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

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

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

4. 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 0,35 - 20 347 0.66 506  10 -18 640  10 -18 Hg 2 Br 2 a=4,640 b=11,100 0,42 - 30 282 0.85 991  10 -18 1804  10 -18 Hg 2 I 2 a=4,920 b=11,610 0,55 - 40 254 1.48 2246  10 -18 4284  10 -18 Hg 2 Hal 2 property

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

6. 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

7. Model of Phase transition g g SoftMode

8. SM at approaching T c

9. Brillouin Zone

10. Experimental technique

11. 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).

12. 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. 050100150200 * 4 3 2 1 x20 Hg 2 I 2 Intensity x10 4 3 2 1 Hg 2 Br 2, cm

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

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

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

16. Domain Structure

17. Concentration dependence of frequency and intensity

18. X-ray analysis Brillouin Zone

19. X-ray experiment

20. Reciprocal Lattice

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

23. Diffuse maxima Hg 2 Br 2 Hg 2 I 2

24. Temperature dependence of integral intensity

25. Halfwidth temperature dependence  =2/  Correlation radius

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

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

28. Temperature dependence of susceptibility in Hg 2 I 2

29. 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.