• R CHITRA

      Articles written in Bulletin of Materials Science

    • A single crystal neutron diffraction study on mixed crystal (K)$_{0.25}(NH$_4$)$_{0.75}H$_2$PO$_4$: tuning of short strong hydrogen bonds by ionic interactions

      RAJUL RANJAN CHOUDHURY R CHITRA

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      One of the most intriguing facts about hydrogen bonds is that bonds formed between the same couple of donorand acceptor atoms can have large variation in the geometry as well as energies; this is mainly because hydrogen bonds arehighly sensitive to their environment.We report here a comparative study of neutron structures of mixed crystals of potassium–ammonium dihydrogen phosphate. Basic structural framework of crystals belonging to potassium dihydrogen phosphatefamily is constructed by a network of zigzag O–H–O hydrogen bonded chains of PO$_2$(OH)$_2$ anions, with cations occupying the interspaces of the zigzagged anionic chains. These crystals having simple crystal structure primarily determined by theO–H–O hydrogen bonds connecting PO$_2$(OH)$_2$ anions provide us with a unique opportunity to tune the hydrogen bondgeometry by varying the cationic occupancy and study the effect of this change on the average structure of the crystal. Thecation–anion interaction is found to have a direct influence on the O–H–O hydrogen bonds of the mixed crystals.

    • Deterioration of hydrogen-bonded superprotonic conductors belonging to CsHSO$_4$–CsH$_2$PO$_4$–H$_2$O salt system: a single-crystal neutron diffraction investigation

      R R CHOUDHURY R CHITRA I P MAKAROVA E V SELEZNEVA V A KOMORNIKOV

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      Single-crystal neutron diffraction investigation on Cs$_4$(HSO$_4$)$_3$(H$_2$PO$_4$) and Cs$_6$H(HSO$_4$)$_3$(H$_2$PO$_4$)$_4$ superprotonic crystals, belonging to CsHSO$_4$–CsH$_2$PO$_4$–H$_2$O salt system, is under taken to elucidate the precise hydrogen atom positions in these crystals. The investigation revealed that these crystals are very sensitive to the ambient conditions and can undergo deterioration due to fluctuation in air moisture content. Cs$_6$H(HSO$_4$)$_3$(H$_2$PO$_4$)$_4$ crystals are more stable as compared to Cs$_4$(HSO$_4$)$_3$(H$_2$PO$_4$). Crystal structure of Cs$_6$H(HSO$_4$)$_3$(H$_2$PO$_4$)$_4$ is obtained both before and after deterioration,it is found that the asymmetric O–HO hydrogen bond between the PO$_4$ and SO$_4$ ions of this crystal becomes stronger after deterioration. This led to the shrinkage of the unit cell, and most likely prevented further deterioration. Diabatic state model for hydrogen bonds is used to obtain the energy contour for the O–H$\ldots$O hydrogen bond of Cs$_6$H(HSO$_4$)$_3$(H$_2$PO$_4$)$_4$ crystal. The influence of this change in the hydrogen bonding on the proton conduction ability ofthe crystal is discussed.

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