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We report the synthesis of double perovskite oxides Sr2(Co1−xFex)TeO6 obtained by solid-state reaction. The changes in the crystal structures of the samples due to iron substitution were studied using XRD, Raman spectroscopy and 57Fe Mössbauer spectroscopy.

We report a new series of DP Sr2(Fe1−xNix)TeO6, which have different transition metal Fe and Ni on B sites, providing an opportunity to investigate their effect on crystal structure, vibrational, optical and magnetic properties.

Currently, thermochemical energy storage (TCES) offers the opportunity to relieve the enormous gap of energy demand and supply in the solar energy field. This method has its specific requirements, such as high energy storage density, good thermal stability, high reaction enthalpy, high heat capacity and switched temperature range of thermochemical storage material (TCM) for low to medium temperature building applications. This paper reports the synthesis of blodite-type structure Na2M(SO4)2·4H2O (M = Zn, Mg) and copper krohnkite phase Na2Cu(SO4)2·2H2O. Their structural and thermal behavior were also investigated to explore a new temperature range using incorporated salts. An evaluation of potential energy storage density, structural reversibility, thermal cycling stability and storage efficiency of double salts compared to pure hydrates salts as TCM was carried out. The preliminary results show a good structural matching of synthesized double salts confirmed by XRD, FTIR, and Raman spectroscopy, while the transition phase investigation since dehydration process was reported by XRD and Raman spectroscopy. Moreover, Na2Zn(SO4)2·4H2O exhibited a suitable charging range temperature (

Efficient energy storage is a bottleneck for nearly every renewable energy technology. Thermal energy storage (TES) is widely considered as a relatively simple and reliable method, particularly for concentrated solar power (CSP) plants. Currently, considerable scientific effort is focused on the development of new molten salt-based nanofluids as storage materials with enhanced thermophysical properties and lower cost for TES purpose. However, an understanding of the effect of nanoparticles on the corrosivity of such nanofluids is practically absent. In the present work, using nanofluids based on eutectic mixture of NaNO3-KNO3 we demonstrate that nanoparticles doping has complex effects on the corrosion rates of carbon steel. In particular, if the negative effect of microbubbles of air trapped between the nanoparticles is not predominant, one can obtain reduced corrosion rates due to the incorporation of the nanoparticles into the oxidation layer. The obtained results are important both for expanding the very limited knowledge on the corrosion aspects of molten salts-based nanofluids, as well as for comprehensive evaluation of the feasibility of such nanofluids for TES applications.

The new double perovskite oxides Sr2Mn1−xNixTeO6 with x = 0.25, 0.5 and 0.75 have been synthesized in polycrystalline form by a conventional solid-state reactions process at 1180 °C, in the air. The structural and vibrational properties of these materials were studied by means XRPD, Raman, and IR spectroscopy. It has been proven that all the materials show typical double perovskite structures with a monoclinic symmetry, but with two different space group, P21/n for the compositions (x = 0.25 and 0.5), while the composition (x = 0.75) crystallizes in the space group I2/m. The lattice parameters obtained are in agreement with those of the two pure extremes Sr2MnTeO6 and Sr2NiTeO6. The monoclinic structural distortion involves long range ordering between Te6+ (in 2b site) and a random mixture (Mn2+/Ni2+) (in 2a site) for the two compositions (x = 0.25 and 0.5) that crystallize in P21/n. For the material (x = 0.75) with I2/m, similar distortion, ordering and mixing occur at the B and B’ double perovskite sites. It was observed that Vegard Law is satisfied, taking into account the cell parameters of both extremes. The effect of the partial substitution of Mn by Ni was also seen in Raman and IR data where a significant mode shift was observed with nickel content increase.

The reaction between the complexes of double perovskite formula Sr2Co1−xNixTeO6 in different stoichiometric proportions (x = 0.25, 0.5 and 0.75) have been processed in polycrystalline form by solid– state reaction mode in air. Based on the Rietveld refinements of x-ray powder diffraction data, the crystal structures and phase transitions, at room temperature of this double perovskite series are reported. The materials crystallize in a monoclinically distorted perovskite structure (the two compositions with x = 0.5 and 0.75 belong the to I2/m space group, while the composition with x = 0.25 crystallize in P21/n space group). We found a good agreement between the lattice parameters of this series and those of the two materials Sr2CoTeO6 and Sr2NiTeO6 with x = 0 and 1, respectively. The linear evolution of crystalline parameters proves the realization of the Vegard Law. The effect of the partial substitution of Co by Ni was also seen in the spectra of Raman and infrared, where a band shift was observed with increased nickel content.

The mid-infrared (3–22 μm) emissivity of high-purity Ni has been studied in its pure state, during an isothermal oxidation in air at 730 °C and in the fully oxidized state. Measurements in pure Ni were performed in Ar between 200 and 800 °C and a change of slope in the temperature dependence of the total normal emissivity around its Curie temperature (354 °C) was observed. An oxidation process was carried out at 730 °C for 33 days, when the emissivity stopped evolving and the results were representative of NiO. During the first stages, the emissivity evolved forming the usual interference patterns of semi-transparent films. A mixture of oscillatory and monotonic behaviours of the emissivity as a function of wavelength and oxide layer thickness was found, which manifests as a non-trivial evolution of the total normal emissivity, different than that reported in previous studies. Finally, the emissivity of NiO was measured from below its Neel temperature (252 °C) to 850 °C. It showed the typical shape of a ceramic material with an extra vibrational mode due to two-phonon processes and an additional absorption band around 5 μm in the antiferromagnetic phase produced by magnons. The temperature dependence of its total normal emissivity differs significantly from that of a lightly oxidized nickel sample from the literature. Overall, the influence of the surface characteristics on the thermal radiative properties of oxidized Ni is thoroughly discussed and highlights the importance of accounting for all possible sources of infrared emissivity evolution in order to make accurate radiative heat transfer calculations.

