Lead niobate

The spaces between the MOe (M=W, Ta, Nb, Ti etc.) form tunnels. In the section shown in Fig. 6.16 the largest tunnels, A2, have five O2 ions round their periphery. The somewhat smaller tunnels, Al, have four peripheral O2 ions, while the smallest, A3, have three. Lower-valency cations are located in sites in these tunnels above and below the plane shown. The MOe groups provide two distinguishable B sites shown as B1 and B2. The general formula for a tungsten bronze is 

The A3 sites are vacant unless small (r6=50-70pm) ions such as Li+ or Mg2+ are present. A large number of compounds are known in which one in six of the Al and A2 sites is also vacant, giving rise to the simpler formula AB206 of which PbNb206 is an example. 

In the majority of ferroelectric tungsten bronzes the polar axes are parallel to the A-site tunnels. PbNb206 is an exception below the Curie point it undergoes an orthorhombic distortion in the plane perpendicular to the A-site tunnels as indicated in Fig. 6.16. It possesses four possible polar directions and can be poled successfully in ceramic form. 

Although it is difficult to obtain a ceramic with less than 7% porosity and the piezoelectric coefficients are smaller than those of BaTi03, PbNb206 has advantages in certain applications. Its Qm value is approximately 11, which is 

The substitution of lead by barium enhances the piezoelectric properties which peak when Pb/Ba I. At this and higher barium contents the structure changes to tetragonal with the polar axis parallel to the A-site tunnels. There is a morphotropic boundary, similar to that found in PZT compositions, and peak values of piezoelectric properties are found near the Pb1/2Ba1/2Nb206 composition. d33 rises to 220 pCN-1 and d31 rises to — 90pCN 1, while Qm increases to 300 and the Curie point falls to 250 °C thus most of the features peculiar to 

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The Nb NMR spectra of a number of alkali and lead niobates have been acquired using MAS, DAS, MQMAS and two-dimensional nutation NMR to improve the resolution and determine values of the quadrupolar products Pq for these materials (Prasad et al. 2001). The 9.4 T Nb MAS NMR spectrum of LiNbOs spun at 25 kHz (Figure 10.16A) shows a lineshape dominated by the second-order quadrupolar interaction, with only marginal improvement in resolution at a field of 14.1 T. For most of the metal niobates, the second-order interaction is not removed by MAS alone, even at high magnetic fields and fast spinning speeds. The expected improvement in Nb resolution provided by DAS spectroscopy is offset by significant homonuclear... 

Detail studies on the reaction sequences in the Pb0-Fe203-Nb205-W03 and PMN systems [27-30] suggested that the perovskite phase was formed through a sequence of reactions involving several lead-niobate pyrochlores. From extensive differential thermal analysis and XRD analysis, Inada [30] proposed the following reactions in PMN ... 

The pure PbMgo.33Nbo.67O3 perovskite phase was crystallized at 1253 K from xerogel obtained by cogelation of Pb-acetate, Mg- and Nb-ethoxides in methoxyethanol [69]. Its purity was controlled by addition of excess Pb and Mg components that prevented formation of the lead niobate pyrochlore phase. 

Lead Niobate. Pb(Nb03)2 a ferroelectric compound having properties that make it useful in high-temperature transducers and in sensing devices. The Curie temperature is 570 = C. 

Geusic J.E., Kurts S.K., Van Uitert L.G., Wemple S.H. Electrooptic properties of some ABO3 perovskite in the paraelectric phase. Appl. Phys. Lett. 1964 4 141-143 Giess E.A., Scott B.A., Burns G., O kane D.F., Segmuller A. Alkali strontiirm-barium-lead niobate systems with a tungsten bronze structure Crystallographic properties and Curie points. J. Am. Ceram. Soc. 1969 52 276-281... 

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