Alexei A. Belik

Tailored Design of Multiple Nanoarchitectures in Metal-Cyanide Hybrid Coordination Polymers

Recently, coordination polymers (CPs) with nanoscale porosity and unique property have demonstrated great potential in many applications. Encouraged by significant progress in the controlled synthesis of nanomaterials, such as metals and semiconductors, the morphologically controlled synthesis of CPs has been considered a potential way to further enhance the inherent properties and develop new functions. In particular, hollow-based CPs are promising nanoarchitectures that can bring several properties derived from crystalline thin shells and interior cavities. Here we demonstrate an exquisite construction method to synthesize CPs with multiple hollow-based nanoarchitectures. Through step-by-step CP crystal growth and subsequent etching processes, various types of CPs with shell-in-shell, yolk-shell, and yolk-double-shell hollow structures can be synthesized for the first time. This type of nanoarchitecture is powerful for the exploration of alternative properties of CPs. The resultant hollow-based nanoarchitectures significantly increase gas adsorption and bring out interesting magnetic properties.

Pressure-Induced Spin-State Transition in BiCoO3

The structural and electronic properties of BiCoO(3) under high pressure have been investigated. Synchrotron X-ray and neutron powder diffraction studies show that the structure changes from a polar PbTiO(3) type to a centrosymmetric GdFeO(3) type above 3 GPa with a large volume decrease of 13% at room temperature revealing a spin-state change. The first-order transition is accompanied by a drop of electrical resistivity. Structural results show that Co(3+) is present in the low spin state at high pressures, but X-ray emission spectra suggest that the intermediate spin state is present. The pressure-temperature phase diagram of BiCoO(3) has been constructed enabling the transition temperature at ambient pressure to be estimated as 800-900 K.

Colossal positive and negative thermal expansion and thermosalient effect in a pentamorphic organometallic martensite

The thermosalient effect is an extremely rare propensity of certain crystalline solids for self-actuation by elastic deformation or by a ballistic event. Here we present direct evidence for the driving force behind this impressive crystal motility. Crystals of a prototypical thermosalient material, (phenylazophenyl)palladium hexafluoroacetylacetonate, can switch between five crystal structures (α-ε) that are related by four phase transitions including one thermosalient transition (α↔γ). The mechanical effect is driven by a uniaxial negative expansion that is compensated by unusually large positive axial expansion (260 × 10(-6)  K(-1)) with volumetric expansion coefficients (≈250 × 10(-6)  K(-1)) that are among the highest values reported in molecular solids thus far. The habit plane advances at ~10(4) times the rate observed with non-thermosalient transitions. This rapid expansion of the crystal following the phase switching is the driving force for occurrence of the thermosalient effect.

Dysnomia, a computer program for maximum-entropy method (MEM) analysis and its performance in the MEM-based pattern fitting

A computer program, Dysnomia, for the maximum-entropy method (MEM) has been tested for the evaluation and advancement of MEM-based pattern fitting (MPF). Dysnomia is a successor to PRIMA, which was the only program integrated with RIETAN-FP for MPF. Two types of MEM algorithms, i.e., 0th-order single-pixel approximation and a variant of the Cambridge algorithm, were implemented in Dysnomia in combination with a linear combination of the “generalized F constraints” and arbitrary weighting factors for them. Dysnomia excels PRIMA in computation speed, memory efficiency, and scalability owing to parallel processing and automatic switching of discrete Fourier transform and fast Fourier transform depending on sizes of grids and observed reflections. These features of Dysnomia were evaluated for MPF analyses from X-ray powder diffraction data of three different types of compounds: taurine, Cu 2 CO 3 (OH) 2 (malachite), and Sr 9 In(PO 4 ) 7 . Reliability indices in MPF analyses proved to have been improved by using multiple F constraints and weighting factors based on lattice-plane spacings, d , in comparison with those obtained with PRIMA.

Indium‐Based Perovskites: A New Class of Near‐Room‐Temperature Multiferroics

These systems may have wide technological applicationsbecause they allow control of electric properties by magneticfield and control of magnetic properties by electric field.Applications might include multiple-state memory elementsand will allow exploitation of the advantages of both ferro-electric and magnetic random-access memories.

