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Structural and optical studies of Eu3+ doped Na3Mg2P5O16 pentaphosphate
Journal of Alloys and Compounds 695 (2017) 21e26
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Structural and optical studies of Eu3þ doped Na3Mg2P5O16 pentaphosphate P. Godlewska a, *, A.J. Pelczarska a, A. Watras b, L. Macalik b, M. Ptak b, I. Szczygieł a, J. Hanuza b a b
Wrocław University of Economics, Faculty of Chemistry and Food Technology, ul. Komandorska 118/120, 53-345, Wrocław, Poland lna 2, 50-422, Wrocław, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Oko
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 April 2016 Received in revised form 7 October 2016 Accepted 10 October 2016 Available online 11 October 2016
Sodium-magnesium pentaphosphate with a formula Na3Mg2P5O16 doped with europium(III) have been synthesized. The concentration of the rare earth ion activators was established to be 1 to 10 at.% in the doped samples. Spectroscopic and structural properties of the obtained samples were investigated by Xray, Raman, infrared, absorption and luminescence studies. The structure of the studied material was confirmed as monoclinic P2/a ¼ C42h (No. 13) similar to this described in the literature. The decay times were measured for all samples monitoring the maximum of emission at lmon ¼ 611 nm and exciting in the 5L6 band (lexc ¼ 396 nm). All decay curves were single exponential and lifetimes remain constant with value t ¼ 2 ms ± 0.2 ms for the all dopant concentration. © 2016 Published by Elsevier B.V.
Keywords: Rare earth alloys and compounds Optical spectroscopy Crystal structure X-ray diffraction Optical properties
1. Introduction Rare earth pentaphosphates LnP5O14 have been a subject of a great technical interest due to their potential applications as lasing materials. They exhibit a high luminescence efficiency and good optical quality [1e8]. The pentaphosphates of the Na3Mg2P5O16 type are less recognized, particularly their physicochemical properties practically have not been reported. This compound crystallizes in a monoclinic structure but some controversy exists in the description of its unit cell. Smolin et al. described its unit cell parameters as a ¼ 18.617 ± 0.005 Å, b ¼ 6.844 ± 0.003 Å, c ¼ 5.174 ± 0.003 Å and b ¼ 90.15 , Z ¼ 2 (S.G. P2/a e No. 13) [9]. According to Majling and Hanic [10] the proper unit cell has the dimensions a ¼ 5.177 ± 0.001 Å, b ¼ 6.853 ± 0.001 Å, c ¼ 18.628 ± 0.005 Å and b ¼ 90.00 ± 0.04 , Z ¼ 2 (S.G. P21/c, No. 14). This compound is stable below 675 C. At this temperature decomposition of the compound takes place, leading to the appearance of secondary phases, mainly NaMg(PO3)3 and Mg2P2O7 [11]. It is interesting which description reflects the real structure of
* Corresponding author. E-mail address: [email protected] (P. Godlewska). http://dx.doi.org/10.1016/j.jallcom.2016.10.087 0925-8388/© 2016 Published by Elsevier B.V.
this compound better. Besides, the question arises: can measurements of IR and Raman spectra be used for arbitrating this structural problem. Therefore, in the present work sodium-magnesium pentaphosphates doped with europium(III) have been synthesized and their spectroscopic properties have been studied. The results of these investigations could be used for recognition of future optical applications of these materials. 2. Experimental The undoped and doped with 0.01e0.10 at.% Eu3þ powders of Na3Mg2P5O16 were obtained by carrying out reactions in the solid state. The following chemicals of analytical purity were used for the synthesis: NH4H2PO4 (POCh Gliwice), MgHPO4 (POCh Gliwice), NaH2PO4 (POCh Gliwice) and Eu2O3 (99.9% POCh, Gliwice). The powders were ground, mixed in an agate mortar with acetone and sintered for 1 h at 400 C and for 15 h at 600 C. The last heating step occurred at 600 C for 20 h. Between heating steps the powders were carefully ground in order to obtain homogeneous powders. XRD patterns of the powders were registered using a Siemens D5000 diffractometer (Siemens, Karlsruhe, Germany) equipped with a copper X-ray tube. The measurements were performed in 2q angle range of 5-55 with a 0.04 step and at least 4 s t time per
