International
Tables for
Crystallography
Volume D
Physical properties of crystals
Edited by A. Authier

© International Union of Crystallography 2006
International Tables for Crystallography (2006). Vol. D. ch. 3.1, pp. 369-370

Section 3.1.5.2.11. Sodium nitrite, NaNO2

J. F. Scottc*

3.1.5.2.11. Sodium nitrite, NaNO2

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Sodium nitrite exhibits a purely order–disorder transition and has been chosen for discussion to contrast with the systems in the sections above, which are largely displacive. The mechanism of its transition dynamics is remarkably simple and is illustrated in Fig. 3.1.5.19[link]. There is a linear array of Na and N ions. At low temperatures, the arrow-shaped NO2 ions (within each domain) point in the same direction; whereas above the Curie temperature they point in random directions with no long-range order. The flopping over of an NO2 ion is a highly nonlinear response. Therefore the response function (spectrum) associated with this NO2 flip-flop mode will consist of two parts: a high-frequency peak that looks like a conventional phonon response (lightly damped Lorentzian), plus a low-frequency Debye relaxation (`central mode' peaking at zero frequency). Most of the temperature dependence for this mode will be associated with the Debye spectrum. The spectrum of sodium nitrite is shown in Fig. 3.1.5.20[link].

[Figure 3.1.5.19]

Figure 3.1.5.19 | top | pdf |

Structure of sodium nitrite, NaNO2. The molecularly bonded NO2 ions are shaped like little boomerangs. At high temperatures they are randomly oriented, pointing up or down along the polar b axis. At low temperatures they are (almost) all pointed in the same direction ([+b] or [-b] domains). Over a small range of intermediate temperatures their directions have a wave-like `incommensurate' modulation with a repeat length L that is not an integral multiple of the lattice constant b.
[Figure 3.1.5.20]

Figure 3.1.5.20 | top | pdf |

Raman spectra of sodium nitrite, showing diffusive Debye-like response due to large-amplitude flopping over of nitrite ions [note that the high-frequency phonon-like response is due to the small-amplitude motion of this same normal mode; thus in this system N ions give rise not to 3N (non-degenerate) peaks in the spectral response function, but to [3N+1]].

Particularly interesting is its phase diagram, relating structure(s) to temperature and `conjugate' field applied along the polar axis. As Fig. 3.1.5.21[link] illustrates somewhat schematically, there are first-order phase boundaries, second-order phase boundaries, a tricritical point and a critical end point (as in a gas–liquid diagram). If the electric field is applied in a direction orthogonal to the polar axis, a Lifshitz point (Fig. 3.1.5.22[link]) may be expected, in which the phase boundaries intersect tangentially. The ionic conductivity of sodium nitrite has made it difficult to make the figures in Figs. 3.1.5.21[link] and 3.1.5.22[link] precise.

[Figure 3.1.5.21]

Figure 3.1.5.21 | top | pdf |

Phase diagram for sodium nitrite for `conjugate' electric fields applied along the polar b axis, showing triple point, tricritical point and critical end point. (a) Schematic; (b) real system.
[Figure 3.1.5.22]

Figure 3.1.5.22 | top | pdf |

Phase diagram for sodium nitrite for electric fields applied perpendicular to the polar b axis. In this situation, a Lifshitz point is possible where phase boundaries `kiss' (touch tangentially).








































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