The directional spectral emissivities of two V-4Cr-4Ti family alloys, candidate structural materials for fusion first wall/blanket applications, were measured between 200 °C and their working temperatures (700–750 °C), with and without a high-temperature treatment. Besides showing the typical metallic behavior, an increase in the emissivity after the heat treatment (1000–1200 °C) was observed in both alloys. This has been attributed to several microstructural changes, which show the important role of microstructure in the thermal radiative properties of these alloys. In order to explain these mechanisms, the samples were analyzed using electron microscopy and X-ray diffraction. These measurements revealed differences in grain size, composition of the main phase and amount and distribution of dispersed secondary phases. X-ray diffraction and X-ray photoelectron spectroscopy were also used in order to check the extent of oxygen penetration. The results of directional spectral emissivity measurements were integrated to calculate the total hemispherical emissivity, the key heat transfer parameter in the high-temperature high-vacuum environments of fusion reactors. It is observed that the strategy of mechanical alloying with oxide and carbide dispersion to improve the mechanical properties also translates into an enhancement of the radiative refrigerating capability of these alloys.

Electrothermal behaviour of 14S5P battery-pack, usually placed in small electric vehicles, is analysed for the first time using multiphysics simulations. An electrochemical model is developed to describe the electric behaviour of LMO 26650 battery (3.5 Ah). NMC active material is added in the LMO cathode (0, 29, and 59 % wt) to check the improvement of the electrical performance of the battery. The heating process of LMO 26650 battery is simulated in a high intensity discharge process (10 A). As a result, the temperature of the cell suffers an important increment of 64 %. Finally, with intensities of usual working conditions, the temperature of 14S5P battery-pack does not increase remarkably.; Normalean ibilgailu elektriko txikietan erabiltzen den 14S5P bateria-multzoaren jokaera elektrotermikoa lehenengo aldiz aztertu dugu simulazio multifisikoen bidez. LMO kimikako 26650 bateriaren (3,5 Ah) jokaera elektrikoa simulatzeko eredu elektrokimikoa garatu dugu. LMOko katodoan NMC material aktiboa gehitu dugu (pisuan, % 0, % 29 eta % 59), bateriaren ezaugarri elektrikoak hobetzen direla egiaztatzeko. LMO kimikadun 26650 bateriaren beroketa simulatu dugu intentsitate altuko deskarga batean (10 A), eta tenperatura % 64 igotzen dela behatu dugu. Azkenik, ohiko lan-baldintzetako korronteetan, 14S5P bateria-multzoaren tenperatura ez dela era nabarmenean handitzen frogatu dugu.

Emisibitate infragorria materialen propietate termofisiko bat da. Erradiazio elektromagnetikoa igortzeko materialek daukaten ahalmena adierazten du parametro horrek. Eguzki-energiaren industrian erabiltzen diren materialen emisibitatea neurtzea oso garrantzitsua da materialen eraginkortasuna handitzeko; izan ere, erradiazio bidez gertatzen diren bero-galeren kalkuluak egiteko eta galera horiek saihesteko, ezinbestekoa da emisibitatea. Emisibitatearen zehaztasun handiko neurketak egin dira UPV/EHUko puntako tresna bat erabiliz Mg-%47Zn-%4Al aleazio hirukoitz eutektikoan, zeina eguzki-energia termikoa metatzeko sistemetarako hautagai den.; Infrared emissivity is a thermophysical property of materials which deter-mines their capability to emit thermal radiation. Emissivity measurements are essential for calculating and minimizing thermal losses by radiation in many technological fields, such as solar energy and thermal storage. Accurate emissivity measurements performed with a singular equipment at the UPV/EHU are presented in the case of a eutectic ternary alloy (Mg-47%Zn-4%Al), which is a candidate for solar thermal-en-ergy storage systems.

In the present work, synchrotron X-ray powder-diffraction measurements at room temperature and laboratory X-ray powder-diffraction measurements at high temperatures of nine members of the antimony family Sr2MSbO6 M = Ln (Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb) and Y ; are reported. Six of them are synthesized for the first time in this work. The crystal structures at room temperature of these compounds are determined and their symmetries are described in the P 2 1 / n ( a - a - b + ) space group (No. 14, non-standard setting). The materials have double perovskite structure with 1:1 ordered arrangement of the B sites. The high-temperature measurements show the existence of two temperature driven phase-transitions: a discontinuous one, from the RT symmetry to R 3 ¯ ( a - a - a - ) and a continuous one from the latter symmetry to the highest one, the cubic Fm 3 ¯ m ( a 0 a 0 a 0 ). The phase transition temperatures are correlated to the M3+ cation size. The analysis of the phase transitions and the structural refinements have done using the symmetry-adapted modes. A step forward in these analysis is performed: parametrization of the refinements by fixing a selected common direction in the sub-space spanned by X 5 + , tri-linearly coupled to the order parameters of the cubic to monoclinic first order phase transition.

