| Summary: | Mineral trioxide aggregate (MTA) is well known as an effective root canal filling material for endodontics therapy. Within MTA, bismuth oxide (Bi<sub>2</sub>O<sub>3</sub>) serving as the radiopacifier still has biocompatibility concerns due to its mild cytotoxicity. In the present study, we tried to modify the Bi<sub>2</sub>O<sub>3</sub> radiopacifier by doping hafnium ions via the sol-gel process and investigated the effects of different doping ratios (Bi<sub>2-x</sub>Hf<sub>x</sub>O<sub>3+x/2</sub>, x = 0–0.3) and calcination temperatures (400–800 °C). We mixed various precursor mixtures of bismuth nitrate (Bi(NO<sub>3</sub>)<sub>3</sub>·5H<sub>2</sub>O) and hafnium sulfate (Hf(SO<sub>4</sub>)<sub>2</sub>) and controlled the calcination temperatures. The as-prepared Hf-doped Bi<sub>2</sub>O<sub>3</sub> radiopacifier powders were investigated by thermogravimetric analysis (TGA), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Portland cement/radiopacifier/calcium sulfate (75/20/5) were mixed and set by deionized water (powder to water ratio = 3:1). Changes in radiopacity, diametral tensile strength (DTS), and in vitro cell viability of the hydrated MTA-like cement were carried out. The experimental results showed that the group containing radiopacifier from sol-gelled Bi/Hf (90/10) exhibited significantly higher radiopacity (6.36 ± 0.34 mmAl), DTS (2.54 ± 0.29 MPa), and cell viability (84.0±8.1%) (<i>p</i> < 0.05) when compared to that of Bi/Hf (100/0) powders. It is suggested that the formation of β-Bi<sub>7.78</sub>Hf<sub>0.22</sub>O<sub>12.11</sub> phase with hafnium addition and calcining at 700 °C can prepare novel bismuth/hafnium composite powder that can be used as an alternative radiopacifier for root canal filling materials.
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