Centrosymmetric Pnma Research Articles

Second-Order Nonlinear Switching and Photoluminescence Properties of Cd-Based Hybrid Perovskite with High-Temperature Phase Transition.

Recently, organic-inorganic hybrid perovskites exhibiting facile structural phase transitions have accumulated significant attention due to their switchable second-order nonlinear optical (NLO) properties, which hold significant promise for next-generation intelligent optoelectronic devices. In this study, we present a novel one-dimensional hexagonal hybrid perovskite, (4-methoxypiperidinium)CdCl3, which undergoes a reversible high-temperature structural phase transition at 389 K. Notably, (4-methoxypiperidinium)CdCl3 demonstrates switchable second-order NLO and dielectric properties, accompanied by symmetry breaking from the centrosymmetric Pnma to noncentrosymmetric Pna21 space group. Variable-temperature structure analyses reveal that this transition is mainly driven by the order-disorder transformation of the 4-methoxypiperidinium cations. Furthermore, it also features a promising photoluminescence performance with blue-light emission and a long lifetime of 25.34 ns. It is anticipated that this study will expand the family of hybrid perovskites exhibiting high-temperature phase transitions and offer valuable guidance for the design of new NLO switching materials with superior optoelectronic properties.

A first-principles study of ternary metal chalcogenides [formula omitted][formula omitted] for efficient thermoelectric applications empowered by machine-learning interatomic potential

Metal chalcogenides have already been recognized as promising candidates for thermoelectric applications in the past decade. This study systematically analyzes the ternary metal chalcogenide Ba2MnX3 (X = Te,Se,S) materials to check their suitability for thermoelectric applications. Ba2MnX3 materials belong to the orthorhombic and centrosymmetric Pnma space group, with Ba atoms distributed inside the three-dimensional framework of MnX4 tetrahedron units, forming a quasi-one-dimensional crystal structure. The electronic band structures of the materials investigated using density functional theory (GGA+U) reveal that the materials have direct band gaps ranging between 1.3 eV to 1.9 eV. The structural stability and phonon transport properties are studied using a combination of density functional theory and machine-learning approach. The study shows that all Ba2MnX3 materials possess low lattice thermal conductivity (κL). For Ba2MnTe3, at 700 K κL∼0.45Wm−1K−1 is the lowest achieved κL in this work. The thermoelectric transport parameters of the materials are estimated by working out the semiclassical Boltzmann transport equation. Finally, the Figure of merit (ZT) and thermoelectric conversion efficiency values of the materials are calculated. The maximum ZT≈ 0.74 at 700 K at a carrier concentration of 2.43×1020cm−3, and an efficiency ∼8% in the temperature interval 300 – 700 K is achieved for Ba2MnSe3.

On the Crystal Chemistry of RE2Si2O7: Revisited Structures, Group–Subgroup Relationship, and Insights of Ce3+-Activated Radioluminescence

Polymorphic rare-earth disilicates RE2Si2O7 (RE = La–Lu, Y, and Sc) are attractive materials as thermal barrier coatings and scintillators; however, the orthorhombic E(δ)-type (RE = Eu–Ho and Y) and the triclinic F-type (RE = Sm and Eu) remain structurally controversial hitherto. In this work, we revisit the crystal structures of E(δ)-RE2Si2O7 (Pnma), F-RE2Si2O7 (P1̅), and the monoclinic G-La2Si2O7 (P21/n) by X-ray single-crystal/powder diffraction. The second-harmonic generation (SHG) and/or the local-structure-sensitive VUV–UV–vis luminescence of Ce3+/Eu3+ validate the structure determination. E(δ)-RE2Si2O7 (RE = Eu–Ho) crystallizes in the centrosymmetric Pnma instead of the previous noncentrosymmetric Pna21 (a subgroup of Pnma). Increasing the RE3+ radius results in structural transformations of the reconstructive-type E(δ) ↔ F and translationengleiche subgroup–group relationship F ↔ G (RE = La–Nd). More importantly, we perform a complete survey of 17 structure-types over 63 compounds of RE2Si2O7 and their related transformations depending on the radius of RE3+. On the basis of these studies, one could (i) predict the potential polymorphs of RE2Si2O7, including the high-pressure phase and the RE2Si2O7 solid solutions; (ii) correlate the coefficient of thermal expansion (CTE) with the crystal structure, and (iii) understand the unusual radioluminescence of (Gd,La)2Si2O7:Ce3+ high-temperature scintillators.

Structuration of Water in Microporous CAU-10-H under Gigapascal Pressure.

