Crystal structures and chemistry of double perovskites Ba2M(II)M′(VI)O6 (M=Ca, Sr, M′=Te, W, U)

Effects of iron substitution and anti-site disorder on crystal structures, vibrational, optical and magnetic properties of double perovskites Sr2(Fe1−xNix)TeO6

We report a new series of DP Sr2(Fe1−xNix)TeO6, which have different transition metal Fe and Ni on B sites, providing an opportunity to investigate their effect on crystal structure, vibrational, optical and magnetic properties.

On how the mechanochemical and co-precipitation synthesis method changes the sensitivity and operating range of the Ba2Mg1-xEuxWO6 optical thermometer

The suitability of Ba₂MgWO₆ (BMW) double perovskite doped with Eu³⁺ for the construction of an optical thermometer was tested. It has been shown that by controlling the conditions of BMW synthesis, the sensitivity of the optical thermometer and the useful range of its work can be changed. Pure BMW and doped with Eu³⁺ samples were prepared using the mechano-chemical and co-precipitation methods. Both the absolute sensitivity and the relative sensitivity in relation to the synthesis route were estimated. The findings proved that the relative sensitivity can be modulated from 1.17%K⁻¹ at 248 K, to 1.5%K⁻¹ at 120 K for the co-precipitation and the mechanochemical samples, respectively. These spectacular results confirm the applicability of the Ba₂MgWO₆: Eu³⁺ for the novel luminescent sensors in high-precision temperature detection devices. The density-functional theory was applied to elucidate the origin of the host emission.

Study of Polycrystalline Bulk Sr3OsO6 Double-Perovskite Insulator: Comparison with 1000 K Ferromagnetic Epitaxial Films

Polycrystalline Sr₃OsO₆, which is an ordered double-perovskite insulator, is synthesized via solid-state reaction under high-temperature and high-pressure conditions of 1200 °C and 6 GPa. The synthesis enables us to conduct a comparative study of the bulk form of Sr₃OsO₆ toward revealing the driving mechanism of 1000 K ferromagnetism, which has recently been discovered for epitaxially grown Sr₃OsO₆ films. Unlike the film, the bulk is dominated by antiferromagnetism rather than ferromagnetism. Therefore, robust ferromagnetic order appears only when Sr₃OsO₆ is under the influence of interfaces. A specific heat capacity of 39.6(9) × 10^-3 J mol^-1 K^-2 is found at low temperatures (<17 K). This value is remarkably high, suggesting the presence of possible Fermionic-like excitations at the magnetic ground state. Although the bulk and film forms of Sr₃OsO₆ share the same lattice basis and electrically insulating state, the magnetism is entirely different between them.

Crystal structures of the triple perovskites Ba2K2Te2O9 and Ba2KNaTe2O9, and redetermination of the double perovskite Ba2CaTeO6

Single crystals of Ba2K2Te2O9 (dibarium dipotassium nona-oxidoditellurate), (I), Ba2KNaTe2O9 (dibarium potassium sodium nona-oxidoditellurate), (II), and Ba2CaTeO6 (dibarium calcium hexa-oxidotellurate), (III), were obtained from KNO3/KI or KNO3/NaNO3 flux syntheses in platinum crucibles for (I) and (II), or porcelain crucibles for (III). (I) and (II) are isotypic and are members of triple perovskites with general formula A2[12co]A'[12co]B2[6o]B'[6o]O9. They crystallize in the 6H-BaTiO3 structure family in space-group type P63/mmc, with the A, A', B and B' sites being occupied by K, Ba, Te and a second Ba in (I), and in (II) by mixed-occupied (Ba/K), Ba, Te and Na sites, respectively. (III) adopts the A2[12co]B'[6o]B''[6o]O6 double perovskite structure in space-group type Fmm, with Ba, Ca and Te located on the A, B' and B'' sites, respectively. The current refinement of (III) is based on single-crystal X-ray data. It confirms the previous refinement from X-ray powder diffraction data [Fu et al. (2008). J. Solid State Chem.181, 2523-2529], but with higher precision.

