Search

In the present work we report results from neutron and synchrotron radiation diffraction measurements that confirm the presence of a very weak phase transition in Sr2CuWO6 at about 600 °C. This phase transition is continuous and changes the symmetry from I4/m at low temperature to another tetragonal phase at high temperature. On the basis of the experimental results we identify I4/mmm as the most probable space group for the high-temperature tetragonal phase.

As a result of a pseudosymmetry search, the double perovskite compounds Sr2MWO6 (M = Ni, Zn, Co, Cu) were identified as good candidates for presenting Landau-type phase transitions at high temperatures. In the present work, the phases observed at different temperatures are reported and the phase transitions are discussed. The Sr2MWO6 (M = Ni, Zn, Co) compounds present I4/m–Fmm phase transitions, while Sr2CuWO6 shows an I4/m–I4/mmm phase transition. All these transitions are second order and are consistent with the pseudosymmetry concept. Sr2ZnWO6 and Sr2CoWO6 also show P21/n–I4/m phase transitions and Sr2CuWO6 presents a phase transition of the type I4/mmm–Fmm. These transitions are first order. Alongside the experimental results a discussion of some aspects of the study of phase transitions in double perovskite materials is presented.

Polar structures with pseudosymmetry related to a hypothetical non-polar configuration are considered as good candidates for displacive ferroelectrics. Recently we have developed a procedure for the systematic search of pseudosymmetric structures in the available databases. The aim of the present contribution is the generalization of the pseudosymmetry search procedure to include possible displacive ferroelectric structures which implies an additional optimization of the continuous parameter(s) describing the location of the non-polar symmetry elements. Here we present the results of the search for possible displacive ferroelectrics among the compounds with space group Pba 2, listed in the Inorganic Crystal Structure Database.

A systematic search for structures having a high-temperature structural phase transition can be carried out through the identification in the structural databases of those structures that can be considered pseudosymmetric. Pseudosymmetry in a crystal structure indicates the possibility of a similar configuration of higher symmetry. If the distortion relating both structures is small enough, it can be expected that the crystal acquires the more symmetric configuration through a Landau-type phase transition at a higher temperature. Here, we present the results of such a search among inorganic structures with space group Pnma retrieved from the Inorganic Crystal Structure Database. Pseudosymmetry has indeed been detected in those compounds with a known (displacive) Landau-type phase transition at higher temperatures. This is measured by a parameter Δ, which measures the maximal atomic displacement relating the pseudosymmetry-transformed structure and the original one. In most of these compounds with a known phase transition, this parameter was smaller than 1.0 Å for at least one minimal supergroup of Pnma. The database contains 144 additional structures with pseudosymmetry features under the same quantitative limit. A comparison of the Δ distributions in both sets of compounds suggests a smaller Δ window (with 0.7 Å as maximal value) for selecting the materials having maximal probability of exhibiting a phase transition at higher temperatures. A set of 58 compounds fulfils this criterion.

A systematic procedure for the determination of supergroups of space groups is developed and used for the search of pseudosymmetric structures within crystal structure databases. The method is applied to P212121 structures listed in the Inorganic Crystal Structure Database. A significant number of compounds with known phase transitions at high temperatures are clearly detected as pseudosymmetric with differences between the transformed and original structure lower than 1·0 A. Taking this value as a threshold about 20 additional compounds are detected having a high probability of exhibiting a phase transition at higher temperatures.

La1-xSrxFeO3-xFx (x = 1, 0.8, 0.5, 0.2, 0) phases are prepared by solid state reaction of stoichiometric mixtures of La2O3, SrCO3, and Fe2O3 (1250 °C, 30 h) followed by fluorination with poly(vinylidenefluoride) at 400 °C for 24 h.

The existence of a pseudosymmetry in a crystal structure is indicative of a slightly distorted structure of higher symmetry. If the distortion is small enough, it can be expected that the crystal acquires this more symmetric configuration at a higher temperature after a phase transition. The aim of the present study is the development of a general procedure for the systematic search of pseudosymmetric structures in the available structural databases. The first step is the determination of all relevant supergroups of the space group associated to the known structure. This problem is reduced to the generation of minimal supergroups of space groups using the existing data on their maximal subgroups and normalizers. The determined pseudosymmetry elements are then applied to the crystal structure and a quantitative comparison of the transformed structure and the original one is performed. Pseudosymmetry is detected when the difference is under a certain threshold. As an example we present the results obtained for inorganic structures with space group P${2}_{1}$${2}_{1}$${2}_{1}$ retrieved from the Inorganic Crystal-Structure Database. A significant number of compounds with known phase transitions at high temperatures have been clearly detected as pseudosymmetric with differences between the transformed and the original structures lower than 1.0 \AA{}. About 20 additional structures were detected with pseudosymmetric features under the same threshold. These compounds are considered as having a high probability of exhibiting a phase transition at higher temperatures. \textcopyright{} 1996 The American Physical Society.

