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Crystals, Volume 7, Issue 5 (May 2017)

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Cover Story (view full-size image) We analysed topologic and holonomic aspects of discrete-time quantum walk architectures. In [...] Read more.
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Open AccessReview Material and Device Architecture Engineering Toward High Performance Two-Dimensional (2D) Photodetectors
Crystals 2017, 7(5), 149; https://doi.org/10.3390/cryst7050149
Received: 1 April 2017 / Revised: 4 May 2017 / Accepted: 10 May 2017 / Published: 22 May 2017
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Abstract
Photodetectors based on two-dimensional (2D) nanostructures have led to a high optical response, and a long photocarrier lifetime because of spatial confinement effects. Since the discovery of graphene, many different 2D semiconductors have been developed and utilized in the ultrafast and ultrasensitive detection
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Photodetectors based on two-dimensional (2D) nanostructures have led to a high optical response, and a long photocarrier lifetime because of spatial confinement effects. Since the discovery of graphene, many different 2D semiconductors have been developed and utilized in the ultrafast and ultrasensitive detection of light in the ultraviolet, visible, infrared and terahertz frequency ranges. This review presents a comprehensive summary of recent breakthroughs in constructing high-performance photodetectors based on 2D materials. First, we give a general overview of 2D photodetectors based on various single-component materials and their operating wavelength (ultraviolet to terahertz regime). Then, we summarize the design and controllable synthesis of heterostructure material systems to promote device photoresponse. Subsequently, special emphasis is put on the accepted methods in rational engineering of device architectures toward the photoresponse improvements. Finally, we conclude with our personal viewpoints on the challenges and promising future directions in this research field. Full article
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Open AccessArticle Synthesis, Crystal Structure and Nonlinear Optical Property of RbHgI3
Crystals 2017, 7(5), 148; https://doi.org/10.3390/cryst7050148
Received: 15 January 2017 / Revised: 15 May 2017 / Accepted: 16 May 2017 / Published: 22 May 2017
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Abstract
Searching for new nonlinear optical (NLO) crystals to be used in the infrared (IR) region is still a challenge. This paper presents the synthesis, crystal structure and properties of a new halide, RbHgI3. Its non-centrosymmetric single crystal can be grown in
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Searching for new nonlinear optical (NLO) crystals to be used in the infrared (IR) region is still a challenge. This paper presents the synthesis, crystal structure and properties of a new halide, RbHgI3. Its non-centrosymmetric single crystal can be grown in solution. In its crystal structure, all the polar [HgI4]2− groups align in such a way that brings a favorable net polarization. The measurement by Kurtz–Perry powder technique indicates that RbHgI3 shows a phase-matchable second harmonic generation (SHG) property seven times stronger than that of KH2PO4 (KDP). RbHgI3 displays excellent transparency in the range of 0.48–25 μm with relatively good thermal stability. The UV absorption implies that this yellow compound’s band gap is about 2.56 eV, close to that of AgGaS2. A preliminary measurement indicates that the laser-induced damage threshold of the crystal is about 28.3 MW/cm2. These preliminary experimental data reveal that RbHgI3 is a new candidate as nonlinear optical material in the infrared region. Full article
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Open AccessReview Crystalline and Spherulitic Morphology of Polymers Crystallized in Confined Systems
Crystals 2017, 7(5), 147; https://doi.org/10.3390/cryst7050147
Received: 10 April 2017 / Revised: 15 May 2017 / Accepted: 15 May 2017 / Published: 19 May 2017
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Abstract
Due to the effects of microphase separation and physical dimensions, confinement widely exists in the multi-component polymer systems (e.g., polymer blends, copolymers) and the polymers having nanoscale dimensions, such as thin films and nanofibers. Semicrystalline polymers usually show different crystallization kinetics, crystalline structure
