Calcium and magnesium carbonates are believed to be the host compounds for most of the oxidized carbon in the Earth's mantle. Here, using evolutionary crystal structure prediction method USPEX, we systematically explore the MgO-CO 2 and CaO-CO 2 systems at pressures ranging from 0 to 160 GPa to search for thermodynamically stable magnesium and calcium carbonates. While MgCO 3 is the only stable magnesium carbonate, three calcium carbonates are stable under pressure: well-known CaCO 3 , and newly predicted Ca 3 CO 5 and CaC 2 O 5 . Ca 3 CO 5 polymorphs are found to contain isolated orthocarbonate CO 4 4tetrahedra, and are stable at relatively low pressures (>11 GPa), whereas CaC 2 O 5 is stable above 33 GPa and its polymorphs feature polymeric motifs made of CO 4 -tetrahedra. Detailed analysis of chemical stability of CaCO 3 , Ca 3 CO 5 and CaC 2 O 5 in the environment typical of the Earth's lower mantle reveals that none of these compounds can exist in the Earth's lower mantle. We conclude that MgCO 3 is the main host of oxidized carbon throughout the lower mantle.

A new form of nanosized SrTiO 3 semiconducting oxide material with an average crystal grain size of 27 nm has been synthesized using the physical high-energy ball milling technique. The unique presence of the singly charged "O -" ions in the lattice oxygen sites of the nano-range crystal grain material has been observed in the XPS study. This unprecedented and interesting discovery has expounded on the new oxygen sensing mechanism for the near 1 / 2 slope gradient (the best value ever reported) in the logarithmic relationship of the electrical conductance with the oxygen partial pressure. The derived SrTiO 3 oxygen gas sensors were found to operate at 40 °C (the lowest temperature ever reported for an SrTiO 3 -based sensor) that is near to the human body temperature.

In the present work, the xBi(Fe 1/3 Mo 2/3 )O 4 -(1Àx)BiVO 4 (0.0 # x # 1.0) ceramics were prepared via the solid state reaction method. All the ceramics can be densified at low sintering temperatures around 820 C. At room temperature, the BiVO 4 type scheelite monoclinic solid solution was formed in ceramic samples with a composition of x # 0.10. When x lies between 0.1 and 0.7, a BiVO 4 scheelite tetragonal phase is formed at room temperature. In the range 0.7 # x < 0.9, the ceramic samples were found to be composites consisting of BiVO 4 type tetragonal and Bi(Fe 1/3 Mo 2/3 )O 4 type monoclinic scheelite phases, and when x $ 0.9, the Bi(Fe 1/3 Mo 2/3 )O 4 type monoclinic scheelite solid solution was formed. In the BiVO 4 type monoclinic solid solution region, the phase transition to tetragonal phase was studied by in situ Raman and Far-Infrared spectroscopies and by thermal expansion analysis. All of these methods indicated that the phase transition temperature almost linearly decreased from 255 C for pure BiVO 4 to about À9 C for x ¼ 0.1 sample. High performance microwave dielectric properties with a high permittivity of about 74.8, high Qf values above 11 500 GHz, and a small temperature coefficient of resonant frequency within +20 ppm per C in a wide temperature range of 20-140 C can be obtained in the composite ceramic sample with 60 mol% x ¼ 0.10 composition and 40 mol% x ¼ 0.02 composition. The xBi(Fe 1/3 Mo 2/3 )O 4 -(1Àx)BiVO 4 (0.0 # x # 1.0) ceramics might provide useful candidate materials for microwave integrated capacitive devices, such as filters, antennas, etc.

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The polarization of polar domain in ferroelectric materials is orientated and reversed with the alternating electric field, and the hysteresis loops of polarization-electric field (P-E) and strain-electric field (S-E) are observed. For electrocaloric (EC) effect, the temperature change with the application and removal of electric field is also attributed to the change of polarization with the applied field. In most reports about EC, the temperature change is shown as an abrupt jump or slump due to the applied electric field that is a pulsed wave. Obviously, it is impossible to observe the hysteresis loop of EC. In our research, both sine wave and pulsed wave electric field are applied to samples in direct measurement, and temperature-electric field hysteresis loop (T-E) is observed only in measurement of sine wave. The T-E hysteresis loop displays a shape of butterfly, just like the shape of S-E. The electric field dependence of EC is also given. The obtained results will be helpful for us to know the electrocaloric effect further.

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PbTiO 3 (PTO) is explored as a versatile and tunable electron-selective layer (ESL) for perovskite solar cells. To demonstrate effectiveness of PTO for electron-hole separation and charge transfer, perovskite solar cells are designed and fabricated in the laboratory with the PTO as the ESL. The cells achieve a power conversion efficiency (PCE) of ≈12.28% upon preliminary optimization. It is found that the PTO ferroelectric layer can not only increase the PCE, but also tune the photocurrent via tuning PTO's ferroelectric polarization. Moreover, to understand the physical mechanism underlying the carrier transport by the ferroelectric polarization, the electronic structure of PTO/CH 3 NH 3 PbI 3 heterostructure is computed using the first-principles methods, for which the triplet state is used to simulate charge transfer in the heterostructure. It is shown that the synergistic effect of type II band alignment and the specific ferroelectric polarization direction provide the effective extraction of electrons from the light absorber, while minimize recombination of photogenerated electronhole pairs. Overall, the ferroelectric PTO is a promising and tunable ESL for optimizing electron transport in the perovskite solar cells. The design offers a different strategy for altering direction of carrier transport in solar cells.

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