## 3 Results and Discussion
### Preparation and Characterization of Sol-Gel Bi\({}_{2x}\)Hf\({}_{x}\)O\({}_{3+x/2}\) Powders
The TEM examinations confirmed the XRD results that the \(\beta\)-Bi\({}_{7.78}\)Hf\({}_{0.22}\)O\({}_{12.11}\) is the major crystalline phase in SGHB100 powders after calci  
Figure 3: The X-ray diffraction (XRD) for various compositions of (a) Bi/Hf (100/0) (b) Bi/Hf (92.5/7.5) (c) Bi/Hf (90/10) (d) Bi/Hf (87.5/12.5) after calcination from 400 \({}^{\circ}\)C to 800 \({}^{\circ}\)C for 1 h.  
Characterization of MTA-Like Cement Comprising Bi\({}_{2\text{-}x}\)Hf\({}_{x}\)O\({}_{3\text{+}\text{/}2}\) Radiopacifiers  
MTA-like cement primarily consists of inorganic particles of Portland cement and radiopactifier. After mixing cement powders and water, the hydrated cement creates a calcium silicate hydrate (C-S-H) based impermeable barrier and high radiopacity. Radiopacity is an essential property to offer the relationships of the radiographic interface between dental materials and tooth substrates. Due to the similar radiopacity equivalence between dentin and aluminum (AI) [32], the radiopacity of dental materials was quantitatively assayed using a graduated aluminum step-wedge for measuring its equivalence thickness of optical radiographic densities under X-ray exposure [33]. According to its intended uses, the minimum radiopacity requirement is 3 mmAl for dental root canal sealing materials based on ISO 6876/2001 [34]. Figure 5 presents the radiopacity (mean \(\pm\) standard deviation (SD) (mmAl)) of hydrated PC associated with various Bi\({}_{2\text{-}x}\)Hf\({}_{x}\)O\({}_{3\text{+}\text{/}2}\) radiopacifiers prepared from different Bi:Hf mole ratios and calcination temperatures. It can be noted that the addition of hafnium into sol-gelled bismuth oxide increased the radiopacity performance. The radiopacity of SGB powder was ranged from 4.59 \(\pm\) 0.19 (SGB_5) to 4.82 \(\pm\) 0.21 mmAl (SGB_7). Whereas the radiopacity was significantly improved with hafnium addition. Generally, the SGBH100 (i.e., x = 0.20, Bi/Hf = 90.0/10.0) group exhibited the higher radiopacity compared with the other groups. In addition, calcination at 700 \({}^{\circ}\)C exhibited the highest radiopacity. All the radiopacity results were summarized in Table 2. Among all the tested sol-gelled powder, the MTA-like cement prepared with SGHB100. 7 powder (i.e., Bi/Hf (90/10), calcining at 700 \({}^{\circ}\)C) showed the highest radiopacity of 6.26 \(\pm\) 0.34 mmAl that is significantly different (\(p<0.05\)) from that (4.56 \(\pm\) 0.13 mmAl) prepared by pristine Bi\({}_{2}\)O\({}_{3}\) powder (i.e., Bi/Hf (100/0)). The experimental error in radiopacity (ranged from 2.4-5.4%) may arise from the sample preparation and X-ray exposure. The increased radiopacity might be attributed to the sol-gel process and the formation of the \(\beta\)-Bi\({}_{7.78}\)Hf\({}_{0.22}\)O\({}_{12.11}\) dominant phase during calcination. The addition of Bi\({}_{2}\)O\({}_{3}\) into PC usually resulted in mechanical strength reduction and porosity increase [35]. Using an effective but less amount of radiopacifiers to satisfy the regulatory radiopacity requirement (\(\geq\)3 mmAl) for dental filling materials might be beneficial to improve their mechanical properties and sealing ability. Figure 4: The TEM image for Bi/Hf (90/10) sintered at 700 \({}^{\circ}\)C (**a**) low magnification TEM (**b**) high magnification TEM (**c**) d-spacing in HRTEM (high resolution transmission electron microscopy) (**d**) SAED (selected area electron diffraction). SGBH100 (Bi/Hf = 90/10, the one with the highest radiopacity and clinical application potential) and its counterpart SGB (Bi/Hf = 100/0) were investigated further by diametral tensile strength and biocompatibility tests. Physical properties usually reflect how materials respond to changes in their usage environments. The mechanical properties might not be crucial because MTA bears no direct occlusal load [36]; however, it is recognized as a representative index for product quality.