Photoluminescence and excitation spectra of microcrystalline and nanocrystalline nickel tungstate (NiWO₄) were measured using UV-VUV synchrotron radiation source. The origin of the bands is interpreted using comparative analysis with isostructural ZnWO₄ tungstate and based on the results of recent first-principles band structure calculations. The influence of the local atomic structure relaxation and of Ni²⁺ intra-ion d–d transitions on the photoluminescence band intensity are discussed

Kalinko, A.

Kotlov, A.

Kuzmin, A.

Pankratov, V.

Popov, A.I.

Shirmane, L.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 7, pp. 694–698 UV-VUV synchrotron radiation spectroscopy of NiWO4 A. Kuzmin1, V. Pankratov1,2, A. Kalinko1,3, A. Kotlov3, L. Shirmane,1 and A.I. Popov1 1Institute of Solid State Physics, University of Latvia, 8 Kengaraga str., LV-1063 Riga, Latvia E-mail: a.kuzmin@cfi.lu.lv 2Research Center of Molecular Materials, University of Oulu, P.O. Box 3000, FIN-90014, Finland 3DESY Photon Science, Notkestraße 85, D-22607 Hamburg, Germany Received January 18, 2016, published online May 25, 2016 Photoluminescence and excitation spectra of microcrystalline and nanocrystalline nickel tungstate (NiWO4) were measured using UV-VUV synchrotron radiation source. The origin of the bands is interpreted using com-parative analysis with isostructural ZnWO4 tungstate and based on the results of recent ﬁrst-principles band structure calculations. The inﬂuence of the local atomic structure relaxation and of Ni2+ intra-ion d–d transitionson the photoluminescence band intensity are discussed. PACS: 78.55.Hx Other solid inorganic materials; 61.05.cj Electron density of states and band structure of crystalline solids; 71.20.–b X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc. Keywords: NiWO4, ZnWO4, UV-VUV spectroscopy, EXAFS, first-principles calculations. Introduction Metal tungstates (MWO4) have received considerable interest from both theoretical and technological point of view due to their excellent combination of the optical, pie-zoelectric, ferroelectric and other properties [1,2]. The op-tical properties of different metal tungstates can be con-trolled by their composition and many wide band-gap tungstates are found to be promising materials for scintilla-tor applications [3–10]. Among them, nickel tungstate (NiWO4) ﬁnds applica-tions in catalysis [11–14], as humidity [15] and gas [16] sensor, a photoanode in photovoltaic electrochemical cell [17], a pigment [18] and in microwave [19] and elect-rochromic [20,21] devices. These applications are tightly connected to its electronic structure being scarcely studied in the past. The electronic band structure of NiWO4 was calcu-lated recently using ﬁrst-principles spin-polarized periodic linear combination of atomic orbitals (LCAO) method in [22]. The photoluminescence properties of NiWO4 powder and sol-gel derived NiWO4 ﬁlms, calcined above 600°C, were investigated at room temperature under pulsed Xenon discharge lamp excitation in [23], indicating the presence of a broad blue-green (2.07–3.54 eV) photoluminescence band. In this work we present original results on the photolu-minescence and excitation spectra of microcrystalline and nanocrystalline NiWO4, obtained using UV-VUV synchro-tron radiation spectroscopy. The experiments were carried out at SUPERLUMI beamline (HASYLAB at DESY, Hamburg) using 4–20 eV synchrotron radiation from the DORIS storage ring for excitation [24]. This experimental set-up is a unique tool for investigations of different types of wide band gap materials [24–33]. The interpretation of the photoluminescence and excitation spectra is given based on the comparative analysis with isomorphous ZnWO4 tungstate [32] and using the results of LCAO cal-culations [22] and the Ni(Zn) K-edge and W L3-edge x-ray absorption spectroscopy studies. Experimental and calculation details NiWO4 and ZnWO4 powders were synthesized using co-precipitation technique by mixing two aqueous solutions of Na2WO4·2H2O and Ni(NO3)2·6H2O or ZnSO4·7H2O salts in bi-distilled water at room temperature (20 °C). The pH value of the solution was equal to eight. The tungstate sedi-ment was subsequently washed, ﬁltrated and, after drying, annealed