CaYGaO 4 ; a fully ordered novel olivine type gallate Ryan Clark, Sarah Jiaxin Zhu, Shou-Tian Zheng, Xianhui Bu, Shahab Derakhshan ⇑ Department of Chemistry and Biochemistry, California State University, Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840-3903, United States article info Article history: Received 12 May 2014 Received in revised form 8 July 2014 Accepted 15 July 2014 Available online 23 July 2014 Keywords: Crystal structure Oxide materials Olivine Gallates Cationic ordering Solid solution abstract CaYGaO 4 was synthesized by conventional solid-state method. Its crystal structure was determined by single-crystal X-ray diffraction technique to be olivine structure type. CaYGaO 4 comprises an orthorhom- bic space group Pnma, with lattice constants of a = 11.3484(1) Å, b= 6.5712(6) Å and c= 5.2818(8) Å. All three cations occupy their specific atomic positions and the structure is fully ordered. The crystallo- graphic ordering scheme in the new gallate system, is different from those reported for the previously studied silicate and germanate olivines. The partial replacement of the Ga 3+ by Fe 3+ ions could be achieved, resulting in a series of solid solutions with the chemical composition of CaYGa 1x Fe x O 4 (0 < x 6 0.50). The unit cell volumes of the doped variants are consistent with Vegard’s law. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Materials with the general formula of ABCO 4 (A = Ca, Sr, Ba, B = Y, Ln, and C = Al, Ga) have been the subjects of many studies due to their various potential technological applications. The vast majority of the above-mentioned family adopts the perovskite related K 2 NiF 4 structure type with the I4/mmm tetragonal space group [1]. The corresponding crystal structure is composed of tetragonally elongated CO 6 octahedra, which share their equatorial corners and form layers within the ab crystallographic plane. The other cationic position (K sites) with ninefold coordination envi- ronment is randomly filled by A 2+ and B 3+ cations, which are responsible for connecting the perovskite layers along the c direc- tion [2]. The possibility of partial ordering between A 2+ and B 3+ cat- ions has been investigated for CaNdAlO 4 , SrLaAlO 4 , and SrNdGaO 4 [3,4]. The lattice parameters and thermal expansion coefficients of these crystals match suitably with those of high temperature superconductors (HTSC), which make these materials promising candidates as substrates for HTSC thin film epitaxy [5–8]. More- over, tetragonal ABCO 4 crystals doped with proper luminescent ions have been considered as effective laser active materials [9]. Several phases with chemical composition of CaLnGaO 4 (Ln = rare earth element) are known which crystallize in olivine structure type with orthorhombic crystal systems. However, under high-temperature or high-pressure condition, many of them undergo an irreversible phase transition to K 2 NiF 4 I4/mmm structure type [2,10]. For instance, Li et al. have successfully syn- thesized both o-CaLaGaO 4 and t-CaLaGaO 4 phases. They obtained the orthorhombic phase in Pna2 1 polar space group by heating the reactants at 1150 °C. However, when the sample was heated at higher temperature (1350 °C) and was subsequently air quenched to room temperature, a distorted form of K 2 NiF 4 struc- ture type with I4mm space group was formed. In the latter variant, the gallium atoms deviate from the center of octahedra, which is the origin of ferroelectric property [11]. On the other hand, CaNd- GaO 4 has been characterized with Cmca space group, where Ca 2+ and Nd 3+ ions are not ordered and are statistically distributed in crystal structure [12]. The olivine itself (Mg 2x Fe x SiO 4 ), which is the main constituent of the upper mantle, crystallizes in Pnma space group. The structure comprises two cationic sites with octa- hedral coordination sphere (M1 on 4a special position with  1 site symmetry and M2 on 4c special position with site symmetry m) and one cationic position with tetrahedral environment, M3, which is also denoted as 4c in Pnma setting. In olivine crystal structure Mg 2+ and Fe 2+ ions randomly occupy two distinct octahedral posi- tions and Si 4+ cations reside in tetrahedral voids. In calcium-based silicate olivines, Ca 1+x M 1x SiO 4 , in ambient condition it is known that the larger divalent cations have stronger tendency to occupy the M2 positions, while smaller cations show more preference for M1 sites [13]. Several work on in-situ experiments to understand the cation ordering in silicate olivines at elevated temperatures have revealed that such orderings are altered by changing the tem- perature. For instance, above 900 K Fe 2+ exhibits higher preference to occupy the M2 site in Mg 2x Fe x SiO 4 [14,15]. Alternatively, Mn 2+ has higher selectivity towards the M2 sites in Mg 2x Co x SiO 4 at low http://dx.doi.org/10.1016/j.jallcom.2014.07.121 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: shahab.derakhshan@csulb.edu (S. Derakhshan). Journal of Alloys and Compounds 616 (2014) 340–344 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom