High resolution x ray diffraction study of phase and domain structures and thermally induced phase transformations in PZN (4 5 9)%PT 2

Chapter 5 Surface Layer in Relaxor Ferroelectric PZN-PT Single Crystals 5.1 Introduction An extremely broad lower 2θ peak adjacent to the major 002 R peak has been frequently reported

Chapter 5 Surface Layer in Relaxor Ferroelectric PZN-PT Single Crystals

5.1 Introduction

An extremely broad lower 2θ peak adjacent to the major (002) R peak has

been frequently reported in standard and high-resolution XRD profiles of mechanically

polished bulk PZN-PT single crystals Lim et al [63] attributed this to the presence of

trapped metastable phases in the crystal Meanwhile, Ohwada et al [56] and Xu et al

[57-59] reported a dual structure in PZN-PT single crystals According to the latter

authors, the lower 2θ diffraction peak indicates a rhombohedrally distorted structure of

10 to 50 µm thick surface layer The bulk structure, in contrast, was found to have an

average C lattice, yet the true symmetry is undetermined Hence, the undetermined

bulk structure is termed the “X”-phase by these authors Recently, a detailed study in

search of the “X”-phase has been performed by Kisi and Forrester [60-62] The

“X”-phase, however, was not detected by the high-resolution neutron powder

diffractometer Not only confirming the R phase as the bulk structure, these authors

also concluded that the structural difference between the surface and the bulk are due

to inter-domain strains, leading to distorted R structure in the surface layer The origin

of the lower 2θ peak, however, remains unanswered

In addition, x-ray penetration depth in high lead content materials remains

controversial Normal x-ray diffraction with energy 8.048 keV performed on

PZN-8%PT sample has a penetration depth of 1 µm [16] Synchrotron x-ray diffraction

of higher energy, being 18 keV, revealed similar penetration depth in PZN-8%PT

sample [32] Meanwhile, using synchrotron x-ray diffraction at 10.7 keV, 32 keV, and

67 keV, the respective penetration depths are reported to be 13 µm, 59 µm, and 412 µm

of a PZN sample [57] This suggests a nonlinear relation between the x-ray energy and

the penetration depth despite similar test samples were used Whether the x-ray

diffraction patterns pertain to bulk structure or a mere surface layer remains

controversial although the general belief is that it reveals more the structure of the thin

surface layer

The experimental results pertaining to the origin of the broad lower 2θ peak

adjacent to the major (002)Rpeak and the effects of surface layer on x-ray diffraction

study of PZN-PT single crystals are presented and discussed in this chapter

5.2 Polished surface vs fractured surface at room temperature

5.2.1 Effect of polished surface on x-ray diffraction results

Typical (002) XRD profile of an as-polished sample of PZN-4.5%PT is given

in Figure 5.1(a) The XRD result, which relates to the surface layer of the as-polished

sample, revealed two (002) diffraction peaks: A major peak at 2θ ≈ 44.65º and a minor

but very broad peak in the range of 2θ ≈ 43.0-44.0º The major peak relates to the (002)

R phase, and will be represented by (002)R in our further discussion The origin of the

minor broad peak lying at the lower side of the major (002)R peak is still not well

understood at present

Since the x-ray used can only penetrate a thin surface layer (of <a few µm at

best) of the lead-based material [16, 63, 74], it is important to check if the observed

x-ray profiles pertain to the surface layer or if it is representative of the entire crystal

To check for this, the same crystal was fractured into two halves to reveal the bulk

phase, as described in Chapter 4 Section 4.2.2 In brief, two matching slots from the

opposite sides of the sample with the cut plane were made roughly parallel to the (010)

crystal plane were made to serve as stress concentrators and the sample was fractured

into two halves by means of an impact blow The fractured surfaces were again x-rayed

Figure 5.1(b) shows the XRD profiles taken from the fractured surfaces of the sample

Only a fine main (002)R peak at 2θ ≈ 44.62° remains and the lower 2θ peak is no

longer present These results strongly suggest that the lower 2θ peak shown in Figure

5.1(a) pertains to the surface phase The bulk phase revealed by the fracturing

technique, has a predominantly R structure

To ascertain the origin of the lower 2θ peak, the fractured surface of the

X-ray X-ray

2θ

42 43 44 45 46 47

b) Fractured surface

44.62o

b)

