(Y0.5Gd0.5) BO3 : Eu phosphor powders are prepared by homogeneous precipitation from a salt solution using a slow increase in pH by the hydrolysis of urea. Powders obtained by urea hydrolysis followed by hydrothermal treatment, with HNO3 as mineralizer, are partially crystallized. Calcination treatment at relatively low temperatures in the range 700-900degreesC leads to complete crystallization. The urea concentration has a very significant influence on the particle morphology and yield. The average particle size of the as-prepared powder is between 1 to 3 mum. The particles are composed of primary particles with crystallite size in the range from 12 to 15 nm. The particle size remains unchanged upon hydrothermal and calcination treatments under the conditions used. Calcination of the powders leads to a large increase in the photoluminescence (PL) intensity due to increased crystallization. The PL peak intensity is at its maximum when the Eu content is 4 atom % and the stoichiometric amount of H3BO3 is used. In the powders calcined at lower temperatures, the contribution of the D-5(0)-F-7(2) transition to the PL intensity is significantly larger, implying a better color purity. (C) 2004 The Electrochemical Society.