The dobrovolskyite-type Na4Ca(SO4)3 compound was obtained by rapid cooling of a melt. Crystal structure was solved and refined using single crystal X-ray diffraction (SCXRD) data (R3, a = 15.7355(3), c = 22.4023(5) Å, V = 4803.8(2) Å3, R1 = 0.053). The thermal behavior of Na4Ca(SO4)3 was investigated using high-temperature powder X-ray diffraction (HTPXRD) from 30 to 840 °C. The HTPXRD data reveal a reversible R3 ↔ P63/mmc phase transition at 280 °C. The crystal structure of the high-temperature polymorph (P63/mmc) was refined via the Rietveld method using data collected at 820 °C (P63/mmc, a = 5.38706(6), c = 7.82774(2) Å, V = 196.731(1) Å3, Z = 2, R1 = 0.060). The low-temperature Na4Ca(SO4)3 (R3) polymorph exhibits highly anisotropic thermal expansion between 30 and 270 °C (αa = 26(1), αc = 53 (1), αV = 105 (2) × 10 −6 °C–1 at 30 °C). Bond-valence energy landscape (BVEL) maps, constructed using the crystal structure data for both polymorphs, predict potential Na+ migration pathways. The calculated activation energies for migration at 30 °C are 1.64 eV within the ab plane and 1.61 eV along the c axis. Experimentally, the electrical conductivity is 5.89 × 10−6 S/cm at 200 °C for the low-temperature modification and increases significantly to 7.06 × 10−2 S/cm at 600 °C for the high-temperature modification. The activation energies Ea for electrical conductivity are 1.21(4) eV and 0.49(2) eV for both polymorphs, respectively. © 2025