Spectral-Luminescent Properties of ZrO₂-Y₂O₃-Pr₂O₃ Crystals

Abstract

We studied the spectral-luminescent properties of (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 and (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 crystals grown by directional melt crystallization in a cold skull. Analysis of the absorption spectra of the crystals suggested the presence of Pr³⁺ and Pr⁴⁺ ions. Measurement of the relative intensities of the luminescence bands corresponding to the ³P₀ → ³H_4,5, ³P₀ → ³F_2,3,4, ³P₁ → ³H₅ and ¹D₂ → ³H₄ optical transitions of the Pr³⁺ ions, and analysis of luminescence extinction kinetics for the ³P₀ and ¹D₂ levels of the Pr³⁺ ions, suggests the presence of cross-relaxation (¹D₂ → ¹G₄) → (³H₄ → ³F₄) of the Pr³⁺ ions in the ZrO₂-Y₂O₃-Pr₂O₃ crystals.

1. Introduction

Visible-wavelength solid-state lasers are intensively being developed nowadays. The interest in these lasers stems from their wide range of potential applications in various fields of science and engineering, medicine, agricultural technologies, etc. [1,2,3,4,5,6].

The trivalent praseodymium ion (Pr³⁺) is the most widely used rare-earth ion for visible-wavelength laser generation because the structure of its energy levels provides the possibility of laser transitions in the entire visible spectral region from blue (³P₁ → ³H₄) to deep red (³P₀ → ³F₃).

Fluoride crystals are the most widely used matrix materials for solid-state visible-wavelength lasers because of the low frequency of their phonon spectrum (E = 400 cm⁻¹) which significantly reduces the rate of multiphonon non-radiative energy relaxation from the excited rare-earth ion levels in these crystals. For example, LiYF₄ crystals with a phonon spectrum frequency of ~460 cm⁻¹ [7] are widely used. The spectral-luminescent and laser properties of Pr³⁺ ion-doped fluoride crystals have been studied before [8,9,10,11,12,13,14,15,16,17,18,19,20].

Oxides exhibit higher stability and mechanical strength in comparison with fluoride materials. Moreover, oxide crystals exhibit higher phonon spectrum frequencies and hence have a higher probability of non-radiative transitions. The spectral-luminescent and laser properties of some Pr³⁺ ion-doped oxide crystals have also been studied before [21,22,23,24,25].

Zirconia-based solid solution single crystals were obtained for the first time with the development of refractory materials technologies in the 1970s. As-crystallized cubic ZrO₂ undergoes a number of polymorphic transformations upon cooling. At 2340 K, there is a cubic–tetragonal transition, and at 1170 K a monoclinic modification forms. To stabilize the cubic phase of zirconia, one can introduce appropriate oxides during synthesis, such as Y₂O₃ and rare-earth (RE) oxides, as well as CaO and MgO. Many studies have dealt with the polymorphic transformations and conditions required for the stabilization of the cubic and tetragonal phases of zirconia [26,27,28,29,30].

Single crystals of stabilized zirconia have a number of unique physical properties. They are optically homogeneous, have a high hardness (8.5–9 Moos), and a wide spectral transmittance region (250–7500 nm). Initially, cubic single crystals of zirconia solid solutions doped with rare-earth ions were considered as active media for lamp-pumped rare-earth lasers but did not find wide application due to their low thermal conductivity. Laser diode pumping changed the requirements of the thermomechanical parameters of laser crystals, and this revived interest in zirconia single crystals. The phonon spectrum frequency of zirconia solid solutions is ~610 cm⁻¹ [31,32,33], which is intermediate between that for Y₃Al₅O₁₂ crystals (~865 cm⁻¹) [7] and that for fluoride crystals (400 cm⁻¹). It should be noted that the crystalline structure of ZrO₂-based solid solutions is disordered; therefore, their absorption and luminescence spectra are broadened quite inhomogeneously. Thus, these materials can be used for the fabrication of tunable and short-pulse laser generators.

