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Solid solutions of BiGaxM1–xO3 (M = Cr, Mn, and Fe) were prepared using the high-pressure high-temperature method at 6 GPa and 1700 K (M = Cr and Fe) and 1300 K (M = Mn). The formation of a large family of polar materials with R3c and Cm symmetries was found. Crystal structures were studied with laboratory X-ray powder diffraction: space group Cm, a = 5.3150(1) A, b = 5.2960(1) A, c = 4.6965(1) A, and β = 92.556(2)° for BiGa0.4Fe0.6O3; space group Cm, a = 5.2798(1) A, b = 5.2577(1) A, c = 4.6465(1) A, and β = 91.974(2)° for BiGa0.7Mn0.3O3; space group R3c, a = 5.51623(8) A and c = 13.61391(17) A for BiGa0.4Cr0.6O3. Samples with the Cm symmetry have square-pyramidal coordination of (Ga,M)3+ ions, and their structure is very similar with the structure of supertetragonal PbVO3, BiCoO3, and Bi2ZnTiO6 materials. Samples with the R3c symmetry are isostructural with BiFeO3 and have comparable calculated (based on the point-charge model) spontaneous polarization (58 μC/cm2 for BiGa0.4Cr0.6O3). The calculated pola...

Our recent work has demonstrated that well-defined hollow interiors can be created inside Prussian Blue (PB) nanoparticles through controlled chemical etching in the presence of poly(vinylpyrrolidone) (Angew. Chem., Int. Ed.2012, 51, 984). By calcination of these PB nanoparticles as starting precursors, we can successfully synthesize nanoporous iron oxides with hollow interiors. From scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the original hollow cavities of PB nanocubes are shown to be retained after crystal transformation to iron oxides. Also, the obtained hollow iron oxides show a very high surface area because of their nanoporous shells, as illustrated by N2 gas adsorption–desorption analysis. By tuning the applied calcination temperatures and selecting the PB nanoparticles with different hollow cavities, crystalline α-Fe2O3, and γ-Fe2O3 can be selectively formed in the products without formation of any impurity phases. Field-dependent magnetization measurements indi...

ScCrO3 and InCrO3 were synthesized at high pressure of 6 GPa and 1500 K. Crystal structures of ScCrO3 and InCrO3 were studied with synchrotron X-ray powder diffraction. They crystallize in the GdFeO3-type perovskite structure (space group Pnma, a = 5.35845(1) A, b = 7.37523(1) A, c = 5.03139(1) A for ScCrO3 and a = 5.35536(1) A, b = 7.54439(1) A, c = 5.16951(1) A for InCrO3). The physical properties of ScCrO3 and InCrO3 were investigated with specific heat, ac/dc magnetization, and dielectric measurements and compared with those of YCrO3 with nonmagnetic Y3+ ions at the A site. Antiferromagnetic transitions occur at TN = 73 K in ScCrO3 and 93 K in InCrO3 in agreement with the general trend of ACrO3 (A = Y and rare earths) where TN decreases with decreasing the radius of the A ions. Extremely weak ferromagnetism was found in ScCrO3 and InCrO3 in contrast to YCrO3. Ac magnetization measurements revealed some peculiarities in behavior of ScCrO3 and InCrO3, namely, double-peak anomalies just below TN. Dielect...

Solid solutions of BiFe1xMnxO3 (0.0 ≤ x ≤ 0.4) were prepared at ambient pressure and at 6 GPa. The ambient-pressure (AP) phases crystallize in space group R3c similarly to BiFeO3. The high-pressure (HP) phases crystallize in space group R3c for x = 0.05 and in space group Pnma for 0.15 ≤ x ≤ 0.4. The structure of HP-BiFe0.75Mn0.25O3 was investigated using synchrotron X-ray powder diffraction, electron diffraction, and transmission electron microscopy. HP-BiFe0.75Mn0.25O3 has a PbZrO3-related √2ap × 4ap × 2√2ap (ap is the parameter of the cubic perovskite subcell) superstructure with a = 5.60125(9) Å, b = 15.6610(2) Å, and c = 11.2515(2) Å similar to that of Bi0.82La0.18FeO3. A remarkable feature of this structure is the unconventional octahedral tilt system, with the primary ab0a tilt superimposed on pairwise clockwise and counterclockwise rotations around the b-axis according to the oioi sequence (o stands for out-of-phase tilt, and i stands for in-phase tilt). The (FeMn)O6 octahedra are distorted, with one longer metaloxygen bond (2.222.23 Å) that can be attributed to a compensation for covalent BiO bonding. Such bonding results in the localization of the lone electron pair on Bi3+ cations, as confirmed by electron localization function analysis. The relationship between HP-BiFe0.75Mn0.25O3 and antiferroelectric structures of PbZrO3 and NaNbO3 is discussed. On heating in air, HP-BiFe0.75Mn0.25O3 irreversibly transforms to AP-BiFe0.75Mn0.25O3 starting from about 600 K. Both AP and HP phases undergo an antiferromagnetic ordering at TN ≈ 485 and 520 K, respectively, and develop a weak net magnetic moment at low temperatures. Additionally, ceramic samples of AP-BiFe0.75Mn0.25O3 show a peculiar phenomenon of magnetization reversal.

