Synthesis and Nd3+ Luminescence Properties of ALa1âxNdxP4O12

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The Journal of Physical Chemistry

Synthesis and Nd3+ Luminescence Properties of ALa1-xNdxP4O12 (A=Li, Na, K, Rb) Tetraphosphate Nanocrystals L.Marciniak1*, W. Strek1, Y.Guyot2, D. Hreniak1, G. Boulon2 1

Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wroclaw, Poland

2

Institute Light Matter (ILM), UMR5306 CNRS-Université Lyon1, Université de Lyon, Bât. Kastler, 69622 Villeurbanne, France * corresponding author: [email protected]

KEYWORDS tetraphosphates, neodymium, stoichiometric phosphors, energy migration

ABSTRACT The nanocrystalline alkali tetraphosphates doped with Nd3+ ions were synthesized using co-precipitation method. The structural and spectroscopic properties of ALa1-xNdxP4O12 (A=Li, Na, K, Rb) nanocrystals were shown. The impact of the dopant concentration on the emission spectra and the kinetics of the 4F3/2 excited state was analyzed. The exponential decay profile for each of the Nd3+ concentrations was discussed in terms of fast energy diffusion with reduced cross-relaxation processes. The comparison of luminescent properties for different alkali cations was presented. INTRODUCTION The neodymium based stoichiometric phosphor crystals were extensively investigated in 70s and 80s of last century as an interesting alternative for well known laser crystal like YAG :Nd etc1-14. This interest was justified by the unique optical properties of these materials, to mention the reduced cross relaxation, low laser action threshold, high absorption cross section. The most investigated stoichiometric phosphors were tetra- and pentaphosphates associated with the large Nd3+-Nd3+ distance (around 5.5-6 Å). However due the tendency to the twinning of the NdP5O14 the tetraphosphates seems to be more applicable as laser crystals. With increase Nd3+ concentration the average distance between active ions reduces. Therefore, enlargement of dopant ions may leads to the luminescence concentration quenching via cross-relaxation process ({4F3/2, 4I9/2}↔{4I15/2, 4I15/2}). In materials with large separation between dopant ions, the probability of the intertion energy transfers, which lead to reduction of the emission intensity, is diminished. Moreover, in case of tetra- and pentaphosphates, the aforementioned cross relaxation process is limited due to the energy mismatch for this process, hence the exponential profile of luminescence decay was reported for these materials with the decay constant of 120 µs for LiNdP4O12 15 and 90 µs for NdP5O14

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16

. High number of optically active ions is a suitable feature for size minimization of lasers.

Therefore the neodymium tetra- and penthaphosphates are especially attractive for laser application, which was confirmed by great number of publications concerning laser action of LiNdP4O12 12-15 and NdP5O14 1-11. Although crystals of neodymium tetraphosphates are well known for a few decades, yet the spectroscopic properties of nanocrystals have not been well described. We have recently reported Stokes and anti-Stokes properties of nanocrystalline LiNdP4O12 3+

17

on the influence of Nd concentration

22

and grain size of the crystals

19-23

, with emphasis

on their luminescent

properties. The characterization of the luminescence of the La1-xNdxP5O14 nanocrystals was given24. In this paper, from the best of our knowledge for the first time, spectroscopic properties of NaLa1-xNdxP4O12, KLa1-xNdxP4O12 and RbLa1-xNdxP4O12 nanocrystals are presented. The comparison of spectroscopic properties of ALa1-xNdxP4O12 (A=Li, Na, K, Rb) nanocrystals is given with special emphasis on both the influence of alkali cation and Nd3+ dopant amount on the structural and luminescent properties. Since these compounds are characterize by high absorption cross section, their nanocrystals can be interesting host for biomedicine applications like photodynamic therapy, luminescent nanothermometers and infrared bio-imaging25-27.

EXPERIMENTAL Nanocrystalline powders of ALa1-xNdxP4O12 (A=Li, Na, K, Rb) tetraphosphates were synthesized by using the co-precipitation method described previously17. Stoichiometric amounts of alkali ions carbonates (Li2CO3 of >99.998 % purity from Fluka Analitical, Na2CO3 of 99.9 % purity from POCH, K2CO3 of 99 % purity from POCH, Rb2CO3 of 99.9 % purity from Alfa Aesar) were dissolved in deionized water. Simultaneously the lanthanum and neodymium oxides (La2O3 of 99.999% purity from Stanford Materials Corporation, Nd2O3 of 99.95% purity from Stanford Materials Corporation) were dissolved in nitric acid in order to obtain lanthanides nitrates. After mixing the water solutions of nitrates with carbonates, the obtained mixture was added to the water solution of diammonium phosphate ((NH4)2HPO4 of >99.99 % purity from Sigma Aldrich). The solution was then dried at 90 oC for 24 h and thermally treated at 450 oC for 6h. The synthesis can be expressed by following chemical equation: (1 − x) La( NO3 )3 + xNd ( NO3 )3 + ANO3 + 4( NH 4 ) 2 HPO4 → ALa1− x Nd x P4O12 (amorph) + 4 NH 4 NO3 + 4 NH 4OH


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450o C (6 h )

ALa1− x Nd x P4O12 (amorph)

