Modified Czochralski growth and characterization of RETiNbO6 crystals

Modified Czochralski growth and characterization of RETiNbO6 crystals

CRYIITAL GROWTH ELSEVIER Journal of Crystal Growth 180 (1997) 73 80 Modified Czochralski growth and characterization of RETiNbO6 crystals X. Qi, H...

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ELSEVIER

Journal of Crystal Growth 180 (1997) 73 80

Modified Czochralski growth and characterization of RETiNbO6 crystals X. Qi, H.G. Gallagher*, T.P.J. Han, B. Henderson Optical Materials Research Centre, Department o['Physics and Applied Ptzvsics, Universi(v of Strathelyde, Glasgow G1 IXN, Scotland, UK

Received 21 January 1997; accepted 6 March 1997

Abstract Bulk crystals of RETiNbO6 (RE = Nd, Pr) with size 2~ 15 x 20mm have been grown by a modified Czochralski method. The RETiNbO6 compounds melt incongruently and the crystals can only be grown from melts containing a small amount of extra TiO2. The structures of bulk crystals were confirmed by X-ray powder diffraction to be the same as mini-crystals grown by the laser heated pedestal growth (LHPG) method. The bulk crystals were characterized by measurement of their optical absorption and photoluminescence spectra. PACS: 42.7; 78.5; 78.55 Kevwords: NdTiNbO6; PrTiNbO6; Laser crystals: Czochralski growth; Phase diagrams; Optical spectroscopy

1. Introduction Stoichiometric compounds with a large concentration of rare-earth ions intrinsic to the crystal structure are ideal gain media in laser-diode-pumped microlasers. In addition to the advantage of miniaturisation, the stoichiometric compounds are naturally able to form better quality crystals. As intrinsic constituents of the crystal rare-earth ions do not further distort the lattice as much as dopant ions, for which mismatch of ionic radii and charge and dopant segregation all contribute to internal and macroscopic strains. Among the most success-

* Corresponding author. Fax: + 44 141 5534162.

ful stoichiometric laser crystals are NdPsO15, LiNd(PO3)4, K3Nd(PO4)2 and NdA13(BO3)4: in these compounds the Nd 3 + concentrations exceed 3 x 1021 ions c m - 3 and the absorption coefficients at ~ 8 0 0 n m are some 50 times larger than in N d : Y A G laser crystals [1]. Since the Nd 3+ absorption lines in such crystals as YAG are rather narrow they are not optimally matched to the 800 nm output of laser diodes which have linewidths of about 2-3 nm. Much current research on diode-pumped lasers seeks to improve the tolerance of the gain medium to the p u m p wavelength by designing materials with broader absorption lines. Recently the authors have synthesized a new family of stoichiometric rare-earth compounds,

0022-0248/97/$17.00 Copyright ~( 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 2 0 5 - 4

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X. Qi et al. /'Journal of C~stal Growth 180 (1997) 73 80

RETiNbO6 (RE = Pr, Nd and Er), and grown miniature crystals of them by the laser heated pedestal growth (LHPG) technique. These crystals are disordered with the component Nb 5+ and Ti 4+ ions randomly distributed on the same crystallographic site [2]. The absorption lines of RE 3+ ions in these crystals are very strong and also broad (5-10 nm) as a consequence of this intrinsic crystalline disorder [2, 3]. The luminescence lines from such compounds are very intense due to the high RE 3+ concentration (~ 1022 ions cm-3), and the enhanced transition rates that result from oddparity distortions of the RE 3+ ion environment. In this paper, the growth and characterization of bulk RETiNbO6 crystals are reported.

2. Experimental techniques

2.1. C~stal growth Crystal growth was carried out in a Crystalox computer controlled medium-frequency induction (20 kHz) heated furnace equipped with a precision pulling head. The growth chamber contained nitrogen gas to protect the iridium crucibles from oxidation at high temperature. The iridium crucibles used in the present study had the dimensions 38 mm diameter × 38 mm height. Starting chemicals for crystal growth were oxides including Nd20 3 (purity 99.9%), Pr203 (99.99%), TiO2 (99.99%) and Nb205 (99.99%). The phase diagrams of the REzO3-TiOz-Nb205 systems were unknown before this study, therefore the growth of NdTiNbO6 crystals was initially attempted from a stoichiometric charge of composition Nd203 + 2TIO2 + Nb2Os. A 150 g charge was well mixed in an agate mortar and pestle before charging the crucible. The charge was completely melted at a temperature ca. 1980 K, estimated by pyrometer. This temperature was kept constant for 30 rain and then the charge was cooled to ca. 1890 K, at which point an iridium wire was dipped into the melt to seed the crystal. The iridium wire was rotated at 9rpm and slowly withdrawn at a speed of 0.2 mm h- a. During pulling the output power was continuously lowered at 0.04% per hour, corresponding to an estimated cooling rate of 0.3 K h- 1.

