Optical properties of Sm3+-doped Y2Te4O11

Optical properties of Sm3+-doped Y2Te4O11

Author’s Accepted Manuscript Optical properties of Sm3+ -doped Y2Te4O11 Marcin Sobczyk, Damian Szymański www.elsevier.com/locate/jlumin PII: DOI: Re...

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Author’s Accepted Manuscript Optical properties of Sm3+ -doped Y2Te4O11 Marcin Sobczyk, Damian Szymański

www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(15)00233-1 http://dx.doi.org/10.1016/j.jlumin.2015.04.035 LUMIN13317

To appear in: Journal of Luminescence Received date: 4 November 2014 Revised date: 16 March 2015 Accepted date: 24 April 2015 Cite this article as: Marcin Sobczyk and Damian Szymański, Optical properties of Sm3+ -doped Y2Te4O11, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.04.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Optical properties of Sm3+ -doped Y2Te4O11 Marcin Sobczyk1 and Damian SzymaĔski *

Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław. Poland.

Abstract Spectroscopic properties of Sm3+ –doped Y2Te4O11 micro–crystals were studied in the 6000 - 18500 cm-1 spectral range. Vibrational, electronic absorption, fluorescence and excitation spectra as well as fluorescence dynamics of the Sm3+ – doped title micro-crystals are presented and analyzed in detail. A Judd–Ofelt intensity analysis of the absorption spectrum at 300 K has been applied for determination of ȍȜ parameters which in turn have been used for calculations of the radiative transition probability factor (A), the fluorescence branching ratios (ȕ), the natural (radiative) lifetime of the 4G(4)5/2 level of Sm3+ and the emission cross-section (ıem). An analysis of the non–radiative decay was based on the cross– relaxation mechanisms. The achieved results indicate that the investigated samples are potentially applicable as a 605 and 652 nm laser host as well as an orange phosphor. Keywords: Sm3+ ion; tellurate; phosphor; laser materials; cross-relaxation.

1

Corresponding author. Tel. +48713757333; fax +48 713282348. e-mail address: [email protected] 1

1. Introduction Crystals and glasses containing trivalent lanthanide ions have attracted great attention because of their extensive applications in such fields as optical fiber communications, information display and optical thermometry as well as in advanced laser materials. TeO2 – based glasses have been extensively investigated over the last two decades, due to their high refractive index, a wide transmission range from IR to UV (typically 5.0–0.35 ȝm) and good thermal stability [1-4]. As compared with borate, phosphate and silicate materials the oxotellurates(IV) exhibit a lower phonon energy (700 – 800 cm-1) which leads to a significant reduction of multiphonon relaxation rates. For that reason the oxotellurates(IV) are attracting hosts in order to obtain from trivalent lanthanide ions an efficient Vis and NIR fluorescence as well as a NIR–to–Vis energy transfer up-conversion. Telluride glasses and oxotellurate(IV) crystals are chemical stable, not hygroscopic and exhibit good mechanical properties. RE3+–oxotellurates(IV) (where RE = lanthanides and scandium group), reveal different formula types depending on the RE2O3:TeO2 molar ratio, e.g. RE2Te4O11. As compared with telluride glasses, the oxotellurate(IV) crystals have been much less explored. Studies on synthesis and structure of the RE2Te4O11 type of compounds were started in 1990 by the work of Castro et al. [5] and afterwards several results on structural and optical properties of the RE–oxotellurates(IV) [6-9] have been also published. The electronic spectra of Pr2Te4O11, Nd2Te4O11, Eu2Te4O11 as well as Pr3+ and Eu3+ –doped Gd2Te4O11 have been already reported and analyzed in relation with the crystal-field effect of the RE3+ ions [10-12]. Some spectral and luminescence characteristics of Eu3+, Er3+ and Yb3+ doped Y2Te4O11 have been also reported [13-15]. To the best of our knowledge, investigation on Sm3+:Y2Te4O11 microcrystals have not been reported so far. Crystallographic investigation by Höss et al. [14] have shown that the Y2Te4O11 crystals are monoclinic, belonging to the space group C2/c, No. 15. The elementary cell comprises

2

four structural units (Z=4) with the following unit cell parameters: a = 12.3876, b = 5.1068, c = 16.0193 Å, β = 106.154° and V = 973.38 Å3. The Y, Te(1), Te(2) and O (1-5) atoms are in the 8f positions (Wyckoff notation) whereas the O(6) atoms are in the 4e positions. The Y atom is coordinated by eight oxygen atoms in a distorted trigonal dodecahedron and has the C1 site symmetry. The Te(1) and Te(2) atoms are coordinated by three oxide anions in the primary coordination sphere building ȥ1-tetrahedra [TeO3]2− together with the lone pairs at the Te4+ cations. Sm3+ ions are widely used as activators of bulk crystals, nanocrystals and glasses, characterized by efficient emission from the 4G(4)5/2 level to the 6HJ (J = 5/2, 7/2, 9/2 and 11/2) states in visible region [16, 17]. Studies on optical absorption and emission spectra of Sm3+:LiNbO3 single crystals have demonstrated the possibility of a laser action at unique orange wavelength [18]. Earlier studies of K5Li2SmF10 and Sm3+ -doped K5Li2LaF10 singlecrystalline samples, due to absorption and excitation energy in the UV-VUV and Vis region, have been mainly focused for possible application as an efficient phosphors of visible light [19]. The 4G(4)5/2 state shows different quenching emission channels which make the Sm3+ ion an interesting case for an analysis of the energy transfer process [5, 20]. Fluorescence of Sm3+ ions in the NIR region, which originate from the 4G(4)5/2 level to the 6H13/2, 6H15/2 and 6FJ (J=1/2, 3/2, 5/2, 7/2, 9/2, 11/2) states, is very rarely presented and discussed [20]. The aim of the present study were investigations of fluorescence properties of Sm3+:Y2Te4O11 micro-crystalline samples which were synthesized by means of a low– temperature solid state reaction method. The absorption (300 K), excitation (300 K) and fluorescence (10 and 300 K) spectra were in detail investigated in the NIR–Vis–UV region along with the electronic structure of Sm3+ and the energy transfer mechanisms between Sm3+–Sm3+ ions. The Judd–Ofelt theory has been applied in order to characterize the most

3

important emission properties of Sm3+ ions. Radiative transition probabilities, fluorescence branching ratios, radiative decay time and emission cross–sections have been calculated. The effect of the Sm3+ ion concentration on decay times is discussed.