[001] T

[010] W

[100] L

PZN-4.5%PT

X-ray

2θ

42 43 44 45 46 47

a) As-polished surface

~43.0-44.0o

44.65o

a)

Figure 5.1 (002) XRD profiles of PZN-4.5%PT single crystal taken from

(a) polished surface and (b) fractured surface A broad lower

2θ peak adjacent to the major (002) R peak at 2θ ≈ 44.65° was

detected from the polished surface Only a fine main (002)R peak at 2θ ≈ 44.62° remains in fractured surface

PZN-4.5%PT sample was polished with SiC paper of increasingly coarse particle sizes

of 1-9 µm and the newly polished surface was again x-rayed Figure 5.2 shows that the

broad lower 2θ peak of both the samples reappeared after the polishing and that its

intensity increased with the particle size of the SiC paper used This experiment

confirms that mechanical polishing has induced the surface domain patterns

There are two plausible causes for the presence of the lower 2θ peak One

likely cause is that, since the samples have been mechanically polished, the lower 2θ

peak may reflect a plastically deformed, highly dislocated, surface layer produced by

the polishing Alternatively, the deformation process of the crystal may occur via

domain switching and structural twinning and transformation such that a lower

symmetry phase may be formed giving rise to the lower 2θ peak

To answer the above question, HR-XRD RSM was performed on both the

as-polished and fractured surfaces of a PZN-4.5%PT crystal Figure 5.3(a) shows the

(002) RSM taken from the fractured surface As expected, only the major (002) peak at

2θ ≈ 44.62º was detected, indicating that the bulk is in an R state Figure 5.3(b) shows

the RSM taken from the as-polished surface of the crystal It is evident from these

figures that other than the main (002)R at 2θ ≈ 44.68º, no other distinct diffraction

peaks can be concluded It should, however, be noted that instead of symmetrical

smearing of the intensity profile around the main (002)R peak, as typically observed

Figure 5.2 Same as Figure 5.1 but after the fractured surface of the

PZN-4.5%PT single crystal sample was polished with SiC papers of different particle sizes The inset gives the intensity of

the lower 2θ peak as a function of particle size of the polishing

medium, suggesting that diffraction arose from polished surface pertains to surface layer

(a) (b)

Figure 5.3 (a) (002) RSMs taken from the fractured surface of PZN-4.5%PT

showing only the main R (002) peak (b) Same as (a) but taken from the as-polished surface, showing the lower 2θ peak in the ω

= 0° plane arising from the spreading (or splitting) of the (002)R

diffraction out of the ω = 0° plane but toward lower 2θ values only The intensity contours are on log scale

in internally stressed solids, the RSM of the “deformed layer” shows consistently the

following: (1) The smearing is only towards lower 2θ values such that the lower 2θ

reflection appears more like a one-sided shoulder extending from the main (002)R peak,

its intensity typically decreasing continuously with decreasing 2θ value in the mapping;

(2) in addition to the extended spreading in the ω = 0° plane, the “shoulder” also

spreads out onto the ω ≠ 0° plane on both sides, a phenomenon resembling peak

splitting as observed in M symmetry except that no clear degeneracy peaks could be

detected in the mapping; (3) the lower 2θ peak observed in the ω = 0° plane (Figure

5.1a) corresponds to a possible saddle point in the mapping except that the said saddle

point remains not well-defined in the mapping in Figure 5.3(b) Lastly, should it

represent a new phase, then (4) there is a wide spread in its lattice constant and (5) the

deduced lattice dimensions normal to the surface of observation is very much larger

than the c axes (i.e., that having the largest lattice parameter) of any known phases of

PZN-PT, as judged from its Bragg’s position and the extreme broadness of the lower

2θ peak in the ω = 0° plane (Figure 5.1a)

Since we have ruled out the lower 2θ peak being arising from a plastically

deformed (or heavily dislocated) material, the above observations strongly indicate

that the lower 2θ peak must correspond to a “highly distorted R phase” owing to the

soft elastic constants of R The mechanical softness of R and hence large piezoelectric

coefficients is believed to be the underlying cause of the distorted R phase [41, 42]

