Emeritus Professor Mike Glazer

Abstract:

The lead transition metal oxides ${\mathrm{PbTiO}}_3, {\mathrm{PbZrO}}_3$ , and ${\mathrm{PbHfO}}_3$ are shown to be extremely similar in their overall properties, such as soft-mode behavior, phase transitions, and long-wavelength related characteristics. However, locally, they develop very different qualities because, as the ${\mathrm{BO}}_6$ mass increases (where B ≡ Ti, Zr, Hf), the anharmonicity of each compound increases significantly, leading to the formation of intrinsically localized modes (ILMS) or discrete breathers (DBS). This explains their exceptionally flat transverse acoustic modes. These nonlinear excitations are observed in birefringence measurements, where their coexistence with the phonon spectrum is demonstrated. While the soft modes, whether normalized by mass or temperature (long-wavelength optic and zone boundary acoustic modes), follow nearly identical temperature dependencies, the transverse acoustic modes exhibit a highly dispersion-free momentum dependence, characteristic of ILMS or DBS. Crucially, our findings support the earlier notion that the unusual dynamics are solely due to the ${\mathrm{BO}}_6$ polarizable unit, with the Pb subarrays acting merely as spectators. Furthermore, these results provide a unique explanation for the appearance of the transient phase below the phase transitions of ${\mathrm{PbZrO}}_3$ and ${\mathrm{PbHfO}}_3$ , which appears to be intrinsic to these two compounds.

Abstract:

Natural optical activity (NOA), the ability of a material to rotate the plane of polarized light, has traditionally been associated with structural chirality. However, this relationship has often been oversimplified, leading to conceptual misunderstandings, particularly when attempts are made to correlate structural handedness directly with optical rotatory power. In reality, the relationship between chirality and NOA is more nuanced: optical activity can arise in both chiral and achiral crystal structures, and the sign of the rotation cannot necessarily be inferred from the handedness of the space group. In this work, we conduct a first-principles investigation of NOA in SiO2 and AlPO4 crystals, focusing on their enantiomorphic structural phase transition from high-symmetry hexagonal (P6422 or P6222) to low-symmetry trigonal (P3121 or P3221) space groups. This transition, driven by the condensation of a zone-centre Γ3 phonon mode, reverses the screw-axis type given by the space-group symbol while leaving the sign of the optical activity unchanged. By following the evolution of the structure and the optical response along the transition pathway, we clarify the microscopic origin of this behaviour. We demonstrate that the sense of optical rotation is determined not by the nominal handedness of the screw axis given in the space-group symbol but by the atomic-scale handedness of the most polarizable atoms of the structure.

Abstract:

It is pointed out that many authors are unaware that the particular choice of unit‐cell origin determines the irreducible representations to which octahedral tilts in perovskites belong. Furthermore, a recommendation is made that the preferred option is with the origin at the B‐cation site rather than that of the A site.

Abstract:

The description of crystal structures is given starting with some fundamental notions of crystal symmetry. The topics of lattices and space groups are briefly introduced and how these can be used with unit cell contents to describe the crystal structure. This leads to crystallographic databases where information on crystal structures is stored and can be searched. A brief discussion on refinement of diffraction information is given, together with the resulting geometric parameters.