Crystallographic study of the phase transition in (Me4P)2ZnBr4

The compound (Me₄P)₂ZnBr₄, a member of the β-K₂SO₄ structure class, undergoes a phase transition at 84°C from the room temperature space group P12₁/c1 to the parent Pmcn structure. The room temperature structure corresponds to a ferrodistortive transition of B_1g 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 Me₄P⁺ cations and the ZnBr₄²⁻ 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.     

Crystal and magnetic structures of Y2CrS4

Chromium(II) sulfide, Y₂CrS₄, prepared by a solid-state reaction of Y₂S₃ and CrS, showed an antiferromagnetic transition at 65 K. The neutron diffraction patterns at 10 and 90 K were both well refined with the space group Pca2₁. At 90 K, cell parameters were a=12.5518(13) Å, b=7.5245(8) Å, and c=12.4918(13) Å. At 10 K, magnetic peaks were observed, which could be indexed on the same unit cell. Magnetic moments of chromium ions were parallel to the b-axis and antiferromagnetically ordered in each set of the 4a sites.     

Structural, vibrational and dielectric properties of new potassium hydrogen sulfate arsenate: K4(SO4)(HSO4)2(H3AsO4)

A new compound, K₄(SO₄)(HSO₄)₂(H₃AsO₄) was synthesized from water solution of KHSO₄/K₃H(SO₄)₂/H₃AsO₄. This compound crystallizes in the triclinic system with space group P1¯ and cell parameters: a=8.9076(2) Å, b=10.1258(2) Å, c=10.6785(3) Å; α=72.5250(14)°, β=66.3990(13)°, γ=65.5159(13)°, V=792.74(3) Å³, Z=2 and ρ_cal=2.466 g cm⁻³. The refinement of 3760 observed reflections (I>2σ(I)) leads to R₁=0.0394 and wR₂=0.0755. The structure is characterized by SO₄²⁻, HSO₄⁻ and H₃AsO₄ tetrahedra connected by hydrogen bridge to form two types of dimer (H(16)S(3)O₄⁻?S(1)O₄²⁻ and H(12)S(2)O₄⁻?H₃AsO₄). These dimers are interconnected along the [1¯ 1 0] direction by the hydrogen bonds O(3)-H(3)?O(6). They are also linked by the hydrogen bridge assured by the hydrogen atoms H(2), H(3) and H(4) of the H₃AsO₄ group to build the chain S(1)O₄?H₃AsO₄ which are parallel to the “a” direction. The potassium cations are coordinated by eight oxygen atoms with K-O distance ranging from 2.678(2) to 3.354(2) Å.Crystals of K₄(SO₄)(HSO₄)₂(H₃AsO₄) undergo one endothermic peak at 436 K. This transition detected by differential scanning calorimetry (DSC) is also analyzed by dielectric and conductivity measurements using the impedance spectroscopy techniques. The obtained results show that this transition is protonic by nature.     

Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(ClH2CCOO)2

The effects of hydrostatic pressure and substitution of Rb⁺for the ammonium cations on the ferroelectric phase transition temperature in NH₄H(ClH₂CCOO)₂ have been studied by electric permittivity measurements. The transition temperature (T_c) decreases with increasing pressure up to 800 MPa and the pressure coefficient dT_c/dp=−1.4×10⁻² [K/MPa] has been experimentally determined. The substitution of Rb⁺ for the ammonium cations has been shown to considerably lower the ferroelectric phase transition temperature T_c. In mixed crystals, additional electric permittivity anomaly has been clearly evidenced. The results are discussed assuming a model, which combines polarizability effects, related to the heavy ion units, with the pseudo-spin tunnelling.     

On asymmetric propagation on transverse acoustic waves near the structural phase transition in RbH3(SeO3)2

Recently ultrasonic measurements of the RbH₃(SeO₃)₂ crystal have shown that a transverse acoustic wave propagating along the modulation axis of the incommensurate structure (c-axis) and polarized along the polar axis (b-axis) exhibits greater temperature anomalies of its velocity and damping than a wave propagating along the b-axis and polarized along the c-axis. The possible explanation of this experimental result is given on the basis of the Landau theory.     