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step. IR spectra were measured using a Nicolet iS50 FT-IR (Thermo Scientific) spectrometer equipped with an Automated Beam splitter exchange system (iS50 ABX containing a DLaTGS KBr detector and a DLaTGS Solid Substrate detector for mid-IR and far-IR regions, respectively), Built-in an all-reflective diamond ATR module (iS50 ATR), Thermo Scientific Polaris™ and a HeNe laser as an IR radiation source. Polycrystalline mid-IR spectra were collected in the 4000400 cm1 range in KBr pellets and far-IR spectra in the 600-50 cm1 range in Nujol mull. Spectral resolution was 4 cm1. FT Raman spectra in the 1000-80 cm1 range were measured with a Bruker FT-Raman RFS 100/S spectrometer and the 1064 nm excitation. The measurements were performed in the back scattering technique. Spectral resolution was 2 cm1. These spectra were compared to those recorded on a Renishaw InVia Raman spectrometer equipped with a confocal DM 2500 Leica optical microscope, a thermoelectrically cooled Ren Cam CCD as a detector and Arþ ion laser operating at 488 nm. Excitation and low temperature emission spectra were measured by the use of a McPherson spectrometer equipped with 150 W Xe lamp for UV-VIS and 150 W D2 lamp for VUV region as an excitation source and a Hamamatsu R928 photomultiplier as a detector. Focal length of monochromator was 30 cm and resolution was 0.05 nm. Room temperature emission spectra were obtained using a Hamamatsu PMA-12 photonic multichannel analyzer. For measuring emission spectra and decay times 396 nm line from a tunable pulsed Ti:Sapphire laser pumped by second harmonic (l ¼ 532 nm) of Nd:YAG laser was used to excite the samples. The decay curves were collected on a digital oscilloscope. Absolute quantum efficiency was measured on Hamamatsu C9920-02G Absolute PL quantum yield spectrometer. 3. Results and discussion 3.1. XRD studies X-ray diffractograms of the studied materials are shown in Fig. 1. They contain the reflexes characteristic for Na3Mg2P5O16 pentaphosphate (ICDD card No 4-9-4605) and a few very weak peaks, which were assigned to the Na7Mg4.5(P2O7)4 phase (ICDD card No 27-738). Additional peaks have the same intensity, which means that contamination does not change in whole range of Eu3þ concentration and should not affect spectroscopic properties.
Fig. 1. The XRD patterns of Na3Mg2P5O16 powders doped with Eu3þ.
3.2. IR and Raman spectra Figs. 2 and 3 present the MIR, FIR and Raman spectra of the studied phosphates. Table 1 list the band wavenumbers of the IR and Raman spectra. In the analysis of the measured IR and Raman spectra of Na3Mg2P5O16 pentaphosphate the P2/a (C42h, Z ¼ 2) space group (No. 13) [9] is considered in the first step. Its primitive unit cell contains six Naþ, four Mg2þ, ten P5þ and thirty two O2 ions. Sodium atoms occupy the 2f and 4g positions (C2 and C1 symmetry), magnesium and oxygen atoms lie at 4g positions (C1 symmetry) and phosphorus atoms occupy the 2e, 4g and 4g positions. Therefore, 52 atoms of the primitive cell give rise to 156 zone-center degrees of freedom described by the irreducible representation: G ¼ 38Ag þ 40Bg þ 38Au þ 40Bu. Three of them, GT ¼ Au þ 2Bu, describe the acoustic phonons and remaining, GO ¼ 38Ag þ 40Bg þ 37Au þ 38Bu, correspond to the optical modes. The optical modes can be further subdivided into the vibrations of the respective components of the primitive cell: Translations of Naþ ions: GT’ (Naþ) ¼ 4Ag þ 5Bg þ 4Au þ 5Bu, Translations of Mg2þ ions: GT’ (Mg2þ) ¼ 3Ag þ3Bg þ 3Au þ 3Bu,
Fig. 2. The MIR and FIR spectra of Na3Mg2P5O16 doped with different concentration of europium ions.
Vibrations of P5þ atoms: Gv (P) ¼ 7Ag þ 8Bg þ 7Au þ 8Bu, Vibrations of O2 atoms: Gv (O) ¼ 24Ag þ 24Bg þ 24Au þ 24Bu. The internal modes coupled with the librations of the whole
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23
Table 1 The Raman and IR wavenumbers (cm1) for Na3MgP5O16 containing 1 mol% of europium ions. Raman
IR
Assignment
nas(PO2/3)
1296 w 1210 w 1180 m 1164 1153 1123 1107 1096 1068 1055 1034
s m vw w sh w sh w
1287 1210 1194 1175 1163
m s s sh s
ns(PO2/3) 1119 s 1097 m 1063 m 1036 w 1007 sh 963 s
nas(POP)
936 w Fig. 3. The Raman spectra of Na3Mg2P5O16 doped with different concentration of europium ions.
