Optical spectra and crystal field calculation for SrB4O7:Sm2+

Journal of Alloys and Compounds 661 (2016) 419e427

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Optical spectra and crystal field calculation for SrB4O7:Sm2þ Piotr Solarz a, *, Mirosław Karbowiak b, Michał Głowacki c, Marek Berkowski c, Ryszard Diduszko c, d, Witold Ryba-Romanowski a
lna 2, Wrocław, 50-422, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Oko Faculty of Chemistry, University of Wroclaw, ul. F. Joliot-Curie 14, Wrocław, 50-383, Poland c
w 32/46, Warsaw, 02-668, Poland Institute of Physics, Polish Academy of Sciences, al. Lotniko d Tele and Radio Research Institute, ul. Ratuszowa 11, Warsaw, 03-450, Poland a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 September 2015 Received in revised form 19 November 2015 Accepted 21 November 2015 Available online 2 December 2015

Undoped and samarium-doped SrB4O7 polycrystalline samples were prepared by a solid state reaction method in air. XRD examination revealed that the undoped sample is consistent with the pure SrB4O7 phase but in samarium-doped sample in addition to the main tetraborate SrB4O7 phase there is a little amount of metaborate. Based on preliminary spectroscopic examination it was ascertained that Sm2þ ions are incorporated without measurable traces of Sm3þ admixture. High resolution luminescence spectra and decay curves of Sm2þ luminescence were measured as a function of temperature in the 5 e550 K temperature region. Energies of crystal field components of the 7FJ (J ¼ 1e5) multiplets at 5 K and those of the 5D1 multiplet at 300 K were determined from luminescence spectra and used subsequently as input data for the crystal field calculation. Performed crystal field calculation predicts the energy level scheme for the 4f6 configuration of Sm2þ in SrB4O7 up to 29000 cm1 and makes it possible to assess the contribution of transitions from the excited states above the 5D1 to luminescence phenomena. Evaluated activation energy Ea ~4600 cm1 characterizing a steep decrease of the 5D0 lifetime of Sm2þ between 320 K and 550 K indicates strongly that electronic origin of the lowest energy 7F0 e 4f5d1 vibronic transitions is located around 19200 cm1, slightly below the 7F0 e 5D3 transition energy. © 2015 Elsevier B.V. All rights reserved.

Keywords: Divalent samarium Luminescence Tetraborate Crystal field Red phosphor

1. Introduction There is a long lasting interest in Sm2þ -doped materials. In early works the laser potential of CaF2:Sm2þ and SrF2:Sm2þ systems has been investigated since the low energy of the 4f5d1 excited configuration of Sm2þ made it possible to use broad vibronic bands related to parity allowed interconfigurational transitions for optical pumping [1,2]. Numerous papers have been devoted to the interpretation of spectroscopic properties of Sm2þ in alkali halide crystals, e.g.: Ref. [3e5]. Attention has been paid to elucidate mechanisms responsible for the 5D0 e 7F0 transition intensity of Sm2þ in solids [6e8]. Interest in oxide compounds doped with Sm2þ ions has been

Abbreviations: FWHM, full width at half maximum; XRD, x-ray diffraction; JCPDS, joint committee on powder diffraction standards; CIE, commission internationale de l'Eclairage; CF, crystal field; CAS, crystallographic axis system; SPM, superposition model; CFPs, crystal-field parameters; RMS, root-mean-square. * Corresponding author. E-mail address: [email protected] (P. Solarz). http://dx.doi.org/10.1016/j.jallcom.2015.11.155 0925-8388/© 2015 Elsevier B.V. All rights reserved.

stimulated by the discovery of intense luminescence in SrB4O7 host doped with Eu2þ [9] and with Sm2þ ions [10]. In has been then demonstrated that divalent rare earth ions show luminescence in both crystalline and glass modification of SrB4O7 host [11]. In these studies the materials have been prepared in a reducing atmosphere consisting of H2/N2 or H2/Ar gas mixtures to induce reduction of rare earth ions to divalent state. Further progress in the chemistry of SrB4O7 host doped with divalent rare earth ions resulting from a successful preparation of these systems in air, without any reducing agents [12], has stimulated the search for other oxide-based hosts [13,14]. Up to now, however, the SrB4O7 is a unique host in which divalent rare earth ions show luminescence with practically useful efficiency, in similar hosts like CaB4O7, BaB4O7, CdB4O7 and PbB4O7 the luminescence of divalent samarium ions has not been observed [13]. Recent papers dealing with the SrB4O7:Sm2þ system have demonstrated its suitability for application as optical pressure sensors at high temperatures [15e18]. Peculiarities of emission spectra related to the intra-configurational transitions of Sm2þ in SrB4O7:Sm2þ have been reported in Refs. [19e21]. Effect of

