| Summary: | Belomarinaite KNaSO<sub>4</sub> (space group <i>P</i>3<i>m</i>1, <i>a</i> = 5.6072(3), <i>c</i> = 7.1781(4) Å and <i>Z</i> = 2) has been studied by high-temperature single crystal X-ray diffraction. The K and Na atoms are disordered, and <i>M</i>1(1<i>c</i>) and <i>M</i>2(1<i>b</i>) sites merge into an <i>M</i>1(2<i>d</i>)(K50%/Na50%) site with increasing temperature, and novel <i>translationsgleiche</i> phase transition of belomarinaite KNaSO<sub>4</sub> (<i>P</i>3<i>m</i>1 → <i>P</i>-3<i>m</i>1) was revealed at 123 °C. Then, the temperature increase leads to phase transition of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>P</mi><mover accent="true"><mrow><mn>3</mn></mrow><mo>¯</mo></mover><mi>m</mi><mn>1</mn></mrow></semantics></math></inline-formula>-polymorph of KNaSO<sub>4</sub> to α-KNaSO<sub>4</sub> (<i>P</i>6<sub>3</sub>/<i>mmc</i>) at 446 °C, and sodium and potassium atoms in K1(1<i>a</i>) and Na1(1<i>b</i>) sites merged into an <i>M</i>4(2<i>a</i>)(K50%/Na50%) site. Crystal structures of KNaSO<sub>4</sub> were refined at 300, 500 and 750 °C to <i>R</i><sub>1</sub> = 0.057, 0.056 and 0.048, respectively. The BVS maps and bond-valence energy landscape (BVEL) have been calculated, and the probability of the Na<sup>+</sup> migration has been predicted using structural data for belomarinaite at 25 °C. The Na<sup>+</sup> ion migration can occur at 2.14 eV in the <i>ab</i> plane and 2.80 eV along the <i>c</i> axis at 25 °C.
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