Calorimetric measurements of Cs2ZnI4 showed four distinct phase transitions at 117, 108, 104 and 94 K. The shape of the observed specific heat peaks indicates that those at 108 and 94 K are first order in nature. This sequence is in agreement with previous results using different experimental techniques, and supports the hypothesis that the phase transition sequence of Cs2ZnI4 is the same as that of Cs2HgBr4 and can be written as Pnma (Z=4) 2nd order to T1=117 K incommensurate 1st order to T2=108 K ferroelastic P21/n (Z=8) 2nd order to T3=104 K ferroelastic P1 1st order to T4=94 K ferroelastic P1.

The crystal and magnetic structures of SrLnFeRuO6 (Ln = La, Pr, Nd) double perovskites have been investigated. All compounds crystallize with an orthorhombic Pbnm structure at room temperature. These materials show complete chemical disorder of Fe and Ru cations for all compounds. The distortion of the structure, relative to the ideal cubic perovskite, has been decomposed into distortion modes. It has been found that the primary modes of the distortion are octahedral tilting modes: R + 4 and M + 3. The crystal structure of SrPrFeRuO6 has been studied from room temperature up to 1200 K by neutron powder diffraction. There is a structural phase transition from orthorhombic (space group Pbnm) to trigonal (space group ) at T = 1075 K. According to group theory no second-order transition is possible between these symmetries. Magnetic ordering for all the compounds is described by the magnetic propagation vector (0,0,0). SrPrFeRuO6 shows ferrimagnetic order below ca 475 K, while SrLaFeRuO6 (below ca 450 K) and SrNdFeRuO6 (below ca 430 K) exhibit canted-antiferromagnetic order. The magnetic moments at low temperatures are m(Fe/Ru) = 1.88 (3)B for SrLaFeRuO6 (2 K), m(Pr) = 0.46 (4)B and m(Fe/Ru) = 2.24B for SrPrFeRuO6 (2 K), and m(Fe/Ru) = 1.92B for SrNdFeRuO6 (10 K). © 2012 International Union of Crystallography., This work has been supported by ILL (grant MBR-GL/09-056 and MBR-GL/11-050), the Spanish Ministry of Science and Innovation (project MAT2008-05839) and the European Commission under the 7th Framework Program through the ‘Research Infrastructures’ action of the ‘Capacities’ Program, Contract No: CP-CSA_INFRA-2008-1.1.1 Number 226507- NMI3. The authors are grateful to FRMII (exp. 4456), ILL (exp. 5-24-458) and PSI (exp. 20101302) for the beamtime allocation and the instrument local contacts Dr Markus Hoelzel (SPODI, FRMII), Dr Gabriel Cuello (D2B, ILL) and Dr Vladimir Pumjakushin (HRPT, SINQ, PSI) for their kind help and technical assistance.

X-ray diffraction and Raman spectroscopy studies of Sr doped double perovskites compound Ba 2− x Sr x ZnWO 6 with (0 ≤ x ≤ 2) were investigated. As a function of composition while increasing strontium amount, the samples show two phase transitions from cubic to tetragonal phase and from tetragonal to monoclinic structure. Both Rietveld refinements and Raman studies showed that the first phase transition occurs between x = 1 and 1.2; the second phase transition is observed around x = 2. Furthermore, increasing the temperature for the compositions (1.2 ≤ x x = 2 two phase transitions were observed as a function of temperature. For this series, the phase transitions are illustrated by considerable changes in the composition and temperature dependence of the modes. All the Raman modes show a linear behavior when the composition or temperature is increased, then the slope change indicating the symmetry change.

In the present work we report X-ray powder diffraction measurements of Sr 2 SmSbO 6 and Sr 2 LaSbO 6 , at different temperatures. The crystal structures at room temperature of both compounds are determined; and results showing the existence of high-temperature phase transitions in them are presented. Both compounds have double perovskite structure with 1:1 ordered arrangement of the B site cations. At room temperature their symmetries are described with the P 2 1 / n space group, that correspond to the ( a + b - b - ) tilt system. The evolution with temperature of the structure of both compounds shows the presence of two phase transitions: a discontinuous one, at 885 and 945 K, for Sr 2 SmSbO 6 and Sr 2 LaSbO 6 , respectively; and a continuous one, at 1170 and 118 K, for Sr 2 SmSbO 6 and Sr 2 LaSbO 6 , respectively, with the following phase transition sequence: P 2 1 / n → R 3 ¯ → Fm 3 ¯ m .

