The aqueous quaternary system K⁺, Rb⁺, Cs⁺//SO₄^2––H₂O is one of the most important subsystems of the sulfate-type salt lake brine located in the Qinghai-Tibet Plateau. The system was investigated at T = 298.2 K by the isothermal dissolution equilibrium method. The space phase diagram, the projected phase diagram, the water content diagram, and the physicochemical properties versus composition diagrams were plotted based on the measured data. The projected phase diagram consists of four invariant points (including three commensurate points and one incommensurate point), nine univariant curves, and six crystallization fields corresponding to three single salts K₂SO₄, Rb₂SO₄, and Cs₂SO₄ and three solid solutions [(K_x, Rb_1–x)₂SO₄], [(K_x, Cs_1–x)₂SO₄], and [(Rb_x, Cs_1–x)₂SO₄]. No double salt was formed in this system at 298.2 K. The size order of the crystalline phase region of salts is [(K_x, Rb_1–x)₂SO₄] > [(K_x, Cs_1–x)₂SO₄] > K₂SO₄ > [(Rb_x, Cs_1–x)₂SO₄] > Rb₂SO₄ > Cs₂SO₄, which indicates that it is hardly possible to directly separate rubidium and cesium as single salt Rb₂SO₄ and Cs₂SO₄ from the sulfate-type old brine. The type of three solid solutions was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the three solid solutions [(K_x, Rb_1–x)₂SO₄], [(K_x, Cs_1–x)₂SO₄], and [(Rb_x, Cs_1–x)₂SO₄] are the complete solid solution in which the solute atoms are dissolved in the solvent lattice while still maintaining the solvent type, and isomorphism phenomenon is observed in the three solid solutions [(K_x, Rb_1–x)₂SO₄], [(K_x, Cs_1–x)₂SO₄], and [(Rb_x, Cs_1–x)₂SO₄].

中文翻译：

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第四水盐系统K ⁺，Rb ⁺，Cs ⁺ // SO ₄ ^{2 –} –H ₂ O在T = 298.2 K时的稳定相图

四元水系K ⁺，Rb ⁺，Cs ⁺ // SO ₄ ^{2 –} –H ₂ O是位于青藏高原的硫酸盐型盐湖卤水最重要的子系统之一。通过等温溶解平衡法研究了在T = 298.2 K时的体系。根据测得的数据绘制了空间相图，投影相图，水含量图以及理化性质与组成图。投影相图由四个不变点（包括三个相等点和一个不相等点），九个单变量曲线和六个结晶场组成，分别对应于三个单盐K₂ SO ₄，Rb ₂ SO ₄和Cs ₂ SO ₄以及三种固溶体[（K _x，Rb _{1– x}）₂ SO ₄ ]，[（K _x，Cs _{1– x}）₂ SO ₄ ]和[ （Rb _x，Cs _{1– x}）₂ SO ₄ ]。该系统在298.2 K时未形成重盐。盐的结晶相区域的大小顺序为[（K _x，Rb _{1– x}）₂ SO ₄]> [（K _X，铯_{1- X}）₂ SO ₄ ]>ķ ₂ SO ₄ > [（RB _X，铯_{1- X}）₂ SO ₄ ]> RB ₂ SO ₄ >铯₂ SO ₄，这表明几乎不可能直接将id和铯分离为单盐Rb ₂ SO ₄和Cs ₂ SO ₄来自硫酸盐型的老盐水。通过X射线衍射（XRD）和扫描电子显微镜（SEM）分析三种固溶体的类型。结果表明，三种固溶体[（K _x，Rb _{1– x}）₂ SO ₄ ]，[（K _x，Cs _{1– x}）₂ SO ₄ ]和[（Rb _x，Cs _{1– x}）₂ SO ₄是完全固溶体，溶质原子溶解在溶剂晶格中，同时仍保持溶剂类型，并且在三种固溶体中观察到同构现象[（K _x，Rb _{1– x}）₂ SO ₄ ]，[（K _x，Cs _{1– x}）₂ SO ₄ ]和[（Rb _x，Cs _{1– x}）₂ SO ₄ ]。