Selective Laser Spectroscopy of the Bixbyite-Type Yttrium Scandate Doped by Rare-Earth Ions

Abstract

Yttrium scandate crystals doped by Nd³⁺ and Tm³⁺ ions have been successfully grown in the form of fibers using the laser-heated pedestal growth (LHPG) technique. The selective laser spectroscopy methods have identified and distinguished three distinct types of optically active centers associated with Nd³⁺ and Tm³⁺ ions. The substitution of Y³⁺ and Sc³⁺ for rare-earth ions in the C₂ structural site leads to the formation of two distinct basic long-time centers. In Nd³⁺:YScO₃, another type of center (a short-lifetime one) is formed known as the Nd³⁺–Nd³⁺ aggregate pair. This center arises from the substitution of Y³⁺ or Sc³⁺ for Nd³⁺ cation in the adjacent MO₆ polyhedra that share an edge. In Tm³⁺:YScO₃, the third optical center is formed as a result of the substitution of Y³⁺ or Sc³⁺ for Tm³⁺ in the MO₆ octahedra with the C_3i site symmetry. The fluorescence decay lifetimes of Nd³⁺ and Tm³⁺ ions in the YScO₃ crystal structure have been accurately measured and estimated. A Stark level diagram illustrating the splitting of ⁴F_3/2, ⁴I_11/2, and ⁴I_9/2 multiplets of Nd³⁺ ions has been constructed to show features of the active optical centers with the C₂ site symmetry.

1. Introduction

Rare-earth (RE³⁺)-doped Sc₂O₃ and Y₂O₃ sesquioxides exhibit high thermal conductivity, approximately twice that of yttrium aluminum (YAG) [1], and have a wide transparency range from 0.25 μm to 9.6 μm [2]. These materials have demonstrated excellent potential for high-power solid-state lasers [3,4,5,6,7,8], particularly in the infrared (IR) spectral region. They could serve as a viable alternative to the well-established YAG crystals. RE³⁺-doped sesquioxide materials show strong potential for a variety of applications, including solid-state light-emitting devices [9,10,11], high-efficiency luminescent materials [12,13], rare-earth magnets [14,15], magneto-optical recording materials [16,17], and so on.

Both Y₂O₃ and Sc₂O₃ are categorized as a cubic bixbyite-type structure [18,19,20,21,22], where there are one position for the oxygen ion and two symmetrically independent positions for cations: the 8b site with C_3i symmetry and 24d site with C₂ symmetry. The unit cell contains three cations occupying the the C₂ site and one cation in the C_3i site. Two types of 6-vertex polyhedra share corners and edges to form a chessboard packing derived from the fluorite structure type [23].

Extensive research has been carried out on rare-earth dopants in Y₂O₃ crystals, revealing that RE³⁺ ions have the capability to occupy Y³⁺ sites [24,25,26,27,28,29,30,31]. However, due to inversion symmetry, the electric dipole–dipole transitions within the 4f-configuration are forbidden for the Kramers RE³⁺ ions (which have an odd number of 4f-electrons) occupying the C_3i sites. Consequently, all electron transitions observed in the fluorescence spectra of these ions should be assigned to electronic transitions in active optical centers located in the C₂ sites. Sc₂O₃ is also a promising host material as it exhibits a strong crystal field effect, primarily caused by the lattice distortions resulting from the substitution of Sc³⁺ for RE³⁺ ions of larger size [4,32,33]. Moreover, Tm³⁺-doped Sc₂O₃ and Y₂O₃ have extraordinary broad gain spectra around 2 μm. In comparison to YAG, the peak emission cross-section of the Tm³⁺ ion at 2 μm is four times greater in both Sc₂O₃ and Y₂O₃ [32].

