TY - JOUR

T1 - Bubnovaite-type K2Na8Ca(SO4)6 sulphate: Crystal structure, phase transitions, thermal expansion and ionic conductivity

AU - Shablinskii, A.p.

AU - Shorets, O.yu.

AU - Melnikova, N.a.

AU - Povolotskiy, A.v.

AU - Biryukov, Y.p.

AU - Avdontceva, M.s.

AU - Bubnova, R.s.

AU - Krzhizhanovskaya, M.g.

AU - Ugolkov, V.l.

AU - Murin, I.v.

AU - Filatov, S.k.

PY - 2025/8/1

Y1 - 2025/8/1

N2 - The bubnovaite-type K2Na8Ca(SO4)6 compound was first obtained by cooling from a melt. Crystal structure was solved and refined from single crystal X-ray diffraction data (R1 = 0.058). K2Na8Ca(SO4)6 crystallizes in space group P31c with unit cell parameters: a = 10.8431(7), c = 22.170(1) Å, V = 2257.4(2) Å3, Z = 4. Thermal behavior of K2Na8Ca(SO4)6 was studied by high-temperature powder X-ray diffraction (HTPXRD) (22–740 °C). According to the data, an increase in temperature leads to the P31c ↔ P63/mmc structural phase transition at 500 °C. Crystal structure of the K2Na8Ca(SO4)6 (P63/mmc) polymorphic modification was refined using HTPXRD data collected at 600 °C by the Rietveld full-profile refinement (R1 = 0.052, a = 5.48704(3), c = 7.78284(5) Å, V = 202.931(4) Å3, Z = 2). Thermal expansion of the K2Na8Ca(SO4)6 (P31c) polymorph is highly anisotropic in the range 22–240°C (αa = 14.9 (7), αc = 2.8 (1), αV = 25 (1) × 10 –6 °C−1 at 22 °C). The bond valence sums (BVS) maps and bond-valence energy landscape (BVEL) were calculated for the both modifications, and a probability of the Na+ migration was predicted using SCXRD data. The Na+ ion migration can occur at 1.78 and 2.75 eV in the ab plane and along the c axis respectively in the K2Na8Ca(SO4)6 (P31c) structure (at 20 °C). The migration can occur at 0.49 and 0.68 eV in the ab plane and along the c axis in the K2Na8Ca(SO4)6 (P63/mmc) crystal structure (at 600 °C). The electrical conductivity was evaluated by impedance spectroscopy. The electrical conductivity value for the low-temperature modification of K2Na8Ca(SO4)6 at a temperature of 400 °C is 5.41·10−3 S/cm, for the high-temperature modification at a temperature of 600 °C – 7.43·10−2 S/cm. The activation energies of electrical conductivity are Ea(I) = 1.04 ± 0.03 eV and Ea(II) = 0.58 ± 0.02 eV for both polymorphic modifications.

AB - The bubnovaite-type K2Na8Ca(SO4)6 compound was first obtained by cooling from a melt. Crystal structure was solved and refined from single crystal X-ray diffraction data (R1 = 0.058). K2Na8Ca(SO4)6 crystallizes in space group P31c with unit cell parameters: a = 10.8431(7), c = 22.170(1) Å, V = 2257.4(2) Å3, Z = 4. Thermal behavior of K2Na8Ca(SO4)6 was studied by high-temperature powder X-ray diffraction (HTPXRD) (22–740 °C). According to the data, an increase in temperature leads to the P31c ↔ P63/mmc structural phase transition at 500 °C. Crystal structure of the K2Na8Ca(SO4)6 (P63/mmc) polymorphic modification was refined using HTPXRD data collected at 600 °C by the Rietveld full-profile refinement (R1 = 0.052, a = 5.48704(3), c = 7.78284(5) Å, V = 202.931(4) Å3, Z = 2). Thermal expansion of the K2Na8Ca(SO4)6 (P31c) polymorph is highly anisotropic in the range 22–240°C (αa = 14.9 (7), αc = 2.8 (1), αV = 25 (1) × 10 –6 °C−1 at 22 °C). The bond valence sums (BVS) maps and bond-valence energy landscape (BVEL) were calculated for the both modifications, and a probability of the Na+ migration was predicted using SCXRD data. The Na+ ion migration can occur at 1.78 and 2.75 eV in the ab plane and along the c axis respectively in the K2Na8Ca(SO4)6 (P31c) structure (at 20 °C). The migration can occur at 0.49 and 0.68 eV in the ab plane and along the c axis in the K2Na8Ca(SO4)6 (P63/mmc) crystal structure (at 600 °C). The electrical conductivity was evaluated by impedance spectroscopy. The electrical conductivity value for the low-temperature modification of K2Na8Ca(SO4)6 at a temperature of 400 °C is 5.41·10−3 S/cm, for the high-temperature modification at a temperature of 600 °C – 7.43·10−2 S/cm. The activation energies of electrical conductivity are Ea(I) = 1.04 ± 0.03 eV and Ea(II) = 0.58 ± 0.02 eV for both polymorphic modifications.

KW - Bubnovaite

KW - Crystal structure

KW - Phase transition

KW - Sulphates

KW - Thermal behavior

UR - https://www.mendeley.com/catalogue/c1cf8e5e-d4d4-396e-8ad8-7e272a913df7/

U2 - 10.1016/j.inoche.2025.114611

DO - 10.1016/j.inoche.2025.114611

M3 - Article

VL - 178

JO - Inorganic Chemistry Communication

JF - Inorganic Chemistry Communication

SN - 1387-7003

M1 - 114611

ER -