Chemical formula of a mullite is A 4+2xB_2-2xO_10-x, where A is trivalent cation, and B is a tetravalent cation[1,2]. The structure is formed by edge-shared octahedra and their interconnection with tetrahedra. When tetravalent B cations are substituted by trivalent cation like Bi, oxygen vacancies are introduced. In that sense, if the B-cation is fully substituted by trivalent Bi, the chemical formular is Bi₂Ga₄O₉. The structure consists of Ga tetrahedra and octahedra networks. Octahedra are connected by edge-sharing, while connection between octahedra and tetrahedra is made by corner-sharing. If the structure is seen along [001], it has open frame as can be seen in Fig. 1(a). With this open structure, oxygen vacancies, the material is expected to have decent ionic conductivity, as typically seen in high ionic conductivity in bismuth-containing oxides[3-6]. In this regard, several approaches including divalent doping were taken place for developing new ionic conductors and single crystal growth to understand diffusion mechanism[4,7-9]. In addition, another use of Bi₂Ga₄O₉ is in its semiconducting property and moderate band gap ( 3 eV) for visible-light-driven water splitting. It has been demonstrated that Bi₂Ga₄O₉ can be a catalyst candidate for water splitting[10-12]. In recent, due to increased interests in this system, creation of high entropy oxides has been attempted[13].

Figure 1. (Color online) (a) Atomic structure of Bi₂Ga₄O₉, (b) Temperature profile as a function of time for single crystal growth (Inset: a Bi₂Ga₄O₉ separated from Bi₂O₃ flux), (c) Topographic image from SEM and elemental mapping from SEM/EDS, (d) Whole EDS spectrum of Bi₂Ga₄O₉.

In this study, we present the synthesis of single crystal Bi₂Ga₄O₉ with flux method and its interesting transport behavior in different gas environments such as pure O₂ flow and diluted H₂ gas flow at high temperature.

We grew Bi₂Ga₄O₉ single crystals by the self-flux method. The compounds were Ga₂O₃(Alfa Aesar, 99.99%) and Bi₂O₃(Afla Aesar, 99.999%), with excess Bi₂O₃ acting as the self-flux. The crystal growth is in the excess of Bi₂O₃. Note that self-flux is rather commonly in the single crystal growth[14]. To see the effect of cooling rate, we varied cooling time in between 1100 °C and 800 °C. The larger and more transparent single crystals were obtained as the cooling time increases to 120 hours from 60 hours, where cooling rate is changed from 5 °C/hrs to 2.5 °C/hrs. The detailed growth diagram is seen in Fig. 1(b).

To analyze the composition of the single crystals, energy dispersive X-ray spectroscopy (EDS) was used. SEM/EDS (JEOL JCM-7000) was used for our compositional analysis. Note that we additionally put the specimen in aqua regia for two hours after separation of crystals from the Pt crucible to remove the residual bismuth oxides. Due to the insulating nature of the single crystals, a Pt coating was made on the surface for SEM observation.

X-ray diffractometers (Panalytical Xpert3 and Bruker D8 Discover) were used to collect x-ray diffraction (XRD) patterns of our single crystals. For structural characterization, some crystals were grounded for powder XRD and Rietveld refinement[15]. With Bruker D8 Discover, we measured XRD of the flat surface as the surface can be seen in the inset of Fig. 1(b).

As an way to confirm Bi₂Ga₄O₉ phase, Raman spectroscopy (Ramantouch, Nano Photon) was used. The excited beam is impinged on (001) of Bi₂Ga₄O₉, and wavelength of the incident beam was 532 nm.

After confirming phase and single crystallinity of the Bi₂Ga₄O₉, to see high-temperature transport behavior depending on reducing and oxidizing conditions at 600 °C, we performed real-time dc transport experiments using environmental chamber (Nextron Inc.) and a source meter (Keithley 2450). With this time-dependent transport measurements, we could see which step is a rate determining step for this experiments. Note this redox activity can be a platform for new phase of matter[16].

As we cooled the crystal like Fig. 1(b), the color of the as-grown crystal is light yellow. This is consistent with the color of Bi₂Ga₄O₉[17]. However, when the cooling rate is higher than this, the color of crystal is dark yellow and rather opaque, indicating the crystal would consist of multiple domains. The obtained crystals are rectangular shaped with typical dimension of 5 mm by 2.5 mm by 1 mm. The plate-like shape is related to crystal growth habit, where the crystal growth slower along [001][17]. The surface and composition of the crystal from the same batch was analyzed with SEM/EDS. Before the analysis, the crystal was dipped in aqua regia for additional two hours to remove bismuth oxide as a way to remove flux residue. Since the crystal is insulating, the crystal was coated with Pt. In Fig. 1(c) shows surface topology and elemental maps of Bi, Ga, and O. We could not find agglomerated area in the elemental mapping. This indicates crystal growth was well-made. Figure 1(d) is EDS spectrum of the crystal, and Bi, Ga, O, and Pt are only shown in the spectrum. So, it is nearly free from any contamination. On the same surface, we collected whole spectra in nine points and calculated atomic ratio between Ga and Bi. The ratio (Ga/Bi) is 1.67 as an average. It is rather smaller than the expected. The value might be related to the excess Bi on the surface.