High-pressure structural properties of multiferroic perovskite-type BiFeO3 have been investigated by high-resolution synchrotron X-ray powder diffraction at room temperature up to 9.7 GPa. BiFeO3 shows rather complicated structural behavior. The ambient-pressure ferroelectric R3c phase transforms to an orthorhombic phase OI at around 4 GPa during compression. The OI phase is characterized by a superstructure √2ap × 3√2ap × ap, where ap is the parameter of the cubic perovskite subcell (a = 5.4939(4) A, b = 16.6896(9) A, c = 3.8728(2) A at 4.9 GPa). The OI phase transforms to an orthorhombic phase OII at around 7 GPa. The OII phase is characterized by a superstructure √2ap × 3√2ap × 2ap (a = 5.5021(3) A, b = 16.2439(11) A, c = 7.6960(4) A at 9.7 GPa). During decompression, significant hysteretic behavior was found. The OII phase was stable down to about 3 GPa. The OII phase then transforms to an orthorhombic phase OIII that is characterized by a superstructure √2ap × 2√2ap × 2ap (a = 5.5617(6) A, b = 11.215...

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) A, b = 7.57250(4) A, and c = 5.27643(2) A 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-x...

The crystal and magnetic structures of polycrystalline BiCoO3 have been determined by the Rietveld method from neutron diffraction data measured at temperatures from 5 to 520 K. BiCoO3 (space group P4mm; Z = 1; a = 3.72937(7) A and c = 4.72382(15) A at room temperature; tetragonality c/a = 1.267) is isotypic with BaTiO3 and PbTiO3 in the whole temperature range. BiCoO3 is an insulator with a Neel temperature of 470 K. A possible model for antiferromagnetic order is proposed with a propagation vector of k = (1/2, 1/2, 0). In this model, magnetic moments of Co3+ ions are parallel to the c direction and align antiferromagnetically in the ab plane. The antiferromagnetic ab layers stack ferromagnetically along the c axis, forming a C-type antiferromagnetic structure. Refined magnetic moments at 5 and 300 K are 3.24(2)μB and 2.93(2)μB, respectively. The structure refinements revealed no deviation from stoichiometry in BiCoO3. BiCoO3 decomposed in air above 720 K to give Co3O4 and sillenite-like Bi25CoO39.

Ferroelectric materials, which have a reversible spontaneous polarization and generate an electric potential when subjected to a mechanical stress (piezoelectricity), have applications in nonvolati...

Solid solutions BiMn1-xScxO3 (0 ≤ x ≤ 1) were prepared at 6 GPa and 1383−1443 K. Selected area and convergent beam electron diffraction showed that BiMn0.9Sc0.1O3 crystallizes in the centrosymmetric space group C2/c at room temperature. The structure parameters of BiMn0.9Sc0.1O3 were refined by the Rietveld method from laboratory X-ray diffraction data (Z = 8; a = 9.6029(3) A, b = 5.60988(14) A, c = 9.7690(3) A, β = 108.775(2°) at 293 K). The Mn−O bond lengths suggest that the orbital order present in BiMnO3 at 300 K disappears in BiMn0.9Sc0.1O3. Therefore, the monoclinic-to-monoclinic phase transition observed in BiMnO3 at 474 K and associated with the orbital melting was not detected in BiMn1-xScxO3 for x ≥ 0.05 down to 133 K. BiMn1-xScxO3 were characterized by dc and ac magnetization, specific heat, and differential scanning calorimetry measurements. The long-range ferromagnetic order seems to survive for x = 0−0.2. For x ≥ 0.4, the samples showed spin-glass-like features. The Weiss temperature deduced...

