Analytical control of the preparation of single crystal materials by inductively coupled plasma atomic emission spectrometry

Spectrochimica Acta Part B 57 (2002) 755–768

Analytical control of the preparation of single crystal materials by inductively coupled plasma atomic emission spectrometry N. Daskalova*, L. Aleksieva, G. Gentsheva, S. Velichkov Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, BG 1113 Sofia, Bulgaria Received 19 June 2001; accepted 31 December 2001

Abstract The present paper shows that inductively coupled plasma atomic emission spectrometry (ICP-AES) and the Q concept, in accordance with Boumans and Vrakking wSpectrochim. Acta Part B 43 (1988) 69x can be used in the determination of a large number of dopants with different characteristics (charge and ionic radius) in the single crystals of potassium titanylphosphate wKTiOPO4 x, some of its structural analogues and potassium gadolinium tungstate wKGd (WO4)2 x. The basic conclusion from the analytical data obtained in this work is that the incorporation of Meq, Me2q, Me3q, Me4q and Me5q ions in the crystal lattice depend on its ionic radii. The effect of the ionic charge of the dopant ions is negligible. The light on the regularities of dopant incorporation in the crystal lattice was thrown and hence on the possibilities of modifying the properties of the single crystal materials. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma atomic emission spectrometry; Single crystals; Dopants; Analysis

1. Introduction Potassium titanylphosphate KTiOPO4 (KTP), together with many of its structural analogues, are some of the most attractive materials for non-linear optical and electro-optical applications. These materials have good non-linear optical properties and high laser damage thresholds w1–4x. Potassium gadolinium tungstate KGd(WO4)2 (KGW) is not non-linear optical material. In recent years, different investigations have been carried out to activate KGW single crystals by rare earth ions and it is *Corresponding author. Tel.: q359-2-979-2543; fax.: q 359-2-705024. E-mail address: [email protected] (N. Daskalova).

supposed that new kinds of solid-state lasers might be obtained on their basis w5x. Doping can modify the properties of single crystal materials, the success of which depends on the type and concentration of the dopants w1–7x. Among with dopants added to achieved specific properties, metal contaminations can unintentionally get into the single crystals from the staring materials or during chemical and powder processing from crucibles, utensils, furnaces or airborne particulates w4x. Impurities can significantly influence the preparation, properties and performance of the crystal materials. Recent studies on nonlinear optical materials have shown that the impurities can adversely affect the optical properties of

0584-8547/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 2 . 0 0 0 0 8 - 3

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single crystals w8x. The content of impurities in single crystals of Nb2O5 change the conductivity of this material w9x. The dopant concentration in the initial solution is known, but that in the crystal is not, since it is a function of its distribution coefficient k (metal concentration in the crystal vs. metal concentration in the initial solution) w2–4x. Evidently, the properties of the doped materials will strongly depend on the concentration of the dopants in the single crystals. The determination of the dopant concentration in a given single crystal is extremely important step of a systematic study since it may differ from that in the initial solution by several orders of magnitude w10x. Hence, interpretations of the relation between the conditions of synthesis and the physical properties of the crystal materials are speculative and imprecise without the knowledge of the real concentration of the dopant in the single crystals. Several instrumental methods have been applied to this purpose: a Direct current arc–atomic emission spectrographic (d.c. arc-AES) method for determination of neodymium in neodymium doped yttrium aluminum garnet single crystals (Y3Al5O12:Nd3q). For calibration polycrystalline yttrium aluminum garnet (ground to 100 mm) mixed with Nd2O3 was used. The calibration samples contain neodymium in desired concentration range w11x; b D.c. arc-AES method for determination of Cr, Ga, Al, Mg, Li and V as dopants in single crystals of potassium titanylphosphate (KTP). Calibration procedure is not shown w3x; c Spark source mass spectrometry for determination of Ga, Al and Si as dopants and large number of impurities in single crystals of KTP. Calibration procedure is not known w4x; d Three atomic emission spectral methods for the determination of impurities in single crystals of TiO2: d.c. arc-AES method for determination of Al, Mg, Cu, Mn, Cr, Si, W, Te, Fe, V, Nb and Mo; ‘graphite arc ’ method for determination of Al, Mg, Cr, Cu, Mn and W. The samples were dissolved in a closed system by using an autoclave with Teflon vessels; two-stage method for

e

f

g

h

determination of Al, Mg, Cr, Cu, Mn and W including preliminary sputtered by means of laser evaporation at the top of electrode with the subsequent excitation of the condensate in the d.c. arc. Calibration procedures are described in the all cases w12x. Flame atomic absorption method (FAAS) for determination of Cr, Mn and Ni as dopants and Fe, Na, Mg, W and Al as trace metal impurities in single crystals of potassium titanylphosphate. The reference solutions for the determination of the analytes were prepared on the basis of a undoped potassium titanylphosphate crystals w13x; Determination of trace elements as impurities in calcium fluoride by direct solid sampling graphite furnace AAS (SS-GF-AAS), the electrothermal vaporization (ETV) combined with the ICP-AES and d.c. arc-AES. Certified reference materials were used for calibration w14–16x; Determination of Er and Nd dopants in bismuth telluride optical crystals by graphite furnace atomic spectrometry techniques w17x; Determination of the composition of the single crystals of copper doped double lithium-barium phosphates. The content of the major component lithium, barium and phosphorus and the dopant copper was determined by means of ICP-AES and FAAS. The accuracy of the results was assessed by the comparison of the data obtained by these two techniques. Owing to the poor sensitivity and precision of the FAAS determination of phosphorus, comparative data for this analyte were obtained by spectrophotometry. On the basis of the information about the concentration of the major components the crystallized phases were identified and the conditions for the synthesis of single crystals of the novel double lithium–barium phosphate Li2BaP2O7 were specified w18x. This polycrystalline Li2BaP2O7 was identified by X-ray diffraction analysis w19x.

