Stabilization of divalent neodymium (Nd2+) in strontium tetraborate

Stabilization of divalent neodymium (Nd2+) in strontium tetraborate

ELSEVIER Journal of Alloys and Compounds 249 (1997) 213-216 Stabilization of divalent neodymium (Nd’ + ) in strontium tetraborate W. Xu”, J.R. Peter...

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ELSEVIER

Journal of Alloys and Compounds 249 (1997) 213-216

Stabilization of divalent neodymium (Nd’ + ) in strontium tetraborate W. Xu”, J.R. Peterson”‘b’* bTransuratzium

Research

“Depnrmwzr of Chemisty, Universify of Termessee, Kttoxvilie, TN 37996- 1600, USA Laboratory (Chemical and Amlytic_nl Sciences Division), Oak Ridge h~ationnl Lnboratoty, P.O. Box 2008, 3 783 I - 63 7.5, USA

Oak Ridge,

TN

Abstract

Luminescence and absorptionspectrafrom SrB,O,:Nd”* samplespreparedfrom NdZO, (1 mol% basedon Sr2* ion mass)in air at high temperatures havebeenrecordedat roomtemperature.A broademissionbandcenteredin the vicinity of 560 nm hasbeenobserved from the samples uponexcitation at 514.5nm.We su,, uuestthat it is dueto emissionfrom the 5d bandin Nd” ion into its groundstate4f level (‘I,). Analysis of absorptionspectrarecordedfrom the samesamplessupportthis conclusion.SrB,O,:Nd’+ samplespreparedin Ar/H,(4%) atmosphereexhibited more strongly the optical characteristicsof Nd*+ ion comparedto samplespreparedin air. Several conditionspromoting the srabilization of Nd” ion in this matrix are discussed. The range of f-element reducibility and stabilization in SrB,O, hasbeenextendedfrom Tm’+ to Nd” in the presentwork; however,the limit of this facile reductionprocesshasnot yet been determined.

1. Introduction Luminescence from divalent lanthanide ions in crystals, powders and glasseshas attracted much attention over the last fifteen years [l-5]. Luminescence from Eu’+ [6], Sm” [7,8], Yb2+ [9] and TmZC [lO,ll] ions in crystailine SrB,O, prepared in air, N,/H,, or Ar/H, atmospherehas been reported. The spectra from these ions show that the 4f”-‘5d electronic configuration is at a relatively high energy. High quantum efficiencies, small Stokes shifts and vibrational structures indicate that non-radiative relaxation from their excited states is restricted [9]. This has been ascribed to the stiffness of the host lattice [6,8]. On the other hand, the reduction of trivalent lanthanide ions in solid state compounds requires a reducing agent [5]; the most commonly used one is N,/H, or H, atmosphere[ 121. Recently the observation of reduction of Sm3+, Eu”‘, Tm3+ and Yb3+ ions to their divalent counterparts in SrB,O, prepared in air and/or in partial H, atmosphereat high temperature has been reported [IO- 121. No similar study with the next most easily reduced trivalent lanthanide ion, Nd3+, has been reported. This report is focused on the preparation of SrB,O,:Nd’+ and the emission and absorption spectral data recorded from divalent *Corresponding author. Tel: ( l-423) e-mail: [email protected] Published by Elsevier Science PII SO925-8388(96)02637-O

S.A.

974-3434;

Fax: (l-423)

974-3454;

neodymium ion in this host. Preparations in air and in Ar/H, were carried out to determine the effect of this variable.

2. Experimental Powder samples of SrB,O,:Nd*’ were prepared as described in the literature [12] by solid state reactions in air or laboratory-safe Ar/H,(4%) atmosphere. Stoichiometric amounts of SrCO, and H,BO, mixed together with a 3 mol% excess of H,BO,, along with 1 mol% (based on Sr2+ mass)Nd’+ ion as Nd,O,, were thoroughly ground together. Portions of this mixture were heated separately at 550, 600, 650, 700 and 800 “C in glazed porcelain boats for 5 h and then reground and heated for 5 h at 650, 700, 750, 800 and 900 “C, respectively. These second heating temperaturesare referred to herein as the sample preparation temperatures. The 514.5 nm line from a Coherent Model 300 argonion laser was used as the excitation source. Luminescence spectra were recorded with a Ramanor Model HG.2S spectrophotometer (Jobin-Yvon Instruments SA), having a resolution of 0.5 cm-’ at 514 nm and a grating which limits our spectral observation range to wavelengths less than or equal to 800 nm. The collected light was detected by a photon counting system, which employed a cooled

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photomultiplier tube (Hamamatsu R636) and a multichannel analyzer (Nicolet 1170) interfaced with an AT personal computer using SPECTRA CALC software (Galactic Industries Corp.). Visible absorption spectra (via the diffuse reflectance method) were recorded with a Cary Model 14H scanning spectrophotometer converted by On-Line Instrument Systems (OLIS) for data acquisition and analysis by an IBM-compatible computer. For luminescence lifetime measurements, the same argon-ion laser was used as the excitation source. The signal from the laser was turned on and off with a mechanical chopper (Stanford Research Systems, SR 540) and impinged on the sample. The luminescence from the sample was characterized using a monochromator (Spex 1404), a photomultiplier tube (Hamamatsu R636) and an amplifier discriminator (Advanced Research Instrument Corp., MF-100) and was recorded by a multichannel scaler (EG and G ORTEC, A68-Bl) system. The luminescence decay was analyzed using Table Curve 2D software (Jandel Industries Corp.), and the luminescence lifetime was extracted from the fitted decay curve. X-ray powder diffraction patterns were obtained using a generator (General Electric XRD-6) equipped with a copper target, Ni foil to filter out the CuKP radiation, a 57.3 mm diameter Debye-Schemer type camera and Kodak DEF-392 film.

3. Results and discussion Emission spectra in the range 520-750 nm recorded at room temperature from samples prepared in air at various temperatures under 514.5 nm light excitation are shown in Fig. 1. Two main emission bands, centered at about 560 and 600 nm, are observed. In accordance with an energy level diagram of Nd3- ion [13], the multicomponent

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emission band at 600 nm is attributed to transitions m Nd3+ ion from the 4G,,, state to the ‘I,,, ground state. Based on the only available energy level diagram of Nd’+ ion, that in single-crystal CaF, [14], the broad, weak emissionband centered at 560 nm is due to allowed 4f-5d transitionsin Nd*’ ion (seeFig. 2). Although this emission band exhibits its maximum intensity near that observed from Tm*+ in SrB,O,:Tm’+ [ll], its bandwidth is narrower than that observed from the Tm*+ ion emission. We have also measuredthe absorption spectrum of the neat SrB,O, host. The low-energy end of a broad absorption band is at 330 nm with the center wavelength of this band at about 270 nm, in agreement with the reports of others [12]. Therefore it is not likely that one would observe emissionthrough a charge transfer band of Nd3+ ion by 514.5 nm light excitation of the SrB,O, host. Also very evident from the data shown in Fig. 1 is the decrease in concentration of Nd’+ ion in the product tetraborate with increasingtemperatureof its preparation in air. With a change in preparation temperature from 650 to 800 “C (Nd*’ ion emission intensity from the tetraborate sample prepared in air at 900 “C was unmeasurable),the emissionintensity from Nd’* ion decreasedby a factor of 3, while that from Nd”’ ion increased by a factor of 10. The most favorable preparation temperaturefor Nd”’ ion production in air is the lowest one studied, 650 “C. It is anticipated that lower temperaturesof samplepreparation are less likely to produce the crystalline strontium tetraborate host. When a sample of SrB,O,:Nd*+ is prepared in an

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Fim 0’ 2. Schematic energy level diagrams for Nd2+ and Nd”+ ions with some indicated absorption and emission processes.