Yttrium scandate (YScO₃) has a polymorphic nature, implying that the laser properties can be varied depending on the host crystal structure and the type of rare-earth ions. Additionally, mixed sesquioxides demonstrate notably lower melting temperatures compared to yttria and scandia. Mixed sesquioxides hold great promise for the generation and amplification of ultrashort pulses [6,34,35]. This is due to their inhomogeneously broadened gain spectra, which is a result of the intrinsic disorder in the crystal structure [35]. Nd lasers have been realized successfully in the YScO₃ and isostructural (Lu_1−xSc_x)₂O₃ mixed seqioxides [36]. The high-temperature modification of YscO₃ belongs to the bixbyite-type structure [18,20,37,38] similar to yttria (Y₂O₃) [21] and scandia (Sc₂O₃) [22]. It can be “frozen” during the crystal growth process, and the synthesis method used plays a critical role. The features of the laser-heated pedestal growth (LHPG) technique are high temperature of heating under laser irradiation followed by rapid cooling. The method allows one to obtain crystals in the form of fibers [39,40].

There is a high probability that certain rare-earth optical centers will be formed in the disordered structure. Selective laser spectroscopy methods are particularly suitable for studying optical properties of rare-earth-doped laser active media such as yttrium scandate.

We have conducted a comprehensive spectral–kinetic analysis of both Nd³⁺:YScO₃ and Tm³⁺:YScO₃ crystal fibers using the methods of selective laser spectroscopy.

2. Materials and Methods

Crystals of 0.1 at. % Nd³⁺:YScO₃ and 0.1 at. % Tm³⁺:YScO₃ have been obtained using the laser-heated pedestal growth (LHPG) technique. Commercial powders of Y₂O₃ and Sc₂O₃ (purity > 99.999%, Sigma Aldrich, Burlington, MA, USA) were used as precursors, while Nd₂O₃ and Tm₂O₃ powders (purity > 99.999%, Sigma Aldrich, Burlington, MA, USA) were used as activators. Both Nd³⁺:YScO₃ and Tm³⁺:YScO₃ crystals have been grown in the form of fibers with a diameter of 0.6 mm and a length of 50 mm.

The structural characteristics of the Nd³⁺- and Tm³⁺-doped YScO₃ crystal fibers have been described in our earlier works [18,37,38]. The X-ray pattern of YScO₃ includes good narrow peaks, which points to a high degree of crystallinity. According to X-ray data, the YScO₃ crystal fiber is homogeneous and belongs to the bixbyite structural type (space group Ia3).

Time-resolved luminescence and excitation spectra and kinetics of luminescence decay of both Nd³⁺ and Tm³⁺ ions in YScO₃ crystal fiber were measured on the MDR-23 monochromator (LOMO, Saint-Petersburg, Russia) at temperatures of 77 and 300 K. The Solar LP-604 parametric generator (Solar LS, Minsk, Belarus) was used as a source of excitation. The luminescence excitation spectra were measured by scanning the irradiation of the Solar LP-604 tunable parametric generator across the absorption bands of both Nd³⁺ and Tm³⁺ ions while keeping the absorption intensity of the laser emission on the studied transitions constant (λ_det. = const). The decay kinetics of luminescence was measured using the double selection method (based on luminescence spectrum and excitation). The luminescence from the studied samples was focused into the slit of the monochromator using a condenser, and the signal was subsequently detected using the FEU-83 and Hamamatsu R5108 photomultipliers (Hamamatsu Photonics, Sendai City, Japan) and a PbS photosensitive resistor. The signal from the detectors was fed into the Tektronix–TDS3052B wideband (0–500 MHz) oscilloscope (Tektronix, Beaverton, OR, USA) connected to a computer.