The results of the analysis of single crystals of Li2BaP2O7 showed that the introduction of higher amounts of copper in the single crystal leads to a decrease in the lithium content whereas the barium and phosphorus content remains at the same level,

N. Daskalova et al. / Spectrochimica Acta Part B 57 (2002) 755–768

i.e. copper substitutes lithium and not barium or phosphorus in the crystal lattice w18x. Along with the average dopant concentration in the single crystals, crystal growers are also interested from the extent of crystal homogeneity, i.e. the distribution of the dopants along the both growth axis and the crystal radius. Two instrumental methods have been used: a The distribution of neodymium in single crystals of Y3Al5O12:Nd3q).The activator (neodymium) distribution was studied length-wise along the crystal and along its radius by laser-microemission spectroscopy. A quartz cylindrical lens was applied for the enhancement of the integral illumination level of the photographic emulsion. Owing to the lack of certified reference materials, the neodymium distribution in the single crystal is represented by the change in the ratio of the intensities of the spectral lines Nd II 401 220 pm and Y I 403 980 pm w11x; b Quantitative results for the dopant content in separate parts (bottom and top) of the crystal of KTP doped with Mn, Ni, Cr, Ge and Zr, respectively were obtained by ICP-AES. The reference solution were prepared on the basis of a potassium titanylphosphate single crystals. Calibration in ICP-AES (analysis of solution) is much easier than for instrumental methods analyzing solid samples w10x. The purpose of the present paper is the experimental demonstration of the ICP-AES and the Qconcept in the determination of a large number of dopants with different characteristics (charge and ionic radius) in order to throw light on the regularities of dopant incorporation in the crystal lattice of (KTP), some of its structural analogues and (KGW). 2. Q-values and true detection limits The quantification of the interferences in terms of Q-values was used in accordance with Boumans and Vrakking w20x. According to this approach Qvalues for line interference {QI (la)} and Q-values for wing (background) interference {Qw (Dla)} for each of the interferents were distinguished. The term QI (la) is expressed as the ratio SI (la)ySA,

757

where SI (la) is the partial sensitivity of the interfering line, defined as the signal per unit interferent concentration produced by the interfering line at the peak wavelength of the analysis line la and SA is the sensitivity of the analysis line. The term Qw (Dla) is expressed as the ratio Sw (Dla)ySA, where Sw (Dla) is the wing sensitivity of the interfering line in the spectral window Dla and SA is as stated above. Using the QI (la) and Qw (Dla) values, the conventional detection limits (CL conv) and the true detection limits (CL true) can be calculated. Optimum line selection implies the choice of the prominent lines with minimum value of the CL true, i.e. these with lowest line interference and background (wing) interference signals in the presence of the given matrix w20x. The detailed experimental study of spectral interferences in the determination of Na, Cr, Mn, Fe, Co, Ni, Al, Mg, Rh, W, Pt, Ge, Rb, Zr, Nb, Eu, Tb, Yb, Ho, Tm and Ga present as dopants or impurities in single crystals of KTP has been shown in our former papers w10,21x. Spectral data for K, Ti and P as interferents around the prominent lines of above mentioned analytes and a quantitative data base of Q-values were obtained. The Q-concept was used for quantification of the spectral interferences in the presence of gadolinium as interferent around the prominent lines of Yb, Er, Ho, Eu and Nd w22x. These analytes present as dopants in single crystals of KGW. Optimum line selection implies the choice of the prominent lines with minimum value of the true detection limits, i.e. these with lowest line interference and background (wing) interference signals in the presence of the matrix w20x. Table 2 summarizes the true detection limits (in %) with respect to the dissolved solid (8 mg mly1 KTP in solution) using the ‘best’ analysis lines of the analytes. The base data are taken from the literature w10,21x. Ho, Tm and Ga were enlisted in addition as dopants in the KTP single crystals and the spectral interferences were studied in separate paper w21x. Table 3 summarizes the true detection limits (in %) with respect to the dissolved solid (2 mg mly1 KGW in solution) using the ‘best’ analysis lines of the analytes w22x.

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Table 1 Operating conditions with the 27 MHz ICP JY 38 (Jobin Yvon, France) Set I

Incident power (kW) Outer argon flow rate (l miny1) Carrier flow rate (l miny1) Liquid uptake rate (ml miny1) Nebulizer pressure flow (lbfiny2) Transport efficiency of ICP system (%) Excitation temperature of ICP plasma (K)

1.0 15 0.5 1.3 30 (1.38 bar) 3 6200

Set II

Incident power (kW) Outer argon flow rate (l miny1) Carrier flow rate (l miny1) Liquid uptake rate (ml miny1) Nebulizer pressure flow (lbfiny2) Transport efficiency of ICP system (%) Excitation temperature of ICP plasma (K)

