Hafnon (hafnium silicate, HfSiO4) ceramics are important candidates for high-temperature refractory materials because of their high thermal-shock resistance; this originates from their thermalexpansion coefficient which is lower than any other oxide ceramic. It is, furthermore, well-known that the thermalexpansion coefficient of hafnon (3.6 × 10 -6, 150~ 1300°C) is smaller than that (5.1 x 10 -6, 25 ~ 1500 °C) of zircon (ZrSiO4) [1], and each of the three forms of silica quartz, cristobalite and vitreous silica when heated with HfO2 at 1550 °C yields hafnon [2]. Nevertheless, the formation mechanism of hafnon is still not well-known. This letter reports the effect of dopants on the formation of hafnon by a sol-gel method because this method has attracted attention as a lowtemperature route to oxide materials. An equimolar solution of hafnium oxychloride (HfOC12.8HeO) dissolved in water containing hydrochloric acid and Si(OC:Hs)4 (tetraethoxyorthosilane, TEOS) was mixed by magnetic-stirrer agitation at room temperature in the presence of a small amount of divalent metal ions (M~÷/TEOS ~ 3 x 10 -~ molmol-~). Thence, the water contents (molar ratio, r ~-H:O/TEOS) was nearly equal to 30. The molar ratio teaches us that the siloxane polymer formed is composed of a three-dimensionalladder-chain structure during hydrolysis and condensation of TEOS [3, 4]. All reagents were of the highest purity grade and they were employed without further purification. Finally, the rotation of the stirrer was terminated due to the increase in viscosity with the agitation time. The gel was allowed to air dry, and then it was dried to a powder and calcined at the increasing temperature rate of 300 °C h -~ with a clean air carrier, kept for 1 h at the desired temperatures (1100-1400 °C), and then cooled in the furnace under air flow. The heat-treated powders were characterized by X-ray diffraction (XRD, Rigaku Denki Company) using CuK~ radiation. Table I shows the starting reagents doped as a solution. Fig. 1 shows representative XRD patterns of powders calcined at 1100 °C. The XRD peaks of the sample in the absence of a dopant suggest the beginning of crystallization, and correspond to badly developed monoclinic hafnium dioxide (m-HfO;) with a = 0.512 nm, b = 0.518 nm, c = 0.525 nm, an extremely small amount of hafnon (that is a zircon