Journal of Luminescence 175 (2016) 260–266Contents lists available at ScienceDirectJournal of Luminescencehttp://d
0022-23
n Corr
E-mjournal homepage: www.elsevier.com/locate/jluminFull Length ArticleStokes and anti‐Stokes luminescence from cubic elpasolite Cs2NaYF6
crystals doped with Er3þand Yb3þ ions
P.A. Loiko a,n, N.M. Khaidukov b, J. Méndez-Ramos c, E.V. Vilejshikova a, N.A. Skoptsov a,
K.V. Yumashev a
a Center for Optical Materials and Technologies (COMT), Belarusian National Technical University, 65/17 Nezavisimosti Ave., Minsk 220013, Belarus
b N.S. Kurnakov Institute of General and Inorganic Chemistry, 31 Leninskii Prospekt, Moscow 119991, Russia
c Departamento de Física, Universidad de La Laguna, Tenerife, 38206 La Laguna, Spaina r t i c l e i n f o
Article history:
Received 29 October 2015
Received in revised form
31 January 2016
Accepted 22 February 2016
Available online 15 March 2016
Keywords:
Elpasolite crystals
Erbium
Ytterbium
Luminescence
Up-conversionx.doi.org/10.1016/j.jlumin.2016.02.025
13/& 2016 Elsevier B.V. All rights reserved.
esponding author.
ail address: kinetic@tut.by (P.A. Loiko).a b s t r a c t
Er3þ and Yb3þ doped cubic elpasolite Cs2NaYF6 crystals including stoichiometric compositions
Cs2NaErF6 and Cs2NaYbF6 have been synthesized under hydrothermal conditions. Absorption,
stimulated-emission and gain cross-sections spectra have been determined for the 2F5/2-
2F7/2 (Yb
3þ)
and 4I13/2-4I15/2 (Er3þ) transitions at room-temperature. The maximum σSE values are 1.81021 cm2 at
993 nm (Yb3þ) and 3.81021 cm2 at 1535 nm (Er3þ). Elpasolite crystals provide exceptionally long
radiative lifetimes of the excited-states for both ions, namely τ(2F5/2)¼6.3 ms and τ(4I13/2) 32 ms for
10 at% Yb3þ:Cs2NaYF6 and 10 at% Er3þ:Cs2NaYF6 which can be used in high pulse energy Q-switched
lasers. Up-conversion luminescence has been studied for Er3þ doped and Er3þ , Yb3þ codoped Cs2NaYF6
crystals.
& 2016 Elsevier B.V. All rights reserved.1. Introduction
Halide compounds having the elpasolite structure can be
described by a general formula A2BMX6 where A and B are
monovalent alkali ions provided that the ionic radius of A is
greater than that of B, M stands for trivalent metal ions, which can
be Al, Sc, transition elements and rare earth elements, and X is a
halogen [1]. The elpasolite compounds crystallize in the face-
centered cubic space group Fm3m [2] and can be described as
double perovskites. In the structure the A alkali cations are located
in the cuboctahedral sites whereas the B alkali cations and the M
trivalent metal ions regularly alternate along the unit cell axes in
octahedral sites. Accordingly, the MX6 octahedra are perfectly
cubic with Oh site symmetry for trivalent metal ions, while the
MX6 octahedra do not share halide ions with each other, i.e. the
elpasolite structure is characterized by the isolated MX6 octahedra.
In addition, as all the halide compounds, elpasolites are materials
with low-phonon energy.
The above mentioned peculiarities of elpasolite hosts lead to a
significant reduction of the multiphonon relaxation rates for
optically active rare earth (RE) ion dopants, which allows for an
increased lifetime of some excited levels that can relax radiativelyor can store energy for further up-conversion and cross-relaxation
as well as provide relatively low concentration quenching of
luminescence, which allows obtaining efficient luminescence in
A2BMX6 containing high trivalent RE ion concentrations up to
100 at%.
To date, some spectroscopy data for several RE ions (Er3þ ,
Yb3þ , Tm3þ , Eu3þ) doped into the Cs2NaYF6 compound have been
published; however, characterization of these materials is far from
completeness mainly due to the difficulty of synthesizing optical
quality crystals of elpasolites in general and Cs2NaYF6 in particular.
