SrFe₂(PO₄)₂: Ab Initio Structure Determination with X-ray Powder Diffraction Data and Unusual Magnetic Properties

Structure of SrFe2(PO4)2 was solved ab initio from X-ray powder diffraction data (space group P21/c (No. 14); Z = 4; a = 9.3647(2) Å, b = 6.8518(1) Å, c = 10.5367(2) Å, and β = 109.5140(8)°). It has almost linear tetrameric units Fe2−Fe1−Fe1−Fe2 which join with each other through common oxygen atoms creating a complicated two-dimensional network parallel to the bc plane. Specific heat measurements revealed two phase transitions at T1 = 7.0 K and T2 = 11.3 K in zero magnetic field. The phase transition at T2 seems to be a structural phase transition. Magnetization measurements showed that, below T1, SrFe2(PO4)2 exhibits weak ferromagnetism and demonstrates clear ferromagnetic hysteresis loops. Above 15 K, Curie−Weiss behavior was observed with an effective magnetic moment of 5.23 μB per Fe2+ ion and Weiss constant of −18.9 K. Weak ferromagnetic properties below T1 can be explained by canting of antiferromagnetically ordered spins. Several field-induced phase transitions were observed in SrFe2(PO4)2 at low temperatures

BiInO₃: A Polar Oxide with GdFeO₃-Type Perovskite Structure

A new oxide, BiInO3, was prepared using a high-pressure high-temperature technique at 6 GPa and 1273 K. BiInO3 has the GdFeO3-type perovskite structure, but crystallizes in the polar space group Pna21. Structure parameters of BiInO3 were refined from laboratory X-ray powder diffraction data (Z = 4; a = 5.95463(7) Å, b = 5.60182(7) Å, and c = 8.38631(11) Å). BiInO3 shows a second-harmonic generation signal of about 120−140 times that of quartz. BiInO3 decomposes at ambient pressure on heating above 873 K to give In2O3 and Bi25InO39. No phase transitions were found between 140 and 873 K using differential scanning calorimetry and differential thermal analysis. Vibrational properties of BiInO3 were studied by Raman spectroscopy

New Noncentrosymmetric Vanadates Sr₉R(VO₄)₇ (R = Tm, Yb, and Lu): Synthesis, Structure Analysis, and Characterization

New vanadates Sr9R(VO4)7 (R = Tm, Yb, and Lu) were synthesized using a standard solid-state method at 1373 K and found to be isotypic with Ca3(VO4)2 at room temperature (RT). Their structure parameters were refined using the Rietveld method from synchrotron X-ray diffraction (XRD) data measured at RT (space group R3c and Z = 6). Sr9R(VO4)7 (R = Y and La−Er) do not form a phase isotypic with Ca3(VO4)2. Sr9R(VO4)7 (R = Tm, Yb, and Lu) were characterized through the magnetic susceptibility (2−400 K), the specific heat (0.45−31 K), thermal analysis (300−1573 K), and high-temperature XRD, second-harmonic generation, and dielectric measurements. The temperature dependence of the dielectric constant and tangent loss suggested that they exhibit a reversible ferroelectric−paraelectric phase transition of the first order near 950−960 K. The high-temperature phases have space group R3̄m and Z = 3. Thermal analysis revealed the presence of an intermediate phase between the R3c and R3̄m phases in a very narrow temperature range. Magnetic susceptibilities of Sr9Tm(VO4)7 and Sr9Yb(VO4)7 are typical of Tm3+ and Yb3+ ions affected by an octahedral crystal field. The effective magnetic moments were 7.39 μB for Tm3+ and 4.59 μB for Yb3+

Redox Reactions in Strontium Iron Phosphates: Synthesis, Structures, and Characterization of Sr₉Fe(PO₄)₇ and Sr₉FeD(PO₄)₇

