G. Calvarin

Monoclinic structure of unpoled morphotropic high piezoelectric PMN-PT and PZN-PT compounds

Evidences of a monoclinic phase in unpoled (PbMg1/3Nb2/3O3)1-x-(PbTiO3)x with x=0.35 (PMN-PT 35%) and unpoled (PbZn1/3Nb2/3O3)1-x-(PbTiO3)x with x=0.09 (PZN-PT 9%) are presented from a neutron Rietveld analysis. This monoclinic phase is different from the phase recently evidenced by Noheda and coworkers in PbZr1-xTixO3 with x=0.481 but is identical to the phase observed in poled PZN-PT with x=0.102 by the same authors. The structural resolutions allowed us to compare both structures and to deduce the direction and magnitude of polarization. In PMN-PT 35% and PZN-PT 9% this phase is characterised by a weak value of polarization, a strong deformation of oxygen polyhedra and weak cationic shifts.

Interlayer Interactions in M(OH)2: A Neutron Diffraction Study of Mg(OH) 2

Magnesium hydroxide, Mg(OH) 2 : M r = 58.3, trigonal, P3m1, a = 3.148(1), c = 4.779(2)A, V = 41.015 A 3 , D x = 2.36 g cm -3 at room temperature ; a = 3.145 (1), c = 4.740 (2) I, V = 40.6021 3 , D x = 2.39 g cm -3 at 70 K ; Z = 1, λ = 0.8330(5) A, v = 1.78 cm -1 , F(000) = 9.503 fm ; final R = 2.42, wR = 2.40, S = 3.22 for 82 unique reflections at room temperature ; R = 1.84, wR = 1.82, S = 2.54 for 81 unique reflections at 70 K. Refinements have been carried out using anisotropic thermal coefficients for all atoms. To interpret the very large thermal motion of the H atom, subsequent refinements have been carried out with an anharmonic model and with a three-site split-atom model, and results are compared with those previously reported for Ca(OH) 2 . By comparing the interatomic distances O...O, H...H and H...O in the interlayer spacing of Mg(OH) 2 and Ca(OH) 2 , as well as their temperature dependence, the nature and the strength of interlayer interactions in both compounds are discussed.

X-ray study of the electric field-induced phase transition in single crystal Pb(Mg1/3Nb2/3)O3

High-resolution X-ray diffraction experiments were performed on a single crystal of PMN in the temperature range of 90-300 K, by applying an electric field (up to E = 3.0 kV cm-1) along the [111] direction. A first order electric field-induced phase transition, from the mean cubic phase to a rhombohedral Phase, was evidenced by discontinuous variation of the d222-spacing upon cooling with E ≥ 1.8 kV cm-1. Results suggest that a tricritical point exists in the (E, Tc) phase diagram of PMN for E ≥ 3.8 kV cm-1. Upon field heating no phase coexistence was detected, unlike the field cooling runs, and the rhombohedral-cubic phase transition appears to be of second order at least for E = 3.0 kV cm-1.

Hydrogen thermal motion in calcium hydroxide: Ca(OH)2

Calcium hydroxide, Ca(OH) 2 , M r =74, trigonal, P3m1, a=3.589 (8), c=4.911 (14) A, V=54.8 (4) A 3 , D x =2.26 cm -1 at room temperature, Z=1, λ=0.8330 (5) A, μ=1.195 cm -1 , F(000)=9.028 fm, final R=0.018, wR=0.019, for 105 unique reflections, at 80 K; R=0.025, wR=0.027, for 211 unique reflections, at room temperature. The structure of calcium hydroxide has been refined using neutron data collected at 80 K and at room temperature. Thermal motion of the H atom has been refined with anharmonic thermal-motion parameters at room temperature and at 80 K

Structural study of a poled PbMg13Nb23O3 ceramic at low temperature

Abstract The evolution of the structure of a poled PbMg 1 3 Nb 2 3 O 3 ceramic is studied by precise X-ray diffraction, between 300 and 5 K. The application of a high electric field at low temperature induces a macrodomain ferroelectric rhombohedral structure ( a = 4.040 A , α = 89.91°). By increasing the temperature, this structure disappears at T t # 200 K and it is transformed into the polar nanodomain structure observed when an unpoled ceramic is cooled. This transformation is irreversible.

