  We report optimization of the synthesis parameters viz. heating temperature
(TH), and hold time (thold) for vacuum (10-5 torr) annealed and LN2 (liquid
nitrogen) quenched MgB2 compound. These are single-phase compounds
crystallizing in the hexagonal structure (space group P6/mmm) at room
temperature. Our XRD results indicated that for phase-pure MgB2, the TH for
10-5 torr annealed and LN2 quenched samples is 750 0C. The right stoichiometry
i.e., MgB2 of the compound corresponding to 10-5 Torr and TH of 750 0C is found
for the hold time (thold) of 2.30 hours. With varying thold from 1- 4 hours at
fixed TH (750 0C) and vacuum (10-5 torr), the c-lattice parameter decreases
first and later increases with thold (hours) before a near saturation, while
the a-lattice parameter first increase and later decreases beyond thold of 2.30
hours. c/a ratio versus thold plot showed an inverted bell shape curve,
touching the lowest value of 1.141 which is reported value for perfect
stoichiometry of MgB2. The optimized stoichimetric MgB2 compound exhibited
superconductivity at 39.2 K with transition width of 0.6 K. In conclusion, the
synthesis parameters for phase pure stoichimetric vacuum annealed MgB2 compound
are optimized and are compared with widely reported Ta tube encapsulated
samples.
