Room Temperature Space Group Research Articles

Structural characterization and dielectric behavior of substituted bismuth niobate Bi17Nb3-4xV2xP2xO33 with fluorite-like structure

Structural, morphological, and dielectric properties of substituted bismuth niobates Bi17Nb3-4xV2xP2xO33 with a fluorite-like structure have been reported. The study focuses on substituting Nb5+ with V5+ and P5+ cations in Bi17Nb3O33. The resulting ceramics were synthesized using the solid-state reaction technique, and their crystal structure was analyzed using X-ray diffraction. The analysis confirmed the stabilization of the compounds in a single phase of a cubic fluorite structure. Structural refinement analysis has been performed for the compound Bi17Nb2.4V0.3P0.3O33 to investigate the details of the crystalline structure which was found to be a cubic fluorite structure at room temperature (space group Fm3‾m). Scanning Electron Microscopy (SEM) was used to examine the surface morphology of the samples, which exhibited irregular shapes and heterogeneous size distribution. The dielectric properties of the ceramics were analyzed, and the dielectric permittivity (ε’) and loss factor (tan δ) were measured at different frequencies and temperatures. The results showed anomalies in each frequency-dependent curve, indicating a relaxation phenomenon. The electrical conductivity (σ) of the ceramics was studied, and it was found that the conductivity displayed a frequency-independent plateau at low frequencies, indicating a semiconductor behavior. The complex impedance spectra and electrical modulus analysis were also conducted to investigate the relaxation processes in the material.

Topotactic synthesis of DyVO3; an investigation of magnetocaloric effect

The orthovanadate DyVO3 has been synthesized by reducing DyVO4 in H2 environment. The material crystallizes in an orthorhombic crystal structure at room temperature (space group Pbnm). The antiferromagnetic transition of the vanadium sublattice is observed near 114 K. The effect of an external magnetic field on spin re-arrangement in the vanadium sublattice has been studied. The magnetic entropy change has been calculated from the isothermal magnetization measured in the magnetic field 0–5 T. At 2 K, the M−H isotherm saturates with the increasing magnetic field. The maximum magnetic entropy change (-ΔSMmax) has been estimated to be 12.9 J kg−1 K−1 at an applied external magnetic field of 5 T around 17 K, and a relative cooling power ∼ 348 J/kg. The maximum entropy change occurs around the ordering temperature of the Dy-sublattice. The significant -ΔSMmax and relative cooling power values suggest it to be a better magnetocaloric material than several other polycrystalline perovskite compounds.

Structural, morphological, dielectric and optical properties of double perovskites RBaFeTiO6 (R = La, Eu).

Double perovskites RBaFeTiO6 (R = La, Eu) were successfully synthesized using a solid state reaction route. Structural refinement analysis has been performed to investigate the details of the crystalline structure which was found to be a cubic double perovskite structure at room temperature (space group Pm3̄m, No. 221). Crystallite size, lattice strain, density, and porosity parameters were also calculated. The morphology and the elemental composition were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and elemental mapping. FTIR and Raman scattering spectroscopy have been performed in order to study the vibrational modes and the various bond formations of the synthesized samples. The optical properties investigated by means of the optical absorbance measurements highlight that both materials present large band gap energies, and are thus potential candidates for various technological applications. The electrical behavior of these double perovskites was also studied by frequency-dependent dielectric measurements and impedance spectroscopy. The electrical conduction follows Jonscher's power law and the conduction mechanisms are identified.

Structural Evolution of Ferroelectric and Ferroelastic Barium Sodium Niobate Tungsten Bronze.

The crystal structure of the ferroelastic and ferroelectric tungsten bronze Ba2NaNb5O15 (BNN) has been debated. Here, we re-examine the crystal structure of BNN by ambient powder X-ray diffraction combined with density functional theory calculations. We demonstrate that the room temperature space group is Cmm2 with significant cation disorder on the Ba and Na cation sublattices. Density functional theory calculations reveal a relatively flat energy landscape between structures of different symmetries, including the energetics of cation disorder. We also study the structural evolution and the ferroelectric and ferroelastic phase transitions by high-temperature X-ray diffraction and dilatometry. The ferroelectric phase transition at 570 °C is of first order and cause the cell to expand in the c direction, while the ferroelastic distortion starting at 270 °C takes place in the ab plane and does not affect the polarization. The phase transitions are not coupled, which means that BNN is a ferroic material with two primary and uncoupled order parameters.

