A crystallographic approach incorporating multinuclear high field solid state NMR (SSNMR), X-ray structure determinations, TEM observation, and density functional theory (DFT) was used to characterize two polymorphs of rubidium cryolite, Rb3AlF6. The room temperature phase was found to be ordered and crystallizes in the Fddd (n°70) space group with a = 37.26491(1) Å, b = 12.45405(4) Å, c = 17.68341(6) Å. Comparison of NMR measurements and computational results revealed the dynamic rotations of the AlF6 octahedra. Using in-situ variable temperature MAS NMR measurements, the chemical exchange between rubidium sites was observed. The β-phase, i.e. high temperature polymorph, adopts the ideal cubic double-perovskite structure, space group 3 ̅ , with a = 8.9930(2) Å at 600 °C. Additionally, a series of polymorphs of K3AlF6 has been further characterized by high field high temperature SSNMR and DFT computation.

Epitaxial NaNbO3 thin films have been grown by pulsed laser deposition on cubic (00l) MgO substrate with epitaxial (La0.5Sr0.5)CoO3 buffer layer. Micro-Raman spectroscopy studies revealed that the ferroelectric Q phase (Pmc21, Z=4) is stable in a 250-nm-thick film in contrast to the antiferroelectric phase P (Pbma, Z=8) known to exist in the bulk single crystals and ceramics of undoped stoichiometric NaNbO3. Temperature-dependent Raman spectra indicate that the Q phase is stable over a wide temperature range (at least from 80 to 600 K), while the low-temperature ferroelectric rhombohedral phase N, typical for NaNbO3 single crystals, is not observed.

Micro‐Raman studies of oxygen‐deficient gray‐colored NaNbO3 single crystals have shown that at room temperature both antiferroelectric P and ferroelectric Q phases are present simultaneously as separate regions tens of micrometers in size. Phase transitions in P and Q regions have been elucidated by dielectric, dilatometric and micro‐Raman studies. For the first time, the sequence of phase transitions between antiferroelectric phase P and ferroelectric Q and N phases has been revealed. Copyright © 2012 John Wiley & Sons, Ltd.

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