Two new double perovskites SrCaCoTeO6 and SrCaNiTeO6 have been successfully synthesized via the conventional solid state reaction method. The crystal structures and phase transitions of both materials have been investigated using conventional X-ray powder diffraction method at room- and high-temperatures. The Rietveld analysis has shown that both compounds crystallize at room temperature in a primitive monoclinic perovskite-type structure with P 2 1 / n (No. 14) space group and 1:1 ordered arrangement of the B-site cations (Co(Ni)/Te). The high temperature analysis has shown that these materials present temperature induced phase transitions following the next phase transition sequence: P 2 1 / n ( a - a - c + ) → I 2 / m ( a - a - c 0 ) → I 4 / m ( a 0 a 0 c - ) → Fm 3 ¯ m ( a 0 a 0 a 0 ) .

were obtained. Their structural and vibrational properties were investigated. From the Rietveld refinement of X Ray diffraction pattern we show that the composition x = 0.2 crystallizes in the cubic symmetry, space group Fm 3m (ITA No. 225) with a lattice parameter a =5.49 A. The reliability factors are: RF =2.151 % and RBragg=2.545 % confirm the goodness of the refinement. From the evolution of Raman bands, we confirm the existence of the solid solution features. Furthermore, comparing the spectra of a-Bi 2 O 3 with the alpha phase, we comfortably suggest that the decrease of the number of Raman bands is a consequence of an increase in the lattice symmetry. Similarly to other fluorite compounds, we show that the structure presents oxygen defects clearly identified in the Raman spectra.

Two Al2O3 based nanofluids have been developed to increase the thermophysical properties of the molten salts used for thermal energy storage application at concentrated solar power plants. A novel simple dry preparation method by using a physical shaking technique was demonstrated in the present research work. A good dispersion of 1 wt% Al2O3 nanomaterials with different shapes (nanospheres and nanorod) in the base eutectic salt, i.e. 51 wt% NaNO3 – 49 wt% KNO3, was achieved by a shaking time of 15 min. Two different shaped Al2O3 nanomaterials, i.e. nanoparticles (Al2O3-NPs) and nanorods (Al2O3-NRs), have been used to prepare the nanofluids. Their stability and thermophysical properties have been studied by means of TGA, DSC, LFA, Rheometry and Dilatometry. The main aim of the present study is to investigate the nanoparticle shape effects on the enhancement of the thermophysical properties of the base inorganic salt. The Al2O3-NPs based nanofluid (Al2O3-NPs-nanofluid) and Al2O3-NRs based nanofluid (Al2O3-NRs-nanofluid) show enhancements of the specific heat capacity (Cp) in solid and liquid states of ~3% and ~6%, respectively. The thermal conductivity analysis demonstrates enhancements of 12% and 20% in solid states and 16% and 12% in liquid state for Al2O3-NPs-nanofluid and Al2O3-NRs-nanofluid, respectively. The viscosity increases of 25% and 37% were observed for Al2O3-NPs-nanofluid and Al2O3-NRs-nanofluid, respectively.

The total hemispherical emissivity of materials used in the solar energy industry is a critical parameter in the calculation of the radiative thermal losses and material efficiency, especially in solar thermal collector absorbing surfaces. This is because the radiative heat losses have a significant economic impact on the final cost of the electricity produced in solar plants. Our laboratory, HAIRL, in the University of the Basque Country (UPV/EHU) in Spain [1] is the first to have published infrared spectral emissivity measurements in Solar Absorber Surfaces (SAS) at working temperature [2]. The laboratory allows measuring between 50 and 1000 ºC in the 0.83-25 μm range and is also capable of doing directional measurements at different angles between 0 and 80 degrees. Therefore, it is suitable for measuring solar selective coatings, for studying high temperature stability and for characterizing thermal energy harvesting materials. In this presentation, we show the specifications of our laboratory, the results of spectral emissivity measurements in air-resistant solar selective coatings and in eutectic alloys for thermal storage and we demonstrate the necessity of measuring at working temperature in order to possess reliable data.

Material batean gertatzen diren trantsizio magnetikoen eta haren egitura kristalinoaren arteko erlazioa aztertzea oso garrantzitsua da, batik bat multiferroikoak diren materialetan. Gehienetan, trantsizio magnetikoetan gertatzen diren egiturazko al-daketak gako dira trantsizioan agertzen edo desagertzen den polarizazio elektrikoaren jatorria zein den ulertzeko. Hala ere, hainbat materialetan aldaketa horiek oso txikiak dira eta horiek detektatzea oso zaila izan daiteke. Hori da aztergai dugun materialean (MnWO4) gertatzen dena. Hori dela eta, ondorio zuzenetara heltzekotan, ezinbestekoa da kristalaren egitura ikertzeko teknika oso sentikorrak erabiltzea. Egiturazko aldaketa oso txikiak aztertzeko neutroien prezesioaz baliatzen den teknika aurkeztuko dugu lan honetan, neutroien bidezko Larmor-en difrakzioa, hain zuzen ere. Nahiz eta lortutako emaitzak behin-behinekoak izan, oso argi ikusten dira zein diren teknika honen indarra eta potentzia, eta trantsizio magnetikoetan gertatzen diren egiturazko aldaketak; The study of the interplay between magnetic transitions and the crystal structure is crucial for the investigation of multiferroic materials. In fact, the structural changes across the magnetic transitions are usually the clue to ascribe the origin of the concomitant appearance of electric polarization. Nevertheless, in some materials, those structural variations are so small that their detection is very difficult. That is the case of MnWO4. Hence, in order to reach appropiate conclusions we need to use experimental techniques which are very sensitive to crystal structure variations. In this piece of work we present an experimental technique that takes profit of the precession of the neutrons in magnetic fields in order to see those small changes in the crystallographic structures, that is, the Larmor diffraction by means of neutrons. Even if the obtained results are rather preliminary, the undoubtedly show the strength and power of this technique and the modifications of the structure across the magnetic transitions.