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Due to the effects of microphase separation and physical dimensions, confinement widely exists in the multi-component polymer systems (e.g., polymer blends, copolymers) and the polymers having nanoscale dimensions, such as thin films and nanofibers. Semicrystalline polymers usually show different crystallization kinetics, crystalline structure and morphology from the bulk when they are confined in the nanoscale environments; this may dramatically influence the physical performances of the resulting materials. Therefore, investigations on the crystalline and spherulitic morphology of semicrystalline polymers in confined systems are essential from both scientific and technological viewpoints; significant progresses have been achieved in this field in recent years. In this article, we will review the recent research progresses on the crystalline and spherulitic morphology of polymers crystallized in the nanoscale confined environments. According to the types of confined systems, crystalline, spherulitic morphology and morphological evolution of semicrystalline polymers in the ultrathin films, miscible polymer blends and block copolymers will be summarized and reviewed. Full article
(This article belongs to the Special Issue Crystal Morphology and Assembly in Spherulites)
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Open AccessArticle AlGaN/GaN MOS-HEMTs with Corona-Discharge Plasma Treatment
Crystals 2017, 7(5), 146; https://doi.org/10.3390/cryst7050146
Received: 29 April 2017 / Revised: 13 May 2017 / Accepted: 16 May 2017 / Published: 18 May 2017
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Abstract
The effects of a corona-discharge plasma treatment on the performance of an AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor fabricated onto Si substrates were studied. The threshold voltage shifted from −8.15 to −4.21 V when the device was treated with an Al2O3
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The effects of a corona-discharge plasma treatment on the performance of an AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor fabricated onto Si substrates were studied. The threshold voltage shifted from −8.15 to −4.21 V when the device was treated with an Al2O3 layer. The leakage current was reduced from 2.9 × 10−5 to 4.2 × 10−7 mA/mm, and the ION/IOFF ratio increased from 8.3 × 106 to 7.3 × 108 using the corona-discharge plasma treatment, which exhibited an increase of about two orders of magnitude. The device exhibited excellent performance with a subthreshold swing of 78 mV/dec and a peak gain of 47.92 mS/mm at VGS = 10 V. Full article
(This article belongs to the Special Issue Advances in GaN Crystals and Their Applications)
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Open AccessReview Polymorphism and Structural Distortions of Mixed-Metal Oxide Photocatalysts Constructed with α-U3O8 Types of Layers
Crystals 2017, 7(5), 145; https://doi.org/10.3390/cryst7050145
Received: 12 April 2017 / Revised: 12 May 2017 / Accepted: 14 May 2017 / Published: 18 May 2017
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Abstract
A series of mixed-metal oxide structures based on the stacking of α-U3O8 type pentagonal bipyramid layers have been investigated for symmetry lowering distortions and photocatalytic activity. The family of structures contains the general composition Am+((n+1)/m)
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A series of mixed-metal oxide structures based on the stacking of α-U3O8 type pentagonal bipyramid layers have been investigated for symmetry lowering distortions and photocatalytic activity. The family of structures contains the general composition Am+((n+1)/m)B(3n+1)O(8n+3) (e.g., A = Ag, Bi, Ca, Cu, Ce, Dy, Eu, Gd K, La, Nd, Pb, Pr, Sr, Y; B = Nb, Ta; m = 1–3; n = 1, 1.5, 2), and the edge-shared BO7 pentagonal pyramid single, double, and/or triple layers are differentiated by the average thickness, (i.e., 1 ≤ n ≤ 2), of the BO7 layers and the local coordination environment of the “A” site cations. Temperature dependent polymorphism has been investigated for structures containing single layered (n = 1) monovalent (m = 1) “A” site cations (e.g., Ag2Nb4O11, Na2Nb4O11, and Cu2Ta4O11). Furthermore, symmetry lowering distortions were observed for the Pb ion-exchange synthesis of Ag2Ta4O11 to yield PbTa4O11. Several members within the subset of the family have been constructed with optical and electronic properties that are suitable for the conversion of solar energy to chemical fuels via water splitting. Full article
(This article belongs to the Special Issue Crystallography of Functional Materials)
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Open AccessArticle Modeling the Neutral-Ionic Transition with Correlated Electrons Coupled to Soft Lattices and Molecules
Crystals 2017, 7(5), 144; https://doi.org/10.3390/cryst7050144
Received: 3 April 2017 / Revised: 2 May 2017 / Accepted: 7 May 2017 / Published: 16 May 2017
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Abstract
Neutral-ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar π-electron donors (D) and acceptors (A) that form mixed stacks ... D+ρAρD+ρAρD+ρAρ ... with
