The wide band gap semiconductor materials have received more and more attention because they render the light-emitting devices possible.^[1–4] As a wide band gap semiconductor, the corundum Mg₄Ta₂O₉ with a high dielectric performance attracts much more attention due to its possible applications in serving as luminescent^[5–7] and microwave dielectric resonator materials.^[8] However, the Mg₄Ta₂O₉ that has been obtained so far is just in the form of powders^[9–11] or thin films.^[8,12,13]

The optical properties of Mg₄Ta₂O₉ are very important, for they can involve many basic physics problems related to the energy levels and show the possibilities in some optical equipment applications.^[14–17] Furthermore, light emitting devices which are based on semiconductor have been found to have many applications, such as in water purification equipment and surface purification, food preservation devices, and new sorts of communication system.^[1,18] Meanwhile, it is significant to well know the intrinsic properties of materials in their application.^[19,20] However, the boundaries and some twin structures in materials can influence their characters and applications significantly. So, the significance for studying well oriented crystals to overcome these extrinsic effects lies not only in their practical applications, but also in their fundamental research.^[21,22]

Previously, high-quality corundum Mg₄Ta₂O₉ single crystals have been achieved by the optical floating zone method.^[23] In this work, we investigate the experimental band gap energy and strong near-infrared (NR) emission from corundum Mg₄Ta₂O₉, which may open the door for a large number of potential applications in many fields including laser diodes, NR light-emitting devices (NRLEDs), etc.

Moreover, it is important to investigate the optical phonon behaviors of Mg₄Ta₂O₉ for its potential applications in NRLED and laser active media. Furthermore, the response of the material to microwave dielectric can also be achieved from the intrinsic contribution of its polarized phonons. In addition, the nonradioactive decay is mainly dependent on the phonons in the laser crystals, which may be harmful effect in some circumstances. The Raman scattering is a useful approach to studying the optical phonon behavior of the materials.^[22,24,25] So, the polarized Raman spectra are also investigated in the present work. Furthermore, a phonon participating in the PL process is exactly observed from room temperature PL.

Transparent and colourless corundum Mg₄Ta₂O₉ crystalswere obtained via the optical floating zone technology, previously. And we have prepared a crystal slice for optical measurements, which was cut and finely polished along the c axis direction into 3.0 mm× 5.1 mm×0.7 mm in dimension, as shown in the insert of Fig. 1(a). And the structure and orientation of the as-grown crystals were identified previously.^[23]

Fig. 1.

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	Fig. 1. (a) Room-temperature transmittance and reflectance spectrum, with insert showing Mg4Ta2O9 crystal wafer, and (b) absorption spectrum with insert showing variation of (α hv)2 with photon energy (hv) of as-grown Mg4Ta2O9 crystal.

The absorption and transmission spectra were measured via Shimadzu UV-VIS-3600 spectrophotometer at room-temperature. Both the PL spectra and polarized Raman spectra measurements were performed by the Jobin Yvon High Resolution 800 Raman spectrometer in the backscattering geometry with polarized plate, half wave plate, the 514.5-nm excitation line from the Spectra Physics Stabilize 2017 Ar ion laser and Linkam THS600 Thermal stage.

Figure 1(a) displays the obtained transmittance spectrum of the Mg₄Ta₂O₉ crystal sample. The transmittance for the Mg₄Ta₂O₉ crystal wafer reaches up to 75% in the visible and near-infrared region (from 325 nm to 1000 nm). But the transmittance becomes lower suddenly at a lower wavelength (< 325 nm), which is called ultraviolet (UV) region. So, we can achieve a lower cutoff wavelength of Mg₄Ta₂O₉, about 325 nm.

Figure 1(b) shows the Mg₄Ta₂O₉ crystal wafer absorption spectrum along the c-axis direction. The crystal has a low absorption coefficient (α) (about 1.4 cm⁻¹) in a wavelength range from 1000 nm to 325 nm, a transparent region. As for ultraviolet region (< 325 nm), the crystal has a strong absorption.

Photon energy hv can be related to the absorption coefficient by the following equation

Within the transparent range of Mg₄Ta₂O₉ crystal, T can be described by the reflectance (R) and α from the crystal wafer by^[27]

The relationship among extinction K, refractive index n and R is related by the following expression:^[28]

Fig. 2.

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	Fig. 2. (a) Refractive index and extinction coefficient versus wavelength, and (b) dielectric constant and dielectric loss factor versus wavelength of Mg4Ta2O9.

Then we introduce the following equation:^[28]

Urbach tail defined as the energy under the band gap (E < E_g). The n versus energy (E) expressed as the Cauchy–Sellmeier function is related by the following expression:^[28]

Fig. 3.

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	Fig. 3. Plot of n and (n2 – 1)−1 versus E2 of Mg4Ta2O9.