New oxides, BiAlO3 and BiGaO3, were prepared using a high-pressure high-temperature technique at 6 GPa and 1273−1473 K. BiAlO3 is isotypic with multiferroic perovskite-like BiFeO3 and has octahedrally coordinated Al3+ ions. Structure parameters of BiAlO3 were refined from laboratory X-ray powder diffraction data (space group R3c; Z = 6; a = 5.37546(5) A and c = 13.3933(1) A). BiGaO3 has the structure closely related to pyroxene-like KVO3. Structure parameters of BiGaO3 were refined from time-of-flight neutron powder diffraction data (space group Pcca; Z = 4; a = 5.4162(2) A, b = 5.1335(3) A, and c = 9.9369(5) A). The GaO4 tetrahedra in BiGaO3 are joined by corners forming infinite (GaO3)3- chains along the a axis. Bi3+ ions in BiGaO3 have 6-fold coordination. Both BiAlO3 and BiGaO3 decompose at ambient pressure on heating above 820 K to give Bi2M4O9 and Bi25MO39 (M = Al and Ga). Vibrational properties of BiAlO3 and BiGaO3 were studied by Raman spectroscopy. In solid solutions of BiAl1-xGaxO3, a C-centered...

Physical and chemical properties of Sr9Fe(PO4)7 and Sr9FeD(PO4)7 were investigated by Mossbauer, 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) A, b = 10.6005(13) A, c = 17.9632(3) A, and β = 112.5053(9)°), and those of the PE phase at 923 K from synchrotron X-ray powder diffraction data (space group R3m; Z = 3; a = 10.70473(13) A and c = 19.8605(2) A). 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...

Crystallographic features and stability of the tetragonal phase of PbVO3 were investigated under ambient and high pressure and compared with those of PbTiO3. PbVO3 is isotypic with PbTiO3 [at 300 K, space group P4mm; a = 3.803 91(5) A and c = 4.676 80(8) A, Z = 1]. Tetragonality (c/a) of PbVO3 was the largest among the PbTiO3-type compounds and increased monotonically with increasing temperature from 12 to 570 K without transition to cubic phase. The large tetragonality and the atomic position determined from synchrotron X-ray powder diffraction data suggest a large ferroelectric polarization above 100 μC/cm2. Above 570 K in air, PbVO3 was oxidized to Pb2V2O7. Application of high pressure prevented the oxidation and thus a tetragonal-to-cubic phase transition accompanied by an insulator-to-metal phase transition was observed from about 2 GPa at room temperature.

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 R3m and Z = 3. Thermal analysis revealed the presence of an intermediate phase between the R3c and R3m phases in a very narrow tempera...

Structure of SrFe 2 (PO 4 ) 2 was solved ab initio from X-ray powder diffraction data (space group P2 1 /c (No. 14); Z = 4; a = 9.3647(2) A, b = 6.8518(1) A, c = 10.5367(2) A, and β = 109.5140(8)°). It has almost linear tetrameric units Fe2-Fel-Fel-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 T 1 = 7.0 K and T 2 = 11.3 K in zero magnetic field. The phase transition at T 2 seems to be a structural phase transition. Magnetization measurements showed that, below T 1 , SrFe 2 -(PO 4 ) 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 μ B per Fe 2+ ion and Weiss constant of -18.9 K. Weak ferromagnetic properties below T 1 can be explained by canting of antiferromagnetically ordered spins. Several field-induced phase transitions were observed in SrFe 2 (PO 4 ) 2 at low temperatures.

Reduction (in H2) and re-oxidation (in air) behavior of Ca9Fe(PO4)7 was studied by X-ray powder diffraction, Mossbauer spectroscopy, thermogravimetry, hydrogen absorption, electrical conductivity, and magnetic-susceptibility measurements. The β-Ca3(PO4)2-like framework of Ca9Fe(PO4)7 was stable in 100% H2 up to 820 K. In the temperature range from 680 to 820 K, reversible redox reactions occurred without changing the stoichiometry of oxygen and phosphorus atoms and destroying the structure. The reduction process in Ca9Fe(PO4)7 included the change of the oxidation state of Fe atoms and incorporation of hydrogen atoms into the structure to form Ca9FeHx(PO4)7 (0 < x ≤ 1). Above 820 K, treatment in 100% H2 was accompanied by loss in mass, partial destruction of the structure, and appearance of FeP and Fe2P phases. Re-oxidation studies in air showed that samples partially lost phosphorus atoms during reduction above 820 K. Behavior of Ca9Fe(PO4)7 under a reduction atmosphere depended on H2 concentration and te...

