The structure of NaNbO3 has been refined from three-dimensional X-ray data. It is a distorted perovskite, with space group Pbma and 8 formula units in an orthorhombic unit cell about 5.5 x 15 x 5.5/l,.Electron-density projections constructed with only the Nb atoms displaced from their ideal positions gave approximate magnitudes for all the larger displacement parameters. With the help of an additional projection, assuming one oxygen parameter to fix the sense of the y axis, the signs of all these displacement parameters could be found by several different arguments allowing cross-checking. The trial model thus set up proved to be a very good approximation and refined easily both by Fourier difference syntheses and by least-squares.The NbO6 octahedra, which are the asymmetric units, are in fact very regular, though tilted appreciably (,-, 10 °) about both the [010] and the [100] directions. The Nb atoms are off-centre, with their displacement vector inclined at about 20 ° to [100], nearly in the (010) plane. There are thus two short, two long, and two intermediate Nb-O bonds; each O has only one short bond to Nb. Deviations from regularity of the octahedron shape, though of second-order magnitude, are systematic and can be correlated with the Nb displacement. Along any column of octahedra parallel to [010], a pair of Nb displacements related by the (010) mirror plane is followed by an antiparallel pair, but in adjacent columns the displacements are not exactly parallel. The two independent Na atoms have very different environments; Na(1) has 3 very near neighbours in a triangle, and 6 distinctly more distant, while Na(2) has 4 nearest in a tetrahedron and 4 slightly more distant. Thermal vibration amplitudes are in general greatest at right angles to strong bonds. The structure is compared with the ferroelectric phase (Phase Q), and some suggestions are made as to how the Na environment may determine the relative stabilities of the two phases.

The lattice parameters of NaNbO3 (which has a perovskite structure and is antiferroelectric at room temperature) have been measured by single-crystal X-ray methods from room temperature to 800°C. While the measurements are in substantial agreement with earlier work, the greater resolution shows up new details, and the interpretation of the earlier work has to be very seriously revised, particularly as concerns the symmetry of the high-temperature phases.Below the 360°C transition a second very similar phase Q can sometimes replace the normal roomtemperature phase P. Q is believed to be the 'forced ferroelectric' known to be produced from P by the action of an electric field; the field-free P-Q transitions are influenced by temperature but have not a reproducible transition temperature.Above 360°C there are a large number of transitions characterized by very different features. The facts from this and earlier work are critically reviewed. While the effects cannot yet be explained in detail, the general reasons for expecting such behaviour are discussed. No detailed structural work is reported, but there is evidence for the persistence of Nb displacements above 360 °C, which is therefore not the paraelectric transition.The transitions in a pseudosymmetric structure cannot be explained or predicted in terms of conventional or macroscopic theories which disregard structural details. A new empirical approach is needed, for which this system provides useful material.

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The temperature dependence of the X- and Q-band EPR spectra of Cs(2)[Zn(H(2)O)(6)](ZrF(6))(2) containing approximately 1% Cu(2+) is reported. All three molecular g-values vary with temperature, and their behavior is interpreted using a model in which the potential surface of the Jahn-Teller distorted Cu(H(2)O)(6)(2+) ion is perturbed by an orthorhombic "strain" induced by interactions with the surrounding lattice. The strain parameters are significantly smaller than those reported previously for the Cu(H(2)O)(6)(2+) ion in similar lattices. The temperature dependence of the two higher g-values suggests that in the present compound the lattice interactions change slightly with temperature. The crystal structure of the Cs(2)[Zn(H(2)O)(6)](ZrF(6))(2) host is reported, and the geometry of the Zn(H(2)O)(6)(2+) ion is correlated with lattice strain parameters derived from the EPR spectrum of the guest Cu(2+) complex.