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temperature on intensities of inter- and intraconfigurational transitions and luminescence lifetime of SrB4O7:Sm2þ have been studied in Ref. [22]. However, there are still open questions regarding the crystal field splitting of excited multiplets involved in luminescence phenomena. In particular, experimental energies of crystal field components of the 5D3, 5D2, 7F5 and 7F6 are missing and few crystal field components of the 7F3 and 7F4 have been located. Available information on energy of the lowest state of the 4f5d1 configuration is rather qualitative and the excited state relaxation dynamics is not fully understood. Intention of this work is to get a more detailed insight into issues mentioned above. Spectroscopic measurements performed in a wide temperature region from 5 to 550 K and the crystal field calculation made it possible to get a new information on the system under study. 2. Experimental section 2.1. Synthesis Undoped and samarium-doped SrB4O7 polycrystalline samples were prepared by solid state reaction method. As starting materials SrCO3 (4N5), B2O3 (4N) and Sm2O3 (4N) were used. Powders of strontium carbonate and diboron trioxide were dried for 10 h at 300  C while samarium oxide was dried at 1000  C for 6 h. Then the appropriate amounts of substrates were weighted to obtain the molar compositions of SrB4O7 and Sr0.99Sm0.01B4O7. Mixed powders were pressed into pellets and heated in air for 10 h at 300  C and then for 10 h at 800  C. The first step of heating is necessary to avoid melting of B2O3 (melting point at 450  C) before getting in reaction with strontium carbonate. When the reaction begins, CO2 starts to volatilize and heated pellets begin to swell. The second step of heating is performed in order to remove CO2 from the material. Thoroughly ground product of the first synthesis was formed again into pellets and heated in air again for 24 h at 800  C in order to get rid of the remains of CO2 and other SrOeB2O3 phases, and obtain pure SrB4O7 phase. After this step the material was ground and formed in shape of pellets of 17 mm in diameter and 2 mm thickness and heated for 24 h at 800  C to complete the synthesis of pure and Sm - doped material. After this procedure the material was ready for structural and spectroscopic investigation. 2.2. Crystallographic structure determination Phase analysis and structural refinement for synthesized samples were performed with a Siemens D5000 diffractometer (Nifiltered CuKa radiation). Data were collected in the range 20e100 with a step of 0.02 and averaging time of 10 s/step. 2.3. Spectroscopic measurements Experiments, in the range 300e550 K, were accomplished using an excitation source consisting of a femtosecond laser (Coherent Model “Libra”) coupled to optical parametric amplifier (Light Conversion Model “OPerA”). The system delivers 100 fs pulses at repetition rate regulated up to 1 kHz at wavelength tuned between 230 and 2800 nm. The pulse energy is comprised between 6 and 150 mJ, depending on the spectral region. Excitation light was focused on single crystal samples using a lens having a focal length of 30 cm. Location of the focus with respect to the sample surface was changed by moving the lens farther or closer to the sample. Luminescence emerging from the sample was observed in the direction perpendicular to the excitation beam. Appropriate longand short-pass filters have been used to eliminate unwanted

radiation. To record preliminary survey luminescence spectra a micro spectrometer (Hamamatsu Multichannel Analyzer PMA e 12) was employed. High resolution luminescence spectra and luminescence decay curves were recorded with a grating spectrograph (Princeton Instr. Model Acton 2500i) coupled to a streak camera (Hamamatsu Model C5680) operating in the 200e1100 nm spectral region with a temporal resolution of 20 ps. To perform measurement at high temperatures the sample was placed in a software controlled heater house with thermocouple. Low temperature and high resolution spectra were recorded upon ion argon laser excitation 5145.31 Å line with DongWoo Optron CO setup composed of 750 mm focal length monochromator DM711 (1 cm1 spectral resolution FWHM for 0.02 mm slits), and detection systems: PDS-1 with photomultiplier tube R3896 (Hamamatsu) or PS/TC-1 controller with H-series receiver module IGA-030-TE2-H (Electro-Optical Systems Inc.). Kinetic measurements were taken with Tektronix TDS 3052 oscilloscope when the Surelite I OPO (Continuum) has been used for excitation. To perform measurements at low temperature, the samples were installed in a continuous flow liquid helium cryostat equipped with a temperature controller. 3. Results and discussion 3.1. Structural considerations XRD patterns shown in Fig. 1 indicate that undoped sample is consistent with pure SrB4O7 phase (JCPDS Card No: 15-0801). In the samples doped with 1at% of samarium one cannot observe the shift of the diffraction peaks due to similar ionic radii of Sr2þ (1.31 Å) and Sm2þ (1.32 Å) and low concentration of the dopant ions. However, in samarium-doped sample in addition to the main SrB4O7 tetraborate phase there is a little amount of metaborate (JCPDS Card No: 15-0779). Similar result for doped SrB4O7 has been reported earlier [20e22]. SrB4O7 crystallizes in the orthogonal space group Pmn21 with the lattice constants (in nm): a ¼ 0.4237, b ¼ 0.4431 and c ¼ 1.0706 [23,24]. The crystal structure is built up from corner-sharing borate tetrahedra forming a three-dimensional network with channels along the b- and c-axis, where the strontium cations are located. The Sr2þ ion is surrounded by nine oxygen atoms at a distance from 0.2525 to 0.2840 nm, forming SrO9 polyhedra, with monoclinic Cs symmetry. The SrB4O7 structure is depicted in Fig. 2. For more information see, Crystal-field calculation section. 3.2. Optical spectra and excited state relaxation dynamics Excitation of samarium-doped SrB4O7 sample at any wavelength below ca 550 nm brings about an intense luminescence whose spectral characteristics, hence color, depend strongly on the sample temperature. Fig. 3 compares luminescence spectra recorded in the visible region at several different temperatures. At 320 K the spectrum consists essentially of relatively narrow lines with the highest energy line peaking at 16000 cm1. When the temperature increases a broad luminescence band arises and grows steadily achieving the maximum intensity at 470 K, and then, both the narrow lines and the broad band tend to decrease. The chromaticity diagram shown in the inset to Fig. 3 provides color coordinates of the sample emission at temperatures indicated. It follows from Fig. 3 that in contrast to majority of available visible phosphors the system under study has an ability to emit intense light at temperatures well above room temperature offering thereby an important practical potential. In addition, the color of emitted light can be adjusted to some degree by varying the temperature of the phosphor. The emission of samarium-doped SrB4O7