916 m 882 vw 768 w 753 vw 745 vs
units are described by the representation: G(int þ L) ¼ 38Ag þ 40Bg þ 37Au þ 38Bu. Because the Ag and Bg modes are Raman active only, 78 bands are expected in the polarized spectra of single crystal. Similarly, 75 bands should be observed in respective IR spectra because the Au and Bu modes are IR active only. The realization of similar analysis for the P21/c ¼ C52h unit cell is impossible because the positions of the atoms are not reported in the paper [10]. However, independently of this problem the expected phonon properties derived from this structure should be very similar to those calculated for the P2/a unit cell. The same C2h factor group and Z ¼ 2 factor made that the same irreducible representation charter table, Raman tensors and selection rules should work for both structural models. Instead of the C2 site symmetry, the Ci should be taken into account in this case. From the presented below electron absorption and emission studies follow that the increase of the Eu3þ content causes higher ordering of the structure and the local surrounding of europium ions become more octahedral. Therefore the slight phase P2/a / P21/c transition could appear in the studied sample. The phosphate anion in both structures forms a P5O16 treedimensional framework built from five PO4 tetrahedra joined by four bend PeOeP oxygen bridges. All PeO bond lengths in this short ribbon are different and range from 1.4829 to 1.6557 Å [9]. Therefore, the set of expected IR and Raman modes of the single P5O16 unit should contain four ns(POP), nas(POP), d(POP) and g(POP) modes and because such four units are included into the primitive cell, all these numbers should be multiplied by four. Remaining two PO3 and three PO2 components of the P5O16 framework give rise to 44 modes consisting of the nas, ns and d vibrations. The assignment of the observed bands to the respective vibrational modes is proposed in Table 1. In general, the proposed in these tables description of the spectra agrees with those published earlier in the papers [12e25]. The particular types of vibrations appear in the typical for them ranges. The nas(PO2/3) vibrations are observed in the range 1160e1300 cm1 where the IR and Raman multiplets totally contain 9 bands. In the range 1000e1155 cm1, characteristic for the ns(PO2/3) vibrations, 13 bands should be assigned to these modes. The ns(POP) and nas(POP) vibrations appear in the ranges 700e770 and 850e970 cm1, respectively. All bending das(PO2/3) and ds(PO2/3) vibrations are observed in the ranges 450e650 and 300e420 cm1, in which 19 and 15 are recorded, respectively. The medium to strong bands in the range
746 vw 720 m
das(PO2/3)
634 sh 621 w
588 vw 567 vw 557 vw 543 517 500 492 483 476 420 407
vw w sh w sh w vw vw
384 372 362 340 326 318 302
w w vw w w sh vw
275 251 241 225 218
sh w w w sh
613 m 594 sh 584 m
550 sh 539 m 522 w 490 m 482 sh
ds(PO2/3) 409 w 399 w 374 360 346 332
w vw w w
290 w
185 w 178 w 169 w 147 134 121 108
w vw vw vw
ns(POP)
d and g(POP) þ L(PO2/3)
258 vw 245 vw 227 vw 208 w 183 vw 175 vw 165 w 151 vw 145 sh 129 w 123 sh 112 w 103 sh 93 vw
T0 (Naþ)
T0 (Mg2þ)
n e stretching, d e in-plane bending, g e out-of-plane bending, T0 e translations, L e librations.
150e290 cm1 were assigned to the bending d(POP) and g(POP) vibrations. Finally, the T0 (Naþ) and T0 (Mg2þ) translations appear in the ranges 120e190 and 50e125 cm1, respectively. It should be noted that the number of the observed IR and Raman bands is significantly smaller than that predicted by the factor group
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analysis. This is a consequence of polycrystallinity of the studied samples. Only 45 and 38 bands are observed in the IR and Raman spectra, respectively. It means that the Ag/Bg and Au//Bu splitting is not observed in the spectra due to their close lying energy levels. Besides, the measurements at room temperature cause that several vibrations are accidentally degenerated or have nearly the same energy. 3.3. Luminescence spectra The room temperature excitation spectra of Na3Mg2P5O16 doped with different concentration of europium ions recorded in the UVVis and NIR region are presented in Fig. 4. All spectra were normalized to maximum of absorption and corrected for apparatus response. One can see that the most intense band is the broad one in UV region, which is ascribed to the charge transfer from O2 to Eu3þ ions (CTB e charge transfer band). The maximum of the CTB is red-shifted when the concentration of Eu3þ ions is increased. The exact position of the CTB is plotted in the inset of Fig. 4. For the lowest concentration the CTB is located at 235.5 nm (42463 cm1) and then it is shifted up to 244.5 nm (40733 cm1) for 7 and 10% of Eu3þ. This phenomenon is caused by substituting a smaller Mg2þ ion with a much bigger Eu3þ, which causes distortion in the structure. Moreover there is also a valence mismatch of these ions, which also influence the local surrounding of europium ions. Beside the CTB there are also much less intense, characteristic for Eu3þ ions 4f-4f bands. They are ascribed to the transitions from 7F0 ground level to the: 5F4 level at 300 nm, 6HJ level at 321 nm, 5D4 level at 365 nm, 5GJ, 5L7 levels at 382.5 nm, 5L6 at 396 nm, 5D3 at 418 nm, 5D2 at 467 nm and 5D1 at 537 nm. Among these transitions the most intense is the 7F0 / 5L6 one. Changing the concentration of dopant does not affect the position of 4f-4f bands since 4f shell is shielded by 5s and 5p shell. The emission spectrum of the sample doped with 7% of Eu3þ ions is presented in Fig. 5. The spectrum consists of characteristic for europium ions bands in red and NIR region. The most intense one is the 5D0 / 7F2 transition and has its maximum placed at l ¼ 611 nm. Besides, there are also visible transitions from the 5D0 level to the: 7F0 level with maximum at l ¼ 578.5 nm, 7F1 level with maximum at l ¼ 589.5 nm, 7F3 level with maximum at l ¼ 652 nm and 7F4 with maximum at l ¼ 698 nm. The integrated intensity in a function of dopant concentration is presented in the inset of Fig. 6. It is clearly visible that an increase of the Eu3þ concentration causes
Fig. 5. The emission spectrum of Na3Mg2P5O16: 7% Eu3þ. In the inset: integrated intensity of the emission in a function of dopant concentration.
