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.

Atomic Motion in Crystals from Neutron Diffraction Experiments

Static or dynamic disorder in crystals causes a decrease in the Bragg peak intensity, and given sufficient number of Bragg peaks over an extended Q-range, atomic displacement parameters (ADPs) can be refined that quantify the intensity reduction due to the mean square displacements of atoms about their average positions. It is becoming increasingly apparent that ADPs constitute equally important structural information as atom positions and site occupancies. Unusual, yet accurately determined, ADPs can provide telltale clues to interesting physical phenomena, e.g., approach of phase transitions, glass-like thermal conductivity, pathways for high ionic conductivity, and a variety positional disorders. Consequently, the demand for high quality ADPs is increasing, owing in part to our desire to understand and tune physical properties of technological materials. Temperature dependent neutron diffraction using single-crystals is perhaps the best possible method to determine precise individual ADPs, yet the number of these studies is surprisingly limited owing to the paucity of neutron sources and dedicated single-crystal neutron diffractometers. ADPs exhibit various temperature dependent behaviors, and can range from harmonic to anharmonic. Examples from work completed and ongoing at Oak Ridge National Laboratory (stephanite, triphylite, amblygonite, petalite, brucite, filled-skutterudites, gas clathrate hydrates, etc.) as well as previously published work will be reviewed with the aim to generalize insights and recommendations. Research conducted at ORNL's High Flux Isotope Reactor and Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.