Resolving the controversies about the 'nearly cubic' and other phases of Sr(1-x)Ca(x)TiO(3) (0≤x≤1): I. Room temperature structures

Results of a powder x-ray diffraction study are presented to show that the so-called 'nearly cubic' phase region of Sr(1-x)Ca(x)TiO(3), in fact, corresponds to two different types of crystal structures. The 'nearly cubic' phase occurring in the composition range 0.35<x<0.41 is antiferroelectric in nature and its most plausible space group is Pbcm, while the crystal structure of the 'nearly cubic' phase for 0.41≤x<0.70 belongs to the space group Pbnm. This space group persists up to x = 1.0, even though the 'nearly cubic'-like features disappear for x≥0.70. There are two other phase boundaries which occur around 0.05<x<0.06 and 0.35<x<0.36 and correspond to the change of structures with space groups Pm3m to Ibmm and Ibmm to Pbcm, respectively.

New mechanism for the alpha to omega martensitic transformation in pure titanium

We propose a new direct mechanism for the pressure driven alpha-->omega martensitic transformation in pure titanium. A systematic algorithm enumerates all possible pathways whose energy barriers are evaluated. A new, homogeneous pathway emerges with a barrier at least 4 times lower than other pathways. The pathway is shown to be favorable in any nucleation model.

Calculation of polarization using a density functional method with localized charge

A density functional method, which represents the total charge density as a sum of self-consistently determined localized densities, is described. While this approach is generally less accurate than conventional band-structure methods, it offers a relatively simple interpretation of polarization and related properties. The method is illustrated with results for NaCl, MgO, and AlP.

Group-Theoretical Analysis of Octahedral Tilting in Perovskites

A group-theoretical analysis is made of the structures derived from the aristotype cubic perovskite (Pm3¯m) by the simple tilting of rigid octahedral units. The tilting is mediated by the irreducible representations R^+_4 and M^+_3 or the two in combination. These result in 15 possible structures, compared with the 23 possibilities suggested previously by Glazer [Acta Cryst. (1972), B28, 3384–3392]. The analysis makes the group–subgroup relationships apparent.

Tuned limiter for receiver amplifier in a fast-recovery pulsed NMR spectrometer

A simple tuned limiter is described which limits the rf voltage amplitude at the input of the receiver amplifier of a pulsed NMR spectrometer. This device prevents overload of the receiver amplifier during the high-power rf pulse and thereby allows fast recovery of the spectrometer following the pulse. When used with Avantek unit amplifiers at 24 MHz, a recovery time of 2.5 mus is achieved.