In this article we comment on the results published by Thompson et al. (, J. Solid State Chem. 219 (2014) 173–178) on the crystal structure of SrFeO 2 F, who claim the compound to crystallize in the cubic space group Pm -3 m . We give a more detailed explanation of the determination of our previously reported structural model with Imma symmetry (Clemens et al., J. Solid State Chem. 206 (2013) 158–169), with addition of variable temperature XRD measurements with high counting time to provide unambiguous evidence for the Imma model being correct for our sample.

Na0.5K0.5NbO3has been synthesized by the conventional solid-state reaction process. The crystal structures and phase transitions, at low and high temperature, determined from the Rietveld refinements of X-ray and neutron powder diffraction data are reported. The structure evolution of Na0.5K0.5NbO3in the temperature range from 2 to 875 K shows the presence of three phase transitions. The first one, at ∼135 K, is discontinuous from the rhombohedralR3c(No. 161) space group to the room-temperature orthorhombicAmm2 (No. 38) space group; the second is discontinuous from the orthorhombic to the tetragonalP4mmspace group (No. 99) at ∼465 K, and the third is continuous from the tetragonal to the cubic Pm\overline{3}m space group (No. 221) at ∼700 K. The obtained phase-transition sequence isR3c→Amm2 →P4mm→Pm\overline{3}m. No previous studies at low temperature have been carried out on the material with composition Na0.5K0.5NbO3. In the course of the determination of the three experimentally found phases, a novel method of refinement is presented. This is a step forward in the use of the symmetry-adapted modes as degrees of freedom in the refinement process: the parameterization of a direction in the internal space of the, in this case, sole irreducible representation, GM4−, responsible for the symmetry breaking from the parent cubic space group to the polar distorted low-symmetry phases. Eventually, this procedure enables the calculation of the spontaneous polarization.

Synthesis and crystal structures are described for the Bi1-xSb1-xTe2xO4 solid solution with 0 ≤ x ≤ 0.1. It crystallizes in the monoclinic system, space group I2/c. Rietveld refinements of X-ray powder diffraction data indicate that the atomic positions are: Bi/Te(2)(4c), Sb/Te(1)(4d). The oxygen occupied two sites, 8f and 8b, respectively. The reliability factors are: Rp = 7.45%, Rwp = 10.6% and Rb = 3.88% for x = 0.1. The structure contains [(Sb/Te(1))O4]n layers formed by (Sb/Te(1))O6 octahedra sharing corners, which are parallel to (001) plan and held together by bismuth and tellurium atoms. The Raman study of this solid solution shows the bands which are assigned to O-Bi3+-O, O-Sb5+-O and connects (Bi/Te(2))O8-(Sb/Te(1))O6 vibration in the crystal.

Hot corrosion is profoundly detrimental to construction elements in Concentrated Solar Power (CSP) plants affecting their lifetime, running costs and safety. In this work we have studied the anticorrosion effect of TiO2 nanoparticles additives on carbon steel using XRD, XPS with depth profiling, EDX and FIB/SEM techniques. The results revealed that the addition of 1 wt% of TiO2 nanoparticles to molten binary nitrate salt reduces the corrosion rate of carbon steel more than twice and stabilizes the corrosion scale at 390 °C. The anticorrosion effect of TiO2 nanoadditive was attributed to the formation of iron-titanium mixed oxide on the carbon steel surface. It was confirmed by XRD and TGA techniques that addition of TiO2 nanoparticles does not alter the stability of the salt. In view of presented results, the feasibility of molten salts based nanofluids in the CSPs can be reconsidered in terms of improved compatibility with construction materials.

Three different sizes of the SiO2@Al2O3 core-shell nanoparticles were synthesized by tuning the silica precursor using the wet chemical route. The precise control of the outer shell diameter of SiO2 was controlled by optimum concertation of the silica precursor in the alumina solution. Gradual increment of the outer shell depth and the chain-like aggregated structure was observed from TEM while increasing the silica precursor during the synthesis of SiO2@Al2O3 core-shell nanoparticle. Respectively, the size of the SiO2@Al2O3-10 nanoparticles is 12 ± 2 nm with no observable SiO2 diameter, SiO2@Al2O3-20 is 14 ± 2 nm with 2 nm of SiO2 diameter and SiO2@Al2O3-35 is 17 ± 2 nm with 5 nm of SiO2 diameter in the outer layer. The interaction and quantitative elemental properties of the core-shell nanoparticles were studied using FTIR, XRD and SEM-EDX analyses. Consequently, the potentiality of the core-shell nanoparticle was analyzed for the prospective of TES application by mixing 1 wt% core-shell nanoparticles with 99 wt% eutectic binary molten nitrate salt (nanofluids) via one-step dry preparation method. The changes in the thermophysical properties were observed with respect to the addition of silica precursor or increasing silica outer diameter in the SiO2@Al2O3 core-shell nanoparticle. From the analysis, 19% higher thermal conductivity was obtained for the nanofluid prepared with SiO2@Al2O3-35 core-shell nanoparticles. A higher viscosity also was observed for the chain-like aggregates of SiO2@Al2O3-35 core shell-based nanofluids, which was 25 to 34% higher compared to the base fluids in the temperature range between 250–400 °C.

