Amorphous-like State Research Articles

High-Pressure Behavior of δ-Phase of Formamidinium Lead Iodide by Optical Spectroscopies

The exceptional photovoltaic properties of hybrid organic–inorganic perovskites have attracted increasing interest in the past decades. Among these materials, FAPbI3 shows two structural phases: the high temperature perovskite α-phase, with direct bandgap close to the Shockley–Queisser limit, and the much less photoactive non-perovskite δ-phase, stable at ambient conditions. Although the presence of the δ-phase has been usually regarded as a limitation for FAPbI3 optoelectronic applications, recent studies have found that devices with increased stability and efficiency can be designed by mixing α- and δ-phases. This has brought out the need for a deeper understanding of the physical properties of δ-FAPbI3. In this paper, we present an original high-pressure Raman and photoluminescence study to address the effects of compression on the lattice and optoelectronic response of the sample. Also, based on the previous findings on different hybrid perovskites, our results for δ-FAPbI3 show that the cation configuration goes from a dynamically disordered regime at ambient conditions to a statically ordered phase at ∼1.5 GPa. On further increasing pressure, above 7 GPa, a statically disordered regime takes place, where the cations are locked at random orientations in the inorganic framework, giving rise to an amorphous-like state. Compared with α- FAPbI3, we found that the hexagonal δ-phase is less affected by external compression, as both the first detectable structural transition and the amorphous-like behavior occur at higher pressures.

Pressure-driven electronic and structural phase transition in intrinsic magnetic topological insulator MnSb2Te4

Intrinsic magnetic topological insulators provide an ideal platform to achieve various exciting physical phenomena. However, this kind of materials and related research are still very rare. In this work, we reported the electronic and structural phase transitions in the intrinsic magnetic topological insulator $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ driven by hydrostatic pressure. Electric transport results revealed that temperature dependent resistance showed a minimum value near short-range antiferromagnetic (AFM) ordering temperature ${{T}_{N}}^{\ensuremath{'}}$, which declines with pressure. The short-range AFM ordering was sensitive to pressure and fully suppressed above 11.5 GPa. The intensity of three Raman vibration modes in $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ declined quickly starting from 7.5 GPa and these modes become undetectable above 9 GPa, suggesting possible insulator-metal transition, which is further confirmed by theoretical calculation. In situ x-ray diffraction demonstrated that an extra diffraction peak appears near 9.1 GPa and $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ started to enter an amorphouslike state above 16.6 GPa, suggesting the structural origin of suppressed AFM ordering and metallization. This work has demonstrated the correlation among interlayer interaction, magnetic ordering, and electric behavior, which could be benefit the understanding of the fundamental properties of this kind of materials and devices.

VERIFICATION OF RABINOWICZ’ CRITERION BY DIRECT MOLECULAR DYNAMICS MODELING

In the paper we use direct molecular dynamics modeling to validate the criterion for formation of wear debris proposed by E. Rabinowicz in 1958. A conventional molecular dynamics using a classical Tersoff’s potential was applied to simulate the sliding behavior within a thin film corresponding to a tribofilm formed from silica nano-particles in amorphous-like state. The simulation was carried out by varying the initial temperature and the spatial size of the simulated crystallite. The results show the change in sliding behavior of silica-based tribofilm depending on the temperature and the size parameter of the system under consideration. Thus increasing the temperature provides smooth sliding while at moderate conditions wear process can occur via debris formation. Our estimations show good correlation between predicted critical size of the simulated system and calculated energetic characteristics.

Decomposition of lanthanum hafnate at high pressures

Our high pressure x-ray diffraction and Raman scattering studies on lanthanum hafnate $({\text{La}}_{2}{\text{Hf}}_{2}{\text{O}}_{7})$ establish that it decomposes into its constituent oxides beyond $\ensuremath{\sim}18\text{ }\text{GPa}$. These results are supported by our first-principles total energy calculations. Raman scattering results also imply that the products of dissociation are in an amorphouslike state beyond 50 GPa but crystallize slowly upon release of pressure.

