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We present a new method to synthesize bulk indium nitride by means of a simple solid-state chemical reaction carried out under hydrostatic high-pressure/high-temperature conditions in a multi-anvil apparatus, not involving gases or solvents during the process. The reaction occurs between the binary oxide In 2 O 3 and the highly reactive Li 3 N as the nitrogen source, in the powder form. The formation of the hexagonal phase of InN, occurring at 350 °C and P ≥ 3 GPa, was successfully confirmed by powder X-ray diffraction, with the presence of Li 2 O as a unique byproduct. A simple washing process in weak acidic solution followed by centrifugation allowed us to obtain pure InN polycrystalline powders as a precipitate. With an analogous procedure, it was possible to obtain pure bulk GaN, from Ga 2 O 3 and Li 3 N at T ≥ 600 °C and P ≥ 2.5 GPa. These results point out, particularly for InN, a clean, and innovative way to produce significant quantities of one of the most promising nitrides in the field of electronics and energy technologies.

We report on the high-pressure solid-state synthesis and the detailed structural characterization of the metastable, CuAu-type CuInS 2 (CA-CIS) phase. Although often present in CIS thin films as unwanted phase, it has been never synthesized in pure form, and its effect on the performance of CIS-based solar cells has been long debated. In this work, pure CA-CIS phase is synthesized in bulk polycrystalline form through a high-pressure-high-temperature solid-state reaction. Single-crystal X-rays diffraction reveals the formation of tetragonal CA-CIS ( a = 3.9324(5), c = 5.4980(7) Å) either in cation-ordered and disordered phase, pointing out the role of the pressure/temperature increase on the Cu/In ordering. The resistivity measurements performed on CA-CIS show low resistivity and a flat trend vs temperature and, in the case of the ordered phase, highlight a bad-metallic behavior, probably due to a high level of doping. These findings clearly rule out the possibility of a beneficial effect of this phase on the CIS-based thin film solar cells.

The temperature behavior of the crystal structure as well as dielectric and magnetic properties of the perovskite bismuth chromate ceramics with the 10 mol % Cr 3+ -to-Sc 3+ substitution were studied and compared with those of the unmodified compound. Using a high-pressure synthesis, BiCrO 3 and BiCr 0.9 Sc 0.1 O 3 were obtained as metastable perovskite phases which are monoclinic C 2/ c with the √6 a p × √2 a p × √6 a p superstructure (where a p is the primitive perovskite unit-cell parameter) under ambient conditions. At room temperature, the unit cell volume of BiCr 0.9 Sc 0.1 O 3 is ∼1.3% larger than that of BiCrO 3 . Both perovskites undergo a reversible structural transition into a nonpolar GdFeO 3 -type phase (orthorhombic Pnma , √2 a p × 2 a p × √2 a p ) in the temperature ranges of 410-420 K (BiCrO 3 ) and 470-520 K (BiCr 0.9 Sc 0.1 O 3 ) with a relative jump of the primitive perovskite unit cell volume of about -1.6 and -2.0%, respectively. Temperature dependences of the complex dielectric permittivity demonstrate anomalies in the phase transition ranges. The Pnma -to- C 2/ c crossover in BiCrO 3 is accompanied by a decrease in the direct current (dc) conductivity, while in BiCr 0.9 Sc 0.1 O 3 the conductivity increases. The onset of an antiferromagnetic order in BiCr 0.9 Sc 0.1 O 3 is observed at the Néel temperature ( T N ) of about 85 K as compared with T N = 110 K in BiCrO 3 . In contrast to BiCrO 3 , which exhibits a spin reorientation at T sr ∼ 72 K, no such a transition occurs in BiCr 0.9 Sc 0.1 O 3 .

By means of single-crystal X-ray diffraction, we give direct crystallographic evidence of a centrosymmetry breaking below T S = 200 K, concomitant with the onset of a commensurate structural modulation in the quadruple perovskite YMn 3 Mn 4 O 12 . This result, which explains the anomalously large thermal coefficient of the Y 3+ ion in previously reported structural models, is attributed to the small size of the Y 3+ ion, which causes its underbonding within the dodecahedral coordination polyhedron. The present data are consistent with a commensurate superstructure described by an I-centered pseudo-orthorhombic cell with polar Ia symmetry and a ≈ a F √2 = 10.4352(7) Å, b ≈ 2 b F = 14.6049(9) Å, c ≈ c F √2 = 10.6961(7) Å, and β = 90.110(3)°, where a F ≈ c F ≈ 7.45 Å, b F ≈ 7.34 Å, and β ≈ 91° are the unit cell parameters of the I 2/ m structure observed at room temperature. Consistent with the above polar structure, at lower temperature, T * = 70 K, we observe in polycrystalline samples an anomaly of the direct current (DC) and alternating current (AC) magnetization, concomitant with the appearance of a net electric polarization, as indicated by pyrocurrent and dielectric constant measurements. These results, complemented by electrical transport measurements, suggest a magnetic ferroelectricity driven by short-range magnetic order in YMn 3 Mn 4 O 12 .

