The Aurivillius family of bismuth layer-structured ferroelectrics (BLSFs) [1,2] has received significant attention in recent times owing to their potential in applications including non-volatile random access memory (NvRAMs) [3][4][5][6]. The BLSFs present relatively low dielectric permittivity, low temperature coefficients of dielectric and piezoelectric properties, low aging rate, and strong anisotropic electromechanical coupling factors. These features, together with their high Curie temperature (T c ), make BLSFs attractive in high temperature and high frequency resonance applications [7][8][9][10][11][12][13][14].However, the piezoelectric activities in high Curie temperature BLSFs are very low, with values less than 20 pC/N found for pure and modified Bi O 15 , even adopting advanced technique, such as hot-forging, templated grain growth, and spark plasma sintering [7][8][9][10][11][12][13][14]. To improve the piezoelectric activity, the introduction of A-site modifications allowing ease of polarization was proposed [15]. The four-layer Aurivillius phase piezoelectric ceramics Na 0.5 Bi 4.5 Ti 4 O 15 (NBT) is chosen herein due to its high Curie temperature, >650 °C. It is well known that CeO 2 is commonly used as an effective additive in enhancing the piezoelectric properties of lead-based ceramics [16], therefore, CeO 2 was selected as dopant to show its effect on improving the piezoelectric properties of the NBT ceramics. In the present letter, ordinarily fired A-site cerium modified NBT BLSFs were fabricated by a conventional solid state processing and the piezoelectric properties were investigated.The pure NBT ceramics were produced using a conventional mixed oxide processing route procedure. The starting materials used were analytical grade Bi 2 O 3 (99.8% purity), Na 2 CO 3 (99.8%), and TiO 2 (99.9%). The compositions were mixed by ball-milling, and then sintered at 800 °C for 3 h. After calcinations, the ball-milled powders were pressed into discs and then sintered at 1130 °C for 3 h by the ordinarily fired method. The Aurivillius phase NBT ceramics with A-site cerium modification were also produced under the same conditions. Cerium at different amount up to 1 wt% was added in the form of CeO 2 (99.9%) to the raw materials after calcination according toThe cerium modified sodium bismuth titanate (Na 0.5 Bi 4.5 Ti 4 O 15 , NBT) piezoelectric ceramics have been prepared by using the conventional mixed oxide method. X-ray diffraction analysis revealed that the cerium modified NBT ceramics have a pure four-layer Aurivillius phase structure. The piezoelectric activity of NBT ceramics was found significantly improved by the modification of cerium. The Curie temperature T c , and piezoelectric coefficient d 33 for the NBT ceramics with 0.50 wt% cerium modification were found to be 655 °C, and 28 pC/N respectively. The Curie temperature gradually decreased from 668 °C to 653 °C with the increase of cerium modification. The dielectric spectroscopy showed that the samples possess stable piezoelectric propertie...

A potential superhard 𝑜-BC4N with 𝐼𝑚𝑚2 space group is identified by ab initio evolutionary methodology using CALYPSO code. The structural, electronic and mechanical properties of 𝑜-BC4N are investigated. The elastic calculations indicate that 𝑜-BC4N is mechanically stable. The phonon dispersions imply that this phase is dynamically stable under ambient conditions. The structure of 𝑜-BC4N is more energetically favorable than 𝑔-BC4N above the pressure of 25.1 GPa. Here 𝑜-BC4N is a semiconductor with an indirect band gap of about 3.95 eV, and the structure is highly incompressible with a bulk modulus of 396.3 GPa and shear modulus of 456.0 GPa. The mechanical failure mode of 𝑜-BC4N is dominated by the shear type. The calculated peak stress of 58.5 GPa in the (100)[001] shear direction sets an upper bound for its ideal strength. The Vickers hardness of 𝑜-BC4N reaches 78.7 GPa, which is greater than that of 𝑡-BC4N and 𝑏𝑐-BC4N proposed recently, confirming that 𝑜-BC4N is a potential superhard material.

Via structural searching methodology and first-principles calculations, we predicted two new BC6N allotropes, a C-centered monoclinic BC6N (Cm-BC6N) and a primitive-centered monoclinic BC6N (Pm-BC6N). The lattice vibrations, elastic properties, ideal strength, theoretical hardness, and electronic structure of the predicted BC6N were investigated systematically. Our results reveal that Cm-BC6N is more favorable energetically than graphite-like g-BC6N above 20.6 GPa, which is lower than the transition pressures of r-BC6N, t-BC6N, and Pm-BC6N. Both Cm-BC6N and Pm-BC6N are indirect semiconductors with band gaps of 2.66 eV and 0.36 eV, respectively. Cm-BC6N exhibits the excellent ideal shear strength of 53.9 GPa in (011)[01 1 ¯ ], much greater than that of Pm-BC6N (25.0 GPa in (010)[101] shear direction), and Cm-BC6N shows a much lower anisotropy in shear strength than Pm-BC6N. The Vickers hardness of Cm-BC6N is estimated to be above 80 GPa, which is more outstanding than those of t-BC6N and r-BC6N.

The microRaman scattering of 4H-SiC films, fabricated by low pressure chemical vapor deposition under different growth conditions, is investigated at temperatures ranging from 80 K to 550 K. The effects of growth conditions on E2(TO), E1(TO) and A1(LO) phonon mode frequencies are negligible. The temperature dependences of phonon linewidth and lifetime of E2(TO) modes are analyzed in terms of an anharmonic damping effect induced by thermal and growth conditions. The results show that the lifetime of E2(TO) mode increases when the quality of the sample improves. Unlike other phone modes, Raman shift of A1 (longitudinal optical plasma coupling (LOPC)) mode does not decrease monotonously when the temperature increases, but tends to blueshift at low temperatures and to redshift at relatively high temperatures. Theoretical analyses are given for the abnormal phenomena of A1(LOPC) mode in 4H-SiC.

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