Igor A. Presniakov

Effects of the order–disorder phase transition on the physical properties of A8Sn44□2 (A = Rb, Cs)

Multi-temperature synchrotron powder diffraction and differential thermal analysis have revealed an enantiotropic order–disorder phase transition in Rb8Sn44□2 and Cs8Sn44□2 at 353 and 363 K, respectively. The low-temperature phases crystallize with a 2 × 2 × 2 superstructure of the conventional type-I clathrate. At higher temperatures both compounds show a phase transition with less ordering of the framework vacancies (□). Rb8Sn44 has a Debye temperature of 152(1) K whereas the Rb atoms in the large cages have Einstein temperatures of 81(1) and 54(1) K for the displacement perpendicular and parallel to the host-structure hexagons. Thermoelectric properties of Rb8Sn44 have been measured from 2 to 400 K. The Seebeck coefficient decreases rapidly above the transition temperature. This is explained by changes in the band structure following the phase transition. The 119Sn Mossbauer spectra for both compounds have also been investigated and their analysis suggests that the transformation occurs without altering the local environment of the tin atoms, but with a variation of the vacancy concentration in the domains of the crystal.

High-pressure synthesis, crystal structures, and properties of CdMn7O12 and SrMn7O12 perovskites.

We synthesize CdMn7O12 and SrMn7-xFexO12 (x = 0, 0.08, and 0.5) perovskites under high pressure (6 GPa) and high temperature (1373-1573 K) conditions and investigate their structural, magnetic, dielectric, and ferroelectric properties. CdMn7O12 and SrMn7O12 are isostructural with CaMn7O12: space group R3̅ (No. 148), Z = 3, and lattice parameters a = 10.45508(2) Å and c = 6.33131(1) Å for CdMn7O12 and a = 10.49807(1) Å and c = 6.37985(1) Å for SrMn7O12 at 295 K. There is a structural phase transition at 493 K in CdMn7O12 and at 404 K in SrMn7O12 to a cubic structure (space group Im3̅), associated with charge ordering as found by the structural analysis and Mössbauer spectroscopy. SrMn6.5Fe0.5O12 crystallizes in space group Im3̅ at 295 K with a = 7.40766(2) Å and exhibits spin-glass magnetic properties below 34 K. There are two magnetic transitions in CdMn7O12 with the Néel temperatures TN2 = 33 K and TN1 = 88 K, and in SrMn7O12 with TN2 = 63 K and TN1 = 87 K. A field-induced transition is found in CdMn7O12 from about 65 kOe, and TN2 = 58 K at 90 kOe. No dielectric anomalies are found at TN1 and TN2 at 0 Oe in both compound, but CdMn7O12 exhibits small anomalies at TN1 and TN2 at 90 kOe. In pyroelectric current measurements, we observe large and broad peaks around magnetic phase transition temperatures in CdMn7O12, SrMn7O12, and SrMn6.5Fe0.5O12; we assign those peaks to extrinsic effects and compare our results with previously reported results on CaMn7O12. We also discuss general tendencies of the AMn7O12 perovskite family (A = Cd, Ca, Sr, and Pb).

Ferromagnetic Order, Strong Magnetocrystalline Anisotropy, and Magnetocaloric Effect in the Layered Telluride Fe3−δGeTe2

The ternary transition-metal compound Fe(3-δ)GeTe2 is formed for 0 < δ < 0.3. X-ray diffraction and Mössbauer spectroscopy reveal its layered crystal structure with occasional Fe vacancies in the Fe2 site, whereas no Fe atoms occupy the interlayer space, so that only van der Waals interactions exist between adjacent layers. We explore magnetic behavior and ensuing functional properties of Fe(2.9)GeTe2 via neutron diffraction, thermodynamic and transport measurements, Mössbauer spectroscopy, and electronic structure calculations. Below T(C) = 225 K, Fe(2.9)GeTe2 is ferromagnetically ordered with the magnetic moments of 1.95(5) and 1.56(4) μ(B) at T = 1.5 K, both directed along c, which is the magnetic easy axis. Electronic structure calculations confirm this magnetic structure and reveal a remarkably high easy-axis anisotropy of 4.2 meV/f.u. Mössbauer spectra reveal the magnetic ordering too, although a drastic influence of Fe vacancies on quadrupolar splittings and local magnetic fields has been observed. A moderate magnetocaloric effect with the magnetic entropy change upon the ferromagnetic ordering transition, -ΔS ∼ 1.1 J·kg(-1)·K(-1) at 5 T, is found.

Sc2NiMnO6: A Double-Perovskite with a Magnetodielectric Response Driven by Multiple Magnetic Orders.