Solid solutions Ca3-xSrx(PO4)2 (0 ≤ x ≤ 16/7) were studied by X-ray powder diffraction, infrared spectroscopy, differential scanning calorimetry, electrical-conductivity measurements, and second-harmonic generation. Phosphates with 0 ≤ x ≤ 12/7 (β-phase) crystallize in space group R3c, Z = 21, a ≈ 10 A, and c ≈ 38 A. Sr-rich phosphates with 13/7 ≤ x ≤ 16/7 (β‘-phase) have a somewhat different crystal structure: space group R3m, Z = 10.5, a ≈ 10 A, and c ≈ 19 A. The solid solutions with 0 ≤ x ≤ 12/7 showed a reversible high-temperature phase transition β ⇄ β‘, where the β and β‘ phases have noncentrosymmetric and centosymmetric structures, respectively. The temperature of the phase transition decreased with increasing Sr content. In Ca3(PO4)2, a new phase transition, β ⇄ β‘, was observed at 920 °C in the electrical-conductivity measurements. The structure parameters of polar β-Ca2Sr(PO4)2 (x = 1) and centrosymmetric β‘-Ca5/7Sr16/7(PO4)2 (x = 16/7) at room temperature were refined by Rietveld refinements ...

The (1 − x)BiFeO3−xLaFeO3 system has been investigated and characterized by room-temperature and high-temperature laboratory and synchrotron powder X-ray diffraction, electron diffraction, high-resolution transmission electron microscopy, differential scanning calorimetry, and magnetization measurements. At room temperature, the ferroelectric R3c phase is observed for 0.0 ≤ x ≤ 0.10. The PbZrO3-related √2ap × 2√2ap × 4ap superstructure (where ap is the parameter of the cubic perovskite subcell) is observed for Bi0.82La0.18FeO3, while an incommensurately modulated phase is formed for 0.19 ≤ x ≤ 0.30 with the √2ap × 2ap × √2ap basic unit cell. The GdFeO3-type phase with space group Pnma (√2ap × 2ap × √2ap) is stable at 0.50 ≤ x ≤ 1. Bi0.82La0.18FeO3 has no detectable homogeneity range (space group Pnam, a = 5.6004(1) Å, b = 11.2493(3) Å, c = 15.6179(3) Å). The incommensurately modulated Bi0.75La0.25FeO3 structure was solved from synchrotron X-ray powder diffraction data (Imma(00γ)s00 superspace group, a = 5.5956(1) Å, b = 7.8171(1) Å, c = 5.62055(8) Å, q = 0.4855(4)c*, RP = 0.023, RwP = 0.033). In this structure, cooperative displacements of the Bi and O atoms occur, which order within the (AO) (where A = Bi, La) layers, resulting in an antipolar structure. Local fluctuations of the intralayer antipolar ordering are compensated by an interaction with the neighboring (AO) layers. A coupling of the antipolar displacements with the cooperative tilting distortion of the perovskite octahedral framework is proposed as the origin of the incommensurability. All the phases transform to the GdFeO3-type structure at high temperatures. Bi0.82La0.18FeO3 shows an intermediate PbZrO3-type phase with √2ap × 2√2ap × 2ap (space group Pbam; a = 5.6154(2) Å, b = 11.2710(4) Å, and c = 7.8248(2) Å at 570 K). The compounds in the compositional range of 0.18 ≤ x ≤ 0.95 are canted antiferromagnets.

A new phosphate, Ca9FeD(PO4)7, with a whitlockite-like structure was synthesized by treating Ca9Fe(PO4)7 with D2 at 820 K. Mossbauer spectroscopy showed that the oxidation state of Fe in Ca9FeD(PO4)7 is +2. Ca9FeD(PO4)7 was paramagnetic in a temperature range from 5 to 300 K with an effective magnetic moment of 4.97 μB per Fe2+ ion. The infrared spectrum of Ca9FeD(PO4)7 exhibited a broad absorption band due to an O−D stretching vibration at 2225 cm-1 in addition to bands assigned to PO4 groups. Ca9FeD(PO4)7 was stable under an Ar atmosphere up to 840 K. Above this temperature, it decomposed to a β-Ca3(PO4)2-like phase, Ca9.333Fe1.167(PO4)7, as well as β-Ca2P2O7 with releasing D2O and keeping the oxidation state of +2 for Fe. Ca9FeD(PO4)7 liberated D2O in air above ca. 720 K to produce Ca9Fe(PO4)7. Second-harmonic-generation signals, I2ω/I2ω(SiO2), in Ca9FeD(PO4)7 (ca. 0.15) were by a factor of ca. 17 less than ca. 2.5 in Ca9Fe(PO4)7. Rietveld analysis of time-of-flight neutron diffraction data for Ca9FeD(...