  In the reported low-fluorine MOD-YBCO studies, the lowest F/Ba mole ratio of
the precursor solution was 4.5. However, further lowering the F/Ba ratio is
important according to the researches of YBCO thick film. On the other hand,
the F/Ba ratio is necessary to be at least 2 for the full conversion of the Ba
precursor to BaF_2 to avoid the formation of BaCO_3, which is detrimental to
the superconducting performance. In this study, a novel solution with the F/Ba
mole ratio of 2 was developed, in which the fluorine content was only about
10.3% of that used in the conventional TFA-MOD method. Attenuated total
reflectance-Fourier transformed-infrared spectra(ATR-FT-IR) revealed that
BaCO_3 was remarkably suppressed in the as-pyrolyzed film and eliminated at 700
Celsius degree. Thus YBCO films with a critical current density (J_c) over 5 MA
cm^{-2} (77 K, 0 T, 200 nm thickness) could be obtained on LAO single crystal
substrates. In-situ FT-IR spectra showed that no obvious fluorinated gaseous
by-products were detected in the pyrolysis step, which indicated that all of
the F atoms might remain in the film as fluorides. X-ray diffraction (XRD)
\theta/2\theta-scan showed that BaF_2, but neither YF_3 nor CuF_2, was detected
in the films quenched at 400 - 800 Celsius degree. The formation priority of
BaF_2 over YF_3 and CuF_2 was interpreted by the chemical equilibrium of the
potential reactions. Our study could enlarge the synthesis window of the
precursor solution for MOD-YBCO fabrication and open a gate to study the
fluorine content in the precursor solution continuously and systematically.