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421

Fig. 1. Diffraction patterns of SrB4O7 and Sr0.99Sm0.01Br4O7 polycrystalline pellets.

is long-lived and after a short pulse excitation the intensities of both the broad band and narrow lines decay with the same time constant (lifetime) value. It follows from the plot of the emission lifetime versus increasing temperature, shown in Fig. 4, that the gentle decrease of lifetime values from about 5 ms to 4.2 ms in the low temperature region between 5 and 350 K becomes abrupt at higher temperatures reducing the lifetime value to about 0.55 ms at 550 K. The activation energy Ea ~4631 cm1 (±120 cm1), characterizing the steep decrease of lifetime in the 450e570 K temperature region, was determined from the Arrhenius plot shown in the inset in Fig. 4, according to formula:

lnðWÞ ¼ lnðAÞ 

Ea kT

(1)

where W is transition rate defined as an inverse of luminescence lifetime and A (s1) represents transition speed not-dependent on the temperature, Ea is the energy of thermal activation, T is the temperature and k is the Boltzmann constant. General understanding of findings described above relies on results gathered during early studies of Sm2þ -doped fluoride or chloride crystals [1,2,5] and more recent investigation of the SrB4O7:Sm2þ system [13,14]. It is based on the assumption that emission features of Sm2þ result from the competition between parity allowed vibronic 4f5d1 e 4f6 transitions and lower energy intra-configurational 4f6 e 4f6 transitions. Owing to small energy difference between the ground and excited configurations the excitation of the 4f5d1 states is followed by fast nonradiative processes feeding excited states of the ground configuration and terminal relaxation gives rise to relatively narrow emission lines characteristic for transitions within the 4fN configurations of rare earth ions. As the temperature increases the lowest energy states of the 4f5d1 configuration become thermally populated and a broad band related to the vibronic 4f5d1 e 4f6 transitions contributes to the emission spectrum. Experimental data gathered thus far for the SrB4O7:Sm2þ system corroborate qualitatively the interpretation summarized above. In fact, room temperature excitation spectra of the 5D0 e 7F0 emission of Sm2þ reported in Refs. [12,14] indicate that the lowest energy state of the 4f5d1 configuration is located above 18000 cm1. The most extensive and thorough spectroscopic investigation of SrB4O7:Sm2þ reported in a recent paper by Sakirzanovas et al. [22] has encompassed measurement of excitation spectra, emission spectra and luminescence lifetimes as a function of temperature in the 100e500 K temperature region. Our results depicted in Fig. 3 agree well with their data. Also, a temperature

dependence of lifetimes reported at the time, revealing the occurrence of an abrupt drop of the lifetime value down to ca 2 ms at 500 K, is fully consistent with our plot presented in Fig. 4. However, Authors of Ref. [22] did not determine the electronic origin of the lowest energy 4f6 e 4f5d1 transition. More attention has been paid to the description of emission spectra related to transitions within the 4f6 configuration of Sm2þ in SrB4O7 host. In several published papers transition energies and crystal field splitting of low energy terminal multiplets have been determined based on high resolution spectra recorded at 5 K [26], 77 K [20], 100 K [22], and 300 K [16]. Reliability of data found depends on intensities of the 5D0 e 7FJ transitions. Available information is restricted to terminal multiplets 7F0-4 but energies of few crystal field levels of the 4F3 and 4F4 multiplets have been reported owing to very weak intensity of related transitions. In addition, dissimilarity of the values determined in different works is quite large. Disparity in energy of crystal field levels of 7F0-2 multiplets reported thus far is reasonable small, however. Table 1 gathers relevant values reported in several published papers. In the last column of this table our experimental data determined from emission spectra recorded at 5 K are included for a comparison. It can be seen that energies of crystal field levels in Table 1 follow from spectra recorded at different temperatures ranging from 5 to 300 K and the disparity of data may result, in principle, from thermal shift of spectral lines. To examine this supposition the emission spectra were recorded as a function of temperature in the 5e500 K temperature region and the line shift was assessed. It has been found that the thermal shift of spectral lines does not exceed 2 cm1 except for lines located at 14387, 14312, 13814 and 13632 cm1 that are strongly affected by temperature, as shown in Fig. 5. Corresponding energies of terminal crystal field levels are indicated by asterisks in Table 1. Accordingly, the thermal line shift can not be considered as a major factor affecting the consistency of data gathered in this Table. Careful optimization of our experimental set-up made it possible to observe and record emission spectra that include weak transition ending on crystal field levels of the 7F4 and 7F5 multiplets. Fig. 6 shows spectra recorded at 5 K. These spectra has been recorded with different monochromator slit widths (given in the figure) to show details that are concealed by the 5D0 / 7F0 transition, which dominate the spectrum. Derived energies of crystal field levels were next used as input data for a crystal field calculation described in the following subsection.

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Fig. 2. SrB4O7 crystal structure seen in three different directions (according to crystallographic data from Ref. [25]).

P. Solarz et al. / Journal of Alloys and Compounds 661 (2016) 419e427

Fig. 3. Influence of temperature on spectral characteristics of Sr0.99Sm0.01B4O7 luminescence. Inset shows corresponding colors of luminescence represented on the CIE chromaticity diagram. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Lifetime of the 5D0 Sm2þ multiplet in Sr0.99Sm0.01B4O7 versus temperature, monitored at the 5D0e7F0 transition at 14593 cm1. The inset represents Arrhenius plot of activation energy.