The directional spectral emissivities of four new copper-alloyed spinel coatings for concentrated solar power applications were measured up to 800 °C and compared Pyromark 2500®, deposited in the same conditions on Inconel 625. Reproducible results were found for all coatings at all temperatures, with similar spectral features at working temperatures. The temperature and angular dependences are related to the morphology and composition of the samples. The total hemispherical emissivity increases up to 400 °C for all coatings and then stabilizes, with similar values for most materials, except for the porous Cu0.5Cr1.1Mn1.4O4 coating. This coating offers a reduced total hemispherical emissivity due to increased semitransparency at high angles arising from its porosity. This porosity is linked to an increase in both the solar absorptance and the emissivity in the normal direction due to enhanced light trapping, which means that this coating shows signs of directional selectivity. These results, together with the data dispersion reported for Pyromark, suggest that structural properties are key for the high-temperature emissivity of the coatings and highlight the importance of direct emissivity characterization. Combined with absorptance measurements, these emissivity measurements allow for accurate calculations of the high-temperature efficiencies of the coatings, which reach values up to 0.929.

X-ray diffraction and Raman spectroscopy studies of Sr doped double perovskites compound Ba 2− x Sr x MWO 6 with (M = Ni, Co and 0 ⩽ x ⩽ 2) were investigated. The samples show a transition from cubic to tetragonal phase as a function of composition while increasing strontium amount; both Rietveld refinements and Raman studies showed that this transition occurs between x = 1.2 and 1.4. Furthermore, increasing temperature for the tetragonal compositions (1.4 ⩽ x ⩽ 2), manifest the tetragonal to cubic phase transition. For this series, the transition from tetragonal ( I 4/ m )/to cubic ( Fm -3 m ) shows considerable changes in the composition and temperature dependence of the modes: all the Raman modes show a linear behavior when temperature is increased, then the slope change dramatically indicating the symmetry change from tetragonal to cubic. To better view this phase transition, the full width at half maximum (FWHM) temperature dependence of the most intense modes were studied; by this study, we have obtained a sensitive guide to the onset of the phase transition. A large increase of the FWHM with increasing temperature is observed for all modes in the spectrum, clear transitional effects were observed: the width behaves in a linear way as a function of temperature; when the temperature reached the transition temperature, a drop in the width of this mode was observed.

The crystal structures of double perovskites SrLaMRuO6(M= Zn and Mg) have been studied using X-ray and neutron diffraction data. Both compounds have monoclinic structure at room temperature with space groupP21/n. A first-order structural phase transitionP21/n→R{\overline 3} has been observed in SrLaZnRuO6and SrLaMgRuO6, at about 800 and 500 K, respectively, with the two phases coexisting in a temperature interval of 150 K. The temperature- and tolerance-factor-dependent phase diagram of the SrLnMRuO6(Ln = La, Nd and Pr;M= Zn, Co, Cu, Mg and Ni) family shows two different phase transition sequences depending on theA-site cation. The structures of the title compounds at room and high temperatures have been analyzed using the symmetry-mode decomposition approach.

Crystal structures of SrNdZnRuO6, SrNdCoRuO6, SrNdMgRuO6 and SrNdNiRuO6 double perovskites have been studied by X-ray, synchrotron radiation and neutron powder diffraction method, at different temperatures, and using the symmetry-mode analysis. All compounds adopt the monoclinic space group P 2 1 / n at room-temperature, and contain a completely ordered array of the tilted MO6 and RuO6 octahedra, whereas Sr/Nd cations are completely disordered. The analysis of the structures in terms of symmetry-adapted modes of the parent phase allows the identification of the modes responsible for the phase-transition. The high-temperature study (300–1250 K) has shown that the compounds present a temperature induced structural phase-transition: P 2 1 / n → P 4 2 / n → Fm 3 ¯ m .

The new ALn 2 CuTi 2 O 9 ( A = Ca, Ba; Ln = La, Pr, Nd, Sm) series have been synthesized and the structures have been characterized by X-ray and neutron powder diffraction techniques. The whole series show a cationic disordered distribution over the A - and B -sites of the perovskite structure. Ca Ln 2 CuTi 2 O 9 samples are isostructural, crystallizing at room temperature in an orthorhombic cell with space group Pbnm (ITA No. 62); and Ba Ln 2 CuTi 2 O 9 samples in the tetragonal I 4/ mcm (ITA No. 140) space group. The homovalent substitution of a larger lanthanide cation leads to an increase in the unite-cell volume, but a decrease in the Cu/TiO 6 octahedra tilting. By comparing the room temperature symmetry structure with an archetype ( Pm 3 ¯ m , ITA, No. 221) phase of Pbnm or I 4/ mcm symmetry, two distortion modes R 4 + and M 3 + were found to be dominant for the orthorhombic phase; whereas for tetragonal phase there is a unique active mode, R 4 + . They correspond to Cu/TiO 6 octahedron tilt modes, commonly found in many perovskite type materials. The high temperature study for CaLa 2 CuTi 2 O 9 , CaNd 2 CuTi 2 O 9 and CaSm 2 CuTi 2 O 9 materials (300–1670 K) showed that the room temperature Pbnm orthorhombic symmetry seems to be stable up to 1475 K; and irreversible phase transition is observed above that temperature.

