The low-temperature phase transition of sodium niobate and the structure of the low-temperature phase, N

Abstract

Sodium niobate undergoes an antiferroelectric-ferroelectric transition (P ~ N) at temperatures below about -110°C. The ferroelectric low-temperature phase (N) is rhombohedral, with space group R3c.Though a perovskite structure, it is isostructural with LiNbO3. Using pseudocubic axes of reference, a=2 × 3"9083 (5) ~, ~= 89 ° 13 (1)' at -150°C. The NbO6 octahedra remain nearly regular, but are tilted about the triad axis; displacements of Nb atoms from the centres of octahedra are parallel to this axis. The temperature dependence of the lattice parameters of both phases, N and P, below room temperature is reported, and also that of the magnitude of the Nb displacement in the antiferroelectric phase P. Experimental details of the low-temperature work are given. Evidence used in the structure determination comes from the juxtaposition of high-angle reflexions from twin components in phase N, and the orientation of domains of phase N relative to the parent phase P. The transformation is more rapid and more complete if the parent crystal is single-domain; if it is twinned, both phases coexist for long periods down to the lowest temperatures studied. The relation of the structure to that of other niobates is discussed, and some general principles are put forward from which the occurrence of different structures and the transitions between them can be understood.

The structure of NaTaO3by X-ray powder diffraction

“…The effect of the smaller tilt angles is most readily seen in Table 3 in the reduction of the theoretical intensities of 301, 321 and 232; 410 difference reflexions as compared with those calculated on the basis of the structure suggested by K & M. Darlington & Megaw (1973) suggested certain empirical generalizations applicable to most distorted perovskites, e.g. that the tilts are such as to provide an environment in which the A cation has nearly equidistant O neighbours holding it in position.…”

“…CaTiO3: Kay & Bailey (1957); NaNbO3, phase P: Sakowski-Cowley,-Lukaszewicz & Megaw (1969); NaNbOa, phase N: Darlington & Megaw (1973); Nal-xKxNbO3, phase Q: Ahtee & Hewat (1975)]. For example, in the room-temperature phase P of NaNbO3 the octahedra are very regular even though the Nb atoms are displaced inside the octahedra; the mean of all twelve edges is 2.80 A, with extreme values 2.76 and 2.83 A.…”

The compositional dependence of octahedral tilting in orthorhombic and tetragonal perovskites

“…It is as if the room-temperature phase is an intergrowth, within each unit cell, of a pseudo-rhombohedral and an orthorhombic structure. Furthermore, it acts as a precursor to the R3c rhombohedral phase which is formed at temperatures below 163 K (Darlington & Megaw, 1973;see Table 10). …”

Competing antiferroelectric and ferroelectric interactions inNaNbO3: Neutron diffraction and theoretical studies

“…Experimental studies of NaNbO 3 using a wide variety of techniques including x-ray and neutron diffraction [8][9][10][11][12][13][14][15] , Raman and infrared spectroscopy [16][17][18][19][20][21] , XAFS 22 , transmission electron microscopy (TEM) 23 , electron paramagnetic resonance (EPR) 24 , nuclear magnetic resonance (NMR) 25 , etc. have been reported and the crystal structure [8][9][10][11][12][13][14][15] , phonon spectrum [16][17][18][19][20][21]26 , dielectric and piezoelectric behavior 4,6,2 6-30 , temperature and pressure driven phase transitions and critical behavior of the order parameter 24,25 studied. First principles calculations of the structural and dynamical properties, epitaxial strain and anomalous LO-TO splittings of NaNbO 3 have also been reported.…”