423

Fig. 5. Thermal shift of spectral lines located at 14387, 14312, 13814 and 13632 cm1 at 5 K.

Fig. 6. Luminescence spectrum of Sr0.99Sm0.01B4O7 at 5 K. Gradual increase of a monochromator slits from 0.02 mm to 1 mm reveals the spectral distribution of the emission intensity. The spectrum recorded with 0.02 mm slits width is inverted for better clarity.

3.3. Crystal-field calculation (CF)

Table 1 Energies of crystal field levels of the 7F1 and 7F2 multiplets at different temperatures with accuracy 1 cm1. Asterisks indicate terminal levels have been analyzed in Fig. 5, as the lines at 14387, 14312, 13814, 13632 cm1. Multiplet

7 7

7

F0 F1

F2

Energy [cm1] 300 K [16]

100 K [22]

77 K [20]

5 K [26]

5 K this work

0 211 276 398/414 743 782 835

0 205 277 399 738 776 833 e

0 209 281 393 747 775 832 e

0 207 279 398 744 779 837 960

0 206* 281* 398 744 779* 835 872

955

964

963

961*

e 957

For crystal-field (CF) calculation we assume that Sm2þ substitute for Sr2þ ion. However, the actual coordination of central ion is not evidently defined, because there are six next-nearest oxygen neighbors, at 0.3042e0.3203 nm distances. Inclusion of these additional six oxygen atoms gives SrO15 polyhedra with the preserved Cs site symmetry of strontium ion. Since next-nearest oxygen neighbors may have some influence on the CF exerted on Sr2þ ions, both types of coordination are considered in our CF calculations. For superposition model (SPM) calculations we adopt the Cartesian axis system (x, y, z) defined w.r.t. the crystallographic axis system (CAS) (a, b, c) in such a way the z-axis coincides with c-axis and the angle between the x- and a-axis is 132.05 . In such defined (x,y,z) axis system the site symmetry of Sr(Sm) is Cs and the z-axis is perpendicular to the sh symmetry plane. So chosen axis system (x, y, z) constitutes the symmetry adapted axis system. The spherical

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Table 2 The spherical polar coordinates (Ri, qi, 4i) of oxygen ligands in SrB4O7 expressed in the Cartesian axis system (x, y, z) in Fig. 7. The SrO9 and SrO15 polyhedra are formed by O1eO9 and O1eO15 oxygen atoms, respectively.

O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 O15

4i[ ]

qi[ ]

Ri[nm]

0.00 66.47 147.56 66.47 147.56 57.84 152.97 57.84 152.97 93.87 102.63 102.44 2.41 102.44 2.41

90.00 53.29 57.89 126.71 122.11 57.19 58.45 122.81 121.55 90.00 90.00 13.58 41.81 166.42 138.19

0.2625 0.2525 0.2840 0.2525 0.2840 0.2668 0.2762 0.2668 0.2762 0.3153 0.3042 0.3051 0.3203 0.3051 0.3203

polar coordinates (Ri, qi, 4i) of oxygen ligands in SrB4O7 expressed in this axis system are given in Table 2, whereas the SrO9 and SrO15 coordination polyhedra are depicted in Fig. 7. b ¼ For the energy level calculations we apply the Hamiltonian H b suitable for 4fN ions in crystals [27e30]. The observed b FI þ H H CF energy levels are fitted to the experimental ones by simultaneous b FI Þ and the crystal field part diagonalization of the free-ion part ð H b CF Þ expressed in the expanded form [31] in the Wybourne noðH tation [27]. The theoretical background for these calculations and notations used for relevant parameters may be found in our previous papers [32,33]. The general SPM [34,35] expressions in the Wybourne notation, which involve the intrinsic parameters Bk , the power law exponents tk, and the reference metal-ligand distance R0 may be found in Refs. [32,33]. In the present study the shortest SreO distance was chosen as the reference distance, i.e. R0 ¼ 0.2525 nm. For monoclinic Cs site symmetry the number of independent crystal-field parameters (CFPs) is reduced from 27 to 15. Since in the adopted axis system (x,y,z) the sh plane coincides with the xy plane (Fig. 7) and is perpendicular to the z(c)-axis, the appropriate b ðCs Þ is that with even q value CFPs ReBkq form of the monoclinic H CF and ImBkq, i.e. with k ¼ 2, 4, 6, and q ¼ 2, 4, 6. In the present calculations the energy matrix of HCF(Cs) was restricted to the subspace spanned by the 360 jSLJMJ > basis vectors, i.e. to energy ranging from 0 to ~30000 cm1. Among parameters of free-ion Hamiltonian only values of F2 and z4f have been previously determined based on experimental data for Sm2þ in MFCl (M ¼ Ca, Sr, Ba)

Table 3 The fitted free-ion and CF parameters for Sm2þ ions in SrB4O7, derived with assumption that the first coordination sphere is formed by nine (SrO9) or fifteen (SrO15) oxygen atoms e all values are in cm1, except for n (dimensionless), experimental uncertainties are given in brackets; Note that B20, ReB22 and ImB22 were obtained from direct fittings, whereas the remaining CFPs are calculated using the fine-tuned values of B4 and B6 obtained using the SPM procedure and the coordination factors; The F4 and F6 were constrained by ratios: F4/F2 ¼ 0.668 and F6/ F2 ¼ 0.494; S is the CF strength parameter defined in Ref. [38].