The crystal structure and ferroelectric properties of ε-Ga 2 O 3 deposited by low-temperature MOCVD on (0001)-sapphire were investigated by single-crystal X-ray diffraction and the dynamic hysteresis measurement technique. A thorough investigation of this relatively unknown polymorph of Ga 2 O 3 showed that it is composed of layers of both octahedrally and tetrahedrally coordinated Ga 3+ sites, which appear to be occupied with a 66% probability. The refinement of the crystal structure in the noncentrosymmetric space group P6 3 mc pointed out the presence of uncompensated electrical dipoles suggesting ferroelectric properties, which were finally demonstrated by independent measurements of the ferroelectric hysteresis. A clear epitaxial relation is observed with respect to the c-oriented sapphire substrate, with the Ga 2 O 3 [10-10] direction being parallel to the Al 2 O 3 direction [11-20], yielding a lattice mismatch of about 4.1%.

We present a comprehensive study of the electrical properties of bulk polycrystalline BiFe0.5Mn0.5O3, a double perovskite synthesized in high-pressure and high-temperature conditions. BiFe0.5Mn0.5O3 shows an antiferromagnetic character with TN = 288 K overlapped with an intrinsic antiferroelectricity due to the Bi(3+) stereochemical effect. Beyond this, the observation of a semiconductor-insulator transition at TP ≈ 140 K allows one to define three distinct temperature ranges with completely different electrical properties. For T > TN, electric transport follows an ordinary thermally activated Arrhenius behavior; the system behaves as a paramagnetic semiconductor. At intermediate temperatures (TP < T < TN), electric transport is best described by Mott's variable range hopping model with lowered dimensionality D = 1, stabilized by the magnetic ordering process and driven by the inhomogeneity of the sample on the B site of the perovskite. Finally, for T < TP, the material becomes a dielectric insulator, showing very unusual poling-induced soft ferroelectricity with high saturation polarization, similar to the parent compound BiFeO3. Under external electric poling, the system irreversibly evolves from antiferroelectric to polar arrangement.

The physical characterization and the extended crystallographic study of the double perovskite system Pb2Mn0.6Co0.4WO6 indicate an improper ferroelectric contribution to the polarization induced by the magnetic ordering. In the paramagnetic phase, the compound displays a centrosymmetric orthorhombic double perovskite structure with the Pmcn1' symmetry. The structure is strongly distorted by the lead stereoactivity. Magnetization measurements show two magnetic transitions at 188 and 9 K, but the time-of-flight neutron diffraction data provide evidence for a long-range magnetic ordering only below the second transition. Quantitative structure refinements combined with a comprehensive symmetry analysis indicate the Pm'c21' magnetic space group to be the adequate symmetry to describe the structural distortions and spin ordering in the ground state of the system. The symmetry implies a coexistence of a spontaneous ferromagnetic moment and a ferroelectric polarization along the orthogonal b- and c-axes, respectively, in the long-range ordered structure. Macroscopic measurements confirm the presence of the spontaneous polarization also below the first transition at 188 K, where only short-range magnetic correlations are evidenced by diffuse scattering in neutron diffraction.

In this paper we describe the new ferri-electric compound Pb2MnWO6 (PMW), a double perovskite that can be considered as a novel structural prototype showing complex nuclear structure and interesting electric properties. According to single-crystal synchrotron data, PMW crystallizes in the noncentrosymmetric polar group Pmc21, in which the two symmetry-independent lead atoms give rise to a ferrielectric arrangement. The accurate crystallographic characterization indicates the presence of a complex distortion of the perovskite lattice driven by the local instability induced by the 6s(2) lone pair of the lead atoms. These peculiar structural features are confirmed by the complete electrical characterization of the system. Dielectric and transport measurements indicate an insulating character of the sample, while pyroelectric measurements point out a ferrielectric state characterized by different contributions. The magnetic transition at 45 K is accompanied by a magnetostrictive effect indicating a probable spin-lattice coupling. The characterizations carried out on PMW, showing the evidence of a coexistence of antiferromagnetism and ferrielectricity at low temperature, could lead to the definition of a new class of multiferroic materials.