Fig. 6. The emission spectrum of Na3Mg2P5O16: 10% Eu3þ measured at 77 K. In the inset the 5D0 / 7F0 transitions.
an increase of the total intensity up to 7% of Eu3þ and then it remains constant. Since the 5D0 / 7F0 transition (so called 0-0 transition) is nondegenerated under any symmetry it is easy to estimate the number of Eu3þ sites in the host. To do this emission spectrum at 77 K was measured and can be seen in Fig. 6. One can see that the 5D0 / 7F0 transition has two components. This means that Eu3þ ions enter at least two different crystallographic sites in Na3Mg2P5O16 host. Another very useful property of Eu3þ ions is the fact that the electric dipole transition 5D0 / 7F2 is very sensitive to changes in local surrounding of europium ions while the magnetic dipole transition 5D0 / 7F1 remains almost independent of local changes and even of a host structure. When Eu3þ ions occupy noncentrosymmetric site the most intense band corresponds to the electric dipole transition [26]. If now we consider the ratio between electric and magnetic dipole transition, we will have information about local symmetry of europium ions. This is called asymmetry parameter (R) and is defined as follows:
Z R¼Z Fig. 4. The excitation spectra of Na3Mg2P5O16: Eu3þ. In the inset: position of the CTB in function of dopant concentration.
5
D0 /7 F2
5
D0 /7 F1
:
(1)
P. Godlewska et al. / Journal of Alloys and Compounds 695 (2017) 21e26
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and relatively short lifetime it can be used for production of phosphor for modern lighting (pc-WLEDS) or LED screens. Acknowledgment This work was supported by Polish Ministry of Science and Higher Education in the frame of the Grant No. N N209 767240. References
Fig. 7. The R parameter and decay times in a function of dopant concentration.
The higher the R is, the more apart from ideal octahedron symmetry is. In Na3Mg2P5O16 phosphate the R parameter value decreases with increasing Eu3þ concentration from 4.74 for 1% of Eu3þ to 3.38 for 10% of Eu3þ (Fig. 7). This means that increasing Eu3þ content causes better organization of the structure and local surrounding of europium ions become more octahedral. The decay times were measured for the all samples monitoring the maximum of emission at lmon ¼ 611 nm and exciting in the 5L6 band (lexc ¼ 396 nm). The all decay curves were single exponential and the lifetimes remain constant with the value t ¼ 2 ms ± 0.2 ms for all dopant concentration. Absolute quantum yield of the highest doped sample was measured with excitation in the 5L6 level (lexc ¼ 396 nm). Its value is QY ¼ 60%, which means that this material can find many applications especially as phosphor for PC-WLEDs (phosphor converted white LEDs) or for modern LCD screens.
