Spectroscopic modifications of Pippard relations: first-order and second-order phase transitions in NH4Cl

In this study we introduce the spectroscopic modifications of Pippard relations and apply them to the disorder-induced Raman modes of NH₄Cl in the first-order (P=0) and second-order (2.8 kbar) phase regions in this crystalline system. We obtain linear variations of the specific heat C_P with our observed frequency shifts [(1/ν)(∂ν/∂T)_P] of those Raman modes studied for the first-order and second-order phase transitions in NH₄Cl. This will be discussed in detail.     

Pippard relations applied to the lambda-phase transition in NH4Br

We establish here a linear variation of the specific heat C(P) with the frequency shifts (1/nu)( partial differentialnu/ partial differentialT)(P) close to the lambda-phase transition in NH(4)Br (T(lambda)=234K). This linear relationship is based on our spectroscopically modified Pippard relations. For this verification of our relations, we use our observed Raman frequencies of the lattice mode of nu(7) (56cm(-1)) and the internal mode of nu(2) (1684cm(-1)) in the NH(4)Br crystalline system. Our study given here indicates that the thermodynamic data can be obtained from the measured frequencies.     

Molar volume calculated at various pressures and the Pippard relations close to the melting point in benzene

The molar volume of solid and liquid benzene was calculated at various pressures (at constant temperatures), and the Pippard relations were examined close to the melting point in this organic molecule.

The molar volume calculated is in good agreement with the observed data, which decreases as the pressure increases up to about 150 MPa. The Pippard relations are also valid within this pressure range at constant temperatures studied here for the solid and liquid phases of benzene.     

Structural phase transitions in the relaxor ferroelectric Pb2Bi4Ti5O18

The relaxor ferroelectric Pb₂Bi₄Ti₅O₁₈ has been studied by Rietveld refinement of powder neutron diffraction data collected at temperatures of 100, 250 and 400 °C. Our refinements are compatible with the ‘average’ crystal structure of Pb₂Bi₄Ti₅O₁₈ undergoing the phase transition sequence F2mm→I4mm→I4/mmm as a function of increasing temperature, with the latter phase being observed above the known ferroelectric Curie temperature, T_m, and the intermediate phase consistent with a previously observed dielectric anomaly around 207 °C. The results are, however, in conflict with both observation of a symmetry lowering (to space group B2eb) in the lowest temperature phase, observed by electron diffraction, and also with electrical property measurements, which suggest both a- and c-axis polarisation up to T_m. Nevertheless, these crystallographic results are consistent with the observation of relaxor behaviour in this material, and underline the importance of considering ‘long-range’ versus ‘local’ structural effects in relaxor materials.     

Spectroscopic modifications of the Pippard relations for NaNO2 in the sinusoidal antiferroelectric phase

We establish in this study a linear variation of the specific heat Cp with the frequency shifts (l/v)(delta v/delta T) in the sinusoidal antiferroelectric phase of NaNO2, according to the first Pippard relation which we have modified spectroscopically. For this linear relationship we use the Brillouin frequencies of acoustic phonon modes for the q[010], q[001] and q[100] modes of NaNO2 in the sinusoidal antiferroelectric phase. We then calculate dT/dP values due to those modes studied for NaNO2, which agree with the literature values.     

Crystal structure of the ‘mixed-layer’ Aurivillius phase Bi5TiNbWO15

The crystal structure of the Aurivillius phase Bi₅TiNbWO₁₅ has been analyzed in detail using powder X-ray and neutron diffraction. The structure can be described as a regular intergrowth of alternating single and double perovskite-like layers sandwiched between fluorite-like bismuth oxide layers, such that the layer sequence is … [WO₄]-[Bi₂O₂]-[BiTiNbO₇]-[Bi₂O₂] …. There is complete ordering of tungsten within the B sites of the single perovskite layer, so that the structure can be described as a direct intergrowth of the ‘component’ Aurivillius phases Bi₂WO₆ and Bi₃TiNbO₉. At 25 °C the structure adopts the polar orthorhombic space group I2cm, , , .     

EPR evidence of possible metastable regions in Mn2+ doped ferroelectric NaNO2

Of the various EPR spectra observed in the ferroelectric phase of NaNO₂, a spectrum designated as spectrum II is observed to have some peculiar properties like dependence of its intensity on growth conditions of the crystal, its disappearance below ≈60°C after heat treatment of the crystal, its reappearance by heating above ≈60°C and also by application of an external dc electric field at ≈30°C. These observations suggest the possible existence, around Mn²⁺-vacancy complex in the ferroelectric phase of NaNO₂, of a metastable structure intermediate to the paraelectric and ferroelectric phase structures.     

Defect and dopant properties of the Aurivillius phase Bi4Ti3O12

Computer modelling techniques have been used to investigate the defect and oxygen transport properties of the Aurivillius phase Bi₄Ti₃O₁₂. A range of cation dopant substitutions has been considered including the incorporation of trivalent ions (M³⁺=Al, Ga and In). The substitution of In³⁺ onto the Bi site in the [Bi₂O₂] layer is predicted to be the most favourable. The calculations suggest that lanthanide (Ln³⁺) doping at the dilute limit preferentially occurs in the [Bi₂O₂] layer, with probable distribution over both the [Bi₂O₂] and the perovskite A-site at higher dopant levels. It is predicted that the reduction process involving Ti³⁺ and oxygen vacancy formation is energetically favourable. The energetics of oxide vacancy migration between various oxygen sites in the structure have been investigated.     