The area below the band-to-band absorption edge, due to their correspondence to the fundamental electronic excitation properties, the refractive index dispersion is very important for us to learn the dielectric characteristics of materials. By using the single-effective oscillator model, many samples in the solid and liquid forms were validated.^[29,30] Using Krammers–Kroning relation, the ε_r can be described as^[28]

In corundum Mg₄Ta₂O₉, the cations are arrayed along the c axis. The MgO₆ octahedrons connect to the other MgO₆ octahedrons by sharing edges and faces. Meanwhile, the TaO₆ octahedrons connect to the other TaO₆ octahedrons via sharing faces and connect to the MgO₆ octahedrons via sharing edges as shown in the insert of Fig. 4.^[31,32] The band structure of Mg₄Ta₂O₉ can be observed through the relations between oxygen and cations in the octahedron. The conduction band Ta⁵⁺ is 5d orbitals. While the valence band of Mg₄Ta₂O₉ is presumed to be O^2– 2p orbitals.^[32]

Fig. 4.

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	Fig. 4. Polarized Raman spectrum in (a) Y(ZZ)-Y and (b) Z(XY)-Z geometry, with inserts showing view direction along b and c axes of Mg4Ta2O9 single crystal, respectively.

The orientation and conformation of single crystal can activate special vibration modes by changing the proper polarization direction of the incident and scattered light. So, the polarized Raman spectrum of single crystal is utilized for assigning the vibrations, which cannot be proved by unpolarized Raman spectrum with the same wavenumber. On the other hand, the space group of Mg₄Ta₂O₉ is P-3c1(165) and the point group is D_3d (-3m). There are 7A_1g and 15E_g that are both Raman active (R) according to the group theory analysis. Meanwhile, Raman tensors can be expressed as follows:

And based on polarization selection rule and the Raman tensor, the A_1g and E_g can be obtained in Y(ZZ)-Y and Z(XY)-Z, respectively, under back scattering geometry. In this study, the angle of XY is 120°, so a half-wave plate is used. The polarized Raman spectrum of Mg₄Ta₂O₉ crystal is obtained as shown in Fig. 4. All the Raman peaks in the spectra can be assigned to in the whole wavenumber range.^[23,33] In the Y(ZZ)-Y configuration (Fig. 4(a)), the spectrum is dominated by A_1g model at 820 cm⁻¹ and all other six A_1g models at 200 cm⁻¹, 250 cm⁻¹, 320 cm⁻¹, 361 cm⁻¹, 454 cm⁻¹, and 639 cm⁻¹ are achieved properly. While in the Z(XY)-Z, all 13 E_g bands (128 cm⁻¹, 254 cm⁻¹, 263 cm⁻¹, 309 cm⁻¹, 326 cm⁻¹, 335 cm⁻¹, 379 cm⁻¹, 400 cm⁻¹, 418 cm⁻¹, 486 cm⁻¹, 580 cm⁻¹, and 672 cm⁻¹) are observed in this range. And the strongest mode in this configuration is E_g model at 128 cm⁻¹ as shown in Fig. 4(b). Meanwhile, there are some other bonds in special polarized configuration marked by related symbols (A_1g and E_g models are marked in blue and red, respectively). The peaks which do not belong to our polarized configuration but appear should be attributed to the complex structure, little disorientation of the Mg₄Ta₂O₉ crystals or the dazzle effect of the lens. In general, these phenomena reflect the Raman selection rules. The occurrence of the highest energy and second strongest peak at 820 cm⁻¹ are due to the symmetry stretching models of terminal Ta–O chain between the face-shared two TaO₆ octahedra. The peak located at 580 cm⁻¹ with a lower energy can be attributed to the stretching mode of the bridged Ta–O bond. We can identify the peaks at 672.7 cm⁻¹ and 637.2 cm⁻¹ as two chain Ta–O bonds, and the close bonding energy and length lead the chain bonds to appear, so their wavenumbers are close to each other. The puzzling peaks from 300 cm⁻¹ to 500 cm⁻¹ come from the vibrations of MgO₆ octahedra. Meanwhile, the mode at 128 cm⁻¹, having the strongest intensity and the lowest energy, can be assigned to an additional mode that forms the Ta–Ta stretching bonds between the two TaO₆ octahedra connected by face-shared oxygens^[34] as shown in Table 1.

Table 1.

Table 1.

Table 1. Vibrational modes of Mg4Ta2O9. . Raman mode Assignment f/cm−1 Eg Ta–Ta stretching 128 A1g bending vibration in the O–Ta–O 200 250 Eg bending vibration in the O–Ta–O 254 263 A1g MgO6 octahedrons 320 361 454 Eg MgO6 octahedrons 309 326 335 361 379 400 418 486 Eg bridged Ta–O stretching 580 A1g two Ta–O chain stretchings 639 Eg Ta–O stretching 672.2 672.7 A1g symmetric Ta–O stretching 820

Table 1. Vibrational modes of Mg4Ta2O9. .

