We found that the high-pressure-synthesized material LiOsO 3 (see Supplementary Information) shows a structural transition at a temperature T s = 140 K. The room-temperature crystal structure of LiOsO 3 was initially examined using powder X-ray diffraction (XRD). The Goldschmidt diagram predicts that LiOsO 3 crystallizes into a LiNbO 3 -type structure 3,10 , and a preliminary refinement of the structure was carried out in the R-3c space group with Os at the 6b site 0,0,0 and O at the 18e siteTo investigate the position of the Li ion we turned to neutron diffraction, which is much more sensitive to Li than XRD. The neutron diffraction patterns collected above T s could be successfully described in the R-3c space group, in agreement with the XRD refinement, with the Li ion at the 6a position 0,0,1/4. Atomic absorption spectrometry (see Supplementary Information) indicated that the average Li mass was 2.77%, which corresponds to the composition Li 0.98 OsO 3 . We have used the stoichiometric composition throughout the structural analysis. The refinement indicated highly anisotropic thermal displacements of the Li ions with considerable extension along the c-axis (Table 1 and Fig. 1), which might indicate that the Li ions are distributed equally among equivalent 12c sites 0,0,z and 0,0,1/2-z either side of the oxygen layer at z = 1/4, as reported for LiNbO 3 and LiTaO 3 (refs 3, 11).The thermal variation of the structure of LiOsO 3 was studied by neutron diffraction for temperatures between 10 and 300 K. Figure 1a-d shows structural data obtained from refinements in the R-3c space group. The lattice parameters ( Fig. 1a) decrease uniformly from 300 K until T s = 140 K, below which the parameter c increases and a decreases with only a small variation in the unit-cell volume. Just below T s , the non-symmetry-breaking strain components e xx + e yy and e zz vary almost linearly (Fig. 1b). These 4 results show that the phase transition is continuous and the strain components behave like a secondary order parameter coupled to a primary one via a linear-quadratic free energy invariant 12 . The primary order parameter must necessarily be symmetry-breaking according to Landau's theory of second-order phase transitions 12 . Furthermore, the anisotropic thermal parameter  33 , which describes Li displacements along the c-axis, increases markedly below T s (Fig. 1c). This indicates that the primary structural instability involves the position of the Li ions along the c-axis (Fig. 1d).Given that the phase transition involves a change in symmetry, we find from representation theory 13 that there are three isotropy subgroups, R-3, R32 and R3c, which maintain the translational invariance of the R-3c space group and allow the transition to be continuous. These space groups were tested by refinement against the neutron diffraction data at 10 K. Note that R-3 and R32 should generate additional reflections below T s which were not observed in the experiment. The refinement in the non-centrosymmetric R3c space group gave the best de...

We successfully synthesized a novel ferroelectric A-site-ordered double perovskite CaMnTi2O6 under high-pressure and investigated its structure, ferroelectric, magnetic and dielectric properties, and high-temperature phase transition behavior. Optical second harmonic generation signal, by frequency doubling 1064 nm radiation to 532 nm, was observed and its efficiency is about 9 times as much as that of SiO2 (α-quartz). This compound possesses a tetragonal polar structure with space group P42 mc. P-E hysteresis measurement demonstrated that CaMnTi2O6 is also ferroelectric. A spontaneous polarization calculated by use of point charge model and the observed remnant polarization are 24 and 3.5 μC/cm2, respectively. CaMnTi2O6 undergoes a ferroelectric–paraelectric order–disorder-type phase transition at 630 K. The structural analysis implies that both the ordering of shift of Mn2+ from the square-planar and the off-center displacement of Ti4+ in TiO6 octahedra are responsible for ferroelectricity. CaMnTi2O6 belongs to a new class of ferroelectrics in which A-site ordering and second-order Jahn–Teller distortion are cooperatively coupled. The finding gave us a new concept for the design of ferroelectric materials.

A polar LiNbO3-type (LN-type) titanate ZnTiO3 has been successfully synthesized using ilmenite-type (IL-type) ZnTiO3 under high pressure and high temperature. The first principles calculation indicates that LN-type ZnTiO3 is a metastable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement using synchrotron powder X-ray diffraction data reveals that LN-type ZnTiO3 crystallizes into a hexagonal structure with a polar space group R3c and exhibits greater intradistortion of the TiO6 octahedron in LN-type ZnTiO3 than that of the SnO6 octahedron in LN-type ZnSnO3. The estimated spontaneous polarization (75 μC/cm(2), 88 μC/cm(2)) using the nominal charge and the Born effective charge (BEC) derived from density functional perturbation theory, respectively, are greater than those of ZnSnO3 (59 μC/cm(2), 65 μC/cm(2)), which is strongly attributed to the great displacement of Ti from the centrosymmetric position along the c-axis and the fact that the BEC of Ti (+6.1) is greater than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-type ZnTiO3 is greater than that of LiNbO3 (62 μC/cm(2), 76 μC/cm(2)), indicating that LN-type ZnTiO3, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-type ZnTiO3 is 24 times greater than that of LN-type ZnSnO3. The findings indicate that the intraoctahedral distortion, spontaneous polarization, and the accompanying SHG response are caused by the stabilization of the polar LiNbO3-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d(0) ions such as Ti(4+).

Superconductivity suppression of Ba 0.5 K 0.5 Fe 2−2x M 2x As 2 single crystals by substitution of transition-metal (M = Mn, Ru, Co, Ni, Cu, and Zn) 5 Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan. 6 Department of Physics, Nagoya University, Furo-cho, Nagoya 464-8602, Japan.(Dated: June 6, 2012)We investigated the doping effects of magnetic and nonmagnetic impurities on the singlecrystalline p-type Ba0.5K0.5Fe2−2xM 2xAs2 (M = Mn, Ru, Co, Ni, Cu and Zn) superconductors. The superconductivity indicates robustly against impurity of Ru, while weakly against the impurities of Mn, Co, Ni, Cu, and Zn. However, the present T c suppression rate of both magnetic and and nonmagnetic impurities remains much lower than what was expected for the s±-wave model. The temperature dependence of resistivity data is observed an obvious low-T upturn for the crystals doped with high-level impurity, which is due to the occurrence of localization. Thus, the relatively weak T c suppression effect from Mn, Co, Ni, Cu, and Zn are considered as a result of localization rather than pair-breaking effect in s±-wave model.

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