Employing metal-organic frameworks (MOFs) as water adsorbents has become a flourishing field in recent years. Aluminum isophthalate [Al(OH)(bdc)]·nH2O, referred to as CAU-10-H, is renowned for its high physical and chemical stability under external mechanical stimuli, which is crucial for use in sorption-based heat exchange. We report pressure-induced structural changes and water uptake/release of CAU-10-H in the gigapascal regime. CAU-10-H displays four different phases during pressurization in water up to 4.08(1) GPa. Upon immersion in water at ambient conditions, the unit cell volume expands by 1.3(1)%, by increasing water content from 78.7(1) to 85.6(1) H2O per unit cell. Upon pressurization to 1.08(1) GPa, the water content increases gradually to 112.5(1) H2O per unit cell and reduces the symmetry to P1 phase. At 1.72(1) GPa, a correlated "gate-opening/channel-closing" transition to a centrosymmetric Pnma phase occurs to increase the unit cell volume by 6.9(1)% and water content to 138.8(1) H2O per unit cell. Further increase in pressure to 4.08(1) GPa then leads to a gradual decrease in water content to 119.2(1) H2O per unit cell and redistribution of the inserted water molecules during a "gate-closing/channel-opening" transition to an I41/amd phase. After pressure removal, CAU-10-H reverts to its initial state in terms of crystallinity and water content. This pressure-induced water adsorption/desorption and concerted movements of gate-constraining ligands provides a detailed understanding of the water-MOF interplay, which can be used as a basis for designing new materials, possibly utilizing pressure as a tool for chemical modifications.

NH4IO2F2 and (NH4)3(IO2F2)3·H2O: A Series of Ammonium-Containing Fluoroiodates with Wide Band Gaps.

A series of ammonium-containing fluoroiodates, NH4IO2F2 and (NH4)3(IO2F2)3·H2O, with isolated [IO2F2] units have been fabricated by a fluorine-oxygen substitution strategy from NH4IO3. The two compounds crystallize in the orthorhombic system, but in different space groups, noncentrosymmetric Pca21 for NH4IO2F2 and centrosymmetric Pnma for (NH4)3(IO2F2)3·H2O, and show wide band gaps of 4.53 eV for (NH4IO2F2) and 4.55 eV for ((NH4)3(IO2F2)3·H2O). In addition, NH4IO2F2 exhibits a 1.2 × KDP second harmonic generation response, a short ultraviolet cutoff edge in iodates, and a good crystal growth habit. The crystal of NH4IO2F2 with a size of 11 × 5 × 2 mm3 was obtained by the aqueous solution method. The results enrich the structural diversity of iodate and supply a greater understanding of the design of new functional materials based on the fluoroiodates.

Relaxor ferroelectric phase transition and ac conduction in polycrystalline Gd0.55Ca0.45MnO3 at low temperature

The present investigation reports the dielectric phase transition studies in polycrystalline Gd0.55Ca0.45MnO3 along with ac conduction and structural property. Polycrystalline Gd0.55Ca0.45MnO3 has been synthesized by the solid-state synthesis route at 1350 °C which crystallized in the orthorhombic phase with centrosymmetric Pnma symmetry at room temperature. The complex dielectric and modulus properties of polycrystalline Gd0.55Ca0.45MnO3 has been explored in the frequency range of 500 Hz to 1 MHz at low temperatures having the range 6 K–200 K. The real part of dielectric permittivity shows the sharp increase up to 30 K–120 K temperature range and then decreases constantly which confirm the characteristic of relaxor ferroelectric material. The low-temperature dielectric study reveals that Gd0.55Ca0.45MnO3 possesses a high dielectric permittivity and low loss below 120 K. The frequency variation of ac conductivity shows the correlated barrier hopping in the temperature range of 10 K–60 K and 100 K–180 K and gives the signature of small polaron hopping at 60 K–100 K. Thermal variation of activation energy for Gd0.55Ca0.45MnO3 gives the activation energy of 0.0634(2) eV for higher frequency 1 MHz.

Cation order imaging and magnetic properties in the Ca2Fe2-xGaxO5 brownmillerite (0 ≤ x ≤ 1)

Polycrystalline Ca2Fe2-xGaxO5 brownmillerite samples of nominal x = 0.5, 0.7, 0.9 and 1 compositions have been synthesized by solid state reaction under normal pressure conditions. Whatever x is, room temperature structural analyses by Rietveld refinements and electron microscopy techniques highlight a centrosymmetric Pnma space group. Furthermore, STEM-HAADF observations point towards a preferential Ga occupation at the tetrahedral sites of the structure in agreement with the Rietveld analyses. This structure remains unchanged up to 1000 K as shown by thermo-diffraction analysis. As the Ga content x increases, the room temperature unit cell volume decreases gradually consistently with the fourfold coordination of the smallest Ga3+ ionic radius than that of high spin Fe3+ in the same tetrahedral coordination. This magnetic dilution by a diamagnetic cation is responsible for the TN decrease revealed by magnetization measurements and allows a spin flop transition to be observed. Despite their similarities with the Ca2Fe2-xAlxO5 samples, no electric polarization could be evidenced for the homologous Ca2Fe2-xGaxO5 series.

