Average Structure, Local Structure, Photoluminescence, and NLO Properties of Scheelite Type NaCe(WO₄)₂

Abstract

Phase pure nanoparticles of NaCe(WO 4 ) 2 green phosphor were synthesized via a template-free solvothermal method using water (NaCeW-a) and ethylene glycol (NaCeWb) as reaction solvents. Rietveld refinements using powder neutron diffraction (NPD) data confirms that both NaCeW-a and NaCeWb crystallize in the Scheelite-like tetragonal I4 1 /a space group. However, local structure analysis using total scattering atomic pair distribution function (PDF) refinements in a shorter 'r' range (1.5 to 10 Å) established the noncentrosymmetric I4̅ space group where Na/CeO 8 polyhedra are slightly distorted, distributed in 2b and 2d sites. The interesting observation of noncentrosymmetry locally in both NaCeW-a and NaCeW-b is supported by optical nonlinear second harmonic generation (SHG) measurements. Closer inspection of the scanning electron micrograms showed a distinct difference in the particle morphology as a function of reaction solvent. Observation of the +3 valence state of the cerium ion in NaCe(WO 4 ) 2 via X-ray photoelectron spectroscopy measurements was further corroborated by magnetic (VSM) measurements. In addition, both the samples emitted bright narrow green emission upon UV excitation with no considerable change in emission intensity with respect to solvent. Hence, the sharp green light emission and SHG properties make the NaCe(WO 4 ) 2 a potential candidate for solid state display and nonlinear optical applications.

Rotational Disorder in Scheelite-Type NaRE(MO4)2 (RE = Rare-Earth, Y; M = Mo, W)

“…Likewise, local deviations from the average symmetry play a crucial role in determining the materials’ ability to function as frequency-doubling crystals. Recently, Munirathnappa and co-workers demonstrated that the combined use of Rietveld and PDF analysis serves to rationalize second harmonic generation in NaCe­(WO 4 ) 2 . Similar to our findings, these authors reported that Na/Ce–O 8 dodecahedra appear locally distorted and that PDF analysis is able to capture these short-range bond distance distortions (Δ (Na/Ce)O 8 : 14 × 10 –3 –25 × 10 –3 ) whereas Rietveld analysis is not (Δ (Na/Ce)O 8 : 3 × 10 –3 –9 × 10 –3 ).…”

“…Refinements were performed in the 1.4−25°2θ range. The following parameters were refined: (1) scale factor; (2) background, which was modeled using a shifted Chebyschev polynomial function; (3) peak shape, which was modeled using a Thompson−Cox−Hastings pseudo-Voigt function; 16 constrained by the site symmetry (U 11 and U 33 for Na, RE, and M; U 11 , U 22 , U 33 , U 12 , U 13 , and U 23 for O). R wp residuals and difference curves were employed to assess the quality of the refined structural models.…”

“…Fits of these models to the experimental PDFs were performed in the interatomic distance range of 1.2−13 Å in order to account for all atom−atom pairs along the largest dimension of the unit cell. The following parameters were refined: (1) scale factor, (2) lattice constants (a and c), (3) fractional atomic coordinates of the oxygen atom (x O , y O , z O ), (4) anisotropic displacement parameters constrained by the site symmetry (U 11 and U 33 for Na, RE, and M; U 11 , U 22 , U 33 , U 12 , U 13 , and U 23 for O), and (5) linear atomic correlation factor. R w residuals and difference curves were employed to assess the quality of the refined structural models.…”

“…The need to go beyond the average bulk picture has been demonstrated in a number of investigations aimed at explaining the luminescence response of AEu(WO 4 ) 2 (A = Li, Na, K) 10 and the observation of second harmonic generation in NaCe(WO 4 ) 2 . 11 These studies employed a combination of Rietveld and pair distribution function (PDF) analyses of X-ray or neutron scattering data to interrogate the local atomic arrangement and identify local deviations from the average structure; specifically, to probe whether rare-earth cations occupy acentric sites thereby leading to local noncentrosymmetry. Much less attention has been devoted to investigating the presence of local rotational disorder of MO 4 tetrahedra in scheelite-type NaRE(MO 4 ) 2 materials.…”

“…Central to the functionality of scheelite-type solids as optical materials is the accurate depiction of their local crystal structure. The need to go beyond the average bulk picture has been demonstrated in a number of investigations aimed at explaining the luminescence response of AEu­(WO 4 ) 2 (A = Li, Na, K) and the observation of second harmonic generation in NaCe­(WO 4 ) 2 . These studies employed a combination of Rietveld and pair distribution function (PDF) analyses of X-ray or neutron scattering data to interrogate the local atomic arrangement and identify local deviations from the average structure; specifically, to probe whether rare-earth cations occupy acentric sites thereby leading to local noncentrosymmetry.…”