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Several Na-S solid phases have been considered to be produced in low-temperature Na/S batteries during chargedischarge reactions. We study structural and electronic properties of ¡-S and Na 2 S x (x = 1, 2, 4, and 5) crystals using first-principles calculations. Calculated structural properties are in good agreement with experiments, and the van der Waals interactions have an essential role on the cohesive properties in ¡-S. The results of energetics and electronic structures show that all the calculated Na-S crystal phases are insulating and are stable relative to the elemental Na and S phases. We estimate voltage curves as a function of the Na concentration in the S-cathode of all-solid Na/S batteries, showing mainly three voltage regions at 2.

Longitudinal piezoelectric constant (e 33 ) values of wurtzite materials, which are listed in a structure database, are calculated and analyzed by using first-principles and statistical learning methods. It is theoretically shown that wurtzite materials with high e 33 generally have small lattice constant ratios (c/a) almost independent of constituent elements, and approximately expressed as e 33 / c/a % (c/a) 0 with ideal lattice constant ratio (c/a) 0 . This relation also holds for highly-piezoelectric ternary materials such as Sc x Al 1%x N. We conducted a search for high-piezoelectric wurtzite materials by identifying materials with smaller c/a values. It is proposed that the piezoelectricity of ZnO can be significantly enhanced by substitutions of Zn with Ca. © 2018 The Japan Society of Applied Physics I n recent times, owing to their suitable mechanical and semiconducting properties, piezoelectric wurtzite semiconductors such as ZnO and GaN have received a lot of attention in the form of piezotronics and piezo-phototronics device materials.1) The wurtzite-type piezoelectric materials, especially AlN, have another advantage that these can be used in high-temperature environments, e.g., as sensors in automobile engines, because their noncentrosymmetric structures are stable even at high temperatures.2,3) However, the piezoelectric constants of wurtzite-type materials are generally much smaller than those of the perovskite-based materials such as Pb(Zr x Ti 1−x )O 3 , known as PZT, by few orders. It remains a challenge to explore better piezoelectric wurtzite materials; there are many reports aiming to enhance piezoelectricity by element doping or codoping into parent materials. 4-7)Among the wurtzite materials, the highest piezoelectricity has been experimentally discovered for Sc x Al 1−x N (about 25 pC=N for x ∼ 0.5).2,3) First-principles calculations have shown the microscopic effects of Sc on the piezoelectricity in Sc x Al 1−x N by demonstrating an enhancement in piezoelectricity with increase in x. 8,9) Tasnádi et al. have revealed that the piezoelectric enhancement comes from the large responses of atomic positions to external strain and the significant softening of elastic constant. 8) However, novel materials which are superior to Sc x Al 1−x N have not been synthesized yet as there are no clear and general material-design criteria practically usable for enhancing the piezoelectricity of wurtzite materials. It has been proposed theoretically that there exists a relationship between piezoelectricity and microscopic structures, 10) but the physical mechanism behind the relation with intricate parameters is not fully understood yet. Therefore, better understanding of piezoelectricity in wurtzite materials might lead to simple design criteria making it possible to find a controllable key parameter describing wurtzite piezoelectricity with the help of first-principles and machine learning techniques which have generated a great interest in the recent times. 11,12) In this study, we calculate longitudinal...

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We theoretically investigate the piezoelectricity of ScxAl1−xN in the entire range of x by first-principles calculations. We find that the piezoelectric constants of wurtzite-type ScxAl1−xN significantly enhance as x increases from 0 to 0.75. However, the energy stability analyses between structure phases show that the cubic-type phases become more stable than the wurtzite-type phases at x of approximately 0.5 and higher, interfering with the ability of wurtzite-type ScxAl1−xN to realize the maximum piezoelectricity. Moreover, our study on element combination dependences on piezoelectricity in A0.5B0.5N (A = Sc, Y, La and B = Al, Ga, In) indicates that Sc, Y, and La have the strongest effect on the enhancement of piezoelectric constants in AlN, GaN, and InN, respectively.

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