Magnetic Properties of Isostructural BaCoP₂O₇, BaNiP₂O₇, and BaCuP₂O₇ Studied with dc and ac Magnetization and Specific Heat

Magnetic properties of three isostructural compounds BaMP2O7 (M = Co, Ni, and Cu) were investigated by dc and ac magnetization and specific heat measurements. BaCuP2O7 was shown to be an excellent quasi-one-dimensional linear-chain Heisenberg antiferromagnet with an exchange constant (J/kB) of 103.8 K (Hamiltonian H = J ∑SiSi+1) and a temperature for the long-range magnetic order (TN) of 0.81 K giving the ratio kBTN/J = 0.78%. BaCoP2O7 and BaNiP2O7 exhibited long-range antiferromagnetic order at TN = 10.4 and 10.1 K, respectively. BaCoP2O7 and BaNiP2O7 showed a large contribution of the short-range correlation above TN. BaNiP2O7 remained in the antiferromagnetic state up to 90 kOe at 2 K, whereas BaCoP2O7 demonstrated two metamagnetic phase transitions at about 52 and 71 kOe at 2 K if the magnetic field was parallel to the easy direction. BaMP2O7 melted incongruently at 1323 K (M = Co), 1344 K (M = Ni), and 1338 K (M = Cu)

Short-Range and Long-Range Magnetic Ordering in SrCuP₂O₇ and PbCuP₂O₇

Magnetic properties of SrCuP2O7 and PbCu1-xZnxP2O7 (x = 0, 0.1, and 0.5) were studied by magnetic susceptibility, χ(T), and specific heat, Cp(T). Both data showed that magnetism of SrCuP2O7 and PbCuP2O7 can be described by the one-dimensional (1D) uniform chain model despite the structural features suggesting the presence of zigzag chains with next-nearest-neighbor interactions. The χ(T) data were fitted by the Bonner−Fisher curve (plus temperature independent and Curie−Weiss terms) with g = 2.20 and J/kB = 9.38 K for SrCuP2O7 and g = 2.17 and J/kB = 8.41 K for PbCuP2O7 (Hamiltonian H = J ΣSiSi+1). Magnetic specific heat, Cm(T), exhibited one broad maximum due to short-range ordering and one sharp peak at TN = 1.64 K for SrCuP2O7 and TN = 1.15 K for PbCuP2O7 due to long-range antiferromagnetic ordering. The characteristic values of the broad maxima on the Cm(T) curves (Cmax and TCmax) were in good agreement with the theoretical calculations for the uniform 1D S = 1/2 Heisenberg chain. Magnetic properties of PbCu0.9Zn0.1P2O7 still obeyed the 1D uniform chain model but those of PbCu0.5Zn0.5P2O7 did not. In air, SrCuP2O7 was stable at least up to 1373 K while PbCuP2O7 melted incongruently above 1180 K

High-Pressure Synthesis and Structure of SrCo₆O₁₁: Pillared Kagomé Lattice System with a 1/3 Magnetization Plateau

High-Pressure Synthesis and Structure of SrCo6O11:  Pillared Kagomé Lattice System with a 1/3 Magnetization Platea

High-Pressure Synthesis and Structure of SrCo₆O₁₁: Pillared Kagomé Lattice System with a 1/3 Magnetization Plateau

High-Pressure Synthesis and Structure of SrCo6O11:  Pillared Kagomé Lattice System with a 1/3 Magnetization Platea

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

Low-Dimensional Ferromagnetic Properties of SrCuV₂O₇ and BaCuV₂O₇

The crystal structure of isostructural SrCuV2O7 and BaCuV2O7 consists of one-dimensional (1D) zigzag chains of Cu atoms with next-nearest-neighbor interaction. The main intrachain interaction was found to be ferromagnetic and estimated at 4.6 K (Hamiltonian H ∼ −2J). SrCuV2O7 and BaCuV2O7 are new examples in the scanty family of 1D ferromagnets. Isothermal magnetization measurements at 0.08 K and specific heat data showed that MCuV2O7 exhibits antiferromagnetic long-range ordering at TN = 1.36 K for SrCuV2O7 and TN = 1.47 K for BaCuV2O7. Spin-flop transitions were observed in the antiferromagnetic state at 0.08 K near 0.5 kOe in SrCuV2O7 and 2 kOe in BaCuV2O7. In air, SrCuV2O7 and BaCuV2O7 melted incongruently above 983 and 1018 K, respectively

