High Pressure X-Ray Diffraction Study of Sodium Oxide (Na2o): Observations of Amorphization and Equation of State Measurements to 15.9 Gpa
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Journal of Alloys and Compounds 823 (2020) 153793 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom High pressure X-ray diffraction study of sodium oxide (Na2O): Observations of amorphization and equation of state measurements to 15.9 GPa Xiaoxin Wu a, b, Yan Zhang a, b, Junkai Zhang a, b, Ran Liu c, Jinghai Yang a, b, Bin Yang d, * Hongxin Xu a, b, Yanz Ma a, b, a Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun, 130103, China b College of Physics, Jilin Normal University, Siping, 136000, China c State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, China d Center for High Pressure Science and Technology Advanced Research (HPSTAR), Changchun, 130012, China article info abstract Article history: We have investigated the high-pressure behavior of sodium oxide (Na2O) up to 30 GPa by synchrotron Received 31 August 2019 angle-dispersive powder X-ray diffraction in a diamond anvil cell at room temperature. Between 15.9 and Received in revised form 17.3 GPa crystalline Na2O transforms to an amorphous state, and on decompression the amorphous 8 January 2020 structure recrystallizes back between 4.9 and 10.3 GPa. The zero-pressure bulk modulus of Na Ois Accepted 9 January 2020 2 obtained in experimental observations for the first time. The pressure-volume data is fitted according to Available online 11 January 2020 the third-order Birch-Murnaghan equation of state and yields four bulk modulus values by setting the V0 or B ’ either as a constant or variable. The bulk modulus values of Na O are smaller than a-Li O, which is Keywords: 0 2 2 High-pressure in good agreement with the previous calculation results. © Alkali-metal oxide 2020 Elsevier B.V. All rights reserved. Bulk modulus Amorphous state 1. Introduction Alkali-metal oxides have already been studied extensively by a variety of theoretical methods to investigate the lattice constants, Alkali-metal oxides are the materials with potential techno- elastic properties, band structures, superionic behavior, structural logical utility due to their remarkable and interesting physical and optical properties [9e13]. Nevertheless, the study based on properties, which could be used in catalytic reactions [1,2], solid- high pressure experiment is limited and always focused on Li2O. state batteries [3,4], gas storage [5], gas detectors [6], and fuel Kunc et al. identified a high pressure phase transition of Li2O in the cells [7]. The X2O(X¼ Li, Na, K and Rb) compounds crystallize in the range of 45e50 GPa by powder X-ray diffraction [14]. Lazicki et al. cubic anti-fluorite structure (space group Fm3m) [8]. The crystal exhibited the concrete data of the anti-fluorite a-Li2O structure and structure and atomic fractional coordinates are shown in Fig. 1 and the anti-cotunnite b-Li2O structure by high pressure powder X-ray Table 1. In alkali metal oxides, the cation and anion sublattices have diffraction and Raman scattering [15]. However, there is hardly any different symmetries. The negative oxygen ions form a face- high pressure experimental study on other alkali-metal oxides. centered (FCC) lattice and occupy the corner positions, while the Sodium oxide (Na2O) plays an important role among the alkali- positive alkali-metal ions form a simple cubic lattice and are situ- metal oxides as the mantle’s sixth abundant oxide [16]. Na2Ois ated in oxygen tetrahedral. Compared with fluorite type-structure not stable in nature, and it is difficult to prepare and preserve at in CaF2, the interchange of cation and anion positions lead to the ambient conditions. Despite the role it plays in geosciences and its formation of large cavities in the crystal structure. potential technological utility like other alkali-metal oxides, little experimental work has been reported on Na2O. Zintl et al. deter- mined that the lattice parameter of Na2O crystal is 5.55 Å using powder diffraction [17]. Barrie et al. reported measurements on Na * Corresponding author. Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun, 130103, metal and on Na2O using XPS, X-ray excited Auger spectroscopy and China. electron excited Auger spectroscopy [18]. More concrete data on E-mail address: [email protected] (Y. Ma). https://doi.org/10.1016/j.jallcom.2020.153793 0925-8388/© 2020 Elsevier B.V. All rights reserved. 