Global Instability Index Research Articles

A new series of low-loss multicomponent oxide microwave dielectrics with a rock salt structure: Li5MgABO8 (A=Ti, Sn; B=Nb, Ta)

In this study, a series of Li5MgABO8 (A=Ti, Sn; B=Nb, Ta) compounds were designed and synthesized through a solid-state reaction method. The XRD patterns showed that a single phase was formed for all the samples. All the peaks were indexed based on the MgO-Like rock salt cubic structure with space group Fm-3m (225) and the peaks indicated that crystals presented different (111) orientation growths in the prepared samples. For all the specimens, the relative density values of 94% and above could be achieved when the sintering temperature exceeded 1300 °C. The εr values of the specimens showed an increasing trend when the sintering temperatures increased. However, the Q × f values firstly increased and then decreased with the rising temperatures. The electronically-induced strains and lattice-induced strains, which were evaluated by the bond strain index (BSI) and global instability index (GII), presented same variation trends as the Q × f values when the sintering temperature increased. The lowest dielectric loss was achieved in Li5MgTiNbO8 ceramics: εr=17.55, Q × f=109,700 GHz (f=7.379 GHz), τf =-32.5 ppm/°C, when sintered at 1290 °C for 4 h.

A2AgCrCl6 (A = Li, Na, K, Rb, Cs) halide double perovskites: a transition metal-based semiconducting material series with appreciable optical characteristics.

We have theoretically examined the geometries, electronic density of states and band structures of cubic and hexagonal A2AgCrCl6 (A = Cs, Rb, K, Na, Li) using meta-GGA SCAN-rVV10. The optimized lattice density was found to vary between 2.68 and 4.08 g cm-3 for cubic-A2AgCrCl6, with the fundamental electronic bandgap (direct) in the range of 0.66-0.69 eV. The cell density of hexagonal A2AgCrCl6 was between 2.97 and 3.93 g cm-3, but with an indirect bandgap of 0.93-1.02 eV. The valence band maximum and the conduction band minimum of A2AgCrCl6 were confirmed to be essentially of Cr(3d) character, but the contributions from the orbital states of Cl(3p) to the VBM were also appreciable. Cubic A2AgCrCl6 (A = Cs, Rb, K) was identified to possess genuine perovskite stoichiometry, evaluated using various geometry-based indices (viz. octahedral factor, tolerance factor, and global instability index). This was not so for A2AgCrCl6 (A = Na, Li), and was due to the small size of Na and Li cations that caused the critical strain of CrCl6 octahedra and a significant decrease in the cell volume. However, all the five A2AgCrCl6 displayed nearly similar optical properties, including the nature of the oscillator peaks in the dielectric function, absorption coefficient, photoconductivity, reflectivity, and Tauc spectra. The zero-limit of the refractive index was calculated around 2.25 and 2.00 for cubic and hexagonal A2AgCrCl6, respectively, and the extinction coefficient was very small for all cases. The nature of the optical bandgap and transition peaks discussed in this study of cubic and hexagonal Cs2AgCrCl6 agreed well with the experiment. The examination of phonon band dispersion led to the conclusion that cubic-A2AgCrCl6 (A = Cs, Rb) are the only halide double perovskites of the entire series that are dynamically stable.

Predicting the Structure Stability of Layered Heteroanionic Materials Exhibiting Anion Order

We report a workflow for heteroanionic materials discovery using Pauling's second rule to filter for and predict new candidate materials for synthesis with reduced computational overhead. Using oxyfluoride and oxynitride n = 1 Ruddlesden-Popper compounds as a use-case, we show that a minimization scheme based on the global instability index (GII) efficiently filters up to 50% of highly unstable candidate compositions based on crystal-chemistry grounds. We then validate the minimization scheme using density functional theory (DFT) calculations and find that unexpectedly the GII of stable heteroanionic materials is higher than that of homoanionic oxides owing to significant charge redistribution in compounds containing more than one anionic species. Using this workflow, we predict Sr2AlO3F to be stable and describe our attempts to synthesize a phase-pure material.