F2 F4 F6

z4f B20 ReB22 ImB22 B40 ReB42 ImB42 ReB44 ImB44 B60 ReB62 ImB62 ReB64 ImB64 ReB66 ImB66 N Rms S

SrO9

SrO15

75050(48) 50133 37075 1073(3) 487(47) 158(54) 268(42) 252 135 45 5 53 632 430 49 569 572 402 126 27 13.4 280.0

75032(51) 50121 37066 1072(3) 475(49) 190(63) 243(52) 313 149 72 33 55 602 334 25 546 514 341 89 27 14.3 289.6

[36] and MFBr (M ¼ Sr, Ba) [37] hosts. Our experimental data enabled identification of energy levels of 7FJ (J ¼ 0e6) and 5DJ (J ¼ 0 and 1) multiplets. Owing to such limited experimental data set only two free-ion parameters, F2 and z4f were optimized in fittings. The F4 and F6 Slater parameters were kept at constant ratios: F4/ F2 ¼ 0.668 and F6/F2 ¼ 0.494, within the approximation of hydrogen-like wavefunctions, whereas the values of other free-ion parameters (Trees parameters e a, b, g, and three-body interaction parameters Ti) were set to zero. It has been shown that such twoparameter model can reasonably describe the free-ion multiplet (7FJ þ 5DJ) schemes of Sm2þ 4f6 configuration, with rms deviation as low as about 10 cm1 [36]. The number of 27 energy levels determined from experiment is not feasible for reliable fitting of 15 CF and 2 FI parameters. Therefore, the energy level calculations have been carried out within the superposition model, which enables to express 15 parameters of HCF(Cs) in terms of 3 intrinsic parameters Bk and 3

Fig. 7. The SrO9 (a) and SrO15 (b) polyhedra represented in the Cartesian (x, y, z) axis system suitable for the Cs symmetry. The orientation of the CAS is also shown.

P. Solarz et al. / Journal of Alloys and Compounds 661 (2016) 419e427 Table 4 The experimental energy levels and the calculated ones by using the Hamiltonian parameter values listed in column SrO15 in Table 3.

Table 4 (continued ) Energy (cm1) SLJ statea

Energy (cm1) SLJ statea 7 7

7

7

7

7

F0 F1

F2

F3

F4

F5

7

F6

5

D(3)0 D(3)1

5

5

D(3)2

5

L6

calcdb 8.1 211.8 271.8 411.6 748.0 788.9 823.7 866.0 931.7 1457.7 1465.0 1487.8 1510.9 1544.8 1573.7 1576.8 2128.1 2212.4 2260.1 2302.0 2332.2 2346.7 2385.8 2417.8 2502.6 3042.8 3053.4 3129.5 3143.5 3192.5 3220.1 3239.9 3294.6 3314.1 3333.1 3336.0 3963.6 3967.9 3968.2 3976.6 4048.1 4069.6 4091.9 4136.0 4143.7 4217.3 4218.8 4328.6 4328.6 14607.9 15902.5 15923.1 15962.4 17869.1 17878.1 17886.0 17891.0 17891.7 19963.5 19995.3 20016.7 20057.3 20068.4 20082.1 20086.6 20118.0 20130.7 20161.8 20178.8 20183.8 20189.4

exptc 0.0 206.4 281.1 397.7 744.0 778.8 835.2 872.0 960.8 * 1460.1 1479.1 * 1541.5 1570.4 * 2120.0 * 2256.9 2294.8 2326.5 2349.0 2402.7 * * 3031.8 3083.0 * * 3188.0 3223.6 * * * * * * * * * * * * * * * * * * 14593.1 15915.2e 15939.8e 15947.6e e e e e e e e e e e e e e e e e e e

Dd 8.1 5.4 9.3 13.9 4.0 10.1 11.5 6.0 29.1 * 4.9 8.7 * 3.3 3.3 * 8.1 * 3.2 7.2 5.7 2.3 16.9 * * 11.0 29.6 * * 4.5 3.5 * * * * * * * * * * * * * * * * * * 14.8 12.7 16.6 14.8 e e e e e e e e e e e e e e e e e e

425

5

D(3)3

calcdb

exptc

Dd

20195.5 20205.0 20207.4 20219.1 20228.8 20247.1 20248.9

e e e e e e e

e e e e e e e

a

Nominal quantum numbers for the atomic state(s) associated with the group. Calculated by using the Hamiltonian parameter values listed in column SrO15 in Table 3. c Experimentally determined positions of energy levels. d Difference between observed and calculated energies. e Values determined from spectra recorded at 300 K. b

