The structures having the minimum interaction energy of the Li₂CO₃ monomer, dimer, trimer and tetramer were derived using the density functional theory (DFT). The structures of the monomer and the dimer agreed with the ab initio molecular orbital calculation results at a Hartree—Fock level. The structures of the trimer and the tetramer model were C_3h and C₂ symmetry, respectively. Then we carried out the harmonic frequency analysis of these geometries. The tetramer model showed the best agreement among these results with the Raman spectrum of the liquid state. On the other hand, we calculated the total structure factor S(Q) of the trimer and tetramer geometries considered from the DFT calculation to examine the local structure of molten Li₂CO₃. The calculation result corresponding to the tetramer model reproduced satisfactorily the position and the height of the S(Q) peaks. It was found that the DFT calculation of the tetramer model reproduced the S(Q) at the small Q region where the intermolecular correlations predominated.

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We developed a new type of an ion-sensitive field-effect transistor (ISFET) with an ion sensing polymerized silsesquioxane (polySQ) membrane carrying a quaternary ammonium salt (QAS) as an ionophore. A noticeable feature of this ISFET is that the metal disc part is designed to be detachable from an FET body. Owing to this structure, the full polymerization of SQ at relatively high temperature became possible; leading to preparation of a durable ion sensing polySQ membrane on a metal disk connected to the FET gate terminal. The ISFET with the polySQ membrane carrying QAS (polySQ-QAS) (optimal content of QAS, 40 wt% in the matrix), prepared at 150°C for 3 h, showed not only the theoretical potential response to the nitrate ion concentration in the range of 1.0 × 10⁻⁶ to 0.10 mol dm⁻³ (M) with the Nernst slop of −61.5 mV·decade⁻¹, but also the rapid response (3 s for 90% of response). Furthermore, the ion sensing property is favorably sustained even after being soaked in methanol for 24 h.

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