2. Experimental Microcrystalline powders of Y2-xSmxTe4O11 (where x = 0.01, 0.02, 0.1, 0.5, 1.0, 2.0 and 5.0 at%) were prepared according to the procedure reported for Y2Te4O11 in Ref. 21. As starting materials the Y2O3 (99.999%), Sm2O3 (99.999%) and TeO2 (99.95+%) oxides were used. For the synthesis of the title compound the oxides were mixed in stoichiometric proportions and subsequently thoroughly homogenized in a ball mill at 1200 rpm. After 24 h of homogenization the mixture was pressed into pellets of 15 mm diameter at 0.5 GPa and put into a platinum–iridium (95% Pt + 5% Ir) crucible with a diameter of 20 mm and height of 60 mm. The pellets were heated at 770°C in an electric furnace for 24 h with one intermittent grinding. The XRD diffractograms of the products were recorded on a D8 ADVANCE X–ray diffractometer equipped with Ni-filtered Cu radiation (λ = 1.5418 Å). The analysis was performed in the 2Θ = 10 – 90° range with 0.03° step. The Y, Sm and Te contents were determined by inductively coupled plasma (ICP) using an ARL spectrometer Model 3410 ICP (Fisons Instruments). Approximately 25 mg of the sample was transferred into a Teflon beaker followed by 20 cm3 2 M HCl and the mixture was heated to 50°C. After cooling the solution was transferred to a 100 cm3 volumetric flask and brought up to the volume with a 1 M HCl. A content of 1.000 mg × cm-3 of each of the Y, Sm and Te single components were used. An ICP analysis of the Sm3+:Y2Te4O11 samples revealed that the Sm:Y:Te ratio of atomic masses is in good agreement with the structural formula. 4

The 300 K infrared powder spectra of un un–doped Y2Te4O11 were measured on a Bruker 113v FTIR spectrometer as Nujol mulls (in the 4000 – 370 cm-1 range) and as polyethylene pellets (in the 600 – 50 cm-1 range). The FT-Raman spectrum was probed using a Nicolet IFS-860 instruments with the Raman attachment and Nd:YAG laser (λ = 1064 nm, P = 300 mW) pumped by the semiconductor laser diode (808 nm). The spectral resolution for both experiments was 2 cm-1. Room–temperature absorption and transmittance spectra for the Y2Te4O11 powder were measured with a Cary 5 (Varian) spectrophotometer. Samples for spectra measurements were placed in a copper mold and then compacted with a quartz rod. The electronic absorption spectrum of the Sm3+(2 at.%):Y2Te4O11 samples was recorded at 300 K on a Cary 5000 SCAN NIR-Vis-UV spectrophotometer (Agilent Technologies). A microcrystalline sample in the form of a compressed and polished disc shaped pellet (ca. 12.63 mm diameter and 0.368 mm thick) was used in order to obtain high-quality spectra. The thickness was measures with a micrometer screw. The

visible

and

near

infrared

fluorescence

spectra

were

obtained

with

a high–resolution HR-4000 OceanOptics spectrofluorometer upon excitation of a 405 nm GaN laser diode with a maximum power of 0.1 W. For low-temperature (10 K) measurements (fluorescence spectra) a closed cycle helium refrigerator (APD-Cryogenic ARS-2HW) equipped with a temperature controller was used. The excitation spectra and fluorescence decay curves were measured with a fluorescence scanning spectrophotometer CaryEclipse (Agilent Technologies) upon excitation from an Xe source.

5

3. Results and discussion The X–ray diffraction pattern of Y2Te4O11 is shown in Fig. 1 together with a theoretical simulation. The absence of additional reflections indicates that in the samples impurity phases were not present. In Y2Te4O11 the Sm3+ ions replace the single crystallographic position of Y3+. The site symmetry of Y3+ and Sm3+ is the C1.

3.1. Analysis of the vibrational spectra In order to set the energy of the phonon in the Y2Te4O11 host, Raman and Infrared measurements have been applied. The vibrational spectra of the Y2Te4O11 micro-crystalline sample are shown in Fig. 2a and b. RM and IR studies indicate that in the 4000–810 cm−1 range the Y2Te4O11 polycrystals are transparent. A relatively low intensity band at ~3440 cm-1, characteristic for vibration of OH- groups, has been attributed to small amounts of water sorption by the sample surface or H2O impurities in the crystals. The structure of the title compound could be also formulated according to the Y2[TeO3]2[Te2O5] formula, with a solely oxoditellurate(IV) unit [Te2O5]2− connecting the [Y2O10]14− layers [14]. Consequently, the bands observed in the 810 – 280 cm-1 region as well as in the RM and IR spectra are due to internal vibrations of [TeO3] and [Te2O5] units. For this reason, the vibrational spectra are very complicated and difficult to interpret. One of the most noteworthy features of the spectra is the presence of bands in the 800 – 590 cm-1, 480– 260 cm-1 and < 260 cm-1 ranges. These bands have been assigned to symmetric (ν1) and asymmetric (ν3) modes of O–Te–O, symmetric (ν2) and asymmetric (ν4) modes of Te–O–Te and the L’ lattice vibrations, respectively. The separation between the stretching and bending modes in Y2Te4O11 is about 110 cm-1. The symmetric stretching modes give rise to strong bands in the Raman spectra and weak in the infrared spectra. The strongest bands, observed at 789 and 711 cm-1 in the RM spectrum, have been assigned to the ν1 mode. The corresponding 6