4. Conclusions 1. The Na3Mg2P5O16 pentaphosphate crystallizes in a stable monoclinic form described in the P2/a ¼ C42h space group. Apart from the main XRD lines in the diffraction patterns a few very weak peaks are observed, which can not be indexed. 2. The IR and Raman spectra contain vibrational patterns typical for other types of pentaphosphates. The strong differences in the band wavenumbers and intensities of some IR and Raman spectra confirm the centrosymmetric C42h structure of the studied material. 3. The electron absorption, excitation and emission spectra are characteristic for Eu3þ ions. An increase of the Eu3þ concentration in the studied host causes an increase of the overall emission intensity. 4. The asymmetry ratio R value shows that the local surrounding of Eu3þ is only slightly distorted from octahedral. The R parameter value decreases with increasing the Eu3þ concentration from 4.74 for 1% of Eu3þ to 3.38 for 10% of Eu3þ. This means that increasing Eu3þ content causes better organization of the structure and local surrounding of europium ions becomes more octahedral. 5. The results of the structural and spectroscopic studies prove that the Na3Mg2P5O16 pentaphosphate is an excellent host material for doping of Ln3þ ions. Because of its high quantum efficiency
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Structural and optical studies of Eu3þ doped Na3Mg2P5O16 pentaphosphate P. Godlewska a, *, A.J. Pelczarska a, A. Watras b, L. Macalik b, M. Ptak b, I. Szczygieł a, J. Hanuza b a b
Wrocław University of Economics, Faculty of Chemistry and Food Technology, ul. Komandorska 118/120, 53-345, Wrocław, Poland lna 2, 50-422, Wrocław, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Oko
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 April 2016 Received in revised form 7 October 2016 Accepted 10 October 2016 Available online 11 October 2016
Sodium-magnesium pentaphosphate with a formula Na3Mg2P5O16 doped with europium(III) have been synthesized. The concentration of the rare earth ion activators was established to be 1 to 10 at.% in the doped samples. Spectroscopic and structural properties of the obtained samples were investigated by Xray, Raman, infrared, absorption and luminescence studies. The structure of the studied material was confirmed as monoclinic P2/a ¼ C42h (No. 13) similar to this described in the literature. The decay times were measured for all samples monitoring the maximum of emission at lmon ¼ 611 nm and exciting in the 5L6 band (lexc ¼ 396 nm). All decay curves were single exponential and lifetimes remain constant with value t ¼ 2 ms ± 0.2 ms for the all dopant concentration. © 2016 Published by Elsevier B.V.
Keywords: Rare earth alloys and compounds Optical spectroscopy Crystal structure X-ray diffraction Optical properties
1. Introduction Rare earth pentaphosphates LnP5O14 have been a subject of a great technical interest due to their potential applications as lasing materials. They exhibit a high luminescence efficiency and good optical quality [1e8]. The pentaphosphates of the Na3Mg2P5O16 type are less recognized, particularly their physicochemical properties practically have not been reported. This compound crystallizes in a monoclinic structure but some controversy exists in the description of its unit cell. Smolin et al. described its unit cell parameters as a ¼ 18.617 ± 0.005 Å, b ¼ 6.844 ± 0.003 Å, c ¼ 5.174 ± 0.003 Å and b ¼ 90.15 , Z ¼ 2 (S.G. P2/a e No. 13) [9]. According to Majling and Hanic [10] the proper unit cell has the dimensions a ¼ 5.177 ± 0.001 Å, b ¼ 6.853 ± 0.001 Å, c ¼ 18.628 ± 0.005 Å and b ¼ 90.00 ± 0.04 , Z ¼ 2 (S.G. P21/c, No. 14). This compound is stable below 675 C. At this temperature decomposition of the compound takes place, leading to the appearance of secondary phases, mainly NaMg(PO3)3 and Mg2P2O7 [11]. It is interesting which description reflects the real structure of
* Corresponding author. E-mail address: [email protected] (P. Godlewska). http://dx.doi.org/10.1016/j.jallcom.2016.10.087 0925-8388/© 2016 Published by Elsevier B.V.
this compound better. Besides, the question arises: can measurements of IR and Raman spectra be used for arbitrating this structural problem. Therefore, in the present work sodium-magnesium pentaphosphates doped with europium(III) have been synthesized and their spectroscopic properties have been studied. The results of these investigations could be used for recognition of future optical applications of these materials. 2. Experimental The undoped and doped with 0.01e0.10 at.% Eu3þ powders of Na3Mg2P5O16 were obtained by carrying out reactions in the solid state. The following chemicals of analytical purity were used for the synthesis: NH4H2PO4 (POCh Gliwice), MgHPO4 (POCh Gliwice), NaH2PO4 (POCh Gliwice) and Eu2O3 (99.9% POCh, Gliwice). The powders were ground, mixed in an agate mortar with acetone and sintered for 1 h at 400 C and for 15 h at 600 C. The last heating step occurred at 600 C for 20 h. Between heating steps the powders were carefully ground in order to obtain homogeneous powders. XRD patterns of the powders were registered using a Siemens D5000 diffractometer (Siemens, Karlsruhe, Germany) equipped with a copper X-ray tube. The measurements were performed in 2q angle range of 5-55 with a 0.04 step and at least 4 s t time per