The results of semiconductor-pumped laser generation experiments for ZrO₂-Y₂O₃ crystals doped with Nd³⁺, Tm³⁺, Ho³⁺, Er³⁺, and Yb³⁺ ions have been reported previously [28,29,30,34,35,36,37]. However, there are no literary data on the properties of lasers based on ZrO₂-Y₂O₃ crystals doped with Pr³⁺ ions. To assess potential application domains of ZrO₂-Y₂O₃-Pr₂O₃ crystals as active media of solid-state lasers, one should carry out detailed studies of the spectral-luminescent properties of these materials. Such experiments have not yet been conducted. Therefore, the aim of this study was to assess the spectral-luminescent properties of Pr³⁺ ion-doped ZrO₂-Y₂O₃ crystals for evaluating their potential as active media for visible-wavelength lasers.

2. Materials and Methods

For the tests, ZrO₂-Y₂O₃ solid solution crystals with different Pr³⁺ ion concentrations were grown by directional melt crystallization in a cold skull with direct inductive heating on a Kristall-407 (Russia) plant at a 5 mm/h growth rate [28,29,30]. The as-grown crystals were transparent and had a light-green color, the intensity of which increased with the praseodymium oxide concentration. The length and cross-section of the crystals were 30–40 and 10–20 mm, respectively. Photographs of the as-grown crystals are shown in Figure 1. For elemental analysis and spectral-luminescent measurements, plane-parallel wafers were cut out from the beginning and the end sections of the crystals and polished.

The (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 and (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 solid solution crystals had a cubic structure, as confirmed by the Raman scattering data for the crystals. By way of example, Figure 2 shows the Raman spectrum of the ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 crystal. The Raman spectrum was recorded with a Renishaw InVia (UK) spectrometer, excitation wavelength 532 nm.

The elemental composition of the crystals was determined using energy dispersion spectroscopy (EDS) with a microanalysis attachment to a Quanta TM 3D 200i electron microscope (FEI Company, Oregon, USA). Elemental analysis data are shown in

Table 1. Elemental analysis data for test crystals.

Specimen *		ZrO2, mol.%	Y2O3, mol.%	Pr2O3, mol.%
a	(1)	80.74 ± 0.52	18.71 ± 0.43	0.55 ± 0.10
	(2)	80.35 ± 0.54	18.87 ± 0.43	0.78 ± 0.13
b	(1)	80.65 ± 0.53	19.12 ± 0.44	0.23 ± 0.15
	(2)	79.86 ± 0.52	19.81 ± 0.45	0.33 ± 0.13

* (1) For wafer cut out from crystal beginning section; (2) for wafer cut out from crystal end section.

.

The Pr³⁺ ion absorption spectra of the beginning and the end of the test crystals were recorded in a double-beam scheme with a Lambda 950 spectrophotometer (Perkin Elmer, USA) at room temperature.

T = 300 K luminescence spectra were recorded with an FHR 1000 spectrophotometer (Horiba, Japan). The luminescence spectra were excited from the ³P₂ level to the underlying levels of the Pr³⁺ ions by a diode solid-state laser, MGL-N-457nm-1W (λ_exc. = 457 nm) (CNI Laser, China). The radiation receiver was a Hamamatsu R928B photoelectron multiplier (Hamamatsu Photonics, Japan).

Extinction kinetic curves of luminescence from the ³P₀ and ¹D₂ levels of the Pr³⁺ ions in the ZrO₂-Y₂O₃-Pr₂O₃ crystals were recorded with a Rohde & Schwarz RTM3004 high-resolution digital oscilloscope (Rohde & Schwarz, Germany). The recording wavelengths were 504 and 612 nm, with luminescence excitation by a Al₂O₃:Ti crystal LX 329 laser (SOLAR Laser Systems, Belarus Republic) with a 450 nm wavelength, a 15 ns pulse duration, and a 10 Hz pulse rate.

3. Results and Discussion

Elemental analysis data (Table 1) suggest an increase in the praseodymium oxide content along the crystal. The difference in the Pr₂O₃ concentrations at the beginning and the end of the crystal is caused by the fact that the Pr³⁺ ions have a relatively large radius in comparison with that of the Zr⁴⁺ ions and are therefore displaced toward the end of the growing crystals.

