Comprehensive understanding of the crystal structure of perovskite-type Ba3Y4O9 with Zr substitution: a theoretical and experimental study.

We previously reported that Zr substitution improves the chemical stability of Ba3Y4O9 and nominally 20 mol% Zr-substituted Ba3Y4O9 is an oxide-ion conductor at intermediate temperatures (500-700 °C). However, the influence of Zr substitution on the structural properties of Ba3Y4O9 was poorly understood. This paper aims to comprehensively understand the crystal structure of Ba3Y4O9 with Zr substitution by powder X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) measurements, and first-principles calculations. From the results, firstly we found that the hexagonal unit cell of Ba3Y4O9 reported in the database should be revised as doubled along the c-axis in terms of the periodicity of oxide-ion positions. The revised unit cell of Ba3Y4O9 consists of 18 layers of BaO3 and 24 layers of Y which periodically stack along the c-axis. In this work, we focused on the cationic lattice and noticed that the periodical stacking of Ba and Y layers comprises a similar sequence to that in the body-centered cubic (BCC) structure. There are two regions in the Ba3Y4O9 structure: one is a hetero-stacking region of Ba and Y layers (Ba-Y-Ba-Y-Ba) and the other is a homo-stacking region (Ba-Y-Y-Ba). It is noteworthy that the former region is similar to a cubic perovskite. In Zr-substituted Ba3Y4O9, Zr ions preferentially substitute for Y ions in the hetero-stacking region, and therefore the local environment of Zr ions in Ba3Y4O9 is quite similar to that in BaZrO3. Besides, the Zr substitution for Y in Ba3Y4O9 increases the fraction of the cubic-perovskite-like region in the stacking sequences. The structural change in the long-range order strongly affects the other material properties such as chemical stability and the ionic-conduction mechanism. Our adopted description of perovskite-related compounds based on the stacking sequence of the BCC structure should help in understanding the complex structure and developing new perovskite-related materials.

Theoretical and Experimental Studies on the Ability of Intracrystalline Pores of ß-La2(SO4)3 To Accommodate Various Gas Species with a Special Focus on Ammonia Insertion Behaviors.

ß-La2(SO4)3 is a microporous inorganic crystal with one-dimensional perforated pores where H2O molecules can be inserted. To evaluate the nature of the pores and extend the application range, we investigate the ability to accommodate various hydrogen compound molecules XHn (CH4, NH3, HF, H2S, HCl, and HI) by insertion. The stable structures of the XHn molecules in the pores of ß-La2(SO4)3 and the change in the Gibbs energy for XHn insertion ΔinsertG (T) are estimated by first-principles calculations. The guest XHn molecules are stabilized by forming H-O and X-La bonds with the ß-La2(SO4)3 host structure. Based on the values of ΔinsertG (T), NH3, H2O, and HF are energetically stable in the crystal even above 0 °C. Correspondingly, thermogravimetry (TG) of ß-La2(SO4)3 in NH3, CH4, and CO2 gases revealed that NH3 can be inserted into ß-La2(SO4)3 below 360 °C, but CH4 and CO2 cannot. Unlike the case of H2O insertion, NH3 insertion proceeds via two steps. The first step is a single-solid-phase reaction of ß-La2(SO4)3·yNH3, where NH3 molecules are inserted into the host structure with a continuously changing nonstoichiometric y value between 0 and 0.1. The second step is a two-solid-phase reaction between ß-La2(SO4)3·0.1NH3 and ß'-La2(SO4)3·0.3NH3, which is a phase formed after further NH3 insertion into ß-La2(SO4)3·0.1NH3 with a minor change in the host structure. The fact that both NH3 and H2O can be inserted confirms that the pores of ß-La2(SO4)3 allow for the insertion of molecules with a strong polarity. This nature is similar to zeolites and metal-organic frameworks (MOFs) with polar surfaces in the pores.

Experimental Study of Hydration/Dehydration Behaviors of Metal Sulfates M2(SO4)3 (M = Sc, Yb, Y, Dy, Al, Ga, Fe, In) in Search of New Low-Temperature Thermochemical Heat Storage Materials.