In this paper we report high-temperature (up to 550 °C) Raman spectroscopy of phase transitions in Sr2MWO6 (M = Zn, Co) double perovskite oxides. For Sr2ZnWO6 modes centered at 137 cm−1, 152 cm−1, merge around 80 °C. The same behavior was observed for the 436 cm−1 and 447 cm−1 modes. The mode centered at 854 cm−1 shows a change in the slope at the same temperature, thus indicating a first order phase transition from the monoclinic (P21/n) to tetragonal (I4/m) structure. With increasing temperature, a little change in the slope of the temperature dependence of mode centered at 855 cm−1 is observed around 480 °C which might be a sign of the second phase transition in Sr2ZnWO6 (tetragonal (I4/m) to cubic (Fm-3m)). The temperature dependence of the full width at half maximum (FWHM) of the most intense modes in Sr2ZnWO6 spectra was studied. The width behaves linearly as a function of temperature with a break observed in the slope of the width of the modes at 80 °C; thus, showing the first phase transition occurrence. For Sr2CoWO6 the transition from tetragonal (I4/m) structure to cubic (Fm-3m) phase is accompanied by considerable changes in the temperature dependence of modes centered at 438 cm−1 and 845 cm−1. The transition was observed around 420 °C.

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.

The crystal structures and phase transitions of the two ordered double perovskites Sr2ScSbO6 and Ca2ScSbO6 were studied using conventional X-ray and neutron powder-diffraction methods. The crystal structures of both compounds have the P 2 1 / n space group symmetry at room temperature, resulting from Sc/Sb ordering. The evolution with temperature of the structure of the Sr containing compound shows the presence of three phase transitions with the following sequence: P 2 1 / n → I 2 / m → I 4 / m → Fm 3 ¯ m , at about 400, 560 and 650 K, respectively. The smaller size of Ca cation, with respect to that of Sr cation, leads to a large distortion of Ca2ScSbO6 at room temperature. This fact in turn causes that the Ca containing compound shows only the first phase transition from P 2 1 / n to I 2 / m at high temperature at about 1440 K. The analysis of the phase transitions and the refinements have done using the symmetry-adapted modes and the tools of the Bilbao Crystallographic Server.

In this work we report on the crystal structure and Raman spectroscopy of BaSrMWO 6 (M Ni, Co, Mg) double perovskite oxides. The results showed that the Ni, Co and Mg containing compounds crystallize in a cubic system with the space group Fm-3 m. The double perovskite structure can be represented as a three-dimensional network of alternating MO 6 and WO 6 octahedra, with Ba- and Sr-atoms occupying the interstitial spaces. The Raman spectra of this crystalline perovskite oxides are interpreted by means of factor group analysis in terms of space group Fm-3 m. Assignments of the W O vibrational stretching and bending modes have been made. High temperature Raman spectroscopy of BaSrNiWO 6 showed that all lattice modes illustrate a linear decrease in wavenumbers while temperature is increased. No discontinuities, in the temperature changes, in wavenumber observed for all modes recorded: indicating that the structure remains cubic and stable up the highest temperature studied here.

We have investigated the crystal structure and phase transitions of new series of ordered double perovskite oxides Ca2−xSrxLaSbO6 and Ca2−xSrxSmSbO6 (x = 0, 0.5, 1). Rietveld refinements of the X-ray powder diffraction data at room temperature shows that these compounds crystallize in monoclinic symmetry with P21/n space group, with cation distribution [Ca1−xSrxR]ACaSbO6 general formula. The La compounds have a total ordering between La and Ca in A- and B-sites, respectively; however, in Sm compounds, the degree of Sm–Ca ordering has been found decreasing with an increase in the Sr content. A linear increasing of the lattice parameters and the volume, at room temperature, has been found with increasing of Sr concentration. The temperature evolution of the structure has revealed that the compounds Ca1.5Sr0.5LaSbO6, CaSrLaSbO6 and CaSrSmSbO6 present two phase transitions, the first one being discontinuous and the second one continuous as a following phase-transition sequence: P 2 1 / n → R 3 ¯ → Fm 3 ¯ m . In Ca2LaSbO6, Ca2SmSbO6 and Ca1.5Sr0.5SmSbO6, no indication of any phase transitions from room temperature to 1770 K was observed.