Crystal and Magnetic Structures and Properties of BiMnO3+δ

Crystal and magnetic structures of BiMnO(3+delta) (delta = 0.03, 0.08, and 0.14) have been determined by the Rietveld method from neutron diffraction data at 8-10 and 290 K. BiMnO(3.03) (= Bi(0.99)Mn(0.99)O(3)) crystallizes in a monoclinic system (the refinement was performed in space group C2/c; Z = 8; a = 9.5313(3) A, b = 5.57791(17) A, c = 9.7375(4) A, beta = 108.951(2) degrees at 290 K). BiMnO(3.08) (= Bi(0.974)Mn(0.974)O(3)) crystallizes in space group P2(1)/c (Z = 8; a = 9.5565(4) A, b = 5.51823(16) A, c = 9.7051(4) A, beta = 109.442(3) degrees at 290 K). It was found that Mn vacancies are localized mainly in one Mn site (among three sites) in Bi(0.974)Mn(0.974)O(3). Vacancy-ordering and charge-ordering scenarios are suggested as possible reasons for the crystal symmetry change compared with Bi(0.99)Mn(0.99)O(3). BiMnO(3.03) and BiMnO(3.08) are ferromagnetic below T(C) = 82 and 68 K, respectively, with magnetic moments along the monoclinic b axes. Refined magnetic moments at 10 K are 2.88(2)micro(B) in BiMnO(3.03) and 2.33(2)micro(B) in BiMnO(3.08). BiMnO(3.14) (= Bi(0.955)Mn(0.955)O(3)) crystallizes in an orthorhombic system (space group Pnma; Z = 4; a = 5.5136(4) A, b = 7.8069(8) A, and c = 5.5454(5) A at 290 K), and its structure is similar to that of LaMnO(3.11)-LaMnO(3.15). No magnetic reflections were found in BiMnO(3.14) down to 8 K, in agreement with its spin-glass magnetic state. Magnetic and chemical properties of BiMnO(3+delta) (0.02 < or = delta < or = 0.14) have also been investigated and compared with those of LaMnO(3+delta). Systematic changes of magnetic parameters in BiMnO(3+delta) were found to depend on delta.

Crystal structures of double calcium and alkali metal phosphates Ca10M(PO4)(7) (M = Li, Na, K)

Crystal structures of double calcium and alkali metal phosphates described by the general formula Ca10M(PO4)7(M = Li, Na, K) have been studied by the Rietveld method. The lattice parameters are a = 10.4203(1) and c = 37.389(1) Å (for M = Li), a = 10.4391(1) and c = 37.310(1) Å (for M = Na), and a = 10.4229(1) and c = 37.279(2) Å (for M = K); sp. gr. R3c, Z = 6. The specific features of the distribution of alkali metal cations over the structure positions are discussed.

Continuous metal-insulator transition of the antiferromagnetic perovskite NaOsO3

Newly synthesized perovskite NaOsO3 shows Curie-Weiss metallic nature at high temperature and suddenly goes into an antiferromagnetically insulating state at 410 K on cooling. Electronic specific heat at the low temperature limit is absent, indicating that the band gap fully opens. In situ observation in electron microscopy undetected any lattice anomalies in the vicinity of the transition temperature. It is most likely that the antiferromagnetic correlation plays an essential role of the gap opening.

Antiferrodistortive phase transition in EuTiO3

X-ray diffraction, dynamical mechanical analysis and infrared reflectivity studies revealed an antiferrodistortive phase transition in EuTiO3 ceramics. Near 300K the perovskite structure changes from cubic Pm-3m to tetragonal I4/mcm due to antiphase tilting of oxygen octahedra along the c axis (a0a0c- in Glazer notation). The phase transition is analogous to SrTiO3. However, some ceramics as well as single crystals of EuTiO3 show different infrared reflectivity spectra bringing evidence of a different crystal structure. In such samples electron diffraction revealed an incommensurate tetragonal structure with modulation wavevector q ~ 0.38 a*. Extra phonons in samples with modulated structure are activated in the IR spectra due to folding of the Brillouin zone. We propose that defects like Eu3+ and oxygen vacancies strongly influence the temperature of the phase transition to antiferrodistortive phase as well as the tendency to incommensurate modulation in EuTiO3.

Observation of persistent centrosymmetricity in the hexagonal manganite family

The controversy regarding the ferroelectric behavior of hexagonal InMnO3 is resolved by using a combination of x-ray diffraction (XRD), piezoresponse force microscopy (PFM), second harmonic generation (SHG), and density functional theory (DFT). While XRD data show a symmetry-lowering unit-cell tripling, which is also found in the multiferroic hexagonal manganites of P6(3)cm symmetry, PFM and SHG do not detect ferroelectricity at ambient or low temperature, in striking contrast to the behavior in the multiferroic counterparts. We propose instead a centrosymmetric P (3) over barc phase as the ground-state structure. Our DFT calculations reveal that the relative energy of the ferroelectric and nonferroelectric structures is determined by a competition between electrostatics and oxygen-rare-earth covalency, with an absence of covalency favoring the ferroelectric phase.