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step. IR spectra were measured using a Nicolet iS50 FT-IR (Thermo Scientific) spectrometer equipped with an Automated Beam splitter exchange system (iS50 ABX containing a DLaTGS KBr detector and a DLaTGS Solid Substrate detector for mid-IR and far-IR regions, respectively), Built-in an all-reflective diamond ATR module (iS50 ATR), Thermo Scientific Polaris™ and a HeNe laser as an IR radiation source. Polycrystalline mid-IR spectra were collected in the 4000400 cm1 range in KBr pellets and far-IR spectra in the 600-50 cm1 range in Nujol mull. Spectral resolution was 4 cm1. FT Raman spectra in the 1000-80 cm1 range were measured with a Bruker FT-Raman RFS 100/S spectrometer and the 1064 nm excitation. The measurements were performed in the back scattering technique. Spectral resolution was 2 cm1. These spectra were compared to those recorded on a Renishaw InVia Raman spectrometer equipped with a confocal DM 2500 Leica optical microscope, a thermoelectrically cooled Ren Cam CCD as a detector and Arþ ion laser operating at 488 nm. Excitation and low temperature emission spectra were measured by the use of a McPherson spectrometer equipped with 150 W Xe lamp for UV-VIS and 150 W D2 lamp for VUV region as an excitation source and a Hamamatsu R928 photomultiplier as a detector. Focal length of monochromator was 30 cm and resolution was 0.05 nm. Room temperature emission spectra were obtained using a Hamamatsu PMA-12 photonic multichannel analyzer. For measuring emission spectra and decay times 396 nm line from a tunable pulsed Ti:Sapphire laser pumped by second harmonic (l ¼ 532 nm) of Nd:YAG laser was used to excite the samples. The decay curves were collected on a digital oscilloscope. Absolute quantum efficiency was measured on Hamamatsu C9920-02G Absolute PL quantum yield spectrometer. 3. Results and discussion 3.1. XRD studies X-ray diffractograms of the studied materials are shown in Fig. 1. They contain the reflexes characteristic for Na3Mg2P5O16 pentaphosphate (ICDD card No 4-9-4605) and a few very weak peaks, which were assigned to the Na7Mg4.5(P2O7)4 phase (ICDD card No 27-738). Additional peaks have the same intensity, which means that contamination does not change in whole range of Eu3þ concentration and should not affect spectroscopic properties.
Fig. 1. The XRD patterns of Na3Mg2P5O16 powders doped with Eu3þ.
3.2. IR and Raman spectra Figs. 2 and 3 present the MIR, FIR and Raman spectra of the studied phosphates. Table 1 list the band wavenumbers of the IR and Raman spectra. In the analysis of the measured IR and Raman spectra of Na3Mg2P5O16 pentaphosphate the P2/a (C42h, Z ¼ 2) space group (No. 13) [9] is considered in the first step. Its primitive unit cell contains six Naþ, four Mg2þ, ten P5þ and thirty two O2 ions. Sodium atoms occupy the 2f and 4g positions (C2 and C1 symmetry), magnesium and oxygen atoms lie at 4g positions (C1 symmetry) and phosphorus atoms occupy the 2e, 4g and 4g positions. Therefore, 52 atoms of the primitive cell give rise to 156 zone-center degrees of freedom described by the irreducible representation: G ¼ 38Ag þ 40Bg þ 38Au þ 40Bu. Three of them, GT ¼ Au þ 2Bu, describe the acoustic phonons and remaining, GO ¼ 38Ag þ 40Bg þ 37Au þ 38Bu, correspond to the optical modes. The optical modes can be further subdivided into the vibrations of the respective components of the primitive cell: Translations of Naþ ions: GT’ (Naþ) ¼ 4Ag þ 5Bg þ 4Au þ 5Bu, Translations of Mg2þ ions: GT’ (Mg2þ) ¼ 3Ag þ3Bg þ 3Au þ 3Bu,
Fig. 2. The MIR and FIR spectra of Na3Mg2P5O16 doped with different concentration of europium ions.
Vibrations of P5þ atoms: Gv (P) ¼ 7Ag þ 8Bg þ 7Au þ 8Bu, Vibrations of O2 atoms: Gv (O) ¼ 24Ag þ 24Bg þ 24Au þ 24Bu. The internal modes coupled with the librations of the whole
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23
Table 1 The Raman and IR wavenumbers (cm1) for Na3MgP5O16 containing 1 mol% of europium ions. Raman
IR
Assignment
nas(PO2/3)
1296 w 1210 w 1180 m 1164 1153 1123 1107 1096 1068 1055 1034
s m vw w sh w sh w
1287 1210 1194 1175 1163
m s s sh s
ns(PO2/3) 1119 s 1097 m 1063 m 1036 w 1007 sh 963 s
nas(POP)
936 w Fig. 3. The Raman spectra of Na3Mg2P5O16 doped with different concentration of europium ions.