To identify potential low-temperature thermochemical heat storage (TCHS) materials, hydration/dehydration reactions of M2(SO4)3 (M = Sc, Yb, Y, Dy, Al, Ga, Fe, In) are investigated by thermogravimetry (TG). These materials have the same rhombohedral crystal structure, and one of them, rhombohedral Y2(SO4)3, has been recently proposed as a promising material. All M2(SO4)3·xH2O hydrate/dehydrate reversibly between 30 and 200 °C at a relatively low p H2O (=0.02 atm). Among them, rare-earth (RE) sulfates RE2(SO4)3·xH2O (RE = Sc, Yb, Y, Dy) show narrower thermal hystereses (less than 50 °C), indicating that they have faster reaction rates than the other sulfates M2(SO4)3·xH2O (M = Al, Ga, Fe, In). As for the heat storage density, Y2(SO4)3·xH2O is most promising due to the largest mass change (>10 mass % anhydrous basis) during the reactions. This is larger than that of the existing candidate CaSO4·0.5H2O (6.6 mass % anhydrous basis). Regarding the reaction temperature of the water insertion into rhombohedral RE2(SO4)3 (RE = Yb, Y, Dy) to form RE2(SO4)3·H2O, it increases as the ionic radius of RE3+ becomes larger. Since such a relationship is also observed in ß-RE2(SO4)3·xH2O, RE(OH)3, and REPO4·xH2O, this empirical knowledge should be useful to expect the dehydration/hydration reaction temperatures of the RE compounds.

Multi-step hydration/dehydration mechanisms of rhombohedral Y2(SO4)3: a candidate material for low-temperature thermochemical heat storage.

To evaluate rhombohedral Y2(SO4)3 as a new potential material for low-temperature thermochemical energy storage, its thermal behavior, phase changes, and hydration/dehydration reaction mechanisms are investigated. Rhombohedral Y2(SO4)3 exhibits reversible hydration/dehydration below 130 °C with relatively small thermal hysteresis (less than 50 °C). The reactions proceed via two reaction steps in approximately 0.02 atm of water vapor pressure, i.e. "high-temperature reaction" at 80-130 °C and "low-temperature reaction" at 30-100 °C. The high-temperature reaction proceeds by water insertion into the rhombohedral Y2(SO4)3 host structure to form rhombohedral Y2(SO4)3·xH2O (x = ∼1). For the low-temperature reaction, rhombohedral Y2(SO4)3·xH2O accommodates additional water molecules (x > 1) and is eventually hydrated to Y2(SO4)3·8H2O (monoclinic) with changes in the host structure. At a water vapor pressure above 0.08 atm, intermediate Y2(SO4)3·3H2O appears. A phase stability diagram of the hydrates is constructed and the potential usage of Y2(SO4)3 for thermal energy upgrades is assessed. The high-temperature reaction may act similarly to an existing candidate, CaSO4·0.5H2O, in terms of reaction temperature and water vapor pressure. Additionally, the hydration of rhombohedral Y2(SO4)3·xH2O to Y2(SO4)3·3H2O should exhibit a larger heat storage capacity. With respect to the reaction kinetics, the initial dehydration of Y2(SO4)3·8H2O to rhombohedral Y2(SO4)3 introduces a microstructure with pores on the micron order, which might enhance the reaction rate.

Discovery of Rapid and Reversible Water Insertion in Rare Earth Sulfates: A New Process for Thermochemical Heat Storage.

Thermal energy storage based on chemical reactions is a prospective technology for the reduction of fossil-fuel consumption by storing and using waste heat. For widespread application, a critical challenge is to identify appropriate reversible reactions that occur below 250 °C, where abundant low-grade waste heat and solar energy might be available. Here, it is shown that lanthanum sulfate monohydrate La2 (SO4 )3 ⋅H2 O undergoes rapid and reversible dehydration/hydration reactions in the temperature range from 50 to 250 °C upon heating/cooling with remarkably small thermal hysteresis (<50 °C), and thus it emerges as a new candidate system for thermal energy storage. Thermogravimetry and X-ray diffraction analyses reveal that the reactions proceed through an unusual mechanism for sulfates: water is removed from, or inserted in La2 (SO4 )3 ⋅H2 O with progressive change in hydration number x without phase change. It is also revealed that only a specific structural modification of La2 (SO4 )3 exhibits this reversible dehydration/hydration behavior.