Phase composition and morphology of nanoparticles of yttrium orthophosphates synthesized by microwave-hydrothermal treatment: The influence of synthetic conditions A.S. Vanetsev a,⇑ , E.V. Samsonova a , O.M. Gaitko b , K. Keevend a , A.V. Popov c , U. Mäeorg d , H. Mändar a , I. Sildos a , Yu.V. Orlovskii a,c a Institute of Physics, University of Tartu, Ravila 14c, Tartu 50411, Estonia b Kurnakov Institute of General and Inorganic Chemistry RAS, Leninskii Prospekt 31, Moscow 119991, Russia c Prokhorov General Physics Institute RAS, Vavilov St. 38, Moscow 119991, Russia d Institute of Chemistry, University of Tartu, Ravila 14a, Tartu 50411, Estonia article info Article history: Received 13 February 2015 Accepted 15 March 2015 Available online 20 March 2015 Keywords: Chemical synthesis Microwave-assisted synthesis Hydrothermal treatment Nanostructured materials Phase transitions Yttrium orthophosphate abstract Herein we report the study of the influence of synthesis conditions during the microwave-hydrothermal crystallization of freshly precipitated gels on the phase composition and morphology of the rare-earth doped yttrium orthophosphates nanoparticles. We characterize the nanoparticles of YPO 4 and YPO 4 0.8H 2 O using X-ray diffraction analysis, TEM, and FT-IR spectroscopy. Furthermore, we argue that for the given phase the degree of crystallinity and thus the sample morphology depend strongly on the synthesis conditions. We establish that the hexagonal hydrate phase can be obtained by means of microwave-hydrothermal method if one uses phosphate anion excess or adjusts pH of the reaction mix- ture. Also we show that the metastable hydrate phase is most likely stabilized by hydroxyl groups at ele- vated temperatures. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction A significant number of papers devoted to the synthesis of YPO 4 nanoparticles doped with rare earth ions has been published recently due to their unique combination of desirable properties. Chemically and thermally stable, highly crystalline, photostable and efficiently emitting, these particles are admitted to be an attractive candidate for a wide range of applications, from optoelectronics (display and LED production) to biomedicine. The biomedical application include such areas as cell labeling [1], the detection of molecules [2], while YPO 4 containing composite nano- particles were suggested for hyperthermia treatment [3]. Apart from high fluorescence intensity and stability, the latter impose additional requirements on the material properties, namely, low toxicity, high biocompatibility and dispersability in aqueous media. However, such characteristics are strongly dependent on the phase composition, morphology of the synthesized nanoparti- cles and the state of their surface. Anhydrous orthophosphates of large rare earth elements can be crystallized in the monoclinic monazite structure (space group P21/n, Z= 4), while orthophosphates of rare-earth elements with small ion radii (including yttrium and scandium) are normally crystallized in tetragonal zircon-type xenotime structure (space group I41/amd, Z= 4). So, in the case of the anhydrous yttrium phosphate, which has tetragonal structure, a distorted dodec- ahedron D2d (bisphenoid) RE-O configuration is formed due to two orthogonal sets of equivalent bond distances, so an yttrium ion is coordinated with eight oxygen atoms [4]. Among hydrate phases there are two that are typical for yttrium phosphate hydrates, namely, the weinschenkite-type YPO 4 2H 2 O and the hexagonal YPO 4 0.8H 2 O. The hexagonal YPO 4 0.8H 2 O has the rhabdophane-type structure (space group P6 2 22, No. 180) with a = 6.833 Å and c = 6.291 Å where Y 3+ ions occupy a D 2 point-group symmetry site in the coordination feeding solution formed by eight oxygen ions (four at 2.257 Å and another four at 2.578 Å) (see Fig. 1). Although the first report on the synthesis of the yttrium orthophosphate hydrate (i.e., YPO 4 0.8H 2 O) with hexagonal struc- ture dates back to 1989 [5], there are very few works concerning the synthesis and properties of this material. While anhydrous yttrium orthophosphate has been obtained both in single crystal and nanocrystal forms by means of variety of techniques, there is only a limited number of works on nanomaterials based on the hydrate phases. In [6] authors suggest the coprecipitation http://dx.doi.org/10.1016/j.jallcom.2015.03.125 0925-8388/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: alexander.vanetsev@ut.ee (A.S. Vanetsev). Journal of Alloys and Compounds 639 (2015) 415–421 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom