Lattice thermal expansion studies of V2GeO4F2: A topaz type oxide-fluoride

A new compound V₂GeO₄F₂ was earlier found to exist in the V₂O₃-VF₃-GeO₂ system and the structure elucidation revealed it to be iso-structural to the mineral topaz. Herein, we report the lattice thermal expansion data of this compound. The lattice thermal expansion of V₂GeO₄F₂ was studied in the temperature range of 298-873 K under a flowing helium atmosphere by the high temperature XRD (HTXRD). The coefficients of axial thermal expansions of V₂GeO₄F₂ were found to be as: α_a = 3.5 × 10⁻⁶, α_b = 6.1 × 10⁻⁶ and α_c = 7.6 × 10⁻⁶ K⁻¹. The coefficient of volume (α_V) thermal expansion was 17.3 × 10⁻⁶ K⁻¹, which is relatively low compared to many analogues silicates.     

Crystal structure, thermal expansion and electrical properties of a new compound Al0.33DyGe2

A new ternary compound Al_0.33DyGe₂ has been synthesized and studied from 298 K to773 K by means of X-ray powder diffraction technique. The crystal structural refinement of Al_0.33DyGe₂ has been performed by using the Rietveld method. The ternary compound Al_0.33DyGe₂ crystallizes in the orthorhombic of the defect CeNiSi₂-type structure (space group Cmcm, a = 0.41018(2)nm, b = 1.62323(6)nm, c = 0.39463(1)nm, Z = 4 and D_calc = 8.004 g/cm³). The average thermal expansion coefficients α_a, α_b and α_c of Al_0.33DyGe₂ are 1.96 × 10^− 5 K^− 1, 0.93 × 10^− 5 K^− 1 and 1.42 × 10^− 5 K^− 1, respectively. The bulk thermal expansion coefficient α_V is 4.31 × 10^− 5 K^− 1. The resistivity is observed to fall from 387 to 308 µΩ cm between room temperature and 25 K.     

Thermal expansion studies of Gd2Mo3O12 and Gd2W3O12

Simultaneous thermogravimetric/differential thermal analysis of Gd₂Mo₃O₁₂ showed an irreversible phase transition at 1178 K where as Gd₂W₃O₁₂ showed reversible phase transition at 1433 K, which were confirmed by powder X-ray diffraction. The thermal expansion behavior of α-Gd₂Mo₃O₁₂ (room temperature phase), β-Gd₂Mo₃O₁₂ (phase obtained by heating Gd₂Mo₃O₁₂ at 1223 K) and Gd₂W₃O₁₂ have been investigated using high temperature X-ray diffractometer. The cell volume of α-Gd₂Mo₃O₁₂, β-Gd₂Mo₃O₁₂ and Gd₂W₃O₁₂, fit into polynomial expression with respect to temperature, showed positive thermal expansion up to 1073, 1173 and 1173 K, respectively. The average volume expansion coefficients for α-Gd₂Mo₃O₁₂, β-Gd₂Mo₃O₁₂ and Gd₂W₃O₁₂ are 39.52 × 10⁻⁶, 21.23 × 10⁻⁶ and 37.96 × 10⁻⁶ K⁻¹, respectively.     

Hygroscopicity and bulk thermal expansion in Y2W3O12

Negative thermal expansion material, Y₂W₃O₁₂ has been synthesized by the solid-state method and bulk thermal expansion of the material has been investigated from 300 to 1100 K. The material reversibly forms a trihydrate composition whose X-ray diffraction pattern can be indexed to an orthorhombic unit cell with a = 10.098(1) Å, b = 13.315(3) Å, c = 9.691(4) Å. The cell volume of the hydrated pattern is 7% smaller than the unhydrated cell volume. According to the dilatometric studies, the material shows a 3-6% increase in the linear strain at about 400 K, which can be attributed to the removal of water. Sintering the material at 1473 K leads to large grain size of >100 μm, which results in a large hysteresis in the bulk thermal expansion behavior. Hot pressing at 1273 K under a uniaxial pressure of 25 MPa results in a fine-grained (2-5 μm) ceramic. Glazing the ceramic prevents moisture pick up and a linear thermal expansion over the entire temperature range 1100-300 K and an average linear thermal expansion co-efficient of −9.65 × 10⁻⁶/K is observed. The effect of water on the thermal expansion behavior of this system is discussed.     

