The invention relates to a powder having a maximum particle size D99,5 of less than 200 microns, a porosity index Ip higher than 2, the porosity index being equal to the ratio Asr/Asg where Asg is the theoretical geometrical specific surface area of the particles of the powder, and Asr is the measurement of the real specific surface area by the BET method, and/or, when all the dimensions of the base particles of the powder are higher than 50 nm, a specific surface area higher than 10m2/g and a total of mesoporous and microporous volumes higher than 0,05 cm3g. More than 20 % of said powder is formed from base particles which are aggregated or not, have dimensions which are all higher than 200 nra, a sphericity index of less than 0.6, and consist of a zirconium oxide and/or a hafnium of formula MOx, M being Zr4+, Hf4+, or a mixture of Zr4+ and Hf4+, and x being a non-zero positive number.