1.0 15 0.4 1.1 20 (2.08 bar) 3 7200

The question arises of how to improve the accuracy and precision of ICP-AES analyte determination in a broad concentration range, owing to the lack of certified reference materials, the restricted possibilities of AAS (analysis of solution) in the determination of rare earth and refractory elements and problems with the calibration by using a given instrumental methods for analysis of solid samples. For improving accuracy and precision it was used at more than two analysis lines, which were characterized with: (a) QI (la)s 0 since line interferences wQI (la))0x may severely endanger accuracy; (b) different sensitivities in ICP w24x in order to estimate the differences of acidity and solid sample concentration between the samples and calibration solutions. In this case analytical results cannot depend on the selected analysis lines. In Daskalova et al. w10x we accurately specified how the accuracy and precision of the analytical results can be ensured. 3. Experimental 3.1. Instrumentation The reader can refer to Daskalova et al. w10x for information about specifications of the equipment. There we noted that the measurements were performed with a Jobin Yvon (Longjumeau, France) system equipped with a 1-m Czerny–Turner monochromator (the practical spectral bandwidth was

measured to be 15.6 pm) and 27 MHz ICP. Two sets of operating conditions were used (Table 1). 3.2. KTP single crystals 3.2.1. Dissolution procedure (according to Ivanova and Havezov w13x) Each single crystal was crushed in pieces and finely ground in an agate mortar. A sample of 200 mg of the crystal material was transferred into a 100-ml PTFE beaker. Aliquots of 2 ml of 96% H2SO4 and 6 ml of 40% HF were added. The beaker was heated on a hot plate until sample dissolution and removal of the excess HF. The beaker was cooled, the walls were washed with several milliliters of water and heating was repeated until vapors of SO3 appeared. Then 0.1 ml of 30% H2O2 was added drop-wise. The resulting orange-colored solution was transferred to a 25-ml polypropylene graduated flask and was brought up to volume with water. The concentration of KTP in the solution was 8 mg mly1, that of H2SO4 142 mg mly1, and that of H2O2 approximately 0.005 mg mly1. 3.2.2. Calibration procedure The reference solutions for the determination of the analytes were prepared on the basis of undoped single crystals dissolved as described above. Undoped single crystals were used as the matrix blank.

N. Daskalova et al. / Spectrochimica Acta Part B 57 (2002) 755–768 Table 2 Detection limits (in %) with respect to the dissolved solid in the solution The ‘best’ analysis lines (pm)

Detection limits with respect to the dissolved solid in the solution (in %) True detection detection limits

Fictitious detection limits

Na I 588 995a Rb I 420 185a Al I 396 152a Fe II 238 204b Cr II 205 552b Ga I 294 364a

1.4=10y5 1.7=10y1 1.5=10y4 8.5=10y5 3.1=10y5 1.3=10y4

1.0=10y5 1.5=10y1 9.4=10y5 7.1=10y5 1.4=10y5 8.8=10y5

Nb II 309 418b Nb II 269 706b

2.0=10y4 1.7=10y4

6.9=10y5 1.0=10y4

Ni II 221 647b Mn II 257 610b Yb II 328 937a

1.6=10y4 2.5=10y5 3.3=10y5

1.0=10y4 2.1=10y5 2.9=10y5

Tm II 317 283a Tm II 313 389a

4.0=10y5 4.4=10y5

2.8=10y5 2.8=10y5

Er II 323 058a

3.2=10y4

4.6=10y5

Ho II 339 898a Ho II 389 102a Ho II 347 426a

4.7=10y5 5.6=10y5 6.3=10y5

3.1=10y5 5.0=10y5 5.0=10y5

Tb II 350 917a Nd II 406 109a Ge I 265 158b Zr II 343 823b

2.6=10y4 6.6=10y4 1.6=10y2 4.6=10y5

1.9=10y4 6.2=10y4 1.2=10y2 1.9=10y5

380a 660a 765a 254a

2.6=10y4 2.6=10y4 2.9=10y4 3.0=10y4

2.1=10y4 2.1=10y4 2.2=10y4 2.2=10y4

Ce Ce Ce Ce

II II II II

413 418 413 395

True detection limits for a solid concentration of 8 mg mly1 KTP using the ‘best’ analysis lines of Na, Rb, Al, Fe, Cr, Ga, Nb, Ni, Mn, Yb, Tm, Er, Ho, Tb and Nd. The fictitious detection limits, which are the detection limits that would be reached with 8 mg mly1 KTP in solution if it does not contribute the both interfering signal at the peak wavelength la of the analysis line and wing background interference in Dlas 200 pm wide windows centered around the given analysis line. a Texcs6200 K (set 1, Table 1). b Texcs7200 K (set 2, Table 1).

The final concentrations KTP in the matrix blank was 8 mg mly1, that of H2SO4 and H2O2 were 142 mg mly1 and 0.02 mg mly1, respectively.

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3.3. KGW single crystals 3.3.1. Dissolution procedure The finely ground crystal material (100 mg) was transferred into a 50-ml PTFE beaker. Then 5 ml of 40% (mym) HF were added. The beaker was heated on a hot plate for 5 min. The crystal material was treated with hydrofluoric acid for destroying the tungstate matrix. After cooling, 1 ml of 96% (mym) H2SO4 were added and the sample was heated again until the white vapors of SO3 appeared. During this treatment the insoluble rare earth fluorides were converted to the soluble sulfates w23x. The yellow precipitate of tungstic acids was obtained. through a dense paper filter. The insoluble residue was separated carefully onto a dense paper filter, collecting the filtrate in a 50ml volumetric flask. The insoluble residue of tungstic acid was washed with a 5 ml HNO3 (190 mg mly1).The solution thus obtained was brought up to the volume with HNO3. Therefore, 0.053 g tungsten was separated as an insoluble residue from the solution before the ICP-AES analysis. The final sample acidity was 190 mg mly1 HNO3. The concentration of potassium in the solution was 113 mg mly1, that of gadolinium was 454 mg mly1. 3.3.2. Calibration procedure The reference solutions for the determination of the analytes were prepared on the basis of undoped single crystals dissolved as described above. Undoped single crystals of KGW were used as the matrix blank. The final concentration of potassium in the matrix blank was 113 mg mly1, that of gadolinium was 454 mg mly1, the final acidity was 190 mg mly1 HNO3. 4. Results and discussion 4.1. Analysis of KTP single crystals and some of its structural analogues 4.1.1. Single crystals of KTP doped with different metal ions In the present study single crystals of KTP doped with representatives of the different metal ions: Meq (Na, Rb), Me2q (Mn, Ni), Me3q (Yb,