IR bands are sited at the same frequencies as in the RM spectrum. The bands between 480 and 260 cm-1 in vibrational spectra are assigned to symmetric and asymmetric deformation modes. The lattice modes, observed below 260 cm-1, characterize a lower intensity in RM spectrum and higher in the IR. The strong bands at 257, 251 and 243 cm-1 in the IR spectrum are assigned to a translation mode of Y3+ [22]. The observed vibrational frequencies are summarized in Table 1. A more precise assignment of the Raman and infrared active modes is not possible without additional data from single crystals. However, the results seem to be sufficient enough for an analysis of relaxation processes from the excited states of Sm3+ doped in the Y2Te4O11 micro-crystals.

3.2. Electronic absorption spectra analysis The absorption spectrum of the micro-crystalline Y2Te4O11 powder has been recorded at 300 K in the UV–Vis spectral region. The presented in Fig. 3 absorption spectrum was calibrated in absorption coefficient units α (in cm-1), using the following equation:

§1· § I0 · § A· ¸ = 2.303¨ ¸ ©l ¹ © I ¹ ©l ¹

α (ω ) = ¨ ¸ ln¨

(1)

where ln(I/I0) corresponds to the absorbance A at the frequency ω and l = 0.160 cm is the thickness of the sample. In the short wavelength absorption area, optical cut–off edges of electric absorption appear. The relation between α(ω) and photon energy of the incident radiation, hω is given by the following relation:

α (ω ) =

B(hω − Eopt ) hω

p

(2)

7

where Eopt is the optical band gap energy (in eV), B is a constant, h is the Planck constant and p is an exponent, which characterizes the type of electronic transition. For directly allowed, directly forbidden, indirectly allowed, or indirectly forbidden transitions p assumes the values of 0.5, 1.5, 2 and 3, respectively. The optical band gap in semiconductor materials is closely related to the energy gap between the valence band and conduction band. In Fig. 3 the absorption coefficient in the form of (αhω)p is plotted versus hω values. The best fit was obtained for p = 2. The Eopt value was found to be equal to 4.02 eV. The refractive index (n) can be calculated based on the optical energy gap, by the Dimitrov and Sakka relation [23]:

Eopt n2 −1 = 1− 2 20 n +2

(3)

The n value was found to be 2.17. The refractive index for the Y2Te4O11 crystal has not been reported, and therefore, our calculated value may be compared only with the observed results of TeO2 –based glasses. The measured refractive index of TeO2-based glasses in the UV–Vis spectral region is reported to achieve values between 1.9 – 2.2 [2, 24]. The absorption spectrum of Sm3+(2 at.%):Y2Te4O11 was measured at room temperature in NIR–Vis–UV spectral region and is shown in Fig. 4 with labeled absorption bands. The spectrum was calibrated in absorption cross section units (in cm2). In the 5950 – 27675 cm-1 range, sharp and relatively well separated bands of intraconfigurational 4f5 Æ 4f5 transitions from the 6H5/2 level of the Sm3+ ion were observed. In the UV range, below 350 nm, the Sm3+ bands are masked by the absorption of the Y2Te4O11 host. The absorption bands of the Sm3+ ion can be classified into two groups: the first group consists of transitions in the NIR spectral region, up to about 10925 cm-1 and the second one is containing high–energy transitions in the Vis–UV range. The absorption bands observed 8

between 5950 – 10925 cm-1 have been assigned to transitions from the 6H5/2 level to the 6F1/2 + 6H15/2 + 6F3/2 (5950 – 7550 cm-1), 6F7/2 (7630 – 8620 cm-1), 6F9/2 (8700 – 9700 cm-1) and 6

F11/2 (10100 – 10925 cm-1) multiplets. The hypersensitive transition of samarium(3+) is that

of 6H5/2 Æ 6F1/2. The bands observed between 17610 – 18200 cm-1, with a maximum at 17682 cm-1, were assigned as transition from the

6

H5/2 ground state to the

4

G(4)5/2 level.

Well separated bands has been observed between 5950 and 10925 cm-1 and 17610 – 27625 cm-1. Among these transitions, 6H5/2 Æ 6P3/2 exhibits the highest intensity and is suitable for pumping by a GaN diode-laser. The line at 24702 cm-1 (404.83 nm) is dominating with absorption cross-section of 3.72×10-19 cm2. The application of the Judd-Ofelt method in the analysis enables the prediction of the lifetime of the 4G(4)5/2 state as well as radiative transition probabilities and fluorescence branching ratios from the 4G(4)5/2 level to all final states (6FJ and 6HJ). The experimental oscillator strengths (fexp) of the absorption transitions were calculated using the expression:

f exp = 4.318 × 10 −9 ³ ε (v )dν ,

(4)

where ε (v ) is the molar extinction coefficient at v (in cm-1). The theoretical oscillator strengths (fcalc) of an electric dipole transitions is given by [25a-c]:

f calc

8πmcν § n 2 + 2 · ¨ ¸ = 3h(2 J + 1) ¨© 9n ¸¹

2

Ω λ (ΨJ U λ Ψ ' J ') , ¦ λ 2

(5)

