powder the thus obtained yttrium oxide powder had an average particle diameter d 50 of 4 1 u03bcm further the electron microscopic observation of the powder revealed that the powder consisted of discrete non agglomerated particles each having a globular particle configuration the procedure for the precipitation of yttrium hydroxide was carried out in just the same manner as in example 1 excepting omission of the triethanolamine in the ammonia water the yttrium hydroxide in the thus obtained aqueous slurry was in a gelatinous form so that a great difficulty was encountered in the filtration of the aqueous slurry a small portion of the slurry was taken and processed in the same manner as in example 1 to give a yttrium oxide powder which however consisted of coarse and irregularly angular particles having a particle diameter exceeding several tens of u03bcm electron microscopic examination of the powder under high magnification revealed that the powder consisted of agglomerates of very fine particles having a particle diameter of 0 5 u03bcm or smaller the procedure for the precipitation of yttrium hydroxide was carried out in just the same manner as in example 1 excepting omission of the triethanolamine in the ammonia water and.