Electrochemical properties of Mg-based hydrogen storage alloys prepared by hydriding combustion synthesis and subsequent mechanical milling (HCS+MM)

In this paper, a novel method, namely hydriding combustion synthesis (HCS) and subsequent mechanical milling (MM) was used to prepare Mg-based hydrogen storage electrode alloy. The phase structures and electrochemical properties of the alloys before and after MM were characterized by X-ray diffraction (XRD) analysis and galvanotactic charge–discharge cycle test, respectively. The XRD results showed that the structure of the as-milled alloys was nanocrystallite or amorphous-like state. Electrochemical measurements showed that the discharge capacity was improved greatly for the products of HCS+MM. The HCS product with only 5h MM showed markedly increased discharge capacity up to 481.5mAh/g for the first cycle, which was 10 times higher than the HCS product (39.4mAh/g). The discharge capacity was further increased to 628.3mAh/g for the HCS product after milling with nickel powder. Besides, the addition of nickel also led to an improved cycling stability of the alloy electrode during cycling in KOH electrolyte. It was indicated that the HCS+MM was promising for preparing Mg-based hydrogen storage electrode alloys.

Deposition of HfO 2 thin films on ZnS substrates

HfO 2 thin films with columnar microstructure were deposited directly on ZnS substrates by electron beam evaporation process. SiO 2 thin films, deposited by reactive magnetron sputtering, were used as buffer layers, HfO 2 thin films of granular microstructure were obtained on SiO 2 interlayer by this process. X-ray diffraction patterns demonstrate that the as-deposited HfO 2 films are in an amorphous-like state with small amount of crystalline phase while the HfO 2 films annealed at 450 °C in O 2 for 30 min and in Ar for 150 min underwent a phase transformation from amorphous-like to monoclinic phase. Antireflection effect in certain infrared wave band, such as 3–6 μm, 4–12 μm, 4–8 μm and 3–10 μm, can be observed, which was dependent on the thickness of thin films. The cross-sectional images of HfO 2 films, obtained by field emission scanning electron microscopy, revealed that there was no distinct morphological change upon annealing.

연료전지의 수소저장용 마그네슘계 합금의 표면제어에 의한 전기화학적 수소화 거동 연구

Mg₂Ni계 합금 입자의 수소저장 특성에 대한 표면처리 효과가 마이크로 전극 측정법에 의해 검토되었다. 카본-섬유로 된 마이크로 전극을 KOH 수용액 속에서 조정자를 사용하여 수소 단일입자에 접촉시켰다. 상온에서 Mg₂Ni 합금의 수소저장 특성은 니켈 도금용액에 의한 표면 처리에 의해 크게 개선되었다. 니켈 도금용액 속에 있는 함유된 나트륨염(sodium phosphate 및 sodium dihydrogen citrate)이 합금을 아몰퍼스와 같은 형태로 만들었으며, 그 결과 상온에서 수소 흡장/방출 용량이 최초의 17[mAh/g]에서 150[mAh/g]로 향상되었다. 합금 입자 내에서 수소원자의 겉보기 화학적 확산계수를 계산하기 위하여 Potential-step 실험을 실시하였으며 데이터 해석을 위해 구형확산 모델을 적용하였다. 실험결과로서 겉보기 확산계수(DSUBapp/SUB)는 수소 흡장 및 방출되는 전 과정에서 10??～10??[㎠/s] 수준인 것으로 확인되었다.

Equation of state and high-pressure irreversible amorphization in Y3Fe5O12

The change of crystal structure in yttrium iron garnet Y3Fe5O12 was studied at room temperature at high pressures up to ∼55 GPa by the x-ray diffraction technique in diamond anvil cells. At a pressure of about ∼50 GPa, a drastic change in the x-ray diffraction pattern was observed indicating the transition into an amorphouslike state. When the pressure was increased, the bulk modulus of YIG was found to be 193 ± 4 GPa. It was also found that the amorphous state was retained after decompression down to ambient pressure. From the shape of x-ray patterns in the “amorphous” phase, it was concluded that the local atomic structure consists of iron-oxygen FeO6 octahedral complexes with disordered orientations of local axis and of randomly arranged others ion fragments with the overall Y3Fe5O12 composition. For the amorphous phase, it was evaluated that the bulk modulus of FeO6 octahedral complexes is about 260 GPa.

Solid phase crystallisation of HfO 2 thin films

In this paper, we report on the solid phase crystallisation of carbon-free HfO 2 thin films deposited by plasma ion assisted deposition (PIAD). After deposition, the HfO 2 films were annealed in N 2 ambient for 3 h at 350, 550 and 750 °C. Several characterisation techniques including X-ray reflectometry (XRR), X-ray diffraction (XRD), spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) were used for the physical characterisation of as-deposited and annealed HfO 2. XRD has revealed that the as-deposited HfO 2 film is in an amorphous-like state with only traces of crystalline phase and that the annealed films are in a highly crystalline state. These results are in good agreement with the SE results showing an increase of refractive index by increasing the annealing temperature. XRR results show a significant density gradient over the as-deposited film thickness, which is characteristic of the PIAD method. The AFM measurements show that the HfO 2 layers have a smooth surface even after annealing at 750 °C. The present study demonstrates that the solid phase crystallisation of HfO 2 PIAD thin films starts at a temperature as low as 550 °C.