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Neutral-ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar π -electron donors (D) and acceptors (A) that form mixed stacks ... D+ρAρD+ρAρD+ρAρ ... with variable ionicity 0 < ρ < 1 and electron transfer t along the stack. The microscopic NIT model presented here combines a modified Hubbard model for strongly correlated electrons delocalized along the stack with Coulomb intermolecular interactions treated in mean field. It also accounts for linear coupling of electrons to a harmonic molecular vibration and to the Peierls phonon. This simple framework captures the observed complexity of NITs with continuous and discontinuous ρ on cooling or under pressure, together with the stack’s instability to dimerization. The interplay of charge, molecular and lattice degrees of freedom at NIT amplifies the nonlinearity of responses, accounts for the dielectric anomaly, and generates strongly anharmonic potential energy surfaces (PES). Dynamics on the ground state PES address vibrational spectra using time correlation functions. When extended to the excited state PES, the NIT model describes the early (<1 ps) dynamics of transient NIT induced by optical CT excitation with a fs pulse. Although phenomenological, the model parameters are broadly consistent with density functional calculations. Full article
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)
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Open AccessReview Structure Property Relationships and Cationic Doping in [Ca24Al28O64]4+ Framework: A Review
Crystals 2017, 7(5), 143; https://doi.org/10.3390/cryst7050143
Received: 20 March 2017 / Revised: 8 May 2017 / Accepted: 10 May 2017 / Published: 16 May 2017
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Abstract
Ca12Al14O33 (C12A7, 12CaO·7Al2O3, or [Ca12Al14O32]2+:O2) is a material with a clathrate cage framework, positively charged and stabilized by anions occluded
[...] Read more.
Ca12Al14O33 (C12A7, 12CaO·7Al2O3, or [ Ca 12 Al 14 O 32 ] 2 + : O 2 ) is a material with a clathrate cage framework, positively charged and stabilized by anions occluded within 17% of the cages. The occluded anion is modular and can be elemental, polyatomic, and electronic in nature. This review focuses on the electride C12A7 ( [ Ca 24 Al 28 O 64 ] 4 + : ( 4 * ) e ( 2 ) O 2 ), where O2− anions are replaced with electrons, and compliments previous structural and electronic property reviews to illuminate the structure–property relationships. Electride formation is updated with new findings in carbonaceous reduction methods. Most importantly, an extensive compilation of cationic doped C12A7 isostructural compounds is presented as motivation to study doped C12A7 electrides. Cationic dopants have profound impacts on the electronic properties due to changes in the density of states, localized electron behavior, and structural distortions. Full article
(This article belongs to the Special Issue Crystallography of Functional Materials)
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Open AccessReview Crystal Structures from Powder Diffraction: Principles, Difficulties and Progress
Crystals 2017, 7(5), 142; https://doi.org/10.3390/cryst7050142
Received: 27 April 2017 / Revised: 10 May 2017 / Accepted: 11 May 2017 / Published: 16 May 2017
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Abstract
The structure solution from powder diffraction has undergone an intense evolution during the last 20 years, but is far from being routine. Current challenges of powder crystallography include ab initio crystal structure determination on real samples of new materials with specific microstructures, characterization
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The structure solution from powder diffraction has undergone an intense evolution during the last 20 years, but is far from being routine. Current challenges of powder crystallography include ab initio crystal structure determination on real samples of new materials with specific microstructures, characterization of intermediate reaction products from in situ, in operando studies and novel phases from in situ studies of phase diagrams. The intense evolution of electron diffraction in recent years, providing an experimental (precession) and theoretical (still under intense development) solution to strong dynamic scattering of electrons, smears the traditional frontier between poly- and single-crystal diffraction. Novel techniques like serial snapshot X-ray crystallography point in the same direction. Finally, for the computational chemistry, theoreticians hand-in-hand with crystallographers develop tools where the theory meets experiment for crystal structure refinement, which becomes an unavoidable step in the validation of crystal structures obtained from powder diffraction. Full article
(This article belongs to the Special Issue Crystal Structure of Electroceramics) Printed Edition available
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Open AccessArticle Synthesis and Thermoelectric Properties of Copper Sulfides via Solution Phase Methods and Spark Plasma Sintering
Crystals 2017, 7(5), 141; https://doi.org/10.3390/cryst7050141
Received: 27 April 2017 / Revised: 12 May 2017 / Accepted: 13 May 2017 / Published: 16 May 2017
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Abstract
Large-scale Cu2S tetradecahedrons microcrystals and sheet-like Cu2S nanocrystals were synthesized by employing a hydrothermal synthesis (HS) method and wet chemistry method (WCM), respectively. The morphology of α-Cu2S powders prepared by the HS method is a tetradecahedron with
[...] Read more.