Perovskite materials provide a large variety of interesting physical properties and applications. Here, we report on unique properties of a fully ordered magnetodielectric double-perovskite, Sc2NiMnO6 (space group P21/n, a = 4.99860 Å, b = 5.35281 Å, c = 7.34496 Å, and β = 90.7915°), exhibiting sequential magnetic transitions at T1 = 35 K and T2 = 17 K. The transition at T1 corresponds to a single-k antiferromagnetic phase with propagation vector k1 = (1/2, 0, 1/2), while the second transition at T2 corresponds to a 2-k magnetic structure with propagation vectors k1 = (1/2, 0, 1/2) and k2 = (0, 1/2, 1/2). Symmetry analysis suggests that the two ordering wave vectors are independent, and calculations imply that k1 is associated with the Mn sublattice and k2 with the Ni sublattice, suggesting that Mn-Ni coupling is very small or absent. A magnetodielectric anomaly at T2 likely arises from an antiferroelectric ordering that results from the exchange-striction between the two magnetic sublattices belonging to k1 and k2. The behavior of Sc2NiMnO6 demonstrates 3d double-perovskites with small A-site cations as a promising avenue in which to search for magnetoelectric materials.

Crystal structure, physical properties, and electronic and magnetic structure of the spin S = 5/2 zigzag chain compound Bi2Fe(SeO3)2OCl3.

We report the synthesis and characterization of the new bismuth iron selenite oxochloride Bi2Fe(SeO3)2OCl3. The main feature of its crystal structure is the presence of a reasonably isolated set of spin S = 5/2 zigzag chains of corner-sharing FeO6 octahedra decorated with BiO4Cl3, BiO3Cl3, and SeO3 groups. When the temperature is lowered, the magnetization passes through a broad maximum at Tmax ≈ 130 K, which indicates the formation of a magnetic short-range correlation regime. The same behavior is demonstrated by the integral electron spin resonance intensity. The absorption is characterized by the isotropic effective factor g ≈ 2 typical for high-spin Fe(3+) ions. The broadening of ESR absorption lines at low temperatures with the critical exponent β = 7/4 is consistent with the divergence of the temperature-dependent correlation length expected for the quasi-one-dimensional antiferromagnetic spin chain upon approaching the long-range ordering transition from above. At TN = 13 K, Bi2Fe(SeO3)2OCl3 exhibits a transition into an antiferromagnetically ordered state, evidenced in the magnetization, specific heat, and Mössbauer spectra. At T < TN, the (57)Fe Mössbauer spectra reveal a low saturated value of the hyperfine field Hhf ≈ 44 T, which indicates a quantum spin reduction of spin-only magnetic moment ΔS/S ≈ 20%. The determination of exchange interaction parameters using first-principles calculations validates the quasi-one-dimensional nature of magnetism in this compound.

Mössbauer investigations of hyperfine interactions features of 57Fe nuclei in BiFeO3 ferrite

New results of 57Fe Mossbauer studies on BiFeO3 powder sample performed at various temperatures above and below magnetic phase transitions point TN ≈ 640K are reported. We have performed self-consistent calculations of the lattice contributions to the EFG tensor, taking into account dipole moments of the O2− and Bi3+ ions. Low-temperature 57Fe Mossbauer spectra recorded at T < TN were analyzed assuming an anharmonic cycloidal modulation of the Fe3+ magnetic moments. The cycloidal modulation of the iron spin was described with the elliptic Jacobi function sn[(±4K(m)/λ)x,m]. The good fit of the experimental spectra was obtained for the anharmonicity m = 0.44 ± 0.04 (T = 4.9K) resulting from the easy-axis magnetic anisotropy.

Perovskite-Structure TlMnO3: A New Manganite with New Properties

We synthesize a new member of the AMnO3 perovskite manganite family (where A is a trivalent cation)--thallium manganite, TlMnO3--under high-pressure (6 GPa) and high-temperature (1500 K) conditions and show that the structural and magnetic properties are distinct from those of all other AMnO3 manganites. The crystal structure of TlMnO3 is solved and refined using single-crystal X-ray diffraction data. We obtain a triclinically distorted structure with space group P1̅ (No. 2), Z = 4, and lattice parameters a = 5.4248(2) Å, b = 7.9403(2) Å, c = 5.28650(10) Å, α = 87.8200(10)°, β = 86.9440(10)°, and γ = 89.3130(10)° at 293 K. There are four crystallographic Mn sites in TlMnO3 forming two groups based on the degree of their Jahn-Teller distortions. Physical properties of insulating TlMnO3 are investigated with Mössbauer spectroscopy and resistivity, specific heat, and magnetization measurements. The orbital ordering, which persists to the decomposition temperature of 820 K, suggests A-type antiferromagnetic ordering with the ferromagnetic planes along the [-101] direction, consistent with the measured collinear antiferromagnetism below the Néel temperature of 92 K. Hybrid density functional calculations are consistent with the experimentally identified structure, insulating ground state, and suggested magnetism, and show that the low symmetry originates from the strongly Jahn-Teller distorted Mn(3+) ions combined with the strong covalency of the Tl(3+)-O bonds.