Growth and properties of UV nonlinear optical crystal ZnCd(SCN)4

A second-order nonlinear optical coordination crystal, zinc cadmium thiocyanate, ZnCd(SCN)₄ (ZCTC) was grown as a frequency doubler, emitting UV light. A large typical single crystal with dimensions up to 15×7×7 mm³ has been obtained by slow solvent-evaporation method for the first time. The infrared (IR) spectroscopy and X-ray powder diffraction (XRPD) of single crystals were performed at room temperature. The specific heat of the crystal has been measured to be 367.9 J/mol K at 300 K. The thermal expansion coefficients a- and c-oriented, have been measured to be −1.69×10⁻⁵ and 1.95×10⁻⁴ K⁻¹, respectively. The second harmonic generation (SHG) efficiency of ZCTC crystal is 51.6 times as high as that of urea reference, and the measured transmittance spectra from 190 to 3200 nm showed that the UV transparency cutoff occurs at 290 nm and the transmission is 73.22% at 380 nm. UV laser light of wavelength 380 nm has been achieved by the frequency doubling of a 760 nm laser diode at room temperature.     

Heat capacity and thermal expansion of samarium titanate

Samarium titanate (Sm₂TiO₅) was prepared by solid-state synthesis and characterized by X-ray diffraction (XRD). Heat capacity measurements were carried out by differential scanning calorimeter (DSC) in the temperature range 298-800 K. Thermal expansion characteristics have been studied by high temperature X-ray diffraction technique (HTXRD) in the temperature range 298-1573 K. The heat capacity value at 298 K is 170 J K^− 1 mol^− 1. The percentage linear thermal expansion in the temperature range 298-1573 K along a, b and c axes are 0.96, 0.89 and 1.07 respectively. The average coefficient of thermal expansion value obtained in the present study for samarium titanate up to 1573 K is 10.8 × 10^− 6 K^− 1.     

Synthesis of nanocrystalline zirconia by amorphous citrate route: structural and thermal (HTXRD) studies

Nanocrystalline zirconia powder with a fairly narrow particle size distribution has been synthesized by the amorphous citrate route. The sample obtained has a high BET surface area of 89 m² g⁻¹. Rietveld refinement of the powder X-ray diffraction (XRD) profile of the zirconia sample confirms stabilization of zirconia in the tetragonal phase with around 8% monoclinic impurity. The data show the presence of both anionic as well as cationic vacancies in the lattice. Crystallite size determined from XRD is 8 nm and is in close agreement with the particle size determined by TEM. The in situ high temperature-X-ray diffraction (HTXRD) study revealed high thermal stability of the mixture till around 1023 K after which the transformation of tetragonal phase into the monoclinic phase has been seen as a function of temperature till 1473 K. This transformation is accompanied by an increase in the crystallite size of the sample from 8 to 55 nm. The thermal expansion coefficients are 9.14 × 10⁻⁶ K⁻¹ along ‘a’- and 15.8 × 10⁻⁶ K⁻¹ along ‘c’-axis. The lattice thermal expansion coefficient in the temperature range 298-1623 K is 34.6 × 10⁻⁶ K⁻¹.     

Growth and properties of Ca0.28Ba0.72Nb2O6 single crystals

Calcium barium niobate Ca_0.28Ba_0.72Nb₂O₆ (CBN-28) crystals were successfully grown by the Czochralski method. X-ray powder diffraction experiments indicated that CBN single crystals are tetragonal with a = 12.432(±0.002) Å and c = 3.957(±0.001) Å, which have almost the same structure as the Sr_0.50Ba_0.50Nb₂O₆ (SBN-50) crystal. The thermal expansion coefficient perpendicular to Z-direction had been measured to be 1.25 × 10⁻⁵ K⁻¹ between 293.15 and 572.15 K, and along Z-axis was negative between 298.15 and 543.15 K. The specific heat of the crystal had been measured by the differential scanning calorimetric experiments. The transmittance spectra from 200 to 3200 nm were also measured. The measured temperature dependence of dielectric constants showed that the Curie temperature of the CBN-28 crystals is 260 °C, which is about 200 °C higher than that of the (SBN) crystal.     

