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Abstract
Cr-doped GdScO3 single crystals were grown by the EFG method. Based on their absorption and emission spectras, the crystal field strength and the crystal field parameters of the octahedrally-coordinated Cr3+ ions were determined: Dq = 1553 cm-1, B = 574 cm-1, C = 3211 cm-1, and Dq/B = 2.71 for 0.5% Cr:GdScO3. Effective phonon energy was calculated to be 412 cm−1, and the Huang–Rhys factor was 3.0. The emission spectra of Cr3+ doped GdScO3 crystal covered the region from 650 nm to 1100 nm, and the emission cross-section at 767 nm was calculated to be 0.32×10−20 cm2. In comparison with the other Cr3+ doped materials, the Cr:GdScO3 crystal has a short fluorescence lifetime, it can be used as a potential tunable laser gain medium.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The rare earth scandate crystals (ReScO3) are generally regarded as multifunctional materials for various applications. ReScO3 Single crystals with high-κ dielectric constants, large optical bandgaps (>5.5eV) and thermodynamic stability in contact with silicon are promising candidates for the replacement of SiO2 in silicon Si-based MOSFETs [1–6]. In addition, ReScO3 single crystals are also suitable substrates for the epitaxial growth of perovskite and perovskite-related films [7–10]. The single crystals of ReScO3 belong to the group of oxide compounds in a perovskite structure with space group Pnma. ReScO3 Single crystals were grown by the conventional Czochralski method [11,12]. Now, the edge-defined film-fed growth (EFG) method was developed as a perspective way to attain high-quality ReScO3 single crystals.
Gadolinium scandate (GdScO3) is a kind of efficient laser host material. GdScO3 crystal has the structure of perovskite belonging to orthorhombic system with the space group of Pnma (no. 62) [13]. Compared with other oxide crystals, GdScO3 crystal has lower phonon energy (∼ 452 cm-1) [14], which decreases nonradiative relaxation between adjacent energy levels and robust thermal stability. GdScO3 crystal is more disordered in the structure than orthoaluminates, and thus larger spectrum broadening can be expected. Meanwhile, low-symmetry distortions in the local symmetry of the activator center in GdScO3 are larger than those in the orthoaluminates, which contribute to the low-symmetry components in the crystalline-field potential expansion [14]. In addition, the thermal-induced birefringence at high pumping energy will be lower due to its high refractive index of 2.1103 at 532nm [15]. In a word, GdScO3 crystal is a kind of new potential host material. As laser host crystal, Nd3+ doped GdScO3 was first reported in 1972 [16]. Dy3+ doped and Dy3+/Tb3+ co-doped GdScO3 crystals were considered to be potential solid-state laser materials [14,16]. As far as we know, the photoluminescence properties of Cr3+ ions doped GdScO3 crystal are not reported yet.
In this work, Cr3+ doped GdScO3 crystal was first reported. The Cr:GdScO3 single crystals were grown by the EFG method. The absorption spectrum and broad-band emission spectra were measured at room temperature. We also analyzed the crystal field of grown Cr:GdScO3 crystal. The spectral properties of Cr:GdScO3 crystal demonstrate that it will be a potential material for tunable laser.
2. Experimental procedure
2.1 Crystal growth
Cr3+ doped GdScO3 crystals were grown by the EFG method under flowing argon atmosphere with the rate of flow 1L/min, which could prevent oxidization of the crucible. The Gd2O3(5N), Sc2O3(5N) and Cr2O3(5N) powders were weighed according to the stoichiometric composition CrxGdSc(1-x)O3 (x = 0.001 and x = 0.005) and mixed. The mixed powders were pressed into bulks and sintered in air atmosphere at 1500℃ for 12 h. A Mo die with an end face of 4×25mm2 was put directly in the crucible. The crystal was grown in tungsten crucible. The growth rate was 15mm/h in growth process. During the cooling process, Cr:GdScO3 crystals strongly tended towards cracking. The polished sample of 0.1at.%Cr:GdScO3 with the dimensions of 15×(5-13)×1.7mm3 and 0.5at.%Cr:GdScO3 crystals with the dimensions of (9-11) ×7×1.7mm3 are shown in Fig. 1.
