Spectral characterization of lanthanum heptamolybdate single crystals

Spectral characterization of lanthanum heptamolybdate single crystals

Materials Science and Engineering B57 (1999) 209 – 213 Spectral characterization of lanthanum heptamolybdate single crystals K.S. Raju a, Sushma Bhat...

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Materials Science and Engineering B57 (1999) 209 – 213

Spectral characterization of lanthanum heptamolybdate single crystals K.S. Raju a, Sushma Bhat b, Sanjay Pandita b, P.N. Kotru b,* a

Department of Crystallography and Biophysics, Uni6ersity of Madras, Guindy Campus, Madras 600 025, India b Department of Physics, Uni6ersity of Jammu, Baba Saheb Ambedkar Road, Jammu 180 004, India Received 25 September 1997

Abstract Using sodium metasilicate gel, lanthanum heptamolybdate (La2Mo7O24·nH2O) single crystals are grown by the diffusion of lanthanum nitrate into the set gel embedded with a mixture of molybdenum trioxide, ammonium hydroxide and concentrated nitric acid. The infrared (IR) absorption spectrum of lanthanum heptamolybdate (LHM) single crystals identifies the presence of molybdate (MoO4)2 − groups and water of crystallization, while energy dispersive X-ray analysis (EDAX) confirms the presence of heavy elements (La and Mo) in LHM. X-ray photoelectron spectroscopic (XPS) studies of LHM re-establish the presence of lanthanum and molybdenum besides their chemical states as their respective oxides. It is shown that the oxygens in the sample are present in three different environments as terminal oxygen (Mo.O), bridging oxygen (MoOLa) and oxygen of lattice water (HOH). The implications are discussed. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Spectral characterization; Lanthanum heptamolybdate; Single crystals

1. Introduction The importance of rare earth molybdates lies in their ferroelectric and ferroelastic properties finding applications in electro- and acousto-optical devices [1–3]. From neutral aqueous solution, tetragonal bipyramidal crystals of barium molybdate are grown by the precipitation of alkaline-earth metal molybdate powders [4]. Rare earth molybdates, having the general formula R2(MoO4)3, have been reported as ferroelectric materials [5]. Brixner [6] has reported the growth of rare earth molybdates employing the Czochralski technique at elevated temperatures but the thermal stresses introduced during the growth make the crystals defective. The crystal growth in gels at room temperature pioneered by Henisch and co-workers [7,8] has been fully exploited in the investigation of growth and characterization of rare earth mixed single crystals of samarium barium molybdate by Isac and Ittyachen [9]. Studies on the growth of pure lanthanum and neodymium and mixed La–Nd heptamolybdate single crystals in silica gels have been carried out, covering aspects like concen* Corresponding author.

tration and ageing of the gel, concentration programming, effect of pH, gel breaking, bubble trapping, seeding and morphology [10–12]. Structural characterization of neodymium copper oxalate has been reported by Raju et al. [13]. It is well known that IR absorption spectral studies throw light on the presence of functional groups present in the sample under investigation, while energy dispersive X-ray analysis (EDAX) identifies the heavy elements in terms of their characteristic binding energies relating to KLM transitions. The X-ray photoelectron spectroscopic (XPS) peaks facilitate in the identification of the elements incorporated in the sample and quantitative data is obtained from the peak heights and/or areas. The identification of the chemical states of the elements can often be made from the exact positions and separations of the peaks, as well as from certain spectral contours. In addition to the emission of photoelectrons in the photoelectric process (XPS), electrons are also emitted by the Auger process (AES), owing to the relaxation of the energetic ions left after the photoelectric event (roughly 10 − 14 s later). Thus, Auger signals relating to the elements will be a complementary consequence of XPS.

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Hence, it is the double assertion of elemental confirmation by both XPS and AES. In order to confirm the presence of elements besides their chemical states, studies of LHM employing XPS and AES are reported in this paper, which consequently shed information on the co-ordination of oxygen as well.

2. Experimental details Lanthanum heptamolybdate (LHM) single crystals were grown by gel technique using the single gel single tube (SGST) system, by the diffusion of lanthanum nitrate into the set gel impregnated with a mixture of molybdenum trioxide, ammonium hydroxide and concentrated nitric acid. The explanation of the mechanism of reactions leading to the formation of LHM along with the details of growth have been previously reported [10]. The IR spectrum of LHM taken in a KBr matrix was recorded on a Perkin-Elmer IR model 530 (UK). For the confirmation of the elemental incorporation such as lanthanum and molybdenum in LHM, EDAX work was taken up. Tiny crystals of LHM were mounted on to an aluminium stub and their surfaces were coated with a thin layer of gold to make them electrically conducting. The surfaces were then examined in an EDAX analyser No 711, an accessory to the Philips SEM model 501. The XPS investigations of LHM were carried out using a system VG scientific ESCALAB MK2 spectrometer (Al K source: photon energy 1486.6 eV) operated at 10 kV and 10 mA. The overall resolution of the system with an analyser chamber is maintained below 10 − 9 torr during the present work. For AES observation, 2 keV (10 mA) electron source is used.

Fig. 1. IR absorption of LHM.

Fig. 2) positioned at 2.3 and 2.4 keV relates to La and Lb of molybdenum and hence is broad. Thus EDAX of LHM establishes the presence of Lanthanum and Mo in the LHM sample.

