Samarium hexaboride was prepared by the borothermal reaction ; 2Sm_2O_3+(6+4x)B=4SmB_x+3B_2O_2. The residual oxygen content amounted to less than 0.03 w/o under optimum reaction conditions heating at 1650β for 2 hr under a high vacuum. It was found by X-ray intensity and density measurements that the samarium hexaboride stayed stable over a large nonstoichiometric region from Sm_B_6 to SmB_6. These extensive nonstoichiometric properties of samarium hexaboride can be explained in terms of the required electron numbers of the SmB_6 network in hexaboride