Crystal structure and thermal behavior of two new organic monosulfates NH3(CH2)5NH3SO4 1.5H2O and NH3(CH2)9NH3SO4 H2O

The chemical preparation, the calorimetric studies and the crystal structure are given for two new organic sulfates NH₃(CH₂)₅NH₃SO₄ 1.5H₂O (DAP-S) and NH₃(CH₂)₉NH₃SO₄·H₂O (DAN-S). DAP-S is monoclinic P2₁/n with unit cell dimensions: a=11.9330(2) Å; b=10.9290(2) Å; c=17.5260(2) Å; β=101.873(1)°; V=2236.77(6) Å³; and Z=8. Its atomic arrangement is described as inorganic layers of units and water molecules separated by organic chains. DAN-S is monoclinic P2₁/c with unit cell parameters: a=5.768(2) Å; b=25.890(10) Å; c=11.177(5) Å; β=115.70(4)°; V=1504.0(11) Å³ and Z=4. Its structure exhibits infinite chains, parallel to the [100] direction where the organic cations are interconnected. In both structures a network of strong and weak hydrogen bonds connects the different components in the building of the crystal.     

Fourier transform infrared study of perchlorate (ClO4 and ClO4) anions isomorphously isolated in potassium permanganate matrix. Vibrational anharmonicity and pseudo-symmetry effects

Fourier transform infrared spectra of isotopomeric ³⁵ClO₄⁻ and ³⁷ClO₄⁻ anions isomorphously isolated in potassium permanganate matrix were recorded at room and low temperature (LT, ∼100 K). On the basis of the detected second-order vibrational transitions involving the dopant species ν₃ mode components, anharmonicity constants and harmonic eigenvalues for these modes were calculated. Although the overall appearance of the region of fundamental vibrational transitions in the spectra of dopant perchlorate anions may be better explained in terms of a pseudo-symmetry (the so-called ‘latent’ symmetry) site group C_2v, corresponding to the pseudo-symmetry space group Imma, instead of the (rigorous) crystallographic C_s one (corresponding to the crystallographic Pnma space group), an opposite statement seems to be valid for the region of the second-order vibrational transitions. The vibrational mode mixing (a ‘Fermi-like’ resonance) of the ClO₄⁻ ν₁ mode with the ν_3a site group component is almost negligible.     

Effects of uniaxial pressure and annealing on the resistivity of Ba(Fe1−xCox)2As2

Single crystals of underdoped Ba(Fe_1−xCo_x)₂As₂ were detwinned by applying uniaxial pressure. The anisotropic in-plane resistivity was measured using the Montgomery method without releasing pressure. The resistivity along the a-axis shows metallic behavior down to 5 K, while the resistivity along the b-axis shows an insulator-like behavior in some temperature range. Annealing the sample radically reduces the residual resistivity for x=0, and at the same time the anisotropy becomes much smaller at low temperatures.     

Synthesis and characterization of a new monophosphate [2,5-(CH3)2C6H3NH3]H2PO4

Chemical preparation, calorimetric studies, crystal structure and spectroscopic investigations are given for a new noncentrosymmetric organic cation monophosphate [2,5-(CH₃)₂C₆H₃NH₃]H₂PO₄. This compound is orthorhombic P2₁2₁2₁ with the following unit-cell parameters: a=5.872(4), b=20.984(3), c=8.465(1) Å, Z=4, V=1043.0(5) Å³ and D_x=1.396 g cm⁻³. Crystal structure has been solved and refined to R=0.048 using 2526 independent reflections. Structure can be described as an inorganic layer parallel to (a,b) planes between which organic groups [2,5-(CH₃)₂C₆H₃NH₃]⁺ are located. Multiple hydrogen bonds connecting the different entities of compound thrust upon three-dimensional network a noncentrosymmetric configuration.     

Phonon dynamics of hexagonal Y Mn1−xFexO3

Phonon dynamics was investigated on Y Mn_1−xFe_xO₃(0≤x≤0.20) hexagonal manganite polycrystals. Phonon modes were properly assigned using results obtained on Y MnO₃ single crystal, by mixing the ab plane and the c axis optical responses. Upon increasing the Fe content, most of the phonon mode frequencies do not vary drastically. However, some modes involving Y and O atom displacements along the c axis are affected by doping. Indeed, a redshift of their transverse optical frequency was interpreted as an elongation of Y-O distance along the c-axis, resulting in a change in yttrium coordination.     