In the first run, some dark-red polycrystalline matter was obtained on the iridium wire, which the X-ray powder diffraction pattern indicated to be polycrystalline NdNbO4 (NdzO3" Nb2Os). Crystal growth was then attempted with the remaining charge: in the third run crystals grown onto the wire were divided into two parts by colour. The dark-red central part was identified to be NdNbO4. The outer part was black and became clear and deep-red after annealing in air for 10 h. This outer section was confirmed to be NdTiNbO6 by X-ray powder diffraction. Based on these results, it was assumed that NdTiNbO6 melted incongruently and decomposed in the TiOe-NdNbO4 pseudobinary system. An approximate phase diagram, Fig. 1, was then proposed, according to which subsequent growths were carried out from charges containing extra TiO2. After several trial runs a suitable composition was defined as: Nd203 + 2.7TIO2 + Nb2Os. NdTiNbO6 single crystals of about ~ 12 × 15 mm 3 in size were obtained from such a melt at a pulling rate of 0.15 mm h -~ and rotation rate of 9 rpm. Oriented single crystal samples with edges parallel to the a-, b- and c-axes were cut from these boules for optical characterisation. The process for growing the PrTiNbO6 crystal was similar to that of NdTiNbO6, and the crystal

2100

Liq.

2000 A

,= ,,,t

Liq.+NdNbO4""~~57% ~'~,,

1900

#

~1873±20 K

1800

¢

E I-

Z

1700 1600 0

NdNbO 4

Liq.+NdTiNbO 6

z I 20

I 40

I 60

Mol.% TiO 2

I 80

100

TiO 2

Fig. 1. A postulated phase diagram for the TiO2 NdNbO4 pseudo-binary system.

X. Qi et al. / Journal of C~stal Growth 180 (1997) 73-80

75

with a grating of 1200 lines/mm blazed at 570 nm. Signal from the output slit of the monochromator was detected by a photomultiplier (Hamamatsu R928) in the wavelengths from 400 to 850 nm or by a liquid N2 cooled germanium detector in the near infrared region. In the low temperature experiment, samples were cooled by a closed-cycle He cryorefrigerator.

3. Experimental results and discussion

3.1. X-ray diffraction and physical properties Fig. 2. An as-grown single crystal and optical sample of PrTiNbO6.

could only be grown from a melt containing excess TiO2. However, PrTiNbO6 crystals were easier to grow and the suitable melt composition for growth deviated less from the stoichiometric composition, i.e. Pr2Oa + 2.5TIO2 + Nb20 5. The pulling rate for successful growth was 0.25 mm h - I and the rotation speed 9 rpm. PrTiNbO6 crystals measuring 15 mm diameter×20mm length have been grown. A photograph of a PrTiNbO6 crystal is shown in Fig. 2. In projection, two orthogonal ridge patterns can be observed on the cone of the crystal in the photograph, indicating that the crystal is orientated along one of the principle axes of the orthorhombic lattice. The as-grown PrTiNbO6 crystals were black in colour, similar to NdTiNbO6. After annealing in air at 1550 K for 10h, the PrTiNbO6 crystal turned green. Such colour changes in RETiNbO6 crystals have been ascribed to oxygen deficiencies in the as-grown crystals [2].

2.2. Spectroscopic measurement Optical absorption spectra were measured using a modified CARY-AVIV 14DS spectrophotometer, capable of operating over the range 200-2500 nm. The laser excited photoluminescenee measurements were made at room temperature and near 10 K using a Spex 500M monochromator operated

