Proton Presence and Motion in Rhenium-Oxide Films and Their Application to Liquid-Crystalline Cells

Disordered solid phases, containing appreciable amounts of hydrogen ions, aregrown at the surface of rhenium oxide crystals, because of the high reactivity ofthis compound with ambient moisture. To investigate such phenomena, acomparative study is performed on ground powder and thermally evaporated orsputtered films using x-ray diffraction and micro-Raman spectroscopy. Two typesof solid phases were found in the films: HxReO3 distorted perovskite structures,based on corner-sharing ReO6 octahedra as in the bulk crystals, and orderedHReO4 crystalline structures, based on tetrahedral perrhenate ions. The compleximpedance measurements on ReO3 films support the hypothesis of mobile hydrogenions in such defective films. Moreover, this relevant protonic conductivityallows the application of these films as active layers inserted into asymmetricnematic liquid-crystalline cells to rectify the electro-optical response of suchdevices, with performances quite similar to previously studied oxides such as WO3.INTRODUCTIONBecause of its conductivity properties, rhenium oxide can be very usefulfor technological applications, such as SOFCs (solid oxide fuelcells), electrochromic devices, solid-state batteries, and other particularapplications.Rhenium trioxide appears red colored and shows metallic conductivityat temperatures less than 500K [1–4]. The bulk rheniumtrioxidecrystal belongs to the space group O1h(Pm3 m) with the latticeparameter a0 ¼ 3.7504A ° [5]. Its crystal lattice is composed of ReO6octahedra joined by corners, and the Bravais unit cell contains oneunit of ReO3. The cubic lattice of ReO3 is very stable at normal pressurefor all the temperatures [5,6]. The Re6þ(5d1) configuration withone d-electron in the conduction band, in excess with respect to tungstentrioxide, justifies the high electronic conductivity, similar to theone of tungsten bronze [7].A recent work [8] on Raman spectroscopy and vibrational dynamicsof ReO3 indicates that a different kind of disorder is always present atthe surface of powders and thin films of ReO3. Both electronic and ionicconductivities can be observed in actual samples of rhenium trioxide,with the ionic one associated to intercalated hydrogen cations comingfrom the moisture reaction with a subsurface layer of the oxide:ð1 þ xÞReO3 þ xH2O ! HxReO3 þ xHReO4:The defective system of HxReO3 and HReO4, formed in this reaction, isdescribed as an amorphous layer [9]. The hydrogen concentration incommercial ReO3 powders is higher in the layers closer to the surface,and when the temperature is greater than 200C, the intercalated protonsleave the