Figure 5(a) shows the room-temperature PL spectrum of Mg₄Ta₂O₉. The PL peak can be well fitted to two emission peaks centered at 821.19 nm (1.51 eV) and 770.19 nm (1.61 eV) by using Gaussian fitting. Significantly, the PL emission centers at 1.51 eV–1.61 eV and instead of the obtained band gap (∼ 4.28 eV) from Mg₄Ta₂O₉ excited by using laser line of 514 nm at room temperature, which cannot be attributed to the normal band-to-band transitions. The mechanism of Mg₄Ta₂O₉ luminescence can be compared to that of the solid with niobate–oxygen octahedron due to their identical outer electronic configuration. In the PbMg_1/3Nb_2/3O₃–PbIn_1/2Nb_1/2O₃ systems, the special luminescence tends to relate to the origin of PL bands with the Nb–O system without influence of Mg and Pb. The two PL peaks at 1.61 eV and 1.51 eV of Mg₄Ta₂O₉ are consistent with the A (with a weaker intensity and higher energy) and B (with a stronger intensity and lower energy), which are mainly determined by the defect and regular Ta–O respectively. To understand the observed phenomenon, we need to consider the non-radiative transition of exciton in the schematic band diagram as shown in Fig. 5(b). The photo-excitation allows electrons in the oxygen state in the valence band (V_o) to transit to the conduction band (CB), leaving holes in the V_o, and then the excited electrons in the CB relax to intrinsic Ta_i after a non-radiative (NR) transition process. Eventually, the relaxed electrons recombine with holes, and thus cause the PL emission. There will be two possible types in this case: one is the B peak and the other is the A peak which is caused by defect states. Moreover, the energy difference between A and B excitons is 0.10 eV, corresponding to the relative vibration energy 820 cm⁻¹ that corresponds to the strongest peak in Raman spectrum and can be assigned to the symmetry stretching model of terminal Ta–O chain between the face-shared two TaO₆ octahedrons. It well illustrates that the phonons participate in the recombination process of excitons. Therefore, the defect emission A is from the electrons recombining with holes directly, while the B originates from the emission and vibration and the relaxing of photon and phonon.

Fig. 5.

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	Fig. 5. (a) PL spectrum of Mg4Ta2O9 crystal at room temperature, (b) schematic band diagram with phonon excitation, A and B emission process and non-radiative transition, (c) temperature-dependent PL spectra of Mg4Ta2O9 crystal, and (d) two temperature-dependent fitted peak centers in a range from 85 K to 605 K.

The temperature-dependent PL spectra of Mg₄Ta₂O₉ crystal in a range from 85 K to 805 K are present at Fig. 5(c). As the temperature increases, the intensity of PL presents a decrease tendency and quenches at 605 K as shown in Fig. 5. One of the main reasons for reducing the intensity can be attributed to the thermodynamic movement of molecules. It is possible to convert the excitation energy into vibrational energy of the ground state, and when the excited molecule receives additional thermal energy, the rapid vibration and relaxation will lead the vibration energy to lose. And the two peaks of temperature-dependent PL emission can also be fitted by Gaussian. The temperature-dependent values of the two peak centers in a temperature range from 85 K to 605 K are shown in Fig. 5(d). As the temperature increases, both of the two centers show blueshifts linearly and we fit the two data linearly. The shift speed of defect A bond (1.2× 10⁻⁴ eV/K) is slightly faster than that of the B bond (0.9× 10⁻⁴ eV/K). The blueshifts can be attributed to the shrinkage of Ta_i–CB and V_o–VB, and resulting in the fact that the broadening of recombination energy between Ta_i and V_o. The lower blueshift speed of regular B bond should be due to the phonons at 820 cm⁻¹ having a lower thermal response.^[23]

The lower cutoff wavelength of the Mg₄Ta₂O₉ is 325 nm obtained from the room-temperature transmittance spectrum. The Mg₄Ta₂O₉ is a direct transition material and band gap energy is determined to be 4.28 eV. The reflectance spectrum of Mg₄Ta₂O₉ is also presented. Useful photoelectricity parameters of Mg₄Ta₂O₉ are obtained. Additionally, the Raman selection rules for the Mg₄Ta₂O₉ crystal are estimated and the vibrations of Raman bonds are assigned. In particular, the PL spectra centered at 1.51 eV and 1.61 eV are observed to be dependent on the regular Ta–O system and defective Ta–O system respectively. The phonons at 820 cm⁻¹ (0.10 eV) are found to participate in the PL process. Furthermore, the temperature-dependent PL spectra are also discussed.