exponents tk (k ¼ 2,4 and 6). Since the 2nd -rank CFPs determined by SPM are least reliable, the 2nd -rank parameters were not expressed in SPM terms but varied freely as the CFPs B2q, which increased the number of freely varied parameters only by one as compared to the pure SPM case. The coordination factors gk,q were calculated for the ligands' positions of Table 2 and in fittings to experimental energy levels the F2, z4f, B20, ReB22, ImB22, B4 and B6 parameters were simultaneously optimized. Since the t4 and t6 power low exponents for Sm2þeO2 bonds have not been yet reported, the values between 1 and 18 have been tested at various combinations in our calculations. The analysis have been performed with assumption that the coordination sphere of Sm2þ is formed by nine (SrO9) or fifteen (SrO15) oxygen atoms. In the nine ligand model the best solution with rms ¼ 13.4 cm1 was obtained for t4 ¼ 11 and t6 ¼ 14, yielding the intrinsic parameters as: B4 ¼ 183(55) cm1 and B6 ¼ 876(73) cm1. In the case when 15 ligands were included, the lowest rms deviation of 14.3 cm1 was obtained for t4 ¼ 9 and t6 ¼ 13, with intrinsic parameters determined as: B4 ¼ 268(66) cm1 and B6 ¼ 781(58) cm1. In both cases the agreement between the experimental energy levels and the calculated ones can be considered as very satisfactory. The values of intrinsic parameters and power low exponents appear very reasonable. The CFPs calculated using the fine-tuned values of Bk and tk obtained using the SPM procedure and the appropriate coordination factors are listed in Table 3, together with B2q, F2 and z4f obtained from direct fittings. The experimental energy levels and calculated ones for SrO15 coordination polyhedra are provided in Table 4. The optimized values of free-ion F2 and z4f parameters are in good accordance with those reported earlier for Sm2þ [36,37]. One may notice relatively small differences between CFPs obtained with assumption that the first coordination sphere is formed by nine ligands (SrO9) and those obtained with inclusion of additional six next-nearest ligands. This suggests that the CF effect is exerted mostly by the nine nearest oxygens with the minor contribution from six oxygens in the next-nearest sphere. The CF affecting Sm2þ ions in SrB4O7 is considerably larger as compared to MFX hosts (M ¼ Sr, Ba; X ¼ Cl, Br). From the calculated Bkq values the CF strength parameter S ¼ 280.0 cm1 and S ¼ 289.6 cm1 was obtained for SrB4O7:Sm2þ assuming SrO9 and SrO15 coordination polyhedra, respectively, whereas for MFX:Sm 2þ the S values are confined in the 90.9e112.6 cm1 range [36,37]. This is in accordance with experimentally observed differences in total splittings of SLJ multiplets: for example, the splitting of the 7F1 multiplet of Sm2þ in SrB4O7 is 192 cm1, whereas for BaFBr:Sm2þ was found equal to 84.5 cm1 [37]. There are no reported data for Sm2þ in oxide hosts available for comparison with the present results. It can be seen in Table 4 that the agreement between calculated

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and experimental energies of crystal field levels is reasonable good. Available experimental data make it possible to compare the overall crystal field splitting of 199 cm1 and 190 cm1 calculated for the 7 F1 and 7F2 multiplets, respectively, to corresponding experimental values amounting to 190 cm1 and 217 cm1. The crystal field calculation provides energies of crystal field levels of Sm2þ in SrB4O7 that were not observed in experiment and predicts the overall crystal field splitting of 121, 368, 292 and 372 cm1 for the 7 F3, 7F4, 7F5 and 7F6 multiplets, respectively. Calculated energies for the 5D2 and 5D3 multiplets agree well with values amounting to 17814.5 cm1 and 20110 cm1 determined in the past for respective multiplets of Sm2þ in BaClF host [5]. However, proposed assignment of sharp lines observed between 18957 cm1 and 19056 cm1 in the 100 K excitation spectrum of SrB4O7:Sm2þ to the 7F0 e 5D2 transition [22] is not consistent with our calculation. Instead, the correlation of calculated energies of crystal field levels and evaluated activation energy Ea ~4600 cm1 indicates strongly that the lines mentioned above are related with the electronic origin of the lowest energy 7F0 e 4f5d1 vibronic transitions. 4. Conclusions In contrast to majority of available visible phosphors the SrB4O7:Sm2þ system has an ability to emit intense light well above room temperature offering thereby an important practical potential. In addition, the color of emitted light can be adjusted to some degree by varying the temperature of the phosphor. Careful optimization of the experimental set-up made it possible to observe and record emission spectra that include weak transition ending on crystal field levels of the 7F4 and 7F5 multiplets providing therefore a set of experimental input data for the crystal field calculation. The calculation predicted energies of crystal field components of excited Sm2þ multiplets up to 25000 cm1 offering a possibility to assess the contribution of transitions from the excited states above the 5D1 to luminescence phenomena. Calculated energy for the 5D2 multiplet of Sm2þ in SrB4O7 agrees well with that reported in the past for the 5D2 multiplet of Sm2þ in BaClF host. The activation energy Ea ~4600 cm1 characterizing a steep decrease of the 5D0 lifetime of Sm2þ between 320 and 550 K indicates strongly that electronic origin of the lowest energy 7F0 e 4f5d1 vibronic transitions is located around 19200 cm1, slightly below the 7F0 e 5D3 transition energy. Funding sources NCN under project DEC-2013/09/D/ST5/03878. Acknowledgment The work was funded by the National Science Centre (NCN http://ncn.gov.pl/) on the basis of the decision number DEC-2013/ 09/D/ST5/03878. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2015.11.155. References [1] W. Kaiser, C.G.B. Garrett, D.L. Wood, Fluorescence and optical maser effects in CaF2: Smþþ, Phys. Rev. 123 (1961) 766e776, http://dx.doi.org/10.1103/ PhysRev.123.766. [2] P.P. Sorokin, M.J. Stevenson, J.R. Lankard, G.D. Pettit, Spectroscopy and optical maser action in SrF2:Sm2þ, Phys. Rev. 127 (1962) 503e508, http://dx.doi.org/ 10.1103/PhysRev.127.503.

