Thermal decomposition of undiluted monosilane at high pressures produces amorphous spherical silicon particles, with a mean diameter of 3 mum, clustered together. The size of the particles remains unchanged regardless of monosilane initial pressures between 0.13 and 6.61 MPa. This may reflect that the particle nucleation process is completed at an early stage of the decomposition reaction and/or demonstrate growth inhibition caused by the high hydrogen pressure resulting from the decomposition. The hydrogen content of the particles goes through a maximum of similar to5at% at similar to2 MPa initial pressure and seems to flatten at some 3.5at% hydrogen at the highest monosilane initial pressures studied. After dehydrogenation by heat treatment, the present raw-silicon powder is easily melted and crystalline silicon obtained on cooling. (C) 2004 Elsevier B.V. All rights reserved.