Section 3.1.5.2.8. Barium sodium niobate

The tungsten bronzes represented by Ba₂NaNb₅O₁₅ have complicated sequences of structural phase transitions. The structure is shown in Fig. 3.1.5.12 and, viewed along the polar axis, consists of triangular, square and pentagonal spaces that may or may not be filled with ions. In barium sodium niobate, the pentagonal channels are filled with Ba ions, the square channels are filled with sodium ions, and the triangular areas are empty.

Figure 3.1.5.12 | top | pdf | Structure of the tungsten bronze barium sodium niobate Ba2NaNb5O15 in its highest-temperature phase above 853 K.

The sequence of phases is shown in Fig. 3.1.5.13. At high temperatures (above  K) the crystal is tetragonal and paraelectric (). When cooled below 853 K it becomes ferroelectric and of space group (still tetragonal). Between ca. 543 and 582 K it undergoes an incommensurate distortion. From 543 to ca. 560 K it is orthorhombic and has a `' modulation along a single orthorhombic axis. From 560 to 582 K it has a `tweed' structure reminiscent of metallic lattices; it is still microscopically orthorhombic but has a short-range modulated order along a second orthorhombic direction and simultaneous short-range modulated order along an orthogonal axis, giving it an incompletely developed `' structure.

Figure 3.1.5.13 | top | pdf | Sequence of phases encountered with raising or lowering the temperature in barium sodium niobate.

As the temperature is lowered still further, the lattice becomes orthorhombic but not incommensurate from 105–546 K; below 105 K it is incommensurate again, but with a microstructure quite different from that at 543–582 K. Finally, below ca. 40 K it becomes macroscopically tetragonal again, with probable space-group symmetry () and a primitive unit cell that is four times that of the high-temperature tetragonal phases above 582 K.

This sequence of phase transitions involves rather subtle distortions that are in most cases continuous or nearly continuous. Their elucidation has required a combination of experimental techniques, emphasizing optical birefringence (Schneck, 1982), Brillouin spectroscopy (Oliver, 1990; Schneck et al., 1977; Tolédano et al., 1986; Errandonea et al., 1984), X-ray scattering, electron microscopy and Raman spectroscopy (Shawabkeh & Scott, 1991), among others. As with the other examples described in this chapter, it would have been difficult and perhaps impossible to establish the sequence of structures via X-ray techniques alone. In most cases, the distortions are very small and involve essentially only the oxygen ions.