[3] J.D. Axe, P.P. Sorokin, Divalent rare earth spectra selection rules and spectroscopy of SrCl2:Sm2þ, Phys. Rev. 130 (1963) 945e952, http://dx.doi.org/ 10.1103/PhysRev.130.945. [4] Francis K. Fong, Lawrence A. Vredevoe, Roger E. De Wames, Symmetry selection rules of electric-dipole transitions of Eu3þ and Sm2þ in crystals: a solid-state infrared quantum counter-amplifier, Phys. Rev. 170 (1968) 412e417, http://dx.doi.org/10.1103/PhysRev.170.412. [5] Z.J. Kiss, H.A. Weakliem, Stark effect of 4f states and linear crystal field in BaCIF: Sm2þ, Phys. Rev. Lett. 15 (1965) 457e460, http://dx.doi.org/10.1103/ PhysRevLett.15.457. [6] L. Smentek, B.A. Hess Jr., Theoretical description of 040 and 041 transitions in the Eu3þ ion in hosts with C2v symmetry, Mol. Phys. 92 (1997) 835e845, http://dx.doi.org/10.1080/002689797169781. [7] Takashi Kushida, Masanori Tanaka, Transition mechanisms and spectral shapes of the 5D0-7F0 line of Eu3þ and Sm2þ in solids, Phys. Rev. B 65 (2002) 195118.1e195118.6, http://dx.doi.org/10.1103/PhysRevB.65.195118. [8] H. Hagemann, F. Kubel, H. Bill, F. Gingl, 5D0/7F0 transitions of Sm2þ in SrMgF4:Sm2þ, J. Alloys Compd. 374 (2004) 194e196, http://dx.doi.org/ 10.1016/j.jallcom.2003.11.091. [9] A. Meijerink, J. Nuyten, G. Blasse, Luminescence and energy migration in (Sr,Eu)B4O7, a system with a 4f7-4f65d crossover in the excited state, J. Luminescence 44 (1989) 19e31, http://dx.doi.org/10.1016/0022-2313(89) 90017-3. [10] A. Lacam, C. Chateau, High-pressure measurements at moderate temperatures in a diamond anvil cell with a new optical sensor: SrB4O7:Sm2þ, J. Appl. Phys. 66 (1989) 366e372, http://dx.doi.org/10.1063/1.343884. [11] J.W.M. Verwey, G.J. Dirksen, G. Blasse, The luminescence of divalent and trivalent rare earth ions in the crystalline and glass modifications of SrB4O7, J. Phys. Chem. Solids 53 (1992) 367e375, http://dx.doi.org/10.1016/00223697(92)90170-I. [12] Zhiwu Pei, Qiang Su, The valence change from RE3þ to RE2þ (RE: Eu, Sm, Yb) in SrB4O7:RE prepared in air and the spectral properties of RE2þ, J. Alloys Compd. 198 (1993) 51e53, http://dx.doi.org/10.1016/0925-8388(93)90143-B. [13] P. Mikhail, J. Hulliger, Sm2þ in oxide lattices: an unexplored chapter of crystal chemistry, Comments Inorg. Chem. 21 (1999) 263e283, http://dx.doi.org/ 10.1080/02603599908012009. [14] Patric Mikhail, Jürg Hulliger, Marc Schnieper, Hans Bill, SrB4O7:Sm2þ: crystal chemistry, Czochralski growth and optical hole burning, J. Mater. Chem. 10 (2000) 987e991, http://dx.doi.org/10.1039/A906056A. [15] F. Datchi, A. Dewaele, P. Loubeyre, R. Letoullec, Y. Le Godec, B. Canny, Optical pressure sensors for high-pressureehigh-temperature studies in a diamond anvil cell, High Press. Res. 27 (2007) 447e463, http://dx.doi.org/10.1080/ 08957950701659593. [16] Sergey V. Rashchenko, Anna Yu Likhacheva, Tatyana B. Bekker, Preparation of a macrocrystalline pressure calibrant SrB4O7:Sm2þ suitable for the HP-HT powder diffraction, High Press. Res. 33 (2013) 720e724, http://dx.doi.org/ 10.1080/08957959.2013.819098. [17] Qiumin Jing, Qiang Wu, Yonggang Liu, Yi Zhang, Shenggang Liu, Lei Liu, Jian Xu, Yan Bi, Effect of pressure and temperature on the wavelength shift of the fluorescence line of SrB4O7:Sm2þ scale, High Press. Res. 33 (2013) 725e733, http://dx.doi.org/10.1080/08957959.2013.845665. [18] Qiumin Jing, Qiang Wu, Lei Liu, Ji-an Xu, Yan Bi, Yonggang Liu, Haihua Chen, Shenggang Liu, Yi Zhang, X. Lun iong, Yanchun Li, Jing Liu, An experimental study on SrB4O7:Sm2þ as a pressure sensor, J. Appl. Phys. 113 (2013) 023507.1e023507.5, http://dx.doi.org/10.1063/1.4774113. [19] Hongbin Liang, Qinghua Zeng, Tiandou Hu, Shubin Wang, Qiang Su, Local structure and valences of samarium in SrB4O7:Sm and SrB6O10:Sm prepared in air, Solid State Sci. 5 (2003) 465e467, http://dx.doi.org/10.1016/S12932558(03)00033-5. [20] R. Stefani, A.D. Maia, E.E.S. Teotonio, M.A.F. Monteiro, M.C.F.C. Felinto, H.F. Brito, Photoluminescent behavior of SrB4O7:RE2þ (RE¼Sm and Eu) prepared by Pechini, combustion and ceramic methods, J. Solid State Chem. 179 (2006) 1086e1092, http://dx.doi.org/10.1016/j.jssc.2006.01.006. [21] Jiayue Sun, Jicheng Zhu, Xiaotang Liu, Haiyan Du, Luminescence properties of SrB4O7:Sm2þ for light conversion agent, J. Rare Earths 30 (2012) 1084e1087, http://dx.doi.org/10.1016/S1002-0721(12)60183-5. [22] Simas Sakirzanovas, Arturas Katelnikovas, Danuta Dutczak, Aivaras Kareiva, Thomas Jüstel, Concentration influence on temperature-dependent luminescence properties of samarium substituted strontium tetraborate, J. Luminescence 132 (2012) 141e146, http://dx.doi.org/10.1016/ j.jlumin.2011.08.011. [23] A. Perloff, S. Block, The crystal structure of the strontium and lead tetraborate, SrO$2B2O3 and PbO$2B2O3, Acta Crystallogr. 20 (1966) 274e279, http:// dx.doi.org/10.1107/S0365110X66000525. [24] W.-D. Stein, J. Liebertz, P. Becker, L. Bohatý, M. Braden, Structural investigations of the tetraborate MB4O7 (M ¼ Pb, Sr, Ba), Eur. Phys. J. B 85 (2012) 236.1e236.5, http://dx.doi.org/10.1140/epjb/e2012-21062-y. [25] J. Krogh-Moe, The Crystal structure of strontium diborate, SrO$2B2O3, Acta Chem. Scand. 18 (1964) 2055e2060, http://dx.doi.org/10.3891/acta.chem.scand.18-2055. [26] Zeng Qinghua, Pei Zhiwu, Wang Shubing, Su Qiang, Lu Shaozhe, The luminescent properties of Sm2þ in strontium tetraborate (SrB4O7:Sm2þ), J. Phys. Chem. Solids 60 (1999) 515e520, http://dx.doi.org/10.1016/S0022-3697(98) 00304-7. [27] B.G. Wybourne, Spectroscopic Properties of Rare Earths, Interscience, New

