Single crystals of Na 1.75 Yb 0.08 SO 4 , Na 1.82 Y 0.06 SO 4 , and Na 1.70 Ln 0.10 SO 4 (Ln 3؉ ؍La, Yb), were grown at 1300 K by the Czochralski method. The Yb 3؉ and Y 3؉ substituted crystals transform upon cooling to a monoclinic phase intergrown in a twin relationship. However, the high-temperature defect structure of Na 2 SO 4 type I can be supercooled by annealing at 1000 K and subsequent quenching. (La,Yb)-bearing crystals preserve even after moderate cooling hexagonal symmetry of Na 2 SO 4 type I but display very weak and di4use superstructure re6ections indicating tripling of the c-axis. After annealing at 1000 K and subsequent quenching the superstructure re6ections disappear and the (La, Yb)-substituted crystals exhibit slightly contracted cell dimensions (a ؍ 5.331(1), c ؍ 7.188(1) A s ) compared to the slowly cooled crystals with the same composition (a ؍ 5.3472(5), c ؍ 7.2102(6) A s ). The crystal structures of all synthesized hexagonal crystals were determined from room temperature singlecrystal X-ray data and re5ned in space group P6 3 /mmc. The corresponding re5ned structures are strongly related to the high temperature phase I of pure Na 2 SO 4 but display additional SO 4 orientations correlated with the amount of substituted Y 3؉ and Ln 3؉ on Na1 and also with the number of cation vacancies on Na2 and Na3. The structures reveal a disordered arrangement of 5ve di4erent SO 4 orientations. If a (La,Yb)-substituted crystal is rapidly quenched from 1000 K, Ln 3؉ is concentrated together with Na at the Na1 site whereas upon slow cooling some Ln 3؉ di4uses to Na2.