Bulk RETiNbO6 crystals grown by a modified Czochralski technique were studied by powder and single crystal X-ray diffraction methods. Laue diffraction showed that the crystals had good single crystal quality and had grown along the b-axis. The powder diffraction patterns confirmed that the bulk crystals have the same structure as the mini-crystals grown by the LHPG technique [-2]. The powder patterns of RETiNbO6 compounds were indexed in Tables 1 and 2 according to an orthorhombic unit cell with lattice parameters a = 1.096 +_ 0.001, b = 0.7508 _+ 0.0007 and c = 5.375 + 0.0005 nm for PrTiNbO6, and a = 1.095 ± 0.001, b = 0.7491 _+ 0.0007 and c = 5.345 +_ 0.0005 nm for NdTiNbO6. The d-spacings calculated from these unit cell parameters are well matched with the measured values for all detected diffraction lines with the RMS deviation less than 0.0005 nm. The peritectic melting temperature of RETiNbO6 crystals was measured using a Pt-PtRh(87%) thermocouple in a muffle furnace which was capable of heating samples up to 1923 K. NdTiNbO6 started to melt at 1873 _+ 20 K, whereas PrTiNbO6 had a higher peritectic melting point of 1943 ± 2 0 K . Although RETiNbO6 crystals do not melt congruently, their incongruent melting temperatures are very close to the liquidus at the stiochiometric composition (Fig. 1). This may be the reason that RETiNbO6 crystals can be grown from the stoichiometric melts using the LHPG technique, where fast pulling rates and high thermal gradients induce a large enough supercooling to allow crystal growth at a lower temperature than the peritectic point.

X. Qi et al, /Journal o/" Co,stal Growth 180 (1997) 73-80

76

Table 1 X-ray powder diffraction data of PrTiNbO6 /7 k /

2 10 0 11 I 11 220 30 1 12 1 3 11 22 1 4 10 1I2 40 I 2 12 230 420 022 302 3 12 222 511 331 040 4 I2 430 240 422 30 3 12 3 53 1 522

d Cxp(nm)

0.4428 0.4364 0.4058 0.3095 0.3018 0.2959 0.2801 0.2685 0.2571 0.2483 0.2439 0.2295 0.2273 0.2212 0.2186 0.2163 0.2080 0.2029 0.1959 0.1927 0.1879 0.1860 0.1850 0.1774 0.1709 0.1608 0.1598 0.1576 0.1550

/~xp

14 9 8 29 59 100 22 28 8 5 7 7 6 11 5 6 6 22 15 6 13 7 7 8 13 6 18 6 13

d TM (nm)

0.4426 0.4370 0.4060 0.3097 0.3021 0.2963 0.2803 0.2683 0.2574 0.2465 0.2441 0.2297 0.2277 0.2213 0.2185 0.2165 0.2080 0.2030 0.1959 0.1927 0.1877 0.1859 0.1848 0.1776 0.1708 0.1609 0.1600 0.1576 0.1548

Table 2 X-ray powder diffraction data of NdTiNbO6 d TM - d cxp (nm)

hk1

- 0.0002 0.0007 0.0001 0.0002 0.0004 0.0004 0.0002 - 0.0002 0.0003 - 0.0017 0.0002 0.0002 0.0003 0.0002 - 0.0001 0.0001 0.0000 0.0000 0.0000 0.0000 - 0.0002 - 0.0001 - 0.0002 0.0001 0.0000 0.0001 0.0002 0.0000 - 0.0002

2 10 0 11 111 220 30 1 12 1 311 22 1 4 10 40 1 2 12 230 420 302 3 12 222 511 33 1 040 4 12 430 24 0 232 422 303 12 3 53 1

RETiNbO6 crystals have very good mechanical properties and chemical stability. The Knoop indention hardness of PrTiNbO6 and NdTiNbO6 was measured by the LEITZ MINILOAD 2 hardness tester to be 1000-1050kg/mm 2, having the same order as Zirconia [4]. They showed no natural cleavage facets and were insoluble in water or in nitric acid solutions.

3.2. Spectroscopic properties The optical absorption spectra of NdTiNbO6 were recorded over the wavelength range 300-2000 nm at 300, 77 and 14 K. Strong absorption transitions from the ground state of Nd 3+ were observed including 419/2 ~ 4Ii5/2 (1800nm),

d exp (nm)

I °xp

dTM (nrn)

d TM -- dexp (nm)

0.4405 0.4341 0.4030 0.3083 0.3007 0.2949 0.2792 0.2674 0.92570 0.2435 0.2283 0.2264 0.2208 0.2155 0.2071 0.2022 0.1956 0.1921 0.1873 0.1851 0.1844 0.1774 0.1730 0.1703 0.1602 0.1592 03 576

20 9 15 32 86 100 22 28 9 8 9 7 10 9 10 23 15 9 14 12 10 9 10 18 10 24 10

0.4420 0.4351 0.4043 0.3091 0.3014 0.2954 0.2796 0.2676 0.2571 0.2437 0.2287 0.2272 0.2210 0.2156 0.2072 0.2022 0.1956 0.1923 0.1873 0.1853 0.1845 0.1772 0.1731 0.1703 0.1601 0.1592 0.1574

