Calorimetric investigations of overcritical behaviour in (TEA)2CuCl4 crystals

The results of experimental investigation of molar heat capacity of [N(C₂H₅)₄]₂CuCl₄ - ferroelasto-ferroelectric crystals in a wide temperature range are presented. The typical anomaly for the first order phase transition of order-disorder type at 258 K and additional anomaly with maximum at 195 K has been observed. We suggest that the low temperature anomalies of physical properties of [N(C₂H₅)₄]₂CuCl₄ crystals can be attributed to over critical traces of isomorphous phase transitions (similar to that observed in liquid-gas system). Results of numerical analysis of phenomenological model of phase transition in [N(C₂H₅)₄]₂CuCl₄ crystals are presented and compared with the experimentally determined temperature dependence of the specific heat and entropy changes.     

Coexistence of polar and non-polar phases in ND4DSeO4 crystals

The temperature variation of the dielectric constant has been measured in ND₄DSeO₄ with over 90 mole percent of deuterium content. The grown crystals having P2₁2₁2₁ symmetry at room temperature, after being heated above 367 K, exhibit irreversible phase transition to I2 symmetry. At room temperature the phase I2 is not stable unlike for the crystal with lower degree of deuteration and the crystal returns to P2₁2₁2₁ phase during about 60 hours. A t 275 K occurs the phase transition to I1 but only from I2 phase.     

Dielectric and Ferroelectric Properties of [(CH3)2NH2]3CuCl5 Single Crystal

The detailed studies of dielectric properties of [(CH₃)₂NH₂]₃CuCl₅ single crystal were performed. Distinct anomalies of permittivity along three crystallographic direction were observed at 277.5 K and 252.7 K on cooling. The observed permittivity behaviour evidenced the first order character of both phase transitions. The hysteresis loops characteristic of a ferroelectric phase were observed in the range of temperature 277.5-252.7 K. An appearance and disappearance of spontaneous polarization confirms the first order phase transitions in this crystal. The permittivity measured along the ferroelectric a-axis under influence of external electric field in paraelectric phase showed clearly a nonlinear increase with increase of electric field intensity applied to the crystalline sample.     

Dieletric and relaxation behaviors of (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal

Dielectric permittivity along the [111] direction has been measured as a function of temperature for a relaxor ferroelectric single crystal (PbMg_1/3Nb_2/3O₃)_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 ε'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.     

A new low-temperature Tcr=101 K phase transition in (NH4)3H(SeO4)2 evidenced by single crystal ESR

Temperature ESR measurements on SeO₃ - radicals in (NH₄)₃H(SeO₄)₂ single crystals disclosed new ESR anomalies at T = 101 K. Below this temperature the hyperfine splitting becomes to be temperature averaged, ESR linewidth shows motional narrowing and an anomaly in forbidden transition intensity is observed. Also the dielectric measurements show an anomaly of the ε at this temperature. We suggest an existence of a new crystal phase VI below T_cr = 101 K with NH₄-groups ordering and slow SeO₄-group dynamics.     

Dielectric indications on the incommensurate - commensurate phase transition in NH4HSeO4 at 250 K

The precise dielectric measurements of NH₄HSeO₄ were performed in vicinity of phase transition at 250 K. The existence of the thermal hysteresis of dielectric permittivity, typical for incomensurate-commensurate phase transition was found.     

Isotope effect in the phase transition of partially d euterated T1H2As 04

The complex dielectric constant of Tl(H_1-X D_X)₂AsO₄ powder pellets has been measured from 100 to 450 K. It is found that the I11 ← 11 phase transition point (T_L) shifts from 251 K(for hydrogenated crystal) to about 340 K(for partially deuterated crystal), while the I1 ← I phase transition point(T_U) goes from about 390 K to 433 K. The order of the I11 ← I1 phase transition changes from a weakly first-order to a second- order, while that of the I1 ← I phase transition from a second-order to a first-order. From the experimental results, the tentative phase diagrams of Tl(H_1-X D_X)₂AsO₄, and Tl(H_1-X D_X)₂PO₄ are proposed.     

