High Temperature XRD of Phase Transition in Piezoelectric PbNb 2 O 6 across its Curie Temperature

Orthorhombic phase of lead metaniobate (PbNb2O6) is piezoelectric with a high Curie temperature (>570°C) with high potential of wide use in important high temperature applications that cannot be covered by the popular piezoelectric materials based on BT and PZT. Difficulty of preparing it in pure phase hindered full characterization and wide use of this long known piezoelectric material. Here, PbNb2O6 pellets have been prepared in pure orthorhombic phase and first characterized by room temperature TEM and XRD techniques, with a satisfying agreement of the two results. Next, we carried out high temperature x-ray diffraction study of its ferroelectric to paraelectric phase transition on heating across the Curie temperature, and also traced the cooling path. On heating the sample to 590°C and cooling back, there is a significant increase of cell volume.


iNTRODUCTiON
Piezoelectr ic mater ials are finding increasing utilization as sensors, actuators, ultrasonic imaging devices, and in many other commercially profitable applications.For applications in critical areas like Fast Breeder Reactors and exhausts of rockets and vehicles, piezoelectric materials with high Curie temperature 1 are essential.Among the few materials in different stages of development for higher temperature piezoelectric devices, less work has been done on the old but inadequately studied piezoelectric material 2 of lead metaniobate (PbNb 2 O 6 or PN).Room temperature structure of PN can be [2][3][4] either rhombohedral or orthorhombic.Only the latter is ferroelectric with piezoelectric properties, attractive because of attractively high Curie temperature 5 (> 570°C).Fewer studies on PN, in spite of this high Curie temperature, have been basically due to the problem of preparing, by quenching 6 , the piezoelectric form (the meta-stable orthorhombic structure) in pure phase.Subsequently, various aspects have not been re-studied in modern times.We decided to investigate with high temperature (HT) x-ray diffraction (XRD) the structural changes during ferroelectric to paraelectric phase transition on heating across the Curie temperature and trace the cooling path also, of orthorhombic PbNb 2 O 6 , characterized at room temperature by XRD and TEM (Transmission Electron Microscopy).We have optimized preparation steps 3 including the quenching (Q) step, and prepared pure orthorhombic phase of PN, to be called PNOQL, with L denoting the supplier (Loba) of Nb 2 O 5 used in the preparation.

Sample Preparation
Pellets from starting chemicals of PbO and Nb 2 O 5 with 2% extra weight of PbO (to compensate for feared loss of Pb during firing) have been calcined first for 3.5 h 1050°C, and then at 1290°C for 1 h.The 3 rd firing (about 5 h) at 1270°C has branched into quenched (PNOQ or PNOQL) 7 and slow-cooled (PNS) samples.

HT XRD study
Using Cu K a radiation with a 1 = 1.54056Å and a 2 = 1.54439Å, generator voltage = 40 V, tube current = 30 A, scan step size = 0.02, and scan step time = 0.50 s, x-ray powder diffraction has been carried out (Figs. 1-5) at different temperatures on PNOQL sample: 30°C, 550°C, 570°C and 590°C.After soaking at 590°C for about 1hour, the powdered sample has been cooled back, and XRD has been taken again at 30°C.Rietveld refinements 3 have next been done, as depicted in Figs. 1 & 3-5, to extract lattice parameters, given in Table 1, and a pictorial representation (Fig. 2).

TeM examination
Practically the same inter-layer separation (d) (in table 8 2) has been obtained by Transmission Electron Microscopy (Fig. 6, d(TEM)), and by x-ray, d(XRD), for the orthorhombic PbNb 2 O 6 sample at room temperature.

ReSUlT aND DiSCUSSiON
The temperature variation of lattice parameters (a, b and c) of the orthorhombic PNOQL sample, from room temperature (RT) to 590°C and return to RT, has been shown graphically in    12 techniques showed the Curie temperature to be above 570°C.
Below Curie temperature, a < b < c, and the sample is ferroelectric with piezoelectric properties.
Loss of piezoelectric properties 11,12 above this temperature is accompanied by increase of a, and decrease of b, to make a = b, as indicated in our (b-a) vs. T plot.Unit cell volume is seen to increase rapidly with temperature in the orthorhombic phase, and only slightly above Curie temperature.It is a new observation that room temperature cell volume increases on heating the sample to 590°C and cooling back to room temperature.

Fig. 7 .
This matches well with the original work of Francombe and Lewis 9 .As the temperature is raised from 30°C to ~ 550°C, the lattice parameter 'a', indicated by open circle, increases (from 17.6515 Å to 17.7587 Å) and the lattice parameter 'b', indicated

Fig. 1 Fig. 2 :Fig. 3 :Fig. 4 :Fig. 6 :Fig. 5 :Fig. 7 :Fig. 8 :Fig. 9 :
Fig. 1 Open circle symbols (black), giving XRD data for PNOQl at 30°C, fit closely the Rietveld refined plot (the continuous red curve in the main graph).Their difference is shown by the continuous curve in the smaller-sized graph (green in color).The vertical bars (blue color) in the main graph are the calculated peak positions.