Cation occupancy at the A1/A2 sites in strontium barium niobate micro-crystals grown from molten NaCl and KCl salts

The cation chemistry in two sets of strontium barium niobate (SBN) micro-crystals, synthesized at 1100 °C via molten NaCl and KCl salts was investigated using analytical electron microscopy. Na+ and K+ ions were found to enter the tungsten bronze (TB) structure to form the filled TB phase Sr4−mBamNa2Nb10O30 or the partially filled TB phase (Sr,Ba)4+nK2−2n□n Nb10O30. Small Na+ ions fill the A2 sites, leaving Sr2+ and Ba2+ ions to fill the A1 sites. The shifts in the actual compositions (m) from the nominal ones imply there may be morphotropic phase boundaries for the given composition range. In contrast, the large K+ ions partially fill two of the A1 sites, creating vacancies and results in the formation of (Sr4−mBam)(BanK2−2n□n) Nb10O30. A correlation between Ba, Sr and K contents reveals that m and n have a linear relation (m ≈ 3n). This leads to the new formula Sr4−3y/4BayK2−2y/4□y/4 Nb10O30 (0 < y < 4), which illustrates a balance in the solution energy.