= 2, 4 , 6

where J is the total angular momentum of the ground state, Ωλ (λ = 2, 4, 6) are the phenomenological Judd–Ofelt intensity parameters (in cm2) and U λ are double reduced 9

matrix elements of the unit tensor operator of rank k = 2, 4 and 6, calculated for the intermediate coupling approximation. The applied in the calculations doubly matrix elements were taken from Ref. 26. The magnetic dipole oscillator strengths (fmd) were currently considered as independent from the host. Appropriate values have been extracted from the relation:

f md = f '×n ,

(6)

where f’ is the calculated magnetic dipole oscillator strengths for a the 4f5 Æ 4f5 transition of the Sm3+ ion [27]. The fexp and fcalc oscillator strengths for Sm3+:Y2Te4O11 are included in Table 2. Since the crystal–field components of the 6F1/2, 6H15/2, 6F3/2 and 6F5/2 levels are overlapped, one could not separate the individual band areas of the SLJ multiplets. In this case one oscillator strength value for the combined four areas was determined. Consequently, the sum of all reduced matrix elements of the appropriate components of the SLJ levels were taken as the proper one, also. Hence, the oscillator strength values could be determined from five well– separated SLJ levels and the above mentioned one group of the joint multiplet [20, 25c]. The following Judd–Ofelt parameters (Ωλ) have been obtained (×10-20 cm2): Ω2 = (3.59±0.23), Ω4 = (3.78±0.28) and Ω6 = (1.86±0.35). The RMS value of 5.3×10-7 indicates of a good agreement between the calculated and experimental oscillator strengths. In general the magnitude of the Ωλ parameters for Sm3+ ions doped in telluride glasses follow the trend

ȍ4>ȍ2>ȍ6 [28]. For the investigated Sm3+:Y2Te4O11 micro-crystals this ratio was found to be the same.

10

3.3. Radiative properties By means of the obtained Ωλ intensity parameters the following fundamental fluorescence parameters have been determined. The radiative transition probabilities (A) were calculated by applying the equation:

A(ΨJ , Ψ ' J ') = Aed + Amd

2 º 64π 4ν 3 ª§ n(n + 2 ) · ¨ ¸ S ed + n 3 S md » , = «¨ ¸ 3h(2 J + 1) «¬© 9 »¼ ¹

(7)

where Aed and Amd are electric (ed) and magnetic (md) dipole transition probablilities. The dipole line strengths, Sed and Smd are expressed by:

2

S ed = e 2 Ω λ f 5 [L, S ]J U λ f 5 [L' , S ']J ' ,

S md

e2h2 = 16π 2 m 2 c 2





2

f [L, S ]J L + 2 S f [L ' , S ']J ' 5

(8)

5

.

(9)

The total radiative transition probability (ATOT) for the 4G(4)5/2 excited state of the Sm3+ ion is given as the sum of the A(ΨJ , Ψ ' J ') terms, calculated over all final states:

ATOT (ΨJ ) = ¦ A(ΨJ , Ψ ' J ') .

(10)

Ψ 'J '

The natural (radiative) lifetimes (τR) of the 4G(4)5/2 emitting state and the fluorescence branching ratio (βR) were calculated from the following equations:

11

τ R (ΨJ ) = ATOT (ΨJ )−1 ,

β R (ΨJ , Ψ ' J ') =

A(ΨJ , Ψ ' J ') . ATOT (ΨJ )

(11)

(12)

The obtained values of the Aed, Amd, ATOT, τR and βR parameters are presented in Table 3. The total transition probability ATOT is found to be 917 s-1 and τR is equal to 1090 µs.

3.4. Fluorescence properties The 10 and 300 K fluorescence spectra of the Sm3+:Y2Te4O11 sample, pumped by the 405 nm GaN laser diode are shown in Fig. 5. The presented spectra were calibrated for the spectral response the applied monochromator and detector. The 405 nm laser line excites the 4f5 electron from the 6H5/2 ground state to the 6P3/2 level. The 4G(4)5/2 emitting level is populated via a non-radiative process. The band with a maximum at 17721 cm-1 (564.30 nm) was attributed to the transition from the 4G(4)5/2 level to the 6H5/2 ground level. The next one with a maximum at 16527 cm-1 (605.07 nm) represents the emission from the 4G(4)5/2 level to the first excited state 6H7/2, of Sm3+. The band with a maximum at 15341 cm-1 (651.85 nm) has been attributed to the 4G(4)5/2 Æ 6H9/2 transition and the last bands in the visible spectrum, observed between 14130 and 13930 cm-1 (771.40 – 717.88 nm), to the 4G(4)5/2 Æ 6H11/2 transition. In the NIR fluorescence spectra the observed bands originate from transitions between the 4G(4)5/2 state and the 6H13/2, 6F1/2, 6F3/2, 6F5/2 and 6F7/2 levels. The spectra were found to be independent of the gate delay time after the excitation pulse. The UV–Vis excitation spectrum of Sm3+:Y2Te4O11 shown in Fig. 6, was recorded at 300 K and monitored at 605 nm, which corresponds to the 4G(4)5/2 Æ 6F7/2 transition from the Sm3+ ion. The spectrum was calibrated for the spectral response the applied monochromator

12

and detector. In the excitation spectrum the 4f5 Æ 4f5 transitions of Sm3+ were observed exclusively; no matrix bands were present. Hence, one may assume that in the samples no important host sensitization occurs. As one can notice from Table 3, the calculated fluorescence branching ratios of the 4

G(4)5/2 Æ 6H7/2 and 4G(4)5/2 Æ 6H9/2 transitions are equal to 35.5 and 35.8 %, respectively.

