College of Letters & Science
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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 Temperature dependent Raman spectra of Rb1-x(ND4)xD2AsO4 mixed crystals(1998) Tu, Chi-Shun; Gao, S.-S.; Jaw, R.-J.; Hwa, L.-G.; Schmidt, V. Hugo; Brandt, Dan; Chien, R.-M.In the mixed ferroelectric (FE)-antiferroelectric (AFE) systemA1−x(ND4)xD2BO4 [A=Rb(orK,Cs)andB=As (or P)], there is competition between the FE and the AFE orderings, each characterized by specific configurations of the acid deuterons. [1–8] The random distribution of the Rb and ND4 ions is the main source to produce frustration which can increase local structural competition such that the long-range order of electric dipole disappears. Instead of a typical sharp FE or AFE phase transition, the phase coexistence becomes a characteristic in this type of mixed compounds. By a group theoretical analysis for the KDP-type structure (which contains two molecular units in a primitive unit cell); at zero wavevector, the vibrational modes in the tetragonal symmetry (space group I¯ 42d − D12 2d) can be decomposed into the following irreducible representations: Γvib = 4A1(R) + 5A2(Silent) + 6B1(R) + 6B2(R,IR) + 12E(R,IR). [9] The symmetry species A1, B1, B2 and E are Raman active. The situation in the mixed system D*RADA-x is more complicated than one in the parent crystals, because some Rb (or ND4) ions have been substituted by ND4 (or Rb) ions. In this case, the selection rule of the free AsO4 group is expected to be broken much easily than in the pure crystal. In the recent years, many measurements in D*RADAx system have been achieved on ferroelectric-side crystals x = 0.1, 0.10 and 0.28. [2–5] However, only a few experiments were done on antiferroelectric-side compounds (x ≥0.50).[6,7]A complete understanding for this mixed system is still lacking. This motivated us to carry out the polarized Raman scattering on D*RADA-0.55, 0.69 and 1.0. Here, we pay special attention to the stretching mode ν1 (near 755 cm−1) and the in-plane bending mode δ(O-D) (near 825 cm−1).Item Electric-field poling effect on thermal stability of monoclinic phase in Pb(Mg1/3Nb2/3)0.74Ti0.26O3 single crystal(2006) Chien, R. R.; Schmidt, V. Hugo; Tu, Chi-ShunPhases and domains in a (1 1 0)-cut Pb(Mg1/3Nb2/3)0.74Ti0.26O3 (PMNT26%) single crystal have been investigated as functions of temperature and direct current (DC) electric (E) field by dielectric permittivity, polarizing microscopy, and electric polarization. The unpoled sample has a dominant rhombohedral (R) phase coexisting with monoclinic (M) phase domains, i.e. R/M at room temperature (RT). With 45 kV/cm DC poling applied along [1 1 0] at RT, a single domain of R phase with polar orientation perpendicular to the poling field, i.e. R, was obtained. No microcracking was observed under such high DC field poling. After the poling was removed, the poled sample has R/M microdomains, where the M distortion is close to the R phase. The zero-field-heating domain patterns in the unpoled and poled samples exhibit continuous polarization rotation via an intrinsic M phase in the regions of 355–373 and 365–378 K, respectively. Orthohombic (O) and tetragonal (T) phases were not observed in the temperature-dependent study. The whole crystal becomes cubic (C) phase near 393 and 399 K in the unpoled and poled sample, respectively. In brief, an R/M→M→C transition sequence takes place upon heating for both unpoled and poled samples.Item Direct observation of ferroelectric domains and phases in (001)-cut Pb(Mg1/3Nb2/3)1-xTixO3 single crystals(2006) Chien, R. R.; Schmidt, V. Hugo; Tu, Chi-Shun; Wang, F.-T.Real-time direct observation of ferroelectric domains and phases under electric-field poling along [0 0 1] at room temperature in Pb(Mg1/3Nb2/3)0.67Ti0.33O3 (PMNT33%) single crystal has been performed by polarizing microscopy. A hysteresis loop of polarization vs. electric field at room temperature was also measured for comparison. By using relations of crystallographic symmetry and optical extinction, polarizing microscopy reveals orientations of the domain polarizations and their corresponding phases. It also provides direct real-time observation of microcracking phenomena. It was found that the monoclinic phase domains play a crucial role in bridging higher symmetry (tetragonal and rhombohedral) phases while field-induced phase transitions take place.Item Electric-field-induced and temperature‑induced phase transitions in high-strain ferroelectric Pb(Mg1/3Nb2/3)0.67Ti0.33O3 single crystal(2006) Chien, R. R.; Tu, Chi-Shun; Schmidt, V. Hugo; Wang, F.-T.This work is to study electric (E)-field-induced and temperature-induced phase transitions in (001)-cut Pb(Mg1/3Nb2/3)0.67Ti0.33O3 (PMNT33%) single crystal, which are critical concerns for piezoelectric applications. Dielectric properties and domain structures (by polarizing microscope) are measured as functions of temperature and E field. The hysteresis loop of the polarization versus E field at room temperature is also measured. Without any E-field application, upon heating a first-order-type phase transition sequence rhombohedral (R) → rhombohedral/monoclinic/[001]tetragonal (R/M/T001) → cubic (C) takes place near 350 and 430 K, respectively. Under a dc E-field application along [001] at room temperature, [001] tetragonal (T001) phase domains are induced by various phase transition sequences, i.e. R → T001,R → M → T001, R → T → T001,andR→ M → T → T001,asthe E-field strength increases. In addition, E-field-induced microcracking is observed in this work.Item Phase transitions and domain structures in relaxor-based ferroelectric (PbZn1/3Nb2/3O3)0.915(PbTiO3)0.085 single crystal(2001) Tu, Chi-Shun; Schmidt, V. Hugo; Shih, I.