₄ Clathrasils with the AST-Type Structure

The thermal expansion behavior of silica and aluminophosphate clathrasil materials that adopt the AST-type structure has been studied in both as-made and calcined forms using X-ray diffraction (single crystal or powder) over the temperature range 100−400 K. The four materials studied all adopt body-centered tetragonal variants of the ideal cubic AST-type structure. In the as-made form, where quinuclidinum cations fill the larger cages and fluoride anions occupy interstitial sites at the center of double four-ring units, both materials have net positive volume thermal expansion coefficients, larger than dense forms of SiO2, but both show highly anisotropic thermal expansivity with positive expansion in the ab plane and negative expansion parallel to c. The anisotropy becomes more pronounced upon removal of the extra-framework species, with larger magnitudes of positive (αa and αb) and negative (αc) coefficients than those before calcination. This indicates a greater structural flexibility once free pore volume is available, although still net positive thermal expansion is seen. In the case of as-made SiO2-AST (that contains template and fluoride ions) and the fully calcined AlPO4 analogue, full structure refinements as a function of temperature have been performed; this reveals that whether or not template and fluoride are present, the double-four-ring composite building units show a volume contraction with temperature, whereas an expansion of linking T−O−T bonds is seen. These opposing effects are responsible for the anisotropy in thermal expansion behavior.本研究针对具有AST型结构（AST-type structure）的二氧化硅（SiO₂）与铝磷酸笼形硅材料，分别以原合成态与焙烧态样品为研究对象，在100−400 K的温度区间内，借助X射线衍射（X-ray diffraction，含单晶与粉末衍射模式）对其热膨胀行为开展了系统表征。本次研究所涉及的四种材料，均属于理想立方AST型结构的体心四方变体。在原合成态样品中，奎宁环阳离子填充于较大笼腔，氟阴离子占据双四元环结构单元中心的间隙位点；两类材料均表现出净正体积热膨胀系数，且其数值高于致密态SiO₂；同时二者均呈现出极强的各向异性热膨胀特性：ab平面内为正膨胀，平行于c轴方向则为负膨胀。随着骨架外物种脱除（即焙烧处理），该各向异性特征愈发显著，αₐ、αᵦ（正膨胀系数）与α_c（负膨胀系数）的绝对值均高于焙烧前，这表明当材料具备可利用的自由孔体积后，其结构柔性进一步提升，但仍保持净正热膨胀特性。针对原合成态SiO₂-AST（含模板剂与氟离子）以及完全焙烧的AlPO₄对应物，本研究完成了基于温度函数的全结构精修；结果显示，无论是否存在模板剂与氟离子，双四元环复合结构基元均随温度升高发生体积收缩，而连接的T-O-T键则表现为膨胀；这两种相反的效应共同导致了热膨胀行为的各向异性。