Thermal stability of the Mobil Five type metallosilicate molecular sieves—An in situ high temperature X-ray diffraction study

We have carried out in situ high temperature X-ray diffraction (HTXRD) studies of silicalite-1 (S-1) and metallosilicate molecular sieves containing iron, titanium and zirconium having Mobil Five (MFI) structure (iron silicalite-1 (FeS-1), titanium silicalite-1 (TS-1) and zirconium silicalite-1 (ZrS-1), respectively) in order to study the thermal stability of these materials. Isomorphous substitution of Si⁴⁺ by metal atoms is confirmed by the expansion of unit cell volume by X-ray diffraction (XRD) and the presence of Si-O-M stretching band at ∼960 cm⁻¹ by Fourier transform infrared (FTIR) spectroscopy. Appearance of cristobalite phase is seen at 1023 and 1173 K in S-1 and FeS-1 samples. While the samples S-1 and FeS-1 decompose completely to cristobalite at 1173 and 1323 K, respectively, the other two samples are thermally stable upto 1623 K. This transformation is irreversible. Although all materials show a negative lattice thermal expansion, their lattice thermal expansion coefficients vary. The thermal expansion behavior in all samples is anisotropic with relative strength of contraction along ‘a’ axes is more than along ‘b’ and ‘c’ axes in S-1, TS-1, ZrS-1 and vice versa in FeS-1. Lattice thermal expansion coefficients (α_v) in the temperature range 298-1023 K were −6.75 × 10⁻⁶ K⁻¹ for S-1, −12.91 × 10⁻⁶ K⁻¹ for FeS-1, −16.02 × 10⁻⁶ K⁻¹ for TS-1 and −17.92 × 10⁻⁶ K⁻¹ for ZrS-1. The highest lattice thermal expansion coefficients (α_v) obtained were −11.53 × 10⁻⁶ K⁻¹ for FeS-1 in temperature range 298-1173 K, −20.86 × 10⁻⁶ K⁻¹ for TS-1 and −25.54 × 10⁻⁶ K⁻¹ for ZrS-1, respectively, in the temperature range 298-1623 K. Tetravalent cation substitution for Si⁴⁺ in the lattice leads to a high thermal stability as compared to substitution by trivalent cations.     

Transport properties and thermal expansion of La2Mo2O9-based solid electrolytes

The oxygen ion transference numbers of La_1.7Bi_0.3Mo₂O₉, La₂Mo_1.7W_0.3O₉ and La₂Mo_1.95V_0.05O₉ ceramics, determined by modified faradaic efficiency and e.m.f. methods at 973-1173 K, vary in the range 0.995-0.977 in air, decreasing when temperature increases. The activation energies for the ionic and electronic transport are 61-71 kJ/mol and 123-141 kJ/mol, respectively. Reducing oxygen chemical potential leads to increasing n-type electronic contribution to the total conductivity, which remains, however, essentially p(O₂)-independent down to oxygen pressures of 10⁻⁴-10⁻³ atm and exhibits reversible drop on further reduction, probably due to phase decomposition. Doping La₂Mo₂O₉ with calcium results in segregation of a CaMoO₄-based phase, accompanied with increasing electronic transport. The average thermal expansion coefficients of La₂Mo₂O₉-based materials, calculated from dilatometric data in air, are (14.4-14.8) × 10⁻⁶ K⁻¹ at 300-700 K and (16.4-22.5) × 10⁻⁶ K⁻¹ at 700-1070 K.     