Evolution and characterization of fluorite-like nano-SrBi2Nb2O9 phase in the SrO-Bi2O3-Nb2O5-Li2B4O7 glass system

Transparent glasses of various compositions in the system (100−x)Li₂B₄O₇−x(SrO-Bi₂O₃-Nb₂O₅) (where x=10, 20, 30, 40, 50 and 60, in molar ratio) were fabricated via splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses. X-ray powder diffraction (XRD) and transmission electron microscopic studies confirmed the amorphous nature of the as-quenched and crystallinity in the heat-treated samples. Fluorite phase formation prior to the perovskite SrBi₂Nb₂O₉ phase was analyzed by both the XRD and high-resolution transmission electron microscopy. Dielectric and the optical properties (transmission, optical band gap and Urbach energy) of these samples have been found to be compositional dependent. Refractive index was measured and compared with the values predicted by Wemple-Didomemenico and Gladstone-Dale relations. The glass nanocomposites comprising nanometer-sized crystallites of fluorite phase were found to be nonlinear optic active.     

Phase formation in the Dy2O3-HfO2-MoO3 system. Crystal structure of the new binary molybdate Dy2Hf2(MoO4)7

Phase relations have been studied for the subsolidus region of the Dy₂O₃-HfO₂-MoO₃ system; binary molybdates, Dy₂Hf(MoO₄)₅ and Dy₂Hf₂(MoO₄)₇, were found to form. The crystal structure of the latter has been solved (a = 20.661(3) , b = 9.816(1) , c = 13.796(3) , = 113.47(1)°, Z = 4, space group C2/c, R = 0.023) and found to be a new type of structure. In the structure, MoO₄ tetrahedra are linked by their vertices with HfO₆ octahedra and DyO₈ tetragonal antiprisms to form a 3D open-work, in which one can isolate double-row tetrahedral and octahedral chains extended down the c axis.Original Russian Text Copyright © 2004 by S. F. Solodovnikov, B. G. Bazarov, E. Yu. Badmaeva, Yu. L. Tushinova, E. S. Zolotova, and Zh. G. BazarovaTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 4, pp. 692–697, July–August, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Phase relations in the CaOIrO2Ir system in air

The equilibrium phase relations for the CaOIrO₂Ir system were determined in air using the quenching technique. The system contains three stable compounds 4CaO·IrO₂, 2CaO·IrO₂, and CaO·IrO₂; they dissociate to an oxide phase, Ir metal, and oxygen at 1240, 1170, and 1135°C, respectively. A metastable form of CaO·IrO₂ occurs at temperatures between 900 and 1100°C. Indexed X-ray diffraction powder patterns of all compounds are given.     

Crystal structure, phase relations and electrochemical properties of monoclinic Li2MnSiO4

Phase relations in the MnO-SiO₂-Li₄SiO₄ subsystem have been investigated by X-ray diffraction after solid-state reactions in hydrogen at 950-1150 °C. Both cation-deficient and cation-excess solid solutions Li_2+2xMn_1−xSiO₄ (−0.2?x?0.2) based on Li₂MnSiO₄ have been found. According to Rietveld analysis, Li₂MnSiO₄ (monoclinic, P2₁/n, a=6.3368(1), b=10.9146(2), c=5.0730(1) Å, β=90.987(1)°) is isostructural with γ_II-Li₂ZnSiO₄ and low-temperature Li₂MgSiO₄. All components are in tetrahedral environment, (MnSiO₄)²⁻ framework is built of four-, six- and eight-member rings of tetrahedra. Testing Li₂MnSiO₄ in an electrochemical cell showed that only 4% Li could be extracted between 3.5 and 5 V against Li metal. These results are discussed in comparison with those for recently reported orthorhombic layered Li₂MnSiO₄ and other tetrahedral Li₂MXO₄ phases.     

Subsolidus phase relations in Na2O-CuO-Sb2On system and crystal structure of new sodium copper antimonate Na3Cu2SbO6

Subsolidus phase relation studies in the NaSb₃O₇-Na₃SbO₄-CuO-CuSb₂O₆ quadrangle of Na₂O-CuO-Sb₂O_n system at 1123-1173 K revealed the formation of one new compound Na₃Cu₂SbO₆. It is a superstructure derived from α-NaFeO₂ type, space group C2/m, lattice constants: a=5.6759(1) Å, b=8.8659(1) Å, c=5.8379(1) Å, β=113.289(1)°. All ions are in octahedral environment, but CuO₆ polyhedron exhibits strong elongation due to Jahn-Teller effect (Cu-O: 2.000(2) Å×2, 2.021(2) Å×2, 2.494(3) Å×2), whereas SbO₆ octahedron is almost regular. The relationship to other similar superlattices is discussed.     

