RF2:Bi (R = Ca and Sr) phosphors were synthesized by solid state reaction method in air and their luminescence properties were investigated. Broad yellow-to-orange emissions peaking at ~550 nm (CaF2:Bi) and ~600 nm (SrF2:Bi) were observed under ~260 nm excitation. The emission centers inRF2:Bi (R = Ca and Sr) phosphors are Bi2+ ions, and the excitation and emission bands ofRF2:Bi (R = Ca and Sr) phosphors can be attributed to 2P1/2→2S1/2 and 2P3/2(1) →2P1/2 transitions of Bi2+ ions, respectively. The phosphors are promising for application in lighting due to broad yellow-to- orange emission.
©2013 Optical Society of America
Lanthanide ions activated luminescent materials as light sources show sharp emission peak, narrow and discontinuous band emissions due to intra-configutational 4f-4f transitions . These luminescence properties may affect their applications in some special fields, such as optical telecommunication etc. Main group metal ions such as Bi activated luminescent materials showing broad emission may improve those properties [2, 3], and they are also cheap and environment friendly. In the past decades, it has been confirmed that Bi ion doped luminescent materials exhibit wonder luminescent properties due to the large number of valence states and strong interaction with the surrounding lattice, and have many potential applications in lighting, tunable laser, optical telecommunication and biomedicine etc [4–12].For instance, (i) Bi3+ion doped luminescent materials emitting near ultraviolet (n-UV) or blue light and can be used in light sources etc . (ii) Bi2+ ion doped luminescent materials emitting orange-red light have potential application in white light-emitting diodes (W-LEDs) etc [14, 15]. (iii) Bi+ ion or Bi0 doped luminescent materials emitting broadband near infrared (NIR) luminescence in the range from 1000 to 1600nm have potential applications in tunable laser and broadband optical amplifier etc . (iv) Bi53+ cluster emitting broadband near to mid infrared luminescence in the range from 1000 to 3000nm have potential applications in laser and biomedicine etc. [17–19].
RF2 (R = Ca, Sr and Ba) fluorides are chemical and thermal stable and usually used as laser matrices due to their low phonon energy and strong ionicity etc. comparing with corresponding oxides etc. BaF2:Bi crystal prepared in reductive atmosphere has been reported to possess broadband NIR emission in the range from 950 to 1650nm . However, there have been few investigations on RF2:Bi (R = Ca and Sr) phosphors.
In the paper, RF2:Bi (R = Ca and Sr) phosphors were synthesized by solid state reaction method in air. Crystal phases and luminescent properties were characterized by X-ray diffractometer (XRD) and FLS920 spectrofluorimeter, respectively. Broad yellow-to-orange emissions were observed from RF2:Bi (R = Ca and Sr) phosphors. The origins of the emissions were discussed, and the effect of matrix on the emission was also examined.
All the chemicals purchased from the Aladdin Chemical Reagent Company were used as raw materials without further purification, such as CaF2 (A.R. 99.9%), SrF2 (A.R. 99.9%), Bi2O3 (99.99%).RF2:Bi (R = Ca and Sr) phosphors were synthesized by solid-state reaction method in air. The stoichiometric amount of raw materials were weighed according to nominal compositions R(1-x)BixF2 (R = Ca and Sr) (x = 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0 mol%) phosphors and mixed thoroughly, then filled into alumina crucibles and sintered at 400°C for 2h, well grounded again, subsequently sintered at 1000°C in air for 2h.In order to obtain the optimal synthesis conditions,R0.95Bi0.05F2 (R = Ca and Sr) phosphors were sintered in the 850 to 1200°C region in air for different times. To analyze the origins of luminescence center, R0.95Bi0.05F2 (R = Ca and Sr) phosphors were also sintered in reducing atmosphere (5%H2 and 95%N2 gas mixture) at 1000°C for 30min for comparison .
