Abstract
This paper reviews the experimental studies of displacive phase transitions in solids. Primary emphasis is upon inelastic light scattering and neutron scattering; related infrared reflectivity measurements, as well as x-ray and EPR analyses are also summarized. Several prototype structures are considered in detail: (1) the rocksalt IV-VI semiconductors PbTe, SnTe, and GeTe; (2) the ferroelectric perovskites exemplified by PbTi and BaTi; (3) perovskites which exhibit cell-doubling transitions, such as LaAl, SrTi, and KMn; (4) crystals having the -quartz structure, including Ge, Si, and AlP; (5) the "improper ferroelectrics" and ; (6) the V-VI-VII semiconductors typified by SbSI; (7) the hydrogen-bonded ferroelectrics of the KP family; (8) Jahn-Teller systems such as DyV and RbCo, in which structural distortions occur as secondary effects; (9) order-disorder systems such as NaN and the ammonium halides (NCl, NBr), in which no "soft mode" occurs in the spectral region ( Hz) probed by ir, Raman, and neutron spectroscopy; (10) -tungsten () structures such as Si and Sn, which exhibit high-temperature superconductivity. These crystal categories are used to illustrate several phenomena of current physical interest: Specifically, we discuss harmonic and anharmonic mode coupling; "critical exponents" differing from one-half in the temperature dependences of the order parameter and of the soft-mode frequency ; and the recently discovered "central" modes centered at zero frequency, which grow in intensity as the transition temperature is approached from above or below. The review covers the period 1940-1972. A few 1973 works are mentioned for which the author had preprints in 1972 or very early 1973. This review is in no sense a comprehensive survey of ferroelectricity. Readers are referred to the following earlier reviews on that subject: Silverman (1966, 1969), Cochran and Cowley (1967), Blinc (1968), Murzin et al. (1968), Nettleton (1970), and Blinc and Zaks (1972).
DOI:https://doi.org/10.1103/RevModPhys.46.83
©1974 American Physical Society