Figure 3 shows the absorption spectra of the test ZrO₂-Y₂O₃-Pr₂O₃ crystals for (a) 350–700 and (b) 900–2500 nm ranges. The digits mark the absorption spectra of the ZrO₂-Y₂O₃-Pr₂O₃ specimens cut out from the crystal sections corresponding to (1) the beginning of growth and (2) end of growth.

The absorption spectra of the ZrO₂-Y₂O₃-Pr₂O₃ crystals contain bands corresponding to the 4f–4f intra-configurational optical transitions of the Pr³⁺ ions from the main state ³H₄ to the ³P₂, ³P₁, ³P₀, ¹D₂, ¹G₄, ³F_{2, 3, 4}, and ³H₆ excited multiplets. These bands are broadened quite inhomogeneously due to the disordering of the crystal structure.

It is noteworthy that the absorption bands of the Pr³⁺ ions are resolved against the tail of a wide band, which seems to originate from the presence of Pr⁴⁺ ions in the ZrO₂-Y₂O₃-Pr₂O₃ crystals. The presence of both trivalent and tetravalent praseodymium ions in the ZrO₂-Y₂O₃-Pr₂O₃ crystals has been reported before [28,29,38,39].

Analysis of the absorption spectra shown in Figure 3 revealed that the intensity of the Pr³⁺ ion-related absorption bands increases as one moves toward the end of the crystal.

Comparing the absorption spectra of the Pr³⁺/Pr⁴⁺ ions in the ZrO₂-Y₂O₃-Pr₂O₃ crystals containing different concentrations of praseodymium ions was also of interest. Figure 4 shows the absorption spectra of the Pr³⁺/Pr⁴⁺ ions in the (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 and (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 crystals recorded under similar conditions.

Analysis of the absorption spectra shown in Figure 4 suggests that both the Pr³⁺ and Pr⁴⁺ ion absorption band intensities for the (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 crystal are higher than those of the respective bands for the (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 crystal.

The luminescence spectra for the ³P₀ → ³H_4,5, ³P₀ → ³F_2,3,4, ³P₁ → ³H₅, and ¹D₂ → ³H₄ optical transitions of the Pr³⁺ ions for different crystal ZrO₂-Y₂O₃-Pr₂O₃ sections, recorded with laser excitation at λ_exc. = 457 nm at T = 300 K, are shown in Figure 5. The digits mark the luminescence spectra of the ZrO₂-Y₂O₃-Pr₂O₃ specimens cut out from the crystal sections corresponding to (1) beginning of growth and (2) end of growth.

The significant inhomogeneous broadening in the luminescence spectra of Pr³⁺ ions in ZrO₂-Y₂O₃-Pr₂O₃ is caused by the disordering of the crystal structure.

It can be seen from Figure 5 that the luminescence spectra of the Pr³⁺ ions taken from different crystal sections exhibit a redistribution of the relative intensities of the ³P₀ → ³H₄ and ¹D₂ → ³H₄ optical transition bands. In the luminescence spectrum taken at the beginning of the (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 crystal, the relative intensity of the ³P₀ → ³H₄ transition bands is comparable with that of the ¹D₂ → ³H₄ transition bands. However, in the luminescence spectrum taken at the end of the crystal, the intensity of the ¹D₂ → ³H₄ transition bands decreases relative to that of the ³P₀ → ³H₄ transition bands. For the (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 crystal with a lower praseodymium concentration, the relative intensity of the ³P₀ → ³H₄ transition bands is lower than that of the ¹D₂ → ³H₄ transition bands. There is also a trend of decreasing intensity of the ¹D₂ → ³H₄ transition bands relative to that of the ³P₀ → ³H₄ transition bands in the luminescence spectrum taken from the crystal section corresponding to the end of growth. The trend of decreasing intensity of the ¹D₂ → ³H₄ transition bands in comparison with that of the ³P₀ → ³H₄ transition bands is also observed in the luminescence spectra of the crystals with a higher concentration of Pr³⁺ ions (Figure 6).