Large-scale Cu2S tetradecahedrons microcrystals and sheet-like Cu2S nanocrystals were synthesized by employing a hydrothermal synthesis (HS) method and wet chemistry method (WCM), respectively. The morphology of α-Cu2S powders prepared by the HS method is a tetradecahedron with the size of 1–7 μm. The morphology of β-Cu2S is a hexagonal sheet-like structure with a thickness of 5–20 nm. The results indicate that the morphologies and phase structures of Cu2S are highly dependent on the reaction temperature and time, even though the precursors are the exact same. The polycrystalline copper sulfides bulk materials were obtained by densifying the as-prepared powders using the spark plasma sintering (SPS) technique. The electrical and thermal transport properties of all bulk samples were measured from 323 K to 773 K. The pure Cu2S bulk samples sintered by using the powders prepared via HS reached the highest thermoelectric figure of merit (ZT) value of 0.38 at 573 K. The main phase of the bulk sample sintered by using the powder prepared via WCM changed from β-Cu2S to Cu1.8S after sintering due to the instability of β-Cu2S during the sintering process. The Cu1.8S bulk sample with a Cu1.96S impurity achieved the highest ZT value of 0.62 at 773 K. Full article
(This article belongs to the Special Issue Materials Processing and Crystal Growth for Thermoelectrics)
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Open AccessArticle A New Hemihydrate of Valacyclovir Hydrochloride
Crystals 2017, 7(5), 140; https://doi.org/10.3390/cryst7050140
Received: 28 March 2017 / Revised: 9 May 2017 / Accepted: 11 May 2017 / Published: 16 May 2017
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Abstract
Several crystal forms of valacyclovir hydrochloride, including two anhydrous and three hydrates, were investigated in this study. At the same time, a new hemihydrate of valacyclovir hydrochloride was first discovered and its properties were characterized by PXRD, TGA, DSC, and Raman in this
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Several crystal forms of valacyclovir hydrochloride, including two anhydrous and three hydrates, were investigated in this study. At the same time, a new hemihydrate of valacyclovir hydrochloride was first discovered and its properties were characterized by PXRD, TGA, DSC, and Raman in this study. The hemihydrate shows a distinctive PXRD pattern and a melting point of 209 °C with a water weight loss of 2.42% from the thermal analysis. The Raman spectra show a few distinctive peaks in the region of 1250–1400 cm−1 due to different crystal forms. The thermostability testing suggests it is a stable crystal form and remain the same for several months under high temperature and humidity. All these crystal forms show good dissolubility in the water at room temperature with excess 100 mg/mL. Full article
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Open AccessArticle Preparation, Crystal structure and Luminescence Properties of Lanthanide Complexes with 2,4,6-tri(pyridin-2-yl)-1,3,5-triazine and Organic Carboxylic Acid
Crystals 2017, 7(5), 139; https://doi.org/10.3390/cryst7050139
Received: 1 April 2017 / Revised: 9 May 2017 / Accepted: 10 May 2017 / Published: 14 May 2017
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Abstract
Five crystal complexes {[Eu2(TPTZ)2(mNBA)6(H2O)2]·2CH3OH}n (1), [Eu(TPTZ)(CF3COO)(H2O)5]·Cl2·CH3CH2OH (2), {[Yb2(TPTZ)2(BDC)3]·2H2O}n (3), [Yb(TPTZ)Cl(H2O)4]·Cl2 (4) and [Er(TPTZ)(TTA)Cl2] (5) (mNBA = m-nitro benzoate, BDC = terephthalate, TTA = thenoyltrifluoroacetone, TPTZ = 2,4,6-tri(2-pyridyl)-1,3,5-triazine) have been synthesized. The single X-ray diffraction reveals that TPTZ is mainly in the trident coordination
[...] Read more.