All measurements were carried out at temperatures of 77 and 300 K. The width of the spectral lines depends on the value of an electron—the phono interaction of the impurity center with the phonon field of the crystal lattice. In spectroscopy, this parameter is characterized by homogeneous broadening. At 77 K, phonons are frozen up to the energy of kT = 51 cm⁻¹, resulting in the narrowing of luminescence lines and a decrease in the value of homogeneous linewidth. At 300 K, phonons with energy up to kT = 210 cm⁻¹ are involved, resulting in an increase in nonradiative relaxation channels, leading to broadening of luminescence lines. This effect is especially pronounced for lines in the long-wavelength region of the luminescence spectrum where inter-Stark transitions of both Nd³⁺ and Tm³⁺ ion levels are involved in nonradiative relaxation processes.

3. Results and Discussion

3.1. Spectroscopy of Nd³⁺:YScO₃

Normalized time-resolved luminescence excitation spectra of Nd³⁺ ions in the YScO₃ host were recorded at a temperature of 77 K, as shown in Figure 1. The spectra have been measured in the spectral range of 780–850 nm. Luminescence excitation spectra have been obtained through the scanning of parametric laser radiation on the ⁴I_9/2 → ⁴F_5/2 + ²H_9/2 electron transitions and selective detection of luminescence on the ⁴F_3/2 → ⁴I_9/2 (λ_det. = 896 nm). The luminescence excitation spectra are modified and intensities between spectral lines, in particular 821.4 and 823.8 nm, are redistributed with changes in delay times of 20 μs to 254 μs relative to the moment of laser excitation. Analysis of the changes in luminescence excitation spectra allows us to assume that two types of Nd³⁺ optical centers exist in the YScO₃ host.

Time-resolved luminescence spectra of Nd³⁺ ion in the YScO₃ crystal fiber have been measured on the ⁴F_3/2 → ⁴I_9/2,11/2 electron transitions at 77 and 300 K. Nd³⁺ ions have been excited at 823.8 and 821.4 nm on the ⁴I_9/2 → ⁴F_5/2 + ²H_9/2 electron transitions. Figure 2a,c show luminescence spectra of Nd³⁺ on the ⁴F_3/2 → ⁴I_9/2 electron transition being measured at excitation of 823.8 and 821.4 nm, respectively. Luminescence spectra of Nd³⁺ ions change with the excitation wavelength. Double selection for excitation wavelength and time leads to shifting luminescence lines. Double selection shows that luminescence spectra of Nd³⁺ ions change with the excitation wavelength and delay times relative to the moment of laser excitation. Analysis of luminescence spectra of Nd³⁺ ions allows us to determine the number of spectral lines being recorded at two different excitation wavelengths. In both cases, the spectra of Nd³⁺ on the ⁴F_3/2 → ⁴I_9/2 electron transition includes five Stark components (T = 77 K) that correspond to the number of lines (2J + 1)/2 being predicted by theory. This points to complete removal of degeneration of the electron transitions.

So, the ⁴I_9/2 electron level splits into five components. ⁴F_3/2 and ⁴I_11/2 split into two and six Stark components, respectively, at each excitation wavelength. Spectral lines are shifting with excitation wavelength, but its number remains the same. This fact points to the presence of two types of Nd³⁺ optical centers. The number of Stark components and values of splitting allow us to conclude that both centers have low site symmetry of C₂.

Normalized luminescence spectra of Nd³⁺ ions in YScO₃ have been measured on the ³F_3/2 → ⁴I_11/2 electron transition at excitation of 823.8 and 821.4 nm and temperatures of 77 and 300 K (Figure 3a,b). Analysis of luminescence spectra of the ³F_3/2 → ⁴I_11/2 shows that the increase in time delay relative to the moment of laser excitation leads to a shift of spectral lines and redistribution of their intensities on the ³F_3/2 → ⁴I_9/2 electron transition. The changes in luminescence spectra allow us to identify spectral lines of Nd³⁺ ions and distinguish two group of lines and hence two types of Nd³⁺ optical centers in the YScO₃ host.

