We report here results based on electrochemical K-alloying/de-alloying and associated in-situ studies with Sn in K ‘half cells’. The as-recorded reversible K-capacity of ∼245 mAh/g agree with the ‘final’ phase (K4Sn4) observed in in-situ synchrotron XRD scans at the end of one K-alloying half cycle. The electrochemical cycling and in-situ XRD results indicate that K4Sn4 forms via one-step phase transformation with no prior Sn-K solid solution formation and reverts back to β-Sn after de-potassiation half cycle (leading to ∼85 ± 6% first cycle coulombic efficiency). Interestingly, no notable evidence for occurrence of irreversible surface reaction could be found during the 1st discharge. However, strong evidences for the same were recorded (at ∼1.3–1.4 V, against K/K+) after completion of one full K-alloying/de-alloying cycle. In-situ monitoring of stress developments in the Sn film electrodes during galvanostatic cycling indicated the occurrence of mechanical instability not only during K-alloying/de-alloying induced phase transformations, but also upon occurrence of the surface reactions (as supported by SEM observations), which also lead to development of compressive stresses by itself. Accordingly, galvanostatic cycling within restricted cell voltage window of 1.2–0.01 V, as against 2–0.01 V, suppressed the irreversible surface phenomena and improved the cyclic stability.

We report Raman spectroscopic studies of the nanosized rare earth sesquioxides, namely yttrium sesquioxide (Y(2)O(3)), gadolinium sesquioxide (Gd(2)O(3)) and samarium sesquioxide (Sm(2)O(3)), under high pressure. The samples were characterized using x-ray diffraction, Raman spectroscopy and atomic force microscopy at atmospheric pressures. Y(2)O(3) and Gd(2)O(3) were found to be cubic at ambient, while Sm(2)O(3) was found to be predominantly cubic with a small fraction of monoclinic phase. The strongest Raman peaks are observed at 379, 344 and 363 cm(-1), respectively, for Y(2)O(3), Sm(2)O(3) and Gd(2)O(3). All the samples were found to be nanosized with 50-90 nm particle sizes. The high pressures were generated using a Mao-Bell type diamond anvil cell and a conventional laser Raman spectrometer is used to monitor the pressure-induced changes. Y(2)O(3) seems to undergo a crystalline to partial amorphous transition when pressurized up to about 19 GPa, with traces of hexagonal phase. However, on release of pressure, the hexagonal phase develops into the dominant phase. Gd(2)O(3) is also seen to develop into a mixture of amorphous and hexagonal phases on pressurizing. However, on release of pressure Gd(2)O(3) does not show any change and the transformation is found to be irreversible. On the other hand, Sm(2)O(3) shows a weakening of cubic phase peaks while monoclinic phase peaks gain intensity up to about a pressure of 6.79 GPa. However, thereafter the monoclinic phase peaks also reduce in intensity and mostly disordering sets in which does not show significant reversal as the pressure is released. The results obtained are discussed in detail.

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The phase transformation in nano-crystalline dysprosium sesquioxide (Dy 2 O 3 ) under high pressures is investigated using in situ Raman spectroscopy. The material at ambient was found to be cubic in structure using X-ray diffraction (XRD) and Raman spectroscopy, while atomic force microscope (AFM) showed the nano-crystalline nature of the material which was further confirmed using XRD. Under ambient conditions the Raman spectrum showed a predominant cubic phase peak at 374 cm −1 , identified as F g mode. With increase in the applied pressure this band steadily shifts to higher wavenumbers. However, around a pressure of about 14.6 GPa, another broad band is seen to be developing around 530 cm −1 which splits into two distinct peaks as the pressure is further increased. In addition, the cubic phase peak also starts losing intensity significantly, and above a pressure of 17.81 GPa this peak almost completely disappears and is replaced by two strong peaks at about 517 and 553 cm −1 . These peaks have been identified as occurring due to the development of hexagonal phase at the expense of cubic phase. Further increase in pressure up to about 25.5 GPa does not lead to any new peaks apart from slight shifting of the hexagonal phase peaks to higher wavenumbers. With release of the applied pressure, these peaks shift to lower wavenumbers and lose their doublet nature. However, the starting cubic phase is not recovered at total release but rather ends up in monoclinic structure. The factors contributing to this anomalous phase evolution would be discussed in detail.