The values of the experimental branching ratio for the 4G(4)5/2 Æ 6H7/2 and 4G(4)5/2 Æ 6H9/2 transitions, determined from the relative area of the fluorescence bands at 300 K were found to be equal 34.7 and 25.6 %, respectively. This irregularity may be attributed to the proximity of the LMCT states, which in Y2Te4O11 appear at low wavenumbers and/or it may be indicative for some uncertainty of the Judd–Ofelt treatment. It has been noticed that the values of the experimental branching ratio are practically independent on the Sm3+ concentration. A fine structure of the

2S+1

LJ multiplets is well manifest at 10 K since at this temperature

only the lowest crystal–field level of the 4G(4)5/2 emitting state is expected to be populated. In agreement with the X-ray investigation, the structures of the 10 K emission manifolds indicate the presence of a single optically active center. The splitting ∆E = 180 cm-1 of the 6H5/2 ground state is determined by the bands located at 17721 and 17541 cm-1. The experimental crystal–field components of the ground and excited state levels of the Sm3+:Y2Te4O11 crystalline sample, determined from the 10 K fluorescence spectra, are collected in Table 4. Some additional lines, assigned to transitions from the second and third crystal–field component of the 4G(4)5/2 level, have been observed at 300 K. Orange light, originating from Sm3+:Y2Te4O11 was observed by naked eyes. Based on the fluorescence spectrum, excited by 405 nm at 300 K, the CIE chromaticity coordinates were calculated to be equal to x = 0.614 and y = 0.384. The Fig. 7 shows the emission spectra at 300 K for the 4G(4)5/2 Æ 6HJ (J = 5/2, 7/2, 9/2, 11/2 and 13/2) transitions under excitation at 405 nm to the absorption bands of the 6H5/2 Æ 13

6

P3/2 transition. The presented spectra were calibrated in stimulated emission cross-section

units, σ em (λ ) (in cm2). For the calculation, the Füchtbauer–Ladenburg formula has been applied [29]:

σ em (λ ) =

ηλ5 I (λ ) , τ f ³ I (λ )dλ f j 8πn 2 c

(

)

(13)

where I (λ ) is the intensity of the emission spectrum, n is the index of refraction of the sample, τf is the fluorescent lifetime of the upper state, η is the radiative quantum efficiency of the upper states and fj is the fraction of the pumped population in the particular j state, given by the Boltzman distribution. If the natural (radiative) lifetime, τR, and fluorescence branching ratio, βR, are know, then σ (λ ) may be expressed by :

σ em (λ ) =

β R λ5 I (λ ) , τ R ³ I (λ )dλ f j 8πn 2 c

(

)

(14)

The highest values of stimulated emission cross–section for the 4G(4)5/2 Æ 6F7/2 (at 605.05 nm) and 4G(4)5/2 Æ 6F9/2 (at 651.96 nm) transitions of Sm3+, recorded at 300 K, are equal to 1.45×10-21 and 1.51×10-21 cm2, respectively. The values of σ (λ ) are higher than those reported for Sm3+:Na3Y(VO4)2 [20] single crystals and Sm3+:REOCl powders (where RE = Y, La and Gd) [30] as well as telluride [3] and silicate glasses [17]. The σ em (λ ) values of the 4

G(4)5/2 Æ 6F7/2 and 4G(4)5/2 Æ 6F9/2 transitions are comparable with that reported for lead

telluroborate glasses [31] and is slightly smaller than that reported for Sm3+: Gd2SiO5 laser crystals [32]. On the basis of the obtained results one may assume that for a lasing action in the orange and reddish orange spectral region, the 4G(4)5/2 Æ 6F7/2 and 4G(4)5/2 Æ 6F9/2 14

transitions are more potential, respectively. The absorption bands in the 414 – 396 nm spectral range enabled an efficient excitation of Sm3+ ions in the Y2Te4O11 host, using a cheap commercial laser diode (GaN) with an emitting wavelength at 405 nm. We are aware that the presented results are subjected to errors arising from an imperfection of the Judd–Ofelt model and will require a verification by laser experiments.

3.5. Decay kinetics Lifetime measurements of the 4G(4)5/2 fluorescent level were recorded at 77 and 300 K in order to obtain additional information about the fluorescence dynamics. The measured decay time (τexp) can be expressed as [28]:

−1 τ exp = τ R−1 + WMPR + WET + WOH + ... , −

(15)

where WMPR , WET and WOH − are the rates of multiphonon relaxation, the energy transfer between two neighboring lanthanide ions and the energy transfer between lanthanide ions and OH- groups, respectively. As a rule of thumb in lanthanide spectroscopy it has been found that WMPR processes are competitive when p =

∆E , the number of phonons ( =ω ) needed to bridge =ω

the electronic energy gap ( ∆E ), is smaller than six [33]. From a vibrational analysis follows, that in Y2Te4O11 the phonon density of states cuts off at 789 cm-1. The energy gap of about 7100 cm-1 between the 4G(4)5/2 fluorescent level and the next lower–laying 6F11/2 level of Sm3+doped in the Y2Te4O11 micro-crystals, corresponds roughly to nine highest energy phonons of the Te4O113- ions, available in the Y2Te4O11 host lattice. Therefore, in the case of Sm3+:Y2Te4O11 the WMPR process is negligible.