Figure 4a,b show luminescence decay kinetics of Nd³⁺:YSO₃ being measured at excitation of 823.8 and 821.4 nm (⁴F_9/2 → ⁴F_5/2 + ²H_9/2 electron transition) and temperatures of 77 and 300 K. The LP-604 (Solar LS) parametric generator has been used as an excitation source. The luminescence kinetics have been recorded on the ⁴F_3/2 → ⁴I_9/2 electron transition. Approximation of luminescence decay curves of Nd³⁺:YScO₃ being measured at excitation of λ_exc = 823.8 nm, detection of λ_det = 896.0 nm, and temperatures of 77 and 300 K shows that each of the decay curves is described by two exponential models, with lifetimes of τ₁(300 K) = 100/250 μs; τ₁(77 K) = 130/290 μs (Figure 4a). At excitation of λ_exc = 821.4 nm and detection of λ_det = 893.5 nm, luminescence decay curves consist of two exponents with lifetimes of τ₂(300 K) = 100/240 μs and τ₂(77 K) = 130/250 μs (Figure 4b).

Total intensity of luminescence decay kinetics of Nd³⁺:YScO₃ is determined by the long-lifetime center (intensity ratio I₁/I₂ of 0.78/0.22, Figure 4a). The short-lifetime center is formed less effectively and its contribution to the total intensity is insignificant (intensity ratio I₁/I₂ of 0.94/0.06, Figure 4b). Figure 4a,b show that the lifetime of both centers is increasing with decreasing temperature down to 77 K. Decreasing temperature probably leads to decreasing electron–phonon interaction and intracenter nonradiative relaxation of the excitation energy.

The crystal structure of Nd³⁺:YScO₃ has been described previously [18]. It was shown that neodymium ions can substitute basic ions of yttrium and scandium. The relation between ionic radii of yttrium (0.9 Å), scandium (0.83 Å), and neodymium (0.98 Å) is near the Goldschmidt criterion. Substitution of yttrium and scandium ions by neodymium does not require charge compensation in YScO₃. This leads to the formation of two basic types of optical center of Nd³⁺ activators with the local symmetry of C₂. However, a slight difference between cation radii leads to a local distortion in octahedra occupied by Nd³⁺. The selective laser spectroscopy method is sensitive to these changes, and local distortion in the ion activator surrounding affects the spectral–kinetic characteristics of neodymium ion.

The third optical center with a short lifetime is probably the pair aggregate (Nd³⁺–Nd³⁺) one. The Nd³⁺–Nd³⁺ center is formed in the neighboring MO₆ polyhedra sharing the O²⁻–O²⁻ edge, where Nd³⁺ substitutes basic ions Y³⁺ and Sc³⁺. The multipole interaction of Nd³⁺ ions in the pair (Nd³⁺–Nd³⁺) leads to luminescence quenching and hence decreasing lifetime of the Nd³⁺ activator. The effect of this interaction depends on activator concentration as well as Nd³⁺–Nd³⁺ distance. This is one of the reasons for the non-exponential character of kinetics at the beginning state of the luminescence quenching.

Spectral lines of the Nd³⁺ ion occupying the C_3i site are not observed because electro-dipole transitions are forbidden in rare-earth ions occupying centro-symmetric positions.

Analysis of luminescence excitation (⁴I _9/2 → ⁴F_5/2 + ²H_9/2 transitions) and luminescence spectra (⁴F_3/2 → ⁴I_9/2, ⁴I_11/2 transitions) allows us to build the scheme of ⁴I_9/2,⁴I_11/2, and ⁴F_3/2 Stark levels of Nd³⁺ ions, which occupy two positions with the site symmetry of C₂ in the YScO₃ crystal structure. Figure 5 shows the Stark energy for each of two basic Nd³⁺ optical centers with site symmetry of C₂. The first center C₂ (I) is formed in the result of substitution of Y³⁺ by Nd³⁺, and the second center C₂ (II) is a result of the substitution of Sc³⁺ by Nd³⁺.