Microstructural changes in silicon thermal oxide induced by high-flux copper negative-ion implantation

Silicon-thermal-oxide (a-SiO 2) films of 115 nm thick on Si wafers are implanted by high-flux Cu negative-ions of 60 keV. Optical reflectivity measurements show that the Si substrates are transformed into an amorphous-like state after the implantation. The “amorphization” is ascribed to recoil atoms, not directly to the incident Cu ions. The crystallinity is recovered after a heat treatment at 900°C for 1 h in Ar gas. The interference fringes in the reflectivity spectra shift to high energy side at 60 μA/ cm 2 and the higher fluxes, indicating decrease of the oxide thickness. At the same fluxes, AFM observation shows a prominent change of surface morphology and a large increase of step-height at implanted–unimplanted area boundaries, indicating enhanced sputtering under high-flux implantation.

Microvoltammetric Study of Electrochemical Hydrogenation of a Surface-Treated Mg2Ni Alloy Single Particle

Abstract The effects of surface treatments on the hydrogen storage properties of a Mg2Ni alloy particle were investigated by the microvoltammetric technique, in which a carbon-filament microelectrode was manipulated to make electrical contact with the particle placed in a KOH aqueous solution. It was found that the hydrogen storage properties of Mg2Ni at room temperature were improved by the surface treatments with a nickel plating solution. The sodium salts (sodium phosphinate and sodium dihydrogen citrate) contained in the nickel plating solution made the alloy form an amorphous-like state, resulting in an improved hydrogen absorption/desorption capacity at room temperature as high as about 150 mA h g−1 from the original value of 17 mA h g−1.

Structural Aspects of Reversible Control of Optical Activities of Bis(dimethylglyoximato)platinum(II) Thin Film

By iodine doping, the structure of Pt(dmg)2 is changed from the orthorhombic with one-dimensional metal chain to an amorphous-like state, which means that the doping induces fluctuation of metal-metal distance along the chain axis. This structural change is reversible; that is, the amorphous state turns back to the orthorhombic crystal by heating at 130°C, corresponding to the reversible change in optical properties.

Structural and Thermal Study of Fe-Ni-Si-B Powdres Prepared by Mechanical Alloying

Fe 39 Ni 39 Si 10 B 12 alloys, in both a nanostructural and a disordered amorphous-like state, were produced from different precursors using the mechanical alloying technique. Products were examined as a function of the milling time as well as the precursors. Milling times up to 80h were used and the progress in the alloying effect was analysed by X-ray diffraction, scanning electron microscopy (with energy dispersive X-ray microanalysis) and differential scanning calorimetry. The results show that at least partial amorphisation takes place. The newly formed amorphous structure and the residual crystalline phases have reached equilibrium after 40h milling. The crystallisation enthalpies were evaluated and the resulting phases were identified. The products of crystallisation include fcc(Ni,Fe), Fe 3 B, Fe 3 Si, Ni 31 Si 12 and Fe 5 Si 3 . The results suggest that the formation of the amorphous structure have been promoted by diffusion of Fe into Ni-rich lattices.

Surface oxidation of selenium treated GaAs(100)

The surface oxidation of Se treated GaAs(100) has been investigated in order to understand in greater detail the degradation of the Se passivated GaAs surface upon exposure to atmosphere. An increase in band bending is initially observed at relatively low exposure times, which corresponds to an increase in the O 2p intensity in the valence band. At this stage, oxygen is thought to weakly physisorb at the Ga vacancy sites. At intermediate exposure levels, the other unadsorbed oxygen atom of O2 attacks the nearest Ga atom. The bond between the nearest Ga atom and Se is then severed, resulting in the formation of Se, which closely resembles amorphous Se. Ultimately, both Se states are converted to this amorphouslike state and at longer exposure times are oxidized. At longer exposure times, the oxidation of Se is also accompanied by As oxidation. In contrast to S treated GaAs, Se/GaAs is relatively resistant to oxidation where only about 10% of the As is oxidized (As2O3) after 180 min of exposure versus oxidation of 35% of the As atoms for S/GaAs after only 20 min of atmosphere exposure. This relative oxidation resistance is attributed to greater penetration of Se into GaAs relative to S into GaAs.