15

The fluorescence decay curves were recorded for samples with the following Sm3+ concentrations: 0.01, 0.02, 0.1, 0.5, 1.0, 2.0 and 5.0 at%. The lifetime were measured by applying a direct, selective excitation. Fig. 8 shows the fluorescence decay curves of the 4

G(4)5/2 emitting level. The fluorescence decays for 0.01–1.0 at.% of Sm3+ ion concentrations

are single exponential. In the case of Y2Te4O11 with a 2 and 5 at.% concentration of Sm3+ ions, the character of decay curves are non–exponential. Therefore the effective decay time (τm) can be calculated using the following formula [34]:

t =∞

τm

³ = ³

tI (t )dt

t =0 t =∞ t =0

I (t )dt

,

(16)

where I is the intensity at the beginning of the decay curve and t represents time. The emission decay times of the fluorescent level moderately dependent on concentration of Sm3+ ions. For example the decay times (at 300 K) for 0.01 at.% (7.15×1016 ions/cm3) and 5 at.% (3.59×1019 ions/cm3) of Sm3+ were assessed as 1030 and 457 µs, respectively. The quantum efficiency, calculated from the formula:

η=

τ exp × 100% , τR

(17)

diminishes from 94% (for 0.01 at.% of Sm3+) to 53% (for 5 at.% of Sm3+). A high value of the quantum efficient can indicate that the concentration of OH- ions in the Y2Te4O11 microcrystals is negligible. One may notice that with an decrease of temperature from 300 to 77 K the lifetimes of the examined samples are practically temperature independent. The lifetime values of the 4G(4)5/2 level as a function of Sm3+ concentration are presented in Table 5.

16

A concentration quenching of the 4G(4)5/2 level has been observed also for other Sm3+ glasses and Sm3+ doped crystals (e.g. Refs. [3, 20, 35, 36]) and was attributed to energy transfer from an excited Sm3+ ion to a nearby unexcited Sm3+ ion, via a cross relaxation process. There are two possible cross-relaxation mechanisms with different cross–relaxation channels: 1. Resonant cross-relaxation: 4

G(4)5/2 Æ 6F11/2 (6854 cm-1) = 6H5/2 + 6H15/2 (6854 cm-1)

4

G(4)5/2 Æ 6F9/2 (8120 cm-1) = 6H5/2 + 6F7/2 (8120 cm-1)

4

G(4)5/2 Æ 6F7/2 (9212 cm-1) = 6H5/2 + 6F9/2 (9212 cm-1)

4

G(4)5/2 Æ 6F3/2 (10412 cm-1) = 6H5/2 + 6F11/2 (10412 cm-1)

4

G(4)5/2 Æ 6H15/2 (10776 cm-1) = 6H5/2 + 6F11/2 (10687 cm-1) + 89 cm-1

2. One phonon assisted cross-relaxation: 4

G(4)5/2 Æ 6F5/2 (10080 cm-1) = 6H5/2 + 6F9/2 (9421 cm-1) + 659 cm-1 4

G(4)5/2 Æ 6F1/2 (11166 cm-1) = 6H5/2 + 6F11/2 (10687 cm-1) + 479 cm-1

The non–radiative transition rate in Y2Te4O11 is relatively low, what indicates on a large probability of a resonant cross-relaxation energy transfer between two neighboring Sm3+ ions. With the increase of Sm3+ concentration, the distance between nearby activator ions becomes shorter resulting in an increase of interaction between the ions. In this case, the Sm3+ ion initially plays the part of a donor and after that is the acceptor of the excitation energy. In order to obtain a more thorough understanding of the energy transfer processes between two neighboring Sm3+ ions, the Inokuti–Hirayama (I–H) model for analysis of the decay curves, has been applied. The fluorescence intensity decay is defined as:

17

3/ S ª− t § t · º Φ(t ) = A exp « − α ¨¨ ¸¸ » , «¬ τ 0 © τ 0 ¹ »¼

(18)

where Φ (t ) is the fluorescence intensity after pulse excitation, A is the fluorescence intensity at t = 0, and τ0 is the intrinsic decay time of the donor ions in the absence of acceptors. S describes the energy transfer mechanisms and is equal to 6 for dipole–dipole (D–D), 8 for

dipole–quadrupole (D–Q), and 10 for quadrupole–quadrupole (Q–Q) interactions. The α parameter is defined as:

α=

4π § 3 · Γ¨1 − ¸ N A R03 , 3 © S¹

(19)

where ī is the Eulers’s gamma function (ī = 1.77, 1.43 and 1.30 for S = 6, 8 and 10, respectively), NA is the concentration of Sm3+ ions in the Y2Te4O11 crystal and R0 is the critical transfer distance, defined as such separation at which the probability of energy transfer between donor and acceptor is equal to τ0. We have fitted the decay curves by means of Eq. (18), considering A, α and τ0 as adjustable parameters. The best fit was obtained for S = 6 in distinction to S = 8 or 10 (see inset of Fig. 7) and the calculated lifetime was determined to be

τ0 = 1032 µs. This indicate that the interaction mechanism of cross-relaxation is of the dipole– dipole type. From a fitting procedure of the 4G(4)5/2 fluorescence decay curve the value

α = 1.27 has been obtained whereas the value of the critical distance R0,= 16.9 Å was determined by means Eq. (19).

18

Hence, the donor–acceptor parameter Cda, which describes the non–radiative loss due to the cross–relaxation process between two neighboring Sm3+ ions, could be estimated from the relation:

6

−1

C da = R0 τ 0 .

(20)

The energy transfer probability Wda was calculated using the following relation:

−6

Wda = C da R0 .

(21)

The Cda and Wda values were found to be 2.29×10-50 m6s-1 and 969.0 s-1, respectively. The calculated Cda parameter is higher than that reported for the Sm3+:Na3Y(VO4)2 single crystal and is smaller than the reported values for Sm3+ –doped telluride glasses [3]. Therefore, Sm3+: Y2Te4O11 exhibit weaker concentration quenching than Sm3+ ions in TeO2 – based glasses.

