Nitrate-citrate Sol-gel Synthesis of Phase Pure <i>Nd</i><sup>3+</sup>-doped Nanocrystalline Yttrium-aluminum-garnet

Phase pure nanopowders of undoped and neodymium (Nd3+)-doped yttrium-aluminum-garnet(YAG) was synthesised by sol-gel process using nitrate-citrate gel method. Complete crystallisation of YAG was studied using Fourier transform infra-red spectroscopy (FTIR) and x-ray diffraction (XRD). Complete phase formation took place at 900 °C for 2 h. The average primary particle size calculated was ~25 nm. Transmission electron microscopy (TEM) showed particles in the range of ~25 nm to 30 nm. Nd-dopant concentration levels of 2 and 4 atomic per cent were studied. Using XRD data the cell parameters of Nd (0, 2 and 4 atomic per cent):YAG were calculated and it was found to increase with increase in dopant concentration.

Keywords:    Phase-pure YAG,   sol-gel process,   Nd-doping,   cell parameter,  

Rare earth-doped Y3Al5O12, (yttrium-aluminum-garnet (YAG)) is an important solid-state material due to its interesting optical and mechanical properties1,2. Single crystals of YAG doped with neodymium ion are used as gain medium in solid-state lasers3. The single crystals of YAG require complex methods of synthesis. However, polycrystalline Nd:YAG powders can be compacted and sintered to give transparent ceramics1. Synthesis of YAG nanopowders by various methods has been reported such as sol-gel4,5, co-precipitation6,7, spray pyrolysis8 and combustion9 technique. Synthesis by solid-state reaction involves lengthy high temperature treatment yielding phase impurities. Due to the formation of intermediate metastable phases such as Y4Al2O9 (YAM), hexagonal YAlO3 and orthorhombic YAlO3, other than Al2O3 and Y2O3 in the yttria-alumina system, phase pure YAG is not easy to obtain10.

The author earlier reported the synthesis of phase pure YAG nanopowders by nitrate-citrate sol-gel route5. Highly doped YAG ceramics are promising materials for high power microchip laser application11. However, in single crystals grown by Czochralski method, more than 1 per cent of Nd3+ cannot be doped homogeneously without deteriorating the lasing properties12. The author in the paper report synthesis of YAG nanopowders doped with 2 and 4 atomic per cent neodymium ion. The benefit of present method is high neodymium ion doping by citrate nitrate sol-gel process during which chemical homogeneity of the system is maintained throughout the process.

Yttrium nitrate, Y(NO3)3.6H2O (99.9 per cent purity Alfa Aesar), neodymium nitrate, Nd(NO3)3.6H2O (99.9 per cent purity Alfa Aesar) aluminum nitrate, Al(NO3)3.9H2O (99.9 per cent purity Alfa Aesar) and citric acid anhydrous (99.5+ per cent purity Alfa Aesar) were used as the starting materials. The metal precursors were taken in the molar ratios of Y2.94Nd0.06Al5 and Y2.88Nd0.12Al5 for 2 atomic per cent and 4 atomic per cent Nd respectively. Citric acid was used as the chelating agent. The total cation to citric acid was taken in the ratio of 1:1. The nitrate-citrate sol in millipore water was prepared by ultrasonic stirring. Gelation took place at 80 °C for 16 h. Drying of the gel was carried out from 110 °C to 150 °C in 24 h. This precursor powder was calcined at different temperatures up to 900 °C.

Thermo-gravimetric analysis (TGA) of the precursor was carried out in air at a heating rate of 10 °C/min from room temperature to 1300 °C by Perkin Elmer Diamond Simultaneous TGA/DTA. Fourier transform infra-red spectroscopy (FTIR) was carried out by Bruker, (Vector 22) to identify the temperature at which Y-O and Al-O bond formation was taking place. X-ray diffraction (XRD) was carried out using Philips x-ray diffractometer, PW 3020 in 2θ range from 15° to 80° for characterisation of phase purity, particle size, and cell parameters. Transmission electron microscopy (TEM) was done on FEI Philips Morgagni 268. The TEM specimens were prepared on copper grids by dispersing the powder ultrasonically in acetone. Nd+3 doping was characterized by EDX using scanning electron microscope (ZEISS EVO Series, EVO 50).

TGA of the xerogel shows total weight loss of ~65 per cent (Fig.1) with maximum weight loss taking place up to 600 0C. The strong exotherm at 421.46 °C along with weight loss depicted by TGA corresponds to decomposition of the citrate network. A small exotherm corresponding to weight loss at ~ 890 °C indicates crystallisation of YAG4 Further after 900 °C the weight loss was almost negligible, indicating that reactions involving weight loss is over by this temperature.