For example, VUV 5d–4f luminescence of Er3þ ions, including a
stoichiometric Cs2NaErF6 compound, has been studied followed by
crystal field calculations for Er3þ ions [3,4]. Near-IR (2.7 μm)
emission of Er3þ ions in Cs2NaYF6 has been also investigated and
the lifetime of the 4I11/2 excited-state has been detected to be as
long as 1 ms at room-temperature for highly-doped (410 at%)
samples [5]. Absorption and emission as well as crystal field
analysis for Yb3þ ions in Cs2NaYbF6 has been reported [6–8]. In
particular, very long lifetime of the Yb3þ excited-state, τ(2F5/2)¼
5.63 ms, at room-temperature is observed for Cs2NaYbF6. Some
spectroscopic peculiarities of Tm3þ ions in Cs2NaYF6 have been
also studied [3,6,9] and crystal field calculations for a Tm:Cs2NaYF6
crystal have been performed [10]. The spectroscopic properties of
Eu3þ and Tb3þ ions doped into Cs2NaYF6 have been investigated
as well [11–13].
P.A. Loiko et al. / Journal of Luminescence 175 (2016) 260–266 261The fluoride cubic elpasolite Cs2NaYF6 is isostructural with the
chloride elpasolite Cs2NaYCl6, that has been extensively studied as
an low phonon energy host for optically active ions of 3d, 4f and 5f
elements, for example Er3þ [14,15]. In particular, Cs2NaYCl6 crystals
doubly doped with Er3þ and Yb3þ have been studied as the near-
IR-to-visible frequency converters [16,17] based on photon upcon-
version processes [18]. As a result of upconversion mechanisms,
Er3þ–Yb3þ ion pairs can convert near-IR (1 μm) emission of
InGaAs laser diodes to green and red light. On the other hand,
Cs2NaYF6 is harder and less moisture-sensitive than Cs2NaYCl6.
Thus, the study of visible emissions in Er3þ and Yb3þ ions doped
Cs2NaYF6 crystals is of practical interest for developing photon
upconverters. High efficiency of up-conversion in the cubic elpa-
solite Cs2NaYF6 can be achieved due to the low value of the max-
imum phonon frequency which is 468 cm1 [11]. This value is
higher than that for Cs2NaYCl6 (285 cm1) [19] but it is acceptable
for energy storage on excited levels. In addition, long lifetimes of
the first excited-states of Er3þ and Yb3þ ions observed for highly-
doped Cs2NaYF6 crystals indicate their potential for generation of
high pulse energies in passively Q-switched near-IR lasers.
In the present paper, the data on optical absorption, near-IR
and visible f–f emissions of Cs2NaYF6 crystals doped with Er3þ and
Yb3þ ions as well as Cs2NaErF6 and Cs2NaYbF6 are reported.Fig. 1. X-ray diffraction patterns for Cs2NaYF6, Cs2NaErF6 and Cs2NaYbF6 crystals;
numbers on the graph represent the Miller indices, (hkl).
Table 1
Lattice parameter (а), unit cell volume (V), calculated density (ρ) and absolute
concentration of rare earth ions (N) for cubic elpasolites (space group: Fm3m, Z¼4).
Composition а, Å V, Å3 ρ, g/cm3 N, at/cm3
Cs2NaYF6 9.057 742.9 4.42 5.381021
Cs2NaErF6 9.045 740.0 5.15 5.411021
Cs2NaYbF6 9.016 732.9 5.24 5.4610212. Experimental
Crystals of cubic elpasolite Cs2NaYF6 doped with Er3þ and
Yb3þ ions as well as Cs2NaErF6 and Cs2NaYbF6 were grown under
hydrothermal conditions. For hydrothermal experiments, copper-
insert lined autoclaves with a volume of 40 cm3 were utilized,
and the inserts were separated by perforated diaphragms into
synthesis and crystallization zones. The fluoride crystals were
synthesized by a direct temperature-gradient method as a result of
the reaction of the aqueous solutions containing 35–40 mol% CsF
and 8–10 mol% NaF with oxide mixtures (1x)Y2O3– xLn2O3
(Ln¼Er and Yb) at a temperature of 750 K in the synthesis zone,
a temperature gradient along the reactor body of up to 3 K/cm, and
a pressure of 100 MPa. Under these conditions, spontaneously
nucleated crystals of up to 0.5 cm3 were grown in the upper
crystallization zone of the autoclave for 200 h. The purities of the
utilized rare earth oxides were 99.99%.
