Article pubs.acs.org/JPCC
Eu2+ Stabilization in YAG Structure: Optical and Electron Paramagnetic Resonance Study L. Havlák,† J. Bárta,†,‡ M. Buryi,† V. Jarý,† E. Mihóková,† V. Laguta,† P. Bohácě k,† and M. Nikl*,† †
Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 1999/2, 182 21 Prague 8, Czech Republic Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Brehova 7, Praha 1 11519, Czech Republic
‡
ABSTRACT: A set of Eu-doped Y3Al5O12 (YAG) phosphors in the powder form was successfully synthesized by multiple-step solid-state reaction under the reducing Ar:5%H2 atmosphere. Their physical properties were investigated by means of X-ray diffraction, time-resolved luminescence spectroscopy, and electron paramagnetic resonance (EPR). Special attention was given to wellgrounded confirmation of Eu2+ occurrence in the YAG structure. Xray diffraction confirmed the presence of a major YAG phase in all samples. The influence of Eu concentration, form of doping (EuS, EuF2, or Eu2O3) and number/form of annealing steps was studied. Corresponding characteristics were measured and evaluated in a broad temperature range (8−800 K). The correlated measurements showed that the single Eu2+ ions stabilized in YAG in the yttrium position are responsible for the 440 nm emission. The corresponding lowest energy excitation band is situated at 300 nm, and the radiative lifetime of Eu2+ emission is about 500 ns. A phenomenological model of excited state dynamics was used to explain the experimentally observed features. Possible practical exploitation of the Eu2+ center in the YAG host is discussed. (CN 6), Eu2+ 1.39 Å (CN 8), Eu3+ 1.21 Å (CN 8), and Y3+ 1.04 Å (CN 6). The crystal radii of anions are F− 1.19 Å, I− 2.06 Å, O2− 1.24 Å, and S2− 1.7 Å, respectively.2 Recently, several papers reporting the Eu2+ center in the YAG garnet structure have been published. The Pechini sol−gel method was employed to prepare the YAG:Eu2+ precursor from aqueous solutions of Y(NO3 ) 3 , KAl(SO 4 ) 2 , and Eu 2 O 3 dissolved in HNO3, followed by adding citric acid and ethylene glycol.3 Subsequently, they annealed the formed mixture in air. According to XRD, they obtained a pure YAG phase in the 1000−1200 °C temperature range. Under the 254 nm excitation, photoluminescence emission (PL) spectra exhibited a broad band in the blue region peaking between 447 and 503 nm, with a maximum at 480 nm.3 However, the PL spectra for wavelengths greater than 550 nm were not shown, and therefore it is not clear if some Eu3+ emission occurred in these YAG samples. As the samples were annealed in air, one would expect Eu3+ to be present with a much higher probability compared to the presence of Eu2+. In ref 3, the discussion concerning the Eu2+ stabilization in YAG structure is missing. The authors simply assigned the emission band at 480 nm to the Eu2+ ions in Y3+ positions in the garnet lattice even though they worked in oxidizing environment and they doped europium in the Eu3+ form. In ref 4, the authors used hydroiodic acid and formation of iodides to stabilize Eu2+ in the
1. INTRODUCTION Garnets are cubic oxide materials with a general formula (A2+)3(B3+)2(C4+)3O12 that crystallize in the Ia3̅d (no. 230) space group. As an example, the symbols A, B, and C can represent Ca, Al, and Si, respectively, providing one particular garnet composition of Ca3Al2Si3O12 (grossular). The ions A2+ (Wyckoff position 24c − 1/8, 0, 1/4), B3+ (16a − 0, 0, 0), and C4+ (24d − 3/8, 0, 1/4) are coordinated by eight, six, and four oxygen atoms, respectively. The polyhedrons corresponding to the structural environment of A2+, B3+, and C4+ cations in the garnet structure are dodecahedron (a slightly distorted cube), octahedron, and tetrahedron, respectively. Crystal chemistry of garnets is described in detail in ref 1. It covers more than a hundred different garnet compositions. Many cations which may enter the garnet structure are discussed, and their preferences in accommodating different structural positions are analyzed. The mentioned structure is well illustrated in ref 1. Cations like Ca, Mg, Mn, Fe, Co, Cd, or Sr willingly enter the position A; Al, V, Cr, Mn, Fe, Ga, In, or Rh occupy the position B; and Si or Ge enter the position C. Yttrium aluminum garnet, Y3Al5O12 (YAG), belongs to synthetic garnets formed solely by trivalent cations. Its general formula can be written as (Y3+)3(Al3+)2(Al3+)3O12. Y3+ cations are coordinated by eight oxygen atoms, 2/5 of Al3+ cations by six oxygen atoms, and 3/5 of Al3+ cations by four oxygen atoms. For the purpose of this work, in particular the stabilization of the Eu2+ ion in the YAG structure, it is useful to list the crystal radii of the ions that will be discussed below: Y3+ 1.16 Å (for coordination number (CN) 8), Al3+ 0.53 Å (CN 4), Al3+ 0.68 Å © 2016 American Chemical Society
Received: June 27, 2016 Revised: August 31, 2016 Published: September 1, 2016 21751
DOI: 10.1021/acs.jpcc.6b06397 J. Phys. Chem. C 2016, 120, 21751−21761
Article
The Journal of Physical Chemistry C
crystals grown under 100% Ar atmosphere in ref 12 were shown only in the 550−750 nm spectral range, and therefore, it is impossible to determine whether there was any Eu2+ emission in the blue region, i.e., below 550 nm. In this case, the information in refs 3 and 12 seems to be in contradiction. In ref 3, the PL spectrum of YAG:Eu is shown only up to 550 nm, and in ref 12, the PL spectrum starts at 550 nm. It is highly probable that in both cases the YAG samples contained both Eu2+ and Eu3+. The Eu2+-doped YAG and Lu3Al5O12 (LuAG) garnets were also prepared by the spark plasma sintering method in the form of ceramic pellets at 1600 °C and under the 100 MPa pressure in a graphite die of 10 mm diameter in ref 16. In radioluminescence emission (RL) spectra excited by 241 Am α-rays, the red emission belonging to the Eu3+ center dominated. Under the 254 nm excitation, green emission of YAG:Eu and blue emission of LuAG:Eu appeared. Emission band peaking at 475 nm in YAG:Eu and double-emission peak between 400 and 500 nm in LuAG:Eu were ascribed to the Eu2+ centers. An emission band at 350 nm was observed in LuAG:Eu due to antisite defect (AD) LuAl. Al2O3:Eu prepared as a reference material in ref 16 exhibited a broad emission in the 350−500 nm region in its RL spectrum, with maximum at around 450 nm. By vacuum sintering, the authors in ref 17 prepared Eu2+-doped Al2O3, which showed blue emission peaking at 440 nm under the 278 and 330 nm UV excitations. At increased Eu content in Al2O3, XRD proved the presence of another phase, namely, EuAl12O19. This Al2O3 was doped by europium in the Eu2O3 form, and that is why the content of Eu2+ can be connected to low pressure (
