Effect of hydrostatic pressure on the crystal structure of sodium oxalate: X-ray diffraction study and ab initio simulations

Abstract

Effect of hydrostatic pressures up to 8 GPa on the crystals of Na 2 C 2 O 4 (sp. gr. P2 1 /c) was studied in situ in the diamond anvil cells a) in neon, b) in methanol-ethanol mixture by high-resolution X-ray powder diffraction (synchrotron radiation, l ¼ 0.7 A, MAR345-detector). Below 3.3-3.8 GPa, anisotropic structural distortion was observed, which was similar to, but not identical with that on cooling. At 3.8 GPa, a reversible isosymmetric first-order phase transition without hysteresis occurred. The orientation of the oxalate anions changed at the transition point by a jump, and so did the coordination of the sodium cations by oxygen atoms. Ab initio simulations based on the generalized gradient approximation of density functional theory have reproduced the main features of the structural changes in the crystals of sodium oxalate with increasing pressure. The theoretical pressure for the isosymmetric phase transition is 3.65 GPa, close to the experimental value; in agreement with experiment the transition was predicted to be reversible. Ab initio calculations gave a pronounced hysteresis for this transition, and have also predicted a further isosymmetric phase transition at 10.9 GPa, also with a hysteresis. The role of temperature in the pressure-induced phase transitions in sodium oxalate is discussed.

Computational Raman spectroscopy of organometallic reaction products in lithium and sodium-based battery systems

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Effect of pressure on two polymorphs of tolazamide: why no interconversion?

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Structure solution and refinement from powder or single-crystal diffraction data? Pros and cons: An example of the high-pressureβ′-polymorph of glycine

“…Structure solution and refinement based on X-ray diffraction data from powder samples is becoming a routine procedure for more and more complex samples (David and Shankland, 2008). Even for the structures with light atoms only, the structural model can be found successfully ab initio and also from the data collected in a diamond anvil cell at high pressures in situ , despite a small amount of the sample, a large background of a complex shape, the necessity to mask reflections from diamonds and ruby, and unavoidable large preferred orientation effects [see Boldyreva et al (2006a, 2006b), Boldyreva (2008), and references therein]. Still, a structure solution and refinement from a single-crystal data set is traditionally considered to be advantageous as compared to that from powder-diffraction data.…”