FTIR spectra of 0, 2, and 4 atomic per cent Nd doped YAG calcined at 900 °C for 2 h are shown in Fig. 2. The metal-oxide (M-O) bonds in YAG show vibrational bands in the region of 400-800 cm-1. Peaks were seen at 786 cm-1, 721 cm-1, 690 cm-1, 565 cm-1, 512 cm-1, 458 cm-1 and 432 cm-1 which may be attributed to the M-O vibrations in YAG13. Formation of YAG at this temperature of 900 °C for 2 h as indicated by FTIR peaks well corresponds to the crystallisation peak of YAG as observed at ~ 890 °C in results of TG-DTA. With Nd3+ doping, slight shifts in peak position of M-O bonds characteristic of YAG towards lower wave number were observed for both 2 and 4 atomic per cent neodymium ion doping. No new peaks were observed on addition of dopant. This accounts for the fact that the dopant ion occupies the lattice positions in YAG without distorting the parent structure14.

XRD patterns of YAG (Fig.3) calcined at 800 °C for 4 h showed the characteristic peaks (JCPDS 33-40) but the background intensity was very high indicating incomplete crystallisation. However at 900 °C for 2 h complete crystallisation of the phase pure cubic YAG structure was obtained. Crystallite size was calculated using Scherrer’s equation15 as

t= 0.9λ ( β sample 2 β inst 2 ) 1/2 cosθ

where t is the crystallite diameter, ë=1.54056Å, è is the diffraction angle, âsample is the FWHM of the diffraction peak and âinst is characteristic of the instrument. The primary particle size of the samples was ~25 nm.

The cell parameters of undoped and doped YAG were calculated with the help of diffraction angles and interplanar spacing from the XRD data. The cell parameter, a of cubic YAG was 12.008 Å (JCPDS 33-40). With the increase in dopant concentration, the value of cell parameter also increased (Fig.4). This observation supported the fact that Nd substitutes the lattice site of Y in YAG cubic structure without disturbing the garnet structure. Further, the absence of any extra peak in the XRD graphs confirmed that the compounds were phase pure and Nd had been uniformly doped in the desired lattice structure. From EDX spectra (Fig.5) it was observed that there was an increase in the area under the peaks of Nd with the increase in dopant concentration. The elemental composition in Tables 1(a) and 1(b) indicate that the concentration of Nd is approximately doubled from 2 atomic per cent to 4 atomic per cent. Elemental composition analysis by EDX becomes difficult when rare earth metal ions are involved. Maybe this is the reason, accurate quantitative analysis of Nd:YAG is never being discussed in the literature. Even in this extensive work on Nd doping7, elemental composition (of Nd) is discussed only by the increase in the peak of Nd in EDX graphs and increase in lattice parameter.

A typical TEM (Fig. 6) showed particles in the size range of 25-50 nm. The particle size and shape was uniform which can lead to versatile application of these Nd:YAG nanopowders.

Phase pure YAG and Nd3+-doped YAG were synthesised by sol-gel process using nitrate-citrate sol.

  • Complete crystallisation of YAG took place at 900 °C for 2 h.

  • On doping Nd3+, there was no change in the phase purity.

  • Homogeneous doping with 2 and 4 atomic per cent neodymium ion took place as indicated by cell parameter increase and confirmed by EDX spectra.

  • Particles were morphologically uniform and very small (in the range of 25-50 nm).

This method is simple and robust for obtaining highly neodymium doped phase pure Nd:YAG at low temperature and can be scaled-up to large batches efficiently.

The authors wish to express sincere thanks to the Director, Laser Science and Technology Centre (LASTEC), Delhi, for his constant inspiration and encouragement for this work. The authors also thank Mrs Neelam Malhan, Head, Chemistry Division, and all the members of the Division, for their support and cooperation. The authors thank Characterisation Division, Solid State Physics Laboratory; Centre for Fire Explosives and Environment Safety, and Indian Institute of Technology Delhi for providing item timely characterisation of samples.

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Ms Kiranmala Laishram obtained her MSc (Physical Chemistry) from Manipur University in 2002. She is working as Scientist ‘C’ at Laser Science and Technology Centre (LASTEC), Delhi. Her areas of interest are material characterisation techniques, laser materials, and nanomaterial-synthesis and applications.

Dr (Mrs)Rekha Mann obtained her MSc and MPhil (Organic Chemistry) from Kurukshetra University, and PhD (Applied Chemistry) from Delhi College of Engineering, Faculty of Technology, Delhi University in 2003. Presently she is working as Scientist ‘D’ at LASTEC, Delhi. Her areas of interest are nanomaterial synthesis and processing, laser materials, organic synthesis, and polymer materials.

Mr Ashok Kumar Satsangi obtained his BSc from Agra University in 1973. He retired as Technical Officer ‘C’ from LASTEC, Delhi, in 2010. His areas of interest include organic synthesis, laser materials, and polymer materials.