In this paper, we report on high temperature Raman spectroscopy studies (up to 550 °C) of the phase transitions in Sr2BWO6 (B = Ni, Mg) double perovskite oxides. For both compounds, the transition from the tetragonal (I4/m) phase to the cubic (Fm-3m) phase is accompanied by considerable changes in the temperature dependence of the modes. To better view this phase transition, the FWHM temperature dependence of the most intense modes were studied; by this study, we have obtained a sensitive guide to the onset of the phase transition. A large increase of the FWHM with increasing temperature is observed for all modes in the spectrum, clear transitional effects were observed: the width behaves in a linear way as a function of temperature; when the temperature reached around 300 °C, a drop in the width of this mode was observed; thus, showing the phase transition occurrence for both compounds. This drop is much more illustrated in Ni-containing compound than in Sr2MgWO6.

Single crystals of Sr2CaWO6have been prepared by sintering at high temperature. Powder samples were compressed into rods and heated up to 1953 K. This seems a promising new route for further studies of the structure and physical properties of double perovskites. The structural model of Sr2CaWO6includes a quantitative description of the twinning shown by the diffraction pattern that should be present in almost any single-crystal specimen for this type of compound.

Sr 2 FeSbO 6 , SrCaFeSbO 6 and Ca 2 FeSbO 6 with the double perovskite structure have been synthesized by the conventional solid state reaction process. The crystal structures and phase transitions at low and high temperature were determined from Rietveld refinements of X-ray and neutron powder diffraction data. Sr 2 FeSbO 6 crystallizes at room temperature in a monoclinic system having a space group I 2 / m and not P 2 1 / n space group as was previously reported. The crystal structures of SrCaFeSbO 6 and Ca 2 FeSbO 6 are described with the P 2 1 / n space group. All compounds show partial order of the B-site Fe and Sb cations, with degree of ordering of 89% for Sr 2 FeSbO 6 and 94% for SrCaFeSbO 6 and Ca 2 FeSbO 6 .

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 .

We present a new effective method for synthesizing Sr 2 MWO 6 double perovskite oxides: the co-precipitation route at 1220 K in nitrogen environment. Using conventional X-ray diffraction methods, we have confirmed the room-temperature space group of Sr2Mn2+ WO 6 to be P 2 1 / n : Z = 2; a = 5.6764 ( 1 ) A ; b = 5.6752 ( 1 ) A ; c = 8.0144 ( 1 ) A ; β = 90.12 ( 1 ) ° and V = 258.18 ( 1 ) A 3 . We show that the compound presents the following temperature induced phase-transition sequence: P 2 1 / n → I 4 / m → Fm 3 ¯ m . After a thermal treatment, 24 h at 870 K in air, the Sr 2 Mn 2 + WO 6 compound transforms irreversibly to Sr 2 Mn 3 + WO 6 + δ . This transformation has been confirmed by EPR and XANES measurements. Using conventional X-ray diffraction methods, we have shown the room-temperature space group of Sr 2 Mn 3 + WO 6 + δ to be I 4 / m : Z = 2; a = 5.6353 ( 1 ) A ; c = 8.0149 ( 1 ) A and V = 254.53 ( 1 ) A 3 . We show that the compound presents the following temperature induced phase-transition sequence: I 4 / m ( → I 4 / mmm ) → Fm 3 ¯ m . The tetragonal-to-tetragonal phase transition is suggested to be present, it is not observed as it is very weak; it is attributed to the presence of the Jahn-Teller active Mn3+ cation.

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 solid solution with double perovskite structure and general chemical formula Sr2Cd1−xCaxWO6 (0 ⩽ x ⩽ 1) has been synthesized by the co-precipitation method. The Cd2+ cation substitution by Ca2+ in the Sr2CdWO6 double perovskite led to a continuous solid solution in the whole (0 ⩽ x ⩽ 1) fraction range. The crystal structures were determined from the Rietveld refinements of laboratory X-ray powder diffraction data. All members of the solid solution crystallize at room temperature in the P21/n space group. The high-temperature study has revealed the following two phase-transition sequence for all the members of the solid solution: P21/n→I4/m→Fm3¯m. The phase-transition temperatures of both successive phase-transitions showed a linear behavior in the whole (0 ⩽ x ⩽ 1) fraction range. Finally, following the results reported for Sr2CdWO6 and Sr2CaWO6, and comparing those temperatures with others of the Sr2MWO6 family reported previously, it was observed that the transition temperatures are higher in compounds with low tolerance factors.

Using synchrotron radiation and Raman spectroscopy and a diamond anvil cell we measured the pressure dependence of the lattice parameters and Raman modes of polycrystalline Sr 2 CoWO 6 . Angle-dispersive X-ray diffraction patterns were indexed and showed that at 2.2 GPa, the material transforms from the I 4/ m tetragonal structure to the P 2 1 / n monoclinic structure. For pressure values between 2.2 and 12.7 GPa only monoclinic symmetries were found. We had difficulties with the convergence of the profile matching of the diffractogram taken at 12.7 GPa, indicating that the second phase-transition took place. To get more informations about these structural changes, an in-situ Raman spectroscopic study was conducted to explore the pressure-induced phase-transition sequence of Sr 2 CoWO 6 to pressures of 40.8 GPa at room-temperature. Group theory yields nine Raman-active modes for Sr 2 CoWO 6 ( I 4/ m ), all the predicted bands are observed at ambient conditions (phase I). The experimental results indicate that structural changes are observed at 2.15 and 11.15 GPa, which we attribute to phase transitions; thus, giving rise to two new phases, named as phase II and phase III, respectively. In the 9.33–14.84 GPa interval a coexistence of phases II and III is observed.

