a-hexathienyl ( a -6 T ) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported. 1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult. 2 " 5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of a -6 T (a-6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Hiickel theory (EHT) band structure calculations show that a -6 T / H T is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at T. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as /c-(ET) 2 Cu(NCS) 2 .Structural studies on single crystals of oligomers of thiophene have so far been carried out only for terthienyl 6 and dimethylquaterthienyl. 7 Previous structural studies of the hexamer are all based on the Rietveld method using powder diffraction data. For systems this complicated, the Rietveld refinement has to be strongly constrained, which may cause the refinement to lock in a local minimum and bias the result. It is therefore highly desirable to use single crystals for any structural study. Crystal growth, on the other hand, has proven very difficult, with little success in the past. From the proposed and refined structures of a-hexathienyl, a variety of different monoclinic unit cells have been reported: a = 45.38 A, b = 7.86 A, c = 6.03 A, and f3 = 99.1 °5; a = 5.98 A, b = 7.80 A, c = 50.28 A, and / 3 = 111.3°; or a = 5.67 A, b = 7.80 A, c = 47.2 A, and /3 = 1OO.6 0 . 4 The main difference is in the variation of the longest axis, indicating a different packing of the molecules and even possible conformational departures from planarity. 2 In addition, a mesophase was reported 8 and analyzed. 5 This high-temperature polymorph, obtained by prolonged heating at 307 °C, shows a very different powder pattern than that of sublimed a -6 T . These puzzling facts prompted us to further investigate the various phases of a -6 T .The starting material a -6 T was synthesized differently from most reported synthesis, in a manner pro-ducing the highest-purity and highest-melting material to date. 1 Crystals of a high-temperature polymorph, a-6T/HT, were prepared by controlled melt growth in a special microscope heating stage, permitting melt...

No abstract

Deviations from stoichiometry occurring during the crystal growth of lithium niobate have been studied by 93Nb NMR and by shifts in the ferroelectric transition temperature. The high-temperature phase equilibria of nonstoichiometric lithium niobate were accurately determined by these techniques together with DTA and crystal-growth experiments. The congruently melting composition is located at 48.6 mole% Li2O and the solid-solution region at room temperature extends from 48% to 50% Li2O. The implications of growth at noncongruently melting compositions on the compositional uniformity of lithium niobate are discussed in detail.

No abstract

No abstract