These “highly distorted R phase” have their c axes (i.e., that having the largest lattice

parameter) lying out of the surface of observation and is placed in intense compression

in the plane of the surface such that the lattice dimension across the specimen

thickness is further elongated due to the Poisson’s ratio effect

The present work shows that the deformed surface layer was produced by the

mechanical polishing It has been known that surface grinding can lead to the 90o

switching of the ferroelastic [86] and ferroelectric [87, 88] tetragonal domains located

near the surface of the sample This effect was explained by the compressive residual

stress parallel to the sample surface generated by grinding In BaTiO3 this residual

stress was estimated (from x-ray diffraction) to be ≈300 MPa [88] Recently, it was

shown that a small <001>−directed compressive stress (<100 MPa) can induce the

transition from R to O phase in PZN-4.5%PT crystal [89]

The above observations confirm that: (a) the surface layer of R PZN-PT single

crystal can be easily deformed by mechanical polishing, and (b) deformation of R

relaxor single crystals with their c axes lying normal to the plane of the surface and are

placed in high in-plane compression Their microscopic sizes and the highly stressed

nature, together with a spread in their lattice parameters, all contribute to the extreme

broadness of the lower 2θ peak in the x-ray profile

Subsequent XRD (Figure 5.4) and HR-XRD (Figure 5.5) measurements were

also performed on as-polished (001)-cut unpoled PZN-7%PT, PZN-8%PT, PZN-9%PT,

and PZN-10.5%PT single crystals To expose the bulk material, the same samples were

later fractured into two halves and were again x-rayed as shown in Figures 5.4 and 5.5

Further evidence on mechanical polishing induced surface layer was shown in

PZN-7%PT sample Figure 5.6 shows that the broad lower 2θ peak of the PZN-7%PT

sample reappeared after the polishing and that its intensity increased with the particle

size of the SiC paper used Compared with the polished surface, the crystal bulk (as

revealed by the fractured surface) shows:

1) absence of the lower 2θ peak

2) narrow FWHM (Full-Width Half Maximum) for the R peak

3) minimum peak convolution when multiple peaks (or phases) are present

5.2.2 Effect of polished surface on polarized light microscopy results

Under polarized light, two different types of domain patterns were found to

exist in as-polished PZN-4.5%PT single crystal plates By varying the depth of focus

with the turning knob of PLM, the top layer was found to be about 20-40 µm thick,

which had elongated domains lying roughly along the [010]pc polishing direction

(Figure 5.7a) In contrast, the domains in the bulk of the crystals are relatively faint but

may also assume any of the following configurations: spindle-like, spearhead-like or

42 43 44 45 46 47

f) PZN-9%PT

Fractured surface

44.28o 44.94

o 44.50o

2θ

42 43 44 45 46 47

e) PZN-9%PT

As-polished surface

44.94o

~43.5-44.8o

2θ

42 43 44 45 46 47

2θ

42 43 44 45 46 47

c) PZN-8%PT

As-polished surface

44.65o

~42.50-44.20o

44.60o

d) PZN-8%PT Fractured surface

2θ

42 43 44 45 46 47

a) PZN-7%PT

As-polished surface

44.70o

~42.50-44.20o

2θ

42 43 44 45 46 47

b) PZN-7%PT

Fractured surface

44.64o

Figure 5.4 (a), (c) and (e) (002) XRD profiles taken from the as-polished

surface of PZN-7%PT, PZN-8%PT and PZN-9%PT, respectively (b), (d) and (f) same as (a), (c) and (e) but taken from the fractured surface

(a) PZN-8%PT

As-polished surface

(b) PZN-8%PT Fractured surface

(e) PZN-9%PT

As-polished surface

(f) PZN-9%PT Fractured surface

(c) PZN-10.5%PT

As-polished surface

(d) PZN-10.5%PT Fractured surface

Figure 5.5 (a), (c) and (e) (002) RSMs taken from the as-polished surface of

PZN-8%PT, PZN-9%PT and PZN-10.5%PT, respectively (b), (d) and (f) same as (a), (c) and (e) but taken from the fractured surface The intensity contours are on log scale