Nanoclusters in model ferroelastics Hg 2 Hal 2 E.M.Roginskii A.F.Ioffe Physical-Technical Institute, Russia

Outline 1.Introduction 2.Model of Phase Transitions 3.Raman scattering investigations 4.X-Ray analysis 5.Conclusion

Hg 2 Hal 2 (Hal=Br,Cl,I) unit cell

Table of basic physical properties Table1. Physical properties of univalent mercury halides Lattice constants, Å Transparency spectral range,  m Transverse (TA) sound velocity along [110] polar. on, m/sec. Birefringence  n( =6328 Å) Acousto-optical coefficient M 2, (CGS) for [100]/[100] and Hg 2 Cl 2 a=4,480 b=10,910 0,   Hg 2 Br 2 a=4,640 b=11,100 0,   Hg 2 I 2 a=4,920 b=11,610 0,   Hg 2 Hal 2 property

Comparison with physical characteristics of often used materials Acousto-optical coefficient for longitudinal wave Sound velocity

Ferroelastic Phase transition Hg 2 Cl 2 T c =186K Hg 2 Br 2 T c =144K Hg 2 I 2 incipient PT P c =9Kbarr

Model of Phase transition g g SoftMode

SM at approaching T c

Brillouin Zone

Experimental technique

Eigen vectors of vibrations in Hg 2 Hal 2 crystals. R refers to Raman-active vibrations, and IR, to vibrations active in infrared absorption (reflection).

Raman spectra of Hg 2 I 2 and Hg 2 Br 2 single crystals taken at room temperature. Dashed lines correspond to XZ(YZ) polarization, and solid lines, to ZZ polarization. Star denotes the 1 overtone * x20 Hg 2 I 2 Intensity x Hg 2 Br 2, cm

Low-Temperature (10K) low frequency Raman Spectra for Hg 2 (Br 1-x I x ) 2 Hg 2 Br 2 Hg 2 I 2

Low-Temperature (10K) high frequency Raman Spectra for Hg 2 (Br 1-x I x ) 2 Hg 2 Br 2 Hg 2 I 2

Soft Mode Raman Spectra (X  ) T c =144 T c =100

Domain Structure

Concentration dependence of frequency and intensity

X-ray analysis Brillouin Zone

X-ray experiment

Reciprocal Lattice

Typical scans for Hg 2 Br 2 and Hg 2 I 2 crystals

Diffuse maxima Hg 2 Br 2 Hg 2 I 2

Temperature dependence of integral intensity

Halfwidth temperature dependence  =2/  Correlation radius

log-log scale Hg 2 Br 2 A~    ~   ~   =(T-T c )/T c – reduce temperature

log-log scale Hg 2 I 2 A~    ~   ~   =(T-T c )/T c – reduce temperature

Temperature dependence of susceptibility in Hg 2 I 2

Conclusion The Effects of the phase transition such as nucleation of low- temperature phase clusters in high-temperature tetragonal matrix and soft mode appearance in ferrophase was observed.The Effects of the phase transition such as nucleation of low- temperature phase clusters in high-temperature tetragonal matrix and soft mode appearance in ferrophase was observed. Appearance of “ferroelectric” and “antiferroelectric” nanoclusters in mixed crystals Hg 2 (Br,I) 2 was investigated. Their appearance induced by Hg 2 (BrI) 2 – mixed molecules existing in these compounds.Appearance of “ferroelectric” and “antiferroelectric” nanoclusters in mixed crystals Hg 2 (Br,I) 2 was investigated. Their appearance induced by Hg 2 (BrI) 2 – mixed molecules existing in these compounds. Anisotropic diffuse X-ray scattering maxima associated with order-parameter fluctuations and nucleation of low-temperature orthorhombic clusters in the high-temperature tetragonal matrix have been found to exist at X-points.Anisotropic diffuse X-ray scattering maxima associated with order-parameter fluctuations and nucleation of low-temperature orthorhombic clusters in the high-temperature tetragonal matrix have been found to exist at X-points. New information has been obtained on the temperature dependence of the susceptibility and correlation length, cluster size shape and anisotropy, and the critical exponents.New information has been obtained on the temperature dependence of the susceptibility and correlation length, cluster size shape and anisotropy, and the critical exponents.