Electron diffraction study of the space group of Bi5Ti3FeO15 multiferroic ceramic.

Bi5Ti3FeO15 (pentabismuth trititanium iron pentadecaoxide), which is a multiferroic four-layer Aurivillius phase compound, has received much attention in recent years. However, three mutually inconsistent orthorhombic space groups, i.e. A21am, Fmm2 and Pnn2, have been reported for the room-temperature phase of Bi5Ti3FeO15 by X-ray and neutron diffraction investigations. Here, electron diffraction results are presented and discussed for the first time to unambiguously clarify the room-temperature space group of ceramic Bi5Ti3FeO15. It has been found that all the observed reflections from the ceramic agree with those expected in A21am, while the observed reflections 011, 013 and 015 should be forbidden in the case of Fmm2, and no 107 and 109 reflections were observed although allowed for Pnn2. The present study has demonstrated that the space group of Bi5Ti3FeO15 ceramic is A21am rather than Fmm2 or Pnn2, an identification that proved to be a challenge for X-ray diffraction. On the basis of the space group A21am, the lattice parameters of the Bi5Ti3FeO15 ceramic were calculated from its X-ray diffraction data.

Hydrogen-bonded and supramolecular ferroelectricity in a new hybrid (C12H25NH3)2CoCl4

A new organic-inorganic layered halide perovskite ((C12H25NH3)2CoCl4) has been synthesized and characterized by: chemical analysis, x-ray powder diffraction, differential scanning calorimetry, and differential thermal analysis thermographs. The dielectric and ferroelectric properties have been investigated. Analysis of the ac conductivity σac (ω, T) measured in the temperature interval from 110 K to 370 K and at selected frequencies in the range 1 kHz < f < 100 kHz and the results show that the Jonscher universal power law is accurately obeyed. The existence of ferroelectricity has been demonstrated by the hysteresis P-E loop. The collective data evidenced that the hybrid behaves as a proper ferroelectric rather than a relaxor one and undergoes an order-disorder ferroelectric phase transition at the Curie temperature Tc (Tc ≈ 360 K) accompanied with a structural distortion from an intercalated to non- intercalated state as the other Co and/or Zn hybrids. Since the room temperature space group of the hybrid is centrosymmetric (P21/c), the ferroelectricity should be due to the supramolecular interaction which results from the induced dipoles. The hydrogen bonding system of the type N–H….Cl plays the key role in the mechanism of supramolecular ferroelectricity.