The crystal structures of RF2:Bi (R = Ca and Sr)phosphors were analyzed by using X-ray diffractometer (XRD) (Philips Model PW1830) with Cu-Kα radiation at 40 kV and 40 mA. The XRD data were collected in the range 2θ = 5 - 90° in 0.02° steps at room temperature. Luminescence properties of RF2:Bi (R = Ca and Sr) phosphors were characterized using FLS920 spectrofluorimeter (Edinburgh Instrument).
3. Results and discussion
The XRD patterns of R(1-x)BixF2 (R = Ca and Sr) (x = 0 and 5 mol%)sintered at 1000°C in air for 2h, JCPDS card no. 75-363 (CaF2) and 86-2418 (SrF2) are shown in Fig. 1. The positions of diffraction peaks of R(1-x)BixF2 (R = Ca and Sr) (x = 0 and 5 mol%)are well matched with those in the XRD patterns of JCPDS card no. 75-363 (CaF2) and 86-2418 (SrF2), respectively. The XRD patterns of other R(1-x)BixF2 (R = Ca and Sr) (x = 0.5 ~6.0 mol%) phosphors are not displayed in the figure, but those patterns are also in line with those of JCPDS card no.75-363 (CaF2) and 86-2418 (SrF2), suggesting that the doping of Bi ions does not cause significant changes of RF2 (R = Ca and Sr) crystalline phases structure. The crystal structure of R(1-x)BixF2 (R = Ca and Sr) (x = 0.5 ~6.0 mol%) phosphors belongs to cubic structure and their space group Fm-3m (no. 225).
Figure 2 shows photoluminescence excitation (PLE), photoluminescence (PL) and pictures of R0.95Bi0.05F2 (R = Ca and Sr) phosphors intered at 1000°C in air and reducing atmosphere. The excitation wavelength locates at ~260 nm. CaF2:Bi phosphor shows an emission peaking at ~550 nm in the range of 380 to 800 nm, and SrF2:Bi phosphor shows an emission peaking at ~600 nm in the range of 400 to 810 nm. The emission intensity of R0.95Bi0.05F2 (R = Ca and Sr) phosphors intered in reducing atmosphere is weaker than those sintered in air. R0.95Bi0.05F2 (R = Ca and Sr) phosphors intered in air are light yellow, while those sintered in reducing atmosphere are light black due to the reduction of Bi3+ ion to Bi atom. For comparison, we also prepared the sample using BiF3 as raw material and sintered under N2 atmosphere and found that the sample had the same luminescent behavior as those using Bi2O3 .For all the samples, typical blue emission due to Bi3+ ion  and NIR emission due to Bi+ ion  or Bi0  were not observed.
Table 1 summarizes the spectroscopic data of Bi2+-doped phosphors reported in literatures [11, 14, 15, 22–24]. The electronic configuration of Bi2+ is [Xe]4f145d106s26p1, and the ground configuration (6p1) is split by spin-orbit coupling interaction into 2P1/2 ground state and 2P3/2excited state. The excited state 2P3/2 is also split into two sublevels 2P3/2(1) and 2P3/2(2) by crystal field, and those states derive from 6s26p1, while the 2S1/2 state mainly derives from 6s27s1. 2P1/2→2S1/2 transition is strongly allowed, and 2P1/2→2P3/2transition is usually forbidden but gains intensity due to mixing in of the higher even parity states by the odd-parity crystal-field components. As shown in the Table 1, the excitation in the range of 230 to 290 nm is due to2P1/2→2S1/2transition and two excitations in the range of 380 to 650 nm due to 2P1/2→2P3/2(2) and 2P1/2→2P3/2(1) transition can be separated from each other in Bi2+-doped phosphors. Accordingly, the emission due to2P3/2(1)→2P1/2 transition appears in the range of 550 to 716 nm. The excitation of2P1/2→2S1/2transition in RF2:Bi (R = Ca and Sr) phosphors is only observed due to 2P1/2→2P3/2transition forbidden, and the emissions are attributed to the 2P3/2(1) →2P1/2 transition of Bi2+ ions substituting Ca2+ and Sr2+ ions in RF2 (R = Ca and Sr) host since Bi2+, Ca2+ and Sr2+ ions have the similar ionic radius (rBi2+ = 1.14Ǻ, rCa2+ = 0.99Ǻ and rSr2+ = 1.13Ǻ) and same charge .
