Phase transitions in vitreous and amorphous SiO1

The relationship between structural and electronic phase transitions in V2O3 thin films is of critical importance for understanding of the mechanism behind metal–insulator transition (MIT) and related technological applications. Despite being extensively studied, there are currently no clear consensus and picture of the relation between structural and electronic phase transitions so far. Using V2O3 thin films grown on r-plane Al2O3 substrates, which exhibit abrupt MIT and structural phase transition, we show that the electronic phase transition occurs concurrently with the structural phase transition as revealed by the electrical transport and Raman spectra measurements. Our result provides experimental evidence for clarifying this issue, which could form the basis of theoretical studies as well as technological applications in V2O3.

This paper focused on the effects of various phases of SiO2 additives on the γ‐Al2O3‐to‐α‐Al2O3 phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta‐to‐α phase transition was elevated by adding amorphous SiO2, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO2, such as quartz and cristobalite. Amorphous SiO2 was considered to retard the γ‐to‐α phase transition by preventing γ‐Al2O3 particles from coming into contact and suppressing heterogeneous nucleation on the γ‐Al2O3 surface. On the other hand, crystalline SiO2 accelerated the α‐Al2O3 transition; thus, this SiO2 may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO2 additives, especially amorphous and crystalline phases, largely influenced the temperature of γ‐Al2O3‐to‐α‐Al2O3 phase transition.

We report on the microwave, terahertz (THz), infrared, and Raman spectroscopic studies of $\mathrm{Bi}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ ceramics, shedding more light onto the nature of two structural phase transitions and their possible relation with ferroelectricity in this compound. We observed a softening of one polar phonon in the THz range on cooling towards 460 and 300 K, i.e., temperatures at which $\mathrm{Bi}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ undergoes subsequent structural phase transitions from monoclinic $I2/m$ to polar monoclinic $\mathit{Im}$ and triclinic $P1$ phases. The soft phonon causes dielectric anomalies typical for displacive ferroelectric phase transitions. Microwave measurements performed at 5.8 GHz up to 400 K qualitatively confirmed not only the dielectric anomaly at 300 K, but also revealed two other weak dielectric anomalies near the magnetic phase transitions at 60 and 28 K. This evidences the multiferroic nature of the low-temperature phases, although the relatively high conductivity in the kHz and Hz spectral range prevented us from directly measuring the permittivity and ferroelectric polarization. Some Raman modes sense the magnetic phase transitions occurring near 60 and 25 K, showing that spin-phonon coupling is relevant in this compound and in this temperature range. The deviation of the Mn-O stretching mode frequency from the anharmonic temperature behavior was successfully explained by the spin correlation function calculated from the magnetic contribution to the specific heat.

