Abstract
Optical, spectroscopic and thermo-mechanical properties of monoclinic Li6Eu1-xGdx(BO3)3 (x = 0,0.25,0.35) bulk single crystals, grown to be used in the design of heat-scintillation cryogenic bolometers (HSCBs), were investigated. The linear thermal expansion was determined along the a, b, c and c* directions over the temperature range 303–873 K, and its tensor principal coefficients were calculated for both x = 0.25 and x = 0. In addition, the anisotropic thermal conductivity was measured over the temperature range 20-400 K in Li6Eu0.75Gd0.25(BO3)3 (LGEB7, x = 0.25), and the principal components of its tensor at 300 K were established. Spectroscopic properties such as polarized absorption, polarized emission, Raman spectroscopy and optical refractive indices are also reported for the first time. Based on the polarized emission spectra, the line and oscillator strengths, the radiative lifetimes and fluorescent branching ratios were obtained. The intensity parameters Ωλ (λ = 2,4) were obtained and then predicted via the Judd–Ofelt theory. The crystal field parameters and the 7F1 level splitting were discussed using the simple overlap model (SOM) and the method of equivalent nearest neighbours (MENN). The 613 nm emission originates from the 5D0→7F2 transition and the associated stimulated emission peak cross section reaches its maximum value in π-polarization, ≈1.07 × 10−20 cm2 in LGEB7. The thermo-mechanical characterizations and spectroscopic analysis in LGEB7 suggest that this crystal has better optical properties than the Eu3+-doped Li6Y(BO3)3 crystals. However, its potential for solid-state laser applications is still quite speculative, unless a clever thermo-mechanical management of the crystal under laser operation is designed.
Highlights
Because of the threefold or fourfold coordination of boron atoms in oxide compounds, borate crystals display a huge variety of structural types, which gives the possibility to select in this family of compounds appropriate materials for chosen applications
An emerging frontier research field where these crystals could be successfully employed is the study of solar axions, hypothetical particles providing an elegant solution of the so-called “strong CP problem” of quantum chromodynamics (QCD) [6]
This technology, which has significantly matured since its invention in 1992 [8,9], works at ultralow temperature (≤ 25 mK) and measures an energy release in a single crystal by means of weak heat and light pulses resulting from the same event
Summary
Because of the threefold (trigonal) or fourfold (tetrahedral) coordination of boron atoms in oxide compounds, borate crystals display a huge variety of structural types, which gives the possibility to select in this family of compounds appropriate materials for chosen applications. An emerging frontier research field where these crystals could be successfully employed is the study of solar axions, hypothetical particles providing an elegant solution of the so-called “strong CP problem” of quantum chromodynamics (QCD) [6] Another major research field where the use of these crystals is currently investigated is the detection and spectroscopy of fast and extremely rare neutrons, which helps understanding the ultimate background of the underground sites dedicated to the direct detection of dark matter, but is per se a promising research area for environmental issues [1,7]. We report for the first time on a detailed study of the polarized emission cross section, JO parameters and radiative lifetime, comparing some of the results with the physical properties of an Y1.866Eu0.134O3 crystal elaborated elsewhere by the flux method [16]
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