A general trend of rare earth ions in the KRE(WO4)2 double tungstates (RE = Y, Yb, Gd, Lu)

doi:10.1016/j.mseb.2010.04.023

Materials Science and Engineering: B

Volume 172, Issue 1, 15 August 2010, Pages 89-95

https://doi.org/10.1016/j.mseb.2010.04.023 Get rights and content

Abstract

Crystal-field energy levels of Er³⁺ ions doped in KYb(WO₄)₂, KY(WO₄)₂ and KLu(WO₄)₂ laser crystals were fitted to the parameters of semi-empirical Hamiltonian including J-mixing. A simulation of the energy level scheme is carried out. The crystal-field effect is introduced through the 14 non-zero crystal-field parameters allowed by C₂ symmetry site occupied by the rare earth. Very satisfactory correlations were obtained between 58, 64 and 15 calculated and experimental Stark energy levels, with an r.m.s. of 8.7 cm⁻¹, 9.3 cm⁻¹ and 2.6 cm⁻¹ for Er³⁺ ions doped in KYb(WO₄)₂, KY(WO₄)₂ and KLu(WO₄)₂ respectively. These results are compared with those reported for other rare earth ions in the same hosts and with Er³⁺ ions in KGd(WO₄)₂ crystal. Furthermore, based on the extended electrostatic point charge model, interesting results are obtained for Yb³⁺ ions doped KYb(WO₄)₂, KY(WO₄)₂, KLu(WO₄)₂ and KGd(WO₄)₂ crystals. The concordance between the crystal-field strength parameters indicates the consistence of our analysis. A general trend is then proposed for the rare earth ions in the KRE(WO₄)₂ double tungstates.

Introduction

In recent years, the Yb³⁺ ion has been recognized as a dopant with interesting potential for diode-pumped solid-state lasers in the 1 μm spectral region. Yb³⁺ doped materials are attractive as efficient active media because they possess several important advantages over the widely used Nd³⁺: a small thermal loading due to a single de-excitation channel, no excited state absorption or up-conversion losses, a large transparency over the visible range, a weaker cross-relaxation process, and a long (μs) radiative lifetime [1], [3]. In other hand, erbium has played an important role in the development of optical communication technology. The erbium ion is suitable for obtaining laser radiation in the near-infrared region after diode pumping because of its ⁴I_11/2 energy level to absorb the pump radiation and its ⁴I_13/2 energy level to emit to the ground state (1.5 μm emission). Its absorption band between 0.9 μm and 1.1 μm (11111–9091 cm⁻¹), corresponding to the ⁴I_11/2 energy level, is the easily available diode-laser emission range, but its low absorption cross section in the above-mentioned spectral range limits pump efficiency. In this field, ytterbium ions are widely used as sensitizer ions for increasing the absorption of light. Ytterbium ions posses a high absorption cross section in the above-mentioned spectral range and its ²F_5/2 energy level overlaps in energy with the ⁴I_11/2 energy level of erbium. This energy overlap derives to a very good resonant energy transfer between these two ions and the consequently greater efficiency of erbium luminescence generation. All of these advantages make ytterbium an ideal sensitizer ion of erbium [4]. The 1.5 μm (6667 cm⁻¹) erbium emission comprises a very efficient-level laser system, trough the ⁴I_15/2, ⁴I_13/2 and ²H_11/2 multiplets. The determination of the possibility of energy transfer between Yb³⁺ and Er³⁺ ions can be illustrated by the theoretical investigations of these ions in order to determinate their Stark energy levels. In this work, we are interested by erbium and ytterbium doped KYb(WO₄)₂, KY(WO₄)₂, KLu(WO₄)₂ and KGd(WO₄)₂ and we estimate a general trend for the rare earth ions in the KRE(WO₄)₂ double tungstates.

