One Ion, Many Facets: Efficient, Structurally and Thermally Sensitive Luminescence of Eu²⁺ in Binary and Ternary Strontium Borohydride Chlorides

Abstract

The Eu 2+ -doped mixed alkaline metal strontium borohydride chlorides ASr(BH4)3-xClx (A = K, Rb, Cs) and Eu 2+ -doped strontium borohydride chloride Sr(BH4)2-xClx have been prepared by mechanochemical synthesis. Intense blue photoluminescence for Sr(BH4)2-xClx (λem= 457 nm) and cyan photoluminescence for the perovskite-type mixed alkaline metal strontium borohydride chlorides ASr(BH4)3-xClx (A = K, Rb, Cs) (λem = 490 nm) is already observable after short milling times. Temperature dependent luminescence measurements reveal an appreciable blue shift with increasing temperature for all ASr(BH4)3-xClx (A = K, Rb, Cs) until 500 K. This extremely large shift, caused by structural relaxation, as well as the vibrationally induced emission band broadening can serve as a sensitive response signal for temperature sensing, and this unique behavior has, to the best of our knowledge, not been reported in any Eu 2+ doped phosphor so far. Additionally, bright luminescence, high quantum efficiencies, and very low thermal quenching of these Eu 2+ -doped borohydrides show that such host materials could serve in solid state lighting applications. xClx:Eu 2+ (A = K, Rb, Cs) for the first time. All discussed ternary compounds show efficient cyan emission at room temperature as well as a strongly temperature-dependent shift of the emission energies. This unusually strong response of the luminescence properties of Eu 2+ with Chemistry of Materials 2019, 31, 8957-8968 https://doi.org/10.1021/acs.chemmater.9b03048regard to temperature changes may be interesting for potential applications in the field of thermometry. Experimental sectionSynthesis. Due to moisture and air sensitivity all compounds were handled in an argon-filled glove box. The mixed metal borohydrides ASr(BH4)3-xClx:Eu 2+ (A = K, Rb, Cs) were prepared via mechanochemical reaction of LiBH4 (Alfa Aesar, 95%), SrCl2 (Alfa Aesar, 99.995%), the corresponding alkaline borohydrides (KBH4, Alfa Aesar 98%; RbBH4 and CsBH4 prepared from Rb or Cs metal and NaBH4 in ethanol according to the procedure described in Ref 34) as well as EuH2 (2 mol% doping; synthesized via hydrogenation of europium metal in an autoclave made from hydrogen-resistant Böhler L718 alloy) in a Fritsch Pulverisette Premium Line 7 in a ZrO2 bowl with an overpressure valve. Sr(BH4)2-xClx:Eu 2+ was prepared via mechanochemical reaction of stoichiometric amounts of SrCl2, LiBH4 and 2 mol% EuH2. 10 ZrO2 balls with a diameter of 10 mm are used together with approximately 0.3 g of reactants. Caution! Overpressure may build up during milling via decomposition of the borohydrides. Reactants were milled during 19 cycles with 2 minutes of milling at 600 rpm followed by 3 minutes pauses to prevent heating of the samples. To increase crystallinity, the samples were annealed at 200 °C in fused silica ampoules for 2 days. Powder x-ray diffraction (XRD). For powder XRD analysis, a STOE STADI P diffractometer (Cu-Kα1 radiation, λ = 1.54051 Å (determined from a silicon standard)), Ge-(111)monochromator) including a DECTRIS Mythen...

A Theoretical Framework for Ratiometric Single Ion Luminescent Thermometers—Thermodynamic and Kinetic Guidelines for Optimized Performance

“…[174][175][176][177][178] Also anomalous thermal quenching effects often found in nanocrystals was demonstrated to be a successful strategy to achieve high relative sensitivities. [118,179,180] For the sake of completion, it is also noteworthy that divalent lanthanides such as Sm 2+ , [181][182][183] Eu 2+ , [184] Tm 2+ [185][186][187][188] and especially Yb 2+ [189][190][191][192] show potential for luminescence thermometry, although their thermal behavior is often not governed by simple Boltzmann statistics anymore, but more complex temperaturedependent excited state dynamics involving a crossover to the 4f n-1 5d 1 configuration. [193,194] Unlike the chemically more challenging examples of divalent lanthanides, Pr 3+ has also recently been introduced as an alternative, more stable luminescent thermometer with the same underlying principle of a 4f 2 -4f 1 5d 1 crossover (with eventual incorporation of thermal ionization into the conduction band).…”

MCaHxF3−x (M = Rb, Cs): Synthesis, Structure, and Bright, Site‐Sensitive Tunable Eu2+ Luminescence

“…A blueshift in the emission of Eu 2+ or Ce 3+ with increasing temperature may have several reasons, among others the increase of the activator–ligand distance and the consequent decrease of the crystal field strength and covalency of the ligand‐activator bond. [ 27,52 ] The observed blueshift of the main emission maximum from Figure 6b (see Figure S8 in the Supporting Information) is ≈370 cm −1 , larger than thermochromic blueshifts of the 4f 6 5d 1 –4f 7 emission of Eu 2+ in other halide hosts, but still significantly smaller than the blueshifts reported for pure borohydride systems. [ 52,53 ]…”

“…[27,52] The observed blueshift of the main emission maximum from Figure 6b (see Figure S8 in the Supporting Information) is ≈370 cm −1 , larger than thermochromic blueshifts of the 4f 6 5d 1 -4f 7 emission of Eu 2+ in other halide hosts, but still significantly smaller than the blueshifts reported for pure borohydride systems. [52,53] The FWHM for the most prominent emission at 647 nm is about 28 nm at 4.2 K and increases till 35 nm at 300 K with the peak position shifting from 647 to 635 nm. This is due to temperature-induced effects, such as site expansion, which leads to a reduced crystal field strength, and an increase in bond length variations.…”

Thermal quenching properties of narrow-band blue-emitting MBe2(PO4)2:Eu2+(M = Ca, Sr) phosphors towards backlight display applications

“…As temperature increases, emission bands gradually broaden and a slight blue shift is observed, and the mechanism of this variation is discussed below based on previous reports. 35 One important cause is thermally induced lattice expansion, which increases the activator-ligand distance, and thus results in a smaller crystal field splitting and decreasing covalency of the Eu-O bond. Furthermore, thermally induced population of higher vibrational states may lead to a higher emission energy with increasing temperature due to the enhanced electron-phonon strength.…”