Origin of the Giant Piezoelectric Properties in the [001] Domain Engineered Relaxor Single Crystals

Relaxor single crystals PZN–9%PT have been cut and poled along [101] direction for which the spontaneous polarization Ps is approximately parallel to the poling field Ep. The piezoelectric matrix of the monoclinic single-domain (1M state) has been determined, in the approximation of an orthorhombic symmetry. The shear mode (15) along [101], gives the highest electromechanical coupling factor k15 (>80%) and the largest piezoelectric coefficient d15 (~3200 pC/N). The properties of the disoriented 1M state have been calculated from a change of axes. The maximum of d33θ is obtained along a direction close to [001]. This is due to the very large value of d15 compared to d33 in the basic 1M state. On the other hand the transverse piezoelectric coefficient along [uv0] for the [001] disoriented Ps single domain presents a strong anisotropy. Finally, in the [001] domain engineered configuration, an important extrinsic contribution of the domain coexistence is evidenced by comparing calculated and measured coefficients.

New low‐temperature crystalline phase of ferrocene: Isomorphous to orthorhombic ruthenocene

A new low‐temperature phase of ferrocene Fe (C5H5)2, stable below 250 K, has been characterized by means of x‐ray diffraction on powder and differential scanning calorimetry experiments. Lattice of this phase is orthorhombic and its molecular packing is isomorphous to ruthenocence one, so configuration of ferrocene molecules is eclipsed (D5h symmetry). Experimental conditions to obtain this orthorhombic phase are discussed in connection with crystallite size.

Experimental evidence for a spontaneous relaxor to ferroelectric phase transition in Pb(Mg13Nb23)O3 - 10% Ti

Abstract From X-ray diffraction, polarization and dielectric measurements, it is shown that a 0.9 Pb(Mg 1 3 Nb 2 3 )O 3 - 0.1 PbTiO3 ceramic undergoes a zero-field rhombohedral phase transition. This material transforms spontaneously from a relaxor into a normal ferroelectric upon cooling. On such an observation, the influence of titanium on Pb(Mg 1 3 Nb 2 3 )O 3 based ceramics is discussed.

Domain Structures in Monoclinic Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 Poled Single Crystals

A tetragonal (T) ?monoclinic (M) phase transition characterized by a wide thermal hysteresis is observed in Pb(Zn1/3Nb2/3)O3?PbTiO3 (PZN?PT) single crystals close to the morphotropic composition PZN?9%PT. The domain structure of crystals, determined by X-ray diffraction and optical observations, is dependent on the poling crystallographic direction. A monoclinic quasi-single domain structure is obtained by poling along the pseudocubic [101] direction whereas an unexpected monoclinic multidomain state with macroscopic 2 mm symmetry can be obtained for [001] poled crystals. Finally it is shown that the largest piezoelectric response corresponds to the monoclinic multidomain state of [001] poled crystals.

Dielectric and crystallographic study of the lead magnotantalate relaxor

Abstract The complex perovskite Pb(Mg 1/3 Ta 2/3 )O 3 is one of the first relaxor ferroelectric compounds that was investigated. The present study reports some new important data of dielectric properties and structure characteristics. A Vogel-Fulcher behavior is observed for the frequency dependence of T max with a freezing temperature T f =144K. Dielectric measurements under high electric field show that the dielectric constant decreases as the field increases, but T max remains unchanged. The x-ray crystallographic study reveals a cubic structure down to liquid nitrogen temperature. The temperature dependence of the cell parameter is determined and the cluster nucleation temperature is thus estimated to be about 573K. The difference in relaxor behavior between Pb(Mg 1/3 Ta 2/3 )O 3 and Pb(Mg 1/3 Nb 2/3 O 3 is discussed.