→

ALa1− x Nd x P4O12 ( nanocrystals )

The X-ray diffraction patterns were collected in Bragg-Brentano geometry on PANalytical X'Pert Pro diffractometer with beta-filtered Cu Kalpha radiation (λ = 0.15418 nm). The visualization of the structures was prepared in Diamond 3.0 software basing on the reference pattern from ICSD database. Transmission electron microscope images were taken by using FEI Tecnai G2 20 X-TWIN microscope, equipped with CCD FEI Eagle 2K camera with HAADF detector, and electron gun with LaB6 cathode. Moreover, the microscope was equipped with X-ray microanalyzer EDAX. The maximal applied voltage was 200 kV; the maximal point resolution of 0.25 nm, and maximal linear resolution of 0.102 nm. The angle range for EDAX was ±12°.Raman spectra were measured on inVia confocal microscope from Renishaw equipped with Si CCD camera as detector and 830 nm excitation line. The spectra were taken in a range of 100-1400 cm-1 at room temperature under 100x objective. Absorption spectra were measured in the back scattering mode using Cary Varian 5E UV-Vis-NIR spectrometer. The emission spectra were measured with a CCD IDUS near-infrared InGaAs camera from ANDOR equipped with a 900 line/mm grating blazed at 1300 nm, and laser diode (LD) 808 nm excitation line.The luminescence kinetics measurements were performed by using Jobin-Yvon HR1000 monochromator equipped with 5108 photomultiplier from Hamamatsu. The decay profiles were collected by using a LeCroy WaveSurfer 400 oscilloscope.

RESULTS AND DISCUSSION The structural properties of ALaP4O12 nanocrystals doped with Nd3+ ions were analyzed in the wide range of dopant concentration x=[0.01, 1]. The visualization of the LiNdP4O12, NaLaP4O12, KLaP4O12 and RbNdP4O12 structures is presented in Fig 1 a; b; c and d, respectively. Although the presented structures reveal a great similarity, from the chemical composition point of view, some crystallographic difference among these compounds can be found. The structures of the first three tetraphosphates are constructed from PO4 tetrahedral, which forms a helical chain. These chains are linked by –La3+ (Nd3+)-A+- chains. On the other hand, in case of RbLaP4O12, the structure of the PO4 groups creates tetrametaphopsphates (P4O12)4- rings sharing two corners of the PO4 tetrahedral. Moreover the structures consists of LnO8 dodecahedrons which are well separated by the PO4 tetrahedral.



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Figure 1. The presentation of the ALaP4O12 structures: LiLaP4O12 –a; NaLaP4O12 –b; KLaP4O12 –c; RbLaP4O12 –d.

Table 1. The comparison between cell parameters of ALnP4O12 structures (ICSD numbers are given in the brackets). β [o ] V [Å3] Host Structure Symmetry a[Å] b[Å] c[Å]

Z

LiLaP4O12 (416877)

monoclinic

C 1 2/c 1 (15)

16.62

7.11

9.90

126.43

943.19

4

LiNdP4O12 (4253)

monoclinic

C 1 2/c 1 (15)

16.40

7.03

9.72

126.38

904.15

4

NaLaP4O12 (415535) NaNdP4O12 (401) KLaP4O12 (33241) KNdP4O12 (4254) RbNdP4O12 (1114)

monoclinic monoclinic monoclinic monoclinic monoclinic

P 1 21/n 1 (14) P 1 21/n 1 (14) P 1 21 1 (4) P 1 21 1 (4) C 1 2/c 1 (15)

7.255 9.907 8.106 7.266 7.845

13.186 13.10 8.551 8.436 12.691

10.067 7.201 7.326 8.007 10.688

90.40 90.51 92.18 91.97 112.34

963.03 934.52 507.43 490.51 984.24

4 4 2 2 4

A comparison of the cell parameters of ALaP4O12 host materials and stoichiometric ANdP4O12 phosphors, based on the corresponding ICSD data, is given in Table 1. All the structures belong to the monoclinic structures with different symmetries. The change of the symmetry is associated to both different relative position of PO4 tetrahedral and the ionic radius of alkali metal ions. For all the aforementioned structures, the substitution of the La3+ ions to Nd3+ ions results in the reduction of the cell parameters due to the differences in the ionic radius between these ions.

Additional significant difference in the angle of the primitive cell can be observed. The value of β can be found as a close to the right angle for NaLaP4O12 and KLaP4O12, while its value is considerably increased for RbLaP4O12, reaching the highest value for LiLaP4O12.


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Structural properties of lanthanides ions can also be determined by the metal-to-oxide and metal-to-metal distances. In Table 2 the Ln3+-O2- and Nd3+-Nd3+ distances for all of described hosts are listed. Especially, the former parameter has some impact on the Nd3+-Nd3+ nearest neighbor interaction probability, hence affects the spectroscopic properties more especially luminescent concentration quenching probability as we shall analyse it later, according to the following rule: shorter is the distance between dopants, higher is the probability of nonradiative relaxation of the excited states, since the crystal field itself could slightly vary. Consequently, the luminescence of KNdP4O12 nanocrystals (Nd3+-Nd3+ = 3.923, 4.051,4.612, 4.963, 6.589 Å respectively) should be mostly affected by nonradiative relaxations. However, due to the differences among the structures described above, their luminescent properties as a function of alkali metal ion is not so obvious, hence only a comparison of the luminescent properties of ALa1-xNdxP4O12 is gathered in this section. Table 2. The Ln3+-O2- and Nd3+- Nd3+ distances (Å) for ANdP4O12 structures.