Section snippets

The KRE(WO₄)₂ double tungstates

Monoclinic potassium rare earth double tungstates, KRE(WO₄)₂ (RE = rare earth = Yb, Y, Gd, Lu, hereafter KREW), belong to the monoclinic system, with space group C2/c and unit cell parameters a = 10.590 Å, b = 10.290 Å, c = 7.478 Å, β = 130.70° for KYbW [5], a = 10.64 Å, b = 10.35 Å, c = 7.54 Å, β = 130.5° for KYW [6], a = 10.652 Å, b = 10.374 Å, c = 7.582 Å, β = 130.80° for KGdW [7] and a = 10.576 Å, b = 10.214 Å, c = 7.487 Å, β = 130.68° for KLuW [8]. The RE is coordinated with 8 oxygen atoms, which form a distorted square antiprism with

Crystal-field calculation procedure

The total model Hamiltonian used for crystal-field analysis consists of the free-ion (H_FI) and crystal-field (H_CF) parts: $\hat{H} = {\hat{H}}_{FI} + {\hat{H}}_{CF}$ where ${\hat{H}}_{FI} = {\hat{H}}_{0} + \sum_{k} E^{k} {\hat{e}}_{k} + ξ_{SO} {\hat{A}}_{SO} + α \hat{L} (\hat{L} + 1) + β \hat{G} (G_{2}) + γ \hat{G} (R_{7}) + \sum_{k} T^{i} {\hat{t}}_{i}$ in which H₀ represents the kinetic energy of 4f interactions with the nucleus, E^k are Racah parameters and ξ_SO is the spin–orbit coupling constant. α, β and γ are the Trees parameters and Tⁱ the three-body interactions parameters [25].

The crystal-field Hamiltonian is expressed as a sum of products of

Free-ion energy levels

The digitalization of the whole matrix associated with ${\hat{H}}_{FI}$ Hamiltonian acting on the basis formed by the |4f¹¹αSLJM〉 coupled states allows us to obtain the eigenstates of erbium ion in the intermediate coupling scheme. In experimental support, we have taken into account centroïds of the ⁴I_15/2, ⁴I_13/2, ⁴I_9/2, ⁴F_9/2, ⁴S_3/2, ²H_11/2, ⁴F_7/2, ⁴F_5/2, ⁴F_3/2, ²H_9/2, ⁴G_9/2 and ⁴G_11/2 levels observed previously for the excited energy levels of erbium observed in KYbW [9]. All calculated parameters are

Conclusion

In summary, Stark energy levels of C₂ symmetry center of Er³⁺ in KREW crystal, calculated by using a whole Hamiltonian including free-ion and crystal-field parameters in an intermediate coupling scheme, are in a good agreement with the experimental energy levels. The r.m.s. deviation of the fit of 8.7 cm⁻¹, 9.3 cm⁻¹ and 2.6 cm⁻¹ respectively for KYbW, KYW and KLuW crystals, indicates the success of the fitting. Besides, the calculated CF parameters are compared with those reported for Er³⁺ ions in