916 m 882 vw 768 w 753 vw 745 vs
units are described by the representation: G(int þ L) ¼ 38Ag þ 40Bg þ 37Au þ 38Bu. Because the Ag and Bg modes are Raman active only, 78 bands are expected in the polarized spectra of single crystal. Similarly, 75 bands should be observed in respective IR spectra because the Au and Bu modes are IR active only. The realization of similar analysis for the P21/c ¼ C52h unit cell is impossible because the positions of the atoms are not reported in the paper [10]. However, independently of this problem the expected phonon properties derived from this structure should be very similar to those calculated for the P2/a unit cell. The same C2h factor group and Z ¼ 2 factor made that the same irreducible representation charter table, Raman tensors and selection rules should work for both structural models. Instead of the C2 site symmetry, the Ci should be taken into account in this case. From the presented below electron absorption and emission studies follow that the increase of the Eu3þ content causes higher ordering of the structure and the local surrounding of europium ions become more octahedral. Therefore the slight phase P2/a / P21/c transition could appear in the studied sample. The phosphate anion in both structures forms a P5O16 treedimensional framework built from five PO4 tetrahedra joined by four bend PeOeP oxygen bridges. All PeO bond lengths in this short ribbon are different and range from 1.4829 to 1.6557 Å [9]. Therefore, the set of expected IR and Raman modes of the single P5O16 unit should contain four ns(POP), nas(POP), d(POP) and g(POP) modes and because such four units are included into the primitive cell, all these numbers should be multiplied by four. Remaining two PO3 and three PO2 components of the P5O16 framework give rise to 44 modes consisting of the nas, ns and d vibrations. The assignment of the observed bands to the respective vibrational modes is proposed in Table 1. In general, the proposed in these tables description of the spectra agrees with those published earlier in the papers [12e25]. The particular types of vibrations appear in the typical for them ranges. The nas(PO2/3) vibrations are observed in the range 1160e1300 cm1 where the IR and Raman multiplets totally contain 9 bands. In the range 1000e1155 cm1, characteristic for the ns(PO2/3) vibrations, 13 bands should be assigned to these modes. The ns(POP) and nas(POP) vibrations appear in the ranges 700e770 and 850e970 cm1, respectively. All bending das(PO2/3) and ds(PO2/3) vibrations are observed in the ranges 450e650 and 300e420 cm1, in which 19 and 15 are recorded, respectively. The medium to strong bands in the range
746 vw 720 m
das(PO2/3)
634 sh 621 w
588 vw 567 vw 557 vw 543 517 500 492 483 476 420 407
vw w sh w sh w vw vw
384 372 362 340 326 318 302
w w vw w w sh vw
275 251 241 225 218
sh w w w sh
613 m 594 sh 584 m
550 sh 539 m 522 w 490 m 482 sh
ds(PO2/3) 409 w 399 w 374 360 346 332
w vw w w
290 w
185 w 178 w 169 w 147 134 121 108
w vw vw vw
ns(POP)
d and g(POP) þ L(PO2/3)
258 vw 245 vw 227 vw 208 w 183 vw 175 vw 165 w 151 vw 145 sh 129 w 123 sh 112 w 103 sh 93 vw
T0 (Naþ)
T0 (Mg2þ)
n e stretching, d e in-plane bending, g e out-of-plane bending, T0 e translations, L e librations.
150e290 cm1 were assigned to the bending d(POP) and g(POP) vibrations. Finally, the T0 (Naþ) and T0 (Mg2þ) translations appear in the ranges 120e190 and 50e125 cm1, respectively. It should be noted that the number of the observed IR and Raman bands is significantly smaller than that predicted by the factor group
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P. Godlewska et al. / Journal of Alloys and Compounds 695 (2017) 21e26
analysis. This is a consequence of polycrystallinity of the studied samples. Only 45 and 38 bands are observed in the IR and Raman spectra, respectively. It means that the Ag/Bg and Au//Bu splitting is not observed in the spectra due to their close lying energy levels. Besides, the measurements at room temperature cause that several vibrations are accidentally degenerated or have nearly the same energy. 3.3. Luminescence spectra The room temperature excitation spectra of Na3Mg2P5O16 doped with different concentration of europium ions recorded in the UVVis and NIR region are presented in Fig. 4. All spectra were normalized to maximum of absorption and corrected for apparatus response. One can see that the most intense band is the broad one in UV region, which is ascribed to the charge transfer from O2 to Eu3þ ions (CTB e charge transfer band). The maximum of the CTB is red-shifted when the concentration of Eu3þ ions is increased. The exact position of the CTB is plotted in the inset of Fig. 4. For the lowest concentration the CTB is located at 235.5 nm (42463 cm1) and then it is shifted up to 244.5 nm (40733 cm1) for 7 and 10% of Eu3þ. This phenomenon is caused by substituting a smaller Mg2þ ion with a much bigger Eu3þ, which causes distortion in the structure. Moreover there is also a valence mismatch of these ions, which also influence the local surrounding of europium ions. Beside the CTB there are also much less intense, characteristic for Eu3þ ions 4f-4f bands. They are ascribed to the transitions from 7F0 ground level to the: 5F4 level at 300 nm, 6HJ level at 321 nm, 5D4 level at 365 nm, 5GJ, 5L7 levels at 382.5 nm, 5L6 at 396 nm, 5D3 at 418 nm, 5D2 at 467 nm and 5D1 at 537 nm. Among these transitions the most intense is the 7F0 / 5L6 one. Changing the concentration of dopant does not affect the position of 4f-4f bands since 4f shell is shielded by 5s and 5p shell. The emission spectrum of the sample doped with 7% of Eu3þ ions is presented in Fig. 5. The spectrum consists of characteristic for europium ions bands in red and NIR region. The most intense one is the 5D0 / 7F2 transition and has its maximum placed at l ¼ 611 nm. Besides, there are also visible transitions from the 5D0 level to the: 7F0 level with maximum at l ¼ 578.5 nm, 7F1 level with maximum at l ¼ 589.5 nm, 7F3 level with maximum at l ¼ 652 nm and 7F4 with maximum at l ¼ 698 nm. The integrated intensity in a function of dopant concentration is presented in the inset of Fig. 6. It is clearly visible that an increase of the Eu3þ concentration causes
Fig. 5. The emission spectrum of Na3Mg2P5O16: 7% Eu3þ. In the inset: integrated intensity of the emission in a function of dopant concentration.