LiNdP4O12

NaNdP4O12

KNdP4O12

RbNdP4O12

Ln -O

2.3849, 2.3849, 2.4200, 2.4200, 2.4887

2.3854, 2.4059, 2.4285, 2.4441, 2.4664, 2.4713

2.4062, 2.4091, 2.4404, 2.4498

Nd3+- Nd3+

5.616, 6.643, 7.120, 8.925

5.719, 6.157, 6.209, 7.201, 8.654, 10.475

2.3487, 2.3687, 2.4289, 2.4604, 2.4732 3.923, 4.051, 4.612, 4.963, 6.589

3+

2-

6.13, 6.256, 7.845, 8.295

In order to determine the phase purity of the tetraphospahtes powders of LiLa1-xNdxP4O12, NaLa1-xNdxP4O12, KLa1-xNdxP4O12 and RbLa1-xNdxP4O12 nanocrystals, the X-ray diffraction patterns were recorded in Fig. 2. One can see a significant difference between the reference data obtained for KLaP4O12 relative to the ALaP4O12 A=(Li, Na, K, Rb) structures associated with the much richer number of reflections. It is associated to the lack of the plane of symmetry perpendicular to the b axis in KLaP4O12 (P121 1 (4)) in comparison to the NaLaP4O12 structure of symmetry (P121/n1(14)). Hence the lowering of the symmetry results in the increase of the number of reflection in the spectra.



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Figure 2. X-ray diffraction data of the ALaP4O12 structures: LiLa1-xNdxP4O12–a; NaLa1-xNdxP4O12 –b; KLa1xNdxP 4O12

–c; RbLa1-xNdxP4O12 –d.

All the samples reveal the pure monoclinic phase characteristic for proper host material. The average grain size for all the ALa1-xNdxP4O12 powders is equal to 40-45 nm. The latter was determined by using the Rietveld refinement method as it is independent from both the type of host material and the dopant concentration. The average grain sizes were determined by using the Rietveld refinement analysis of transmission electron microscope (TEM) images (Fig. 3). One can see that the powders of tetraphosphates consist of well crystallized high agglomerated grains. In the case of nanocrystalline powders, there was a problem to obtain the high resolution TEM images due to low stability of the crystals under high voltage electron beam, hence justifying the reduced quality of some of the TEM images.


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Figure 3. The transmission electron microscopy (TEM) images of the ALaP4O12 structures: KLaP4O12 –a; NaLaP4O12 –b; the grain size distribution for KLaP4O12 nanocrystals –c and the average grain size for phosphates with different cation –d.

The Raman spectra of ALaP4O12 nanocrystals (A=Li, Na, K, Rb) were measured in range 1400-100 cm-1 (see Fig. 4). The spectra show typical characteristics of tetraphosphates. The most intense of them are localized around 1190 cm-1 and 690 cm-1 which can be attributed to the symmetric oscillation of O-P-O and P-O-P group, respectively. The former of these two peaks is almost independent from the type of cation which constitutes the host. On the other hand, the latter is shifted to higher wavenumber as follows: 697 cm-1, 694 cm-1, 689 cm-1, and 677 cm-1 for Li, Na, K and Rb structures, respectively. This change is associated with the proximity of the alkali ion to the phosphorous one, hence the change of the cation size induces the change of the P-O-P angle in the following order: 132.5o, 136.3o, 138.0o and 139.7o. The fact that the maximal phonon energy is very close for all the structures suggests the nonradiative luminescence quenching will be affected in a similar way.



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Figure 4. Raman spectra of the ALaP4O12 structures: LiLaP4O12 –a; NaLaP4O12 –b; KLaP4O12 –c; RbLaP4O12 – d.

A comparison of the absorption spectra of the stoichiometric ANdP4O12 nanocrystals is presented in Fig. 5. The spectra are associated with the f-f transitions of Nd3+ optical ions of ANdP4O12 nanocrystals which are assigned directly in this figure. One can see that the intensities at room temperature of particular 4I9/2→2S+1LJ absorption transitions do not differ significantly with respect of different alkali cations. We have only noticed that the relative intensities of the 4I9/2→4F5/2,2H9/2 transition band lines for the LiNdP4O12 nanocrystals are much lower in comparison to the other absorption bands of A(=Na, K, Rb) NdP4O12 nanocrystals.


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Figure 5. Absorption spectra of LiNdP4O12 , NaNdP4O12 , KNdP4O12 , RbNdP4O12 nanocrystals.