(CH3) 4NHgBrI2—A new polar organic-inorganic double halide

(CH₃)₄NHgBrI₂ exhibits at room temperature a polar columnar structure based on edge-sharing anion tetrahedra with a double anion occupancy. Its orthorhombic space group is Pb2_lm. An unusual multiple phase transition sequence revealed by differential scanning calorimetry, dilatometry and permittivity vs. temperature measurements leads to a non-polar high temperature phase with a trigonal mercury coordination and the monoclinic space group P2_l/n.     

Ferroelectricity and phase transitions in highly deuterated NH4HSeO4 crystals

Investigations of dielectric and pyroelectric properties as well as domain structure of 70% deuterated ammonium hydrogen selenate crystals have been carried out. Physical properties of 70% dauterated crystals grown in 12 phase differ considerably from that grown in P2₁2₁2₁ phase. Dielectric anomaly is observed that the paraelectric-incommensurate phase transition.     

Ultrasonic investigations of [(CH3)2NH2]5Cd3Cl11

The temperature dependences of ultrasound velocities along the crystallographic a, b and c axis in [(CH₃)₂NH₂]₅Cd₃Cl₁₁ were studied in the vicinity of phase transition at 178.5 K. The results obtained for the longitudinal and shear ultrasonic velocities confirmed clearly the existence of the second-order phase transition at this temperature and the quadratic relation between δV₄ and δV₆ and phase transition parameter was observed.     

Phase transition in Sol-Gel processed barium titanate ceramics

Ferroelectric barium titanate (BaTiO₃) ceramics have been prepared from barium acetate [Ba(CH₃COO)₂] and titanium isopropoxide [Ti(CH₃)₂CHO)₄] precursors by sol-gel technique The as-grown powder was found to be amorphous which crystallized to tetragonal phase after annealing at 700°C in air for one hour. The ceramics showed well saturated polarization-field (P-E) hysteresis loops at room temperature. The values of spontaneous polarization(P_s), remanent polarization (P_r) and coercive field (E_c) of the ceramics were found to be19.0, 12.66μC-cm^-2 and 30KVcm^-1 respectively. The room temperature dielectric constant (ε) and loss tangent (tanδ) of the ceramics were found to be 1135 and 0.012 respectively. The dielectric constant and tanδ showed anomaly peaks at 125°C showing ferroelectric to paraelectric phase transition. The polarization parameters also vanish at 125°C confirming the phase transition.     

Uniaxial stress dependence of the elastic wave propagation near normal—incommensurate phase transition in (NH4)2ZnCl4

The effect of uniaxial σ₂ stress on the velocity and attenuation of longitudinal ultrasonic wave propagating along the [001] direction in an NH₄)₂ZnCl₄ crystal was studied in the vicinity of the orthorhombic — incommensurate phase transition. A stress induced shift of the transition temperature is dT₁/dσ₂= 0.2 K/MPa. Application of stress greater that 5 MPa results in the appearance of additional anomalies of velocity and attenuation, which are interpreted as manifestations of a new phase induced by uniaxial Stress.     

Phase transitions in R2 SnCl6-type crystals (R = NH3 C2H3, NH3C2H5 and N(CH3)4)

The thermal dilatation in (NH₃ ·CH₃) SnCl₆, (NH₃ · C₂H₅) SnCl₆ and [N(CH₃)] SnCl₆ was measured, and as the results it has turned out that (NH₃ ⁶·C₂H₅) SnCl₆ and [N(CH₃)₄]₂ SnCl₆ undergo the first order transitions at 128 K and 158 K, respectively. The low temperature phases of (NH · C₂H₅) SnCl₆ and [N(CH₃)₄]₂ SnC1₆ are found to be monoclinic and tetragonal, respectively, No phase transition was observed in (NH₃ ·CH₃)₂ SnCl₆ down to 77 K.     

Raman spectra of (NH4)4H2(SeO4)3 superionic single crystal

Raman spectra of (NH₄)₄H₂(SeO₄)₃ protonic conductor were studied at room temperature using a 1064 nm - excited Fourier transform (FT) - Raman spectroscopy. A tentative assignment of the external and internal vibrations is proposed.     

Optical second harmonic study of the ieicommensuirate phase in K2SeO4 and (NH4)2BeF4 crystals

Second harmonic generation was studied in the region of phase transition in K₂SeO₄ and (NH₄)₂BeF₄. Symmetry-forbidden harmonic generation was observed in the incommensurate phases near T_c. The experimental results are explained under assumption that the unipolar domain-like structure with C_2v symmetry appears in the incommensurate phase.     