3. Results and discussion Fig. 1 is the IR spectrum of LHM. The presence of molybdate (MoO4)2 − ions is identified by the presence of a broad peak at 820 cm − 1 relating to symmetric or antisymmetric stretching modes of vibrations [14], while the peak at 380 cm − 1 relates to the antisymmetric bending modes of vibrations and the peaks at 325 and 290 cm − 1 relate to the symmetric bending modes of vibrations [15]. That the LHM sample has water of crystallization is established by the presence of a broad peak at 3000–3600 cm − 1 corresponding to HOH bending [16]. The EDAX recorded spectrum of LHM is shown in Fig. 2. The four prominent peaks (on the right hand side of Fig. 2) positioned at 4.65, 5.04, 5.38 and 5.78 keV relates to La, Lb1, Lb2 and Ln energies of lanthanum, while the short peak (on the left hand side of

Fig. 2. EDAX spectrum of LHM.

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Fig. 3. XPS peaks of 3d levels of lanthanum.

The XPS results of LHM are recorded in Figs. 3–5. The XPS peak of C ‘‘1s’’ observed at the binding energy of around 284.6 eV agrees quite well with the reported value in the literature [17] and hence the binding energy scale (x-axis) is calibrated. The contribution of carbon in the present investigation is attributed to the adventitious hydrocarbon nearly always present on samples introduced from the laboratory environment or from a glove box. The XPS peaks relating to the 3d5/2 and 3d3/2 states of lanthanum in LHM have binding energies positioned around 835 and 851.8 eV, respectively, with a separation of 16.8 eV. Similarly the XPS peaks relating to the molybdenum 3d states (Fig. 4) are positioned at the binding energies around 232.5 and 235.7 eV with a separation of 3.2 eV. These binding energy values and their separations confirm that lanthanum and molybdenum are in their oxide states, La2O3 and MoO3, in the LHM sample [17]. It is interesting that the peak relating to oxygen 1s (Fig. 5) has a complex spectral contour and is a broad peak comprising of three peaks with the binding energies around 526, 531, and 535 eV, respectively, giving the clue that the oxygen in LHM have three different co-ordinations.

Fig. 4. XPS peaks of 3d levels of molybdenum.

Fig. 5. XPS peaks of 1s levels of oxygen.

The Auger spectra for LHM are shown in Figs. 6–8. The Auger signals relating to Mo(M4,5N45N45) and Mo(M4,5N23V) transitions (Fig. 6) have kinetic energies around 1268 and 1300 eV, (M4,5N45N45) transitions relating to lanthanum (Fig. 7) at 864 eV and oxygen

Fig. 6. AES signal relating to molybdenum.

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Fig. 7. AES signal relating to lanthanum.

(KL23L23) transition (Fig. 8) at 976 eV, respectively, thereby re-establishing the presence of molybdenum, lanthanum and oxygen in LHM, supplementing the XPS results.

Attention is drawn to the following fact that the chemical formula for LHM as derived from the mechanism of reaction kinetics is La2Mo7O24·nH2O [18]. It indeed seems reasonable to speculate that LHM is constituted of the entities, La2O3, MoO3 and H2O. One of the possible oxygen linkages with MoO3 and La2O3 in LHM is shown schematically in Fig. 9. A careful examination of Fig. 9 reveals that oxygen has two types of linkages, namely, terminal oxygen (MoO) and bridging oxygens (MoOLa). Taking into account the third entity, i.e water of crystallization, the third peak in the complex oxygen ‘1s’ (Fig. 5) may be attributed to the oxygen being co-ordinated to the hydrogens of the lattice water, whose presence is confirmed by the IR absorption spectrum of LHM. Thus XPS and AES studies throw light on the presence of molybdenum, lanthanum and oxygen, as well as their chemical states, confirming the speculation of the entities of LHM as La2O3, MoO3 and lattice water. Besides this, the complex spectral contour of the XPS peak of O 1s (Fig. 5) also helps in understanding the oxygen linkages in LHM (Fig. 9). In XPS the heights and/or areas of the peaks, meaning their intensities, are measures of the quantities of the species present in the material [19]. It is obviously evident from Fig. 5 that the intensity of oxygen 1s relating to the bridging oxygens is higher than that of the terminal oxygens. Hence, from the structural view point, the bridging oxygens are more significant than the terminal oxygens in LHM. It has been reported to be so even in the case of neodymium heptamolybdate [20].

4. Conclusions The presence of the molybdate groups and water of crystallization in LHM is identified by the IR absorption spectrum. The EDAX pattern of LHM confirms the presence of lanthanum and molybdenum present in it. The incorporation of molybdenum, lanthanum and oxygen in LHM has been established by XPS and AES, besides confirming the existence of molybdenum and lanthanum as their oxides. These conclusions are strongly supported by the binding energy values (XPS) and the kinetic energy values of Auger electrons (AES) of molybdenum, lanthanum and oxygen, which are in excellent agreement with the literature findings [17].

Acknowledgements

Fig. 8. AES signal relating to oxygen.

The authors thank the Regional Sophisticated Instrumentation Centre, IIT Powai, Bombay, India, and the Physics Department, IIT, Delhi, India, for the ESCA facility. The authors owe their thanks to Professor B. Vishanathan, Department of Chemistry, IIT, Madras,

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Fig. 9. Schematic of oxygen co-ordinations in LHM: molybdenum (dark circle), lanthanum (dotted circles) and oxygen (big blank circles).

India, for valuable discussions. One of the authors (SB) thanks CSIR, New Delhi, India, for providing a Research Associateship.

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