High-temperature X-ray Powder Diffraction Study Research Articles

Crystal structure and high-temperature thermodynamic properties of Pr-doped barium zirconates, BaZr1–xPrxO3 (x = 0.1, 0.5)

The crystal structure of BaZr1–xPrxO3 (x = 0.1, 0.5) perovskites was studied by means of X-ray powder diffraction (XRD). It was found that, at room temperature, BaZr0.9Pr0.1O3 possesses cubic (Pm3‾m) crystal structure, whereas BaZr0.5Pr0.5O3 is rhombohedral (R3‾c). High-temperature XRD study revealed that rhombohedral distortions in the BaZr0.5Pr0.5O3 lattice gradually decrease upon heating. As a result, BaZr0.5Pr0.5O3 becomes ideally cubic (Pm3‾m) at temperatures higher than 723 K. The enthalpy increments, Δ298.15TH○, of the perovskites BaZr1–xPrxO3 (x = 0.1, 0.5) were measured by high-temperature drop calorimetry in the temperature range (373–1273) K in air. The Δ298.15TH○ data for BaZr1–xPrxO3 (x = 0.1, 0.5) were approximated using one Einstein term with, for BaZr0.5Pr0.5O3, an addition of the exponential terms describing the influence of the R3‾c→Pm3‾m transition. The high- and low-temperature thermodynamic properties of BaZrO3 reported in the literature were reevaluated and its heat capacity, Cp(T), was approximated with one Einstein term as well. Thus, analytical Cp(T) dependences are reported for BaZr1–xPrxO3 (x = 0, 0.1, 0.5). While the Cp(T) of BaZrO3 and BaZr0.9Pr0.1O3 are very close to each other, the values of Cp(T) of BaZr0.5Pr0.5O3 are higher, mostly because of the phase transition and the differences in the crystal structure. Using the thermodynamic data obtained in this work along with those reported earlier, it was shown that addition of praseodymium leads to decreasing relative stability of BaZr1–xPrxO3 towards interaction with CO2 and gaseous H2O.

Рентгеноструктурное исследование аморфно-кристаллического фазового перехода в Ni

ave been obtained by high-temperature X-ray powder diffraction study. The amorphous structure of nanosized Ni particles is stable up to 200 ℃. Ni nanocrystals are formed in the temperature range of 300–600 ℃. The sizes of the coherent scattering regions (CSR) depend on the temperature of isothermal annealing and they are in the 5–15 nm interval. The activation energy of nanocrystal growth has been estimated. It is 67.3 kJ/mol. The relationship between the unit cell parameter of nanocrystalline Ni and the size of the CSR has been determined. An increase in the CSR of Ni nanoparticles causes the increase in the cell metric.

High-temperature behaviour of astrophyllite, K2NaFe7 2+Ti2(Si4O12)2O2(OH)4F: a combined X-ray diffraction and Mössbauer spectroscopic study

High-temperature X-ray powder-diffraction study of astrophyllite, K2NaFe7 2+Ti2(Si4O12)2O2(OH)4F, and investigation of the samples annealed at 600 and 700 °C, reveal the occurrence of a phase transformation due to the thermal iron oxidation coupled with (1) deprotonation according to the scheme Fe2+ + OH− → Fe3+ + O2− + ½H2 ↑, and (2) defluorination according to the scheme Fe2+ + F− → Fe3+ + O2−. The phase transformation occurs at 500 °C, it is irreversible and without symmetry changes. The mineral decomposes at 775 °C. Both astrophyllite and its high-temperature dehydroxylated (HT) modification are triclinic, P-1. The unit-cell parameters are a = 5.3752(1), b = 11.8956(3), c = 11.6554(3) A, α = 113.157(3), β = 94.531(2), γ = 103.112(2)o, V = 655.47(3) A3 for unheated astrophyllite, and a = 5.3287(4), b = 11.790(1), c = 11.4332(9) A, α = 112.530(8), β = 94.539(6), γ = 103.683(7)o, V = 633.01(9) A3 for the HT (annealed) modification of astrophyllite. The oxidation of iron is confirmed: (1) by the presence of an exothermic effect at 584 °C in the DTA/TG curves in an Ar–O atmosphere and its absence in an Ar–Ar atmosphere and (2) by ex situ Mossbauer spectroscopy that showed the oxidation of Fe2+ to Fe3+ in the samples heated to 700 °C. Deprotonation was detected by the evolution of IR spectra in the region 3600–3000 cm−1 for astrophyllite and its HT modification. Defluorination was detected by the presence of F in the electron microprobe analysis of unheated astrophyllite and the absence of F in the analysis of unpolished heated astrophyllite. The significant difference between astrophyllite and its HT modification is in the reduction of the M–O interatomic distances after heating to 500 °C and the distortion indices of the MO6 and Dφ6 octahedra. Thermal behaviour of astrophyllite in the 25–475 °C temperature range can be described as a volume thermal expansion with maximal coefficient of thermal expansion in the direction perpendicular to the plane of the HOH layers. In contrast, the HT phase experiences a strong contraction in the 600–775 °C temperature range, again in the direction perpendicular to the plane of the HOH layers.