The structure type and phase purity of synthesized samples
were characterized with conventional powder X-ray diffraction
(XRD) technique and powder XRD patterns were obtained by using
a Bruker D8 Advance X-Ray powder diffractometer with Cu Kα
radiation.
Absorption spectrum was measured for 10 at% Er3þ and 10 at%
Yb3þ doped Cs2NaYF6 crystals with a Varian CARY 5000 spectro-
photometer (the spectral bandwidth, SBW, was 0.1 nm). Thin
polished platelets (1 mm) of crystals were used.
Luminescence spectra were measured with a lock-in amplifier,
a monochromator MDR-23 (SBW 0.1 nm) and Hamamatsu
G5851 and C5460-01 photodetectors. The spectral sensitivity of
the set-up was accurately determined with a halogen lamp with
calibrated spectral power density. Near-IR luminescence and up-
conversion luminescence (UCL) of Er3þ and Yb3þ ions was excited
by InGaAs laser diode emitting at 960 nm (excitation to the
Er3þ4I11/2 state or Yb3þ2F5/2 state). Excitation light was focused on
a sample in a 100 μm spot; the maximum power density was
40 kW/cm2. Luminescence was also excited at 355 and
520 nm (excitation to the 2G9/2 and 2H11/2 states of Er3þ ,
respectively) and a ns optical parametric oscillator (OPO) Lotis TII
LT-2214 was used. CIE chromaticity coordinates of phosphors were
calculated by using the photoluminescence data.For the studies of luminescence decay of Yb3þ ions, OPO with
the pulse duration of 20 ns was tuned to 960 nm. Luminescence
was collected by a wide-aperture lens and re-imaged to the input
slit of a monochromator MDR-12 (tuned to 1020 nm, SBW
1 nm); then it was detected with a fast Hamamatsu G5851
photodetector (response time,o100 ns) and a 500 MHz Textronix
TDS-3052B digital oscilloscope. To avoid the reabsorption loss,
studied crystals were finely powdered and immersed in glycerin
(5 wt% powder content).
All spectroscopic studies were performed at room temperature.3. Results and discussion
The X-ray phase analysis has confirmed that all the synthesized
compounds are single-phase samples containing only the cubic
elpasolite phases, space group Fm3m, Z¼4 (ICDD PDF Cards: 00-
020-1214; 00-021-0221; 00-021-0224) [1]. X-ray diffraction pat-
terns of Cs2NaYF6, Cs2NaErF6 and Cs2NaYbF6 are showed in Fig. 1
and they can be indexed with a cubic cell with the lattice para-
meter, a¼9.057 Å, 9.045 Å and 9.016 Å, respectively. Lattice para-
meter (a), volume of the unit cell (V), density (ρ) and concentra-
tion (N) of the Y3þ , Er3þ and Yb3þ ions which have been obtained
from the X-ray diffraction studies for these compounds are sum-
marized in Table 1. One can see that there is a marginal change in
cell parameters due to differences in rare earth ionic radii and it
should be also noted that the unite cell parameters determined in
this study are notably less than those tabulated in [1].
An absorption spectrum measured for a 10 at% Er3þ:Cs2NaYF6
crystal is shown in Fig. 2. For the 4I15/2-4I11/2 transition, the peak
absorption cross-section is σabs¼0.071021 cm2 at 963 nm. The
full width at half maximum (FWHM) for this band is 24 nm. Such
low absorption determines low efficiency for the excitation of Er3þ
ions by using InGaAs diodes. Indeed, for the stoichiometric
Cs2NaErF6 crystal, the corresponding peak absorption coefficient is
α 0.37 cm1, which means that the absorption of diode emission
is not complete even for a few mm-thick crystal. For the 4I15/2-4I13/2
transition that is typically used for the in-band excitation of
Fig. 2. Absorption cross-section spectra for a 10 at% Er3þ:Cs2NaYF6 crystal.
Fig. 3. Near-IR emission spectra of Cs2NaErF6 (a) and Cs2NaYbF6 (b) crystals;
excitation wavelength is denoted by arrow (960 nm).
Table 2
Energies of Stark sub-levels for the ground-states and the first excited-states of
Er3þ and Yb3þ in Cs2NaYF6 crystals [4,8].
Multiplet (Yb3þ) Sub-levels, cm1 Multiplet (Er3þ) Sub-levels, cm1
2F7/2 3 4I15/2 0
324 25
656 57
259
287
2F5/2 10,389 4I13/2 6492
11,109 6517
6532
6682
6686
Fig. 4. (a) Stimulated-emission cross-sections spectra calculated with the reci-
procity and F-L methods for a 10 at% Er3þ:Cs2NaYF6 crystal, (b) gain cross-sections
[σg¼βσSE–(1–β)σabs] spectra, β¼N2/N is the inversion ratio.