2.2 Spectra measurements
The crystal structure was studied by X-ray powder diffraction (XRD) using Ultima IV diffractometer, with data of 2θ from 10° to 80°. The concentration of the Cr3+ ions was measured by inductively coupled plasma-atomic emission spectrometry (ICPAES,Ultima2, Jobin-Yvon).The absorption spectra was recorded with a Cary-Varian 5000 spectrophotometer in the region 400-900 nm. The emission spectra and fluorescence decay curves were measured excited at 446nm pumped with a high resolution spectrofluorometer (FLSP 920, Edinburgh Instruments Inc., English) equipped with a red sensitive single photon counting photomultiplier (HamamatsuR928P) in Peltier air-cooled ousing. A microsecond pulsed Xenon flash lamp µF900H with an average power of 60 W was used to measure the decay curves of Cr3+:GdScO3 crystal, which can measure decays from 1 µs to 10s. All the measurements were performed at room temperature.
3. Results and discussion
3.1 Crystal structure
The crystal structure was measured by XRD with data of 2θ from 10° to 80°. The XRD patterns of grown Cr:GdScO3 single crystals were shown in Fig. 2, and the lattice parameters of the Cr:GdScO3 crystals are listed in Table 1. All the diffraction peaks of Cr:GdScO3 consistent well with the standard patterns of JSPDF #27-0220 of pure GdScO3, which indicates that the doping Cr3+ ions do not change the phase of GdScO3 crystal. The XRD patterns exhibit a small shift of the refraction peaks towards higher 2θ values compared with the undoped host materials due to a small decrease of the lattice parameters. The unit cell of the host becomes slightly smaller as a consequence of the Cr-substitution on the Sc-site, consistent with the small ionic radius of Cr3+.
Table 1. The lattice parameters of the Cr:GdScO3 crystals.
The concentration of Cr3+ ion in 0.5% Cr:GdScO3 crystal was measured to be 2.06×1019 cm3 by ICP-AES. Then, the segregation coefficient η of Cr3+ ion in crystal is defined as following formula:
where Ccrystal is the actual concentration of Cr3+ ion in Cr:GdScO3 crystal, Cmelt is the concentration of Cr3+ ion in the melt. Thus, the segregation coefficient η of Cr3+ ion in Cr:GdScO3 crystal is 0.26. The value is lower than 1.0, which indicates that Cr3+ is difficult to incorporate into the Sc3+ sites. The reason of the low segregation coefficient in the crystal is the difference of the ionic radii between Cr3+ (550 pm) and Sc3+ (750 pm) ions.3.2. Optical absorption
Figure 3 shows the absorption spectra of the Cr:GdScO3 crystals measured at room temperature. There are two broad bands in both absorption spectra of Cr:GdScO3 crystals, which centered at 461 nm and 647 nm for 0.1% Cr:GdScO3, 468 nm and 644 nm for 0.5% Cr:GdScO3, respectively. The absorption coefficient became larger with the increase of Cr3+ concentration. The two broad absorption bands could be attributed to the electron-vibronic transitions from 4A2 ground state to 4T1 and 4T2 excited states of Cr3+ ion in an octahedral (or near octahedral) crystal field respectively. The absorption cross section σa were determined using σa=α/Nc, where α is the absorption coefficient, Nc is the concentration of Cr3+ ions in Cr:GdScO3, which is 2.06 × 1019 cm3 of 0.5% Cr:GdScO3. Then the absorption cross sections σa of 0.5% Cr:GdScO3 are 9.31×10−20 cm−2 at 468 nm for the 4A2→4T1 transition and 8.02 × 10−20 cm−2 at 644 nm for the 4A2→4T2 transition, respectively. The peak at 717 nm is attributed to 4A2→2E transition (the R-line) of Cr3+ ion.
3.3. Crystal field strength
The local crystal field Dq, Racah parameters B and C [17,18], and normalized Dq/B can be derived from the absorption spectrum of the Cr:GdScO3 crystal. The relative energy between 4A2 and 4T1, 4T2, 2E levels were used in the calculation and these parameters were calculated by the following equations:
Table 2. Spectroscopic parameters of Cr3+ ions in various crystals. Dq is the crystal field splitting, B and C are the Racah parameters.