Sr2CrSbO6 was synthesized by the conventional solid-state reaction process. X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD) has been used to reinvestigate the structure at room temperature and to study the phase transitions at high- and low-temperature. Rietveld analysis revealed that Sr2CrSbO6 crystallizes at room temperature in a monoclinic system having a space group I2/m, with a=5.5574(1) A; b=5.5782(1) A; c=7.8506(2) A and β=90.06(2), no P21/n space group as was previously reported. The high-temperature study (300–870 K) has shown that the compound presents the following temperature induced phase-transition sequence: I2/m–I4/m–Fm-3m. The low-temperature study (100–300 K) demonstrated that the room-temperature I2/m monoclinic symmetry seems to be stable down to 100 K.

The crystal structure of Sr 2 CdWO 6 , prepared by solid state reaction, was determined by high-resolution X-ray diffraction at different temperatures. At room temperature, this compound has a monoclinic structure (space group P 2 1 / n ) with a = 5.7463 ( 1 ) , b = 5.8189 ( 1 ) , c = 8.1465 ( 1 ) , β = 90.071 ( 1 ) . At 1105 K the structure is converted to tetragonal (space group I 4 / m ). Diffraction data also suggest that a cubic phase exists above 1220 K. Comparing the phase transition temperatures of Sr 2 CdWO 6 with those of other compounds of the Sr 2 MWO 6 family reported previously, it was observed that the transition temperatures are higher in compounds with low-tolerance factors. At the same time, the temperature range in which the intermediate tetragonal phase exists is reduced.

Powder SrPrZnRuO6, SrPrCoRuO6, SrPrMgRuO6 and SrPrNiRuO6 double perovskites were synthesized by the solid-state reaction method, and their crystal structure was investigated by Rietveld analysis (using the symmetry-mode procedure) by x-ray, synchrotron and neutron powder diffraction data. SrPrMRuO6 materials are monoclinic perovskites at room temperature, adopting the space group P21/n, , c ≈ 2ap, Z = 2. The unit cell parameters increase through the series as the B-cation size increases. The tilting of the octahedra is associated with irreducible representations GM+4 and X+3. In the studied series, Sr2+/Pr3+ are in eight-fold coordination and are displaced from the center of the Sr/PrO8 polyhedron along [010] by the X+5(A10) mode. The size of the first coordination sphere of Sr/Pr increases and the second coordination sphere reduces with temperature, suggesting an actual change in coordination. While changing the interatomic distances, the distortion of the structure diminishes, as observed in distortion mode amplitudes and octahedral tilt angles. The temperature driven phase transition was observed for Ni and Mg materials at ≈1025 K and 950 K, respectively. Tilting of the octahedra in the trigonal phase is associated with GM+4, which is the unique active mode associated with the not experimentally observed phase transition.

The phase-transition sequence undergone by [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{ZnI}}_{4}$ has been studied by adiabatic calorimetry. The specific-heat results show the P${112}_{1}$/a-P${2}_{1}$ca phase transformation around 215 K to take place in two independent steps. The corresponding specific-heat anomalies differ by only \ensuremath{\approxeq}0.4 \ifmmode^\circ\else\textdegree\fi{}C on heating, and \ensuremath{\approxeq}3 \ifmmode^\circ\else\textdegree\fi{}C on cooling. An unusual thermal hysteresis effect has been found to be present, which leads to the reversal in the order of appearance of the two specific-heat peaks. This fact has been unambiguously confirmed by means of an exhaustive experimental specific-heat study and by the use of the thermodynamic-functions values associated with this anomalous phase transformation.

The problem of the determination of the supergroups of a given space group is of rather general interest. It is useful in phase-transition problems, in the search for overlooked symmetries in a crystal structure determination, or in the detection of pseudosymmetries as a tool for predicting higher-temperature phase transitions (Igartua et al., 1996). In all these applications, it is not suf®cient to know the space-group types of the supergroups of a given group; rather, it is necessary to have available all the different supergroups Gj > H which are isomorphic to G, and are of the same index [i]. In the literature, there are very few papers treating the supergroups of space groups in any detail (Koch, 1984; Wondratschek & Aroyo, 2001). In the International Tables for Crystallography, Vol. A, Space Group Symmetry (1983), one ®nds only listings of minimal supergroups of space groups, which, in addition, are not explicit: they only provide for each space group H the list of those space-group types in which H occurs as a maximal subgroup, i.e to which minimal supergroups of H belong. It is rather dif®cult to determine all supergroups Gj > H if only the types of the minimal supergroups are known. The program SUPERGROUPS solves this problem for a given ®nite index [i].