The ferroelectric phase of CdTiO3: A powder neutron diffraction study

The synthesis of bulk samples of polycrystalline CdTiO₃ in both the rhombohedral ilmenite and orthorhombic perovskite forms is described and the structures of these have been refined using powder neutron diffraction data. This involved the preparation of samples enriched in ¹¹⁴Cd. Cooling perovskite-type CdTiO₃ to 4 K induces a ferroelectric phase transition, with the neutron data suggesting the low temperature structure is in Pna2₁. Mode analysis shows the polar mode to be dominant at low temperatures. The ilmenite-structure of CdTiO₃ is compared with that of ZnTiO₃. The refined scattering length of the ¹¹⁴Cd is estimated to be 5.56 fm. Attempts to dope CdTiO₃ with Ca and Sr are described.     

Electric field gradients in the ionic crystals NaNO2, NaBF4, NaNO3, and Ba(NO3)2

The electric field gradients (EFG) at the sites of the cations and the “central” atoms of the anions in the ionic crystals NaNO₂, NaBF₄, NaNO₃ and Ba(NO₃)₂ are calculated by a method based on a combination of the semi-empirical INDO method for the charge distribution and the intramolecular EFG with a lattice summation in the framework of the extended multipole model. At some lattice sites the contribution of the induced dipole and quadrupole moments to the EFG is comparable with the contribution of the point charges. The charge distribution within the molecular ions is found by adjusting either the calculated asymmetry parameter η or the z-component of the EFG to the experimental value deduced from nuclear quadrupole coupling constants. These charge distributions are in good agreement with those gained from INDO calculations. The calculated and experimental quadrupole coupling constants of nuclei in anions and cations are compared.     

Subsolidus phase relations in the BaTiO3TiO2 system

Ba₆Ti₁₇O₄₀, Ba₄Ti₁₃O₃₀, BaTi₄O₉, and Ba₂Ti₉O₂₀ are the only compounds which were found to have a stability range in the subsolidus of the BaTiO₃TiO₂ system. BaTi₂O₅ and BaTi₅O₁₁, reported in other studies, apparently are not stable. The compound reported as Ba₂Ti₅O₁₂ appears to have been mistaken for Ba₆Ti₁₇O₄₀. X-Ray diffraction powder data are given for this phase which is monoclinic with a = 9.890, b = 17.117, c = 18.933 Å and β=98°42.6′. The phase formulated previously as BaTi₃O₇ is shown to be Ba₄Ti₁₃O₃₀ based on structural and density considerations, phase equilibria, and single crystal and powder X-ray diffraction data. This compound is orthorhombic with a = 17.072, b = 9.862, and c = 14.059 Å, probable space group, Cmca. An idealized structure for this phase is proposed. Ba₂Ti₉O₂₀ decomposes above 1300°C in the solid state to BaTi₄O₉ plus rutile. Single crystals were grown using BaF₂ as a mineralizer.     

Formation of copper oxide through NaNO3–KNO3 eutectic melt and its catalytic activity in the decomposition of ammonium perchlorate

The decomposition of basic copper carbonate in the presence and absence of NaNO₃–KNO₃ eutectic melt has been studied by employing isothermal TG and dynamic TG/DSC techniques. The rate constants for the decomposition in the presence of eutectic melt were found to be higher than when carbonate was heated alone. In both the cases copper oxide was found to be the end product. Catalytic activity of copper oxide obtained by the two methods were tested for the decomposition of ammonium perchlorate.     

High-pressure phase relations of CsD2PO4

The high-pressure phase diagram of CsD₂PO₄ to 4.5 GPa and temperatures between 0 and 470°C is reported. Comparisons are made with CsH₂PO₄ and correlated with the isotope effect on the high-temperature high-pressure phase relations of KH₂PO₄.     

High-pressure modifications of CaZn2, SrZn2, SrAl2, and BaAl2: Implications for Laves phase structural trends

High-pressure forms of intermetallic compounds with the composition CaZn₂, SrZn₂, SrAl₂, and BaAl₂ were synthesized from CeCu₂-type precursors (CaZn₂, SrZn₂, SrAl₂) and Ba₂₁Al₄₀ by multi-anvil techniques and investigated by X-ray powder diffraction (SrAl₂ and BaAl₂), X-ray single-crystal diffraction (CaZn₂), and electron microscopy (SrZn₂). Their structures correspond to that of Laves phases. Whereas the dialuminides crystallize in the cubic MgCu₂ (C15) structure, the dizincides adopt the hexagonal MgZn₂ (C14) structure. This trend is in agreement with the structural relationship displayed by sp bonded Laves phase systems at ambient conditions.     

Differential scanning calorimetric study of the solid—solid transitions in NaNO2-base binary alloys with NaNO3 and KNO2

Up to ～1 mol % maximum solid solubility of KNO₂ or NaNO₃, the two solid—solid transition temperatures in NaNO₂-base alloys decrease ～11±4 deg per mol % solute, except that the upper transition temperature is hardly altered by alloying with KNO₂. From the data on alloys with KNO₂, the ratio of the entropy of the upper transition to the lower transition in NaNO₂ is estimated as 0.35±0.07.