Absence of a polar phase in perovskite chromite RCrO3 (R=La and Pr)

Magnetic and dielectric properties have been studied for LaCrO3, PrCrO3, and their solid solution La0.5Pr0.5CrO3, which belong to a family of the ferroelectric orthochromite series RCrO3 (R: rare earths). The magnetic measurements confirm that the materials show canted antiferromagnetic ordering at 240–288 K. Neutron diffraction patterns could be fitted with the centrosymmetric Pnma, which is different from the non-centrosymmetric structure proposed for the ferroelectric phase of NdCrO3. The large dielectric constants are likely due to the hopping of charge carriers as proposed previously for other chromites.

Magnetoelectric coupling and exchange bias effects in multiferroic NdCrO3

We report ferroelectricity around ∼88 K that appears well below TN (∼25 K), unlike other members of RCrO3 series. A synchrotron diffraction study suggests that the occurrence of ferroelectricity in NdCrO3 is coupled to the structural transformation from centrosymmetric Pnma to a non-centrosymmetric Pna21 space group. A strong magnetoelectric coupling is observed in the electric polarization [P(T)]. This coupling is significantly influenced by the magnetic field cooling effect, suggesting an exchange bias effect in P(T). This exchange bias effect is also revealed by the systematic shift of the magnetic hysteresis loops below TN. The rare occurrence of an exchange bias effect in both the magnetic and electric polarizations associated with a strong magnetoelectric coupling is of fundamental interest, as well as being attractive for technological applications close to liquid nitrogen temperature.

Small polaron conduction in lead modified lanthanum ferrite ceramics

In the present work we have illustrated the physics of the electrical characteristics of nanocrystalline La1−xPbxFeO3, (0⩽x⩽0.2) powder prepared using auto-combustion synthesis. The effect of lead doping on the dielectric, impedance and ac conductivity characteristics of lanthanum ferrite has systematically been investigated. The synthesized powders were phase pure and crystallized into centro-symmetric Pnma space group. As compared to pure LaFeO3 ceramics (dielectric constant∼14,000), the dielectric constant is grossly increased (∼30,000) in Pb doped LaFeO3. The temperature dependence of dielectric constant of 10.0 at.% Pb doped LaFeO3 exhibits dielectric maxima similar to that observed in ferroelectric ceramics with non-centrosymmetric point group. For La0.8Pb0.2FeO3 ceramics, the frequency dependence of the dielectric constant and loss tangent at various temperatures (300–450K) exhibit typical colossal dielectric constant (CDC) like behavior. From the impedance spectroscopy we have estimated the grain and grain boundary resistance and capacitance of Pb doped LaFeO3 that follow a small polaron hopping conduction model. Long range movement of the charge carriers govern the CDC behavior.

In[IO3](OH)2 – New member of hydrous and anhydrous iodate family with indium

A new indium iodate hydrate In[IO3](OH)2 was synthesized by the hydrothermal methods. Single crystal X-ray diffraction revealed centrosymmetric Pnma space group. [IO3]-groups have typical umbrella-like configuration: iodine atom and three oxygens with I–O distances ∼1.8 Å. In-octahedra have 4 equatorial OH-groups and 2 apical O-atoms of 2 [IO3]-groups and are connected into the layers via OH-groups. It is found that indium iodate family without any other metals is a regular row of compounds: anhydrous In[IO3]3, one hydrated In[IO3]2(OH)·H2O, new two hydrated In[IO3](OH)2 and “end member” represented by three hydrated hydroxide In(OH)3. Chemical relations are parallel to the structural: In-octahedral framework in In-hydroxide, In-octahedral layer as a fragment of the framework, In-octahedral chain as a fragment of the layer, isolated In-octahedra as a fragment of the chain with no In-octahedral condensation via vertices but only via [IO3]-groups in anhydrous In-iodate. If alkali metals are introduced in anhydrous In-iodates, their influence is different. Large metals as K, Rb, Cs hinder condensation of {In[IO3]6}3− blocks which are isolated in the structures whereas smaller Li, Na metals allows condensation up to bands {In[IO3]4}1−∞. Octahedral chains selected in hydrated In-iodates are similar to chains in nonlinear optical compound TiO[IO3]2. The reasons of polarity or non-polarity and possible elements substitutions promising for properties are discussed.

