Elsevier

Optical Materials

Volume 47, September 2015, Pages 30-33
Optical Materials

Temperature dependent infrared absorption, crystal-field and intensity analysis of Ce3+ doped LiYF4

https://doi.org/10.1016/j.optmat.2015.06.046Get rights and content

Highlights

  • 10 K infrared absorption spectrum of LiYF4:Ce3+.

  • Comprehensive crystal-field analysis with intensity analysis for the intra-4f transitions.

  • Temperature dependent line broadening studies which reveal coupling to low frequency phonon modes.

Abstract

Infrared absorption has been used to determine the crystal-field levels of the 2F7/2 excited multiplet of trivalent cerium doped into scheelite structure LiYF4 single crystals. A crystal-field analysis well accounts for a total of six experimentally observed energy levels and the ground state g-values as previously determined by electron paramagnetic resonance, whilst intensity simulations confirm the experimentally assigned level symmetries. Temperature dependent spectral line broadening measurements highlight the importance of coupling to low frequency phonon modes of the YF8 tetrahedron.

Introduction

There has been much interest in trivalent cerium doped fluoride crystals, both fundamental spectroscopy (see for example, [1], [2], [3]) and with a view to applications such as the development of UV tunable lasers [4] or fast scintillators [5]. As a consequence, LiYF4:Ce3+ has been the focus of many investigations, albeit nearly exclusively optical measurements of interconfigurational transitions [6] or ground state electron paramagnetic resonance (EPR) studies [7]. However a large number of Ce3+ doped solid state gain media suffer from either strong excited state absorption (ESA) or ESA followed by the formation of pump induced photochromic centres [8] leading to poor laser performance in some cases.

LiYF4 is a tetragonal crystal of the Scheelite structure (CaWO4) having the C4h6 space group. In this material, each Li+ ion is positioned at the centre of a tetrahedron of F ions and the Y3+ ions are each surrounded by eight F ions forming a tetragonal dodecaheron yielding D2d point group symmetry. However, owing to a modest 2.3° distortion of the F cage the local Y3+ symmetry is in fact reduced to S4.

In this work, we focus on the much less studied intra-4f transitions occurring between the spin–orbit split 2F5/2 and 2F7/2 multiplets in the infrared region of the spectrum.

Section snippets

Experimental details

Large single crystals of LiYF4 doped with trivalent cerium were grown using the vertical Bridgman–Stockbarger technique [9], [10]. The furnace growth chamber was filled with greater than 1 atm pressure of highly purified argon gas to minimise evaporative losses. LiYF4 incongruently melts at a temperature of 819 °C with a composition of 49 mol% YF3 and 51 mol% LiF. The crystal growth was unseeded in graphite crucibles and the as grown boules were unoriented.

Infrared absorption spectra were recorded

Infrared absorption spectra

Fig. 1 shows the 10 K infrared absorption spectrum of a 4.3 mm thick sample of LiYF4 doped with 0.5% Ce3+. Transitions to the 2F7/2 multiplet are observable in the 2000–3000 cm−1 region due to the spin–orbit splitting of the degenerate 4f1 configuration superimposed upon the crystal-field splitting of the excited multiplet. Under S4 point group symmetry the J=7/2 multiplet is expected to break up into two states of γ5,6 symmetry and another two states of γ7,8 symmetry. As such, we label the

Conclusions

Combining infrared absorption as well as previously measured high resolution inter-configurational emission and EPR data, we have performed a crystal-field analysis for the 4f levels of Ce3+ in LiYF4. Excellent agreement is obtained for fits to the six experimentally determined energy levels as well as the ground state g-values, with the resultant crystal-field parameters in line with those for other lanthanide ions in this host crystal. An intensity analysis has been employed to confirm the

Acknowledgements

This work was supported by the Marsden Fund of the Royal Society of New Zealand under research contract 09-UOC-080. S.P.H. acknowledges the support of the University of Canterbury through the award of a postgraduate scholarship.

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