-

-

-

0.0015 0.0010 0.0014 0.0008 0.0007 0.0005 0.0005 0.0002 0.0001 0.0001 0.0004 0.0008 0.0002 0.0001 0.0001 0.0001 0.0000 0.0002 0.0000 0.0002 0.0001 0.0002 0.0001 0.0000 0.0001 0.0000 0.0003

4F3/2 (880 nm), 4F5/2 + 2H9/2 (810 nm) 4F7/2 + 483/2 (755 nm) 4F9/2 (685 nm), 2Hll/2 (630 nm), 4 G 5 / 2 + 2G7/2 (590 nm), 4 G 7 / 2 + 2G9/2 (525 n m ) , 2 0 3 / 2 + 4Gll/2 (470nm), 2 p l / 2 (433 nm), 2 0 5 / 2 (425 nm), 2p3/2 (385 rim), and 4D1/2+ 2I~1/2 + 4 D 3 / 2 (355 nm). The background absorption increased at

wavelengths below 380 nm and more rapidly above 350 nm, indicating the occurrence of the absorption edge of the host crystal. Fig. 3 shows representative spectra measured at 300, 77 and 14 K for the 4F3/2 and 2pl/2 manifolds. The 419/2 ---*4F3/2 absorption spectrum at 14 K consists of two lines, Fig. 3a, due to the removal of orbital degeneracy of the 4F3/2 state by the crystal field. The 2Pl/2 level is not split by the crystal field and only a single line is observed in Fig. 3b at 14 K. These absorption patterns evidently indicate the

X. Qi et al. / Journal of C~. stal Growth 180 (1997) 73-80

2pit2

14 K

77

(b)

15

2Dsl2

!

A I I

~ 2o 14K

10

~_

5

77 K

0

850

,

I

860

870

880

I

I

890

900

Wavelength (nm) Fig. 3. A b s o r p t i o n spectra of N d T i N b O 6 at 300, 77, 14 K.

single site occupancy of Nd 3 + in the NdTiNbO6 crystal. The splitting of the energy levels of the 419/2 multiplet in the crystal field is very small: the second-lowest Stark level is only 80cm-~ above the ground state. Even at 77 K this level is well populated, resulting in an extra set of absorption lines in the spectra. At 300 K, the transition from the second-lowest crystalfield level of 419/2 to the 2P1/2 state is even stronger than that from the ground level due to a stronger oscillation strength for this transition (Fig. 3b). The photoluminescence spectra of the 4F3/2 ~ aI11/2 transition recorded at 14 K is shown

in Fig. 4. There are six lines clearly resolved in agreement with the six-fold degeneracy of the 4Ill/2 multiplet, further confirming the single site occupancy of Nd 3 + in NdTiNbO6. Crystals containing a single site for rare-earth ions are preferred in applications as laser gain media, since undesirable energy transfers between different sites may be avoided, thereby increasing the efficiency of the radiative transition. As expected from crystal structure considerations, the disordered crystal environment of Nd 3÷ results in large inhomogeneous broadening of the transition lines: the profiles of the transition lines in Fig. 4 have perfect Gaussian shapes with typical F W H M = 20 cm -1 at 14 K.

78

)5 Qi et al. / Journal of Crystal Growth 180 (1997) 73-80 25000

4F312->411112at 14 K 20000 A

4 c

15000

/

E//b-axis

j

E//c-axis

10000

m

5000

• ....

-1.

0

1050

t| . . . .

1

....

J

!

I

I

I

I

I

I

I

1060

1070

1080

1090

1100

1110

1120

Wavelength (nrn) Fig. 4. Photoluminescence spectra of NdTiNbO6 at 14 K.

Temperature-dependent homogeneous broadening makes the line-widths even larger at 300 K: the absorption transition o f 4"I9/2 ~ 4F5/2 + 2H9/2 at 800 nm has a line-width of 50 cm-1 (4 nm). Such a large line-width is ideally suited for diode laser pumping, since it becomes less important to temperature stabilise the output wavelengths of the diode laser. The optical absorption spectra of PrTiNbO6 were measured from 350 to 2500 nm at 300 and 77 K. Transitions were observed from 3H4 to 3H 6 -F 3F 2 (2350-1850 nm), 3F 3 -q- 3F 4 (16001400 nm) and all other levels up to 3p2,1 (450 nm). The absorption edge of the host crystal occurred at c a 400nm, about 50nm longer than the NdTiNbO6 crystal. The absorption spectra of bulk PrTiNbO6 crystals at wavelengths l l 0 ( O 0 0 n m are identical to those measured with mini-crystals grown by LHPG [1, 2-1. This also confirms that the bulk crystals have the same crystal structure as the mini-crystals. Fig. 5a shows the absorption spectra over the wavelength range 1200-2500 nm,