W. Xu, J.R. Peterson

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Fig. 3. Room temperature emission spectra in the wavelength region 520-800 nm from SrB,O,:Nd” samples prepared in (a) air and (b) Ar/H, (4%) atmosphere under otherwise identical conditions.

Fig. 4. Room temperature absorption spectra in the wavelength region 495-630 nm from SrB,O,:Nd’+ samples prepared at 650 “C in (a) air and (b) Ar/H, (4%) atmosphere.

Ar/H,(4%) atmosphere, the emission spectrum recorded from the sample at room temperature (seeFig. 3) exhibits much increasedintensity in the 560 nm Nd’+ ion emission band compared to that from an identically prepared sample in air. The spectral characteristics (peak shape, position and relative intensity) of this Nd2+ ion emission,however, are the same, independent of the preparation route. Also the emissionbandat 600 nm, attributed to Nd3’ ion and present in samples prepared in air, is absent in samples prepared in Ar/H,(4%). To substantiatefurther our interpretation of the luminescence data, luminescence lifetime measurements were made as well. The lifetime of the Nd2+ ion emissionat 560 nm from a SrB,07:Nd2+ samplewas too fast (510 us) for our measurement capabilities. Then the same SrB,0,:Nd2+ sample was heated in air to 800 “C to promote oxidation to SrB,0,:Nd3+, which was confirmed by emission spectral analysis (no NdZC ion emission was observed). The decay of the 600 nm emission from the Nd3+ ion in this samplewas fitted to a single exponential, and the corresponding luminescencelifetime was found to be 135 ps, more than a factor of ten slower than that of Nd2+ ion in the samehost. This difference in lifetimes is consistent with the allowed 5d-4f transition in Nd2+ ion versus the Laporte-forbidden 4f-4f transition in Nd3’ ion. The room-temperature absorption spectra in the wavelength region 495-630 nm from SrB,0,:Nd2+ prepared at 650 “C in (a) air and (b) Ar/H,(4%) are shown in Fig. 4. The broad absorption manifold centered at 586 nm corresponds to transitions from the 41,,2 ground state to the 4%* excited states in Nd3+ ion [13]. According to the energy levels of Nd*+ ion in single-crystal CaF, [14], the absorption band centered at 527 nm in Fig. 4 stems from allowed 4f-5d transitions in Nd’+ ion. Note that the most intense absorption band of Nd3+ ion is not observed when

the SrB,O,:Nd’+ sampleis prepared in Ar/H,(4%). These absorption spectra results are consistent with those obtained from the emission spectra from SrB,O,:Nd”+ samplesunder 514.5 nm excitation. Analysis of representative diffraction patterns revealed that, although the sampleswere crystalline, purely singlephase materials were not prepared. From a SrB,O,:Nd*+ sample prepared in air at 650 “C, thirty diffraction lines were indexed according to Powder Diffraction File [ 151 Card No. 15-801 for orthorhombic SrB,O, (a=0.44263, b= 1.07074, c=O.42338 nm [15]) and lattice parameters(S.D.), refined by least squaresanalysis [ 161,calculated to be n=0.4427(8), b=1.062(2), c=O.422(1) nm. In addition, nine diffraction lines (four coincident with those from SrB,O,; only one of the other five lines exhibited an intensity above “weak”) were indexed on the basis of orthorhombic NdB03 (a=O.5729, b=0.8076, c=O.5037 nm [17]) and yielded lattice parameters(S.D.) a= 0.5733(g), b=0.812(1). c=O.5043(7) nm. In contrast, analysis of the powder diffraction pattern obtained from a SrB,0,:Nd2+ sample prepared in Ar/H,(4%) at 850 “C revealed the prominence of the orthorhombic SrB,O, phase,but five additional lines, not identified asthose from NdBO,, were observed as well. It is significant that NdBO, could not be identified in the sample prepared in Ar/H,(4%) and that the presence of Nd3’ ion in the various samplesprepared in air is probably due to NdBO,. Additional work, outside the scope of the present paper, will be necessaryto characterize completely (on the basis of X-ray diffraction analysis) these samplesas a function of their preparation conditions. There are several probable reasonsfor the reduction of Nd3’ ion to Nd2+ ion in SrB,O,. When Nd3+ ion replaces Sr2+ ion in the lattice, somedefects will occur in the solid to create a charge balance. It may be the transfer of an