Bismuth titanate Bi4Ti3O12 (BTO) is a member of the layered Aurivillius phase perovskite ferroelectrics and has a structural phase transition at Tc of 675 C. Below Tc, BTO shows ferroelectricity with a large spontaneous polarization (30 C/cm) parallel to the layered structure. BTO attracted much attention as candidate material for ferroelectric memories because of the fatigue-free nature of the BTO-based mixed crystal Bi4 xLaxTi3O12 (BLT-x). Although the space group of BTO at room temperature is determined to be B2cb ð1⁄4 Aba2 1⁄4 C2cbÞ (Z 1⁄4 4) only from the X-ray and neutron diffraction patterns, its space group has finally been concluded to be Pc (Z 1⁄4 2) with monoclinic symmetry, taking into account the existence of the small spontaneous polarization (4 C/cm) perpendicular to the layered structure. Here, Z is the number of the chemical formula in the conventional unit cell. Various physical properties of BTO, such as a soft mode behavior and domain wall structures, have been intensively investigated. The Landau theory of phase transition in BTO was presented by assuming the point group 4=mmm in the hightemperature phase, where the phase transition of BTO from the tetragonal (4=mmm) phase to the monoclinic (m) phase was named the triggered phase transition. The splitting of the triggered phase transition, that is the existence of the intermediate orthorhombic phase (B2cb), was observed in the mixed crystal systems in Bi4 xRxTi3O12 (R = La, Nd, Sm, and Gd). It was reported that the prototype structure of BTO shows the space group I4=mmm (Z 1⁄4 4). It is conjectured, however, that neither the orthorhombic (B2cb) nor the monoclinic (Pc) phase can be directly induced from the tetragonal (I4=mmm) phase, on the basis of the group theoretical consideration. Even the primitive unit cell size in I4=mmm and B2cb phases is different. Note that the proper ferroelectric phase transition from I4=mmm to B2cb is impossible, because the latter is not any subgroup of the former at the -point. On the other hand, it was reported, on the basis of experimental results, that the Curie–Weiss behavior of the dielectric constant in the high-temperature phase is observed in BTO, indicating the proper ferroelectric phase transition. This seems to be in contradiction. In order to solve the above contradiction, Rae et al. and Perez-Mato et al. assumed the Fmmm symmetry to be the parent phase, where Fmmm is a subgroup of I4=mmm. If the high-temperature parent phase above 675 C is indeed orthorhombic as they claim, however, the Landau theory so far proposed of the triggered phase transition in BTO, which is based on the assumption that it is tetragonal, may fail. Thus, the symmetry of the high-temperature parent phase seems to be of crucial importance. Under these circumstances, to clarify the structural phase transition at 675 C and the symmetry of the parent phase of BTO, we investigate the domain wall structure in the high-temperature phase and discuss its symmetry on the basis of the group theory. Single crystals of BTO were grown by the flux method from a Bi2O3–TiO2 system. For domain wall observation, some c-plate crystals with an area of about 5mm and thicknesses of about 10–50 m were selected among asgrown crystals as samples. The temperature dependence of the domain wall structure in the c-plate of the BTO crystal was observed using a polarizing microscope with a hightemperature sample stage (Linkam TS-1500), where the possible temperature range of this stage is from room temperature to a maximum of 1500 C. Figures 1(a) and 1(b) show photographs of the c-plate crystal in BTO under crossed-Nicols below and above Tc, respectively, where the sensitive plate was used in Fig. 1(b). In Fig. 1(a), the 90 domain wall structure is observed, while no domain wall structure appears in Fig. 1(b), indicating that the c-plate sample of BTO is optically isotropic. Thus, it was confirmed that the symmetry of the high-temperature phase is tetragonal. In our experiment, the phase front was also observed near Tc, indicating the first-order phase transition. 200 μm A P

GeTe Thermoelectrics

The results of studying the dielectric constant σ’ and the third harmonic coefficient γ3ω of a new organic ferroelectric diisopropylammonium bromide (C6H16NBr, DIPAB), embedded in porous glasses with an average pore size of 100 nm are presented. Diffusion and shifting of the phase transition to low temperatures by 5 K were detected in comparison with bulk DIPAB. A decrease in the phase transition temperature in nanocomposites with DIPAB nanoparticles is qualitatively consistent with theoretical models of the size effects on the structural phase transition.

Ground state, structural, and magnetic phase transitions of strained BiVO3 were investigated using the first-principles calculation based on the self-interaction corrected local density approximation + U functional. The calculated total energy indicated that the ground state structure for BiVO3 was a monoclinic P21/a phase and ferromagnetic ordering is predicted to be the magnetic ground state. Furthermore, it was found that BiVO3 are metastable compounds under strain effects, where several structural phase transitions were predicted from the monoclinic P 21 / a phase to the orthorhombic P b n m phase (at compressive volumetric strain) and to monoclinic C m and C c phases (at expansive volumetric strain). Furthermore, calculations on electronic band structure and density of states for the monoclinic P21/a phase and the monoclinic C c phase of BiVO3 with ferromagnetic ordering were conducted. The calculated band structure predicted a small energy gap with a direct energy bandgap of 0.456 eV and an indirect energy gap of 0.466 eV. Interestingly, the obtained results revealed that the BiVO3 calculated density of states exhibited Mott-Hubbard insulator type. Therefore, the ground state monoclinic C c phase of strained BiVO3 with ferromagnetic ordering is suggested as a new candidate for multiferroic materials at room temperature and a visible light driven photocatalytic/photovoltaic effect. The obtained results in this study could provide valuable theoretical information on BiVO3 under strain effects, which could open new opportunities for future research and technological applications.