4. Summary Spectroscopic properties and excited state relaxation dynamics of Sm3+ ions in the Y2Te4O11 crystalline host lattice are for the first time reported. The electronic absorption (300 K), excitation (300 K) and fluorescence (10 and 300 K) spectra as well as the decay kinetics are presented and discussed. The Judd–Ofelt theory was used for the analysis of the 300 K absorption spectrum and subsequently for the calculation of radiative properties. The decay curves of fluorescence originating from the 4G(4)5/2 excited state were observed to be non– exponential for concentrations of Sm3+ ions higher than 1 at.% and were concentration dependant. An analysis of the cross-relaxation process in the frame of the Inokuti–Hirayama

19

model has been performed. The calculated results indicate that dipole–dipole interactions plays a major role in the concentration quenching mechanism of Sm3+ in the Y2Te4O11 host. The results show that Sm3+:Y2Te4O11 crystals may be considered as a potential laser material for orange or reddish orange laser output.

References: [1] K. Selvaraju, K. Marimuthu, J. Alloys Compd. 553 (2013) 273. [2] M. Sobczyk, J. Quant Spectrosc. Radiat. Transfer 119 (2013) 128. [3] T. Sasikala, L. Rama Moorthy, A. Mohan Babu, Spectrochim. Acta A 104 (2013) 445. [4] D. Yin, Y. Qi, S. Peng, S. Zheng, F. Chen, G. Yang, X. Wang, Y. Zhou, J. Lumin. 146 (2014) 141. [5] A. Castro, R. Enjalbert, D. Lloyd, I. Rasines, J. Galy, J. Solid State Chem. 85 (1) (1990) 100. [6] I. Ijjaali, C. Flaschenriem, J.A. Ibers, J. Alloys Compd. 354 (2003) 115. [7] Y.-L. Shen, J.-G. Mao, J. Alloys Compd. 385 (2004) 86. [8] P. Höss, S.F. Meier, Th. Schleid, Z. Kristallogr. Suppl. 21 (2004) 162. [9] P. Höss, G. Starkulla, Th. Schleid, Acta Crystallogr. E 61 (2005) i113. [10] C. Cascales, E. Antic-Fidancev, M. Lemaitre-Blaise, P. Porcher, J. Alloys Compd. 180 (1992) 111. [11] C. Cascales, E. Antic-Fidancev, M. Lemaitre-Blaise, P. Porcher, J. Phys.: Condens. Matter 4 (1992) 2721. [12] M. Karbowiak, C. Rudowicz, P. Gnutek, Opt. Mater. 33 (2011) 1147. [13] T. Endo, A. Shibuya, H. Takizawa, M. Shimada, J. Alloys Compd. 192 (1993) 50. [14] P. Höss, A. Osvet, F. Meister, M. Batentschuk, A. Winnacker, T. Schleid, J. Solid State Chem. 181 (2008) 2783. [15] Y. Dwivedi, Kavita Mishra, S.B. Rai, J. Alloys Compd. 572 (2013) 90. 20

[16] A. StrzĊp, W. Ryba-Romanowski, M. Berkowski, J. Lumin. 153 (2014) 242. [17] F. Fu, B. Chen, L. Shen, E. Yue Bun Pun, H. Lin, J. Alloys Compd. 582 (2014) 265. [18] G. Dominiak-Dzik, J. Alloys Compd. 391 (2005) 26. [19] P. Solarz, G. Dominiak-Dzik, W. Ryba-Romanowski, J. Alloys Compd. 362 (2004) 61. [20] M. Sobczyk, D. SzymaĔski, J. Lumin. 142 (2013) 96. [21] O. Noguera, J. Jouin, O. Masson, B. Jancar, P. Thomas, J. Eur. Ceram. Soc. 32 (2012) 4263. [22] M. Sobczyk, Opt. Mater. 35 (2013) 852. [23] V. Dimitrov, S. Sakka, J. Appl. Phys. 79 (3) (1996) 1736. [24] T. Komatsu, N. Ito, T. Honma, V. Dimitrov, Solid State Sci. 14 (10) (2012) 1419. [25] a) B.R. Judd, Phys. Rev. 127 (1962) 750 b) G.S. Ofelt, J.Chem. Phys. 37 (1962) 511 c) R.D. Peacock, Struct. Bond. 22 (1975) 83. [26] C.K. Jayasankar, P. Babu, J. Alloys Compd. 307 (2000) 82 [27] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412. [28] A. Kumar, D.K. Rai, S.B. Rai, Spectrochim. Acta A 59 (5) (2003) 917. [29] B.F. Aull, H.P. Jenssen, IEEE J. Quantum Electron. 18 (5) (1982) 925. [30] U. Rambabu, K. Rajamohan Reddy, K. Annapurna, T. Balaji, J.V. Satyanarayana, S. Buddhudu, Mater. Lett. 27 (1-2) (1996) 59 [31] B.C. Jamalaiah, M.V. Vijaya Kumar, K. Rama Gopal, Opt. Mater. 33 (2011) 1643. [32] A. StrzĊp, R. Lisiecki, P. Solarz, G. Dominiak-Dzik, W. Ryba-Romanowski, M. Berkowski, Appl. Phys. B 106 (1) (2012) 85. [33] L.A. Riseberg, H. W. Moos, Phys. Rev. 174 (2) (1968) 429 [34] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978.

21

[35] M. Sobczyk, P. Starynowicz, R. Lisiecki, W. Ryba-Romanowski, Opt. Mater. 30 (2008) 1571. [36] P. Solarz, M. Sobczyk, Opt. Mater. 34 (2012) 1826.