The role of Yb3+ concentrations on Er3+ doped SrLaMgTaO6 double perovskite phosphors

The upconversion photoluminescence (UCPL) spectra of SrLaMgTaO₆:Er³⁺/Yb³⁺ phosphor with different concentration of Yb³⁺ ion under a 975 nm excitation and the insert shows the downconversion photoluminescence (DCPL) spectra under a 355 nm excitation.

Raman studies of A2MWO6 tungstate double perovskites

The Raman spectra of seven A(2)MWO(6) tungstate double perovskites are analysed. Ba(2)MgWO(6) is a cubic double perovskite with Fm3[combining macron]m symmetry and its Raman spectrum contain three modes that can be assigned in a straightforward manner. A fourth mode, the asymmetric stretch of the [WO(6)](6-) octahedron, is too weak to be observed. The symmetry of Ba(2)CaWO(6) is lowered to tetragonal I4/m due to octahedral tilting, but the distortion is sufficiently subtle that the extra bands predicted to appear in the Raman spectrum are not observed. The remaining five compounds have additional octahedral tilts that lower the symmetry to monoclinic P2(1)/n. The further reduction of symmetry leads to the appearance of additional lattice modes involving translations of the A-site cations and librations of the octahedra. Comparing the Raman spectra of fourteen different A(2)MWO(6) tungstate double perovskites shows that the frequency of the symmetric stretch (ν(1)) of the [WO(6)](6-) octahedron is relatively low for cubic perovskites with tolerance factors greater than one due to underbonding of the tungsten and/or M cation. The frequency of this mode increases rapidly as the tolerance factor drops below one, before decreasing gradually as the octahedral tilting gets larger. The frequency of the oxygen bending mode (ν(5)) is shown to be dependent on the mass of the A-site cation due to coupling of the internal bending mode with external A-site cation translation modes.

The role of π-bonding on the high temperature structure of the double perovskites Ba2CaUO6and BaSrCaUO6

High temperature synchrotron X-ray and neutron diffraction powder diffraction studies of the uranium perovskites Ba₂CaUO₆and BaSrCaUO₆reveal unusual phase transition behavior associated with the progressive loss of cooperative octahedral tilting.

Bi2Te(IO3)O5Cl: a novel polar iodate oxychloride exhibiting a second-order nonlinear optical response

A novel iodate oxychloride Bi₂Te(IO₃)O₅Cl with a second-order nonlinear optical response originating from constructive stacking of dipole moments of IO₃.

Sr3TeO6 and Ba3TeO6: double perovskites with pronounced superstructures

The orthotellurates (VI) of strontium and barium were synthesized at 1373 K in air by oxidation of TeO2 in a melt of SrBr2 and BaBr2, respectively. Their unprecedented structures were determined from room temperature single crystal X-ray diffraction data. Sr3TeO6, [F-1, Z = 32, a = 16.5338(2) Å, b = 16.6308(2) Å, c = 16.7112(3) Å, α = 90.031(2)°, β = 90.035(2)°, γ = 90.1370(10)°, R[F 2 > 2σ(F 2)] = 0.035, 19195 F 2 data, 375 variable parameters] and Ba3TeO6 [I41/a, Z = 80, a = 19.3878(10) Å, c = 34.909(2) Å, R[F 2 > 2σ(F 2)] = 0.034, 19576 F 2 data, 453 variable parameters] are eight- and 40-fold superstructures which can be described as a pseudo-cubic close-packing of [TeO6]6– octahedra with M 2+ (M = Ba, Sr) atoms located in the tetrahedral and octahedral voids. Sr3TeO6 and Ba3TeO6 are hettotypes of the cubic aristotype with the double perovskite structure crystallizing in space group Fm-3m. A comparative discussion of the crystal structures and their group-subgroup relations are given, including different approaches for the structure descriptions.