Fluorescence decay curves of R0.95Bi0.05F2 (R = Ca and Sr) phosphors are shown in Fig. 3.The monitoring wavelengths are 550 nm (Ca0.95Bi0.05F2) and 600 nm (Sr0.95Bi0.05F2) with~260 nm excitation. The luminescence decay curves are well fitted by a first-order exponential function  and fluorescence lifetimes of Ca0.95Bi0.05F2and Sr0.95Bi0.05F2 phosphors are ~17.4 and 34 μs, respectively.
Figure 4(A) shows time resolved luminescence spectra of R0.95Bi0.05F2 (R = Ca and Sr) phosphors with excitation at ~260nm at different delay time, respectively. The emission intensity monotonically decreases, and the shape of emissions remains unchanged with increasing delay time. Combining with data in Fig. 3, these results indicate that it is only a single type of Bi2+ ion luminescent center in RF2:Bi (R = Ca and Sr) phosphors. PL of R0.95Bi0.05F2 (R = Ca and Sr) phosphors in the range of 10 to 300K and influences of the temperature on the emission intensity and fluorescence lifetime of R0.95Bi0.05F2 (R = Ca and Sr) phosphors are shown in Fig. 4(B). The shape of the PL spectra of R0.95Bi0.05F2 (R = Ca and Sr) phosphors remains almost unchanged in the range of 10 to 300K, and their emission intensity and fluorescence lifetime increase at first with increasing temperature, and then decrease with further increasing temperature.
To understand the observed influences of the temperature on the emission intensity and fluorescence lifetime of RF2:Bi(R = Ca and Sr)phosphors, a schematic configurational coordinate diagram with emission and excitation transitions inBi2+ ion is shown in Fig. 5. Electrons are only lifted from the ground state “GS” to the excited state “ES3”due to 2P1/2→2P3/2transition forbidden. After these electrons reach “ES3”, they seek to return to the equilibrium state A at the bottom of the potential well of “ES3”. These excited electrons which lands in A may return the point of D via path 1 and 2, then transit to O in “GS” and emitting visible photons. When the temperature rises, thermal excitation may lead to more electrons in state A via B return to D, more electrons will finally transit to O in “GS”, then both the lifetime and emission intensity increase. Further increasing temperature will push some electrons at state D predominantly towards path 3, where more energy is lost nonradiatively via the crossing point of E to O. Then both the lifetime and emission intensity decrease.
In summary, RF2:Bi(R = Ca and Sr) phosphors have been synthesized by solid state reaction method in air. Broad yellow-to-orange emissions peaking at ~550 nm (CaF2:Bi) and ~600 nm (SrF2:Bi) were observed under ~260 nm excitation at room temperature. Bi2+ ions as the only luminescent center substitute for the Ca2+ and Sr2+ ions in the RF2(R = Ca and Sr) host. The excitation and emission bands ofRF2:Bi (R = Ca and Sr) phosphors can be assigned to 2P1/2→2S1/2 and 2P3/2(1) →2P1/2 transitions of Bi2+ ions, respectively. The influences of the temperature on the fluorescence lifetime and emission intensity of RF2:Bi(R = Ca and Sr)phosphors can be interpreted with the help of the configurational coordinate diagram of Bi2+ ions. These phosphors may be promising for application in lighting due to broad yellow-to-orange emission.
This work was financially supported by the National Natural Science Foundation of China (Grants no. 51072054, 51072060, 51132004), Guangdong Natural Science Foundation (Grant no. S2011030001349).
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