The emission spectra of the ALa1-xNdxP4O12 nanocrystals were measured at 300 K and 77 K (Fig. 6). Fig. 6a and b shows the comparison of the emission spectra for different cations for x=0.01 and stoichiometric phosphors, respectively. The spectra consist of three emission lines localized at 890 nm, 1060 nm and 1350 nm attributed to the following electronic transitions: 4

F3/2→4I9/2, 4F3/2→4I11/2, and 4F3/2→4I13/2, respectively. The emission intensities of two

dominant emission lines assigned to the 4F3/2→4I9/2 and 4F3/2→4I11/2 transitions are of comparable in the case of the most diluted Nd3+ concentration, whereas for stoichiometric phosphor the 4F3/2→4I11/2 it becomes dominant. The impact of the Nd3+ concentration on the relative emission intensities of those two lines was previously discussed in our paper17 in terms of energy reabsorption process of 4F3/2→4I9/2 resonant transition. In the case of all the host materials, the same trend can be observed. However, some differences can be inferred: the highest 4F3/2→4I11/2 to 4F3/2→4I9/2 intensity ratio was found for K, whereas the lowest is observed for Na in case of diluted concentration. On the other hand, for stoichiometric phosphors in LiNdP4O12, laser 4F3/2→4I11/2 emission line reveals the highest intensity,



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compared to the 4F3/2→4I9/2 emission line. Moreover, significant changes among relative intensities of the emission lines, associated with the transition from 4F3/2 state to the particular Stark component of 4I9/2 state can be found. This effect is associated with different reabsorption rates for ALaP4O12 hosts17. In case of 4F3/2→4I11/2 in LiNdP4O12 only one line localized at 1048 nm is dominant, whereas for other hosts a unique line cannot be distinguished. Therefore the best laser properties were reported for LiNdP4O12 single crystals15.

Figure 6. Emission spectra of ALa1-xNdxP4O12 nanocrystals: comparison of emission spectra for ALa0.99Nd0.01P4O12 – a; and ANdP4O12 –b; the 4F3/2→4I9/2 emission band of ALa0.99Nd0.01P4O12-c and ANdP4O12 – d; and the 4F3/2→4I11/2 emission band of ALa0.99Nd0.01P4O12-e and ANdP4O12 –f.

The impact of Nd3+ concentration on the relative emission intensities can be inferred from changes of luminescent branching ratios of 4F3/2 excited state (Fig. 8). The luminescence branching ratio for emission from J manifold to J’ manifolds is expressed as a probability of spontanous emission for particular transition AJJ’ to the sum of the all transition probabilities from the J manifold28:

β JJ ' = AJJ ' / ∑ AJJ '

(2)

J'


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The increase of the Nd3+ content causes significant changes of β9/2 and β11/2, especially in LiLa1-xNdxP4O12 nanocrystals. Initially the dominant β9/2 undergoes a reduction from 0.6 to 0.31 with Nd3+ concentration, whereas β11/2 behaves in the opposite way. The branching ratio of β13/2 is independent on the dopant concentration. The changes of the branching ratios are due to the decrease of the resonant transition due to the energy reabsorption. For NaLa1xNdxP4O12

nanocrystals the initial values of β11/2 and β9/2 are 0.5 and 0.4, respectively,

becoming almost constant up to x=0.2. Above this value of dopant concentration, the β11/2 starts to rise up to 0.6 at the expense of the β9/2. It is worth mentioning β13/2 slightly increases with Nd3+ concentration up to around 0.15. For KLa1-xNdxP4O12 in case of most diluted concentration the β11/2 and β11/2 are almost equal (0.45 and 0.41, respectively). The value of β11/2 increases faster with Nd3+ amount compared to LiLa1-xNdxP4O12, and above x=0.2 values of β11/2 and β9/2 becomes constant with values of 0.58 and 0.31, respectively. The dependence of branching ratios of RbLa1-xNdxP4O12 nanocrystals reveals similar trend as in the case of NaLa1-xNdxP4O12. However, the branching ratios change above x=0.1. .

Figure 7. Luminescent branching ratios as a function of dopant concentrations of LiLa1-xNdxP4O12-a, NaLa1xNdxP 4O12-b,

KLa1-xNdxP4O12-c, and RbLa1-xNdxP4O12-d.

The splitting of the excited and 4IJ manifolds in ALa1-xNdxP4O12 nanocrystals is listed in Table 3. The highest splitting of the ground 4I9/2 state is observed for the Na sample (365 cm-



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), whereas the lowest for the Li sample (325 cm-1). However the splitting of the 4F3/2

excavated state is the lowest in case of K phosphate (58 cm-1) and the highest for Na phosphate (81cm-1).

Table 3. Splitting of the 2S+1LJ manifolds in ALa1-xNdxP4O12 nanocrystals 2S+1

Li

Na

K

Rb

(cm-1)

(cm-1)

(cm-1)

(cm-1)

I9/2

0, 100, 187, 257, 325

0, 108, 169, 222, 365

4

I11/2

1930, 1997, 2055, 2132, 2202, 2275

1962, 1959, 2071, 2129, 2209, 2256

0, 101, 163, 221, 343 1951, 1988, 2077, 2144, 2203, 2272

0, 128, 184, 224, 348 1950, 1983, 2058, 2097, 2178, 2268

4

I13/2

3893, 3964, 4009, 4053, 4073, 4120, 4132

3921, 3955, 4033, 4061, 4143, 4168, 4185

3914, 3956, 4037, 4078, 4118, 4145, 4203

3910, 3948, 4031, 4080, 4111, 4141, 4180

11478, 11548

11478, 11559

11481, 11539

11474, 11538

4

4

LJ

F3/2

A very important parameter characterizing luminescent properties of phosphors is the socalled spectroscopic-quality parameter, defined as follows: X Nd =