While numerous experimental results support the conventional BCS electron-phonon mechanism, quite a number of specific data like the specific heat jump and the critical magnetic fields lie up to a factor of two above the predictions of the strong coupling BCS/Eliashberg theory using all of the experimental phonons. In particular the observed dependence of the electron-phonon matrix element on the vibrational amplitude for strong coupling superconductors violates the BCS theory. It is proposed to account for high temperature superconductivity in terms of a largely conventional BCS /Eliashberg-theory, extended to include additionally the effects of a nearby structural phase transition (SPT). Experimental data show that the SPT at the temperature Tp causes a resonance like enhancement of the phonon population numbers with energies near kBTp. In many respects, these additional vibrations of the SPT act like an additional broad phonon with an integrated intensity comparable to that of the narrow normal mode phonons. In addition in the HTS systems, there are several broad “phonons” associated with spatial gaps in the structure. These are the explanation for the “electronic” states in the gap. Hence in the calculation of the superconducting and thermodynamic quantities using the Eliashberg theory all of these have to be taken into account. The total phonon spectrum F(e) includes the narrow regular normal mode phonons and these broad gap modes as evidenced by diffuse X-ray scattering. These additional phonons provide the factor of more than two in the measure of strength of the electron-phonon interaction A =∫α2F(e)de required to account for the Tc of the HTS and give the low value of λ ab = 1.4. The physical reasons for the choice of this ansatz are discussed. It corresponds to the Frohlich /Weisskopf picture, extended by including the effect of a nearby SPT. The specific heat of the superconducting phase transition sits on the tail of that of the SPT. When this is allowed for, one obtains the expected exponential shape for s-wave superconductivity with a fitted scalar gap twice the value as calculated from Tc. The specific heat jump △Cp is due not only to the usual electronic part, but also to an ionic part of comparable magnitude. Hence the critical fields and the gap are also twice the BCS value as confirmed by data. Results are presented for the calculation of Tc and the isotope effect as function of the Sr-fraction for La-214 and as function of the oxygen content for Y-123 and are in good agreement with the data. The enhancement through the SPT leads to the coupling of the superconducting and the structural order parameters. The qualitative agreement of numerous further experimental data with the present ansatz confirms it.

In this study, the structural properties, phase transition, relaxor behavior, and strain properties of (1−x)BaTiO3‒xBiYO3 (x= 0.02‒0.15 mol) ceramics were investigated. The room temperature x-ray diffraction and Raman spectroscopy results reveal that (1−x)BaTiO3‒xBiYO3 ceramics undergo phase transition from tetragonal to pseudo-cubic structure with x increasing. The curves of dielectric constant and loss tangent as a function of temperature and frequency show that the dielectric constant was changing from being dependent on temperature to being independent of it upon increasing BiYO3 amount, which is induced obvious dielectric relaxation behavior. Slimmer polarization–electric field (P–E) loops and lower remnant polarization (Pr ) were observed for samples with x ⩾ 0.08. The transition between the ferroelectric and relaxor states leads to the narrow strain–electric field (S–E) loops, which exhibit a high electric field-induced strain of 0.192% and an ultra-low strain hysteresis of 10.4% at an electric field of 70 kV cm−1 for x= 0.04. This excellent performance indicates that 0.96BaTiO3‒0.04BiYO3 ceramic may be promising lead-free materials for high-precision displacement actuators applications.

Reports detailed experimental studies of the onset of hard-mode Raman activity near the cubic-tetragonal structural phase transitions in KMnF3 and RbCaF3. The results are discussed in the context of a theory taking into account the influence not only of the long-range order, reflected in the macroscopic order parameter, but also that of the precursor order reflected in the central peak and phonon sideband of the soft-mode spectral function. The precursor order, dominant near the phase transition, is responsible for the persistence of quasi-first-order hard-mode features above the transition temperature. The extent of this persistence is used as a basis for specific estimates of the degree of precursor order in KMnF3 and RbCaF3.