Five crystal complexes {[Eu2(TPTZ)2(mNBA)6(H2O)2]·2CH3OH}n (1), [Eu(TPTZ)(CF3COO)(H2O)5]·Cl2·CH3CH2OH (2), {[Yb2(TPTZ)2(BDC)3]·2H2O}n (3), [Yb(TPTZ)Cl(H2O)4]·Cl2 (4) and [Er(TPTZ)(TTA)Cl2] (5) (mNBA = m-nitro benzoate, BDC = terephthalate, TTA = thenoyltrifluoroacetone, TPTZ = 2,4,6-tri(2-pyridyl)-1,3,5-triazine) have been synthesized. The single X-ray diffraction reveals that TPTZ is mainly in the trident coordination mode and organic aromatic carboxylic acids are in the multiple coordination modes in the crystals. The composition of solvents, reaction temperature and reactant ratios all affect the composition and structure of the formed crystals. Crystals 1 and 3 belong to triclinic system, while the other three belong to monoclinic system. Among them, Crystal complexes 1 and 3 are coordination polymers. The other three crystals are mononuclear complexes with LnШ ions in the asymmetric environment. Both of the Crystal complexes 1 and 2 show strong luminescence emissions of Eu3+. The luminescence lifetimes of the two complexes are 0.761 ms and 0.447 ms, respectively. In addition, their luminescence quantum efficiencies are 25.0% and 16.7%, respectively. Full article
(This article belongs to the Special Issue Luminescent Properties of Lanthanoid Doped Crystals)
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Open AccessArticle Using Neutron Diffraction to Investigate Texture Evolution During Consolidation of Deuterated Triaminotrinitrobenzene (d-TATB) Explosive Powder
Crystals 2017, 7(5), 138; https://doi.org/10.3390/cryst7050138
Received: 10 April 2017 / Revised: 9 May 2017 / Accepted: 10 May 2017 / Published: 14 May 2017
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Abstract
Triaminotrinitrobenzene (TATB) is a highly anisotropic molecular crystal used in several plastic-bonded explosive (PBX) formulations. A complete understanding of the orientation distribution of TATB particles throughout a PBX charge is required to understand spatially variable, anisotropic macroscale properties of the charge. Although texture
[...] Read more.
Triaminotrinitrobenzene (TATB) is a highly anisotropic molecular crystal used in several plastic-bonded explosive (PBX) formulations. A complete understanding of the orientation distribution of TATB particles throughout a PBX charge is required to understand spatially variable, anisotropic macroscale properties of the charge. Although texture of these materials can be measured after they have been subjected to mechanical or thermal loads, measuring texture evolution in situ is important in order to identify mechanisms of crystal deformation and reorientation used to better inform thermomechanical models. Neutron diffraction measurements were used to estimate crystallographic reorientation while deuterated TATB (d-TATB) powder was consolidated into a cylindrical pellet via a uniaxial die-pressing operation at room temperature. Both the final texture of the pressed pellet and the in situ evolution of texture during pressing were measured, showing that the d-TATB grains reorient such that (001) poles become preferentially aligned with the pressing direction. A compaction model is used to predict the evolution of texture in the pellet during the pressing process, finding that the original model overpredicted the texture strength compared to these measurements. The theory was extended to account for initial particle shape and pore space, bringing the results into good agreement with the data. Full article
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Open AccessArticle Domain Patterning in Ion-Sliced LiNbO3 Films by Atomic Force Microscopy
Crystals 2017, 7(5), 137; https://doi.org/10.3390/cryst7050137
Received: 7 April 2017 / Revised: 2 May 2017 / Accepted: 11 May 2017 / Published: 14 May 2017
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Abstract
Photonic structures denoted as LNOI (LiNbO3-on-insulator) are of considerable interest for integrated optics due to a high refractive-index contrast provided by the interface LiNbO3/insulator. A topical problem for LNOI-based optical waveguides is optical-frequency conversion, in particular realized on ferroelectric
[...] Read more.