Ω4 Ω6

(3)

where Ω4 and Ω6 are the intensity parameters of Judd-Ofelt theory29,30. The experimental value of the spectroscopic parameter can be determined from emission spectra according to following relation28:

X Nd = 0.765YNd − 2.96

(4)

where YNd represents the ratio of emission intensities of 4F3/2→4I11/2 to 4F3/2→4I13/2 electronic transitions. The values of spectroscopic parameters strongly depend on the type of host material (Table 4). The highest value was found for Na (6.35), and its value decreases in the order K (5.0) > Li (4.31) > Rb (4.13).

Table 4. The values of the spectroscopic parameter XNd for different ANdP4O12 nanocrystals host

LiNdP4O12

NaNdP4O12

KNdP4O12

RbNdP4O12

XNd

4.31

6.35

4.98

4.13


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At last, the influence of the cation on the excited state kinetics of the luminescent decay times was measured as a function of Nd3+ concentration at room temperature by using λexc=532 nm (Fig. 8a-d). The longest decay time was observed for the most diluted samples (x=0.01) of NaLa0.99Nd0.01P4O12 to be around 397 µs, and the value of the decay time decreased as follows: 290 µs, 281 µs and 267 µs for LiLa0.99Nd0.01P4O12, RbLa0.99Nd0.01P4O12, and KLa0.99Nd0.01P4O12, respectively. The decay luminescent constant is associated with the Nd3+-Nd3+ distance whose value was found to be the smallest for KLa0.99Nd0.01P4O12 , i.e., ~3.92 Å. For LiLa0.99Nd0.01P4O12, the metal-to-metal distance is 5.6 Å and for NaLa0.99Nd0.01P4O12 is 5.7 Å. The longest distance 6.13 Å was found for RbLa0.99Nd0.01P4O12. A strong correlation between the shortest distance among dopant ions and the decay constant for each of the host which consists of (PO4) tetrahedral chains, can be inferred. However, RbLaP4O12 is constituted of (P4O12)4- rings, hence, changes in the relative orientations of the PO4 tetrahedrals, results in the deviation of the decay constant trends. With increasing the dopant concentration, the profiles of the luminescence decay become slightly nonexponential, showing an exponential trend for x=1 with the decay constant: 78 µs, 80 µs, 67 µs and 58 µs. The decay time as a function of dopant concentration is presented in Fig. 8e. The most affected by on the luminescence quenching is the RbNdP4O12 host, and for the series of the PO4 tetrahedral chain based hosts, the correlation between the Nd3+-Nd3+ distance with decay time can be observed.



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Figure 8. Luminescence decay profiles ALa1-xNdxP4O12: A=Li – a; A=Na –b; A=K – c; A= Rb – d; and the decay times of ALa1-xNdxP4O12 – e as a function of dopants concentration.

The energy transfers between Nd3+ ions in the systems with the reduced cross-relaxation are well described by Yokota-Tanimoto model31:   t   t  I ( t ) = I 0 exp  −  − α     τ0  τ0 

3/S

 1 + 10.866 Z + 15.50Z 2  ×  1 + 8.749Z  

( S − 3) / ( S − 2 ) 

 +b 

(5)

where I(t) is the intensity of the donor emission, I0 is the initial emission intensity (t=0), τ0 is the radiative decay time, S is the parameter describing the type of multipole interaction (S=6 for dipole-dipole interaction, S=8 for the dipole-quadrupole and S=8 for the quadrupole-quadrupole interactions) and b is the offset. The parameters α is expressed by the relation of the concentration of the donors ND and the critical distance R0 according to the formula:

4

 

3

α = πΓ 1 −  N D R03 3 s 

(6)

where Γ () is the gamma function and R0 is the critical distance. Z merge the donor-donor interaction parameter CDD and the diffusion rate D as follows: −2/ S 1− 2/ S Z = DCDD t

(7)

In case of low dopant concentration where the energy fast diffusion is not yet dominant the donor-acceptor interaction constant CDA can be determined from the critical distance as follows:

CDA = R06 ⋅τ 0

(8)

In order to describe the energy transfers which take place in the nanocrystalline ALa1xNdxP4O12

hosts following calculation procedure has been conducted. From the luminescent

decay profile measured for the samples with relatively low dopant concentrations (x=0.2) for which eq. 4 can be reduced to the Inokuti-Hirayama relation32 the CDA parameter was determined, taking into account the values of the radiative times τ0 these measured for x=0.01. Using these values of CDA parameters the luminescent decay profiles for stoichiometric phosphors were fitted using eq. 4. Due to this procedure the CDD parameters were determined for all of ANdP4O12 hosts. From the comparison of the CDA and CDD parameters presented in Table 5 one can see that in case of each of the cations the CDD interation parameter is


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dominant what is expectable since the luminescent decay profiles are exponential even for stoichiometric phosphors. The lowest value of the CDD can be found for RbNdP4O12 and the highest one for LiNdP4O12 what is in agreement with the changes of the luminescent lifetimes for x=1 which becomes the shortest for RbNdP4O12 and the highest for LiNdP4O12.