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electron from such a defect to a Nd3+ ion that reduces it to a Nd2+ ion. The temperature of sample preparation is also quite important, especially when the sample is prepared in air. Divalent lanthanide ions are easily oxidized to trivalent ones at elevated temperatures in an oxidizing atmosphere [ 181. It was confirmed in our experiments that the emission intensity from Nd2+ ion in the tetraborate decreases with increasing temperature of sample preparation in air, while that from Nd3’ ion increases with increasing preparation temperature. When a SrB,0,:Nd2+ sample is prepared in Ar/H,(4%), the main emission band observed is from Nd” ion regardless of the preparation temperature. This implies that a reducing agent (H,) is necessary for complete/facile reduction of Nd3+ ion to Ndzi ion. The crystal structure of SrB,O, is also conducive to the stabilization of divalent lanthanide ions substituting for Sr2+ ion. In SrB,O, tetrahedral (BOJunits build up a three-dimensional [(B407)2-]m network structure [19], in which it has been found in this work that even Nd3+ ion can be reduced partially to Nd2+ ion and in the work of others [12], that Eu3’, Yb”‘, Sm3’ and Tm3+ [ll] ions are reduced, more or less completely, to their divalent counterparts in air. The three-dimensional network structure of SrB,O, provides a metal ion- site completely surrounded by the tetrahedral (BO,)‘units of the [(B,O,)‘-1, network and of a size comparable to the ionic radii of these divalent lanthanide ions. Thus, the ability of this matrix to stabilize the less easily reduced Sm3+, Tm3+ and Nd3+ ions is facilitated by its structural framework and its acidic (covalent) nature. In the present case where [20]) sites are occupied Sr2+ ion (0.118 nm, CN=6 partially by Nd2+ ions (0.122 nm, CN=6 [21]), the Nd*+ ion is completely surrounded by tetrahedral (B0,)5units of the [(B407)*-], network and therefore protected from attack by oxygen. In summary we have observed, for the first time, divalent neodymium in SrB,O, prepared in air and in Ar/H,(4%) at elevated temperature and characterized it preliminarily via absorption and emission spectroscopy. Previously divalent neodymium was usually obtained only in alkaline earth fluorides (CaF,, BaF,) following irradiation with gamma (7) rays or by reaction of neodymium metal with a neodymium trihalide. It is significant that in the present work some divalent neodymium was obtained even in air in a straightforward laboratory procedure. This usually difficult reduction is facilitated in an Ar/H,(4%) atmosphere. In this work the range of f-element reducibility and stabilization in SrB,O, has been extended from Tm*+ [E”(Tm3+/Tm2+)=-2.3 V] to Nd2’ [E”(Nd3+/ Nd2+)= -2.6 V]; however the limit of this facile f-element reduction process has not yet been determined. Extension of this work to still less easily reduced lanthanide ions and to selected actinide ions is also of great interest in order to

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be able to characterize more fully their divalent oxidation states. In addition the limit of f-element reducibility and stabilization in this tetraborate matrix should be determined, as well as the degrees to which divalent f-elements can be substituted for Sr” ions.

Acknowledgments The authors thank Dr. C. Bamberger of ORNL for his helpful discussions and Dr. S. Dai of ORNL for his assistance with the absorption spectral measurements. This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under grant DE-FG0588ER13865 to the University of Tennessee, Knoxville and contract DEAC05960R22464 with Lockheed Martin Energy Research Corp.

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