Synthesis, Structure, and Physical Properties of <i>A</i>-site Ordered Perovskites <i>A</i>Cu₃Co₄O₁₂ (<i>A</i> = Ca and Y)

A-site ordered perovskites CaCu3Co4O12 and YCu3Co4O12, and their solid solutions Ca1−xYxCu3Co4O12 (x = 0.25, 0.50, and 0.75), were synthesized under high pressure (9 GPa) and high temperature (1273 K). They all have a CaCu3Ti4O12-type structure (cubic, space group: Im3̅) and the lattice constant a slightly decreased from 7.1226(5) Å of CaCu3Co4O12 to 7.1195(3) Å of YCu3Co4O12. Rietveld refinement based on synchrotron powder X-ray diffraction suggests that the valence state of CaCu3Co4O12 is close to CaCu3+3Co3.25+4O12, which differs from those of other CaCu2+3B4+4O12 perovskites. Substitution of Ca with Y at the A-site changes the metallic CaCu3Co4O12 to an insulating YCu3Co4O12, and a metal−insulator transition occurs at x = 0.50−0.75. The electrical resistivity, thermoelectricity, and specific heat results reveal that electrons are doped into the Co 3d band and that the valence state changes from CaCu3+3Co3.25+4O12 to YCu3+3Co3+4O12

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+

Structural and Physical Properties of Heavily Doped Yttrium Vanadate: Y_0.6Cd_0.4VO₃

Structural properties of Y0.6Cd0.4VO3 were investigated by electron diffraction and laboratory and synchrotron X-ray powder diffraction methods. Y0.6Cd0.4VO3 crystallizes in space group Pnma (GdFeO3-type perovskite structure) between 12 and 300 K (a = 5.45887(3) Å, b = 7.57250(4) Å, and c = 5.27643(2) Å at 300 K). The lattice parameters showed anomalous behavior on temperature. The c parameter linearly decreased from 12 to 120 K, and then it lineally increased from 160 to 300 K. The b parameter was constant between 12 and 120 K, demonstrated a drop from 120 to 200 K, and then lineally increased from 200 to 300 K. The c/a ratio had a rather sharp maximum at 150 K. In Y0.6Cd0.4VO3 the V−O distances in the ac plane began to split to shorter and longer ones below 150 K, indicating that orbital fluctuations are involved. The phase transition near 150 K in Y0.6Cd0.4VO3 is accompanied by a broad anomaly on the specific heat and change of the slope of the inverse magnetic susceptibility. Other members of the Y1-xCdxVO3 solid solution with x = 0.3, 1/3, and 0.5 did not show this kind of phase transition. This kind of a phase transition has never been detected in other doped vanadates, R1-xMxVO3 (R = Y and rare earths and M = Ca and Sr)

Alchemy in the Art of Traditional Japanese Ceramics: Microstructure and Formation Mechanism of Gold-Colored Bizen Stoneware

The microstructure and formation process of the golden color on traditional Japanese Bizen stoneware was investigated through model experiments. The current compositional and structural research of pottery fragments has revealed that the golden color comes from Fe oxide consisting of approximately 100 nm thick agglomerates of Al-substituted hematite (α-(Fe_1–<i>x</i>Al_<i>x</i>)₂O₃, <i>x</i> ≈ 0.05). The color is reproducible in the laboratory by sequential heat treatments of Bizen clay pellets under oxidizing and reducing atmospheres with an amount of potassium supplied as a melting point depressant. Lustrous colors such as silver and gold in Bizen stoneware have generally been attributed to the optical interference in superficial carbon films produced by burning wood fuel. Here, we show that the golden color is caused by the formation of Al-substituted hematite, not by the formation of carbon