Fig. 6. The emission spectrum of Na3Mg2P5O16: 10% Eu3þ measured at 77 K. In the inset the 5D0 / 7F0 transitions.
an increase of the total intensity up to 7% of Eu3þ and then it remains constant. Since the 5D0 / 7F0 transition (so called 0-0 transition) is nondegenerated under any symmetry it is easy to estimate the number of Eu3þ sites in the host. To do this emission spectrum at 77 K was measured and can be seen in Fig. 6. One can see that the 5D0 / 7F0 transition has two components. This means that Eu3þ ions enter at least two different crystallographic sites in Na3Mg2P5O16 host. Another very useful property of Eu3þ ions is the fact that the electric dipole transition 5D0 / 7F2 is very sensitive to changes in local surrounding of europium ions while the magnetic dipole transition 5D0 / 7F1 remains almost independent of local changes and even of a host structure. When Eu3þ ions occupy noncentrosymmetric site the most intense band corresponds to the electric dipole transition [26]. If now we consider the ratio between electric and magnetic dipole transition, we will have information about local symmetry of europium ions. This is called asymmetry parameter (R) and is defined as follows:
Z R¼Z Fig. 4. The excitation spectra of Na3Mg2P5O16: Eu3þ. In the inset: position of the CTB in function of dopant concentration.
5
D0 /7 F2
5
D0 /7 F1
:
(1)
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25
and relatively short lifetime it can be used for production of phosphor for modern lighting (pc-WLEDS) or LED screens. Acknowledgment This work was supported by Polish Ministry of Science and Higher Education in the frame of the Grant No. N N209 767240. References
Fig. 7. The R parameter and decay times in a function of dopant concentration.
The higher the R is, the more apart from ideal octahedron symmetry is. In Na3Mg2P5O16 phosphate the R parameter value decreases with increasing Eu3þ concentration from 4.74 for 1% of Eu3þ to 3.38 for 10% of Eu3þ (Fig. 7). This means that increasing Eu3þ content causes better organization of the structure and local surrounding of europium ions become more octahedral. The decay times were measured for the all samples monitoring the maximum of emission at lmon ¼ 611 nm and exciting in the 5L6 band (lexc ¼ 396 nm). The all decay curves were single exponential and the lifetimes remain constant with the value t ¼ 2 ms ± 0.2 ms for all dopant concentration. Absolute quantum yield of the highest doped sample was measured with excitation in the 5L6 level (lexc ¼ 396 nm). Its value is QY ¼ 60%, which means that this material can find many applications especially as phosphor for PC-WLEDs (phosphor converted white LEDs) or for modern LCD screens.
4. Conclusions 1. The Na3Mg2P5O16 pentaphosphate crystallizes in a stable monoclinic form described in the P2/a ¼ C42h space group. Apart from the main XRD lines in the diffraction patterns a few very weak peaks are observed, which can not be indexed. 2. The IR and Raman spectra contain vibrational patterns typical for other types of pentaphosphates. The strong differences in the band wavenumbers and intensities of some IR and Raman spectra confirm the centrosymmetric C42h structure of the studied material. 3. The electron absorption, excitation and emission spectra are characteristic for Eu3þ ions. An increase of the Eu3þ concentration in the studied host causes an increase of the overall emission intensity. 4. The asymmetry ratio R value shows that the local surrounding of Eu3þ is only slightly distorted from octahedral. The R parameter value decreases with increasing the Eu3þ concentration from 4.74 for 1% of Eu3þ to 3.38 for 10% of Eu3þ. This means that increasing Eu3þ content causes better organization of the structure and local surrounding of europium ions becomes more octahedral. 5. The results of the structural and spectroscopic studies prove that the Na3Mg2P5O16 pentaphosphate is an excellent host material for doping of Ln3þ ions. Because of its high quantum efficiency
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