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Item Optical properties of RbTiOAsO4 single crystal(1998) Tu, Chi-Shun; Yeh, Y.-L.; Katiyar, R. S.; Guo, Ruqian; Schmidt, V. Hugo; Chien, R.-M.; Guo, Ruyan; Bhalla, A.S.Rubidium titanyl arsenate (RbTiOAsO4) belongs to the family of nonlinear optical crystals with the general formula M1+TiOX5+O4, where M = K, Rb, Tl, Cs and X = P, As. [1–6] The high damage threshold and broad angular acceptance have made such crystals attractive materials for frequency doubling of Nd-based lasers at λ=1.064 and 1.32 µm, and for optical parametric oscillators (OPO). In addition, the ion exchange properties also make them one of the best candidates for waveguide applications. Potassium titanyl phosphate, KTiOPO4 (KTP), is the most popular among such materials and has been used successfully in different applications. However, the orthophosphate absorption at ∼4.3 and ∼3.5 µm in KTP severely limits the oscillator output power. In contrast, RTA has a broad infrared transparency (∼0.35-5.3 µm) and exhibits no overtone absorption between 3 and 5 µm. [4] This makes the RTA crystal a potential candidate for nonlinear optical applications. At room temperature, KTP-type crystals have an orthorhombic structure with non- centrosymmetric point group C2v (mm2) and space group Pna2 (Z=8). The crystal framework is a three-dimensional structure made from corner-linked TiO6 octahedra and PO4 tetrahedra. Four oxygen ions of the TiO6 belong to PO4 tetrahedral groups which link the TiO6 groups. In our earlier Raman results, a slight softening was exhibited by several LO and TO vibrational modes of RTA. [1,2] However, there is no typical soft mode observed in the lowfrequency modes of the Raman spectra. This motivated usto carryoutBrillouinscatteringmeasurements tolook for softening in the acoustic modes. We report here both the temperature-dependent acoustic phonon spectra and wavelength- dependent refractive indices. The Cauchy equations [n(λ) = A + B/λ2 + C/λ4] of nx, ny and nz are obtained. In particular, the first direct evidence for acoustic phonon soft mode is presented.Item Origin of low-frequency dielectric permittivity in BiFeO3 multiferroic ceramics(2011) Tu, Chi-Shun; Wang, T.-H.; Schmidt, V. Hugo; Chien, R. R.A one-dimensional conductivity barrier model is introduced to describe the dielectric response and conductivity of BiFeO 3 (BFO) and (Bi 0.95 Nd 0.05 )FeO 3 (BFO-5%Nd) ceramics as functions of temperature and frequency. Good qualitative fits of conductivity and dielectric permittivity in the intermediate-temperature region (500-800 K) are obtained with intrinsic barriers of B=8700 K (for BFO) and B=8400 K (for BFO-5%Nd), and extrinsic barriers of Δ=2500 K (for BFO and BFO-5%Nd). The phase-shifted conductivity is responsible for a step-like dielectric relaxation in the region of 500-800 K. The experimental conductivity departs from the conductivity-barrier-model fit below 650 K. This deviation is likely caused by the magneto-electric coupling near the antiferromagnetic-paramagnetic transition. This work suggests that the 5% mole Nd-substitution can stabilize the perovskite structure in BFO ceramic.Item Raman vibrations and photovoltaic conversion in rare earth doped (Bi 0.93 RE 0.07 )FeO 3 (RE=Dy, Gd, Eu, Sm) ceramics(2016-01) Chang, L.-Y.; Tu, Chi-Shun; Chen, Pin-Yi; Chen, Cheng-Sao; Schmidt, V. Hugo; Wei, H.-H.; Huang, D.-J.; Chan, T.-S.High-resolution Raman spectra, X-ray diffraction, oxygen vacancies, synchrotron X-ray absorption spectroscopy, magnetization, optical band gap, and photovoltaic (PV) conversion have been studied in BiFeO3 (BFO) and (Bi0.93RE0.07)FeO3 (RE=Dy, Gd, Eu and Sm) multiferroic ceramics (7%Dy–BFO, 7%Gd–BFO, 7%Eu–BFO, and 7%Sm–BFO). 7%Dy–BFO exhibits a weak ferromagnetic behavior instead of the linear antiferromagnetic responses found in the other compounds. Optical transmissions reveal band gaps of 2.20–2.21 eV, which are slightly smaller than 2.24 eV in pure BFO. The current vs. voltage (I–V) characteristic curves of indium tin oxide (ITO)/(Bi0.93RE0.07)FeO3 ceramics/Au heterostructures suggest a p–n-junction-like behavior. The maximal PV power-conversion efficiencies under illumination of λ=405 nm in ITO/7%Dy–BFO/Au, ITO/7%Gd–BFO/Au, ITO/7%Eu–BFO/Au, and ITO/7%Sm–BFO/Au respectively reach 0.22%, 0.35%, 0.27%, and 0.24%, which are much larger than 0.017% in ITO/BFO/Au. The PV open-circuit voltage and short-circuit current can be reasonably described by a junction model as a function of illumination intensity.Item Raman vibrations, domain structures, and photovoltaic effects in A-site La-modified BiFeO 3 multiferroic ceramics(2015) Tu, Chi-Shun; Chen, Cheng-Sao; Chen, Pin-Yi; Xu, Zhe-Rui; Idzerda, Yves U.; Schmidt, V. Hugo; Lyu, M.