Both Nd³⁺ optical centers have a site symmetry of C₂ but depend on Nd–O distance in a structural polyhedra where Nd³⁺ substitutes Y³⁺ (center I) or Sc³⁺ (center II). Isomorphic substitution of basic structural cations (Y³⁺ and Sc³⁺) by Nd³⁺ in a local position leads to changes in the crystal field around Nd³⁺ ions, but the type of local symmetry remains C₂. So, we can distinguish two different optical centers C₂ (I) and C₂ (II) with almost equal lifetimes and values of Stark splitting of ⁴I _9/2, ⁴I _11/2, and ⁴F_3/2 levels (Figure 4 and Figure 5).

3.2. Spectroscopy of Tm³⁺:YScO₃

Figure 6 shows low-temperature (T = 77 K) time-resolved luminescence excitation spectra of the 0.1 at. % Tm³⁺:YScO₃ crystal fiber. The luminescence excitation spectra have been recorded at the LP-604 parametric laser radiation in the spectral range of 1160 to 1230 nm (³H₆ → ³H₅ transition) and registration of luminescence at 1945.6 nm (³F₄ → ³H₆ transition).

Changes in time delay of 0.6 ms to 12 ms relative to the moment of laser excitation lead to changes in luminescence excitation spectra, intensity redistribution between spectral lines of 1202.4 and 1209.4 nm. Changes being observed in luminescence excitation spectra allow us to assume that two types of optical centers of Tm³⁺ ions exist in the YScO₃ crystal structure.

The luminescence of Tm³⁺:YScO₃ has been measured on the ³F₄ → ³H₆ electron transition (spectral range of 1600–2240 nm) at selective laser excitation of 1202.4 and 1209.4 nm (³H₆ → ³H₅ electron transition). Figure 7a,c show time-resolved luminescence spectra of Tm³⁺:YSO₃ being measured at temperatures of 77 and 300 K and laser excitation of 1202.35 nm. The luminescence spectra of Tm³⁺ ions show slight changes in spectral line profile. The luminescence line intensities are redistributing, depending on the time delay relative to the moment of laser excitation (0.6 to 12 ms). However, the Stark structure is poorly resolved. The low-temperature (77 K) luminescence spectra being measured on the ³F₄ → ³H₆ electron transition are non-structured (Figure 7c), as are the room-temperature (300 K) ones. To identify and assign the luminescence lines to specific optical centers has become a difficult task. First, the luminescence spectra of the Tm³⁺ ion include many Stark components of ³F₄ (9 ones) and ³H₆ (13 ones), and the mean values of Stark splitting are sufficiently low. Second, the spectral lines of the Tm³⁺ ion are broadened and overlapped due to high values of electron–phonon interaction.

Figure 7b,d show luminescence quenching kinetics of Tm³⁺:YScO₃ being measured at excitation of 1202.4 nm (³H₆ → ³H₅ electron transition) and registration on 1944 nm in the ³F₄ → ³H₆ electron transition at temperatures of 300 and 77 K. Each luminescence quenching curve is described by two exponential models with lifetimes of τ (300 K) = 3.8/17 ms (I₁/I₂ = 0.96/0.04) and τ (77 K) = 4.25/19 ms (I₁/I₂ = 0.9/0.1). Short-time exponents (τ (300 K) = 3.8 and τ (77 K) = 4.25 ms) make a major contribution to luminescence quenching kinetics of the Tm³⁺ ion. Decreasing temperature down to 77 K leads to increasing the lifetime of the Tm³⁺ ion up to 11%. The contribution of the long-time component in the total intensity of luminescence quenching kinetics is low.

Figure 8a,c show normalized time-resolved luminescence spectra of Tm³⁺:YScO₃ being measured at excitation of 1209.4 nm (³H₆ → ³H₅ electron transition) and temperatures of 77 and 300 K. The luminescence spectra are modified, and the line intensities are redistributed with time delay. Decreasing the temperature down to 77 K does not improve structurization of luminescence spectra in the ³F₄ → ³H₆ electron transition in the Tm³⁺ ion (Figure 8c). Due to the poorly resolved Stark structure, identifying and assigning lines to specific centers is difficult. This is the result of the high value of electron–phonon interaction, overlapping spectral lines, and low values of splitting of ³F₄ and ³H₆ Stark levels.