Thermal behaviour of cinnabar, α-HgS, and the kinetics of the β-HgS (metacinnabar) - α-HgS conversion at room temperature

Cinnabar use on painting is historically documented from ancient times, together with its tendency to darken. The most accepted mechanism for this kind of deterioration considers cinnabar (α-HgS) transformation into its polymorph metacinnabar, β-HgS, but recent studies have cast doubt on this hypothesis. On this background, an in situ high-temperature X-ray powder diffraction study of the thermal behaviour of cinnabar has been carried out. Data, measured in transmission geometry on a non-hermetically sealed capillary, indicate that the α-HgS (trigonal)→β-HgS (cubic) phase transition occurs at 673 K and is completed at 698 K due to kinetics reasons. The thermal expansion of cinnabar is fairly isotropic, the a -parameter being slightly softer against heating. A fast cooling of metacinnabar at room temperature ( T ) produced a mixture of both polymorphs. The analysis of the kinetics of the β-HgS→α-HgS transformation at room- T shows a low rate of conversion, but not low enough to be consistent with a persistence of metacinnabar for a time span of 2000 years as hypothesized in reference data for Pompeian frescoes.

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

The dehydroxylation reactions of chrysotile Mg3Si2O5(OH)4 and brucite Mg(OH)2 were studied under inert nitrogen atmosphere using isothermal and non-isothermal approaches. The brucite decomposition was additionally studied under CO2 in order to check the influence of a competing dehydroxylation/carbonation/decarbonisation reaction on the reaction kinetics. Isothermal experiments were conducted using in situ high-temperature X-ray powder diffraction, whereas non-isothermal experiments were performed by thermogravimetric analyses. All data were treated by model-free, isoconversional approaches (‘time to a given fraction’ and Friedman method) to avoid the influence of kinetic misinterpretation caused by model-fitting techniques. All examined reactions are characterised by a dynamic, non-constant reaction-progress-resolved (‘α’-resolved) course of the apparent activation energy E a and indicate, therefore, multi-step reaction scenarios in case of the three studied reactions. The dehydroxylation kinetics of chrysotile can be subdivided into three different stages characterised by a steadily increasing E a (α ≤ 15 %, 240–300 kJ/mol), before coming down and forming a plateau (15 % ≤ α ≤ 60 %, 300–260 kJ/mol). The reaction ends with an increasing E a (α ≥ 60 %, 260–290 kJ/mol). The dehydroxylation of brucite under nitrogen shows a less dynamic, but generally decreasing trend in E a versus α (160–110 kJ/mol). In contrast to that, the decomposition of brucite under CO2 delivers a dynamic course with a much higher apparent E a characterised by an initial stage of around 290 kJ/mol. Afterwards, the apparent E a comes down to around 250 kJ/mol at α ~ 65 % before rising up to around 400 kJ/mol. The delivered kinetic data have been investigated by the z(α) master plot and generalised time master plot methods in order to discriminate the reaction mechanism. Resulting data verify the multi-step reaction scenarios (reactions governed by more than one rate-determining step) already visible in E a versus α plots.

Thermal behavior of realgar As4S4, and of arsenolite As2O3 and non-stoichiometric As8S8+x crystals produced from As4S4 melt recrystallization

An in situ high-temperature X-ray powder diffraction study of the thermal behavior of realgar (α-As 4 S 4 ) has been carried out. Data, measured in transmission geometry on a non-hermetically sealed capillary, indicate that the realgar → β-As 4 S 4 phase transition starts at 558 K and is completed at 573 K due to kinetics. Melting starts at 578 K and is completed at 588 K. Thermal expansion of realgar is significant and fairly isotropic. In fact, the a -and b -parameters expand almost at the same rate, whereas the c -parameter is slightly softer against heating. Moreover, the β-angle contracts as temperature is raised. The geometry of the As 4 S 4 molecule is largely independent from heating. The lengthening of a few As-S and As-As contacts above or near the sum of the As,S van der Waals radii represents the driving force of the phase transition. In addition, the thermal behavior of arsenolite As 2 O 3 and non-stoichiometric As 8 S 8+x crystals produced from As 4 S 4 melt recrystallization has been investigated. Two members located along the β-As 4 S 4 -alacranite (As 8 S 9 ) series joint were identified at RT: a term close to the β-As 4 S 4 end-member (As 8 S 8+x : x = ca. 0.1) and one term of approximate As 8 S 8.3 composition. The thermal expansion of β-As 4 S 4 is significantly anisotropic following the α b > α a > α c relationship. This is clearly the result of the different packing scheme of the As 4 S 4 cages in β-As 4 S 4 with respect to realgar. The dependence of cell parameters and volume of As 8 S 8.3 is more complicated. In fact, a strong discontinuity on the dependence of cell parameters and volume is observed in the 403–443 K thermal range, i.e., that at which As 8 S 8.3 converts partly to realgar. A significant volume expansion is observed as a result of a change of composition to As 8 S 8.7 .