-Q.; Chan, T.-S.; Liu, C.-Y.Micro‐Raman spectroscopy, X‐ray diffraction, high‐resolution transmission electron microscopy (TEM), oxygen vacancies, synchrotron X‐ray absorption spectroscopy, magnetizations, optical band gaps, and photovoltaic (PV) effects have been studied in (Bi1−xLax)FeO3 (BFO100xL) ceramics for x = 0.0, 0.05, 0.10, and 0.15. XRD, Raman spectra, and TEM confirm a rhombohedral R3c symmetry with the tilted FeO6 oxygen octahedra in all compounds. The low‐frequency Raman vibrations become broader and shift toward higher frequency as La3+ increases. Fe K‐edge synchrotron X‐ray absorptions reveal that Fe3+ valence and Fe–O–Fe bond angle are not modified by the La3+ substitution. All compounds exhibit a linear antiferromagnetic feature. Optical transmission reveals band gaps in the range of 2.22–2.24 eV. The heterostructures of indium tin oxide (ITO) film/(Bi1−xLax)FeO3 ceramics/Au film show a p–n junction‐like I–V characteristic behavior. The maximal PV power conversion efficiency can reach 0.19% in ITO/BFO15L/Au under illumination of λ = 405 nm. A junction‐like theoretical model can reasonably describe open‐circuit voltage and short‐circuit current as a function of illumination intensity.Item A-site strontium doping effects on structure, magnetic, and photovoltaic properties of (Bi 1-x Sr x )FeO 3-δ multiferroic ceramics(2015) Tu, Chi-Shun; Xu, Zhe-Rui; Schmidt, V. Hugo; Chan, T.-S.; Chien, R. R.; Son, H.Raman spectroscopy, X-ray diffraction (XRD), magnetization hysteresis loop, synchrotron X-ray absorption spectroscopy, and photovoltaic effects have been measured in (Bi1−xSrx)FeO3−δ (BFO100xSr) ceramics for x=0.0, 0.05, 0.10, and 0.15. Raman spectra and XRD reveal a rhombohedral R3c structure in all compounds. A-site Sr2+ doping increases fluctuations in cation-site occupancy and causes broadening in Raman modes. BFO15Sr exhibits a strong ferromagnetic feature due to reduction of FeOFe bond angle evidenced by the extended synchrotron X-ray absorption fine structure. The heterostructure of indium tin oxide (ITO) film/(Bi1−xSrx)FeO3−δ ceramic/Au film exhibit clear photovoltaic (PV) responses under blue illumination of λ=405 nm. The maximal power-conversion efficiency and external quantum efficiency in ITO/BFO5Sr/Au are about 0.004% and 0.2%, respectively. A model based on optically excited charges in the depletion region between ITO and (Bi1−xSrx)FeO3−δ can well describe open-circuit voltage and short-circuit current as a function of illumination intensity.Item Structural stability and depolarization of manganese-doped (Bi 0.5 Na 0.5 ) 1-x Ba x TiO 3 relaxor ferroelectrics(2014-10) Wang, S.-F.; Tu, Chi-Shun; Chang, T.-L.; Chen, Pin-Yi; Chen, Cheng-Sao; Schmidt, V. Hugo; Anthoniappen, J.This work reveals that 0.5 mol. % manganese (Mn) doping in (Bi0.5Na0.5)1−xBaxTiO3 (x = 0 and 0.075) solid solutions can increase structural thermal stability, depolarization temperature (Td), piezoelectric coefficient (d33), and electromechanical coupling factor (kt). High-resolution X-ray diffraction and transmission electron microscopy reveal coexistence of rhombohedral (R) R3c and tetragonal (T) P4bm phases in (Bi0.5Na0.5)0.925Ba0.075TiO3 (BN7.5BT) and 0.5 mol. % Mn-doped BN7.5BT (BN7.5BT-0.5Mn). (Bi0.5Na0.5)TiO3 (BNT) and BN7.5BT show an R − R + T phase transition, which does not occur in 0.5 mol. % Mn-doped BNT (BNT-0.5Mn) and BN7.5BT-0.5Mn. Dielectric permittivity (ε′) follows the Curie-Weiss equation, ε′ = C/(T − To), above the Burns temperature (TB), below which polar nanoregions begin to develop. The direct piezoelectric coefficient (d33) and electromechanical coupling factor (kt) of BN7.5BT-0.5Mn reach 190 pC/N and 47%.Item Structural and dielectric properties of (Bi 1-x Ba x )(Fe 1-x Ti x )O 3 multiferroic ceramics(2014) Wang, T.