Polymorphism in Photoluminescent KNdW 2 O 8 : Synthesis, Neutron Diffraction, and Raman Study Swetha S. M. Bhat, † Diptikanta Swain, ‡ Chandrabhas Narayana, ‡ Mikhail Feygenson, § Joerg C. Neuefeind, § and Nalini G. Sundaram* ,† † Materials Science Division, Poornaprajna Institute of Scientific Research, Bidalur Near Devanahalli, Bengaluru, Karnataka, India ‡ CPMU, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru, Karnataka, India § Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States * S Supporting Information ABSTRACT: Polymorphs of KNdW 2 O 8 (α-KNdW 2 O 8 and β-KNdW 2 O 8 ) phosphors were synthesized by an efficient solution combustion technique for the first time. The crystal structure of the polymorphs analyzed from Rietveld refinement of neutron diffraction data confirms that α-KNdW 2 O 8 crystallizes in the tetragonal system (space group I4̅), and β-KNdW 2 O 8 crystallizes in the monoclinic system (space group C2/m). The local structure of both polymorphs was elucidated using combined neutron pair distribution function (PDF) and Raman scattering techniques. Photoluminescence measurements of the polymorphs showed broadened emission line width and increased intensity for β-KNdW 2 O 8 in the visible region compared to α-KNdW 2 O 8 . This phenomenon is attributed to the increased distortion in the coordination environment of the luminescing Nd 3+ ion. Combined PDF, Rietveld, and Raman measurements reveal distortions of the WO 6 octahedra and NdO 8 polyhedra in β-KNdW 2 O 8 . This crystal structure-photoluminescence study suggests that this class of tungstates can be exploited for visible light emitting devices by tuning the crystal symmetry. 1. INTRODUCTION Photoluminescent materials have drawn much attention in recent years, due to their potential applications in optoelec- tronics. The alkali metal double tungstate materials with general formula AR(WO 4 ) 2 (A = Li + , Na + , Rb + , Cs + and R = Y 3+ , Yb 3+ , Nd 3+ , etc.) are multifunctional, disordered materials, used as active solid-state luminescent hosts, and their properties could be tailored by different compositions. Among rare earths, neodymium-doped tungstates are widely used for lasing applications owing to their high absorption and emission cross sections. 1 Recently these materials have also been investigated for visible and ultraviolet upconversion from its near-infrared (NIR) origin. 2,3 Crystallographic structures of these materials are related to that of tetragonal Scheelite CaWO 4 , where Ca 2+ is substituted by an A + R 3+ pair. 4 Moreover, distribution of R 3+ ions in host materials leads to distortion in the structure, lowering of symmetry of the parent compound, local environmental disorder, and phase transformations, which eventually results in interesting physical properties as a function of crystal structure. Interestingly, most of the potassium, rubidium, and cesium rare earth tungstates undergo high temperature polymorphic phase transitions. 5 Klevstova et al. 6 have compiled a detailed review on polymorphism in rare earth double molybdates and tungstates which exhibit a wide range of crystal structures. This structural diversity in the polymorphs results in a difference in luminescence properties which could be exploited in different optical devices without substantial fluorescence quenching. 7 Recently, Zaldo et al. 5 studied polymorphs of AgNdW 2 O 8 in detail and correlated their crystal structure with optical bandwidths. On the basis of crystal field analysis, they concluded that the presence of two Nd 3+ sites contributed to the experimental bandwidths. KNdW 2 O 8 has also been reported to exist in two polymorphic phases: 6 a high temperature tetragonal phase (α- KNdW 2 O 8 ), which exists above 1000 °C, and a monoclinic phase (β-KNdW 2 O 8 ) at 770 °C. While the crystal structure of the β-KNdW 2 O 8 phase has been elucidated, there have been no crystal structure reports for the α-KNdW 2 O 8 phase. Kolesov et al. 8 have carried out a Raman study on the monoclinic phase of KNdW 2 O 8 to illustrate the size and charge effect of K + and Nd 3+ ions on the ordered distribution of cations. Kato et al. 9 has compared the charge compensation effect on photo- luminescence spectra of KNdW 2 O 8 with other smaller alkali rare earth tungstates. KNdW 2 O 8 exhibits efficient photo- luminescence with narrow emission spectra in the IR region which is close to that of Nd:YAG at room temperature. Recently, Quereshi et al. 10 demonstrated strong visible light Received: November 15, 2013 Revised: December 24, 2013 Published: January 3, 2014 Article pubs.acs.org/crystal © 2014 American Chemical Society 835 dx.doi.org/10.1021/cg4017068 | Cryst. Growth Des. 2014, 14, 835-843