Put Your Trust in Powder: Negative and Zero Thermal Expansion Materials

Powder diffraction is one of the most powerful structural probes available to the materials chemist. It invariably plays a critical role in the preparation of new phases and is increasingly exploited via in-situ studies to understand and control complex synthetic routes to fleetingly stable materials. Ab initio structure solution followed by Rietveld refinement frequently provides the first structural information on new functional materials, and powder diffraction is often the method of choice for probing structure-property relationships under non-ambient or in-operando conditions, or for following the structures of samples undergoing structural phase transitions. In this presentation I'll show how powder diffraction has been crucial in understanding the properties of so-called negative thermal expansion (NTE) materials - here inorganic oxides which contract on heating. Powder methods have provided key information on the thermodynamic stability of these materials, the low temperature synthetic routes required to prepare them, the average and local distortions that lead to NTE, and on the often complex phase transitions they undergo. I'll also discuss how powder diffraction can probe glass-like relaxation processes which occur over remarkably long timescales in some of these materials and how the synthetic control enabled by in-situ studies has led to the preparation of single-phase isotropic materials whose expansion properties can be systematically tuned from negative to zero to positive values (alpha -8 to +6 * 10-6 K-1) [1]. I'll emphasise how new techniques such as Parametric- [2] and Symmetry-Mode-Rietveld [3] refinement has been crucial in the study of these materials.

Phase behavior and thermoelastic properties of SnMo₂O₈ under hydrostatic pressure

SnMo₂O₈ has been shown to exhibit very different phase behavior and thermal expansion from previously studied members of the AM₂O₈ family.¹ At high temperatures, SnMo₂O₈, ZrW₂O₈, and ZrMo₂O₈ assume cubic structures with orientationally disordered MO₄ tetrahedra; however, their behavior is widely divergent at lower temperatures. ZrMo₂O₈ maintains its disordered structure and continues to display negative thermal expansion (NTE). While cubic symmetry is retained when cooling ZrW₂O₈, its WO₄ tetrahedra become ordered, and its NTE increases in magnitude. Rapid cooling of SnMo₂O₈ leads to a cubic structure that only minimally differs from its high temperature form.¹ Slowly heating this cubic phase results in a transformation to a rhombohedral (γ) structure with ordered MoO₄ tetrahedra that is not isostructural to any known phases of ZrW₂O₈ and ZrMo₂O₈.¹ In stark contrast to ZrW₂O₈, and ZrMo₂O₈, all SnMo₂O₈ phases exhibit positive thermal expansion.¹ In the current work, the phase behavior and thermoelastic properties of cubic SnMo₂O₈ under hydrostatic conditions were investigated via in situ synchrotron x-ray powder diffraction in a recently designed sample environment.² Previous studies of ZrW₂O₈ and ZrMo₂O₈ in this environment have shown that pressure-induced disordering of MO₄ tetrahedra, which only occurred in the orientationally ordered low temperature ZrW₂O₈ phase, was linked to both elastic softening on heating and enhancement of NTE.³ At 298K, cubic SnMo₂O₈ is significantly softer (κT =30GPa) than ZrW2O8 (64GPa) and ZrMo₂O₈ (43GPa).³ Unlike ZrW₂O₈, which softens upon heating to 516K (ΔκT = -9GPa), SnMo₂O₈ stiffens (+5GPa) more than ZrMo₂O₈ (+2GPa).³ The phase behavior of SnMo₂O₈ under pressure also differs from that of ZrW₂O₈ and ZrMo₂O₈. Compression elevated the γ->cubic transition temperature significantly: at ambient temperature, this transition occurs at ~435K; at 310MPa, it occurs at ~490K.

The structural relaxation properties of the amorphous ices

Despite the importance of low-density amorphous ice (LDA) in critical cosmological processes and its prominence as one of the polyamorphs of water there is still an incomplete picture of the processes that take place upon thermal annealing. We show that a gradual structural relaxation process takes place upon heating vapor-deposited LDA, also called amorphous solid water, and LDAs obtained from several different states of high-density amorphous ice. The structural relaxation leads to an increase in structural order on local and more extended length scales as the average O-O distance shortens and the O-O distance distribution narrows. The relaxation process is separate from crystallization and it does not seem to reach completion before crystallization sets in. Our findings are therefore difficult to reconcile with the postulated glass transition of LDA to the supercooled and highly viscous liquid prior to crystallization. On the basis of a comparison of the calorimetric data of LDA with those of some of the crystalline phases of ice we propose that the calorimetric feature of LDA prior to crystallization may in fact be connected to the kinetic unfreezing of defect-migration mediated reorientation dynamics. We finally discuss the relaxation properties of the various kinds of high-density amorphous ice in the context of these new findings.