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Quaternary metal‐chalcogenides combining rare‐earth cations with late transition metal cations are attracting growing attention for their optical properties, such as for solar energy conversion or second harmonic generation. Synthetic explorations of theII3‐I2‐IV2‐Ch8family (II = Eu;I = Cu or Ag;IV = Si, Ch = S or Se) have yielded Eu3Ag2Si2S8(1) and Eu3Cu1.08(1)Si2.42(1)Se8(2). Their structures have been characterized by X‐ray diffraction to form in the noncentrosymmetric space groupI3dand to exhibit two distinct types of mixed‐site occupancies, for the Ag(I) cations in1and mixed Cu(I)/Si(IV) cations in2. In both, the cation disorder occurs to achieve charge balancing with the chalcogenide anions. A high yield of1can be achievedwith optical measurements showing indirect and direct band transitions of ≈2.2(1) and ≈2.4(1) eV, respectively. Its second harmonic generation response is found to be relatively strong, approximately 0.9 × AgGaS2, confirming its noncentrosymmetric structure. Band structure calculations reveal the valence and conduction band edges stem predominantly from the filled Ag(I)/Cu(I)‐based states and empty Si(IV)‐based states, respectively, with additional contributions from the chalcogenide anions. Calculation results also show that cation disorder facilitates a reduction in the antibonding interactions between the Ag(I)/Cu(I)d‐based and chalcogenidep‐based states.
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Cation exchange route to a Eu(II)-containing tantalum oxide
Traditional synthetic efforts to prepare Eu(II)-containing oxides have principally involved the use of high temperature reactions starting from EuO or a controlled, highly-reducing, atmosphere. Conversely, chimie douce approaches that are more amenable to the targeted syntheses of new, and potentially metastable, Eu(II)-oxides have yet to be explored. Herein, a cation-exchange route to new Eu(II)-containing oxides, e.g., EuTa4-xO11 (x = 0.04), has been discovered and its structure determined by powder X-ray diffraction (Space group P6322 (#182), a = 6.2539(2) Å; c = 12.3417(2) Å). The compound derives from the cation exchange of Na2Ta4O11, via a reaction with EuBr2 at 1173 K, and replacement by half the number of divalent Eu cations. Rietveld refinements show preferential ordering of the Eu cations over one of the two possible cation sites, i.e., Wyckoff site 2d (~94%; Eu1) versus 2b (~6%; Eu2). Total energy calculations confirm an energetic preference of the Eu cation in the 2d site. Tantalum vacancies of ~1% occur within the layer of Eu cations and TaO6 octahedra, and ~20% partial oxidation of Eu(II) to Eu(III) cations from charge balance considerations. 151Eu M¨ossbauer spectroscopy measured at 78 K found a Eu(II):Eu(III) ratio of 69:31, with a relatively broad line width of the former signal of Γ = 7.6(2) mm s–1. Also, the temperature-dependent magnetic susceptibility could be fitted to a Curie Weiss expression, giving a μeff = 6.2 μB and θCW = 10 K and confirming a mixture of Eu(II)/Eu(III) cations. The optical bandgap of EuTa4-xO11 was found to be ~1.5 eV (indirect), significantly redshifted as compared to ~4.1 eV for Na2Ta4O11. Spin-polarized electronic structure calculations show that this redshift stems from the addition of Eu 4f7 states as a higher-energy valence band. Thus, these results demonstrate a new cation-exchange approach that represents a useful synthetic pathway to new Eu(II)-containing ox- ides for tunable magnetic and optical properties.
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- Award ID(s):
- 2317605
- PAR ID:
- 10469613
- Publisher / Repository:
- ScienceDirect
- Date Published:
- Journal Name:
- Journal of Solid State Chemistry
- Volume:
- 328
- Issue:
- C
- ISSN:
- 0022-4596
- Page Range / eLocation ID:
- 124338
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.jssc.2023.124338