Preparation of FeNi based metal-metalloid amorphous powders by mechanical alloying

Fe 40Ni 40B 6P 14 and Fe 40Ni 40Si 6P 14 alloys, in both a nanostructural and a disordered amorphous-like state, were produced from different precursors using the ball milling technique. The development of such metastable phases was investigated by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy. After the first 20 h, the diffraction peaks of the precursors disappeared, and the deformation during milling reduced the crystallite size of the alloys to 3–6 nm. The final crystallite size of the powders depended on the composition of the material as well as on the precursors utilized. The differential scanning calorimetry results revealed several exothermic peaks, indicating the crystallization of the amorphous-like state. The crystallization enthalpy was evaluated and the values obtained presented a maximum after 35 h of milling. The results were compared with data for rapidly quenched materials. The effect of the milling conditions on phase formation was analyzed.

Low temperature annealing of ball-milled Fe66B34

High energy ball-milling of Fe and B powders of starting composition Fe66B34 resulted in an alloy which revealed both a nanostructural state and a disordered amorphous-like state and a majority of unreacted components. The disordered phase is located in the interfacial regions between the Fe and B atoms. The ball-milled sample was annealed at low temperatures (250–350°C) to examine further the proposition that solid-state reaction is the mechanism of amorphization by mechanical alloying. A slight reaction between the Fe and B grains which were previously unreacted, probably limited to their contacting boundaries, has been detected.

Mechanical alloying of Fe-B

Fe-B alloys in both a nanostructural state and a disordered amorphous-like state have been produced using a novel ball-mill method. Mossbauer measurements at room temperature and 4.2 K on materials of nominal starting compositions Fe80B20 to Fe40B60 combined with X-ray diffraction and differential scanning calorimetry, revealed that samples with low B-content (<or approximately=34 at.% B) were formed mainly in a nanostructural state while samples with high B-content (>or approximately=50 at.% B) formed primarily in an amorphous-like state. The detectable B content of the mechanically alloyed samples was found to be lower than the nominal composition for all samples. The presence of unreacted B was confirmed by annealing experiments in which the expected fractions of phases alpha -Fe, Fe2B and FeB were obtained as crystallization products. In the samples, which partially exhibit a nanostructural state, B seems to be present only in the disordered interfacial regions.

Temperature effects in compression structures: carbon monoxide on Pd(110) by rairs

High resolution FT-RAIRS and LEED data have been obtained for CO on Pd(110) at 140 and 300 K. At 300 K the following behaviour is deduced. At low coverages, a single infrared band is seen corresponding to the formation of a c(2 × 2) structure. As θ is increased beyond 0.5, there is a striking transformation of the infrared spectrum where a broad absorption band with distinctive features over a 300 cm-1 frequency region is seen, attributed to small domains of p(4 × 2) structure separated by high density domain boundaries. As θ approaches 0.75, the p(4 × 2) domains increase in size and the infrared spectrum shows only two bands which are attributed to this structure. Finally at ambient pressures, islands of a (2 × l)plgl structure coexist with islands of p(4 × 2) structure. At 140 K, immobility of the adlayer leads to different behaviour. At low coverages, islands of CO molecules in B2 sites exist, surrounded by a disordered sea. With increasing coverage a “frozen-in” amorphous-like state is formed in which a number of domains with varying local densities (0.5 < θ < 1) coexist. At saturation a single, sharp, infrared band at 2007 cm-1 arises as the only ordered LEED structure (the (2 × l)plgl) observed at this temperature is formed.

Temperature effects in compression structures: Carbon monoxide on Pd(110) by RAIRS

High resolution FT-RAIRS and LEED data have been obtained for CO on Pd(110) at 140 and 300 K. At 300 K the following behaviour is deduced. At low coverages, a single infrared band is seen corresponding to the formation of a c(2 × 2) structure. As θ is increased beyond 0.5, there is a striking transformation of the infrared spectrum where a broad absorption band with distinctive features over a 300 cm -1 frequency region is seen, attributed to small domains of p(4 × 2) structure separated by high density domain boundaries. As θ approaches 0.75, the p(4 × 2) domains increase in size and the infrared spectrum shows only two bands which are attributed to this structure. Finally at ambient pressures, islands of a (2 × l)plgl structure coexist with islands of p(4 × 2) structure. At 140 K, immobility of the adlayer leads to different behaviour. At low coverages, islands of CO molecules in B2 sites exist, surrounded by a disordered sea. With increasing coverage a “frozen-in” amorphous-like state is formed in which a number of domains with varying local densities (0.5 < θ < 1) coexist. At saturation a single, sharp, infrared band at 2007 cm -1 arises as the only ordered LEED structure (the (2 × l)plgl) observed at this temperature is formed.