P. Solarz et al. / Journal of Alloys and Compounds 661 (2016) 419e427 York, 1965. [28] C.A. Morrison, Angular Momentum Theory Applied to Interactions in Solids, Springer, Berlin, 1988. [29] J. Mulak, Z. Gajek, The Effective Crystal Field Potential, Elsevier, Amsterdam, 2000. [30] W.T. Carnall, H. Crosswhite, H.M. Crosswhite, J.P. Hessler, N.M. Edelstein, J.G. Conway, G.V. Shalimoff, R. Sarup, Energy level analysis of Np3þ:LaCl3 and Np3þ:LaBr3, J. Chem. Phys. 72 (1980) 5089e5102, http://dx.doi.org/10.1063/ 1.439797. [31] Czesław Rudowicz, Concept of spin Hamiltonian, forms of zero field splitting and electronic Zeeman Hamiltonians and relations between parameters used in EPR. A critical review, Magn. Reson. Rev. 13 (1987) 1e89. Rudowicz, C. ibid 13(1988) 335e355 (erratum). [32] M. Karbowiak, P. Gnutek, C. Rudowicz, Reanalysis of energy levels and crystal field parameters for Er3þ and Tm3þ ions at C2 symmetry sites in hexahydrated trichloride crystalsdIntricate aspects of multiple solutions for monoclinic symmetry, Phys. B Phys. Condens. Master 405 (2010) 1927e1940, http:// dx.doi.org/10.1016/j.physb.2010.01.086. [33] M. Karbowiak, C. Rudowicz, P. Gnutek, Energy levels and crystal-field

[34] [35] [36]

[37]

[38]

427

parameters for Pr3þ and Nd3þ ions in rare earth (RE) tellurium oxides RE2Te4O11 revisited e ascent/descent in symmetry method applied for triclinic site symmetry, Opt. Mater. 33 (2011) 1147e1161, http://dx.doi.org/ 10.1016/j.optmat.2011.01.027. D.J. Newman, B. Ng, The superposition model of crystal fields, Rep. Prog. Phys. 52 (1989) 699e762, http://dx.doi.org/10.1088/0034-4885/52/6/002. D.J. Newman, B. Ng, 2000 Superposition model, in: D.J. Newman, B. Ng (Eds.), Crystal Field Handbook, University Press, Cambridge, 2000. Ch. 5. Y.R. Shen, W.B. Holzapfel, Nephelauxetic effects on Sm2þ and Eu3þ in ternary MYX compounds, Phys. Rev. B 52 (1995) 12618e12626, http://dx.doi.org/ 10.1103/PhysRevB.52.12618. €t, Vincenza D'Anna, Hans Hagemann, Effect of Prodipta Pal, Tiphaine Penhoue pressure on the free ion and crystal field parameters of Sm2þ in BaFBr and SrFBr hosts, J. Luminescence 134 (2013) 678e685, http://dx.doi.org/10.1016/ j.jlumin.2012.07.010. N.C. Chang, J.B. Gruber, R.P. Leavitt, C.A. Morrison, Optical spectra, energy levels, and crystal-field analysis of tripositive rare earth ions in Y2O3. I. Kramers ions in C2 sites, J. Chem. Phys. 76 (1982) 3877e3889, http:// dx.doi.org/10.1063/1.443530.