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Al, Fe, Ga, Cr, Tm, Er, Ho, Tb, Nd), Me4q(Ge, Zr and Ce) and Me5q(Nb) series were analyzed. The samples were brought into solution as described in Section 3.2.1. Single crystals were grown by the top seeded solution growth (TSSG) technique w25,26x. Table 4 shows the average dopant concentration in the crystal (column 2) in comparison with its concentration in the initial solid solution prior to crystal growth (column 1). Column 3 presents the values of ionic radii of the investigated metal ions w27x. As Naq and Rbq ions (set 1) are known to substitute Kq in KTP the ionic radii for a co-ordination number of 8, i.e. that of Kq were used w27x. Me2q, Me3q, Me4q and Me5q ions are supposed to substitute for Ti4q in crystal lattice of KTP, the ionic radii for a co-ordination number of 6, i.e. that of Ti4q were listed (Table 4, sets 2 and 3)w27x. The data presented in column 2 are the means of three replicates. These data were used by Sole´ et al. w25x and Nikolov et al. w26x for calculation of the distribution coefficients, listed in column 4. Column 5 shows the distribution coefficients from Morris w2x and McGee et al. w3x. The data in Table 4 show the following: 1. The dopants incorporated vary in a broad concentration range, i.e. the concentrations of Ce, Nd and Rb are close to their detection limits (see Table 2). The concentrations of the other dopants substantially exceed the detection limits. It should be noted that the dynamic range of both the ICP and JY38 detection system permit determination of these concentration levels by using the ‘best’ analysis lines of the analytes (Table 2); 2. The closer the ionic radius of the Meq ion to that of Kq a higher concentration of the Meq dopant can be accommodated in the crystal lattice, and the higher the corresponding distribution coefficient (Table 4, set 1). The best substitution for Kq in the examined Meq series is Rbq. The ionic radius of Naq being smaller than that of Kq, substitution of the latter with Naq is accompanied by a compression of the unit cell, while substitution with the larger Rbq leads to its expansion w25x;

3. The dopants of Me2q, Me3q, Me4q and Me5q ions are supposed for Ti4q in the crystal lattice of KTP w25,26x. It is evident from the presented data (Table 4, sets 2 and 3), that the highest concentrations (column 2) and the corresponding distribution coefficients in this series (column 4) are displayed by metal ions with ionic radii close to that of Ti4q. This result does not depend on whether the dopant ionic radius is smaller or bigger than that of Ti4q. Our data show one exception only: Ga3q was introduced to the initial solution in the same oxidation state as well as Cr3q and Al3q The ionic radii of Cr3q and Ga3q are similar (Table 4, set 2, column 3). Hence, both should be a good fit; however, their distribution coefficients are much different (Table 4, set 2, column 4). Conversely, the distribution coefficients for Ga3q and Al 3q are similar, but the ionic radius of Al3q are much different. The distribution coefficients of Ga3q, calculated by using the concentration in KTP, obtained in this work are in agreement with the data from Morris w2x and McGee et al. w3x (Table 4, set 2, column 5). The crystal growers consider that the differences in the ability to incorporate the Ga may be a function of some complex occurring in the initial solution w3x. 4. According to some published results w3,4x, the substitution of Ti4q by trivalent ions helps the charge compensation and reduce the potassium vacancies and ionic conductivity. For all Me3q substitutions that reduce the ionic conductivity, the distribution coefficients are rather low (Table 4, set 2). Only Cr3q shows a relatively high distribution coefficient (the ionic conductivity is reduced), but, in this case undesired absorption bands are detected in the visible region w3x and this material is not appropriate for waveguide application w26x. 5. The higher values of the concentration levels and of the distribution coefficients of the Me4q dopants (Table 4, set 3) may be related to the easier substitution of Ti4q by metal ions of the same ionic charge. The distribution coefficients of Ge4q, Zr4q and Ce4q slightly depend on the dopant concentration in the

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single crystal. From the experimental data (Table 4, set 3) it has been concluded that, of all the dopants studied here, the values of the distribution coefficient of Zr4q is higher and approximates 1, i.e. gives the most suitable distribution coefficient. The values of the distribution coefficients of Ge4q and Ce4q strongly deviate from 1. The homogeneity of KTP single crystals doped with Ge4q, Ce4q and Zr4q was investigated by microprobe analysis w26x. The results show that Zr is homogeneously distributed along growth axis. Furthermore, Zr significantly reduces the ionic conductivity (two orders of magnitude), does not introduce problematic absorption bands in the visible region and hence this material is appropriate for waveguide application w26x. Particular attention should be paid to the introduction of rare earth elements (REE3q) as dopants into the crystal lattice of KTP. In this case, along with lowering the ionic conductivity of the optical crystals, the broadening of the range of second harmonic generation was reached. By this way the self-frequency doubling lasers can be obtained w25x. The highest REE3q concentrations in crystals change from 0.0052% in the case of Nd to 0.058% in the case of Yb, i.e. rather low amounts of dopants are incorporated (Table 4, set 2, column 2). These concentrations are at the level at which changes in the ionic conductivity as well as in the absorption spectra may occur w4x. The coefficient of distribution depends on the REE3q ionic radii and increases by nearly one order of magnitude, from 5.6=10y4 for Nd3q to 5.8=10y3 for Yb3q, i.e. as the REE3q ionic radii increase (Table 3, set 2, column 3), the coefficient of distributions decrease (Table 4, set 2, column 4). The crystal growers investigate the cell volume of the same KTP single crystals. The results show that the cell volume of doped KTP single crystals is slightly larger than that of undoped KTP. The change in the cell parameters depends on the concentration and ionic radius of the rare earth dopants w25x. The noticeable changes in the optical and laser properties of the single crystals of KTP can be reached in the presence of higher dopant concen-