Ba0.95Ca0.05Ce0.9Y0.1O3 as an electrolyte for proton-conducting ceramic fuel cells

Comparative studies of the physicochemical and electrochemical properties of BaCe0.9Y0.1O3 and Ba0.95Ca0.05Ce0.9Y0.1O3 sintered ceramics are described. The orthorhombic (Pnma) space group was determined for both investigated samples within the temperature range 25–800 °C, using the X-ray diffraction method. A decrease in the cell volume of Ba0.95Ca0.05Ce0.9Y0.1O3 compared to that of BaCe0.9Y0.1O3 was observed within the investigated temperature range.It was found that the partial substitution of calcium for barium, resulting in Ba0.95Ca0.05Ce0.9Y0.1O3, caused an increase in specific free volume and global instability index compared to the original composition BaCe0.9Y0.1O3. Improvement of the chemical resistance of Ba0.95Ca0.05Ce0.9Y0.1O3 in a CO2 gas atmosphere was observed. It was found that Ba0.95Ca0.05Ce0.9Y0.1O3 exhibited a slightly lower level of electrical conductivity than BaCe0.9Y0.1O3 in air as well as in ArH2 gas atmospheres.The greater chemical resistance of Ba0.95Ca0.05Ce0.9Y0.1O3 in a CO2 gas atmosphere enhances the suitability of the modified material as an electrolyte for fuel cells with proton-conducting ceramics. The electrochemical response of the interface (La0.60Sr0.40)0.95Co0.20Fe0.80O3−δ|Ba0.95Ca0.05Ce0.9Y0.1O3 was also investigated within the applied potential range from −0.05 to −0.5 V in SOFC mode as well as from 0.05 to 0.5 V in SOEC mode); it was found that the current reached a fairly stable value over time.

Complementary evaluation of structure stability of perovskite oxides using bond-valence and density-functional-theory calculations

Estimation of structure stability is an essential issue in materials design and synthesis. Global instability index (GII) based on bond-valence method is applied as a simple indication, while density functional theory calculation is adopted for accurate evaluation of formation energy. We compare the GII and total energy of typical ABO3-type perovskite oxides and rationalize their relationship, proposing that the criteria for empirically unstable structures (GII > 0.2 valence unit) correspond to the difference in total energy of 50–200 meV per formula unit.

Room temperature multiferroicity in a new Ba2FeMnO6 double perovskite material

Abstract A new double perovskite Ba 2 FeMnO 6 with global instability index (GII) of about 0.2 v.u. was prepared using a simple chemical combustion route. The material was characterized by powder X-Ray diffraction (PXRD). Structural Rietveld refinement using FULLPROF showed Ba 2 FeMnO 6 to have a distorted F m 3 ?? m space group. The refined parameters confirmed the distorted pseudo-cubic structure, which agrees with the observed GII factor from the SPuDS program. Energy dispersive spectra (EDS) and X-Ray absorption study (XAS) were used to support the stoichoimetric formation and valance states of Mn and Fe ions. The magnetic study revealed a room temperature ferromagnetism due to the exchange and superexchange mechanism of Fe-O-Fe and Mn-O-Mn. The Curie temperature was over 800 K, and Ba2FeMnO6 exhibited hysteresis polarization. Because of co-existence of magnetic and electric hysteresis, this material would be a promising multiferroic candidate for spintronics.

Chemical and physical characterizations of the n = 1 Ruddlesden–Popper phases: Nd2 − y Sr y Ni1 − x Co x O4 ± δ (y = 1 and 0.1 ≤ x ≤ 0.9)

The structural stability and physical properties of NdSrNi1 − xCoxO4 ± δ (0.1 ≤ x ≤ 0.9) mixed oxides, elaborated by conventional sol–gel process, have been investigated and obtained results show that substitution of nickel by cobalt at x = 0.5 enhances conductivity at room temperature; σ = 17.24 Ω−1 cm−1 coinciding with minimum activation energy (Ea = 0.05 eV). Rietveld refinements of X-ray powder diffraction patterns at room temperature indicate that all compositions crystallize in a tetragonal system with I4/mmm space group and exhibit K2NiF4-type structure. Variations of a and c parameters display various behavior with increasing cobalt content. Changes in cell parameters are discussed in terms of crystal field theory. In addition, transition metal oxidation state is investigated on the basis of the Brown bond valence calculation. The deduced Global Instability Index (GII) value decreases when cobalt substance increases, indicating that the structure becomes more stable once cobalt is introduced. Oxygen stoichiometry of these compounds was determined from thermogravimetric analyses (TGA) followed by reduction in 5% H2 in N2 gas. Conductivity of NdSrNi1 − xCoxO4 ± δ (0.1 ≤ x ≤ 0.9) oxides was measured by an ac four-probe method. Oxygen vacancies are the possible ionic charge carriers. Specimens exhibit a semiconducting behavior in the whole range of temperature. The electrical transport mechanism agrees with an adiabatic small polaron hopping (ASPH) model. [Figure not available: see fulltext.]. © 2017, Springer-Verlag Berlin Heidelberg.

