invention also provides a method of preparing the crystalline polymorph comprising the steps of slurrying excess amount of the compound of formula i as prepared in the method of example 1 in the mixed solvent of acetonitrile h 2 o 1 1 or thf h 2 o at room temperature for at least 48 hrs and recovering the resulted crystalline polymorph or comprising the steps of adding the crystalline polymorph as prepared in example 5 as a crystal seed into a solution of the compound of formula i as prepared in example 1 in the mixed solvent of methanol ethyl acetate followed by a spontaneous precipitation and recovering the resulted crystalline polymorph the present invention further provides a crystalline polymorph of the compound of formula i that exhibits an x ray powder diffraction pattern having characteristic peaks at diffraction angles 2 u03b8 of approximately 6 9 u00b0 11 7 u00b0 and 21 1 u00b0 preferably the x ray powder diffraction pattern has characteristic peaks expressed in terms of the interplanar distance at 12 8 u00e5 7 5 u00e5 and 4 2 u00e5 preferably the x ray powder diffraction pattern has characteristic peaks at diffraction angles 2 u03b8 of approximately 6 9 u00b0 11 7 u00b0 15 1 u00b0 21 1 u00b0 and 25.