Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Synthesis and luminescence properties of Er 3+ doped La 3 NbO 7 ceramic powder A. Egaña a , E. Cantelar b , M. Tardío a , J.E. Muñoz Santiuste a,∗ a Departamento de Física, Escuela Politécnica Superior, Universidad Carlos III de Madrid, Avda. de la Universidad, 30, 28911, Leganés (Madrid), Spain b Departamento de Física de Materiales C-04, Universidad Autónoma de Madrid, Cantoblanco, 28045, Madrid, Spain ARTICLE INFO Keywords: Rare earth ions Er3+ La3NbO7 weberite-type compound Absorption Photoluminescence Up-conversion ABSTRACT La 3 NbO 7 compounds doped with Er 3+ were synthesized by conventional solid state reaction. X-ray diffraction (XRD) and scanning microscopy (SEM-EDS) were used to analyze the morphology and microstructure of this ceramic powder. Rietveld's refinements show that the samples exhibit an orthorhombic weberite structure that can be described using the Cmcm spatial group. A gradual reduction of the unit cell (lattice parameters) with the erbium content evidences the effective substitution in the structure of La 3+ ions with the smaller Er 3+ ions. The erbium luminescence capabilities were studied using absorption and photoluminescence techniques in the visible (VIS) and near infrared (NIR) regions. The characteristic green, red and NIR Er 3+ luminescence bands were obtained under excitation with several wavelengths in the VIS-NIR region, being characterized by a strong multicenter structure. Up-conversion luminescence channels have also been studied to put in value the po- tentiality of Er 3+ doped La 3 NbO 7 as a new phosphor. 1. Introduction Materials with general formula Ln 3 BO 7 have recently attracted the interest of the scientific community due to their interesting dielectric [1], catalytic [2] and magnetic properties [3]. In particular, La 3 TaO 7 and La 3 NbO 7 are object of study as possible electrolytes for solid state fuel cells due to their chemical stability and proton conductivity at high temperatures [4,5]. A prominent characteristic of these structures is that La ions can be substituted by other rare earth (RE) ions. The La 3 NbO 7 crystalline structure was first reported by Rosell [6] in 1979 under the orthorhombic space group Cmcm. Although many other works supports this assignation [3,7–10], other authors have assigned the structure to the space group Pmcn [11–13] due to some low in- tensity reflections which cannot be easily distinguished from the background. In either case, the structure can be described as a one- dimensional chain, oriented along the c-axis, of tilted NbO 6 octahedra, with shared corners. One third of the La cations, surrounded by 8 oxygen ions, lies in (001) rows alternating with parallel rows of nio- bium octahedra within slabs parallel to (100). The remaining La cations lie in between these layers with seven-fold oxygen coordination. A schematic representation of the structure showing the two different La environments is depicted in Fig. 1. Over the last years, there has been a renewed interest in the un- derstanding of the role played by the structure and the particle size in the luminescent properties of the crystals doped with trivalent rare earth ions for several applications, including IR-to-visible converters, fiber optic communication and white light generators [14–16]. Re- cently, the Eu 3+ doped La 3 NbO 7 compound has been studied [8], as an efficient luminescent material in the orange-red region. To our knowl- edge, Er 3+ doped La 3 NbO 7 hasn't been reported yet. Among rare earth ions, trivalent erbium exhibit some unique luminescent properties and has been widely used as fiber optic amplifiers and temperature sensors [17,18] and could be potentially used for fluorescence bioimaging or anticancer therapies due to its emission bands centered in the near infrared biological windows (NIR-BW) [19,20]. In this work we have prepared a set of ultrafine powder samples with nominal compositions La 3-x Er x NbO 7 (x = 0.01, 0.05 and 0.1). Apparently, the solubility limit of the Er 3+ ions in the weberite-type structure has not been reached for values of x = 0.1. This fact points that this system could be an interesting material to be used as optically active media. The spectroscopy of Er 3+ ions in the VIS-NIR region is presented, showing the optical transitions obtained under visible (520 nm) or infrared (980 nm) UC processes. Excitation spectra has also been obtained to point out the complex local structure of luminescent centers. Our experimental results show that La 3 NbO 7 :Er 3+ phosphors display luminescence in two of the biological windows (850 nm and 1550 nm). In addition, a change from green to red in the dominant visible luminescence is obtained under VIS excitation (standard or https://doi.org/10.1016/j.optmat.2019.109393 Received 6 May 2019; Received in revised form 13 September 2019; Accepted 15 September 2019 ∗ Corresponding author. E-mail address: jems@fis.uc3m.es (J.E. Muñoz Santiuste). Optical Materials 97 (2019) 109393 0925-3467/ © 2019 Elsevier B.V. All rights reserved. T