X-ray diffraction, Cs and H NMR and thermal studies of CdZrCs1.5(H3O)0.5(C2O4)4·xH2O displaying Cs and water dynamic behavior

A novel mixed cadmium zirconium cesium oxalate with an open architecture has been synthesized from precipitation methods at room pressure. It crystallizes with an hexagonal symmetry, space group P3₁12 (no. 151), a=9.105(5) Å, c=23.656(5) Å, V=1698(1) Å³ and Z=3. The structure displays a [CdZr(C₂O₄)₄]²⁻ helicoidal framework built from CdO₈ and ZrO₈ square-based antiprisms connected through bichelating oxalates, which generates channels along different directions. Cesium cations, hydronium ions and water molecules are located inside the voids of the anionic framework. They exhibit a dynamic disorder which has been further investigated by ¹H and ¹³³Cs solid-state NMR. Moreover a phase transition depending both upon ambient temperature and water vapor pressure was evidenced for the title compound. The thermal decomposition has been studied in situ by temperature-dependent X-ray diffraction and thermogravimetry. The final product is a mixture of cadmium oxide, zirconium oxide and cesium carbonate.     

Raman spectroscopy of RuSr2(Eu1.5Ce0.5)Cu2O10 magneto-superconductor

Raman spectroscopy studies are reported for the RuSr₂Eu_1.5Ce_0.5Cu₂O₁₀ (Ru-1222) compound at various temperatures of 300, 250, 200 and 90 K. Three distinct vibrational bands: the first at 110, 140, and 160 cm⁻¹, the second at 295 and 347 cm⁻¹, and third one at 651 cm⁻¹ are seen in Raman spectra of the compound at room temperature. These bands are attached to the Cu atoms’ c-direction, the Ru atoms’ ab-plane stretching and Ru atoms’ c-direction anti-stretching modes. Below 200 K, an extra vibrational mode is also seen at 260 cm⁻¹. Also, with a decrease in temperature, though the Cu vibrational modes remain intact, the Ru atoms’ ab-plane stretching (295 cm⁻¹) and c-direction anti-stretching (651 cm⁻¹) modes shift gradually to higher wave number positions. The frequencies of modes at 260 and 651 cm⁻¹ showed anomalous softening and line-width broadening below 100 K that corroborates well with the spin ordering seen in susceptibility studies. The studied compound is a ferromagnetic superconductor with magnetic ordering of the Ru spins at 200 K and superconductivity below 30 K. A magnetic and electrical transport characterization of the compound is also presented briefly.     

Magnetic phase diagram of double-layered La2−2xCa1+2xMn2O7 manganite

Double-layered manganite La_2−2xCa_1+2xMn₂O₇ have been synthesized for compositions ‘x’=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 by solid state reaction method. From X-ray diffraction study, their crystal structures were found to be tetragonal perovskite with lattice parameters decreasing with increasing ‘x’. The decreasing lattice parameters affect the balance between in-plane, intra-bilayer and inter-bilayer exchange interactions, which is reflected on magnetotransport properties. The metal-to-insulator transition temperature is found to vary with composition and peaked around ‘x’=0.3. From ac-susceptibility study, 2D-ferromagnetic ordering was observed at higher temperatures for all compositions whereas 3D-ferromagnetic ordering was observed at quite low temperatures. In low-temperature region, decreasing susceptibility shows antiferromagnetic state for all compositions. On the basis of electrical and magnetic properties, a magnetic phase diagram is given.     

Effect of 3d metal ion doping on the structure and superconductivity of (Tl0.5Pb0.5)Sr2CaCu2O7

(Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ (M=Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single ‘1212’ phase. They crystallize in a tetragonal unit cell with a=3.8028-3.8040 Å and c=12.0748-12.1558 Å. In (Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ system (M=Co or Ni), the superconducting critical temperature T_c decreases linearly with both Co and Ni concentrations and the rate of T_c decrease is around −6.5 and −7.0 K/at%, respectively. For (Tl_0.5Pb_0.5)Sr₂Ca (Cu_2−xZn_x)O₇ system, the dependence of T_c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses T_c much less (−2.5 K/at%) than the Co and Ni substitutions. The suppression in T_c in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of T_c in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu d_x²−y² state due to the full d band of Zn ions.     