-H.; Tu, Chi-Shun; Schmidt, V. Hugo; Chien, R. R.; Ding, Y.Phase transitions, dielectric permittivity, and conductivity of (Bi1-xBax)(Fe1-xTix)O3 (x = 0.05 and 0.1) [BFO-(Ba,Ti)] multiferroic ceramics have been studied as functions of temperature and frequency. In situ synchrotron x-ray diffraction revealed rhombohedral–cubic transitions in the temperature ranges 760–780°C in BFO-5%(Ba,Ti), and 720–750°C in BFO-10%(Ba,Ti). A one-dimensional barrier model with intrinsic barriers B every lattice constant a and extrinsic barriers B + Δ is introduced to describe the dielectric response and conductivity. This work revealed that (Ba,Ti) substitutions can enhance the intrinsic barriers and reduce the hopping rate of charge carriers, thereby giving the desired effect of decreasing the conductivity.Item Enhanced photovoltaic effects in A-site samarium doped BiFeO 3 ceramics: The roles of domain structure and electronic state(2015) Tu, Chi-Shun; Chen, Cheng-Sao; Chen, Pin-Yi; Wei, H.-H.; Schmidt, V. Hugo; Lin, C.-Y.; Anthoniappen, J.; Lee, J.-M.This work reports enhanced photovoltaic (PV) responses of (Bi1 − xSmx)FeO3 (x = 0.0, 0.05, 0.10) ceramics (BFO100xSm) with ITO film under near-ultraviolet irradiation (λ = 405 nm). The ceramics were characterized by micro-Raman scattering, high-resolution transmission electron microscopy, and synchrotron X-ray absorption spectroscopy (XAS). A rhombohedral R3c symmetry with tilted FeO6 octahedra has been confirmed. The Fe K-edge absorption spectra reveal a slight shift toward higher energy as A-site Sm3+ substitution increases. The oxygen K-edge XAS reveals an enhancement of hybridization between the O 2p and unoccupied Fe 3d states due to Sm doping. The optical band gaps are in the range of 2.15–2.24 eV. The maximal PV power-conversion and external quantum efficiencies respectively reach 0.37% and 4.1% in the ITO/BFO5Sm/Au heterostructure. The PV responses can be described quantitatively by a p-n-junction-like model. The domain structures and hybridization between the O 2p and Fe 3d states play important roles for the PV responses.Item Dielectric permittivity and magnetoelectric coupling in multiferroic BiFeO3 and (Bi0.95La0.05)FeO3 ceramics(2011) Tu, Chi-Shun; Ding, Y.; Yang, W.-C.; Wang, T.-H.; Chien, R. R.; Schmidt, V. Hugo; Yao, Y.-D.; Wu, K.-T.Dielectric permittivity and loss of BiFeO 3 (BFO) and 5 mol% lanthanum-substituted BFO [(Bi 0.95 La 0.05 )FeO 3 or BFO-5% La] ceramics have been carried out as functions of temperature and frequency. A frequency-dependent and broad dielectric shoulder and maximum were observed in BFO and BFO-5% La near 600-700 K. These dielectric responses are likely due to the magnetoelectric coupling while the antiferromagnetic-paramagnetic transition takes place near the Néel temperature. As an approximation, a barrier model with intrinsic barriers B (in temperature unit) every lattice constant a and extrinsic barriers B +Δ every distance d is introduced to describe the low-frequency upturn in dielectric loss in the high-temperature region. Good qualitative fits are obtained for BFO and BFO-5% La. This work suggests that 5 mol% La substitution can enhance dielectric response and considerably reduce electric conductivity.Item The origin of photovoltaic responses in BiFeO3 multiferroic ceramics(2012) Tu, Chi-Shun; Hung, C.-M.; Schmidt, V. Hugo; Chien, R. R.; Jiang, M.-D.; Anthoninappen, J.Multiferroic BiFeO3 (BFO) ceramics with electrodes of indium tin oxide (ITO) and Au thin films exhibit significant photovoltaic effects under near-ultraviolet illumination (λ = 405 nm) and show strong dependences on light wavelength, illumination intensity, and sample thickness. The correlation between photovoltaic responses and illumination intensity can be attributed to photo-excited and thermally generated charge carriers in the interface depletion region between BFO ceramic and ITO thin film. A theoretical model is developed to describe the open-circuit photovoltage and short-circuit photocurrent density as a function of illumination intensity. This model can be applied to the photovoltaic effects in p–n junction type BFO thin films and other systems. The BFO ceramic exhibits stronger photovoltaic responses than the ferroelectric Pb1−xLax(ZryTi1−y)1−x/4O3 (PLZT) ceramics under near-ultraviolet illumination. Comparisons are made with other systems and models for the photovoltaic effect.