P.A. Loiko et al. / Journal of Luminescence 175 (2016) 260–266262Er3þ ions, the absorption is much stronger, σabs¼4.61021 cm2 at
1532.2 nm. The FWHM of this band is 5.4 nm.
Near-IR emissions of Er3þ ions have been studied under
960 nm excitation by using a stoichiometric Cs2NaErF6 crystal. The
luminescence band related to the Er3þ4I13/2-4I15/2 transition spans
from 1.4 to 1.68 μm, see Fig. 3(a). The band in the 920–1060 nm
spectral range is attributed to the 4I11/2-4I15/2 transition of
Er3þ ions.
Emission of Er3þ ions at 1.5 μm is eye-safe and interesting for
laser operation. To calculate the corresponding stimulated-
emission cross-sections σSE spectra, two methods can be used.
The first one is the reciprocity method [20]:
σSEðλÞ ¼ σabsðλÞ
Z1
Z2
exp hc=λEZL
kT

 
; ð1Þwhere Z1 and Z2 are the lower and upper manifold partition
functions, respectively, EZL is the energy corresponding to the zero
phonon line, h is the Planck constant, c is the speed of light, λ is the
light wavelength, k is the Boltzmann constant and T is the crystal
temperature (room-temperature). Partition functions are deter-
mined as:
Zm ¼
X
k
gmk expðEmk =kTÞ; ð2Þ
where m¼1, 2; gmk is the degeneration of the sublevel having the
number k and the energy Emk measured from the lower sublevel of
the corresponding multiplet. The set of Emk values for the 4I15/2 and
4I13/2 states of Er3þ ions in the Cs2NaYF6 crystal [4] is listed in
Table 2.
Calculation of σSE with the reciprocity method is beneficial as it
does not require the information about the radiative lifetime τrad
of the emitting state as well as direct measurement of the emis-
sion spectrum which can be changed as a result of the reabsorp-
tion loss. The obtained results are shown in Fig. 4(a). The max-
imum σSE value is 3.81021 cm2 at 1532.2 nm. This value is
lower than the typical peak σSE values for the Er3þ4I13/2-4I15/2
transition in widespread oxide and fluoride laser hosts, 0.5–
11020 cm2 [21]. In the spectral range where laser operation is
expected, the maximum σSE value is 1.51021 cm2 at 1574.1 nm.
Radiative lifetime of the Er3þ emitting state (4I13/2) can be
P.A. Loiko et al. / Journal of Luminescence 175 (2016) 260–266 263estimated from the so-called modified reciprocity method (n is the
crystal refractive index) [22]:
τrad ¼
1
8πn2c
Z2
Z1
expðEZL=kTÞR
λ4σabsðλÞexpðhc=ðkTλÞÞdλ
: ð3Þ
For Er3þ:Cs2NaYF6 crystal, τrad(4I13/2) has been determined to
be 3272 ms. Previously, lifetime of this state was measured for
1 at% Er3þ:Cs2NaYF6 crystal only at cryogenic temperature (4 K), τ
(4I13/2) 100 ms [5] and this value can be considered as a radiative
one at low temperatures, as the reabsorption for such crystal
doping is nearly vanishing. Such long values are inherent for all
the elpasolite crystals [14]. In particular, τ(4I13/2)¼22 ms for a
cubic Cs2NaErCl6 crystal [23]. Independent estimation of the τrad
for Er3þ:Cs2NaYF6 crystal can be done with the Judd–Ofelt mod-
eling that will be performed in a separate study.
Using the measured emission spectrum W(λ), Fig. 3(a), and the
determined radiative lifetime τrad(4I13/2), it is possible to calculate
a stimulated-emission cross-sections spectrum for the 4I11/2-4I15/2
transition with the Füchtbauer–Ladenburg (F–L) equation [24]:
σSEðλÞ ¼
λ5
8πn2τradc
WðλÞR
λWðλÞdλ: ð4Þ
The obtained results are shown in Fig. 4(a) and they are in good
agreement with the ones from the reciprocity method, considering
strong reabsorption at short wavelengths.