Figure 8b,d show luminescence quenching kinetics in Tm³⁺:YScO₃ at excitation of 1209.4 nm (³H₆ → ³H₅ electron transition) and registration of luminescence at 1944 nm (³F₄ → ³H₆ electron transition) at temperatures of 77 and 300 K. The luminescence quenching curves are described by a single exponent law with lifetimes of τ (300 K) = 3.2 and τ (77 K) = 3.7 ms at temperatures of 300 and 77 K, respectively. The difference in lifetime can be explained by the decrease in the intercenteral relaxation with temperature (down to 77 K).

Analysis of luminescence quenching kinetics of Tm³⁺:YScO₃ (Figure 7b,d and Figure 8b,d) shows three types of Tm³⁺ optical centers in the crystal structure. Two of them are basic with short lifetimes of τ₁ (300 K) = 3.2 ms, τ₁ (77 K) = 3.7 ms and τ₂ (300 K) = 3.8 ms, τ₂ (77 K) = 4.25 ms. These centers belong to the low local symmetry of C₂. One of them is formed as the result of the local substitution of Y³⁺ by Tm³⁺ and another one is due to the local substitution of Sc³⁺ by Tm³⁺. The third one has a local site of C_3i and lifetimes of τ₃ (300 K) = 17 ms (I = 0.04) and τ₃ (77 K) = 19 ms (I = 0.1) at 300 and 77 K, respectively. The third center has longer lifetime and higher local symmetry (C_3i).

4. Conclusions

The substitution of Y³⁺ and Sc³⁺ by Nd³⁺ ions in the crystal structure of the YScO₃ crystal fiber leads to the formation of three types of Nd³⁺ optical centers. Two of these dominate and exhibit a local symmetry of C₂. The lifetimes of the Nd³⁺ optical centers in the YScO₃ crystal fiber were determined at two different temperatures. For the center I, the lifetimes were measured to be τ₁(300 K) = 250 μs and τ₁(77 K) = 290 μs. For the center II, the lifetimes were determined to be τ₂(300 K) = 240 μs and τ₂(77 K) = 250 μs. In addition to the two dominant optical centers, a third optical center with shorter lifetimes has been observed in the YScO3 crystal fiber. The third center, likely a Nd³⁺–Nd³⁺ aggregate pair, exhibits lifetimes of τ₃(300 K) = 100 μs and τ₃(77 K) = 130 μs. This aggregate pair is believed to form as a result of the local substitution of Nd³⁺ for the basic structural cation (Y³⁺ and Sc³⁺) in the neighboring octahedra that share an edge. The quenching of Nd³⁺ luminescence is primarily ascribed to a combination of interaction between neighboring Nd³⁺ ions and the process of nonradiative relaxation of the excitation energy. The Stark component scheme for ⁴F_3/2, ⁴I_11/2, and ⁴I_9/2 electron levels in the Nd³⁺ ion has been built for both basic optical centers with the local symmetry of C₂.

Two short-lifetime optical centers of Tm³⁺ ions with the local symmetry of C₂ are formed in the YScO₃ crystal fiber through the substitution of Y³⁺ and Sc³⁺ with Tm³⁺ ions. The lifetimes of these optical centers have been estimated as τ₁ (300 K) = 3.8 ms and τ₁(77 K) = 4.25 ms for center I, and τ₂ (300 K) = 3.2 ms and τ₂ (77 K) = 3.7 ms for center II. The third center with long lifetimes of τ₃ (300 K) = 17 ms and τ₃ (77 K) = 19 ms has a local symmetry classified as C_3i.