In Situ Characterization of Elusive Salt HydratesThe Crystal Structures of the Heptahydrate and Octahydrate of Sodium Sulfate

An important intermediate phase in the crystallization of aqueous solutions of sodium sulfate is the highly metastable sodium sulfate heptahydrate (Na2SO4·7H2O). This has been structurally characterized for the first time by in situ single crystal X-ray diffraction. The crystal structure shows that each sodium cation is octahedrally coordinated to water molecules, with a slight distortion due to one of the water molecules being disordered. The hydrated sodium cations are hydrogen-bonded to form a three-dimensional bonded network, which is markedly different from the architecture of one-dimensional bonded chains observed in sodium sulfate decahydrate (mirabilite). This major structural difference explains the reconstructive nature of the transformation observed between the heptahydrate and mirabilite. High-pressure crystallization of a 3.41 mol/kg water aqueous solution of sodium sulfate at 1.54 GPa in a diamond-anvil cell resulted in the formation of a previously unknown sodium sulfate hydrate, which we have determined by single crystal X-ray diffraction methods to be an octahydrate, Na2SO4·8H2O. In this structure the sulfate ions are coordinated directly to sodium ions. This resembles anhydrous sodium sulfate (thenardite) but contrasts with the heptahydrate and decahydrate in which the sodium ions are coordinated exclusively by water molecules. This observation demonstrates how the delicate balance of inter- and intramolecular bonds in the crystal structure can be significantly altered by the application of pressure.

硫酸钠水溶液结晶过程中的重要中间相为高度亚稳态的七水合硫酸钠(Na₂SO₄·7H₂O)。该化合物的结构首次通过原位单晶X射线衍射（in situ single crystal X-ray diffraction）技术得以解析表征。晶体结构分析表明，每个钠离子均以八面体构型与水分子配位，其中一个水分子存在无序分布，导致配位八面体产生轻微畸变。水合钠离子通过氢键相互作用形成三维键合网络，这与十水合硫酸钠（芒硝，mirabilite）中观测到的一维键合链结构存在显著差异。这一显著的结构差异，正是七水合硫酸钠与芒硝之间发生重构型转变的原因。在金刚石对顶砧（diamond-anvil cell）装置中，对浓度为3.41 mol/kg的硫酸钠水溶液于1.54 GPa压力下开展高压结晶实验，得到了一种此前未知的硫酸钠水合物；经单晶X射线衍射方法测定，该产物为八水合硫酸钠(Na₂SO₄·8H₂O)。该结构中，硫酸根离子可直接与钠离子配位。这一特征与无水硫酸钠（无水芒硝，thenardite）相似，但与七水合硫酸钠和十水合硫酸钠截然不同——后两者中的钠离子仅由水分子配位。该观测结果表明，晶体结构中分子间与分子内键合的微妙平衡，可通过施加压力发生显著改变。