Ferroelectric Ceramics in the PbMg1/3Nb2/3O3-PbCd1/3Nb2/3O3 System

Solid solutions (1-x)PbMg_1/3Nb_2/3O₃ + xPbCd_1/3Nb_2/3O₃ with x = 0-0.30 are investigated with purpose to work out a capacitor ceramics with good dielectric properties and low sintering temperature. It is found that the perovskite phase forms at sintering near to 980°C and begins to decompose at higher temperatures. When x grows from 0 to 0.30, the Curie temperature linearly grows from -10°C to +25°C, the dielectric permittivity ε_m in the Curie point T_C decreases from 18000 to 6800 and the phase transition becomes more diffused. The dielectric permittivity at room temperature is rather high and the temperature stability is improved. The system is of interest, because it can serve as a base for working out some ceramic materials for capacitors with low sintering temperature, which needs of no special atmosphere at burning.     

钴离子取代对Ca5Mg4–xCox(VO4)6陶瓷微波介电性能的影响

采用固相反应法制备低温烧结Ca₅Mg₄(VO₄)₆微波介质陶瓷,研究了Co掺杂取代Mg对微波介电性能的影响。结果表明:Co取代后样品形成了固溶体,导致烧结温度降低。当相对密度较低时,致密度主导着εr的变化;当相对密度较高时,εr主要受单位体积离子极化率的影响。此外,Co取代使得晶粒尺寸减小,进而促进了Q×f值的提升。由于τf与键能有着密切的联系,而V—O键的键能最大,τf随着V—O键键能的变化而改变。Ca5Mg4(VO₄)₆具有最佳的微波介电性能:εr=9.65,Q×f=54 685 GHz,τf=–55.3×10^–6℃^–1。     

Sintering and dielectric properties of Cu2Ta4O12 ceramics

In this work, Cu₂Ta₄O₁₂ ceramic was investigated as a promising, lead-free, nonferroelectric material with high dielectric permittivity. The results of impedance spectroscopy studies carried out at frequencies 10 Hz to 2 MHz over a wide temperature range from −55 to 700 °C were analyzed in the impedance, dielectric permittivity and electric modulus formalisms. In complex impedance plots two distinct arcs were distinguished, ascribed to the semiconducting grains and to the insulating grain boundaries. Cu₂Ta₄O₁₂ ceramic was found to exhibit a high dielectric permittivity exceeding 10,000 at low frequencies in the temperature range 150–740 °C. High permittivity of this material was attributed to the formation of internal (grain boundary) barrier layer capacitors. The influence of sintering conditions on microstructure, composition and dielectric properties of Cu₂Ta₄O₁₂ ceramics was also studied.     

Microwave dielectric dispersion in RbD2PO4 crystals

The paper presents the results of the investigation of the dielectric dispersion in RbD₂PO₄ in the vicinity of the low-temperature ferroelectric phase transition. It is shown that the dielectric dispersion of Debye type along the b-axis in frequency range 50 MHz to 50 GHz is caused by the single relaxational soft mode which at T_c2=324 K softens to 0.4 GHz. The results confirm the quasi-one-dimensional nature and the order-disorder type of the ferroelectric phase transition.     

Dielectric Behavior and Cofiring with Silver of Monoclinic BiSbO4 Ceramic

Pure monoclinic phase of BiSbO₄ ceramic was synthesized using the solid-state reaction method. BiSbO₄ ceramics can be well sintered between 960° and 1080°C. The dielectric constant of BiSbO₄ ceramic was about 20.1 and dielectric loss was about 0.055–2.5 × 10⁻⁴ at 1 MHz. With the measured frequencies increasing from 100 to 900 kHz, the temperature coefficients of permittivity decreased from +224.1 to +95.9 ppm/°C. At the microwave range, the best microwave dielectric properties were obtained in the ceramic sintered at 1080°C/2 h with a permittivity of 19.3, a Q _f value of about 70 000 GHz, and a temperature coefficient of resonant frequency of −62 ppm/°C. Cofiring between BiSbO₄ ceramic and 20 wt% Ag was also investigated, and it was found that after cofiring at 900°C for 5 h only very little unknown phase containing a little amount of Ag was formed and most Ag was scattered in grain boundaries.