-C.Polarization-electric field (P-E) hysteresis loops and domain structures have been measured as a function of temperature in relaxor-based ferroelectric single crystal (PbZn,/3Nb2/303)0.9~j(PbTi03)o.o~s (PZN-8.5%PT). In order of increasing temperature, PZN- 8.5%PT undergoes successive phase transitions: rhombohedra1 phase (below -375 K) + coex- istence of rhombohedral and tetragonal phases (between -375 and -390 K) --f tetragonal phase (between -390 and -420 K) -+ coexistence of tetragonal and cubic phases (between -420 and -460 K) -+ cubic phase (above -460 K). Phase coexistence suggests an inhomogeneous distribution of Ti4+ concentration in the PZN-8.5%PT crystal.Item Phases and domain structures in relaxor-based ferroelectric (PbMg1/3Nb2/3O3)0.69(PbTiO3)0.31 single crystal(2001) Tu, Chi-Shun; Chen, L. -F.; Schmidt, V. Hugo; Tsai, Chih-LongThe Brillouin back-scattering spectra, dielectric permittivities, polarization–electric field (P–E) hysteresis loops and domain structures have been measured as a function of temperature in a relaxor-based ferroelectric single crystal (PbMg1/3Nb2/3O3)0.69(PbTiO3)0.31 (PMN–31%PT). In order of increasing temperature, PMN–31%PT undergoes succes-sive phase transitions: rhombohedral phase (below ∼ 370 K) → coexistence of rhombohedral and tetragonal phases (between ∼ 370 and ∼ 380 K) → tetragonal phase (between ∼ 380 and ∼ 400 K) → coexistence of tetragonal and cubic phases (betwee ∼ 400 and ∼ 420 K) → cubic phase (above ∼ 420 K). An extra ferroelectric anomaly of the dielectric per-mittivity appears at 370 K possibly due to the percolating polar cluster induced by an external electric field. It was found that different individual domain regions have different transition temperatures. This phenomenon suggests an inhomogeneous dis-tribution of Ti4+ concentration in the PMN–31%PT crystal. The dielectric permittivity ε' of PMN–31%PT obeys the empirical relation, ε'm/εγ ( f, T ) = 1 +[T − Tm( f )]'/2δγ2 , above the temperature of permittivity maximum Tm.Item Dielectric and relaxation behaviors of (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal(2006) Tu, Chi-Shun; Schmidt, V. Hugo; Luo, H.; Chao, F.-C.Dielectric permittivity along the [111] direction has been measured as a function of temperature for a relaxor ferroelectric single crystal (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 (PMN-33%PT). A sharp ferroelectric phase transition was observed near 425 K and 429 K for cooling and heating processes, respectively. As temperature decreases, a diffuse phase transition (which begins near 330 K upon cooling) was detected. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 330 and 425 K upon cooling) are diffuse first-order and first-order, respectively. The frequency-dependent dielectric data ∊′111 (ƒ, T) prove the existence of an electric dipolar relaxation process between 350 and 400 K. The activation energy, the Vogel-Fulcher temperature and attempt frequency corresponding to this relaxation process are also calculated.Item Nanotwin and phase transformation in tetragonal Pb(Fe1/2Nb1/2)1-xTixO3 single crystal(2008) Tu, Chi-Shun; Tseng, C.-T.; Chien, R.R.; Schmidt, V. Hugo; Hsieh, C.-M.This work is a study of phase transformation in (001)-cut Pb(Fe1/2Nb1/2)1−xTixO3Pb(Fe1/2Nb1/2)1−xTixO3 (x=48%)(x=48%) single crystals by means of dielectric permittivity, domain structure, and in situ x-ray diffraction. A first-order T(TNT)-C(TNT)T(TNT)-C(TNT) phase transition was observed at the Curie temperature TC≅518KTC≅518 K upon zero-field heating. T, TNTTNT, and C are tetragonal, tetragonal nanotwin, and cubic phases, respectively. T(TNT)T(TNT) and C(TNT)C(TNT) indicate that minor TNTTNT domains reside in the T and C matrices. Nanotwins, which can cause broad diffraction peak, remain above TC≅518KTC≅518 K and give an average microscopic cubic symmetry in the polarizing microscopy. Colossal dielectric permittivity (>104)(>104) was observed above room temperature with strong frequency dispersion. This study suggests that nanotwins can play an important role in relaxor ferroelectric crystals while phase transition takes place. The Fe ion is a potential candidate as a BB-site dopant for enhancing dielectric permittivity.Item Dielectric permittivity in paraelectric/ferroelectric coexistence region in several proton glasses(1995) Howell, Francis L.; Fundaun, I. L.; Stadler, S.; Meschia, Steven C. L.; Tu, Chi-Shun; Schmidt, V. HugoWe present results for the complex permittivities of several proton glasses of the form M/sub 1-x/(NZ/sub 4/)/sub x/Z/sub 2/AO/sub 4/, where M=Rb, Z=H or D, and A=As or P. All measurements were made perpendicular to the ferroelectric axis, so no effects of domain wall motion occurred. Phase coexistence was apparent in all species. The phosphate glasses exhibited a much narrower coexistence composition range than the arsenate glasses.