Physical and chemical properties of Sr9Fe(PO4)7 and Sr9FeD(PO4)7 were investigated by Mössbauer, infrared, and Raman spectroscopy, magnetization and dielectric measurements, differential scanning calorimetry, and thermal analysis. Sr9Fe(PO4)7 undergoes an antiferroelectric-paraelectric (AFE-PE) phase transition at Tc = 740 K. Structure parameters of the AFE phase at 293 K were refined from time-of-flight (TOF) neutron powder diffraction data (space group C2/c; Z = 4; a = 14.4971(2) Å, b = 10.6005(13) Å, c = 17.9632(3) Å, and β = 112.5053(9)°), and those of the PE phase at 923 K from synchrotron X-ray powder diffraction data (space group R3̄m; Z = 3; a = 10.70473(13) Å and c = 19.8605(2) Å). Parts of Sr atoms and PO4 tetrahedra are highly disordered in the PE phase. Sr9FeD(PO4)7 was prepared by treating Sr9Fe(PO4)7 with D2 at 820 K. The incorporation of D atoms above Tc kept the structure of the high-temperature modification of Sr9Fe(PO4)7. Therefore, Sr9FeD(PO4)7 is isotypic with the PE phase of Sr9Fe(PO4)7. Rietveld refinements from TOF neutron diffraction data and synchrotron XRD data of Sr9FeD(PO4)7 (at 293 K) on the basis of space group R3̄m gave lattice parameters a = 10.67744(13) Å and c = 19.5810(2) Å, making it possible to locate D atoms at two positions. Oxidation of Sr9FeD(PO4)7 in air above 673 K regenerated Sr9Fe(PO4)7. When Sr9FeD(PO4)7 was heated above 940 K in the absence of oxygen and when Sr9Fe(PO4)7 was treated by 5% H2 + 95% N2 above 1100 K, they decomposed to Sr2P2O7 and Sr9.333Fe1.167(PO4)7

K₅Eu_1–<i>x</i>Tb<i>_x</i>(MoO₄)₄ Phosphors for Solid-State Lighting Applications: Aperiodic Structures and the Tb³⁺ → Eu³⁺ Energy Transfer

This paper describes the influence of sintering conditions and Eu3+/Tb3+ content on the structure and luminescent properties of K5Eu1–xTbx(MoO4)4 (KETMO). KETMO samples were synthesized under two different heating and cooling conditions. A K5Tb­(MoO4)4 (KTMO) colorless transparent single crystal was grown by the Czochralski technique. A continuous range of solid solutions with a trigonal palmierite-type structure (α-phase, space group R3̅m) were presented only for the high-temperature (HT or α-) KETMO (0 ≤ x ≤ 1) prepared at 1123 K followed by quenching to liquid nitrogen temperature. The reversibility of the β ↔ α phase transition for KTMO was revealed by a differential scanning calorimetry (DSC) study. The low-temperature (LT)­LT-K5Eu0.6Tb0.4(MoO4)4 structure was refined in the C2/m space group. Additional extra reflections besides the reflections of the basic palmierite-type R-subcell were present in synchrotron X-ray diffraction (XRD) patterns of LT-KTMO. LT-KTMO was refined as an incommensurately modulated structure with (3 + 1)­D superspace group C2/m(0β0)­00 and the modulation vector q = 0.684b*. The luminescent properties of KETMO prepared at different conditions were studied and related to their structures. The luminescence spectra of KTMO samples were represented by a group of narrow lines ascribed to 5D4 → 7FJ (J = 3–6) Tb3+ transitions with the most intense emission line at 547 nm. The KTMO single crystal demonstrated the highest luminescence intensity, which was ∼20 times higher than that of LT-KTMO. The quantum yield λex = 481 nm for the KTMO single crystal was measured as 50%. The intensity of the 5D4 → 7F5 Tb3+ transition increased with the increase of x from 0.2 to 1 for LT and HT-KETMO. Emission spectra of KETMO samples with x = 0.2–0.9 at λex = 377 nm exhibited an intense red emission at ∼615 nm due to the 5D0 → 7F2 Eu3+ transition, thus indicating an efficient energy transfer from Tb3+ to Eu3+

Crystal Structure and Luminescent Properties of R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) Red Phosphors

The R2(MoO4)3 (R = rare earth elements) molybdates doped with Eu3+ cations are interesting red-emitting materials for display and solid-state lighting applications. The structure and luminescent properties of the R2–xEux(MoO4)3 (R = Gd, Sm) solid solutions have been investigated as a function of chemical composition and preparation conditions. Monoclinic (α) and orthorhombic (β′) R2–xEux(MoO4)3 (R = Gd, Sm; 0 ≤ x ≤ 2) modifications were prepared by solid-state reaction, and their structures were investigated using synchrotron powder X-ray diffraction and transmission electron microscopy. The pure orthorhombic β′-phases could be synthesized only by quenching from high temperature to room temperature for Gd2–xEux(MoO4)3 in the Eu3+-rich part (x > 1) and for all Sm2–xEux(MoO4)3 solid solutions. The transformation from the α-phase to the β′-phase results in a notable increase (∼24%) of the unit cell volume for all R2–xEux(MoO4)3 (R = Sm, Gd) solid solutions. The luminescent properties of all R2–xEux(MoO4)3 (R = Gd, Sm; 0 ≤ x ≤ 2) solid solutions were measured, and their optical properties were related to their structural properties. All R2–xEux(MoO4)3 (R = Gd, Sm; 0 ≤ x ≤ 2) phosphors emit intense red light dominated by the 5D0→​7F2 transition at ∼616 nm. However, a change in the multiplet splitting is observed when switching from the monoclinic to the orthorhombic structure, as a consequence of the change in coordination polyhedron of the luminescent ion from RO8 to RO7 for the α- and β′-modification, respectively. The Gd2–xEux(MoO4)3 solid solutions are the most efficient emitters in the range of 0 x < 1.5, but their emission intensity is comparable to or even significantly lower than that of Sm2–xEux(MoO4)3 for higher Eu3+ concentrations (1.5 ≤ x ≤ 1.75). Electron energy loss spectroscopy (EELS) measurements revealed the influence of the structure and element content on the number and positions of bands in the ultraviolet–visible–infrared regions of the EELS spectrum