This redistribution of the relative luminescence band intensities for the ³P₀ → ³H₄ and ¹D₂ → ³H₄ optical transitions of the Pr³⁺ ions has been observed before for La_1-xPr_xGa_0.5Sb_1.5O₆ and YPO₄ particles and BaGd₂(MoO₄)₄ crystals doped with Pr³⁺ ions [40,41,42].

The redistribution of the relative luminescence band intensities for the ³P₀ → ³H₄ and ¹D₂ → ³H₄ optical transitions of the Pr³⁺ ions was attributed by the authors [40,41,42] to the presence of cross-relaxation of the Pr³⁺ ions (¹D₂ → ¹G₄) → (³H₄ → ³F₄), as shown in Figure 7.

The redistribution of the relative luminescence band intensities for the ³P₀ → ³H₄ and ¹D₂ → ³H₄ optical transitions of the Pr³⁺ ions in the luminescence spectra of the (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 and (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 crystals revealed in this study is also attributable to the presence of non-radiative energy exchange between the Pr³⁺ ions in the excited state ¹D₂ and the Pr³⁺ ions in the main state ³H₄.

In order to confirm this assumption, extinction kinetic curves of luminescence from the ³P₀ and ¹D₂ levels of the Pr³⁺ ions in the (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 and (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 crystals were recorded. Figure 8 shows the 504 and 612 nm luminescence extinction kinetic curves for these levels (excitation by second harmonic of an Al₂O₃:Ti crystal laser, 450 nm wavelength, 15 ns pulse duration, 10 Hz pulse rate).

It can be seen from Figure 8a that the ³P₀ level luminescence extinction curves are non-exponential both for the (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 and the (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 crystals. With an increase in the Pr³⁺ ion concentration in the test crystals, the curve patterns change, but only slightly. Therefore, the non-exponential pattern of the respective decomposition curves can be attributed either to the presence of various optical centers of the Pr³⁺ ions differing in the ³P₀ level lifetime or to the interaction between the Pr³⁺ and the Pr⁴⁺ ions.

The ¹D₂ level luminescence extinction kinetic curves for the (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 and (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 crystals (Figure 8b) are also non-exponential. However, with an increase in the Pr³⁺ ion concentration in these crystals, the decomposition of the ¹D₂ level becomes faster, confirming the earlier assumption of the presence of non-radiative energy transfer between the Pr³⁺ ions (¹D₂ → ¹G₄) → (³H₄ → ³F₄). The non-exponential pattern of the ¹D₂ level luminescence kinetics for the (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 crystals seems to originate from the non-radiative energy transfer as per the schematic Pr³⁺ (¹D₂ → ¹S₀) → (¹D₂ → ³H₄).

The presence of non-radiative energy transfer between the Pr³⁺ ions (¹D₂ → ¹G₄) → (³H₄ → ³F₄) is confirmed by the pattern of the ³P₀ and ¹D₂ level luminescence extinction curves for the Pr³⁺ ions taken from the beginning and the end of the (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 crystals (Figure 9).

4. Summary

In this study, (ZrO₂)_0.802(Y₂O₃)_0.195(Pr₂O₃)_0.003 and (ZrO₂)_0.805(Y₂O₃)_0.188(Pr₂O₃)_0.007 solid solution single crystals were synthesized by directional melt crystallization in a cold skull. The spectral-luminescent properties of praseodymium ions in these crystals were studied in detail for the first time. Analysis of the absorption spectra of the (ZrO₂)_0.803(Y₂O₃)_0.190(Pr₂O₃)_0.007 and (ZrO₂)_0.804(Y₂O₃)_0.193(Pr₂O₃)_0.003 crystals suggests the presence of both Pr³⁺ and Pr⁴⁺ ions in the crystals. Analysis of the luminescence spectra and the ³P₀ and ¹D₂ level luminescence extinction curves for Pr³⁺ ions in the test crystals revealed the presence of concentration quenching of the luminescence from the excited state ¹D₂ in the Pr³⁺ ions due to cross-relaxation (¹D₂ → ¹G₄) → (³H₄ → ³F₄).