The two double perovskite oxides Sr{sub 2}AlSbO{sub 6} and Sr{sub 2}CoSbO{sub 6} were prepared and their structures studied with the X-ray powder diffraction method. At room temperature the crystal structure of Sr{sub 2}AlSbO{sub 6} is cubic (Fm3-barm), with a=5.6058(1)A. It was found that depending on the preparation conditions, the Al{sup 3+} and Sb{sup 5+} cations can be either entirely or partially ordered. In the case of the partially ordered Sr{sub 2}AlSbO{sub 6} sample, the extension of cation ordering was estimated from the hkl-dependent broadening of the diffraction peaks and the results were interpreted as evidence of the formation of anti-phase domains in the material. Low-temperature Raman spectroscopic measurements demonstrated that the cubic phase of Sr{sub 2}AlSbO{sub 6} is stable down to 79 K. The room-temperature crystal structure of Sr{sub 2}CoSbO{sub 6} is trigonal (space group R3-bar) with a=5.6058(1)A and c=13.6758(3)A. At 470 K, however, the material undergoes a continuous phase transition and its structure is converted to cubic (space group Fm3{sup -}barm). The studied Sr{sub 2}CoSbO{sub 6} sample was partially ordered, but unlike Sr{sub 2}AlSbO{sub 6}, no indication of the formation of anti-phase domains was observed. - Williamson-Hall plot of the diffraction peaks in Sr2AlSbO6, samples 1 and 2, andmore » in Sr{sub 2}CoSbO{sub 6}. {beta} denotes the integral breadth corrected for instrumental effects, {theta} is the diffraction angle. It can be seen that in the case of sample 1, {beta} of the superstructure reflections (solid squares) are clearly larger than those of the rest of the peaks. It was found that depending on the preparation conditions, the Al{sup 3+} and Sb{sup 5+} cations can be either entirely or partially ordered. In the case of the partially ordered Sr{sub 2}AlSbO{sub 6} sample, the extension of cation ordering was estimated from the hkl-dependent broadening of the diffraction peaks and the results were interpreted as evidence of the formation of anti-phase domains in the material. In the case of Sr{sub 2}CoSbO{sub 6}, despite the high synthesis temperature (1770 K), the degree of ordering is relatively low. This can be explained by the small difference between the radii of the B-site cations. Another interesting fact is that the sample-related broadening of the diffraction peaks of Sr{sub 2}CoSbO{sub 6} does not show any significant hkl-dependence. This suggests that no extended anti-phase boundaries are formed within the crystallites and the observed low degree of ordering is caused by randomly distributed anti-site defects. In the case of trigonal perovskites the effect of the cation ordering on the peak widths cannot be visualized easily as in the case of cubic perovskites due to the overlapping of the order-related reflections with sub-cell reflections.« less

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.

Adiabatic calorimetry measurements have been performed on Cs2CdBr4 from 90 to 270 K. Phase transitions at 252.03, 237.02, and 154.88 K were found. The thermal expansion coefficients and the elastic constant values were used to separate the harmonic and anharmonic contributions to the specific heat, which permitted us to establish a baseline for the precise determination of the thermodynamic functions associated with the phase transitions. Anharmonic quantities, such as the Gruneisen parameter and the isothermal compressibility, were also calculated. Finally, no experimental evidence of the suggested transitions at 208 and 130 K was found.

An in situ Raman spectroscopic study was conducted to explore the pressure-induced phase transformation of Sr2CaWO6 to pressures of 50.8 GPa and room temperature. Group theory yields 24 Raman active modes for Sr2CaWO6, of which 17 are observed at ambient conditions. Upon elevation of pressure to 3.2 GPa, the results suggest some structural changes, possibly a phase transition. At 10.4 GPa, new modes at high frequencies (824 and 903 cm−1) come into existence. As the material is compressed further, the intensities of the bands, especially the W–O stretching modes, observed at 824 and 903 cm−1 become stronger, while those centred at 863 and 937 cm−1 (at 10.4 GPa) decrease significantly, until disappearance. These changes in the spectrum indicate clearly that Sr2CaWO6 undergoes a phase transition at 9.7 ± 0.7 GPa. Upon release of pressure to ambient conditions, a new phase is observed with ten bands compared to the 17 observed in the original phase.

In the present work we report x-ray powder diffraction measurements of Sr2CaWO6 and Sr2MgWO6, at different temperatures. The crystal structures at room temperature of both compounds are determined; and results showing the existence of phase transitions in them are presented. For the structure at room temperature of the Ca containing compound, the space group P 21/n is obtained, which is different from that previously suggested. The evolution with temperature of the structure of this compound shows the presence of two phase transitions, a discontinuous one at 1130 K and a continuous one at 1250 K, with the following phase transition sequence: . The crystal structure of the Mg containing compound at room temperature has the I4/m space group. The temperature evolution of the structure has revealed a continuous phase transition at 570 K, changing the symmetry from tetragonal (I4/m) to cubic ().

The crystal structures of SrLaCuRuO6 and SrLaNiRuO6, were studied using synchrotron radiation powder diffraction method. The structures of both compounds have the P21/n space group at room temperature and transform to R3 at higher temperatures. The observed phase transiton temperatures are 730 and 670 K for SrLaCuRuO6 and SrLaNiRuO6, respectively.