Ferroelectricity, ionic conductivity and structural paths for large cation migration in Ca10.5−xPbx(VO4)7 single crystals, x = 1.9, 3.5, 4.9. Issue 8 (5th February 2019)

• Record Type: Journal Article
• Authors: Deyneko, Dina V.; Petrova, Daria A.; Aksenov, Sergey M.; Stefanovich, Sergey Yu.; Baryshnikova, Oksana V.; Fedotov, Stanislav S.; Burns, Peter C.; Kosmyna, Miron B.; Shekhovtsov, Aleksey N.; Lazoryak, Bogdan I.
• Is Part Of: CrystEngComm. Volume 21:Issue 8(2019)
• Page Start: 1309
• Page End: 1319
• Publication Date: 2019-02-05
• Subjects: Crystals -- Periodicals; Crystal growth -- Periodicals; Crystallography -- Periodicals; Cristaux -- Périodiques; Cristaux -- Croissance -- Périodiques; Cristallographie -- Périodiques; 548
• DOI: 10.1039/c8ce01843j
• Languages: English
• ISSNs: 1466-8033

Abstract

Abstract : Crystal structure, thermal, ionic conductivity of large cations (Ca and Pb), dielectric and non-linear optical properties were investigated for Ca10.5− x Pb x (VO4 )7 single crystals ( x = 1.9, 3.5, 4.9). Abstract : Crystal structure, and thermal, dielectric and optical second-harmonic activities were investigated for whitlockite-type Ca8.6 Pb1.9 (VO4 )7 (1 ), Ca7 Pb3.5 (VO4 )7 (2 ) and Ca5.6 Pb4.9 (VO4 )7 (3 ) single crystals. The refinement of the structure revealed splitting of the M3 and M4 sites into two and three sub-positions, respectively. Splitting of the M3 and M4 sites strongly enhances nonlinear optical activity, measured on powders, with the effect vanishing at ferroelectric phase transitions at elevated temperatures. Single crystals (1 ) and (2 ) undergo two structural transformations: ferroelectric one between 750–1150 K and a subsequent phase transition at higher temperatures. The structural mechanism of the ferroelectric phase transition is analyzed with respect to the rearrangement of Ca 2+ and Pb 2+ over their positions in the ferroelectric and paraelectric phases. Large Pb 2+ ion substitution for calcium results in unit cell expansion with consequent extending of ion migration channels, thus leading to enhanced ionic conductivity in the Pb-rich materials. It is concluded that in spite of steric hindrances for lead cations, they effectively move between positions M2 → M4 → M1 or M1 → M4 → M2 together with calcium cations across the crystal. The cation conduction pathways as well as the migration energies of Ca 2+ and Pb 2+ cations were calculated by the bond valence energy landscape (BVEL) method. Among the obtained single crystals, (3 ) demonstrated the highest ionic conductivity, maximum second harmonic generation (SHS) signal and the lowest temperature of ferroelectric phase transition.