Highlights • Sm3+:Y2Te4O11 microcrystals were synthesized via a solid-state reaction. • The spectroscopic properties of the samples were investigated at 300 and 77 K. • Concentration quenching mechanisms and possible relaxation channels were

discussed. • The emission cross-sections for visible laser applications are calculated. • Sm3+:Y2Te4O11 is a promising laser material for 405 nm GAN pumped visible laser.

22

Table 1

295 (w) 263 (w)

307 (m)

326 (vw)

422 (vw) 394 (w) 380 (w) 370 (w)

443 (vw)

Raman

Bending modes

vs: very strong, s: strong, m: medium, w: weak, vw: very weak, sh: shoulder

597 (vw)

628 (vs) 605 (vs)

681 (vs)

789 (sh) 763 (vs) 744 (sh) 711 (sh)

789 (vs) 762 (w) 744 (vw) 711 (m) 694 (vw)

641 (w) 633 (w)

Infrared

Raman

Stretching modes

Table 1. Vibrational frequencies (in cm-1) of the Y2Te4O11.

394 (vs) 380 (s) 372 (vs) 343 (s) 338 (s) 332 (s) 326 (sh) 320 (s) 307 (s) 300 (m) 295 (sh) 263 (m)

433 (m)

472 (m)

Infrared

159 (vw)

194 (vw) 190 (vw)

212 (w)

230 (w)

243 (w)

Raman

216 (sh) 210 (vs) 207 (vs) 198 (s)

257 (sh) 251 (sh) 247 (s) 243 (s) 236 (s)

Infrared

189 (s) 175 (s), 168 (s) 160 (m) 154 (m), 147 (m) 136 (s) 129 (m) 118 (s) 114 (s), 112 (s) 85 (s), 78 (sh), 68 (sh), 58 (sh)

Lattice modes

Table 2

Table 2. Experimental and calculated oscillator strength (f ´ 10-6) for Sm3+ ion -doped Y2Te4O11 crystal and the W l intensity parameters (cm2).

No.

Level

Baricenter

fexp

fcalc

fexp – fcalc

(cm-1) 0.0022 (md) 1

6

F1/2 + 6H15/2 + 6F3/2 + 6F5/2

6803

8.25 (ed)

8.25

0.00

2

6

F7/2

8083

4.50 (ed)

4.50

0.00

3

6

F9/2

9259

2.72 (ed)

2.72

0.00

4

6

F11/2

10612

0.847 (ed)

0.430

0.417

G(4)5/2

17721

0.114

0.025

0.089

18846

0.070

0.008

0.062

5

6

4

4

F3/2

W2 = (3.59 ± 0.23) ´ 10-20 W4 = (3.78 ± 0.28) ´ 10-20 W6 = (1.86 ± 0.35) ´ 10-20 ed – electric dipole, md – magnetic dipole.

Table 3

Table 3. Energy transitions (in cm-1), radiative transitions probabilities (Aed, Amd and ATOT), radiative lifetime (tR) and branching ratios (bR) for

4

G5/2 emitting level of

Sm3+ ion -doped Y2Te4O11 crystal.

4

G5/2 à

2S+1

LJ

Energy

Aed

Amd

bR

transition

(s-1)

(s-1)

(%)

6

H5/2

0

26.8

39.3

7.2

6

H7/2

1065

292.8

32.6

35.5

6

H9/2

2270

327.9

0.0

35.8

6

H11/2

3592

86.6

0.0

9.4

6

H13/2

4983

10.9

0.0

1.2

6

F1/2

6275

6.9

0.0

0.8

H15/2

6582

0.7

0.0

0.1

6

F3/2

7053

6.7

13.7

2.2

6

F5/2

7426

45.2

9.3

5.9

6

F7/2

8063

7.9

2.9

1.2

6

F9/2

9195

6.3

0.0

0.7

F11/2

10521

0.6

0.0

0.1

6

6

AT = 917 s-1 tR = 1090 ms

Table 4

4



F7/2

F5/2

F3/2

G(4)5/2

6

6

6

F1/2

H15/2

H13/2

H11/2

H9/2

H7/2

H5/2

6

6

6

6

6

6

6

LJ

2S+1

spectrum.

17721

7976

7129, 7155, 7177

6631, 6650

6375

6202, 6287, 6408, 6433, 6476

4968, 5018, 5046, 5060, 5097 , 5170, 5191

3597, 3642, 3670, 3718, 3746, 3783

2269, 2327, 2380, 2409, 2465

1053, 1151, 1194, 1263

0, 121, 180

Energy (in cm-1)

1/3

1/3

3/3

2/2

1/1

5/8

7/7

6/6

5/5

4/4

3/3

Experimental/ Theoretical





7154

6641

6375

6361

5079

3693

2370

1165

100

Baricenter (cm-1)





48

19

0

274

173

186

196

210

180

Splitting, DE (cm-1)

Table 4. Experimental crystal-field components of the 2S+1LJ multiplets of Sm3+ ion -doped Y2Te4O11 determined from 10 K fluorescence

Table 5

Table 5. Lifetime of the 4G(4)5/2 level of Sm3+ in Y2Te4O11 at 300 K.

Sm3+ concentration

Quantum efficiency, ha

(at. %)

(ions/cm3)

(ms)

(%)

0.01

7.15´1016

1030

94

0.02

17

1020

94

17

984

90

18

823

75

1.0

18

7.13´10

751

69

2.0

1.42´1019

685

63

5.0

19

574

53

0.1 0.5

a

Lifetime

See equation (17)

1.43´10 7.15´10 3.57´10

3.52´10

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8