Synthesis, crystal structure and thermal behavior of a novel oxoborate SrBi 2B 4O 10

A new compound, SrBi 2B 4O 10, has been grown by cooling a melt with the stoichiometric composition. It is triclinic, P−1, a=6.819(1), b=6.856(1), c=9.812(2) Å, α=96.09(1), β=109.11(1), γ=101.94(1)°, V=416.5(1) Å 3, Z=2. The crystal structure of the compound has been solved by direct methods and refined to R 1=0.050 (w R 2=0.128). The structure contains Bi–O pseudolayers build up from Bi–O chains involving oxocentred OBi 3 triangles. Sr atoms and [B 4O 9] 6− isolated anions (4B:3 Δ□:<2 Δ□> Δ) are located between the Bi–O packages. The thermal treatment as well as DSC experiment showed that the compound melts above 800 °C presumably according to the peritectic reaction: SrBi 2B 4O 10 ↔ SrB 2O 4+SrB 4O 7+ Liquid. According to high-temperature X-ray powder diffraction study thermal expansion of SrBi 2B 4O 10 structure is anisotropic ( α 11=13, α 22=9, α 33=2, α V =24×10 −6 °C −1).

High-temperature X-ray powder diffraction study of the tetragonal-cubic phase transition in nanocrystalline, compositionally homogeneous ZrO2–CeO2 solid solutions

Crystal structure of compositionally homogeneous, nanocrystalline ZrO2–CeO2 solutions was investigated by X-ray powder diffraction as a function of temperature for compositions between 50 and 65 mol % CeO2. ZrO2-50 and 60 mol % CeO2 solid solutions, which exhibit the t′-form of the tetragonal phase at room temperature, transform into the cubic phase in two steps: t′-to-t″ followed by t″-to-cubic. But the ZrO2-65 mol % CeO2, which exhibits the t″-form, transforms directly to the cubic phase. The results suggest that t′-to-t″ transition is of first order, but t″-to-cubic seems to be of second order.

High-Temperature X-Ray Powder Diffraction Study on the Phase Transition between the α-Quartz and the β-Cristobalite-Like Phase of GaPO&lt;sub&gt;4&lt;/sub&gt;

High-Temperature X-Ray Powder Diffraction Study on the Phase Transition between the α-Quartz and the β-Cristobalite-Like Phase of GaPO&lt;sub&gt;4&lt;/sub&gt;

Diffuse ferroelectric transition and relaxational dipolar freezing in : III. Role of order parameter fluctuations

Results of a high-temperature x-ray powder diffraction study of the phase transition between cubic and tetragonal phases of are presented for x = 0.08 and 0.12 which exhibit sharp and diffuse dielectric anomalies, respectively. Experimental evidence and arguments are advanced to show that the cubic and tetragonal phases coexist over a wide range of temperatures and that this coexistence is due to the fluctuation of the order parameter which is coupled electrostrictively to strains. For x = 0.08, the phase coexistence disappears below the dielectric anomaly temperature , indicating the critical nature of the fluctuations. For x = 0.12, these fluctuations are non-critical since the phase coexistence persists even below . The complete conversion to the tetragonal phase occurs at which is nearly lower than ). It is shown that the spontaneous polarization also increases gradually below and levels off at . The existence of structural and polarization anomalies well below the frequency-independent dielectric anomaly temperature cannot be rationalized either in terms of the Landau-like theories for ferroelectric transitions which predict or in terms of dipole glass transitions and/or relaxor ferroelectric transitions for which should be frequency dependent.

Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction, thermal expansion and heat capacity

High-temperature x-ray powder diffraction study by the full pattern Rietveld method of orthorhombic CaGeO3 (Pbnm at ambient condition) perovskite confirms the previously observed phase transition at Tc=520 K. The measured volumetric thermal expansion coefficients are 3.1 x 10-5 (K-1) below Tc and 3.5x 10-5 (K-1) above Tc. The space group at T>Tc has been tentatively identified as Cmcm. Such a transition involves the disappearance of one of the two octahedral rotations in the (001) plane, and the doubling of the unit cell volume, with c axis unchanged. Although this transition should be of first order from symmetry considerations, the distortion of the Pbnm phase decreases continuously as the temperate approaches Tc and there is no observable volume discontinuity at Tc. The measured heat capacity places an upper limit on the enthalpy of transition of 50 J/mol, which is quite reasonable in terms of the crystallographic nature of this phase transition.