Table 5. CDD and CDA parameters determined for the ANdP4O12 hosts Host

τ0 [µs]

CDA [m6/s]

CDD[m6/s]

LiNdP4O12

290

2.04⋅10-43

4.06⋅10-43

KNdP4O12

267

1.10⋅10-44

2.04⋅10-44

NaNdP4O12

397

5.59⋅10-44

8.11⋅10-44

RbNdP4O12

281

2.25⋅10-45

9.81⋅10-45

CONCLUSIONS In this work the comparative studies of structural and spectroscopic properties of Nd3+doped (ALa1-xNdxP4O12) (A=Li, Na, K, Rb) tetraphosphate nanocrystals are presented. Crystallographic differences among these compounds have been found. It was shown that the type of alkali ions has a strong influence on the structure of tetraphosphates like P-O-P angles, size of the primitive cell and the metal-to-metal, as well as metal-to-oxide distances. Moreover, further differences between these structures are associated with PO4 tetrahedral, since for Li, Na and K phosphates tetrahedrals builds long helical chains for Rb phosphates the P4O12 rings can be found in the structure. Due to these differences only the comparison of the spectroscopic properties of these structures can be given. As it was shown, the relative emission intensities of the 4F3/2 strongly depend on the type of the alkali ion. The luminescent branching ratios β9/2 and β11/2 depends on the dopant concentration, and, with increase of Nd3+ content, the decrease of β9/2 with increase of β11/2 was observed in each of the tetraphosphates host material due to the reabsorption process which takes place for 4F3/2→4I9/2 electronic transition. The rate of this change depends on the type of the host material. In the case of LiL1-xNdxP4O12 the initially dominant β9/2 parameter decreases with the Nd3+ content from 0.6 to 0.3, whereas β11/2 behave in the opposite way. Above x=0.3, additional changes in the branching ratios are negligible. The faster rate of the changes was observed for KLa1xNdxP4O12,

and above x=0.2 the values of the branching ratios becomes stable. A different

trend of the branching ratios was observed in NaLa1-xNdxP4O12 and RbLa1-xNdxP4O12 which a gradual increase/decrease of β11/2 and β9/2 branching ratios was observed. As tetraphosphtate



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materials belong to the group of relatively high distance between optically active ions which governs the self-trapping effect by resonant re-absorption between 4F3/2→4I9/2 electronic transition, the Nd3+ luminescence is not as strongly quenched by the interion Nd3+- Nd3+ interaction as in case of other materials. Therefore the luminescent decay profiles for whole range of Nd3+ concentration reveal exponential decays. Due to the fact of different Nd3+-Nd3+ distances, the luminescence is characterized by different decay constants. The most affected on the concentration quenching is the RbNdP4O12 structure having the longest Nd3+-Nd3+ distances, which shows the shortest decay constant (63 µs). For structures with helical PO4 chains the shortest decay was found for KNdP4O12 - 67 µs (Nd3+-Nd3+ =3.923 Å), it followed by LiNdP4O12 - 78 µs (Nd3+-Nd3+ =5.6 Å) and finally NaNdP4O12 -80 µs (Nd3+-Nd3+ =5.7 Å). This result confirms the correlation of the Nd3+-Nd3+ distance with the luminescent decay time. Moreover the donor-acceptor (CDA) and donor-donor (CDD) interaction parameters were determined for all of ANdP4O12 stoichiometric nanocrystals. For each of these hosts the CDD parameters are dominant and the smallest value of CDD was found for RbNdP4O12 and the highest for LiNdP4O12 nanocrystals

REFERENCES 1. Gruber, J. B.; Sardar, D. K.; Allik, T. H.; Zandi, B. Spectra and Energy Levels of Nd3+(4f3) in Stoichiometric NdP5O14. Opt. Mater. 2004, 27, 351–358 2. Tofield, B. C.; Bridenbaugh, P. M.; Weber, H. P. NdxLa1−xP5O14 Single Crystal Fibers. Mater. Res. Bull.

1975, 10, 1091–1096 3. Unger, W. K.; Raman Scattering From Soft Phonons in NdP5O14. Solid State Commun. 1979, 29, 601–605 4. Chinn, S. R.; Zwicker, W. K.; Colak, S. Thermal Behavior of NdP5O14 Lasers. J. Appl. Phys. 1982, 53 5471-5478. 5. Flaherty, J. M.; Powell, R. C.; Laser Site-Selection Time-Resolved Spectroscopy of NdP5O14. Solid State Commun. 1978, 26, 503–506 6. Powell, R. C.; Neikirk, D. P.; Flaherty, J. M.; Gualtieri, J. G. Lifetime Measurements, Infrared and Photoacoustic Spectroscopy of NdP5O14. J. Phys. Chem. Solids 1980, 41, 345–350 7. Danielmeyer, H. G.; Jeser, J. P.; Schönherr, E.; Stetter, W. The Growth of Laser Quality NdP5O14 crystals. J. Cryst. Growth 1974, 22, 298–302 8. Schulz, H.; Thiemann, K.-H.; Fenner, J. The High-Temperature Phase of NdP5O14. Mater. Res. Bull. 1974, 9, 1525–1530 9. Yoshimura, M.; Fujii, K.; Sōmiya, S. Phase Equilibria in the System Nd2O3|P2O5|H2O at 500°C Under 100 MPa and Synthesis of NdP5O14 Crystals. Mater. Res. Bull. 1981, 16, 327–333


