Chemical Physics Letters 618 (2015) 108–113 Contents lists available at ScienceDirect Chemical Physics Letters jou rn al h om epa ge: www.elsevier.com/locate/cplett X-ray induced color change on dense yttria samples obtained by spark plasma sintering Mohammed A.A. Attia a , Sebastiano Garroni b , Daniele Chiriu c , Carlo Ricci c , Francesco Delogu a , Roberto Orrù a,∗ , Giacomo Cao a a Dipartimento di Ingegneria Meccanica, Chimica, e dei Materiali, Università degli Studi di Cagliari, via Marengo 2, 09123 Cagliari, Italy b Dipartimento di Chimica e Farmacia, Università degli Studi di Sassari, via Vienna 2, 07100 Sassari, Italy c Dipartimento di Fisica, Università degli Studi di Cagliari, S.P. Monserrato-Sestu km 0.7, 09042 Monserrato (CA), Italy a r t i c l e i n f o Article history: Received 24 July 2014 In final form 5 November 2014 Available online 11 November 2014 a b s t r a c t This work focuses on a particular behavior of dense yttria samples fabricated by spark plasma sintering, which display a reddish coloration when subjected to X-ray irradiation. The reddish coloration appears exclusively in sintered material. Optical measurements, X-ray photoelectron spectroscopy and chem- ical methods have been used to investigate the properties of consolidated yttria. The results obtained suggest that the sintered material exhibits a relatively high concentration of oxygen vacancies. Ther- moluminescence measurements indicate that they act as electronic trapping sites. A generalized-order kinetic analysis suggests that at least five different shallow traps can be identified. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Yttria is a dielectric solid with chemical formula Y 2 O 3 and IUPAC name yttrium (III) oxide [1]. In its allotropic form, it exhibits a body-centered cubic cI80 crystalline lattice, Mn 2 O 3 bixbyite type, belonging to the Ia3 (Th7) (n. 206) space group [2–5]. The unit cell contains 40 atoms, hosting the two different 8b and 24d cation sites [2–5]. Six O atoms surround each Y atom enabling a local octahedral coordination [2–5]. The arrangement of atomic species on lattice sites gives rise to remarkable electronic properties, which include high refractive index, high dielectric constant, large band gap, low absorption, and superior electrical breakdown strength [6–10]. Furthermore, Y 2 O 3 exhibits excellent thermal conductivity and a low dominant phonon energy [11,12], is used as a waveguide in optics and can be easily doped with rare earth ions, thus acting as an efficient laser host [6]. The transparency over a broad spectral range, roughly between 0.2 and 8 m [10], makes Y 2 O 3 appealing for various electrical, optical, and electro-optical applications. In particular, it shows the promise of enabling a significant enhancement of the optical ∗ Corresponding author. E-mail address: roberto.orru@dimcm.unica.it (R. Orrù). transmission confinement via a suitable tuning of its refractive index and extinction coefficient, which would allow a more effi- cient pumping and amplification of optical waveguides [10]. In the light of the sensitivity of Y 2 O 3 optical constants to microstructure, one of the possible strategies to achieve such objective is repre- sented by microstructural engineering. Coherently with this observation, the present work focuses on the utilization of spark plasma sintering (SPS) to obtain dense Y 2 O 3 with controlled microstructure. SPS is an advanced powder den- sification process [13]. Powder is processed inside a cylindrical volume defined by a die and two coaxial plungers that can run inside it to apply uniaxial stress [13]. Since die and plungers are made of graphite, the electric current can freely flow across them as well as across the processing powder if conductive [13]. The simultaneous application of uniaxial stress and pulsed electric cur- rent results in significantly shorter sintering times and definitely milder processing conditions compared with the ones typically experienced by powders in other consolidation methods [14–19]. The available experimental data indicate that SPS-fabricated monolithic Y 2 O 3 generally benefits from such less severe processing conditions [20–28]. This work supports this claim, showing that highly dense Y 2 O 3 solids can be obtained at rel- atively mild sintering temperatures and short processing times. It is also shown that its interaction with X-rays induces a reversible coloration. A combination of optical measurements, http://dx.doi.org/10.1016/j.cplett.2014.11.009 0009-2614/© 2014 Elsevier B.V. All rights reserved.