Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Synthesis, characterization and sintering of Gd 2 Hf 2 O 7 powders synthesized by solid state displacement reaction at low temperature Branko Matovic a, ⁎ , Jelena Maletaskic a,b , Dusan Bucevac a , Jelena Zagorac a , Muhammad Fajar b , Katsumi Yoshida b , Toyohiko Yano b a Centre of Excellence-CextremeLab Vinca, Institute of Nuclear Sciences Vinca, University of Belgrade, Mike Petrovica Alasa 12-14, 11000 Belgrade, Serbia b Laboratory for Advanced Nuclear Energy, Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8550 Tokyo, Japan ARTICLE INFO Keywords: Pyrochlore, Gd 2 Hf 2 O 7 Low-temperature synthesis Phase evolution Sinterability Thermal conductivity ABSTRACT Pyrochlore compound (Gd 2 Hf 2 O 7 ) powder was prepared by reacting gadolinium nitrate and hafnium chloride with NaOH during solid state displacement reaction at low temperature (SSDR). The SSDR process at room temperature initially yielded amorphous powders, which crystallized after subsequent calcination to form crystalline ceramics. The formation of crystalline Gd 2 Hf 2 O 7 took place at temperature as low as 600 °C. The phase evolution with thermal treatment as well as powder properties such as crystallite size, lattice strain and lattice parameter were studied by X-ray powder diffraction (XRPD) at room temperature. High-density ceramic pellets free of any additives were obtained after compaction of the obtained powders and subsequent sintering at 1600 °C for 4 h in air. Rietveld analysis of X-ray diffraction (XRD) pattern of sintered sample showed that the unit cell parameter of the obtained Gd 2 Hf 2 O 7 is 10.5501 (2) Å with x value = 0.345(2) in Wyckoff positions, in- dicating small distortion of octahedra. Hardness of the sintered samples was found to be 7.1 GPa. The thermal conductivity measurements performed in temperature range from room temperature to 1000 °C showed that thermal diffusivity of sintered samples was between 0.5 and 1 mm 2 /s whereas thermal conductivity was between 4 and 7 W/(m K). 1. Introduction Ternary compounds with the general chemical formula (A 2 B 2 O 7 ) are normally called pyrochlores as they are isostructural to mineral pyro- chlore. Over the past two decades pyrochlores have gained the atten- tion of scientific community owing to their interesting combination of properties such as high melting point, good phase stability and che- mical resistance, low thermal conductivity, high thermal expansion coefficient and excellent resistance to radiation induced amorphisation [1–5]. In addition, ceramics with pyrochlore-type structure is also consider as promising host material for phosphorus and actinide waste [6,7]. Letter A in pyrochlore-type formula represents the site occupied by larger trivalent 8-coordinated cation, typically rare earth elements, whereas B represents the smaller tetravalent 6-coordinated cations of transition metals, i.e., Ti, Zr, Hf [8]. The compound can exist in two closely related structures known as ordered pyrochlore structure (space group 227) and disordered fluorite structure (space group 227) [1]. The crystalline structure and properties of pyrochlores are highly dependent on composition and the ratio of radii of A-site cation (rA 3+ ) and B-site cation (rB 4+ ). The pyrochlore-type structure is normally observed for radius ratio rA 3+ /rB 4+ = 1.46–1.78. For this reason the properties of pyrochlores such as ionic conductivity [9,10], geome- trically frustrated magnetism [11], neutron absorption [12], nuclear waste storage capacity [13] and thermal conductivity [14–16] can be easily controlled by changing the constituent ions. Among the pyrochlore-type compounds, Gd 2 Hf 2 O 7 is known to have a strong resistance to ion-beam irradiation and the highest melting point [17]. Instead of becoming amorphous at high temperature, Gd 2 Hf 2 O 7 transforms into a disordered fluorite structure [18] which makes this compound a promising material for fabrication of high- temperature ceramic composites and thermal barrier coatings. Several methods have been used to fabricate pyrochlore com- pounds. Combustion synthesis [19], precipitation method [20], hy- drothermal route [21], solid state reaction method [22,23], sol-gel method [24], combination of sol–gel processing and complex pre- cipitation [25], mechanical milling [26] and molten salt reaction [27] to name a few. However, the use of solid state displacement reaction for this purpose has not been reported. This is quite surprising knowing that the solid state displacement reaction is a convenient method owing to its high efficiency, simplicity, low energy consumption and uniform https://doi.org/10.1016/j.ceramint.2018.06.138 Received 14 May 2018; Received in revised form 15 June 2018; Accepted 16 June 2018 ⁎ Corresponding author. E-mail address: mato@vinca.rs (B. Matovic). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2018 Published by Elsevier Ltd. Please cite this article as: Matovic, B., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.06.138