For assessing the prospects of elpasolite crystals to lasers, a
useful parameter is the gain cross-section, σg:
σgðλÞ ¼ βσSEðλÞð1βÞσabsðλÞ; ð5Þ
where β is the inversion ratio, β¼N2/N0 where N2 and N0 are the
numbers of ions in the upper laser level and the overall number of
ions, respectively. Gain spectra for a 10 at% Er3þ:Cs2NaYF6 crystal
are shown in Fig. 4(b). For these calculations, we used stimulated-
emission cross-sections spectra obtained with the reciprocity
method. For low inversion ratios (βo0.4), laser operation is
expected at several wavelength, 1624, 1658 or 1694 nm. For high
inversion ratios, the peak centered at 1574 nm dominates in the
spectrum.
An absorption spectrum measured for a 10 at% Yb3þ:Cs2NaYF6
crystal is shown in Fig. 5(a) representing the 2F5/2-2F7/2 transition of
Yb3þ ions. The peak absorption cross-section is σabs¼1.31021 cm2Fig. 5. (a) Absorption and stimulated-emission cross-sections spectra for a 10 at%
Yb3þ:Cs2NaYF6 crystal, (b) gain cross-sections [σg¼βσSE–(1–β)σabs] spectra, β¼N2/N
is the inversion ratio.at 962.0 nm. Thus, Cs2NaYF6 crystals containing both Er3þ and Yb3þ
ions provide significant improvement of the excitation efficiency
under using InGaAs laser diodes. In particular, the peak absorption
coefficient for the stoichiometric Cs2NaYbF6 crystal, α¼6.9 cm1 and
FWHM for this band is 5.2 nm.
The stimulated-emission cross-sections spectrum for a 10 at%
Yb3þ: Cs2NaYF6 crystal has been calculated with the reciprocity
method, Eq. (1). The spectrum is shown in Fig. 5(a) and the
energies of Stark sub-levels for the 2F5/2 and 2F7/2 states of Yb3þ
ions used for calculations are listed in Table 2 [8]. The maximum
σSE value is 1.81021 cm2 at 992.4 nm. Emission of Yb3þ ions
spans from 920 to 1080 nm. Thus, the tuning range wider than
100 nm is expected under using this crystal as a laser gain med-
ium. This is in agreement with the near-IR emission spectrum of
the Cs2NaYbF6 crystal measured directly under 960 nm excitation
and shown in Fig. 3(b).
The radiative lifetime of the 2F5/2 state of Yb3þ ions in the
Cs2NaYF6 crystal determined from the modified reciprocity
method, Eq. (3), is τrad(4I13/2)¼6.4 ms. This value is in close
agreement with the decay time of Yb3þ emission measured
directly for a powdered 10 at% Yb3þ:Cs2NaYF6 composition,
τ¼6.3 ms. The corresponding decay curve is shown in Fig. 6 and it
is clearly single-exponential. For a stoichiometric Cs2NaYbF6
crystal, similar measurement yields τ¼5.7 ms, which is close to
the previously reported value [6], τ¼5.63 ms. Thus, the anom-
alously weak concentration quenching is observed for Yb3þ:
Cs2NaYF6 crystals. Determined lifetimes are at least two times
longer than those for typical Yb3þ-doped fluoride laser hosts with
τ ranging typically from 1.5 to 3 ms. This indicates the potential of
highly-doped Yb3þ:Cs2NaYF6 crystals (up to 100 at%) for genera-
tion of high pulse energies in the Q-switched operation mode.
Gain cross-sections spectra for a 10 at% Yb3þ:Cs2NaYF6 crystal are
shown in Fig. 5(b). For low inversion ratios (βo0.1), the gain spec-
trum is flat and it spans from 1000 to 1080 nm, so the multi-peak
spectral behavior is expected. For high inversion, two intense local
peaks centered at 992 nm and 1009 nm are observed. This indicates
the possibility of dual-wavelength laser operation for this crystal.
The lifetimes of the Yb3þ2F5/2 excited-state and the 4I13/2 and
4I11/2 excited-states for Er3þ ions are presented in Table 3. In this
context, it is worth mentioning that even for a stoichiometric
Cs2NaErF6 crystal, the value of τ(4I11/2) is as long as 0.7 ms at room-
temperature [5], which indicates that it can be promising medium
for 3 μm laser operation on the 4I11/2-4I13/2 transition [25].