Synthesis and structural characterization of the hexagonal anti-perovskite Na2CaVO4F

The structural details of the ordered hexagonal oxyfluoride Na2CaVO4F prepared by solid-state synthesis using stoichiometric amounts of V2O5, CaCO3, Na2CO3 and NaF were characterized using high-resolution neutron powder diffraction. The structural changes between 25°C and 750°C revealed that the two structural subunits in this material behave different when heated: there is an expansion of the face-shared FNa4Ca2 octahedra while the VO4 tetrahedra due to increased thermal disorder reveal marginal bond contractions. Bond valences and the global instability index point to significant structural disorder at 750°C.

Theoretical and experimental study of octahedral tilting of Ca1−xGdxMnO3 (x = 0.05, 0.1, 0.15, 0.2) nanometric powders

In order to estimate theoretical stability of the perovskite structure for synthesized Ca1−xGdxMnO3 (x = 0.05, 0.1, 0.15, 0.2) nanopowders, the Goldschmidt tolerance factor Gt and global instability index GII were calculated. Furthermore, we have performed structure prediction of Ca1−xGdxMnO3 perovskites and found several possible perovskite-related phases. The influence of gadolinium amount on Mn─O bond angles and distances, tilting of MnO6 octahedra around all three axes and deformation due to the presence of the Jahn–Teller distortion around Mn3+ cation, as well as the influence of the amount of Mn3+ cation on Ca1−xGdxMnO3 compound, was examined. Ion Mn valence states were determined by bond valence calculations (BVC). Infrared active phonon modes in Ca1−xGdxMnO3 were studied by infrared reflection spectroscopy and magnetic properties were studied by using EPR (electron paramagnetic resonance) measurements.

Complementary method to locate atomic coordinates by combined searching method of structure-sensitive indexes based on bond valence method**Project supported by the National Natural Science Foundation of China (Grant No. 51272027).

Bond valence method illustrates the relation between valence and length of a particular bond type. This theory has been used to predict structure information, but the effect is very limited. In this paper, two indexes, i.e., global instability index (GII) and bond strain index (BSI), are adopted as a judgment of a search-match program for prediction. The results show that with GII and BSI combined as judgment, the predicted atom positions are very close to real ones. The mechanism and validity of this searching program are also discussed. The GII & BSI distribution contour map reveals that the predicted function is a reflection of exponential feature of bond valence formula. This combined searching method may be integrated with other structure-determination method, and may be helpful in refining and testifying light atom positions.

Use of bond-valence sums in modelling the diffuse scattering from PZN (PbZn1/3Nb2/3O3)

This work extends previous efforts to model diffuse scattering from PZN (PbZn1/3Nb2/3O3). Earlier work [Welberryet al.(2005).J. Appl. Cryst.38, 639–647; Welberryet al.(2006).Phys. Rev. B,74, 224108] is highly prescriptive, using Monte Carlo simulation with very artificial potentials to induce short-range-order structures which were deduced as necessary from inspection of the data. While this gives valid results for the nature of the local structure, it does not strongly relate these structures to underlying crystal chemistry. In that work, the idea of the bond-valence sum was used as a guide to the expected behaviour of the atoms. This paper extends the use of the bond-valence sum from a qualitative guide to becoming a key aspect of the potential experienced by the atoms, through the idea of the global instability index, whose square has been shown to be proportional to the density functional theory energy of some systems when close to the minimum energy configuration.

Heat capacity of Ln 2Cu2O5 (Ln = Tb-Lu) cuprates

The correlation between the heat capacity of heavy lanthanide cuprates and the specific features of their structure has been analyzed. It has been shown that the heat capacity regularly changes depending on the Ln3+ ion radius and the global instability index (GII) within the limits of corresponding tetrads (Tb-Ho, Er-Lu).

