High-temperature structural phase transition in Na3H(SO4)(2) crystal

X-ray diffraction was used to study the structural phase transition of Na3H(SO4)(2) crystals. Laue and precession photographs were taken with Mo radiation at room temperature and at 523 K. The X-ray diffraction intensities were modified drastically on heating through the structural phase transition. The crystal system changed from monoclinic to hexagonal. The possible space groups are P6(3)/mmc, P6(3)mc and <P(6)over bar 2c> in the high-temperature phase. The structural phase transition may be caused by disorder of the sodium atoms and reorientation of SO4 tetrahedral groups. The crystal structure of Na3H(SO4)(2) is probably isostructural with that of K2SO4 and K2SeO4 crystals in the high-temperature phase.