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Table 3 Detection limits (in %) with respect to the dissolved solid in the solution The ‘best’ analysis lines (pm)

Detection limits with respect to the dissolved solid in the solution (in %) True detection detection limits

Fictitious detection limits

Nd II 401 225

2.4=10y3

2.0=10y3

Eu II 381 967

2.3=10y4

3.5=10y5

Ho II 345 600

7.0=10

y4

3.5=10y4

Er II 337 271 Er II 369 265

3.2=10y4 4.0=10y4

1.5=10y4 2.0=10y4

Yb II 328 937 Yb II 289 138

2.0=10y4 3.3=10y4

1.0=10y4 3.0=10y4

True detection limits for a solid concentration of 2 mg mly1 KGW using the ‘best’ analysis lines of Nd, Eu, Ho, Er and Yb. The fictitious detection limits, which are the detection limits that would be reached with 2 mg mly1 KGW in solution if it does not contribute the both interfering signal at the peak wavelength la of the analysis line and wing background interference in Dlas200 pm wide windows centered around the given analysis line. Texcs6200 K (set 1, Table 1).

tration of REEs in comparison with the concentration levels, shown in Table 4, set 2. An increase in the amount of the REEs as dopants cannot be achieved by increasing the concentration of the corresponding rare earth oxide in the initial solution due to the crystallization of high-temperature rare earth phosphates. An approach in this respect is the double doping w25x. 4.1.2. Doubly doped single crystals of KTP The samples were brought into solution as described in Section 3.2.1. Single crystals of KTP doped with Nd3q as a model rare earth metal ion, and in combination with a second dopant ion for KTP (double substitution) were submitted for analysis. The second dopant ions were selected on the basis of their ionic radius and ionic charge: (Nd3qqAl3q), (Nd3qqNb5q), (Nd3qqRbq) and (Nd3qqNaq). Table 5 shows the average dopant concentration in the crystal (column 2) in comparison with its concentration in the initial solid solution prior to crystal growth (column 1). The data presented in column 2 are the means of

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762

Table 4 Average dopant concentration in the KTP single crystals Dopant ions

Added to the initial solid solutions (%)

Found in the crystal (%)

Ionic radii ˚ A w27x

Distribution coefficient, Kd, obtained from our data

Distribution coefficient, Kd in accordance with refs.

4.0=10y2 – 5.0=10y1

No data

1.7=10y2 w3x 2.6=10y2 w2x No data

Set 1 Naq Kq Rbq

3.47 – 3.66

0.140"0.004 1.830"0.05

1.180 1.510 1.610

Set 2 Al3q

2.12

0.024"0.002

0.5350

1.1=10y2

Fe3q Ti4q Cr3q Ga3q

1.62

0.080"0.004

5.0=10y2

0.16 1.77

0.080"0.004 0.030"0.004

0.5500 0.605 0.6150 0.6300

Nb5q Ni2q Mn2q Yb3q Tm3q Er3q Ho3q Tb3q Nd3q

0.70 3.94 3.16 5.27 5.25 4.80 6.96 10.00 0.93

0.530"0.004 0.110"0.003 0.048"0.003 0.058"0.005 0.030"0.002 0.024"0.002 0.034"0.002 0.020"0.002 0.052"0.0004

0.6400 0.6900 0.8300 0.8680 0.8800 0.8900 0.9010 0.9230 0.9830

7.6=10y1 2.8=10y2 1.5=10y2 5.8=10y3 5.1=10y3 3.8=10y3 2.7=10y3 2.2=10y3 5.6=10y4

0.37 1.10 1.80

1.20"0.02 3.50"0.02 5.82"0.02

0.52

3.2 3.2 3.1

0.46 1.40 2.30 0.70 2.10 3.50

0.68"0.02 1.80"0.02 2.80"0.02 0.0032"0.0002 0.0084"0.0002 0.0122"0.0002

Set 3 Ge4q

Ti4q Zr4q

Ce4q

0.605 0.80

0.92

No data

5.0=10y1 1.7=10y2 w25x

w25x w25x w25x w25x w25x w25x

7.3=10y1 w3x 8.0=10y2 w3x 1.7=10y2 w2x No data No data No data No data No data No data No data No data No data

w26x w26x w26x

No data

1.5 w26x 1.4 w26x 1.2 w26x 4.30=10y3 3.80=10y3 3.40=10y3

No data

w26x w26x w26x

No data

The crystals were doped with: set 1 — Meq ions (Na, Rb); set 2 — Me2q ions (Mn, Ni), Me3q ions (Yb, Al, Fe, Ga, Cr, Tm, Er, Ho, Tb, Nd), and Me 5q ions ( Nb); set 3 — Me4q ions (Ge, Zr, Ce) wmean of three replicatesx.

three replicates. Table 5 shows the values of ionic radii of the REE3q dopants (column 3) w27x and the distribution coefficients (column 4) w25x. The presence of co-doping element generally induces a slight enhancement of the distribution coefficient of Nd, only the co-doping with Naq induces a decrease. The distribution coefficient of Nd increases four-fold by co-doping with Rbq and five-fold by co-doping with Nbq (column 4, these values are printed in bold). On the basis of the Nd concentrations and co-dopant concentration in KTP

single crystals can be draw the following conclusions: 1. The enhancement of the Nd incorporation to the KTP lattice, co-doping should be carried out with an element with ionic radius as near as possible to that to be substituted but with larger charge. For example Nb5q (ionic radi˚ and an ionic charge that is higher uss0.64 A 4q that of Ti ) is more suitable as a co-doping ˚ and a lower than Al3q (ionic radius 0.535 A