in air for 4 h at 80 °C (nanocrystalline sample with crystallites size below 2 nm) and 900 °C (microcrystalline sample). More details can be found in [32]. The photoluminescence (Fig. 1) and excitation (Fig. 2) spectra were collected at 7–300 K exploiting ultraviolet (UV) and vacuum ultraviolet (VUV) synchrotron radiation © A. Kuzmin, V. Pankratov, A. Kalinko, A. Kotlov, L. Shirmane, and A.I. Popov, 2016 UV-VUV synchrotron radiation spectroscopy of NiWO4 (3.6–20.0 eV) emitted from DORIS III storage ring at SUPERLUMI station (HASYLAB DESY, Hamburg). The low-temperature (10 K) Ni(Zn) K-edge and W L3-edge x-ray absorption spectra were measured in transmis-sion mode at the HASYLAB/DESY C bending-magnet beamline. The x-ray radiation was monochromatized by a detuned Si(111) double-crystal monochromator, and the beam intensity was measured using two ionization cham-bers ﬁlled with argon and krypton gases. The x-ray ab-sorption spectra were analyzed using the EDA software package [34] following conventional procedure [35]. Ra-dial distribution functions (RDFs) G(R) (Fig. 3) for Fig. 1. The photoluminescence spectra of microcrystalline (left panel) and nanocrystalline (right panel) Ni(Zn)WO4 powders excited by 13.8 eV photons. Fig. 2. Left panel: Excitation spectra of microcrystalline and nanocrystalline NiWO4 at T = 7 and 80 K. The bands due to the one-electron transitions are indicated by vertical arrows. Right panel: Electronic band structure diagram for wolframite-type NiWO4 from [22]. The energy zero is set at the Fermi-energy level. The one-electron transitions, corresponding to the peaks at 6, 9, 12, 13.5 and 16 eV in the excitation spectrum of NiWO4 are indicated by vertical arrows. Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 7 695 A. Kuzmin, V. Pankratov, A. Kalinko, A. Kotlov, L. Shirmane, and A.I. Popov Ni(Zn)–O and W–O bonds in microcrystalline and nanocrystalline Ni(Zn)WO4 were obtained by the regular-ization-like method [34,36,37] from the Ni(Zn) K-edge and W L3-edge extended x-ray absorption ﬁne structure (EXAFS) spectra. Theoretical scattering amplitude and phase shift functions, employed in the EXAFS simula-tions, were calculated for NiWO4 and ZnWO4 crystallo-graphic structures by the ab initio FEFF8 code [38,39] using complex exchange-correlation Hedin–Lundqvist potential. Results and discussion Photoluminescence spectra Both tungstates NiWO4 and ZnWO4 have monoclinic wolframite-type structure (space group P2/c) with two for-mula units (Z = 2) per primitive cell [40]. All metal atoms (Ni, Zn and W) are six-fold octahedrally coordinated by oxygen atoms. The presence of two non-equivalent oxygen atoms results in metal-oxygen octahedron distortion [41], which is the largest for WO6 octahedron and the smallest for NiO6 octahedron (Fig. 3). The photoluminescence spectrum of microcrystalline ZnWO4 powders consists of a broad band, peaked at about 2.5 eV (Fig. 1). The origin of the band has been previously assigned to radiative electron transitions within the [WO6]6–molecular complex [42,43]. In NiWO4, the same photolumi-nescence band has irregular asymmetric shape, which is close to that observed previously in solid solutions ZncNi1–cWO4 [32]. The origin of such band shape can be attributed [32] to the self-absorption eﬀect, i.e., to a modulation of optical ab-sorption by the intense intra-ion transition within Ni2+ (3d8) ions from the ground state 3A2g to the excited state 3T1 [44]. The maximum of the photoluminescence band in nano-NiWO4 is located at 2.7 eV, thus being shifted by ~0.32 eV to higher energy compared to nano-ZnWO4 [45]. Such blue-shift can be explained by a diﬀerence in the relaxation of WO6 octahedra in the two tungstates, which is directly evi-denced by our W L3-edge EXAFS data (Fig. 3). Radial distribution functions G(R) for Ni/Zn–O and W–O bonds in microcrystalline and nanocrystalline Ni(Zn)WO4 are shown in Fig. 3. The shape of the RDFs conﬁrms unam-biguously that the WO6 octahedra in both tungstates are strongly distorted. The relaxation of the ﬁrst shell WO6 and Ni(Zn)O6 octahedra in nanocrystalline Ni(Zn)WO4 is clearly observed. Note that in the case of WO6 octahedra the relaxa-tion aﬀects both shortest (at 1.8–1.9 Å) and longest (at 2.1–2.2 Å) W–O bonds. The nearest group of the W–O bonds relaxes stronger in NiWO4 (by ~0.07 Å) than in ZnWO4 (by ~0.02 Å), thus being responsible for a diﬀerence in the posi-tion of the photoluminescence bands in the two nanotungstates. Finally, we would like to note that no signiﬁcant temper-ature eﬀect has been observed on the photoluminescence spectra of microcrystalline and nanocrystalline NiWO4 in the temperature range 7–80 K. Also the photoluminescence spectra of micro-NiWO4 at T = 80 K show weak depend-ence on the excitation wavelength (4.96 or 13.8 eV). Excitation spectra The excitation spectra of microcrystalline NiWO4 show some temperature dependence in the range from 7 to 80 K due to the lattice expansion (left panel in Fig. 2). They diﬀer from that for ZnWO4 from [32] due to the diﬀerence of about 1 eV in the band gaps: Eg = 3.6 eV [46] for NiWO4, but Eg = 4.6 eV for ZnWO4 [47]. As a result, the strong excitonic band clearly visible at ~4.5 eV for ZnWO4 [32] is not observed in the present data for NiWO4 due to the spectrometer range limitations: the excitonic band is expected to be located below 3.7 eV. The interpretation of the excitation spectra can be done using the electronic band structure diagram calculated for Fig. 3. Radial distribution functions (RDFs) G(R) for Ni(Zn)–O and W–O bonds within the ﬁrst coordination shells of metal atoms in mi-crocrystalline (solid lines) and nanocrystalline (dashed lines) Ni(Zn)WO4, obtained from the analysis of the Ni(Zn) K-edge and W L3-edge EXAFS spectra at 10 K. 696 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 7 UV-VUV synchrotron radiation spectroscopy of NiWO4 NiWO4 in Ref. 22 using the ﬁrst-principles spin-polarized periodic linear combination of atomic orbital (LCAO) method. The valence band of NiWO4 is dominated by oxy-gen 2p-states hybridized with nickel 3d(t2g, eg↑)-states, whereas tungsten 5d-states and nickel 3d(eg↓)-states cont-ribute largely into the bottom of conduction band [22]. The broad bands at 5–6, 8–10, 12, 13.5 and 16 eV are due to the one-electron transitions (arrows in the right pan-el of Fig. 2) from the top of the valence band. These transi-tions are even more pronounced in the case of nano-NiWO4 due to quasi-localized nature of the involved elec-tronic states. Note that such transitions have been also ob-served previously in nano-ZnWO4 [32]. Conclusions Microcrystalline and nanocrystalline NiWO4 powders were studied by UV-VUV synchrotron radiation spectro-scopy in comparison with isomorphous ZnWO4 tungstate. The photoluminescence and excitation spectra were inter-preted using the results of the ﬁrst-principles LCAO calcu-lations [22] and the local structural information from the Ni(Zn) K-edge and W L3-edge x-ray absorption spectro-scopy studies. Similar to the case of ZnWO4 [42,43], the photolumi-nescence spectra of both microcrystalline and nano-crystalline NiWO4 powders originate in the [WO6]6– mo-lecular complex. However, their intensity is strongly modulated by the optical absorption of Ni2+ ions (intra-ion d–d transitions). In nano-NiWO4, the photoluminescence band maximum is shifted to shorter wavelengths due to the strong relaxation of WO6 octahedra. The excitation spectra are similar in microcrystalline and nanocrystalline NiWO4 powders. They consist of a number of bands due to the one-electron transitions across the band gap from the top of the valence band to the elec-tron states in the conduction band and above. The bands are more pronounced in nano-NiWO4 due to quasi-loca-lized character of the involved electronic states. Acknowledgments This work was supported by Latvian Science Council Grant No. 187/2012. The experiments at DESY leading to these results have received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 226716. V.P. acknow-ledges the financial supports from Oulu University Strategic Funding and Research Council for Natural Sciences of the Academy of Finland.  1. B.I. Kidyarov and V.V. 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UV-VUV synchrotron radiation spectroscopy of NiWO₄

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UV-VUV synchrotron radiation spectroscopy of NiWO₄

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)