Negative thermal expansion and electrical properties of Mn3(Cu0.6NbxGe0.4 − x)N (x = 0.05-0.25) compounds

Bulk materials with the general formula of Mn₃(Cu_0.6Nb_xGe_0.4 − x)N (x = 0.05, 0.1, 0.15, 0.2, 0.25), Mn₃(Cu_0.6Ge_0.4)N and Mn₃(Cu_0.7Ge_0.3)N were fabricated by mechanical ball milling and solid state sintering. Their thermal expansion coefficients and electrical conductivities were investigated in the temperature range of 80-300 K. It is found that the temperature interval of negative temperature expansion behavior is about 95 K in the samples of Mn₃(Cu_0.6Nb_0.15Ge_0.25)N and Mn₃(Cu_0.6 Nb_0.2Ge_0.2)N, which is twice as large as that of Mn₃(Cu_0.7Ge_0.3)N. The negative thermal expansion of Mn₃(Cu_0.6Nb_0.15Ge_0.25)N can reach to − 19.5 × 10⁻⁶ K^− 1 in the temperature range of 165 to 210 K. The electrical conductivity of this series materials is in a level of about 2.5 × 10⁶ (Ω m)^− 1.     

New cathode compositions based on La0.84Sr0.16Mn1−xMxO3, where M = Al, Ga for solid oxide fuel cell

In order to identify new cathode compositions for the high temperature solid oxide fuel cell, we have investigated the effect of the trivalent cations Al and Ga at the Mn site of the well-studied cathode composition La_0.84Sr_0.16MnO₃. All the compositions have been prepared by the low temperature citrate-nitrate auto-ignition process and sintered within the temperature range of 1150-1350 °C for 4 h. In order to understand the compatibility of the prepared samples as alternative cathode materials, we compared their electrical conductivity and thermal expansion coefficient with those of La_0.84Sr_0.16MnO₃ and yttria-stabilized zirconia. A 10 mol% Al doped La_0.84Sr_0.16MnO₃ composition exhibited a conductivity of around 122 S cm⁻¹ at 950 °C and a thermal expansion coefficient of 11.04 × 10⁻⁶ K with a minimum reactivity towards yttria-stabilized zirconia. Though the conductivity of the new composition is lower than that of La_0.84Sr_0.16MnO₃ (169 S cm⁻¹ at 950 °C), it is still high enough for use as a cathode material.     

Structure and properties of ternary manganese nitride Mn3CuNy thin films fabricated by facing target magnetron sputtering

Deposition of Mn₃CuN_y thin films on single crystal Si (1 0 0) at various substrate temperatures (T_sub) by facing target magnetron sputtering is reported. The crystal structure and composition were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results confirmed that the crystalline antiperovskite Mn₃CuN_y thin film with (2 0 0) highly preferred texture had been obtained at T_sub = 180 °C. Furthermore, for the resulting Mn₃CuN_y thin film, it showed different properties compared with the bulk counterpart. There was a paramagnetic to ferrimagnetic transition at 225 K with decreasing temperature. The change of the lattice constant with temperature presented positive thermal expansion behavior and no structural transition was observed. The average linear thermal expansion coefficient (α) is 2.49 × 10⁻⁵ K⁻¹ from 123 K to 298 K. More interestingly, the temperature dependence of resistivity displayed a semiconductor-like behavior, i.e. an obvious monotonous decrease of resistivity with increasing temperature.     

High temperature thermoelectric properties of (Ca, Ce)4Mn3O10

Thermoelectric (TE) properties such as resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) of Ca_4−3xCe_3xMn₃O₁₀ (0<x≤0.03) polycrystalline samples were measured from room temperature to 1000 K. ρ shows an obvious decrease with the increment of Ce content. The hopping conduction mechanism is used to explain the conduction behavior of these samples. The negative S values indicate that these materials are n-type. The sample of x=0.03 has the largest power factor, 0.52×10⁻⁴ Wm⁻¹ K⁻² at 1000 K. The value of κ and the dimensionless figure of merit of this sample is 1.51 Wm⁻¹ K⁻¹ and 0.034 at 1000 K, respectively.     

Physicochemical properties and theoretical explanation of ZnCd(SCN)4 crystal

The crystal density and Mohs hardness of zinc cadmium thiocyanate (ZCTC), ZnCd(SCN)₄ have been measured at room temperature. The specific heat of the crystal is 699.5 J mol⁻¹ K⁻¹ at 300 K. The thermal expansion coefficient (TEC) along the a and c axes, respectively, is interpreted on the basis of crystal structure. The thermal decomposition process is characterized by thermogravimetry analysis and differential scanning calorimetry (TGA/DSC). The intermediates and final products of the thermal decomposition were identified by X-ray powder diffraction at room temperature. The high-temperature effect in air on the optical transmission of the crystal was also studied.     