N. Daskalova et al. / Spectrochimica Acta Part B 57 (2002) 755–768

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Table 5 Average dopant concentration in the KTP single crystals doped with Nd3q ions and doubly doped with (Nd3qqAl3), (Nd3qq Nb5q), (Nd3qqRbq), (Nd3qqNaq) ions (mean of three replicates) Dopants

Added to the initial solid solutions (%)

Found in the crystal (%)

Ionic radii ˚ (A) w27x

Distribution coefficient w25x

Nd

0.86

0.0052"0.0004

0.9830

5.6=10y4

Nd q Al

3.40

0.0069"0.0004

0.983

1.0=10y3

2.10

0.0240"0.02

0.537

1.8=10y2

Nd q Nb

0.86

0.0053"0.0004

0.983

3.5=10y3

0.70

0.7300"0.040

0.640

7.6=10y1

Nd q Na

4.30

0.0026"0.0004

0.983

3.4=10y4

3.50

0.1400"0.040

1.180

3.7=10y2

Nd q Rb

3.40

0.0140"0.0004

0.983

2.0=10y3

3.70

1.800"0.050

1.610

1.5=10y1

ionic charge than of Ti4q). The ionic radius ˚ (Table 4, set 2). Likewise, of Ti4q is 0.605 A their ionic charges being the same, Rbq (ionic ˚ seems to be a better co-dopant radiuss1.61 A) ˚ when Kq than Naq (ionic radius s1.18 A) ˚ has to be substituted. (ionic radiuss1.51 A) 2. The relation among the concentration of neodymium in KTP single crystal, the size of codopant ions and the cell volume were established. When the second dopant is a large ion (like Rbq substituting Kq) the cell volume tends to increase. If small ions are used as second dopant (Al3q substituting Ti4q or Naq substituting Kq) the cell volume decreases w25x. 4.1.3. REE3q-doped KTP and its structural analogues The data for the concentration of Nd3q, Er3q, Yb3q dopants and the corresponding distribution coefficients in KTP and its structural analogues obtained by partial or complete substitution of Kq with Naq or Rbq are shown in Table 6. As can be seen, all three REE3q metal ions exhibit the same dependence. The substitution of Kq with the smaller Naq ion accompanied by a compression of the unit cell. This is less favourable for

the incorporation of the REE3q dopants w25x. For the maximum admissible KyNa ratio at which the KTP structure is still preserved (0.73y0.27), there is a decrease in the distribution coefficients of the all three rare earth dopants. The reverse tendency is observed when Kq is partially or completely substituted with the larger Rbq as a result of expansion of the unit cell w25x. Hence, the higher values of the distribution coefficients of the REE3q ions can be expected by doping of RbTiOPO4 (RTP) wstructural analogue of KTPx, because the cell volume of RTP tends to increase in comparison with the cell volume of KTP w25x. 4.1.4. REE3q-doped RTP Single crystals of RTP were doped with Nd3q, 3q Er , (Nd3qqNb5q) and (Er3qqNb5q) ions. It follows from the data presented in Table 7 that by co-doping with Nb5q the distribution coefficient of Nd3qand Er3q, respectively increase. 4.1.5. Determination of the critical concentration of Er2O3 or Nd2O3 in the initial solutions in case of doubly doped single crystals of RTP For growing rare earth-doped RTP crystals, the critical concentration of the rare earth below which

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N. Daskalova et al. / Spectrochimica Acta Part B 57 (2002) 755–768

Table 6 Concentrations of Yb3q, Er3q and Nd3q (column 2) and corresponding distribution coefficients in KTP single crystals (column 4) and its structural analogues obtained by partial or complete substitution of Kq with Naq and Rbq Single crystal

Dopant

Found in the crystal (%)

Ionic ˚ radii (A) w27x

Distribution coefficient w25x

KTiOPO4

Yb Er Nd

0.058"0.005 0.024"0.002 0.0052"0.0004

0.8680 0.8900 0.9830

5.8=10y3 3.8=10y3 5.6=10y4

K0.89Na0.11TiOPO4

Yb Er Nd

0.0200"0.005 0.0220"0.002 0.0014"0.0004

0.8900 0.8680 0.9830

4.4=10y3 3.9=10y3 3.2=10y4

K0.73Na0.27TiOPO4

Yb Er Nd

0.0072"0.005 0.0064"0.002 0.0021"0.0004

0.8680 0.8900 0.9830

1.4=10y3 1.4=10y3 2.8=10y4

K0.64Rb0.36TiOPO4

Yb Er Nd

0.1200"0.005 0.0633"0.002 0.0030"0.0004

0.8680 0.8900 0.9830

2.5=10y2 1.3=10y2 7.5=10y4

RbTiOPO4

Yb Er Nd

0.2100"0.005 0.0780"0.002 0.0420"0.0004

0.8680 0.8900 0.9830

5.0=10y2 1.9=10y2 1.2=10y2

it is possible to obtain the RTP phase and above which other phases appear is a very important parameter. Two kinds of experiments were carried out in this series. In the first one, Er or Nd were used, while in the second one, Er or Nd were investigated, but in combination with Nb (double substitution). Table 8 presents the concentration of Er and Nb in the initial solutions and in RTP single crystals doped with Er or (ErqNb) for the different EryNb concentration proportions. Table 9 shows the con-

centration of Nd and Nb in the initial solutions and in RTP single crystals doped with Nd or (NdqNb) for the different NdyNb concentration proportions. The following conclusions may be drawn: 1. The highest (critical) concentration by doping with Er is 4.35% (5% Er2O3) in the initial solution. Above this concentration of Er in initial solutions the erbium concentration in crystal material increases significantly (Table

