Stable and metastable structures of tin (IV) oxide at high pressure

Abstract

We have analysed SnO 2 with a combination of synchrotron X-ray diffraction and X-ray absorption spectroscopy across a pressure range of 0 → 82.9 GPa with thermal annealing by a CO 2 laser allowing access to all of the known high-density polymorphs of SnO 2 , and here report their crystallographic information. The metastability of the post-rutile α - PbO 2 and PdF 2 structures in SnO 2 are investigated by experiment and PW-DFT simulations, revealing a complex energetic landscape and suggesting a significant dependence of the observed phases on the pressure–temperature pathway taken in experiment. This article is part of the theme issue ‘Exploring the length scales, timescales and chemistry of challenging materials (Part 1)’.

High Pressure–Temperature Study of MgF2, CaF2, and BaF2 by Raman Spectroscopy: Phase Transitions and Vibrational Properties of AF2 Difluorides

“… 3 − 5 , 7 , 10 , 12 , 22 At 300 K, rutile-type difluorides such as MgF 2 with a small cation radius of 0.72 Å undergo a second-order transformation to the orthorhombic CaCl 2 -type structure ( Pnnm , Z = 2) at ∼9 GPa and then transforms to the cubic HP-PdF 2 -type structure ( Pa 3̅, Z = 4, also misidentified as a “pyrite-type” structure in some studies) at ∼14 GPa. 10 , 23 , 24 However, an additional α-PbO 2 -type phase was predicted by theoretical calculation to be stable between CaCl 2 - and HP-PdF 2 -type phases. 10 Further elevating pressure to ∼38 GPa leads to the transition to the cotunnite-type structure ( Pnma , Z = 4).…”

“…The phase transition sequence of difluorides at high pressures is strongly dependent on the cation radius. − ,,,, At 300 K, rutile-type difluorides such as MgF 2 with a small cation radius of 0.72 Å undergo a second-order transformation to the orthorhombic CaCl 2 -type structure ( Pnnm , Z = 2) at ∼9 GPa and then transforms to the cubic HP-PdF 2 -type structure ( Pa 3̅, Z = 4, also misidentified as a “pyrite-type” structure in some studies) at ∼14 GPa. ,, However, an additional α-PbO 2 -type phase was predicted by theoretical calculation to be stable between CaCl 2 - and HP-PdF 2 -type phases . Further elevating pressure to ∼38 GPa leads to the transition to the cotunnite-type structure ( Pnma , Z = 4). ,− Increasing the cation radius leads to a decrease in the phase transition pressure to CaCl 2 - and HP-PdF 2 -type structures. , For example, ZnF 2 with a cation radius of 0.74 Å transitions to the CaCl 2 -type phase at 4.5 GPa and to the HP-PdF 2 -type structure at 10 GPa, which are much lower than those of MgF 2 at 300 K. , Of particular interest is the rutile-type MnF 2 , which transforms to the ZrO 2 - or SrI 2 -type rather than CaCl 2 -type structure at ∼3 GPa. , In contrast to the HP-PdF 2 -type MgF 2 , which directly transitions into the cotunnite structure at higher pressures, some transition metal difluorides like ZnF 2 and CoF 2 will transform into the AgF 2 - or cubic fluorite-type structures before changing to the cotunnite structure. , …”

Exploring the length scales, timescales and chemistry of challenging materials (Part 1)

“…Binary metal oxides have a large number of different polymorphs, and pressure-induced changes in oxygen positions can have a dramatic effect on electrical conductivity. Two synchrotron X-ray techniques, combined with DFT modelling, are used by Sneed et al [ 25 ] in their high-pressure study of SnO 2 using a CO 2 laser-heated diamond anvil cell. Diffraction and X-ray absorption spectroscopy data were collected at the APS and show that the ambient-pressure rutile-structured SnO 2 undergoes a series of phase transitions, first adopting a CaCls structure and then at the highest pressure a PbF 2 cubic structure.…”