Characterization of competing distortions inYFe2O4

We report the structural changes of three $\mathrm{YF}{\mathrm{e}}_{2}{\mathrm{O}}_{4\ensuremath{-}\ensuremath{\delta}}$ ($\ensuremath{\delta}l0.1$) specimens using high resolution synchrotron x-ray powder diffraction between 80 and 300 K. All samples adopt a rhombohedral cell at room temperature (space group $R\overline{3}m$). This cell becomes unstable for the three samples on cooling, and the oxygen-poor specimen ($\ensuremath{\delta}\ensuremath{\sim}0.1$) shows a single transition at 240 K. The nearly stoichiometric ($\ensuremath{\delta}\ensuremath{\le}0.03$) compounds exhibit two structural transitions with decreasing temperature at about 240 and 200 K. Each transition is revealed by an anomaly in the heat capacity measurements and a jump in the electric resistivity. Below 240 K, a strong splitting of some diffraction peaks is accompanied by the occurrence of superstructure peaks that follow the propagation vector $\mathbf{k}=(1/7,\ensuremath{-}2/7,9/7)$. The cell symmetry is then triclinic, and the structural transition is characterized by an expansion of the $c$ axis coupled to a contraction of the other two lattice parameters. There are 49 nonequivalent sites for Fe atoms with a maximum charge disproportionation of $\ensuremath{\sim}0.5\phantom{\rule{0.16em}{0ex}}{\mathrm{e}}^{\ensuremath{-}}$. Upon cooling at 200 K, the previous superstructure peaks begin to vanish, and finally they are replaced by a new set of superstructure peaks following the propagation vector $\mathbf{k}=(1/4,1/2,1/4)$ with respect to the rhombohedral cell. The transition is also reflected in sudden changes in the lattice parameters that seem to smooth the changes observed in the previous transition. The new cell is also triclinic, and there are 48 nonequivalent Fe sites with a maximum charge disproportionation of $\ensuremath{\sim}0.7\phantom{\rule{0.16em}{0ex}}{\mathrm{e}}^{\ensuremath{-}}$. Both phases coexist in a wide temperature range because this second transition is not completed at 80 K. A symmetry mode analysis indicates a complicated pattern for the charge distribution in the Fe sublattice of both distorted structures but clearly discard any bimodal distribution of only two types of Fe cations. Therefore, the sharp jumps in the electric resistivity at the phase transitions are clearly correlated with two different structural changes. Finally, the oxygen stoichiometry seems to be a key factor in the stabilization of the different distorted structures.

Structure and Magnetic Properties of Triclinic Ni0.6Co0.4TiO3 Ilmenite Oxide

NiTiO3 and CoTiO3 ilmenites (space group ▪) form a homogeneous mixture with remarkably low crystal symmetry. According to neutron and synchrotron X-ray powder diffraction measurements, the room-temperature space group symmetry of the Ni0.6Co0.4TiO3 sample is triclinic ▪. This structural distortion is the first known case in ilmenites and opens up ways to modify functional properties of magnetic oxides. The origin of the distortion is discussed. Transitions from a paramagnetic to a weak ferromagnetic and further to a ferrimagnetic phase occur at 69 and 25K, respectively. The ferromagnetic phase is characteristic of the solid-solution and is not found in the constituents.

Hydrogen-bond-dynamics-based switching of conductivity and magnetism: a phase transition caused by deuterium and electron transfer in a hydrogen-bonded purely organic conductor crystal.

A hydrogen bond (H-bond) is one of the most fundamental and important noncovalent interactions in chemistry, biology, physics, and all other molecular sciences. Especially, the dynamics of a proton or a hydrogen atom in the H-bond has attracted increasing attention, because it plays a crucial role in (bio)chemical reactions and some physical properties, such as dielectricity and proton conductivity. Here we report unprecedented H-bond-dynamics-based switching of electrical conductivity and magnetism in a H-bonded purely organic conductor crystal, κ-D3(Cat-EDT-TTF)2 (abbreviated as κ-D). This novel crystal κ-D, a deuterated analogue of κ-H3(Cat-EDT-TTF)2 (abbreviated as κ-H), is composed only of a H-bonded molecular unit, in which two crystallographically equivalent catechol-fused ethylenedithiotetrathiafulvalene (Cat-EDT-TTF) skeletons with a +0.5 charge are linked by a symmetric anionic [O···D···O](-1)-type strong H-bond. Although the deuterated and parent hydrogen systems, κ-D and κ-H, are isostructural paramagnetic semiconductors with a dimer-Mott-type electronic structure at room temperature (space group: C2/c), only κ-D undergoes a phase transition at 185 K, to change to a nonmagnetic insulator with a charge-ordered electronic structure (space group: P1). The X-ray crystal structure analysis demonstrates that this dramatic switching of the electronic structure and physical properties originates from deuterium transfer or displacement within the H-bond accompanied by electron transfer between the Cat-EDT-TTF π-systems, proving that the H-bonded deuterium dynamics and the conducting TTF π-electron are cooperatively coupled. Furthermore, the reason why this unique phase transition occurs only in κ-D is qualitatively discussed in terms of the H/D isotope effect on the H-bond geometry and potential energy curve.

Spin and orbital order in the vanadium spinelMgV2O4

We present a unique study of the frustrated spinel MgV2O4 which possesses highly coupled spin, lattice and orbital degrees of freedom. Using large single-crystal and powder samples, we find a distortion from spinel at room temperature (space group F-43m) which allows for a greater trigonal distortion of the VO6 octahedra and a low temperature space group (I-4m2) that maintains the mirror plane symmetry. The magnetic structure that develops below 42 K consists of antiferromagnetic chains with a strongly reduced moment while inelastic neutron scattering reveals one-dimensional behavior and a single band of excitations. The implications of these results are discussed in terms of various orbital ordering scenarios. We conclude that although spin-orbit coupling must be significant to maintain the mirror plane symmetry, the trigonal distortion is large enough to mix the 3d levels leading to a wave function of mixed real and complex orbitals.

Low-temperature synthesis and structural properties of ferroelectric K 3WO 3F 3 elpasolite

Low-temperature ferroelectric G2 polymorph of K 3WO 3F 3 has been prepared by chemical synthesis. Structural and chemical properties of the final product have been evaluated with X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Structure parameters of G2–K 3WO 3F 3 are refined by the Rietveld method from XRD data measured at room temperature (space group Cm, Z = 2, a = 8.7350(3) Å, b = 8.6808(5) Å, c = 6.1581(3) Å, β = 135.124(3) Å, V = 329.46(3) Å 3; R B = 2.47%). Partial ordering of oxygen and fluorine atoms has been found over anion positions. Mechanism of ferroelectric phase transition in A 2BMO 3F 3 oxyfluorides is discussed.

Re-investigation of the structure and crystal chemistry of the Bi2O3–W2O6 `type (Ib)' solid solution using single-crystal neutron and synchrotron X-ray diffraction

Single crystals of composition Bi(35.66)W(4.34)O(66.51) (or Bi(8.2)WO(15.3), bismuth tungsten oxide), within the type (Ib) solid-solution region of the Bi(2)O(3)-WO(3) system, were synthesized using the floating-zone furnace method. Synchrotron X-ray and neutron single-crystal diffraction data were used to confirm the previously tentative assignment of the room-temperature space group as I4(1). Fourier analysis of the combined X-ray and neutron datasets was used to elucidate and refine fully the cation and anion arrays for the first time. The mixed cation site M1 is shown to be coordinated by eight O atoms in an irregular cube when M = Bi, and by six O atoms in an octahedron when M = W. The resulting disorder in the average structure around M1 is discussed in the context of experimentally observed oxide-ion conductivity.

Crystal structures and temperature-induced phase transitions of [formula omitted], and of its transformation to [formula omitted

We present a new effective method for synthesizing Sr 2 MWO 6 double perovskite oxides: the co-precipitation route at 1220 K in nitrogen environment. Using conventional X-ray diffraction methods, we have confirmed the room-temperature space group of Sr 2Mn 2+ WO 6 to be P 2 1 / n : Z = 2; a = 5.6764 ( 1 ) Å ; b = 5.6752 ( 1 ) Å ; c = 8.0144 ( 1 ) Å ; β = 90.12 ( 1 ) ° and V = 258.18 ( 1 ) Å 3 . We show that the compound presents the following temperature induced phase-transition sequence: P 2 1 / n → I 4 / m → Fm 3 ¯ m . After a thermal treatment, 24 h at 870 K in air, the Sr 2 Mn 2 + WO 6 compound transforms irreversibly to Sr 2 Mn 3 + WO 6 + δ . This transformation has been confirmed by EPR and XANES measurements. Using conventional X-ray diffraction methods, we have shown the room-temperature space group of Sr 2 Mn 3 + WO 6 + δ to be I 4 / m : Z = 2; a = 5.6353 ( 1 ) Å ; c = 8.0149 ( 1 ) Å and V = 254.53 ( 1 ) Å 3 . We show that the compound presents the following temperature induced phase-transition sequence: I 4 / m ( → I 4 / mmm ) → Fm 3 ¯ m . The tetragonal-to-tetragonal phase transition is suggested to be present, it is not observed as it is very weak; it is attributed to the presence of the Jahn-Teller active Mn 3+ cation.