Annealing temperature influence on electrical properties of ion beam sputtered Bi2Te3 thin films

Ion beam sputtering process was used to deposit n-type fine-grained Bi₂Te₃ thin films on BK7 glass substrates at room temperature. In order to enhance the thermoelectric properties, thin films are annealed at the temperatures ranging from 100 to 400 °C. X-ray diffraction (XRD) shows that the films have preferred orientations in the c-axis direction. It is confirmed that grain growth and crystallization along the c-axis are enhanced as the annealing temperature increased. However, broad impurity peaks related to some oxygen traces increase when the annealing temperature reached 400 °C. Thermoelectric properties of Bi₂Te₃ thin films were investigated at room temperature. The Bi₂Te₃ thin films, including as-deposited, exhibit the Seebeck coefficients of −90 to −168 μV K⁻¹ and the electrical conductivities of 3.92×10²-7.20×10² S cm⁻¹ after annealing. The Bi₂Te₃ film with a maximum power factor of 1.10×10⁻³ Wm⁻¹ K⁻² is achieved when annealed at 300 °C. As a result, both structural and transport properties have been found to be strongly affected by annealing treatment. It was considered that the annealing conditions reduce the number of potential scattering sites at grain boundaries and defects, thus improving the thermoelectric properties.     

Neutron powder diffraction study of the Nd(Ba1−xNdx)2Cu3O7+δ solid solution

The Nd(Ba_1−xNd_x)₂Cu₃O_7+δ solid solution, Nd123ss, has been investigated by neutron powder diffraction and Rietveld analysis. It is confirmed that the crystal structure of its Nd-rich limit, Nd(Ba_0.55Nd_0.45)₂Cu₃O_7.33, is satisfactorily described in the space group Bmmm (a=7.7679(3), b=3.8542(1), and c=22.9590(9) Å). The fourfold superstructure with respect to the orthorhombic cell of YBCO is due to ordering between Ba and Nd atoms in the bridging layer. Differences with previous works concern exclusively the distribution of O atoms in the ‘chain’ layer. Our results give strong indications that ordering also occurs for lower Nd contents.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).     

Synthesis and crystal structure of [3,5-(CH3)2C6H3NH3]4P4O12·3H2O

Chemical preparation and crystal structure are given for a new cyclotetraphosphate: [3,5-(CH₃)₂C₆H₃NH₃]₄P₄O₁₂·3H₂O. This compound is triclinic P with the following unit-cell parameters: a=8.298(3), b=8.299(3), c=17.242(7)Å, α=97.13(3), β=102.72(3), γ=64.55(3)°, Z=1 and V=1045.2(8)ų. The crystal structure has been solved and refined to R=0.040 using 6086 independent reflections. The atomic arrangement can be described as layers organization. Layers built by P₄O₁₂ ring anions, ammonium groups and water molecules parallel to the plan (001), between which the organic groups are located. Characterization by X-ray diffraction, IR absorption, and thermal analysis are described.     

Dielectric and piezoelectric properties of barium-modified Aurivillius-type Na0.5Bi4.5Ti4O15

In this study, monophasic Ba_x(Na_0.5Bi_0.5)_1−xBi₄Ti₄O₁₅ (x=0.03, 0.06, 0.09 and 0.12) ceramics fabricated from the powders synthesized via the solid-state reaction route exhibited relaxor behavior. X-ray diffraction analysis revealed that the barium-modified Na_0.5Bi_4.5Ti₄O₁₅ ceramics have a pure four-layer Aurivillius phase structure. Dielectric properties and phase transitions were studied and are explained in terms of lattice response of these ceramics. A shift in ferroelectric–paraelectric phase transition (T_c) to lower temperatures and a corresponding increase in permittivity peak with increasing concentration of Ba²⁺ are also observed. The decrease of orthorhombicity in the lattice structure by the larger Ba²⁺ ion incorporation, indicating an approach of a and b parameters, results in lower Curie temperature. The piezoelectric activity of Na_0.5Bi_4.5Ti₄O₁₅ (NBT) ceramics was significantly improved by the modification of barium. The Curie temperature T_c and piezoelectric coefficient d₃₃ for the composition with x=0.12 were found to be 635 °C and 21 pC/N, respectively. The relationship of polarization with lattice response is discussed.