UCL spectra of Er3þ:Cs2NaYF6 crystals under the excitation at
960 nm to the Er3þ4I11/2 state are shown in Fig. 7(a). For a 10 at%
Er3þdoped crystal, the green emission band spanning from 510 to
580 nm and related to the transitions from the closely located and
thermalized state 2H11/2 and 4S3/2 to the ground state 4I15/2Fig. 6. Luminescence decay curve for a 10 at% Yb3þ:Cs2NaYF6 crystal:
λexc¼960 nm; luminescence is detected at 1020 nm.
Fig. 7. Up-conversion luminescence (UCL) spectra for (a) 10 at% Er3þ:Cs2NaYF6 and
Cs2NaErF6, (b) 1 at% Er3þ , 10 at% Yb3þ:Cs2NaYF6 and (c) Cs2NaYbF6 crystals;
λexc¼960 nm; insets show images of the excited samples (a), (b) and details of blue
emission from the Cs2NaYbF6 crystal (c): 1-measured UCL spectrum, 2-calculated
spectrum of cooperative emission, Eq. (6).
Table 4
CIE 1931 color coordinates x, y, dominant wavelength λd and color purity p for
luminescence from Er3þdoped elpasolite crystals.
Crystal x y λd, nm p Color
Exc. 960 nm
10 at% Er3þ:Cs2NaYF6 0.385 0.603 562 98% Yellow–green
Cs2NaErF6 0.343 0.656 556 99% Yellowish–green
1 at% Er3þ , 10 at% Yb3þ:
Cs2NaYF6
0.340 0.660 555 99% Yellowish–green
Exc. 355 nm
10 at% Er3þ:Cs2NaYF6 0.333 0.650 555 99% Yellowish–green
Cs2NaErF6 0.325 0.586 553 75% Yellowish–green
Table 3
Lifetimes of the excited-states of Er3þ and Yb3þ ions in Cs2NaYF6 crystals*.
Ion Yb3þ ion Er3þ ion Er3þ ion
concentration τ(2F5/2), ms τ(4I13/2), ms τ(4I11/2), ms
This work/
RT
[6]/RT This work/
RT
[5]/4 K [5]/RT [5]/4 K
1 at% – – – 100 12 21
10 at% 6.3; 6.4rec – 32rec – 1.1 1.4
100 at% 5.7 5.63 – – 0.7 1.1
* RT - room temperature, REC - estimated with the reciprocity method.
P.A. Loiko et al. / Journal of Luminescence 175 (2016) 260–266264dominates in the spectrum. Four intense peaks at 540, 551, 557
and 566 nm are observed within this emission band. The red
emission in the range 630–700 nm related to the transition 4F9/2-
4I15/2 is much weaker than the green one. The ratio for integrated
intensities of these bands (R/G ratio) is 0.27 which is referred to
the domination of the excited-state absorption (ESA) over the
cross-relaxation (CR) mechanism in generation of UCL, however,
further time-resolved studies of UCL are required to support this
[26]. In the blue region there is a weak emission band which is due
to the transition from the higher-lying excited state 2H9/2 to the
4I15/2 ground-state.
These features determine the yellow-green color of UCL from
10 at% Er3þ:Cs2NaYF6 with x¼0.385 and y¼0.603 in the 1931 CIE
chromaticity coordinates as well as high color purity, p498%,
caused by the dominant wavelength at 562 nm. Color purity p was
calculated from the CIE 1931 chromaticity diagram with respect tothe CIE-E illuminant (uniform white, x¼0.333 and y¼0.333). It is
defined as p¼a/b where a is the distance from the white point to
the sample color point and b is the distance from the white point
to the point on the spectral locus corresponding to the dominant
wavelength. The color purity is a quantitative measure of the
saturation of a particular color. For a stoichiometric
Cs2NaErF6 crystal and a 1 at% Er3þ , 10 at% Yb3þ:Cs2NaYF6 crystals,
very similar UCL spectra are observed, see Fig. 7. Color character-
istics for UCL from the studied crystals are summarized in Table 4.