2 X. Wu et al. / Journal of Alloys and Compounds 823 (2020) 153793 6e13.7 GPa, and 200s above 15.9 GPa, respectively. All measure- ments were made at room temperature. The software FIT2D was used to integrate the diffraction pattern image into a 2-theta rela- tive intensity. Material Studio software was used to build the crystal structure and calculate the XRD patterns of Na2O and NaOH-I. Selected high-pressure synchrotron XRD patterns were fitted by Rietveld method [27], using the GSAS software [28]. The Rietveld refinement allows crystal structural information to be obtained from powder diffraction data. The peaks of the gasket have been removed before refnements using peakfit software. 3. Results and discussion Representative high-pressure XRD patterns of the sample at 0.5 GPa are shown in Fig. 2. The diffraction peaks of the Na2O with cubic Fm3m symmetry and the NaOH-I (marked with I in Fig. 2) with orthorhombic Cmcm symmetry are denoted with Miller Fig. 1. FCC crystal structure of X2O(X¼ Li, Na, K and Rb). indices. A small portion of NaOH is contained in the sample and several peaks are merged into the XRD patterns of gasket, only a part of diffraction peaks of NaOH-I can be observed. The corre- Table 1 sponding lattice constants are a ¼ 5.544 ± 0.002 Å for Na O and ¼ 2 Atomic Wyckoff positions and fractional coordinates of X2O(X Li, Na, K and Rb) at ¼ ± ¼ ± ¼ ± ambient conditions. a 3.304 0.003 Å, b 3.304 0.003 Å, c 11.32 0.02 Å for NaOH-I, in consistence with the previously reported values under Atom Wyckoff x y z ambient conditions [17,29]. X2OO 4a 000 The sample was compressed up to 30.1 GPa at room tempera- X8h 0.25 0.25 0.25 ture. Fig. 3 displays the representative XRD patterns at selected pressures. The diffraction peaks of Na2O (marked with Miller indices) are observable up to 15.9 GPa but all disappear at 17.3 GPa. cohesive energy, equilibrium geometry, lattice constant, bulk No new diffraction peak is observed above 15.9 GPa, except the modulus, and G-point phonon, electronic structure, ground-state remaining diffraction peaks of NaOH-II and the gasket, which in- properties and quasiparticle band structures of Na2Ohavebeen dicates Na2O transforms from a crystalline structure to an amor- obtained from theoretical calculations by various methods including phous state between 15.9 and 17.3 GPa. The transformation e LCAO, HF, DFT, TB-LMTO, LDA, GGA, etc [9,10,19 26]. These theo- pressure (15.9 GPae17.3 GPa) of Na2O is lower than that of Li2O retical calculation results still need the experimental support. (50 ± 5 GPa) [15], and analogous to alkali-metal sulfides, of which In this work, we present the high pressure structural behavior of transformation pressures decrease with increasing cation size (Li2S Na2O by angle-dispersive X-ray diffraction at room temperature up (12.5 GPa) [30], Na2S(7GPa)[31], K2S (2.7 GPa) [32], and Rb2S to 30.1 GPa. An amorphous transformation of Na2O has been (~2 GPa) [32]). However, the above-mentioned alkali-metal sulfides fl observed. The bulk modulus of cubic anti- uorite Na2O has been and Li2O have been shown or predicted to undergo a trans- fi investigated by experiments for the rst time and compared with formation from antifluorite to anticotunnite structure (Cs2S has an previous calculations. anticotunnite structure at ambient conditions). But in our work, no new diffraction peak or appreciable drop in signal can be observed 2. Experimental (the intensity of NaOH diffraction peaks show no sign of abating). A similar trend among two compounds with cations from the same The sample is acquired commercially from Alfa Aesar. It consists main group and the same anions, which one changes into a of 80% Na2O and 20% Na2O2 in the powdered form. The Na2O2 amorphous state but another dose not can also be observed in transformed to NaOH during the sample loading process due to the Ca(OH)2 and Mg(OH)2 [33]. The crystalline Ca(OH)2 transforms into presence of moisture (2Na2O2 þ 2H2O ¼ 4NaOH þ O2[). The a glassy state between 10.7 and 15.4 GPa, but Mg(OH)2 does not vaporized oxygen has been detected in the golvebox used for amorphize up to the highest pressures [34]. Therefore, unlike Li2O, loading; and NaOH has been confirmed by its diffraction patterns as it could be possible for Na2O to transforms into an amorphous state shown in the later section of this paper. High-pressure experiments at the pressure between 15.9 and 17.3 GPa. were performed in a symmetric diamond anvil cell (DAC) with flat The diffraction peaks marked I and II represent NaOH-I and anvil of 500 mm in diameter. A T301 steel sheet was served as gasket NaOH-II, respectively. Beck [29] and Loveday [35] reported that with a drilled hole of 200 mm in diameter and 50 mm in thickness. NaOH exhibits a structural phase transition from orthorhombic The Na2O samples and a ruby ball were placed in the sample (Cmcm, NaOH-I) to orthorhombic (Pbcm, NaOH-II) structure at chamber.