Single crystals of the mixed valence compound Pb3Ni1.5Mn5.5O15 were successfully grown. Polarized Raman spectroscopy revealed that they belong to the point group D3d, in accordance with single-crystal x-ray diffraction data which were refined within the trigonal space group (No. 165), with lattice parameters a = 9.941(3) A and c = 13.543(3) A. Strongly anisotropic long range magnetic order is established below 65 K.

The program PSEUDO provides tools for the systematic search of pseudosymmetry, based on group±subgroup relations between space groups (Igartua et al., 1996). For a crystal structure L, speci®ed by its space group U, the cell parameters and the coordinates of the atoms in the asymmetric unit, the program searches for pseudosymmetry among all minimal supergroups Gk > U of the group U. The interpretation of a structural pseudosymmetry as a small distortion of a higher symmetric (prototype) structure H allows the following. (i) The prediction of phase transitions at higher temperature. If the distortion is small enough, it can be expected that the crystal acquires the more symmetric con®guration at a higher temperature after a phase transition (Igartua et al., 1996, 1999). (ii) The search for new ferroelastic and ferroelectric materials. Polar structures having atomic displacements smaller than 1 AE with respect to a hypothetical non-polar con®guration are considered as possible ferroelectrics (Abrahams & Keve, 1971; Kroumova et al., 2000). (iii) The detection of false symmetry assignments (overlooked symmetry) in crystal structure determination.

We report here a detailed study of the system La 1− x Sr x FeO 3− x F x , by neutron powder diffraction- and magnetic-measurements. All the compounds are robust antiferromagnetics with ordering temperatures well above room temperature. Magnetic moments are shown to align parallel to the c -axis. FC-ZFC measurements indicate a small canting of the magnetic moments, resulting in a ferromagnetic component with a maximum for La 0.5 Sr 0.5 FeO 2.5 F 0.5 . We show that the system exhibits a composition-driven transition from a phase, for low fluorination levels ( x ≤0.5), with Pnma symmetry and the usual system of octahedral tiltings, to a phase with space group Imma for higher fluorine contents, where a correlated distortion of the oxygen octahedra plays a significant role. The consistency of the structural models, with respect to the expected continuity of the amplitudes of the different distortion modes and the invariance of their internal form, was monitored through the symmetry mode decomposition of the structures.

The use of thermal energy storage (TES) for electricity production at concentrated solar power (CSP) plants has provided several important benefits, like lower environmental impact and higher dispatchability, compared to other methods. Many investigations have been focused on the enhancement of thermophysical properties of molten salts (TES material for CSP) by the addition of minor percentage of nanoparticles. In the present work, two nanofluids were developed based on the binary eutectic mixture of NaNO3 and KNO3 (mole ratio of 55:45) as base fluid with by addition of a small amount (1wt.%) of commercial Al2O3 and SiO2 nanoparticles. The thermophysical properties of the developed nanofluids have been investigated in terms of enhancement of their heat capacity and thermal conductivity. Firstly, an effective dry method is proposed and validated for the preparation of molten salt based nanofluids. The obtained nanofluids were characterized by different techniques such as scanning electron microscopy (SEM) and X-ray powder diffraction (XRPD). Secondly, their thermophysical properties have been investigated by means of differential scanning calorimetry (DSC) and laser flash apparatus (LFA). The developed nanofluids present significant enhancements of the specific heat capacity and thermal conductivity as compared to the base fluid ones. Finally, immersion corrosion tests were conducted under atmospheric conditions for carbon steel (CS) A516.Gr70 in contact with nanofluids at 390 °C, for exposure times of 250, 500, 1000 and 1500 hours. Carbon steel samples were analysed by means of SEM-EDX (surface and cross section), XRD and mass variation. Regarding the corrosion study, the oxidation was determined as the main mechanism of carbon steel degradation upon direct contact with molten nitrate salt. It was found that adding 1%wt of Al2O3 or SiO2 nanoparticles decreases the corrosion layer thickness more than twice as compared to the pure salt.