Petalite Under Pressure

The lithium aluminosilicate mineral petalite (LiAlSi₄O₁₀) has been studied using high-pressure single-crystal X-ray diffraction up to 5 GPa. Petalite is a layered silicate mineral. The layers comprise puckered double-sheets of corner-sharing SiO₄ tetrahedra. Corner-sharing AlO₄ tetrahedra bridge neighboring layers and complete the 3D architecture. The charge is balanced by lithium cations that reside within channels that propagate through the structure. Petalite undergoes two pressure-induced phase transitions at ca. 1.5 and 2.5 GPa. The first of these transforms the low-pressure α-phase of petalite (P2/c) to an intermediate β′ phase that then fully converts to the high-pressure β phase at ca. 2.5 GPa. The α→β transition is isomorphic with a commensurate modulation that triples the unit cell volume. Measurement of the unit cell parameters of petalite as a function of pressure, and fitting of the data with 3rd order Birch-Murnaghan equations of state, has provided revised elastic constants for petalite. The bulk moduli of the α- and β-phases are 49(1) and 35(3) GPa, respectively. These values indicate that petalite is one of the most compressible lithium aluminosilicate minerals. The α-phase structure has been refined at five different pressures, revealing a compression mechanism that is driven by the rigid body movement of the Si₂O₇ units from which the silicate double-layers are constructed. The structure of the β′ phase was not determined. The structure of the β phase was determined at 2.71 GPa. Although the fundamental structural features of petalite are retained in the α → β phase transition, subtle alterations occur in the internal structure of the silicate double-layers.

Lanthanide Oxalates Under Pressure: Structural and Photoluminescence Properties

Three dimensional lanthanide (Ln) framework compounds are renowned for their excellent photoluminescence properties, and there is growing interest in the development of this class of metal-organic framework (MOFs) materials for a diverse range of applications ranging from size-selective sensor technology to bio-imaging. Yet, although the physical and structural properties of Ln-MOFs under ambient conditions are well documented, there remains a distinct lack of information pertaining to the behaviour of these materials under non-ambient conditions. In this contribution we present both variable pressure (0-4 GPa) and temperature (100-300 K) single-crystal X-ray diffraction (XRD) studies of several Nd and Pr oxalate MOFs with different topologies (Fig. 1). Furthermore, these extensive XRD investigations have been complemented by variable pressure spectroscopic measurements that allow for evaluation of the influence of pressure on the photoluminescent emissions of these Ln-MOF compounds. This combined diffraction and spectroscopy study has enabled the structure-property relationships, which are so critical to the development of Ln-MOFs for practical usage, to be evaluated comprehensively. We will also show how the framework topology influences the structural behaviour of the Ln-MOF in response to pressure, resulting in the occurrence of unusual phenomena such as negative linear compression (NLC) in which one of the crystallographic axes expands, rather than contracts, with increasing pressure. Analysis of the high-pressure single-crystal XRD data has enabled the NLC mechanism to be elucidated and this will be presented.