Photonic structures denoted as LNOI (LiNbO3-on-insulator) are of considerable interest for integrated optics due to a high refractive-index contrast provided by the interface LiNbO3/insulator. A topical problem for LNOI-based optical waveguides is optical-frequency conversion, in particular realized on ferroelectric domains on the basis of quasi phase-matching principle. This paper presents extended studies on the fabrication of domain patterns by atomic force microscopy (AFM) methods (raster lithography, piezo-force microscopy, conductive AFM) in single-crystal ion-sliced LiNbO3 films forming LNOI sandwiches. A body of data obtained on writing characteristics of domains and specified 1D and 2D domain patterns permitted us to manipulate the domain sizes and shapes. Of special importance is the stability of created patterns, which persist with no degradation during observation times of months. The domain coalescence leading to the transformation of a discrete domain pattern to a continuous one was investigated. This specific effect—found in thin LiNbO3 layers for the first time—was attributed to the grounding of space-charges accumulated on domain walls. Observations of an enhanced static conduction at domain walls exceeding that in surrounding areas by not less than by five orders of magnitude supports this assumption. AFM domain writing in ion-sliced films serves as a basis for studies in nonlinear photonic crystals in integrated optical schemes. Full article
(This article belongs to the Special Issue Lithium Niobate Crystals)
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Open AccessArticle Phase Transition Behavior of the Layered Perovskite CsBi0.6La0.4Nb2O7: A Hybrid Improper Ferroelectric
Crystals 2017, 7(5), 135; https://doi.org/10.3390/cryst7050135
Received: 20 April 2017 / Revised: 9 May 2017 / Accepted: 10 May 2017 / Published: 13 May 2017
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Abstract
The phase behavior of the layered perovskite CsBi0.6La0.4Nb2O7, of the Dion-Jacobson family, has been studied by high-resolution powder neutron diffraction between the temperatures of 25 < T < 850 °C. At ambient temperature, this material
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The phase behavior of the layered perovskite CsBi0.6La0.4Nb2O7, of the Dion-Jacobson family, has been studied by high-resolution powder neutron diffraction between the temperatures of 25 < T < 850 °C. At ambient temperature, this material adopts the polar space group P21am; this represents an example of hybrid improper ferroelectricity caused by the interaction of two distinct octahedral tilt modes. Within the limits of our data resolution, the thermal evolution of the crystal structure is consistent with a first-order transition between 700 and 750 °C, with both tilt modes vanishing simultaneously, leading to the aristotype space group P4/mmm. This apparent “avalanche transition” behavior resembles that seen in the related Aurivillius phase SrBi2Nb2O9. Full article
(This article belongs to the Special Issue Crystal Structure of Electroceramics) Printed Edition available
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Open AccessArticle On the Role of AlN Insertion Layer in Stress Control of GaN on 150-mm Si (111) Substrate
Crystals 2017, 7(5), 134; https://doi.org/10.3390/cryst7050134
Received: 12 April 2017 / Revised: 5 May 2017 / Accepted: 9 May 2017 / Published: 12 May 2017
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Abstract
In this study, low-temperature (LT) and high-temperature (HT) AlN insertion layers (ILs) grown at 680 and 970 °C were integrated with 3.7-μm GaN-based heterostructure grown on 150-mm Si (111) substrates by metalorganic chemical vapor deposition. Under a V/III flow ratio of 1960, the
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In this study, low-temperature (LT) and high-temperature (HT) AlN insertion layers (ILs) grown at 680 and 970 °C were integrated with 3.7-μm GaN-based heterostructure grown on 150-mm Si (111) substrates by metalorganic chemical vapor deposition. Under a V/III flow ratio of 1960, the GaN epilayer with a continuous interface resulting from the LT AlN IL was subject to a compressive stress of −0.109 GPa. However, the GaN epilayer with discontinuous interfaces resulting from the HT AlN IL growth under the same flow ratio was subject to a tensile stress of 0.174 GPa. To realize continuous interfaces between the GaN epilayer and HT AlN IL, a higher V/III ratio of 5960 was utilized to suppress the decomposition of GaN. It results in changing the stress state of the GaN-based heterostructure from tensile to compressive. This strategic finding indicates that a stress-controllable GaN on Si can be achieved via the incorporation of HT AlN ILs. A minimum curvature at 5 km−1 is demonstrated for the 3.7-μm GaN-based heterostructure on a 150-mm Si (111) substrate, which has high potential for power switching device applications. Full article
(This article belongs to the Special Issue Advances in GaN Crystals and Their Applications)
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