Crystal chemistry of fluorite-like clathrates AB 3F10

Crystal chemical analysis of the structures of clathrates AB3F10 derived from the structural type KY3F10 was carried out. As the structure-forming fragment, a truncated octahedron is considered obtained by connecting 24 F− anions of the outer faces of {YF8} antiprisms that form the cluster {Y6F32}. Four modes of connection of these truncated octahedra giving B3F10 frameworks were established, two modes involving common edges and two modes involving common hexagonal faces. Edge connection corresponds to known structural types in which the {B6F32} clusters are arranged as cubic close packing of spheres (structural type KY3F10) or as hexagonal close packing of spheres (structural type β-KYb3F10). Face connection gives rise to motifs similar to those formed by carbon atoms in diamond and lonsdaleite structures. The former motif corresponds to the recently established structure of (H3O)Yb3F10 · H2O. For the latter motif, a model of the hypothetical hexagonal phase ThEr2F10 was proposed (space group P63/mmc, a = 1.1058 nm, b = 1.8007 nm, Z = 8), which was characterzed, in terms of the bond valence method, the global instability index GII = 0.065.

A redetermination of the structure of poly[[μ4-(R)-2-ammonio-3-sulfonatopropanoato]aquasodium], originally reported as poly[[μ7-L-cysteato(2−)]disodium

The structure originally reported as poly[[μ7-l-cysteato(2−)]disodium], [Na2(C3H5NO5S)]n [Liu (2002). Acta Cryst. E67, m1346–m1347], has been redetermined with one of the sodium atoms replaced with a water mol­ecule and an additional proton attached to the amine group, resulting in the revised formula [Na{CO2CH(CH2SO3)NH3}(H2O)]n. The agreement index, wR, has been reduced from 0.159 to 0.087 and the global instability index from 0.56 vu (valence units) to the acceptable value of 0.11 vu.

Stability of divalent/trivalent oxidation state of europium in some Sr-based inorganic compounds

This work investigates the stability of Eu2+ and Eu3+ in some Sr-based inorganic compounds. Generally reducing condition is adopted in order to obtain Eu2+, however, the Eu doped SrAl2O4/SrLaAlO4 case indicates that for some compounds Eu3+ is stabilized even in reducing atmosphere. Bond valence method is applied to explain this phenomenon and it reveals that crystal structure also determines the valence state of europium cations along with reducing/oxidizing condition. An analysis of other Eu doped Sr-based materials is performed which shows the relationship between Eu2+/Eu3+ stability and the Global Instability Index (GII). This research provides a guideline for synthesizing specific novel Eu2+/Eu3+ phosphors.

“A” Cation Polarity Control in ACuTe2O7 (A = Sr2+, Ba2+, or Pb2+)

The synthesis and characterization of ACuTe(2)O(7) (A = Sr(2+), Ba(2+), or Pb(2+)) have been carried out. Interestingly, SrCuTe(2)O(7) and PbCuTe(2)O(7) are centrosymmetric and isostructural, whereas BaCuTe(2)O(7) is noncentrosymmetric and polar. All of the materials contain [CuTe(2)O(7)](2-) layers stacked along the b-axis direction that are separated by the "A" cations. The layers are composed of corner-shared CuO(5), TeO(6), and TeO(4) polyhedra. The influence of the "A" cation on the polarity is described by bond valence concepts, including the bond strain index and global instability index. Infrared, UV-vis, thermogravimetric, differential thermal analysis, and magnetic measurements were performed on all three materials. For BaCuTe(2)O(7), second-harmonic generation (SHG), piezoelectric, and polarization measurements were performed. A moderate SHG efficiency of approximately 70 × α-SiO(2) was measured. In addition, we determined that BaCuTe(2)O(7) is not ferroelectric; that is, the macroscopic polarization is not reversible. For BaCuTe(2)O(7), a pyroelectric coefficient of -9.5 μC/m(2)·K at 90 °C and a piezoelectric charge coefficient of 49 pm/V were determined. Crystal data are the following: SrCuTe(2)O(7), orthorhombic, space group Pbcm (No. 57), a = 7.1464(7) Å, b = 15.0609(15) Å, c = 5.4380(5) Å, V = 585.30(10) Å(3), and Z = 4; PbCuTe(2)O(7), orthorhombic, space group Pbcm (No. 57), a = 7.2033(5) Å, b = 15.0468(10) Å, c = 5.4691(4) Å, V = 592.78(7) Å(3), and Z = 4.