which were not studied previously. The broad absorption spreading from 1800 to 2400nm is identified with the 3H 4 ~ 3H 6 and 3H 4 ~ 3F 2 absorption transitions. The crystal-field splittings of 3H 6 and 3F 2 manifolds are so large that they overlap in the 1380 nm spectral region. The absorption lines clustered at 1500nm are due to the 3H4 ~ 3F3, 3F 4 absorption transitions. At 77 K the lines are still quite broad and individual components from each manifolds are not well resolved. The photoluminescence spectrum of PrTiNbO6 at 14 K is shown in Fig. 5b. When excited by the 458 nm line of Ar laser, all the transitions observed at 14 K came from the 3Po emitting level of Pr 3+, including 3P o ~ 3H4 (500nm), 3H5 (555nm), 3H 6 (625nm), 3F 2 (655nm), 3F 3 (720 nm), and 3F 4 (745 nm). The 3P 0 ~ 3F3, 3F4 transitions were not observed at 300 and 77 K due to the competition of the nonradiative decay from 3P o to the ~D2 level at high temperature.

X. Qi et al. / Journal of C~. stal Growth 180 (1997) 73 80 4.0 3.5

79

(b) Fluorescences from 3P 0 at 14 K 3F 2

---

3.0

<~

2.5

Igl e~

C --

3H 6

2.0

1.5 1.0 0.5

~

I

. ~

3F

0.0 450 4.5 4.0

I

500

550

600

O r-

2.5

.Q

2.0

700

750

800

(a) Absorption from 3H 4 at 77 K 3F 2

3.5 3.0

650

3F 4

3F 3

l_

O u) .O

<

1.5 1.0 0.5 0.0 1200

1400

1600

1800

2000

2200

2400

Wavelength (nm) Fig. 5. Optical absorption and photoluminescence spectra of PrTiNbO6.

4. Concluding remarks Although RETiNbO6 crystals do not melt congruently, their peritectic compositions are very close to the stoichiometric compositions and the crystals can be grown from melts containing small amounts of extra TiO2. By suitable choice of growth parameters, good quality crystals of size 15×20mm 3 have been obtained using the modified Czochralski technique. The bulk crystals were confirmed by X-ray powder diffraction to have the same phase as mini-crystals grown by the LHPG technique.

The optical absorption and photoluminescence spectra of bulk crystals are identical with those measured using mini-crystals grown by LHPG. There is clear evidence from both absorption and photoluminescence spectra that the rare-earth ions in these crystals occupy single sites only. Very high absorption coefficients for the rare-earth ions and strong luminescence signals were observed, encouraging the expectation of efficient laser action. The transition lines of RE 3÷ in these crystals are largely broadened by the random distribution of Nb 5+ and Ti 4+ at the same crystallographic site. Strong upconverted luminescence has also been

80

X Qi et al. /Journal of C~stal Growth 180 (1997) 73 80

observed [53, exhibiting the abundance of possible transition channels for laser operation. Of particular importance in these RETiNbO6 crystals is the very strong, broad absorption lines at wavelengths close to the laser output of commercially available laser diodes. These optical properties combined with excellent chemical and mechanical stability make them very promising for applications as diode pumped solid-state laser sources.

Acknowledgements The authors are indebted to the EPSRC and Ministry of Defence (DRA Malvern) for the award of a grant (GR/G/42051 and GR/K/04392) on which this research are based. X.Q. is grateful to the

CVCP for their ORS award and the University of Strathclyde for their award of a John Anderson Research Scholarship and RDF Grant No. 792.

References [13 M.J. Weber (Ed.), CRC Handbook of Laser Science and Technology, Lasers and Masers, vol. 1, CRC Press, Boca Raton, FL, 1982. [2] X. Qi, R. Illingworth, H.G. Gallagher, T.P.J. Ham B. Henderson, J. Crystal Growth 160 (1996) 111. [3] X. Qi, T.P.J. Ham H.G. Gallagher, B. Henderson, R. lllingworth, I.S. Ruddok, J. Phys: Condens. Matter 8 11996) 4837. [4] R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 60th ed., 2nd printing. CRC Press, Boca Ration, FL, 1980, p. F-24. [5] X. Qi, T.P.J. Han, H.G. Gallagher, B. Henderson, Optics Commun., accepted for publication.