Electrical transport and magnetism in Mo-substituted R2Ti3Ge4 (R=Tb,Er) compounds

The effect of Mo substitution at Ti site of orthorhombic Sm5Ge4-type R2Ti3Ge4 compounds on the magnetic and electrical transport properties has been studied. The Tb2Ti3−xMoxGe4 (x=0.3,0.75) and Er2Ti2.7Mo0.3Ge4 compounds have been synthesized and it is found that these compounds retain parent crystal structure at room temperature (space group Pnma, No. 62). Mo substitution decreases the antiferromagnetic ordering temperature (TN) of Tb2Ti3Ge4 compound from ∼18 to ∼13 and ∼10 K, respectively, for x=0.3 and 0.75. The Er2Ti2.7Mo0.3Ge4 compound shows a tendency to order at ∼2 K, whereas the parent Er2Ti3Ge4 is magnetically ordered at 3 K. Magnetization versus field data of Tb2Ti3−xMoxGe4 (x=0.3,0.75) reveal soft ferromagnetic nature. The metamagnetic transition that is present in parent Tb2Ti3Ge4 is found to disappear with Mo substitution. Magnetization value reaches ∼6.2μB/Tb3+ at 2 K in fields of 8 T, indicating incomplete ferromagnetic ordering with or without an antiferromagnetic component. Electrical resistivity of the Tb-based compounds has a linear variation with temperature from 300 to ∼50 K and shows a prominent slope change at temperatures much above TN, supporting the presence of competing short range ferromagnetic interactions.

Structural and magnetic phase transitions of theV4-cluster compoundGeV4S8

The phase transitions of the strongly correlated tetrahedral ${\text{V}}_{4}$-cluster compound ${\text{GeV}}_{4}{\text{S}}_{8}$ have been studied by low-temperature powder x-ray diffraction, magnetic susceptibility, and specific heat measurements. The crystal structure is cubic at room temperature (space group $F\overline{4}3m$) and transforms to orthorhombic (space group $Imm2$) at ${T}_{S}=30\text{ }\text{K}$. A Jahn--Teller distortion reduces the symmetry of the ${\text{V}}_{4}$-cluster from $\overline{4}3m$ to $mm2$. The second transition at 18 K is the onset of antiferromagnetic ordering without symmetry change but with a certain increase in the distortion. The latter reflects a strong magnetoelastic coupling at ${T}_{N}$. Specific heat anomalies at 30 and 18 K confirm the two phase transitions.

Raman Scattering Studies of the Ba2MnWO6 and Sr2MnWO6 Double Perovskites.

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Raman Scattering Studies of the Ba2MnWO6 and Sr2MnWO6 Double Perovskites

Polycrystalline Ba2MnWO6 (BMW) and Sr2MnWO6 (SMW) samples were studied between 80 and 1200 K by Raman scattering spectroscopy. In the case of BMW (space group Fmm), four Raman active vibrational modes, predicted by factor group analysis, were identified. Raman scattering studies with different wavelengths revealed a resonant bands between 300 and 800 cm-1. The origin of these bands was related to the Franck-Condon process. Line broadening versus temperature and phonon frequency were studied, and a qualitative explanation was proposed. SMW samples had considerably more complex Raman spectra. It was found that SMW transformed from tetragonal (room-temperature space group P42/n) to the cubic phase between 670 and 690 K; the phase transition temperature was dependent on sample preparation conditions, and it was considerably lower than in the case of large grain size powders. The role of grain size in phase transition is discussed. Mn ions were found to have a crucial role in the lattice dynamics of both materials.

Sodium percarbonate between 293 and 100 K.