Page 16 of 28

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10. Watanabe, K.; Tsunoda, S. Growth of NdP5O14 Single Crystals by the Cold-Finger Technique. J. Cryst. Growth 1986, 79, 953–962 11. Fox, D. L.; Scott, J. F.; Bridenbaugh, P. M. Soft Modes in Ferroelastic LaP5O14 and NdP5O14. Solid State Commun. 1976, 18, 111–113 12. Kubodera, K.; Otsuka, K. Single-Transverse-Mode LiNdP4O12 Slab Waveguide Laser. J. Appl. Phys. 1979, 50, 653-659. 13. Kubodera, K.; Otsuka, K.; Laser Performance of a Glassclad LiNdP4O12 Rectangular Waveguide. J. Appl. Phys. 1978, 49, 65-68 14. Webber, H. P.; Nd Pentaphosphate Lasers. Opt. Quant. Electron. 1975, 7, 431-442 15. Otsuka, K.; Yamada, T.; Saruwatari, M.; Kimura, T. Spectroscopy and Laser Oscillation Properties of Lithium Neodymium Tetraphosphate. IEEE J. Quantum Elect. 1975, 11, 330-335 16. Huber, G.; Danielmeyer, H. G.; NdP5O14 and NdAl3(BO3)4 Lasers at 1.3 µm. Appl. Phys. 1979, 18, 77-80 17. Gruber, J. B.; Sardar, D. K.; Allik, T. H.; Zandi, B. Spectra and Energy Levels of Nd3+(4f3) in Stoichiometric NdP5O14. Opt. Mater. 2004, 27, 351–358 18. Strek, W.; Marciniak, L.; Lukowiak, A.; Bednarkiewicz, A.; Hreniak, D.; Wiglusz, R. Synthesis and Luminescence Properties of LiLa1-xNdxP4O12 Nanocrystals. Opt. Mater. 2010, 33, 131-135 19. Strek, W.; Marciniak, L.; Bednarkiewicz, A.; Lukowiak, A.; Hreniak, D.; Wiglusz, R. The Effect of Pumping Power on Fluorescence Behavior of LiNdP4O12 Nanocrystals. Opt. Mater. 2010, 33, 1097-1101 20. Marciniak, L.; Strek, W.; Bednarkiewicz, A.; Lukowiak, A.; Hreniak, D. Bright Upconversion Emission of Nd3+ in LiLa1−xNdxP4O12 Nanocrystalline Powders. Opt. Mater. 2011, 33, 1492-1494 21. Marciniak, L.; Strek, W.; Hreniak, D. Subresonantly Excited Nd3+ Fluorescence in LiLa1−xNdxP4O12 Nanocrystals. Chem. Phys. Lett. 2013, 583, 151-154 22. Marciniak, L.; Strek, W.; Hreniak, D. Cooperative Absorption Transitions in LiLa1-xNdxP4O12 Nanocrystals. J. Lumin. 2014, 148, 214-218 23. Marciniak, L.; Stefanski, M.; Tomala, R.; Hreniak, D.; Strek, W. Size Effect in Luminescent Properties of LiNdP4O12 Nanocrystals. Opt. Mater. DOI: 10.1016/j.optmat.2014.09.023 24. Marciniak, L.; Strek, W.; Guyot, Y.; Hreniak, D. Synthesis and Luminescent Properties of La1−x NdxP5O14 Nanocrystals. Phys. Chem. Chem. Phys. 2014, 16, 18004-18009 25. Chen, G.; Ohulchanskyy, T. Y.; Liu, S.; Law, W.-C.; Wu, F.; Swihart, M. T.; Ågren, H.; Prasad, P. N. Core/Shell NaGdF4:Nd3+/NaGdF4 Nanocrystals with Efficient Near-Infrared to Near-Infrared Downconversion Photoluminescence for Bioimaging Applications. ACS Nano 2012, 6, 2969–2977 26. Rocha, U.; Kumar, K. U.; Jacinto, C.; Villa, I.; Sanz-Rodríguez, F.; Iglesias de la Cruz Mdel, C.; Juarranz, A.; Carrasco, E.; van Veggel, F. C.; Bovero, E.; Solé, J. G.; Jaque, D. Neodymium-Doped LaF3 Nanoparticles for Fluorescence Bioimaging in the Second Biological Window. Small 2014, 10, 1141-1154



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Page 18 of 28

27. Rocha, U.; Silva, C. J.; Ferreira Silva, W.; Guedes, I.; Benayas, A.; Martínez Maestro, L.; Acosta Elias, M.; Bovero, E.; van Veggel, F. C. J. M.; García Solé, J. A.; Jaque, D. Subtissue Thermal Sensing Based on Neodymium-Doped LaF3 Nanoparticles, ACS Nano 2013, 7, 1188–1199 28. Kaminskii, A. Crystalline Lasers: Physical Processes and Operating Schemes, CRC Press; 1996 29. Judd, B. R. Optical Absorption Intensities of Rare-Earth Ions. Phys. Rev. 1962, 127, 750-761 30. Ofelt, G. S. Intensities of Crystal Spectra of Rare‐Earth Ions. J. Chem. Phys. 1962, 37, 511-520 31. Yokota, T.; Tanimoto, O. Effects of Diffusion on Energy Transfer by Resonance. J. Phys. Soc. Japan

1967, 22, 779-784 32. Inokuti, M.; Hirayama, F. Influence of Energy Transfer by the Exchange Mechanism on Donor Luminescence. J. Chem. Phys., 1965, 43, 1978-1989

AUTHOR INFORMATION Corresponding Author * [email protected]

+48 713435188 Author Contributions The manuscript was written through contributions of all authors.