Table 7 Average dopant concentration in the RTP single crystals Single crystal

Dopants

Added to the initial solid solutions (%)

Found in crystal (%)

Distribution coefficient

RTPqNd

Nd

2.57

0.0320"0.0004

1.2=10y2

RTPqNdqNb

Nd Nb

1.72 2.80

0.0490"0.0004 0.880"0.040

2.8=10y2 3.1=10y1

RTPqNb

Nb

3.50

0.780"0.040

2.2=10y1

RTPqEr

Er

3.48

0.300"0.002

8.6=10y2

RTPqErqNb

Er Nb

3.48 1.40

1.000"0.002 0.620"0.040

2.9=10y1 4.4=10y1

Crystals were doped with Nd 3 and Er3q and doubly doped with (Nd3q qNb5q ) and (Er3q qNb5q ) ions wmean of three replicatesx.

N. Daskalova et al. / Spectrochimica Acta Part B 57 (2002) 755–768 Table 8 Determination of critical concentration of Er2 O3 in the RTP single crystals Dopants

Added to the initial solid solutions (%)

Found in the crystal (%)

Er q Nb

1.74

0.18

0

0

1.74

0.62

1.40

1.10

1.74

0.75

2.10

1.77

1.74

0.72

2.80

2.53

2.61

0.27

0

0

2.61

1.39

1.40

1.23

2.61

1.05

2.10

1.80

3.48

0.27

0

0

3.48

1.46

1.40

1.20

Er q Nb Er q Nb Er q Nb Er q Nb Er q Nb Er q Nb Er q Nb Er q Nb Er q Nb

4.35

4.30

0

0

Crystals were doped with Er3q, Nb5 and doubly doped with (Er3qqNb5q) ions wmean of three replicatesx.

8, column 2) because ErPO4 appears as a new phase instead of RTP phase w23x; 2. The highest (critical) concentration by doping with Nd is 2.58% Nd (3% Nd2O3) in initial solution. Above this concentration of Nd in the initial solutions the neodymium concentration in crystal material increases significantly (Table 9, column 2) because NdPO4 appears as a new phase instead of RTP phase ErPO4 and NdPO4 phases were established by the X-

765

ray powder diffraction method w23x. 3. The optimal concentration for Nb as co-dopant is approximately 1.40% Nb (2% Nb2O5). Above this concentration of Nb in initial solutions the erbium concentration or neodymium concentration in crystal material increases unimportant or decreases (Tables 8 and 9, column 2). 4.2. Analysis of KGW single crystals 4.2.1. REE3q doped KGW single crystals The single crystals of KGW doped with REE3qions (Yb, Er, Ho, Eu and Nd) series were analyzed. The samples were brought into solution as described earlier. Single crystals were grown by Table 9 Determination of critical concentration of Nd2 O3 in the RTP single crystals Dopants

Added to the initial solid solution (%)

Found in the crystal (%)

Nd q Nb

0.86

0

0

0

Nd q Nb

0.86

0.0047

0.70

0.60

Nd q Nb

0.86

0.0063

1.40

0.95

Nd q Nb

1.72

0.0028

0

0

Nd q Nb

1.72

0.0210

1.40

1.00

Nd q Nb

1.72

0.0092

2.10

1.38

Nd q Nb

1.72

0.0083

3.50

2.0

Nd q Nb

2.58

2.50

0

0

Crystals were doped with Nd3q , Nb5 and doubly doped with (Nd3qqNb5q) ions wmean of three replicatesx.

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Table 10 Average dopant concentration in the KGW single crystals Dopants

Added to the initial solid solutions (%)

Found in the crystal (%)

Ionic radii ˚ (A) w27x

Distribution coefficient

Yb

0.87 2.60 4.35 8.70 4.35 8.70 1.053 4.30 8.60 0.86

0.65"0.03 1.95"0.03 3.48"0.05 6.90"0.05 3.70"0.05 7.50"0.05

0.985 0.75 1.004 0.79 1.015 0.86

0.75

3.96"0.05 7.70"0.05 0.54"0.03

1.066 0.90 1.109

0.92

Er Ho Gd Eu Nd

Crystals were doped with REE

3q

0.80 0.85

0.63

ions: Yb, Er, Ho, Eu and Nd wmean of three replicatesx.

the TSSG technique w28,29x. Table 10 shows the average dopant concentration in the crystal (column 2) in comparison with its concentration in the initial solid solution prior to crystal growth (column 1). The data presented in column 2 are the means of three replicates. Table 10 presents the values of ionic radii of the REE3q dopants (column 3) w27x. As REE3q ions are known to substitute Gd3q in KGW w26x the ionic radii for a co-ordination number of 8, i.e. that of Gd3q was used w27x. The concentration of the rare earth dopants in single crystals of KGW (Table 10, column 2) and the distribution coefficients (Table 10, column 4), respectively, decrease as the REE3q ionic radius is further from that corresponding to Gd3q. This result does not depend on whether the REE3q ionic radius is smaller or greater than that of Gd3q. The distribution coefficients, of REE3q ions substituting Gd3q in KGW have been calculated and from the rare earth concentration in the crystals, obtained by electron probe microanalysis using CAMECA Camebax SX 50 equipment and the concentration in the solution growth w28x. The results shown in this paper (Table 12, column 4) are in accordance with the corresponding values presented in Fig. 1 in Pujol et al. w28x. There should be noted that the identical dependence were established and in the case of KTP single crystals (Table 4). The homogeneity of KGW single crystals doped with REE3q was investigated by microprobe anal-

