Phase and Microstructural Development of Sol-gel-derived Strontium Barium Niobate Thin Films
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Microstructural changes in sol-gel-derived SrxBa1−x Nb2O6 (SBN) thin films were monitored as a function of Ba-to-Sr ratio (from x ⳱ 0 to x ⳱ 1), choice of substrate (Si or MgO), and processing variations. Sols were created using Ba, Sr, and Nb alkoxides dissolved in acetic acid. The relatively high decomposition temperature for the organics led to a tendency to form defects, but careful control of thermal process parameters could be used to produce a uniform film microstructure. An unexpected phase, interpreted as a hexagonal (pseudo-orthorhombic) variant of hexagonal BaNb2O6, was encountered in Ba-rich sol-gel-derived SBN powders and thin films annealed at 750 °C. Increased (001) orientation was observed for SBN thin films deposited on (100) MgO when fast thermal processing was used.
I. INTRODUCTION
Strontium barium niobate (Srx Ba1−x Nb2O6, SBN) is a ferroelectric phase with a tungsten bronze structure that is formed between BaNb2O6 and SrNb2O6. The region of solid solution for the tungsten bronze structure is generally accepted to be 0.25 < x < 0.75.1 Two different lattice types and symmetries have been observed in the solid solution. The first phase is tetragonal, with space group P4bm, having approximate lattice parameters of a,b ⳱ 1.24 nm, c ⳱ 0.39 nm. Numerous members of the ferroelectric tungsten bronze type of oxides all have a related tetragonal symmetry with a unit cell of approximately 1.2 × 12 × 0.39 nm containing 10 corner-sharing MO6 (M ⳱ penta valent cation) octahedra.2 The tungsten bronze structure is a layered type, with the oxygen ions forming slightly puckered sheets at approximately z ⳱ 0 and 1⁄2, the M ions at approximately z ⳱ 0 and the remaining cations (Ba, Sr, Na, etc.) at z ⳱ 1⁄2. A closely related structure to the tetragonal tungsten bronze family has orthorhombic symmetry, with a unit cell of approximately 1.75 × 1.75 × 0.8 nm. The orthorhombic variant is related to the tetragonal unit cell by the following equations: aorth ≅ borth ≅ √2atetragonal ,
(1)
corth ≅ 2ctetr .
(2)
Only a few researchers have identified this orthorhombic structure (space group Cmm2) in the BaNb2O6–SrNb2O6 system and assigned approximate lattice parameters, a,b ⳱ 1.76 nm, c ⳱ 0.78.3–6 J. Mater. Res., Vol. 15, No. 6, Jun 2000
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The basic formula for a ferroelectric tetragonal tungsten bronze is [(A1)2 (A2)4 C4] [(M1)2 (M2)8] O30.2 The skeletal framework of the tungsten bronze structure is formed by MO6 octahedra, which share corners to form cavities of A1, A2, and C (Fig. 1). The smaller cations prefer A1 sites because of the relatively smaller size of A1 sites compared to A2 sites. Jamieson and Bernstein’s5 atom distribution model for SBN suggests that Ba cations are only found in large A2 sites and Sr atoms preferably occupy the small A1 sites. Nevertheless, some of the Sr atoms can also occupy A2 sites, especially in Sr-rich solid solutions. The C site is the smallest of the three cavities created by the framework of octahedra. Very small cations that
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