BiScO₃: Centrosymmetric BiMnO₃-type Oxide

With neutron powder diffraction, electron diffraction, and second-harmonic generation, we have shown that BiScO3 has a structure closely related to that of multiferroic BiMnO3, but BiScO3 crystallizes in the centrosymmetric space group of C2/c. These results bring up a question about the origin of ferroelectricity in BiMnO3. BiScO3 may serve as a model system to understand the role of Mn3+ ions in the ferroelectricity of BiMnO3

Crystal Structure and Luminescent Properties of R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) Red Phosphors

The R₂(MoO₄)₃ (R = rare earth elements) molybdates doped with Eu³⁺ cations are interesting red-emitting materials for display and solid-state lighting applications. The structure and luminescent properties of the R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) solid solutions have been investigated as a function of chemical composition and preparation conditions. Monoclinic (α) and orthorhombic (β′) R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) modifications were prepared by solid-state reaction, and their structures were investigated using synchrotron powder X-ray diffraction and transmission electron microscopy. The pure orthorhombic β′-phases could be synthesized only by quenching from high temperature to room temperature for Gd_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ in the Eu³⁺-rich part (<i>x</i> > 1) and for all Sm_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ solid solutions. The transformation from the α-phase to the β′-phase results in a notable increase (∼24%) of the unit cell volume for all R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Sm, Gd) solid solutions. The luminescent properties of all R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) solid solutions were measured, and their optical properties were related to their structural properties. All R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) phosphors emit intense red light dominated by the ⁵D₀→​⁷F₂ transition at ∼616 nm. However, a change in the multiplet splitting is observed when switching from the monoclinic to the orthorhombic structure, as a consequence of the change in coordination polyhedron of the luminescent ion from RO₈ to RO₇ for the α- and β′-modification, respectively. The Gd_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ solid solutions are the most efficient emitters in the range of 0 < <i>x</i> < 1.5, but their emission intensity is comparable to or even significantly lower than that of Sm_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ for higher Eu³⁺ concentrations (1.5 ≤ <i>x</i> ≤ 1.75). Electron energy loss spectroscopy (EELS) measurements revealed the influence of the structure and element content on the number and positions of bands in the ultraviolet–visible–infrared regions of the EELS spectrum

Crystal Structure and Luminescent Properties of R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) Red Phosphors

The R₂(MoO₄)₃ (R = rare earth elements) molybdates doped with Eu³⁺ cations are interesting red-emitting materials for display and solid-state lighting applications. The structure and luminescent properties of the R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm) solid solutions have been investigated as a function of chemical composition and preparation conditions. Monoclinic (α) and orthorhombic (β′) R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) modifications were prepared by solid-state reaction, and their structures were investigated using synchrotron powder X-ray diffraction and transmission electron microscopy. The pure orthorhombic β′-phases could be synthesized only by quenching from high temperature to room temperature for Gd_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ in the Eu³⁺-rich part (<i>x</i> > 1) and for all Sm_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ solid solutions. The transformation from the α-phase to the β′-phase results in a notable increase (∼24%) of the unit cell volume for all R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Sm, Gd) solid solutions. The luminescent properties of all R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) solid solutions were measured, and their optical properties were related to their structural properties. All R_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ (R = Gd, Sm; 0 ≤ <i>x</i> ≤ 2) phosphors emit intense red light dominated by the ⁵D₀→​⁷F₂ transition at ∼616 nm. However, a change in the multiplet splitting is observed when switching from the monoclinic to the orthorhombic structure, as a consequence of the change in coordination polyhedron of the luminescent ion from RO₈ to RO₇ for the α- and β′-modification, respectively. The Gd_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ solid solutions are the most efficient emitters in the range of 0 < <i>x</i> < 1.5, but their emission intensity is comparable to or even significantly lower than that of Sm_2–<i>x</i>Eu_<i>x</i>(MoO₄)₃ for higher Eu³⁺ concentrations (1.5 ≤ <i>x</i> ≤ 1.75). Electron energy loss spectroscopy (EELS) measurements revealed the influence of the structure and element content on the number and positions of bands in the ultraviolet–visible–infrared regions of the EELS spectrum