Local structure and hyperfine interactions of 57Fe in NaFeAs studied by Mössbauer spectroscopy.

Detailed 57Fe Mössbauer spectroscopy measurements on superconducting NaFeAs powder samples have been performed in the temperature range 13 K ≤ T < 300 K. The 57Fe spectra recorded in the paramagnetic range (T > TN ≈ 46 K) are discussed supposing that most of the Fe2+ ions are located in distorted (FeAs4) tetrahedra of NaFeAs phase, while an additional minor (<10%) component of the spectra corresponds to impurity or intergrowth NaFe2As2 phase with a nominal composition near NaFe2As2. Our results reveal that the structural transition (TS ≈ 55 K) has a weak effect on the electronic structure of iron ions, while at T ≤ TN the spectra show a continuous distribution of hyperfine fields HFe. The shape of these spectra is analyzed in terms of two models: (i) an incommensurate spin density wave modulation of iron magnetic structure, (ii) formation of a microdomain structure or phase separation. It is shown that the hyperfine parameters obtained using these two methods have very similar values over the whole temperature range. Analysis of the temperature dependence HFe(T) with the Bean–Rodbell model leads to ζ = 1.16 ± 0.05, suggesting that the magnetic phase transition is first order in nature. A sharp evolution of the VZZ(T) and η(T) parameters of the full Hamiltonian of hyperfine interactions near T ≈ (TN,TS) is interpreted as a manifestation of the anisotropic electron redistribution between the dxz-, dyz- and dxy-orbitals of the iron ions.

Evidence through Mössbauer spectroscopy of two different states for 57Fe probe atoms in RNiO3 perovskites with intermediate-size rare earths, R = Sm, Eu, Gd, Dy

In the present work, 57Fe probe Mossbauer spectroscopy was developed to study the nickelates RNi0.98Fe0.02O3 (R = Sm,Eu,Gd,Dy) with the perovskite-like structure. The restoration method for a distribution function P(v) of the positions (v) involving individual Lorentzian lines has been used for processing and analysing the Mossbauer spectra. The P(v) profile for the nickelates, at T<TIM (TIM corresponding to the transition temperature from insulator to metal), can be described as a superposition of two symmetric peaks with different intensities. The distance between these peaks monotonically increases with the decreasing ionic radius value of the R3+ cations . The observed asymmetry of the P(v) profile indicates that 57Fe atoms used as a Mossbauer probe are simultaneously stabilized in two non-equivalent crystallographic positions. This result is an indirect evidence for the existence of two types of nickel position in the insulating state of nickelate RNiO3 lattices with intermediate R3+ size, which remained questionable from diffraction methods.

57Fe Mössbauer study of unusual magnetic structure of multiferroic 3R-AgFeO2

We report new results of a 57Fe Mössbauer study of hyperfine magnetic interactions in the layered multiferroic 3R-AgFeO2 demonstrating two magnetic phase transitions at T N1 and T N2. The asymptotic value β *  ≈  0.34 for the critical exponent obtained from the temperature dependence of the hyperfine field H hf(T) at 57Fe the nuclei below T N1  ≈  14 K indicates that 3R-AgFeO2 shows quasi-3D critical behavior. The spectra just above T N1 (T N1  <  T  <  T  *  ≈  41 K) demonstrate a relaxation behavior due to critical spin fluctuations which indicates the occurrence of short-range correlations. At the intermediate temperature range, T N2  <  T  <  T N1, the 57Fe Mössbauer spectra are described in terms of collinear spin-density-waves (SDW) with the inclusion of many high-order harmonics, indicating that the real magnetic structure of the ferrite appears to be more complicated than a pure sinusoidally modulated SDW. Below T  <  T N2  ≈  9 K, the hyperfine field H hf reveals a large spatial anisotropy (ΔH anis  ≈  30 kOe) which is related with a local intra-cluster (FeO6) spin-dipole term that implies a conventional contribution of the polarized oxygen ions. We proposed a simple two-parametric formula to describe the dependence of H anis on the distortions of the (FeO6) clusters. Analysis of different mechanisms of spin and hyperfine interactions in 3R-AgFeO2 and its structural analogue CuFeO2 points to a specific role played by the topology of the exchange coupling and the oxygen polarization in the delafossite-like structures.