A. M. Glazer

A nitrogen-gas-stream cryostat for general X-ray diffraction studies

A continuous nitrogen-flow cooling device, generally applicable to X-ray diffraction studies, is described. The device works in the range 77.4 to 323.0 K with a precision of ±0.1 K and a liquid-nitrogen constant consumption rate of 0.5 l h−1 over the whole temperature range. The supply vessel is unpressurized, so that refilling can be done without any observable influence on the cooling of the sample.

Simple ways of determining perovskite structures

A simple technique is described for ascertaining trial models for the structures of perovskites. The method relies on an understanding of the fundamental components of the structure. Rules are given for determining trial models rapidly.

An x-ray diffraction and Raman spectroscopy investigation of A-site substituted perovskite compounds: the (Na1-xKx)0.5Bi0.5TiO3 (0le xle1) solid solution

The (Na1 - x Kx )0.5 Bi0.5 TiO3 perovskite solid solution is investigated using x-ray diffraction (XRD) and Raman spectroscopy in order to follow the structural evolution between the end members Na0.5 Bi0.5 TiO3 (rhombohedral at 300 K) and K0.5 Bi0.5 TiO3 (tetragonal at 300 K). The Raman spectra are analysed with special regard to the hard modes and suggest the existence of nano-sized Bi3+ TiO3 and (Na1 - 2x K2x )+ TiO3 clusters. The complementary use of XRD and Raman spectroscopy suggests, in contrast to previous reported results, that the rhombohedral tetragonal phase transition goes through an intermediate phase, located at 0.5 x 0.80. The structural character of the intermediate phase is discussed in the light of sub- and super-group relations.

Lattice parameters and tilted octahedra in sodium–potassium niobate solid solutions

Measurements of lattice parameters and intensities of difference reflexions have been made for all compositions of Nal - xKxNbO3, both at room temperature and above. Structural models are proposed for each phase found.

AN AUTOMATIC OPTICAL IMAGING SYSTEM FOR BIREFRINGENT MEDIA

A new optical microscope imaging system is described. The instrument is capable of producing coded colour images of an optically birefringent material, where the colour represents either |sin δ|, a function of the optical retardation; ɸ, the orientation of a section of the optical indicatrix; or Io, the transmittance, at any place within the image. Thus the contrast normally seen in a standard crossed-polars experiment is separated out into its components. The technique is both qualitative and quantitative. Examples of the application of this system to crystals, a rock section and a biological specimen are given.

Ferroelastic phase transition in BiVO4 I. Birefringence measurements using the rotating-analyser method

A brief review of methods of measuring birefringence is given. Apparatus for the accurate measurement of optical birefringence at high temperatures using a modulation (rotating-analyser) method is described in detail, together with suggestions for further applications of the equipment. The technique is applied to the particular case of the ferroelastic monoclinic–tetragonal phase transition in BiVO4. The birefringence was found to show mean-field behaviour over a wide temperature range with a mean-field transition temperature of 519.9 (3) K.

A comprehensive study of the phase diagram of KxNa1−xNbO3

The phase diagram of lead-free piezoelectric KxNa1−xNbO3 has been studied, with particular focus on the proposed morphotropic phase boundaries, by powder and single crystal x-ray diffraction. The tilt system and cation displacement has been mapped out as a function of temperature and composition, highlighting changes in the oxygen octahedra at x=0.2 and x=0.4 at room temperature. The orthorhombic to monoclinic boundary at x=0.5 has been investigated, with a subtle change in the structure observed. The conclusion is that KxNa1−xNbO3 does not display a morphotropic phase boundary comparable with that in lead zirconate titanate, and that the most significant structural change as a function of composition occurs at x=0.2 because of the change of the tilt system.

Structural phase transitions in lead zirconate

The structural phase transitions occurring in PbZrO3 have been investigated using X-ray diffraction and dielectric constant measurements. The lattice parameters, Pb ion displacements and ZrO6 octahedral tilts in the antiferroelectric, orthorhombic phase have been determined as functions of temperature between 20 and 228 degrees C using continuously recording X-ray diffraction photography. The temperature dependence of the Pb ion displacement is discussed with reference to the Kittel theory of antiferroelectricity. The lattice parameters and structural data derived from these results are used to describe the temperature dependence of the birefringence in PbZrO3.

On the origin of optical activity in crystal structures

Close examination of published measurements of absolute configuration in inorganic crystals has revealed many mistakes, and only a small number of published correlations between optical rotation and chirality of crystal structure can be trusted. The sense of rotation of light is here traced to the hand of specific helices of atoms, provided that the latter are correctly chosen. Four principles are given for choosing the helices. The crystals described are: low-quartz, α-AlPO4, α-HgS, Ca2Sr(C2H5COO)6, Bi12SiO20, NaClO3, NaBrO3 and α-LiIO3. The principles suggest that the published absolute configuration of α-LiIO3 must be incorrect {subsequently proved experimentally by Stadnicka, Glazer & Moxon [J. Appl. Cryst. (1985), 18, 237–240]}.

The missing boundary in the phase diagram of PbZr1−xTixO3

PbZr(1-x)Ti(x)O3 (PZT) is one of the most important and widely used piezoelectric materials. The study of its local and average structures is of fundamental importance in understanding the origin of its high-performance piezoelectricity. Pair distribution function analysis and Rietveld refinement have been carried out to study both the short- and long-range order in the Zr-rich rhombohedral region of the PZT phase diagram. The nature of the monoclinic phase across the Zr-rich and morphotropic phase boundary area of PZT is clarified. Evidence is found that long-range average rhombohedral and both long- and short-range monoclinic regions coexist at all compositions. In addition, a boundary between a monoclinic (M(A)) structure and another monoclinic (M(B)) structure has been found. The general advantage of a particular monoclinic distortion (M(A)) for high piezoactivity is discussed from a spatial structural model of susceptibility to stress and electric field, which is applicable across the wide field of perovskite materials science.