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Magnetic properties of NaY<inf>1−x−y</inf>Ho<inf>x</inf>Yb<inf>y</inf>(WO<inf>4</inf>)<inf>2</inf>: x = 0.05, y = 0.02 and KY<inf>1−x−y</inf>Ho<inf>x</inf>Yb<inf>y</inf>(WO<inf>4</inf>)<inf>2</inf>: x = 0.02, y = 0.01 nanopowders obtained by Pechini and hydrothermal methods
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Citation Excerpt :
In the KRe(WO4)2 single crystals, where Re is Kramers ion, the introduction of a small paramagnetic dopant may not affect the EPR spectrum. As shown in [27–32], a strong EPR signal from the paramagnetic host ions prevails. The host ions are in C2 symmetry but in non-equivalent sites, as well as strong dipolar and exchange interactions take place.
Potassium- and sodium-yttrium double tungstate nanopowders co-doped with Yb³⁺, Ho³⁺ ions were investigated for their magnetic properties for the first time. We performed the analysis of the compounds using two methods: Electron Paramagnetic Resonance (EPR) and static Magnetic Susceptibility (MS). We have described the symmetry of doped ions environment and corresponding magnetic, optical and structural properties of the above compounds obtained by hydrothermal and Pechini methods. It was performed for the compounds underwent to different calcination temperatures in the range from 600 °C to 800 °C. For potassium and sodium double tungstates the local symmetry of ytterbium ions is found to be C₂ and does not change with the calcination temperature rise. Based on the finite-size effect we determined the relative size of the nanoparticles agglomeration, observing changes in the values of Curie-Weiss temperature and the intensity of the EPR signal for various calcination temperatures. The magnetic and structural properties of nanoparticles were compared to corresponding ones registered for single crystals and crystalline powders obtained using other methods.
The crystal structure and luminescence quenching of poly- and single-crystalline KYW<inf>2</inf>O<inf>8</inf>:Tb3+
2015, Journal of Luminescence
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Furthermore, KYW2O8 was studied with europium as a radiation converter in LEDs [10]. Especially for laser applications the yttrium site is partly replaced by ytterbium or erbium [11,12]. To our surprise, terbium was not studied in this host so far, even though it is an interesting candidate for efficient luminescence due to its small multi-phonon relaxation [13].
Terbium-substituted KYW₂O₈ single crystals of high optical quality were grown by the top seeded solution growth technique. The degree of yttrium–terbium mixed occupancy was determined for two samples through structure refinements on the basis of single crystal X-ray diffractometer data. Temperature dependent magnetic susceptibility data underline the paramagnetic nature of terbium doped crystals. No magnetic ordering is evident down to 2 K. Luminescence measurements yield the typical excitation and emission spectra as expected for Tb³⁺ activated materials. The decay time of Tb³⁺ decreases linearly with the Tb³⁺ concentration, while the excess of thermal quenching does not change significantly. At about 405 K the decay time is reduced by roughly 50% relative to the low-temperature value, both for the powders as for the single crystals.
Crystal-field study for the orthorhombic Yb3+ centers in Ca<inf>3</inf>Sc<inf>2</inf>Ge<inf>3</inf>O<inf>12</inf> and Lu<inf>3</inf>Ga<inf>5</inf>O<inf>12</inf> garnets
2015, Journal of Alloys and Compounds
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Therefore, several ways of tackling this problem were taken by different groups. For example, the extrapolation method based on the point charge model (PCM) was utilized by Trabelsi et al. [21]; keeping ratios between CFPs fixed to reduce free variables was adopted by Kamińska et al. [18] and more works [22–26] are based on effective crystal-field models such as simple overlap model (SOM) [27], exchange charge model (ECM) [28] and superposition model (SPM) [29]. In this study, the combination of complete diagonalization (of energy matrix) method (CDM) [30] and SPM are employed to explain optical spectra for CaSGG:Yb3+ and LuGG:Yb3+ garnets and EPR g factors of such two garnets are also investigated (see Section 2).
The crystal-field modeling based on superposition model (SPM) for the orthorhombic Yb³⁺ centers in Ca₃Sc₂Ge₃O₁₂ (CaSGG) and Lu₃Ga₅O₁₂ (LuGG) garnets has been employed to explain their recent experimental optical spectroscopy. Furthermore, the electron paramagnetic resonance (EPR) principal g values for orthorhombic Yb³⁺ centers in LuGG garnet have been interpreted and the predicted g factors for CaSGG:Yb³⁺ garnet are also given. Both calculated optical spectra and EPR g factors for the two Yb³⁺-doped garnets could be regarded as in reasonable agreement with the experimental ones. Two important facts about reliability and standardization of the fitted phenomenological crystal-field parameters (CFPs) have been discussed and the unexpected unavailability of direct parametric modeling by SPM for LuGG:Yb³⁺ garnet is also found and analyzed briefly.
Crystal-field study for Yb3+ doped KY(WO<inf>4</inf>) <inf>2</inf> crystal - Revisited based on superposition model
2013, Optical Materials
Citation Excerpt :
Three shortcomings in a recent work [9] concerning: (i) settling the ‘over-fit’ problem, (ii) performing monoclinic standardization transformations and (iii) explaining experimental EPR parameters have been clarified and solved in the present work. According to our results, the CFPs of Yb3+:KYW crystal given in [9] are unable to reasonably interpret EPR experiments, though they have reached the global minimum in CFP space. Generally, it should be recognized that the best fitted CFPs to explain successfully optical spectroscopy of a paramagnetic rare earth ion in crystal can not always do well in predicting EPR parameters of the studied system.
The optical spectra and electron paramagnetic resonance (EPR) parameters (g_i and hyperfine structure constants A_i, i = x, y, z) for Yb³⁺:KY(WO₄)₂ crystal have been simultaneously explained by a crystal-field study based on superposition model (SPM). The deficiencies in a recent paper by Trabelsi et al. (2010) have been clarified and solved in the present work. The more reliable fitted phenomenological crystal-field parameters (CFPs) have been confirmed and monoclinic standardization of the obtained CFPs for Yb³⁺:KY(WO₄)₂ crystal is performed to ensure direct comparison with other rare earth ion doped potassium rare-earth double tungstates. By SPM calculations in this work, two individual EPR experiments with greatly different y-directional principal g factors for Yb³⁺:KY(WO₄)₂ crystal have been analyzed and possible reason for this large difference is also given. It indicates that magnetic dipolar and exchange interactions among different rare earth ions will play an important role in determining their magnetic properties in crystal under low temperature and more investigations should be carried out. The present procedures for SPM analysis can be applied to other crystal field environment with low site symmetry.
In Situ Neutron Diffraction Studies of LiCe(WO <inf>4</inf> ) <inf>2</inf> Polymorphs: Phase Transition and Structure-Property Correlation
2019, Journal of Physical Chemistry C
Effect of Solvent on the Red Luminescence of Novel Lanthanide NaEu(WO<inf>4</inf>)<inf>2</inf> Nanophosphor for Theranostic Applications
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Materials Science and Engineering: B