After checking the composition of Bi₂Ga₄O₉, x-ray diffraction of the single crystal seen in Fig. 1(b) was performed. Six-circle x-ray diffractometer was used and flat region of the crystal was measured. As can be seen in Fig. 2(a), well-defined XRD peaks were observed, and the peaks are corresponded to (00l). We confirmed the flat surface is (00l) and as typical crystal habit crystal growth is faster along [100] and [010] compared to (001). Note that full width half maximum (FWHM) of rocking curve of (002) reflection is only 0.017°. The value is comparable to that of commercially available oxide substrates.

Figure 2. (Color online) (a) X-ray diffraction of the as grown Bi₂Ga₄O₉ single crystal. (b) Rocking curve along (002) reflection, (c) Powder XRD and Rietveld refinement of powdered Bi₂Ga₄O₉ single crystals.

Then, as-grown Bi₂Ga₄O₉ crystals were grounded to powder for the powder x-ray diffraction. After collecting XRD patterns as shown in Fig. 2(c), we performed Rietveld refinement with FullProf software package. Even if the diffraction pattern is rather noisy, we could fit the data. The fitting was performed in space group Pbam, which is the space group of Bi₂Ga₄O₉[18]. The raw data and the fitted data are well-matched. The resultant lattice constants are 7.95 Å, 8.35 Å, and 5.94 Å along a, b, and c directions, respectively.

To additionally confirm the phase formation of Bi₂Ga₄O₉, we performed Raman spectroscopy. We labeled characteristic peaks in the Fig. 3. At the first three strongest peaks were observed at 98.6 cm^-1 (label a), 328.6 cm^-1 (label f), and 364.4 cm^-1 (label g), where the peaks correspond to low energy bending modes from octahedra and tetrahedra, O–Ga–O bending vibration, and stretching mode of GaO₆, respectively. The peak assignment is based on the literature[17]. Note that the three peaks were also the stronger signals in the literature. Another distinct peaks can be found at 208.3 (label c), 245.5 (label d), and 595.6 cm^-1 (label j), where the first two peaks are related to Bi-O stretching, while the last peak Ga–O–Ga bending modes. Other smaller peaks, which were labeled as b, e, h, i, k, and l, were previously identified from the literature[17]. In broad sense, the modes below 300 cm^-1 are related to vibration associated with Bi, while the peaks above 300 cm^-1 are deeply related stretching and bending of Ga–O.

Figure 3. (Color online) Raman spectrum of Bi₂Ga₄O₉ (Inset: optical image of the crystal surface).

In order to see dc resistance changes upon reducing and oxidizing conditions, we performed real-time dc transport measurements (See Fig. 4(a)). Due to instrumental limit, we performed dc transport measurements at 600 °C. Below 500 °C, it was not possible to obtain reliable results. As can be seen in Fig. 4(b), two contacts were made with silver paints and dried at room temperature for the two hours. Two spring-loaded probe tips were contacted on the contact pads. Since ceramic heater was used, the current conduction path is only made through the Bi₂Ga₄O₉. The Bi₂Ga₄O₉ crystal was ramped in 3% H₂ gas. We monitored the measured voltage under 1 µA of the applied current. When we reached 600 °C, the measured voltage was stable. Then we switched the gas to pure oxygen with 20 sccm. Immediately, the resistance ramped up and continuously increased above 10 MΩ and then we switched back to 3% H₂ gas. Even if the chamber is small, the resistance further increases up to 15 MΩ. It took about 40 min. to downturn in resistance. We believe that oxygen concentration is much sensitive factor to governing in transport behavior. However, the transport response in reducing condition is rather slow. It took a longer time to return the initial state. This indicates the sluggish reaction. Once we reached the initial state, we switched to oxygen environment. It is also clearly seen that immediate increase of resistance is clearly seen, and prolonged increase of resistance even after change of gas type to 3% H₂. However, as the case of the first cycle, transport data indicates the reaction with H₂ is slow. After the reaction in the reducing condition, when we took out the sample, the sample’s color is changed black. This might be due to oxygen vacancy formation in the lattice. This is also seen in other crystals[19]. Note that we measure XRD of the darkened Bi₂Ga₄O_9-x (data not shown). Changes in the lattice constant is changed 0.03 Å, which is negligibly small. This indicates the oxygen vacancy formation may happen at the surface of the Bi₂Ga₄O₉.

Figure 4. (Color online) (a) Real-time transport results in oxidizing and reducing conditions and Photos (b) before and (c) after the transport measurements.

In conclusion, relatively large single crystals of Bi₂Ga₄O₉ were obtained by Bi-rich self flux method. The crystals were characterized with structural, chemical, and optical methods. The crystal is twin-free, chemically homogenous, and typical Raman mode as reported from the literatures. After the successful synthesis of Bi₂Ga₄O₉ single crystals, high temperature transport in different gas environments was performed. The compound has faster response in the case of O₂ introduction.

This work was supported by a 2-Year Research Grant of Pusan National University.