Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Short CommunicationNd3 +, Ho3 +-codoped garnet-related Li7La3Hf2O12 phosphor with NIR luminescence
Graphical abstract
Introduction
Near-infrared lasers operating in the 0.9–3.0 μm spectral range are in great demand for a variety of applications including free-space optical communication, medical surgery, eye-safe laser radar system engineering, remote sensing, atmosphere pollution monitoring, etc. [1], [2], [3], [4], [5], [6]. Production of this kind of lasers requires elaboration of new materials. Luminescence in this wavelength region can be excited due to the 4f–4f transitions in lanthanide ions (Ho3 +, Er3 +, Tm3 +, Dy3 +, etc.). Since these ions have no absorption bands or have a small cross section at 808 or 980 nm, commercial laser diodes cannot be used as pump sources. In order to achieve strong emission in the near-infrared region, codoping of other active ions as sensitizer ions, e.g. Nd3 + or Yb3 +, has been used [7], [8], [9]. Different energy transfer processes occurring between lanthanide ions can greatly enhance the absorption of excitation energy by Er3 + or Ho3 + ions. A significant improvement of sensitization has been recently demonstrated in Ho3 +/Yb3 + [9], [10], [11], [12], Er3 +/Nd3 + double-doped [13], [14], [15], [16], Er3 +/Ho3 +/Nd3 + [17], Er3 +/Ho3 +/Yb3 + [18], and Ho3 +/Tm3 +/Yb3 + triply-doped systems [9], [19]. The above mentioned lanthanide ions can either be introduced in an optical host as codopants in form of the corresponding individual educts directly in the synthesis procedure, or exist already in a lanthanide-containing initial reagent of interest according to stoichiometric relation for the chosen method of preparation. The second variant in which, for example, residual concentration of neodymium ions in scandium, yttrium or lanthanum oxide (even of > 99.99% purity) is precisely defined before preparation of a tailored phosphor can be experimentally considered from the technological point of view. On the one hand, while global demand of rare earths is known to continue growing in recent decade, the rare earth elements (REE) market remains small. It represents in fact only local and regional markets for a number of REEs. Due to trade restrictions to a few suppliers who have no long-term contracts, it is almost impossible to predict the potential prices with any certainty [20], [21], [22]. On the other hand, a permanent challenging task in the REE industry and related fields is in production of high purity lanthanide-containing educts which are critical for further research and development of materials for optics and photonics in particular. The mentioned circumstances keep being problematic for years and, as a result, the prices of many end-products of optical engineering become unacceptably high.
Recent decades have witnessed a great development of various crystals and glasses doped with lanthanide ions such as fluorides, YAGs, tellurides, silicates, germanates, etc. [2], [3], [9], [19]. When considering the hosts to be used in infrared lasers, both good mechanical properties and transmittance in a broad spectral range are required. Since the effective phonon energy value for zirconates and hafnates is usually small [23], [24], [25], these compounds are appropriate for application in infrared-emitting phosphors. Meanwhile, Eu3 +-doped LixA3M2O12 (A = Y, La; M = Zr, Nb, Te, Hf; x = 3–7) compounds, which may exist in the form of a cubic or tetragonal polymorph, have been earlier considered as prospective phosphors for white LEDs [25], [26], [27]. However, no near-infrared luminescence properties of Li7La3M2O12 (M = Zr, Nb, Te, Hf) doped with lanthanide ions have been reported. Generally, the concentration of lanthanide ions is about tenths of mol% in different near-infrared emitting materials [13], [14], [17]. Trace amounts of lanthanide dopants can transform luminescence properties of a phosphor. The residual thulium ions have been previously shown to cause up-conversion emission with a maximum at 475 nm in Nd-doped YVO4 and KGd(WO4)2 crystals [28]. Furthermore, similar green emission can be observed in KGd(WO4)2 crystal comprising erbium ions with a concentration of 10− 5–10− 6 wt.% [29].
Given the above listed observations, the study of multiband near-infrared luminescence of garnet-related Li7La3Hf2O12 codoped with Nd3 + and Ho3 +, in particular, in the form of the solid solution of Li7La3 − xNdxHf2O12 comprising residual content of holmium ions, and possible energy transfer processes between Nd3 + and Ho3 + belongs to topical tasks both in optical spectroscopy and applied spectrochemistry, and is related to the subject of this short paper.
Section snippets
Reagents and Synthesis
The polycrystalline samples of Li7La3 − xNdxHf2O12:Ho3 + (x = 0.00–0.15) were prepared by a modified solid-state method close to that described by Awaka et al. [30]. La2O3 (99.99%) and Nd2O3 (99.99%) calcined previously at 700–900 °С for 5 h, HfO2 (99.99%) and Li2CO3 (99.99%) were used as raw chemicals. The lanthanum and neodymium oxides contained trace impurities of holmium, which were accurately defined by mass spectrometry on a PerkinElmer Elan 9000 ICP-MS: 1.4 ∙ 10− 3 mol% Nd3 + and 2.0 ∙ 10− 6 mol% Ho3 +
Results and Discussion
Powder XRD patterns of the solid solution Li7La3 − xNdxHf2O12:Ho3 + (x = 0.00–0.15) are shown in Figs. S2–S6 (Supplementary material). No additional peaks from any intermediate phases such as lanthanum oxide or lanthanum hafnate are observed. The diffraction peaks on the Li7La3 − xNdxHf2O12:Ho3 + patterns match well with the tetragonal Li7La3Hf2O12 phase, FIZ ICSD card #174202, as proved by the powder neutron and X-ray diffraction data [30].
The powder XRD study shows that the solid solutions Li7La3 − xNdx
Conclusions
Powder XRD analysis shows that Li7La3Hf2O12:Nd3 +,Ho3 + compounds crystallize in the tetragonal space group I41/acd, Z = 8, with a unit cell of garnet-type structure. Luminescence in the range from 2 to 3 μm in Nd3 +/Ho3 + codoped Li7La3Hf2O12 is observed under 808 nm laser diode excitation. The trace amount of holmium ions and the low doping ratio of Ho3 + and Nd3 + allow revealing of intensity emission at 2.1 and 2.7 μm. The concentration of neodymium ions strongly affects the intensity of the infrared
Acknowledgements
This work was partially supported by the FASO Program No. 0397–2015–0020 and RFBR Grant No. 15–03–03951a. The crystallographic study was carried out at the multiple–access center for X–ray structure analysis at the Institute of Solid State Chemistry, UB RAS (Ekaterinburg, Russia).
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