Abstract

Introduction

Section snippets

The KRE(WO₄)₂ double tungstates

Crystal-field calculation procedure

Free-ion energy levels

Conclusion

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Cited by (6)

Magnetic properties of NaY<inf>1−x−y</inf>Ho<inf>x</inf>Yb<inf>y</inf>(WO<inf>4</inf>)<inf>2</inf>: x = 0.05, y = 0.02 and KY<inf>1−x−y</inf>Ho<inf>x</inf>Yb<inf>y</inf>(WO<inf>4</inf>)<inf>2</inf>: x = 0.02, y = 0.01 nanopowders obtained by Pechini and hydrothermal methods

The crystal structure and luminescence quenching of poly- and single-crystalline KYW<inf>2</inf>O<inf>8</inf>:Tb<sup>3+</sup>

Crystal-field study for the orthorhombic Yb<sup>3+</sup> centers in Ca<inf>3</inf>Sc<inf>2</inf>Ge<inf>3</inf>O<inf>12</inf> and Lu<inf>3</inf>Ga<inf>5</inf>O<inf>12</inf> garnets

Crystal-field study for Yb<sup>3+</sup> doped KY(WO<inf>4</inf>) <inf>2</inf> crystal - Revisited based on superposition model

In Situ Neutron Diffraction Studies of LiCe(WO <inf>4</inf> ) <inf>2</inf> Polymorphs: Phase Transition and Structure-Property Correlation

Effect of Solvent on the Red Luminescence of Novel Lanthanide NaEu(WO<inf>4</inf>)<inf>2</inf> Nanophosphor for Theranostic Applications

A general trend of rare earth ions in the KRE(WO4)2 double tungstates (RE = Y, Yb, Gd, Lu)

Abstract

Introduction

Section snippets

The KRE(WO4)2 double tungstates

Crystal-field calculation procedure

Free-ion energy levels

Conclusion

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J. Opt. Mater.

J. Alloys Compd.

IEEE J. Quantum Electron.

IEEE J. Sel. Top. Quantum Electron.

Phys. Rev. B

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J. Appl. Crystallogr.

A general trend of rare earth ions in the KRE(WO₄)₂ double tungstates (RE = Y, Yb, Gd, Lu)

The KRE(WO₄)₂ double tungstates