Coupling lattice and spin degrees of freedom without the use of external magnetic fields allows for energy-efficient spintronic devices. In this context, hybrid materials composed of an oxide layer undergoing a first-order, structural phase transition (SPT), such as V2O3, and a ferromagnetic layer, such as Ni, constitute a very promising paradigm for the development of such applications. In the case of a Ni/V2O3 heterostructure, the structural domains inherent to the first-order phase transition in the V2O3 cause a differential strain pattern in the Ni layer. By virtue of the inverse magnetorestrictive effect, this strain pattern is converted into a magnetic anisotropy distribution in the Ni. This offers a promising alternative to voltage-controlled magnetism or other switching mechanisms without a magnetic field [1,2].Here we show by direct imaging of the thermal evolution of the Ni spin structure that the magnetic domains can be tuned in both size and orientation upon crossing the structural phase transition (SPT) of the proximal V2O3 layer [3]. We find a drastic temperature-driven reorientation of the Ni magnetic domains across the SPT which is responsible for an abrupt increase in the coercive field (up to 500%). By synchrotron-based X-ray microscopy we show a reconfiguration of the ferromagnetic domain pattern in the Ni layer across the V2O3 SPT together with changes in magnitude and direction of the magnetic anisotropy. These observations are in good agreement with both angular dependent ferromagnetic resonance measurements and micromagnetic simulations. Our simulations successfully replicate the change in the coercivity as well as the reorientation of the magnetization of the Ni domains after the implementation of a simple model of strain-dependent magnetic anisotropies. Furthermore, simulations also exhibit a phase coexistence of different magnetic domains across the SPT in the V2O3. Direct observations of the lateral correlation length of the Ni domains show an increase of almost an order of magnitude at the SPT compared to room temperature as well as a broad spatial distribution of the local transition temperatures. This corroborates the phase coexistence of Ni anisotropies due to the V2O3 structural domains coexistence across the SPT. Our data reveal that the reorientation of Ni domains is controlled by the reconfiguration of the structural domains of the oxide layer across the SPT [4,5], due to strain induced proximity.Our findings reveal a novel pathway to control magnetic domains without a magnetic field through proximity to a material undergoing a first-order structural phase transition, which allows for engineering of coercive fields for novel data storage architectures and novel device concepts based on “straintronics”. Acknowledgements:This work was supported by the Spanish MINECO projects MAT2015-68772-P and PGC2018-097789-B-I00 and European Union FEDER funds. A.F.R. and M.G.M acknowledge financial support from the EU CALIPSO Transnational Access programme. J.G.R. acknowledges support from Colciencias under grant 120471250659. The work by I.V., C.W. and I.K.S. at UCSD were supported by the Office of Basic Energy Science, U.S. Department of Energy, BES-DMS funded by the Department of Energy’s Office of Basic Energy Science, DMR under grant DE FG02 87ER-45332. **

Investigation of structural phase transition behavior of SrZrO3 by thermal analyses and high-temperature X-ray diffraction

Die IR‐Spektren zeigen, daß Metakaolinit durch Dehydroxylierung von Kaolinit bei etwa 500°C gebildet wird.

Phase transitions from and to the quasicrystalline state show a typical signature due to some structural peculiarities. Both quasicrystals and most of their translationally periodic transformation products, the approximants, consist of the same basic structural units ('clusters'). This is the cause of low-energy interfaces between the newly forming phase and the parent phase. It is also the origin of the rather high stability of the frequently resulting orientationally twinned nanodomain structures. Owing to topological incompatibilities between quasiperiodic and periodic structures, purely displacive phase transitions are impossible. Diffusion of a significant fraction of atoms, at least on the scale of the cluster diameters, always has to take place. Locally similar icosahedral structural ordering between parent phase and nucleating phase is also responsible for the frequently occurring formation of icosahedral quasicrystals from undercooled liquid alloys or during devitrification of metallic glasses. The different types of experimentally observed phase transitions are discussed, from amorphous to quasicrystalline, quasicrystalline to ordered/disordered quasicrystalline and quasicrystalline to crystalline as a function of temperature, pressure, irradiation and high-energy ball milling, respectively.

Investigation of Li-induced structural disorder and phase transition in ZnO by Raman spectroscopy