Funding Sources

L.

M.

acknowledges

support

from

the

NCN

under

grant


no.

2012/05/N/ST5/02327.

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Table of Contents Graphic: Figure 1. The presentation of the ALaP4O12 structures: LiLaP4O12 –a; NaLaP4O12 –b; KLaP4O12 –c; RbLaP4O12 –d. Figure 2. X-ray diffraction data of the ALaP4O12 structures: LiLa1-xNdxP4O12–a; NaLa1-xNdxP4O12 –b; KLa1xNdxP 4O12

–c; RbLa1-xNdxP4O12 –d.

Figure 3. The transmission electron microscopy (TEM) images of the ALaP4O12 structures: KLaP4O12 –a; NaLaP4O12 –b; the grain size distribution for KLaP4O12 nanocrystals –c and the average grain size for phosphates with different cation –d. Figure 4. Raman spectra of the ALaP4O12 structures: LiLaP4O12 –a; NaLaP4O12 –b; KLaP4O12 –c; RbLaP4O12 – d. Figure 5. Absorption spectra of LiNdP4O12 , NaNdP4O12 , KNdP4O12 , RbNdP4O12 nanocrystals. Figure 6. Emission spectra of ALa1-xNdxP4O12 nanocrystals: comparison of emission spectra for ALa0.99Nd0.01P4O12 – a; and ANdP4O12 –b; the 4F3/2→4I9/2 emission band of ALa0.99Nd0.01P4O12-c and ANdP4O12 – d; and the 4F3/2→4I11/2 emission band of ALa0.99Nd0.01P4O12-e and ANdP4O12 –f. Figure 7. Luminescent branching ratios as a function of dopant concentrations of LiLa1-xNdxP4O12-a, NaLa1xNdxP 4O12-b,

KLa1-xNdxP4O12-c, and RbLa1-xNdxP4O12-d.

Figure 8. Luminescence decay profiles ALa1-xNdxP4O12: A=Li – a; A=Na –b; A=K – c; A= Rb – d; and the decay times of ALa1-xNdxP4O12 – e as a function of dopants concentration.



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Figure 1. The presentation of the ALaP4O12 structures: LiLaP4O12 –a; NaLaP4O12 –b; KLaP4O12 –c; RbLaP4O12 –d. 230x135mm (96 x 96 DPI)


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Figure 2. X-ray diffraction data of the ALaP4O12 structures: LiLa1-xNdxP4O12–a; NaLa1-xNdxP4O12 –b; KLa1-xNdxP4O12 –c; RbLa1-xNdxP4O12 –d. 288x201mm (300 x 300 DPI)



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Figure 3. The transmission electron microscopy (TEM) images of the ALaP4O12 structures: KLaP4O12 –a; NaLaP4O12 –b; the grain size distribution for KLaP4O12 nanocrystals –c and the average grain size for phosphates with different cation –d. 225x174mm (96 x 96 DPI)


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Figure 4. Raman spectra of the ALaP4O12 structures: LiLaP4O12 –a; NaLaP4O12 –b; KLaP4O12 –c; RbLaP4O12 –d. 288x201mm (300 x 300 DPI)



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Figure 5. Absorption spectra of LiNdP4O12 , NaNdP4O12 , KNdP4O12 , RbNdP4O12 nanocrystals. 198x167mm (300 x 300 DPI)


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Emission spectra of ALa1-xNdxP4O12 nanocrystals: comparison of emission spectra for ALa0.99Nd0.01P4O12 – a; and ANdP4O12 –b; the 4F3/2→4I9/2 emission band of ALa0.99Nd0.01P4O12-c and ANdP4O12 –d; and the 4F3/2→4I11/2 emission band of ALa0.99Nd0.01P4O12-e and ANdP4O12 –f. 288x201mm (300 x 300 DPI)



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Luminescent branching ratios as a function of dopant concentrations of LiLa1-xNdxP4O12-a, NaLa1xNdxP4O12-b, KLa1-xNdxP4O12-c, and RbLa1-xNdxP4O12-d. 288x201mm (300 x 300 DPI)


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Luminescence decay profiles ALa1-xNdxP4O12: A=Li – a; A=Na –b; A=K – c; A= Rb – d; and the decay times of ALa1-xNdxP4O12 – e as a function of dopants concentration. 288x201mm (300 x 300 DPI)



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50x50mm (300 x 300 DPI)


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Synthesis and Nd3+ Luminescence Properties of ALa1âxNdxP4O12

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