In addition, UCL from a stoichiometric Cs2NaYbF6 crystal has
been studied under the excitation at 960 nm to the Yb3þ2F5/2
state. The emission bands observed in Fig. 7(c) are due to ET from
Yb3þ to impurity Tm3þ and Er3þ ions and UCL from Tm3þ
dominates. Both Tm and Er are presented as trace impurities in
ytterbium oxide, Yb2O3, reagent and thus their amount in stoi-
chiometric crystal is enough to provide detectable UCL. The bands
at 450–505, 640–680 and 750–820 nm are related to the
transitions 1G4-3H6, 1G4-3F4 and 3H4-3H6 for Tm3þ , respectively,
whereas weak green Er3þ emission from the 2H11/2 and 4S3/2 states
to the 4I15/2 ground state is detected at 550 nm. The red Er3þ
emission from the 4F9/2 state at 650 nm can overlap with the red
Tm3þ one, so it is not discovered. Accordingly, UCL from the
Cs2NaYbF6 crystal is blue–violet.
In the previous study of a Cs2NaYbF6 crystal [6], the strong
emission bad at 480 nm is at least partially attributed to coop-
erative emission from Yb3þ–Yb3þ ion pairs [27]. This effect is
known for materials with the strong clustering of Yb3þ ions [28].
In this case, a pair of closely located ions can form a virtual excited
state with the energy 2E(2F5/2) 20,000 cm1. The emission from
this state can be observed in the region 480–500 nm [29]. The
emission spectrum can be calculated as the convolution of the
near-IR emission spectrum of Yb3þ ions related to the 2F5/2-2F7/2
transition [27]:
FcoopðEÞ ¼
Z
FIRðE0ÞFIRðEE0ÞdE0: ð6Þ
The result of this calculation is presented in the inset of Fig. 7(c).
The cooperative emission should occur in the spectral range
475–510 nm with a maximum at 484 nm that is very close to the
maximum of the 1G4-3H6 Tm3þ emission in the Cs2NaYF6 crystal
[3]. Thus, it is difficult to distinguish these effects spectrally. In [6] it
has been found that the decay time of blue emission from a
Cs2NaYbF6 crystal is 2.73 ms, which is almost exactly half of τ(2F5/2)
measured at 1 μm for Yb3þ ions. Such a correlation is expected for
cooperative emission, τcoop¼τIR/2. Thus, blue UCL may be attributed
to both Tm3þ impurity centers and cooperative emission. It should
be also noticed that such intense UV up-conversion emissions can
contribute to enhance the spectral response of photocatalytic
semiconductor electrodes used in water-splitting processes for
sustainable production of hydrogen, since they can help in bridging
large bands gaps of efficient photocatalysts, like α-Fe2O3 (hematite),
located at around 560 nm and eventually increase the efficiency of
photoelectrochemical devices.
Fig. 9. Luminescence of Cs2NaErF6 (a) and 1 at% Er3þ , 10 at% Yb3þ:Cs2NaYF6
(b) crystals after excitation into the 2G9/2 state at 355 nm and into the 2H11/2 state at
520 nm; the spectrum of Yb3þ emission from the Cs2NaYbF6 crystal is added in
(b) for comparison.
P.A. Loiko et al. / Journal of Luminescence 175 (2016) 260–266 265In Fig. 8(a), log–log plots for the UCL intensity (IUCL) versus the
excitation power P are shown for a Cs2NaErF6 crystal. For the up-
conversion process, IUCL is proportional to the nth power of P, i.e.
IUCLPn [30] where n is the number of pump photons absorbed
per up-converted photon emitted. A plot of log IUCL versus log P
yields a straight line with slope n. For green emissions that occur
from the 2H11/2 and 4S3/2 states, for the slope of this dependence
n¼2.1 (524 nm) and 1.9 (557 nm), which means that two pump
photons are required to populate the above mentioned states.
There is the same situation for red emission from the 4F9/2 state at
660 nm where n¼2.1. Normally, excitation of Er3þ ions to the
higher lying excited states is achieved in two steps, for example
ground-state absorption (GSA) 4I15/2-4I11/2 followed by excited-
state absorption (ESA) from the 4I11/2 or the 4I13/2 states. For
Cs2NaErF6, cross-relaxation (CR) between the adjacent Er3þ ions,
which can participate as an additional mechanism for the popu-
lation of the 4F9/2 state, seems to be very weak. Indeed, the R/G
ratio for UCL is not enhanced when the Er3þ concentration in
Cs2NaYF6 increases from 10 to 100 at%, Fig. 7(a). This well corre-
lates with the large distance between the isolated Y3þ sites in the
Er3þ:Cs2NaYF6 crystal, so the probability of CR is not varied with
the increase of the Er3þ concentration in the elpasolite hosts. No
significant dependence of R/G ratio on the excitation power was
detected.
