Polarized Light Microscopy (PLM) of Ferroelectric and Ferroelastic Domains in Transmitted and Reflected Light

Abstract

The importance of polarized light microscopy (PLM) for the study of the domains and phase transitions of fe rroelectric a nd ferroelastic single crystals is pin-pointed and the e xperimental set-ups for observations and me as ure ments between 4 and 1600K are outlined. The contrast formation between domains by spontaneous birefringence and reciprocal rotatory power in transmission. and by bireOectance. specular reflection and e tchin g in re flec ted light are co ns ide re d. A s ymme try classificati o n for th e birefringe nce and rotatory power contras t formation is proposed. The use of PLM for ceramic materials e ncounters difficulties, whic h can be overcome hy e nhancing grain g rowth and re ducing sources o f light scattering. So me e xamples are give n, demonstratin g the great powe r of PLM , if used in conjunction with other physical measurements. ~'"""1 IntroductionFerroe lectrics and other materials with phase transiti ons split up into "domains" (Brezina et al. , 1989) upon passage to the low s ymmetry phase. The ir identificati on and the s tudy of their optical properties by polarized light mic rosc opy (PLM) is the main subject o f this contribution. The resolution o f a light microscope is in the mos t favourable case of the order of half a wavelength of light, whe re as with a high resolution e lectron microscope o ne c an see ranges of atom s, incommensurabilities and many mo re Enc de tail, including do mains and domain wa lls.Info rm ati on on the bulk and anisotropy properties may, however, getlosL Fortunately there exists a resolution ran ge, spanning from about half a microme ter to the resolution of the bare human eye (about l OO~m ) , which is most brilliantly covered for anisotropic crystals by the polarizing li ght microscope, permitting great pene tration depths in the trans mission regi me, unequaled in e lectron mic roscopy. For PLM s tudies the availability o f single crystals is o f utmos t importance.

Large electrostrictive actuation of barium titanate single crystals

“…The most important e ect for obtaining contrast between 90 • domains is the spontaneous linear birefringence of the material (Schmid, 1993). When a barium titanate crystal with (0 0 1) or out-of-plane spontaneous polarization is placed between the cross polarizers and illuminated in transmission as shown on the right side of Fig.…”

“…where I 0 is the incident intensity, n and n are the principal refractive indices (or n is the ordinary refractive index and n is the extraordinary refractive index), Â is the angle between the plane of vibration of the polarizer and the direction of n , is the wavelength of the light, d is the thickness of the in-plane polarized domain, and n − n is the birefringence (Schmid, 1993). The value of birefringence for barium titanate at room temperature is −0:073.…”

The observation of ferroelastic/ferroelectric domains in synthetic Mn3B7O13X single‐crystal boracites by light and electron microscopy

“…However, it was the vapour transport method of Schmid (1965) which produced most of the halogen boracites in single-crystal form and it is still the most successful technique for growing single crystals of good quality and size for physical characterization. The family of halogen boracites as now known was recently completed with the synthesis of Cu-I (Berset et al, 1985) With the exception of Cr-Br and Co-I boracites, which remain cubic down to low temperatures, all other halogen boracites transform from a cubic high-temperature phase (point group 43m) to one or to a sequence of 'fully ferroelectric/fully ferroelastic' phases with point groups mm2, m and 3m (Toledano et al, 1985;Schmid, 1992). Phase transitions accompanied by a change in the pointgroup symmetry are generally called ferroic.…”

“…Under this scheme, by 'ferroelastic' we mean a crystal which has two, or more, domains differing in spontaneous strain and which can be transformed from one to another of these states by an external mechanical stress (Salje, 1993). For full nomenclature see, for example, Aizu (1970) and Schmid (1992).…”

“…A certain number of techniques has been devised (for a review of known methods see, for example, Kahmann et al, 1992) for observing the domain structure of ferroelectric crystals, each of which offers advantages and disadvantages, depending on the ferroelectric material in hand. In the case of most boracites, for example, which undergo a phase transition from a high-temperature cubic phase (optically isotropic) to a low-temperature orthorhombic, tetragonal, monoclinic and/or trigonal (all optically anisotropic) phase, their domain structure has been traditionally studied by polarized-light microscopy (PLM) in both transmitted and reflected mode (Schmid, 1992). It should be remarked that this crystal family is considered as 'fully ferroelectric/fully ferroelastic' which means, among other things, that birefringence contrast is allowed between all the possible domains states (Aizu, 1970;Schmid, 1992).…”