Material Design and High-Pressure Synthesis of Novel A-Site-Ordered Perovskites AMn3Al4O12 (A = Y, Yb, and Dy) with Square-Planar-Coordinated Mn3+

Abstract Structural stability of A-site-ordered perovskites AMn3Al4O12 was evaluated by the global instability index calculated from the SPuDS program. Based on the results, we “designed” new compounds AMn3Al4O12 (A = Y, Yb, and Dy), and these compounds were synthesized with a high-pressure technique. The obtained compounds were ionic crystals, A3+Mn3+3Al3+4O2−12, with fourfold square-planar coordination for Mn3+ at the originally twelvefold-coordinated A site of the ABO3 simple perovskite structure. Structural parameters obtained from the structure refinement well agreed with the “predicted” values. The synthesized compounds contained magnetic Mn3+ ions with S = 2 spins at the A′ site, and the A′–A′ interaction resulted in antiferromagnetic ordering of the Mn3+ spins at temperatures ranging from 29 to 40 K. The Yb3+/Dy3+ moments in (Yb/Dy)Mn3Al4O12 were found to be paramagnetic even below the antiferromagnetic transition temperatures.

Analysis of instability indices during the development of a Mediterranean tropical-like cyclone using MSG-SEVIRI products and the LAPS model

A tropical-like cyclone that affected southeastern Italy in September 2006 is analyzed by means of instability parameters. The Lifted Index and K-Index generated by two different tools have been tested: the “Global Instability Index” (GII) developed at the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), mainly based on Meteosat Second Generation (MSG) observed radiances, and the Local Analysis and Prediction System (LAPS), a numerical diagnostic model developed at the National Oceanic and Atmospheric Administration (NOAA). The indices are very similar in the regions where they are both available. However, as the GII algorithm works only in cloud-free conditions and provides data with a coarse resolution (about 60–75 km over the Mediterranean regions), the LAPS products are selected as more appropriate for the analysis of this small scale cyclone. The analysis of the K-Index and Lifted Index during the evolution of the cyclone suggests that the areas identified as favorable for the development of severe convection are different according to the two indices. In fact, very large values of the K-index are found in the vicinity of a frontal system crossing the central Mediterranean Sea, and in particular in its northern, most active side. At the same time, the Lifted Index identifies as area potentially favorable to the development of severe convection a region of thermodynamic instability between the warm and moist air at low levels and the cold and dry air aloft. These two areas overlap just before the explosive deepening of the cyclone. Thus, at least for this case study, the co-location of the extreme values of the two indices may be regarded as an indication for the occurrence of severe convection.

The combined instability index: a new very‐short range convection forecasting technique for southern Africa

AbstractThunderstorms, due to their high frequency of occurrence over southern Africa and their dominant contribution to summer rainfall, are the primary focus of very short range forecasting and nowcasting efforts in South Africa. Most southern African countries are heavily reliant on satellite technology due to the limited number of surface and upper‐air observations and the limited availability of numerical model output. In developing tools for the first 12 forecast hours, the South African Weather Service has to address both national and regional needs. The blending of techniques in an optimal manner is essential in achieving this. In this paper a description is given of how the Global Instability Index product derived from the European Meteosat Second Generation Satellite was adapted to fit to South African circumstances using a different numerical model as background information, creating the Regional Instability Indices (RII). The focus of the study is the development of a new convection indicator, called the Combined Instability Index (CII), which calculates the probability of convection based on satellite derived instability indices and moisture as well as orographic lift early in the morning when the sky is as cloud free as possible. The CII is an objective way for operational forecasters to calculate the probability for convection later in the day based on satellite and model data. Early morning CII values were evaluated statistically against the occurrence of lightning as well as against satellite derived precipitation later in the same day. It is shown that the CII not only performs well, but also outperforms the individual RII. The CII will be beneficial to operational forecasters to focus their attention on the area which is most favourable for the development of convection later in the day. Copyright © 2010 Royal Meteorological Society

Crystal structures of carbonates Cs2Sr2(CO3)3 and Rb2Sr2(CO3)3 from powder data

Two carbonates Cs2Sr2(CO3)3 and Rb2Sr2(CO3)3 were synthesized by solid-state reaction at high temperatures, and their crystal structures were determined from X-ray powder diffraction data. Because of the presence of heavy atoms and heavy peak overlapping, proper restrains on light-atom groups were found essential to obtain satisfactory results. The title compounds are isostructural to an early reported compound Cs2Ba2(CO3)3 and crystallize in cubic space group I213, with unit-cell dimensions a=10.072 64(2) Å for the cesium phase and a=9.855 56(7) Å for the rubidium phase. Unlike most carbonates, the title compounds feature in mutually perpendicular CO3 atomic groups. Our results also indicate that the global instability index is a more sensitive parameter in evaluating the structure rationality over the agreement factors. Moreover, differential thermal analysis and thermogravimetric analysis measurements reveal that Cs2Sr2(CO3)3 and Rb2Sr2(CO3)3 are stable in air up to 796 and 880 °C, respectively.