Similar slopes (n2) for the log–og plots of the UCL intensity vs.
the excitation power are observed also for a 1 at% Er3þ , 10 at% Yb3þ:
Cs2NaYF6 crystal, Fig. 8(b). In this case, the first step of the UCL
mechanism is the excitation of Er3þ ions through the GSA
2F7/2-2F5/2 of Yb3þ ions and the energy-transfer from Yb3þ to Er3þ ,
2F5/2(Yb3þ)-4I11/2(Er3þ). On the other hand, in any case, 2 pump
infrared photons are required for the generation of one visible
photon. More detailed discussion about the mechanisms of UCL in
hosts containing Er3þ and Yb3þ can be found elsewhere [31].
In Fig. 8(a), the IUCL vs P plot for the blue UCL from a Cs2NaYbF6
crystal is also presented and the slope of this straight line is 2.4.
This value is intermediate between the theoretical value of 2 for
pure cooperative emission and 3 for pure Tm3þ1G4-3H6 emission.
This supports our idea that the blue emission from the Cs2NaYbF6
crystal is due to the both processes.
Under excitation into the 2G9/2 or the 2H11/2 states of Er3þ in
Cs2NaYF6 at 355 nm or 520 nm, respectively, all the crystals singly
doped with Er3þ ions demonstrate emission spectra similar to
those obtained under near-IR excitation, Figs. 9(a), 3(a) and 7(a).
The corresponding color coordinates can be found in Table 4.
However, for a 1 at% Er3þ , 10 at% Yb3þ:Cs2NaYF6 crystal the
spectra are different from those of Er3þ doped Cs2NaYF6 crystals.
In particular, a broad emission band spanning from 940 to
1050 nm is detected. This band is clearly different from theFig. 8. Dependences of the UCL intensity on the excitation power in a log–log scale fo
λexc¼960 nm; n is the number of pump photons absorbed per up-converted photon emluminescent band related to the 4I11/2-4I15/2 transition of Er3þ
ions and has the spectral features of emission on the Yb3þ2F5/
2-
2F7/2 transition. Excitation of Yb3þ ions is due to the so-called
down-conversion (“quantum cutting”) process for the Er3þ–Yb3þ
ion pairs [32,33].
It should be also noted that enhancement of the efficiency of
the up- and down- conversion processes in cubic elpasolites is
possible with the ytterbium compositions Cs2NaErxYb1xF6 con-
taining relatively high Er3þconcentrations.4. Conclusions
To conclude, we have studied absorption and luminescence of
Er3þ and Yb3þ ions in cubic elpasolite Cs2NaYF6 crystals as well as
in crystals of Cs2NaErF6 and Cs2NaYbF6 synthesized under hydro-
thermal conditions. Stimulated-emission and gain cross-sections
have been determined for the Yb3þ2F5/2-2F7/2 and the Er3þ4I13/2-
4I15/2 transitions in 10 at% Yb3þ:Cs2NaYF6 and 10 at% Er3þ:
Cs2NaYF6 at room temperature. The maximum σSE values are
1.81021 cm2 at 993 nm (Yb3þ) and 3.81021 cm2 at 1535 nm
(Er3þ). The possibility of dual-wavelength laser operation and
wide tuning range exceeding 100 nm for laser emission arer (a) Cs2NaErF6, Cs2NaYbF6 and (b) 1 at% Er3þ , 10 at% Yb3þ:Cs2NaYF6 (b) crystals;
itted.
P.A. Loiko et al. / Journal of Luminescence 175 (2016) 260–266266expected for Yb3þ:Cs2NaYF6 crystals. Due to their structure
peculiarities, elpasolite crystals provide exceptionally long radia-
tive lifetimes of the excited-states for both Yb3þand Er3þ ions, τ
(2F5/2)¼6.3 ms and τ(4I13/2) 32 ms, respectively, together with
relatively weak UCL, which makes them attractive for high pulse
energy Q-switched lasers. Under excitation at 960 nm, Cs2NaErF6
crystals provide yellowish–green UCL with CIE coordinates
x¼0.353, y¼0.656. Down-conversion is detected for Er3þ ,Yb3þ:
Cs2NaYF6 crystals.Acknowledgments
This research was partially supported by the Russian Founda-
tion for Basic Research (Research Project no. 15-03-02507a);
Fundación CajaCanarias (AYE06) within the Project MAGEC
(Materials for Advanced Generation of Energy at Canary Islands)
and the Spanish Ministry of Economy and Competitiveness (Pro-
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