Sodium carbonate perhydrate, Na(2)CO(3).1.5H(2)O(2), commonly known as sodium percarbonate, has been structurally characterized at temperatures ranging from 293 down to 100 K. As the temperature drops, there is a reversible phase change between 250 and 240 K, in which the crystallographic mirror plane of the room-temperature space group, Cmca, is lost, transforming it to Pbca. This transformation is accompanied by a reversal in the expansion coefficient of cell side b, which, like the other two sides, initially contracts on cooling. The two crystallographically independent hydrogen peroxide sites undergo the largest change, where the disorder seen at room temperature gradually diminishes and is almost completely absent at 100 K, thus indicating dynamic rather than static disorder. It is noteworthy that, despite Cmca being the preferred room-temperature space group in the current work, two previous single-crystal studies rejected this space group in favour of non-centrosymmetric Aba2 [Carrondo et al. (1977). J. Chem. Soc. Dalton Trans. pp. 2323-2331; Adams & Pritchard (1977). Acta Cryst. B33, 3650-3653]. A re-evaluation of these determinations indicates that they are identical to the current structure and should be re-assigned to Cmca.

Ferroelectric and Ionic-Conductive Properties of Nonlinear-Optical Vanadate, Ca9Bi(VO4)7

Structural, chemical, and physical properties of whitlockite-type Ca9Bi(VO4)7 were studied by X-ray powder diffraction (XRD), electron diffraction (ED), second-harmonic generation (SHG), thermogravimetry, differential scanning calorimetry, dielectric, and electrical-conductivity measurements. A new phase-transition of the ferroelectric type was found in Ca9Bi(VO4)7 with a transition temperature, Tc, of 1053 ± 3 K. The polar phase, β-Ca9Bi(VO4)7, is stable below Tc down to at least 160 K. The centrosymmetric β‘-phase is stable above Tc up to 1273 ± 5 K. Above 1273 K, it decomposes to give BiVO4 and whitlockite-type solid solutions of Ca9+1.5xBi1-x(VO4)7. The β ↔ β‘ phase transition is reversible and of second order. Electrical conductivity of β‘-Ca9Bi(VO4)7 is rather high (σ = 0.6 × 10-3 S/cm at 1200 K) and obeys the Arrhenius law with an activation energy of 1.0 eV. Structure parameters of Ca9Bi(VO4)7 are refined by the Rietveld method from XRD data measured at room temperature (space group R3c; Z = 6; a = 10.8992(1) Å, c = 38.1192(4) Å, and V = 3921.6(1) Å3; Rwp = 3.06% and Rp = 2.36%). Bi3+ ions together with Ca2+ ions are statistically distributed among the M1, M2, M3, and M5 sites. Ca9Bi(VO4)7 has a SHG efficiency of about 140 times that of quartz. Through the powder SHG measurements, we estimated the nonlinear optical susceptibility, 〈d〉, at about 6.1−7.2 pm/V. This value for Ca9Bi(VO4)7 is comparable with that for known nonlinear optical materials such as LiNbO3 and LiTaO3.

Crystallographic study of the phase transition in (Me 4P) 2ZnBr 4

The compound (Me 4P) 2ZnBr 4, a member of the β-K 2SO 4 structure class, undergoes a phase transition at 84°C from the room temperature space group P12 1/ c1 to the parent Pmcn structure. The room temperature structure corresponds to a ferrodistortive transition of B 1 g symmetry at the zone center. At room temperature, the compound has lattice constants a=9.501(1), b=16.055(2), c=13.127(2) Å and β=90.43(1)°. For the high temperature phase, the orthorhombic cell has dimensions a=9.466(2), b=16.351(3) and c=13.284(2) Å. The structures consist of two crystallographically independent Me 4P + cations and the ZnBr 4 2− anions. In the room temperature phase, all three ionic species show substantial displacement from the mirror plane perpendicular to the a-axis that exists in the high temperature phase, as well as rotations out of that plane. The thermal parameters of the cations are indicative of substantial librational motion. Measurements of lattice parameters have been made at 2–5°C intervals over the temperature range 40–140°C. The changes in the lattice constants appear continuous at T c (within experimental limits) indicating that the phase transition is likely second-order. The a lattice constant shows an anomalous shortening as T c is approached. Thermal expansion coefficients are calculated from this data. An application of Landau theory is used to derive the temperature dependencies of spontaneous shear strain and corresponding elastic stiffness constants associated with the primary order parameter.