Maximizing completeness in single-crystal high-pressure diffraction experiments: phase transitions in 2°AP.

D Tchoń,A Makal

doi:10.1107/s2052252521009532

Abstract

Sufficiently high completeness of diffraction data is necessary to correctly determine the space group, observe solid-state structural transformations or investigate charge density distribution under pressure. Regrettably, experiments performed at high pressure in a diamond anvil cell (DAC) yield inherently incomplete datasets. The present work systematizes the combined influence of radiation wavelength, DAC opening angle and sample orientation in a DAC on the completeness of diffraction data collected in a single-crystal high-pressure (HP) experiment with the help of dedicated software. In particular, the impact of the sample orientation on the achievable data completeness is quantified and proved to be substantial. Graphical guides for estimating the most beneficial sample orientation depending on the sample Laue class and assuming a few commonly used experimental setups are proposed. The usefulness of these guides has been tested in the case of luminescent 1,3-diacetylpyrene, suspected to undergo transitions from the α phase (Pnma) to the γ phase (Pn21 a) and δ phase (P1121/a) under pressure. Effective sample orientation has ensured over 90% coverage even for the monoclinic system and enabled unrestrained structure refinements and access to complete systematic extinction patterns.

Highlights

Insufficient reciprocal space coverage in diffraction experiments may impede space group determination (Merli et al, 2002; Friese & Grzechnik, 2014; Sheldrick, 2015a), render solution of a crystal structure (Yogavel et al, 2007) or determination of absolute configuration (Friese & Grzechnik, 2014) impossible, and conceal or misrepresent fine details such as disorder or unusual charge density distribution (Takata & Sakata, 1996)
We present a comprehensive set of statistics describing data completeness in HP experiments, calculated using a series of numerical simulations performed with dedicated software
The aim of this work was to systematize the combined impact of X-ray wavelength, diamond anvil cell (DAC) opening angle and the sample orientation, with the focus on the achievable completeness of HP diffraction data, and to quantify how much can the latter be improved by proper sample placement depending on the

Summary

Introduction

Insufficient reciprocal space coverage in diffraction experiments may impede space group determination (Merli et al, 2002; Friese & Grzechnik, 2014; Sheldrick, 2015a), render solution of a crystal structure (Yogavel et al, 2007) or determination of absolute configuration (Friese & Grzechnik, 2014) impossible, and conceal or misrepresent fine details such as disorder or unusual charge density distribution (Takata & Sakata, 1996). Data completeness of 40% or below generally poses difficulties for most structure-solution methods and results in numerous artifacts on Fourier maps. In the majority of X-ray diffraction experiments modern area detectors and multi-axis goniometers allow all reflections to be recorded, the signal will not be observed if the beam does not have access to the sample altogether. This is the case when a diamond anvil cell (DAC) is used in HP structural analysis (Merrill & Bassett, 1974; Dziubek & Katrusiak, 2002; Dawson et al, 2004). Recent publications indicate a variety of applications in the analysis of phase diagrams, polymorphism and relative stability of pharmaceutical compounds (Guerain, 2020); investigation of structure-property relationships in metal-

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