Physics of the multi-functionality of lanthanum ferrite ceramics

In the present work, we have illustrated the physics of the multifunctional characteristics of nano-crystalline LaFeO3 powder prepared using auto-combustion synthesis. The synthesized powders were phase pure and crystallized into centro-symmetric Pnma space group. The temperature dependence of dielectric constant of pure LaFeO3 exhibits dielectric maxima similar to that observed in ferroelectric ceramics with non-centrosymmetric point group. The dielectric relaxation of LaFeO3 correlates well with small polaron conduction. The occurrence of polarization hysteresis in LaFeO3 (with centro-symmetric Pnma space group) is thought to be spin current induced type. The canting of the Fe3+ spins induce weak ferromagnetism in nano-crystalline LaFeO3. Room temperature saturation magnetization of pure LaFeO3 is reported to be 3.0 emu/g. Due to the presence of both ferromagnetic as well as polarization ordering, LaFeO3 behaves like a single phase multiferroic ceramics. The magneto-electric coupling in this system has been demonstrated through the magneto-dielectric measurements which yield about 0.8% dielectric tuning (at 10 kHz) with the application of 2 T magnetic field. As a typical application of the synthesized nano-crystalline LaFeO3 powder, we have studied its butane sensing characteristics. The efficient butane sensing characteristics have been correlated to their catalytic activity towards oxidation of butane. Through X-ray photoelectron spectroscopy analyses, we detect the surface adsorbed oxygen species on LaFeO3 surface. Surface adsorbed oxygen species play major role in their low temperature butane sensing. Finally, we have hypothesized that the desorbed H2O and O2 (originate from surface adsorbed hydroxyl and oxygen) initiate the catalytic oxidative dehydrogenation of n-butane resulting in weakening of the electrostatics of the gas molecules.

Neutron-diffraction study of CePtSn

We report a neutron- and X-ray-diffraction study of single crystals of CePtSn. As for the crystallographic structure, we have determined that the space group is centrosymmetric Pnma, contrary to the previously reported non-centrosymmetric e-TiNiSi-type structure. At low temperatures two magnetic phase transitions have been observed at TN≈7.5 K and TM≈5.2 K. Our neutron-diffraction data support the previously proposed ‘spin-slip’ magnetic structures with the basic propagation vector q=(0,1/2,0) and sample-dependent spin-slips resulting in various observed incommensurate propagation vectors. Two separately prepared CePtSn single crystals were studied. In one of them we have found coexistence of the propagation vectors q1=(0,0.470,0) and q2=(0,0.422,0) below TM, while only q2 exists between TN and TM. In the other sample the propagation vector q2 also exists for the whole range T<TN and coexists with q1 below TM, but the intensity of q2-related reflections is strongly suppressed with decreasing temperature.

A New Refinement of α-Sr2P2O7

The crystal structure of α-Sr2P2O7, distrontium diphosphate, has been refined by X-ray diffraction using single crystals grown from a SrLiPO4-P2O5 melt cooled between 1323 and 1073 K. The fully anisotropic refinement confirms the centrosymmetric Pnma space group, but also reveals a strong anisotropy along the b direction for two oxygen atoms. This anisotropy suggests atomic scale disorder associated with the local loss of mirror symmetry and the formation of a noncentrosymmetric structural arrangement.

Ferrocenium Tetrachloroferrate Revisited, and Ferrocenium Tetrachloroaluminate

The structure of the title compound, [Fe(Cp)2][FeCl4] (Cp = cyclopentadienyl, C5H5), was reported in 1978 in the non-centrosymmetric space group Pna21 at room temperature [Paulus & Schäfer (1978). J. Organomet. Chem. 144, 205–213]. A low-temperature data set indicates a phase transition from the non-centrosymmetric space group to the centrosymmetric Pnma. Crystals of [Fe(Cp)2][AlCl4] have also been studied at low temperature and are isomorphous with the FeCl4− complex.

Crystal structure analysis of Ca4YFe5O13by combining 1 MeV high-resolution electron microscopy with convergent-beam electron diffraction

By combining 1 MeV high-voltage-high-resolution electron microscopy with convergent-beam electron diffraction (CBED), the crystal structure of a perovskite-related compound, Ca4YFe5O13, is determined. The crystal has orthorhombic symmetry with lattice parameters a = 5.46, b = 37.4 and c = 5.54 A. The space group is determined to be centrosymmetric Pnma by selecting the point group from the CBED patterns. The structure images taken from two principle-axis directions, which are interpreted on the basis of the calculated images, reveal that the structure consists of a succession of the stacking of FeO6 octahedra (O layer) and FeO4 tetrahedra (T layer) and is represented by the stacking sequence of ...OTOOTOTOOTO... along the b axis repeated at intervals of 37.4 A. It is shown that the Ca4YFe5O13 crystal structure can be described in terms of unit-cell-level twinning of Ca2Fe2O5.