The Tanabe–Sugano diagram calculated from the above obtained values is given in Fig. 4. According to the Tanabe and Sugano diagram [17], a strong crystal field is present when Dq/B > 2.3, which implies that the energy level of 4T2 is higher than that of 2E (the lowest exciting level).
The degree of the electron and lattice vibration coupling in the crystal are also connected with the effective phonon energy, which is involved in the lattice vibration induced non-radiative procedure, ћω and the Huang–Rays parameter S. For oxides the ћω can be expressed in the following equation:
where Ea is the peak energy of absorption spectrum and Ee the peak energy of emission spectrum of 4A2→4T2 transition and 4T2→4A2 transition, respectively. The Huang–Rays parameter is related to the difference in energy between Ea and Ee (called Stokes shift Es) by equation: Then ћω = 412 cm-1 and S = 3.0 could be obtained from the experimental data Ea = 15528 cm-1 and Ee = 13038 cm-1.3.4. Fluorescence spectra
The emission spectrum of Cr:GdScO3 crystals excited with 446 nm at room temperature are shown in Fig. 5. The dominant feature of Cr:GdScO3 crystals is broad emission band with peak at 767 nm, the emission covers the spectral range from 650 nm to 1100 nm. The emission spectrum is very broad and is comparable with that of Cr:Sc2O3 [22]. Corresponding to the 4T2 → 4A2 transition, Dq/B = 2.71 > 2.3 of Cr:GdScO3 crystal implies that the energy level of 4T2 is higher than that of 2E. Therefore, the line emission of 2E → 4A2 transition should be visible. However, only the 4T2 → 4A2 transition is observed on the emission spectrum at room temperature. This could be explained by the thermal population of the 4T2 level at room temperature as well as by two orders of magnitude greater probability for the 4T2→4A2 transition than that for the 2E→4A2 transition to take place [26].
The peak emission cross sections of the Cr3+-ion in the investigated materials can be estimated by the following formula [25]:
where n is refractive index which was estimated to be 2.11 by Abbe Refractometer at 532 nm wavelength, λ is the emission wavelength, the Δν is the frequency at FWHM and τf is fluorescence lifetime. The fluorescence lifetimes of 4T2 → 4A2 transition were measured to be 150 µs and 161 µs at room temperature, as shown in Fig. 6. Then, the emission cross section of 0.5% Cr:GdScO3 at 767 nm is 0.32 ×10−20 cm2. In comparison with the other Cr3+ doped materials (see Table 3), 0.5% Cr:GdScO3 crystal has a broad emission band, larger absorption cross sections and high thermal conductivity. The Cr:GdScO3 crystal has a short fluorescence lifetime, it can be used as a potential tunable laser gain medium.Table 3. Spectral parameters and thermal conductivity of Cr:GdScO3 crystal and other Cr3+ doped materials.
4. Conclusion
Cr-doped GdScO3 crystals were grown by the EFG method with different Cr concentrations. Powder XRD measurements determined that a crystal structure of Cr:GdScO3 corresponded to a perovskite phase. In addition, the absorption spectrum had two broad absorption-bands centered around 468 nm and 644 nm wavelengths. These bands originated from d–d transitions of Cr3+ from the ground state (4A2) to its excited states 4T1 and 4T2 for the 468 nm and 644 nm bands, respectively. The photo-luminescence peaks were detected around 767 nm with emission cross section σe of 4T2 → 4A2 transition in 0.5% Cr:GdScO3 is 0.32 ×10−20 cm2 at 767 nm and FWHM of 128 nm.The luminescence lifetime of 4T2→4A2 transition was measured to be 161µs. In comparison with the other Cr3+ doped materials, 0.5% Cr:GdScO3 crystal has a broad emission band, larger absorption cross sections and high thermal conductivity. In conclusion, the investigated results show that Cr:GdScO3 crystal may be regarded as a potential tunable laser crystal material.
Funding
National Natural Science Foundation of China (61605069, 61805177).
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