Oxygen ionic conduction in brownmillerite CaAl0.5Fe0.5O2.5+δ

The oxygen permeability of CaAl_0.5Fe_0.5O_2.5+δ brownmillerite membranes at 1123-1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2×10⁻³ to 5×10⁻³. The analysis of structural parameters showed that the ionic transport in the CaAl_0.5Fe_0.5O_2.5+δ lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron-oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700-800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl_0.5Fe_0.5O_2.5+δ lattice. The average thermal expansion coefficients of CaAl_0.5Fe_0.5O_2.5+δ ceramics in air are 16.7×10⁻⁶ and 12.6×10⁻⁶ K⁻¹ at 370-850 and 930-1300 K, respectively.     

Thermal and mechanical properties of novel substrate crystal Mg0.4Al2.4O4

Mg_0.4Al_2.4O₄ single crystal was grown by the Czochralski method. The measured specific heat values are 0.804-1.06 J g^− 1 K^− 1 in the temperature range from 298.15 to 573.15 K. The calculated thermal conductivity components are 11.37, 11.47 and 10.77 W m^− 1 K^− 1 along the [111], [004] and [22?0] direction at 298.15 K. The Vickers microhardness values are 1328-1414 kg mm^− 2. These experimental results show that Mg_0.4Al_2.4O₄ crystal is a promising substrate for GaN-based LEDs.     

The first bulk synthesis of ReO3-type tungsten trioxide, WO3, from nanometric precursors

A perovskite form of WO₃ has been synthesized in bulk for the first time at 0.66 GPa and 973 K with a=3.7823(4) Å [a₀=3.7719(4) Å, at ambient conditions] from nanometric powder of WO₃ with an average crystallite size of 35 nm. Data collected during tests to determine both the likelihood of retaining the structure at room temperature and the effect of high pressure on distortion have afforded analysis of thermal expansivity and compressibility of this phase. These result in V_T=53.407(5)exp(−3.9(12)×10⁻⁶(T−298)+1.91(9)×10⁻⁸(T²−298²)) Å³ and equation of state parameters of V₀=53.67(4) Å³, K₀=41.8(19) GPa with ∂K/∂P=K′=5.6(12).     

Optical properties of transparent Dy doped Ba2TiSi2O8 glass ceramic

The Ba₂TiSi₂O₈ is a well known piezoelectric, ferroelectric and non-linear crystal. Nanocrystals of Ba₂TiSi₂O₈ doped with 1.5 Dy³⁺ have been obtained by thermal treatment of a precursor glass and their optical properties have been studied. X-ray diffraction patterns and optical measurements have been carried out on the precursor glass and glass ceramic samples. The emission spectra corresponding to the Dy³⁺: ⁴F_9/2 → ⁶H_13/2 (575 nm), ⁴F_9/2 → ⁶H_11/2 (670 nm) and ⁴F_9/2 → ⁶H_9/2 (757 nm) transitions have been obtained under laser excitation at 473 nm. These measurements confirm the incorporation of the Dy³⁺ ions into the Ba₂TiSi₂O₈ nanocrystals which produces an enhancement of luminescence at 575 nm. At this wavelength has been demonstrated a maximum optical amplification around 1.9 cm⁻¹ (∼8.2 dB/cm).     

Ferroelectric properties of lithia-doped (Bi0.95Na0.75K0.20)0.5Ba0.05TiO3 ceramics

Lead-free piezoelectric (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics have been prepared by conventional process for different lithium substitutions. The SEM images show that the ceramics are well sintered at 1428 K. Dielectric and ferroelectric measurements have been performed. With the increasing of lithium substitution, the Curie temperature of the (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics shifts from 570 K to 620 K, but the maximum value of the dielectric constant decreases from 6700 to 4700 correspondingly. A relatively larger remanent polarization of 36.8 μC/cm² has been found in the x = 0.05 sample. The coercive field decreases as the lithium substitution amount increases. An optimized d₃₃ = 194 × 10^− 12 C/N and a relative dielectric constant ε_r = 1510 have been obtained in (Bi_0.95Na_0.75K_0.15Li_0.05)_0.5Ba_0.05TiO₃.