ysis w29x. In this case the distribution coefficients of REE3q approximate 1 (Table 10, column 4). The results show that REEs are homogeneously distributed along the growth axis and this single crystals are appropriate for laser applications w28,29x. 5. Conclusions The determination of the dopant concentration in the single crystals is an extremely important step of a systematic study since it may differ from that in the initial solutions by several orders of magnitude (Tables 4 and 10). Hence, interpretations of the relation between the conditions of synthesis and the physical properties of the crystal materials are speculative and imprecise without the knowledge of the real concentration of the dopant in the single crystal. In this paper the quantitative data about the concentration of a large number of dopants were obtained by ICP-AES. Owing to the lack of certified reference materials and the restricted possibilities of atomic absorption spectrometry in the determination of rare earth and refractory elements, the only way out for improving accuracy and precision was the use of the possibilities of the Qconcept: measuring at more than two analysis lines with QI(la)s0 since line interferences QI (la)) 0 may endanger accuracy w10x. The following

N. Daskalova et al. / Spectrochimica Acta Part B 57 (2002) 755–768

conclusions from the analytical data presented in this paper can be drawn: ●

●

●

The incorporation of dopant ions in the obtained crystals of KTP or KGW depend on its ionic radii. This does not depend on whether the ionic radii of dopants are smaller or greater than that of Kq, Ti4q (Table 4) or Gd3q (Table 10); The properties of the doped materials strongly depends on the type and concentration of the dopants: Zr4qdoping gives the suitable concentration in KTP single crystals and significantly reduces the ionic conductivity of this material; the cell volume of KTP single crystals doped with REEs is slightly larger than that of undoped KTP; The desired optical and laser properties of the single crystals of KTP can be reached in the presence of higher dopant concentration of REEs in comparison with the concentration levels, shown in Table 4, set 2. An increasing in the amounts of the REEs in single crystals cannot be achieved by increasing the concentration of the corresponding rare earth oxides in the initial solutions due to the crystallization of high temperature rare earth phosphates (Tables 8 and 9). An approach in this respect is the doubly doped single crystals of KTP (Table 5). The higher values of the concentration levels of the REEs were reached by doping RTP (Tables 6 and 7), because the cell volume of RTP as a structural analogue of KTP tends to increase in comparison with the cell volume of KTP.

The single crystal materials were derived from crystal growers in Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria, Laboratory of Applied Physics and Crystallography, University Rovira I Virgili, Tarragona, Spain and Department of Crystallography and Mineralogy, University of Barcelona, Spain. The large number of distribution coefficients was calculated. The relation among the concentration of dopants incorporated in the crystal lattice, its distribution coefficients and optical and laser properties of the single crystal materials was established. On this basis were assessed the pos-

767

sibilities for obtaining of optical materials with desired properties by appropriate doping w22,23,25,26x. Acknowledgments The financial support from the National Fund for Scientific Research of the Ministry of Science and Education of Bulgaria under registration No X-629 is gratefully acknowledged. References w1x E.J. Millett, Progress in the analysis of crystalline solids, J. Cryst. Growth 48 (1980) 666–682. w2x P.A. Morris, Impurities in nonlinear optical oxide crystals, J. Cryst. Growth 106 (1990) 76–88. w3x T.F. McGee, G.M. Bloom, G. Kostecky, Growth and characterization of doped KTP crystals, J. Cryst. Growth 109 (1991) 361–366. w4x P.A. Morris, A. Ferretti, J.D. Bierlein, Reduction of the ionic conductivity of flux grown KTiOPO4, J. Cryst. Growth 109 (1991) 367–375. w5x C. Tu, Z. Luo, G. Chen, T. Zhao, Crystal growth of KGd(WO4)2, J. Cryst. Growth 152 (1995) 235–237. w6x N. Manuilov, V. Nikolov, G. Gentsheva, P. Peshev, Study of some K2W2O7(KGd1yxLnx (WO4 )2 systems to be used for flux growth of doped KGd(WO4)2 single crystals, J. Cryst. Growth 196 (1999) 181–184. w7x H.M. Ortner, P. Wilhartitz, The characterisation of hightech materials: perspectives, challenges, trends, Microchim. Acta (Wien) II (1991) 177–214. w8x D.K. Fox, R. Mazelsky, Impurities: curse and blessing for crystal growers, J. Cryst. Growth 106 (1990) 1–5. w9x V.S. Grunin, Z.N. Zonn, I.B. Patrina, Problems of theoretical crystallochemistry in complex oxides, L., 66, 1882 (Russia). w10x N. Daskalova, S. Velichkov, P. Slavova, E. Ivanova, L. Aleksieva, Application of inductively coupled plasma atomic emission spectrometry in analytical control of single crystals of potassium titanylphosphate: spectral interferences, line selection, detection limits and trace element determination, Spectrochim. Acta Part B 52 (1997) 257–278. w11x N. Krasnobaeva, N. Nedyalkova, P. Peshev, E. Kirkova, L. Nikolova, S. Velichkov, Some examples on the use of atomic emission spectral analysis in analytical control of the preparation of oxide single crystals, Spectrochim. Acta Part B 37 (1982) 829–833. w12x A.A. Boitzov, I.B. Gornushkin, N.N. Daskalova, Ch.I. Zilbershtein, M.V. Razumeenko, Emission spectral analysis of impurities in TiO2 single crystals, Zh. Prikl. Spectr. (Russ.) 47 (1987) 554. w13x E. Ivanova, I. Havezov, Flame AAS determination of dopants and trace metal impurities in single crystals of

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